Effect of catalysts on dehydrogenation of LiAlH₄+MgH₂ composite and catalytic mechanism

The LiAlH₄+MgH₂ composites doped with different catalysts were prepared by mechanical milling to improve the hydrogen desorption properties. Structural and dehydrogenation properties of the composites were investigated by XRD, SEM, EDS, XPS and Sieverts method. The catalytic mechanism of TiF₃ was also discussed. The results show that doping with TiF₃ decreases effectively the onset hydrogen desorption temperature, and increases the dehydrogenation kinetics of the composite. The TiF₃-doped system exhibits the onset hydrogen desorption temperature of 84.1°C and hydrogen desorption capacity (mass fraction) of 8.0%. The unknown Ti^x+-containing compound forms during thermal desorption of the composite, which promotes effectively the synergetic dehydrogenation of LiAlH₄ and MgH₂. In addition, the dehydrogenation activation energy E_a of the composite doped with TiF₃ is calculated by JMA equation to be 79.1 kJ/mol, which is much lower than that of the composite without TiF₃ addition (91.3 kJ/mol), indicating that TiF₃ addition decreases greatly the kinetic barrier for dehydrogenation.