Photocatalytic Decolorization of Organic Dyes in Titanium Dioxide-Air Systems

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Title: Photocatalytic Decolorization of Organic Dyes in Titanium Dioxide-Air Systems
Author: Julson, Alison Jean
Advisors: Dr. David F. Ollis, Committee Chair
Dr. Michael R. Overcash, Committee Member
Dr. H. Henry Lamb, Committee Member
Abstract: The photocatalytic degradation of adsorbed organic dyes in air is reported with the goal of color removal for potential catalyst characterization and application in self-cleaning surfaces. In this study, a laboratory photoreactor equipped with 8 GE Blacklight Blue 20W lamps (peak emission at 365 nm) was the source for near-UV exposure of dye-coated Degussa P25 titanium dioxide (TiO₂) particles as well as dye coated photo-inert aluminum oxide (Al₂O₃) particles used for control experiments. The average UVA intensity of exposure for all experiments was 1.3 mW cm⁻². Dyes were adsorbed onto catalyst particles and inert supports in aqueous solution, and the coated particles were captured by centrifugation, dried, and hand ground to powder before near-UV exposure. The dyes included in this study were Acid Blue 9, Acid Orange 7, Reactive Black 5 and Reactive Blue 19. Sub-monolayer initial dye coverage of support particles was used. Visual evidence of color removal was recorded with digital photographic images of samples before near-UV illumination and after exposure. Two methods, Indirect and Direct Analysis, were employed to quantitatively examine the decolorization of organic dyes with near-UV exposure. During illumination with the Indirect method, dye-coated powder samples were subject to ambient humidity and were shaken in uncovered Petri dishes in attempt to achieve uniform near-UV exposure of particles. Separate samples for each exposure time were used. Reaction products were desorbed from the support particles into basic aqueous solution, support particles were removed by centrifugation, and the resulting solution was analyzed by UV-visible transmission spectroscopy to examine the photoreaction progress. The Direct Analysis method allowed for the examination of the same sample throughout the reaction with near-UV exposure. Dye-coated sample powder was pressed into a sealable sample holder with a quartz window which allowed for both UV exposure and analysis by diffuse reflectance UV-visible spectroscopy. UV-visible spectroscopy was used to monitor changes in unreacted dye concentration with near-UV illumination compared to unexposed controls. Absorbance spectra obtained during Indirect Analysis was proportional to dye concentration following the Beer-Lambert law. The reflectance spectra acquired by the Direct Analysis method was transformed to Kubelka-Munk units, proportional to concentration, for all of the dye-coated TiO₂ and Al₂O₃ samples studied. For both analytic methods, a decrease in dye concentration was observed with near-UV illumination of the dye-coated TiO₂ powders for the four dyes studied. The dye did not photodegrade significantly in the absence of TiO₂. The data obtained by UV-visible spectroscopy, for both analytic methods, was used to model the kinetics of the photocatalytic degradation. Several kinetic models were developed and applied to the reaction data. The most convincing mechanism developed for the photocatalytic degradation of organic dyes adsorbed to TiO₂ assumes a series reaction. The first step is the conversion of colored dye to colored intermediate by a first order reaction with respect to dye concentration, and the second step is conversion to colorless product by another first order reaction with respect to intermediate concentration. The rate constants for the first reaction step were of similar magnitude for all dyes, and the average rate constant, k₁, for the first step of all experiments was 0.13 min⁻¹ and for the second step, k₂, was 0.0014 min⁻¹.
Date: 2005-01-27
Degree: MS
Discipline: Chemical Engineering

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