Photocatalysis by Titanium Dioxide and Polyoxometalate/TiO₂ Cocatalysts. Intermediates and Mechanistic Study

A representative polyoxometalate, alpha-12-tungstophosphatic acid (PW12(3-), POM), is loaded on the surface of TiO2 particles used as a cocatalyst to gain further insights into the underlying reaction mechanism and to estimate the feasibility of using the new POM/TiO2 cocatalyst in the photocatalytic degradation of 2,4-dichlorophenol (DCP) in aqueous media. Loading the PW12(3-) species on the surface of TiO2 enhances charge separation in the UV-illuminated TiO2, thereby accelerating the hydroxylation of the initial DCP substrate but not the mineralization of DCP, which is somewhat suppressed in the presence of the polyoxometalate. An increase in the load of POM increases the concentration of aromatic intermediates, and more toxic intermediates, such as 2,6-dichlorodibenzo-p-dioxin, 2,4,6-trichlorophenol, are detected in the PW12(3-)/TiO2 system. By contrast, cleavage of the whole conjugated structure of DCP predominates in TiO2 only dispersions. Strong ESR signals for the superoxide radical anionic species, O2*- (HO2* radicals in acidic media; pH < 5), are detected in TiO2 only dispersions; signals of O2*- are much weaker in the TiO2/ POM composite system under otherwise identical conditions. Experimental results infers that enhancement of charge separation in TiO2 photocatalysis does not always result in improvement of the efficiency of mineralization of organic substrates, and the reaction between organic radical cations and the formed superoxide radical anions may be responsible forthe mineralization of the chlorophenol.