CO oxidation over Pd and Cu catalysts IV. Prereduced Al 2O 3-supported copper

Abstract

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 O 2 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/Al 2O 3 catalysts were more active than Pd/Al 2O 3 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.