Interaction of Hydrogen with Cu-Modified Cerium Oxide Surfaces

We investigate the interaction between molecular hydrogen and ultrathin epitaxial CeO₂films modified with a 2% concentration of Cu atoms using X-ray photoemission spectroscopy (XPS) during thermal reduction cycles in H₂. The XPS measurements are combined with density functional theory calculations to obtain further insight into the observed modifications of the film surface. Our results show that the presence of Cu atoms decreases the barrier for H₂dissociation in comparison to that on pure ceria surfaces, leading to the formation of surface OH groups after exposure to H₂. Moreover, surface oxygen vacancies are generated already at mild temperatures (470 K), most likely via water formation and desorption. The presence of surface oxygen vacancies and hydroxyls contributes to the observed large increase in surface Ce³⁺concentration with increasing reduction temperature. In spite of these atomic scale modifications, the surface morphology observed by scanning tunneling microscopy remains substantially unchanged on the length scale of tens of nm.