Aluminum tetracarboxyphthalocyanine adsorbed on TiO2 has been examined as a sensitizer for degradation of 4-chlorophenol (4-CP) in water under visible light irradiation (λ ≥ 450 nm). It was observed that 4-CP was mostly decomposed into CO2 and chloride ions, whereas the reaction rate was greatly influenced by several parameters. The optimum loading of the dye on TiO2 was about 1.0 wt %, nearly independent of TiO2 used. The solution pH which favored 4-CP degradation over the dye loaded TiO2 was about pH 7. An increase in 4-CP concentration below 0.27 mM resulted in increased rate of 4-CP degradation, the kinetics well fitted into the Langmuir−Hinshelwood equation. In the flow of N2, the reaction was almost completely inhibited. Experiments through EPR spin trapping, methanol quenching, and silver ions as electron acceptors revealed the involvement of superoxide, hydroxyl and dye cation radicals in the dye sensitized reaction. Evidence is well interpreted in terms of the electron transfer from the excited dye to TiO2, followed by generation of the reactive species. However, among different sensitizers, the observed difference in activity is hardly correlated with their differences in the particle size, surface area, crystalline structure, and crystallinity of bare TiO2, and in the dye absorbance and aggregation as well, whereas it is well in accordance with the trend in pore volume, measured for these samples through O2 adsorption−desorption at 77 K. The result demonstrates for the first time the predominant role of O2 to the dye sensitized degradation of organic pollutants over TiO2 in aqueous suspension. Furthermore, the dye loaded TiO2 under visible light was also active for degradation of other aromatic pollutants such as sulfosalicylic acid, from which the dye cation radical with a redox potential of about 1.25 V vs NHE at pH 6.5 is inferred.

Various methods that aim to improve the photocatalytic activity of TiO2 have been reported in the literature. Herein, we report that addition of CuWO4 into the aqueous suspension of TiO2 can result in significant enhancement in the rate of phenol degradation. As the amount of CuWO4 increased, the rate of phenol degradation increased and then decreased. A maximum rate of phenol degradation observed with 2 wt % CuWO4 was about 2.83 times that in the absence of CuWO4. A similar result was also observed with CuO. However, six consecutive tests showed that CuWO4/TiO2 was much more stable than CuO/TiO2, due to the very high stability of CuWO4 against photocorrosion. The improved activity of TiO2 is not due to CuWO4 and CuO themselves and also does not match their solubility in aqueous solution. Moreover, for the generation of OH radicals, and for the decomposition of H2O2 in aqueous solution, CuWO4/TiO2 was also more active than TiO2. Through a (photo) electrochemical measurement, a possible mechanism is proposed, involving electron transfer from the irradiated TiO2 to CuWO4 that facilitates the charge separation of TiO2 and consequently accelerates reactions at interfaces.

In thermodynamics, the one-electron reduction of O2 by the conduction band electrons of Bi2WO6 or WO3 is not allowed. However, many studies have reported that Bi2WO6 is photocatalytically active for organic degradation in aerated aqueous suspension. In this work, the photocatalytic activities of Bi2WO6 and WO3 under visible light have been compared by using phenol degradation as a model reaction. In aerated aqueous solution, Bi2WO6 and WO3 were indeed active and inactive, respectively, as reported. However, by using Pt as a catalyst for O2 reduction, or by using H2O2 as an electron scavenger, Bi2WO6 became much less active than WO3. Similar results were also obtained in the production of H2O2 under visible light, and in the generation of •OH radicals under UV light, measured by a spin-trapping electron paramagnetic resonance (EPR) spectroscopy. Moreover, in the presence of catalase to completely remove H2O2, the EPR signal due to •OH radical was reduced, but not disappeared. These observations indicate that the irradiated Bi2WO6 is not only active for water oxidation to •OH but also active for the two-electron reduction of O2 to H2O2, the latter of which hardly occurs with the irradiated WO3.Keywords: Bi2WO6; EPR signal; flat band potential; platinization; two-electron reduction

The sensitized oxidation of Fe2+ ions by I2 or I3− in acidic aqueous solution has been examined. The reactions could occur either under UV or visible light, with the stoichiometric formation of Fe3+ and I−. However, the I3−-sensitized reaction was fast and complete only in the presence of excess Fe2+. Through a kinetics study, it becomes clear that I˙ and I2−˙ are the main reactive species for the I2 and I3−- sensitized oxidation of Fe2+, respectively. Moreover, the quantum yields determined with the I2 and I3−-sensitized formation of Fe3+ at 456 nm were 0.119 and 0.118, respectively. The I2-sensitized reaction was first order only in I2, and had an Arrhenius activation energy of 29.3 kJ mol−1. It is proposed that the process of Fe2+ oxidation by I˙ is fast, while the rate-determining step is the formation and self-recombination of I˙ radicals.

