Quantitative STEM and HRTEM Studies on Au Metallic Nano-Catalysts
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1026-C09-06
Quantitative STEM and HRTEM Studies on Au Metallic Nano-Catalysts Long Li1, Laurent D. Menard2, Fengting Xu1, Ralph G. Nuzzo2, and Judith C. Yang1 1 Mechanical Engineering and Materials Science Department, University of Pittsburgh, Pittsburgh, PA, 15261 2 Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL, 61801 ABSTRACT Nano-catalysts, Au nano-particles on TiO2 (anatase), were studied by quantitative scanning transmission electron microscopy (Q-STEM) and high-resolution transmission electron microscopy (HRTEM). The Au13 precursor, Au13[PPh3]4[S(CH2)11CH3]4, was dispersed onto TiO2 supports. Three different post-treatments were used to remove the ligands from the Au nanoparticles: (1) thermal heating in air at 400°C for 2 hours, (2) exposure to ozone (O3) at room temperature, and (3) exposure to atomic oxygen (AO) with an AO dose of 7.3 ×1018 atom/cm2 at room temperature. Both reactive oxygen species O3 and AO produced significantly smaller sizes of Au particles as compared to those from heating in air (2.7 ± 0.6 nm, average 324 atoms). Ozone produced the smallest particles (1.2 ± 0.5 nm, average 40 atoms), whereas AO produced smaller Au particles (2.1 ± 0.7 nm, average 72 atoms), with a broad size distribution and a variety of shapes. HRTEM studies on the AO treated Au/TiO2 samples revealed that Au nanoparticles with diameters < 3nm were raft-like, whereas those with diameters > 5nm were spherical, indicating a size dependant interaction with the TiO2 supports. INTRODUCTION Although gold is a noble metal, it shows remarkably high activity as a catalyst for CO oxidation when it is finely dispersed with diameters down to nano-scale on selected metal oxide supports.1-4 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 agglomeration, and Au clusters 6, 8, 9, 10, 11, 13, 25, 39, and 55 atoms, etc. have been successfully synthesized.5-8 We prepared the Au/TiO2 nanocatalysts by depositing ligand-protected Au13 nanoparticles onto anatase TiO2 followed by either thermal heating, ozone exposure at room temperature, or exposures with a reactive atomic oxygen species at room temperature in order to remove the ligands while maintaining small particle sizes.3,9,10 We have previously reported our results on the ozone treatment in comparison to annealing, or calcinations, where the reactive oxygen significantly reduced the amount of agglomeration. As inspired by this observation, we report here the affects of atomic oxygen, which also reduced the amount of agglomeration in comparison to annealing alone but not as much as ozone. The atomic oxygen treatment produced a broad size distributions and a variety of shapes of Au particles on TiO2 support.11 Since heterogeneous catalysis is a surface chemical reaction, the surface structure of the supported nanoparticles is needed for fundamental understanding in heterogeneous catalysis .12 Furthermore, the interactions of the supported
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