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FF5: Poster Session: Ex-situ and In-situ Studies of Nanomaterials
Session Chairs
Tuesday PM, April 26, 2011
Exhibition Hall (Moscone West)
1:00 AM - FF5: Poster
FF5.17 Transferred To FF16.10
Show AbstractFF1: In-situ Studies of Catalysts using Electron Spectroscopy
Session Chairs
Tuesday PM, April 26, 2011
Room 3022 (Moscone West)
9:00 AM - FF1.1
Advances in High-resolution Transmission Electron Microscopy for Catalysis.
Stig Helveg 1 , Alfons Molenbroek 1
1 , Haldor Topsoe A/S, Kgs. Lyngby Denmark
Show AbstractImprovements in the understanding of structure-reactivity relationships in heterogeneous catalysis strongly correlate with new developments in characterization techniques [1]. Especially, advancements of high-resolution transmission electron microscopy (HRTEM) techniques have established powerful and versatile tools for the real-space examination of nanostructured catalysts at the atomic-level. Here, the application of two recent advancements of HRTEM for catalysis will be discussed. The first advancement is focussed on HRTEM imaging of the shape and structure of industrial-style prepared graphite-supported MoS2 nanocatalysts for hydrotreating reactions [2]. Previously, it was difficult to obtain atomic-resolved TEM images of the MoS2 nanocatalysts due to insufficient image contrast or resolution. However, the introduction of aberration-corrected HRTEM has now made it possible to obtain atomically resolved images with single-atom sensitivity. The second advancement is the introduction of MEMS (micro-electro-mechanical system) nanoreactors for in situ HRTEM of nanocatalysts during the exposure to reactive gases at ambient pressures and high temperatures. The pressure of 1 atm. exceeds that of existing in situ HRTEM systems by a factor of one hundred and is at a level of relevance for catalyst testing. The nanoreactor integrates a micro-meter sized gas-flow channel with a microheater and with an array of electron-transparent windows of silicon nitride. The nanoreactor performance is demonstrated on a methanol-synthesis catalyst by the time-resolved observation of the formation of Cu nanoparticles on a ZnO support with atomic-scale resolution [3,4].[1]A. Stierle and A.M. Molenbroek, Guest editors, MRS Bulletin, 32-12, 1001 (2007).[2] C. Kisielowski, Q.M. Ramasse, L.P. Hansen, M. Brorson, A.Carlsson, A.M. Molenbroek, H. Topsøe and S. Helveg, Angewandte Chemie, 15 (2010).[3] J.F. Creemer, S. Helveg, G.H. Hoveling, S. Ullmann, A.M. Molenbroek, P.M. Sarro, H.W. Zandbergen, Ultramicroscopy 108, 993-998 (2008).[4] J. F. Creemer, S. Helveg, P.J. Kooyman, A.M. Molenbroek, H.W. Zandbergen, and P.M. Sarro, Journal of Microelectromechanical Systems, IEEE Early Access, Issue 99, 1-11(2010).
9:15 AM - FF1.2
In-situ Environmental TEM Study of the Shape Change of Pt Nanoparticles Supported on CeO2 in Gases.
Hideto Yoshida 1 , Yasufumi Kuwauchi 2 , Hideo Kohno 2 , Satoshi Shimada 3 , Masatake Haruta 3 , Seiji Takeda 1
1 The Institute of Scientific and Industrial Research, Osaka University, Osaka Japan, 2 Graduate School of Science, Osaka University, Osaka Japan, 3 Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, Tokyo Japan
Show AbstractThe catalytic activity of noble metals is strongly influenced by their surface atomic structure. In general, atomic steps and kinks are active sites for various chemical reactions, and therefore high-index planes of single crystal surface show higher catalytic activity than low-index planes. Since noble metal nanoparticles have a lot of atomic steps and kinks, they show higher catalytic activity. Thus, it is important to clarify the atomic structure of noble metal nanoparticles catalysts under actual catalytic reaction conditions for future applications of these materials. Environmental transmission electron microscope (ETEM) is one of the powerful tools for the in-situ study of solid gas reactions at atomic scale [1,2]. In this study, Pt nanoparticles supported on CeO2 (Pt/CeO2) catalysts were observed in N2 and CO/Air (CO 1 vol%, O2 21 vol%, N2 78 vol%) gas by a high-resolution ETEM (FEI Tecnai F20 equipped with an environmental-cell) operated at 200 kV. We have found that the shape change of Pt nanoparticles supported on CeO2 depending on the gas environment. Pt/CeO2 catalysts were prepared by solid grinding method which can produce halogen-free supported metal catalysts [3]. The Pt content in the catalyst was 10 wt%. Pt nanoparticles were supported on various plans of CeO2 and the mean diameter of Pt nanoparticles was estimated to be 3.6 nm with a standard deviation of 1.1 nm by TEM. In a vacuum, the shape of Pt nanoparticles is polyhedron that is enclosed by low index crystal planes such as (111) and (100). When N2 gas of 100 Pa was introduced in the ETEM, no significant change in their shape and structure was detected. In general, N2 molecules are not adsorbed on the surface of Pt. This means that the impact of N2 molecules does not cause the detectable shape change of the nanoparticles. On the other hand, the shape of Pt nanoparticles becomes round in CO/Air gas of 100 Pa. Based on ETEM observations, including recent results using a Cs-corrected Titan ETEM, we will discuss the effect of reactant gases on Pt nanoparticles [4].[1] H. Yoshida et al., Nano Lett. 8 (2008) 2082.[2] H. Yoshida et al., Nano Lett. 9 (2009) 3810.[3] T. Ishida et al., Chem. Eur. J., 14, (2008) 8456.[4] This research was supported by Grant-in-Aid for Specially Promoted Research Grant No. 19001005 from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
9:30 AM - **FF1.3
In situ Environmental Transmission Electron Microscopy in Thermal and Light Induced Catalytic Reactions.
Peter Crozier 1
1 School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona, United States
Show AbstractUnderstanding the relationship between catalyst nanostructure and activity is a major objective for fundamental research on heterogeneous catalysts. In situ environmental transmission electron microscopy (ETEM) is a powerful technique for revealing the structure of catalytic nanomaterials under near reactor conditions. ETEM allows materials to be observed in the presence of reactive gases at pressures of 0.01 – 1 atmosphere and temperatures up to 800oC. Electron imaging techniques allow the internal and surface structures of nanoparticles to be observed at atomic resolution. Electron energy-loss spectroscopy(EELS) and energy dispersive x-ray spectroscopy allow the elemental and electronic structures to be determined at subnanometer resolution and atomic resolution in favorable cases. This combination of imaging and nanospectroscopy allows the composition and structure of the catalysts can be explored at elevated temperatures in the presence of reactive gases. Differences (such as pressure, mass transport, catalysts loading, conversion etc…) may exist between the ETEM reactor and the reactors typically employed to determine catalytic properties. These differences must be taken into account in the design of ETEM experiments to ensure that the nanostructures observed in the microscope are relevant to those that may form in a typical reactor. These considerations are discussed and illustrated on supported Ni catalysts for partial oxidation of methane where we correlate ETEM observations with reactor data to give a detailed description of the catalyst evolution during activation[1]. We are continuing to develop the ETEM technique so that a wider range of problems in heterogeneous catalysis can be explored. Recently, using in situ EELS, we have development methods for performing quantitative analysis of the gas volume in direct contact with the TEM sample[2]. This opens up the possibility of performing operando ETEM because we can simultaneously measure the catalytic activity and selectivity while monitoring the catalyst structure. We are also interested in applying ETEM methods to photocatalytic materials of relevance to solar fuels. To determine structure-activity relations in these catalysts it is necessary to have in situ capabilities where the sample is exposed to gas and light. We are currently installing a variable wavelength light source in our ETEM which will allow us to expose materials to photons over the wavelength range 250 - 800 nm. We will discuss progress on this project and show preliminary results. 1. S. Chenna, R. Banerjee, P.A. Crozier, ChemCatChem, (2010). DOI: 10.1002/cctc.201000238 2. Peter A. Crozier and Santhosh Chenna, Ultramicroscopy (2011) DOI: 10.1016/j.ultramic.2010.11.005
10:00 AM - FF1.4
In-situ Microscopy of PEM Fuel Cell Catalyst and Catalyst-support Degradation.
Karren More 1 , Ray Unocic 1 , Kelly Perry 1 , K. Shawn Reeves 1
1 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractHigh-resolution imaging and spectroscopy of the structural and chemical changes for Pt-based catalyst nanoparticles supported on carbon blacks (such as Vulcan XC-72 or Ketjen Black) using in-situ transmission and scanning transmission electron microscopy (S/TEM) are being used to directly understand the mechanism(s) of catalyst particle growth/coalescence and carbon corrosion under relevant environmental conditions employed to simulate fuel cell operation. Specialized in-situ microscopy holders that incorporate fluid and gas flow-cells are being developed to image catalyst particles and their support structures during electrochemical aging at low temperatures within thin liquid electrolyte or gas layers. Such in-situ exposures of fuel cell nano-scale structures in the S/TEM permits the “capture” of microstructural changes at high spatial and temporal resolution to elucidate the predominant corrosion mechanisms contributing to materials degradation during the operation of PEM fuel cells. Results will be presented that focus primarily on (1) the nanoparticle and carbon support microstructures that contribute to the durability of fuel cell cathode materials and (2) observed catalyst-ionomer-support interactions during in-situ exposure in the S/TEM. The in-situ microscopy observations are compared with microscopy results from the post-mortem (ex-situ) examination of similar fuel cell materials aged via different accelerated stress tests (ASTs). The laboratory-based ASTs are designed to degrade specific components of the fuel cell cathode. The combination of in-situ and ex-situ microscopy provides unprecedented insight regarding the electrochemical processes controlling materials durability in fuel cell electrodes._______________________Research sponsored by (1) the Fuel Cell Technologies Program, Office of Energy Efficiency and Renewable Energy, the U.S. Department of Energy and (2) ORNL’s Shared Research Equipment (SHaRE) User Facility, Office of Basic Energy Sciences, the U.S. Department of Energy.
10:15 AM - FF1.5
Dynamic Phase Transformation Observations in Ni/SiO2 Catalyst for Partial Oxidation of Methane Using In-situ Environmental TEM.
Santhosh Chenna 1 , Peter Crozier 1
1 School of Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona, United States
Show AbstractUnderstanding the fundamental processes that take place in a catalytic material during catalytic reactions is necessary to develop structure-activity relationships. During a reaction, the catalyst may undergo several structural and chemical changes due to interactions between the material and the gases in the reactor. Thus to develop structure-activity relationships it is necessary to study the catalyst under working conditions. In-situ environmental transmission electron microscopy (ETEM) can allow us to probe the nanoscale structural and chemical changes taking place in the catalyst under reacting gas conditions.[1]We are applying in-situ ETEM studies to follow the fundamental processes taking place in mono and bi-metallic supported metal catalysts during energy-related catalytic reactions. Catalytic activity measurements in parallel with in-situ ETEM studies were performed on Ni/SiO2 catalysts for the partial oxidation of methane to syngas (CO+H2) reaction. From the catalytic measurements we observe that, with increasing temperature, the gas composition changes along the catalyst bed from oxidizing (2CH4 + O2 mixture) to reducing conditions (CH4 rich). ETEM experiments under different gas environments were performed to simulate the reaction conditions that exist in the rector.From the in-situ experiments we observed that the phase transformation of Ni metal to void structured NiO and NiO back to Ni metal takes place as the gas environment changes from 2CH4 + O2 to pure CH4 respectively. The mechanism for phase transformation during both oxidation and reduction involves preferential migration of Ni cations along the NiO grain boundaries and extended defects. This transformation pathway suppresses the formation Ni metal crystallites on the catalyst surface during the initial reduction suppressing the syngas formation. Syngas formation will only take place during the final stage of NiO reduction when Ni metal nanoparticles break through the NiO shell. A detailed discussion of phase transformation mechanisms and its affect on the activation of Ni catalyst for partial oxidation of methane will be presented. Results on Ni-Ru bimetallic catalysts for partial oxidation of methane will also be presented.References:1.S. Chenna, R. Banerjee and P.A. Crozier, ChemCatChem, n/a. doi: 10.1002/cctc.201000238.
10:30 AM - FF1.6
Environmental Transmission Electron Microscopy of Electron-beam-induced Changes of Au/TiO2 Catalysts in Reactant Gases.
Yasufumi Kuwauchi 1 , Hideto Yoshida 2 , Tetsuya Uchiyama 1 , Tomoki Akita 3 , Hideo Kohno 1 , Seiji Takeda 2
1 Physics, Graduate School of Science, Osaka University, Osaka Japan, 2 Nanoscience and Nanotechnology Center, Institute for Industrial and Scientific Research, Osaka University, Osaka Japan, 3 , National Institute of Advanced Industrial Science and Technology, Osaka Japan
Show AbstractGold nanoparticles exhibit high catalytic activity when they are supported on selected metal oxides. Au/TiO2 catalysts show great activity for CO oxidation even at low temperature. Although it is well known that the catalytic activity of gold nanoparticles depends on the average size of them and support selection, the origin of the catalysis is still under debate. To reveal the processes of reactions on nanoparticle catalysts, atomic-resolution in situ observations by environmental transmission electron microscopy (ETEM) make important contributions. However, it is indispensable in ETEM observations to distinguish the effects of catalysis and those of electron irradiation. In this study, we performed systematic observations of Au/TiO2 catalysts in reactant gases under electron irradiation with several different intensities.Au/TiO2 catalysts were prepared by the deposition precipitation method and show catalytic activity for CO oxidation below room temperature. As a specimen for ETEM observation, Au/TiO2 catalysts were dispersed over a carbon microgrid and transferred into ETEM with a specimen holder. An FEI Tecnai F20 transmission electron microscope equipped with an environmental cell was used for our observations. A differentially pumped system is adopted for the environmental cell, enabling us to perform high resolution observations in gases up to 2500 Pa (for N2). Accelerating voltage was fixed at 200 kV.We have observed formation of pillar-like structures and capsules of TiO2 around Au nanoparticles during ETEM observations of Au/TiO2 catalysts. Pillars and capsules often show the crystalline structures, which are identified as rutile-TiO2 epitaxial to the support rutile-TiO2. During the formation of pillars and capsules, Au particles were rotating and reconstructing the shape. Our systematic ETEM observations of Au/TiO2 catalysts in high vacuum and pure O2 gas with several different intensities of electron irradiation revealed that the shape changes mentioned above are mainly caused by effects of electron irradiation and have no direct relationship with catalytic reactions by Au/TiO2. Detailed processes of pillar formation and encapsulation with high resolution ETEM images will be demonstrated in the meeting.
