Yttria-Stabilized Zirconia Supported Copper Oxide Catalyst: I. Effect of Oxygen Vacancy of Support on Copper Oxide Reduction

Abstract

Copper oxide was supported on yttria-stabilized zirconia (YSZ) and γ-alumina, respectively, using impregnation methods. The reducibility and characteristics of the supported copper oxide catalysts with various copper loadings were revealed and determined by temperature-programmed reduction (TPR) and electron paramagnetic resonance (EPR), respectively. For CuO/γ–alumina catalyst, only two peaks were found in the TPR patterns. One with lower peak temperature has been attributed to the reduction of highly dispersed copper ions; the other with higher peak temperature has been ascribed to the reduction of bulk-like copper oxide. For CuO/YSZ catalyst, four TPR peaks could be observed. Two TPR peaks with lower peak temperatures (namely, α1and α2) have been attributed to the hydrogen uptake of nested oxygen ions (NOIs) and temptable oxygen ions (TOIs), respectively. These are interface-boundary terminal oxygen ions of the supported copper oxide but have different environment and interaction with the surface oxygen vacancies of the YSZ support. The other two TPR peaks of CuO/YSZ catalyst were formed with the same reason as occurring on the CuO/γ–alumina. EPR has confirmed these structures and environments of these supported copper oxides. Two reduction mechanisms have been proposed for hydrogen reacting with the NOIs and the TOIs, respectively. The former case has been attributed to the function of oxygen-ionic transport of the YSZ support. The latter case was owing to the oxygen vacancies of YSZ acting as Lewis acid sites. In addition, a small but sharp TPR peak occurring at ca. 657°C was found in the TPR pattern of YSZ support. This peak has been attributed to the hydrogen uptake of surface capping oxygen ions of YSZ support which are coordinated beside the surface oxygen vacancies of YSZ.