(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.