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