Vlasta Brezová

Phenol decomposition using Mn+ /TiO2 photocatalysts supported by the sol-gel technique on glass fibres

The photoactivity of supported TiO2 and Mn+/TiO2 (Mn4  Li+, Zn2+, Cd2+, Ce3+, Co3+, Cr3+, Fe3+, Al3+, Mn2+ and Pt0) layers, fixed on glass fibres by the sol-gel technique, was tested using the reaction of phenol degradation. The concentration of metal dopant in the immobilized photocatalysts was 5 mol.% Mn+ :Ti4+. In addition, the effect of platinum loading in Pt0/TiO2 samples (Pt0 content in the range 0.2–5.0 mol.% Pt0:Ti4+) was investigated. The prepared materials were also characterized by their isoelectric points and reflectance spectra. The photoactivity of the materials prepared is strongly dependent on the character and concentration of the dopant. The best results in terms of phenol decomposition were obtained for dopant-free TiO2 Li+/TiO2Zn2+ /TiO2 and Pt0/TiO2 (Pt content, less than 5 mol.% Pt0:Ti4+). However, using Pt0/TiO2 layers, we observed a slower decomposition of hydroquinone, which is produced as an intermediate by phenol hydroxylation. The presence of Co3+, Cr3+, Ce3+, Mn2+, Al3+ and Fe3+ ions in the TiO2 photocatalyst (5 mol.% Nn+ :Ti4+) has a detrimental effect on its photoactivity.

Antioxidant properties of tea investigated by EPR spectroscopy

The antioxidant properties of green, black and mixed (fruit) tea samples of different origin were investigated by means of EPR spectroscopy. A six line EPR spectrum of solid tea samples indicates the presence of Mn(II) ions and it is superimposed with a sharp singlet line attributed to semiquinone radical species (Delta H(pp)=1 mT; g=2.0022). Antioxidant properties of aqueous tea extracts in H(2)O(2)/NaOH/dimethylsulfoxide system generating reactive radicals (*OH, O(2)*-), *CH(3)) were followed by spin trapping technique. In addition, antioxidant capacity of these samples was assessed using stable radicals 1,1-diphenyl-2-picrylhydrazyl (DPPH) and 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPOL). Typically, the highest antioxidant potential to terminate superoxide radicals was found in green teas, followed by black and fruity teas. The pro-oxidant activity of green teas evidenced by spin traps was promoted in samples with higher Mn(II) and ascorbic acid concentrations. Various sources of free radicals used in the antioxidant tests due to their specific action show different termination rates in the presence of the individual tea samples.

The influence of dissolved metal ions on the photocatalytic degradation of phenol in aqueous TiO2 suspensions

Abstract The photocatalytic degradation of phenol was investigated in the oxygen-saturated titanium dioxide suspensions in the presence of dissolved metal ions Ca 2+ , Mg 2+ , Zn 2+ , Ni 2+ , Mn 2+ , Co 2+ , Cu 2+ , Cr 3+ and Fe 3+ in the concentration range 0.28–1.1 mM. The most photoactive systems for phenol degradation were found in the presence of ferric ions, while the addition of Cr 3+ strongly inhibited the photocatalytic decomposition of phenol. The role of dissolved metal ions in the photocatalytic processes on TiO 2 surface is discussed.

Study of the oxidative stress in a rat model of chronic brain hypoperfusion

A multiple analysis of the cerebral oxidative stress was performed on a physiological model of dementia accomplished by three-vessel occlusion in aged rats. The forward rate constant of creatine kinase, k(for), was studied by saturation transfer (31)P magnetic resonance spectroscopy in adult and aged rat brain during chronic hypoperfusion. In addition, free radicals in aging rat brain homogenates before and/or after occlusion were investigated by spin-trapping electron paramagnetic resonance spectroscopy (EPR). Finally, biochemical measurements of oxidative phosphorylation parameters in the above physiological model were performed. The significant reduction of k(for) in rat brain compared to controls 2 and 10 weeks after occlusion indicates a disorder in brain energy metabolism. This result is consistent with the decrease of the coefficient of oxidative phosphorylation (ADP:O), and the oxidative phosphorylation rate measured in vitro on brain mitochondria. The EPR study showed a significant increase of the ascorbyl free radical concentration in this animal model. Application of alpha-phenyl-N-tert-butylnitrone (PBN) and 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin traps revealed formation of highly reactive hydroxyl radical (.OH) trapped in DMSO as the .CH(3) adduct. It was concluded that the ascorbate as a major antioxidant in brain seems to be useful in monitoring chronic cerebral hypoperfusion.

Solvent effect on the photocatalytic reduction of 4-nitrophenol in titanium dioxide suspensions

The selective photocatalytic reduction of 4-nitrophenol to 4-aminophenol in titanium dioxide suspensions prepared in aliphatic alcohols (methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, i-butanol) was investigated. The photoreduction rate is significantly affected by the solvent parameters, such as viscosity, polarity, polarisability, and polarity/polarisability.

