Symposium EN06—Development in Catalytic Materials for Sustainable Energy—Bridging the Homogeneous/Heterogeneous Divide

Immobilized molecular complexes are an atomically precise platform on which to systematically interrogate and design electrocatalysts. This is a particularly exciting prospect because in electrocatalysis, the interplay between electrode, solvent, electrolyte, and reactants is often complex, and well-defined active sites could be leveraged to clarify some of that complexity. Cobalt tetrapyrroles, such as cobalt phthalocyanine (CoPc), are well known to be active and selective for the reduction of carbon dioxide (CO₂) into carbon monoxide (CO) when immobilized on carbonaceous supports. To date, many studies of CoPc have focused on achieving performance, and indeed, current densities upward of 100 mA/cm² with near unity Faradaic efficiencies to CO have been reported. However, in order to take deeper lessons from this successful catalyst, and use them to guide rational design of future catalysts, it is important to understand intrinsic catalytic properties of CoPc. Towards this more fundamental goal, we measured the influence of bicarbonate buffer concentration on CO₂ reduction to CO at well-dispersed, immobilized CoPc. We found that the apparent bicarbonate order dependence shifted from negative to positive as the applied potential became more reductive. Tafel slopes were also found to vary depending on bicarbonate concentration. Coupled reaction-diffusion models suggested that transport limitations do not account for these observations. We propose a mechanism in which CoPc is deactivated either via coordination to bicarbonate anions or partial reduction, which is able to explain the measured bicarbonate order dependencies and Tafel data. This work provides evidence that the rate determining step in the CoPc catalytic cycle is in fact an initial proton coupled electron transfer and that the presence of various “spectating” ionic species affect intrinsic activity. Such a complete set of mechanistic measurements on immobilized CoPc has not been reported before, and these surprising results have important fundamental implications for elucidating reaction mechanism and practical implications for designing and evaluating high-performance devices.

To discover new alloy catalysts that might dramatically improve the efficiency of the Haber–Bosch (HB) process for ammonia synthesis from hydrogen and nitrogen, we employed quantum mechanics (QM) to determine the reaction mechanism and free energy reaction barriers for the two most highly reactive Fe surfaces (Fe(211)R and Fe(111)) under experimental single crystal reaction conditions (400°C and 20 atm). We determined the rates for all 10 important surface reactions on both surfaces. Then we used this information in a full kinetic Monte Carlo (kMC) simulation (60 minutes) to determine the kinetics. We predict that the Turn Over Frequency (TOF) for Fe(111) surface is TOF =18.7 s⁻¹ per 2 × 2 surface site for 1.5 Torr NH₃ pressure in excellent agreement with experiment (9.7 NH₃/sec for (111) and 7.3 NH₃/sec for (211)R) which is consistent with experiment (~1:0.75). The excellent agreement between theory and experiment on two very different surfaces and reaction mechanisms is a testament to the completeness and accuracy of our QM and kMC modelling. To dramatically improve the efficiency of HB reactions on these highly reactive surfaces, we applied hierarchical high throughput screening approach to sift the promising doping elements on these two highly reactive surfaces. We found several promising dopants that can significantly improve the TOF.

We report an experimental-computational study of mechanistic reaction pathways during the electrochemical reduction of CO₂ to CH₄, catalyzed by solid-state, single-site Fe-N-C catalysts. Fe-N-C catalysts feature molecularly dispersed catalytically active Fe-N motifs and represent a type of non-Cu-based catalysts that yield “beyond CO” hydrocarbon products. The various multi-step mechanistic pathways toward hydrocarbons with these catalysts has never been studied before and is the focus of this study. A number of different reactant molecules with varying formal carbon redox states, more specifically CO₂, CO, CH₂O, CH₃OH and formate were electrochemically converted at the Fe-N sites, yet only CO₂, CO and CH₂O could be protonated into methane. Also, we observed a distinctly different pH dependence of the catalytic CH₄ evolution from CO and CH₂O, suggesting differences in the proton participation of rate determining steps. In comparing the experimental observations with Density Functional Theory (DFT) -derived Free Energy Diagrams of reactive intermediates along the reaction coordinates, we unraveled the distinctly different dominant mechanistic pathways and roles of CO and CH₂O along the catalytic CO₂-to-CH₄ cascade and their rate-determine-steps (RDS). We close with the first comprehensive reaction network of the CO₂ electroreduction on a M-N-C catalyst. Our findings offer valuable insights in the catalysis of the CO₂RR on single site Fe-N-C catalysts that may prove useful in developing efficient, non-Cu-based catalysts for direct electrochemical hydrocarbons production.

I present the design and synthesis of highly active TiO₂ photocatalysts incorporated with site-specific single copper atoms (Cu/TiO₂) that exhibit reversible and cooperative photoactivation process.¹ I believe that this study represents one of the important breakthroughs in atomically defined photocatalysts that share the similar mechanism with metalloenzymes. The reversible and cooperative activation process is one of the most important characteristics of biological enzymes, and has been frequently used in designing homogeneous catalysts. However, there is nearly no report on industrially important heterogeneous catalysts with these enzyme-like characteristics. Herein we designed single atom catalysts (SACs) with well-dispersed copper atoms exclusively occupied in the most stable Ti vacancies in the hollow TiO₂ nanoparticles, and identified atomic-level reversible and cooperative photoactivation process during dynamic photocatalysis. Furthermore, we demonstrate that in single-atom Cu/TiO₂, the dynamic cooperation between site-exclusive Cu atoms and TiO₂ can trigger the unprecedented reversible photo-activation process that includes valence state change of Cu atom and reversible modulation of surface electronic structure of the local TiO₂ environment. This unique photo-activation process has led to the extremely strong light absorption in the whole visible range and resulting highly enhanced H₂ generation activity that far exceeds state-of-the-art TiO₂-based photocatalysts incorporated with earth-abundant transition metal-based cocatalysts. This first demonstration of heterogeneous catalysts with the enzyme-like characteristics is a monumental discovery in realizing the reversible electronic structure modulation in heterogeneous catalysts. I will also report various catalytic applications of SACs based on nanomaterials of CeO₂, TiO₂, and graphene oxide.
1. “Reversible and cooperative photoactivation of single-atom Cu/TiO₂ photocatalysts,” Nature Mater. 2019, 18, 620.

Supported metallic nanoclusters (NCs, < 2 nm) are of great interests in various catalytic reactions with enhanced activities and selectivity, yet it is still challenging to efficiently and controllably synthesize ultra-small NCs with a high-dispersal density. Herein, we report the in situ synthesis of surfactant-free, ultra-small, and uniform NCs via a rapid thermal shock on defective substrates. This is achieved by using high temperature synthesis with extremely fast kinetics while limiting the synthesis time down to milliseconds (e.g. ~1800 K for 55 ms) to avoid aggregation. Through defect engineering and optimized loading, the particle size can be robustly tuned from > 50 nm particles to < 1 nm uniform NCs with a high-dispersal density. We can also achieve high-density single atom dispersion using a similar high-temperature method. We demonstrate that the ultra-small NCs exhibit drastically improved activities for catalytic CO oxidation as compared to their nanoparticulated counterparts. In addition, the reported method shows generality in synthesizing most metallic NCs (e.g. Pt, Ru, Ir, Ni) in an extremely facile and efficient manner. The ultrafast and robust synthesis of uniform, high-density, and size-controllable NCs paves the way for the utilization and nanomanufacturing of NCs for a range of catalytic reactions.

Atomically dispersed clusters comprising single and pairs of atoms are gaining increasing attention for heterogeneous catalysis as they are structurally different from bulk metal and metal nanoparticles. The different catalytic activity stems from both surface effects, where the number of unsaturated bonds increase with decreasing cluster size, and electronic effects that result in discrete energy levels of the electrons thus enabling greater orbital overlap with substrate molecules. In recent years, the structure and catalytic activity of single atom catalysts has been extended to selectively investigate the stability of dimers on different supports by wet chemical deposition.^1,2 In this study, we present that dinuclear rhodium complexes could be well-dispersed on layered titanate supports (H_0.13Ti_0.87O₂ and H₂Ti₄O₉ nanosheets). Nanosheets derived from layered metal oxides represent a unique class of catalyst supports as they can be imaged by electron microscopy and were previously shown to resist sintering of late transition metal oxide nanoparticles.³ The stability of dinuclear clusters on layered titanate supports are elucidated by a combination of aberration-corrected transmission electron microscopy, DRIFT spectroscopy, and X-ray absorption (EXAFS and XANES) spectroscopy.
References
1. Zhao, Y.; Yang, K. R.; Wang, Z.; Yan, X.; Cao, S.; Ye, Y.; Dong, Q.; Zhang, X.; Thorne, J. E.; Jin, L.; et al. Stable Iridium Dinuclear Heterogeneous Catalysts Supported on Metal-Oxide Substrate for Solar Water Oxidation. Proc. Natl. Acad. Sci. 2018, 115 (12), 2902–2907.
2. Tian, S.; Fu, Q.; Chen, W.; Feng, Q.; Chen, Z.; Zhang, J.; Cheong, W.-C.; Yu, R.; Gu, L.; Dong, J.; et al. Carbon Nitride Supported Fe₂ Cluster Catalysts with Superior Performance for Alkene Epoxidation. Nat. Commun. 2018, 9 (1), 2353.
3. Strayer, M. E.; Binz, J. M.; Tanase, M.; Shahri, S. M. K.; Sharma, R.; Rioux, R. M.; Mallouk, T. E. J. Am. Chem. Soc. 2014, 136, 5687-96.

Single metal atoms anchored on N doped graphene (M-N4-G) have attracted extensive interest as catalysts for carbon dioxide reduction. Although DFT calculations based on computational hydrogen electrode mode have been performed to understand the atomistic mechanisms, several important questions remain unexplained. For example, the DFT calculations show that the formation free energy of *COOH, a critical intermediate during CO2R, is rather high (> 1 eV), contradicting with the experimental report of high activity. Here, using ab initio molecular dynamics (AIMD) simulations with explicit water molecules and surface charge (which are neglected in conventional DFT calculations), we are able to elucidate the atomistic mechanisms, and explain the difference in catalytic activity of different metal atoms

The current rate of energy consumption has shifted the balance of environment especially owing to the exponentially increasing concentration of carbon dioxide. Reduction of carbon dioxide into fuels or useful chemicals is probed as a new way to clean the air, lessen the release of carbon dioxide in the atmosphere (especially from point sources) and store renewable energy like electricity as chemical energy. Different approaches have been used to reduce carbon dioxide, like electrochemical, photochemical, thermochemical, biological or combination of any of these approaches. The electrochemical route is attractive given many renewable energies are stored in electric form and hence are readily available. Copper metal has been found unique pertaining to its ability to reduce carbon dioxide into hydrocarbons which can be used as fuels or industrial feedstock. Given the affinity of MOFs to absorb gases like carbon dioxide many groups developed different MOFs to study ERC ( electrochemical reduction of carbon dioxide). Copper variants of MOFs have shown promising activity for ERC.
Taking cues from the gas absorbing capability of MOFs and promising performance of copper towards ERC, we synthesized different copper MOFs like CuBDC (copper benzyl-di-carboxylic acid) and CuBTC( copper benzyl-tri-carboxylic acid) to study ERC. The materials were characterized by XRD, FESEM, TEM, IR, Raman, XPS and TGA. All analysis confirmed the formation of impurity-free successful synthesis of CuMOFs. We found that both MOFs are active for ERC but CuBDC is more selective for hydrocarbons than CuBTC. Further, it was found and thoroughly analyzed that the active site for ERC is actually ' in situ ' formed metastable copper and copper (I) oxides which are formed over negative potentials applied. structural changes were also found on the application of negative potentials, metal and metal oxides were embossed in the carbon backbone. Earlier reports suggest that MOFs are proven to convert CO₂ into alcohols while we found they convert CO₂ considerably into ethylene and some methane.
Recent studies have proved that associated organic groups with metal catalysts as ligands or stabilizers are also involved in the alteration of catalytic activity of the materials, especially in nano-domain. We altered the free -COOH groups in terephthalate groups of an organic framework with amine groups to check the effect of functional groups on ERC associated with the metal centers. It was found that -NH₂ groups aid in catalyzing the ERC with greater selectivity towards hydrocarbons and higher cumulative faradaic efficiencies towards ERC. Also, the stability of the catalyst increased with keeping hydrogen evolution limited under 30%.

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is the most studied reactions for clean energy devices for energy conversion devices such as fuel cells as result of slow kinetics. Transmission metal oxides (TMO) such as Co₃O₄, Fe₃O₄, and MnO_x [1] have been extensively reasearched as way to replace platinum and its alloys for both sustability and cost effectiveness. To combat the intrinsic low conductivity of TMOs, they are often incorporated into a highly conductive carbon nanostructure such as metal organic frameworks (MOFs). MOFs have garnered wide attention for their ability to efficiently and effectively improve adsorption of reactants, stability, and conductivity,

In this study a bimetallic MOF was synthesized via a solvothermal method and pyrolysis using both cerium and cobalt nitrate salts with a ligand called 1,3,5-Benezenetricarboxylic acid to create MOF-Co-Ce with extreme surface area and excellent electronic conductivity. Though this structure is conductive, this can be enhanced with the functionalization of cerium particles. Specifically, the Ce(IV) complexes has shown superior rates of phosphonate ester bond cleavage compared to other metal oxides. This unprecedented activity of the Ce(IV) complex has been attributed to it’s 4f orbital being able to “mix” with the orbitals of the P=O bond to form hybrid orbitals[2]. To functionalize cerium, phosphoric acid is employed to induce the binding of phosphate groups onto cerium oxide. This gives rise to the pentacoordinate intermediate being susceptible to a nucleophilic attack [2]. The functionalized MOF-Co-Ce is subjected to the solvothermal method at 160°C for 24 h with the precursor P25, thus creating MOF-Co-Ce with TiO₂ attached (MOF-Co-Ce-Ti).

Results have shown an enhanced performance in terms of current density and onset/halfwave potential for both CV and LSV data as result of phosphate groups absorbing cerium nanoparticles. FTIR data indicates the absorption of 1040 cm^-1 (phosphate group peak) appeared after MOF-Ce-Co treatment of phosphoric acid. X-ray analysis indicated that both samples (MOF-Co-Ce and MOF-Co-Ce-Ti) had cerium and cobalt particles, with MOF-Co-Ce-Ti having an additional peak at 56°. Thus, indicating the presnece of TiO₂ (anatase). Therefore, the improved performance of the treated MOF is the result of its functionalizationl

This project was funded by NASA Advanced STEM Training and Research (ASTAR) Fellowship.

References:

[1] G. Wu and P. Zelenay, “Nanostructured nonprecious metal catalysts for oxygen reduction reaction.,” Acc. Chem. Res., vol. 46, no. 8, pp. 1878– 89, Aug. 2013.

[2] T. Islamoglu, A. Atilgan, S. Moon, G. Peterson, J. DeCoste, M. Hall, J. Hupp, and O. Farha, "Cerium(IV) vs Zirconium(IV) Based Metal Organic Framwork for Detoxification of a Nerve Agent." Chemistry of Materials.

Confinement of metal centres is a powerful tool to manipulate reactivity of catalytic centres and tune selectivity in chemical transformations. While aluminum as a foil is inactive for CO₂ reduction and shows high selectivity for hydrogen evolution reaction, here we show that aluminum centres confined in a metal organic framework, MIL-53(Al), suppress hydrogen evolution reaction activity and interestingly enhance carbon dioxide reduction. Aluminum confined in MIL-53 can electrochemically reduce CO₂ to both carbon monoxide and formic acid, as valuable carbon-based products. Control experiments support that the catalytic activity is related to the presence of aluminium and that the confinement effect promoted by the organic ligands is key to obtaining carbon dioxide reduction behaviour. The observation of Al⁰ after electrochemical CO₂ reduction supports that the active form of the catalyst may involve the presence of aluminium in this oxidation state. This aluminum MOFs can produce up to 40% faradaic efficiency for carbon monoxide and formic acid, and with a turnover frequency of 174 h^-1. This study demonstrates that the confined reaction space enables changes in reaction selectivity and can impart atypical catalytic capabilities to metals.

A frontier challenge in the materials chemistry of heterogeneous catalysis is to obtain designed, stable arrays of desired catalysts that are uniform in structure and composition. Catalyst organization in high-density, yet reactant accessible, form enables catalysts to out-perform lower density structures simply because rates of chemical transformations generally scale as the quantity of accessible catalyst employed. Thus, heterogeneuous electrocatalysts that are perhaps marginal in sub-monolayer amounts can yield kinetic overpotentials that are hundreds of millivolts smaller when the same catalysts are presented as hundreds or thousands of redox-addressable, and reactant- and product-permeable monolayers at electrode/solution interfaces.^1,2 Related arguments are relevant to heterogeneous catalysis of gas phase chemical reactions.

Uniformity of catalyst structure and composition, while standard for molecular catalysts, can be difficult to achieve -- or, once achieved, difficult to sustain -- for non-molecular, heterogeneous catalysts. To the extent that uniformity can be achieved, however, hypothesis-driven approaches to advancing Catalysis Science can be pursued. For heterogeneous catalysis, pertinent hypotheses may include the dependence of catalyst activity, selectivity, and long-term stability on, for example: a) catalyst-catalyst site-isolation versus reactant accessibility, b) the nature of the environment beyond the catalyst active-site,³ c) cluster-type-catalyst metal nuclearity, d) the nature and extent of catalyst/support interactions, e) the chemical basis for conversion of synthesized precatalysts to functional catalysts, and f) the ability of the surrounding infrastructure, for example the porous MOF, to supply or sustain essential ancillary properties or components such as local pH, reaction-space confinement, suitably reactive oxidizing or reducing equivalents, charge-compensating ions, or reactive photo-generated molecular excitons.

This presentation will focus on synthesis, characterization, and chemical/photochemical/electrochemical application of cluster-based catalysts that are constructed in automated, atom-by-atom fashion and supported on crystallographically well-defined metal-oxide nodes within high porosity, metal-organic frameworks.^4,5 By way of illustration, two or three case studies will be presented.

1) “Metal–Organic Frameworks as Platform Materials for Solar Fuels Catalysis.” Majewski, et al., ACS Energy Lett. 2018, 3 (3), 598-611
2) “Fe-Porphyrin Based MOF Films as High-Surface-Concentration, Heterogeneous Catalysts for Electrochemical Reduction of CO₂,” Hod, et al., ACS Catal. 2015, 5 (11), 6302-6309
3) “Beyond the Active Site: Tuning the Activity and Selectivity of a Metal–Organic Framework-Supported Ni Catalyst for Ethylene Dimerization,” Liu, et al., J. Am. Chem. Soc. 2018, 140 (36), 11174-11178
4) “Redox-Mediator-Assisted Electrocatalytic Hydrogen Evolution from Water by a Molybdenum Sulfide-Functionalized Metal–Organic Framework,” Noh, et al., ACS Catal. 2018, 8 (10), 9848-9858
5) “Metal–Organic Framework Supported Cobalt Catalysts for the Oxidative Dehydrogenation of Propane at Low Temperature.” Li, et al., ACS Cent Sci 2017, 3 (1), 31-38.

Novel catalyst designs aiming at the development of energy-efficient, low-cost and sustainable processes are of great interest for applications to fuels and chemicals production, and to environmental pollution abatement. Identification of the active catalytic site and design of catalysts with 100% atom efficiency has been a long-standing goal in heterogeneous catalysis. For catalytic metals, atomic dispersion affords better utilization, different (often better) selectivity than the extended metal, and new prospects for low-cost and green process development. It is crucial to understand the mechanisms behind catalysis by supported single metal atoms, as this will guide new, improved catalyst designs.
A new class of single-atom heterogeneous catalysts will be presented, namely Single-Atom Alloys that comprises catalytically active elements like Pt, Pd and Ni alloyed in a more inert host metal like Cu, Au or Ag at the single-atom limit. Single-atom alloys offer a unique approach towards rational catalyst design, one that combines surface science, catalysis and theory in a most efficient way. Model surfaces and nanoparticles that can host isolated atoms in the surface layers behave similarly in escaping the linear scaling relationships and allowing for the rational fine-tuning of activity and selectivity. Good stability is imparted by the strong metal-metal bonds between the host and the minority metal, and atomic dispersion can be maintained at high temperatures. Resistance to CO poisoning and to coking are additional advantages of these promising materials, as will be shown in the presentation drawing examples from alkyne and alkadiene hydrogenation and alkane dehydrogenation. Novel synthesis methods and the stability of single-atom metal catalysts in various supports and reaction environments will be discussed.

Numerous global challenges, such as the development of environmentally friendly energy and fuel technologies, can be addressed by the development of advanced heterogeneous catalysts. Increasingly often, such catalysts are developed by designing metal-oxide interactions that can boost the catalytic activity by several orders of magnitude. However, such interactions are notoriously challenging to characterize experimentally, which puts electronic structure simulations to the forefront of research on complex nanostructured catalysts.

In this contribution, first I will discuss pioneering density functional simulations of Pd and/or Pt nanocrystallites up to 1.5 nm in size supported on MgO(100), cubic ZrO₂(111) and/or CeO₂(111) supports. The electron transfer from Pt nanoparticles to reducible CeO₂(111) support was quantified by experiments and simulations, which showed that it can reach up to 0.1 electrons per Pt atom and significantly change the reactivity of Pt particles.[1] In turn, the effect of irreducible oxide supports on the nanoparticle reactivity is more complicated. On the one hand, MgO(100) support was calculated to insignificantly affect H adsorption and absorption in supported Pd and Pt nanoparticles.[2] On the other hand, perimeters sites on MgO(100) and ZrO₂(111) interfaces with Pd particles were calculated to have increased resistance to CO poisoning, which was observed to significantly increase overall CO tolerance of μm-large Pd aggregates in societally important CO oxidation reaction.[3]

At the same time, interactions with metallic supports also affect the structure and the catalytic properties of the oxides. For example, indium oxide films on Co supports are calculated to contain more O vacancies, which are active sites for CO₂ reduction, and have higher methanol selectivity compared to pristine indium oxide surfaces, which is also shown experimentally.[4] In another study, we show how MoO_xH_y clusters interacting with Ni support adopt the structure and the degree of oxidation that makes them highly active in electrochemical hydrogen evolution reaction (HER) at neutral pH, which is in line with the experimental characterization of Ni-Mo catalysts.[5]

The discussed studies shed light on the electronic aspects of metal-oxide interactions and offer strategies for using such interactions to improve the activity and the selectivity of catalysts.

[1] Lykhach, Kozlov, Skála, Tovt, Stetsovych, Tsud, Dvorák, Johánek, Neitzel, Mysliveček, Fabris, Matolín, Neyman, Libuda, Counting Electrons on Supported Nanoparticles, Nat. Mater. 2016, 15, 284
[2] Kozlov, Aleksandrov, Neyman, Energetic Stability of Absorbed H in Pd and Pt Nanoparticles in a More Realistic Environment, J. Phys. Chem. C 2015, 119, 5180
[3] Suchorski, Kozlov, Bespalov, Datler, Vogel, Budinska, Neyman, Rupprechter, The role of metal-oxide interfaces for long range metal particle activation during CO oxidation, Nat. Mater. 2018, 17, 519
[4] Bavykina, Yarulina, Al Abdulghani, Gevers, Hedhili, Miao, Ramírez Galilea, Pustovarenko, Dikhtiarenko, Cadiau, Kozlov, Oud-Chikh, Cavallo, Gascon, Turning a methanation Co catalyst into an In-Co methanol producer, under revision
[5] Bau, Kozlov, Azofra, Oud-Chikh, Cavallo, Takanabe, Mo3+ active sites for Hydrogen Evolution Reaction on NiMo-based catalysts, in preparation

The electrochemical CO₂ reduction reaction (CO2RR) valorizes CO₂ directly into chemicals and fuels. In addition to energy efficiency and selectivity challenges, the supply of CO₂ in sufficiently high purity is of great concern. Industrial CO₂ feeds are often contaminated with CO, which calls for an investigation of the catalytic CO2RR under CO₂/CO co-feed (“dirty CO₂”) conditions. In this talk, we present new mechanistic insights into the electroreduction of external (mixed) CO₂/CO co-feeds on Cu-based nanocatalysts, which exhibits significantly promoted ethylene production rate over a wide range of electrode potentials and co-feeding ratios. Kinetic isotope labelling experiments using ¹²CO₂ and ¹³CO – performed in a novel operando differential electrochemical mass spectrometry (DEMS) capillary flow cell with milli-second time-resolution – showed that in both cathodic and anodic scan direction the enhanced C₂H₄ production originated to almost 50% from the C-C coupling of a ¹²CO with a ¹³CO molecule, that is from a cross-coupled “¹²CO₂-¹³CO” reactive pathway, and to a lower degree from the ¹²CO₂-¹²CO₂ or ¹³CO -¹³CO pathways. These transit results with DEMS setup identify the stationary results with gas chromatography and further evidence the absence of a kinetic competition between CO₂ and CO molecules for surface adsorption sites, implying distinct surface sites for CO₂ and CO adsorption/reduction and suppressed surface adsorbate scrambling. Based on these fundametnal mechanistic insights, the practical significance of sustained local interfacial CO partial pressures under local CO₂ depletion is then demonstrated using the family of non-metallic/metallic and metallic/metallic tandem CO2RR catalysts with improved and stable ethylene yields. Together, our findings uncover the mechanistic origin of improved C-C product selectivities under co-feeding, but also highlight technological opportunities of impure CO₂/CO process feeds for use in the electrochemical CO₂ reduction process.

Strategies to enhance catalytic activity are manifold and often rely on exploiting features of seemingly disjoint system. Examples are electrochemistry based catalytic approaches (e.g. electro-catalysis or photo-catalytic electrochemistry processes), nano-catalyst, supported catalyst and others.
We will discuss two examples, in which density-theory calculations provide insight into features relevant for catalysis, thereby suggesting routes to tailor catalytic activities. The first example is concerned with ZnO surfaces, where the contact with an aqueous environment leads to the stabilization of catalytically active step and kink sites [1]. The second example, focuses on gold nanocatalysts supported on reduced TiO₂ surfaces, for which we discuss the alignment of the respective band structures and its consequences for the catalytic activity of the overall system [2].

[1] S. Yoo, M. Todorova, and J. Neugebauer, Phys. Rev. Lett. 120, 066101 (2018).
[2] S. Yoo, N. Siemer, M. Todorova, D. Marx, and J. NeugebauerJ. Phys. Chem. C 123, 5495-5506 (2019).

The activity, selectivity, and stability of alloy catalysts can be optimized by tailoring their bulk lattice parameters (the strain effect) and near-surface atomic order (the ensemble effect). However this same compositional and structural flexibility makes it difficult to predict the structure and catalytic properties of alloys from first principles. We will present an approach for simultaneously accounting for the strain and ensemble effects in alloy surfaces that enables the rapid prediction of the surface structure and catalytic properties of stable and metastable alloy materials. We use our approach to introduce complete maps of catalytic activity for thermodynamically stable phases across compositions and temperatures, making it possible to identify the synthesis conditions likely to produce the most active catalysts. By analyzing these maps, we will discuss in particular the roles of intermetallic phases, nanoparticle sizes, and atomic ensembles in determining the catalytic activity of alloy surfaces. We will present examples from Pt-based alloys for the oxygen reduction reaction, but the approach we present is applicable to a wide range of reactions on alloy surfaces.

Surface segregation in bimetallic systems plays a crucial role in heterogeneous catalysis. However, mechanistic studies of such restructuring phenomena remain scarce due to the large configurational space that makes it challenging to establish the most relevant set of mechanisms. To this end, we develop an automated method to discover elementary surface restructuring processes in an unbiased fashion, and apply it to Pd/Ag single-atom alloy as an example.

We employ high-temperature classical molecular dynamics (MD) to rapidly detect restructuring events, isolate them, and optimize using density functional theory (DFT). Our approach, combining MD and ab initio calculations, has two main advantages over conventional static transition state modeling methods. First, our approach is unbiased and does not rely on human intuition, instead discovering the relevant transition state pathways from the real-time surface dynamics. Second, the process is streamlined, quickly and automatically detecting restructuring events from MD and optimizing every event with DFT.

In addition to finding the previously proposed mechanism of exchange descent, our approach has revealed three new general classes of concerted restructuring events, dominant at step edges of close-packed surfaces: (1) vacancy insertion; (2) direct exchange; (3) interlayer exchange. Our results highlight the importance of vacancies at steps and kinks in facilitating Pd-Ag exchange processes, an insight that could not be established from previous DFT calculations on model terraces. The restructuring events discovered through our automated scheme enable us to go beyond thermodynamics and construct a complete set of mechanisms by which Pd is incorporated into the Ag host in vacuum at the single-atom limit. Furthermore, the approach establishes an important foundation to investigate more complex restructuring phenomena, such as Pd/Ag heteroepitaxy and adsorbate-induced reverse segregation. Our automated method is transferable to other alloy systems of interest and allows the dynamic nature of surfaces to be studied at the microscopic level with ab initio level of accuracy. The atomistic insights presented in our work provide a step toward systematic understanding and engineering of surface segregation dynamics in bimetallic catalysts.

Catalysis is one of the main tenants of green chemistry and the optimization of catalytic processes to improve the stability and reusability of the catalyst is an important research endeavor. Nanocatalysts in particular have been widely used and can uniquely catalyze more than one reaction on their large amounts of surface area – potentially minimizing solvent, substrate, and catalyst waste if reactions are engineered properly. In this work, we report the synthesis of a series of AuPd alloy nanoparticles with careful control of nanoparticle size and composition. We highlight AuPd’s high activity for ammonia borane (a green alternative hydrogen source) dehydrogenation. With a TOF of 160 min^-1 for AB methanolysis, Au₆₆Pd₃₄ has the highest activity of all Pd-based catalysts reported for H₂ production with a very low activation energy, 22.4 kJ/mol. Not only is the AuPd active for the formation of hydrogen (H₂), but, we can then use this generated H₂ in-situ for the dehalogenation of polychlorinated phenols (tough to degrade chemicals that have been used as pesticides in the past). Beyond this, ambient pressure H₂ formed can also be used for the mild reductive amination of levulinic acid/ethyl levulinate, biomass derivatives, to value-added pyrrolidones, over the same AuPd catalyst, simply recycled and used for both reactions, over ten times with no change in catalyst activity.

