H₂ is recognized as the environmentally desirable clean fuel of the future and a primary technical challenge is its safe generation, delivery and storage. Borohydrides can be considered as the most promising H₂ storage systems. Aqueous NaBH₄ solutions in contact with selected catalysts hydrolyze to H₂ gas and sodium metaborate NaBO₂. In terms of activity/price ratio Co-based catalysts appear as one of the best solutions. However, in spite of many research works published on the subject, the mechanism of the formation and stability of the catalyticaly active phase is still not well understood.

In this work the mechanism of the active phase formation and evolution during hydrolysis of NaBH₄ solution has been successfully resolved using liquid phase reaction calorimetry coupled with a volumetric measurement of the hydrogen evolved, and followed by a detailed characterization of the catalyst. This work was carried out within the framework of CASTAFHYOR project, funded by the French National Research Agency (ANR), and aimed at the understanding of catalysis and stability of the phases formed during the hydrolysis of borohydrides. The active phase, composed of nanoparticles of Co₂B, was observed to be generated in situ by reaction of the stabilized NaBH₄ solution using either a cobalt chloride solution or formed from different Co containing solid materials. The characterisation of the remaining catalyst after test by XRD, magnetism, TEM and TGA-MS, permitted to elucidate the evolution of the phase composition during the hydrolysis reaction. It was shown that, whatever the Co precursor was, cobalt boride was formed in situ in the presence of NaBH₄ solution. However the Co-B nanoparticles were not stable in alkaline solution after NaBH₄ consumption, and were coated by a hydroxide surface layer which re-transformed into cobalt boride in the presence of additional NaBH₄ permitting an easy regeneration of the catalyst.

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