The water-gas shift (WGS) reaction (CO + H2O → CO2 + H2) is an important industrial chemical process for hydrogen production. Our work focuses on determining the 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 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 (TOR’s), normalized by surface atoms, of Pt/ZrO2, 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 do not vary with particle size. Thus, all surface Pt atoms exhibit the same rate.
The WGS 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) varies as Pt/Al2O3 (0.90) ~ Pt/SiO2 (0.85) < Pt/TiO2 ~ Pt/ZrO2 ~ Pt/CeO2 (0.65-0.70) at 300°C. We interpret this data to show that the support plays a direct role in activating H2O.
The effect of promoters like K, Na, Li, Fe and Mo on the catalytic performance on Pt/Al2O3 catalyst is determined. A comparison of WGS reaction kinetic parameters and TOR’s on supported Au, Pt and promoted Pt catalysts serve as a guideline to determine the descriptors that govern their WGS catalytic performance. These will be discussed.