Insights into the oxidation mechanism of millerite exposed to O-2 and H2O using DFT study

Millerite (NiS) as the source of nickel extraction, is often transformed into the corresponding metal oxide in hydrometallurgical process, which involves both oxygen and water. To elucidate the oxidation mechanism, periodic density functional theory (DFT) calculations were performed to investigate the adsorption of oxygen and water on the NiS(1 0 0) surface. First, the isolated H2O molecule shows a relatively weak interaction with the NiS (100) surface. Furthermore, the adsorption of oxygen on the NiS(1 0 0) surface has been comprehensively discussed. The results of adsorption energies and energy barrier for oxygen dissociation show that the interaction between oxygen and the NiS(1 0 0) surface is favored thermodynamically and kinetically. Finally, it was found that the adsorbed oxygen atoms promote H2O dissociation significantly by diminishing the energy barrier. This work provides a microscopic insight into the oxidation mechanism of millerite in the presence of oxygen and water.