Adsorption and desorption of hydrogen in Mg nanoclusters: Combined effects of size and Ti doping

Using density functional theory formalism, we have investigated the interaction of hydrogen with pure and Ti doped Mg clusters. The objective of this study is two folds: (i) the reactivity of small Mg clusters in comparison to the extended Mg surface and (ii) the catalytic effect of Ti on the hydrogenation behavior. For Mg55 cluster, the activation energy of hydrogenation is calculated to be 0.72 eV, which is 30% less than the bulk value of 1.04 eV. The interaction of hydrogen with Mg55 and TiMg54 clusters gives the binding energy of 0.217 and 0.164 eV, respectively. Moreover, the activation energy calculated by the elastic band method reveals that the dissociation barrier of hydrogen is 0.72 and 0.58 eV for Mg55 and TiMg54, respectively. Thus we could show a significant reduction in the activation barrier (almost 40%) of hydrogen dissociation in small clusters than the bulk. This has been attributed to the combined effects of the finite size of Mg clusters and the catalytic influence of Ti substitution. Further to underscore the hydrogen desorption mechanism, we have calculated the onset temperature of hydrogen diffusion using ab initio molecular dynamics simulation study on the hydrogenated Mg55 cluster. The results reveal that at room temperature, the hydrogen atoms starts toggling from one Mg to another, which has been ascribed as the onset of hydrogen desorption.