Insights into the C–H Bond Activation on NiO Surfaces: The Role of Nickel and Oxygen Vacancies and of Low Valent Dopants on the Reactivity and Energetics

For the development of nickel oxide (NiO) as an oxidation catalyst, a fundamental understanding of the role of surface morphology and of nickel and oxygen vacancy defects is essential, since they govern the reactivity of the surface. Using density functional theory (DFT) calculations, we investigated the reactivity of two different crystal facets of NiO and reveal the contribution of the coordinatively unsaturated Ni–O pairs, nickel and oxygen vacancies, and low valent dopant Li in determining and altering the reactivity of the surfaces. The most stable surface, NiO(100), is relatively inactive for methane C–H activation with an activation barrier of 136.6 kJ mol–1. However, the relatively less stable NiO(110) surface is extremely active and can dissociate methane with an activation barrier of 57.1 kJ mol–1. The coordinative unsaturation and comparatively low binding strength of the four-coordinated surface lattice oxygen on the NiO(110) surface leads to strong chemisorption of the dissociated H, facilitating extremely low activation barriers for methane dissociation. The presence of a Ni vacancy on the inactive NiO(100) surface brings down the activation barrier for methane dissociation to 90 kJ mol–1. This is a result of weakening of the binding strength of the oxygen, allowing strong chemisorption of the dissociated H. In this work, we predict that an equivalent increase in the surface reactivity can be achieved by doping the inactive NiO(100) surface with low valent metals like Li, which also weakens the binding strength of surface oxygen. The hydrogen chemisorption energy on the oxygen site is identified as a descriptor for estimating the reactivity of surfaces.