Role of ZnO and CeO_x 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 H₂O and CO₂ 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 H₂O and CO₂. The Ce³⁺ ions are identified as the most efficient sites for H₂O and CO₂ dissociation, while Cu⁰ keeps Ce³⁺ stable by promoting reduction of Ce⁴⁺ 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.