On the potential role of hydroxyl groups in CO oxidation over Au/Al₂O₃

The deuterium isotope effect in the steady state CO oxidation rate over Au/γ-Al₂O₃ in the presence of H₂ or H₂O and the effect of pretreatment on an uncalcined catalyst were studied. In a reaction feed containing 1% CO, 0.5% O₂, and 40.5% H₂ at room temperature, CO oxidation exhibited a deuterium isotope effect (k_H/k_D) of 1.4 ± 0.2. The rate of D₂ oxidation was also slower than the oxidation of H₂, such that the selectivity for CO oxidation was 86% in the presence of D₂ versus 77% in the presence of H₂. In contrast, there was no deuterium isotope effect in a feed containing 1% CO, 0.5% O₂, and 1.5% H₂O. H₂ was also more effective in regenerating a CO oxidation reaction deactivated catalyst than D₂, whereas H₂O and D₂O were equally effective. The difference was attributed to the different mechanisms with which H₂ or H₂O prevented deactivation of the catalyst during CO oxidation. An uncalcined Au/γ-Al₂O₃ was rather inactive. It could be activated by treatment with a mixture of H₂ and H₂O at 100°C, although treatment by either H₂ or H₂O alone was ineffective. The observations are consistent with the model of the active site consisting of an ensemble of metallic Au atoms and a cationic Au with a hydroxyl group.