In this work we have found that simple mixing of WO3 and Fe2O3 can result into significant enhancement in activity for phenol degradation in the presence of H2O2 either under UV or visible light. The enhanced activity varied with Fe2O3 content in the mixed oxide. The best catalyst was 1.0 wt% Fe2O3/WO3, whose activity, in relative to bare WO3, increased to 3.8-fold under UV light, and 2.1-fold under visible light. Control experiments indicated that phenol degradation was mainly initiated by the excited WO3 in Fe2O3/WO3, and that there was a synergistic effect between two oxides. A spin trapping electron paramagnetic resourance spectroscopy revealed that the production of hydroxyl radicals over Fe2O3/WO3 was greatly enhanced. Solid characterization with X-ray diffraction, N2 adsorption and element mapping showed that Fe2O3 particles were highly dispersed in the sample, with a small size and high surface area. We propose that there occurs an electron transfer from the conduction band of WO3 to low energy trapping sites of Fe2O3, which promotes the separation of WO3 charge carriers, and consequently accelerates phenol degradation.Graphical abstractDevelopment of a highly visible-light-driven photocatalyst is a challenge for chemical use of solar energy. In this work, we find that simple mixing of WO3 and Fe2O3 can result into significant enhancement in activity for phenol degradation in water in the presence of H2O2 under either UV or visible light. The enhanced activity greatly varied with the weight percent of Fe2O3 in the mixed oxide. The best catalyst was 1.0 wt% Fe2O3/WO3, whose activity, in relative to bare WO3, increased to 3.8-fold under UV light, and 2.1-fold under visible light, respectively. We propose that the observed synergistic effect between two oxides is due to the electron transfer from the conduction band of WO3 to low energy trapping sites of Fe2O3. Such interfacial charge transfer between two oxides would facilitate the separation of WO3 charge carriers, and consequently accelerate the surface reaction on both oxides for organic degradation.Highlights► Simple mixing of WO3 and Fe2O3 can significantly improve the photoactivity. ► The enhanced photoactivity greatly varied with Fe2O3 content in the mixed oxide. ► Fe2O3 particles were highly dispersed with a small size and high surface area. ► Synergism between Fe2O3 and WO3 are observed under UV or visible light.

Molecular iodine has been studied, for the first time, as a sensitizer for the degradation of 2,4,6-trichlorophenol (TCP) in aqueous solution under visible light (λ ≥ 450 nm). TCP was degraded in the presence of commercial I2, but the reaction rate decreased significantly after 2 h. When a solution of NaI and H2O2 was used as an iodine source with phosphotungstic acid (PW) as a catalyst, TCP degradation was not only fast but also followed zero-order kinetics. Importantly, the I2 concentration remained unchanged with time, indicative of I2 recycling as a kind of photocatalyst. During TCP degradation, 2,6-dichloro-1,4-benzoquinone was produced as the main intermediate (76%), which slowly degraded in the irradiated solution. For every equivalent of TCP consumed at the 2 h time point, approximately 1.7 equivalents of chloride ions were produced. Further study of the effect of variables including the type of polyoxometalates (POM) and the initial concentration of each component revealed that the rate of TCP degradation under visible light was determined by the rate of I2 production in the dark. The optimum pH and apparent activation energy for TCP disappearance were 4.5 and 42.8 kJ/mol, respectively. It is proposed that TCP degradation is initiated by iodine radicals produced from I2 photolysis, followed by I2 regeneration through a POM-catalyzed oxidation of I3– by H2O2.