10:45 AM - FF1: TEM
BREAK
FF3: In-situ STM Studies of Catalysts
Session Chairs
Tuesday PM, April 26, 2011
Room 3022 (Moscone West)
2:30 PM - **FF3.1
Catalytic Model Systems Studied by High-resolution, Fast-scanning STM.
Flemming Besenbacher 1
1 Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Aarhus Denmark
Show AbstractFor decades single-crystal surfaces have been studied under ultrahigh vacuum (UHV) conditions as model systems for elementary surface processes underlying phenomena such as heterogeneous catalysis, epitaxial growth, corrosion, etc. This “surface science approach” has contributed sub-stantially to our understanding of the processes involved in especially catalysis, and has even led to the design of improved catalysts in some cases. Recently, much attention has been paid to the so-called gaps between surface science and industrial catalysis. One of these gaps is the pressure gap, which arises because of the 13 orders of magnitude pressure difference between UHV base pressures and atmospheric pressure. The other gap is the structural gap relating to the difference in reactivity on single-crystal surfaces as opposed to small nanoclusters. In this talk I will show how one can use the unique capabilities of Scanning Tunneling Microsco-py (STM) to reveal fundamental processes in relation to catalysis. STM has proven to be a fasci-nating and powerful technique for revealing the atomic scale realm of matter, and the unique as-pect of our Aarhus STM has allowed us to record time-resolved, high-resolution STM images, visualized in the form of STM movies (see www.phys.au.dk/spm) [1-8]. I will show how we can obtain unique new insight into diffusion and transport phenomena associated with surface processes of nanostructures which allows us to identify active sites and new nanostructures with new novel catalytic properties. Finally, I will show how our fundamental atomic-scale studies may lead to the design of new improved catalysts [9]. References1. F. Besenbacher, Reports on Progress in Physics 59, 1737 (1996)2. T. Linderoth et al., Phys. Rev. Lett. 78, 4978 (1997) 3. S. Horch et al., Nature 398, 1344 (1999)4. S. Wendt et al., Phys. Rev. Lett. 96, 066107 (2006)5. S. Wendt et al. Science 320, 1755 (2008) 6. D. Matthey et al., Science 315, 1692 (2007) 7. J. V. Lauritsen et al., Nature Nanotechnology, 2, 53 (2007)8. F. Besenbacher et al., Surf. Sci.603 (2009) 1325-13279. F. Besenbacher et al., Science 279, 1913 (1998)
3:00 PM - FF3.2
Thin-film Mixed Oxide Model Catalysts for Methane Reforming.
Kathrin Mueller 1 , Dario Stacchiola 2 , David Starr 1
1 Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York, United States, 2 Chemistry Department, Brookhaven National Laboratory, Upton, New York, United States
Show AbstractDry-reforming of methane (CH4) is a potential route to produce syn-gas – a mixture of H2 and CO used for Fischer-Tropsch and methanol synthesis – from two greenhouse gases, CO2 and CH4. Ni-based catalysts are commonly used for methane reforming reactions. A common problem with Ni catalysts, however, is carbon deposition that leads to deactivation. Investigations of reduced mixed oxide catalysts (NiO/MgO) have demonstrated long term stability as well as high yield for dry reforming of methane [1,2]. The long term stability for the reduced mixed oxide catalyst, compared to Ni catalysts synthesized by more traditional methods, is explained by smaller Ni particles which are less susceptible to coking. Additionally, reduction and re-oxidation of Ni in the mixed oxide during the reaction may lead to re-dispersion of the catalyst and may improve its long-term stability. In order to explore the favorable properties of these mixed-oxide catalysts we have taken a model catalyst approach: MgO plus NiO or CoO thin films were deposited onto Mo(100) and the intermixing of the oxides followed by their reduction was investigated with normal and grazing emission x-ray photoelectron spectroscopy (XPS) as well as with scanning tunneling microscopy (STM). By heating the thin films the two oxides intermix forming a solid solution. Heating in hydrogen leads to partial reduction of the mixed-oxides to form metallic Ni or Co. Additionally, we have used ambient-pressure X-ray photoelectron spectroscopy to investigate the reduction and re-oxidation of the NiO, providing insight into the reduction and re-oxidation mechanism for the mixed-oxide catalysts. [1] Y.H. Hu, E. Ruckenstein, Catal. Lett. 36, 145 (1996).[2] E. Ruckenstein, Y.H. Hu, Appl. Catal. A : General 133, 149 (1995).
3:15 PM - FF3.3
Size Dependent Vacancy Formation and Reactivity in MoS2 Nanoclusters.
Eeva Niemi 1 2 , Anders Tuxen 3 , Jeppe Lauritsen 3 , Adam Foster 1 2
1 Department of Physics, Tampere University of Technology, Tampere Finland, 2 Department of Applied Physics, Aalto University, Helsinki Finland, 3 Interdisciplinary Nanoscience Center, Department of Physics and Astronomy, Aarhus University, Aarhus Denmark
Show Abstract Detailed control of nanoparticles' size and form offers new possibilities for the design of materials with novel functional properties, such as heterogenous catalysts with highly controlled reactivity and selectivity. STM studies following the structural progression of triangular MoS2 nanocrystals as a function of their size[1], have shown how the equilibrium structures are altered when the cluster size is reduced, and that certain ’magic’ clusters are preferred. In recent STM experiments on Au(111), the catalytic properties of single layer MoS2 nanoclusters are found to vary remarkably according to cluster size[2]. During hydrogenation, defects are created on cluster edges, and, depending on cluster form, different edge defects sites are preferred with different activity on catalysis. This control of vacancy forming can be used in e.g. cleaning sulpur from the fossil fuel residues. First principles calculations show the effects of structural modifications in the electronic structure of the clusters adsorbed on the surface, as well as the stability and energetics of the vacancies. We investigate how the formation energy of vacancies depend on size and edge termination of the clusters. We further explore how the vacancies influence the adsorption of DBT molecules, which are a common impurity in fossil fuels.[1] J. V. Lauritsen et al., Nat. Nanotechnol. (2007) vol. 2 (1) pp. 53-58.[2] A. Tuxen, J. Kibsgaard, H. Goebel, E. Laegsgaard, H. Topsoe, J.V. Lauritsen and F. besenbacher, ACS Nano (2010) 4 pp. 4677-4682
3:30 PM - FF3.4
Fischer-Tropsch Synthesis Followed at High Pressures with STM and SXRD.
Violeta Navarro 1 , Sander Roobol 1 , Richard van Rijn 1 2 , Qian Liu 1 , Olivier Balmes 2 , Didier Wermeille 2 , Andrea Resta 2 , Roberto Felici 2 , Joost Frenken 1
1 Kamerlingh Onnes Lab, University of Leiden, Leiden Netherlands, 2 Beam line ID03, ESRF, Grenoble France
Show AbstractFischer-Tropsch synthesis (FTS) is the catalytic reaction that leads to the production of hydrocarbons from a mixture of H2 and CO under conditions of high temperature and pressure. In the recent years it has stimulated renewed interest as an alternative source for the creation of synthetic fuel. Most of the scientific studies about this reaction have been performed from the point of view of the industry. However, there has been much less basic research towards understanding the atomic and molecular aspects of the reaction mechanisms under industrial conditions [1].The techniques used in surface science can shed light onto the fundamental mechanisms that govern the catalytic reactions. However most of these studies are performed under typical surface science conditions which differ significantly from the commercial processes. In surface science the working pressures are several orders of magnitude lower than the pressures at which the industrial FTS takes place. Also the structure, composition and morphology of the catalyst can be very different. The pressure and the material gaps between science and industry can prevent the research on the FTS to understand the real catalytic processes that take place.Cobalt nanoparticles are one of the most popular catalysts used in the FTS industry. However in our studies, we use a Co(0001) single crystal as a model catalyst. In order to study the FT reaction from the atomic or molecular point of view, we need a system with a controlled amount of defects (steps, kinks, vacancy islands etc.) as these might play a critical role in the catalytic reaction. The single crystal is a first approach to understand the basic mechanisms that take place on the catalytic surface. We perform studies on the FTS with two innovative surface science techniques that approach the conditions at which the reaction takes place in industry. STM [2] and surface X-ray diffraction (SXRD) [3] are used to study in situ the catalytic surface under conditions of high pressure and high temperature. Both techniques are complementary since STM gives local information about the changes in the surface, and the SXRD gives long range information of the periodic structures present on the catalyst. In both cases the instruments are configured in the form of a small flow reactor which can mimic industrial conditions. The reactors are embedded inside a UHV chamber so surface science techniques can be used to prepare and characterize the samples. These techniques allow us to study the structural changes on the surface of the catalyst while the reaction takes place. First results will be presented.References:1- J. Wilson et al., J. Phys. Chem. 99, 7860-7866 (1995).2- C.T. Herbschleb et al., to be submitted. B.L.M. Hendriksen, et al., Topics in Catalysis, 36, 1–4 (2005).3- R. van Rijn et al., Rev. Sci. Instrum. 81, 014101 (2010).
FF4: Synthesis and Catalysis of Metal and Alloy Nanoparticle Catalysts
Session Chairs
Tuesday PM, April 26, 2011
Room 3022 (Moscone West)
4:15 PM - **FF4.1
In-situ Infrared Absorption Spectroscopy Characterization of the Adsorption Properties of Catalysts Made Using Novel Synthetic Techniques.
Ilkeun Lee 1 , Manuel Albiter 1 , Junghyun Hong 1 , Jobong Joo 1 , Yadong Yin 1 , Francisco Zaera 1
1 , University of California, Riverside, California, United States
Show AbstractSelf-assembly, in particular colloidal and sol-gel procedures, has been used to prepare heterogeneous catalysts with novel architectures. The adsorption properties of those catalysts have then been tested using infrared absorption spectroscopy. Several examples of this approach will be illustrated in this talk. In one case, platinum nanoparticles were first dispersed on the surface of a support material, a silica bead, and then covered by a freshly grown layer of mesoporous silica in order to physically fix their positions and avoid sintering. Access to the metal particles is regained via a novel etching process, as indicated by the return of the ability of the platinum surface to uptake carbon monoxide and to promote the catalytic conversion of olefins. A second example involves the use of dendrimer encapsulated nanoparticles (DENs) for the making of catalysts with small but well-defined metal particles. The resulting material shows a very narrow distribution of particles sizes, in our example around a diameter of 1.5 nm. The level of access of adsorbates to the surface of the metal was tested in both gas- and liquid-phases, and the activation of supported DENs towards hydrocarbon conversion catalysis evaluated as a function of the pretreatment used. In a third example, the anchoring of molecular functionality to solid supports was explored. In this case, cinchona alkaloids were covalently tethered to silica surfaces to promote enantioselective reactions. Finally, the characterization of metal@titania nanostructures, developed for photocatalysis, will be briefly discussed.
4:45 PM - FF4.2
Design of Catalysts at the Sub-nanometer and nm Scale: The Role of In Situ and Ex Situ Characterization Techniques in the Tuning of Catalytic Properties via Size, Composition and Support.
Stefan Vajda 1 2 , Marcel Di Vece 2 , Sungsik Lee 1 , Byeongdu Lee 1 , Soenke Seifert 1 , Randall Winans 1 , Glen Ferguson 1 , Larry Curtiss 1 , Jeffrey Greeley 1 , Zhiwei Wang 3 , Richard Palmer 3 , Qiang Qian 4 , Mathew Neurock 4 , Simone Goergen 5 , Rui Si 5 , Maria Flytzani-Stephanopoulos 5 , Xiaoming Wang 2 , Gary Haller 2 , Lisa Pfefferle 2
1 , Argonne National Laboratory, Argonne, Illinois, United States, 2 , Yale University, New Haven, Connecticut, United States, 3 , University of Birmingham, Birmingham United Kingdom, 4 , University of Virginia, Martinsville, Virginia, United States, 5 , Tufts University, Medford, Massachusetts, United States
Show AbstractOn the way to the design of new classes of catalysts, a fundamental understanding of the size/composition/shape/structure and function correlation, along with the effect of support is necessary. In order to identify nanoscale materials with tailored properties, a highly interdisciplinary joint experimental and theoretical approach is desirable. First, highly uniform nanoparticles dispersed on technologically relevant supports are prerequisites for such studies.(1) Second, the synthesized nanomaterials are to be imaged with highest precision available and their nature, size and shape be monitored under working conditions of pressure and temperature. Third, state of the art density functional calculations are necessary to gain mechanistic understanding of the function of these new materials. Finally, a close-coupling between experiment and theory will be needed to design custom materials in an iterative feed-back loop fashion. The experimental studies presented in this paper are based on 1) use of chemically uniform supports 2) functionalization of the support to fine-tune the properties of the supported nanoparticles, 3) synthesis of well defined nanocatalysts by size-selected sub-nm cluster deposition or refined wet-chemical synthesis of small nanoparticles, 4) imaging the nanoclusters by electron microscopies (i.e. ex situ TEM and HAADF-STEM) and 5) performing in situ X-ray scattering and absorption characterization of the nanocatalysts under working conditions, combined with mass spectroscopy analysis of reaction products. (2-4) Density functional theory calculations are used to understand the activity of clusters and the underlying reaction mechanisms. The potential of this approach is illustrated on the example of the dehydrogenation of cyclohexane on sub-nm to nm size Pt and CoOx clusters. The results show that efficient dehydrogenation of cyclohexane can be performed on subnanometer and nanometer size catalysts, including at low temperatures. The activity and selectivity of the studied systems can be tuned by varying composition and functionalization of the support material, the size and doping of the nanocatalyst. DFT studies are used to help understand the structures and reaction pathways.(1)A.T. Bell, Science 2003 299, 1688 (2) Lei, Y.; Mehmood, F.; Lee, S.; Greeley, J.; Lee, B.; Seifert, S.; Winans, R.E.; Elam, J.W.; Meyer, R.J.; Redfern, P.C.; Teschner,D.; Schlogl, R.; Pellin, M.J.; Curtiss, L.A.; Vajda, S. Science 2010, 328, 224. (3) Li, Z. Y., Young, N.P., Di Vece, M., Palomba,S., Palmer, R. E., Bleloch, A. L., Curley,B. C., Johnston, R. L., Jiang,J. , Yuan,J., Nature 2007 451, 46 (4) Wyrzgol, S.A.; Schafer, S.; Lee, S.; Lee, B.; Di Vece, M.; Li, X.; Seifert, S.; Winans, R.E.; Stutzmann, M.; Lercher, J.A.; Vajda, S. Phys Chem Chem Phys 2010, 12, 5585.
