Water-induced formation of cobalt oxides over supported cobalt/ceria–zirconia catalysts under ethanol-steam conditions

The water-induced formation of cobalt oxides by the re-oxidation of metallic cobalt in pre-reduced 10% Co/CeO2–ZrO2 catalyst was verified by in situ temperature-programmed reduction (TPR) and in situ X-ray Photoelectron Spectroscopy (XPS) studies under various ethanol-steam conditions at 450 °C. The formation and transformation of water-induced cobalt oxide species during the reaction were influenced by the pre-reduction conditions as well as the feed stream composition. Our results suggest that the surface composition of the cobalt species (e.g. Co, CoO, and CoOx) in the 10% Co/CeO2–ZrO2 catalyst, initially determined by the catalyst pre-treatment conditions, changed toward an equilibrium state governed by the feed stream composition as the reaction proceeded. In addition, the reducibility of the ceria sites may play a significant role in the formation of such water-induced cobalt oxide species, as the redox process involves both cobalt and ceria sites in the ethanol-steam environment. Finally, the effect of the water-induced cobalt oxides in the 10% Co/CeO2–ZrO2 catalyst was investigated under ethanol and ethanol-steam conditions, particularly for the carbon–carbon (C–C) bond cleavage of ethanol. The water-induced formation of cobalt oxides in the catalyst was not observed to affect the cleavage, in either ethanol-rich (e.g. ethanol decomposition) or steam-rich (e.g. ethanol steam reforming) conditions. This result could be explained by our in situ studies, which revealed the redox exchange between reduced cobalt and oxidized cobalt in a feed stream with a changing water-to-ethanol ratio.