Hydrogen release through catalyzed methanolysis of solid sodium borohydride

Abstract

Methanolysis of sodium borohydride (NaBH4) is a way of recovering the hydrogen stored in the hydride. Though the reaction is spontaneous, it can be accelerated by virtue of Co-TiO2 or Ru-TiO2 catalysts. Under our experimental conditions, Co-TiO2 shows high catalytic performances, higher than those of Ru-TiO2. Hydrogen generation rates of 144 to 644 L(H2) min−1 g−1(Co) were measured as the Co content was decreased. The kinetic parameters of the catalyzed reaction were determined. The Co-TiO2-catalyzed methanolysis follows a power law, i.e. r20 = k·[NaBH4]1.3·[CH3OH]0.9 with k = 1.8 × 10−2 s−1. The Langmuir–Hinshelwood bimolecular mechanism accounts for the kinetics. The apparent activation energy was found to be 20.4 kJ mol−1 whereas that of the catalyzed hydrolysis was 49.4 kJ mol−1. Indeed, the catalyzed methanolysis was compared to the catalyzed hydrolysis as well as the catalyzed ethanolysis. For instance, it was remarked that water in methanol has a detrimental effect on the H2 release kinetics. In parallel, the gravimetric hydrogen density of the system NaBH4-CH3OH has been optimized. Under our experimental conditions, it was found that the highest capacity that can be achieved is 3.4 wt%.