The Promoting Role of Ni on In(2)O(3) for CO(2) Hydrogenation to Methanol

[Image: see text] Ni-promoted indium oxide (In(2)O(3)) is a promising catalyst for the selective hydrogenation of CO(2) to CH(3)OH, but the nature of the active Ni sites remains unknown. By employing density functional theory and microkinetic modeling, we elucidate the promoting role of Ni in In(2)O(3)-catalyzed CO(2) hydrogenation. Three representative models have been investigated: (i) a single Ni atom doped in the In(2)O(3)(111) surface, (ii) a Ni atom adsorbed on In(2)O(3)(111), and (iii) a small cluster of eight Ni atoms adsorbed on In(2)O(3)(111). Genetic algorithms (GAs) are used to identify the optimum structure of the Ni(8) clusters on the In(2)O(3) surface. Compared to the pristine In(2)O(3)(111) surface, the Ni(8)-cluster model offers a lower overall barrier to oxygen vacancy formation, whereas, on both single-atom models, higher overall barriers are found. Microkinetic simulations reveal that only the supported Ni(8) cluster can lead to high methanol selectivity, whereas single Ni atoms either doped in or adsorbed on the In(2)O(3) surface mainly catalyze CO formation. Hydride species obtained by facile H(2) dissociation on the Ni(8) cluster are involved in the hydrogenation of adsorbed CO(2) to formate intermediates and methanol. At higher temperatures, the decreasing hydride coverage shifts the selectivity to CO. On the Ni(8)-cluster model, the formation of methane is inhibited by high barriers associated with either direct or H-assisted CO activation. A comparison with a smaller Ni(6) cluster also obtained with GAs exhibits similar barriers for key rate-limiting steps for the formation of CO, CH(4), and CH(3)OH. Further microkinetic simulations show that this model also has appreciable selectivity to methanol at low temperatures. The formation of CO over single Ni atoms either doped in or adsorbed on the In(2)O(3) surface takes place via a redox pathway involving the formation of oxygen vacancies and direct CO(2) dissociation.