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The Synthesis, Characterization, and Catalytic Activity of Uranium Compounds Supported on Liquid Crystal Templated Mesoporous Silica

The Synthesis, Characterization, and Catalytic Activity of Uranium Compounds Supported on Liquid Crystal Templated Mesoporous Silica
The Synthesis, Characterization, and Catalytic Activity of Uranium Compounds Supported on Liquid Crystal Templated Mesoporous Silica

Mesoporous silica with a regular hexagonal array of pores has been synthesised by a liquid crystal templating route, using a variety of surfactants. The mesostructure in all cases is retained on surfactant removal. The structure of these materials is typified by the Brij76 surfactant-templated silica, for which the nitrogen adsorption data showed a type IV isotherm with no hysteresis, giving a specific surface area of 870m2g-1, a pore volume of 0.7cm3g-1 and an average pore diameter of 3.6nm. X-ray powder diffraction patterns for this silica show an intense peak at low 2θ value corresponding to a d100 value of 4.9nm, as well as secondary peaks indicative of a hexagonal array of pores and extended long range order. The hydroxyl concentration is found to be 1.21 OHnm-2. Heating in oxygen at temperatures up to 950°C reveals a slow decline in surface area and pore size up to 800°C, whereafter these values fall sharply to 300m2g-1, and 2.2nm respectively at 850°C. A similar effect on the (100) reflection intensity is noted by X-ray powder diffraction. The surface hydroxyl concentration undergoes a similar non-linear reduction with calculation temperature, but this occurs between 700 and 800°C.

The activity of these systems towards the catalytic oxidation of CO, and the selective catalytic reduction of NO with CO has been determined. For both reactions, the activity of supported uranyl nitrate materials increases with decreasing catalyst pre-treatment temperature, whereas the uranium tetrachloride derivatives typically achieve a maximum in activity after pre-treatment at 600°C in oxygen. Generally, the activity increases with a decrease in either the size of supported uranium oxide particles, or, for the tetrachloride derivatives, the amount of chlorine retained. For the NO reduction reaction, the most active catalysts achieve 100% selectivity towards nitrogen at 360°C, and 90% NO conversion at 450°C. This reaction was found to be zero order with respect to NO, and have an order of 1.4 with respect to CO.

The organometallic complexes [(C5H5)3UC1], {N[CH2-CH2N(SiMe2tBu)]3UOBut}, and {N[CH2-CH2N(SiMe2tBu)]3UNEt2}, have been deposited on mesoporous silica at 6wt% U. The supported species have been characterised, and the effect of thermal treatment investigated.

University of Southampton
Campbell, Tom
57689594-8890-4064-b0e0-3eae0099a6b0
Campbell, Tom
57689594-8890-4064-b0e0-3eae0099a6b0

Campbell, Tom (2001) The Synthesis, Characterization, and Catalytic Activity of Uranium Compounds Supported on Liquid Crystal Templated Mesoporous Silica. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

Mesoporous silica with a regular hexagonal array of pores has been synthesised by a liquid crystal templating route, using a variety of surfactants. The mesostructure in all cases is retained on surfactant removal. The structure of these materials is typified by the Brij76 surfactant-templated silica, for which the nitrogen adsorption data showed a type IV isotherm with no hysteresis, giving a specific surface area of 870m2g-1, a pore volume of 0.7cm3g-1 and an average pore diameter of 3.6nm. X-ray powder diffraction patterns for this silica show an intense peak at low 2θ value corresponding to a d100 value of 4.9nm, as well as secondary peaks indicative of a hexagonal array of pores and extended long range order. The hydroxyl concentration is found to be 1.21 OHnm-2. Heating in oxygen at temperatures up to 950°C reveals a slow decline in surface area and pore size up to 800°C, whereafter these values fall sharply to 300m2g-1, and 2.2nm respectively at 850°C. A similar effect on the (100) reflection intensity is noted by X-ray powder diffraction. The surface hydroxyl concentration undergoes a similar non-linear reduction with calculation temperature, but this occurs between 700 and 800°C.

The activity of these systems towards the catalytic oxidation of CO, and the selective catalytic reduction of NO with CO has been determined. For both reactions, the activity of supported uranyl nitrate materials increases with decreasing catalyst pre-treatment temperature, whereas the uranium tetrachloride derivatives typically achieve a maximum in activity after pre-treatment at 600°C in oxygen. Generally, the activity increases with a decrease in either the size of supported uranium oxide particles, or, for the tetrachloride derivatives, the amount of chlorine retained. For the NO reduction reaction, the most active catalysts achieve 100% selectivity towards nitrogen at 360°C, and 90% NO conversion at 450°C. This reaction was found to be zero order with respect to NO, and have an order of 1.4 with respect to CO.

The organometallic complexes [(C5H5)3UC1], {N[CH2-CH2N(SiMe2tBu)]3UOBut}, and {N[CH2-CH2N(SiMe2tBu)]3UNEt2}, have been deposited on mesoporous silica at 6wt% U. The supported species have been characterised, and the effect of thermal treatment investigated.

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Published date: 2001

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Local EPrints ID: 464659
URI: http://eprints.soton.ac.uk/id/eprint/464659
PURE UUID: 77566a01-fbe5-43cf-acc5-dd4c54d75ef8

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Date deposited: 04 Jul 2022 23:54
Last modified: 05 Jul 2022 02:50

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Author: Tom Campbell

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