The theoretical study of dehydrogenation mechanism from Sr(NH2BH3)2

Abstract

A first-principle study of dehydrogenation mechanism from Sr(NH2BH3)2 was performed. Three different pathways were explored for Sr(NH2BH3)2 monomer, and the most favorable dehydrogenation pathway involved the formation of the Sr-H group firstly, and then becoming the key hydride source for the consecutive H2 evolution. The intrinsic activation free energy barriers were 29.2, 30.7, 42.7, and 42.4 kcal/mol for four equivalent of H2 release at 298.15 K, which were slightly decreased by 0.3~2 kcal/mol at experimental temperature 333 and 366 K. For Sr(NH2BH3)2 dimer, the estimated intrinsic free energy barriers are 22.7, 30.4, 40.1, and 45.4 kcal/mol for four equivalent of H2 release, respectively. Although it seemed that the energy barriers for the second equivalent of H2 were still higher, considering the energy gain in the formation of dimer, − 18.1 kcal/mol, the first two equivalents of H2 release easily took place at experiment temperature. Thus, our calculations indicated that the most favorable dehydrogenation mechanism involved the strongly basic H− from Sr-H acting as a hydride source to accelerate the H2 release, and the effect of neighboring molecules in the solid phase should not be neglected. These results will shed some light on the design of the future hydrogen storage media.