Showing papers in "Journal of Photochemistry and Photobiology A-chemistry in 2003"

Photocatalytic degradation of azo dye acid red 14 in water: investigation of the effect of operational parameters

Abstract: Acid Red 14 (AR14), commonly used as a textile dye, could be photocatalytically degraded using TiO 2 suspensions irradiated by a UV-C lamp (30 W). The experiments showed that TiO 2 and UV light had a negligible effect when they were used on their own. The semi-log plot of dye concentration versus time was linear, suggesting first order reaction ( K =1.41×10 −2 min −1 ). The effects of some parameters such as pH, the amount of TiO 2 and initial dye concentration were also examined. The photodegradation of AR14 was enhanced by the addition of proper amount of hydrogen peroxide, but it was inhibited by ethanol. From the inhibitive effect of ethanol it was deducted that hydroxyl radicals played a significant role in the photodegradation of dye, but a direct oxidation by positive holes was probably not negligible. Accordingly, it could be stated that the complete removal of color, after selecting optimal operational parameters could be achieved in a relatively short time, about 3.5 h.

Studies on photokilling of bacteria on TiO2 thin film

Abstract: In order to elucidate the mechanism for photokilling of Escherichia coli (E. coli) cells on titanium dioxide (TiO2) thin film, the survival of intact cells and the spheroplasts was investigated as a function of photo-illumination time. The photokilling reaction for intact cells was observed to involve two steps, an initial lower rate photokilling step followed by a higher rate one. In contrast, the reaction for spheroplasts, which do not have cell wall, exhibited only a single step kinetics with a higher rate, suggesting that the cell wall of E. coli cell acts as a barrier to the photokilling process. Changes in concentration of the cell wall components during illumination further showed that the outer membrane serves as a barrier, while the peptidoglycan layer does not have a barrier function. Moreover, atomic force microscopy measurements of cells on illuminated TiO2 film showed that the outer membrane decomposed first, and with further illumination, the cells completely decomposed. These results suggest that the photokilling reaction is initiated by a partial decomposition of the outer membrane, followed by disordering of the cytoplasmic membrane, resulting in cell death.

Measurements of molecular absorption spectra with the SCIAMACHY pre-flight model: instrument characterization and reference data for atmospheric remote-sensing in the 230–2380 nm region

Abstract: Using the scanning imaging absorption spectrometer for atmospheric chartography (SCIAMACHY) pre-flight model satellite spectrometer, gas-phase absorption spectra of the most important atmospheric trace gases (O3, NO2, SO2, O2, OClO, H2CO, H2O, CO, CO2, CH4, and N2O) have been measured in the 230–2380 nm range at medium spectral resolution and at several temperatures between 203 and 293 K. The spectra show high signal-to-noise ratio (between 200 up to a few thousands), high baseline stability (better than 10−2) and an accurate wavelength calibration (better than 0.01 nm) and were scaled to absolute absorption cross-sections using previously published data. The results are important as reference data for atmospheric remote-sensing and physical chemistry. Amongst other results, the first measurements of the Wulf bands of O3 up to their origin above 1000 nm were made at five different temperatures between 203 and 293 K, the first UV-Vis absorption cross-sections of NO2 in gas-phase equilibrium at 203 K were recorded, and the ultraviolet absorption cross-sections of SO2 were measured at five different temperatures between 203 and 296 K. In addition, the molecular absorption spectra were used to improve the wavelength calibration of the SCIAMACHY spectrometer and to characterize the instrumental line shape (ILS) and straylight properties of the instrument. It is demonstrated that laboratory measurements of molecular trace gas absorption spectra prior to launch are important for satellite instrument characterization and to validate and improve the spectroscopic database.

