Theoretical insight into adsorption and dissociation of water on NiCr binary alloy surfaces: Early-stage oxidation mechanism

The effects of alloy surface composition and pre-adsorbed oxygen on the behaviors of H2O over Ni–Cr binary alloy surfaces were investigated by using the first-principles method. The surface energies and work functions for a series of Ni–Cr (111) alloy surfaces with different Cr concentrations were addressed to track the surface reactivities. An enhancement effect on the surface reactivity from Cr doping in the top-surface layer (TSL) of nickel substrates was identified. The locations of Cr in the TSL dramatically promoted exothermic adsorption of H2O and its decomposed products, including OH, O, and H. The calculated potential dissociation pathways further demonstrated that the successive dissociation of H2O molecules was substantially triggered in the presence of Cr doping in the TSL. By contrast, the Cr doping in the sub-surface layer hindered the dehydrogenation of H2O with a relatively higher energy barrier for OH dissociation. Moreover, with pre-adsorbed oxygen atoms closer to Cr, the first elementary step of H2O dissociation was easily fostered, whereas the OH dissociation was hindered. The Cr doping and O pre-adsorption accelerated the dissociation of H2O, which plays a critical role in the initial oxidation of nickel-based alloys in water- or oxygen-bearing environments.