Catalytic effects of subsurface carbon in the chemisorption of hydrogen on a Mg(0001) surface: An ab-initio study

Ab initio density functional theory (DFT) calculations are performed to explore possible catalytic effects on the dissociative chemisorption of hydrogen on a Mg(0001) surface when carbon is incorporated into Mg materials. The computational results imply that a C atom located initially on a Mg(0001) surface can migrate into the subsurface and occupy an fee interstitial site, with charge transfer to the C atom from neighboring Mg atoms. The effect of subsurface C on the dissociation of H₂ on the Mg(0001) surface is found to be relatively marginal: a perfect sublayer of interstitial C is calculated to lower the barrier by 0.16 eV compared with that on a pure Mg(0001) surface. Further calculations reveal, however, that sublayer C may have a significant effect in enhancing the diffusion of atomic hydrogen into the sublayers through fee channels. This contributes new physical understanding toward rationalizing the experimentally observed improvement in absorption kinetics of H₂ when graphite or single walled carbon nanotubes (SWCNT) are introduced into the Mg powder during ball milling.