Influence of chemical structure of dyes, of pH and of inorganic salts on their photocatalytic degradation by TiO2 comparison of the efficiency of powder and supported TiO2

Abstract: Anionic (Alizarin S (AS), azo-Methyl Red (MR), Congo Red (CR), Orange G (OG)) and cationic (Methylene Blue (MB)) dyes were degraded, either individually or in mixtures, by using UV-irradiated TiO 2 in suspension or supported on glass and on paper. The influence of the chemical structure of different dyes as well as that of pH and of the presence of inorganic salts on the photocatalytic properties of TiO 2 has been discussed. The role of adsorption is suggested, indicating that the reaction occurs at the TiO 2 surface and not in the solution. S and N hetero-atoms are respectively mineralized into SO 4 2− , NO 3 − and NH 4 + , except azo-groups which mainly formed N 2 which represents an ideal case for a decontamination reaction. The fate of nitrogen strongly depends on its initial oxidation degree. High photocatalytic activities have been found for TiO 2 coated on glass by the sol–gel method. Its efficiency was intermediate between those of PC-500 and P-25 powders. The efficiency of PC-500 TiO 2 sample, fixed on paper by using a binder, is slightly less important than that of the powder. The presence of a silica-binder with an acidic pzc is suggested to be at the origin of the decrease in efficiency.

Photochemical and photocatalytic degradation of an azo dye in aqueous solution by UV irradiation

Abstract: The photochemical and photocatalytic degradation of aqueous solutions of Solophenyl Green (SG) BLE 155%, an azo dye preparation very persistent in heavy colored textile waters, has been investigated by means of ultraviolet (UV) irradiation. The pure photochemical process demonstrated to be very efficient for low initial concentrations of the dyestuff. For higher concentrations the photocatalyitic degradation was carried on using commercial titanium dioxide, and mixtures of this semiconductor with different activated carbons (AC) suspended in the solution. The kinetics of photocatalytic dyestuff degradation were found to follow a first-order rate law. It was observed that the presence of the activated carbon enhanced the photoefficiency of the titanium dioxide catalyst. Differently activated carbon materials induced different increases in the apparent first-order rate constant of the process. The effect was quantified in terms of a synergy factor (R) already described in the literature. The kinetic behavior could be described in terms of a modified Langmuir–Hinshelwood model. The values of the adsorption equilibrium constants for the organic molecules, KC, and for the reaction rate constants, kC, were 0.0923 l mg−1 and 1.58 mg l−1 min−1 for the TiO2/UV process and 0.0928 l mg−1 and 2.64 mg l−1 min−1 for the TiO2+AC/UV system with highest synergy factor, respectively. The mechanism of degradation was discussed in terms of the titanium dioxide photosensitization by the activated carbon.

TiO2 photocatalytic oxidation of nitric oxide: transient behavior and reaction kinetics

Abstract: Photocatalytic oxidation (PCO) of nitric oxide (NO) over TiO 2 catalyst was studied at source levels (5–60 ppm) The PCO process involves a series of oxidation steps by the OH radical: NO→HNO 2 →NO 2 →HNO 3 The product NO 2 can be collected in an adsorbent bed and either recycled back to the combustion chamber or recovered as nitric acid The ratio of NO 2 − to NO 3 − from spent catalyst liquor drops with irradiation time until a steady state is reached The reactions are limited by thermodynamic equilibrium after ∼12 s space time The steady-state experimental data from space time and inlet concentration effects can be described with the Langmuir–Hinshelwood (L–H) kinetic model with R 2 =09208

Characterisation of the photocatalyst Pilkington Activ (TM): a reference film photocatalyst?

Abstract: Pilkington Glass Activ™ represents a possible suitable successor to P25 TiO2, especially as a benchmark photocatalyst film for comparing other photocatalyst or PSH self-cleaning films. Activ™ is a glass product with a clear, colourless, effectively invisible, photocatalytic coating of titania that also exhibits PSH. Although not as active as a film of P25 TiO2, Activ™ vastly superior mechanical stability, very reproducible activity and widespread commercial availability makes it highly attractive as a reference photocatalytic film. The photocatalytic and photo-induced superhydrophilitic (PSH) properties of Activ™ are studied in some detail and the results reported. Thus, the kinetics of stearic acid destruction (a 104 electron process) are zero order over the stearic acid range 4-129 monolayers and exhibit formal quantum efficiencies (FQE) of 0.7×10−5 and 10.2×10−5 molecules per photon when irradiated with light of 365±20 and 254 nm, respectively; the latter appears also to be the quantum yield for Activ™ at 254 nm. The kinetics of stearic acid destruction exhibit Langmuir-Hinshelwood-like saturation type kinetics as a function of oxygen partial pressure, with no destruction occurring in the absence of oxygen and the rate of destruction appearing the same in air and oxygen atmospheres. Further kinetic work revealed a Langmuir adsorption type constant for oxygen of 0.45±0.16 kPa−1 and an activation energy of 19±1 kJ mol−1. A study of the PSH properties of Activ™ reveals a high water contact angle (67°) before ultra-bandgap irradiation reduced to 0° after prolonged irradiation. The kinetics of PSH are similar to those reported by others for sol-gel films using a low level of UV light. The kinetics of contact angle recovery in the dark appear monophasic and different to the biphasic kinetics reported recently by others for sol-gel films [J. Phys. Chem. B 107 (2003) 1028]. Overall, Activ™ appears a very suitable reference material for semiconductor film photocatalysis.

