Jinghua Guo Lawrence Berkeley National Laboratory
Hendrik Bluhm Lawrence Berkeley National Laboratory
Michael Haevecker Fritz Haber Institute of the Max Planck Society
Shu Yamaguchi University of Tokyo
TT1: Nanoparticles and Catalysts
Tuesday AM, November 30, 2010
9:30 AM - **TT1.1
In-situ Scanning Transmission X-ray Microscopy of Catalytic Solids and Nanomaterials.
Frank de Groot 1 , Emiel de Smit 1 , Matti van Schooneveld 1 , Luis Aramburo 1 , Bert Weckhuysen 1 Show Abstract
1 Chemistry, Utrecht University, Utrecht Netherlands
The present status of in-situ Scanning Transmission X-ray Microscopy (STXM) is discussed, with an emphasis on the abilities of the STXM technique in comparison with Transmission Electron Microscopy (TEM) . Nanoscale chemical imaging of catalysts under working conditions is outlined using cobalt and iron [2,3] Fischer-Tropsch catalysts as an example. We show that STXM-XAS can image a catalytic system under relevant reaction conditions and provides detailed information on the morphology and composition of the catalyst material in situ. The nanometer resolution combined with powerful chemical speciation by XAS and the ability to image materials under reaction conditions opens up new opportunities to study many chemical processes. The spatial resolution can be as good as 0.2 nm for Electron Energy Loss Spectroscopy (EELS) in a TEM, which is a factor 100 better than the spatial resolution in STXM-XAS. An interesting option is the combination of ex-situ TEM and in-situ STXM-XAS measurements using the same nanoreactor and looking at the same parts of the material. We have performed such experiment in the study of the synthesis, reduction and catalysis of Cu/ZnO, a catalyst industrially used for the conversion of synthesis gas to methanol. For example, the reduction experiment shows a steady transition from Cu2+ and Cu+ to Cu0 in a spatially inhomogeneous manner, which reveals that the reduction from Cu2+ to Cu+ is fast compared to the further reduction to Cu0. STEM–EELS and STXM–XAS are transmission experiments and the required contrast for a core level signal implies that spectral shapes of dilute species can not be measured. The concentration required to measure reliable and quantitative spectra is strongly dependent on the cores state and on the sample composition. As a rule, one can state that concentrations above 5000 ppm are required for good spectra and above 1000 ppm for elemental maps. We critically compare STXM-XAS and STEM-EELS measurements and indicate some future directions of in-situ nanoscale imaging of catalytic solids and related nanomaterials .F. de Groot, E. de Smit, M. van Schooneveld, L. Aramburo, Bert Weckhuysen, ChemPhysChem, 2010, 11, 951 E. de Smit, I. Swart, J. F. Creemer, G. H. Hoveling, M. K. Gilles, T. Tyliszczak, P. J. Kooyman, H. W. Zandbergen, C. Morin, B. M. Weckhuysen, F. M. F. de Groot Nature. 2008, 456, 222-U239. E. de Smit, I. Swart, J. F. Creemer, C. Karunakaran, D. Bertwistle, H. W. Zandbergen, F. M. F. de Groot, B. M. Weckhuysen Angew. Chem.. 2009, 48, 3632-3636.F. de Groot, A. Kotani A, Core level spectroscopy of solids. (CRC Press 2008) M.M. van Schooneveld, A. Gloter, O. Stephan, L.F. Zagonel, R. Koole, A. Meijerink, W.J M. Mulder, F.M.F. de Groot, Nature Nanotechnology Online 6 June, DOI 10.1038/NNANO.2010.105 (2010)
10:00 AM - TT1.2
The Study of d-band Structure of the Gold Nano-clusters Supported on Amorphous Carbon.
Anton Visikovskiy 1 , Hisashi Matsumoto 1 , Kei Mitsuhara 1 , Tomoki Akita 2 , Yoshiaki Kido 1 Show Abstract
1 Department of Physics, Ritsumeikan University, Kusatsu, Shiga, Japan, 2 , National Institute of Advanced Industrial Science and Technology, Ikeda, Osaka, Japan
Gold (Au) is the most inert metal in periodic table. In the form of the small, nanometer-sized particles, however, Au exhibits high catalytic activity. Although, many efforts are applied in recent years to clarify the mechanism of emerging catalytic activity of Au nano-particles, the results are still contradictory. Several explanations have been proposed, including charge transfer between Au nano-particle and support material , and size-dependent changes in electronic structure of Au . In a present work we analyze the d-band structure and Au 4f core-level spectra of Au nano-clusters deposited on amorphous carbon depending on particle size.Thin layer of amorphous carbon (~10 nm) was deposited on NaCl, KCl and Si(111) substrates. Gold was thermally evaporated in ultra-high vacuum onto these substrates. The size and shape of the Au clusters have been determined by means of medium energy ion scattering spectroscopy combined with transmission electron microscopy. The valence band and Au 4f core-level spectra of the nano-clusters were measured by photoelectron spectroscopy using synchrotron-radiation-light. After subtraction of carbon background, parameters of Au d-band were measured (d-band width, value of spin-orbit splitting, and d-band center position).When the average number of Au atoms in cluster drops below ~200, the width and spin-orbit splitting of the d-band start to decrease rapidly, due to increased number of under-coordinated Au atoms. The position of the d-band center moves away from Fermi level with cluster size decreasing. The latter observation contradicts to the d-band model proposed by Hammer et al. , according to which the increase in catalytic activity of the d-metals is owing to the movement of the d-band towards the Fermi level expected for under-coordinated atoms. It has been shown in a number of studies that dynamic final-state effect in photoemission plays an important role in smearing and shifting the spectra to higher binding energies for metal nano-clusters. Indeed, we have observed higher binding energy shifts of the Fermi edge in the valence spectra and the Au 4f core-level with decreasing Au-cluster size. The movement of the d-band center depending on size may be partially attributed to this phenomenon. Exact estimate of the contribution from the final-state effect allows us to evaluate the d-band center of the Au nano-clusters.References: A.Sanchez, et.al., J. Phys. Chem. A 103 (1999) 9573 B. Hammer, J.K. Nørskov, Nature 376 (1995) 238
10:15 AM - TT1.3
Electronic Structure and Optical Activity of Original Titanium Oxide Nanoparticles Synthesized By Laser Pyrolysis.
Nathalie Herlin Boime 1 , Pardis Simon 1 , Bruno Pignon 1 , Baoji Miao 1 , Anne Marie Flank 3 , Sylvie Marguet 1 , Servane Coste Leconte 2 , Yann Leconte 1 , Cecile Reynaud 1 Show Abstract
1 Service des Photons, Atomes et Molécules, Laboratoire Francis Perrin , CEA, Gif/Yvette cedex France, 3 LUCIA Beam line, Synchrotron Soleil, BP48, 91192 Gif/Yvette cedex France, 2 INSTN-UESMS, CEA, Gif/Yvette cedex France
Over the past decades, titanium dioxide has received great interest thanks to its potential applications in various fields such as photocatalysis, photovoltaic and photoabsorption. In view of such applications, TiO2 is appreciated due to its high photocatalytic activity form towards harmful organic compounds. However, in its anatase form, it absorbs light mainly from the UV which is only a very small fraction of the energy received on earth. Several ways have been explored in order to extend the absorption range and the performances of TiO2. In particular, a shift of the bad gap toward the visible range would be interesting.In this context, using the laser pyrolysis set up in reducing conditions (high power + presence of a reducing agent), it was possible to synthesize original TiO nanoparticles exhibiting a cubic phase from XRD data. This sample shows peculiar optical properties, with a strong visible shift (more than 1.2 eV) in its bandgap. Moreover, from XRD data, the TiO cubic phase survives in air under annealing up to 400°C, with a progressive conversion to TiO2. However, from by X-ray absorption measurements (EXAFS and XANES) on Synchrotron Soleil (LUCIA beam light), the structure does not appear so simple. XANES indeed shows the presence of peaks specific to nanostructured particles. Even for the as formed nanoparticles, the main phase is attributed to Ti4+ (TiO2 environment) with a minor Ti2+ (TiO) component. Upon annealing at the lowest temperature, the Ti4+ becomes the only component. We will present here the correlation between the different characterization methods and evolution of optical properties. For first evaluation of activity under light irradiation, a common test of evolution of methylene blue (MB) signal (detected at 664 nm). Compared to P25, TiO appears more active when irradiated in the UV range (340 nm) and is still active in the visible till 530 nm while P25 does not absorb anymore in this range.
10:30 AM - TT1.4
In situ Real-time X-ray Diffraction Study of Metallic Nanocatalyst Growth and Phase Transformations.
Oana Malis 1 , D. Mott 2 , B. Wanjala 2 , J. Luo 2 , C. Zhong 2 Show Abstract
1 Physics Dept., Purdue University, West Lafayette, Indiana, United States, 2 Chemistry Dept., Binghamton University, Binghamton, New York, United States
Nanomaterials exhibit unique structural and morphological properties that often stand in stark contrast to the properties of their bulk counterparts. Using in situ, real-time, synchrotron-based x-ray diffraction (XRD) we investigate temperature-induced phase and morphological transformations in multi-metallic nanoparticles of interest for catalysis, in particular for fuel cell applications. Pt-containing binary and ternary nanoparticles have been demonstrated to have unprecedented catalytic activity for the oxygen reduction reaction. Au nanoparticles also exhibit catalytic activity for CO and methanol oxidation. Moreover, Au clusters have been shown to improve the stability of Pt electrocatalysts without loss of activity. It is therefore believed that Au-Pt nanoparticles can be tailored to exhibit synergistic catalytic activity and superior stability, qualities necessary to make them viable candidates for replacing pure Pt in fuel cell applications. In order to understand the fundamental processes involved in nanocatalyst synthesis, we have performed a detailed study of low-temperature heat treatment of Au, Pt, and Au65Pt35 nanoparticles. 1-3-nm nanoparticles encapsulated with alkanethiolate monolayer shells were synthesized in solution by a modified two-phase protocol and dispersed on silicon substrates. Synchrotron-based XRD indicates that the starting Au and AuPt nanoparticles have an amorphous structure, while the Pt nanoparticles have a crystalline structure. Heat treatment of nanoparticles at temperatures around 300°C is a typical processing step employed to activate the catalyst by removing the organic capping shells. By following the real-time evolution of the XRD pattern, we found that the nanoparticles undergo dramatic structural changes at temperatures as low as 120°C. During low-temperature annealing, the Au and AuPt nanoparticles first melt and then immediately coalesce into 4-5-nm crystalline structures. The Pt nanoparticles also aggregate and grow to similar sizes, but with limited intermediate melting. Solidification of high-density AuPt nanoparticles is followed by a transient structural transformation that affects only the surface of the particles. At low temperatures the Au-Pt nanoparticles are found to be nano-alloyed with an average lattice constant closer to that of Au, than to the lattice constant of a random alloy. In contrast to the phase segregation observed in bulk, and also previously reported by us on other substrates (alumina, carbon black), Au-Pt nanoparticles on silicon undergo only partial phase segregation upon extensive annealing above 700°C.
