Revision of the NaBO_2-H_2O phase diagram for optimized yield in the H_2 generation through NaBH_4 hydrolysis

摘要

The binary phase diagram NaBO_2-H_2O at ambient pressure, which defines the different phase equilibria that could be formed between borates, end-products of NaBH_4 hydrolysis, has been reviewed. Five different solid borates phases have been identified: NaBO_2·4H_2O (Na[B(OH)_4]·2H_2O), NaBO_2·2H_2O (Na[B(OH)_4]), NaBO_2·2/3H_2O (Na_3[B_3O_4(OH)_4]), NaBO_2·1/ 3H_2O (Na_3[B_3O_5(OH)_2]) and NaBO_2 (Na_3[B_3O_6]), and their thermal stabilities have been studied. The boundaries of the different Liquid + Solid equilibria for the temperature range from -10 to 80 ℃ have been determined, confirming literature data at low temperature (20-50 ℃). Moreover the following eutectic transformation, Liq. → Ice + NaBO_2·4H_2O, occurring at -7 ℃, has been determined by DSC. The Liquid-Vapour domain has been studied by ebullioscopy. The invariant transformation Liq.→ Vap. + NaBO_2·2/3H_2O has been estimated at 131.6 ℃. This knowledge is paramount in the field of hydrogen storage through NaBH_4 hydrolysis, in which borate compounds were obtained as hydrolysis reaction products. As a consequence, the authors propose a comparison with previous NaBO_2-H_2O binary phase diagrams and its consequence related to hydrogen storage through NaBH_4 hydrolysis.