Optical detection of a photoinduced thermal transient in titanium dioxide powder by diffuse reflectance laser flash photolysis

Laser flash photolysis of titanium dioxide suspensions for the evaluation of solvent-mediated radical reactions

The chemistry originating from the scavenging of the highly electrophilic hole in TiO₂ can be readily monitored using laser flash photolysis techniques. Dilute suspensions are sufficiently transparent in the UV region that long lived signals from reactions of solvent radicals with 1,1-diphenylethylene can be readily monitored. Transient signals originating from hole, electron and trapped radicals are extremely long lived showing stretched exponentials (nanoseconds to milliseconds), adequately described by fractal models.

Laser-flash-photolysis-spectroscopy: a nondestructive method?

Herein, we report the effect of the laser illumination during the diffuse-reflectance laser-flash-photolysis measurements on the morphological and optical properties of TiO₂ powders. A grey-blue coloration of the TiO₂ nanoparticles has been observed after intense laser illumination. This is explained by the formation of nonreactive trapped electrons accompanied by the release of oxygen atoms from the TiO₂ matrix as detected by means of UV-vis and EPR spectroscopy. Moreover, in the case of the pure anatase sample a phase transition of some TiO₂ nanoparticles located in the inner region from anatase to rutile occurred. It is suggested that these structural changes in TiO₂ are caused by an energy and charge transfer to the TiO₂ lattice.

Surface photochemistry of pesticides: an approach using diffuse reflectance and chromatography techniques

The photochemistry of pesticides triadimenol and triadimefon was studied on cellulose and beta-cyclodextrin (beta-CD) in controlled and natural conditions, using diffuse reflectance techniques and chromatographic analysis. The photochemistry of triadimenol occurs from the chlorophenoxyl moiety, while the photodegradation of triadimefon also involves the carbonyl group. The formation of 4-chlorophenoxyl radical is one of the major reaction pathways for both pesticides and leads to 4-chlorophenol. Triadimenol also undergoes photooxidation and dechlorination, leading to triadimefon and dechlorinated triadimenol, respectively. The other main reaction process of triadimefon involves alpha-cleavage from the carbonyl group, leading to decarbonylated compounds. Triadimenol undergoes photodegradation at 254 nm but was found to be stable at 313 nm, while triadimefon degradates in both conditions. Both pesticides undergo photochemical decomposition under solar radiation, being the initial degradation of rate per unit area of triadimefon 1 order of magnitude higher than the observed for triadimenol in both supports. The degradation rates of the pesticides were somewhat lower in beta-CD than on cellulose. Photoproduct distribution of triadimenol and triadimefon is similar for the different irradiation conditions, indicating an intramolecular energy transfer from the chlorophenoxyl moiety to the carbonyl group in the latter pesticide.

Photochemistry of 4-chlorophenol on cellulose and silica

The photochemistry of 4-chlorophenol (4-CP) was studied on silica and cellulose, using time-resolved diffuse reflectance techniques and product degradation analysis. The results have shown that the photochemistry of 4-CP depends on the support, on the concentration, and also on the sample preparation method. Transient absorption and photoproduct results can be understood by assuming the formation of the carbene 4-oxocyclohexa-2,5-dienylidene in both supports. On cellulose, at concentrations lower than 10 micromol g(-1), the carbene leads to the unsubstituted phenoxyl radical, and phenol is the main degradation product. At higher concentrations a new transient resulting from phenoxyl radicals coupling was also observed, and dihydroxybiphenyls are also formed. The reaction of the carbene with ground-state 4-CP was also detected through the formation of 5-chloro-2,4'-dihydroxybiphenyl. 4-Chlorophenoxyl radical and degradations products resulting from its coupling were also detected. Oxygen has little effect on the photochemistry of 4-CP on cellulose. On silica the transient benzoquinone O-oxide was formed in the presence of oxygen. Benzoquinone and hydroquinone are the main degradation products. In well-dried samples the formation of hydroquinone is reduced. At higher concentrations the same products as detected on cellulose were observed. 4-CP undergoes slow photochemical decomposition under solar radiation in both supports. The same main degradation products were observed in these conditions.

Sonophotochemical destruction of aqueous solution of 2,4,6-trichlorophenol

The combination of ultrasound and photochemistry have been used to degrade an aqueous solution of 2,4,6-trichlorophenol. An ultrasonic probe of 22 kHz frequency and a UV tube of 15 W have been used. Anatase grade TiO2 was used as the semiconductor catalyst. The effect of parameters such as ultrasonic intensity, operating conditions, type of ultrasonic equipment, and mode of UV transmission have been studied. The sonophotochemical degradation has been found to be dependent on the intensity of sonication, temperature of the reaction, and the type of ultrasonic equipment used, but was independent of the mode of UV transmission. Enhancement in the degradation rate has been observed at a higher sonication intensity and temperature of the solution.