Figure
4. Time evolution of the potential energies and representative
hydrogen-hydrogen pair distance of (a), (c)
MgmH2m and (b), (d)
MgmH2m+1 clusters at 300K.
As discussed above, the hydrogen dissociation reactions for saturated
MgmH2m clusters were not observed in our
AIMD simulations. To further understand hydrogen dissociation reactions
of the saturated MgmH2m clusters, we
perform hydrogen desorption energies and barrier heights of hydrogen
dissociation reactions of the saturated
MgmH2m clusters. Fig. 5a shows the
average desorption energy
(<Ed>)
as a function of cluster size. Overall, the average desorption energy
increases monotonically for larger size of clusters and approaches the
experimental value of 0.81 eV for bulk MgH2. The
inclusion of ZPE slightly shifts down the average desorption energy for
larger clusters. Figure 5b shows the stepwise desorption energies
(∆Ed) of MgmHn (m = 3-6)
clusters. From m=4 to 6, the stepwise desorption energy first rises
slightly and then decreases abruptly with the lower content of hydrogen
in the clusters. This indicates that less and less energy is needed for
the dissociation of hydrogen molecule as the desorption reaction
proceeds. Interestingly, the production of the last H2is exothermic reaction, in consistent with endothermic reaction for the
adsorption of the first H2 in the clusters.