Spectral Selectivity of Photocatalyzed Reactions Occurring in Liquid−Solid Photosystems

The photocatalyzed transformations of phenol (PhOH) and 4-chlorophenol (4ClPhOH) in irradiated TiO 2 (P25) aqueous dispersions were reexamined to assess the spectral dependencies of the selectivities of the primary oxidative and/or reductive steps in the formation of the primary intermediates catechol (Cat), hydroquinone (HQ), and benzoquinone (BQ) for PhOH and chlorocatechol (ClCat), HQ, and BQ for 4ClPhOH. These selectivities depend on the relative surface concentrations of the electrons and holes at the various wavelengths, which in turn hinge on the spectral variations of the absorption coefficients of the photocatalyst. The rates of degradation of PhOH and of 4ClPhOH, together with the rates of formation of the primary intermediate species, scaled linearly with photon flow from the irradiation source, as required if quantum yields and selectivities of formation of the intermediates are to be correctly evaluated. As expected, the selectivities of formation of the intermediates in phenol were spectrally invariant, whereas those for the 4-chlorophenol were spectrally variant. The average of the spectrally integrated selectivities for the formation of Cat, HQ, and BQ for phenol ranged from 0.18 to 0.24 (error ca. ±0.02), whereas the corresponding ones for the formation of ClCat, HQ, and BQ from 4-chlorophenol ranged between 0.18 and 0.32. With a minor exception, these selectivities compare remarkably well with those determined under broadband irradiation (0.20−0.24 for phenol and 0.15−0.26 for 4-chlorophenol).