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/ -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 (kH/kD) 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/ -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. © 2002 Elsevier Science B.V. All rights reserved.