Molecular models of the MH 2 binding sites of experimentally characterised amorphousanadium hydrazide gels are studied computationally using gradient corrected density functional theory, to probe the coordination number of the vanadium in the material and the nature of the interaction between the metal and the H2 molecules. The H2 is found to bind to the vanadium through the Kubas interaction, and the first quantum theory of atoms-nmolecules analysis of this type of interaction is reported. Strong correlation is observed between the electron density at the HH bond critical point and the MH2 interaction energy. Four coordinate models give the best reproduction of the experimental data, suggesting that the experimental sites are four coordinate. The VH2 interaction is shown to be greater when the non-hydrazine based ligand, THF, of the experimental system is altered to a poorer p -acceptor ligand. Upon altering the metal to Ti or Cr the MH2 interaction energy changes little but the number of H2 which may be bound decreases from four (Ti) to two (Cr). It is proposed that changing the metal from V to Ti may increase the hydrogen storage capacity of the experimental system. A 9.9 wt% maximum storage capacity at the ideal binding enthalpy for room temperature performance is predicted when the Ti metal is combined with a coordination sphere containing 2 hydride ligands.
- density functional calculat6ions
- hydrogen storage
- transition metals