5:00 PM - FF4.3
Amorphization of Supported Pt Nanoparticles: Dependency on Size, Support and Adsorbates.
Long Li 6 , Linlin Wang 4 , Zhongfan Zhang 6 , Eric Stach 5 , Anatoly Frenkel 2 5 , Duane Johnson 4 , Ralph Nuzzo 3 , Judith Yang 1
6 Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania, United States, 4 , Ames Laboratory, Ames, Iowa, United States, 5 , Brookhaven National Laboratory, Long Island, New York, United States, 2 , Yehsiva University, New York, New York, United States, 3 , University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 1 Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania, United States
Show AbstractHere we reveal an amorphous state of Pt nanoparticles, where its manifestation is necessarily then due to the mesoscopic nature of clusters interacting with their environment, since the amorphous state of an elemental metal is not stable in bulk. We use complementary experimental methods, high-resolution transmission electron microscopy (HRTEM), and in situ X-ray absorption spectroscopy (XAS), combined with first-principles simulations of relevant model systems. The samples were prepared by impregnating the Pt2+ precursor, Pt(NH3)4(OH)2.H2O, on γ-Al2O3, reducing in H2 gas at 573 K to remove the ligands. The sizes of Pt particle were controlled by the loading amount, where 1 wt% produced an average Pt sizes of ~1nm, 3 wt% produced in average Pt sizes of 2.1 nm and heavy loading of 5 wt% produced sizes of ~2.7 nm. We discovered that the onset of the crystalline state is found to be statistical, not abrupt, where the all Pt particles supported on γ-Al2O3 smaller than 1 nm adopted disordered structure and Pt particles larger than 2.5 nm possess crystalline structure. The amorphous-to-crystalline transition is narrower for the Pt/γ-Al2O3 zone than that of Pt/C. Our results also demonstrate that absorbates have a dramatic impact on NP crystallinity, with H stabilizing the truncated fcc structure of ultra-small supported clusters. This data indicate the existence of several metastable phases that depend intimately on the size, adsorbates, and support material. This work establishes a richer, albeit more complex, picture of the local structures in supported metal nanoclusters, ones that well model structural habits present in the real, technologically-relevant materials used as heterogeneous catalysis.
5:15 PM - FF4.4
Structural and Compositional Stability of Nanoporous Pd/Rh Alloy Powders.
Joshua Sugar 1 , Benjamin Jacobs 2 , Patrick Cappillino 2 , Michael Grass 4 , Markus Ong 2 3 , David Robinson 2
1 Materials Physics Department, Sandia National Laboratories, Livermore, California, United States, 2 Energy Nanomaterials Department, Sandia National Laboratories, Livermore, California, United States, 4 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Physics Department, Whitworth University, Spokane, Washington, United States
Show AbstractThe large surface area of nanoporous noble metal particles offers improvements in kinetics and capacity in catalysis, hydrogen storage, and electrical energy storage. We synthesize nanoporous Pd alloys in a scalable fashion using surfactant templates, which results in particles with diameters that range from 0.1–3 µm with closely packed, interconnected 3 nm pores. The alloying components enhance storage properties and thermal stability. In situ heating TEM, TEM-EDS spectrum imaging, ambient-pressure XPS, and porosimetry show that the as-fabricated Pd-Rh alloy particles have a gradient both in the pore size and compositional distribution within each particle. In general, the particles exhibit a core/shell structure with smaller, more ordered pores at the Rh-rich shell and larger, disordered pores at the Pd-rich core. At elevated temperature, the Rh-rich regions of the nanoporous structure are stabilized against changes in pore morphology, but Pd-rich regions can experience significant pore changes that include bubble formation, coalescence, and/or Ostwald ripening, which affects the hydrogen storage capacity. We use XPS and TEM to confirm earlier reports [1] that when heated in a reducing atmosphere, the surface gradually becomes more Pd-rich. This process can be analyzed by a simple transport model. The advanced characterization techniques used here provide a route for us to understand complicated nanostructures in their operating environments, which is crucial to the further development and implementation of these materials in real technological applications.[1] F. Tao et al., Science, 322, p. 932(2008).
5:30 PM - FF4.5
Surface Structure and Catalytic Evolution of AuxPd1-x Nanoparticles under CO/O2 Reaction in Torr Pressure Regime.
Selim Alayoglu 1 2 , Franklin Tao 1 2 3 , Colin Specht 1 2 , Zhongwei Zhu 1 2 , Gabor Somorjai 1 2
1 Department of Chemistry, University of California,Berkeley, Berkeley, California, United States, 2 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Department of Chemistry and Biochemistry , University of Notre Dame, Notre Dame, Indiana, United States
Show AbstractAuxPd1-x (xx=0, 0.25, 0.5, 0.75, 1) nanoparticle (NP) catalysts (8-11 nm) were synthesized by a one-pot reaction strategy using colloidal chemistry. XPS depth profiles using X-rays with different energies and scanning transmission electron microscopy (STEM) analyses show that the as-synthesized AuxPd1-x (x=0.25, 0.5, 0.75) bimetallic NPs have gradient alloy structures with Au-rich cores and Pd-rich shells. The evolution of composition and structure in the surface region corresponding to a mean free path of 0.6-0.8 nm (i.e. 2-3 layers to the bulk from the particle surface) was studied with ambient pressure x-ray photoelectron spectroscopy (AP-XPS) under CO/O2 reaction in the Torr pressure regime. Under the reaction conditions of 80 mTorr CO and 200 mTorr O2 at 200oC, the surface region of Au0.75Pd0.25 NPs with Pd enrichment (~80% by Pd) in the as-synthesized NPs is restructured into Au-rich surface (~ 70% by Au). In contrast, the surface structures of the Au0.25Pd0.75 and Au0.5Pd0.5 NPs remain fundamentally unchanged under similar reaction conditions. AuxPd1-x (xx=0, 0.25, 0.5, 0.75, 1) NP catalysts exhibit a ‘volcano’ behavior under CO/O2 reaction for the reaction mixture of 80 mTorr CO and 200 mTorr O2 at 200oC. The Pd-rich Au0.25Pd0.75 NPs show the highest turnover rates and the Au-rich Au0.75Pd0.25 NPs the lowest turnover rates prior to the surface restructuring performed at 200oC. Upon temperature cycling at 200oC, Au0.75Pd0.25 NPs exhibit almost an enhancement by five times in the catalytic activity at 200 oC, in parallel with its restructuring due to segregation of Au atoms in the cycling. However, there is no such an enhancement in the cases of AuxPd1-x (x=0.25 and 0.5) NPs upon the same temperature cycling, consistent with the absence of restructuring in their surface regions. The difference in catalytic and structural evolution between Au0.75Pd0.25 and Au0.5Pd0.5 and Au0.25Pd0.75 indicates an alloy effect in Au0.75Pd0.25 catalysts.
5:45 PM - FF4.6
Evolution of Oxidation State and Structure of Co in Co and CoPt Nanoparticles under the Reducing and Oxidizing Environment.
Fan Zheng 1 , Selim Alayoglu 1 , Vladimir Pushkarev 1 , Jinghua Guo 1 , Gabor Somorjai 1
1 Materials Science Division, Lawrence Berkeley National Lab, Berkeley, California, United States
Show AbstractTwo gaps exist for extrapolating the results from the single crystals under the UHV in order to understand the real catalysts at the working conditions1. One is the material gap, where model single crystal catalysts could not identify all of the active sites that are important for the catalytic properties since catalysts are usually nanoparticles supported on the oxide surface. The other is the pressure gap, where under the high pressure, the surface structure and composition undergo dramatic transformations, which may not occur under the UHV conditions. Thus studying the catalytic reaction with nanoparticles at the ambient pressure is important for understanding the active phases of the catalyst. Co-Pt is of great interest because of its high oxygen reduction reaction (ORR) activity. The application of the three-way catalysts and the shortage of Rhodium also motivate the investigation of promoted Pt catalysts. It is well known that Co-Pt alloy decreases the CO poisoning on the Pt surface and Co itself is a low temperature catalyst for CO oxidation. In situ X-ray absorption experiments were performed at the undulator beamline 7.0.1 of the Advanced Light Source (Berkeley, CA). A gas flow cell is used to collect the total electron yield (TEY) at the pressure range from millitorr to one atmosphere pressure. When introducing H2 at 38C, the Co is almost completely reduced in CoPt NPs. While for Co in Co NP, one bar H2 at 250C still could not reduce Co to the metal state. This dramatic comparison demonstrates that Pt facilitates Co to be reduced in the form of bimetallic nanoparticles. Co and CoPt NPs are also studied in the oxidizing conditions. The gas is composed of He and O2 with He:O2=20:1 at 125C. The total pressure is changed from millitorr to one atmosphere pressure. Through least square component fitting with references with definite oxidation states, fraction of Co oxidation states at each oxidizing condition can be obtained. Reducibility and oxidation states of Co in Co and CoPt NPs were compared. Pt has a substantial impact on the oxidation state and structure of Co. Pt keeps Co in a more reduced state in the low pressure oxidizing conditions. When pressure is increased above 100 torr, Co and Co2+ fractions in Co and CoPt NPs tend to converge while Co has a much larger amount of Co3+ fraction in Co NPs. Specifically in the Co2+ oxidation state, Co2+ Td fraction drops for both Co and CoPt NPs at higher pressure while it is always higher for Co than CoPt NPs; Co2+ Oh fraction increases with increasing pressure for both Co and CoPt NPs while at high pressure it is higher for CoPt than Co NPs. Catalytic property of Co is closely related to the oxidation state of Co. By in-situ mapping the oxidation states distribution of Co at various conditions important information for strategy on optimizing the reaction condition is provided.
FF5: Poster Session: Ex-situ and In-situ Studies of Nanomaterials
Session Chairs
Tuesday PM, April 26, 2011
Exhibition Hall (Moscone West)
6:00 PM - FF5.1
Synthesis and Optical Properties of Branched Au/Pd Nanocrystals.
Christopher DeSantis 1 , Sara Skrabalak 1
1 Chemistry, Indiana University, Bloomington, Indiana, United States
Show AbstractThere is an increasing interest in the synthesis of bimetallic nanoparticles with well-defined shapes and/or three dimensional structures as controlling these features can enhance properties for use in optical, catalytic, and biological applications. Much of the synthetic focus has been on epitaxial deposition to achieve shape-controlled core@shell particles. However, anisotropic particles are also of interest due to their enhanced surface areas, unique optical signatures, and expression of higher index facets. Using a seed-mediated method, Au/Pd structures with eight arms are readily synthesized. Characterization of these octopods by TEM and EDX elemental mapping reveals that the arms are comprised primarily of Au while the Pd deposits mainly at the tips of these arms. Mechanistic studies indicate that both the Pd content and seed structure are important to the formation of octopods. The surface plasmon resonance of these Au/Pd octopods can be tuned from 600 to 1100 nm and is dependent on the Au-to-Pd ratio used in the synthesis and the specific branching structure achieved.
6:00 PM - FF5.10
Porosity Control and Ex-situ Reaction Study of Palladium-based Nanocatalysts with Nickel Phyllosilicate Shells for Suzuki Coupling Reactions.
Mijong Kim 1 , Hyunjoon Song 1
1 Chemistry, Korea Advanced Institute of Science and Technology, Daejeon Korea (the Republic of)
Show Abstract The reactions on the surface of heterogeneous catalysts typically involve atom-molecule interactions and the active sites of catalyst particles are placed on or surrounded with highly porous supports where the access to the active sites are in the range of small dimension. In order to investigate such catalytic systems, we fabricated four palladium-based catalysts with branched nickel phyllosilicate shells (abbreviated as Niphy) which adopt the different degree of the porosity. We named each of the catalysts as Pd@SiO2, Pd@SiO2-Niphy 1, Pd@SiO2-Niphy 2 and Pd@Niphy, and applied to the Suzuki coupling reaction at room temperature to monitor trends of the catalytic activities by screening each of the catalysts. We controlled the portion of nickel phyllosilicate branches stretched toward the Pd core as well as outside silica shell in the catalyst structures by the amount of nickel precursor (Ni(OAc)2 4H2O) added during the synthetic process, and especially Pd@Niphy was designed to have the hollow layer by etching the residual silica for larger porosity. We expected Pd@Niphy having hollow space and dense Niphy branches would show the best catalytic activity considering its short diffusion pathway only through the Niphy shell and the complete exposure of the active sites to the diffused reactants. As well, when Pd@SiO2 with no Niphy branches takes part in the catalytic reaction, the reactivity would be worse. Interestingly, the best catalytic activity was shown with Pd@Niphy followed by Pd@SiO2-Niphy 2, Pd@SiO2-Niphy 1 and Pd@SiO2, and the ranking trend was the same as expected. The results demonstrate that the large surface area and pore volume of heterogeneous catalysts fundamentally play a key role in the catalytic performance of materials, especially in the small systems such as hollow nanostructures.
