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.