Synergic catalytic effect of Ti hydride and Nb nanoparticles for improving hydrogenation and dehydrogenation kinetics of Mg-based nanocomposite

The Mg-9.3 wt%(TiH_(1.971)-TiH)-0.7 wt% Nb nanocomposite has been synthesized by hydrogen plasma-metal reaction(HPMR) approach to enhance the hydrogen sorption kinetics of Mg at moderate temperatures by providing nanosizing effect of increasing H "diffusion channels" and adding transition metallic catalysts. The Mg nanoparticles(NPs) were in hexagonal shape range from 50 to 350 nm and the average size of the NPs was 177 nm. The small spherical TiH_(1.971), TiH and Nb NPs of about 25 nm uniformly decorated on the surface of the big Mg NPs. The Mg-TiH_(1.971)-TiH-Nb nanocomposite could quickly absorb 5.6 wt% H_2 within 5 min at 573 K and 4.5 wt% H_2 within 5 min at 523 K, whereas the pure Mg prepared by HPMR could only absorb 4 and 1.5 wt% H_2 at the same temperatures. TiH_(1.971), TiH and Nb NPs transformed into TiH_2 and Nb H during hydrogenation and recovered after dehydrogenation process. The apparent activation energies of the nanocomposite for hydrogenation and dehydrogenation were 45.0 and 50.7 kJ mol~(-1), which are much smaller than those of pure Mg NPs, 123.8 and 127.7 kJ mol~(-1). The improved sorption kinetics of the Mg-based nanocomposite at moderate temperatures and the small activation energy can be interpreted by the nanostructure of Mg and the synergic catalytic effects of Ti hydrides and Nb NPs.