Here, we report a robust platform for sustainable photobiocatalysis using an ene-reductase from Thermus scotoductus SA-01 (TsOYE) and a light-harhevsting dye (rose bengal, RB) co-immobilized alginate hydrogel. TsOYE-encapsulated alginate hydrogel was prepared by simple mixing and gelling with high efficiency. Highly efficient encapsulation of RB in alginate hydrogel was followed using the intrinsic affinity between TsOYE and RB, which allowed for the construction of robust RB-TsOYE-loaded alginate capsules. A series of spectroscopic and electrochemical assays verified spontaneous binding of RB molecules to the flavin prosthetic group of TsOYE with a higher affinity than NADH, and direct electron transfer and co-reduction of TsOYE and RB in alginate hydrogel. In the absence of NADH, the photobiocatalytic system facilitated asymmetric hydrogenation of 2-methylcyclohexenone to an enantiopure (R)-2-methylcyclohexanone (max. conversion: 70.4%, turnover number: 300.2, turnover frequency: 1.54 min^-1) under illumination of a white light-emitting diode (LED) (power: 5.22 mW/cm²). While the limited mobility of immobilized TsOYE and substrate accessibility reduced the overall catalytic performances, we revealed a significant enhancement of TsOYE's robustness against heat and chemical denaturants and reusability while immobilized in alginate hydrogel. This study provides insight into a greener and sustainable approach of cofactor-free OYE biocatalysis for solar-driven synthesis of value-added chemicals.

Selective methane oxidation to methanol represents an important challenge in catalysis due to the difficulty in activating the strong C–H bond of methane (bond dissociation energy = 104 kcal mol⁻¹), giving rise to selectivity and activity problems. Finding a solution to this impediment is a key toward the direct synthesis of methanol from methane.

In nature, particulate methane monooxygenase (pMMO) is known as an effective catalyst for the oxidation of methane to methanol. Inspired by pMMO, molecular complexes have been developed utilizing the tunability of ligand design in pursuit of duplicating the structure and reactivity in a synthetic system. However, it has been a challenge to incorporate copper active sites bound to imidazole units in the pore of heterogeneous catalysts for methane oxidation.

We envisaged that metal–organic framework (MOF) can serve as a scaffold akin to the polypeptide chains in enzymes whose arrangement of secondary and tertiary structures can be achieved by judicious choice of structure and topology of MOFs. From structural analysis of various MOFs, MOF-808 was used to postsynthetically install ligands bearing imidazole units for subsequent metalation with Cu(I) in the presence of dioxygen. The catalysts show high selectivity for methane oxidation to methanol under isothermal conditions at 150 °C. We identified the structure of the active sites using solid-state UV−vis, resonance Raman, and X-ray absorption spectroscopies complemented by density functional theory (DFT) calculations. The results led us to conclude the most probable active site as bis(μ-oxo) dicopper species. We also demonstrated the modularity of our system by incorporating three different ligands bearing imidazole groups with different levels of rigidity and studied their effects on the catalytic properties.

Hydrogen peroxide (H₂O₂) has been mainly produced by the sequential hydrogenation and oxidation process of anthraquinone molecules, which is called as an indirect synthesis method. However, this process requires large volume facilities and a large quantity of energy. So, a direct synthesis method from H₂ and O₂ molecules in the presence of Pd catalyst has been considered for small-scale and on-site H₂O₂ production. However, the disadvantages of the prototypical Pd is that water is easily generated during the synthesis process. In order to improve the catalytic performance, many researchers have focused on Pd-based bimetallic alloys that can be easily formed thermodynamically such as Pd-Pt, Pd-Au, and Pd-Sn. In other words, the combinations of immiscible elements have been thoroughly ignored so far. Therefore, it has been a big challenge to broaden the scope of catalyst research into a wider area.
In this study, we report solid-solution alloy nanoparticles (NPs) of Pt and Au with a face-centered cubic (fcc) structure, although they are completely immiscible in the bulk phase. X-ray diffraction, transmission electron microscopy (TEM), energy-dispersive X-ray measurements, and density functional theory (DFT) calculations revealed that Pt and Au atoms were homogeneously distributed in a wide composition range of the Pt_xAu_100-x NPs (7≤x≤82). Out of all tested compositions, Pt₇Au₉₃ NPs exhibited superior catalytic performances both in productivity (2,327 mmol /g_cat h) and H₂O₂ selectivity (99.2%) and even under ambient conditions (20 °C, 1 atm) and without halide-ion additives. This exceptional catalytic property of the solid-solution Pt-Au alloys was clearly proved with the reaction mechanism obtained from the DFT calculation. In addition, we have checked that the introduction of inactive Au atoms likely suppresses dissociations of O₂* and OOH*, which leads to enhance the H₂O₂ selectivity. To our knowledge, this study is the first demonstration of stabilizing non-equilibrium solid-solution NPs as H₂O₂ direct synthesis catalysts. In addition, this work suggests that active utilization of other bulk-immiscible elements is highly desirable for catalyst development.

One problem that most solar based devices have in common is the discontinuous energy production and the concomitant mismatch between energy production and energy demand. This problem can be circumvented by using molecular solar thermal (MOST) energy storage and release systems. These systems are capable of harvesting the energy and transforming it into a storable form on a molecular level. When needed the stored energy can then be released as thermal energy.^[1] The valence isomerization of norbornadiene (NBD) to its metastable and energy rich isomer quadricyclane (QC) is regarded a promising candidate for MOST applications.^[2]
Within the scope of this project, we designed and synthesized NBD and QC interconversion couples with red-shifted absorption profiles. These NBD derivatives can undergo photoisomerization to their metastable QC isomers upon direct irradiation without the need for a triplet photosensitizer. We investigated each interconversion couple regarding their photophysical properties, stability of the energy rich isomer (t_1/2 ranging from hours to over a year at room temperature) and their potential energy storage capacity. Finally, the most promising derivatives were treated with a magnetic nanoparticle catalyst, designed in our group, to facilitate strain energy release via back-conversion of QC to NBD followed by straightforward purification by the action of an external magnetic field.

[1] K. Moth-Poulsen, D. Ćoso, K. Börjesson, N. Vinokurov, S. K. Meier, A. Majumdar, K. P. C. Vollhardt, R. A. Segalman, Energy Environ. Sci. 2012, 5, 8534.
[2] A. D. Dubonosov, V. A. Bren, V. A. Chernoivanov, Russ. Chem. Rev. 2002, 71, 917.

Abstract: Sequestration of carbon dioxide (CO₂) through electrocatalytic reduction to produce high-value industrial precursors, such as, CO is a promising avenue for sustainable development. However, the choice of an energy efficient and stable electrocatalysts for the selective reduction of CO₂ is a major challenge. Here, we prepare highly active, stable and cost effective CuO/In₂O₃ nanocomposites. We demonstrate a novel approach that utilizes oxygen vacancy (V_o), in CuO/In₂O₃ nanocomposites for the reduction of CO₂ to CO. Here, we control the concentration of V_o by varying the calcination environment during synthesis of the CuO/In₂O₃ nanocomposites. Photoluminescence, X-ray photoelectron and electron paramagnetic resonance spectroscopy measurements indicate that CuO/In₂O₃ nanocomposites prepared in an argon environment exhibit higher concentrations of V_o defects in comparison with samples that are prepared in air ambient. Moreover, CuO/In₂O₃ nanocomposites with less V_o defects exhibit 63% faradaic efficiency at -0.645V, while, CuO/In₂O₃ nanocomposites that are rich in V_o defects yield CO with 85% faradaic efficiency at -0.895V w.r.t. RHE suggesting that V_o serves as the active site for CO₂ adsorption. Through a combination of electrochemical measurements, physical characterization and density functional theory (DFT) calculations, we establish that the enhancement in CO yield is directly correlated to the concentration of V_o defects in the CuO/In₂O₃ nanocomposites and propose a plausible mechanism for electrocatalytic CO₂ reduction.
References:
[1] Larrazabal, G.O.; Martin, A.J.; Mitchell, S.; Hauert, R.; Ramirez, J.P.; ACS Catal., 2016, 6, 6265.
[2] Rasul, S.; Anjum, D.H.; Jedidi, A.; Minenkov, Y.; Cavallo, L.; Takanabe, K.; Angew. Chem. Int. Ed. 2015, 54, 2146.
[3] Luo, W.; Xie, W.; Mutschler, R.; Oveisi, E.; Gregorio, G.L.D.; Buonsanti, R.; Zuttel, A.; ACS Catal. 2018, 8, 6571.
[4] Kim, D.; Resasco, J.; Yu, Y.; Asiri, A.M.; Yang, P.; Nature, 2014, 5, 5948.
[5] Kulkarni, S. C.; Patil, D. S.; J. Mater. Sci. Mater. Electron. 2016, 27 (4), 3731–3735.

Semiconductor quantum dots have emerged in diverse practical applications including bioimaging and labeling, solar energy harvesting, light-emitting diodes, and next-generation displays. Most recently semiconductor quantum dots have attracted more and more attention in the field of photocatalysis, owing to their suitable optoelectronic properties for photocatalytic reactions, including high absorption coefficient and long carrier lifetime. Herein, we report the visible-light-mediated stereoselective C-C oxidative coupling reactions photocatalyzed by semiconductor quantum dots. High quality of halide perovskite and CdSe-based quantum dots have been synthesized and characterized, and then been applied to the model reaction to afford optimized reaction conditions. The desired product can be generated exhibiting high yield with high stereoselectivity. The reaction can be generalized to different starting materials with various substituents on the phenyl ring and varied functional moieties. Furthermore, we study the reaction pathway by using radical trapping agent and electron paramagnetic resonance (EPR) spectroscopy. We propose our stereoselectivity is caused by steric hindered radical reactions. Our study shows semiconductor quantum dots are expected to bring new insights in photochemical synthesis.

Ammonia is used in various industries as an essential chemical in the modern industrial economy. Haber-Bosch process has been widely used to produce ammonia, which involves extremely high temperature and pressure, and hydrogen gas produced from steam reforming. As a result, this process emits more than 1.9 ton CO₂ per ton NH₃. Due to the high global demand of ammonia, 2% of the world’s total energy supply is used for ammonia production, and thus is responsible for more than 400 Mt of emitted CO₂, which is 1.6 % of total CO₂ emissions. Therefore, electrochemical nitrogen reduction reaction (NRR) is attracting attention as an alternative to this process, because electrochemical NRR could serve as an environmentally friendly approach to synthesize ammonia with lower or potentially zero CO₂ emission depending on the electrical source_. However, it has many drawbacks. One of them is the low activity for NRR, because nitrogen is a very stable compound due to its triple bond. Catalysts are critical to achieve better activity and selectivity. Single atom catalyst is one of the attractive materials not only for electrochemical ammonia production, but also for various electrocatalysis where high product selectivity could be achieved.
Here, we present the use of nickel single atom catalyst (Ni-SAC) as an efficient electrocatalyst for NRR. Ni-SAC is derived from Ni²⁺-exchanged ZIF-8 (Ni-ZIF), which involved the ionic exchange between Zn²⁺ and Ni²⁺. Pyrolysis of Ni-ZIF at 950°C yielded Ni-SAC, in which the Ni-atom in this material bonds with 3 nitrogen atoms and 1 carbon atom. The as-prepared Ni-SAC shows high ammonia production rate (3.9 µg h^-1 mg^-1) and faradaic efficiency (0.27 %) at -1.2 vs. RHE in a flow cell. These values are independent on ambient ammonia which are obtained by subtracting the amount of ammonia present during NRR in Ar atmosphere. The high performance can be attributed to the ensemble effect and electronic effect of the isolated Ni²⁺ ion in Ni-SAC. Also, it has high surface area and porosity, which translates to more catalytic sites being accessible for NRR. In this conference, we will present a more holistic evaluation of the high performance observed for NRR and discuss the differences of NRR performances using Ni-SAC in H-cell and in flow cell.

In recent years, there is an unprecedented interest of worldwide organizations in replacing fossil fuel-based energies with other renewable energy sources to reduce the emissions of greenhouse gases released into the atmosphere and to overcome the serious problem of climate change. A viable alternative for this purpose is the use of electrocatalysts for the production of hydrogen (Hydrogen Evolution Reaction - HER) and oxygen (Oxygen Evolution Reaction - OER). In general, the main attributes of efficient electrocatalysts for practical applications include use of low cost and non-toxic materials, high chemical and mechanical stability, durability and low overpotential. Particularly, the lanthanum nickelate (LaNiO₃) is a promising candidate as electrocatalyst because of its high conductivity, low charge transfer resistance, and interesting electrocatalytic properties for OER. The intrinsic OER activity of LaNiO₃ has been attributed to the existence of a single electron in the e_g orbital belonging to Ni³⁺. However, Ni³⁺ is easily reduced to Ni²⁺, leading the e_g orbital to exhibit more than a single electron. The commonly employed strategy to solve this, it’s the partial replacement of crystallographic sites occupied by Ni³⁺ with other transition metals. Therefore, the main aim of our study was to investigate the electrocatalytic properties of LaNi_1-xFe_xO₃ (x = 0.0, 0.3, 0.6, and 0.9 mol%) for OER, mainly analyzing the influences provoked by Fe³⁺ ions in the lattice and the use of pyrolytic graphite sheet (PGS) as a potential substrate for electrocatalysis. LaNi_1-xFe_xO₃ powders were synthesized by the co-precipitation method followed by post-heat treatment (900 οC for 4 h). The thin film electrodes were prepared depositing the obtained powders on PGS substrates via drop-casting. In relation to the powders, X-ray diffraction patterns revealed that the Fe³⁺ level was responsible for a structural phase transition from rhombohedral (x = 0.0 to x = 0.3 mol%) to orthorhombic (x = 0.6 to x = 0.9 mol%). The substitution of Ni³⁺ by Fe³⁺ in the crystalline structure favored a morphological change from irregular cube- to spherical-like microparticles. A superior electrocatalytic activity for OER was detected for Fe-doped electrodes in relation to phase pure (LaNiO₃), so that the LaNi_0.4Fe_0.6O₃ was the most efficient for this purpose. Besides the low charge transfer resistance, LaNi_0.4Fe_0.6O₃ demonstrated a lower overpotential (439 mV) and a smaller Tafel slope (52 mV dec^-1) than the LaNiO₃ (465 mV and 76 mV dec^-1). In addition, a current density above 10 mA cm^-2 was registered for this electrocatalyst in chronoamperometry tests, confirming its effectiveness over long times. The potentiality of PGS substrate for our electrocatalysts was confirmed by comparing with other electrocatalytic tests performed on glassy carbon substrate.

Supported precious metal catalysts are extensively employed in industries for several chemical processes. Size of the metal species is a decisive factor in controlling the performance of such catalysts. Smaller the size, higher is the activity. So, huge interest has been directed towards the formation, stabilization and characterization of single atom catalysts and sub-nanometer clusters in the past few years. However, Supersaturation controlled, wet chemical methods have been hardly explored to synthesize such catalysts on various supports. We describe a robust wet chemical method to synthesize catalyst that consists of stable, isolated single atoms and sub-nanometer clusters of Pt anchored to the surfaces of ZnO nanorods. The thermodynamic and kinetic aspects of reduction of Pt salt under microwave heating conditions and heterogeneous nucleation of Pt atoms/clusters on ZnO have been understood. Driving force of the reaction has been optimized to obtain as small nuclei as possible with no post-nucleation growth and to have only heterogeneously nucleated ones. Heterogeneous nucleation also helps in further reduction of the cluster size by truncating the clusters. Facets on the ZnO rods having highest surface energy possess the lowest kinetic barrier for nucleation and thus such sites are always the most favored for nucleation. Having a very low driving force for nucleation, all the observed Pt clusters sit preferentially on the basal planes of the nanorods. Also, it is desired to produce catalysts that are resistant to sintering. These Pt clusters are stable under elevated temperatures both in air and CO atmosphere at least till 220°C. So, these can be efficiently utilized for CO oxidation which happens below 200°C. Electron microscopy HAADF-STEM technique, which is the only viable tool to visualize atomically distributed species, has been exploited extensively to characterize the ZnO/Pt hybrid structures. Having the finer size of Pt nucleated on ZnO, we expect to achieve higher catalytic activity as compared to Pt nanoparticles on ZnO.

Catalytic upgrading of biomass derived pyrolysis vapors, by passing them over a zeolite catalyst has been explore in the last years. In the fibers incorporated zeolite HZSM-5 as a catalyst, have shown that the Zeolite HZSM-5 has the best yields in the deoxygenation of pyrolytic oil. The synthesis of a catalyst using fibers could increase the accessibility to the active sites and improving the deoxygenation of the pyrolytic oil and to promote green chemistry practice using biopolymers instead of traditional polymers derived from fuel. The use of fibers with zeolite/lignin is proposed in this work to promote de-oxygenation reactions, improving the characteristics of pyrolytic oil such as pH, calorific value, among other properties in the soybean hulls pyrolytic bio-oils. The aim is the fabrication of fibers using lignin as polymer, reducing the use of traditional polymers such as polyvinylpyrrolidone (PVP) and polyacrylonitrile (bread), polymers of high economic value and harmful to environment, compared to lignin. The fibers and zeolite will be characterized by the techniques of XRD, SEM, and ATP. Pyrolytic oils are characterized by GC/MS. This research will contribute to the synthesis of a new catalyst, as well as to use in obtaining biofuels.

The characterization of HZSM zeolite and the thermal characterization of the polymers has been carried out. the XRD Analysis is obtained the crystalline phases of the HZSM-5 zeolite, with sharp reflections corresponding to the MFI structure, the results of nitrogen physisorption Analysis was obtained TYPE IV isotherm and hysteresis characteristic from Mesopores, according to the IUPAC. The analysis BET obtained a surface area of 289.2 m²/g, was obtained with a promised pore diameter of 36.1 nm, characteristic of structures mesoporous, these structures promote the obtaining of bio-oil and the reduction of biochar. The Obtained the different signals from the acid sites, the desorption signals of ammonia less than 200 °C correspond to sites with weak superficial acidity, while desorption signals exhibited between 200-400 °C are associated with medium-strength acid sites, and the signals found to temperatures higher than 400 °C, correspond to sites with strong acidity, weakacid sites are in greater proportion, the results are similar to those obtained by Jia Lui et al. 2015 (Liu et al. 2015). The next step is to manufacture fibers with zeolite HZSM-5 to favor accessibility to acid sites and to increase the deoxygenation of pyrolytic oil, which until now was obtained 18% of deoxygenation using a zeolite catalyst

Liu, Jia et al. 2015. “Hierarchical Macro-Meso-Microporous ZSM-5 Zeolite Hollow Fibers With Highly Efficient Catalytic Cracking Capability.” Scientific Reports 4(1): 7276. http://www.nature.com/articles/srep07276 (February 16, 2019).

Pollution caused by CO₂ emissions in the atmosphere causes a number of environmental damages, such as the so-called greenhouse effect.¹ Thus, in recent years the search for technologies capable of bringing environmental and economic benefits has been intensified, converting CO₂ into value-added molecules, such as ethanol, ethane and formic acid.¹ Among the efficient ways of promoting this conversion of CO₂, we highlight the electrocatalytic and photocatalytic processes, mainly using semiconductors. Niobium pentoxide (Nb₂O₅) and graphite carbon nitride (g-C₃N₄) demonstrate good catalytic performances for degradation of organic pollutants and water splitting^2-4. However, these catalysts have as limitation their high recombination of the photogenerated charge carriers. One way to overcome this negative effect in photocatalytic processes is the formation of heterojunctions between the two materials (Nb₂O₅ and g-C₃N₄). Therefore, in this work the photo and electrocatalytic efficiency of the Nb₂O₅/g-C₃N₄ heterostructures was verified for conversion of CO₂ into CH₄. In addition, we investigated the electrical properties of the synthesized materials, such as flatband values. Thus, this properties change the selectivity of the formed products, obtaining highly efficient processes. In the electrochemical and photoelectrochemical tests, the Nafion and PEDOT: PSS⁵ was used with binder to increase the contact between the synthesized samples (Nb₂O₅, g-C₃N₄, and Nb₂O₅/g-C₃N₄) and de substrate (Fluorine doped Tin Oxide Glass). The photoelectrocatalytic tests shown that the electrode with the heterostructure Nb₂O₅/g-C₃N₄ and PEDOT:PSS has 10 times higher current values than the value obtained to the electrode without PEDOT:PSS (12 μA cm^-2 and 1.5 μA cm^-2) to 0,4V under irradiation. However , both samples showed increase of current in the presence of light, double for samples with nafion and triple for samples with PEDOT. The flatband values obtained were -0.55V and -0.77V, for the Nb₂O₅/g-C₃N₄ samples with and without PEDOT through the Motty-Schottky analyzes using the frequency of 100 Hz, showing that PEDOT decreased the flat band value. It was possible to verify that the synthesized samples are photoactive and that they have great chances of being possible catalysts for photo electroreduction of CO₂ and will be tested later with quantification of the products through GC and HPLC measurements, and the studies in different potential will be perfomed to analyse the effect of the potencial in the selectivity.
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APPEL, A. M. et al. Chemical Reviews, [s.l.], v. 113, n^o 8, p. 6621–58, 2013.
Zaiwang Zhao , Yanjuan Sun and Fan Dong. Nanoscale 2015, 7, 15-37.
Gelson T.S.T.da Silva; André E.Nogueira; Jéssica A.Oliveira; Juliana A.Torres; Osmando F.Lopes; Caue Ribeiro. Applied Catalysis B: Environmental, v. 242, p. 349-357, march 2019
S. C. Yan; Z. S. Li; Z. G. Zou. ACS Publications, July 16, 2009.
G. Greczynski, Th. Kugler, M. Keil, W. Osikowicz, M. Fahlman, W.R. Salaneck. Journal of Electron Spectroscopy and Related Phenomena, 121 (2001) 1–17.

In electrochemical reduction of CO₂ in aqueous media, it is well-known that CO is generated as a main product over Ag cathodes. Several research groups reported that anodic oxidation (anodization) improves the current density and faradaic efficiency toward CO evolution due to the crystal orientation and the valence of Ag cathode. On the other hand, ozone (O₃) is generally used for chemical oxidation. In this study, we investigated the current density and faradaic efficiency toward CO evolution for the electrochemical reduction of CO₂ by using the O₃-pretreated Ag cathode.
Ag foil (Nilaco Co., 99.98%, 6 cm²) electrodes were sonicated in hexane, ethanol, and Milli-Q water, prior to the electrochemical measurements. The O₃ pretreatment of Ag electrodes were performed in an O₃ generator (ASM401N, Asumi Giken Ltd.). Front and back sides of Ag foil were exposed to O₃ atmosphere for 10 hours each. The electrochemical CO₂ reduction experiments were conducted in a H-type two-glass chamber with using Pt wire and Ag/AgCl as a counter electrode and a reference electrode, respectively. The electrolyte was 0.1 M KHCO₃ aqueous solution under flowing of CO₂ gas (99.999 %) at a flow rate of 20 mL min⁻¹. The CO₂ reduction activity was evaluated at constant potential (−1.6 V vs. Ag/AgCl). The electrochemical surface area (ECSA) of Ag electrodes were determined by using underpotential deposition of lead (Pb UPD) in an aqueous solution containing 0.1 M HClO₄, 0.1 M NaClO₄, 1 mM Pb(ClO₄)₂. The crystal structure was confirmed by X-ray diffraction (XRD) analysis.
The XRD patterns of O₃-pretreated Ag electrode were assigned to the typical diffraction pattern of AgO. This electrode exhibited the current density of 3.9 mA cm⁻² and faradaic efficiency of 95%, although these were 1.2 mA cm⁻² and 68% over Ag foil, respectively. After the electrochemical reduction of CO_2, only the diffraction patterns of fcc structure of Ag metal were observed, suggesting that AgO species on O₃-pretreated Ag electrode were reduced to Ag metal during the reaction. The amount of underpotentially deposited Pb atoms on O₃-pretreated Ag electrode (13 nmol) was almost two times larger than that on Ag foil (6.2 nmol). The current density and faradaic efficiency toward CO evolution were not dependent on only ECSA. After the reaction, the ratio of (111) to (200) facets of O₃-pretreated Ag electrode was much higher than that of Ag foil in the XRD pattern. We concluded that the high current density and faradaic efficiency toward CO evolution were attributed to not only the high ECSA but also the face orientation of O₃-pretreated Ag electrode.

Realizing highly efficient and selective electrochemical CO₂ reduction by using cost-effective catalysts is key to large-scale commercialization of this technology. In this work, a nanoporous amalgam catalyst with tuneable amount of Ag-Hg bonding sites has been developed for selective CO₂ electroreduction to formate or CO via a one-step wet-chemistry method. This catalyst is composed of Ag(Hg) solid solution phase and Ag-Hg intermetallic compound, exhibiting a loosened nanorod-channel networks morphology. Ag₇₀Hg₃₀ alloy drives efficient formate generation with a high Faradaic efficiency of 85% at a moderate overpotential of 790 mV while Ag₉₁Hg₉ alloy promotes CO₂ reduction to CO with a Faradaic efficiency of 58% at a low overpotential of 590 mV. The switchable product selectivity has been correlated to a preferable formation of HCOO* or *COOH intermediates at Ag-Hg and Ag-Ag bonding sites. This work highlights amalgams as novel electrocatalysts for CO₂ reduction to valuable products on demand.

Surface sulfurization strategy is utilized to construct Co₃O₄@CoS_x heterogeneous catalysts with a highly efficient activity towards the oxygen evolution reaction (OER), and the enhanced catalytic activity is dependent on the degree of surface sulfurization. It is found that forming amorphous CoS_x on the Co₃O₄ surface through surface anion exchange can effectively generate more Co²⁺ ions and oxygen vacancies in the lattice structure, not only inducing the production of more CoOOH but also facilitating water adsorption during the water oxidation process.

Since the performance is highly dependent on the electronic structure of the material, it is crucial to adjust electron distribution to achieve various improved properties for the utilization of the material in cross-functional applications. Here, precise control of the local electronic migration of carbon nitride via molecular grafting has been successfully developed. The electronic structure could be carefully tailored by the incorporation of π-conjugated molecules and electronwithdrawing groups. Experimental results and theoretical calculation reveal the possible electronic migration path to reset the local electronic configuration, leading to charge separation and red-shift emission. As a result, H₂ evolution of 5 mmol h^-1 g^-1 with a high AQY of 20.98 % at 420 nm and in vitro phototherapy for the anti-cancer effect can be realized. These findings provide a new opportunity for modifying the properties of emerging materials for extended applications through careful control of the electronic structure process.

Photocatalytic hydrogen (H₂) production of ZnS catalysts still remains unsatisfactory because of the fast charge carrier recombination originated from the inherent characteristics of the direct-bandgap semiconductor as well as the photocorrosion by the oxidative holes in the valence band. In this study, we developed the strongly coupled ZnS/polydopamine (ZnS/PDA) photocatalysts via a facile in-situ polymerization method with the precisely controlled thickness of PDA layer onto the ZnS nanorods. The introducing of thin PDA shell on the surface of ZnS nanorods exhibits enhanced photocatalytic H₂ production activity and stability than a single component of pristine ZnS or PDA itself. The synergistic effect responsible for the enhancement of catalytic performance can be attributed to the favored effective charge separation/transport at the ZnS/PDA heterojunction interface as well as extended light absorption of photocatalysts due to the origin of partially formed ZnO. Moreover, the higher surface wettability of ZnS/PDA nanocomposites could be desired properties for photocatalytic water splitting H₂ production than that of the pristine one. The density functional theory (DFT) calculation reveals advantageous type-II band alignment of ZnS/PDA, hole accepter levels of PDA can facilitate the charge-carrier separation and long-periods stability of ZnS via lowering the oxidative capacity of holes in the valence band state. The optimized catalyst of ZnS/PDA hybrid nanorods with PDA shell thickness of 1.2 nm, exhibits excellent photocatalytic H₂ production rate of 2162.5 mmol h^-1 g^-1 (10 mg catalysts) without using a Pt cocatalyst as well as enhanced photostability over long periods.

Co-Fe-P catalysts for fast hydrogen generation from hydrolysis of NH₃BH₃ were fabricated by an electro-deposition. Effects of cathodic current density and plating time on the microstructures of the Co-Fe-P catalysts and their catalytic activities for hydrolysis of NH₃BH₃ were explored. The microstructure, surface morphologies, and composition of the Co-Fe-P catalysts were analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), respectively. Their hydrogen generation kinetics were measured using mass flow meter (MFM). The particle sizes and composition of Co-Fe-P catalysts were depended significantly on the current densities and deposition times during an electro-deposition. As a current density increase, the particle sizes of the Co-Fe-P catalysts increased. With an increase in deposition time, the particles of the Co-Fe-P catalyst get agglomerated densely. The hydrogen generation kinetics of the catalysts was depended largely on their microstructures, which was affected by the electro-deposition conditions such as current density and deposition time. In particular, the Co-Fe-P catalysts, which was deposited at 50 mA for 5min, exhibits the best hydrogen generation rate of 2858.min^-1g^-1-catalyst in 1 wt% NH₃BH₃ solution at 30 ^oC. With an increase in a solution temperature from 10 to 60 ^oC, the hydrogen generation rate increases dramatically. Specially, at 60 ^oC, which is the optimal temperature for the best PEMFC operation, the hydrogen generation rate was 8915 ml.min^-1g^-1-catalyst in 1 wt% NH₃BH₃ solution at 60 ^oC. With increasing the concentration of NH₃BH₃ from 0.5wt% to 3wt% , the hydrogen generation rate of the Co-Fe-P catalyst increased gradually from 1900 ml.min^-1g^-1-catalyst to 8105 ml.min^-1g^-1-catalyst . This behavior of the Co-Fe-P catalyst was in contrast with that of noble metals such as Ru and Pt. Since it is necessary to increase the concentration of NH₃BH₃ in the solution for high hydrogen storage density, it is considered that the Co-Fe-P catalyst is more suitable than noble metal catalysts for a practical use.

