Photocatalytic dechlorination of aqueous carbon tetrachloride solutions in TiO{sub 2} layer systems: A chain reaction mechanism
Quantum yields of chloride ions exceeding unity are obtained upon illumination of TiO{sub 2} layers in aqueous solutions containing CCl{sub 4} and methanol. The TiO{sub 2} layers were prepared by spin coating using concentrated colloidal solution (prepared by hydrolysis of the propoxide). Absorbed light intensities were 7 x 10{sup {minus}11} to 2 x 10{sup {minus}8} in cm{sup {minus}2}s{sup {minus}1}. There is only a small, if any, effect of methanol concentration on the Cl{sup {minus}} yield in the range 0.2--5 M. Above 5 M, the yield remains nearly constant in the absence of air but decreases in aerated solutions. Only negligible Cl{sup {minus}} build up is proportional to the square root of the light intensity, as expected in a chain reaction. A similar study in oxygen-free solutions shows [Cl{sup {minus}}] leveling off at the higher light intensities. The quantum yield increases with pH in both the presence and absence of oxygen, reaching values {phi} {approximately} 7 at pH 12.2, at the lowest light intensity. In the absence of oxygen, there is no observable effect of CCl{sub 4} concentration on the Cl{sup {minus}} yield above 1 x 10{sup {minus}3} M, while the oxygen-containing systems show nearly linear increase of the yield upon increasing [CCl{sub 4}]. Thermal catalyzed formation of chloride is observed in the absence of oxygen. In the absence of oxygen, removal of absorbed hydroxyl radicals, OH{sm{underscore}bullet}{sub ads}, by the methanol enables the electrons that have escaped recombination to react with CCl{sub 4} producing chloride ions and CCL{sub 3}{sm{underscore}bullet} radicals. This is followed by electron injection to the TiO{sub 2} conduction band and subsequent hydrolysis of the CCl{sub 3}{sup +} intermediate to carbon dioxide and HCl. A chain reaction mechanism is proposed also for oxygen-containing systems. The CCl{sub 4} - O{sub 2}{center{underscore}dot}{minus} adduct is the chain carrier and the termination involves dismutation of O{sub 2}{sup {sm{underscore}bullet}{minus}} radical ions.
- Research Organization:
- Hebrew Univ. of Jerusalem (IL)
- OSTI ID:
- 20001143
- Journal Information:
- Journal of Physical Chemistry B: Materials, Surfaces, Interfaces, amp Biophysical, Vol. 103, Issue 40; Other Information: PBD: 7 Oct 1999; ISSN 1089-5647
- Country of Publication:
- United States
- Language:
- English
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