Reactivity of Hydroxyls and Water on a CeO₂(111) Thin Film Surface: The Role of Oxygen Vacancy

Abstract

The interaction of atomic hydrogen and H2O with stoichiometric and partially reduced CeO2(111) thin films deposited on a Cu(111) substrate was investigated by temperature programmed desorption and X-ray photoelectron spectroscopy. On stoichiometric CeO2(111) surface, the adsorption of atomic H(g) leads to the formation of surface hydroxyl (OH(a)) and H2O(a) as well as the reduction of Ce4+ into Ce3+. On reduced CeO2(111) surfaces, the stability of OH(a) was enhanced by the presence of oxygen vacancies. Upon heating, surface hydroxyls undergo two competing reaction pathways: one is the associative desorption of OH(a) releasing H2O and creating oxygen vacancies (OH(a) + OH(a) → H2O(g) + Olattice + Ovacancy), and the other one is to produce H2 via OH(a) + OH(a) → H2(g) + 2Olattice. The presence of oxygen vacancies in CeO2 favors the reaction pathway of H2 formation. At 115 K, reversible dissociation and molecular adsorption of H2O occur on stoichiometric CeO2(111) surface, but irreversible dissociation of H2O occurs on reduced CeO2(111) surfaces. These results deepen the fundamental understanding of the influence of oxygen vacancies on the reactivity of surface hydroxyls and water on CeO2 surface.