Surface modification of TiO2 with CuO or calcium phosphate (CaP) can result in enhancement in the photocatalytic activity for organic degradation. In this work, we report on a synergism between the cation and anion of copper phosphate (CuP) on the photocatalytic activity of TiO2, for phenol degradation in aerated aqueous suspension under UV light at wavelengths longer than 320 nm. Photocatalysts were prepared by mixing TiO2 and CuP powders in isopropyl alcohol, followed by drying at 90 °C. As CuP loading increased, the activity of the modified TiO2 first increased and then decreased. The maximum activity was observed with the catalyst containing 0.1 wt % CuP, which was about 1.9–3.4 times that of bare TiO2 (anatase, rutile, and their mixture) and also exceeded that of the modified TiO2 with CuO or CaP. During five repeated tests, the catalyst activity was stable, without detectable leaching of cupric and phosphate ions into aqueous solution. Solid characterization with several techniques including electron paramagnetic resonance (EPR) spectroscopy revealed that CuP particles at low loading were highly dispersed onto TiO2 as a kind of clusters, whereas the TiO2 phase in different samples remained nearly unchanged in terms of the crystal structure, surface area, and crystallinity. Upon exposure to UV light, the EPR signal of Cu(II) in CuP or CuO-modifed TiO2 was unchanged in air but slightly decreased in N2. Moreover, CuP-modified TiO2 showed a higher capacity than bare TiO2 and CuO- or CaP-modified TiO2 for the uptake of 2,4-dichlorophenol from water. It is proposed that cupric and phosphate ions act as an electron scavenger and organic sorbent, which facilitate electron and hole transfer, respectively. Their co-operation would significantly improve the efficiency of charge separation, and thus increase the rate of phenol degradation.

The synergistic effect between anatase and rutile TiO2 particles for organic photodegradation in an aerated aqueous suspension has been widely reported, ascribed to the charge transfer between two oxides increasing the efficiency of charge separation for the surface reaction. In this work, we have found different explanation. Four anatase and rutile samples, pretreated at different temperatures, were used as a starting material, and the biphase oxide was prepared by mixing them at various ratios in an alcoholic aqueous solvent, followed by sintering at 450 °C. For phenol photodegradation under air, the synergistic effect was observed with one set of the mixed oxides but not with others. When the reaction was carried out under N2 in the presence of AgNO3, no synergistic effect was observed with any mixed oxide. For methylene blue photodegradation under air, the synergistic effect was also observed as reported, but such an effect was absent when dye adsorption on the oxide was taken into account. The difference in activity among differently prepared samples correlates well with the combined effect of oxide crystallinity and its sorption capacity toward O2. We propose that the observed synergistic effect is due to O2 transfer from anatase phase to rutile that explores the masked photoactivity of rutile particles.

Photocatalytic degradation of organic substrates over WO3 in an aerated aqueous suspension is very slow due to the difficulty of O2 reduction by the conduction band electron on WO3. In this work, we report on H2O2 as an electron scavenger significantly accelerating the photodegradation of phenol and azo-dye X3B in water under UV or visible light. More importantly, an iron-containing WO3 (FeW) synthesized through thermal decomposition of a ferrotungstenic acid displayed a much higher activity than pure WO3 (HW) prepared in parallel. As the sintering temperature increased, both FeW and HW showed an exponential increase in activity. The maximum rate constant of phenol degradation obtained with FeW at 400 °C was about 2 times larger than that with HW at 600 °C. Sample characterization with electron paramagnetic resonance (EPR) spectroscopy and other techniques revealed that ferric species (0.3 wt % Fe2O3) were mainly present as clusters on the oxide surface at 120 °C and then they diffused toward the lattice sites of WO3 at high temperature, which was detrimental to the photocatalytic reaction. 5,5-Dimethyl-1-pyrroline N-oxide spin-trapping EPR showed that the production of hydroxyl radicals was greatly enhanced upon the addition of H2O2, the trend of which among different catalysts was the same as that of the rate of phenol degradation. The catalysts after excitation at 350 nm displayed a blue emission centered at 469 nm, the intensity of which varied with the catalyst activity nearly as expected. A possible mechanism for the improved photoactivity of WO3 is proposed involving the electron transfer from WO3 to Fe2O3 and the reaction of the reduced oxide with H2O2 to generate hydroxyl radicals.

