Initial reduction of the NiO(100) surface in hydrogen

Abstract

The reduction of NiO in hydrogen, a reaction with many industrial applications, has not received sufficient attention from theoretical standpoint because the complexity of the material properties and the process present considerable computational challenges. We report here the results of a systematic study on the hydrogen reduction of an ideal NiO(100) surface that produces a water molecule and an NiO(100) surface with an oxygen vacancy, using the Hubbard U corrected density functional theory method, with some of the key results verified by the hybrid density functional method. The major findings are: (1) the O vacancy in the NiO(100) surface slab is stabilized in the subsurface layer, although the vacancy is likely to remain on the outermost surface layer because the barrier for O vacancy migration from the surface to the second layer is as high as 3.02 eV; (2) regarding the energetics of hydrogen interaction with the ideal NiO(100) surface, water formation, and concomitant reduction of NiO is favored at higher H coverage even though surface hydrogenation is energetically more favorable than water formation at lower H coverage; (3) kinetically, the pull-off of the surface oxygen atom and simultaneous activation of the nearby Ni atoms play key roles in hydrogen reduction of NiO(100); and (4) a dual role of hydrogen is revealed as both a reactant and a mediator, which reduces the maximum kinetic barrier from 2.41 eV to 1.86 eV.