Methanolysis of ammonia borane catalyzed by NiO–CuO heterostructured nanosheets: cooperation of visible light and oxygen vacancy

Abstract

Developing cost-effective and high-activity catalysts for the methanolysis of ammonia borane (AB) has attracted great attention in the field of hydrogen energy recently. Besides the modification of the electronic structure of the catalysts, external factors such as visible light irradiation can improve the efficiency of hydrogen production as well. In the present study, a Z-scheme heterostructured V_O–Cu_0.5Ni_0.5O catalysts were constructed by introducing a plenteous phase interface and oxygen vacancy (Vo). The catalytic activity of as-prepared V_O–Cu_0.5Ni_0.5O toward AB methanolysis has been improved dramatically with the assistance of visible light irradiation. The turnover frequency (TOF) under visible light irradiation was measured to be 29 mol_H2·mol_cat.⁻¹·min⁻¹, which is 1.4 times larger than the TOF in the absence of visible light. Systematic characterization experiments and density functional theory (DFT) calculations were conducted to unveil the causation of enhanced catalytic activity. The results demonstrated that the enhancement of the catalytic activity of V_O–Cu_0.5Ni_0.5O originated from the electronic structure modification induced by the formation of heterojunctions, the introduction of oxygen vacancies, and the assistance of visible light cooperatively. The formation of heterojunction and the introduction of oxygen vacancies provoked the upshift of the d-band center; while the visible light irradiation induced the photogenerated electrons to transfer from Cu to Ni sites at the interface. Such electron structure modulation is beneficial for the construction of abundant active sites, thereby enhancing the adsorption of methanol on the Ni sites, which is considered as the rate-determine step for the methanolysis of AB. The strong interaction between Ni and O weakened the O–H bond of methanol, accelerating the methanolysis of AB. These results demonstrate the utilization of combined heterojunction, oxygen vacancy, and visible light to explore highly active AB methanolysis catalysts, which should shed light on the exploration of more effective catalysts for AB methanolysis.

Graphical Abstract