Various factors that influence the photocatalytic activity of TiO2 for organic degradation in aerated aqueous solution have been reported. For instance, anatase is considered to be much more active than rutile. However, no attention has been paid to the difference in their sorption capacities toward O2 in water, which might be critical to the activity determination. In this work, Ag+ has been used as an electron scavenger, for the photocatalytic degradation of 4-chlorophenol in the N2-purged aqueous suspension of TiO2 under UV light. Three different TiO2 samples in the crystal forms of anatase and rutile, containing micro- and mesopores, were prepared, followed by sintering at different temperatures (Ts). The initial rate of 4-chlorophenol photodegradation, per the initial amount of Ag+ adsorbed, increased exponentially with Ts. Such Ts-dependent normalized rates were observed with three differently prepared TiO2 samples, and three curves were almost overlapped. Comparatively, in the aerated aqueous suspension of TiO2, the initial rate of 4-chlorophenol photodegradation, per surface area of the catalyst, first increased and then decreased with Ts, the trend similar to those widely reported. Moreover, from the literature data of water photosplitting over TiO2, the initial rate of O2 evolution, per the initial amount of Ag+ adsorbed, increased exponentially with Ts. Evidence clearly shows that with the same amount of electron scavenger on the catalyst surfaces, anatase and rutile actually have a similar photocatalytic activity at a given Ts, for organic degradation or water oxidation. It is recommended that to evaluate the photocatalytic activities of different TiO2 samples in an aerated aqueous solution, the difference in O2 adsorption needs to be taken into account.

The effect of sintering temperature (150−900 °C) on the photocatalytic activities of ferric oxide and silica-supported ferric oxide for Orange II degradation in water has been examined under UV light irradiation in the absence and presence of H2O2. The solids are characterized by X-ray powder diffraction, nitrogen adsorption, UV/vis diffuse reflectance spectroscopy, FTIR, and Raman spectroscopy. It was observed that the amount of dye adsorbed and the rate of dye photodegradation on these catalysts were a function of sintering temperature and the suspension pH. Evidence appears to correlate with the crystallinity, particle size, and flat-band potential of hematite, in agreement with the model of semiconductor photocatalysis. The recycling experiment showed that bare hematite was relatively stable, whereas silica-supported ferric oxide experienced a progressive degradation, due to preferential deposition of the dissolved ferric species onto silica, possibly with formation of amorphous and low photoactive ferric (hydr)oxide.

Photodegradation of a textile dye X3B and photoreduction of dichromate (Cr(VI)) in an acidic aqueous solution were studied under 320 nm cut-off UV light irradiation in the presence of two polyoxometalates (POM), H3PW12O40 (PW), and H4SiW12O40 (SiW). The reactions in POM-X3B-Cr(VI) system were faster than those in POM-X3B, POM-Cr(VI), and X3B-Cr(VI) systems. For all reactions, PW was more photoactive than SiW. The reaction rates were proportional to the initial concentration of each component. The effects of N2, O2, and air were small but regular, indicating Cr(VI) photoreduction by a reduced POM. Quenching experiments with H2O2 and ethanol revealed that X3B photodegradation mainly occurred through hydroxyl radical (OH). It was proposed that the production of OH and a reduced POM− by the reaction between H2O and excited POM* was the rate determining step, with which all evidence could be well interpreted. Different effects of POM concentration in a two- or three-component system on the reaction rates suggested that the reaction between H2O and excited POM* was reversible.

The stability of eleven metal phthalocyanine sulfonates against UV (λ > 320 nm) or visible light (λ > 450 nm) in the presence of TiO2 semiconductor was studied in an aqueous medium. Although all the dyes were quite photostable in a homogeneous solution, they underwent notably photobleaching in the presence of TiO2. The degree of dye bleaching was strongly dependent on the central metal in the complex, whereas for each complex the bleaching rate under UV irradiation was much faster than that under visible light irradiation. The spectral analysis showed that the dye photobleaching led to complete destruction of the phthalocyanine ring. In addition, the visible light stability of the dye was greatly affected by physical properties of TiO2 semiconductor, and the dye photostability could be improved by addition of electron sacrifice such as 4-chlorophenol.

The Langmuir–Hinshelwood (L–H) kinetic model has been used to describe semiconductor photocatalysis. In this report, the L–H rate constant (kL–H)and the Langmuir adsorption constant (K) have been determined under different light intensity for the photocatalytic degradation of poorly adsorbed acetophenone over TiO2 of Degussa P25 in aqueous medium (pH 6.2). The result shows that K decreases when the irradiation is performed at higher light intensity, while kL–H increases expectedly. It is also demonstrated that the initial time interval selected for the initial rate calculation is quite critical to the final determination for the constants.