Electrocatalytic conversion of carbon dioxide (CO₂) to multicarbon alcohols (C₂H₅OH and C₃H₇OH) in a selective and efficient manner remains a great challenge in renewable and sustainable energy research. Herein, we report a series of hybrid catalysts of Cu nanoparticles supported on N-doped porous carbon with different contents of pyridinic N and demonstrate that the selectivity to the multicarbon alcohols could be tuned by introducing the cocatalyst of N-doped porous carbon materials. By varying the contents of pyridinic N in hybrid catalysts, the highly selective production of multicarbon alcohols is achieved with notable total Faradaic efficiencies of multicarbon alcohols up to the value of 73.3 % at – 1.05 V (vs RHE) for both C₂H₅OH and C₃H₇OH products. We hypothesize in this work that pyridinic N as a CO-producing site together with Cu catalytic sites can act cooperatively to produce C₂H₅OH and C₃H₇OH. Furthermore, a two-site mechanism to rationalize the efficient CO₂ reduction to multicarbon alcohols on the hybrid catalysts is proposed and discussed. These findings provide a new guideline for the rational design of electrocatalysts for tuning catalytic activity and selectivity toward multicarbon alcohols.

Utilization of carbon dioxide conversion to chemical fuels using an artificial photocatalyst under solar irradiation is an ideal goal in renewable energy studies. To date, a variety of strategies have been demonstrated to boost the quantum efficiency of CO₂ photoreduction. Coupling of semiconductors considered one of the key challenges to designing a good photocatalyst that can activate the CO₂ molecule with smallest possible activation energy and produce selective fuels. Since the activity and selectivity are mainly determined by the suitable band structures and surface states of photocatalysts as well as the photoreaction conditions. In this study, we fabricate ZnS/ZnIn₂S₄ composite for CO₂ reduction under visible light irradiation. When the mass ratio between Zinc and Indium is varied, the composite catalyst shows dramatically increasing of acetaldehyde and methanol formation as major products of CO₂ conversion. The optimum ratio exhibits a 200 times enhancement over a single phase of ZnS. The as synthesized samples were characterized by XRD, SEM, TEM, Photoluminescence, UV-Visible diffuse reflectance spectroscopy, XPS, UPS, surface area and CO₂ adsorption measurements. Based on the experimental evidence, the interface between both phases is the crucial role of interfacial charge transfer and subsequent enhancing of catalytic activity. In-situ FTIR study has been done to get a deeper insight into the product formation and gain a detailed comprehension of the fundamental reaction steps during the photocatalysis. In conclusion, my presentation will focus on how to efficiently manage the photo-generated charge carriers and surface reactions of semiconductor photocatalysts to boost the photoreduction activity.

Catalysts embedded in porous structures are widely used in materials research and industry. Understanding the dominant step of catalytic chemical reaction rate is crucial for the development of such catalysts. However, the coupling of mass transport and kinetic effects with reactions presents a significant computational challenge. Here, we show that by using a Lattice Boltzmann approach we can develop a model that will allow us to include kinetic effects such as adsorption and desorption of reactants/products, mass transport bottlenecks in the support, and possible pore blocking effects. Further, we show that our model can be used to determine possible synergistic interactions when bi-metallic catalysts are used. We use a model system composed of Ni-Pt bimetallic catalysts, that are used to catalyze the reverse water gas shift reaction and methanation through a nanostructured support as an case study for our approach. We compare our results to experimental results and show that our model can be used to explain synergistic effects in this system.

Nitrogen-functionalized onion-like carbon (N-OLC) was synthesized using an arc discharge in a liquid phase(a so- called Solution Plasma Process) in the presence of different ammonia concentrations. The N-OLC was investigated as a catalyst support material for a catalytic oxygen reduction reaction (ORR). With increasing ammonia concentrations up to 5 M, the nitrogen doping degree increased to 1.7 at.%, which created a highly defective onion-like structure. The edge and defective sites induced from nitrogen atoms effectively facilitated the nucleation of Pt clusters, which led to the stable dispersion and size-reduction of Pt nanoparticles (PtNPs), as revealed via XPS, XRD, Raman spectroscopy and electron microscopy. From the results of the electrochemical characterization, the PtNPs supported on the N-OLC with a nitrogen content of 1.7 at.% exhibited the highest electrochemically active surface area (ECSA), specific current density and half-wave potential. The enhanced (ORR) could be ascribed to the defective outermost layers and the electronic modification. Moreover, even after an accelerated durability test (ADT) with 5,000 cycles, the N-OLC with a nitrogen of 1.7 at.% exhibited less degradation, indicating the excellent ORR durability.
Acknowledgement
This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korea Government (NRF-2016R1D1A1A09918072)

Based on the concept of electronic effect used in the development of homogeneous catalysts, we have prepared the heterogeneous systems, in which metal-support interaction leads to methane activation at room temperature, enabling the conversion of this hydrocarbon into value-added chemicals. In-situ characterization techniques, such as Time-Resolved X-ray Diffraction (TR-XRD), X-ray Absorption Fine Structure(XAFS) and Ambient Pressure X-ray Photoelectron Spectroscopy (AP-XPS) were employed to investigate changes in chemical state and structural transformations occurring in a series of MO_x/CeO₂ (M = Fe, Co, Ni and Cu) powder catalysts upon reaction with methane and under the methane dry reforming condition. XPS shows the presence of adsorbed CH_x and CO_x species after exposing the powder catalysts to methane at room temperature. Temperature-programmed reduction (TPR) measurements point to the reaction of the samples with methane and formation of CO, CO₂, and H₂O gas at temperatures as low as 100 °C. The TR-XRD results show that all of the transition-metal oxides in the as-prepared catalysts can be reduced to their metallic phase during the CH₄-TPR process with the ceria support undergoing significant reduction from the surface to bulk, yet the reduction temperature varies for different MO_x/ CeO₂ samples. Among these samples, CuO_x/CeO₂ shows the lowest reduction temperature (below 260 °C) for both the oxide overlayer and the ceria support. The NiO_x/CeO₂ and CoO_x/CeO₂ powder catalysts also activate CH₄ at relatively low temperatures (below 350 °C), and the oxide overlayers undergo NiO → Ni and Co₃O₄ → CoO → Co transformations. In the case of FeO_x/CeO₂, a series of complex changes in the chemical state is observed for the oxide overlayer, Fe₂O₃ → Fe₃O₄ → FeO → Fe → Fe₃C. The ceria support of all of the three MO_x/CeO₂ (M = Cu, Ni, and Fe) samples undergoes severe reduction and form Ce₂O₃ at high temperature (> 650 °C) during the CH₄-TPR reaction. The in-situ TR-XRD results highlight rich and complex chemistry for methane on these MOx/CeO₂ mixed oxide systems. The strong metal-support interactions seen in the Ni and Co/CeO₂(111) model catalysts also determine the good performance of the MO_x/CeO₂ (M = Ni, Co) powders for the methane dry reforming reaction.

The sale of value-added byproducts from hydrogen-generating reactions is a strategic approach to lower the costs of hydrogen fuel in realizing a truly-sustainable hydrogen economy. Metal hydrolysis is a chemical process that produces hydrogen together with a metal hydroxide species; however, this reaction is rarely observed without addition of chemical catalysts or extreme reaction conditions. In this talk, I will demonstrate the use ultra-fine nanoporous aluminum to create hydrogen at standard conditions for temperature and pressure via hydrolysis without any catalyst additives.[1] The advantage of this method is the co-production of pure aluminum hydroxide (Al(OH)₃) during the process. I will demonstrate the transformation of this hydrolysis byproduct into valuable materials to elucidate measures in reducing the overall cost of hydrogen. Our results show that (i) the synthesis of nanoporous aluminum is scalable to meet the needs of large scale production for a hydrogen gas economy, and (ii) the transformation of the hydrolysis byproduct Al(OH)₃ to create high surface-area “activated alumina” (Al₂O₃) as a commercially viable product.

[1] J.S. Corsi, J. Fu, Z. Wang, T. Lee, A. K. Ng, and E. Detsi
ACS Sustainable Chemistry & Engineering 2019; https://doi.org/10.1021/acssuschemeng.9b00481

Optimal nanodesign of catalysts can surely be obtained benefiting from the functionalization of carbon based materials. By this approach various functionalities can be strategically anchored on the backbone of these materials, aiming at specific applications such as catalytic. In the context of catalysis, the development of heterogeneous nanocatalysts allies not only the interesting intrinsic properties of nanomaterials but also the facile separation and reuse of the catalysts. The main challenges involve how to design the desired functionalities on heterogeneous surfaces, which many times is inspired on homogenous reactions. Herein, we pursued various functionalization of graphene oxide (GO) and carbon nanotube (NTC) with functional groups and metallic nanoparticles (NPs) towards several application, highlighting the efficient production of hydrogen. In this sense, we obtained nanocatalysts derived from GO, NTC filled with Co, Ag NPs and Ni NPs obtained as thin films, which were employed in the hydrogen production from the hydrolysis reaction of NaBH₄. The maximum rate of hydrogen production reached over 50 x10³ mL^-1 min^-1 g^-1, among the highest reported. Hence, high volumes of hydrogen could be produced using low amounts of catalysts (~0.1–0.8 mg) and could be reused for 10 consecutive cycles without losing activity. The facile use of the nanocatalyst thim film is the most promising aspect towards such an important field: sustainable energy. We can produce high amounts of hydrogen, in a controlled system (ceasing the production is easy) and easily remove the thin film (deposited on glass substrate), wash and reuse it again. Furthermore, another catalytic application is regarding the monitoring and destruction of toxic agrochemical in the field or our food and also chemical warfare in terrorist attacks or war zones, mainly concerned in promoting worldwide chemical security. We have been using the functionalization GO and NTC with various moieties covalently (imidazole, thiol, hydroxamate) and non-covalently (Ni and Ag NPs) linked , obtained as powder and thin film. An innovative interfacial fuctionalization method is proposed for obtaining the thin films. Bifunctionalization has also been carried out by anchoring two different neighbouring groups. Expressive rate enhancements up to 10⁸-fold were obtained in the presence of the nanocatalysts compared to the spontaneous reaction. The nanocatalysts also showed a dual function: could be used as sensors for detecting these toxic agents. These results can be further extended for alerting pesticide contamination and chemical attacks. In summary, indeed the nanodesign of heterogeneous catalysts opens a myriad of possibilities for achieving prominent catalysis.

The authors acknowledge UFPR, CAPES, CNPq, PhosAgro/UNESCO/IUPAC, L’Oréal-UNESCO-ABC, National Institute of Science and Technology of Carbon Nanomaterials (INCT-Nanocarbon) and Fundação Araucária.

Hostert, L., Blaskievicz, S. F., Fonsaca, J. E., Domingues, S. H., Zarbin, A. J., Orth, E. S. J. Cat. 356 (2017), 75.
Hostert, L., Neiva, E. G., Zarbin, A. J., Orth, E. S. J. Mat. Chem. A. 6 (2018), 22226.

Abstract

Cobalt oxide nano-arrays structured catalysts had been demonstrated to exhibit high NO oxidation performance with porous nature of nanowires and relative amount of Co³⁺ on the surface ¹, and ZSM-5 was studied as a porous catalytic material for trapping hydrocarbon during the cold-start process ². In this work, hybrid Co₃O₄/ZSM-5 monolithic catalysts were synthesized on honeycomb cordierite substrate by hydrothermal methods. Two configurations were adopted: Co₃O₄ nanoarray was grown on substrate first followed by the growth of ZSM-5 (CZ), and ZSM-5 layer was grown first followed by the growth of Co₃O₄ nanoarray (ZC). The adsorption/oxidation performance was evaluated under low temperature combustion of diesel (LTC-D) condition with feed gas of CO, C₂H₄, C₃H₆, C₃H₈, and NO. The results showed that within the tested temperature range (up to 300 °C), although the adsorption/oxidation behavior for CO, C₂H₄ and C₃H₈ were similar, ZC configuration exhibited higher adsorption capacity for C₃H₆ and NO than those of CZ. In ZC, C₃H₆ was not released and oxidized directly below 300 °C while NO was desorbed at 280 °C, which was higher than that in CZ.

References
1. Zheng Ren, Y. G., Zhonghua Zhang, Caihong Liu and Pu-Xian Gao, Nonprecious catalytic honeycombs structured with three dimensional hierarchical Co3O4 nano-arrays for high performance nitric oxide oxidation. Journal of Materials Chemistry A 2013, 1.
2. Kim, H.; Jang, E.; Jeong, Y.; Kim, J.; Kang, C. Y.; Kim, C. H.; Baik, H.; Lee, K.-Y.; Choi, J., On the synthesis of a hierarchically-structured ZSM-5 zeolite and the effect of its physicochemical properties with Cu impregnation on cold-start hydrocarbon trap performance. Catalysis Today 2018.

Platinum is an important catalyst for many fuel cell applications. Due to the high cost of platinum, there is continuing research interest in developing nanostructures to reduce the amount of platinum used while maintaining performance. Bimetallic nanoparticles containing platinum are a promising approach to improve the efficiency of fuel cells at a reduced cost. In this work, we report on the formation of gold-platinum nanoparticles through pulsed laser dewetting of metal thin films. Important parameters in laser dewetting, such as laser fluence and irradiation time, were explored. The sputtering order and thickness of the gold and platinum thin films were also investigated. Elemental analysis confirms that both gold and platinum are found within the nanoparticles after dewetting. The results from cyclic voltammetry of the produced bimetallic gold-platinum nanoparticles will also be presented.

Recent development of two-dimensional layered materials including graphene-family and related nanomaterials have arisen as potential game changer for energy, water and sensing applications. While graphene is a form of carbon arranged hexagonally within atomic thin sheet, MoS2 is becoming a popular, efficient, and cost-effective catalyst for electrochemical energy devices, in contrast to expensive platinum and palladium catalysts. In this work, we electrochemically desulfurize few-layer molybdenum disulfide (MoS2) and aerogels with reduced graphene oxide (rGO) prepared under hydrothermal conditions ((P< 20 bar, T< 200 oC), for improving hydrogen evolution reaction (HER) activity via point defects (S-vacancy). Moreover, the interactions between rGO and MoS2 components create emergent heterostructures with desirable physicochemical properties (specific surface area, mechanical strength, faster diffusion, facile electron and ion transport) enabled by chemically bridged (covalently) tailored interfaces. We demonstrate that with an optimized number defect density, particularly by exposing the edges of MoS2 layers and nanowalls in graphene-MoS2 hybrid aerogels, interfacial processes during catalytic reactions are accelerated. To understand the effects of defects on HER activity, we varied the applied potential and operating duration for optimized defect density. This study offers a unique method for tuning the properties of layered MoS2 and hybrids as promising, cost-effective and efficient nanocatalysts. In addition,it establishes the structure–catalytic activity relationships via scanning electrochemical microscopy at electrode/electrolyte interface besides mapping electrochemical (re)activity and electro-active site distribution.

Proton exchange membrane fuel cells (PEMFC) are a promising power technology for vehicles. However, their commercialization has been hindered due to high-cost platinum and platinum-based electrocatalyst which accounts for approximately 20% of the PEMFC total cost. Alkaline membrane fuel cells (AMFC) have emerged as an alternative to PEMFC as ORR kinetics are greatly improved in alkaline conditions. Alkaline media also enables the use of first row transition metal thus allowing for more cost-effective catalysts to be develop. Here we present the use of first row transition metal ferrites as ORR electrocatalyst in alkaline media and study their electrocatalytic mechanism using synchrotron-based X-ray absorption (XAS) techniques. We use a facile co-precipitation method to prepare zinc, cobalt and manganese ferrites. X-ray diffraction patterns suggest a spinel crystal phase is formed for cobalt and manganese ferrites. However, when zinc is used, two crystal phases are observed; zinc oxide and zinc ferrite. Electrocatalytic activity toward ORR was performed in 1.0 M NaOH using a glassy carbon rotary disk electrode (RDE). The highest onset potential was observed for MnFe₂O₄ followed by CoFe₂O₄ and ZnFe₂O₄. Beyond just evaluating the ORR activity of the materials, our work seeks to gain an understanding of the in situ electrocatalytic reactions taking place during ORR. To this end, efforts to conduct in situ XAS electrochemical characterization at are underway. ACKNOWLEDGMENTS: This work was supported by the National Science Foundation NSF-PREM: Center for Interfacial Electrochemistry of Energy Materials (CiE²M) grant number DMR-1827622.

Recently, single atom catalysts (SACs) have gained a great deal of attention as heterogeneous electro-catalysts, owing to their high surface area, abundant active sites, stability, and selectivity. Here, we report the fabrication of Pd and Cu single atom co-doped two-dimensional 2D Ti₃C₂T_x MXene nanosheets (Pd/Cu/Ti₃C₂T_x). This is achieved by the selective chemical etching of Ti₃AlC₂ (MAX phase) followed by ultrasonic irradiation at room temperature and then co-doping with Pd and Cu. This drove the formation of well-defined, ordered, and exfoliated Ti₃C₂T_x nanosheets with average dimensions of 300nm in length and 150nm in width as well as homogenously co-doped with Pd and Cu as single atom (nearly 1wt. % each). The catalytic activity and durability of the typically prepared Pd/Cu/Ti₃C₂T_x were investigated relative to metal-free Ti₃C₂T_x nanosheets. The results displayed the significant activity of Pd/Cu/Ti₃C₂T_x as reflected in its earlier onset potential, lower reduction potential, and higher reduction current density. This is in addition to the high tolerance to the intermediate poisonings owing to the combination between the unique properties of MXene and impressive electro-catalytic properties of Pd/Cu single atom. Intriguingly, Pd-Cu/Ti₃C₂T_x nanosheets showed a high propensity to reduce CO₂ into formic acid with a great faradaic efficiency. We believe that the results obtained from this work may open the door for new efficient and well-designated SACs/MXenes catalysts for further electrochemical applications.

Accumulation and excessive release of CO₂ into the atmosphere resulting from the consumption of fossil fuels are considered as serious debate on harmful environmental changes, such as global warming, desertification, and ocean acidification. To tackle these problems, electrochemical reduction of CO₂ has been regarded as a paramount demand in order to enable the recycling and conversion of CO₂ to useful fuels. Herein, we controlled fabrication of Cu and Zn single atom doped MXene denoted as (Cu/Zn/Ti₃C₂T_x). The typically prepared Cu/Zn/Ti₃C₂T_x were obtained in a high yield of uniform and exfoliated two-dimensional nanosheets with average diameters of (400 nm) in the length and (200 nm) in the width and coherently doped with Cu (1 Wt. %) and Zn (1 Wt. %) in the form of single atom. Cu/Zn/Ti₃C₂T_x showed a significant higher electrochemical CO2 reduction activity and stability compared to Ti₃C₂T_x at room temperature. The obtained current density on Cu/Zn/Ti₃C₂T_x was around 3 times higher than that of Ti₃C₂T_x as well as faster reduction kinetics. Intestinally enough, Cu/Zn/Ti₃C₂T_x revealed higher selectivity and tendency to reduce CO₂ into ethanol. The insights gained from this work would shed the light on designing novel bimetallic systems with MXene support for the electrochemical reduction of CO2.

The transition-metal/nitrogen carbon hybrids with the features of low-cost and high-stability are emerging candidates to homogeneous catalysts used to efficiently catalyse novel sustainable chemical transformation process. The relatively low activity of active metal sites has been an insurmountable dilemma to inhibit development of heterogeneous metal/nitrogen carbon catalyst for their practical application. Attractively, the electron transfer effect of metal/nitrogen carbon interface was wide available to enhance reactivity towards thermal and electrochemical conversions. However, the exact relationship between electron population of metal/carbonic support boundary and specific catalytic performance still remains elusive. We initially used nanoscale assembly of metal-organic complexes to realize the construction of high-loading Co/carbon interfaces. The as-obtained Co nanoparticles embedded into carbon nanofibers with high conductivity exhibited superior activity for electrochemical hydrogen evolution reaction (Nano Energy, 2016, 16, 79). To further enhance the catalytic activity of Co nanoparticles, the Mott-Schottky catalyst of nitrogen-rich carbon-coated cobalt nanoparticles (Co@NC) was prepared through direct polycondensation of simple organic molecules and inorganic metal salts in the presence of polymeric carbon nitride powder. The presented electron-deficiency Co particles induced Mott-Schottky effect can trigger the synthesis of a series of alkyl esters and even diesters in high yields via base-free aerobic of alcohol (J. Am. Chem. Soc., 2017, 139, 811). More importantly, high electron density of single Ni-N₄ sites anchored into nitrogen-doped carbon was fabricated via ligand-stabilized polycondensation method. The experiment and theorical caculation results elucidates that the enhanced electron density at each single Ni-N₄ site by a collective grouping effect substantially decreases the binding energy of H radicals to Ni, promoting a highly efficient hydrogen transfer coupling of benzyl alcohol and aniline into N-benzyl aniline (Angew. Chem. Int. Ed. 2018, 57, 15194).

Electrochemical reduction of emitted CO2 to fuels and complex hydrocarbons represents a promising solution to efficiently store intermittent, renewable energies-generated electricity in the form of chemical energy of carbon-based fuels. Copper is one of the most interesting CO2 reduction electrocatalysts, thanks to its ability to reduce carbon dioxide to complex hydrocarbons with good efficiency, however when in bulk polycrystalline form it has low activity and requires a high overpotential for the reduction reaction.
Obtaining Cu catalysts by the in-situ reduction of a copper oxide precursor at CO2 reduction potentials - with the so-called Oxide-derived Cu catalysts - is considered one of the most efficient way to increase the intrinsic activity of Cu: this class of materials have higher selectivity for complex hydrocarbons and longer stability. Moreover, their CO2 reduction activity and selectivity are strongly affected by the morphology, which controls the mass transport of the reactants to the active sites and the residence time of the reaction intermediates near the CO2 reduction active sites.

In this contribution, the CO2 reduction activity and selectivity of Cu catalysts derived from the reduction of Copper Oxide (CuOx) hierarchical nanostructures with different structural features are investigated.
We firstly employ a nanocrystalline CuOx material, which presents a higher availability of undercoordinated surface sites which, coupled with the high surface area, increases the activity. We synthesize nanostructures for this highly-active material by Pulsed Laser Deposition (PLD), a synthesis method that allows us to control the morphology of the deposited nanostructures down to the nanoscale: this technique, in fact, allows for a fine control over the deposit morphology by finely tuning the deposition parameters. In this way, the chemical composition and thickness of the hierarchical quasi-1D nanostructures are kept constant and it is possible to modify their aspect ratios, their nanostructuring and their micro- and nanopores diameters.
Characterization of the PLD-synthesized nanostructures shows an amorphous matrix with embedded crystalline seeds of CuO and Cu2O, differently from other reported oxide-derived Cu catalysts. CO2 reduction performance of the oxide-derived Cu nanostructures is assessed through chronoamperometric measurements in aqueous electrolyte, showing good current densities and a good stability for at least one hour of prolonged operation. Liquid products analysis, performed via 1H NMR spectroscopy, shows a high productivity of ethanol at limited potentials applied (V = -0.4 VRHE). Overall, the CuOx-derived nanostructures show a good faradaic efficiency at low overpotentials applied, with an FE for CH4 of 40% and an FE for liquids of 55% at -0.4 VRHE.

This contribution shows how coupling an amorphous, oxide-derived Cu catalyst with a fine control over morphology can be an efficient way to increase the activity of CO2 reduction catalysts.

Electrochemical water splitting has been recognized as a promising route for hydrogen fuel generation at the industrial scale. However, due to the sluggish kinetics of both half cathodic and anodic reactions of the electrolyzer (hydrogen/oxygen evolution reactions-HER/OER), the water electrolysis usually requires large input energy that strongly inhibits its application in practice. Therefore, facile and scalable construction of catalytic active, durable and low-cost electrocatalysts to largely boost the sluggish kinetics during electrocatalytic process remains a huge challenge.
Hybridization of a catalytic active material with a large surface area and good electrical conductivity with a hydrophilic substrate is a wise strategy to enhance the electrocatalytic performance by achieving the synergistic effects. Recently, two-dimensional (2D) titanium carbide MXene-based composites applied for energy conversion applications have widely attracted researches due to their unique electronic structure and surface properties over other carbonaceous materials such as reduced graphene oxide (rGO), carbon nanotubes (CNTs), etc. However, MXenes suffer from sluggish ionic accessibility and mass transport due to the strong van der Waals attraction and hydrogen bonding between inter-sheets, which seriously inhibits the realization of the full potential for these materials.
In this work, we develop an advanced electrocatalyst system composed of iridium cobalt (IrCo) hollow structure homogeneously distributed on the highly porous activated titanium carbide MXene (ac-Ti₃C₂) matrix via a wet chemical method at room temperature. We exploit the OER performance of the as-prepared catalyst (IrCo@ac-Ti₃C₂), which shows significantly enhanced catalytic activity with a low overpotential of 220 mV at a current density of 10 mA cm^-2, surpassing the benchmark IrO₂ and stability for 30 h of continuous testing in alkaline media. We also demonstrate the effectiveness of the electrode architecture through the rapid charge/mass transport in the internal electrode by tripling the mass loading on the rotating disk electrode. As expected, the novel catalyst displays a negligible dependence of the current density on the mass loading. Finally, when employed as a bifunctional catalyst in overall water splitting reaction, the optimal IrCo@ac-Ti₃C₂ also shows superior activity with the low voltage cell and excellent long-term durability, standing among the best catalytic activity of the reported electrocatalysts.
The outstanding performance can be attributed to the synergistic effects below: (1) the excellent electrical conductivity of the substrate and the smooth metal-support electronic interaction can ensure fast electron conductance; and (2) the hierarchically porous nanoarchitecture of the obtained compound with abundant of macro/meso/micropores and edges/defects sites not only benefits the ionic accessibility, increases the electrode wettability, boosts the migration of reactant-related species, and fastens gas evolution/evacuation by shortened mass-transport pathways, but also assures uniform distribution and maximization of the number of active sites exposed to the electrolyte, preventing the aggregation of active materials during electrochemical process.

Stable and affordable electrocatalysts are urgently needed to accelerate the transition from conventional fossil fuels to sustainable energy resources such as hydrogen derived from renewable energy sources such as solar and wind. Converting said harvested energy electrochemically into chemical fuels and feed stocks is also highly desirable. Over the last two decades, the electrocatalytic splitting of water, and carbon dioxide reduction to hydrocarbons have received great attention, however approximately 90% of the electricity input is consumed in the anodic oxygen evolution reaction (OER)¹ due to poor reaction kinetics. Precious metal based catalysts (Pt, Ir, Ru) are commonly used as OER catalysts, but prove uneconomic for large scale industrial deployment. New low cost, stable, earth-abundant catalysts such as those based on transition-metal alloys (Ni,Co) offer great promise to overcome these kinetic limitations.

In this work, we used a two-step novel milling process to produce NiCo-based amorphous nano-electrocatalysts. Cryo-milling (mechanical milling of precursors at cryogenic temperatures to achieve alloying) followed by surfactant-assisted ball milling (SABM for particle size reduction) create stable amorphous alloys with high surface areas and coordinatively unsaturated active sites for the reaction of OER intermediates. The improved stability of the amorphous alloys is yet another exciting feature that can significantly enhance the overall catalytic performance of the catalyst.

Using density functional theory (DFT) calculations, two potential elements V and Se were predicted to reduce the overpotential of the catalyst when incorporated in these structure. Alloys of NiCoV and NiCoSe were milled under various conditions and the structural evolution of the system monitored using x-ray diffraction (XRD) and electron microscopy. Our results confirm the production of two fully alloyed ternary systems after 6 hours of milling time. Furthermore, a wide composition range could be investigated using this synthesis technique to accordingly tailor the activity of the catalyst. The electrocatalytic activity and stability of the catalysts was evaluated by Tafel measurements obtained from linear sweep voltammetry (LSV) and cyclic voltammetry (CV) experiments. We found that V in NiCo-based alloys helped the amorphous structures to stabilize by forming non-transitional clusters² while Se significantly reduced the size of the produced particles. Operando X-ray Absorption Spectroscopy (XAS) was conducted to reveal the role of adding V and Se on the chemical-structural transformation and bonding environment of surface species during the reaction. This work suggests that milling can potentially be used to produce OER catalysts for industrial applications.

1. S. Verma, S. Lu, and P. J. A. Kenis, Nat. Energy (2019).
2. J.-F. Deng, H. Li, and W. Wang, Catal. Today, 51, 113–125 (1999).

We are facing energy crisis because of the limitation of the fossil fuel and the pollution caused by burning it. Clean energy technologies, such as fuel cells and metal-air batteries, are studied extensively because of their high efficiency and less pollution. Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are essential in the process of energy storage and conversion, and noble metals (e.g. Pt) are needed to catalyze the critical chemical reactions in these devices. Functionalized metal coordinated frameworks such as metal organic frameworks, covalent organic frameworks, and zeolitic imidazolate frameworks can be used as efficient catalysts to replace expensive and scarce platinum-based catalysts for energy storage and conversion. Traditionally, experimental studies on the catalytic performance of metal coordinated frameworks have been conducted extensively, however, there is lack of computational studies to guide the experiments for rapid search for the best catalysts. In addition, catalytic mechanism and rational design principles towards ORR and OER also needed to be fully understood.

Density functional theory calculations are performed to calculate the thermodynamic and electrochemical properties of metal coordinated frameworks for ORR and OER. Gibb’s free energy, overpotential, charge transfer and ligands effect are evaluated. The charge transfer analysis shows the positive charges on the metal coordinated frameworks play an essential role in improving the electrochemical properties of the metal coordinated frameworks. Based on the calculations, design principles are introduced to rationally design and predict the electrochemical properties of metal coordinated frameworks as efficient catalysts for ORR and OER. An intrinsic descriptor is discovered for the first time, which can be used as a materials parameter for rational design of the metal coordinated frameworks for energy storage and conversion. The success of the design principles provides a better understanding of the mechanism behind ORR and OER and a screening approach for the best catalyst for energy storage and conversion.

