Figure 1. Lowest-energy structures of MgmHn(n ≥ 2m) clusters calculated at M062X/def2TZVP level. The green and white balls represent Mg and H atoms, respectively.
3.2 Hydrogen Adsorption of MgmHnClusters
The adsorption energies (Ea) of MgmHn (m = 3-6, n = 1-13) clusters are shown in Figure 2a. The adsorption energy of these structures was further verified with CCSD (T)/def2TZVP and shown in Table S1 in Supporting Information. Thermodynamically, the lower is the adsorption energy, the more stable the structure is. For MgmHn (n < 2m), as the number of adsorbed hydrogen atoms increases, the adsorption energies of the clusters tend to be more negative until Mg:H = 1:2. The clusters with the stoichiometric ratio of Mg:H of 1:2 are the most stable. When the hydrogen atoms are further adsorbed, the adsorption energy of oversaturated MgmHn (n = 2m+1) structures rises significantly, which indicates that MgmHn (n = 2m+1) are less stable than saturated MgmHn (n = 2m). However, it is noted that the hydrogen adsorption reaction in reality usually occurs under the high-pressure condition [38, 39], in which the hydrogen-enriched clusters can exist. As shown in Figure 2b, as the size of Mgm clusters increases, the adsorption energy of MgmHn (n = 2m+1) gradually decreases and the structure becomes relatively more stable. The adsorption energies of Mg3H7, Mg4H9 are all above zero, while for Mg5H11, Mg6H13, the adsorption energies are negative. These indicate that the formation of Mg3H7, Mg4H9 is endothermic reaction while the formation of Mg5H11, Mg6H13 is exothermic reaction.