Mechanistic photochemistry: From reactive intermediates to environmental remediation

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2010-01-01
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Rockafellow, Erin
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William S. Jenks
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Chemistry

The Department of Chemistry seeks to provide students with a foundation in the fundamentals and application of chemical theories and processes of the lab. Thus prepared they me pursue careers as teachers, industry supervisors, or research chemists in a variety of domains (governmental, academic, etc).

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The Department of Chemistry was founded in 1880.

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1880-present

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Abstract

This dissertation exemplifies the utility of light in chemical research and technology, specifically focusing on oxene generation and environmental remediation.

The first study compares direct and sensitized photolysis of dibenzothiophene-S-oxide (DBTO) and dibenzoselenophene-Se-oxide (DBSeO). Quantum yield and solvent oxidation data are used to separate the direct irradiation conditions, plus benzophenone-sensitized and anthraquinone-sensitized irradiation of DBSeO, into one mechanistic class. Acridine-sensitized photolysis of DBSeO and triplet sensizitization of DBTO result in deoxygenation, but go by different mechanisms than the direct irradations. The two sensitized cases that appear mechanistically linked to direct DBTO photolysis are ones in which the spectroscopic triplet of DBSeO, which is very likely of comparable energy to the selenium-oxygen BDE, is populated by triplet energy transfer.

The main focus of the work presented in this dissertation is on exploration of modifications to TiO2 in order to improve absorption of terrestrial sunlight.. Aromatic organic probe molecules are used in these chapters to evaluate the oxidative chemistry of doped TiO2 and test the efficacy of the catalyst with visible irradiation.

Tungsten-modified titanium dioxide catalysts are compared for their photocatalytic activity. No special visible absorbance is apparent for the sol-gel catalyts. However, an increase in the single-electron transfer chemistry with the presence of WOx is noted, and a distinct wavelength dependence on the product ratios.

13C-modified TiO2 was prepared in order to facilitate study of the dopant atoms and trace their chemical fate throughout the process. Several different carbon species are identified. Some variation in the chemical degradation of quinoline is noted among the catalysts, but coke-containing TiO2 catalysts are not qualitatively better catalysts for use with visible light with this substrate.

S-TiO2 was produced by a literature method, and was shown to facilitate the degradation of organic molecules under UV and visible light. Visible irradiation of sulfur-doped TiO2 did not produce hydroxyl-type chemistry, but could accomplish single-electron transfers in favorable cases. The utility of sulfur-doped TiO2 as a photocatalyst over undoped titania depends entirely whether the requirement for visible-light functionality, even if at low efficiency, outweighs a modest drop in the efficiency of catalysis using UV light.

Selenium-modified TiO2 is capable of degrading quinoline at a slightly faster rate than undoped TiO2 under UV light. Irradiation with > 435 nm light shows no evidence for efficient production of hydroxyl-like species, but single-electron transfer chemistry is still operative. Examinations of Se-TiO2 under hypoxic conditions show that the Se atoms are capable of trapping photogenerated electrons, as evidenced by XPS.

The last study investigates the feasibility of using deep UV treatment for abatement of ammonia in livestock and poultry barn exhaust in a series of laboratory scale experiments. These experiments simulated moving exhaust air with controlled UV wavelength and dose, NH3 concentrations, humidity, and presence of H2S. Reactions were monitored using chemiluminescence detection, GC-MS, and high resolution FTIR. The data show that removal of NH3 is plausible, but highlights concerns over ozone and N2O emission.

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Fri Jan 01 00:00:00 UTC 2010