Date of Award

2010

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Chemistry and Biochemistry

First Advisor

Antonelli, David (Chemistry and Biochemistry)

Keywords

Chemistry, Inorganic.

Rights

CC BY-NC-ND 4.0

Abstract

Hexagonally-packed mesoporous silicas (HMS) grafted with low-valent titanium, vanadium, and chromium organometallic fragments possessing oxidation states between (II) and (IV) were synthesized, tested for their hydrogen storage capacities, and characterized by XRD, nitrogen adsorption, XPS, and EA. The effects of the variation in the surface area, pore size, transition metal type, and metal oxidation state, the organometallic loading levels, as well as the ligand environment on the H2 adsorption capacity and the binding enthalpies of these systems were investigated. This study demonstrated that titanium is more effective at hydrogen binding than vanadium and chromium. HMS silica grafted with benzyl titanium (III) fragments can accommodate up to 4.85 H2 per Ti center. This compares to 2.74 H2 per vanadium center as for the HMS grafted with tris(mesityl) vanadium, and to 1.82 and 2.20 H2 per chromium as for the HMS treated with the tris[bis(trimethylsilyl)methyl] chromium and bis[(trimethylsilyl)methyl]chromium respectively. The hydrogenation of the metal centers had a pronounced effect on the adsorption capacity of the Cr-grafted HMS. This capacity increased from 1.82 to 3.20 H2 per chromium in the case of HMS treated with tris[bis(trimethylsilyl)methyl] chromium, and from 2.20 H2 to 3.50 H2 per chromium in the case of HMS treated with tris[(trimethylsilyl)methyl] chromium. The investigated systems in the first part were used as modules in the design of novel chromium hydrazide gels that use low valent chromium centers as H2 binding sites. These materials were synthesized at various Cr to hydrazine molar ratios and have room temperature excess hydrogen storage of up to 1.01 wt%, and binding enthalpy of up to viii 22.9 kJ/mol. However, the hydrogenation of these materials induced an amplification of the performance and binding enthalpies. The excess room temperature hydrogen storage of the hydrogenated samples goes up to 1.65 wt%, with the room absolute volumetric density goes up to 29.92 Kg/m3, and with binding enthalpy goes up to 51.59 kJ/mol. These materials would use pressure instead of temperature as a toggle and can thus be used in compressed gas tanks, to increase the amount of hydrogen stored, and therefore the driving range of any vehicle.

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