Coverage dependent structure and energy of water dissociative adsorption on clean and O-pre-covered Ni (100) and Ni(110)

Abstract

Due to its importance in energy related catalytic reactions, H₂O dissociative adsorption on clean and O pre-covered Ni(100) and Ni(110) surfaces has been computed systematically on the basis of periodic density functional theory and ab initio atomistic thermodynamics. On clean surfaces, H₂O adsorption prefers the top site at the lowest coverage and forms clusters stabilized by more dominant H-bonding interaction with the increase in coverage; H₂O dissociation [H₂O = OH + H] has a lower barrier than OH dissociation [OH + H = O + 2H], and both steps are exothermic on Ni(100) and Ni(111), while on Ni(110) the first step is exothermic and the second step is endothermic, indicating that surface OH should represent the most preferred species. Co-adsorbed H₂O can lower the barrier of H₂O dissociation, in agreement with the experiment. On O pre-covered surfaces, co-adsorption of O and H₂O prefers the remote configuration without H-bonding on Ni(100), the adjacent configuration with H-bonding on Ni(111) and barrier-less dissociation on Ni(110). Using H₂O as an oxidant, surface OH saturation coverage on Ni(111), Ni(100) and Ni(110) is 0.625, 0.83 and higher than 1 ML, respectively, and surface O saturation coverage on Ni(111), Ni(100) and Ni(110) is 0.25, 0.42 and 0 ML, respectively. The desorption temperature of molecularly adsorbed H₂O clusters and H₂O from OH disproportionation [2OH = O + H₂O(g)] agrees very well with the experimental values. The desorption temperature of H₂ from H₂O dissociation [OH + H = OH + 0.5H₂(g) and O + 2H = O + H₂(g)] agrees very well with the experimental value on Ni(100), but is lower than that on Ni(110). The computed desorption order of H₂ and H₂O from OH disproportionation, i.e., H₂O prior to H₂ on Ni(100) and H₂ prior to H₂O on Ni(110), agrees with the experimental finding. These systematic results show mechanistic insight into H₂O dissociative adsorption on Ni surfaces and provide the basis for investigating water-involving reactions catalyzed by nickel.