First-principles investigation of adsorption and dissociation of hydrogen on Mg2Si surfaces

Silicon can be used to destabilize MgH2 by producing Mg2Si and H-2 as products. This process does not appear to be easily reversible. We have used density functional theory to investigate the adsorption and dissociation of H-2 on clean and oxidized Mg2Si surfaces as a first step toward understanding the hydrogenation cycle. The surface energies of different low-index surfaces of Mg2Si have been examined and the ((1) over bar 10) surface has the lowest surface energy of all the surfaces investigated. The energy barriers for hydrogen dissociation on the clean ((1) over bar 10) surface along two different pathways were found to be 39.8 and 47.2 kJ/mol. Hydrogen dissociation should therefore be facile at room temperature. Oxide formation on Mg2Si((1) over bar 10) was calculated to be very exothermic. Our calculations indicate that the equilibrium coverage of oxygen on Mg2Si is 1.75 monolayer down to very low partial pressures of oxygen. Our calculations indicate that hydrogen dissociation is strongly inhibited on the oxide surface.