6:00 PM - FF5.11
Uniformly-assembled Metal Nanoparticles on Anodic Aluminum Oxide (AAO) Applied in Surface-enhanced Raman Spectroscopy.
Boon Loo 1 , Zhixun Luo 2 , Jiannian Yao 2
1 Chemistry, Towson University, Towson, Maryland, United States, 2 Institute of Chemistry, Chinese Academy of Sciences, Beijing China
Show AbstractWe extend the applications of the anodic aluminum oxide (AAO) templates onto the filtration and assembly for metal nanoparticles. It was found that the colloidal Au nanoparticles can be arranged like a nanonet along the edge of the 100-nm AAO pores. But, they form monolayer assembly on the membrane with smaller AAO pores, such as 20 nm. Similar behavior was also observed for the Ag nanoparticles. Both of the supported Au and Ag nanoparticles on the AAO membranes are closely packed and exhibited localized surface plasmon resonance (LSPR). As a result, taking fullerene molecules C60 & C70 as the probe molecules, and the filtrated Au nanoparticles as the substrate, high quality surface-enhanced Raman scattering (SERS) spectra were obtained. It is demonstrated that the AAO membrance filtrated with noble metal nanoparticles is a highly SERS active substrate.The spherical fullerene molecules C60 & C70 have also been employed as probe molecules for SERS by some groups. However, the measured SERS spectra of fullerene C60 and C70 from the net-assembled Au nanoparticles not only show the sharp differences from their usual SERS, but also differ much from SERS obtained from the layer-coating assembly of the same Au nanoparticles on the AAO templates. The SERS spectra from the monolayer coatings of Au nanoparticles on AAO templates show small differences from those observed from the gold-C60/C70 clusters on various substrates. However, it differs substantially when compared to the SERS from the net-assembled Au nanoparticles. Especially for the C60 system, the two SERS spectra show substantial differences with each other. It is noted that the fullerene C60 and C70 are stable molecules which did not form chemical bonds with the Au nanoparticles. The appearance of additional modes resulted from a symmetry lowering and selection rule relaxation by the adsorption of C60 & C70 on the Au-coated substrates. In addition, the possible weak interaction (Coulombic force) in the ground state of C60 & C70 on the net-like assembly with respect to monolayer coatings of Au nanoparticles may also contribute to the observed difference in the SERS spectra.
6:00 PM - FF5.12
Electric-field Assisted Catalytic Ni and Cu Nanostructures on Silicon Micro Grasses Suitable for CO Combustion.
Aida Ebrahimi 1 , Shamsoddin Mohajerzadeh 1 , Amir Alihosseinzadeh 2 , Abbas-Ali Khodadadi 2 , Yadollah Mortazavi 2
1 ECE, University of Tehran, Tehran Iran (the Islamic Republic of), 2 Chemical Eng., University of Tehran, Tehran Iran (the Islamic Republic of)
Show AbstractWe have fabricated novel electric-field induced CO catalysts based on nickel and copper nanostructures formed on tip of textured silicon micro-grasses. The fabrication process is achieved on (100) Si wafers. The silicon surface is textured to realize micro- and nano-conical grasses using a RF (13.56 MHz) reactive ion etching (RIE) process with SF6 and H2/O2 as inlet gases.Then a 10-20nm thick layer of Ni or Cu is deposited using E-beam evaporation on top of the textured Si substrate with a surface area of about 1cm2. The textured substrate offers a significant increase in the useful surface of the catalyst. The formation of catalytic nano-sized structures is achieved by applying a DC voltage (320-420V) between sample and another plate, placed 1cm away from it and in a chamber with 30sccm flow of H2, pressure of 20mtorr and annealed at 350οC for 90mins. Based on high-resolution SEM images it is seen that most of the deposited layer is pulled up and agglomerated at the sharp tips of grasses, forming sphere-like nanostructures. It is believed that higher electric field densities around sharp tips and annealing in an elevated temperature are the main parameters in forming such structures. By increasing the applied voltage, H2 plasma is ignited and ion bombardment creates nanometric islands. Depending on the voltage and initial thickness of deposited material, nanostructures with diameters of 50-100 nm are formed. SEM images illustrate that in case of plasma formation, in addition to the tip regions, the deposited material on the walls of grasses is converted into nanometric structures, leading to more conversion of CO to CO2 at a given temperature.Preliminary tests on catalytic behavior of these structures were performed in a Gas Chromatograph unit (GC8A) with input feed of %1 CO, %1 O2, %40 H2, and Ar as carrier, in a temperature range of 0-350oC and with a flow of 1cc/min. For the grassy sample with 15nm Cu, nanostructures with an average size of 70nm are formed after proper treatment. For this sample, CO conversion starts at 90oC which reaches %85 at 300oC. For the sample with the same area and Cu thickness, deposited on flat Si substrate, conversion starts at about 130oC. For 15nm Ni-on-grassy substrate, an average size of 60nm and starting conversion temperature of about 190oC have been achieved after the electro-thermal treatment. Such a sample shows a high CO conversion of about %40, whereas no conversion takes place for samples with Ni deposited on flat Si wafers. The process reported in this paper, presents a novel method to realize Cu and Ni nano-structures on nano-textured silicon. Although Ni on Si, unlike Cu-, Pt-, Au-based catalytic gels/powders, is not a common catalyst for PrOx of CO, such nano-structured features serve as conversion sites to detect and burn CO species. In addition, incorporation of these Ni nanostructured catalysts in SnO2-based H2S sensors is expected to have a profound effect on the performance of such devices.
6:00 PM - FF5.13
Charge Transport Properties of Graphene under Strain Probed with Atomic Force Microscopy.
Sangku Kwon 1 , Sunghyun Choi 1 , Hyunjong Chung 2 , Sunae Seo 2 , Jeong Park 1
1 Graduate School of EEWS (WCU), Kaist, Daejeon Korea (the Republic of), 2 , Samsung Advanced Institute of Technology, Yongin Korea (the Republic of)
Show AbstractGraphene, a quasi-two dimensional material, which is an allotrope of carbon, has been attracting great interest due to its remarkable electronic and mechanical properties and its possible applications in electronic devices such as semiconducting material and flexible transparent electrodes. In this study, we report the electrical transport properties of graphene under mechanical deformation and varied chemical environments. We present the study of the correlation between electrical transport and nanomechanical properties of a graphene layer grown on Cu substrate with CVD (Chemical Vapor Deposition) method using the combined apparatus of conductive probe atomic force microscopy (AFM)/friction force microscopy (FFM) operated in the pressure range between ultrahigh vacuum and ambient pressure. The current and friction were simultaneously measured as a function of the applied load between tip and graphene layer. We found that the current density decreased as increasing tip-sample pressure over the pressure range of 0-2 GPa. The influence of chemical environments (reducing and oxidizing condition) on the mechanical and electrical properties has been studied, and the results will be discussed in light of interactions between graphene and atomic scale defects.
6:00 PM - FF5.14
A Combined Photoemission and DFT Study of Cu/CeO2 Model Catalysts.
Lucie Szabova 1 , Iva Matolinova 1 , Matteo Farnesi Camellone 2 3 , Tomas Skala 4 , Vladimir Matolin 1 , Stefano Fabris 2 3
1 Department of Surface and Plasma Science, Faculty of Mathematics and Physics, Charles University, Prague, Prague 8 - Liben Czech Republic, 2 Theory@Elettra group, CNR DEMOCRITOS, IOM, Trieste Italy, 3 , SISSA, Trieste Italy, 4 , Sincrotrone Trieste, Trieste Italy
Show AbstractReducible oxides such as ceria are important in environmental applications for their ability to store and release oxygen as well as for acting as active support for metal particles in catalytic systems. The activity of the catalyst can be influenced by the metal-support interaction and by the size of the metal clusters. In particular Cu/CeO2 catalytic systems are known for promoting several important reactions, including CO oxidation [1-3] or NO reduction [4,5]. The present work is a photoelectron spectroscopy study of the Cu interaction with the CeO2(111) surface combined with the theoretical analysis of copper/ceria systems based on the numerical DFT+U simulations.Resonance photoelectron spectroscopy at the Ce 4d absorption edge is a sensitive tool of ceria oxidation state investigation. We deposited increasing amount of copper on stoichiometric CeO2(111) thin film epitaxially grown on the Ru(0001) surface. Cu 2p spectra showed the binding energy shift to higher values and a simultaneous increase of the Ce3+ resonance indicating ceria reduction and Cu oxidation for very small amounts of Cu.DFT+U calculations were used to investigate structural, thermodynamic and electronic properties of Cu/CeO2 model catalysts. In particular we focused on the bonding of copper adatom on stoichiometric and defective ceria surfaces as well as on the effects at the Cu(111)/CeO2(111) interface. Core level shifts of Cu atoms were calculated and compared to the experimental value. Charge transfer between copper and ceria was observed upon contact of the materials resulting in reduction of cerium atoms in proximity of copper in agreement with experimental results. This study provides clear evidence of the reduction of cerium ions in the presence of copper. Since cerium reduction is suggested to play an important role into the catalytic activity of ceria-based catalysts, the atomistic insight provided by our work can be functional to developing more efficient and stable Cu/CeO2 catalysts.[1] W. Liu and M. Flytzani-Stephanopoulos, J. Catal. 153 (1995).[2] W. Liu and M. Flytzani-Stephanopoulos, Chem. Eng. J. and Biochem. Eng. J. 64, 283 (1996).[3] C. R. Jung, J. Han, S. W. Nam, T.-H. Lim, S.-A. Hong and H.-I. Lee, Catal. Today 93, 183 (2004).[4] B. Wen and M. He, Appl. Catal., B 37, 75 (2002).[5] P. Bera, S. T. Aruna, K. C. Patil and M. S. Hegde, J. Catal. 186, 36 (1999).[6] L. Szabova, V. Matolin, M. Farnesi Camellone, M. Huang and S. Fabris, J. Chem. Phys., in press.
6:00 PM - FF5.15
Tuning Catalytic Activity of Ru and Rh Nanoparticles in CO Oxidation with Engineering Surface Oxide.
Sunmi Kim 1 , Kamran Qadir 1 , Sukyoung Jin 1 , Kyoungmin Jung 1 , A. Reddy 1 , Sang Hoon Joo 2 , Jeong Park 1
1 Graduate School of EEWS (WCU), Kaist, Daejeon Korea (the Republic of), 2 School of Nano-Bioscience and Chemical Engineering, UNIST, Ulsan Korea (the Republic of)
Show AbstractThe catalytic oxidation of carbon monoxide (CO) to carbon dioxide (CO2) using noble metals has long been a subject of interest and recent progress in colloid nanoscience provides an opportunity to develop new model systems of catalysts in this field. Recent studies suggest that surface oxides on transition metal nanoparticles play an important role in determining the catalytic activity of CO oxidation. It was shown that the surface oxide layers surrounding the Ru and Rh nanoparticles are catalytically active, leading to enhancing the catalytic activity of CO oxidation. In this study, we show the influence of surface oxide on Ru and Rh nanoparticles on the catalytic activity of CO oxidation using chemical treatments including oxidation, reduction and UV-Ozone surface treatment. The changes occurring during UV-Ozone surface treatment were characterized with X-ray photoelectron spectroscopy. The catalytic activity before and after the chemical modification were measured. We discuss the trend of catalytic activity in light of the formation of core-shell type oxide on nanoparticle surfaces. The dependence of surface oxide on nanoparticles on the turnover rate suggests an intriguing way to tune catalytic activity using surface engineering.
6:00 PM - FF5.16
Size Effect of Ruthenium Nanoparticle on Catalytic Carbon Monoxide Oxidation.
Sang Hoon Joo 1 , Jeong Park 2 , J. Russell Renzas 3 , Derek Butcher 3 , Wenyu Huang 3 , Gabor Somorjai 3
1 School of Nano-Bioscience and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan Korea (the Republic of), 2 Graduate School of EEWS (WCU), KAIST, Daejeon Korea (the Republic of), 3 Department of Chemistry, University of California, Berkeley, California, United States
Show AbstractCarbon monoxide oxidation over ruthenium catalysts has shown an unusual catalytic behavior. In this work, a particle size effect on CO oxidation over Ru nanoparticle (NP) catalysts has been investigated. Uniform Ru NPs with a tunable particle size from 2 to 6 nm were synthesized by a polyol reduction of Ru(acac)3 precursor in the presence of poly(vinylpyrrolidone) stabilizer. The measurement of catalytic activity of CO oxidation over Ru NP model catalysts under oxidizing reaction conditions showed an activity dependence on the Ru NP size. The CO oxidation activity increases with NP size, and the 6 nm Ru NP catalyst shows 8-fold higher activity than the 2 nm catalysts. The observed trend of CO oxidation activity can be correlated with the stability of catalytically active core-shell particles composed of RuO2 species thin layer formed on the Ru metallic core, which will be scrutinized in the near future. The results gained from this study will provide the scientific basis for the design of Ru-based oxidation catalysts.
6:00 PM - FF5.18
In-situ Characterization of Nanoparticle Catalysts during CVD Growth of Carbon Nanotubes by Environmental TEM.
