Photochemical transformations of sodium anthracene-1-sulfonate in oxygen-saturated aqueous titanium dioxide suspensions (original) (raw)
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A New Method To Determine the Generation of Hydroxyl Radicals in Illuminated TiO2 Suspensions
The Journal of Physical Chemistry B, 1997
The generation of hydroxyl (• OH) radicals produced by irradiation of aqueous TiO 2 suspensions was investigated by electron paramagnetic resonance (EPR) and product analysis employing a stable free nitroxide radical (3-carboxyproxyl) as a spin trap of • OH. Product analysis demonstrated that the attack of • OH on 3-carboxyproxyl produces a diamagnetic product (proxyl-NH) with a trapping efficiency of ca. 80%. It could also be shown that a small amount of the nitroxides is reduced by conducting band electrons. The rate of formation of • OH could be determined by monitoring the time dependence of the decrease in the concentration of 3-carboxyproxyl monitored by EPR spectroscopy or the time dependence of the increase in the concentration of proxyl-NH monitored by gas chromatography analysis. The rate of formation of • OH serves as a mechanistic parameter to investigate the mechanism of formation of this reactive species by the photoexcitation of TiO 2. The dependence of the rate of formation of • OH was shown to be a linear function of light intensity at low intensities. It was observed that molecular oxygen, a good scavenger of conduction band electrons, only slightly influences the rate of formation of • OH and supports the production of • OH from photogenerated holes. Addition of selected anions to the photolysis mixtures results in a diminished rate of disappearance of the spin trap as the result of a competition between the spin trap and the anion for • OH. From a Stern-Volmer analysis of the data, the rate constant for the reaction of hydroxyl radicals with the anions was determined. (1) h vb + and e cb-can react directly with organic species adsorbed on the surface of the particle, (2) h vb + and e cb-can be transformed into high-energy chemical species which react with the absorbed organic species, or (3) h vb + and e cb-can be transformed into high energy chemical species which can diffuse into the aqueous phase and then react with dissolved organic
Radical Intermediates in Photoinduced Reactions on TiO2 (An EPR Spin Trapping Study)
Molecules, 2014
The radical intermediates formed upon UVA irradiation of titanium dioxide suspensions in aqueous and non-aqueous environments were investigated applying the EPR spin trapping technique. The results showed that the generation of reactive species and their consecutive reactions are influenced by the solvent properties (e.g., polarity, solubility of molecular oxygen, rate constant for the reaction of hydroxyl radicals with the solvent). The formation of hydroxyl radicals, evidenced as the corresponding spin-adducts, dominated in the irradiated TiO2 aqueous suspensions. The addition of 17 O-enriched water caused changes in the EPR spectra reflecting the interaction of an unpaired electron with the 17 O nucleus. The photoexcitation of TiO2 in non-aqueous solvents (dimethylsulfoxide, acetonitrile, methanol and ethanol) in the presence of 5,5-dimethyl-1-pyrroline N-oxide spin trap displayed a stabilization of the superoxide radical anions generated via electron transfer reaction to molecular oxygen, and various oxygen-and carbon-centered radicals from the solvents were generated. The character and origin of the carbon-centered spin-adducts was confirmed using nitroso spin trapping agents.
Photocatalytic ?OH radical formation in TiO2 aqueous suspension studied by several detection methods
Physical Chemistry Chemical Physics, 2003
Photoinduced reaction with TiO 2 semiconductor photocatalysts was investigated by using nitroxide radicals as spin probes for OH radicals. The effects of some additives such as I À , Cl À , ClO 4 À , methanol and 2-propanol on the photocatalytic decay of nitroxide radicals, 3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidine-1-oxyl and 4-carboxy-2,2,6,6-tetramethylpiperidine-1-oxyl, were investigated. Among the additives, only iodide ions that can be oxidized by trapped holes prohibited significantly the decay of the nitroxide radicals, indicating that trapped holes oxidize nitroxide probe radicals. For several TiO 2 photocatalysts, the apparent quantum efficiency of OH radical formation was calculated and compared with those obtained by the other detection methods, such as DMPO spin trapping and terephthalic acid fluorescence methods. The experimental observations suggest that the photocatalytic oxidation should be caused more preferably by trapped holes or adsorbed OH radicals, rather than the photoinduced valence band holes in semiconductor and the free OH radicals in solution.
