Photoelectrochemical degradation of selected aromatic molecules

The photoelectrocatalytic degradation of selected aromatic phenols and acids, amongst them substances of environmental concern like bisphenol-A (BPA), ethyl-paraben and phthalic acid, was studied on thin layers of semiconducting titanium dioxide deposited on transparent, electrically conducting substrates. A module of nine backside solar illuminated flow-through parallel plate photoelectrochemical reactors was used and electrical bias for suppressing charge carrier recombination was applied. The progress of oxidative degradation on the illuminated semiconductor electrode was followed by UV/vis absorption and COD (chemical oxygen demand) analysis. From the latter, kinetic data were calculated and compared on the basis of apparent first-order rate constants, related to current efficiencies which reflect the competition between solute and solvent (water) oxidation. The efficiencies (COD based) for 4-chlorophenol, salicylic acid, benzoic acid, phthalic acid, ethyl-paraben, BPA, p-cresol and 4-methoxyphenol as well as model compounds for aliphatic transient species in the course of the degradations – acetate, oxalic acid and maleic acid, lie between 10 and 70 M−1, a rather narrow range illustrating the rather unspecific oxidation on TiO2, which can be ascribed to the high redox potential of light generated oxidizing species. In many cases, these values are close to values obtained from the decrease of UV absorption of bands characteristic for the aromatic system. Long time irradiation experiments using solar light yielded conversion efficiencies of up to 90 percent in batch recirculation mode.