Light intensity-dependence studies on the role of surface deposits for titania-photocatalyzed oxygen evolution: Are they really cocatalysts?

The effects of surface loading of iridium(II) oxide (IrO2), manganese(IV) oxide (MnO2), and cobalt(II) phosphate (CoPi) on the rate of photocatalytic oxygen evolution by anatase or rutile titania particles suspended in aqueous solutions of an electron acceptor, iodate ions, were studied by light intensity-dependence (LID) kinetic analyses. Although the role of these deposits has been reported to be a cocatalyst without showing results of any kinetic analysis, the present LID kinetic study suggested that the deposits may act as a positive-hole capturer for oxygen evolution, not as a cocatalyst for both metal oxides and CoPi. Further studies on the effects of loading amount, deposit types, titania crystalline type, and titania-particle size on the reaction order were also performed by LID analysis based on the proposed kinetic model. The observed LID behaviors could be interpreted consistently using a virtual parameter of effective volume as the target volume of photoabsorption by one positive-hole-bearing titania particle to govern the probability of two positive-hole accumulations in a titania particle.