Magnesium alanate hydride is a potential material for hydrogen storage because of its high hydrogen capacity(9.3 wt%).Some investigations on the kinetic and thermodynamic properties of Mg(AlH4)2 have verified that the addition of suitable catalysts such as TiCl3 and TiB2 [1] is an effective method for reducing its dehydriding temperature.However,the mechanisms of the catalytic effect of Ti or Ni additives on the dehydrogenation properties of Mg(AlH4)2 are still not fully understood.The first-principles calculations show that the Ti/Ni dopants mainly affect the geometric and electronic structures of their vicinal AlH4 units.In the Ti-doped case,Ti prefers to occupy the 13-hedral interstice(TiiA)and substitute for the Al atom(TiAl),to form a high-coordination structure TiHn(n = 6,7).The Ti 3d electrons hybridize markedly with the H 1s electrons in TiAl and with the Al 3p electrons in TiiA,which weakens the Al-H bond of adjacent AlH4 units and facilitates the hydrogen dissociation.A TiAl3H13 intermediate in TiiA is inferred as the precursor of Mg(AlH4)2 dehydrogenation[2].In contrast,Ni tends to occupy the octahedral interstice to form the NiH4 tetrahedron.The tight bind of the Ni with its surrounding H atoms inhibits their dissociation though the nearby Al-H bond also becomes weak.Therefore,Ti is the better dopant candidate than Ni for improving the dehydrogenation properties of Mg(AlH4)2 because of its abundant activated hydrogen atoms and low hydrogen removal energy.