10:45 AM - TT1.5
Environment Controlled De-wetting Kinetics of Rh-Pd Bilayer/Alloy Thin Films on Silica: A Physical Approach to Synthesize Core-shell Nanoparticles.
Gintautas Abrasonis 1 , Sebastian Wintz 1 , Maciej Liedke 1 , Funda Aksoy 2 , Zhi Liu 2 , Karsten Kuepper 3 , Matthias Krause 1 Show Abstract
1 Institute of Ion Beam Physics and Materials Research, Forschungszentrum Dresden-Rossendorf, Dresden Germany, 2 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Institut für Festkörperphysik, Universität Ulm, Ulm Germany
The control of morphology and surface composition of nanoalloys is the key factor in order to tune or to extend the range of their optical, magnetic and chemical properties. Therefore it is one of the major tasks in nanoalloy materials science. As the chemical environment has a profound influence on the structure of supported metallic nanoparticles, it can be used as a powerful tool to tune their structure and properties. This study concerns the in-situ investigation of the oxidizing/reducing environment influence on the de-wetting dynamics and kinetics of a Rh-Pd bilayer/alloy thin film model system. Thin films of ~3nm thickness were grown at RT by molecular beam epitaxy electron beam evaporation on thermally oxidized silicon substrates. These films were subsequently subjected to different combinations of heating and chemical environment (CO and NO) treatments. The film surface composition and the chemical state was determined in-situ by ambient pressure environmental x-ray photoelectron spectroscopy. Independently on whether the initial state is an alloy or a bilayer with Rh on the top, the film surface shows an enrichment of Pd upon heating in vacuum. Exposure to NO or CO at ~250-300°C results in the surface enrichment with either Rh or Pd, respectively, and subsequent film rupture. The metallic islands, produced by the film de-wetting, also show a similar response to the environment changes. On the other hand, de-wetting caused by heating in NO or CO shows significant differences in the surface chemical composition evolution and, consequently, in the de-wetting onset temperature.The results are discussed on the basis of the interplay between thermodynamic and kinetic factors, e.g. the energetics of the metallic surfaces in response to the NO/CO adsorbates, the atomic mobility of metallic and oxide surfaces, and the role of mobile Pd/Rh carbonyl or nitrosyl species. The study demonstrates the effect of the chemical environment on the morphology as well as on the composition of supported nanostructures. The nanoalloy/ bilayer approach presented here opens a new route for the fabrication of supported arrays of core-shell nanoparticles.
TT2: Fuel Cell
Tuesday AM, November 30, 2010
11:30 AM - **TT2.1
In-situ Soft X-ray Characterization of Electrochemical Interfacial Processes in Energy Storage/Conversion Devices.
Farid El Gabaly 1 Show Abstract
1 , Sandia National Labs, Berkeley, California, United States
Electrochemical technologies offer very efficient (40%-95%) routes to convert and store energy while not introducing carbon-containing species into the atmosphere. Thus, it is widely anticipated that electrochemical technologies will be increasingly used to provide energy that does not contribute to climate change, i.e., carbon-neutral storage and conversion. Batteries, ultra-capacitors, fuel cells and electrolyzers are the most important electrochemical devices used to inter-convert electrical and chemical energy for both transportation and stationary applications.Perhaps the most important phenomenon to understand in electrochemical energy storage/conversion is how electric charge is transferred across interfaces. Ideally, one wants to know: 1) what chemical reactions transfer charge, 2) which reaction limits the rate and 3) where the reactions occur in the heterogeneous devices. To help answer these questions we have spectroscopically characterized electrochemical charge-transfer as it occurs. We have done this primarily using new diagnostics based on X-ray spectroscopies that we have been developing through a Sandia LDRD in collaboration with the Advanced Light Source (ALS, LBL). In-situ and in-operando characterization of solid-oxide fuel cells and Ni-based batteries gave us fundamental information about how charge-transfer occurs.Using element-specific XPS peaks and a new 1D spatial imaging capability at ALS, the potential over the entire device is mapped. Analyzing how the overpotentials depend on cell current allowed us to understand the charge-transfer and rate-limiting reaction steps for H2 oxidation in a solid-oxide fuel cell.Charge transfer in batteries and ultracapacitors leads to phase changes in materials. Our approach has the ability to identify the in-situ electrochemically formed phases that store charge –which are only stable under electrical bias– to gain new understanding of electrochemically driven phase changes. We have study the Ni metal to Ni-hydroxide phase transformation in a Ni-based battery configuration.
12:00 PM - **TT2.2
Rate Determining Step of SOFC Cathode Reaction Studied by in situ Electrochemical X-ray Absorption Spectroscopy.
Yoshiharu Uchimoto 1 , Koji Amezawa 2 , Yuki Orikasa 1 , Tatsuya Kawada 2 Show Abstract
1 Graduate School of Human and Environmental Studies, Kyoto University, Kyoto, Kyoto, Japan, 2 Graduate School of Environmental Studies, Tohoku University, Sendai, Miyagi, Japan
In order to understand electrode reaction mechanism for solid oxide fuel cell cathodes, AC impedance and DC polarization measurement usually have been used. In these methods, the mechanism is presumed from the relationship between the energy and the rate. It is difficult to grasp the physical / chemical states of the electrode and electrolyte under operating conditions of devices, which give us important and directly information for understanding the reaction mechanism. Cathode reaction contains many elementary steps such as surface reaction, bulk diffusion and boundary reaction. In this work, we applied in situ X-ray absorption spectroscopy technique to investigate oxygen chemical potentials for La(Sr)Co(Fe)O3 and/or La(Sr)MnO3 perovskite electrode, which show good catalytic activity for high temperature cathodes. The oxygen chemical potential profile around the electrode was evaluated by means of in situ electrochemical XAS. Furthermore, we applied in-situ depth resolved X-ray absorption spectroscopy as the novel technique to clarify the reaction mechanism at interface between electrode / electrolyte.
12:30 PM - TT2.3
Synchrotron X-ray Study of Strontium-doped Lanthanum Manganite (LSM) Solid Oxide Fuel Cell Cathode under Operating Temperature and Pressure.
Jacob Davis 1 , Lincoln Miara 1 , Soumendra Basu 1 , Srikanth Gopalan 1 , Uday Pal 1 , Karl Ludwig 1 , Laxmikant Saraf 2 , Tiffany Kaspar 2 Show Abstract
1 Division of Materials Science and Engineering, Boston University, Boston, Massachusetts, United States, 2 Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington, United States
Total Reflection X-ray Fluorescence (TXRF) and Extended X-ray Absorption Fine Structure (EXAFS) spectrum studies of heteroepitaxial thin-film solid oxide fuel cell (SOFC) cathode material were performed at the National Synchrotron Light Source of Brookhaven National Laboratory. A chamber was developed enabling samples to be heated to 800°C in a variable pressure environment. Samples studied here are 20% strontium doped lanthanum manganite (LSM) deposited by pulsed laser deposition (PLD) on single crystal lanthanum aluminate (LAO) and single crystal yttria-stabalized zirconia (YSZ) substrates. Depositions were characterized using Atomic Force Microscopy (AFM) and transmission electron microscopy (TEM). TXRF and EXAFS data were taken as a function of incoming angle to probe composition ratios as a function of depth. We see evidence of significant strontium enrichment on the surface upon annealing, where the surface enrichment is “frozen in” as samples are cooled in the time scales studied.
TT3: Water Splitting
Tuesday PM, November 30, 2010
2:30 PM - **TT3.1
In-situ Soft X-ray Spectroscopy of Materials for Electrochemical Water Splitting.
Lothar Weinhardt 1 Show Abstract
1 Experimentelle Physik VII, University of Würzburg, Würzburg Germany
In the recent years, soft x-ray photon-in-photon-out techniques, namely x-ray absorption spectroscopy (XAS) and soft x-ray emission spectroscopy (XES), have been successfully used for the study of liquids. These experiments require a third-generation synchrotron light source as well as the development of sophisticated experimental setups. While most existing experiments have been of fundamental nature, several groups are now taking the step towards the in-situ investigation of applied systems. This new approach promises to give a detailed insight into the electronic and chemical structure of the relevant materials and how their properties are influenced by the gas or liquid environment they have to face in the device in particular with respect to surface chemistry/catalysis. In this contribution, a synchrotron endstation dedicated to the investigation of gases, liquids, soilds, and their interfaces (gas/solid andliquid/solid) by using soft x-rays will be presented. In addition, selected experimental results with relevance for electrochemical hydrogen production will be discussed. It will be shown how powerful this experimental approach can be to understand the device under investigation, in particular in combination with complementary vacuum-based electron-spectroscopic measurements of the electronic level position at the surface of the device.
3:00 PM - TT3.2
In situ Soft X-ray Spectroscopy Investigation of Electrochemical Reaction.
Peng Jiang 1 , Jeng-Lung Chen 1 2 , Ferenc Borondics 1 , Per-Anders Glans 1 , Mark West 1 , Ching-Lin Chang 2 , Miquel Salmeron 1 , Jinghua Guo 1 Show Abstract
1 , LBNL, Berkeley, California, United States, 2 Department of Physics, Tamkang University, Tamsui Taiwan
A novel electrochemical setup has been developed for soft x-ray spectroscopy studies of the electronic structure of electrode materials during electrochemical cycling. In this presentation we illustrate the operation of the cell with a study of the corrosion behavior of metallic films in aqueous electrolyte solution via the electrochemically induced changes of their electronic structure. This development opens the way for in situ investigations of electrochemical processes, photovoltaics, batteries, fuel cells, water splitting, corrosion, electrodeposition, and a variety of important biological processes.