Antibacterial textiles prepared by RF-plasma and vacuum-UV mediated deposition of silver

Abstract: The bacterial inactivation efficiencies of silver metal and oxides and their combinations on textile fabrics was investigated to evaluate the disinfectant action on airborne bacteria. The inactivation performance was seen to depend on the amount of silver on the textile surface. The preparation of the polyester-polyamide Ag-loaded textiles was carried out by RF-plasma and vacuum-UV (V-UV) surface activation followed by chemical reduction of silver salts. The rate of bacterial inactivation by the silver loaded textile was tested on Escherichia coli K-12 and showed long lasting residual effect. Specular reflectance has been employed to assess the optical properties of the Ag-loaded fabrics. By elemental analysis it was found that levels of Ag loading >0.118% (w/w) for the vacuum-UV samples lead to complete inhibition of bacterial growth. X-ray photoelectron spectroscopy (XPS) shows that for textiles activated by RF or V-UV methods, the silver in the topmost layer increases with increasing concentration of the Ag used in the precursor solution. The exact determination of the oxidation state of the Ag-clusters on the textile is difficult because of the variation of particle size and electrostatic charging of the supported particles. Ag metal was found to be the main component of the Ag-clusters and not Ag2O and AgO as identified by the binding peak energies (BE). By transmission electron spectroscopy (TEM) it was seen that the Ag-clusters were deposited on the two polymer components of the textile fabric but having widely different sizes. (C) 2003 Elsevier Science B.V. All rights reserved.

Oxidation of commercial reactive azo dye aqueous solutions by the photo-Fenton and Fenton-like processes

Abstract: This paper evaluates the degradation of two azo reactive dyes, C.I. Reactive Yellow 84 (RY84) and C.I. Reactive Red 120 (RR120) by photo-Fenton and Fenton-like oxidation. All experiments were performed on a laboratory scale set-up. The effects of different reaction parameters such as initial pH, contact time, effect of light and hydrogen peroxide concentrations on the oxidation of the dye aqueous solutions have been assessed. Effective system conditions were found to be pH of 3, hydrogen peroxide-to-iron molar ratio of 20:1 and UV or solar irradiation. The color removal efficiency at the optimum conditions during different Fenton-like processes was also evaluated. The results show that the color removal of RY84 after 15 min reaction time follows the decreasing order: solar/Fe(II)/ H 2 O 2 >UV/Fe(II)/ H 2 O 2 >UV/Cu(II)/Fe(III)/ H 2 O 2 >UV/Fe(III)oxalate/H 2 O 2 >UV/Fe(III)/ H 2 O 2 >dark/Fe(II)/ H 2 O 2 >solar/Fe(III)oxalat/H 2 O 2 >UV/H 2 O 2 >UV/Fe(II)=UV. During the same reaction period the relative order for RR120 removal rate was slightly different: solar/Fe(II)/ H 2 O 2 >UV/Fe(II)/ H 2 O 2 >UV/Fe(III)/ H 2 O 2 =UV/Cu(II)/Fe(III)/ H 2 O 2 >UV/Fe(III)oxalate/H 2 O 2 =UV/H 2 O 2 >UV. The toxic potential of the dye’s degradation was investigated by the bioluminescence test using the LUMIStox 300 instrument and results were expressed as the percentage inhibition of the luminescence of the bacteria Vibrio fisheri . Formate and oxalate, identified as fragmental oxidation products of investigated dyes, could also be detected after 15 min irradiation time.

