CO Oxidation Kinetics over Au/TiO2 and Au/Al2O3 Catalysts: Evidence for a Common Water-Assisted Mechanism

The mechanism of CO oxidation over supported gold catalysts has long been debated, with two prevailing mechanisms dominating the discussion: a water-assisted mechanism and a mechanism involving O-defect sites. In this study, we directly address this debate through a kinetic and mechanistic investigation of the role of water in CO oxidation over Au/TiO2 and Au/Al2O3 catalysts; the results clearly indicate a common water-assisted mechanism to be at work. Water adsorption isotherms were determined with infrared spectroscopy; the extracted equilibrium constant was essentially the same for both catalysts. Added water decreases CO adsorption on Au/TiO2, likely by blocking CO binding sites at the metal–support interface. Reaction kinetics (CO, O2, and H2O reaction orders) were essentially the same for both catalysts, as were measured O–H(D) kinetic isotope effects. These data indicate that the two catalysts operate by essentially the same mechanism under the conditions of these experiments (ambient temperature, ...