(127f) Supported Au and Pt Catalysts for the Water-Gas Shift Reaction
The water-gas shift (WGS) reaction (CO + H2O → CO2 + H2) is used as a probe reaction to determine the number and identity of active sites, the effect of support and the effect of promoters on supported Au and Pt catalysts. The WGS rate per total mole of Au varies with the average Au particle size (d) as d-2.7±0.1 and d-2.2±0.2 for Au/TiO2 and Au/Al2O3 catalysts respectively. The variation of reaction rate and apparent reaction orders with particle size was used in conjunction with a physical model of Au clusters and operando IR to show that the active sites are low coordinated metallic corner and perimeter Au atoms. On the other hand, the WGS turnover rates, normalized by surface atoms, of Pt/ZrO2, Pt/Al2O3, Pt/CeO2, Pt/TiO2 and Pt/SiO2 catalysts are independent of average Pt particle size. Unlike Au catalysts, the apparent reaction orders for Pt/Al2O3 catalysts do not vary with particle size. Thus, all surface Pt atoms exhibit the same rate. For both Au and Pt catalysts, the apparent water orders and the apparent activation energies vary with the support while the apparent CO, CO2 and H2 orders do not. We interpret these data to show that the support plays a direct role in activating H2O.