Photochemical conversion of triclosan to 2,8-dichlorodibenzo-p-dioxin in aqueous solution

Abstract: The direct photolysis of triclosan (5-chloro-2-(2,4-dichlorophenoxy)phenol), an antimicrobial additive commonly detected in surface waters, is studied. It is found that 2,8-dichlorodibenzo-p-dioxin (2,8-DCDD) is produced in both buffered and natural (Mississippi River) water with yields ranging from 1 to 12% under a variety of conditions. This result indicates that triclosan is likely converted to 2,8-DCDD in sunlight-irradiated surface waters.

Direct water splitting into H2 and O2 under visible light irradiation with a new series of mixed oxide semiconductor photocatalysts

Abstract: The research on a new series of solid photocatalysts with different crystal structures was reviewed. The first system is A2B2O7 pyrochlore-crystal type: Bi2MNbO7 (M=Al, Ga, In and Y, rare earth, and Fe), which is cubic system and space group Fd3m. The second system is ABO4 stibotantalite-crystal type: BiMO4 (M=Nb5+, Ta5+), in which both the triclinic system with space group P1 in the case of M=Ta and the orthorhombic system with space group Pnna in the case of M=Nb. The third system is ABO4 wolframite-crystal type: InMO4 (M=Nb5+, Ta5+), which is monoclinic system and space group P2/a. Although these photocatalysts crystallize in the different crystal structure, they contain the same octahedral TaO6 and/or NbO6 in the different photocatalysts. The band structure of the photocatalysts is defined by Ta/Nb d-level for a conduction band and O 2p-level for a valence band. The band gaps of the photocatalysts were estimated to be between 2.7 and 2.4 eV. Metal doped InTaO4 photocatalysts were also investigated. Under visible light (λ>420 nm) or ultra-violet irradiation, the H2 and/or O2 evolutions were observed from pure water as well as aqueous CH3OH/H2O and AgNO3 solutions. The photocatalytic activity increases significantly by loading co-catalysts such as Pt, RuO2 and NiOx on the surface of the photocatalysts. Finally, direct water splitting into H2 and O2 under visible light irradiation was firstly established using newly synthesized NiOx (partly oxidized nickel) promoted In0.9Ni0.1TaO4 photocatalyst.

Synthesis of photocatalytic TiO2 nanoparticles: optimization of the preparation conditions

Abstract: Nanosized titanium oxide (TiO 2 ) powders were prepared by sol–gel route The preparation parameters are optimized by means of Malachite green oxalate degradations All catalysts were analyzed by X-ray diffraction (XRD) and BET technique The best catalyst was compared to TiO 2 -P25 using two other types of pollutant, 4-hydroxy benzoic and benzamide (BZ) and their performance was found to be strongly dependant on the pollutant’s type

Molecular design of ruthenium(II) polypyridyl photosensitizers for efficient nanocrystalline TiO2 solar cells

Abstract: Transition metal complexes using polypyridine ligands are finding increasing use in the dye sensitized solar cells. To improve further the efficiency of this cell, an enhanced spectral response of the sensitizer in the lower energies is required. In this paper we review our effort in the molecular design of the ruthenium polypyridyl complexes for nanocrystalline TiO 2 -based solar cells. The poor cell efficiency in Ru(4,4'-dicarboxy-2,2'-biquinoline) 2 (NCS) 2 /TiO 2 system may be ascribed to the low excited-state oxidation potential, which plays a crucial role in the electron-transfer process. Ru(2-(2-(4-carboxypyridyl))-4-carboxyquinoline) 2 (NCS) 2 , when anchored to nanocrystalline TiO 2 films, achieves efficient sensitization over the whole visible range extending up to 900 nm, yielding incident photon-to-current conversion efficiency (IPCE) of 55%. The low cell efficiency of Ru(4-4'-dicarboxy-2,2'-bypyridine) 2 (ethyl-2-cyano-3,3-dimercaptoacrylate) may be due to slow regeneration of the dye by electron donation from iodide following charge injection into the TiO 2 . Tuning of HOMO and LUMO energy level show that an efficient sensitizer should possess ground-state and excited-state redox potentials of 0.5 and -0.8 V vs. SCE, respectively. Transient absorption studies of Ru phenanthroline complexes show that the efficiency of electron injection is strongly affected by the number of carboxyl groups of the sensitizing dye. The p-diketonate complex [Ru(4-4'-dicarboxy-2,2'-bypyridine) 2 (acetylacetonato)]Cl, when anchored to nanocrystalline TiO 2 films, achieves very efficient sensitization across the entire visible region, yielding 60% IPCE. A new series of panchromatic sensitizers of Ru(tricarboxyterpyridine)(β-diketonato)(NCS) type have been developed. Ru(4,4',4-tricarboxy-2,2': 6',2-terpyridine)(1,1,1-trifluoropentane-2,4-dionato)(NCS) achieved an efficient sensitization of nanocrystalline TiO 2 solar cells over the whole visible range extending into near IR region and displaying a maximum around 600 nm, where IPCE approaches a high value of 70%.

