Comparative study of alumina-supported CuO and CuCr₂O₄ as catalysts for CO oxidation

Alumina-supported CuO and CuCr₂O₄ catalysts of various compositions were prepared and their activity for CO oxidation measured. Fresh, pretreated with CO, and reoxidized catalysts were studied. In general, the activity increased with the CO pretreatment. The extent of the activation depended on the catalyst composition. Thus, copper catalysts were more active than copper chromite at low metal concentrations (<12 wt%), but the opposite was observed at concentrations higher than about 12 wt%. The results on activity behavior, together with TPR and XRD spectra, suggest that the active species are related to copper cations. In the low concentration copper catalysts, the most active surface is generated after CO pretreatment, followed by a fast reoxidation occurring at the first stages of the reaction. In the high concentration copper catalysts, the CO pretreatment produced an induction period as a result of excessive reduction. It is suggested that the role of chromium is to limit the extent of reduction through the formation of the CuCr₂O₄ phase. The presence of this phase also resulted in catalysts less prone to deactivation, as compared to copper on alumina catalysts.