Engineering the electronic properties of catalysts to facilitate electron transfer and activation of dynamically stable peroxydisulfate (PDS) offers a promising strategy for efficient remediation of recalcitrant organic pollutants through heterogeneous Fenton-like processes, but it also presents challenges due to a lack of facile methods. In this presentation, we report a simple defect engineering strategy that significantly increases the efficiency of PDS adsorption and activation by introducing oxygen vacancies (VO) into NiO to produce an electron-rich surface. Experimental studies and density functional theory (DFT) calculations confirmed that VO confined in NiO could successfully cause synergetic effects of lower adsorption energy and more exposed active sites, thus facilitating the bonding with PDS molecules and promoting the reactivity of PDS-NiO complex, giving rise to dramatic enhancement of catalytic performance with removal rate as high as 3.978 mmol(phenol) min⁻¹ g(NiO)⁻¹. Mechanistic studies further reveal that the surface-activated PDS-NiO complex mediate oxidative polymerization of phenol (a model pollutant) involved in the generation of phenoxyl radicals and subsequent coupling reactions, providing the potential to convert recalcitrant organic pollutants into value-added products. We believe this work pave the way toward practical fabrication of highly efficient “defect-type” Fenton-like catalysts, and the present method also provides new opportunities to achieve sustainable wastewater treatment through reuse of organic pollutants.
This presentation is based on our recent publication:
Applied Catalysis B: Environmental, 2019, 254, 166-173
https://doi.org/10.1016/j.apcatb.2019.04.094

Nanocarbons are outstanding candidates for electrocatalysis, due to their electrical transport properties (theoretical graphene charge carrier mobility, 200,000 cm² V^-1 s^-1), high specific surface areas (SSA of activated carbons, >2000 m² g^-1), and hierarchical arrangements of electrochemically-active surface functionalities (topological defects and heteroatom dopants). A significant challenge for carbon-based nanomaterials is the structural and chemical disorder introduced during synthesis (e.g. pyrolysis-based catalyst site formation) that makes experimental clarification of the quantity/nature of catalytic sites untenable. To address this issue, new methods will be needed to synthesize nanocarbons with well-defined catalyst site placement. Here, we report a highly-tunable graphene-based electrocatalyst, nanowire-templated three-dimensional fuzzy graphene (NT-3DFG), with promising performance for 2 e^– oxygen reduction reactions (ORR) to form H₂O₂ (~30 mV onset potential and 93 ± 3 % selectivity) after process optimization for high densities of graphene edges. Using spectroscopic surface characterization techniques and density functional theory calculations, we find that the edge sites become saturated by carbonyl (C=O) or hydroxyl (C–OH) groups under alkaline reaction conditions, and that the functionalized zigzag edge sites with high coverage of carbonyl groups could lead to selective, two-electron ORR. We also demonstrate control over ORR pathway (2 e^– vs. 4 e^–) on NT-3DFG electrodes by anchoring metalloporphyrins to out-of-plane graphene surfaces via facile surface treatment, and these research efforts allow experimental study of both graphene edges and heterogenized molecular ORR catalysts. The incorporation of both NT-3DFG’s edges and coordinated transition metals may allow a synergistic, hybrid active site(s) approach to improve ORR activity. Our combined synthesis-, spectroscopy-, and model-based approach accelerates active site characterization and carbon-based electrocatalyst discovery.

Rutile Ruthenium Dioxide (RuO₂) is a highly active catalyst for water oxidation in acidic solutions^1-3. It can also undergo fast surface redox reactions in the electrochemically stable potential window of water, making it an ideal material for electrochemical capacitors that can charge and discharge in a much shorter time scale than batteries⁴. However, there is a lack of fundamental understanding of the active sites, atomic scale processes occurring at the oxide-electrolyte interface and the kinetics of the reaction. In this work, we combine in situ surface sensitive techniques, electrochemical mass spectrometry and density functional theory calculations to elucidate the nature of the active sites, reaction mechanism and Tafel (current-potential) behavior of the oxygen evolution reaction (OER) on ruthenium dioxide surfaces.
First, using synchrotron based ambient pressure X-ray photoelectron spectroscopy and in situ surface diffraction on a single crystal (110) surface coupled with density functional theory calculations, we show that with increasing potential from 0.5 V_RHE, adsorbed water on the coordinatively unsaturated sites (CUS) is successively deprotonated. At oxygen evolution potentials (1.5 V_RHE), surface diffraction measurements reveal the presence of an –OO species on the CUS site. Computation results show that this –OO group is stabilized by a neighboring –OH group suggesting that the final deprotonation step is rate-limiting. We then extend the mechanistic understanding obtained using single crystal surfaces to polycrystalline RuO₂ surfaces using surface enhanced infrared spectroscopy. A vibration mode corresponding to the presence of an –OO like species was detected at potentials relevant to oxygen evolution, thus validating the applicability of single crystal surfaces to understand the OER mechanism on polycrystalline surfaces.
In order to understand the kinetic behavior of oxygen evolution of RuO₂ surfaces, electrochemical activity measurements were combined with highly sensitive electrochemical mass spectrometry to quantify the amount of evolved oxygen at low overpotentials. The combined electrochemical and mass spectrometry results indicate that the current-potential dependence goes through three distinct Tafel regimes, where the Tafel slope increases with electrochemical potential from 30 mV/decade at low potentials to 120 mV/decade at high potentials. We rationalize this kinetic behavior by modeling the coverage of the reaction intermediates as a function of potential, based on their energetics proposed on the (110) surface. Finally, using oriented RuO₂ surfaces, which have different binding energetics for oxygenated species, we highlight differences in Tafel behavior as a function of potential.
Thus, by using different experimental and theoretical techniques as well as model surfaces, this study identifies the active sites and reaction mechanism for oxygen electrocatalysis. We also highlight the importance of understanding and controlling kinetics of OER on highly active catalysts, in order to achieve higher current densities at low overpotentials.

References:
[1] Trasatti S. Electrochimica Acta. 1984;29(11):1503-1512.
[2] Lee Y, Suntivich J, May KJ, Perry EE, Shao-Horn Y. The Journal of Physical Chemistry Letters. 2012;3(3):399-404.
[3] Stoerzinger KA, Qiao L, Biegalski MD, Shao-Horn Y. The Journal of Physical Chemistry Letters. 2014;5:1636-1641.
[4] Over H. Chemical Reviews. 2012;112(6):3356-3426.

As metal-oxygen octahedra in ABO₃-type perovskite oxides are regarded as active sites for notable oxygen electrocatalysis, many studies have been carried out seeking higher catalytic activity by experimenting with various transition metals and their combinations on the B site, as well as the influence of oxygen vacancies. In contrast to the substantial amount of research on different compositions and their impact on overall catalytic performance, the correlation between the structure of the oxygen octahedra in perovskites and electrocatalysis performance has been overlooked, resulting in a limited understanding of the effect of structural variation. From a structural viewpoint, the ABO₃ perovskite framework consists of corner-shared [BO₆] octahedral units, which are catalytically active sites, together with A-centered dodecahedra. Consequently, identifying variations in the shape of the [BO₆] octahedra and understanding under what circumstances structural change takes place remain as key issues for the accurate prediction of electronic structure and subsequent oxygen evolution reaction (OER) activity. In particular, since the shape of the [BO₆] octahedra is determined largely by the location of oxygen anions in the lattice, visualization of the oxygen positions is an essential part of that direct observation of structure (J. Bak et al., Nano Lett. 17, 3126 (2017)), despite being very challenging.
In the first part, by using both sintered polycrystals with various grain sizes and thin films on bicrystal substrates, we directly verify that surface-terminating grain boundaries in LaCoO₃ and LaMnO₃ are exceptional in oxygen evolution electrocatalysis, showing more than an order of magnitude higher activity. A combination of atomic-scale structure observation and density functional theory calculations demonstrates that the displacement of atoms in metal-oxygen octahedra correlates with significant splitting of the degenerate transition-metal 3d orbitals, and subsequently higher covalency for rapid charge transfer between metals and oxygen is attained (Y. Heo et al., Adv. Energy Mater. 8, 1802481 (2018)). In the second part, by utilizing epitaxial LnNiO₃ (Ln = La, Pr, Nd) thin films, we also demonstrate that order-of magnitude higher catalytic activity for the OER can be achieved by selective electrochemical exchange of Fe in the surface region. In addition, we directly establish that strong distortion of oxygen octahedra is induced during the Fe exchange, and that this structural perturbation correlates with the high density of Ni/Fe 3d and O 2p states near the Fermi level, permitting easier charge transfer (J. Bak et al., Nature Commun. (in press)). By demonstrating the remarkable structure-property relationship in oxygen electrocatalysis, our findings suggest that symmetry breaking by atom displacements in metal-oxygen octahedra can be a new and efficient approach to remarkably enhance the oxygen electrocatalytic efficiency in perovskite oxides.

The discovery of new materials with high oxygen reduction reaction (ORR) catalytic activity and high thermodynamic stability under typical cathode operating conditions are crucial for the development of improved solid oxide fuel cell (SOFC) and related technologies. To aid in this discovery we have performed high-throughput Density Functional Theory (DFT) calculations of ~2150 perovskite materials,[1] assessed bulk thermodynamic stability via multicomponent phase stability analysis using the Python Materials Genomics (Pymatgen) package[2], and predicted oxygen reduction reaction (ORR) catalytic activity with the O p-band center. We found that BaCo_xFe_yZr_zO_3-d (BFCZ) (x+y+z=1) was a promising family of highly active and stable ORR cathode materials.[1]

Here, perovskite-type oxides with compositions BaCo_xFe_yZr_zO_3-d (BFCZ) (x+y+z=1, z=0.2, 0.4, 0.6 and 0.75 Zr fraction) were synthesized and characterized via X-ray diffraction (XRD) analysis and electrical conductivity relaxation (ECR) measurements. The bulk stability of BFCZ was analyzed with XRD and atomic force microscopy (AFM) after accelerated high temperature testing. The performance of BFCZ as an electrode was characterized by monitoring the impedance over time of dense spin-coated BFCZ films on a Sm-doped ceria (SDC) protected YSZ electrolyte.

All of the synthesized BFCZ compositions exhibit the cubic perovskite structure. ECR measurements indicate exceptionally high oxygen surface exchange rates and bulk oxygen diffusivity values, on par or better than other state-of-the-art high surface exchange materials like Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃ (BSCF).[3,4] Comparisons between the experimentally measured activities and the model predicted activity results in good qualitative agreement. Chemical trends of activity and stability as a function of Zr content are also in agreement between the computational model and the experimental results. These results collectively demonstrate (1) the power of the developed modeling scheme in predicting and rationalizing the stability and activity of BFCZ and perovskite catalysts more broadly and (2) experimental confirmation of the promise of BFCZ with high Zr content as both a highly active and stable ORR catalyst material.

References:
[1] Jacobs, R., et al. Advanced Energy Materials 8 (2018).
[2] Ong, S. P., et al. Computational Materials Science 68 (2013).
[3] Shao, Z. and Haile, S. M. Nature 431 (2004).
[4] Duan, C., et al. Energy and Environmental Science 10 (2016).

In general, ‘Task Parallelization’ is the simplest and ultimate solution for bottle-neck problems of serial multi-step tasks. Catalytic reactions for energy conversion and storage are in many cases, serial multi-step reactions having bottle-neck steps. In this work, we first suggest the concept of ‘Task Parallelization’ in multi-step catalytic reactions. As the prototype study, we applied this concept to oxygen evolution reaction (OER) on electrocatalytic materials. Since the catalytic activity of a material is the result of moderate adsorption energies of reactants and intermediates, mixture of multiple phases having different bottle-neck reaction steps can lead to excellent catalytic material composite, even though each phase is not much good. In this study, we used first-principles calculations and thermodynamic modeling to predict reaction pathways and facile synthesis of multiphase nanostructure. Our well-designed experiments prove this concept very well for OER electrocatalysis.

Transition metal phosphides (TMPs) are a well-known category of catalysts with good activity and durability for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Most TMPs are found to possess a naturally oxidized surface under ambient condition. However, how the oxidized surface adapts to the electrochemical conditions and leads to the observed catalytic properties are poorly understood. To gain insights into their structure-properties correlations, quasi-in-situ X-ray Photoelectron Spectroscopy (XPS) is employed to study the surface restructuring of TMPs under electrochemical conditions. It is found that the naturally oxidized and phosphorus-rich surface of cobalt phosphide (CoP) will be removed in acidic electrolyte under HER conditions, exposing a fresh CoP surface. Under HER conditions in alkaline electrolyte, electrochemical reduction of the oxidized species and dissolution of the oxidized P species will together render a Co-rich phosphide surface. Under OER conditions in alkaline electrolyte, a cobalt hydroxide/oxide surface will be formed as a result of combined oxidation and anion substitution. In order to explore the dependence of catalytic reactivity on substitutional doping, cation- and anion- substituted CoP nanoparticles are also investigated. For Fe_0.5Co_0.5P, the Fe substituents are more easily oxidized than Co in the air; they are also more difficult to be reduced under HER conditions in alkaline electrolyte and the remaining Fe-OH species on the surface hampers the activity for HER. For CoP_0.5S_0.5, the S substituents are less oxidized than P in the air; they are also more difficult to be oxidized under OER conditions in alkaline electrolyte, and the remaining sulfate like species enhance the activity for OER.

The use of novel anion exchange membranes (AEM) in alkaline water electrolysers is now of particular interest since it eliminates the aqueous KOH electrolyte and can provide improved form factors and much higher current densities that rival proton exchange membrane (PEM) electrolysers.¹ The use of AEM water electrolysis also enables the use of less expensive, earth-abundant, non-platinum group metals (PGM) materials such as nickel-based alloys as electrocatalysts.¹
Conventional Ni electrocatalysts for the oxygen evolution reaction (OER) still display large overpotentials and slow reaction kinetics. The addition of Co to Ni-based alloys has been noted to increase the activity by stabilizing the β-NiOOH phase over γ-NiOOH, but this often leads to an increase in the oxygen overpotential.^2,3 The issue of increased overpotential can be mitigated with the use of amorphous structures to reduce oxygen overpotentials with the addition of Co.^3,4 This prior research has been limited to structures not ideal for catalysts in AEM water electrolysis.⁴
In this work, amorphous Ni_79.2-xCo_xNb_12.5Y_8.3 (x = 0 and 5 at.% Co) nanoparticles were synthesized using cryogenic mechanical alloying followed by surfactant-assisted high energy ball milling (SA-HEBM).⁵ This two-stage ball milling process provided a novel processing route for the production of nanostructured / amorphous materials with a wide range of possible compositions not achievable through rapid solidification, electrodeposition, or chemical reduction techniques. The effect of milling temperature and time on the resulting powder microstructure was characterized using a combination of x-ray diffraction and electron microscopy.
The electrocatalytic activity and stability of amorphous nanoparticles on the alkaline oxygen evolution reaction were investigated through cyclic voltammetry and Tafel measurements. Cyclic voltammagrams demonstrated stable and reproducible curves for amorphous alloys up to 10000 cycles while crystalline Ni and Ni₉₅Co₅ showed signs of deactivation with cycling. Initially the addition of Co to crystalline Ni resulted in enhanced OER catalytic performance, but the performance drastically reduced beyond 500 cycles. A similar increase in performance was observed when adding Co to amorphous Ni_79.2Nb_12.5Y_8.3, except the activity and stability was maintained throughout the 10000 cycles. The presence of Co in Ni-based materials is shown to enhance the OER activity with amorphous structure providing excellent long-term cycling stability. These features, along with the high surface area achieved through SA-HEBM, provide a cost-effective and simple method for producing stable and active amorphous electrocatalysts for the oxygen evolution reaction in AEM water electrolysis.

1. V. R. Stamenkovic, D. Strmcnik, P. P. Lopes, and N. M. Markovic, Nat. Mater., 16, 57–69 (2016).
2. T. N. Lambert et al., Chem. Commun., 51, 9511–9514 (2015).
3. K. M. Cole, D. W. Kirk, and S. J. Thorpe, J. Electrochem. Soc., 165, 3122–3129 (2018).
4. R. D. L. Smith, M. S. Prévot, R. D. Fagan, S. Trudel, and C. P. Berlinguette, J. Am. Chem. Soc., 135, 11580–11586 (2013).
5. S. Ghobrial, K. M. Cole, D. W. Kirk, and S. J. Thorpe, Electrocatalysis, In Review (2019).

Clean energy sources such as hydro, wind and solar energy represent some of the alternatives to diminish our dependency on fossil fuels. Fluctuations in their availability due to geographical features and seasonal weather limit their use. Finding reliable ways to store the energy produced by these clean energy sources will help mitigate these issues.
Chemical energy storage and energy conversion are two ways to approach this problematic. Hydrogen is a clean fuel source, but currently it is mostly obtained from fossil fuels. Water splitting for the production of hydrogen in alkaline water electrolysers arise as a viable option. However, commercial electrolysers perform at efficiencies of 60 to 70%, which is insufficient to supply for the demand of the fast-growing hydrogen economy.
Hydrogen production by electrochemical water splitting is limited by the oxygen evolution reaction (OER). This reaction takes place at the anode, sluggish kinetics and accumulation of oxygen bubbles can block active sites on the surface of the electrode. Electrodes are typically made of expensive precious metals, however nickel-based metal oxides are a promising and robust alternative. Nickel electrodes have shown high activity and reduced activation energies for alkaline OER. However, the surface wettability and the accumulation of oxygen bubbles blocking active sites remain as an issue.
Our work explored the inclusion of boron containing species that are conductive, improve wettability, are stable under alkaline conditions and are inert at OER potentials. The decorated nickel surfaces showed improved OER activity. The electrodes were characterized using electron microscopy, electrochemical testing and X-ray diffraction measurements. The study was complemented with theoretical calculations of the electronic effects of the boron species and reaction intermediaries at the interface of the electrode and the electrolyte.

A key challenge for the design of efficient photoelectrochemical devices that employ nanoscaled interfacial designs for solar energy-to-fuels conversion lies in resolving atomic structures and dynamics at the active interfaces, and relating these to the complex cascade of events which includes excited-state charge separation, charge-accumulation, and multi-step energy conversion and catalysis. Our research programs have been developing in-situ, time-resolved X-ray techniques aimed at the resolution of atomic-scale structures that underlie interfacial charge transfer and photo-driven water-splitting catalysis under conditions relevant to photoelectrochemical function for both interfacial thin-films and molecular photosensitizers and catalysts bound to semiconductor surfaces.

This presentation will show how high energy (>50 keV) X-ray scattering and pair distribution function (PDF) analysis provides atomic structure characterization that usefully complements electronic structure characterization derived from a combination of soft X-ray absorption (XAS), resonant X-ray emission (RXES), resonant inelastic X-ray scattering (RIXS) techniques for structure-function analysis of transition metal semi-conductor and catalyst thin films. Microporous high surface area electrodes were designed that allowed PDF analysis to be accomplished 0.2 Å spatial resolution for amorphous transition metal oxide thin films having thickness of down to 60 nm. The combined electronic and structural analyses demonstrate that hole transfer to catalytic sites in amorphous cobalt oxide thin films is the rate-limiting step for electrochemical water-splitting rather than the multi-step catalytic events themselves. The macroscopic catalytic properties of the thin films were found to be correlated to the electronic structures measured at the atomic scale for the metal-oxo cluster domains. These results show the interplay between intrinsic catalytic activity and charge transport properties of semiconductor thin-film catalysts.

We have extended this approach by the development of nanoporous electrode architectures which enable the use of combined PDF and X-ray spectroscopy analyses to resolve differences in coordination structures for molecular photosensitizers and catalysts in solution and when bound to semiconductor oxide surfaces. Measurements on ruthenium photosensitizer complexes bound to the TiO₂ surface resolve distortions in the surface-coordinating ligand structures. On-going work is investigating the correlation between ligand structural distortion and the metal-to-ligand charge-transfer (MLCT) states with ultrafast interfacial charge injection function.

The intermitting nature of renewable energy sources, i.e. wind and solar radiation, requires storage solutions to match the energy demand of human activities. Among the chemical fuels, hydrogen is an interesting energy vector due to its high gravimetric energy density. Energy can be stored in the chemical bonds of hydrogen by water electrolysis and electric energy can be obtained with the related back reaction by using a fuel cell.
The oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR), involved respectively in the forward and backward reaction, are associated with higher conversion efficiency losses in comparison with their half-reaction counterparts. Therefore, catalysts with superior activity for these reactions are of high interest. Understanding the reaction mechanism and identifying the catalytically active sites is important in order to improve the performance of the state-of-the-art catalysts. However, the atomic structure under reaction condition can be different than in the as synthesized phase. In this contribution, we will present operando wide angle X-ray scattering (WAXS) results providing insights into important structural changes leading to formation of catalytic active phases or associated to degradation mechanisms.
For the OER, NiFe layered double hydroxides (LDHs) are recognized among the most active catalysts in alkaline electrolytes [1]. Their crystal structure is known and is composed of layers of edge sharing metal oxygen octahedra that are intercalated with water molecules and charge balancing anions. Despite their structure is well characterized, there are evidences that the OER active structure of LDH catalysts during OER is different than the one of the as prepared materials [1]. Using operando WAXS we identified the structure of the catalytically active catalyst state and observed that both the interlayer and in-plane distances contract under operating conditions [2]. We further proved that this structure is also exhibited by other Fe containing LDHs under OER conditions. These results are important to validate the structures used in theoretical models to predict the OER mechanism and ultimately to develop strategies to improve the OER activity of NiFe LDH catalysts.
In a second study, we show that operando WAXS can also be applied to study degradation mechanisms. For ORR, octahedral PtNi alloy based nanoparticles shows very high activities in thin film rotating disk electrode measurements in acid [3]. However, the prevention of Ni loss and morphological stability under operation are still challenging. Using a combination of operando WAXS and ex situ X-ray fluorescence spectroscopy (XRF) we found that a structural transformation is involved in the degradation mechanism occurring while cycling the applied potential. The catalyst crystalline structure was modeled using multiple metal phases and the dealloying of the Ni richer phase with associated Ni loss is proposed as the main degradation mechanism [4]. The sensitivity of this process to operating conditions such as the upper potential limit of the cycling protocol was investigated. The binary PtNi alloy based nanoparticles were also compared with surface doped ternary compositions as potential method to prevent structural changes.




[1] F. Dionigi, P. Strasser, Adv. Energy Mater., 2016, 6.
[2] F. Dionigi, Z. Zeng, I. Sinev, T. Merzdorf, S. Deshpande, M. Bernal Lopez, S. Kunze, I. Zegkinoglou, H. Sarodnik, D. Fan, A. Bergmann, J. Drnec, J. Ferreira de Araujo, M. Gliech, D. Teschner, J. Greeley, B. Roldan Cuenya, P. Strasser, submitted, 2019.
[3] P. Strasser, Science, 2015, 349, 379.
[4] F. Dionigi, C. C. Weber, M. Primbs, M. Gocyla, A. Martinez Bonastre, C. Spöri, H. Schmies, E. Hornberger, S. Kühl, J. Drnec, M. Heggen, J. Sharman, R. Edward Dunin-Borkowski, P. Strasser, submitted, 2019.

Water electrolysis has been accentuated important building blocks for energy conversion and storage concepts in aqueous system. In particular, oxygen evolution reaction (OER) is important counter reaction for not only hydrogen evolution reaction but also various electrochemical synthetic reaction including CO₂ and N₂ process. Durable electrocatalyst designs are an urgent problem in both acid and alkaline media because even noble metal based Ru and Ir materials suffer significant activity loss due to the highly corrosive thermodynamic potential window. In order to obtain highly durable electrocatalyst with high activity, it is urgent to link the activity-stability relationship during reaction. Here, we will discuss activity-stability relationship in water electrolysis reaction using our well-defined model system from simple monometallic system to bimetallic and complicated system. In the model system approach, we will discuss the general relationship between activity and stability what we observed and suggest which concept appeal to the future direction for achieving high activity and stability in OER electrocatalysis.

Tailoring active sites position and concentration on the catalyst surface has been shown to effect activity and stability of a catalyst. Specifically, the surface coverage of transition metals on CeO₂-ZrO₂ (CZO) nanoparticles (NPs) is known to directly affect the redox property and ability to utilize lattice oxygens. Recently, Ni incorporation in the core of a CZO NPs has caused a two-fold increase in catalytic activity per surface area due an increased oxygen storage capacity. However, the activity of the particle is dependent on the Ni surface concentration and is traditionally deposited via the insipid wetness impregnation technique, forming Ni on the surface of the particle. In this work, the role of Ni position on catalytic activity is probed to develop a two-step synthesis process which allows for spatially controlled dopant distribution for improved catalytic activity.

A two-step process, co-precipitation/molten salt synthesis (MSS), has been explored for the catalyst core, and urea deposition synthesis for the shell of CZO:Ni NPs. The concentration of oxygen vacancies in these NPs was studied using Raman spectroscopy, which shows an order of magnitude increase in oxygen vacancies based on the change in intensity of defect-induced (D) band at 600 cm^-1 as the position of the active site is modified. High resolution transmission electron microscopy (HRTEM) and energy dispersive x-ray spectroscopy (EDX) identify that a core-shell structure is obtained with spatially controlled Ni concentrations To further correlate the dopant position and oxygen vacancies, local bond structure and geometry of the active site (Ni) were studied by performing L edge X-ray Absorption Spectroscopy (XAS) measurements. Geometric distortions due to the e_g and t_2g orbitals is dependent on the distribution of Ni within the NPs. The change in the geometric structure of Ni was observed via in situ CO chemisorption XAS measurements and correlated to the orbital structures. These preliminary results on the spatially controlled CZO core-shell NPs demonstrate that controlling the position of the active site (Ni) modifies the oxygen vacancies of the NPs which is the first step to understand the diffusion of the active site during a reaction and deactivation pattern of the catalyst.

Engineer Metal Oxide–Metal–Carbon Interface for Highly Efficient Oxygen Reduction
Jun Chen
[email protected]
Assistant Professor in Department of Bioengineering,
University of California, Los Angeles, CA 90095, United States
Abstract:
Oxygen is the most abundant element in the Earth’s crust. The oxygen reduction reaction (ORR) is also the most important reaction in life processes and energy converting/storage systems. Developing techniques toward high-efficiency ORR remains highly desired and a challenge. Here, we report a N-doped carbon (NC) encapsulated CeO2/Co interfacial hollow structure (CeO2–Co–NC) via a generalized strategy for largely increased oxygen species adsorption and improved ORR activities. First, the metallic Co nanoparticles not only provide high conductivity but also serve as electron donors to largely create oxygen vacancies in CeO2. Second, the outer carbon layer can effectively protect cobalt from oxidation and dissociation in alkaline media and as well imparts its higher ORR activity. In the meanwhile, the electronic interactions between CeO2 and Co in the CeO2/Co interface are unveiled theoretically by density functional theory calculations to justify the increased oxygen absorption for ORR activity improvement. The reported CeO2–Co–NC hollow nanospheres not only exhibit decent ORR performance with a high onset potential (922 mV vs RHE), half-wave potential (797 mV vs RHE), and small Tafel slope (60 mV dec–1) comparable to those of the state-of-the-art Pt/C catalysts but also possess long-term stability with a negative shift of only 7 mV of the half-wave potential after 2000 cycles and strong tolerance against methanol. This work represents a solid step toward high-efficient oxygen reduction.
References (# Equal contribution author; * Corresponding author)
[1] L. Lv, J. Chen* et. al. “A Universal Method to Engineer Metal Oxide-Metal-Carbon Interface for Highly Efficient Oxygen Reduction”, ACS Nano, 12, 3042 (2018)
[2] J. Chen, Y. Huang, N. Zhang, H. Zou, R. Liu, C. Tao, X. Fan and Z. L. Wang. “Micro-Cable Structured Textile for Simultaneously Harvesting Solar and Mechanical Energy”, Nature Energy, 1, 16138 (2016)
[3] J. Chen and Z. L. Wang. “Reviving Vibration Energy Harvesting and Self-Powered Sensing by a Triboelectric Nanogenerator”, Joule,1, 480 (2017)

High entropy oxides (HEOs) have been a recently emerging class of materials with promising catalytic activities and electrochemical energy storage capabilities.¹ By incorporating five or more cations, an entropy-stabilized single-phase crystal structure can be formed with various combinations of atomic elements. However, there have been a limited number of reports of HEOs being utilized for catalytic reactions. Here, HEOs with fluorite and rock-salt structures were synthesized via solid state synthesis, and their catalytic activities for low-temperature carbon monoxide (CO) oxidation and thermochemical water splitting (TWS) were explored. HEOs allow incorporation of different elements while maintaining the same crystal structure. Thus, unprecedented chemical combinations can be synthesized to engineer chemical and defect structure of oxides for improved catalysis. Moreover, the phase stability at different temperatures and atmospheres will be investigated to provide insight into the extent of entropy stabilization in HEOs. Current research focuses on HEOs with fluorite-type and rock-salt-type structures but will expand to perovskite-type and spinel-type structures with multiple cation sites.

1. Sarkar, A.; Wang, Q.; Schiele, A.; Chellali, M. R.; Bhattacharya, S. S.; Wang, D.; Brezesinski, T.; Hahn, H.; Velasco, L.; Breitung, B. Adv. Mater. 2019, 1806236.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Increasing the performance of Solid Oxide Electrolysis Cells for the production of hydrogen (H₂) from water (H₂O) splitting requires the use of electrodes with low polarization resistance. To effectively enhance their splitting rate, catalysts are infiltrated into the porous electrodes. Traditional catalyst deposition methods such as wet impregnation or atomic layer deposition are expensive and the resulting catalysts suffer from agglomeration due to operation at high temperature that reduces the long-term performance. To improve upon the aforementioned methods and reduce the catalyst deposition cost, the in-situ growth of catalysts from perovskite oxides (termed catalyst exsolution) has been proposed as a means to decorate the porous electrode with highly dispersed, nanometer-sized catalytic particles [1-3]. These particles are highly active towards reactions of interest and show increased resistance to agglomeration due to their socketed nature [4].
Exsolution occurs when the oxygen chemical potential is reduced significantly and hence can be triggered either thermochemically (i.e. reduction of a perovskite oxide at elevated temperatures in the presence of fuel, usually H₂) or electrochemically (i.e. application of a voltage/current at elevated temperatures with/without the presence of fuel) [5]. In this work, we study the dependency of the exsolution mode (thermochemical vs. electrochemical) on the particle morphology, distribution, dispersion and activity towards H₂O-splitting to H₂. To investigate this dependency, exsolution of metallic Iron (Fe⁰) from an A-site deficient La_0.75Sr_0.2Cr_0.95Fe_0.05O_3-δ (LSCF) electrode was examined. Exsolution was triggered at different temperatures (in the 700C-900C temperature range) either thermochemically using 3%H₂-N₂ or electrochemically under an applied bias ranging between 0-2V in a 3% H₂O – 3% H₂ – N₂ environment. The polarization resistance before and after exsolution is measured using Electrochemical Impedance Spectroscopy (EIS) on asymmetric cells employing La_0.8Sr_0.2CrO_3-δ as the counter electrode. Electrode polarization resistances under Open Circuit Voltage (OCV) in 3% H₂O – 3% H₂ – N₂ show that the exsolution mode plays a significant role in the particle growth and electrode activity. We demonstrate that, under the same environment, applying a bias to trigger exsolution generates more particles on the surface of the electrode compared to thermochemical exsolution. Characterization of the samples using X-Ray Diffraction (XRD), X-Ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDS) confirm the presence of Fe⁰ nanoparticles on the surface of the LSCF electrode and their existence decreases the polarization resistance as observed from EIS data.

