Quantitative evaluation of catalytic effect of metal chlorides on the decomposition reaction of NaAlH₄

The hydrogen desorption (or decomposition) reaction of NaAlH₄ is expressed as NaAlH₄ → 1/3 Na₃AlH₆ + 2/3 Al + H₂, and its desorption rate is accelerated by mixing metal chloride catalysts (e.g., TiCl₃). This catalytic effect of metal chlorides, MCl_n, is theoretically estimated in a quantitative way using atomization energy concept. The atomization energies, δE_M for metal ion and δE_Cl 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 MCl_n interacts mainly with hydrogen or [AlH ₄]^- complex anion in NaAlH₄. To confirm this calculated result, experiments are performed using NaAlH₄ mixed with Ti-based catalysts. The hydrogen desorption rate is enhanced in the order, TiCl₃ > TiO₂ > Ti metal nanopowder, indicating that the Ti ions in TiCl₃ or TiO₂ work to promote the catalytic reaction more effectively than the neutral Ti atoms in Ti metal nanopowder.