Surface and Particle-Size Effects on Hydrogen Desorption from Catalyst-Doped MgH₂

With their high capacity, light-metal hydrides like MgH₂ remain under scrutiny as reversible H-storage materials, especially to develop control of H-desorption properties by decreasing size (ball-milling) and/or adding catalysts. By employing density functional theory and simulated annealing, we study initial H₂ desorption from semi-infinite stepped rutile (110) surface and Mg₃₁H₆₂ nanoclusters, with­(out) transition-metal catalyst dopants (Ti or Fe). While Mg₃₁H₆₂ structures are disordered (amorphous), the semi-infinite surfaces and nanoclusters have similar single, double, and triple H-to-metal bond configurations that yield similar H-desorption energies. Hence, there is no size effect on desorption energetics with reduction in sample size, but dopants do reduce the H-desorption energy. All desorption energies are endothermic, in contrast to a recent report.