Hydrogen production via nanocatalyzed ammonia borane hydrolysis: State of the art, recent progress and perspectives

Abstract

The production, transport and utilization of hydrogen (H₂), a green energy source, are now essential to our modern society in order to face ecological issues involved with fossil fuels that generate CO₂ and toxic particles. Therefore, the search of safe H₂ carriers that are stable in water and have a high hydrogen atom content is promising. Ammonia borane hydrolysis (ABH) is now considered as a possible candidate process to safely generate H₂ upon nanocatalysis. After recalling the historical context and early mechanistic proposals, this review presents the compared efficiencies of various nanomaterials, including a choice of sophisticated supports in room temperature catalysis and photocatalysis of H₂ generation upon ABH. Pioneering works in the late 2000s and most remarkable reports from the 2010s are recalled, and the articles from the last three years are reviewed. The turnover frequency (TOF) and activation energy (E_a) of various nanocatalysts are compared, and the mechanisms are discussed, in particular the electronic interactions involved between the metals, heteroatoms and supports are examined in the light of Density Functional Theory (DFT) calculations. The review also includes discussions of the authors' groups results and catalytic proposals on the particularly efficient thermal and photolytic ABH reactions catalyzed by metal and alloy nanocatalysts embedded in the zeolitic imidazolate framework (ZIF)-8. As a complement, the main catalyst performances (TOF and E_a) during the last five years are summarized in tables. In conclusion, the main progress in H₂ generation from ABH is summarized in this review, and perspectives are provided concerning the possible utilization of H₂ generated from catalyzed ABH as portable H₂ storage devices. Recent progress in ABH has shown that, with the best catalyst designs, H₂ is release at RT within a few seconds at room temperature (TOFs of several thousands mol_H2 mol_cat⁻¹ min⁻¹), which is far superior for H₂ storage to physisorption that requires large material amounts or other chemisorption materials such as Mg, since high temperatures such as 300 °C are necessary for H₂ release from MgH₂. Much work involving AB recycling is still necessary in the near future, however, in order to commercially extend ABH to Hydrogen Fuel Cell powering.