Factors influencing relative efficiency in photo-oxidations of organic molecules by Cs3PW12O40 and TiO2 colloidal photocatalysts

This paper examines the factors influencing the efficiency of photo-oxidative degradation of N-methylpyrrolidinone (NMP) and atrazine by TiO2 and Cs3PW12O40 colloids, especially the roles of mechanism and substrate-photocatalyst interaction at the colloid surface. For both photocatalysts, major primary intermediates generated by the photo-oxidation of atrazine are the same. However, cyanuric acid appears to be oxidized by the UV irradiation of PW12O403− in deaerated solution as evidenced by the formation of the heteropoly blue PW12O404−, suggesting that (in contrast to TiO2) photo-oxidation is not solely due to hydroxyl radicals. Picosecond spectroscopy in water and simple alcohols indicates the importance of direct hydrogen abstraction by the excited tungstate. Quantum yields for the photo-oxidation of NMP by TiO2 fit a Langmuir–Hinshelwood model, with a limiting quantum yield at high surface coverage of 0.029±0.005 and an adsorption/desorption coefficient K of 2900±600 M−1. For Cs3PW12O40 under the same conditions, the quantum yield is 0.004–0.009 over the concentration range 10−5–10−2 M. In the case of atrazine, an analogous treatment for Cs3PW12O40 yields K=1.4(±0.1)×104 M−1 while the value for TiO2 is at least an order of magnitude less. The quantum yields indicate the importance of heterogeneity of the photocatalyst surface for Cs3PW12O40, where the concentration dependence of the quantum yields can be explained by substrate adsorption at a small number of reaction sites, analogous to preassociation of dissolved polyoxotungstates and organic molecules.