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

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