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Photooxidative and photoreductive degradation of chlorinated hydrocarbons on aqueous titanium dioxide colloids

Citation

Choi, Wonyong (1996) Photooxidative and photoreductive degradation of chlorinated hydrocarbons on aqueous titanium dioxide colloids. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-05052006-084215

Abstract

Studies on photocatalytic degradation reactions of chlorinated hydrocarbons on TiO2 colloids are presented in this thesis. Photoreactivities of metal-ion doped quantum-sized TiO2 colloids and photochemical mechanisms of CHCI3 and CCI4 degradation are investigated in detail. A systematic study of 21 metal-ion doped quantum-sized (2-4 nm) TiO2 colloids is performed by measuring their photoreactivities and the transient charge-carrier recombination dynamics. Doping with Fe3+, Mo5+, Ru3+, Os3+, Re5+, V4+, and Rh3+ at 0.1-0.5 atom% significantly increases the photoreactivity for both CHCI3 oxidation and CCI4 reduction while Co3+ and AI3+ doping decreases the photo reactivity. The quantum yields obtained during CW photolyses are quantitatively correlated with the measured transient absorption signals of the charge-carriers. The photoreductive degradation of CCI4 in TiO2 particulate suspensions in the presence of a variety of organic electron donors (alcohols, carboxylic acids, and benzene derivatives) has been examined. The rate of CCI4 dechlorination can be enhanced significantly when alcohols and organic acids are used as electron donors. It is demonstrated that CCI4 can be fully degraded under both oxic and anoxic conditions. A photodegradation mechanism of CCI4 that includes both one-electron and two-electron transfer is proposed. The mechanism of photoreduction of CCI4 on illuminated TiO2 surfaces is investigated by selectively trapping transient free radical intermediates. Dichlorocarbene and trichloromethyl radical are trapped with 2,3-dimethyl-2butene during the photocatalytic degradation of CCI4. The rate of formation of trapped CCI2 and CCI3 is found to be a function of [H2O], pH, [CCI4], the nature of the dissolved gas, and light intensity. A two-electron photoreductive pathway (via dichlorocarbene formation) is suggested to be the dominant mechanism leading to the full degradation of CCI4. The photocatalytic degradation reactions of CHCI3, CHBr3, CCI4, and CCI3CO2 are investigated in aqueous TiO2 suspensions. CHCI3 and CHBr3 are degraded into carbon monoxide and halide ion in the absence of dissolved oxygen. The anoxic degradation proceeds through a dihalocarbene intermediate which is produced by sequential reactions of the haloform molecule with a valence band hole and a conduction band electron. Degradation of haloform is enhanced dramatically at pH >11. This enhancement is ascribed to "photoenhanced hydrolysis".

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Chemistry
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemistry
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Hoffmann, Michael R.
Thesis Committee:
  • Unknown, Unknown
Defense Date:22 February 1996
Author Email:wchoi (AT) caltech.edu
Record Number:CaltechETD:etd-05052006-084215
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-05052006-084215
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:1627
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:05 May 2006
Last Modified:03 Dec 2014 00:02

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