3:15 PM - **TT3.3
Structural and Electronic Characterization of High-performance Hematite (alpha-Fe2O3) for Solar Water Splitting.
Kevin Sivula 1 , Michael Gratzel 1 Show Abstract
1 , Ecole Polytechnique Federale de Lausanne, Lausanne Switzerland
Hematite is a promising material for solar energy conversion due to its stability, abundance, and relatively low band gap (2.1 eV). Recently we have demonstrated major advances in the performance of hematite electrodes using nanostructuring techniques including atmospheric pressures chemical vapor deposition and solution-based colloidal approaches. However, how these preparation methods affect the structural and electronic properties is not fully understood. Here we report the latest advances on the doping, morphology control, and performance of hematite electrodes for solar water oxidation. In addition, we specifically describe how x-ray diffraction spectroscopy and NEXAFS (Near Edge X-ray Absorption Fine Structure) spectroscopy have led to new insights into the important parameters that control light absorption, charge transport, and photo-activity in this promising material.
3:45 PM - TT3.4
Ultra Small Angle X-ray Scattering Studies to Devise Nanostructuring Strategies for Hydrogen Storage Materials.
Shathabish NaraseGowda 1 , Jan Ilavsky 2 , Tabbetha Dobbins 1 Show Abstract
1 Institute for Micromanufacturing, Louisiana Tech University, Ruston, Louisiana, United States, 2 Advanced Photon Source, Argonne National Laboratory, Chicago, Illinois, United States
Complex metal hydrides such as NaAlH4 and LiBH4 are some promising materials proposed for hydrogen storage. However, these materials have poor desorption kinetics at practical temperatures of operation and nanostructuring has evolved as a viable approach to tackle this challenge. This work focuses on revisiting the advantages of using nano-scaffolds, with the goal of developing a set of morphological considerations to substantiate various strategies to nanostructuring (such as choice of pore sizes) and approaches to identify confined material from unconfined material. In this work, NaAlH4 is encapsulated in porous alumina and studied using ultra small angle x-ray scattering (USAXS). USAXS technique yields information on the morphology (i.e., specific surface area and radius of colloidal particles) of the material on a range of length scales from 100nm to 1μm. Morphological changes occurring during desorption were tracked using USAXS. A two fold increase in particle size was observed after desorption which suggests a significant loss in surface area. This effect could reduce the uptake capacity in subsequent cycles. USAXS has also been used to differentiate confined and unconfined NaAlH4 by virtue of shape. The Porod regime of the scattering curve reveals the presence of elongated rod-like particles and flat disc-like particles. These shapes can be interpreted as confined material acquiring the rod-like shape of the pore well and unconfined material resting on the surface in disc-like morphologies. USAXS analysis on bulk powders milled for different durations showed that the 5min milled powders yielded more surface area than the 15 min milled powders. This finding has been used to decide on the effective pore size needed for nano-scaffolding. The radius of particles in the 5min milled powders is around 8.4nm, and it is proposed that 8-10nm sized pores would be most effective in retaining the high surface area through the cycling process.
TT4: Heterogeneous Catalysts I
Tuesday PM, November 30, 2010
4:30 PM - **TT4.1
Revealing Dynamical and Chemical Processes in X-ray Absorption Spectra of Molecules and Condensed Phases from First-principles.
David Prendergast 1 Show Abstract
1 Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California, United States
There are clear advantages to using element-specific probes, such as x-ray absorption spectroscopy (XAS), to characterize the evolution of working electrochemical, photochemical and catalytic devices. However, clear interpretations of XAS presuppose the existence of candidate structural models with known spectral fingerprints and, typically, dynamical processes are not considered in the analysis. In our work, we have found that explicit inclusion of dynamical effects (vibrations, conformational variation, or chemical changes) via molecular dynamics sampling, can lead to predictive approaches to simulating XAS [1-3]. More recently, we have been using first-principles molecular dynamics to sample spectral variations in energy-related materials such as Li-ion battery electrodes. We have also explored new chemistry induced by x-ray probes themselves, which can make for challenging measurements but also may reveal new chemical pathways for radiation-induced catalysis. J. S. Uejio et al., Chemical Physics Letters 467, 195 (2008). C. P. Schwartz et al., Journal of Chemical Physics 131, 114509 (2009). J. S. Uejio et al., Journal of Physical Chemistry B 114, 4702 (2010).
5:00 PM - TT4.2
In-Situ EXAFS and DFT Studies of a Model Heterogeneous Catalyst: WOx/α-Fe2O3.
Martin McBriarty 1 2 , Zhenxing Feng 1 2 , Joseph Libera 3 , Donald Ellis 2 4 5 , Michael Bedzyk 1 2 3 Show Abstract
1 Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States, 2 Institute for Catalysis in Energy Processes, Northwestern University, Evanston, Illinois, United States, 3 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States, 4 Department of Physics, Northwestern University, Evanston, Illinois, United States, 5 Department of Chemistry, Northwestern University, Evanston, Illinois, United States
For the rational design and engineering of catalysts, it is fundamental to understand the morphology of the active surface under reaction conditions. In this study, surface-sensitive experimental techniques are combined with first-principles calculations to elucidate the surface structure and chemistry of a model oxide-supported heterogeneous catalyst, WOx on hematite (α-Fe2O3). WOx is deposited by atomic layer deposition (ALD) onto hematite single crystals and nanopowders. Extended X-ray absorption fine structure (EXAFS) is employed to investigate the surface structures of WOx/α-Fe2O3 single crystals and powders in-situ under oxidizing and reducing conditions. This method reveals W coordination and chemical state as well as W-O and W-Fe bond lengths. Additionally, in-situ synchrotron X-ray fluorescence (XRF) with X-ray standing wave (XSW) imaging pinpoints surface W positions relative to bulk Fe cation lattice sites on single crystal surfaces, revealing redox-reversible migration of surface W. EXAFS and XSW results are compared with structures predicted by density functional theory (DFT) calculations of various surface configurations. Calculated atomic partial densities of states, volume-integrated charges, and bond lengths provide insight on the chemical state and activity of W. The surface chemistry model is further refined by comparing core-hole embedded cluster DFT results with X-ray photoelectron spectra. This study demonstrates that pairing high-throughput theoretical calculations with synchrotron X-ray methods results in detailed structural and chemical models of complex surface structures.
5:15 PM - **TT4.3
CO Adsorption on PtRu/Ru(0001) Near Surface Alloys Using Ambient Pressure Photoelectron Spectroscopy.
David Starr 1 , Hendrik Bluhm 2 Show Abstract
1 Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York, United States, 2 Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
We have investigated the adsorption of CO at 300 K on Ru(0001) and PtRu/Ru(0001) near surface alloys with Pt concentrations of 0.36 ML, 0.73 ML and 0.94 ML from ultra-high vacuum up to CO pressures of 0.04 Torr using ambient pressure X-ray photoelectron spectroscopy (AP-XPS). We have used the attenuation of the Ru 3d5/2 surface core level shifted peak to calibrate the areas of the O 1s from adsorbed CO to provide coverage estimates of CO on Ru a-top, Ru bridge, Pt a-top, and Pt bridge sites. We observe a smaller amount of CO adsorbed on Pt sites in the alloy surfaces than what has been observed on Pt(111) at all pressures to 0.04 Torr. Further the fraction of Pt sites covered by CO on the 0.36 ML Pt and 0.73 ML Pt NSA surfaces are both similar but lower than on the 0.94 ML Pt NSA. This supports the concept in a decrease in the adsorption energy of CO on Pt sites in alloy surfaces compared to when it is pure. Comparison of the Pt 4f7/2 binding energies for the Pt in the NSA surfaces shows that the fraction of Pt covered by CO at steady-state can be correlated with the binding energy of the Pt 4f7/2 core level; the fraction of Pt covered by CO decreases with increasing Pt 4f7/2 binding energy. This may provide a simple analytical test for CO tolerance in PtRu alloy catalysts used in polymer electrolyte fuel cells.
5:45 PM - TT4.4
Differences on Stability of Cu-based Industrial Catalysts for the Water-gas shift Reaction Probed by in-situ Time-resolved Synchrotron Techniques.
Daniela Zanchet 1 , Cristiane Rodella 1 , Daniela Oliveira 1 , Marco Logli 2 , Valeria Vicentini 2 Show Abstract
1 , LNLS, Campinas Brazil, 2 , OXITENO Ind. e Com. S/A – GEDEC , Mauá Brazil
The water gas shift (WGS) reaction, forming H2 and CO2 from CO and steam is an important step in several industrial processes; it maximizes the H2 production and removes the residual CO that acts as poison for fuel cells. In industry, the WGS is carried out in two stages, known as HTS (High Temperature Shift) reaction (350-450oC) and LTS (Low Temperature Shift) reaction (200-250 oC). The most common industrial LTS catalyst is CuO/ZnO, the same catalyst used for methanol synthesis. Despite being largely studied, the variety of composition and formulation of Cu-based catalysts makes difficult the correlation of several data found in the literature with the industrial process. In this work, the main target was to compare two generations of catalysts (LTS1 and LTS2) produced by an industrial partner, with similar chemical composition (CuO/ZnO/Al2O3) but showing different stability on stream. For that, a detailed analysis of the activation process, where copper is reduced, was performed. We address the activation process by in situ time-resolved X-ray diffraction (TR-XRD) and X Ray Absorption Spectroscopy (TR-XAFS). Complementary ex-situ data were obtained by transmission electron microscopy (TEM) and X Ray Photoelectron Spectroscopy (XPS). For the XPS measurements, the catalysts were treated in a furnace attached to the main chamber and the analysis were carried out without sample exposition to air. The activation of the catalysts was performed in situ under H2 flow under isothermal conditions, at 230oC, and under temperature ramp up to 500 oC. We show that initially both samples present a disordered oxide structure that evolves with the activation. Crystallization of ZnO nanometric phase (~5 nm) takes place while part of CuO (Cu2+) is reduced to Cu2O (Cu1+) and then to Cu0 also forming nanometric domains. For LTS1, however, the Cu0 crystalline domains are larger (16 nm compared to 10 nm for LTS1 one) while the ZnO phase is similar. XAFS analyses suggest that a disorder Cu0 phase may coexist for both samples. More interesting, the segregation and formation of Cu0 phase was slower for LTS2 indicating a stronger interaction with the ZnO/Al2O3 phase. XPS data corroborates these results indicating that Cu species first migrate to the catalyst surface and then grow forming the metallic phase; this process was slower for LTS2 sample. A stronger interaction with the ZnO/Al2O3 phase, which slowdown the formation of metallic Cu species on LTS2 catalyst during the activation, may explain its better stability on stream.
