Cobalt in NaBH4hydrolysis

Abstract

Cobalt-catalyzed hydrolysis of sodium borohydride (NaBH₄) has attracted great attention since the hydride is believed to be promising hydrogen storage material. Cobalt is an efficient metal catalyst and has already proven to be a potential alternative to noble metals. Nevertheless it is not stable. Indeed it transforms into a Co- and B-based material when on contact with NaBH₄. Through ex situ characterizations (e.g. ICP, XRD, XPS and SEM), the Co- and B-based material has been supposed to be either a cobalt boride Co_xB (with x from 1 to 3) or a Co–B alloy. This contradiction is the topic of the present paper. Herein, the literature dedicated to the Co-catalyzed NaBH₄ hydrolysis is exhaustively surveyed. The results of the ex situ characterizations are largely discussed, for example that: (i) the ex situ characterized Co_xB or Co–B might be different from the in situ formed Co- and B-based catalyst; (ii) there is no clear evidence of the formation of either Co_xB or Co–B; (iii) the in situ formed catalyst would change in accordance with a cycle in the course of the hydrolysis; and (iv) in situ characterizations are clearly required but their setting up is a challenge. These conclusions, among others, are argued.