Effect of Doped Transition Metal on Reversible Hydrogen Release/Uptake from NaAlH₄

Abstract

Periodic density functional theory calculations with plane-wave basis set and projector-augmented wave potentials have been carried out to investigate the stability and hydrogen interaction in the NaAlH(4)(001) surfaces doped with 3d transition-metal (TM) elements. A complex structure, TMAl(3)H(12), in which the TM atom occupies the interstitial position formed from three AlH(4)(-) groups, is the most stable structure for TM = Sc to Co. The stability of the complex structure, as well as the hydrogen desorption energies from different positions of the complex structure, was found to follow the 18-electron rule in general. The electron-deficient TMAl(3)H(x) tends to get more electrons by coordinating with the surrounding Al-H bonds and H-H bond, or by losing the "outside" hydrogen atoms. On the other hand, the electron-rich complex loses its excess electrons easily by releasing AlH(x), which resulted in the formation of a new catalytic center, or by desorbing H(2). By cycling between the electron-deficient and electron-rich states, TMAl(3)H(x) acted as an active center in reversible hydrogen release/uptake processes. Electronic structure analysis revealed that the electron transfer between hydrogen and Al groups mediated by the d orbitals of TMs played important roles in hydrogen release/uptake from alanate-based materials. The exchange of ligands can be described as a sigma-bond metathesis process catalyzed by transition metals through a dihydrogen complex. Early transition metals are more efficient to reduce hydrogen desorption energy and break H-H and Al-H bonds as a result of balanced electron-accepting/backdonating abilities, making them better candidates as catalysts. The present analyses are consistent with the experimental observations.

A multifaceted approach to hydrogen storage

“…The chemistry of Z 2 -H 2 coordination complexes has been described previously, 66 and their role in the context of dehydrogenation of the TM doped NaAlH 4 was also analysed in a recent study. 67 Although the observed mechanism was completely analogous for the Ti and Sc centres, some considerable differences were seen in the reaction energy profiles. 62 We next confirmed that the model systems exhibit unambiguous kinetic stability when fitted into the (001) surface of the NaAlH 4 crystal.…”

Mechanisms for the enhanced hydrogen desorption performance of the TiF4-catalyzed Na2LiAlH6 used for hydrogen storage

“…3,26 Others believed that the formation of catalytic Al x Ti y complexes on the surface was critical for the improvement of the dissociation/recombination of molecular hydrogen and mass transport. [27][28][29][30] Recently, Kang et al determined that the anion of the catalyst precursor also played an important role in the de-/hydrogenation process of alanates with thermodynamic modification originating from the F À substitution in the hydride lattice. 31,32 Despite intense experimental and theoretical efforts towards mechanistic understanding, the catalytic role of Ti species for the significant improvement in de-/hydrogenation kinetics of alanates is still elusive, due to the multiplicity of the effects.…”

Probing the properties of the (111) and (100) surfaces of LaB6 through infrared spectroscopy of adsorbed CO

“…The bottom two layers were then fixed at those relaxed positions whereas the remaining atoms in the slabs together with the adsorbed CO molecules were allowed to optimize according to the calculated Hellman-Feynman forces. Similar parameters and settings have been applied in previous studies of a range of systems [55][56][57][58]. Fig.…”