The water-gas shift (WGS) reaction rate per total mole of Au varies with number average Au particle size (d) as d-2.7�0.1 for Au/TiO2, d-3.2�0.4 for Au/ZrO2, d-2.9�0.2 for Au/CeO2, d-2.6�0.2 for Au/ZnO, and d-2.2�0.2 for Au/Al2O3 catalysts. The variations of reaction rate and apparent reaction orders with particle size and operando infrared (IR) experiments were used to show that the active sites are low coordinated metallic corner and perimeter Au atoms in contact with the support. Addition of Br, corresponding to the number of corner and perimeter Au atoms, resulted in an undetectable WGS rate for Au/TiO2 catalysts, confirming the low coordinated metallic corner and perimeter Au atoms as active sites. On the other hand, the WGS turn over rates (TOR's), normalized by surface Pt atoms, of Pt/Al2O3 and Pt/SiO2 catalysts are independent of average Pt particle size. Unlike for Au catalysts, the apparent reaction orders for Pt/Al2O3 catalysts at 300�C do not vary with particle size. Thus, all surface Pt atoms exhibit the same rate.The WGS reaction rate per total mole of Au and H2O order (in parenthesis) varies as Au/Al2O3(~0.6) < Au/CeO2(~0.3) < Au/ZrO2(~0.0) < Au/TiO2(~-0.3) at the same Au particle size at 120�C. Similarly, for Pt catalysts, the WGS TOR and H2O order (in parenthesis) vary as Pt/Al2O3 (0.90) ~ Pt/SiO2 (0.85) < Pt/TiO2 ~ Pt/ZrO2 ~ Pt/CeO2 (0.65-0.70) at 300�C. The CO, CO2 and H2 orders do not vary with the support. The kinetic parameters suggest a direct role of support in activation of H2O molecules.

Water-gas shift catalysis on supported Au and Pt nanoparticles

243rd National Meeting (2012)

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