TT5: Poster Session
Tuesday PM, November 30, 2010
Exhibition Hall D (Hynes)
9:00 PM - TT5.10
Probing Structure and Diffusion in Semiconducting Polymer Blends with Synchrotron Radiation.
Paul Dastoor 1 Show Abstract
1 Centre for Organic Electronics, University of Newcastle, Callaghan, New South Wales, Australia
Near-edge X-ray absorption fine structure spectroscopy (NEXAFS) and, its 2D mapping counterpart, scanning transmission X-ray microscopy (STXM) are extremely powerful techniques for probing the structure, alignment and morphology of thin organic films on the nanometer scale. Previous work by us has shown how these techniques can be applied to reveal insights into the morphology and property relationships for the typical semi-conducting polymer blends commonly used in organic electronic applications and, in addition, STXM’s unique chemical sensitivity means that it is ideally suited to probing the lateral composition of a range of semiconducting polymer blends [1-3]. More recently, vertical phase segregation and structure within OPV devices has been recognised to be at least as important as the lateral phase segregation, which has received much more attention to date. In this paper,we show that variation of the post-annealing cooling rate can be used to create a series of “snapshots” of the vertical and interfacial reorganization processes that occur upon annealing. The data show that slower cooling rates result in significantly enhanced device efficiencies and synchrotron radiation is used to probe the origin of these improvements. Our results show evidence for a distinct and changing vertical stratification and interfacial structure in the device throughout the annealing process, with both composition and crystallinity varying through the active layer. The implications of these changes are discussed in terms of device properties.References1. K.B. Burke, W.J. Belcher, L.Thomsen, B. Watts, C.R. McNeill, H. Ade, P.C. Dastoor, Macromolecules, 42(8), 3098-3103, (2009).2. C.R. McNeill, B. Watts, L. Thomsen, W.J. Belcher, N.C. Greenham, P.C. Dastoor, H. Ade, Macromolecules, 42(9), 3347-3352, (2009).3. B. Watts, W.J. Belcher, L. Thomsen, H. Ade, P.C. Dastoor, Macromolecules, 42 (21),8392–8397, (2009).
9:00 PM - TT5.11
Microstructural Analysis of Hydrogen Absorption in 2NaH+MgB2 System.
Christopher Nwakwuo 1 , Claudio Pistidda 2 , Martin Dornheim 2 , John Hutchison 1 , John Sykes 1 Show Abstract
1 Materials, Oxford University, Oxford United Kingdom, 2 Materials Technology, GKSS Research Centre, Geesthacht Germany
Transmission electron microscopy and In situ synchrotron X-ray diffraction has been used to characterize the hydrogenation mechanism of ball milled 2NaH+MgB2 system. Absorption was performed in 50 bar hydrogen pressure and heated from room temperature to 400 oC at 3 oC/minute. An unknown hydride phase is observed at about 280 oC. This phase remains stable up to 320 oC and subsequently disappears, followed by the formation of perovskite-type NaMgH3 at about 330 oC. At 380 oC, crystals of NaBH4 begin to appear up to 400 oC. The observed formation and evolution of these new and intermediate phases is crucial in understanding the rate limiting steps of hydrogen sorption reaction in this complex hydride system.
9:00 PM - TT5.2
Noble Metal Nanoparticle-loaded Mesoporous Metal Oxide Microspheres for Catalytic Applications.
Zhao Jin 1 , Jianfang Wang 1 Show Abstract
1 , The Chinese Universtiy of Hong Kong, Hong Kong China
Noble metal nanoparticles have attracted much attention as catalysts due to their unique characteristics, which are not pocessed by the bulk counterparts. However, because of the high surface energies, nanoparticles often endure size and shape changes during catalytic reactions, which lead to a loss of their catalytic activity. A facile method to prepare noble metal nanoparticle catalysts with high efficiency and stability is highly demanded.Here we use ultrasonic spray pyrolysis to produce hybrid microspheres that are composed of noble metal nanoparticles embedded in mesoporous metal oxide matrices. The mesoporous metal oxide structure allows for the fast diffusion of reactants and products as well as confining noble metal nanoparticles. Three types of noble metals (Au, Pt, Pd) and three types of metal oxide substrates (TiO2, ZrO2, Al2O3) are selected for demonstration. By trying every possible combination of the noble metals and substrates, nine types of catalysts are produced. We characterized the structures of these catalysts and their catalytic performance through a representative reduction reaction from nitrophenol to aminophenol. By comparing the catalytic results, the effects of the different noble metals and substrates on the catalytic abilities are ascertained. For this particular reaction, we find that Pd nanoparticles supported on mesoporous TiO2 gives the best catalytic performance. In addition, the effects of the noble metal concentrations are also investigated. Our low-cost and high-productivity synthesis method can be extended to prepare other catalysts with different metals and oxide substrates, which will find use in industrial applications and provide a platform for the studies of the synergetic catalytic effect between different oxide substrates and metals.
9:00 PM - TT5.3
A Reaction Path for CO Oxidation on TiO2(110) and Au/TiO2(110).
Kei Mitsuhara 1 , Hideki Okumura 1 , Anton Visikovskiy 1 , Yoshiaki Kido 1 Show Abstract
1 Physics, Ritsumeikan University, Kusatsu Japan
It is well known that Au is the noblest of all the metals. However, Haruta found that Au na-no-clusters dispersed on oxide supports become active pronouncedly for oxidation of CO even at low temperatures, if the size ranges below 5 nm. So far, several models have been proposed to ex-plain the emerging catalytic activities for Au nano-clusters on oxide supports, in particular for Au on TiO2(110). In spite of many efforts, however, the mechanism leading to strong catalytic activities of Au nano-clusters on TiO2(110) is still a debatable issue. In this study, we have found a reaction path for oxidation of CO on pseu-do-stoichiometric(S*)-TiO2(110) surface which was prepared by exposing a reduced surface to O2 at room temperature (RT). High-resolution medium energy ion scattering (MEIS) coupled with pho-toelectron spectroscopy reveals that a CO molecule reacts with a bridging O to form CO2, via being mediated by underlying Ti interstitials. Indeed, the Ti 3d defect state intensity increases with CO exposure and clearly correlates with the increased intensity of OH 3σ, which was unintentionally formed by adsorption of H2O molecules in the created bridging O vacancies even under ultrahigh vacuum condition (base pressure: Torr). The CO oxidation at the bridging O site was analyzed quantitatively by high resolution MEIS, which showed presence of 18O about atoms/cm2 for the S*-TiO2(110) after exposure to CO and then 18O2 at RT. The activation barrier for CO oxidation at the bridging O site is estimated to be ~0.22 eV. We also found that Au nano-clusters on S*-TiO2(110) enhances remarkably the CO oxidation rate by a factor more than 2, probably due to the role of Au clusters prolonging the duration time of CO near the periphery. M.Haruta, T.Kobayashi, H.Sano, N.Ymada, Chem.Lett.2,405(1987)
9:00 PM - TT5.4
Ultraviolet Photoelectron Spectroscopy of Nb-doped TiO2 Layers for Dye-sensitized Solar Cells with Improved Performance.
Tsvetkov Nikolay 1 , Liudmila Larina 1 2 , Oleg Shevaleevskiy 3 , Byung Tae Ahn 1 Show Abstract
1 National Research Center for Photovoltaic Materials, Department of Materials Science & Engineering, Korea Advanced Institute of Science and Technology, Daejon Korea (the Republic of), 2 Center for NanoInterface Technology, Department of Materials Science & Engineering, Korea Advanced Institute of Science and Technology, Daejon Korea (the Republic of), 3 Solar Energy Conversion Laboratory, Institute of Biochemical Physics RAS, Moscow Russian Federation
The improvement of dye-sensitized solar cell (DSSC) performance can be gained by optimizing of the structural and electronic properties of nanocrystalline TiO2 working electrodes by doping. Previously we investigated the behavior of DSSCs based on TiO2 single crystals doped with niobium (Nb) and have shown the correlation between Nb-doping level and the cell performance [1, 2]. In a present study we prepared nanocrystalline TiO2 layers doped with Nb in the range of 0.5 to 3.0 mol%. These layers were applied as working electrodes for fabrication of DSSCs. Ultraviolet photoelectron spectroscopy (UPS) provides important information on the surface electronic properties of material. By mean of UPS we have determined directly the valence band maximum positions of the TiO2 layers with different Nb content. The conduction band minimum (CBM) positions were obtained by combination of the results of UPS measurements with the optical absorption spectroscopy data. The solar cell parameters were determined from the current density-voltage measurements of DSSCs based on undoped and doped TiO2 working electrodes. We have found that the decrease in energy of the CBM position of TiO2 initiates the decrease of the open circuit voltage in DSSC due to decrease in the band bending at the TiO2-electrolyte interface. At the same time, we observed an ongoing rise in the short-circuit current (ISC) with the increase of the Nb concentration. The raise of the ISC can be ascribed to the enhanced electron injection from dye molecules to TiO2 and to more effective charge transport throw the TiO2 layer. Our investigations provided us with an optima value of Nb doping concentration that leads to the highest value of the energy conversion efficiency in Nb-doped DSSCs.References:  O.I. Shevaleevskii, V.P. Poponin, L.L. Larina, Mater. Sc. Forum 173-174 (1995) 117.  O.I. Shevaleevskii, L.L. Larina, E.M. Trukhan, Solid St. Phenomena 51-52 (1996) 547.
9:00 PM - TT5.5
Milli Fluidic Reactor Systems for Gold Nano Cluster Synthesis– Bridge Between Bulk and Micro Fluidic Systems.