References:
[1] G. Tsekouras et. al. Energy Environ. Sci., 2013, 6, 256–266
[2] Y. Sun et. al. J. Mater. Chem. A, 2015, 3, 11048–11056
[3] G. Dimitrakopoulos et. al. Sustainable Energy & Fuels, in press
[4] D. Neagu et. al. Nature Communications 8:1855 (2017)
[5] J. Myung et. al. Nature 537 (2016) 528-531

As the energetic demands of our society keep rising, any emerging technology faces the challenge of matching the commercial benefits presented by fossil fuels in terms of energy storage and mobility. Since a vast part of the modern energetic infrastructure already relies on fuels, their production in a sustainable manner stands out as an obvious solution. Therefore, the light-driven conversion of small molecules such as water and CO₂ into so-called solar fuels (e.g. H₂, CO) represents an attractive alternative for simultaneous energy harvesting and storage.^[1,2] While great progress has been made in the development of light absorbers maximizing the solar spectrum coverage, their integration with catalysts into photoelectrochemical (PEC) devices for fuel production still poses challenges. Beside the performance and stability of common PEC prototypes, their scalability and choice of catalyst are also major factors which must be considered for commercial applications.
In this work, we address those issues by developing approaches to synthesize and characterize up-scaled PEC devices, which are able to produce fuels autonomously in the absence of external bias. To overcome the overpotential losses of electroreduction, we introduce state-of-the-art photocathodes obtained by protecting triple cation mixed halide perovskite solar cells with a Field’s metal encapsulant. Their PEC performance is benchmarked using a platinum nanoparticle catalyst for proton reduction, reaching photocurrents as high as -14 mA cm^-2 at 0 V against the reversible hydrogen electrode (RHE) under 1 Sun irradiation.^[3,4,5] By combining the perovskite photocathodes with robust BiVO₄ photoanodes, tandem PEC devices can be obtained which sustain unassisted water splitting at a solar-to-hydrogen conversion of approximately 0.3%. The PEC devices present remarkable stabilities of up to 20 h under operation in an aqueous medium,^[4,5] revealing key general insights for the encapsulation of perovskite optoelectronic devices.
To investigate the scalability of our “artificial leaves”, we prepare devices of sizes up to 10 cm² which reveal a similar performance to their 0.25 cm² counterparts. The PEC tandems are characterized in a versatile 3D-printed PEC cell, which can accommodate a wide array of samples due to its modular design.^[5] The potential for further device up-scaling is revealed by fabricating 25 and 300 cm² doped BiVO₄ panels from bismuth (transition metal) polyoxovanadate single-source precursors.^[6] The overall findings are applicable to a wide range of photoelectrochemical systems employing various photoabsorbers.^[7] Looking beyond water splitting, we will also discuss our recent progress on the development of PEC devices that can couple the more challenging CO₂ reduction to water oxidation, with the ultimate goal of contributing towards a circular carbon economy via photoelectrocatalysis.

[1] N. S. Lewis, D. G. Nocera, Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15729.
[2] V. Andrei, K. Bethke, K. Rademann, Energy Environ. Sci. 2016, 9, 1528–1532.
[3] M. Crespo-Quesada, L. M. Pazos-Outón, J. Warnan, M. F. Kuehnel, R. H. Friend, E. Reisner, Nat. Commun. 2016, 7, 12555.
[4] S. Ahmad, A. Sadhanala, R. L. Z. Hoye, V. Andrei, M. H. Modarres, B. Zhao, J. Rongé, R. H. Friend, M. de Volder, ACS Appl. Mater. Interfaces 2019. DOI: 10.1021/acsami.9b04963.
[5] V. Andrei, R. L. Z. Hoye, M. Crespo-Quesada, M. Bajada, S. Ahmad, M. De Volder, R. Friend, E. Reisner, Adv. Energy Mater. 2018, 8, 1801403.
[6] H. Lu, V. Andrei, K. J. Jenkinson, A. Regoutz, N. Li, C. E. Creissen, A. E. H. Wheatley, H. Hao, E. Reisner, D. S. Wright, S. D. Pike, Adv. Mater. 2018, 30, 1804033.
[7] D. H. Nam, J. Zhang, V. Andrei, N. Kornienko, N. Heidary, A. Wagner, K. Nakanishi, K. Sokol, B. Slater, I. Zebger, S. Hofmann, J. Fontecilla-Campos, C. B. Park, E. Reisner, Angew. Chem. Int. Ed. Engl. 2018, 57, 10595-10599.

That there is an environmental and energy crisis due to civilization is beyond question. Utilization of full solar light spectrum to harvest energy is considered to be one of the most efficient techniques to overcome the energy crisis. The photothermal effect and photocatalytic effect are considered to be two of the most suitable strategies for solar light energy conversion. Metal oxide semiconductors are extensively used in photocatalysis because of their ability to absorb light from different part of the solar spectrum (depends on their band gap). Besides they also have other beneficial properties like high surface area, morphology, stability, non-toxic nature etc. We have tailored the surface defect state of ZnO nanoparticle (NP) using halide ion to enhance the charge separation property of ZnO. Particular halide ion (namely chlorine) shows enhance photocatalytic activity in presence of UV light as well as room temperature magnetic activity. In our other work, we have prepared a new nanohybrid heterojunction using one low band gap semiconductor (CuS) with a wide bandgap semiconductor (ZnO) to form an efficient NIR harvesting photocatalyst. This newly prepared nanohybrid exhibit efficient photocatalytic performance using the NIR fraction of the solar light spectrum. An increment of significant charge separation from the conduction bands of CuS to those of ZnO is the key mechanism behind the enhanced photocatalysis. To utilize visible light for photocatalytic activity, we have prepared porphyrin dye sensitized metal oxide nanohybrid. Porphyrin dye is very much advantageous for visible light based photocatalytic activity due to their additional absorption band that resembles with the visible range of the solar spectrum. Here we have functionalized protoporphyrin IX with copper (II) ion and sensitized a porous metal oxide microsphere TiO₂ (~1.5 µm diameter) in order to prepare a unique functional nanohybrid for visible light photocatalysis application. We have applied the nanohybrid for an efficient reduction of a toxic metal ion Cr (VI) to non-toxic Cr (III) in water under visible light illumination. For a prototype application, we have developed an active filter (having physical and chemical filtering capability) by depositing the nanohybrid on an extended surface of a stainless-steel metal mesh. The prototype active filter exhibits significant potential for chemical filtering of toxic Cr (VI) ions along with physical cleaning of suspended particulates present in water. Various microscopic and spectroscopic characterizations have been carried out to confirm the heterojunction formation. We have utilized first principles investigations to understand the modulation of electronic properties and obtain the insight to the photocatalytic process.

Carbon dots (CDs) are a versatile nanomaterial with attractive photoluminescent and photocatalytic properties. Their complex internal structure, considered to comprise sp²-hybridized domains embedded in an amorphous sp³-hybridized matrix, results in an intricate interplay between the constituent parts with multiple charge and energy transfer pathways determining the photocatalytic properties.[1,2] In effect, the mechanism of the photocatalytic hydrogen generation with CDs remains the subject of a debate. Here we demonstrate that the mode of nitrogen inclusion in the CDs within the aromatic domains controls the charge transfer and separation and thereby the photocatalytic activity for water reduction. [3,4] Furthermore, we propose that the photobasic effect, that is the abstraction of a proton from water upon excitation of CDs by light, facilitates the photoexcited electron transfer to the proton. In this context, we studied the effect of controlled inclusion in CDs of photobasic molecules which resemble the molecular moieties found in photocatalytically active CDs. We show that such addition strongly increases the hydrogen generation. Ultrafast spectroscopy measurements corroborated these findings, revealing the dynamics of the relevant electron and excited state proton transfer processes. The study demonstrates the critical role of understanding of the structure and dynamics of the CDs in designing the materials with improved photocatalytic efficiency.

[1] Fu et al., Carbon Dots: A Unique Fluorescent Cocktail of Polycyclic Aromatic Hydrocarbons, Nano Lett. 15, 6030−6035 (2015)
[2] Ehrat et al., Tracking the Source of Carbon Dot Photoluminescence: Aromatic Domains versus Molecular Fluorophores, Nano Lett. 17, 7710–7716 (2017)
[3] Bhattacharyya et al., Effect of Nitrogen Atom-Positioning on the Trade-off between Emissive and Photocatalytic Properties of Carbon Dots, Nature Commun. 8, 1401 (2017)
[4] Lau et al., Urea-modified Carbon Nitrides: Enhancing Photocatalytic Hydrogen Evolution by Rational Defect Engineering, Adv. Energy Mater. 7, 1602251 (2017)

Devising cost effective methods for efficiently capturing and storing solar energy is among the grand challenges of science. We are using insights from studies of natural photosynthetic systems to develop bioinspired molecular catalysts for photo- and electrochemical water oxidation necessary for solar fuel production. Molecular catalysts are known for their high activity and tunability, but their solubility and limited stability often restrict their use in practical applications. We are developing anchoring chemistry to attach molecular water-oxidation catalysts to metal oxide surfaces, which not only greatly increases the stability of the molecular catalyst but also improves the catalytic performance of the oxide material. Our progress on the development and characterization of molecular iridium, copper and manganese water-oxidation catalysts, along with their application for photoelectrochemical water oxidation and solar fuel production, will be discussed.

The synthesis of solar fuels and chemicals through artificial photosynthesis allows the direct pairing of light absorption to drive chemical redox processes. This approach is a one-step and versatile alternative to the more indirect coupling of a photovoltaic cell with electrolysis and enables potentially the synthesis of a wide range of fuels and feedstock chemicals. A common drawback in most artificial photosynthetic systems and organic photocatalysis is their reliance on expensive materials and device architectures, which challenges the development of ultimately scalable devices. Another limitation in many approaches is their inefficiency and reliance on sacrificial redox reagents, which may be system damaging and often prevent truly energy-storing chemistry to proceed. This presentation will give an overview about our recent progress in developing molecular-semiconductor hybrid systems to perform efficient full redox cycle solar fuel catalysis with inexpensive components, and our steps in extending this approach for sustainable waste polymer photoreforming and fine chemical synthesis.

Representative recent publications
(1) “Electro- and solar-driven fuel synthesis with first row transition metal complexes”
Dalle, Warnan, Leung, Reuillard, Karmel, Reisner, Chem. Rev., 2019, 119, 1752.
(2) “Photoreforming of lignocellulose into H₂ using nanoengineered carbon nitride under benign conditions”
Kasap, Achilleos, Huang, Reisner, J. Am. Chem. Soc., 2018, 140, 11604.
(3) “Plastic waste as a feedstock for solar-driven H₂ generation”
Uekert, Kuehnel, Wakerley, Reisner, Energy Environ. Sci., 2018, 11, 2853.
(4) “Solar Hydrogen Generation from Lignocellulose”
Kuehnel, Reisner, Angew. Chem. Int. Ed., 2018, 57, 3290.
(5) “Solar-driven reforming of lignocellulose to H₂ with a CdS/CdO_x photocatalyst”
Wakerley, Kuehnel, Orchard, Ly, Rosser, Reisner, Nature Energy, 2017, 2, 17021.
(6) “Carbon dots as photosensitisers for solar-driven catalysis”
Hutton, Martindale, Reisner, Chem. Soc. Rev., 2017, 47, 6111.
(7) “Solar-driven Reduction of Protons Coupled to Alcohol Oxidation with a Carbon Nitride-Catalyst System”
Kasap, Caputo, Martindale, Godin, Lau, Lotsch, Durrant, Reisner, J. Am. Chem. Soc., 2016, 138, 9183.

Photocatalytic reduction of CO₂ into solar fuels is regarded as a promising method to address global warming and energy crisis problems. Although heterostructured hybrid metal oxide catalysts have been used for CO₂ reduction, selective control for CO production-only remains the subject of debate. In this work, we report an absolute selectivity for CO production-only with enhanced photocatalytic ability using Ag-decorated reduced titanium oxide/tungsten hybrid nanoparticles (blue TiO₂/WO₃-Ag HNPs) at 1166.72 μmol g⁻¹ h⁻¹ with an apparent quantum yield of 34.8%. The construction of a Z-scheme between blue TiO₂ and WO₃ domains with an excellent band alignment provided remarkably improved separation of photoinduced charges. Importantly, the presence of novel Ag not only produces the highest selectivity up to 100% CO production-only, but also increases the photocatalytic electron reaction rate (2333.44 μmol g⁻¹ h⁻¹).

Coupling of oxo-bridged heterobinuclear light absorbers with metal nanoparticle catalysts on silica supports offer an opportunity to combine the precision of molecularly defined chromophores with the power of multimetal catalyst clusters for visible light driven carbon dioxide reduction to fuels. Such light absorber-catalyst units are components of an inorganic core-shell nanotube assembly for the photoreduction of vapor phase CO₂ by H₂O under membrane separation.
The spatial directionality of visible light driven metal-to-metal charge transfer of Zr^IVOCo^II units was utilized to assemble bimetallic nanoclusters coupled at the Zr acceptor end of the binuclear light absorber. Building on our previous observation of spatially directed photodeposition of Cu clusters coupled to ZrOCo centers, assembly of Cu_xSn_yO_z nanoclusters was explored. Characteristic infrared absorptions of CO probe molecules confirmed the formation of bimetallic clusters (2142, 2135 cm^-1), with the surface metal centers reduced to Cu(I) and Sn(II) (2128, 2111 cm^-1) by illumination of ZrOCo – cluster units in the presence of adsorbed diethylamine electron donor. Formate was detected by FT-IR as sole photoreduction product of vapor phase CO₂ upon excitation with 405 nm light. Photocatalytic units consisting of bimetallic nanocatalyst cluster of Cu with Pd coupled to ZrOCo have been prepared and characterized for exploring CO₂ photoreduction to more energy dense products beyond 2-electron species. Photocatalytic results will be discussed.

The localized surface plasmon resonance (LSPR) of noble metal nanostructures provides appealing strategies to enhance photoinduced charge transfer in photocatalysis. While the plasmonic hot charge carrier production mechanism is well-characterized for initiating direct catalysis by plasmonic metals or heterostructures, the intense but evanescent Electric (E-) field generated by the nanoantennas provides a powerful alternative pathway to magnify resonant intramolecular transition processes in plasmonic catalysis. Harnessing plasmonic E-fields for photocatalysis, however, is fundamentally challenged by the trade-off between distance-dependent local field intensity enhancement and excited state quenching through the metal. Here, we explore quantitative optimization of photocatalysis mediated by an E-field enhanced metal-to-ligand charge transfer (MLCT) process. We present a hierarchical nanopigment architecture that localizes a transition metal photocatalyst, [Ru(bpy)₃]²⁺, in an electromagnetic “sweet spot” for efficient plasmonic enhancement of photocatalysis. A phospholipid membrane self-assembled around a Ag nanoparticle (NP) binds [Ru(bpy)₃]²⁺ as well as serves as a spacer between the transition metal complex and the NP, whose LSPR overlaps with the [Ru(bpy)₃]²⁺ MLCT. The nanopigments allow for substantial absorption enhancement but avoid quenching of photoexcited Ru*(II). We demonstrate direct photocatalytic urea oxidation with the nanopigments, and implement a visible light-driven Direct Urea Fuel Cell (LDUFC) to achieve simultaneous solar energy harvesting and waste water treatment. Our approach provides great promise as an effective and broadly applicable strategy to enhance intramolecular charge or energy transfer processes with metal plasmon resonances, and it depicts a blueprint for designs of bio-mimetic efficient and specific nanoreactors.

With record high emissions of greenhouse gases, the global demand for renewable energies increases. Therefore, also the need for efficient and flexible energy storage systems arises. On the molecular level photoswitches that can undergo reversible transformation to a high energy isomer display a promising class of storage systems.^[1] We report on a novel, inexpensive and effective process for the repeatable photoisomerization of norbornadiene (NBD) to its metastable isomer quadricyclane (QC), followed by catalytically induced strain energy release via back-conversion of QC to NBD. By implementing a quasi homogeneous catalyst system based on magnetic iron oxide (Fe₃O₄) nanoparticles that were covalently functionalized with a Cobalt (II) complex, tedious purification steps can be avoided. These core-shell nanoparticles combine a high active surface area with the intrinsic magnetic properties of the iron oxide core to enable facile separation by the action of an external magnetic field. Together with the interconversion couple NBD1-QC1 that provides an extraordinary stability of the high energy isomer QC1 (t_1/2 = 450 days at room temperature), as well as a high energy storage capacity (88.3 kJ/mol) these core-shell nanoparticles present a promising system for photochemical energy storage.

[1] T. J. Kucharski, Y. Tian, S. Akbulatov, R. Boulatov, Energy Environ. Sci. 2011, 4, 4449-4472.

BiFeO₃ is a material that is of interest for many applications due to its combination of visible bandgap (2.3-2.7 eV) with multiferroic properties (ferroelectricity combined with antiferromagnetism). Bi₂Fe₄O₉ is normally considered as an unwanted secondary phase in the synthesis of BiFeO₃ and therefore has received little attention. However, modelling has shown that it should also possess a bandgap in the visible range [1], and one report has shown that powders of the material are active for photocatalysis, showing degradation of organic pollutants [2].

Here we report the synthesis of Bi₂Fe₄O₉ thin films via simple sol-gel deposition with, for the first time, photoelectrochemical (PEC) properties of the films demonstrating their suitability for use as photoanodes for solar water splitting. UV-Vis absorption show a highly suitable bandgap (2.05 eV) with strong absorption from 600 nm. Phase and composition are also confirmed using XRD and XPS. Furthermore, measurement of the energetics of the material show that this band gap straddles the water reduction and oxidation potentials, with the material displaying slightly n-type behavior.

PEC measurements in the 0 V to 1.5 V range (vs Ag/AgCl) show almost twice the photocurrent compared to BiFeO₃. Further insight is achieved by testing using a hole scavenger (H₂O₂), which leads to an increased photocurrent from 0.05 mA/cm² to 0.1 mA/cm² at 1.2 V (vs NHE), a shift in onset potential from 0.5 V to 0 V (vs Ag/AgCl), and the disappearance of positive/negative photocurrent transients upon light chopping. These results indicate that hole injection barriers leading to slow kinetics and surface recombination limit the photocatalytic performance of this material, thus combination with appropriate water oxidation catalysts may lead to further improved performance. To test this, Co-Pi was deposited on the surface of the thin-film photocatalyst, leading to a 5-fold increase in photocurrent. To confirm water splitting was taking place, oxygen evolution was successfully measured from the photocatalyst, and by comparing to the measured photocurrent a Faradaic efficiency of 82.1% was calculated – over 10% higher than BiFeO₃ photocatalysts.

Overall these results indicate that Bi₂Fe₄O₉ may be a promising new material for PEC water oxidation, with the potential for significant improvements to be made in the future through routes such as nanostructuring and reduction of surface defects. Also, the measured band positions indicate that PEC water reduction may also be possible with this visible-bandgap oxide material via suitable p-type doping and stabilization at cathodic potentials.

[1] Irshad et al., J. Alloys Compd., 2015, 624, 131.
[2] Yang et al., Sci. Rep. 2017, 7, 768.

Many metal oxide semiconductors, such as TiO₂, ZrO₂, and CeO₂, require UV excitation in order to drive photocatalytic reactions. However, UV photons account for only 3-5% of the solar spectrum and thus strategies to extend the range of harvested photons into the visible wavelength range are required. One potential way is to use triplet-triplet annihilation photon up-conversion (TTA-UC). TTA-UC systems generate higher energy photons from lower energy, low intensity, and non-coherent excitation. In this work, we synthesized several 9,10-diphenylanthracene derivatives and attached them to different wide-bandgap metal oxide nanoparticles, which were then dispersed in solutions containing triplet sensitizers. Triplet energy transfer leading to triplet-triplet annihilation photon up-conversion and charge generation were probed through steady-state and transient spectroscopy techniques such as photo-induced absorption, time-resolved photoluminescence, and transient absorption spectroscopy. As a proof of concept, we apply these TTA-UC systems in photocatalytic processes.

Producing sustainable fuels from sunlight is an important objective for mitigating climate changes. We report here results for a solar-driven CO₂ reduction device for CO generation using a flow-by type gas diffusion electrode (GDE), employing a Ag nanoparticle catalyst layer and a GaInP/GaInAs/Ge triple junction solar cell. The GDE is designed to utilize high CO₂ concentration and low overpotential catalysts for the CO₂ reduction reaction. Near-unity Faradic efficiency is obtained for CO₂-to-CO conversion and a solar-to-CO efficiency of 19.5 % can be realized with AM 1.5G illumination at 3.25 Suns. The device was shown to be stable for >20 h with no loss in efficiency, suggesting that high efficiency GDEs are a promising path to producing a chemical fuel from CO₂. Outdoor measurements were performed under ambient solar illumination in Pasadena, CA, resulting in a solar-to-CO conversion efficiency of 18.9 % during noontime, a diurnal-averaged solar-to-CO conversion efficiency of 6.1 %, and yielded a CO production rate of 50 mg/cm² per day. This work gives an outlook for the potential of solar driven carbon dioxide reduction. Pathways to further improvements beyond this record efficiency will be discussed,including the optimization of band gaps and further catalyst engineering. The system opens a new avenue for research into very high efficiency and stable devices for solar CO₂ conversion.

Photoelectrochemical (PEC) water splitting is a promising technique to directly convert solar energy into hydrogen energy for storage and transportation. PEC devices need to efficiently absorb sunlight and separate and collect the photocarriers. Herein we will present our recent work on the development of photonic structures for light trapping, PEC electrodes and electrocatalysts for water splitting. The importance of understanding both the light harvesting and the charge collection characteristics will be highlighted for the light trapping structural design. For electrocatalysts, we will focus on two-dimensional (2D) materials, and combine experimental and theoretical techniques to reveal the importance of the specific nanostructures for hydrogen evolution reaction and oxygen evolution reaction in electrolytes of different pH values. Our overall goal is to develop cost-efficient and robust photocatalysts and electrocatalyst to efficiently convert the abundant solar energy to clean, storable and transportable chemical fuels.

Tandem water splitting with wide-bandgap metal oxides functionalized with chromophore-catalyst assemblies is a potential low-cost solution to synthesize solar fuels. In recent years, high performing molecular catalysts have been developed for reducing protons to H₂ and reducing CO₂ to CO, CH₄, and other useful carbon derivatives. However, many of these dye-sensitized photocathodes are based on NiO scaffolds which exhibit poor performance and limit the catalytic efficiency in aqueous conditions and at potentials needed for tandem water splitting. Here, we elucidate that in the presence of water, intra-bandgap surface states exist that are associated with hydroxyl and oxygen moieties adjacent to Ni vacancies. Through a combination of electrochemical measurements and First-principles calculations, we show that these trap states give rise to a surface capacitance which can be almost completely eliminated by targeted atomic deposition (TAD) of Al treatment. TAD-treated electrodes functionalized with chromophore-catalyst assemblies sustain cathodic photocurrent at positive electrochemical potentials and yield a champion Faradaic efficiency of 46.0 ± 5.9% for H₂ evolution compared to 3.1 ± 0.4% for untreated films at +0.83 V vs. RHE. To the best of our knowledge, these results are the most positive potentials at which H₂ evolution has been observed for a dye-sensitized photoelectrosynthesis cell to date, and they represent an important step toward designing tandem water splitting devices with high efficiencies. This work demonstrates the utility of TAD passivation in suppressing recombination and side reactions that impede aqueous photocatalytic performance, especially at the positive electrochemical potentials needed for wireless operation of tandem solar fuel devices.

Efficient photocatalytic conversion of carbon dioxide to solar fuels presents an attractive option to reduce the problem of energy storage present in today’s renewable energy technology. Until today, most effort has been put into either homogeneous or heterogeneous approaches, working separately on molecular catalysts in solution or metal and semiconductor surfaces, respectively. By anchoring molecular catalysts on semiconductor surfaces and therefore combining approaches of heterogeneous and homogeneous catalysis, one might be able to profit from advantages of both, leading to enhanced efficiency and stability. A major challenge of this idea is the lack of fundamental knowledge about the interaction of the molecular catalyst with the semiconductor.
In order to gain more information specifically on the energy level alignment and charge transport between catalytically active molecules and semiconductor surfaces, a simple model system containing a protoporphyrin IX (PPIX) monolayer deposited on different semiconductor substrates via Langmuir-Blodgett deposition is investigated. Monolayer morphology as well as molecular packing density are studied using atomic force microscopy and surface pressure isotherms, indicating a closely packed monolayer with a slightly tilted orientation on Silicon and Gallium Nitride surfaces. The potential landscape at the interfaces of Silicon and Aluminum Gallium Nitride surfaces with varying Aluminum to Gallium ratio and PPIX are investigated by surface photovoltage analysis, showing indications of a surface potential shift due to an interface dipole. Photoluminescence (PL) studies of these PPIX monolayers reveal a strong dependence of the PL intensity on the band gap of the underlying substrate and excitation wavelength. Shorter excitation wavelengths and higher electron affinities of AlGaN substrates yield lower PL intensity, giving rise to the assumption that electrons in excited molecular states are transferred to the substrate’s conduction band, if the excited electron has sufficient energy compared to the conduction band minimum. This leads to a quenching of the luminescence in this model system and would lead to a decrease in efficiency for a photocatalytically reducing device, highlighting the necessity of matching the choice of the semiconductor support to the specific catalytic molecule in order to ensure beneficial interaction.

Plasmonic catalysts where a single material combines plasmonic and catalytic functionalities can improve reaction rates, improve catalyst lifetimes, and alter product selectivity. These catalytic processes are largely affected by energy transfer between a plasmonically excited active-site and reactants. To access a broader range of reactions, catalysts with weak or no plasmonic properties have been utilized and coupled to a plasmonic absorber. This work addresses the more complex nature of the interaction between a plasmonic absorber, a catalyst, and reactant molecules. Specifically, though alteration of the nanostructure of bimetallic plasmonic-catalytic particles, we analyze the light enhanced reaction rates and separate out thermal and hot-electron transfer effects. Alloy, co-impregnated, and core-frame Pt-Rh catalysts were examined with findings suggest a guiding set of nanostructure design principles which can be used to maximize the efficiency of bimetallic plasmonic catalysts.

Significant optical absorption in the blue-green spectral range, high intralayer carrier mobility, and band alignment suitable for water splitting suggest tin disulfide (SnS₂) as a candidate material for photoelectrochemical applications. In this work, vertically aligned SnS₂ nanoflakes are synthesized directly on transparent conductive substrates using a scalable close space sublimation (CSS) method. Detailed characterization by time-resolved terahertz and time-resolved photoluminescence spectroscopies reveals high intrinsic carrier mobility of 330 cm²V^-1s^-1 and photoexcited carrier lifetimes of 1.3 ns in these nanoflakes, resulting in a long vertical diffusion length of ~1 μm. The highest photoelectrochemical performance is achieved by growing SnS₂ nanoflakes with heights that are between this diffusion length and the optical absorption depth of ~2 μm, which balances the competing requirements of charge transport and light absorption. Moreover, the unique stepped morphology of these CSS-grown nanoflakes improves photocurrent by exposing multiple edge sites in every nanoflake. The optimized vertical SnS₂ nanoflake photoanodes produce record photocurrents of 4.5 mAcm^-2 for oxidation of a sulfite hole scavenger and 2.6 mAcm^-2 for water oxidation without any hole scavenger, both at 1.23 V_RHE in neutral electrolyte under simulated AM1.5G sunlight, and stable photocurrents for iodide oxidation in acidic electrolyte.

Electrocatalytic carbon dioxide (CO₂) reduction is an attractive approach to provide renewable energy from atmospheric CO₂. For achieving efficient CO₂ reduction reaction, electrocatalysts are required to have high selectivity to target gas product, and their enormous conversion rate. The developments of nanostructures achieved high energy efficiency and selectivity for reusable carbon-containing chemicals. However, the complicated nanostructures have restricted the CO₂ conversion rates because of poor mass transport of reactants. Herein, we propose a three-dimensional (3D) hierarchically porous Au consisting of continuous macroporous channels (200-300 nm) and nanopores (<10 nm) fabricated by Proximity-field nanoPatterning (PnP). This hierarchically porous nanostructures could be fabricated involving three sequential processes: i) fabrication of 3D epoxy-based periodic nanostructures using PnP technique on an electrically-conductive substrate, ii) electroplating the Au-Ag alloy into the 3D nanostructured template, and iii) selective removal of Ag components and 3D epoxy template using dealloying and plasma etching machine, respectively. The fabricated macroporous channels have well-ordered pores with the periodicity is approximately 600 nm along the x, y-axis and 1 µm along the z-axis. The roles of each pore network are investigated using reliable 3D nanostructures possessing controlled pore distribution and size: nanoporous Au (N-Au), macroporous Au (M-Au), and hierarchically porous Au (N/M-Au). The continuous, perfectly periodic macropores and nanopores enable efficient mass transport and large active areas, respectively. Numerous nanopores in N/M-Au produce a large electrochemical surface area that is 66.1 times larger than that of electroplated Au film, and high index facet results in high CO selectivity of 85.5 % at a low overpotential of 264 mV. For analysis of the structural effect on CO₂ conversion rates, the specific j_CO is measured using nanostructured electrodes with various thickness at a different applied potential of -0.274 V_RHE and -0.574 V_RHE. The Au nanostructures having macroporous channels show a noteworthy tendency that has higher specific j_CO at the relatively high applied potential of -0.574 V_RHE. The 3D hierarchically porous electrodes consequent allow efficient mass transport during vigorous electrolysis resulting in maximum 3.96 times higher j_CO per mass than that of dealloyed nanoporous Au film. Thus, the systematic model study shows the proposed hierarchical nanostructured electrodes have critical value in increasing the efficiency of expensive Au. These investigations are also useful for other electrochemical applications in an aqueous base solution including batteries, flow cell, and electrocatalysts.