Hideto Yoshida 1 , Yoshikazu Homma 2 , Seiji Takeda 1
1 The Institute of Scientific and Industrial Research, Osaka University, Osaka Japan, 2 Department of Physics, Tokyo University of Science, Tokyo Japan
Show AbstractIt is much needed to control the structure, growth site, and growth direction of carbon nanotubes (CNTs) for their applications. In this point, catalytic chemical vapor deposition (CVD) is the most promising method. In recent years, CVD growth of CNTs has been developed and the growth conditions have been investigated carefully. It is reliable that nanoparticle catalysts play a crucial role in CVD growth of CNTs. The growth site of CNTs can be controlled by careful deposition of the nanoparticle catalysts at predefined positions on substrates. However, no one has yet grown CNTs of a specific structure. Moreover, the growth direction of CNTs changes during the growth [1,2] and could not be controlled except for particular growth methods, for example, electric or magnetic field directed growth and template growth. Therefore, the key to the control growth of CNTs is deep understanding of the role of nanoparticle catalysts. In this study, we have clarified the atomic structure of nanoparticle catalysts during the CVD growth of CNTs using in-situ environmental transmission electron microscopy (ETEM) [3,4].As catalysts, Fe and/or Mo were deposited on a SiO2/Si substrate by vacuum evaporation. After the deposition, the substrate was set on a heating specimen holder and transferred to an ETEM (FEI Tecnai F20 equipped with an environmental-cell) operated at 200 kV. The sample was heated to 600 °C in vacuum, and then a mixture of C2H2:H2=1:1 of 10 Pa was introduced into the ETEM for the CVD.We have found that structurally fluctuating iron carbide (Fe3C) nanoparticles act as catalysts for the growth of CNTs when only Fe was deposited on the substrate. Bulk diffusion of carbon atoms in carbide nanoparticles is very likely. In the Fe-Mo catalyzed CVD growth, CNTs grows from nanoparticles of structurally fluctuating (Fe,Mo)23C6 in addition to Fe3C. Moreover, it has been clarified that Mo suppresses the nucleation of Fe2SiO4 that exhibits no catalytic activity for the growth of CNTs. It is well-known that the yield of CNTs is increased by adding Mo to Fe, Co and Ni. We have concluded that the synergetic roles of Mo are the formation of (Fe,Mo)23C6 nanoparticle catalysts and the suppression of the nucleation of Fe2SiO4. In addition, we will introduce the recent ETEM observation of defect formation in CNTs during their growth [5].[1] Y. Homma et al., Appl. Phys. Lett. 88 (2006) 023115.[2] H. Yoshida et al., Jpn. J. Appl. Phys. 46 (2007) L917.[3] H. Yoshida et al., Nano Lett. 8 (2008) 2082.[4] H. Yoshida et al., Nano Lett. 9 (2009) 3810.[5] This research was partly supported by Grant-in-Aid for Specially Promoted Research Grant No. 19001005 from Ministry of Education, Culture, Sports, Science and Technology, Japan.
6:00 PM - FF5.19
Sonochemically Synthesized PtNi and PtCu Nano Bimetallic Electrocatalysts for Oxygen Reduction Reaction in Proton Exchange Membrane Fuel Cells.
Cenk Gumeci 1 , Deois Cearnaigh 1 , Carol Korzeniewski 1 , Dominick Casadonte 1
1 Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas, United States
Show AbstractBimetallic nano sized particles of PtNi (3:1) and PtCu (1:1 and 1:3) have been synthesized by sonochemical method and applied as electrocatalysts for oxygen reduction reaction. Physical characterizations were made by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Analysis (EDX) and Transmission Electron Microscopy (TEM). Cyclic Voltammetry (CV) and Rotating Disc measurements were conducted to probe the electrochemical activity of the Pt3Ni and PtCu nano catalysts. SEM results suggested the nano particles have a porous structure and particles are interconnected with each other. EDX analysis confirmed the catalyst composition almost same as that of the nominal values. X-Ray diffraction and TEM results indicated that Pt3Ni nano particles have a crystallite size of around 2 nm while PtCu nano particles have a 4 nm crystallite size. As prepared Pt3Ni nano particles exhibited approximately 2 fold enhancement in oxygen reduction reaction activity comparing commercially available Pt-C. After potential cycling (20 cycles, upto 1.2 V (RHE) with a 100 mV/s scan rate) Pt3Ni nano particles showed 3-fold enhancement comparing the Pt-C which may be the results of Pt-skin formation of the Pt3Ni- nano particles. Moreover, durability test revealed that Pt3Ni- nano particles have great stability after 1000 potential cycles. For PtCu3 nano particles electrochemical de-alloying treatment was applied to obtain Cu core Pt shell nano structure. Preliminary results suggested de-alloyed PtCu nano particles have promising oxygen reduction activity. In this study, we showed that sonochemically synthesized PtNi and PtCu nano particles are highly active and durable electrocatalysts for oxygen reduction reaction in proton exchange fuel cells.
6:00 PM - FF5.2
One-pot Synthesis of Dendritic Pd-Au/C Electrocatalysts with Enhanced Activity and Durability toward Ethanol Oxidation.
Shin Wook Kang 1 , Sang Woo Han 1
1 , KAIST, Daejeon Korea (the Republic of)
Show AbstractCatalyst-electrode design is very important for widespread use of direct alcohol fuel cells (DAFCs) because the catalytic properties could depend on the size and morphology of the metal nanoparticles. For improving the catalytic activities, morphology and/or composition-controlled nanoparticle catalysts have been commonly loaded onto commercial carbon materials through the mixing of nanoparticle dispersion with carbon. However, repeated catalysis cycles have frequently induced the change of the nanoparticle shape and composition or sintering/aggregation of catalytic particles, which greatly hinder the long-term usage of the catalysts. On the contrary, direct formation of catalytic nanoparticles on the carbon by widely-used wet impregnation-reduction method cannot provide sufficient control over particle shape and size. Therefore, development of an efficient synthesis method for the preparation of catalyst-electrode system with desirable activity and stability is highly demanding. Here we report a facile one-pot synthesis of carbon-supported Pd-Au alloy nanoparticles with well-defined dendritic shape. The synthesized carbon-supported dendritic Pd-Au nanoparticles exhibited significantly enhanced performance in the electro-oxidation of ethanol compared with dendritic Pd nanoparticles supported on carbon and a commercial Pd/C catalyst. Moreover, the one-pot synthesized carbon-supported dendritic Pd-Au nanoparticles even showed higher durability in ethanol oxidation than the supported catalyst prepared by the deposition of pre-synthesized dendritic Pd-Au nanoparticles on the carbon support. Our results clearly indicate that enhanced interaction between nanoparticles and carbon support through the one-pot synthesis protocol can improve the durability of the electrocatalysts. This method can be applied to the other carbon materials and further improving the catalytic performance and applying to other catalytic reactions can be expected through the tailoring the morphology and composition of NPs.
6:00 PM - FF5.20
Oxidation of Monometallic and Bimetallic Iron-based Nanoparticles.
Lauren Greenlee 1 , Robert Usselman 2 , Stephanie Hooker 1
1 Materials Reliability Division, NIST, Boulder, Colorado, United States, 2 Electromagnetics Division, NIST, Boulder, Colorado, United States
Show AbstractThere has been a growing interest in using iron-based nanoparticles in water treatment due to their catalytic properties. Monometallic and bimetallic iron nanoparticles are reactive in an aqueous system and can be designed for targeted removal of specific organic and inorganic water contaminants. Iron metal nanoparticles undergo surface oxidation through reactions with both water and oxygen and produce reactive species that are then utilized to oxidize or reduce a specific contaminant. Through the oxidation process, iron metal is oxidized to ferrous iron, which in turn can participate in the Fenton reactions and produce reactive oxygen species and hydroxyl radicals. Iron metal oxidation also produces hydrogen gas (through reaction with water), which can react with organic compounds through reduction reactions. The catalytic properties of iron nanoparticles can be enhanced through particle stabilization by organic chelating compounds and through secondary metal coatings. Both organic stabilizers and secondary metals (e.g., palladium, copper) affect the oxidation of the nanoparticles and ultimately, nanoparticle reactivity.This research investigates iron nanoparticle oxidation for monometallic iron nanoparticles and bimetallic iron-palladium nanoparticles. Several organic stabilizers are compared in their ability to control particle aggregation and size during nanoparticle synthesis, as well as control particle oxidation over time. Techniques such as thermogravimetric analysis and electron microscopy are used to evaluate particle morphology and stabilizer surface coverage on the nanoparticles. Quartz crystal microbalance techniques are used to evaluate the mass change of the nanoparticles during surface oxidation in an aqueous environment. Finally, x-ray diffraction, SQUID magnetometry, and electron paramagnetic resonance (EPR) are used to evaluate changes in particle crystallinity, magnetism and radical production as the particles are oxidized.
6:00 PM - FF5.21
Spatial Structure Effect on the Nano Catalystic Activity.
Chun Zhang 1 , Xiaohong Wang 1 , Zhiyu Hu 1
1 , Institute of NanoMicroEnergy, Shanghai University, Shanghai China
Show AbstractAlumina – powder, alumina – fiber and alumina – membrane supported Pt catalyst and single Si substrate supported Pt membrane have been prepared by incipient-wetness impregnation method and radio frequency magnetron sputtering method. The catalytic activity of these different materials for methanol combustion at room temperature has been studied and the technology of the UV–Vis spectra, Raman spectra, FT-IR spectra, TEM, SEM, XPS and so on are also used to study the spatial structure effect on the catalytic activity deeply, which gives a important hint to the design of the catalyst.The results showed that all these materials can catalyze the methanol combustion at room temperature. However, both the difference of the support and active metal’s spatial structure can affect the catalytic activity of the material. Among the above support, alumina- membrane is the best one, alumina – powder is the second one and alumina – fiber is the worst one. The catalytic activity of alumina - membrane supported Pt catalyst for methanol combustion is about 6 times higher than alumina – fiber supported Pt catalyst. This may be ascribed to the reason that the structure of the support not only can affect the active metal particle’s shape, size and dispersion, but also can affect the interaction between the support and the active metal, which has led to the different catalytic activity. Among the above active metal spatial structure, the catalytic activity of the particles is much higher.
6:00 PM - FF5.22
TEM Characterization of Industry-style Rh Nanocatalysts in the Presence of Acoustic Phonons.
Petra Specht 1 , R. Gulotty 3 , D. Barton 3 , R. Cieslinski 3 , S. Rozeveld 3 , J. Kang 3 , Oscar Dubon 1 , Christian Kisielowski 2
1 Materials Science and Engineering, UC Berkeley, Berkeley, California, United States, 3 , The DOW Chemical Company, Midland, Michigan, United States, 2 NCEM, Lawrence Berkeley National Lab, Berkeley, California, United States
Show AbstractAberration-corrected electron microscopy provides single atom sensitivity which will enable the observation of single atom arrangements such as the location of promoter atoms on catalyst particles. However, quantitative procedures must be developed to account for dynamic contrast changes due to beam-sample interactions and incoherent instrument aberrations. The additional introduction of low acceleration voltage microscopy (here at 80kV) facilitates the material evaluation due to suppressed knock-on damage. In the absence of such displacement damage, the contrast of atoms and columns in thin crystals is significantly altered by phonons that are excited by the impingent electron beam yielding contrast fluctuations that can be captured at video rate. Calculated image contrasts can match experimental values if three-dimensional phonon excitations, incoherent aberrations, and modulation functions of CCD cameras are considered. As a result it is possible to distinguish beam-induced potential fluctuations from atom loss. In 3-4 nm small [110] Rh crystals displacement damage and atom loss occur at 300 kV but is almost absent from images recorded at 80 kV in agreement with theoretical predictions. Instead, the observed contrast fluctuations are shown to be dominated by the generation of phonons with beam currents of 1-2 * 106 electrons/nm2/s. The exclusion of atom loss at 80 kV allows for measuring the local atom column height in thin crystals from one projection only. We characterized two [110] oriented Rh nanoparticles on alumina that were extracted from chemical reactors to be platelets with an aspect ratio of ~0.2 and a surface roughness of ± 1 atom.
6:00 PM - FF5.23
Synthesis of Nitrogen Oxide-storage Catalysts by Chemical Vapor Infiltration and Catalyst Activity at Low Temperatures.
Martin Busch 1 2 3 , Uta Sager 5 , Ulf Bergmann 1 3 , Wolfgang Schmidt 4 , Frank Schmidt 2 3 , Burak Atakan 1 3 , Markus Winterer 2 3
1 Thermodynamics, Faculty of Engineering, University Duisburg-Essen, Duisburg Germany, 2 Nanoparticle process technology, Faculty of Engineering, University of Duisburg-Essen, Duisburg Germany, 3 Center for Nanointegration Duisburg-Essen, CeNIDE, Duisburg Germany, 5 , Institut für Energie- und Umwelttechnik e.V., Duisburg Germany, 4 , Max-Planck-Institut für Kohlenforschung, Muehlheim an der Ruhr Germany
Show AbstractDespite good progress in emission reduction, nitrogen oxides remain a major threat to human health and environmental pollution. Established technologies of NOx-reduction include SCR/DeNOx-methods by addition of ammonia or hydrocarbons. Known additive-free nitrogen oxide-storage-reduction catalysts for broad application are highly sensitive to poisoning, e.g. by SO2. New nitrogen oxide-storage catalysts based on microporous substrates were synthesized and investigated as regards their activity at low temperatures. Iron oxide, copper oxide and manganese oxide nanoclusters were deposited in active carbon and zeolithes by chemical vapor infiltration. Parameters of deposition (time-pressure-temperature program and precursor-substrate ratios) were studied in relation to NO2-storage. Various precursors were tested for homogeneous infiltration of microporous materials. Detailed investigation of produced catalysts included nitrogen adsorption, X-ray diffraction and energy-dispersive X-ray spectroscopy. Catalytic activity was studied in a recycle-reactor by time-resolved mass spectrometry in a temperature range of 303 to 523 K. Chemical vapor infiltration of active carbon showed comparable homogeneity to wet impregnation technique. Active carbon-based catalysts revealed high NO2-storage capacities while decomposing at higher temperatures. Zeolithe-based catalysts showed reduced storage performance without decomposition.
6:00 PM - FF5.24
Hydrogen Evolving Nanorod Heterostructure Photocatalysts Probed One Nanoparticle At a Time.
