The effects of vacancies and surface hydroxyls on the interactions of hydrogen with rare earth oxides – DFT calculations on gadolinium oxide

The interactions of gaseous hydrogen with rare earth oxides surfaces are important in certain practical processes such as hydrogenation reactions catalysis and metal hydrides formation (for “real life” metals coated with thin oxidation overlayers). It has experimentally been observed that such interactions are significantly affected by the presence of imperfections on these oxide surfaces, especially oxygen vacancies and hydroxyls. In the present DFT calculations, the effects of vacancies and of hydroxyl groups on the chemisorption process of H₂ were studied for the GdO_1.5, as a model system. Two oxide plane orientations, the (110) and the (111) were compared. It was found that the presence of surface hydroxyls significantly impedes the dissociative chemisorptions step. For the perfect, defect-free oxide surface, hydrogen dissociates heterolytically, forming an hydroxyl and a negatively charged H^−δ species. The latter is likely to be the more mobile moiety which penetrates the oxide overlayer and reacts with the metal beneath to form the hydride. However, accumulation of hydroxyls occurs during this chemisorptions step, which finally leads to the blocking of the H₂ heterolytic dissociation route. On the other hand, for the defected, vacancies containing, oxide surfaces, a new type of homolytic dissociation route has been found. This route yields two negatively charged H^−δ moieties. This route does not produce any hydroxyls and therefore provides a sustainable channel for H₂ dissociation and penetration into the metal. The above calculation may account for some experimental observations regarding the initial steps of hydrides development.