Search

Some TiO2 powders, prepared from titanium(IV)tetraisopropoxide by the sol-gel method and thermally treated between 100 and 1000?C, have been characterized by X-ray powder diffraction and by nitrogen adsorption and desorption at 77 K to calculate the BET-specific surface area, from which the micropore volume and the external surface area can be derived. The photocatalytic activity (ka) of the above powders has been evaluated considering the TiO2-sensitized photo-oxidation of 4-methoxybenzyl alcohol in CH3CN as the test reaction. The decrease of ka have been related to the decrease of the BET surface area, the micropore volume, and the external surface area of the TiO2 powders, but a satisfactory linear correlation is observed only for the last superficial parameter. [ABSTRACT FROM AUTHOR]

Through current efficiency measurements, obtained from the photoelectrochemical oxidation at TiO2 (rutile)/Ti anodes, further mechanistic information has been obtained regarding the TiO2 photosensitized oxidation of benzylic alcohols, ethers and 1,2-diols in CH3CN. In deaerated medium, two electrons are captured by the semiconductor from all the considered substrates (one from the substrate, the second from the intermediate benzylic radical). In contrast, in aerated CH3CN, the number of TiO2-captured electrons can be reduced to one because, depending on its oxidizability, the benzylic radical can be competitively captured by oxygen. Copyright © 2008 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

A mechanistic study, principally based on product analysis, relative to the TiO2-photosensitized oxidation of indane and some of its hetero-analogues, in deaerated CH3CN and in the presence of Ag2SO4, was performed. In particular: (i) 1-acetamidoindan (principal product), indene, 1-indanol and 1-indanone were obtained from indan; (ii) 5-methoxyindan gave 6-methoxyindene (principal product) and 5-methoxy-1-indanone; (iii) 2,3-dihydrobenzofuran, 2,3-dihydroindole and 2,3-dihydrobenzothiophene produced benzofuran, indole and benzothiophen (the last one accompanied by minor amounts of 2,3-dihydrobenzothiophene-1-oxide), respectively. Considering the previous studies on photo-oxidation of analogous substrates as benzylic derivatives (arenes, alcohols and ethers) and from reaction product profiles, an electron-transfer mechanism (from the substrate to the photogenerated hole) is suggested, where the radical cation intermediate should deprotonate to a benzylic radical. The carbocation obtained from the oxidation of this radical should competitively evolve to alkene, alcohol and acetamide. H218O labelling photo-oxidation experiments suggest that the ketone, when present, should derive from the substrate, through the alcohol as intermediate. Copyright © 2006 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

TiO2-sensitised photo-oxidation of α-alkylbenzyl methyl ethers in deaerated CH3CN gives the expected corresponding ketone, whereas in an aerated medium a dealkylation compound (methyl benzoate) is obtained as the principal product, with respect to the ketone, or the exclusive product (when the alkyl group is tert-butyl). The product analysis and distribution, together with a qualitative estimate of the relative reactivity, suggest that the mechanism in deaerated CH3CN is the same as that previously hypothesized for non-α-alkylated benzyl ethers; in particular, the carbonyl compound should be formed through oxidation of the benzylic radical (obtained by deprotonation of the cation radical) to the corresponding cation. To justify the formation of methyl benzoate in an aerated medium, a reasonable hypothesis is that the α-alkyl-α-methoxybenzylperoxy radical (obtained from the competitive attack of oxygen on the benzylic radical) undergoes a dealkylation process; in particular, this intermediate, as a tertiary peroxy radical, could form a dimer that evolves into the corresponding oxy radical, giving the ester through a β-scission process. The general mechanism suggested in aerated medium was confirmed by evaluating the relationship between the ester/carbonyl (E/C) molar ratio and the adiabatic ionization potential of the benzylic radical intermediate. To evaluate the influence of medium heterogeneity, the E/C ratio data were compared with those obtained from an electron-transfer photosensitised [by 9,10-dicyanoanthracene (DCA)] oxidation in a homogeneous phase. A significant confirmation of the mechanism, in both deaerated and aerated media, was obtained by the reaction performed from 4-methoxy-α-ethylbenzyl methyl ether (as a model) in the presence of H218O, sensitised by either TiO2 or DCA. Copyright © 2005 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

