Deactivation and reactivation of cobalt in hydrolysis of sodium borohydride

Cobalt is an efficient metal catalyst in hydrolysis of sodium borohydride NaBH4 owing to its tunable reactivity and cost-effectiveness. However, it is sensitive to the reaction medium, deactivating over cycles. The present work is a contribution in highlighting the fundamental importance of the in situ forming borate-based compounds in the deactivation. Cobalt electrodeposited over nickel foam was used as supported catalyst. Its reactivity and its surface evolution (by SEM, EDS, XRD, XPS, TGA and 11B NMR) were followed up after reaction. It stands out that a surface coating consisting of borates forms already after the first hydrolysis. Actually, the borates are strongly surface-adsorbed, being resistant to thorough washing with deionized water, and are clearly at the origin of the catalyst deactivation. To regain the initial reactivity of cobalt, it is proposed to wash it with a slightly acid solution, which can be done after each hydrolysis. In doing so, the catalyst shows a good stability over cycles. Interestingly, it was observed by SEM that the coating peels off and, by NMR, that the coating is made of borates. Our main results and the main SEM images are reported herein.