Lilac Amirav 1 2 , Jennifer Dionne 1 2 , A.Paul Alivisatos 1 2
1 Chemistry, University of California at Berkeley, Berkeley, California, United States, 2 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractPhotocatalysis presents an attractive and promising solution for both renewable energy generation and other environmental applications, such as water treatment and air purification. A prime example is the solar-driven photocatalytic splitting of water into hydrogen and oxygen, which provide potentially clean and renewable fuels. Such solar-to-fuel energy conversion alleviates the energy storage problem, since fuel (chemical energy) can be stored more easily than either electricity or heat. However, in spite of extensive research devoted to photocatalysis and to its important applications, there are still many material-related obstacles to its widespread use, and many equally alluring and challenging scientific queries related to the physical chemistry of the process.Here we present the design of a multi-component nanorod heterostructure aimed at photocatalytic production of hydrogen, and a new technique utilizing residual photoluminescence from this particle as a probe, enabling in-situ measurements of discreet charge transfer process on the single particle level.Our nanoheterostructure is composed of a platinum-tipped cadmium sulfide rod with an embedded cadmium selenide seed. In such structure holes are three-dimensionally confined to the cadmium selenide, whereas the delocalized electrons are transferred to the metal tip. Consequently, the electrons are now separated from the holes over three different components, and by a tunable physical length. This enables efficient long lasting charge carriers' separation, and the formation of distinct reaction sites, which are further apart, thus minimizing back reaction of intermediates. This structure was found to be highly active for hydrogen production, with an apparent quantum yield of 20% at 450 nm, was active under orange light illumination, and demonstrated improved stability.The efficiency of the catalytic reaction is directly determined by the quantity of photogenerated charges and the rate of charge transfer processes across the different interfaces present in the system(semiconductor/metal and photocatalyst/solution). However, these quantities are difficult to determine with traditional ensemble characterization methods, where essential information on dynamics and heterogeneity is lost. Thus, we present here new technique for in-situ measurements of photocatalytic reactions on the single particle level. Using far-field optical microscopy we explore the fluence-dependent particle photoluminescence. We find that individual catalysts exhibit strong optical nonlinearities that might allow direct determination of the number of charges participating in catalytic reaction and the rate of photoinduced charge transfer. This information will significantly benefit our ability to realize efficient photocatalysts for renewable direct solar-to-fuel energy conversion.
6:00 PM - FF5.3
One-Pot Synthesis of Au-Pd Octapodal Nanoparticles with Enhanced Electrocatalytic Activity and Stability toward Ethanol Oxidation.
Jong Wook Hong 1 , Sang Woo Han 1
1 , KAIST, Daejeon Korea (the Republic of)
Show AbstractThe catalytic activity and selectivity of bimetallic nanoparticles (NPs) with core-shell and alloy structures can be tuned by controlling the morphology because the exposed surfaces of the NPs have distinct crystallographic planes which could determine the overall catalytic properties. Accordingly, shape-controlled synthesis of bimetallic NPs has been extensively studied to optimize their properties. However, shape-controlled synthesis of alloy NPs has been very limited compared to the core-shell NPs. Therefore, the development of strategies for the facile preparation of bimetallic alloy NPs with desirable structures and exploration of their properties have been quite required for their potential applications, especially in catalysis. In our work, we present the one-pot aqueous synthesis of bimetallic alloy Au-Pd NPs with an unprecedented octapodal shape which has eight square rod-shaped pods. To fabricate Au-Pd octapodal NPs, the alloy Au-Pd nanooctahedra should be first prepared as seed NPs. These unique and complex nanostructures have been evolved through the selective etching of {100} faces by the in situ generated Br- ion which was generated by the reduction of Au precursor (AuBr4-). We successfully showed each of the selective etching processes on the basis of the time-based morphology pattern.Interestingly, the synthesized Au-Pd octapodal NPs have exhibited as much as 4 times higher electrocatalytic activity than that of the commercial Pd catalyst owing to their specific morphological features. The present work, mixed metal catalysts with controlled morphology, might provide a new strategy for developing efficient anode catalysts of the fuel cells.
6:00 PM - FF5.4
Growth Mode, Morphology, and Reducibility of CeO2(111) Thin Films on Cu(111).
Filip Dvorak 1 , Oleksandr Stetsovych 1 , Michael Steger 2 , Elmiloudi Cherradi 2 , Iva Matolinova 1 , Josef Myslivecek 1 , Vladimir Matolin 1
1 Department of Surface and Plasma Science, Faculty of Mathematics and Physics , Charles University in Prague, Prague 8 Czech Republic, 2 Institut fuer Experimentelle Physik der kondensierten Materie, Heinrich-Heine-Universitaet, Düsseldorf Germany
Show AbstractCeria films on Cu(111) are becoming increasingly important as model systems in the research of catalysis over ceria. The morphology of the ceria layers represents an important factor determining the catalytical behavior of the ceria-based systems. However, with ceria on Cu(111), the morphology remains largely unknown. We examine growth mechanisms in CeO2(111) thin films on Cu(111) based on scanning tunneling microscopy observations of the film morphology. While the growth mode is essentially three-dimensional, we identify growth mechanisms resembling of other growth modes - formation of an interfacial layer, and growth of CeO2(111) pyramids by stacking of monolayer-high islands. These growth mechanisms are active in the temperature range from 150°C to 650°C and their knowledge allows us to control the coverage, the number of open monolayers, and the step density of CeO2(111) thin films on Cu(111). We collect morphological evidence for further effects, namely for CeO2(111)/Cu(111) lattice mismatch, and for existence of two stable step heights in the CeO2(111) layer. Using photoelectron spectroscopy, we determine the stoichiometry and thermal stability of CeO2(111) films with different morphology. We find a correlation between surface reduction and morphological stability in ceria layers during annealing in vacuum. Annealing in vacuum allows us to control besides the morphology also the degree of surface reduction in CeO2(111) thin films on Cu(111). With increasing temperature during annealing, both the morphology and the degree of reduction in ceria films readily change. This must be accounted for in evaluating temperature-programmed experiments with ceria on Cu(111).
6:00 PM - FF5.6
Self-recharging Palladia Nanoparticles on Ceria Nanotube Support for Catalytic Oxidation.
Chin Li Cheung 1 , Yunyun Zhou 1 , Neil Lawrence 1 , Patrick Kent 2
1 , University of Nebraska-Lincoln, Lincoln, Nebraska, United States, 2 , Creighton University, Omaha, Nebraska, United States
Show AbstractWe report our study to enhance the oxidative catalytic reactivity of palladia nanoparticles through coupling with highly defective ceria nanotube support. Transmission electron microscopy study revealed that the average diameters of these palladia nanoparticles on the cerium nanotubes are ca. 2-5 nanometers. These palladia nanoparticles decorated ceria nanotubes were found to have drastically increased oxidative catalytic activity towards carbon monoxide (CO) oxidation when compared to those of the bulk ceria, ceria nanotubes, and carbon supported palladia nanoparticles. The combined treatments of ceria nanotubes using low vacuum activation and decoration with palladia nanoparticles led to a large decrease in their light-off temperature, T50, from 350°C (bulk ceria), 175°C (ceria nanotubes) to 30°C for CO oxidation. The catalytic activity of this hybrid catalyst was found to be stable over 4 hours and then slowly drop to 50% of its catalytic activity in 120 hours under the bench-mark CO oxidation test. The catalytic activity of this catalyst was also found to be self-rechargeable to the original activity under ambient conditions in 4 hours. A postulated role of the ceria support in the self-recharging capability of the hybrid catalyst will be discussed.
6:00 PM - FF5.7
Surface Regulation of Gold Polyhedral Nanocrystals Using Polymer Surfactants.
Seon Joo Lee 1 , Hyunjoon Song 1
1 Chemistry, KAIST, Daejeon Korea (the Republic of)
Show AbstractResearch into the synthesis and application of various kinds of gold nanocrystals has intensified over the past several years due to their unique electronic, magnetic, and optical properties. When we use gold nanocrystals to catalytic reaction, it is the surface condition that plays the most important role in determining physical and chemical properties of nanocrystals, including solubility, reactivity, and stability. Poly(vinyl pyrrolidone) (PVP) is the most well-known polymer which has been widely used to control size and shape of metallic nanostructures by attaching selectively on the metal surfaces. Although it is believed that the tertiary amide group of PVP interacts with metal surfaces to make shaped nanocrystals, little is known about how PVP adsorbs onto metal surfaces. In this study, we synthesized gold polyhedral nanocrystals using PVP and PVP analogues: poly(vinylcaprolactam) (PVCL) and poly(N,N-dimethyl acrylamide) (PDMAm), and noticed the differences between resulting gold nanocrystals to reveal the coordination mechanism. Based on X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) data, we discovered that it is not N atoms but O atoms of the tertiary amide group that mainly contribute to the interaction with the metal surfaces by donating their electrons. Also we could fabricate smaller gold polyhedral nanocrystals using PVP alternatives with stronger electron-donating ability, which promote gold-based catalytic reaction by increasing their surface area. In particular, using PDMAm, we incorporated functionalities in the surfaces of gold nanocrystals through hydrolysis of PDMAm during the reaction, which can be expected to facilitate the reduction of o-nitroaniline by exposing carboxylate anions on the surfaces. We believe that through this approach for surface regulation we are able to make highly efficient gold nanocrystals which can be available for many catalytic reactions.
6:00 PM - FF5.8
Spectroscopic Characterization of Ultra-thin Al2O3/SiO2/Si Films by SR XPS.
Thithi Lay 1 , Motoyasu Imamura 2 , Nobuyuki Matsubayashi 2
1 Analysis Laboratory, Omicron nanotechology Japan, Tokyo Japan, 2 National Metrology Institute of Japan,, AIST, Tsukuba, Ibaraki, Japan
Show AbstractIntroductionAl2O3 is one of the prominent materials in various applications such as catalysis, coating, and microelectronics. Especially, in microelectronics it has been consider as one of the candidates for next generation high-k dielectrics in CMOS gate electrodes and metal-insulator-metal (MIM) electron emitter devices. In nano-scale film growth, analysis of interfacial reactions between substrate and film are important in considering the device abilities. SR-XPS has advantages in analyzing the chemical states changes and elemental bonding at interface due to its energy variability with ultra-brilliant. In the present work, ultra-thin Al2O3 films with thickness ranging from 2-10 nm were deposited on SiO2/Si substrate and there interfacial analysis was carried out. .ExperimentalThe experiment was carried out at KEK-PF BL-13C. Samples used were Al2O3 films with thickness 2, 3, 5, 10nm deposited on SiO2/Si (100) n-type substrate with thickness 525μm. XPS spectra were obtained using the SR excitation energy from 130-1000eV with CMA analyzer PHI model 1600C. The analyzer was set normal to sample while the excitation beam was set at 55°. Analysis area was 800φμm with solid angle ±7°. The system based pressure during measurement was 2.8 x 10-8 Pa.Results and discussion XPS spectra obtained for different film thickness were fitted by Shirley type background using Multipack XPS software. Photoelectron peaks penetrated from Al2O3, substrate Si and interfacial layer SiO2 were observed. At lower excitation energy, Al2O3 photoelectron peak appeared strongly. Energy loss appeared as surface Plasmon peak at higher binding energy around 14eV from main peak. At higher excitation energy the penetrated photoelectrons peaks showed much prominent from SiO2/Si substrate and no Plasmon like peaks were observed. The characteristic of Plasmon peak according to penetration depth and film thickness can be important information in considering the interfacial behavior of top gate dielectric.
6:00 PM - FF5.9
Two Versatile Transmission Electron Microscopy Specimen Holders for In Situ Illumination of Photoactive Materials.
Mathias Kobylko 1 , Giorgio Divitini 1 , Caterina Ducati 1
1 Department of Materials Science and Metallurgy, University of Cambridge, Cambridge United Kingdom
Show AbstractPhoton-induced activation of surfaces in nanostructures plays an important role in several physical processes, such as catalysis and solar energy harvesting. Gaining an understanding of the phenomena happening on the nanoscale, including modifications to the electronic structure, can contribute to the development of more efficient photovoltaic devices for the conversion of sunlight into electricity and provide more effective solutions for the removal of toxic gases from the environment. Nanoparticles are particularly interesting for these applications because of their high surface to volume ratio, the photovoltaic or photocatalytic yield being proportional to the surface of the photoactive material.The study of the influence of the phase, morphology and size of a nanoparticle on its photoresponse requires the simultaneous performance of the photoexcitation of the particle and of its electronic and structural analysis. A transmission electron microscope (TEM) would be suited for this kind of experiment if a well-defined light source was integrated into its column. In this contribution, we will present a solution to this problem by demonstrating the possibility of incorporating a light emitting diode (LED) with a well-defined emission wavelength into a TEM specimen holder. We have designed and constructed two in situ illumination TEM specimen holders for Philips and FEI TEMs which allow suitable materials to be irradiated with photons of a chosen wavelength under the electron beam. The first design we demonstrate is based on a single tilt Philips EM300 heating holder. It has the advantage of being able to accommodate specimens of different geometries, in particular thicker and/or wider specimens, e.g. silicon oxide or silicon nitride membranes. The second design is based on the Fischione electrical biasing tomography holder [1]. In this design, the LED is placed on a custom-made removable cartridge and thus allows a quick and easy exchange for another LED of a different emission wavelength. Furthermore, this second design is capable of high-resolution imaging under photon irradiation, as verified using a FEI Tecnai F20. We expect holders of this kind, derived from widely available biasing holders, to become a useful tool in the research on photovoltaic and photocatalytic properties of nanomaterials.[1] R.E. Dunin-Borkowski et al., Microscopy and Microanalysis (2004), 10 (Suppl 2), 1012-1013
Symposium Support
FEI Company
RHK Technology Inc
SPECS Surface Nano Anaylsis GmbH
FF6: Application of Ambient Pressure XPS to Studies of Catalysis
Session Chairs
Wednesday AM, April 27, 2011
Room 3022 (Moscone West)
9:00 AM - FF6.1
Au on Pd(111) for the Selective Oxidation of Allylic Alcohols.
