Influence of the support on the preferential oxidation of CO in hydrogen-rich steam reformates over the CuO–CeO2–ZrO2 system

Abstract

Preferential oxidation (PROX) of CO in hydrogen-rich steam reformates was investigated using CuO–CeO2, CuO–CeO2–ZrO2, and CuO–ZrO2 catalysts. CuO (1–10 wt%) samples supported over high-surface-area (SBET=117–172 m2/g), cubic CeO2, CeO2–ZrO2, and ZrO2 were synthesized by coprecipitation. The composition of the support markedly influenced the PROX activity. Both CuO–CeO2 and CuO–CeO2–ZrO2 exhibited higher activity and selectivity in CO oxidation than CuO–ZrO2. The adverse influence of H2O was accentuated in catalysts containing ZrO2. Below 423 K and over CuO–CeO2 with less than 5 wt% CuO, the presence of H2O in the feed suppressed CO oxidation. H2O had a negligible effect on H2 oxidation. The catalysts showed stable activity in long-term experiments with the realistic feeds. The catalysts were characterized by XRD, surface area, TPR, diffuse reflectance UV–visible, EPR, and magnetic-susceptibility techniques. While a small amount of copper might be incorporated in the CeO2/ZrO2 fluorite lattice (forming a solid solution), most of it was at the surface of the support as isolated monomeric (types I and II) and dimeric (type IV) copper oxo species, nano-sized copper clusters containing magnetically interacting copper ions (type III) and a CuO-like, bulk phase. While the isolated copper oxo species exhibited reversible reduction–oxidation behavior, the interacting copper and CuO-like phases exhibited irreversible reduction behavior. The amount and reducibility of CuO on different supports correlated with their CO oxidation activities and increased in the order CuO–ZrO2CuO–CeO2–ZrO2<CuO–CeO2.