UV-irradiated TiO2 Suspensions Research Articles

Photocatalytic green synthesis of piperonal in aqueous TiO2 suspension

Piperonal (heliotropine or 3,4-methylenedioxybenzaldehyde) has been synthesized by oxidizing piperonyl alcohol in aqueous UV-irradiated TiO2 suspensions. This compound was identified by GC–MS chromatography, 1H NMR and melting point determination. The other products of the photoprocess were 1,3-bis(3,4-(methylenedioxy)benzyl) ether (found in traces) and CO2, derived from the parallel pathway of photo-mineralization. Commercial and home-prepared TiO2 samples have been tested and the best selectivity (ca. 35%) was obtained by using the home-prepared ones. The reported green process allows to obtain an added value product (piperonal), upon partial oxidation of a cheap reagent.

Photocatalytic reduction of 6-chloro-3-nitrotoluene-4-sulfonic acid in presence of surfactants

Photocatalytic reduction of 6-chloro-3-nitrotoluene-4-sulfonic acid (CNSA) was studied in UV-irradiated TiO2 suspensions in the presence of methanol and surfactants. A mixture of CNSA, TiO2, water, additives and surfactants was put into a quartz glass reactor with a jacket, which was irradiated with a high pressure Hg lamp in the purging of nitrogen gas. With methanol, the conversion of CNSA increased from 7.7% to 34.6%. Three surfactants significantly promoted the photocatalytic reduction conversion in reduced order of sodium dodecylbenzenesulfonate (DBS), cetyltrimethylammonium bromide (CTAB) and sodium dodecylsulfate (SDS). In suspensions involving DBS and CTAB respectively, CNSA conversion increased in consistence with the adsorption ratio with methanol, but varied inversely with the adsorption ratio without methanol. But no obvious influence on the adsorption ratio was observed with or without methanol added when the SDS concentration was critical micelle concentration (cmc). The photocatalytic reduction of CNSA was enhanced in UV-irradiated TiO2 suspensions in the presence of methanol and surfactants. Methanol inhibited the recombination of photogenerated holes and electrons efficiently. Surfactants around 1 cmc led to the highest reduction conversion.

TiO2-Graphene Nanocomposites. UV-Assisted Photocatalytic Reduction of Graphene Oxide

Graphene oxide suspended in ethanol undergoes reduction as it accepts electrons from UV-irradiated TiO(2) suspensions. The reduction is accompanied by changes in the absorption of the graphene oxide, as the color of the suspension shifts from brown to black. The direct interaction between TiO(2) particles and graphene sheets hinders the collapse of exfoliated sheets of graphene. Solid films cast on a borosilicate glass gap separated by gold-sputtered terminations show an order of magnitude decrease in lateral resistance following reduction with the TiO(2) photocatalyst. The photocatalytic methodology not only provides an on-demand UV-assisted reduction technique but also opens up new ways to obtain photoactive graphene-semiconductor composites.

Visible-Light-Mediated TiO2 Photocatalysis of Fluoroquinolone Antibacterial Agents

This study reports on the photocatalytic transformation of fluoroquinolone antibacterial agents (ciprofloxacin, enrofloxacin, norfloxacin, and flumequine) in aqueous titanium dioxide (TiO2) suspensions irradiated with ultraviolet (UV; lambda > 324 nm) or visible light (lambda > 400, > 420, or > 450 nm). Visible-light-mediated fluoroquinolone degradation is unexpected from direct photolysis or established TiO2 band gap photoexcitation mechanisms, which both require UV light. Visible-light-mediated photocatalysis requires an appropriate conduction band electron acceptor (e.g., O2, BrO3-), but is not dependent upon hydroxyl radical, superoxide, or other reactive oxygen species generated upon TiO2 band gap excitation. The process slows considerably when fluoroquinolone adsorption is inhibited. Whereas fluoroquinolone decomposition in UV-irradiated TiO2 suspensions is accompanied by mineralization, no changes in dissolved organic carbon occur during visible-light-photocatalyzed degradation. Results are consistent with a proposed charge-transfer mechanism initiated by photoexcitation of surface-complexed fluoroquinolone molecules. Complexation to the TiO2 surface causes a red shift in the fluoroquinolone absorption spectrum (via ligand-to-metal charge transfer), enabling photoexcitation by visible light. Fluoroquinolone oxidation then occurs by electron transfer into the TiO2 conduction band, which delivers the electron to an adsorbed electron acceptor. The lack of organic carbon mineralization indicates formation of stable organic byproducts that are resistant to further degradation by visible light. In UV-irradiated TiO2 suspensions, the charge-transfer mechanism acts in parallel with the semiconductor band gap photoexcitation mechanism.

