Spontaneous hydrolysis of sodium borohydride in harsh conditions

摘要

The present study reports fundamental results about the spontaneous hydrolysis of sodium borohydride NaBH_4, which is a potential hydrogen storage material for small, portable applications. The reaction (without stabilizing agent or catalyst) was carried out at temperatures of 30-80 ℃, initial NaBH_4 concentrations of 0.63-6.19 mol L~(-1) (2.3-18.9 wt%), and for unbuffered solutions, which are harsher experimental conditions than those reported so far. The H_2 evolution and the subsequent pH variation were observed to determine the reaction kinetic parameters and characterize the hydrolysis intermediates, i.e. the hydroxyborates BH_(4-z)(OH)_z~-, by XRD, IR and ~(11)B NMR. It was found that: (i) the apparent activation energy of the reaction was 98 ± 10 kJ mol~(-1) and the reaction order versus the initial NaBH4 concentration was 0; (ii) all of the reactions BH_4~-→ BH_3(OH)~-, BH_3(OH)~- →BH_2(OH)_2~-, BH_2(OH)_2~-→ BH(OH)_3~- and BH(OH)_3~-→B(OH)_4~- took place simultaneously; (iii) only 25 mol% of B(OH)_4~- and 75 mol% of BHJ were found at 25% of conversion; (iii) the hydroxyborates are very short-lived intermediates and only traces of BH_3(OH)~-were detected.