The catalytic reaction conversion efficiency over the monolithic three-way catalysts (TWC) is usually limited by the kinetic resistance of the catalytic reactions at low temperatures, internal mass transfer resistance at the intermediate temperatures, and the external mass transfer resistance at high temperatures.¹The state-of-the-art washcoat configured monolithic catalysts have been intensively studied to enhance the catalytic activity and to reduce the external mass transfer resistance. However, the internal mass transfer resistance tends to be hard to reduce as it requires the increase of the macropores portion while maintaining high specific surface area as well as the mechanical and thermal stability of the catalysts.² In order to mitigate this challenge, a novel three-dimensional (3D) nanoarray-based monolithic catalyst configuration has been developed by integrating hierarchically arranged mesoporous metal oxide nanoarrays onto the channel surfaces of commercial honeycomb monoliths in the past decade.^3-4 Compared with the conventional particulate washcoat catalysts, the 3D nanoarray-based monolithic catalysts show advantages in material utilization efficiency, mechanical robustness as well as tunable structures and catalytic performance.⁵ In this work, the kinetics and mass transport during the oxidation of CH₄ and C₂H₄ over the Pt/TiO₂ nanoarray-based monolithic catalysts were experimentally measured and compared with the washcoat counterparts. First, we determined the global kinetics of the oxidation of CH₄ and C₂H₄ over both types of catalysts. Then, using the low-dimensional model developed by Joshi et al.,⁶ we quantified the relative contributions of the chemical kinetics, internal mass transfer and external mass transfer as functions of various catalyst design and operating parameters. The analysis indicates that the nanoarray-based monolithic catalysts show decreased internal mass transfer resistance compared to the washcoat-based catalysts, which originates from the ordered nanoarray configuration and decreased catalyst layer thickness, which facilitate the diffusion of the reactant gas molecules through the nanoarray layers. Furthermore, the mesoporous TiO₂ nanoarrays provide sufficient specific surface area and catalytically active sites for reactions, and the adherent interface between the in-situ grown nanoarrays and substrate channel surfaces promoted the mechanical robustness of the monolithic catalysts. This study sheds light on the design of novel monolithic catalysts for various gas phase reactions.

(1) Joshi, S. Y.; Harold, M. P.; Balakotaiah, V. Overall mass transfer coefficients and controlling regimes in catalytic monoliths. Chemical Engineering Science 2010, 65, 1729-1747.
(2) Santos, H.; Costa, M. Influence of the three way catalytic converter substrate cell density on the mass transfer and reaction resistances. Chemical Engineering Science 2014, 107, 181-191.
(3) Guo, Y.; Ren, Z.; Xiao, W.; Liu, C.; Sharma, H.; Gao, H.; Mhadeshwar, A.; Gao, P.-X. Robust 3-D configurated metal oxide nanoarray based monolithic catalysts with ultrahigh materials usage efficiency and catalytic performance tunability. Nano Energy 2013, 2, 873-881.
(4) Lu, X.; Tang, W.; Gao, P.-X. Nanostructured TiO2 Support Effect on Hydrothermal Stability of Platinum based Catalysts. Microscopy and Microanalysis 2018, 24, 1642-1643.
(5) Weng, J.; Lu, X.; Gao, P.-X. Nanoarray Integrated Structured Catalysts: A New Paradigm upon Conventional Wash-Coated Monolithic Catalysts? Catalysts 2017, 7, 253.
(6) Joshi, S. Y.; Harold, M. P.; Balakotaiah, V. Low-dimensional models for real time simulations of catalytic monoliths. AIChE Journal 2009, 55, 1771-1783.

As demand for energy increases, it is inevitable to develop more efficient, stable, and long-life energy conversion/storage systems. Accordingly, devising highly active and stable electrodes for oxygen reduction reaction (ORR) has attracted a great attention. Heteroatom doped mesoporous carbon is one of the most promising substitutes for Pt-based ORR electrocatalysts with outstanding stability, activity, and electric conductivity. Furthermore, hybridization with transition metal is known to be one of the most efficient ways to improve ORR catalytic activity of mesoporous carbon (MC). Herein, we demonstrate a novel synthetic method of ZnS nanoparticle embedded N, S-codoped mesoporous carbon (NSMC) and its enhanced electrochemical property compared to the commercial Pt/C catalyst. NSMC was synthesized by one-step pyrolysis of a novel metal-organic framework (MOF), S-modified Zn-MOF-74, which was additionally doped with thiourea. The as-prepared NSMC exhibited high catalytic activity (limiting current density (J_L) = -5.19 mA cm^-2), superior stability (91.7% of J_L retention after 25000 s), and a great methanol tolerance in ORR. The enhanced ORR properties might be attributed to the ZnS nanoparticles as catalytic active sites and well-defined structure of N, S-codoped mesoporous carbon. Our results suggest that the introduction of ZnS nanoparticle and N, S-codoping could be a promising strategy for enhancing ORR in basic medium.

The current proton exchange fuel cells (PEFC) employ platinum group metal (PGM) catalysts in their electrode for oxygen reduction reaction (ORR). However, the excessive cost of PGM catalysts is a major hurdle for realizing the potential of PEFCs, thus necessitating the development of inexpensive non-PGM catalysts. Amongst the different PGM-free catalysts, a complex mixture of carbon (C), nitrogen (N), and transition metals (M) forming M-N-C clusters (M=Co, Fe, Ni, Mn, etc.) are currently the leading candidates due to their comparable activity to PGM catalysts and cost-effectiveness [1]. This complex is generally synthesized by the pyrolysis of carbon-based support with metal and nitrogen precursors [2]; however, the carbon-based support suffers from catastrophic failure during repeated start-up/shut-down events wherein the high cathode interfacial potential results in the irreversible corrosion of carbon and hence to the loss of active centers [3]. More distressingly, the reactions typically yield a complex structure, where the active site and reaction mechanism is still under the debate.
To overcome this issue, we employed an approach based on first-principles calculations to rapidly screen through a large set of TMN clusters (TM = Mn, Fe, Co, Ni, Zn, Zr, and Mo) on corrosion-resistant TiC supports that have a good combination of stability, surface area and conductivity. We predict FeN₄ clusters on TiC support to be an inexpensive and durable catalyst having better performance than TMN clusters on carbon (graphene) supports. This catalyst formulation was obtained by developing a scaling relation based on the electrostatic interactions between the TiC support and the TMN clusters. To validate our theoretical predictions, we synthesized various TMN catalysts on TiC supports. We characterized their structure and composition using a suite of techniques including XPS, SEM, STEM-EELS to show successful incorporation of the catalytically active clusters onto the TiC support. The measured ORR activity of these catalysts precisely confirmed our electrostatics-based scaling relation. Moreover, upon comparing an electrocatalyst comprising FeN4 clusters on TiC with a baseline carbon-supported FeN4 cluster electrocatalyst synthesized using the same method, we found that FeN4/TiC exhibited an ORR activity (2.3mA.cm^-2 at the half-wave potential E_1/2) that was 20% greater than FeN₄/C (1.85mA.cm^-2 at E_1/2) and comparable to the state-of-the-art [4, 5]. Furthermore, it showed high stability under 1000 cycles of the stringent accelerated start-up/shut-down stability protocol [6] adopted by the US Department of Energy and most major automotive companies.

1. Wu, G., Current challenge and perspective of PGM-free cathode catalysts for PEM fuel cells. Frontiers in Energy, 2017. 11(3): p. 286-298.
2. Kramm, U.I., et al., Structure of the catalytic sites in Fe/N/C-catalysts for O2-reduction in PEM fuel cells. Phys Chem Chem Phys, 2012. 14(33): p. 11673-88.
3. Castanheira, L., et al., Carbon Corrosion in Proton-Exchange Membrane Fuel Cells: Effect of the Carbon Structure, the Degradation Protocol, and the Gas Atmosphere. ACS Catalysis, 2015. 5(4): p. 2184-2194.
4. Li, J., et al., Structural and mechanistic basis for the high activity of Fe–N–C catalysts toward oxygen reduction. Energy & Environmental Science, 2016. 9(7): p. 2418-2432.
5. Strickland, K., et al., Highly active oxygen reduction non-platinum group metal electrocatalyst without direct metal–nitrogen coordination. Nature Communications, 2015. 6: p. 7343.
6. Cell Component Accelerated Stress Test and Polarization Curve Protocols for PEM Fuel Cells D.o. Energy, Editor. 2013.

It is urgent to develop cost-effective and highly efficient oxygen evolution reaction (OER) catalysts for the water splitting processes in neutral or near-neutral pH. Compared to alkaline or acidic conditions, pH-neutral or pH-near-neutral conditions are expected to be less corrosive and safer. In addition, electrolytes from natural sources such as sea water could be used directly. Here, we discovered for the first time that the anion-exchange properties of large organic anions intercalated NiFe LDHs can be exploited to make them excellent OER catalysts for pH-near-neutral borate electrolyte (K-B_i, pH=9.2). In this work, NiFe LDHs intercalated with different dicarboxylate anions were synthesized and were studied as OER catalysts in the K-B_i electrolyte (pH=9.2). Sebacate anion intercalated NiFe LDH (NiFe LDH Seb) and suberate anion intercalated NiFe LDH (NiFe LDH Sub) were found to be excellent OER catalysts which outperformed RuO₂ in the K-B_i electrolyte (pH=9.2). The NiFe LDH Seb needed an overpotential of 376mV and NiFe LDH Sub needed an overpotential of 387mV to reach a current density of 1 mA cm^-2 which are the lowest among all the reported earth-abundant OER catalysts and lower than that of RuO₂ (393mV). Moreover, these materials also showed great stability for at least 24 hours. Besides, all the NiFe LDHs intercalated with dicarboxylate anions showed good OER performances in K-B_i (pH=9.2). The good OER performances of the dicarboxylate anions intercalated NiFe LDHs can be attributed to the "structural instability" induced in-situ anion exchange process during the OER in which borate anions can enter the interlayer of LDH and facilitate the activation of more Ni sites. This study shows that the "structural instability" caused by pillaring of transition metal LDHs with large organic anions could be a promising way to make highly efficient OER catalysts in the pH-near-neutral K-B_i electrolyte.

Electrocatalytic water splitting is a highly efficient and sustainable way for hydrogen production.¹ For the development of electrocatalysts there have been countless studies, however, the choice of electrocatalysts for oxygen evolution reaction (OER) in acidic condition is quite limited by harsh working condition given by the low pH and high overvoltage.² Iridium oxide is by far the most durable and active performance anode catalyst, but its restricted reserves and increasing price requires a decrease in the usage of Iridium. Many approaches have been done, for example, synthesis of multi-metallic catalysts,³ designing of hollow structures, and more. Herein, we developed a facile, in one-pot preparation core-shell Ir-Ni_x nanoparticles which further etched to form a defined nanoframe structure.
The Ir-Ni nanostructures growth mechanism was studied to control the Ir layer. The as prepared Ir-Ni_x and the nanoframe structure were tested as electrocatalysts for OER in acidic conditions. The nanoframe structure showed better activity than the pristine structure with lower onset potential compared with commercial IrO₂.
This work provides a deep insight into the design of bimetallic electrocatalysts with controlled shape and shape dependent catalytic performance.

The properties of elements are correlated directly with their electronic states. If we can realize a concept of “density-of-states (DOS) engineering” ¹ for the design of materials having the most suitable electronic structure for the target properties, we will be able to make full use of available elements and relieve the resource problems. To freely control an electronic state of a material, the solid-solution alloy is advantageous because its electronic state can be continuously controlled by tuning the compositions and/or combinations of the constituent elements. However, the majority of bulk alloy systems are the phase-separated type under ambient conditions. Recently, we demonstrated that the nanosize effect offers a chance to find a way out of this metallurgical difficulty; that is, we can obtain metal nanoparticles (NPs) having new phases that do not exist in bulk states^2-9. As one of the examples, we found that Pd_0.5Ru_0.5 NPs having a similar electronic structure to Rh exhibit comparable NO_x reduction activity, even though monometallic Ru or Pd NPs do not show high activity.^{5, 10} However, this alloy structure is destroyed after the reaction due to the nature of the alloy system, resulting in the degradation of catalytic activity. In this work, we designed and synthesized ternary PdRuM alloy systems to develop a stable and highly active NO_x reduction property. DFT calculations predicted suitable third metal candidates to improve PdRu systems. The designed ternary solid-solution alloy NPs were successfully synthesized and characterized structures by synchrotron X-ray powder diffraction technique and scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy. The designed ternary PdRuM exhibited higher NO_x reduction than Rh or PdRu NPs and maintained its structure after the reaction.
1) H. Kobayashi, K. Kusada, H. Kitagawa, Acc. Chem. Res., 2015, 48, 1551. 2) K. Kusada, H. Kitagawa. Adv. Mater., 2016, 28, 1129. 3) K. Kusada, et al. J. Am. Chem. Soc., 2010, 132, 15896. 4) K. Kusada, et al. J. Am. Chem. Soc., 2013, 135, 5403. 5) K. Kusada, et al. J. Am. Chem. Soc., 2014, 136, 1864. 6) F. Wang et al., Angew. Chem. Int. Ed., 2018, 395, 4505. 7) Q. Zhang et al., Nat. Commun., 2018, 9, 510. 8) K. Kusada et al., Chem. Sci., 2019, 10, 652. 9) Q. Zhang et al., Chem. Sci., in press. 10) K. Sato, et al., Sci. Rep., 2016, 6, 28265.

Recently, carbon shell-encapsulated metal nanoparticles have been widely studied to enhance the durability of metal catalysts in proton exchange membrane fuel cells (PEMFCs) and anion exchange membrane fuel cells (AEMFCs). However, through the interfacial interaction between the carbon shells and the core metal nanoparticles, the carbon shells can also serve as the main and exclusive active sites to boost electrochemcial catalytic activity instead of the core metal nanoparticles. Therefore, we propose the distinctive funtionality of carbon shells coated on metal nanopaticles for oxygen reduction reaction (ORR) in AEMFCs. As a model study to prove the application feasibility of the ideal carbon-coated metal system for the ORR, the physical and electrochemical properties of Pd nanoparticles encapsulated with carbon shells are invetigated. Furthermore, to practically apply the carbon shell-encapsulated metal nanoparticles to the AEMFC system, less expensive Co nanoparticles coated with carbon shells are intensively studied. From the experimental results, it is conluded that the transition metal core materials can boost the ORR activity of carbon shells and the catalyst durability can be also enhanced due to the protection of the core metal nanoparticles. The present work will provide an interesting strategy for designing new types of carbon-based catalysts for the ORR in AEMFCs.

Clusters and nanoparticles of late transition group metals have shown promising catalytic properties for reactions for sustainable fuels and chemicals (Seh et al.,”Combining Theory and Experiment in Electrocatalysis: Insights into Materials Design”, Science 2017). Entities in this size range (<10 nm) display an increased number of under-coordinated sites and yield a higher surface area per volume. This is advantageous since under-coordinated sites of these materials have been shown to be highly active towards certain reactions e.g. Au for CO oxidation (Janssens et al.,”Insights into the Reactivity of Supported Au Nanoparticles: Combining Theory and Experiments”, Topics in Catalysis, 2007). Furthermore, the increase in surface area allows for a lower volumetric loading of these expensive elements. Altogether, this decreases the cost of using non-abundant materials as catalysts in industrial processes.

However, these small entities are prone to deactivation due to their high surface energy and increased reactivity. Deactivation can be due to particle agglomeration via sintering or due to particle oxidation via the reactive sites (Wallace et al., “The Stabilization of Gold Particles by Surface Defects”, Journal of Molecular Catalysis A, 2005). These deactivation phenomena are known to depend on several catalyst properties such as particle size, coverage and the support material. Therefore, it is vital to control these parameters when studying deactivation phenomena of small entities. However, it is worth considering other factors that might influence catalyst deactivation, recently the edge-edge interparticle distance has received growing attention (Nesselberger et al., “The Effect of Particle Proximity on the Oxygen Reduction Rate of Size-Selected Platinum Clusters”, Nature Materials, 2013). Additionally, one might consider new design strategies to counteract deactivation; it is well known that some supports have anchoring sites that stabilize small entities, but perhaps it is possible to design self-anchoring particles with increased stability.

Using a mass-selected magnetron sputter source it is possible to create well-defined model systems of clusters and nanoparticles, where particle size and coverage are controlled in detail (Roy et al.,”Impact of Nanoparticle Size and Lattice Oxygen on Water Oxidation in NiFeOxHy”, Nature Catalysis 2018). Furthermore, a suitable substrate can be chosen to allow a variety of characterization techniques including ISS, XPS and electron microscopy followed by catalytic testing either electrochemically or using micro-reactors for gas phase catalysis.

I will present the results of studying how catalyst design, size, coverage or even the edge-edge inter-particle distance affects relevant catalytic properties of small entities of Au or Pt for different reactions such as CO oxidation or the Oxygen Reduction Reaction. The well-defined model systems are thoroughly characterized before probing their activity/stability leading to a novel insight into the deactivation that typically occurs when using these materials in commercial catalysts.

Platinum exhibits the best currently known HER catalytic properties, but its sparsity and price prevent widespread use. Transition metal chalcogenides, especially MoS₂ have been shown also to catalyze the hydrogen evolution reaction. Sulfur vacancies in the crystal lattice have a low free hydrogen adsorption energy and therefore easily binds to H⁺ in the solution. This results in the catalysis of the adsorption (Volmer) step of the hydrogen evolution reaction, and these materials generally exhibit Tafel slopes similar to those of platinum (~ 40 mV/dec). Therefore, transitional metal chalcogenides are currently an area of active research.
Although there are several different methods to obtain MoS₂ materials of various crystallographic structures, in this study we focused on electrodeposition, which results in amorphous MoS_2. Previous research has shown, that amorphous MoS₂ has excellent HER catalytic activity due to the number of defect sites, which act as active sites for cathodic hydrogen evolution. MoS₂ films were electrodeposited cathodically onto copper wire substrates, from a solution of tetrathiomolybdate (MoS₄^2-) which acts as a common Mo and S ion precursor. EDX compositional measurements have shown, that the films have a varying stoichiometry, but are deficient in sulfur – MoS_2-x. Polarization experiments in acidic media (0.5 M H₂SO₄) showed, that the deposited films have excellent HER activity, and could reach 10 mA/cm² at -0.18 V overpotential.
As a further modification of the process, we attempted to incorporate W (as a substitute to Mo) and Se (as a substitute to S) into the films during electrodeposition. Due to the difference in their atomic radii and electronegativity, this could have an effect on the electrodes’ electrocatalytic properties. This was realized by mixing a small amount of the respective material into the precursor solution during synthesis. Subsequent electrodepositions were carried out in the same way. Compositional analysis showed, that a small amount of the heteroatoms were incorporated into the material; stoichiometry, such as MoS_1.91Se_0.03, or Mo_0.97W_0.03S_1.74 were obtained. The modified electrodes have Tafel slopes of ~ 40 mV/dec, which shows good HER catalytic activity. Electrochemical impedance spectroscopy was also used to characterize the catalyst-solution interface.
Acknowledgement: Authors acknowledge funding from H2020 project SMARTELECTRODES (No.778357) and by Research Council of Lithuania No 09.3.3-LMT-K-712-08-0003.

Heterogeneous catalysts have been used in all fields of chemical processes, such as the petrochemical industry, pharmaceutical fabrication, energy conversion, etc. Among these catalysts, Pt and its alloys play important roles due to their unique electronic structures and resultant special catalytic properties. However, the low abundance of Pt poses a grand challenge on the scalability of Pt-based catalysts. As a catalytic process only occurs on the surface of a catalyst, the surface and sub-surface atomic structures greatly impact the catalytic activity and stability. As a result, surface structure modification is expected as one of the significant routes that can improve the catalytic performance. Herein, we report a facile method of Pt-Ni catalyst surface modification through a thermal annealing process with carbon monoxide (CO). Literally, the Ni atoms on the catalyst surface can be etched with CO by forming a gaseous intermediate, Ni(CO)₄, at an elevated temperature. This method is utilized to modify the Pt-Ni octahedral and plane-tetrahexahedral (p-THH) catalysts towards oxygen reduction reaction (ORR). EDS mapping and ICP elemental analysis are used to identify the structure evolution. Catalytic results indicate that the ORR activity of Pt-Ni octahedra is improved (E_1/2 increased from 0.8 V to 0.88 V vs. RHE) due to a d-band center shift when Ni is removed from the surface lattice. Simultaneously, it is further determined that the electrocatalytic stability is enhanced (only a 10 % decrease in mass activity after 3000 electrochemical potential cycles in HClO₄) since most of the vacancy defects formed after removing the Ni atoms are diminished due to the thermal annealing effect. In addition, it is observed that this gaseous manipulation can alter the reaction environments from Ni-dominated ORR to Pt-dominated ORR on the Pt-Ni p-THH catalyst, resulting in a change of onset potential from 0.86 V to 1.0 V vs. RHE.

Compared with noble metals, the transition metal nanocatalysts including Cu, Fe, Co, and Ni have received considerable attention in a number of catalytic reactions such as hydrogenation and organic reductions, due to their low cost and high earth-abundance. Like the behavior of noble metals, it is believed that their surface structure heavily governs the catalytic process. However, only a few studies reported the crystal facet-dependent catalytic performance of these transition metal nanocatalysts. In this work, we report a facile colloidal method to fabricate shape-controlled CuNi nanocatalysts. The {111}-terminated CuNi nano-octahedra and {100}-bounded CuNi nanocubes were prepared using a group of synergetic capping ligands in a hot organic solvent system. Borane morpholine was adopted as a reducing agent in both syntheses. It was determined that the {100} surfaces of CuNi nanocrystals could be stabilized by chloride ions released from the precursors, leading to the formation of nanocubes; whereas the CuNi nano-octahedra could be achieved through a thermodynamic control. The resultant CuNi nano-octahedra and nanocubes were further loaded on carbon black, respectively, and reduction of 4-nitrophenol (4-NP) using sodium borohydride (NaBH₄) as a reducing reagent on these nanocatalysts at room temperature was investigated. Both the CuNi nano-octahedra and nanocubes exhibited remarkable catalytic activities with high rate constants over mass (k/m). Specifically, the CuNi nanocubes (k/m= 235 s^-1●g^-1) showed better catalytic performance than CuNi nano-octahedra (k/m= 160 s^-1●g^-1). Results suggest that 4-NP molecules are adsorbed on the {100} facets of CuNi much easier than on the {111}, indicating a strong facet-dependence due to different surface energies.

The electrochemical CO₂ reduction (CO₂R) has received great attention in converting CO₂ into valuable chemical products. Recently, it was found experimentally that MoS₂ converts CO₂ to 1-propanol as a major product. It is also reported that the current density decreases when increasing the portion of edge sites, which indicates that the active sites locate on the basal plane rather than the edges. However, the practical utilization of MoS₂ is limited because Faradaic efficiency of CO₂R is only about 5% because of the poor selectivity compared to hydrogen evolution reaction (HER).
In this study, we first investigate the mechanism of CO₂R at sulfur vacancy (V_S) of MoS₂ by calculating free energies through ab initio calculations and computational hydrogen electrode model.[1] We identify favorable C₁, C₂, C₃, and C₄ pathways starting from *OCHO. HCHO is found to be a key intermediate for C_N pathways by coupling of sp² carbons. The predicted products and corresponding onset potentials are in good agreement with the experiment. To discover novel materials with better selectivity than MoS₂, we perform computational screening of the catalytic activity of CO₂R at atomic defects of various transition-metal dichalcogenides (TMDs). As a result of screening, several TMDs are suggested as promising candidates for CO₂R catalysts with good selectivity.
[1] S. Kang, S. Han, and Y. Kang, Unveiling electrochemical reaction pathways of CO₂ reduction to C_N species at S-vacancies of MoS₂ ChemSusChem, 2019, DOI:10.1002/cssc.201900779.

We present the use of ab-initio calculations to calculate the catalytic activity of alloy nanoparticles with experimentally relevant sizes (5 nm-10 nm) by explicitly predicting atomic-scale structures and adsorbates binding energies. We demonstrate our approach using PtNi nanoparticles as catalysts for the oxygen reduction reaction (ORR). We achieve our results by constructing a quaternary Pt-Ni-OH-Vacancy cluster expansion model to explicitly predict OH adsorption energies on nanoparticles of varying shape, size, and atomic structure. These adsorption energies are then used to predict ORR activity through a volcano plot. This model enables us to accurately investigate the catalytic activity of various surface sites with different coordination numbers and local atomic environments. Using this model, we evaluate how different parameters affect the ORR activity of Pt-Ni nanoparticles, including size (2 nm-10 nm), Pt composition (50%-100%), and shape. Through the use of kinetic Monte Carlo simulations we evaluate how the activities of the particles change due to Ni dissolution. Our approach provides theoretical insights into how to tune the structures of alloy nanoparticles to optimize catalytic activity.

The direct conversion of methane at room temperature is a great challenge to replace the existing Fisher-Tropsch process. However, thermodynamic and kinetic mechanisms have hardly been proposed for the electrochemical reactions of methane. In this study, first-principles calculations were performed on the electrochemical oxidation of methane at β-NiOOH electrode surface. An active oxygen species on the surface play an important role in the CH₄ activation reaction. The effect of the implicit solvation model at constant potential was considered in this calculation. The partial charge of the active oxygen is calculated at each onset potental, and how this change influences on the reaction pathways was investigated. Possible reaction paths were suggested and the reaction rates of the anticipated products were calculated based on the microkinetic models.

Electrochemical reduction of carbon dioxide (CO₂) into value-added chemicals and fuels provides a promising pathway for environmental and energy sustainability. Copper (Cu) demonstrates a unique ability to catalyze the electrochemical conversion of CO₂ into valuable multicarbon products. However, developing a rapid, scalable and cost-effective method to synthesize efficient and stable Cu catalysts with high selectivity towards multicarbon products at a low overpotential is still hard to achieve and highly desirable. In this work, we present a facile wet chemistry approach to yield well-defined cuprous halide (CuX, X=Cl, Br or I) microcrystals with different degrees of truncations at edges/vertices, which can be ascribed to the oxidative etching mechanism of halide ions. More importantly, the as-obtained cuprous halides can be electrochemically transformed into varied Cu nano-architectures, thus exhibiting distinct CO₂ reduction behaviors. The CuI-derived Cu nanofibers composed of self-assembled nanoparticles are reported for the first time, which favor the formation of C₂₊₃ products at a low overpotential with a particular selectivity towards ethane. In comparison, the Cu nanocubes evolved from CuCl are highly selective towards C₁ products. For CuBr-derived Cu nanodendrites, C₁ products are subject to form at a low overpotential, while C₂₊₃ products gradually become dominant with a favorable formation of ethylene when the potential turns more negative. This work explicitly reveals the critical morphology effect of halide-derived Cu nanostructures on the CO₂ product selectivity, and also provides an ideal platform to investigate the structure-property relationship for CO₂ electroreduction.

Today, one of the focusing issues in graphene technology is the development of functionalized (doped) graphene monolayers since the pristine graphene is chemically inert, hydrophobic and zero band gap material [1]. As a result, the potential applications of graphene are limited to a few fields of science and technology. Recently, functionalization of graphene layers with different materials has received great attention as an alternative route to tailor the physical properties of graphene layers [2, 3]. The surface functionalized graphene layers find applications in various fields including the development of paints, corrosion resistive tools, plastics, electronics, flexible devices, LEDs, sensors, energy harvesting devices, wearable devices, bio-imaging, drug-delivery systems [4]. In order to adopt these unique 2-dimensional structures for energy harvesting applications, we have developed surface functionalized graphene monolayers, transferred on to SiO₂/Si substrates, by covalent attachment of metal-oxide particles. Influence of covalently attached metal-oxide particles on the surface morphology, carbon hybridization, and electrical properties of graphene monolayers was investigated. Further, the surface functionalized structures were adopted for the development of electrocatalytic devices and then, their water-splitting performance in different electrolyte solutions was studied.

Acknowledgments:
We acknowledge the financial support of the National Research Foundation of Korea (NRF- 2018R1A5A1025511 and 2017R1A2B4012119).

[1] Georgakilas et al. Chem. Rev. 112 (2012) 6156.
[2] Wagner et al. J. Phys. Chem. C 117 (2013) 26790.
[3] Li et al. Nanoscale 4 (2012) 6205.
[4] Kuila et al. Prog. Mater. Sci. 57 (2012) 1061.

