Oxygen-exchange reactions during CO oxidation over titania- and alumina-supported Au nanoparticles

Gold nanoparticles supported on titania and alumina were used as catalysts for CO oxidation. The oxidation of C16O (2 mol%) with 18O2 (2 mol%) in He at ambient temperature and 1.2 atm total pressure was performed in a single-pass, fixed-bed reactor. A mixture of CO2 isotopomers was observed during oxidation over Au/TiO2 and Au/Al2O3. However, no oxygen scrambling was observed in and between O2 and CO. Moreover, no evidence for direct exchange between O2 and CO2 was found. Isotopic transient analysis of a switch between C16O2 and C18O2 showed that oxygen exchange occurred between the metal-oxide supports and CO2. Therefore, the CO2 isotopomer distribution observed during CO oxidation could not be attributed solely to the oxidation reaction on Au. Comparison of the isotopic transients from 13C and 18O studies suggests the primary product of C16O oxidation with 18O2 is C16O18O. The presence of 0.16 mol% H2O co-fed with the reactants significantly promoted the rate of CO oxidation by factors of 2.5 for Au/TiO2 and 10 for Au/Al2O3. Despite the promotional effect of H2O on the rate of oxidation, no significant differences in the level of oxygen exchange in the CO2 were observed. In addition, the presence of H2O did not result in scrambling of oxygen in O2. Labeled oxygen found in the H2O exiting the reactor appeared to originate from 18O associated with the CO2.