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Initial quantum yields φ for the formation of DMPO-OH adducts in clear, aerated, unbuffered TiO 2 sols irradiated at 295 nm were determined by kinetic electron paramagnetic resonance spectrometry as function of the 5,5-dimethyl-1-pyrroline N -oxide (DMPO) spin trap concentration, photon irradiance I o, and added dichlorobenzene DCB, at 298 K. φ steadily increases with DMPO in the range 0.3 mM to 0.3 M, extrapolating to φ DMPO→∞ = 0.54 ± 0.10, in contrast with the constant yields obtained for the homogeneous generation of OH radicals in the photodissociation of H 2 O 2 at 254 nm under similar conditions. In TiO 2 sols at DMPO = 0.6 mM, φ (≈0.002) remains constant over a 20 - fold variation of I o, but decreases upon addition of comparable DCB concentrations. These observations prove that (1) DMPO reacts with photogenerated holes and/or OH radicals on the surface of TiO 2 particles rather than in the solution bulk, (2) it is possible to approach quantitative carrier trapping, (3) DCB is as reactive as DMPO toward some of the photogenerated carriers, and (4) the recombination of a single carrier pair the usual event in small particles at moderate photon fluxes is a pseudo-first-order process. Photocatalysis in semiconductor sols is a stochastic process not amenable to conventional kinetic analysis based on rate expressions involving continuous concentration variables. Quantum yields are largely controlled by the anodic ( h + D → D + ), rather than by the slower ( e + O 2 → O 2 •- ) cathodic charge transfer reaction.
Grela et al. (Mon,) studied this question.