James Naughton 1 2 , Adam Lee 3 , Karen Wilson 3 , Sarah Thompson 1
1 Physics, University of York, York, North Yorkshire, United Kingdom, 2 Chemistry, University of York, York, North Yorkshire, United Kingdom, 3 Chemistry, Cardiff University, Cardiff United Kingdom
Show AbstractAllylic aldehydes are important intermediates in the synthesis of fine chemicals, and are often also used directly as fragrances and flavourings. Commercial synthesis of these compounds relies on using either hazardous stoichiometric oxidants such as CrVI or peroxides, or expensive homogeneous complexes which are extremely difficult to recover. Alternatively, the use of powerful heterogeneous catalysts for the selective oxidation (selox) of alcohols, would offer great process, safety, and environmental benefits[1].Palladium catalysts display high activity for the selox of primary alcohols, but are prone to rapid on-stream deactivation, meaning their uptake into the fine chemicals sector has been hindered. Our recent in-situ XPS studies of crotyl alcohol (2-buten-1-ol) oxidative dehydrogenation to crotonaldehyde over Pd(111) have shown that secondary decarbonylation reactions produce irreversibly bound side products, including CO, which are thought to cause fast self-deactivation[2].Bimetallic systems often display enhanced activity and/or selectivity when compared to monometallic counterparts. Hutchings and co-workers discovered a range of bimetallic Au/Pd catalysts that show exceptional activity and selectivity towards the oxidative dehydrogenation of a diverse range of alcohols[3]. The exact details of the active surface ensemble and role of Au in promoting selox remain uncertain, making such systems ideal for investigation by temperature-programmed reaction (TPR) and in-situ X-ray Photoelectron spectroscopy (XPS) methods.A thick (~ 4 ML) Au over-layer was deposited onto a Pd(111) crystal. Annealing this layer to various temperatures leads to a wide range of Au/Pd surface compositions[4]. XPS and CO titrations demonstrate that these surface alloys show extremely high selectivity towards crotonaldehyde production. They also dramatically reduce the extent of hydrocarbon decomposition and eventual carbon lay-down compared with Pd(111). The surface chemistry of crotonaldehyde and propene, was also studied by TPR over the same Au/Pd(111) surface alloys in order to elucidate the full selective oxidation reaction pathway. Gold strongly promotes crotonaldehyde and propene desorption over decomposition at mole fractions ≥ 0.4 and ≥ 0.8 respectively; only ~5 % of the chemisorbed aldehyde and alkene react over Au-rich alloys. Co-adsorbed oxygen further suppresses crotonaldehyde decomposition over alloy surfaces, while propene combustion, an important side-reaction over un-promoted Pd(111), is also moderated by Au [5].[1] B.Z. Zhan and A. Thompson, Tetrahedron, 60, 2917, (2004)[2] A. F. Lee et al, J. Phys. Chem. C, 111, 18844, (2007)[3] D. I. Enache et al, Science, 311, 362 (2006)[4] Z. Li et al, Surf. Sci., 601, 1898 (2007)[5] J. Naughton et al, PCCP, 12, 2679 (2010)
9:15 AM - FF6.2
In Situ XPS Study of the Influence of Water Vapor on the Catalytic Decomposition of Hydrocarbons.
Placidus Amama 1 2 , Tyson Back 1 3 , P. Terry Murray 1 2 , Steven Fairchild 1 , Benji Maruyama 1
1 RXB, Air Force Research Laboratory, WPAFB, Ohio, United States, 2 UDRI, University of Dayton, Dayton, Ohio, United States, 3 , Universal Technology Corporation, Dayton, Ohio, United States
Show AbstractWater-assisted chemical vapor deposition (CVD) has become one of the leading methods for producing vertically aligned carpets of single-walled carbon nanotube (SWNT) due to its high catalytic efficiency. The mechanism for the dissociative adsorption of the hydrocarbon on catalyst surfaces in the presence of water vapor is still not fully understood. In this work, we present critical evidence from in situ X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) experiments using ethylene decomposition over Fe catalyst that further rationalizes the growth enhancement observed during water-assisted CVD growth of nanotubes.
9:30 AM - **FF6.3
In-situ Studies of Surface Reactions by Photoelectron Spectroscopy.
Reinhard Denecke 1
1 Wilhelm-Ostwald-Institut, Universität Leipzig, Leipzig Germany
Show AbstractUsing high-resolution and time-dependent x-ray photoelectron spectroscopy (XPS), adsorption and reaction processes on surfaces can be followed in-situ. From the data, adsorbed species as well as reaction intermediates and surface products can be determined and quantitatively analyzed. Such time-dependent data allow for the determination of kinetic parameters. Besides the necessity to model technologically relevant systems, such studies call for flexibility in parameters such as pressure, energy of incoming molecules, surfaces temperature and surface complexity. We follow different strategies to satisfy such conditions while trying to keep the high and detailed information level of model surface science studies.One strategy involves the usage of a supersonic molecular beam to deliver the reactant molecules to the surface. By the ability to control and vary the kinetic (and also internal) energy of the molecules, activated processes can be studied at UHV conditions. In addition, the local impingement rate of the molecular beam can be nicely controlled and ranges up to equivalent pressures of 10-6 mbar, enabling kinetic studies of selected reactions. I will discuss examples involving methane and CO using this strategy.Another strategy utilizes regularly stepped surfaces in order to investigate the influence of defects on surface reactions. As high-resolution XPS can distinguish between step and terrace adsorbed molecules by their core-level binding energy, the reactivity of the different adsorption sites can be studied separately. By decorating the steps with either less or more reactive (metal) atoms, we have another handle to address these sites. Thus, besides poisoning effects also changes in the lateral confinement of molecules on terraces can be observed and analyzed. Results for adsorption and reactions on stepped Platinum surfaces without and with step decoration will be discussed, both for small molecules such as CO and for larger hydrocarbons such as benzene and pyridine.A third strategy aims at raising the local pressure during experiments close to ambient conditions (up to a few ten mbar). In this regime, a feedback of the reaction conditions on the reacting surface can be found in a lot of cases. I will highlight our approach to such an electron spectrometer setup.
10:00 AM - FF6.4
Enhanced Activity of Fe-oxide Nanoparticles on Au(111) Studied with In Situ X-ray Photoelectron Spectroscopy and In Situ Scanning Tunneling Microscopy.
Xingyi Deng 1 2 , Junseok Lee 1 2 , Congjun Wang 1 2 , Christopher Matranga 1 , Funda Aksoy 3 , Zhi Liu 3
1 , National Energy Technology Laboratory, Pittsburgh, Pennsylvania, United States, 2 , URS, Pittsburgh, Pennsylvania, United States, 3 , Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractWe have studied the activity of Fe-oxide nanoparticles, including FeO and α-Fe2O3 grown on Au(111), using in situ X-ray photoelectron spectroscopy (XPS) and in situ scanning tunneling microscopy (STM). At room temperature, in situ XPS shows that adsorbed hydroxyl groups, arising from the dissociation of water, are present on FeO/Au(111) at H2O pressures ranging from 3 × 10-8 to 0.1 Torr. In situ STM with atomic resolution illustrates that these adsorbed OH groups are formed at the edges of the FeO particles with the O atom incorporating itself into the atomic lattice of the oxide. In situ XPS studies also show that these OH groups on FeO nanoparticles are rather stable and exhibit no apparent reactivity in a 1:1 atmosphere of H2O and CO at 200 mTorr. For comparison, the activity of α-Fe2O3 nanoparticles on Au(111) was also investigated. This system again shows a high activity towards H2O with adsorbed OH groups being detected by in situ XPS at partial pressures of H2O as low as ~ 10-8 torr. The surface bound OH groups on α-Fe2O3 nanoparticles react in a 1:1 atmosphere of H2O and CO at 200 mTorr creating a formate intermediate through an associative type water gas shift (WGS) mechanism. Continuous films of α-Fe2O3 with the same O-terminated (0001) surface as the nanoparticles show no measurable reactivity towards H2O or CO. Our results demonstrate that the particle edge and interface with the Au(111) substrate that exists for both Fe-oxide nanoparticle systems are the structural features most likely responsible for the low temperature activity of these nanoparticles. The reactivity differences seen for the FeO and α-Fe2O3 nanoparticles will also be discussed.
10:15 AM - FF6.5
Novel Applications in Surface Science – In Situ Sample Analysis in Extreme Environments.
Andreas Thissen 1 , Thomas Schulmeyer 1 , Oliver Schaff 1
1 , SPECS Surface Nano Analysis GmbH, Berlin Germany
Show AbstractModern devices are often only functional in environments far away from ultrahigh vacuum, still being the standard operation conditions for all Surface Science techniques. In parallel the importance of surfaces for the correct device operation is continuously increasing due to miniaturiziation down to the nanoscale. To contribute to advanced materials analysis in future means using Photoelectron spectroscopy, Scanning Probe Microscopies and related techniques in the generic or near generic device environments. This means high, elevated or near ambient pressures of defined working gas mixtures, liquid media, potentials or magnetic fields applied. Also extremely low or high temperatures might be necessary. In past all standard Surface Science Techniques did not work under these extreme environments. This work summarizes and presents existing solutions nowadays and future development routes to new instruments and materials analysis methods being functional under these working conditions. Opportunities and limits will be discussed. from the perspective of a supplier of scientific instruments. Finally applications, examples and results from existing In situ methods like high pressure treatments cells, complete High Pressure or Near Ambient Pressure Photoelectron Spectroscopy or Scanning Probe Microscopy Systems (NAP-PES or NAP-SPM), liquid and electrochemical cells, Liquid sample “manipulators“, and concepts and status of equipment working in highest or lowest temperatures, high magnetic fields and static or dynamic potentials will be demonstrated.
10:30 AM - FF6.6
In-situ Spectroscopic Study of the Oxidation of Cu(110) and Water Adsorption on CuOx at Near Ambient Conditions.
Peng Jiang 1 , Ferenc Borondics 1 , Lisandro Giovanetti 1 , John Newberg 1 , Elzbieta Pach 1 , Hendrik Bluhm 1 , Miquel Salmeron 1
1 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractThe oxidation of the Cu(110) surface has been studied by in situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS) at ambient oxygen pressures and high temperatures. Different CuOx species have been identified unambiguously. The different adsorption behaviors of H2O on these CuOx surfaces have been investigated in situ systematically at near ambient conditions.
10:45 AM - FF6: APXPS
BREAK
FF7: Catalysis of 2D Model Catalysts
Session Chairs
Wednesday PM, April 27, 2011
Room 3022 (Moscone West)
11:15 AM - **FF7.1
In-situ and Ex-situ Investigation of Ultrathin Oxide Films and Their Chemical Reactivity.
Hans-Joachim Freund 1
1 Chemical Physics, Fritz Haber Institute of the Max Planck Society, Berlin Germany
Show AbstractThin oxide films have been used to successfully model heterogeneous catalysts in the past. Recently, it has been found that ultrathin films, of a few atomic layers exhibit interesting chemical properties, which are of interest with respect to heterogeneous catalysis in general but also with respect to specific aspects such as the famous strong metal support interaction (SMSI) which I soften thought to be detrimental to catalytic activity. I will discuss properties of a double layer iron oxide film grown over magnetite supported Pt nanoparticles as an example where SMSI is observed but where catalytic properties are actually enhanced. We shall use the conclusions drawn from this study to discuss in more general terms how properties of such ultrathin films could be used to predict chemical activity. The results discussed are based on in-situ and ex-situ experimental techniques, such as thermal desorption spectroscopy, scanning tunneling microscopy, polarization-modulation infrared spectroscopy and electron spin resonance spectroscopy.
11:45 AM - FF7.2
Pulsed Laser Deposition and In Situ Scanning Tunneling Microscopy of Pd Clusters Supported on Alumina.
Carlo Casari 1 2 , Andrea Li Bassi 1 2 , Stefano Foglio 1 , Marco Corbetta 1 , Matteo Passoni 1 , Carlo Bottani 1 2
1 Dept. of Energy, Politecnico di Milano, Milan Italy, 2 Center for NanoScience and Technology CNST of IIT@PoliMI, IIT Italian Institute of Technology, Milan Italy
Show AbstractWith the aim of addressing the material gap issue in heterogeneous catalysis between model and real systems, we exploited Pulsed Laser Deposition (PLD) to produce Pd clusters supported on ultrathin alumina surfaces (Pd/Al2O3/NiAl(001)) and ultrathin films (Pd/Al2O3-x/HOPG). The structural and electronic properties have been investigated by in situ Scanning Tunneling Microscopy (STM) and Spectroscopy (STS) in ultra high vacuum (UHV).At first, Pd clusters were deposited on Al2O3 surfaces grown by thermal oxidation of NiAl(001). With respect to the widely studied systems of evaporated metals on Al2O3/NiAl(110), our system shows a good thermal stability up to 650 K. PLD of Pd clusters in the presence of a background gas pressure (10-100 Pa Argon) allowed us to vary the cluster mean size in the 2-4 nm range with a size-dependent energy gap as revealed by STS spectra. In addition, the use of O2 as the background gas allowed to control composition of the ablated species and to deposit PdO clusters.We then realized a Pd/alumina system entirely produced by PLD and addressable by in situ STM. Ultrathin (thickness < 1 nm) alumina films have been deposited by ablating an Al2O3 target in an O2 background atmosphere. STM images of this alumina layer show a high smoothness (0.17 nm rms roughness) with the presence of ordered domains of a few nm. Stoichiometry, morphology and density of alumina films can be tuned by controlling the O2 background gas pressure [F. Di Fonzo D. Tonini, A. Li Bassi, C.S. Casari et al. Applied Physics A 93 765 (2008)]. Thanks to the Pd cluster size control achieved in the previous step, subsequent PLD of Pd leads to a uniform dispersion of Pd clusters. dI/dV differential conductivity maps show a peculiar contrast indicating the presence of Pd clusters with different electronic properties. We finally demonstrate that, by exploiting the same deposition technique, it is possible to produce both a model system to be investigated in UHV by in situ STM or by other surface science techniques and a thick film (> 10 µm) closer to real systems employed in technological applications.
12:00 PM - FF7.3
High Throughput Approaches to Study of Alloy Catalysis and Structure Senstivity.
