Date of Award

2007

Publication Type

Doctoral Thesis

Degree Name

Ph.D.

Department

Chemistry and Biochemistry

Keywords

Pure sciences, Bis(toluene), Hydrogen storage, Titanium oxide

Supervisor

David Antonelli

Rights

info:eu-repo/semantics/openAccess

Abstract

In this work, mesoporous and microporous titanium oxides were reduced and/or impregnated by a variety of reducing agents, such as alkali metals, organometallic sandwich compounds of Ti, V, and Cr, as well as alkali fullerides. These new composite materials were characterized by nitrogen adsorption, powder X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy and elemental analysis. The hydrogen sorption properties were investigated as a function of surface area, pore size, and reducing agent for these new composite materials at 77 K. Unlike MOFs and porous carbons, the hydrogen sorption performance of these new composite materials does not depend greatly on surface area; however, the reduction in the surface Ti species seems to be the crucial factor in determining hydrogen sorption capacities. For example, microporous Ti oxide reduced with bis(toluene) Ti possesses a surface area of 208 m2/g, but exhibits an overall volumetric storage capacity of 40.46 kg/m3 at 77 K and 100 atm. This volumetric storage capacity is higher than that of pristine material, which has a surface area of 942 m2/g. The improved performance for these reduced composite materials relative to the untreated sample was attributed to the increased reduction level of the metal centers in the framework of the structure, which allows for more facile π-back donation to the H-H σ bond, a factor known to strengthen hydrogen binding to metals. Another surprising feature in these reduced materials is the unusual trend in enthalpies, which show an unprecedented increase in binding strength as the surface coverage increase. The binding enthalpies also increase on progressive reduction, from 4.21 to 8.35 kJ/mol. This highly unusual behavior reflects a different mechanism of surface binding than simple physisorption, and indicates that further efforts are required to find a suitable reducing reagent in order to reach even higher volumetric storage densities and tune the hydrogen binding enthalpies to over 20 kJ/mol, which is proposed to be ideal value for porous samples operating at ambient temperature.

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