Quantitative evaluation of catalytic effect of metal chlorides on the decomposition reaction of NaAlH_4

摘要

The hydrogen desorption (or decomposition) reaction of NaAlH_4 is expressed as NaAlH_4 → 1/3 Na_3AlH_6 + 2/3 Al + H_2, and its desorption rate is accelerated by mixing metal chloride catalysts (e.g., TiCl_3). This catalytic effect of metal chlorides, MCln, is theoretically estimated in a quantitative way using atomization energy concept. The atomization energies, δE_M for metal ion and δECl for chloride ion in various metal chlorides are evaluated using the energy density analysis of the total energy. It is shown that the hydrogen desorption reaction rate increases with increasing n × δE_M values of metal chlorides. This indicates that the metal ion in MCln interacts mainly with hydrogen or [AlH _4]- complex anion in NaAlH_4. To confirm this calculated result, experiments are performed using NaAlH_4 mixed with Ti-based catalysts. The hydrogen desorption rate is enhanced in the order, TiCl_3 > TiO_2 > Ti metal nanopowder, indicating that the Ti ions in TiCl_3 or TiO_2 work to promote the catalytic reaction more effectively than the neutral Ti atoms in Ti metal nanopowder.