Localized Electron Redistribution in Methanol Molecules over the Sea Urchin-like Tricobalt Tetroxide/Copper Oxide Nanostructures for Fast Hydrogen Release.

Abstract

Catalytic methanolysis of ammonia borane (NH₃BH₃) is a prospective technology in the field of hydrogen energy in which hydrogen production and hydrogen storage can be integrated together. The splitting of the O-H bond is identified as the rate-determining step (RDS) in this reaction. Thus, a deep understanding of the relationship between the electronic structure of the catalyst, especially the localized electron density of active sites, and the breaking behaviors of the O-H bond is of extreme importance for the rational design of robust catalysts for the reaction. In this work, sea urchin-like tricobalt tetroxide/copper oxide (Co₃O₄/CuO) nanostructures with rich oxygen vacancies (O_v) were fabricated by a simple synthetic route. In NH₃BH₃ methanolysis, the optimal Co₃O₄/CuO sample exhibited ultrahigh catalytic activity with a turnover frequency (TOF) of 87.5 min^-1. Interestingly, when NH₃BH₃ methanolysis was carried out under visible-light illumination, the TOF further increased to 116.4 min^-1, which is the highest TOF value among those of the noble-metal-free catalysts ever documented in the literature. Theoretical calculation results evidenced that the Cu site in the Co₃O₄/CuO sample was in charge of the adsorption and activation of methanol molecules. Both the O_v and visible-light illumination can help electrons on the Cu site flow to the adsorbed methanol molecule, thus leading to localized electron redistribution of the methanol molecule and the extension of the O-H bond. The cooperation of O_v and visible light makes splitting of the O-H bond easier, which is favorable for fast hydrogen release from NH₃BH₃ methanolysis. This study helps us to gain an insight into the influence of localized electron redistribution of methanol molecules on the RDS, which conduces to the rational design of highly effective nanocatalysts. Moreover, the coinduction strategy for localized electron redistribution with oxygen vacancy engineering and visible-light illumination opens up a route to boost catalytic activity in NH₃BH₃ methanolysis.