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The photocatalyzed transformations of phenol (PhOH) and 4-chlorophenol (4ClPhOH) in irradiated TiO2 (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).

The photooxidative degradation of sulforhodamine-B dye (SRB) taking place in visible-light illuminated platinized titania dispersions is revisited to examine the influence of metallic platinum doped on TiO2 particles (Degussa P25 titania) by photocatalytic deposition from a hexachloroplatinic acid solution. The various TiO2/Pt specimens were characterized by transmission electron microscopy (TEM), UV−vis diffuse reflectance spectroscopy, and X-ray photoelectron spectroscopy (XPS). The self-photosensitized degradation of SRB mediated by TiO2/Pt specimens is 3-fold faster than that occurring on TiO2 alone under visible light irradiation and under otherwise identical conditions. That is, the TiO2/Pt system exhibited greater catalytic activity than P25 TiO2 alone, which earlier proved to be an effective photocatalyst in the degradation of dyes under visible illumination. A reaction mechanism is proposed on the basis of a series of experiments that included spin−trap electron spin resonance (ESR) spectral results, measurement of quantities of H2O2 formed, chemical oxygen demand (CODCr), and total organic carbon (TOC) data. The platinum dopant acts as an electron sink from which molecular oxygen scavenges the electrons to yield superoxide radical anions (O2^-•) first and then ^•OH radicals, which are known to cause the ultimate self-destruction of the SRB dye.

The effect of metal ions (Cu²⁺, Fe³⁺, Zn²⁺, Al³⁺, and Cd²⁺) on the photodegradation of several dyes:  sulfo-rhodamine B (SRB), alizarin red (AR), and malachite green (MG) has been investigated in aqueous TiO2 dispersions under visible irradiation (λ > 420 nm). Trace quantities of transition metal ions such as Cu²⁺ and Fe³⁺ having suitable redox potentials alter the electron-transfer pathway involving the dye, O2 and TiO2 particles, and markedly depress the photodegradation of all three dyes under visible irradiation. Other metal ions, such as Zn²⁺, Cd²⁺, and Al³⁺, have only a slight influence on the photoreaction by altering the adsorption of dyes. Photogeneration of H2O2 and reactive radicals, and the changes in fluorescence emission of SRB in TiO2 aqueous dispersions were examined to elucidate the role of the metal ions. Addition of Cu²⁺ or Fe³⁺ decreases the reduction of O2 by the conduction electrons, subsequently blocks the formation of reactive oxygen species (O^-•, ^•OH), and depresses the degradation of dyes under visible irradiation. We deduce that the reduction of O2 is essential for the photodegradation of dyes under visible irradiation.

The spectral dependencies of quantum yields of photostimulated adsorption of oxygen on KBr and ZrO2 specimens were assessed during photoexcitation into the F centers produced by preirradiation with UV light. The spectral limits of this adsorption were shifted to wavelengths longer than the absorption band edge of the specimens. However, of great import are the spectral dependencies of the quantum yields of photochemical processes for the original state(s) of metal oxides irradiated in the intrinsic and extrinsic absorption bands. Such dependencies for the photoadsorption of donor (H2 and CH4) and acceptor (O2) molecules in gas/solid systems are reported for a variety of metal oxides of varying band gaps (Eg), namely TiO2 (rutile form, Eg = 3.0 eV), ZnO (3.0 eV), ZrO2 (5.0), Sc2 O3 (5.4 eV), MgO (8.7 eV), and CaO (7.5 eV) as well as for Bi2 O3 (3.2 eV), CeO2 (3.4 eV), Ga2 O3 (4.4 eV), In2 O3 (3.1 eV), Nb2 O5 (3.3 eV), and Y2 O3 (5.5 eV). The approach taken to model and compare with experimental results was reported earlier (ref 4). For the first six metal oxides, the dependencies are rationalized on the basis of the absorption spectra of the specimens for which the origins of the extrinsic light absorption are known. The corresponding dependencies for the remaining oxides in the series are nonetheless also reported albeit they await characterization of the origins of the spectral features for a complete understanding to be achieved.

Oxidation potentials of resorcinol, 4-chlororesorcinol, 4,6-dichlororesorcinol, catechol, 3,4-dihydroxybenzoic acid and 1,2,4-trihydroxybenzene were measured on particulate TiO2(Degussa P-25) thin films, immobilized on optically transparent SnO2conducting glass electrodes, by cyclic voltammetry in 0.5M KCl aqueous electrolyte solutions. The effect of adsorption on oxidation potentials was examined with the compounds adsorbed on the TiO2particle surface. Scan rate dependencies of oxidation peak currents indicate that adsorbed species are consistently characterized by less positive oxidation potentials compared to those attributed to solution free species; the difference ranges from about 0.2 to 0.8V. Results show that depending on the nature of the working electrode, associating a single oxidation potential to such compounds does not adequately describe their electrochemical behavior. Such observations have relevance in heterogeneous photocatalysis in that predictions of whether a substance will be photooxidized or photoreduced cannot be based on Fermi levels (redox potentials) of the redox couples in homogeneous solutions.