A jointed experimental and theoretical investigation pointing out new insights about the microscopic mechanism of the volatile organic compounds (VOCs) photocatalytic elimination by TiO2 was done. Methane, hexane, isooctane, acetone and methanol were photomineralized in a batch reactor. Values of K (adsorption constant on TiO2) and k (mineralization rate constant) of the five VOCs (treating the kinetic data through a Langmuir–Hinshelwood approach) were determined. Recorded K (in the range of 0.74 × 10−2–1.11 × 10−2 ppm−1) and k (in the range of 1.9–9.9 ppm min−1) values and performed theoretical calculations allowed us to suggest the involvement of an electron transfer step between the VOC and the hole, TiO2(h+), as the rate-determining one. [ABSTRACT FROM AUTHOR]

Abstract: The photosensitized electrochemical oxidation at a TiO2/Ti anode is, in itself, a “green” chemical strategy as the semiconductor remains unchanged during the experiments, the metal is safe, low energies are involved and experimental conditions are mild. In this context, the oxidation of 4-methoxybenzyl alcohol to 4-methoxybenzaldehyde has been carried out on the light of further “green chemistry” requirements: water as medium, NaCl as supporting electrolyte, exclusion of the reference electrode, Ti cathode (less expensive than the traditional Pt one) and Ti mesh (to optimize the cell geometry). In this study some chemical (alcohol conversion to aldehyde and material recovery) and electrochemical (current efficiency and density) parameters have been used to select the reaction conditions that are more suitable for this purpose. As concern the cell, the best geometry was realized with an immersion lamp, coaxial with a mesh cylindrical TiO2/Ti anode and a mesh Ti cathode. [Copyright &y& Elsevier]

Highlights: [•] TiO2 powders are obtained by sol–gel method, annealing at different temperatures. [•] Powders photocatalytic efficiency is tested by photodegradation of gaseous acetone. [•] Efficiency is evaluated by formed CO2 (by gas-chromatography or IR gas sensor). [•] At different conditions, the same CO2/annealing temperature profile is obtained. [•] Efficiency depends on crystalline form, crystallinity and physisorbed water. [ABSTRACT FROM AUTHOR]

Abstract: The TiO2 photosensitized oxidation of 4-dodecyloxybenzyl alcohol (1), water insoluble, was investigated in aqueous surfactant solutions. A variety of surfactants of different charge type was used and in some cases the pH effect was investigated. The considered surfactants are: cationic (cetyltrimethylammonium with various counterions, CTAX with X=Cl, Br and OMs), zwitterionic (dimethyldodecylamine oxide, DDAO, tetradecyltrimethylammonium propane sulfonate, SB3-14, and tetradecyltripropylammonium propane sulfonate, SBPr3-14) and anionic (sodium dodecylsulfate, SDS, sodium dodecylbenzenesulfonate, SDBS). The reaction, that is practically absent in water, is greatly enhanced by several surfactants at concentrations higher than cmc and the effect is strongly dependent on the nature of the surfactant. A rationale for that is suggested, with micellar aggregates rather than monomeric surfactants playing a main role. The effects of surfactant concentration on the conversion to the corresponding benzaldehyde were studied. We suggest that the increase of the surfactant concentration leads to an increase of the number of substrate molecules, solubilized (through co-micellization) in micelles, which can be transported close to the TiO2 particle surface where they react. After a certain concentration which varies with the nature of the surfactant, the presence of the competitive partition between the co-micellized substrate 1 adsorbed on TiO2 (where reaction occurs) and the co-micellized substrate in the bulk tends to limit more and more the beneficial kinetic effect of the surfactant. The surfactant that gives in water the best performance in terms of 4-dodecyloxybenzaldehyde yield (42%) is SB3-14 0.015M. Moreover, a better yield (55%) is obtained in the presence of surfactant CTABr 0.05M with added HCl 0.1M. [Copyright &y& Elsevier]