(495g) Water-Gas Shift Catalysis Over Supported Gold Nanoparticles

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 (1) active sites and (2) the effect of the support for supported Au nanoparticles.

The WGS reaction rate per total mole of Au varies with the average Au particle size (d) as ~ d^-3 for Au/TiO₂, Au/ZrO₂, Au/CeO₂, Au/ZnO,and Au/Al₂O₃ catalysts. The variations of reaction rate and apparent reaction orders with particle size were used to show that the active sites are low coordinated metallic corner and perimeter Au atoms. The addition of Br at a level of 16% of the surface moles of Au to a 2.3%Au/TiO₂ catalyst decreased its WGS reaction rate by six times but did not result in an appreciable change in the average Au particle size, apparent activation energy, or the reaction orders. From operando Fourier transform infrared (FTIR) spectroscopy experiments, we found that the WGS reaction rate is proportional to the normalized peak area of CO adsorbed on metallic Au (IR peak at 2100 cm^‑1). Corner Au atoms were counted as the active sites by transient isotopic switch experiments. Thus, it was confirmed that metallic corner Au atoms are the dominant active sites for Au/TiO₂catalysts.

The WGS reaction rate per total mole of Au and H₂O order (in parenthesis) vary 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. The CO, CO₂ and H₂ orders do not vary with the support. Density functional theory (DFT) results show that the H₂O activation barrier on rutile sites adjacent to Au is lower than on corner sites of unsupported Au nanoparticles and clean rutile TiO₂ (110) surface. We interpret these data to show that the support plays a direct role in activating H₂O. Thus, the supported Au catalysts are bifunctional in nature, where CO activation occurs on Au and the H₂O activation occurs on the support.