Coverage-Dependent Water Dissociative Adsorption Properties on Nickel Surfaces

Periodic density functional theory calculations were employed to study the mechanisms of H₂O dissociative adsorption on the Ni(211) surface at different coverages. Due to stronger H-bonding, H₂O clustering is more favored than direct Ni–O interaction. The first dissociation step has a lower barrier and is much more exothermic than the second dissociation step (0.91/–0.89 vs 1.45/–0.01 eV). Surface OH represents the most stable and important intermediate on the clean and O-precovered surfaces. Taking H₂O as oxidant [H₂O = OH + 1/2H₂(g); H₂O = O + H₂(g)], the saturation coverage of OH and O is 0.67 and 0.33 ML, respectively. Comparing the results of H₂O dissociative adsorption on the Ni(111), Ni(100), Ni(110), and Ni(211) surfaces, which can represent a nickel nanocluster or nanoparticle having different facets, shows that there is no general correlation of adsorption energy (OH, H, and O) and saturation coverage (OH and O) with surface stability (energy). There is also no direct Brønsted–Evans–Polanyi relation between reaction barriers and reaction energies (H₂O = HO + H, 2OH = O + H₂O), but they correlate well for OH + H = H₂O. This diversity offers the mechanistic insight of the H₂O dissociative adsorption of nickel nanoparticles and provides the basis for investigating water-involved reactions catalyzed by nickel having different facets.