EPR study of photoinduced reduction of nitroso compounds in titanium dioxide suspensions

Abstract The radical intermediates produced upon UV irradiation of deoxygenated alcoholic titanium dioxide suspensions of nitrosobenzene, nitrobenzene, 2-nitrosotoluene, 2,3,5,6-tetramethylnitrosobenzene, 3,5-di-bromo-4-nitrosobenzenesulfonate (sodium salt), 2,4,6-tri-t-butyl-nitroso-benzene, and 2-methyl-2-nitrosopropane were investigated using in situ EPR technique. Nitrosobenzene is efficiently photoreduced in TiO2 suspensions (toluene/alcohol, 1:1 (v/v)) forming exclusively one stable radical intermediate corresponding to C6H5N OH species. The formation of this radical species is consistent with the proposed photocatalytic reduction mechanism, occurring from the primary generated nitrosobenzene mono-anion by the hydrogen abstraction from surroundings. The origin of hydrogen added to the nitroso group was demonstrated by the photocatalytic experiments using deuterated methanol, where the production of C6H5N OD was established. Additionally, an identical radical C6H5N OH was detected, when nitrobenzene was reduced under analogous experimental conditions. The photoinduced electron transfer from TiO2 to nitroso compounds is accompanied by alcohol oxidation via the photogenerated titanium dioxide valance band holes forming alkoxy and hydroxyalkyl radicals. Production of hydroxyalkyl radicals ( CH2OH, CH(OH)CH3, C(OH)(CH3)2) with redox potentials suitable for a direct electron transfer to nitroso compounds represents an alternative reaction pathway for their reduction. On the other hand, the investigated nitroso derivatives are efficient spin-trapping agents, therefore, formation of nitroxyl radical spin adducts was observed in the photocatalytic experiments. The EPR spectra monitored upon irradiation of substituted nitrosobenzene derivatives in alcoholic TiO2 suspensions reveal the correlation between nitrosobenzene derivative first step reduction potentials and yield of radical species produced.

Radical Intermediates in Photoinduced Reactions on TiO2 (An EPR Spin Trapping Study)

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 17O-enriched water caused changes in the EPR spectra reflecting the interaction of an unpaired electron with the 17O 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.

Characterization of titanium dioxide photoactivity following the formation of radicals by EPR spectroscopy

In order to find ways to characterize oxygen-saturated aqueous TiO2 suspensions, the formation of photo-induced free radicals was followed by EPR spectroscopy, using as indicators N-oxide and nitrone spin trapping agents, 5,5-dimethyl-1-pyrroline N-oxide (DMPO), 3,3,5,5-tetramethyl-1-pyrroline N-oxide (TMPO), α-(4-pyridyl-1-oxide)-N-tert-butylnitrone (POB N), 4-(N-methylpyridyl)-N-tert-butylnitrone (MePyBN), as well as semi-stable free radicals, 4-hydroxy-2,2,6,6-tetramethylpiperidine N-oxyl (TEMPOL), cation radical of 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid), diammonium salt (ABTS) and 1,1-diphenyl-2-picrylhydrazyl (DPPH). DMPO and TMPO are efficiently oxidized to the EPR-silent products via radical in termediates. Conversely, the nitrone spin traps (POBN and MePyBN) showed selective formation of hydroxyl radical spin adducts upon continuous irradiation of oxygenated TiO2 suspensions. Their concentrations increased proportionally with the amount of photocatalyst and irradiation time. The EPR spectrum of the semi-stable free radicals TEMPOL, ABTS·+ or DPPH is gradually eliminated during irradiation, and this system represents a simple technique for the evaluation of TiO2 activity.

The potential pitfalls of using 1,1-diphenyl-2-picrylhydrazyl to characterize antioxidants in mixed water solvents

Approaching living systems, aqueous solutions are appropriate to characterize antioxidants, whereas the frequently used standard 1,1-diphenyl-2-picrylhydrazyl (DPPH) is insoluble in water. Therefore, mixed water–ethanol solvents were investigated using the electron paramagnetic resonance (EPR) spectroscopy. Two forms of DPPH were identified: at higher ethanol ratios a quintet spectrum characteristic of solutions, and at lower ratios, a singlet spectrum typical for solid DPPH, were found. Mixed solvents with 0–50% (v/v) water reproduced the same antioxidant equivalent points well and the reaction rate between DPPH and the antioxidant may increase considerably with increasing water ratios, as demonstrated using vitamin E as an antioxidant. But at still higher water ratios (70–90% (v/v)) the antioxidant activities dropped, since a part of the DPPH in the aggregated form does not react sufficiently with the antioxidants. Characteristics of the most common antioxidants were determined in ethanol or its 50% (v/v) aqueous solution.

Photocatalytic hydroxylation of benzoic acid in aqueous titanium dioxide suspension

The photocatalytic hydroxylation of benzoic acid in aqueous TiO2 suspension is suggested as evidence for the generation of hydroxyl radicals in the H2O(l)—TiO2(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.