Theoretical Study Of Hydrogen Adsorption On Mg Surface With Co-doping Of Transition Metals

Hydrogen would be undoubtedly a phenomenal choice due to its high energy density andpollution free. However, to store hydrogen fuel economically and conveniently is still abottle-neck in its practical application. MgH2is a potential hydrogen storage material to breakthe bottle-neck, with a gravimetric storage capacity of7.6%and relatively inexpensive cost.Yet, two serious limits are ahead for MgH2. One is its hydride stability because the release ofhydrogen occurs at temperatures in excess of300°C for MgH2The other is slow kinetics ofdehydrogenation and hydrogenation. An important way to overcome the limits is to dopetransition metals (TMs) into the material.The present work, the interactions of a hydrogen atom with clean, vacancied, andtransition metal-doped (TM=Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru,Rh, Pd, Ag, Cd, Au, Pt) Mg(0001) surfaces are investigated using first-principles calculations.The H adsorption on Mg(0001) with TMs doped within the second layer is generally morestable than that on clean Mg, but clearly weaker than that on Mg surfaces with TM in the firstlayer. We found, however, that all these TM atoms prefer to substitute for the Mg atoms in thesecond layer rather than for those in the outermost layer of the Mg surface. To enhance thecatalytic effect of the TM dopants, we investigated various co-doping conditions of TMs, andwe found that i) Ti is a good “assistant” that stabilizes co-doped Co, Ni, Pd, Ag, Pt, and Auwithin the first layers and that ii) Ni and Co are more easily incorporated into the first layer ofa Mg surface when co-doped with Ti, V, and Nb. These observations may lead to a possibleapproach to stabilize the TM dopants within the first layer and thus promote thehydrogenation of Mg accordingly.Then, the dissociation and diffusion of H2with clean, Ni and Nb doped Mg(0001) surfaceare investigated. Individual Ni and Nb atoms within the outermost surface can reduce thedissociation barrier of the hydrogen molecule. Individual Ni or Nb, however, prefers tosubstitute for the Mg atoms within the second layer, leading to a weaker catalytic effect forthe dissociation of H2, a bottleneck for the hydriding of MgH2. Interestingly, co-doping of Niand Nb stabilizes Ni at the first layer, and results in a significant reduction of the dissociationbarrier of H2on the Mg surface, coupled with an increase of the diffusion barrier of H. This implies that “modest” transition metal catalysts are required to optimize the hydriding of Mgwith balanced barriers for dissociation of H2and diffusion of H on Mg surfaces.Finally, we studied Pd covered Mg(0001) surface, and found (i) The distance between Pdlayer and second Mg layer is smaller than that of pure Mg surface. It indicated that theinteraction between Pd and Mg is very strong;(ii) As d orbit of Pd, the surface of Pd coveredMg(0001) is benefited for dissociation of H2.