Characterization of supported cobalt and cobalt-rhodium catalysts : II. Temperature-Programmed Reduction (TPR) and Oxidation (TPO) of Co/TiO2 and Co---Rh/TiO2

The reduction-oxidation behavior of cobalt supported on titania has been studied by temperature-programmed reduction and oxidation (TPR and TPO). Cobalt, supported on titania, was rather easy to oxidize and subsequently hard to reduce. Because of this ease of oxidation, during passivation a considerable amount of cobalt was oxidized. The reduction of Co3O4 supported on TiO2 proceeded in two stages, namely a primary reduction of Co3O4 to CoO and a subsequent reduction of CoO to Co. The addition of rhodium to cobalt caused the resulting bimetallic catalyst to be more difficult to oxidize, while the reducibility of the catalyst depended on the oxidation temperature. When oxidized below 800 K, the reduction proceeded at low temperatures, indicating that Rh2O3 was present in the surface of the bimetallic particles. To complete the oxidation, however, higher temperatures were needed and under these circumstances cobalt rhodate, CoRh2O4, was formed. The reduction behavior of a thoroughly oxidized catalyst revealed that only cobalt oxide was exposed, demonstrating that oxygen-induced surface enrichment of cobalt had occurred