Spectroscopic Study of a Cu/CeO2Catalyst Subjected to Redox Treatments in Carbon Monoxide and Oxygen

Abstract

Redox processes induced by interaction of a calcined Cu/CeO2catalyst with CO and reoxidation with O2have been investigated by CO-TPR, EPR, FTIR of adsorbed CO, and XPS. The initial calcined sample shows the presence of dispersed Cu2+species, which give rise in the EPR spectrum to signals due to isolated entities, a somewhat more aggregated Cu2+-containing phase, and copper ionic pairs, in coexistence with an EPR-silent CuO-type phase, revealed by XPS. A significant reduction of copper is produced already by contact with CO at room temperature, EPR results suggesting that reducibility of Cu2+species decreases with their aggregation degree. Simultaneously, the ceria surface is also reduced by this interaction, copper acting as a strong promoter of this process. A singular consequence of the synergistic reduction of both components is observed by subjecting the catalyst to CO atTr≥ 473 K, at which the CO adsorption capability of copper is apparently suppressed, in view of the absence of copper carbonyls in the FTIR spectrum. This is attributed to the establishment of electronic interactions between reduced ceria and small metallic copper particles generated by the reduction process. Contact of the CO-reduced sample with O2at room or higher temperature produces an important reoxidation of both copper and ceria, revealed by FTIR and EPR. The synergetic effects between copper and ceria in the reduction process and the easy reoxidation of deeply reduced ceria are thought to be crucial to explaining the high catalytic activity shown by this system for CO oxidation.