Effect of Adsorbed Oxygen on the Dissociation of Water over Gadolinium Oxide Surfaces: Density Functional Theory Calculations and Experimental Results

Abstract

The interactions of water vapor with rare earth oxide surfaces play a major role in many important practical processes, such as heterogeneous catalysis and corrosion phenomena, especially for “real life” metals (metals with thin oxide overlays). Generally, these interactions take place by two different routes, a pure ionic dissociation, producing only hydroxyls, and a redox reaction, producing atomic hydrogen and an oxidic oxygen (or hydroxyl). In the presence of oxygen, however, the redox route is eliminated and only the pure ionic dissociation prevails. In the present study, the effect of oxygen on the dissociation of water vapor over GdO1.5 was investigated experimentally and theoretically using density functional theory (DFT) calculations. The DFT calculations revealed that water vapor will follow the redox route only on nonstoichiometric, oxygen-deficient surfaces by producing a hydroxyl and a reduced H-δ moiety that relaxes into an oxygen vacancy site. This moiety may diffuse into the oxide-metal interface (for oxide-coated metals) to form hydrides or it may associate on the oxide surface into H2. In the presence of oxygen, the formation of H-δ is prevented and therefore the formation of hydrides on oxide-coated metals is not expected. The experimental results are in agreement with the DFT analysis for the reaction of gadolinium (coated by its native oxide) with humidity to form gadolinium hydride islands on the surface. However, in the presence of oxygen, the extent of this reaction was very limited.