Today, the study of novel photocatalyst and photo-electrocatalysts (PEC) for solar energy has gained great attention, aiming to use solar energy to directly split water into hydrogen and oxygen to produce solar fuels. To achieve unassisted water splitting, there is a need for low-cost, stable and high- efficiency PEC systems.
Therefore, recent research works have focused on a wide range of materials such as Fe₂O₃, BiVO₄, CuWO₄, with bandgaps in the range of 2-2.5 eV, which allow visible light absorption compared to the traditional photocatalyst, TiO₂. However, these materials generally present high level of surface or bulk recombination and low carrier mobility. Recently, polarization has emerged as novel strategy in photocatalysis to facilitate charge migration towards different sites, promoting selective redox reactions in the respective locations. Thus, in the present work, we develop and investigate a nanostructured, composite thin film, obtained by combining ferroelectric BaTiO₃ with the photocatalyst Fe₂O₃ in parallel at the nanoscale to improve the photocatalytic performance. The polarization of ferroelectrics can aid effective charge separation while narrow bandgaps and good charge transport properties of the photocatalyst are needed for light absorption and carrier extraction. The BaTiO₃ thin films are synthesized by solution-based synthesis, which has been optimized in order to obtain suitable thin films for photocatalyst integration. The morphology of BaTiO₃ is investigated by SEM, which confirms the successfully preparation of nanostructured thin films. The crystalline properties of BaTiO₃ thin films are analysed by X-ray diffraction (XRD) and Raman spectroscopy, in order to verify the transition to the tetragonal phase, which is responsible to the ferroelectric polarization. In addition, the switching of spontaneous polarization of BaTiO₃ thin films by an electric field is investigated by piezoresponse force microscopy (PFM) .The optical properties of BaTiO₃ thin film, Fe₂O₃ thin film and BaTiO₃/Fe₂O₃ composite photocatalyst thin film are analysed by UV-vis spectroscopy, which allow the estimation of the optical bandgap. The PEC water splitting performance was measured by using the BaTiO₃/Fe₂O₃ composite photocatalyst thin film as working electrode in a 3-electrode half cell configuration and illuminating with AM 1.5 simulated solar light.
The BaTiO₃/Fe₂O₃ composite photocatalyst thin film shows an enhancement of the photocurrent density compared to either the bare BaTiO₃ or Fe₂O₃ thin films, which indicates a promising performance of the ferroelectric/photocatalyst composite thin film in solar energy conversion systems.

Earth-abundant Sn/Cu catalysts are highly selective for the electrocatalytic reduction of CO₂ to CO in aqueous electrolyte. However, CO₂ mass transport limitations, resulting from the low solubility of CO₂ in water, so far limit the CO partial current density to about 10 mA cm^-2.[1-2] To enhance CO₂ mass transport, we developed a process to fabricate Sn/Cu-coated polymer nanofiber networks, and demonstrate the materials as gas diffusion electrodes (GDEs) for electrochemically converting gaseous CO₂ to CO.[3] The Sn/Cu-coated PVDF (Sn/Cu-PVDF) nanofiber GDEs achieve CO faradaic efficiencies (FEs) above 80%, and maintain high CO partial current densities of up to 104 mA cm^-2. The Sn/Cu-PVDF GDE remains highly stable during extended operation at -1.0 V for 135 hours, with an average FE for CO of > 85%. These results represent an important step towards an economically viable pathway to CO2RR.

[1] W. Ju, J. Zeng, K. Bejtka, H. Ma, D. Rentsch, M. Castellino, A. Sacco, C. F. Pirri, C. Battaglia, ACS Appl. Energy Mater. 2019, 2 867
[2] J. Zeng, K. Bejtka, W. Ju, M. Castellini, A. Chiodoni, A. Sacco, M. A. Farkhondehfal, S. Hernandez, D. Rentsch, C. Battaglia, C. F. Pirri, Appl. Cat. B.: Env. 2018, 236, 475
[3] W. Ju, F. Jiang, H. Ma, Z. Pan, Y. Zhao, F. Pagani, D. Rentsch, J. Wang, C. Battaglia, submitted

Carbon dioxide is the most abundant greenhouse gas produced as byproduct of fossil fuels exploitation and it is the main responsible for the global climate crisis of current days. The recycling of CO₂ back into chemicals via photo-electrochemical cells represents a viable route to mitigate such problem and great research efforts have been focusing on finding and optimizing functional materials able to catalyze the CO₂ reduction reaction [1, 2]. Here we focus on the copper-iron delafossite oxide, CuFeO₂, which has been proven to have suitable physico-chemical properties to photo-catalyze the carbon dioxide reduction to formic acid [3]. We address the specific electronic and structural features that allow activating the highly stable CO₂ molecule. In particular, by exploiting state-of-the art first principles methods, we dissect the effects of surface oxygen vacancies as well as the different roles of copper and iron on the adsorption and the activation of carbon dioxide at the CuFeO₂ most stable surface. Our results highlight the key role of oxygen defects to allow the CO₂ adsorption and to promote the formation of a radical anion or a carbonate-like adsorbate. While the formation of both intermediates is found to be a downhill process, our data show the second one to be the most probable, paving the route to further mechanistic and kinetic studies on the full pathway for the CO₂ reduction reaction.

References

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[3] J. Gu, A. Wutting, J. W. Krizan, Y. Hu, Z. M. Detweiler, R. J. Cava and A. B. Bocarsly, J. Phys. Chem. C, 2013, 117, 12415-12422.

The versatile properties of cobalt based perovskites such as chemical stability in alkaline media and high electron mobility have led to detailed investigations of LaFe_xCo_1-xO₃ concerning their utilization as potential catalysts for replacing costly and low-abundant materials (e.g. RuO₂, IrO₂ and Pt). A specific interest is in electrochemical water splitting / anodic oxygen evolution reaction (OER) in alkaline electrolytes. Typically, the synthesis of the perovskite materials is done in time-consuming wet-chemical batch processes followed by a calcination step. Thus, the particle size of the raw materials increases, affecting the effective number of active sites and hence, the catalytic performance.
We utilized the scalable spray-flame synthesis (SFS) method to directly synthesize high surface area LaFe_xCo_1-xO₃ perovskites. Typically, high surface area nanoparticles with sizes below 10 nm could be produced. Their catalytic activity can be tuned by adjusting the Fe-to-Co ratio. materials containing 20% Fe performed best for the oxygen evolution reaction (OER) than the samples with lower (10%) and higher (30%) iron substitution concentrations. A potential of 1.53 V vs. RHE was achieved for the OER in alkaline at 10 mA/cm², which is lower than the commercial IrO₂ reference material (1.59 V vs. RHE at 10 mA/cm²). To the best of our knowledge, our high surface area spray-flame made LaFe_0.2Co_0.8O₃ perovskites belong to the best materials for OER and are comparable to the top reported materials for this reaction (e.g. Ba_0.5Sr_0.5Co_0.8 Fe_0.2O_3-d).

In single-atom catalysis, the atomic structure of single metal atoms on a supporting material is one of fundamental questions[1]. We experimentally solved these problems for single Pt atoms dispersed on free-standing graphene using aberration-corrected transmission electron microscopy (TEM) [2]. Graphene was synthesized by the chemical vapor deposition (CVD) method on a copper substrate (purity 99.8%; thickness 25 μm; Alfa Aesar #46365)[3]. We dipped the H₂SO₄ and H₂O₂ mixed solution (H₂SO₄ : H₂O₂ = 3 : 1) to remove the misplaced graphene grown on the back of the Cu substrate. After etching the Cu substrate with 100-mM ammonium peroxodisulfate solution for 4 h, the specimen was rinsed thoroughly with distilled water, and then transferred onto a carbon-supported Cu TEM grid. The graphene was verified as a single layer by electron diffraction. Single Pt atoms were uniformly dispersed on graphene by plasma sputtering method. Sputtering in plasma is well established to form thin films, but this method has also been used to synthesize nanoparticles. Several studies have also attempted to downsize the resulting nanoparticles. In this study, we used this method to disperse single atoms on a substrate. We used an aberration-corrected TEM/STEM (Titan Cubed 60-300, FEI) to verify that the graphene was a single layer and to clarify the atomic arrangements of single Pt atoms. Dispersed single atoms with a size less than 2 Å are clearly discernible in the image of a sputtering time of 1 s. The atom was identified as Pt by energy-dispersive X-ray spectroscopy (EDX), and separation of Pt atoms of 99% was confirmed by measuring the nearest-neighbor distance between Pt atoms. High-resolution imaging using TEM was performed in order to clarify the atomic arrangement of Pt atoms adsorbed on graphene. Although the atomic arrangement above the third layers was hardly realized due to the complicated contrast and atomic migration, typical examples could be clarified. Single Pt atom anchored at the step edge of graphene. The first layer showed a pristine hexagonal lattice, whereas the second layer had a polycrystalline structure, e.g., five- and seven-membered rings. Although this polycrystalline graphene has previously been considered to represent amorphous-like hydrocarbons or contaminations, we revealed the contamination was identified as the aggregation of small islands consisting of several layers of nanographene flakes stacked on a single layer of graphene[4]. The nanographene has a rich area of step edges that serve as anchors and active sites for chemical reactions such as adsorption and catalysis. We reported dense and highly selective formation of single Pt atoms on graphene using nitrogen gas atmosphere during plasma sputtering[5]. We also conducted XPS measurements using the same sample, and observed a large core-level shift for single Pt atoms.
References
[1] J. Liu, ACS Catal. 7 (2017) 34–59.
[2] K. Yamazaki, et al., J. Phys. Chem. C. 122 (2018) 27292–27300.
[3] K. Yamazaki, et al., J. Phys. Soc. Jpn. 87 (2018) 061011.
[4] Y. Maehara, et al., Carbon, 143 (2019) 669–677.
[5] K. Yamazaki, et al., Appl. Phys. Express. 11 (2018) 095101.

Graphene oxide (GO) having oxygen-containing groups is expected as metal-free and environment-compatible catalyst. The research on GO catalyst progressed [1 - 3], although the detail mechanism for GO catalytic activity is not clear yet. Investigating Graphene Oxide (GO) properties before / after the catalytic reaction is a promising strategy for clarification of the mechanism for GO catalytic activity. Additionally, it is known that removing oxidative debris on GO by base-treatment is important factor to get the higher yield for the following catalytic reaction [1]. In this study, the principle for reactivity of GO is investigated by conducting the oxidative amine coupling reaction of benzylamine (1) to N-benzylidenebenzylamine (2) using baGO and thermally reduced one (rbaGO) as catalysts. baGO-Pristine was obtained by washing GO, synthesized by Hummers method, with NaOH. rbaGO was prepared by reducing baGO at 450C under Ar gas flow. After the reaction, the reaction media filtered for removing the catalyst was characterized by GC-MS. After the 1st cycle reaction, the recovered catalyst was used as that for the 2nd cycle (baGO-1st cycle, rbaGO-1st cycle). The chemical structure of each catalyst after the reaction was evaluated by XPS (Al Ka), where it was washed with ethanol after the reaction. The atomic ratio O/C of catalyst much decreases after the reduction or 1st cycle reaction and rbaGO shows poor yield, suggesting roles of baGO / rbaGO not as catalyst but as oxidant. However, baGO-1st cycle exhibits the highest yield in spite of C1s spectrum and O/C similar to that of rbaGO and rbaGO-1st cycle, which also indicates no more reduction for rbaGO in the 1st cycle reaction. Taking these results and appearance of nitrogen content after the 1st cycle reaction into consideration, a different mechanism in the 2nd cycle reaction is suggested, where nitrogen incorporated into rbaGO / baGO in the 1st cycle reaction plays an important role for the reactivity. To clarify the most responsible functional groups in rbaGO / baGO for the higher catalytic reactivity in the 2nd cycle reaction, we have derived the amount of nitrogen-containing functional group per 100 mg in each GO sample for the 2nd cycle reaction. By plotting the amount of nitrogen-containing functional group and the GC-yield of the product, the correlation coefficient r is calculated for each functional groups. In particular, the GC-yield tends to increase as Graphitic N (correlation coefficient r = 0.94) and Oxidized N (correlation coefficient r = 0.87) increase. The important roles of those functional groups in the oxidation amine coupling reaction could be explained by the enhancement in the oxygen adsorption ability, which is important to produce O2*- species from ambient oxygen molecules for proceeding the oxidative coupling reaction.

References
[1] C. Su, M. Acik, K. Takai, J. Lu, S. Hao, Y. Zheng, P. Wu, Q. Bao, T. Enoki, Y. J. Chabal, and K. P. Loh, Nature Communications, 3, 1298 (2012).
[2] A. Shaabani, M. Mahyari, and F. Hajishaabanha, Res. Chem. Intermed., 40, 8, 2799–2810 (2014).
[3] K. Savaram, M. Li, K. Tajima, K. Takai, T. Hayashi, G. Hall, E. Garfunkel, V. Osipov, and H. He, Carbon,139, 861-871(2018).

Single metal sites embedded in nitrogen doped carbon materials have recently demonstrated superior performance as electrocatalysts for oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO₂RR) and so on. However, it's still a challenge to construct abundant single metal atoms during the high temperature processing due to easy aggregation of metal spcies. Herein, we present a facile method to construct N-doped carbon with prevalent single metal sites by employing metal phthalcyanine derivatives as metal precursors. First, iron phthalcyanine (FePc) molecules are encapsulated during the synthesis of zeolitic imidazolate frameworks (ZIFs), and the subsequent pyrolysis yields the Fe-N/C materials. Cyano substitution on FePc can facilitate the introduction of phthalocyanine molecules into ZIFs and formation of more active sites in the carboneous network, thus affording higher activity for oxygen reduction reaction (ORR) than the FePc counterpart. It is also found that the ORR activities in acidic conditions are closely correlated with the surface Fe-N_x contents. Adding iron salt with cyano substituted FePc can afford a better catalyst with enhanced surface Fe-N_x content, which exhibits high activities with a half-wave potential of 0.910 and 0.811 V versus reversible hydrogen electrode in alkaline and acidic conditions, respectively. Moreover, the catalyst derived from cyano substituted FePc exhibits good activities with a reduction current density of 5.0 mA/cm² and a turnover frequency of 0.13 s^-1 and high selectivities with Faradaic efficiency for CO production of around 94% at -0.46 V for carbon dioxide reduction (CO₂RR). DFT caculations are conducted that the bulk Fe-N_x sites are superior to the surface Fe-Nx sites with better catalytic stability in CO₂RR.

Metal phthalocyanines (MePcs) are interesting molecular catalysts for CO₂ electroreduction due to well-defined structural and high turnover frequency (TOF). However, MePcs are semiconducting and have strong intermolecular interactions due to the large π system, which often lead to molecular aggregation on electrodes. As a result, their catalytic performance of MePcs reported in literature could vary significantly and often limited by low current density. Here, we report a carbon nanotube (CNT) hybridization approach and prepare MePcs(Me = Co, Fe and Mn)/CNT hybrids to study the CO₂ reduce electrocatalytic performance of MePcs molecular catalyst. MePc/CNT hybrids are synthesized with MePc molecules anchored on the surface of CNTs without noticeable molecular aggregations. Both activities and stabilities of MePc are significantly enhanced by hybridization with carbon nanotubes. The activity of FePc/CNT is slightly less than CoPc/CNT, but it could deliver higher Faradaic efficiencies for CO production over 98% at low overpotentials. In contrast, among the neat molecules, only CoPc shows appreciable activities for CO₂ electroreduction . Further, the TOF results indicate that CNTs hybridization is helpful to reveal the intrinsic activities of MePc molecules and deliver higher current densities.

Electrochemical conversion of carbon dioxide (CO₂) to renewable chemical feedstocks and fuels could play an important role for ending the use of fossil fuels. To amortize equipment costs over large product volumes, commercial applications will require high current density devices. Due to solubility and diffusion limitations of CO₂ in liquid electrolytes, this requires electrodes to be near the liquid/gas interface, which is accomplished by the gas diffusion electrode (GDE) cell architecture. In designing such electrodes, the question of where the gas/liquid layer is positioned within the GDE becomes important. For example, it has been suggested that the CO₂ reduction reaction occurs at a triple-phase-point, where the gaseous CO₂, aqueous protons, and electrons from a solid metal electrocatalyst can interact. However, from various literature, it seems more likely that the metal is fully wetted and a two-phase reaction occurs with the CO₂ dissolved in the electrolyte. Likewise, it is unclear whether product gas forms nanobubbles on the electrocatalyst surface, or dissolves into the electrolyte.

Here, we use an electrochemical atomic force microscope (EC-AFM) to make operando measurements of the electrocatalyst surface in a GDE CO₂ reduction cell, while operating at 1, 10, and 100 mA/cm². Using peak-force tapping, we map the topography and nanomechanical information. No nanobubbles or macroscopic bubbles are observed. Comparison with a solid metal foil shows bubble formation on only the macroscopic scale, supporting the idea that gaseous reactants and products interacting with the electrocatalyst are primarily, dissolved in the electrolyte.

Photocatalytic hydrogen production through water splitting has recently gained significant attention for efficient solar-to-chemical energy conversion.¹ However, despite immense research efforts on semiconductor materials, key challenges of obtaining catalysts with high activity and long-term stability still remain. Recently, the synthesis of highly crystalline mesoporous semiconductors has been the focus in the field of electro- and photocatalysis. These materials combine the high reactivity of semiconductor nanoparticles with mesoporosity, offering new perspectives in designing novel catalysts with improved efficiency and reliability. Unlike to bulk-like microstructures and individual nanoparticles, 3D mesoporous networks of connected nanoparticles can benefit from the large and accessible surface area within the assembled structure and enhanced light-harvesting efficiency arising from multiple scattering of light inside the pores.
Herein, we present the synthesis of chemically stable and robust mesoporous networks of β-Ni(OH)₂ and ZnS decorated CdS nanoparticles (ca. 4–5 nm) that show high photocatalytic activity for hydrogen evolution under visible light irradiation.² We utilize polymer-assisted self-assembly of colloidal nanoparticles to assemble mesoporous CdS nanoparticle-linked frameworks with high interstitial porosity. This method utilizes the nanoparticles as building block units and based on the coassembly of these nanoscopic units with organic structure-directing agents to construct 3D structures with open-pore morphology.³ The resulting materials feature a nanometer-sized framework perforated by uniform pores, which allows rapid mass transport and provides plenty of exposed surface-active sites available for reaction. In addition, using a combination of photocatalytic experiments and optical absorption, luminescence and electrochemical impedance spectroscopy studies, we highlight the effect of β-Ni(OH)₂ and ZnS additives on the electronic structure and photochemical performance of CdS. For instance, our results indicate that the p-type β-Ni(OH)₂ acts as hole collector, favoring efficient charge transfer and separation across the β-Ni(OH)₂/CdS nano-heterojunctions. As thus, the β-Ni(OH)₂/CdS catalyst containing 10 wt% Ni reached a photocatalytic H₂-evolution rate of 1.4 mmol/h with an apparent QY of 72% at 420 nm, while demonstrating excellent stability in alkaline (5M NaOH) ethanol solution (10% v/v). The results of these studies can offer new insights into the design and development of efficient photocatalysts for energy conversion and environmental remediation applications.

References
(1) H. Ahmad, S.K. Kamarudin, L.J. Minggu, M. Kassim, Renew. Sust. Energ. Rev. 2015, 43, 599.
(2) a) I. Vamvasakis, I.T. Papadas, Th. Tzanoudakis, Ch. Drivas, S.A. Choulis, S. Kennou, G.S. Armatas, ACS Catal. 2018, 8, 8726. b) I. Vamvasakis, K.S. Subrahmanyam, M.G. Kanatzidis, G.S Armatas, ACS Nano 2015, 9, 4419.
(3) I.T. Papadas, I. Vamvasakis, I. Tamiolakis, G.S Armatas, Chem. Mater. 2016, 28, 2886.
(4) We gratefully acknowledge financial support from the University of Crete–Special Account for Research Funds (KA 10138).

Electrochemical CO₂ reduction has been considered as a potential route to convert harmful CO₂ to valuable hydrocarbons for sustainable carbon cycles. Since late 1980s, copper catalyst has been recognized as one and the only catalyst which produced hydrocarbons as primary products;¹ however, the poor selectivity obstructs the applications. According to DFT calculations, surface CO is the most common and important intermediate during CO₂ reduction reaction,² but further reaction mechanisms are still unclear, e.g. protonation and dimerization of CO. Cu(I) oxide was correlated to the formation of C2 products during reactions,³ yet there is lack of solid intermediate evidence to distinguish different reaction routes.
In this talk, we used in situ surface enhanced infrared absorption spectroscopy, in situ X-ray absorption spectroscopy (Cu L-edge) and on-line GC to study the electrochemical CO₂ reduction mechanism occurred in different copper catalysts. We observed various CO intermediates such as CO_atop and CO_bridge, on copper surface during electrochemical CO₂ reduction and the formation of CO_atop/CO_bridge can be correlated to the existence of surface Cu(I) and Cu(0) sites, respectively. Our results suggested that the existence of CO_atop can result in the formation of methane during further reduction. In addition, ethylene can be produced in the presence of CO_atop and CO_bridge. Our results both prove theoretical predictions ⁴ and meet the gap to illustrate the detail reaction mechanisms from CO₂ to hydrocarbons. We next propose a method to create Cu(I) sites efficiently on copper catalysts.

References
1. Y. Hori, K. Kikuchi, S. Suzuki, Chem. Lett. 1985, 1695.
2. T. Cheng, H. Xiao, W. A. Goddard, J. Am. Chem. Soc. 2016, 138, 13802.
3. H. Mistry, A. S. Varela, C. S. Bonifacio, I. Zegkinoglou, I. Sinev, Y. W. Choi, K. Kisslinger, E. A. Stach, J. C. Yang, P. Strasser, B. R. Cuenya, Nature Comm. 2016, 7, 12123.
4. H. Xiao, W. A. Goddard, T. Cheng, Y. Liu, Proc. Natl. Acad. Sci. USA 2017, 28, 201702405-6688.

In recent years, many achievements have been made in improving photocatalytic activity, but the solar conversion efficiency of photocatalysts is still far below the expected value. According to the reaction mechanism of photocatalysis, the factors affecting the conversion efficiency of photocatalyst can be summarized as follows: the band gap, the position of conduction band and valence band, the efficiency of carrier separation and transfer. The magnetic field can be introduced into the photocatalytic process and enhance the solar conversion efficiency: on the one hand, in some molecular catalytic reactions, the ferromagnetic arrangement of catalyst materials will align the spin orientation of the electrons accumulated on the surface, effectively promoting the occurrence of oxygen evolution reaction, on the other hand, magnetic field can change the conductivity of some materials with specific properties, and increase the carrier transport and separation. In addition, the external magnetic field does not require direct contact with the reactor vessel, and does not affect the morphology and own characteristics of photocatalysts, therefore the magnetic field will not cause secondary pollution.
In this work, the composite photocatalysts ZnFe₂O₄/Ag and ZnFe₂O₄/AgBr based on spintronic material ZnFe₂O₄ are constructed. The interface between ZnFe₂O₄ and Ag or AgBr can induce the magnetic moments in Ag or AgBr due to the magnetic proximity effect, emerging negative magnetoresistance (MR) effect in composite photocatalysts. Under the magnetic field, the organic dyes can be removed more efficiently compared to that without magnetic field. The measurement results demonstrate that the magnetic field plays two kinds of roles during the photocatalytic process, firstly, the magnetic field promote the transfer and separation of electrons and holes to enhance the photocatalytic activity because of the negative MR of the composite photocatalysts. Secondly, the magnetic field can replace the function of electron transmission medium in Z-scheme structure, which can simply the structure of the Z-scheme photocatalyst ZnFe₂O₄/AgBr. The combination of spintronics and photocatalysis is of great scientific significance in understanding charge transfer, physical mechanism of photocatalysis, mutual promotion between spin characteristics and photocatalytic performance, meanwhile, this combination provides an important reference for the development of multifunctional electronic devices.

The sluggish kinetics of the oxygen reduction reaction (ORR), insufficient durability and the use of high-cost Pt-based catalyst restrict the comprehensive performance as well as the widespread commercialization of fuel cells (FC). For the development of cost-effective catalyst possessing low noble-metal loading, outstanding ORR activity and long-term stability, we propose novel model nanocatalysts with size-variable Pt clusters decorated Co@Pd core-shell configurations use for the fuel cells cathode. In this work, the effects of Pt-cluster size on ORR catalytic activity for Co@Pd core-shell structure were studied via tuning the number of Pt atoms from 3 to 9 (in size of 0.3-1 nm), 16 and 32 (regard as 1 cover layer and 2 cover layers), based on density functional theory (DFT) calculations. Our results show that the size of Pt_n (n=3-9, 16 and 32) decoration significantly effects the ORR catalytic activity, the active sites selectivity and the stability of Co@Pd model nanocatalyst, which are attributed to their modified electronic structures giving rise to the strong electronic coupling and unique charge transfer process. The moderate ORR-species (O, O₂, H₂O and OH) adsorption energies on different stable active sites and moderate O₂ dissociation barriers corresponding to the Pt_n-cluster model catalysts exhibit the better ORR activity compare to the pure Pt and Pd models. The influence of d-band center shift, strain and ligand effects on the adsorption energies on different surfaces are also discussed. In particular, small platinum-trimer (Pt₃) decorated Co@Pd model nanocatalyst exhibits the most significant enhancement in the ORR kinetics and the predicted structural stability, due to the electron transfer from Pd shell to Pt₃ cluster and then to the adsorbates giving the Pt-trimer a role as ‘charge transfer station’ during the whole ORR process. As a consequence, the ‘charge transfer station’ avoids the valence electrons transfer of adsorption-site Pt atoms to the direct-contact ORR-species, but proceeding the indirect charge transfer between the Pt-neighboring Pd atoms and ORR-species atoms instead, so that reducing the risks of the noble metals oxidation and poisoning. Therefore, the Pt-associated sites are selected spontaneously as the preferred stable active sites, and meanwhile the atomic scale Pt-trimer provids the most number of the optimal active sites. These results are consistent with our latest experimental results.^[1] The effect on the ORR performance of our size-tunable clusters from atomic scale to thin film scale decrorated on the core-shell models, can be further extented to other ternary or multi-metallic systems with most types of 3d-5d transition metals in the three (core-shell-cluster) regions. This conclusion can be applied to guide the selection of candidate systems and predict the performance of the ORR in studies for the catalyst development toward low cost, excellent activity, high durability and environment-friendly designs.
Reference:
1. Sheng Dai, Jyh-Pin Chou, Kuan-Wen Wang, Yang-Yang Hsu, Alice Hu, Xiaoqing Pan & Tsan-Yao Chen, Platinum-trimer decorated cobalt-palladium core-shell nanocatalyst with promising performance for oxygen reduction reaction, Nat. Commun., 2019, 10, 440.

Continuous accumulation of carbon dioxide in the atmosphere is a threat to the environment and its symptoms are increasingly visible every passing year in the form of impulsive whether changes. Therefore, converting carbon dioxide has turned out to be promising to produce fuel and chemicals. Among the various chemicals synthesized from CO₂, dimethylcarbonate (DMC), a simplest organic carbonate, has attracted considerable attention due to its applications as green solvent, fuel additive for octane booster, electrolyte in lithium-ion batteries, and monomer for polycarbonate synthesis etc. Conventionally, DMC is produced either by the reaction of phosgene with methanol or oxidative carbonylation of methanol using CO. The use of phosgene and Co, which are highly toxic and corrosive in nature, makes these processes undesirable. Therefore, much importance has been given to the DMC synthesis from CO₂. In this context, various catalytic systems (homogeneous and heterogeneous) have been reported. However, most of the catalyst afford lower yield of the DMC because of the water generation during the reaction as side product, which shifts reaction equilibrium backwards providing lower yield of DMC. Therefore, the use of dehydrating agent is needed for better conversion. However, these dehydrating agents are difficult to separate from the reaction mixture after the reaction and further increase the cost of the process. Moreover, most of the methods are batch process, which restricts their direct application in industries.
Hence, It is desirable to synthesize DMC directly from the methanol and carbon dioxide in a continuous flow without using any external dehydrating agent. In the present paper, we have synthesized copper sulfide (Cu₂S) impregnated two-dimensional organic polymer (Cu-OP) by simple mechanical mixing of CuCl₂.H₂O, 1, 3, 5-trybenzenecarboxyldehyde and 2, 5-diaminobenzene sulfonic acid. Synthesized catalyst was then used to perform the reaction of methanol and carbon dioxide on the high-pressure fixed bed flow reactor without using any dehydrating agent and exhibited excellent catalytic activity, selectivity reusability affording up to 43% yield of DMC. Moreover, we also investigated the effect of various reaction parameters including flow rate, pressure, temperature and catalyst amount on the DMC yield. It was observed that the high pressure and lower flow rate facilitates the reaction at desired temperature. In addition, the presence of -SO₃H and -C=N functional groups on the catalyst surface was found to be crucial, which not only helps in trapping the water formed during the reaction, shifting reaction equilibrium forward direction, but also activate the carbon dioxide, respectively.

2D nanomaterials decorated with single atom catalyst (SACs) are efficient and low-cost electrocatalyst candidate for clean energy technologies. The 2D supports and coordination environments play an important role in optimizing the catalytic activity and reaction mechanism of single atom centers. Here we summarize the suitable synthetic strategies of single atom on graphene supports. We used recently different methods that facilitate the formation of homogeneous structure and enabled the regulation of surrounding atomic of environment center. We recently used new laser ablation techniques for making defects in 2D nanomaterials, and then used it as supporting material to grow SAC for catalytic applications. Here, we elucidate our latest results about SAC on laser ablated 2D defective nanomaterials for different electrochemical reactions, including HER, ORR and OER. Uniting advanced experimental and theoretical approaches including aberration-corrected scanning transmission electron microscopy (AC-STEM), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and density functional theory (DFT).

Structural evolution of catalytic active sites in methane dehydroaromatization (MDA) reaction in non-oxidative conditions over the Mo/HZSM-5 catalyst is a critical factor in deciding the reactivity and stability of the catalyst as discussed by Khan et al. [J. Phys. Chem. C 2018, 122, 11754-11764]. Molybdenum oxide species, which are anchored on the zeolite framework are transformed into oxycarbide and carbide clusters during the carburization process. The molybdenum carbide clusters anchored on the zeolite framework are considered to be the active sites for MDA. On prolong operation, the clusters tend to migrate and agglomerate at the external zeolite surface, leading to catalyst deactivation. Therefore, a digilent and realistic choice of carburization conditions to control the formation and anchoring of the carbide cluster on the zeolite framework essentially holds the key for obtaining high stability and activity of the catalyst in the MDA process. This thoughtful pretreatment of as-synthesized Mo/HZSM-5 catalyst is demonstrated in two different carburization conditions; using pure methane and a mixture of hydrocarbons representing the natural gas. The effect of carburization process under the two different carburization conditions is monitored with catalyst characterization using the X-ray photoelectron spectroscopy (XPS), which is specifically pointing towards the formation of active molybdenum carbide species under natural gas. In contrast, under pure methane, molybdenum species are observed to transform into an oxycarbide phase. This significant change in the structural evolution of active molybdenum species in the two different carburization conditions reflected into a significant change in the reactivity and deactivation behavior of the Mo/HZSM-5 catalyst. The catalyst treated in natural gas showed a higher benzene formation rate of 3199 nmol/g_cat.s as compared to the one treated in pure methane (2280 nmol/g_cat.s) after 2 hours time-on-stream. This is correlated with high level of BrØnsted acidity measured for natural gas carburized Mo/HZSM-5 catalyst (1.22 mmol/g_cat) as compared to the methane-carburized Mo/HZSM-5 catalyst (0.37 mmol/g_cat). TPO, TGA, XPS and Raman analysis of the carburized and spent catalysts further pointed towards lesser coke formation for the catalyst pretreated under natural gas environments as compared to pure methane environment. Experimental results are correlated with ab-initio thermodynamic phase diagrams constructed under reaction conditions to understand the stability of different phases of molybdenum in different hydrocarbon environments. Overall the results are elucidating a microscopic level control on active sites in the MDA reaction by macroscopic process conditions.