Vanga Reddy 1 , Dawit Yemani 1 , Jost Goettert 1 , Yuehao Li 2 , Nanda Kumar Krishnaswamy 2 , Ashwin Sanampudi 1 2 , Challa Kumar 1 Show Abstract
1 Center for Advanced Microstructures and Devices, Louisiana State University, Baton Rouge, Louisiana, United States, 2 Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana, United States
The wet-chemical synthesis of Au clusters can be classified into either flask based or micro fluidic based processes. In the case of micro fluidics one can see several advantages for synthesis of Au clusters, as compared to flask based reactions, simply due to confined space available for cluster formation as well as controllability of flow rates and mixing processes. However, there is an intermediate stage, between micro fluidic and flask based synthesis, that can be termed as a milli fluidic reactor system that could offer unique opportunities and interesting challenges. Surprisingly, the milli fluidic systems, so far, have not been exploited for small metal cluster synthesis in general and for Au cluster synthesis in particular. Herein we present a case for milli fluidic systems for synthesis of metal nano clusters. One of the potential advantages with a milli fluidic reactor is that the fluid volume is many folds higher than that of a micro reactor while offering similar controllable flow rates and the ability to manipulate reagents to control the reaction as opposed to flask based methods. In addition, the milli fluidic reactors provide a better opportunity for insitu characterization of nanoclusters with various spectroscopy tools because of larger channel size. This is especially valuable when carrying out time-resolved insitu analysis using synchrotron radiation-based X-ray absorption spectroscopy where larger probe dimensions will provide better signal to noise ratio. Therefore, for the first time we investigated the utility of a millifluidic reactor, with a snake mixer, having channel dimensions 2mm width and 125 micrometer depth for synthesis of Au nanoclusters and compared the results with those obtained from micro fluidic and flask reactors. For Au nanoclusters synthesis, Au salt solution and DMSA (meso-2,3-dimercaptosuccinic acid) solutions were mixed in the channels. The DMSA acts as a reducing agent as well as capping ligand.Surface Plasmon absorption band around 530 nm was observed in our case for flow rate of 1 ml/hr, which is similar to that obtained with Au nanoparticles prepared using micro fluidics. For higher flow rate (5- 3ml /hr) the absence of surface Plasmon but presence of a fine structure in the UV-VIS spectrum indicated small Au clusters rather than gold naoparticles. Similar to UV-VIS data, particle size analysis of Au clusters obtained under the three different flow rates showed similarities with those obtained from a micro fluidic reactor. In addition to the similarities in the characteristics of Au clusters synthesized from micro and milli fluidic reactor, our computational simulations provided a strong support for the utilization of milli fluidic systems for the synthesis of nano clusters.
9:00 PM - TT5.9
Electronic Properties at C60/Copper Phthalocyanine/MoO3 Interfaces.
Sang Wan Cho 1 , L. Piper 1 , A. DeMasi 1 , A. Preston 1 , K. Smith 1 , K. Chauhan 2 , R. Hatton 2 , T. Jones 2 Show Abstract
1 Physics, Boston University, Boston, Massachusetts, United States, 2 Chemistry, University of Warwick, Coventry United Kingdom
The interfacial electronic structure of C60/Copper Phthalocyanine (CuPc)/molybdenum trioxide (MoO3) thin films grown in-situ on indium tin oxide (ITO) substrates has been studied using synchrotron radiation-excited photoelectron spectroscopy in an attempt to understand the influence of oxide interlayers on the performance of small molecule organic photovoltaic devices. The MoO3 layer on ITO is found to significantly increase the work function of the substrate, and induces large interface dipoles and band bending at the CuPc/MoO3 interface. The large band bending confirms the formation of an internal potential that assists hole extraction from the CuPc layer to the electrode. The electronic structure of the MoO3 layer on ITO was also examined using various soft x-ray spectroscopies in order to probe the conductive nature of the MoO3 thin film.This work was supported in part by the NSF under Grant No. CHE-0807368. The NSLS is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. Financial support from the EPSRC, UK is also acknowledged through the SUPERGEN Excitonic Solar Cell Consortium programme.
Jinghua Guo Lawrence Berkeley National Laboratory
Hendrik Bluhm Lawrence Berkeley National Laboratory
Michael Haevecker Fritz Haber Institute of the Max Planck Society
Shu Yamaguchi University of Tokyo
Wednesday AM, December 01, 2010
Fairfax B (Sheraton)
9:30 AM - **TT6.1
Electronic Structures of non-Pt Carbon Alloy Catalysts for Polymer Electrolyte Fuel Cells Revealed by Synchrotron Radiation Analysis.
Masaharu Oshima 1 Show Abstract
1 Applied Chemistry, The University of Tokyo, Tokyo Japan
Among various attempts to develop less energy consumption devices, electricity generation devices such as solar cells and fuel cells, energy storage devices for low carbon society, we have been concentrating on developing highly efficient Polymer Electrolyte Fuel Cells (PEFC) without using noble metals which are rare, expensive and degrading membrane and electrode supports. So far, several carbon-based materials such as carbon alloy catalysts showing high oxygen reduction reaction (ORR) activities have been reported [1,2] as cathode catalysts alternative to Pt for PEFC. In this study, we have analyzed electronic structures of carbon alloy catalysts using synchrotron radiation in order to elucidate the ORR mechanism in order to further enhance the ORR activities. We have prepared metal phthalocyanine-based carbon alloy catalysts with 1-2% nitrogen and less than 0.1 % of Fe for PEFC, and have investigated the oxygen reduction reaction mechanism using synchrotron radiation analysis. Photoelectron spectroscopy, X-ray absorption spectroscopy and first principles calculation revealed that graphite-like nitrogen and the neighboring carbons at a zigzag edge of graphite are ORR active sites [3, 4], and that zigzag edges of graphene have well evolved for catalysts synthesized from iron phthalocyanine / phenolic resin pyrolyzed at 600 °C showing the maximum ORR activity . Based on these analyses, we have succeeded in fabricating MEA stack for PEFC which showed almost the same performance as Pt catalysts. Furthermore, we have constructed a new in situ soft X-ray emission spectroscopy system for fuel cell cathode catalysts during operation at the University-of-Tokyo beamline BL07LSU in SPring-8. This work has been done with the financial support from New Energy and Industrial Technology Development Organization, Japan, in collaboration with S. Miyata, J. Ozaki, Y. Nabae, K. Terakura, T. Ikeda, H. Niwa, M. Kobayashi and Y. Harada.References:  Y. Shao et al., Appl. Catal. B: Environ. 79, 89 (2008).  J. Ozaki et al., Carbon 45, 1847 (2007).  H. Niwa et al., J. Power Sources 187, 93 (2009).  T. Ikeda et al., J. Phys. Chem. C, 112, 14706 (2008).  Y. Nabae et al., ECS Transaction 25 (1), 463 (2009).
10:00 AM - TT6.2
Soft X-ray Spectroscopic Study of Strontium-doped Lanthanum Manganite (La0.8Sr0.2MnO3) Cathodes for Solid Oxide Fuel Cell Applications
Louis Piper 1 , Andrew Preston 1 , Sang-Wan Cho 1 , Bo Chen 1 , Jude Laverock 1 , Kevin Smith 1 , Lincoln Miara 2 , Jacob Davis 2 , Soumendra Basu 2 , Uday Pal 2 , Srikanth Gopalan 2 , Laxmikant Saraf 3 , Tiffany Kasper 3 , Per-Anders Glans 4 , Jinghua Guo 4 Show Abstract
1 Department of Physics, Boston University, Boston , Massachusetts, United States, 2 Division of Materials Science and Engineering, Boston University, Boston , Massachusetts, United States, 3 Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington, United States, 4 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States
The modification of the Mn charge-state, chemical composition and electronic structure of La0.8Sr0.2MnO3 (LSMO) cathodes for solid oxide fuel cell (SOFC) applications remains an area of interest, due to the poorly understood enhanced catalytic activity observed after many hours of operation (often referred to as the "burn-in" phenomenon). Using a combination of core-level X-ray photoemission spectroscopy (XPS), X-ray emission/absorption spectroscopy (XES/XAS), resonant inelastic X-ray scattering (RIXS) and resonant photoemission spectroscopy (RPES), we have monitored the evolution of these properties in LSMO at various stages of fabrication and operation. By rapidly quenching and sealing in vacuum, we were able to directly compare the pristine (as-fabricated) LSMO with both "heat-treated" (800°C in air, and no bias) and "burnt-in" (800°C in air, -1 V bias) LSMO cathodes i.e. before and after the activation observed in our electrochemical impedance spectroscopy measurements. Our core-level XPS, Mn L3,2-edge RIXS and Mn L3 RPES studies of “heat-treated” and pristine LSMO determined that SOFC atmospheric environment results in La-deficiency (severest near the surface) and stronger Mn4+ contribution, leading to the increased insulating character (hole doping > 0.55) of the cathode prior to activation. In addition, we report dramatic changes in the oxygen environment with the likely formation of passive SrO and Mn3O4 species (possibly SrxMnyOz) before the application of a bias. After the “burn-in”, the greatest changes occur in the Mn charge state with only subtle changes in the oxygen environment. These findings are discussed in terms of the various "burn-in" mechanisms that have been promoted.
10:15 AM - **TT6.3
Lithium Ion Batteries – a Challenge For in situ Characterisation.
Torbjorn Gustafsson 1 , Kristina Edstrom 1 Show Abstract
1 Materials Chemistry, Uppsala University, Uppsala Sweden
The Li-ion battery, today the battery of choice for portable electronics and tomorrow the power source for environmentally benign vehicles, is still a rapidly developing product. The fundamental properties of a series of new chemistries, tailored for energy, power, large scale storage, durability etc. are studied. It is still a major challenge to explain macroscopic properties in terms of atomic level observations.The mechanism for lithium extraction from (or insertion into) LiFePO4 has been under debate since the first publication by Padhi1. Most studies published have, however, been based on materials in relaxed and equilibrated state. We will present some in situ, synchrotron based, diffraction studies where the material is studied under high current conditions.Photoelectron Spectroscopy and sof X-ray spectroscopy ares extremely well suited to study the surface reactions in Li-ion batteries. The battery conditions, with a thick liquid film covering the electrode surface, are not compatible with the high vacuum requirements for PES. The use of safe transfer vessels, where the samples can be transferred from the battery into the analysis instrument without air contact, and the use of excitation energies up to 10keV are steps on the way towards in situ PES on batteries. Spectroscopic PES and soft X-ray spectroscopy results for some important cathode materials will be discussed in the light of the their function in a Li-ion battery context.Ref. 1)Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. J.Electrochem. Soc. 1997, 144, 1188
10:45 AM - TT6.4
In situ X-ray Absorption Spectroscopy of the Oxygen K edge in a Lithiated Transition Metal Oxide Battery.