Journal of Photochemistry and Photobiology A: Chemistry, 2016
The TiO 2 photosensitized oxidation of benzyl methyl sulfides (X-C 6 H 4 CH 2 SCH 3) and benzyl thiols (X-C 6 H 4 CH 2 SH) has been investigated in Ar-saturated CH 3 CN. Steady-state irradiation produced benzaldehydes or dibenzylsulfides as oxidation products with sulfides and thiols, respectively. The results obtained through kinetic competitive experiments, aimed to evaluate the ring substituent effect on the reactivity, suggested the involvement of radical cation intermediates, formed by the favorable electron transfer from the substrate to the TiO 2 photogenerated hole, which reasonably deprotonate to give the final products. This process was poorly affected by the adsorption of the substrate at the TiO 2 surface, as demonstrated by similar results, both in terms of products and reactivity, collected for the homogeneous photooxidation of the same substrates sensitized by N-methoxyphenanthridinium hexafluorophosphate (MeOP + PF 6 À). This behavior is likely due to the low hydrogen-bond acceptor ability of divalent sulfur systems. Density functional theory calculations pointed out that the most stable conformations of X-C 6 H 4 CH 2 SH + are characterized by having the CÀ ÀS bond almost collinear with the p system of the aromatic ring and by a significant charge and spin delocalization involving both the phenyl ring and the sulfur atom. 2016 Elsevier B.V. All rights reserved.
Photocatalysed reaction of few selected organic systems in presence of titanium dioxide
Applied Catalysis A: General, 2005
The photocatalysed reaction of five selected organic systems, namely 2,2 0-dinitro biphenyl (1), N,N 0-dimethyl-4-nitroso aniline (2), 4dimethyl amino benzaldehyde (3), phthalaldehyde (4) and tetramethyl benzoquinone (5) has been investigated in acetonitrile/water mixture in the presence of titanium dioxide and oxygen. An attempt has been made to identify the products formed during the photooxidation process through GC/MS analysis technique. The photolysis of 2,2 0-dinitro biphenyl (1) leads to the formation of 2-hydroxy-2 0-nitro biphenyl (6), 2,2 0dihydroxy biphenyl (7) and 2-biphenyl amine (11); whereas, N,N 0-dimethyl-4-nitroso aniline (2) gave rise to N,N 0-dimethyl-4-nitro aniline (14) and N,N 0-dimethyl-1,4-benzenediamine (15). However, the photolysis of 4-dimethyl amino benzaldehyde (3) yielded only a demethylated product (18). The illumination of phthalaldehyde (4), on the other hand, led to the formation of two products such as 1(3H)-isobenzofuranone (24) and 2-formyl-benzoic acid (21). Two intermediate products, tetramethyl hydroquinone (35) and 2,5-furandione-3,4-dimethyl (34) have been identified in case of tetramethyl benzoquinone (5). The products have been identified by comparing the molecular ion and mass fragmentation peaks of the products with those reported in the GC/MS library. A probable mechanism for the formation of the products has been proposed.
Photocatalytic hydroxylation of benzoic acid in aqueous titanium dioxide suspension
Journal of Photochemistry and Photobiology A: Chemistry, 1991
The photocatalytic hydroxylation of benzoic acid in aqueous TiOl suspension is suggested as evidence for the generation of hydroxyl radicals in the H,O(l)-TiO*(s) system. The hydroxylation reaction was studied by UV spectroscopy and high performance liquid chromatography (HPLC). The experimental results are discussed in terms of the formal kinetic analysis and the aromatic carbon charges calculated by the INDO method.
Oxygen photosensitization in the presence of sodium anthracene-1-sulphonate
Journal of Photochemistry and Photobiology A: Chemistry, 1993
Sodium anthracene-1-sulphonate (ANlS), UV irradiated in aqueous solutions, forms the intermediate anthracene-1-sulphonyl radical (AN-l-SO;). In the presence of oxygen, ANlS is converted to quinone and oxygen is activated to superoxide as indicated by the 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) spin trap adduct, 'DMPO-02-. The products of oxygen activation are OH' radicals identified as 'DMPO-OH adducts. The time dependence of 'DMPO-OH formation during and after irradiation implies the production of an activated complex between oxygen and anthracene quinones. Quantum chemical calculations indicate a considerable weakening of the C-S bond in the photochemically excited state (T,,S,) of ANlS. As a consequence, the splitting of 'S03Na is probable which, in a subsequent reaction with the sulphonate group in ANlS, may be responsible for the formation of AN-I-SO;.
