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%.