Enhanced hydrogen release by catalyzed hydrolysis of sodium borohydride–ammonia borane mixtures: a solution-state 11B NMR study

Abstract

Hydrolysis of mixtures consisting of sodium borohydride NaBH₄ (SB) and ammonia borane NH₃BH₃ (AB) was studied in the absence/presence of a Co catalyst. The kinetics of the H₂ evolutions was measured. The reactions were followed in situ by solution-state ¹¹B NMR and the hydrolysis by-products characterized by NMR, XRD and IR. It is demonstrated that the combination of the two compounds gives a synergetic effect. SB rapidly reduces the Co catalyst precursor and the NH₄⁺ ions from AB contribute in the dispersion of the in situ formed Co nanoparticles. As a result, the kinetics of H₂ evolution is greatly improved. For instance, a hydrogen generation rate of 29.6 L min⁻¹ g⁻¹(Co) was found for a mixture consisting of 81 wt% NH₃BH₃, 9 wt% NaBH₄ and 10 wt% CoCl₂. By ¹¹B NMR, it was showed that the reaction mechanisms are quite trivial. As soon as the Co catalyst forms in situ, SB, rather than AB, hydrolyzes until it is totally converted. Then, the overall hydrolysis continues with that of AB. Both reactions follow a bimolecular Langmuir–Hinshelwood mechanism; no reaction intermediates were observed during the process. In fact, SB and AB convert directly into B(OH)₄⁻, which comes in equilibrium with a polyborate compound identified as B₃O₃(OH)₄⁻. All of these results are discussed herein.