Heterogeneous photocatalytic reduction of Fe(VI) in UV-irradiated titania suspensions: effect of ammonia

Results of the heterogeneous photocatalytic reduction of Fe(VI) in UV-irradiated TiO2 suspensions in the presence of ammonia are presented. The initial rate of Fe(VI) reduction, R, may be expressed as R = k Fe(VI)[Fe(VI)]1.25 where k Fe(VI) = a[Ammonia]+b), a = 6.0 × 103 μm 0.25 s and b = 4.1 × 106 μm −1.25s−1. The rate constant, k Fe(VI), increases with the ammonia concentration. The photocatalytic oxidation of ammonia is enhanced in the presence of Fe(VI). A mechanism involving Fe(V) as a reactive intermediate is presented which explains the faster photocatalytic oxidation of ammonia in the presence of Fe(VI).

Heterogeneous Photocatalytic Reduction of Ferrate(VI) in UV-Irradiated Titania Suspensions

The heterogeneous photocatalytic reduction of Fe(VI) in UV-irradiated TiO2 suspensions was conducted in a basic media. Fe(VI) reduction as a function of TiO2 suspensions, Fe(VI) concentrations, and pH was studied. The initial rate of Fe(VI) reduction adheres to the simple Langmuir−Hinshelwood kinetics. This and the fractional order with respect to initial Fe(VI) concentrations suggests that the surface reaction is the rate-controlling step. The photocatalytic oxidation of ammonia using TiO2 in the presence and absence of Fe(VI) was also investigated at pH 9.0. Enhancements in the rates of ammonia oxidation and Fe(VI) reduction were observed. The results indicate the formation of Fe(V) by a TiO2 conduction band electron. Fe(V) selectively and rapidly oxidizes compounds; therefore, production of Fe(V) by a photocatalytic technique has a role in synthesis and remediation processes.

Heterogeneous Photocatalytic Reduction of Cr(VI) in UV-Irradiated Titania Suspensions: Effect of Protons, Ammonium Ions, and Other Interfacial Aspects

Heterogeneous photocatalytic reactions in UV-irradiated TiO2 suspensions are comprised of two conjugate reaction pathways involving the photogenerated electrons and holes, respectively. The role of the hole reaction pathway in the dynamics of the photocatalytic reduction of Cr(VI) is a major focus of this study. It is shown that proton supply plays a crucial role in this reduction reaction. Thus, the Cr(VI) photoreduction kinetics switch from first order to zero order as the proton concentration is systematically increased in the aqueous suspensions. Ammonium ions are also shown to exert a dramatic accelerating influence on Cr(VI) reduction in media of initial pH 6. This new observation is rationalized by considering that these species act as hole scavengers. The consequent improvement in quantum yield combines with the facile proton generation upon NH4+ photooxidation, to result in the observed rate enhancement. Other interfacial aspects (i.e., adsorption) are also discussed.

Degradation of phenyltrifluoromethylketone in water by separate or simultaneous use of TiO2 photocatalysis and 30 or 515 kHz ultrasound

TiO2 photocatalysis and ultrasound are emerging technologies for the mineralization of pollutants in water. To further investigate these technologies and to assess whether advantages and synergy can be expected from their differences, phenyltrifluoromethylketone (PTMK) was selected as a test compound for pollutants generating CF3COOH, an undesirable final product. The PTMK first-order removal rate constant k was ca. 14 times higher when the ultrasound frequency was 515 kHz instead of 30 kHz for the same energy, and ca. 2.5 times higher when a TiO2 sample we synthesized was used instead of TiO2 Degussa P25. On simultaneous photocatalytic and ultrasonic treatment an increase in k by a factor between 1.4 and 1.9, depending on the TiO2 sample, was observed at 30 kHz but not at 515 kHz. On the basis of catalase enzymatic effect upon k, these observations are tentatively explained by a photocatalytic OH• radical production from sonochemically formed H2O2, provided that the H2O2 residence time on TiO2 is sufficient. PTMK ultrasonic pyrolysis was demonstrated by product analysis. The amount of CF3COOH was ca. 8 times lower in sonicated solutions than in UV-irradiated TiO2 suspensions, for both frequencies and both TiO2 samples. Therefore, because of a higher k value, a high frequency ultrasonic (pre)treatment is preferable to minimize CF3COOH formation.