Computational Study of Silica-Supported Transition Metal Fragments for Kubas-type Hydrogen Storage

Abstract

To verify the role of the Kubas interaction in transition metal grafted mesoporous silicas, and to rationalize unusual rising enthalpy trends with surface coverage by hydrogen in these systems, computational studies have been performed. Thus, the interaction of H2 with the titanium centers in molecular models for experimentally characterized mesoporous silica-based H2 absorption materials has been studied quantum chemically using gradient corrected density functional theory. The interaction between the titanium and the H2 molecules is found to be of a synergic, Kubas type, and a maximum of four H2 molecules can be bound to each titanium, in good agreement with previous experiments. The average Ti-H2 interaction energies in molecules incorporating benzyl ancillary ligands (models of the experimental systems) increase as the number of bound H2 units increases from two to four, in agreement with the experimental observation that the H2 adsorption enthalpy increases as the number of adsorbed H2 molecules increases. The Ti-H2 interaction is shown to be greater when the titanium is bound to ancillary ligands, which are poor π-acceptors, and when the ancillary ligand causes the least steric hindrance to the metal. Extension of the target systems to vanadium and chromium shows that, for molecules containing hydride ancillary ligands, a good relationship is found between the energies of the frontier molecular orbitals of the molecular fragments, which interact with incoming H2 molecules, and the strength of the M-H2 interaction. For the benzyl systems, both the differences in M-H2 interaction energies and the energy differences in frontier orbital energies are smaller than those in the hydrides, such that conclusions based on frontier orbital energies are less robust than for the hydride systems. Because of the high enthalpies predicted for organometallic fragments containing hydride ligands, and the low affinity of Cr(III) for hydrogen in this study, these features may not be ideal for a practical hydrogen storage system.

The Kubas interaction in M(ii) (M = Ti, V, Cr) hydrazine-based hydrogen storage materials: a DFT study

“…This functional was chosen because of its success in our previous studies on the benzyl titanium binding sites of a mesoporous silica 14 and on the vanadium binding sites of a hydrazine linked material, 17 and because it has been shown by Sun et al to be the best functional to balance computational speed and accuracy when looking at dihydrogen bound to metal centres. This functional was chosen because of its success in our previous studies on the benzyl titanium binding sites of a mesoporous silica 14 and on the vanadium binding sites of a hydrazine linked material, 17 and because it has been shown by Sun et al to be the best functional to balance computational speed and accuracy when looking at dihydrogen bound to metal centres.…”

“…2,[11][12][13][14][15][16][17] Our first computational study was of a storage material based on an experimentally confirmed mesoporous silicate with titanium benzyl binding sites, 11 and the second was a hydrazine linked system containing vanadium binding sites. 2,[11][12][13][14][15][16][17] Our first computational study was of a storage material based on an experimentally confirmed mesoporous silicate with titanium benzyl binding sites, 11 and the second was a hydrazine linked system containing vanadium binding sites.…”

“…2,[11][12][13][14][15][16][17] Our first computational study was of a storage material based on an experimentally confirmed mesoporous silicate with titanium benzyl binding sites, 11 and the second was a hydrazine linked system containing vanadium binding sites. In both of these systems the experimental enthalpies in the 20-40 kJ mol −1 range are believed to be due to the Kubas interaction, and this was verified by our calculations, 14,17 which also showed that the highest H 2 binding enthalpies would occur in systems based on early transition metals with hydride ancillary ligands. In both of these systems the experimental enthalpies in the 20-40 kJ mol −1 range are believed to be due to the Kubas interaction, and this was verified by our calculations, 14,17 which also showed that the highest H 2 binding enthalpies would occur in systems based on early transition metals with hydride ancillary ligands.…”

Comprehensive review on synthesis and adsorption behaviors of graphene-based materials

“…Thus, by use gen molecules are basically non-polar, but the strong interaction of the metal particles leads to the dipole inducing effects of the hydrogen molecules. The third group and any remaining hydrogen molecules are adsorbed by the same mechanism of high gas pressure, but this equilibrium can be broken when the interaction of oxidized graphite surfaces and dipole-induced hydrogen molecules is stronger than metal-hydrogen interactions [98][99][100][101][102]. This mechanism can be applied only under higher pressure, and when graphite supports are charged strong electron acceptors.…”

Spin-State Effects on the Thermal Dihydrogen Release from Solid-State [MH(η2-H2)dppe2]+ (M = Fe, Ru, Os) Organometallic Complexes for Hydrogen Storage Applications

“…11,12 While this behavior has since been disproven by examination of the electronic exchange interaction occurring in similar metal–organic frameworks, 13−15 interest in Kubas-type binding has remained for hydrogen storage applications and has led to renewed experimental and theoretical efforts to develop materials exhibiting this effect. 16−18 …”