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Photocatalysis under periodic illumination

Citation

Cornu, Catherine J. G. (2002) Photocatalysis under periodic illumination. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-05082006-143046

Abstract

In the first part of this work, the use of periodic illumination is investigated as a means to improve the quantum yield of aqueous phase photocatalysis. We find that quantum yield enhancements are obtained under periodic illumination compared to those under continuous illumination with the same maximum photon absorption rate. This result is in agreement with previous literature reports. However, no enhancements are obtained when compared to those under continuous illumination with the same average photon absorption rate. Ways in which the use of periodic illumination in remediation systems might nevertheless be more cost-effective than continuous operation are discussed. In the second part of this work, periodic illumination is applied as a tool to investigate the kinetic behavior of the intermediates involved in photocatalysis. Two transitions are observed in the quantum yield as a function of the active light period, corresponding to separate lifetimes of two intermediates. These lifetimes are found to be relatively insensitive to the physical properties of the TiO2 photocatalyst particles, the compound being photocatalytically oxidized, or the oxygen concentration. However, a strong correlation with the pH of the solution is observed. For example, one intermediate's lifetime increases exponentially with pH, while the other one's decreases exponentially. Principles from electrochemistry, namely Nernst's law and the Butler-Volmer equation, allow us to conclude that the intermediate with a decreasing lifetime with pH is a reducing species, while the other one is an oxidizing species. The intermediates' lifetimes are equal at a pH ~ 8, concurrent with a minimum in quantum yield. Below pH 8 the reducing species has a longer lifetime, indicating that interfacial electron transfer to oxygen is the slowest step under these conditions.

The observed lifetimes are compatible with those previously measured for superoxide radicals (reducing species), and hydroxyl radicals, surface trapped holes, or surface bound hydroxyl radicals (oxidizing species).

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Environmental Science and Engineering
Thesis Availability:Restricted to Caltech community only
Research Advisor(s):
  • Hoffmann, Michael R.
Thesis Committee:
  • Hoffmann, Michael R. (chair)
  • Blake, Geoffrey A.
  • Hering, Janet G.
  • Lewis, Nathan Saul
Defense Date:24 September 2001
Record Number:CaltechETD:etd-05082006-143046
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-05082006-143046
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:1680
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:08 May 2006
Last Modified:26 Dec 2012 02:40

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