Visible-Light-Driven Multichannel Regulation of Local Electron Density to Accelerate Activation of O-H and B-H Bonds for Ammonia Borane Hydrolysis

Regulating electron density at the active site by integrating contributions from multiple channels is an effective strategy to accelerate the reaction rate. Herein, the hydrolysis of ammonia borane (AB) is systematically studied on the NiCu alloy-loaded carbon nitride nanosheets (NixCuy/CNS). The TOF of AB hydrolysis for NiCu/CNS catalyst under visible light irradiation was nearly 3.5 times higher than that in the dark. Photoelectrochemical characterizations indicate that the improved photocatalytic activities originate from a combination of alloying effect, Mott-Schottky junction at the metal-semiconductor interface as well as the localized surface plasmon resonance induced under the visible light irradiation, which synergistically increase the local electron density at the active Ni sites. More importantly, the infrared spectra and isotope labeling-mass spectrometry methods were used to establish the source of hydrogen and unravel the reaction mechanism. It is suggested that the cleavages of B-H and O-H bonds are the initial steps of AB hydrolysis, which lead to the formation of intermediates M-H- (metal and electronegative H- from -BH3) and M-H+ (metal and electropositive H+ from H2O), respectively. The H-2 molecule could form through three main paths as (1) two H atoms from -BH3, (2) two H atoms from H2O, and (3) one H atom from -BH3 and another from H2O. Results of density functional theory (DFT) calculations are consistent with the formation of electron-rich Ni sites in the NiCu, and the activation of H2O is a rate-limiting step (RLS). Redistribution of electrons in NiCu significantly enhances the adsorption of AB, the activation of H2O molecules, and the associative desorption of H adatoms as well, which effectively promote the cleavage of B-H and O-H bonds and release of H-2. This work gives a systematic mechanistic study on photocatalytic AB hydrolysis involving multichannel electron-transfer pathways, which will provide a powerful guidance for the rational design of active catalysts for AB hydrolysis through multipronged effects.