Mechanism for H2 release from potential hydrogen storage materials of phosphine alane and phosphine borane in the presence or absence of alane or borane: a theoretical study

Abstract

Electronic structure calculations have been employed to probe the reaction mechanisms of hydrogen release from phosphine alane and phosphine borane in the presence or absence of alane and borane. Geometries of stationary points are optimized at the MP2/aug-cc-pVDZ level. Then, the energy profiles are refined at the CCSD(T)/aug-cc-pVTZ level based on the MP2 optimized geometries. In both AlH₃PH₃ and BH₃PH₃ monomer, H₂ elimination cannot be compared with the Al–P and B–P bonds dissociation. The theoretical results demonstrate that both AlH₃ and BH₃ can facilitate H₂ dissociation from AlH₃PH₃. However, the H₂ production process becomes more difficult once the strong adduct is formed. Similarly, AlH₃ and BH₃ can also take part in a catalytic process for H₂ loss from BH₃PH₃ and induce a substantial reduction of the energy barrier for H₂ release. Comparison with the reaction mechanisms of H₂ elimination from BH₃PH₃ with AlH₃ or BH₃ as the catalyst shows that BH₃ is a better catalyst than AlH₃. Moreover, BH₃PH₃ is more favored as a potential hydrogen storage material than AlH₃PH₃ in the presence of AlH₃ or BH₃. So a catalyst is required as the generation of H₂ from phosphorous systems proceeds with an energy barrier much higher than the X–P (X = Al and B) bond energy.