Influence of Oxygen Vacancy and Dopant in NiO Catalysts on the Selective Catalytic Reduction of NO with NH3

A detailed mechanistic investigation of the selective catalytic reduction (SCR) of NO with NH3 was carried out on NiO(100) and NiO(110) surfaces using density functional theory (DFT) simulations. On stoichiometric NiO surfaces, the high barrier for O-2 dissociation resulted in a NO-assisted O-2 dissociation to form NO2, NH3 dehydrogenation by active oxygen, NO2-NH2 coupling, and N2O formation, ultimately leading to the product N-2 via N-O bond cleavage of N2O. On NiO(100), the N-O bond cleavage in N2O, and on NiO(110), the N-N coupling had high barriers of 1.95 eV. On NiO(100), N2O was a likely side product aside from N-2. On partially reduced NiO surfaces, facile direct O-2 activation at the oxygen vacancy led to a mechanistic shift to a NO-mediated pathway. This enabled selective N-2 formation on partially reduced NiO(100) and NiO(110) surfaces, with the highest activation barrier of less than 0.89 eV. Substitutional doping of NiO with Ru facilitated enhanced reducibility of the surface and strong interaction of O with the Ru site and reduced the N-O breaking barrier, enhancing the catalytic activity compared to undoped NiO. The partially reduced Ru-doped NiO surface exhibited activation barriers lower than those on the stoichiometric Ru-doped NiO surface and formed N-2. These results indicate the possibility of enhancing catalytic activity and selectivity of NiO catalysts for low-temperature SCR by modulating the reducibility of the surface and by substitutional doping, which enhances the Lewis acidity of the catalyst by modulating the local electronic structure around the metal sites.