Cole Petersburg 1 , Robert Daniel 1 , Faisal Alamgir 1 , Cherno Jaye 2 3 , Daniel Fischer 2 3 Show Abstract
1 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 Ceramics Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 3 National Synchrotron Light Source, Brookhaven National Laboratory, Upton, New York, United States
ObjectivesNovel lithium ion battery cathode compounds are difficult to design because intercalation electrochemistry is not well understood. Whereas the changes in crystallography of transition metal oxides as a function of lithium content have been thoroughly measured in situ, the oxidation states of all the components of mixed oxides including manganese, cobalt, nickel, and especially oxygen as a function of lithium content have not been fully characterized. Such information is necessary in understanding not only the desired charge compensation reactions, but also the deleterious oxidation of liquid electrolytes, the release of oxygen gas, and the formation of inactive surface oxides due to cycling. The possible contribution of univalent oxygen ions to both processes is still unknown.MethodOur soft x-ray synchrotron-based X-ray Absorption Spectroscopy (XAS) characterization method uses a solid state battery with an oxygen-free chalcogenide glass electrolyte and a graphite) anode to reverse-titrate an intercalation cathode in situ and in vacuo while measuring the occupation of oxygen and metal molecular orbitals.Partial electron yield (PEY) and fluorescence yield (FY) data were collected simultaneously at NIST beamline U7A at the National Synchrotron Light Source at Brookhaven National Laboratory.New ResultsIn the early stages of the deintercalation of lithium cobalt oxide an increase in absorption is seen at the Fermi level in the oxygen K edge spectrum. The new peak is located over half an electron volt below the pre-existing peak and is not seen in the cobalt L edge spectrum. In fact, no changes are seen in the cobalt L edge spectrum between 0% and 24% delithiation. The same trends are seen in both surface-sensitive PEY spectra and in bulk-sensitive FY spectra.These trends match those in the ex situ lithium cobalt oxide work of Yoon et al., although with much higher precision in delithiation. In addition, small absorption features in the oxygen K edge spectrum, not previously studied, were seen to change rapidly at low levels of delithiation. These surface features were found not to derive from carbonate contamination and instead correlate with defect states seen in a separate in situ annealing study.ConclusionsWe have demonstrated a novel experimental method for the in situ detection of reversible and irreversible chemical reactions in the bulk and surface. This information can be used to generate an electrochemical road map for any transition metal oxide electrode in terms of lithium content and cell voltage. The ability to use a single battery allows for the fine-grained monitoring of changes in the electronic structure due to delithiation.References C. Petersburg, R. Daniel, F. Alamgir, C. Jaye, D. Fischer, (2009) Journal of Synchrotron Radiation 16, 5, p610. W.-S. Yoon, K.-B. Kim, M.-G. Kim, M.-K. Lee, H.-J. Shin, J.-M. Lee, J.-S. Lee, C.-H. Yo (2002) J. Phys. Chem. B 106, p2526.
Wednesday AM, December 01, 2010
Fairfax B (Sheraton)
11:30 AM - **TT7.1
Quantum Rods and Dots-based Structures and Devices: Low Cost Aqueous Synthesis and Bandgap Engineering for Solar Hydrogen and Solar Cells Applications.
Lionel Vayssieres 1 Show Abstract
1 MANA, NIMS, Tsukuba Japan
Our strategy to fulfill the drastic requirements of novel materials development for direct solar water splitting is the ability to design metal oxide semiconductor based on vertically oriented anisotropic nanostructures with intermediate bands and highly quantized band structure such as quantum rods and dots to enable high efficiency in the visible range as well as tuning bandgap and band edges by quantum confinement effects. Such unique characteristics, combined with in-depth investigation and modeling of their electronic structure and large scale and low cost fabrication methods provide this research a substantial advance in the field of semiconductor technology and materials for solar energy conversion. Such studies include polarization dependent soft x-ray absorption and emission measurements, as well as energy dependent resonant inelastic x-ray scattering performed at synchrotron radiation facilities. The outcome has brought important information on bandgaps, Fermi energies, work functions, orbital character and symmetry as well as band edges characteristics and confinement effects of important large bandgap semiconductors. Such advanced knowledge of the electronic structure, structural properties, and surface chemistry of metal oxide nanostructures and heterostructures has allowed better fundamental understanding of the structure-property relationships as well as efficiency optimization of novel nanodevices based on metal oxide nanostructures and heterostructures for renewable hydrogen generation.
12:00 PM - TT7.2
Generation of Pd Model Catalyst Nanoparticles by Spark Discharge.
Maria Messing 1 , Rasmus Westerstrom 2 , Bengt Meuller 1 , Sara Blomberg 2 , Johan Gustafson 2 , Jesper Andersen 2 , Edvin Lundgren 2 , Richard van Rijn 3 4 , Olivier Balmes 3 , Hendrik Bluhm 5 , Knut Deppert 1 Show Abstract
1 Solid State Physics, Lund University, Lund Sweden, 2 Synchrotron Radiation Research, Lund University, Lund Sweden, 3 , ESRF, Grenoble France, 4 Kamerlingh Onnes Laboratory, Leiden University, Leiden Netherlands, 5 Chemical Sciences, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Heterogeneous catalysts used for industrial purposes or for cleaning of exhaust gases are complicated systems of materials usually consisting of an insulating oxide support with dispersed metal nanoparticles of the active catalyst, as well as a wide range of additives to promote or poisoning specific reactions. Due to the material complexity, atomic scale information on the inner workings of the catalyst is at best limited, and as a consequence catalyst development is for a large part based on a trial and error approach.Because of the need to obtain fundamental information on catalytic reaction pathways, model systems have been developed. Studies of processes related to heterogeneous catalysis under Ultra High Vacuum (UHV) conditions on well defined single crystal surfaces have been a major part of surface science for decades. In recent years, more complex material model systems have been developed by depositing pure metal or alloy nanoparticles by Molecular Beam Epitaxy (MBE) or by wet chemical methods on a thin oxide film formed on a conducting material.In this work we present a different route to a model system; size selected aerosol palladium particles generated by spark discharge. The system presented consists of Pd nanoparticles with a diameter of 15 or 35 nm deposited on HF-etched SiOx or on Al2O3 substrates. The conducting SiOx is used for the sake of characterization using x-ray photoelectron spectroscopy (XPS), but the type of metal deposit or substrate can be chosen almost arbitrarily. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray energy dispersive spectroscopy (XEDS), x-ray photoelectron spectroscopy (XPS) and x-ray diffraction (XRD) measurements have been carried out to thorough investigate the as-produced catalyst model particles. From these investigations it was found that the particles consist of a crystalline core surrounded by an amorphous shell. Furthermore, the main contaminant of the particles is carbon, most likely contained in the amorphous particle shell. The carbon is believed to be incorporated during particle production and can be removed by oxidation by O2 of the particles at elevated temperatures.The present study shows that aerosol deposition is a useful method for deposition of metal nanoparticles on virtually any substrate to be used as a model system for studies of catalytic properties. By this method, a well-defined size of the particles can be chosen and a suitable well-controlled coverage can be selected, determined by the technique to be used in investigations of the catalytic properties.
12:15 PM - **TT7.3
Optical Property of Semiconductor Nanowires Studied by Synchrotron Induced X-ray Excited Optical Luminescence.
Wenjun Zhang 1 2 , Xuhui Sun 1 , Xintai Zhou 3 , Richard Rosenberg 4 , Tsun-Kong Sham 5 Show Abstract
1 , The City University of Hong Kong, Hong Kong Hong Kong, 2 Functional Nano and Soft Materials Laboratory, Soochow University, Suzhou China, 3 , Shanghai Institute of Applied Physics, Shanghai China, 4 Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois, United States, 5 Department of Chemistry, University of Western Ontario, London, Ontario, Canada
X-ray excited optical luminescence (XEOL) is a spectroscopic technique that monitors light emission from matter in the optical region (ultraviolet, visible, and near-infrared) on absorption of X-rays, often tunable X-rays from a synchrotron light source . XEOL has been shown to be a powerful tool for investigating the local chemical environment of a site that gives rise to a particular luminescent band by examining the X-ray energy dependence of the various luminescence bands across various absorption edges. In the past, it has been successfully used to study the chemical origin of luminescent bands in porous Si . Recently, taking advantage of the pulsed nature of the third generation of synchrotron radiation enables the study of temporal behavior, and hence further characterization of the luminescence. Time-resolved XEOL monitors the time dependence of the emission intensity (decay curve) following absorption of a synchrotron X-ray pulse (typically sub nanosecond bandwidth and 10–102 ns repetition rates). From the decay curve, one can obtain the lifetime of the excited state and set a desired time window for luminescence spectral measurements. In this talk, we present the recent XEOL and TRXEOL studies of nanomaterials, especially one-dimensional semiconductor nanomaterials such as Si, ZnO, ZnS, Ga2O3, SnO2 nanowires, Si-CdSe core-shell nanowires, etc. The optical luminescence and associated electronic structures of such structures have been investigated and the chemical origins of the light from the nanostructures have been determined. 1. A. Rogalev and J. Goulon, in Chemical applications of synchrotron radiation: X-ray Applications, edited by T. K. Sham, World Science Publishing Co., Singapor, part II, p. 707. 2. T. K. Sham, D. T. Jiang, I. Coulthard, J. W. Lorimer, X. H. Feng, K. H. Tan, S. P. Frigo, R. A. Rosenberg, D. C. Houghton, and B. Bryskiewicz, Natur 363, 331, 199.
12:45 PM - TT7.4
Subnanometer Size-Selected Cobalt and Nickel Cluster Based Nanomaterials for Fischer-Tropsch Synthesis.
