Fundamental studies about the interaction of water with perfect, oxygen-vacancy and pre-covered oxygen Cu_2O(111) surfaces:Thermochemistry, barrier, product

摘要

The adsorption and dissociation of H_2O on three types of Cu_2O(111) surfaces, including perfect, oxygen-vacancy and pre-covered oxygen surfaces have been systematically investigated using periodic density functional slab model calculations. Different kinds of possible modes of H_2O adsorbed on those surfaces are identified. Our results first show that the bonding of dissociated species (HO, H and O) for H_2O is substantially stronger than that of H_2O on Cu_2O(111) surface. Then, on perfect surface, H_2O mainly exist in the form of molecular adsorption, in which Cu_(CUS) site is chemisorption, O_(SUF) site is physisorption with a weak hydrogen bond. On oxygen-vacancy surface, dissociative adsorption of H_2O occurs predominantly, suggesting that oxygen-vacancy exhibits a strong catalytic activity toward H_2O dissociation; meanwhile, a small quantity of chemisorption H_2O adsorbed at Cucus site also exists. On pre-covered oxygen surface, H_2O is typical of physisorption due to a hydrogen bond interaction between H of adsorbed H_2O and pre-covered oxygen. Further, the dissociation mechanisms of molecular adsorption H_2O, OH from dissociative adsorption H_2O, as well as single OH group on different surfaces, leading to the final products H and O atoms, give out the thermochemistry, barrier of each elementary reaction, which indicates that OH species is energetically the most stable product on those surfaces. Pre-covered oxygen on Cu_2O(111) surface can act as a promoter to facilitate the first dehydrogenation of molecular adsorption H_2O leading to OH species, which is more favorable both thermodynamically and kinetically than that of molecular adsorption H_2O on other two surfaces. Finally, the vibrational frequencies for the adsorbed H_2O and OH species on Cu_2O(111) surfaces can be applied to guide surface vibrational spectroscopy in experiment. Our calculation may be a worthwhile theoretical example for the interaction of H_2O with other metal oxide surface.