Accentuating the Competency of $3d$ Transition Metal Doped $Mg_4$ Clusters towards Molecular Hydrogen Adsorption: A DFT Study

Density functional theory $(DFT)$ studies show that doping of $3d TM$ atoms into $Mg_4^{0,+} (TMMg_3^{0,+})$ can alter the endothermic nature of molecular hydrogen adsorption on bare $Mg_4^{0,+}$. $H_2$ adsorption on $TMMg_3^{0,+}$ clusters depends on the $TM(3d)$ orbital contribution to the frontier molecular orbitals of the clusters. In $H_2TMMg_3^{0,+}$ complexes, $H_2$ is adsorbed through donation and back donation of electronic charges with $TMMg_3^{0,+}$ clusters. $H_2$ binding energy is the maximum for $NiMg_3$ (-0.76 eV) and $FeMg_3^+$ (-0.48 eV) among the neutral and cationic $TMMg_3$ clusters, respectively. The geometry of $H_2$ adsorption complex entirely depends on the geometry of the host cluster. The lowest energy symmetrical structures of both $NiMg_3$ and $FeMg_3^+$ clusters are less efficient than some slightly higher energy low symmetrical geometrical isomers for adsorption of multiple hydrogen molecules (13.28 $H_2$ $wt\%$ for $NiMg_3$ and 7.89 $H_2$ $wt\%$ for $FeMg_3^+$). Greater exposer of $TM^{0,+}$ along with its lower Mg coordination number make host clusters more suitable for hydrogen storage.