(483b) The Contribution of Nickel and Oxygen Vacancies and of Low Valent Dopants in Altering the Surface Reactivity of NiO

Metal oxides are increasingly being used as oxidation catalysts for biomass and hydrocarbon derived feedstocks. Nickel oxide (NiO) is an effective oxidation catalyst and is used in the chemical looping combustion and reforming of methane, in addition to its application in photo and electrochemical systems. Since oxidation reactions involve the direct participation of lattice oxygen, its reactivity on different exposed facets of typical NiO morphologies determine the overall catalytic performance. Using Density functional theory (DFT) calculations incorporating the Hubbard U correction (DFT + U method), we reveal 1) the intrinsic differences in the reactivity of two different crystal facets of NiO, NiO(100) and NiO(110), in C-H activation of methane and 2) the contribution of surface nickel and oxygen vacancies, and of a low valent dopants like Li in altering the mechanism of methane dissociation and the reactivity of the surfaces. Methane dissociation by the synergistic participation of the 5-coordinated Ni-O pairs on the most stable surface of NiO, NiO(100), proceeds with a large activation barrier of 136.6 kJ mol^-1 while the 4-coordinated Ni-O pairs on the relatively less stable NiO(110) surface can dissociate methane with an activation barrier of 57.1 kJ mol^-1. The presence of a surface Ni vacancy on the inactive NiO(100) surface increases its reactivity to enable C-H dissociation by the hydrogen abstraction mechanism by oxygen alone (activation barrier 90 kJ mol^-1),in addition to the Ni-O pair mediated mechanism (activation barrier 97 kJ mol^-1). This increase in the surface reactivity results from the weakening of the binding strength of lattice oxygen, increasing the surface reducibility, and allowing strong chemisorption of the dissociated H. An equivalent increase in the surface reactivity can be achieved by doping the inactive NiO(100) surface with low valent metals like Li. Low valent metals lower the surface oxygen binding strength, marginally increasing surface reducibility, although not to the extent of surface Ni vacancy. The hydrogen chemisorption energy on the oxygen sites is identified as a descriptor for estimating the reactivity of surfaces, irrespective of the mechanism of dissociation. The experimentally determinable H chemisorption energy as a descriptor, together with the technique of doping, makes a useful tool set to screen metal oxides, their different surfaces, and potential dopants to tune the reactivity of the oxides for desired reactions.