Engineering Ni³⁺ Cations in NiO Lattice at the Atomic Level by Li⁺ Doping: The Roles of Ni³⁺ and Oxygen Species for CO Oxidation

Abstract

To investigate the Li+ doping effect on the structure and reactivity of NiO, a series of NiO catalysts doped by Li+ cations have been synthesized and probed by using CO oxidation as a model reaction. With a combination of experimental methods and DFT calculations, it has been revealed that the Li+ cations preferentially replace the lattice Ni2+ cations instead of directly refilling the Ni2+ vacancies in the cubic NiO lattice to form a solid solution structure below the lattice capacity. For samples possessing a pure solid solution phase, the Ni3+ cation amount increases with the increasing of lattice Li+ cation content, hence inducing the formation of larger quantities of surface mobile oxygen species. In addition, the surface reducibility and the CO adsorption and activation ability can be enhanced, accompanying the easier formation of surface oxygen vacancies and the extraction of surface active oxygen. Therefore, the intrinsic CO oxidation activity can be remarkably enhanced. In contrast, by the addition of excess Li+ cations above the lattice capacity, Li+ is present as an additional surface Li2CO3 phase, which evidently degrades the activity of the catalysts because of the loss of lattice Ni3+ cations and active oxygen sites. It is concluded in this paper that the best catalyst can be tailored at the atomic level by engineering the maximum amount of Ni3+ cations in the NiO lattice matrix with a pure solid solution phase by Li+ addition.