Sodium borohydride (NaBH4) is an interesting material for future energy applications, and in particular for hydrogen storage [1]. Its hydrolysis allows the production of hydrogen that can be readily used in fuel cells. In order to better understand the chemistry of this reaction, we have used liquid-phase calorimetry to determine the reaction mechanism for hydrogen liberation on nanoparticles of cobalt boride synthesized in situ.
The experiments were carried out in a DRC reaction calorimeter (Setaram) coupled to measurements of the volume of hydrogen evolved using a volumetric flowmeter (Ritter TG01). The catalytic material is generated in situ by reaction of sodium borohydride with cobalt chloride. The resulting catalyst is composed of nanoparticles of Co2B of diameter ca. 10 nm.

Catalytic tests realized with low concentrations of NaBH4 have confirmed the catalytic activity of the synthesized materials. The tests performed at higher concentrations have made it possible to show that the reaction mechanism over this type of catalyst consists of three elementary steps, namely:

1) Exothermic reduction of the surface layer of cobalt borate to cobalt boride.

2) The catalytic reaction proper, in which the evolved energy depends on the hydration state of the metaborates generated as reaction products, between -217 kJ/molNaBH4 at high hydration and -249 kJ/molNaBH4 at low hydration.
3) Endothermic regeneration of the protective borate layer.

[1] L. Schlappbach, A. Züttel, Nature, 414 (2001), 353-358.