Stefan Vajda 1 2 , Sungsik Lee 3 , Byeongdu Lee 3 , Soenke Seifert 3 , Jeffrey Elam 4 , Michael Pellin 5 , Randall Winans 3 Show Abstract
1 Materials Science Division and Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, United States, 2 Department of Chemical Engineering, Yale University, New Haven, Connecticut, United States, 3 X-ray Sciences Division, Argonne National Laboratory, Argonne, Illinois, United States, 4 Energy Systems Division, Argonne National Laboratory, Argonne, Illinois, United States, 5 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States
The elucidation of the effect of catalyst size and composition along with the effect of support on catalyst’s performance are instrumental to pave the way towards the design of new classes of catalytic materials. Uniform particles on technologically relevant supports are prerequisites for such studies , making size-selected clusters of few atoms as ideal model systems.[2-4] Our experimental studies are based on 1) chemically uniform support fabrication, 2) size-selected cluster deposition, and 3) in situ synchrotron X-ray characterization of clusters under working conditions, combined with mass spectroscopy analysis of the reaction products. This contribution focuses on the studies of new catalytic materials designed at the atomic, subnanometer scale for Fischer-Tropsch synthesis of fuels. Size-selected cobalt and nickel clusters in the range of 4 to 32 atoms were deposited on various oxide and carbon-based supports and their reactivity studied with a mixture of carbon monoxide and hydrogen. A pronounced effect of cluster size and support chemistry on catalyst performance was observed. In situ X-ray scattering (GISAXS) revealed stable clusters, without an indication of agglomeration. The evolution of the oxidation state of the nanocatalyst under working conditions followed by X-ray absorption (XANES) will be discussed.1. A.T. Bell, Science 299, 1688 (2003)2. S. Lee et al, Angew. Chemie. Int. Ed. 48, 1467 (2009)3. S. Vajda et al., Nat. Mater. 8, 213 (2009)4. Y. Lei et al, Science 328, 224 (2010)5. S. Wyrzgol et al, Phys. Chem. Chem. Phys. 12,, 5585 (2010)6. S. Lee et al, in preparation
TT8: Heterogeneous Catalysts II
Wednesday PM, December 01, 2010
Fairfax B (Sheraton)
2:30 PM - **TT8.1
In-situ Photoelectron Spectroscopy and its Applications in Energy Research.
Zhi Liu 1 , Michael Grass 1 Show Abstract
1 Advanced Light Source, Lawrence Berkeley National Lab, Berkeley, California, United States
Many unwanted electrochemically driven processes, such as corrosion and the degradation of catalysts, fuel cell, and batteries, are determined by slow surface chemical reactions. In-situ surface sensitive tools are ideal in detecting the small changes at the gas-solid/liquid-solid interface. By investigating within a few nanometers of the interface, we can evaluate and predict the long-term, large-dimension changes within a reasonable time span.The newly designed ambient pressure x-ray photoelectron spectroscopy endstations at ALS based on differentially pumped electron energy analyzers along with other synchrotron based techniques have been recognized by scientific communities beyond the ALS as an important in-situ tool to study catalysis, electrochemical cells, and other energy related fields. We will give an overview of science projects at BL9.3.2 in heterogeneous catalysis and electrochemical cells (fuel cell and battery). Acknowledgement: The ALS is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
3:00 PM - TT8.2
Three-dimensional Atomic Structure of Supported Nanoparticles Determined by Complementary in situ X-ray Absorption Spectroscopy and Transmission Electron Microscopy.
Long Li 1 , Lin-Lin Wang 2 , Zhongfan Zhang 1 , Sergio Sanchez 3 , Joo Kang 3 , Ralph Nuzzo 3 , Qi Wang 4 , Anatoly Frenkel 4 , Duane Johnson 2 , Beatriz Roldan Cuenya 5 , Jason Croy 5 , Simon Mostafa 5 , Farzad Behafarid 5 , Judith Yang 1 Show Abstract
1 Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania, United States, 2 Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 3 Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 4 Department of Physics, Yeshiva University, New York, New York, United States, 5 Department of Physics, University of Central Florida, Orlando, Florida, United States
Recent advances in nanotechnology have led to the remarkable ability to synthesize a wide variety of nanostructures. Yet, critical to the development of novel nanomaterials is the corresponding improvements of nanocharacterization tools, because one needs to “see” the synthesized material to confirm the structure and/or provide insights into how to alter the synthesis method to create the desired nanostructure for specific properties. Here we focus on the complementary methods of X-ray absorption fine-structure spectroscopy (XAFS) and transmission electron microscopy (TEM) to determine the 3-dimensional shape of supported nanoparticles (NPs) for applications in heterogeneous catalysis. Two examples will be presented. (1) the discovery of size-dependent crystallinity of supported Pt NPs synthesized by impregnating γ-Al2O3 powders with different loadings of a Pt2+ precursor, Pt(NH3)4(OH)2 H2O. Ligand removal and nanoparticle formation were induced by reduction in 4% H2 (g), balance He or ultrahigh-purity H2 (g) at elevated temperatures.1 Samples with monodispersive sizes were measured with temperature-resolved XAFS in an in situ cell with H2 or He gas environments to attain the static disorder parameters. It shows an increasing distribution of disordered Pt-Pt bond lengths with decreasing nanoparticle sizes. In complementary, a focal-series reconstruction (FSR) of HRTEM was employed to characterize the crystallinity of thousands of individual Pt NPs. Statistical results revealed an order-disorder transition substantially exist in the Pt/γ-Al2O3 system. Pt NPs <1 nm have disordered structures while those >2.5 nm prefer an ordered structure and a transition zone of mixed ordered/disordered NPs in between. DFT/MD simulations of a 1.1-nm Pt37/γ-Al2O3 (100) model demonstrated that the groundstate structure is disordered, being energetically more favorable than the fcc-ordered structure by 1.53 eV.(2) Using PS(x)-b-P2VP(y) diblock copolymers and an H2PtCl6 precursor, a novel micelle encapsulation technique allowed the synthesis of monodisperse size- and shape-selected Pt NPs supported on nanocrystalline γ-Al2O3 powders. The samples were characterized by XAFS, TEM, scanning tunneling microscopy (STM), atomic force microscopy (AFM), and computer modeling to extract information on NP shapes. In contrast to conventional deposition-precipitation methods, which typically produce 2D raft-like structures, 3D, faceted shapes having long range crystalline order were found even for ~1 nm NPs.2 These large surface area, faceted NPs may be ideal candidates for catalysis applications.References:1.J. H. Kang, L. D. Menard, R. G. Nuzzo, and A. I. Frenkel, J. Am. Chem. Soc. 128 (37), 12068 (2006).2.B. Roldan Cuenya, J. R. Croy, S. Mostafa, F. Behafarid, L. Li, Z. Zhang, J. C. Yang, Q. Wang, and A. I. Frenkel, J. Am. Chem. Soc. (2010) online ASAP paper.
3:15 PM - TT8.3
WITHDRAWN 12/27/10 Ni-Substituted Ba-β-Alumina Solid Oxide Catalysts: Structural and Performance Characteristics during CO2-CH4 Reforming.
Todd Gardner 1 , Edwin Kugler 2 , James Spivey 3 , Victor Abdelsayed 1 Show Abstract
1 , National Energy Technology Laboratory, Morgantown, West Virginia, United States, 2 , West Virginia University, Morgantown, West Virginia, United States, 3 , Lousiana State University, Baton Rouge, Louisiana, United States
Anthropogenic CO2 emissions are an important global issue due to the significant and continuous rise in atmospheric CO2 concentrations. One method of utilizing CO2 is through its re-use as an oxidant at point sources. A series of Ba0.75NiyAl12-yO19-δ (y = 0.2, 0.4, 0.6, 0.8 and 1.0) catalysts exhibit CO2 adsorption characteristics that are directly related to both activity in the CO2-CH4 reaction and the extent of Ni-substitution into the Ba-β-alumina structure. The relationship between catalyst activity and structure is characterized by EXAFS, XANES, XRD, XPS and TPR. Although there is no clear trend in the average Ni coordination number and bond distance with Ni substitution, the deposition of coke, a primary mechanism of deactivation in this reaction, is greatly reduced by the extent of Ni substitution into the Ba-β-alumina lattice.
3:30 PM - TT8.4
A Simple Strategy for Obtaining Au38 Clusters Stabilized by Different Thiols.
Daniel Stellwagen 1 2 , Andrew Weber 1 , Challa Kumar 1 Show Abstract
1 Center for Advanced Microstructures and Devices, Louisiana State University, Baton Rouge, Louisiana, United States, 2 Chemistry, Utrecht University , Utrecht Netherlands
Ultrasmall (< 2nm), atomically monodisperse, clusters of gold protected by thiolate ligands are gaining lot of prominence due to their promise in a number of applications in catalysis, biological labeling, and nano-electronics. [1,2,3] Unlike larger gold colloids whose electronic properties resemble the bulk state, gold clusters with a core diameter less than 2 nm are characterized by distinct quantum confinement effects. As a result discrete electronic structure and molecular type properties such as HUMO-LUMO transitions and intrinsic magnetism are observed. A major challenge in gold cluster synthesis using wet chemistry is the ability to synthesize same cluster size with different stabilizers. The obvious reason for this problem is that the nature of the stabilizer influences the nucleation and growth processes and therefore influences both size of the cluster as well stablizer. Using a well established ‘thiol-etching’ procedure , known to yield Au38(SC12H25)24, we have investigated the influence of thiol on the etching process. A wide variety of thiols, both aliphatic (CnH(2n+1)SH [n= 4,6,8,12,16]) and aromatic (4-tertbutyl benzylmercaptan) were employed. The resulting thiol-capped gold clusters were analyzed with various techniques such as MALDI, HR-TEM, UV-VIS, IR, WAXS and Sulfur K-edge XANES. Based on these analyses, we report obtaining Au38 clusters stabilized by thiols, both aliphatic (CnH(2n+1)SH [n= 4,6,8,12,16]) and aromatic (4-tertbutyl benzylmercaptan). More specifically, the synchrotron radiation-based small angle X-ray scattering analysis provides a strong supporting evidence for cluster size in agreement with MALDI analysis. However, on the contrary, it is very interesting to note that the HRTEM results indicate systematic variation in cluster size with the chain length of the aliphatic thiols. It is obvious that for ultrasmall metal clusters HRTEM size determination is not a reliable technique. The opportunity to synthesize Au38 clusters stabilized by different thiols opens up a number of avenues of research investigations with applications ranging from novel magnetic materials, catalysis to biomedicine. With recent demonstration of Au38 clusters as an effective catalyst for low-temperature ambient CO oxidation, our results could help in fine tuning the Au38-based catalytic systems for practical applications in green chemistry. Similarly, coupled with recent reports about permanent magnetism in some of the thiol-stabilized Au clusters will lead to the understanding of influence of the nature of thiol on magnetic properties of Au38 clusters. References: Angew. Chem. Int. Ed. 2010, 49, 1295-1298 Nature Chemistry. 2010, 2, 329-334 Nanoscale. 2010, 2, 343  J. Phys. Chem. A 2009, 113, 4281-4284
3:45 PM - TT8.5
Studies of CO Adsorption on Supported Catalysts by Combination of Quick XAFS/Rapid-Scan DRIFTS.
