Reactivity and sintering kinetics of Au/TiO2(110) model catalysts: particle size effects |
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Authors: | Stephen C Parker Charles T Campbell |
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Affiliation: | (1) Department of Physics and Astronomy, Carleton College, Northfield, MN 55057, USA;(2) Department of Chemistry, University of Washington, Seattle, WA 98195-1700, USA |
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Abstract: | We review here our studies of the reactivity and sintering kinetics of model catalysts consisting of gold nanoparticles dispersed
on TiO2(110). First, the nucleation and growth of vapor-deposited gold on this surface was experimentally examined using x-ray photoelectron
spectroscopy and low energy ion scattering. Gold initially grows as two-dimensional islands up to a critical coverage, θ
cr, after which 3D gold nanoparticles grow. The results at different temperatures are fitted well with a kinetic model, which
includes various energetic parameters for Au adatom migration. Oxygen was dosed onto the resulting gold nanoparticles using
a hot filament technique. The desorption energy of Oa was examined using temperature programmed desorption (TPD). The Oa is bonded ~40% more strongly to smaller (thinner) Au islands. Gaseous CO reacts rapidly with this Oa to make CO2, probably via adsorbed CO. The reactivity of Oa with CO increases with increasing particle size, as expected based on Br?nsted relations. Propene adsorption leads to TPD
peaks for three different molecularly adsorbed states on Au/TiO2(110), corresponding to propene adsorbed on gold islands, to Ti sites on the substrate, and to the perimeter of gold islands,
with adsorption energies of 40, 52 and 73 kJ/mol, respectively. Thermal sintering of the gold nanoparticles was explored using
temperature-programmed low-energy ion scattering. These sintering rates for a range of Au loadings at temperatures from 200 to
700 K were well fitted by a theoretical model which takes into consideration the dramatic effect of particle size on metal
chemical potential using a modified bond additivity model. When extrapolated to simulate isothermal sintering at 700 K for
1 year, the resulting particle size distribution becomes very narrow. These results question claims that the shape of particle
size distributions reveal their sintering mechanisms. They also suggest why the growth of colloidal nanoparticles in liquid
solutions can result in very narrow particle size distributions. |
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Keywords: | Catalysis gold TiO2 sintering nanoparticles particle growth selective oxidation |
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