Quantitative Study of Au Catalytic Nanoparticles by Stem and Hrtem
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QUANTITATIVE STUDY OF Au CATALYTIC NANOPARTICLES BY STEM AND HRTEM F.T. Xu1, L. Menard2, H. P. Xu1, J. Kang2, S. P. Gao1, L. L. Wang3, A. Frenkel4, R. Nuzzo2, D. D. Johnson3, J. C. Yang1 1
Department of Materials Science and Engineering, University of Pittsburgh, Pittsburgh, PA 15261. 2 Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801. 3 Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801 4 Department of Physics, Yeshiva University, New York, NY 10016. ABSTRACT The highly dispersed metal (e.g. Au) nanoparticles have exhibited exceptional catalytic activity for several reactions, including CO oxidation. Their high catalytic activity has been attributed to nanoparticles nano-structural effects (including cluster thickness, shape, chemical information, and number of atoms of the cluster). The three dimensional exact structure and chemical bonding state of these supported nanoparticles is still challenging to be quantified by conventional methods due to their limitations in understanding size distribution of supported metal nanoparticles that are usually less than 1 nm (< 100 atoms). In this paper, the structure of Au heterogeneous catalysts has been successfully characterized by High Resolution Electron Microscopy (HREM), Z-contrast Scanning Transmission Electron Microscopy (STEM). The ligand protected Au13 nanoparticles on TiO2 support have been studied by ozone and thermal treatments to remove the ligands. The ozone removal method results in the truncated cuboctahedral structure while the thermal treatment results in the cuboctahedral structure. The ozone treatment yields less Au nanoparticles sintering than thermal treatment. Their FCC structure was confirmed by quantified Z-contrast STEM, HREM and its Fourier transformation. KEY WORDS: HAADF-STEM, Catalysis, Au Nanoparticle INTRODUCTION Due to their unique physical and chemical properties, especially catalytic behaviors, supported, nanometer-sized, metal particles (such as Au) continue to draw considerable attention in research. [1-4] The properties of such ensembles of atoms, for all but the most special cases, remain very poorly defined. The structural habits that are not present in the bulk metal may emerge while the size decreases. [5, 6] The small metal nanoparticles can exhibit patterns of reactivity and catalytic activity that are dramatically distinct than behaviors seen with larger nanoparticles. It is therefore important in research to fundamentally understand and predict the local structure and
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stability of the catalytic particles. A popular method to produce and stabilize the catalytic particles is to use phosphines or thiols to form a ligand shell around the metal cores to prevent them from aggregating. Gold nanoparticles containing 6, 8, 9, 10, 11, 13, 39, and 55 atoms have been reported by using this method. [7-9] In order to realize the high activity for CO oxidation, we deposited ligand-protected Au13 nanoparticles onto an
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