Résumé
Cobalt-catalyzed hydrolysis of sodium borohydride (NaBH
4
) 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 NaBH
4
. 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
x
B (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 NaBH
4
hydrolysis is exhaustively surveyed. The results of the
ex situ
characterizations are largely discussed, for example that: (i) the
ex situ
characterized Co
x
B 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
x
B 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.
Cobalt is largely used as a catalyst in hydrolysis of sodium borohydride NaBH
4
but its catalytically active form has not been clearly evidenced yet, various forms having been suggested: namely, cobalt borides, cobalt-boron alloy, metal cobalt and cobalt hydroxide.