CO OXIDATION OVER PD AND CU CATALYSTS .4. PREREDUCED AL2O3-SUPPORTED COPPER

A study of the kinetics for CO oxidation over supported, prereduced Cu crystallites showed that it is close to first order in CO and near zero order in O2 at temperatures around 400 K. In situ IR measurements under reaction conditions revealed a clear first-order dependence on chemisorbed CO and an absence of activity when no adsorbed CO was detected; thus an Eley-Rideal mechanism is eliminated and a Langmuir-Hinshelwood surface reaction between adsorbed CO molecules and chemisorbed oxygen is strongly supported. These Cu/Al2O3 catalysts were more active than Pd/Al2O3 catalysts, and the average turnover frequency of 0.016 s-1 at 400 K and 26 Torr CO was about four times higher than the average value on Pd. An invariant IR band at 2118 cm-1 for CO was very near that of 2115 cm-1 reported for CO adsorbed on an oxidized Cu(111) surface and therefore was consistent with the near-saturation oxygen coverage indicated from the kinetic behavior. Analysis of the IR spectra at various temperatures allowed a heat of adsorption of 7.2 kcal/mol to be determined under reaction conditions. An activation energy of 22-27 kcal/mol was estimated for the rate-determining step, i.e., the surface reaction, which is somewhat higher than that reported for adsorbed CO and submonolayer coverages of oxygen on Cu single crystals. © 1991.