Andrew Gellman 1 , James Miller 1 , Peter Kondratyuk 1 , Esteban Broitman 1 , Deepika Priyadarshini 1
1 Chemical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
Show AbstractA suite of tools has been developed and is now being applied for the high throughput study of catalytic surface chemistry on composition spread alloy films (CSAFs) and on surface structure spread single crystals (S4Cs). A fairly simple offset filament tools is being used for deposition of PdxCuyAu1-x-y CSAFs. These alloys have been chosen for study because of their use as dense metal membranes for hydrogen separation and purification from coal gas. The PdxCuyAu1-x-y CSAFs are being used as libraries for the study of surface segregation in both binary and ternary alloys. Segregation impacts the ability of these PdxCuyAu1-x-y alloys to dissociatively adsorb hydrogen, the first step in hydrogen transport through their bulk. X-ray photoemission spectroscopy (XPS) and low energy ion scattering (LEIS) are being used to determine the near surface and the topmost surface compositions as functions of the bulk film composition. Surface energetics favors the segregation of Cu and Au to the alloy surfaces. In parallel with these efforts, a multichannel microreactor has been developed for high throughput measurements of catalytic surface chemistry. It is being used to study to composition dependence of the kinetics of H2-D2 exchange on the PdxCuyAu1-x-y alloys.A set of surface single crystal surfaces exposing a continuous distribution of all possible Cu(hkl) surfaces has been prepared and is being used for study of the structure sensitivity of various surface reactions. Imaging XPS has been used to measure the kinetics of oxygen adsorption these surfaces at room temperature. The S4C platform has allowed measurement of oxygen uptake along the Cu(hhk) directions of the surface and allows quantitative evaluation of the kinetics of oxygen adsorption at the (100) and (110) step edges.
12:15 PM - FF7.4
Adsorption Dynamics in the Selective Oxidation of Crotyl alcohol over Pd(111).
James Naughton 1 2 , Andy Pratt 1 3 , Chris Eames 2 , Sarah Thompson 1 , Steve Tear 1 , Adam Lee 4 , Karen Wilson 4
1 Physics, University of York, York, North Yorkshire, United Kingdom, 2 Chemistry, University of York, York, North Yorkshire, United Kingdom, 3 York Institute for Materials Research, University of York, York, North Yorkshire, United Kingdom, 4 Chemistry, Cardiff University, Cardiff United Kingdom
Show AbstractThe selective aerobic oxidation (selox) of allylic alcohols via heterogeneous catalysis presents a green and economically attractive route to corresponding aldehydes. Such compounds are important fine chemicals used as food preservatives, fragrances and flavourings[1]. Despite obvious process, safety and environmental advantages that a heterogeneous approach affords, due to a dearth of knowledge regarding the adsorption mode, optimal reaction conditions, activation protocols and deactivation pathways, such aldehydes are prepared industrially by either harmful stoichiometric oxidants or expensive homogeneous complexes. To understand more about the surface reactions, deactivation mechanisms and nature of active site involved, a variety of UHV techniques have been employed in order to probe the chemistry of allylic alcohols on various metal surfaces. Promising heterogeneous catalysts are derived from platinum group and noble metal clusters, although these are prone to rapid onstream-deactivation[2]. Crotonaldehyde (but-2-enal, CrCHO) is an important precursor to sorbic acid and vitamin E, and is prepared via the selox of crotyl alcohol (but-2-en-1ol, CrOH). Supported Pd nanoparticle catalysts have been identified as a particularly green alternative to traditional reagents[2]. In the case of the selox of CrOH over Pd(111), our understanding of the deactivation has been enhanced through the use of temperature programmed X-ray photoelectron spectroscopy (XPS) combined with near edge X-ray absorption fine structure (NEXAFS)[3]. An additional—although much less common—technique that is well suited to the study of molecular adsorbates is metastable de-excitation spectroscopy (MDS) which makes use of the energy associated with the 23S state of metastable helium to induce electron emission from a surface or molecule. Lack of surface penetration of the probe He* beam means MDS is extremely surface sensitive and ideal for the in-situ study of interaction dynamics in heterogeneous catalysis. As well as giving an overview of this unique technique, we present results from an MDS study of low-temperature crotyl alcohol adsorption on Pd(111). Using supporting DFT calculations, we find the molecule to adsorb in a side-on state with the C=C bonds oriented parallel to the surface, in agreement with previous NEXAFS studies. Additionally, direct emission from O lone-pair 2p n0 states associated with the –H2C-OH functionality is observed with the relative intensity and coverage dependence of this feature suggesting that the C-O bond also aligns somewhat parallel to the substrate. Results from a temperature-dependent MDS study performed to gain insight into the selox of CrOH, and subsequent deactivation mechanism, are also presented. [1] T. Mallat and A. Baiker, Chem. Rev. 104, 3037 (2004). [2] A. F. Lee et al, Green Chem. 8, 549 (2006). [3] A. F. Lee et al, J. Phys. Chem. C 111, 18844 (2007).
12:30 PM - FF7.5
Combined TPRx, in situ GISAXS and GIXAS Study of GaN-supported Platinum Model Catalysts.
Sonja Wyrzgol 1 , Susanne Schaefer 2 , Xuebing Li 1 , Martin Stutzmann 2 , Sungsik Lee 3 , Marcel Di Vece 4 , Stefan Vajda 3 4 , Johannes Lercher 1
1 Department of Chemistry, Catalysis Research Center, Technische Universität München, Garching Germany, 2 Walter Schottky Institute, Technische Universität München, Garching Germany, 3 Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois, United States, 4 Department of Chemical Engineering, Yale University, New Haven, Connecticut, United States
Show AbstractHeterogeneous catalysis combined with wide-band gap semiconductor physics provides a potential approach to explore electronic reaction control. Semiconducting supports may influence the electronic structure of metal nanoparticles via charge transfer and would add unexplored flexibility for small devices. As practical example, the properties of Pt particles on GaN support are probed in this contribution.Size- and shape-selective poly(vinylpyrrolidone)-protected Pt nanoparticles were synthesized by reduction of the metal precursor with an alcohol or ethylene glycol and were spin-coated onto GaN surfaces. Subsequent evaporation of the polymer in oxygen plasma and heat treatment at elevated temperatures resulted in homogeneously distributed and mechanically stable particles on the surface. Material properties during catalysis were investigated by combining temperature programmed reaction (TPRx) with in situ grazing incidence small-angle X-ray scattering (GISAXS) and X-ray absorption spectroscopy (GIXAS). Small spherical Pt particles of 1.8 nm and large cubic particles of 6.7 nm applied to GaN were found to be sintering resistant at elevated temperatures as well as during reduction and ethene hydrogenation. In contrast to the large particles, small particles responded to their gaseous environment and demonstrated dynamic shape changes in reaction atmosphere with a simultaneous increase in electron density. Deactivation of small particles within 10 min was observed under ethene hydrogenation, while the activity remained constant for large particles. A turnover rate of 63 molecules/Pt (exposed)/s and 53 molecules/Pt (exposed)/s was determined for spherical and cubic particles, respectively. The temperature-dependent X-ray absorbance of small particles indicated variations in electronic structure by altering the state of reduction due to metal-semiconductor interaction. The combination of the presented techniques allows an advanced understanding of geometric and electronic effects of metal particles on their catalytic behavior under well controlled reaction conditions in a single experiment.
12:45 PM - FF7.6
Photochemistry of Methanol on TiO2(110) Studied by Two-photon Photoemission and Scanning Tunneling Microscopy Method.
Chuanyao Zhou 1 , Zefeng Ren 1 , Shijing Tan 3 , Zhibo Ma 1 , Hongjun Fan 1 , Alec Wodtke 2 , Bing Wang 3 , Xueming Yang 1
1 State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian, Liaoning Province, China, 3 Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui Province, China, 2 Department of Chemistry and Biochemistry , University of California at Santa Barbara, Santa Barbara, California, United States
Show AbstractMethanol can act as a strong enhancement agent for the photocatalytic H2 production in water/methanol mixture over TiO2.[1] The detailed physics for such strong enhancement in photocatalysis on TiO2, however, has not been clarified thus far.Here, we report the first clear evidence of photocatalyzed splitting of methanol on TiO2 derived from time-dependent two-photon photoemission (TD-2PPE) results in combination with scanning tunneling microscopy (STM). Methanol adsorbed on the Ti4+ sites was dissociated through the cleavage of O-H bond, and the proton was transferred to the nearby bridge bonded oxygen(BBO). Isotope effect has been found in this photoinduced proton transfer process. TD-2PPE reveals that the kinetics of methanol photodissociation is clearly not of single exponential, an important characteristic of this intrinsically heterogeneous photoreaction. This photocatalytic process is also found to be excitation wavelength dependent.[1] T. Kawai and T. Sakata, J. Chem. Soc., Chem. Commun., 1980, 694.
FF8: In-situ and Ex-situ Studies for Electrocatalysts
Session Chairs
Wednesday PM, April 27, 2011
Room 3022 (Moscone West)
2:30 PM - **FF8.1
Application of Ambient Pressure XPS to Catalysis and Electrochemistry.
Hendrik Bluhm 1
1 Chemical Sciences Division, Lawrence Berkeley Lab, Berkeley, California, United States
Show AbstractSince the early work by Hans and Kai Siegbahn in the 1970ies, ambientpressure XPS has become an increasingly popular tool for theinvestigation of heterogeneous chemical processes at liquid/vapor andsolid/vapor interfaces. This talk will give a short introduction intothe basics of ambient pressure XPS and present examples of itsapplication from the fields of heterogeneous catalysis and electrochemistry.
3:00 PM - FF8.2
Structure Determination and Structure/Property Mapping of Nanoscale Platinum Electrocatalysts.
Amanda Barnard 1 , Lan-Yun Chang 2
1 Materials Science & Engineering, CSIRO, Clayton, Victoria, Australia, 2 Monash Centre for Electron Microscopy and School of Chemistry, Monash University, Clayton, Victoria, Australia
Show AbstractThe development of the next generation of nanosized heterogenous electrocatalysts requires precise control of the size and shape of individual particles, coupled with an accurate understanding of the structure of active sites. We present the highly accurate experimental characterization of the atomic structures of surface monoatomic steps on industrial platinum nanoparticles, and show that the edges of nanoparticles can significantly alter the atomic positions of monoatomic steps in the proximity, and must be included along with surface steps and kinks in any robust prediction of activity and performance. Based on these results, we further develop a structure/property map for the catalytic activity of platinum nanoparticles using a multi-scale thermodynamic model and parameters obtained using density functional theory. This map predicts the size and temperature dependence of the particle morphology, and shows how relatively modest annealing can increase the number of active sites and more than double the activity of particles at industrially relevant sizes.
3:15 PM - FF8.3
In-situ XAS Study of PtxPd1-x Nanoparticles Under H2 Reduction: Evidence of Charge Transfer Effects.
Jonder Morais 1 , Fabiano Bernardi 2 , Alex Kilian 1 , Jocenir Boita 1 , Adriana Rodrigues 1 , Agnes Traverse 3 , Maria Martins Alves 4
1 Physics, UFRGS, Porto Alegre, RS, Brazil, 2 ALS, LBNL, Berkeley, California, United States, 3 LCP, Univ. Paris-Sud, Orsay France, 4 Chemistry, UFRGS, Porto Alegre, RS, Brazil
Show AbstractNumerous efforts have been directed towards the search of new and more efficient catalysts for their use in the petroleum refinement processes. However, many catalysts are very susceptible to sulfur poisoning and their use is limited, unless sulfur tolerance can be greatly improved. X-ray absorption spectroscopy (XAS) measurements are an important tool to extract atom specific electronic and structural properties of catalysts, and may be performed under reaction conditions. In special for Pt-based materials, L3 and L2 near edge (XANES) measurements allow to quantitatively extract the fractional change in the number of d-band holes relative to a reference material. In previous investigations [1], non-supported PtxPd1-x (x = 1, 0.7 or 0.5) nanoparticles were studied during H2 reduction and sulfidation under H2S atmosphere. Our results revealed that the presence of sulfur atoms increases with the amount of palladium and the formation of a core-shell structure [2]. In this work, we have used in-situ XANES to investigate PtxPd1-x nanoparticles subjected to H2 reduction at 300oC, before and after the thermal treatment. The results show that the degree of sulfidation is proportional to the decrease of the fractional change in the number of d-band holes relative to the as-prepared sample after the reduction process.[1] F. Bernardi, G. H. Fecher, M. C. M. Alves and J. Morais, J. Phys. Chem. C 113(10), 3909 (2009).[2] F. Bernardi ,M. C. M. Alves,A. Traverse, D. O. Silva,C. W. Scheeren, J. Dupont and J. Morais, J. Phys. Chem. Letters 1, 912 (2010).
3:30 PM - FF8.4
In Situ DXAS Study of Pt Nanoparticles Formation by H2 Reduction of PtO2.
Jonder Morais 1 , Fabiano Bernardi 2 , Maria Martins Alves 3
1 Physics, UFRGS, Porto Alegre, RS, Brazil, 2 ALS, LBNL, Berkeley, California, United States, 3 Chemistry, UFRGS, Porto Alegre, RS, Brazil
Show AbstractThe reduction process is commonly used on metal based heterogeneous catalysts to bring them into an active form prior to a catalytic reaction. In order to improve the performance of the catalysts, useful chemical detailed information of this process is required through in situ measurements. X-ray absorption spectroscopy (XAS) measurements are an important tool to extract atom specific electronic and structural properties of catalysts, and may be performed under reaction conditions [1, 2]. In this work, in situ dispersive XAS (DXAS) measurements were used to monitor the Pt nanoparticles formation by hydrogen reduction of PtO2 at 150 oC. The kinetics of the reduction process, as well its dependence on the percentage of H2 in the reductive gas mixture, was investigated. The results reveal that a complete reduction of the PtO2 was obtained using a gas mixture with 55% of H2 in the time of 20 min, resulting in nanoparticles with an estimated mean diameter of 2.8 nm. In addition, the in situ DXAS investigation of the H2S sulfidation of the formed Pt nanoparticles at 150 oC revealed low degree of sulfidation, in opposition to the Pt-Pd bimetallic systems [1].[1] F. Bernardi, G. H. Fecher, M. C. M. Alves and J. Morais, J. Phys. Chem. C 113(10), 3909 (2009).[2] F. Bernardi ,M. C. M. Alves,A. Traverse, D. O. Silva,C. W. Scheeren, J. Dupont and J. Morais, J. Phys. Chem. Letters 1, 912 (2010).
3:45 PM - FF8: Electro
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