Transition metal (TM) atoms prefer to substitute for the Mg atoms in the second layer rather than for those in the outermost layer of Mg(0001) surface.However, the H adsorption on Mg surface with TM doped at the second layer are clearly weaker than that with TMs within the first layer, though generally more stable than that on clean Mg.To enhance the catalytic effect of the TM dopants, we have investigated various co-doping conditions of TMs, and found that: i).Ti is a good "assistant" that stabilizes co-doped Co,Ni, Pd, Ag, Pt, and Au within the first layers; ii) Ni and Co are more easily incorporated into the first layer of a Mg surface when co-doped with Ti, V, and Nb; iii).The co-doping of TMs, exemplified with (Ni, Nb), results in a significant reduction of the dissociation barrier for hydrogen molecules on the Mg surface.These observations may lead to a possible approach to stabilize the TM dopants within the first layer and thus promote the hydrogenation of Mg accordingly.Meanwhile, we will briefly introduce our recent progresses in the dehydrogenation mechanism of NaAlH4.