Fundamental studies of H2 interaction with MAl3 clusters [M = Li, Sc, Ti, Zr]

• A first-principles calculation on H2 interaction and saturation on stable MAl3 clusters [M = Li, Sc, Ti, Zr] is performed.
• Transition metals have negative electron charge density while Al atom has positive in MAl3 from NBO charge analysis.
• H2 undergoes dissociative chemisorption on the M atop site resulting in M–H–Al bridge bond.
• H2 saturation on M atop site results initially chemisorption and then physisorption with 3-centered Kubas interaction.
• External electric field applied on TiAl3H4 leads to more polarization, H2 bond elongation and increased adsorption energy.

Complex metal hydride is a promising hydrogen storage material for automobile applications due to its reversible storage capacity. The presence of transition metal halide is found to improve significantly the kinetics of H2 adsorption and desorption processes. Experimental studies have indicated the formation of distorted MAl3 phase where M = Sc, Ti, Zr. In this study, a first-principles density functional study has been performed to investigate the hydrogen interaction and saturation on stable tetrahedral MAl3 clusters [M = Li, Sc, Ti, and Zr] by employing spin-polarized hybrid and non-local density functionals. On saturation, the first H2 molecule undergoes chemisorption in the transition metals while further loading results in physisorption with the Kubas-type H2 interaction. Activation energy barrier for the H2 dissociation over the cluster is calculated to be ∼0.2 eV for the transition metals. Effect of external electric field on the MAl3H4 cluster with molecular H2 is studied which leads to polarization of physisorbed H2 and the cluster. The results offer an explanation for catalysts role in improving the kinetics of H2 sorption process in complex metal hydrides.

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