Role of ZnO and CeOx in Cu-Based Model Catalysts in Activation of H2O and CO2 Dynamics Studied by in Situ Ultraviolet–Visible and X-ray Photoelectron Spectroscopy

Flat model and powder Cu, ZnO/Cu, and CeO x /Cu catalysts were studied by focusing on the role of the oxide phase as a promoter in the water gas shift (WGS) and its reverse reaction (RWGS). Activity measurements of the powder catalysts showed that both oxides enhance Cu reactivity, with CeO x /Cu being more active than ZnO/Cu in the WGS reaction. In situ ultraviolet–visible spectroscopy, exploiting the localized surface plasmon resonances of metallic Cu nanoparticles, together with X-ray photoelectron spectroscopy was then used to elucidate the origin of the enhanced reactivity on flat model catalysts. These experiments showed that ZnO and CeO x promote H 2 O and CO 2 dissociation, leading to oxidation of the Cu nanoparticles. CeO x performs better in this respect than ZnO. This is important because the reactivity in the WGS and RWGS reactions is related to the ability to activate H 2 O and CO 2 . The Ce 3+ ions are identified as the most efficient sites for H 2 O and CO 2 dissociation, while Cu 0 keeps Ce 3+ stable by promoting reduction of Ce 4+ during the dissociation process. In this sense, the CeO x /Cu catalyst forms a bifunctional catalyst, which is more active in the (R)WGS than CeO x and Cu catalysts separately.