Influence of the support on the preferential oxidation of CO in hydrogen-rich steam reformates over the CuO–CeO 2–ZrO 2 system

Abstract

Preferential oxidation (PROX) of CO in hydrogen-rich steam reformates was investigated using CuO–CeO 2, CuO–CeO 2–ZrO 2, and CuO–ZrO 2 catalysts. CuO (1–10 wt%) samples supported over high-surface-area ( S BET=117–172 m 2/g), cubic CeO 2, CeO 2–ZrO 2, and ZrO 2 were synthesized by coprecipitation. The composition of the support markedly influenced the PROX activity. Both CuO–CeO 2 and CuO–CeO 2–ZrO 2 exhibited higher activity and selectivity in CO oxidation than CuO–ZrO 2. The adverse influence of H 2O was accentuated in catalysts containing ZrO 2. Below 423 K and over CuO–CeO 2 with less than 5 wt% CuO, the presence of H 2O in the feed suppressed CO oxidation. H 2O had a negligible effect on H 2 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 CeO 2/ZrO 2 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–ZrO 2⩽CuO–CeO 2–ZrO 2<CuO–CeO 2.