Qi Wang 1 , Nebojsa Marinkovic 1 , Laura Barrio 2 , Chana Cooper 3 , Anatoly Frenkel 3 Show Abstract
1 Synchrotron Catalysis Consortium, University of Delaware, Newark, Delaware, United States, 2 Chemistry Department, Brookhaven National Laboratory, Upton, New York, United States, 3 Physics Department, Yeshiva University, New York, New York, United States
X-ray Absorption Fine-Structure (XAFS) Spectroscopy and Infrared (IR) Spectroscopy are two essential techniques in catalyst studies, in which XAFS probes structure and electronic properties of the catalysts while IR monitors the surface interaction of catalysts with adsorbates. One of the main challenges in catalysis science is to analyze these properties at the same conditions and at the same state of the reaction, which is rarely possible if studied by these two techniques independently. We have designed and tested a new setup for combined, time-resolved XAFS and Diffuse Reflectance Fourier Transform Infrared Spectroscopy (DRIFTS) measurements within the same in-situ experiment to study catalytic reactions in real time. The additional capability of quickly (with the scan duration of less than a second) scanning the x-ray energy, as well as the use of the rapid-scan mode of the infrared spectrometer enables investigation of the catalytic reactions with sub-second time resolution. Here, we present an in-situ synchronous study of the CO adsorption by a model catalyst (5 wt. % Pt-Al2O3) to demonstrate the capability of the combined Quick XAFS/ rapid-scan DRIFTS setup. This setup was developed at the Synchrotron Catalysis Consortium at the National Synchrotron Light Source of Brookhaven National Laboratory. The marriage of the two rapid, complementary spectroscopic techniques enables kinetic and mechanistic studies of a large number of important catalytic reactions.
TT9: Charge Transfer
Wednesday PM, December 01, 2010
Fairfax B (Sheraton)
4:30 PM - **TT9.1
Visualizing Photoinduced Ligation and Interfacial Charger Transfer Using X-ray Transient Absorption Spectroscopy.
Lin Chen 1 2 Show Abstract
1 Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois, United States, 2 Department of Chemistry, Northwestern University, Evanston, Illinois, United States
Ultrafast excited-state structural dynamic of metalloporphyrin has been revealed by ultrafast optical and X-ray absorption spectroscopy. The complementary studies provide new insight into effects of structural constraints on excited state potential surfaces.Using both x-ray transient absorption spectroscopy (XTA) and optical transient absorption spectroscopy (OTA), we previously studied electronic and nuclear structures of the T1 excited state of NiTMP without the axial ligation and observed the direct evidence of singly occupied 3dz2 and 3dx2-y2 MOs in the T1 excited state, the energy difference between 1s →3dz2 or 3dx2-y2 transitions, as well as the energy shifts of 3dx2-y2 and 4pz MOs in the T1 state from that of the S0 state. Meanwhile, the Ni-N bond elongation in the T1 state was also resolved, suggesting an increased ionic radius for Ni(II) in the excited state and a more planar macrocycle conformation. We now focus on the molecular structural dynamics of the ground and the excited state during the axial ligation process. Both the electronic and nuclear configurations have been monitored, and the results not only shed light on the time sequence of excited state structural changes, but also reveal a unified mechanism for Ni(II) porphyrin ligation in the T1 and S0 state. In addition, we performed a successful XTA measurement on a heterogeneous system – interfacial photoinduced charge injection from a transition metal complex to semiconductor nanocrystals mimicking the dye-sensitized solar cell (DSSC) and captured the structural changes of RuN3 dye adsorbed to TiO2 nanoparticle surface accompanying the electron injection from the dye to TiO2 using XTA. This work has expanded XTA from homogeneous systems to a heterogeneous interfacial system in studying transient molecular structures in solar energy conversion processes which is an important and necessary step for future development of photovoltaic devices. This work has demonstrated the great potential of using XTA to study fundamental structural-functional correlations in solar cells and solar fuel generation in heterogeneous and interfacial systems. The work was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract Nos. DE-AC02-06CH11357.
5:00 PM - TT9.2
Shining the X-rays on the Pt:SnO2 Based Gas Sensors in Operando Conditions.
Michael Huebner 3 , Dorota Koziej 1 2 , Nicolae Barsan 3 , Matthias Bauer 4 , Kristina Kvashnina 5 , Marta Rossel 2 , Udo Weimar 3 , Jan-Dierk Grunwaldt 4 Show Abstract
3 Institute of Physical Chemistry, Univeristy of Tuebingen, Tuebingen Germany, 1 , Harvard University, Cambridge, Massachusetts, United States, 2 Department of Materials, ETH Zurich, Zurich Switzerland, 4 Institute for Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology, Karlsruhe Germany, 5 , European Synchrotron Radiation Facility, Grenoble France
Nobel metals play a fundamental role in improving the sensing performance of tin dioxide based gas sensors. In the past, efforts have been undertaken to understand the relation between their structure and an electrical signal. Analogous to catalysis, where the goal is to study catalysts inside a reactor by spectroscopic means, the interest for adapting synchrotron based spectroscopic techniques to investigate the sensors at the operation conditions is high. In particular, recent in-situ XAS studies on the role of Pd and Au in real sensors have questioned the originally anticipated mechanism of noble metal dopants like spillover and Fermi-control. However in case of Pt the lack of knowledge is not only due to difficulties in gaining detail in-situ experimental information about the structure of metal during sensing but also due to the complexity of the sensing device. This is mainly due to the fact that platinum is only present in very low concentrations (0.2-1 wt% Pt in SnO2) incorporated in relatively thick and high porous layers of about 50 μm, which presents a typical question in solid state chemistry. To catch the state and structure of Pt in SnO2 based sensors under these demanding working conditions we have to revisit the sensor design and apply advanced spectroscopic tools allowing structural identification on an atomic level even at low concentration range of 0.2 wt% of Pt in a porous thick film SnO2 matrix. For the latter purpose an element specific X-ray absorption spectroscopy was applied, giving structural information even at high dispersion. A new sensor design was required to decouple the XAS signal for the minute amounts of Pt in the SnO2 matrix from the usually used Al2O3 substrates with massive Pt electrodes - for the readout of the resistance - and Pt heater – for operation of the sensor at well controlled temperatures. Firstly, the sensor was successfully modified in a way that platinum was only present in the sensing layer. Secondly, the gold fluorescence could be efficiently eliminated by using a high energy resolved fluorescence detector. This did not only result in proper XANES but even in range-extended EXAFS data since the sensor layer is located on top of the gold electrode. The whiteline intensity of >> 6 uncovered that platinum is in a surprisingly high oxidized state. Platinum is furthermore very difficult to reduce, which shows that it is strongly incorporated into the matrix as also supported by detailed EXAFS analysis. These results are surprising but important to elucidate the role of Pt in the detection mechanism of Pt-based SnO2 sensors. They further go in a similar direction as in the case for Pd-based sensors, although Pd species in SnO2 are easier to reduce than the corresponding Pt species, incorporated in the SnO2 lattice. This paper demonstrates the potential of using new synchrotron-based technique together with sensor design and structure-function relationships.
5:15 PM - **TT9.3
Electronic Structure of Actinide Materials Probed by Resonant Inelastic X-ray Scattering Spectroscopy.
Sergei Butorin 1 Show Abstract
1 Department of Physics and Astronomy, Uppsala University, Uppsala Sweden
By doing resonant inelastic x-ray scattering (RIXS) measurements at d-edges of actinides, the localized and delocalized 5f state components can be probed selectively in material in question. By tuning energies of incident photons the cross-section of inelastic scattering for transition to localized versus delocalized 5f states are enhanced making it easier to study these components separately. The results of RIXS experiments for a number of materials of actinides from the beginning and middle of the actinide-row are presented and discussed along with results of model calculations.
5:45 PM - TT9.4
Soft X-ray Synchrotron Radiation Studies of Materials for Nuclear Energy.
David Shuh 1 , Per-Anders Glans 1 , Jinghua Guo 1 , Tolek Tyliszczak 1 , Sergei Butorin 2 , Kristina Kvashnina 2 , Joseph Nordgren 2 , Lars Werme 2 , Tsuyoshi Yaita 3 Show Abstract
1 , LBNL, Berkeley, California, United States, 2 , Uppsala University, Uppsala Sweden, 3 , JAEA, Hyogo Japan
Soft X-ray synchrotron radiation approaches can impact the understanding of bonding in a range of materials systems relevant to nuclear energy and has the potential to aid in the design of special-purpose actinide materials and ligands critical to the future implementation of improved nuclear energy systems. The characteristic capabilities of the scanning transmission X-ray microscope (STXM) to examine radioactive materials in an in-situ manner, with and without extensive sample preparation, provides a mechanism to probe a range of difficult materials by soft X-ray absorption spectromicroscopy. STXM investigations have included model actinide materials, uranium complexes with the 2,6-Bis(2-benzimidazyl)pyridine (BBP) ligand to give insight into light element bonding with the actinides, actinide alloys, and actinide materials systems with light element constituents. Similarly, complementary information about bonding in radioactive materials without the need for extensive sample preparation is obtained from soft X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS) methods. XES is particularly useful since it provides the partial density of occupied states and when coupled with pre-requisite absorption spectroscopy, yields detailed information about the electronic states involved in bonding. This information is particularly useful for understanding light element interactions in f-element complexes and ligands designed for the trivalent separation of actinides from lanthanides. Furthermore in the actinide systems, RIXS can elucidate the role of the actinide 5f electrons under high resolution. A key and ever-developing attribute of all these soft X-ray synchrotron radiation approaches is the ability to successfully simulate the resulting spectra.