Generation of Active Hydrogen and Its Application to CO Hydrogenation on the ZnCr₂O₄(111)-O_V Surface

The reduced ZnCr₂O₄(111) surface is an important catalytic material for CO hydrogenation, but the mechanism underlying the generation of active hydrogen species and its role for CO hydrogenation on this surface are controversial. In this work, we systematically calculate the activation of the most stable H₂ species on the ZnCr₂O₄(111)-O_V surface, and the calculated results show that the heterolytic H₂ dissociation producing O_V-H⁻ and O-H species on the ZnCr₂O₄(111)-O_V surface is the optimal activation pathway. The heterolytic H₂ dissociation can form the highly active and stable O_V-H⁻ species, which is mainly attributed to the flexible 3d orbitals of the Zn species on the ZnCr₂O₄(111)-O_V surface, which favours the stabilization of hydride species due to the high symmetry of the Zn 3d orbitals. Moreover, we systematically investigate the reaction mechanism of CO-selective hydrogenation on the ZnCr₂O₄(111) surface, and the results show the energy barriers of the O_V-H⁻/O-H species attacking with the C^δ+/O^δ− of CO to generate HCO species are lower than those generating COH species. When the O-H species is used as a hydrogen source to initiate the CO hydrogenation reaction, the H-species is converted into radicals during the reaction; whereas, when the O_V-H⁻ species is used as an H-source, the hydride species can be directly used as a hydrogen source to participate in the highly selective hydrogenation of CO. We also find that the O_V-H⁻ species activates CO to produce HCO species with the lowest energy barriers, indicating that O_V-H⁻ species also possesses high activity and selectivity. This work can provide some theoretical guidance for the rational design of Zn-based catalytic materials for selective hydrogenation of syngas.