(236a) First Principles Investigation of Adsorption and Dissociation of Hydrogen on the Mg2si Surface

MgH₂ is a light-weight metal hydride having a hydrogen storage capacity of 7.7 wt%. However, the heat of reaction for this hydride is too high so that hydrogen is not released until unacceptably high temperatures. For example, MgH₂ has a H₂ vapor pressure of 1 bar at 275 °C. Vajo and coworkers (JPCB, 108, 13977, 2004) have demonstrated that the thermodynamics of MgH₂ can be modified by adding Si to form a destabilized metal hydride, resulting in the reaction 2MgH₂ + Si = Mg₂Si + 2H₂. This destabilized reaction has a heat of reaction about one-half that of direct decomposition of MgH₂, on a per mole of H₂ basis. However, Vajo et al. were unable to observe the reverse reaction, Mg₂Si + 2H₂ = 2MgH₂ + Si, even though the computed thermodynamics indicate that the reaction is favorable. Recently Janot et al. (Intermetallics, 14, 163, 2006) have demonstrated that Mg₂Si can be hydrogenated to form MgH₂ and Si when ball-milled under a hydrogen atmosphere.

We present theoretical calculations modeling the first steps of the hydrogenation of Mg₂Si. Our goal is to understand why ball milling is required in order to hydrogenate Mg₂Si. Electronic density functional theory (DFT) calculations were used to investigate the adsorption and dissociation of H₂ on the clean and oxidized Mg₂Si surfaces. The surface energies of different low-index surfaces are examined. It was found that the (-110) surface is the most energetically favorable among all the surfaces tested. The adsorption energies are calculated for H atoms and H₂ molecules on the surface to identify the most favorable binding sites and geometries. The energy barriers for hydrogen dissociation on the clean Mg₂Si surface along two different pathways have been computed; the calculated barriers are 39.8 kJ/mol and 47.2 kJ/mol, after zero point vibrational energy corrections. Hydrogen dissociation processes are investigated on Mg₂Si(-110) surfaces with varying amounts of oxygen contamination. The energy barrier for hydrogen dissociation on the surface with 2/3 ML coverage of oxygen increases to 80.0 kJ/mol. More importantly, there are no energeticly favorable sites for hydrogen atoms to adsorb when the oxygen coverage is 5/4 ML or greater, indicating that hydrogen can not dissociate on oxidized Mg₂Si surfaces. Oxide formation on the Mg₂Si surface is highly exothermic, having an energy of about 4.5 eV per oxygen atom. The pure Mg₂Si surface is therefore expected to be highly reactive to oxidation. Our calculations indicate that hydrogen decomposition should be facile on the clean Mg₂Si surface at room temperature, but that surface oxidation will prevent dissociation under ambient conditions. Our calculations can therefore explain the experimental observations regarding hydrogenation of Mg₂Si.