Unlocking the catalytic role of oxygen functionalities on carbon-based catalysts for hydrogen generation from ammonia...

Carbon-based materials are steadily emerging as a potential solution to reduce the use of metals in hydrogen generation. The role of the amount of different oxygen-based functional groups in the ammonia borane hydrolytic decomposition to hydrogen was investigated by functionalising with a strong oxidising agent, HNO₃, carbon materials with different graphitisation degrees, i.e., pyrolitically stripped carbon nanofibers (CNFs), graphite and graphene nanoplates (GNP). Results obtained with XPS and Raman spectroscopy studies indicate that the oxygen content increases due to an increase in the quantity of defects in the original materials: O-GNP < O-Graphite ≪ O-CNFs. In order to further refine the results in terms of the nature of the oxygen functionalities introduced, a mild oxidising agent was also employed on CNFs, i.e. hydrogen peroxide (H₂O₂). Indeed, carbonyl and carboxyl groups are predominant under strong oxidizing conditions, while ether-like and hydroxyl groups were introduced using H₂O₂. The presence of CO bonds has a positive effect on hydrogen production, resulting in an increase of approximately one order of magnitude from O-Graphene to O-CNFs. Quantum chemical density functional theory calculations were used to obtain some atomistic insight into the activation of ammonia borane and to assess the effect of oxygen functionality. The results of this study provide evidence of the positive effects induced by the presence of C-O double bonds, suggesting a viable method for further catalytic optimization toward the generation of hydrogen from ammonia borane.