  The kinetics of hydrogen absorption by magnesium bulk is affected by two main
activated processes: the dissociation of the H$_2$ molecule and the diffusion
of atomic H into the bulk. In order to have fast absorption kinetics both
activated processed need to have a low barrier. Here we report a systematic
ab-initio density functional theory investigation of H$_2$ dissociation and
subsequent atomic H diffusion on TM(=Ti,V,Zr,Fe,Ru,Co,Rh,Ni,Pd,Cu,Ag)-doped
Mg(0001) surfaces. The calculations show that doping the surface with TM's on
the left of the periodic table eliminates the barrier for the dissociation of
the molecule, but the H atoms bind very strongly to the TM, therefore hindering
diffusion. Conversely, TM's on the right of the periodic table don't bind H,
however, they do not reduce the barrier to dissociate H$_2$ significantly. Our
results show that Fe, Ni and Rh, and to some extent Co and Pd, are all
exceptions, combining low activation barriers for both processes, with Ni being
the best possible choice.