Well-defined catalytic nanoparticles consisting of bimetallic Pt-Ni alloys were prepared with homogeneity and characterized by high-resolution transmission electron microscopy (HRTEM), x-ray photoelectron spectroscopy (XPS), and energy dispersive x-ray spectroscopy (EDS). The effect of the support material on the catalytic activity of the nanoparticles was investigated with the aim of optimizing the catalyst activity. Nickel and platinum supported on silica (SiO₂,), graphene oxide (GO), and molybdenum sulfide (MoS₂) with the ratio of the Pt:Ni being 1:1, 1:2, and 2:1 were prepared and their catalytic efficiency was evaluated using a model oxime reaction. The catalysts were optimized to allow the reaction to proceed at ambient temperature and in the aqueous phase. The concentration of each metal ion was determined by employing inductively coupled plasma−optical emission spectroscopy (ICP-OES). The turnover frequency (TOF) was calculated to evaluate the catalytic performance of Pt/Ni@SiO₂, Pt/Ni@GO, and Pt/Ni@MoS₂toward oximation reactions at 50% and 90% conversion. The TOF of Pt/Ni@MoS₂ was found to be higher than of Pt@SiO₂and Pt/Ni@GO.

Carbon dioxide can be electrochemically converted into feedstocks for many industrial processes such as the manufacturing of synthetic fuels and chemicals. In order for this technology to be capable of matching and ultimately supplanting any industrial process, it needs the capability to convert CO₂ at an industrial scale. This means the reaction needs to happen at a fast rate for long periods of time and make products with high selectivity and high energy efficiency. In order to achieve these objectives, the electrocatalysts and electrochemical systems need to be studied while operating at high current densities.
This talk will focus on the structure-functionality relationship between Au, Sn, and bimetallic AuSn catalysts, and their CO₂ reduction performance at high current densities.
X-Ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), and atomic force microscopy (AFM) were used to determine the crystal structure, surface topography and composition of compositionally variant alloy thin films of Au-Sn before and after electrolysis. The electrochemical activity for each bimetallic film was measured in terms of electrode potential/current and product selectivity as a function of applied current density/potential and catalyst composition. To achieve high current density operation (>100mA/cm²), gas diffusion electrode’s (GDE’s) were used as the working electrode in a gas-fed electrolyser.
Finally, the reaction’s activity and selectivity were examined in an aqueous H-cell reactor to compare the performance of the bimetallic catalyst’s with liquid and gas-phase feedstock.
The results of this work provide fundamental insights to reaction mechanisms in aqueous and gas-fed CO₂ electrolysers, while also providing a template for assessing electrochemical performance across these two reactor configurations. Electrochemical trends at low current density were found to not always translate to high current density operation, and therefore future work is needed to understand the differences in these two architectures.

Electrochemical CO₂ reduction has attracted significant attention not only for its technological implications but also for the scientific knowledge it holds. One critical challenge is to suppress the kinetically favored hydrogen evolution reaction in an aqueous electrolyte while achieving high selectivities toward carbonaceous products. Here we report a new way to realize such a goal. Our strategy involves a unique system where the hydrogen formation can be controlled. Such a platform offers the opportunity to study the role of electrolyte component in electrochemical CO₂ reduction, which is critical to the understanding of the reaction mechanisms and the performance matrix. Using Au as a prototypical catalyst, we revealed that the selectivity toward CO can be greatly improved by manipulating the electrolyte properties. It was also found that the overall performance can be significantly influenced by mass transport. The results shed new light on the mechanistic understanding of electrochemical CO₂ reduction, and offered a new way to achieve high selectivities for practical applications.

A major challenge in electrocatalysis is the rational control of forming and stabilizing reaction intermediates to guide specific reaction pathways. In this work, we present a model system introducing the concept of surface-bound host-guest chemistry in CO₂ electrocatalysis. The functionalization of pristine gold (Au) with cucurbit[6]uril (CB[6]) nanocavities was studied as a hybrid organic-inorganic model system that utilizes host-guest chemistry to influence heterogeneous electrocatalytic reactions.
The combination of surface-enhanced infrared absorption (SEIRA) spectroscopy and electrocatalytic experiments in conjunction with theoretical calculations support capture and reduction of CO₂ inside the hydrophobic cavity of CB[6] on the gold surface in aqueous KHCO₃ at negative potentials. SEIRA spectroscopic experiments show that the decoration of gold with the supramolecular host CB[6] leads to an increased local CO₂ concentration close to the gold interface. Electrocatalytic CO₂ reduction on a CB[6]-coated gold electrode indicates differences in the specific interactions between CO₂ reduction intermediates within and outside the CB[6] molecular cavity, illustrated by a decrease in CO current density, but almost invariant H₂ production.
Based on the obtained results, design criteria of molecular cavities that allow to overcome current limitations in heterogeneous CO₂ electrocatalysis are discussed. We believe that the methodology and molecular insights in the presented work will provide fruitful basis for future design concepts of molecularly engineered catalytic environments through interfacial host-guest chemistry.

It has long been recognized that the reaction rates of the hydrogen oxidation and hydrogen evolution reactions (HOR /HER) are up to 200 times slower in basic than acidic electrolytes, even though the surface intermediate of adsorbed hydrogen is independent of solution pH [1]. One explanation for this difference is a proposed `bifunctional mechanism’, in which slow water dissociation is overcome by mediation of adsorbed hydroxide [2]. We have completed detailed microkinetic models of this pathway for single- and two-site catalysts, and compared these models to single-crystal voltammetry on Pt(110) and transition-metal-doped Pt(111) [3-4]. We find that any chemical dissociation steps result in anomalous trends with potential, and that oxophilicity has no effect on hydrogen coverage or HER/HOR rates. Comparison with experiment strongly indicates that the observed bifunctional mechanism is unrelated to hydroxide binding strength [4].
Our work resolves a long-standing paradox in electrocatalysis and surface science by determining that oxophilicity is not an accurate descriptor for alkaline hydrogen electrocatalysts. Other parameters, such as water orientation and non-covalent interactions, must play a greater role in overall activity. In efforts to identify and measure these parameters, we combine a series of techniques from catalysis, classical electrochemistry, and colloid science. These experiments yield new understanding how the solution-phase double-layer interacts with solid-phase surface structure to govern electrocatalytic activity, thus enabling rational design of next-generation catalytic interfaces [5].
[1] Durst et al. Energy Environ. Sci. 2014, 2255.
[2] Li et al. Angew. Chem. Int. Ed. 2017, 15594.
[3] Intikhab et al, ACS Catal., 2017, 8314.
[4] Rebollar-Tercero et al, J. Electrochem. Soc, 2018, J3209.
[5] Intikhab et al, under review, 2019.

The utilization of fossil fuels as the main energy source gives rise to serious environmental issues, including global warming caused by the continuously increasing level of atmospheric CO₂. Recently, the electrochemical conversion of CO₂ (CO₂RR) to chemicals and fuels driven by electricity derived from renewable energy has been recognized as a promising strategy towards sustainable energy.
I will provide examples of recent advances in the development of highly active plasma-modified single crystals, nanostructured thin films and nanoparticle (NP) electrocatalysts (Cu, Ag, Zn, Ag-Sn and Cu-M with M = Co, Zn, Ag, Pd) and how their structure, oxidation state and composition influence their selectivity in CO₂RR. Additionally, the determining role of the electrolyte in the reaction activity and selectivity will be illustrated by considering the role of different cations and anions (Cs⁺, Li⁺, Na⁺, K⁺, I^-, Br^-, Cl^-) in aqueous electrolytes. Finally, the importance of in situ and operando characterization methods (e.g. EC-AFM, Liquid-TEM, XAS, XPS) to gain in depth understanding on the structure- and electrolyte-sensitivity of real CO₂RR catalysts under working conditions will be demonstrated. Our results are expected to open up new routes for the reutilization of CO₂ through its direct selective conversion into higher value products.

I will discuss advances in increasing the selectivity, improving the energetic efficiency, and improve the rate (current density) of heterogeneous CO2 electrocatalysts and systems.

A key challenge in heterogeneous catalysis has been developing technologies that can selectively convert CO₂ or CO into short-chain multi-carbon oxygenates. Here, I will show how to improve selectivity to multi-carbon oxygenates by rationally controlling a variety of properties that are intrinsic and extrinsic to the catalyst. First, I will discuss how extrinsic properties such as the electrochemical potential and electrolyte pH impact reactivity on polycrystalline Cu electrodes. By comparing CO and CO₂ reduction from similar reactors, we demonstrate that alkaline reaction conditions increase the energy efficiency towards multi-carbon products. Furthermore, we reveal selectivity trends that are common to both CO and CO₂ reduction, indicating both the production of oxygenates and the growth of longer carbon chains are favored at lower overpotentials. Next, I will show how proper control of these extrinsic properties combined with increasing the electrode surface area of polycrystalline Cu leads to high selectivity for multi-carbon oxygenates. We demonstrate that by increasing the electrode surface area, nearly 100% selectivity for CO electroreduction to multi-carbon oxygenates is achieved at a very low overpotential of ~200 mV, from a concomitant suppression of the competing hydrogen evolution reaction. Furthermore, we reveal the generalizability of this surface area effect by using a variety of methods to synthesize polycrystalline Cu electrodes of many different morphologies and roughness factors, and comparing their electrocatalytic performance. Finally, I will show how combining all of these design principles with controlling the surface composition, an intrinsic property of the catalyst, leads to improved selectivity for a single multi-carbon oxygenate. Under alkaline reaction conditions, we observed high selectivity for CO reduction to acetaldehyde on CuAg bimetalic electrodes at low overpotentials, reaching a maximum total Faradaic efficiency of >70% and selectivity of >90% on a carbon basis.

It is of both scientific and societal importance to advance the sustainable energy technology that enables the conversion of CO₂ into fuels and value-added chemicals via cost-effective electrocatalysis. At present, precious metals (e.g. Au, Ag, and Pd) exhibit the best catalytic activity for the electrochemical reduction reaction of CO₂ (CO2RR) to fuels. However, their high cost and lack of selectivity hamper the wide implementation of the technology. Development of alternative catalysts based on earth-abundant elements (i.e., free of precious group metals (PGM-free)) is therefore highly demanded. In recent years, transition metal (TM=Fe, Co, or Ni) and/or nitrogen (N) doped carbon (denoted as TM-N-C) catalysts have been demonstrated to be capable of catalyzing CO₂ reduction to CO with a reasonable activity and amazingly high selectivity. However, knowledge about how the activity and selectivity of TM-N-C electrocatalysts for CO2RR are dependent on their chemical composition, structure, morphology, and synthetic chemistry is seriously lacking. For a rational design of these TM-N-C catalysts, we have performed density functional theory (DFT) calculations to investigate CO2RR on various TM-N₄ active sites (TM = Fe, Co, Ni) with different moieties. Specifically, three different TM-N₄ moieties are studied, such as the TM-N₄ moiety embedded in the graphene basal plane, the porphyrin-like moiety and/or the TM-N₄ moiety bridging the edges of two adjacent graphene layers. We calculated the adsorption energies of all the possible chemical species and the activation energies for C-O bond dissociation reactions involved in CO2RR on the active sites. Our DFT calculations predicted that the CO2RR could happen at the proposed active sites under an electrode potential more positive than that for competing hydrogen evolution reaction (HER). Consequently, these TM-N-C catalysts exhibit good selectivity by diminishing parasitic HER. Moreover, the TM-N₄ moiety bridging the edges of two adjacent graphene layers was predicted to be capable of breaking the C-O bond efficiently. These theoretical results are in agreement with experimental observations. Furthermore, we investigated how the doping of S would affect the CO2RR on Fe-N-C catalysts. Our DFT calculation reveals that the adjacent S atoms engineering increases the charge density of centered Fe in a Fe-N₄ site, and thus predicts that endowing S-modified Fe−N₄ with lower formation energy barriers for the key COOH* intermediate in CO2RR. These results suggest that adding S into the TM-N-C catalysts is an effective approach to enhance their catalytic activity for CO2RR.

The quest for the design of active and stable (electro)catalysts hinges on the ability to unequivocally identify the reaction mechanism under realistic operating conditions. The past few decades of research have focused on developing a range of spectroscopic and imagining tools to enable the study of the chemical and structural changes at the solid/liquid interface.
In this talk, we will bring the most recent understanding of the surface (electro)chemistry of CO₂ on Pt surface. The molecular-level understanding of the interfacial (electro)chemical processes on Pt surfaces under CO₂-contained aqueous system by in situ surface enhanced infrared spectroscopy (SEIRAS) will be particularly discussed. We will also show the integration of those experimental approaches and density functional theory (DFT) calculation further strengthen the understanding of the reaction mechanism. It will be also demonstrated that the holistic information about the nature of the adsorbed species can provide additional knobs to tune the kinetics of electrochemical reaction, leading further improvements in performance for energy conversion and storage devices.

Increasing the selectivity of the electroreduction of carbon dioxide on Cu electrodes for valuable hydrocarbons is a great challenge. A promising approach for improving the product selectivity is through modification of the Cu/electrolyte interface with organic additives. For example, it has been shown that the decoration of the Cu electrode surface with amino acids or polyamide films can enhance the selectivity of ethylene by approximately a factor of two. Recently, it has been reported that addition of N-substituted arylpyridinium salts can greatly alter the product selectivity of the electroreduction of CO₂ on Cu electrodes [Han et al. ACS Cent. Sci. (2017) 3, 853-859]. Han et al. found that the product spectrum sensitively depends on the structure of the added pyridinium cation. For example, 1-(4-tolyl)pyridinium (T-Pyr) increases the Faradaic efficiency of C_≥2 products to ≈80% (compared to ≈25% observed for unmodified Cu/electrolyte interface). By contrast, in the presence of 1-(4-pyridyl)pyridinium (P-Pyr), hydrogen is the major product. Although it has been shown that thin films derived from the reduced cation species form on the surface of Cu electrode, the mechanisms of how product selectivity is altered remain poorly understood. To further improve the selectivity of CO₂ reduction, mechanistic insights are essential. Herein, using surface enhanced infrared spectroscopy (SEIRS) during the electroreduction of CO₂, we reveal the differential impact of T-Pyr and P-Pyr on the properties of the Cu/electrolyte contact. We find that under reduction conditions both cations form thin films that are composed of the reduced dimers of the respective cation. In the presence of T-Pyr, we observe a substantial build-up of solution-phase carbonate at the interface. By contrast, no such increase is observed at the bare Cu and P-Pyr-modified interfaces. These observations demonstrate that, in the presence of T-Pyr, the local pH in the vicinity of the electrode measurably increases. Because a high local pH is known to change the selectivity towards C_≥2 products, our measurements suggest that the higher local pH in the presence of T-Pyr is the underlying mechanism of the selectivity changes (compared to bare Cu). We propose that the T-Pyr films impose mass transport limitations that give rise to the observed high local pH. In addition, we find that in the presence of P-Pyr, the CO stretch frequency of surface adsorbed CO is significantly red-shifted by several tens of wavenumbers (with respect to the frequencies observed at the unmodified and T-Pyr-modified electrodes). This observation suggests that that the CO adsorption on Cu is negatively impacted by the presence of this additive, explaining the poor selectivity for hydrocarbons in its presence. We will discuss the implications of these findings for the design of catalytic interfaces with organic thin films.

Electrocatalytic CO₂ reduction is often carried out in a solution electrolyte such as KHCO₃ (aq) for ion conduction. Therefore, liquid products that form are in a mixture with the dissolved salts, requiring energy-intensive downstream separation. Here we report continuous electrocatalytic conversion of CO₂ to pure liquid fuel solutions in cells that utilize solid electrolytes, where electrochemically generated cations and anions are combined to form pure product solutions without mixing with other ions. Using a HCOOH-selective (Faradaic efficiencies > 90%) and easily-scaled catalyst at the cathode, we demonstrate production of pure HCOOH solutions with concentrations up to 12 M. We also show 100-hour continuous and stable generation of 0.1 M HCOOH with negligible degradation in selectivity and activity.

The development of the next generation technologies for CO₂ reductions benefits from investigating a wide range of approaches in order to move catalytic CO₂ reduction towards a path to commercialization. This presentation focuses on lessons learned from the development of a novel PdPt bimetallic catalyst with a ‘nanoclam’ morphology for electrocatalytic CO₂ reduction. This presentation will be divided into two parts: study of the electrochemical properties of this catalysts and efforts to use this catalyst synergistically with microorganisms to achieve high selectivity to a single multi-electron reduction product.

In terms of catalyst development, the PdPt nanoclams have a unique tapered morphology that combines high surface area with exposure of numerous undercoordinated sites for CO₂ reduction with activity exceeding that of either Pd or Pt for CO₂ reduction to formate at - 0.2 V vs RHE. In comparison with bulk Pd, PdPt, and Pt systems, we find that the interplay of multiple trends affects the activity:

1. The PdPt alloy composition offers improved activity by disfavoring hydride intercalation into the lattice
2. The incorporation of Pt affects increases the faradaic efficiency for HER over formate production
3. The nanostructured morphology appears to increase the faradaic efficiency for formate production over HER

This PdPt nanoclam catalyst is uniquely suited to integration with microorganisms owing to its high activity for the formation of 2-electron products from CO₂ reduction at a mild overpotential of – 0.2 V vs RHE as well as reduced deactivation from CO poisoning. Thus, we have developed a hybrid electrochemical-biological CO₂ reduction system in which formate and hydrogen are produced through electrochemical CO₂ reduction by PdPt nanoclams. These products are metabolized by methanogens in combination with CO₂ to yield ~100% faradaic efficiency to methane. The integration of microbial communities with these nanostructured PdPt catalysts has the potential to combine the best-of-both-worlds from electrochemical and biological systems to achieve a regenerative catalytic system with high-selectivity, high activity, and low overpotential.

The utilization of renewable energy sources to drive electrochemical transformations that allow for the storage of energy in chemical bonds is important for the realization of a more sustainable energy infrastructure. The conversion of abundant small molecules into fuels and/or high-value chemicals driven by renewable electrons is a promising strategy to reduce the reliance on traditional fossil fuels in the transportation and industrial sectors, provide liquid energy carriers for energy storage, and improve grid management. To selectively promote these difficult conversions, electrocatalysts that operate at high rates and efficiencies and low overpotentials are needed. Nanoparticle metal phosphides have emerged as a promising class of electrocatalysts for a variety of energy conversion reactions. Significant advances in the solution phase synthesis of nanomaterials for energy conversion applications have enabled a high level of control over the size, shape, and composition of nanostructured catalysts, which has revealed the importance of these properties on the observed electrocatalytic performance. In this work, we present a novel and facile approach utilizing air-stable and inexpensive metal triphenylphosphine complexes as single-source molecular precursors to access phase-pure metal phosphide nanoparticles that have controlled stoichiometry, size, and shape. The metal phosphide nanoparticles are evaluated as electrocatalysts for several energy converting reactions and important performance metrics, i.e. faradaic efficiency, overpotential, and product selectivity, are discussed.

Electrochemical CO₂ reduction reactions promise the interconversion of carbon emissions to fuels and value-added chemicals with renewable electricity. Exploration of multi-component electrocatalysts with unconventional reactivity (e.g. uncommon mechanistic pathway, bifunctionality or new product) is not only scientifically interesting, but could also lead to new device-level functionality. As a case in point, we report a new Pb-Pd alloy system for CO₂-to-formate interconversion that enables controllable utilization of alternative energy by integrating the functions of a CO₂ electrolyzer, a liquid fuel container and a fuel cell in one device. The alloy system reversibly restructures its phase, composition, and morphology and thus alters its catalytic properties under controlled electrochemical conditions. Under cathodic conditions, the catalyst is relatively Pb-rich and is active for CO₂-to-formate conversion over a wide potential range; under anodic conditions, it becomes relatively Pd-rich and gains stable catalytic activity for formate-to-CO₂ conversion. The bifunctional activity and superior durability of our Pb-Pd catalyst leads to the first proof-of-concept demonstration of an electrochemical cell that can switch between the CO₂ electrolyzer/formate fuel cell modes with stable operation for twelve days.

Approaches to make fuel and fine chemicals from carbon dioxide, water, and renewable electricity are being widely researched. One advantage of technologies that can produce chemicals in this manner is their potential for on-site production. This is a key value proposition currently being realized by companies that sell on-site electrochemical hydrogen generators. Translating this value proposition and design feature of water electrolyzers for on-site production of chemicals such as carbon monoxide, formic acid, ethylene, and ethanol requires understanding the unit economics and early-stage markets for small-scale carbon dioxide conversion systems. With an understanding of what an integrated electrochemical carbon dioxide conversion system looks like, specifically capital requirements and balance of plant, the metrics required for carbon dioxide conversion catalysts can be determined. Starting with outlining the real-world requirements for carbon dioxide conversion systems, we will discuss the important design considerations, metrics, and cost factors to define targets for heterogeneous and homogeneous catalysts for carbon dioxide conversion.

Electrochemical synthesis methods can provide sustainable routes to our most coveted chemicals. In this contribution, I will discuss our recent progress in the heterogeneous electrocatalysis of H₂O₂ and NH₃ production. Both chemicals are currently synthesised in enormous quantities, in large centralised reactors, far from the point-of-consumption. The logistical and safety challenges intrinsic to the transportation of these chemicals set them beyond the reach of many end users. In contrast, their electrochemical synthesis is far more attractive.

Efficient electrochemical H₂O₂ production requires a catalyst that can steer the reaction away from the formation of the most thermodynamically stable product, H₂O. To this end, porphyrin like structures offer advantages over pure metal structures by lieu of their isolated reactive sites. We emulated the reactivity of porphyrins by producing mercury-based alloys, which demonstrated unprecedented selectivity, activity and stability.^1,2 I will provide an overview of the factors controlling H₂O₂ production, focussing on the role of (i) catalyst structure at the atomic scale (ii) electrode mesotructure and (iii) electrolyte pH. I will also compare results from model rotating ring disk electrode to tests in real electrolytic devices.³

Current ammonia production via the Haber Bosch process requires high pressures, high temperatures and consumes >1% of our current fossil fuel production. To the contrary, nitrogenase in nature catalyses N₂ reduction at room temperature and atmospheric pressures. At present, sold electrodes are rather inactive and unselective. The translation of the activity and selectivity of nitrogenase to a solid inorganic surface would enable the efficient on-site on demand synthesis of ammonia, powered by renewable electricity.

While we have made huge progress in the electrocatalysis of H₂O₂ production, there remains much to be learnt about N₂ electroreduction. We recently used quantitative isotopic labelling experiments to provide unequivocal proof that Li-based electrodes are able to reduce N₂ to NH₃ in non-aqueous electrolytes.⁴Our work provides a solid foundation for further progress, and enables scientists to distinguish false positives from true breakthroughs. Nonetheless, the efficiency we reported is rather low, i.e. there is still ample room for improvement. Drawing insight from the nitrogenase enzyme, I will discuss different avenues towards more efficient N₂ reduction.


1 Siahrostami, S., Verdaguer-Casadevall, A., Karamad, M. R., Deiana, D., Malacrida, P., Wickman, B., Escudero-Escribano, M., Paoli, E. A., Frydendal, R., Hansen, T. W., Chorkendorff, I., Stephens, I. E. L. & Rossmeisl, J. Nature Materials12, 1137, (2013).
2 Verdaguer-Casadevall, A., Deiana, D., Karamad, M., Siahrostami, S., Malacrida, P., Hansen, T. W., Rossmeisl, J., Chorkendorff, I. & Stephens, I. E. L. Nano Letters 14, 1603, (2014).
3 Yang, S., Verdaguer-Casadevall, A., Arnarson, L., Silvioli, L., Čolić, V., Frydendal, R., Rossmeisl, J., Chorkendorff, I. & Stephens, I. E. L. ACS Catalysis, 4064, (2018).
4 Andersen, S. Z., Čolić, V., Yang, S., Schwalbe, J. A., Nielander, A. C., McEnaney, J. M., Enemark-Rasmussen, K., Baker, J. G., Singh, A. R., Rohr, B. A., Statt, M. J., Blair, S. J., Mezzavilla, S., Kibsgaard, J., Vesborg, P. C. K., Cargnello, M., Bent, S. F., Jaramillo, T. F., Stephens, I. E. L., Nørskov, J. K. & Chorkendorff, I. Nature (2019) DOI: 10.1038/s41586-019-1260-x.

The Haber Bosch process has provided a large-scale method of synthesizing ammonia for a century. The large carbon footprint and process conditions which limit modularity have led to new efforts to uncover routes through which ammonia can be produced at ambient conditions using potential. Lithium-mediated routes have emerged as a set of methods which can lead to high selectivities for ammonia synthesis. We are particularly interested in molecular-level understanding of the kinetics of the cathodic reaction through which the nitrogen triple bond is broken and subsequent nitrogen-hydrogen bonds are formed. We have uncovered the reaction network through which ammonia is produced in competition with hydrogen. Our work has shown the importance of transport in controlling this reaction, providing critical insights that have led to high rates of ammonia synthesis and a path towards future increases in both rates and selectivities.

Among the many strategies proposed to convert generated CO2 into value-added chemicals and energy carriers, its electrochemical conversion using bioelectrochemical systems (BES)[1] is extremely promising. The advantage of this new technology lies in the possibility of exploiting the enzyme properties catalysing the reduction reaction with a high selectivity and specificity toward products with a low overpotential applied, overcoming the limited selectivity problem that affects the usually-employed metallic electrocatalysts for CO2 reduction.
In this work, we present a novel BES, where the NAD-dependent enzyme Formate Dehydrogenase (FDH) from Thiobacillus sp. KNK65MA [2] is expressed by heterologous production in E. coli BL21 (DE3) and deposited on a nanostructured mesoporous support of Titanium Nitride (TiN) realized by Pulsed Laser Deposition. Thanks to PLD, we can realize a nanostructured support with high surface area and tree-like morphology. By optimizing the synthesis parameters, it is possible to obtain a nanostructured, hierarchical support that maximizes the available surface area for catalyst absorption, thanks to its BET area of 169 m² g^-1, and enhances the bio-interface between enzyme and inorganic electrode by exposing a high density of binding sites for the FDH. We quantify the amount of immobilized enzymatic catalyst on the nanostructure through standard enzymatic assays, demonstrating that the nanostructuration of the TiN support increases the surface area available for enzyme immobilization, achieving a maximum enzyme adsorption of 59 µg cm^-2 for the BES. The TiN support is firstly characterized electrochemically, to verify its mechanical and chemical stability in the electrolyte solution. Subsequently, the enzymatic electrosynthesis of formic acid from CO2 is investigated at different applied potentials, showing a productivity for formic acid that ranges from 1.5 to 3.7 mmol mg^-1_enzyme h^-1 according to the applied overpotentials. Finally, post-catalysis characterization of the hybrid system shows that the amorphous nanostructure does not undergo any important modifications in its morphology and composition, thus demonstrating the mechanical stability of the TiN scaffold.
This performance, which is unparalleled in previous studies involving enzymes of the FDH family immobilized on inorganic supports, stems from a combination of are the best for inorganic support-immobilized enzymes of the FDH family, achieved thanks to the high reducing activity of TsFDH and the high contact area offered by the nanostructured TiN support, and demonstrates the feasibility and as well as the potential for a biotechnological device in terms offeaturing product specificity and stability.

References

[1] Y. Amao, Formate dehydrogenase for CO2 utilization and its application. Journal of CO₂ Utilization (2018)
[2] H. Choe et al, Efficient CO2-Reducing Activity of NAD-Dependent Formate Dehydrogenase from Thiobacillus sp. KNK65MA for Formate Production from CO Gas. PLOS ONE (2014)

The abundant yet widely distributed natural gas resources require efficient conversion of methane into liquid chemicals at ambient conditions with minimal infrastructure support. Here we report an electrochemical catalytic system that is capable to functionalize natural gas at room temperature and ambient pressure. This electricity-driven system is capable to liquifies natural gas under ambient conditions with 90% selectivity and turnover numbers beyond 100,000.

CO₂ electrolysis into value-added fuels represents a renewable-electricity-powered strategy to reduce today’s dependence on conventional fossil fuels, mitigating net CO₂ emissions. However, the sluggish electroreduction of CO₂ (CO₂RR) and water oxidation reaction (WOR) limit the performance for CO₂ electrolysis. In this talk, I will describe my research efforts to develop, understand, and eventually design new catalysts for CO₂RR and WOR using computational materials science, in situ/ex situ synchrotron X-ray spectroscopies, and advances in materials chemistry. First, I will report a concept to modulate electronic structures of active sites for CO₂RR by incorporating the third element in an intimately integrated fashion. Specifically, guided by theoretical predications, we synthesized the designed CuSn₃ alloy and sulfur-modulated-Sn catalysts, and found they selectively produced formate from CO₂RR at low overpotentials. Next, I will describe the development of a room temperature sol-gel technology to produce atomically dispersed metal oxy-hydroxides with high WOR activity. Herein, we showed the gelled FeCoW and NiCoFeP oxy-hydroxide materials has potential industrial applications. In situ/Ex situ X-ray absorption and computational studies, taken together, reveal that the enhanced energetics for CO₂RR and WOR resulted from the ready promotion of favorable local coordination environments and electronic structures.