Hydrogen storage in chemically reducible mesoporous and microporous Ti oxides

Chemically reducible micro- and mesoporous Ti oxides with controlled pore sizes from 12 to 26 A were synthesized. The hydrogen storage and adsorption capacity at 77 K was tested as a function of surface area, pore size, and reducing agent. Surprisingly, the oxidation state of the surface Ti species had an even greater effect on the storage densities than surface area or pore size. For example, the 12 A material reduced with bis(toluene) Ti possesses a surface area of less than 300 m2/g, but absorbs up to 4.94 wt % and 40.46 kg/m3 of H2 reversibly at 77 K and 100 atm. This volumetric storage capacity is higher than that of AX-21, which has a much higher surface area. The H2 binding enthalpies increased from 4.21 kJ/mol to 8.08 kJ/mol as the surface oxidation state of the Ti decreased. These results suggest that a Kubas-type sigma H2 complex may be involved and that further tuning of the H2 binding enthalpies through use of appropriate organometallic reagents may achieve even higher storage levels at more moderate temperature.