Journal of Photochemistry and Photobiology A: Chemistry, 1995
2,3-Benzofuran (BZF), a mutagenic and carcinogenic compound, was chosen as a model water pollutant to study the TiO2 photocatalytic degradation of molecules which have two aromatic rings and a built-in oxygen heteroatom. For BZF and its two main degradation intermediates, salicylaldehyde (SA) and 2-coumaranone (2-CO), a relatively good correlation was found between the apparent first-order rate constant of disappearance (approximately 0.059 min -1, 0.0365 min -1 and 0.031 min -1 respectively) and the measured extent of dark adsorption (about 0.43, 0.134 and 0.108 molecules per square nanometre of TiO2 respectively) or the logarithm of the 1-octanol-water partition coefficient (2.67~xp, 1.75exp and 1.35taLc respectively). This illustrates the importance of surface phenomena for these classes of compounds. From the degradation of BZF, 2-CO and SA (studied separately), it was inferred that the larger number of BZF degradation intermediates resulted from the oxidation of the furan ring to produce 2-CO (and perhaps 3-CO), 2,3-dioxobenzofuran and SA, which was further oxidized/hydroxylated to salicylic acid, catechol and dihydroxybenzaldehydes (and perhaps to dioxobenzaldehydes and hydroxyparabenzoquinone or isomers). However, the identification of other compounds with one hydroxyl substituent on the benzene ring and an unopened furan ring demonstrated that the degradation proceeded by a variety of pathways. Although most of these intermediates could be formed by hydroxyl radicals, the involvement of other species cannot be ruled out; in particular, the direct formation of SA from BZF may result from the interaction of superoxide with BZF "+. The efficiency of the photocatalytic pollutant degradation was illustrated by the observation that the maximum quantity of each intermediate was less than a few per cent of eliminated BZF.
Environmental Toxicology and Chemistry, 1994
Heterogeneous photocatalysis via Ti0, suspensions is effective in anthraquinone 2 sulfonic acid sodium salt (ASS) degradation The presence of oxygen promotes the observed semiconductor powder oxidation of the substrate Degradation of ASS in aerated dispersions of Ti0, needed about 24 h at 60°C Added H,O, enhances the degradation rate considerably As the pH increases, the H,02 produced also increases Also, peroxide production was observed as the concentration of Ti0, is increased owing to the availability of more surface states for the reaction The Langmuir adsorption isotherm of ASS on Ti0, Degussa P-25 shows that ASS adsorbs readily on the Ti0, particles The observed rate for H,O, production decreased as the concentration of isopropanol (hole scavenger) is increased, suggesting an oxidative pathway as the main reaction responsible for H,O, photoproduction The degradation of ASS is a result of indirect action due to light mainly absorbed by Ti0, The experiments described here demonstrate the feasibility of dyestuff decomposition via semiconductor powders
Environmental Science & Technology, 1992
rn The technique of spin trapping, with EPR detection of spin adducts, has been applied to the study of the photodegradation of benzene, chlorobenzene, bromobenzene, iodobenzene, o-dichlorobenzene, m-dichlorobenzene, and p-dichlorobenzene sensitized by the photolysis of HzOz in aqueous solution. By employing a competition kinetic scheme and relative initial slopes or signal amplitudes, plus published rate constants for the reaction of *OH radicals with the spin trap (DMPO), it has been possible to obtain rate constants for the reaction of *OH radicals with benzene and its halo derivatives. Rate constants obtained by this method at neutral and acidic solutions are similar to published methods using pulse radiolysis, where available. Significant differences were found for measurements carried out in basic solutions. The rate constants do not vary much among the halobenzene derivatives studied, and an average rate constant of -5.0 X lo9 M-l could be used for any of these compounds.