Adsorption and diffusion of H and O on an Ni(1 1 1) surface containing different amounts of Cr

Abstract

Density functional theory (DFT) is used to calculate the adsorption and diffusion of atomic hydrogen and oxygen on Ni(111) and NiCr(111) surfaces. The calculated results show that the adsorption energy (Ead) of H and O gradually decreases when increasing their coverage of the surface, but the rate is very slow for H. H in the interstitial site significantly modifies the geometrical structure of the surface, and as a result, the metal–metal bond length is elongated. At high H coverage, the NiCr bond length is noticeably elongated by interstitial H, indicating that Cr atoms are preferentially moved outward. Additionally, the activation energy of H is increased by Cr. Doping of Cr modifies the surface electronic structures, which can increase the energy barrier. For O, Ead increases with increasing Cr content on top of the surface. The highest energy is attained by the surface with the most Cr on top. Metallic atoms are less mobile from the Ni or Cr rich region, and the mobility is high in less concentrated regions, especially at high O coverage. The activation energy of O is 0.41 eV for the path III, i.e., over the NiCr bridge, which is 0.26 eV less than that of the pristine Ni(111) surface. The overall activation energy of O is increased on the NiCr(111) surface compared to the pristine Ni(111) surface, which is consistent with experimental results. This study suggests that Cr atoms pulled away from the surface by H results in a cation vacancy on the surface. The process can accelerate surface oxidation at a very early stage. In contrast, Cr can trap atomic O and reduces its surface diffusivity, resulting in the formation of passive film in the Cr-rich region.