Photocatalytic bactericidal effect of TiO2 on Enterobacter cloacae: Comparative study with other Gram (−) bacteria

Abstract: The bactericidal action of heterogeneous photocatalysis (UV-A/TiO2) has been tested on Enterobacter cloacae, a microorganism very resistant to UV-A irradiation. Results have been compared with other representative strains of Gram (−) bacilli of different photosensitivity like E. coli, Pseudomonas aeruginosa and Salmonella typhimurium. The TiO2 photocatalytic technology can inactivate bacteria resistant to oxidative membrane damage caused by direct UV irradiation, like E. cloacae, a common soil and aquatic microorganism, which normally is not affected by low UV-A irradiation intensity. In all cases, sublethal UV-A doses provoked an important lethality in the presence of TiO2. Inactivation rates of the microorganisms are compared and some clues on the mechanism of bacteria destruction are discussed.

Free radical macrophotoinitiators: an overview on recent advances

Abstract: This paper provides an overview of the recent advances on polymeric photoinitiators for UV curing. During the last decade, significant developments have been achieved in the synthesis of macrophotoinitiators, due to the advantages derived of their macromolecular nature, in comparison with their corresponding low molecular weight analogues. In particular, a variety of macromolecules containing the two main types of free radical photoinitiators: hydrogen-abstracting (thioxanthone, benzil, anthraquinone, camphorquinone) and photofragmenting chromophores (benzoin ether, acylphosphine oxides) are described. For hydrogen-abstracting photoinitiator, the photoinitiation activity have been examined in terms of volume and nature of substituent in the polymeric coil, and their influence to prevent the recombination of radicals favouring their reaction with the monomer. Also, copolymers bearing chromophore and amine groups with potential synergistic effects of activity are reported. It has been found that the approach of the tertiary amine to the chromophore to produce the corresponding exciplexes is dependent on both the monomeric or polymeric nature of chromophore and the tertiary amine. Type II polymeric photoinitiators, such as benzoin ether derivatives having the benzoin methyl ether moieties connected to the main chain through the benzyl aromatic are reported. And a fragmentation mechanism involving the formation of an stable quinoid structure and aliphatic acyl radical is proposed for the above copolymers, which would justified their lower initiating efficiency than the corresponding low molecular weight model. In addition, polymers bearing phosphine oxide moieties are described. The efficiency in the polymerisation of all photoinitiators was found to be similar and irrespective of the presence of flexible spacer in their structure. However, it was found that the flexible oligomethylene spacer enhanced the compatibility of the new polymeric photoinitiators in acrylic adhesive formulations. Finally, polysilanes as photoinitiators are reported. Under UV irradiation, polysilanes undergo main-chain scission leading to free silyl radicals capable of reacting with olefinic monomers. The silyl radicals generated by photolysis can be oxidised by appropriate onium salts to yield cationic initiating species (photoinitiated radical promoted cationic polymerisation). The photoinitiation efficiency of polysilanes having different aliphatic and aromatic side groups has been investigated and compared with commercial low molecular weight photoinitiators as benzoin.

Thick titanium dioxide films for semiconductor photocatalysis

Abstract: Thick paste TiO2 films are prepared and tested for photocatalytic and photoinduced superhydrophilic (PSH) activity. The films are effective photocatalysts for the destruction of stearic acid using near or far UV and all the sol–gel films tested exhibited a quantum yield for this process of typically 0.15%. These quantum yields are significantly greater (4–8-fold) than those for titania films produced by an APCVD technique, including the commercial self-cleaning glass product Activ™. The films are mechanically robust and optically clear and, as photocatalysts for stearic acid removal, are photochemically stable and reproducible. The kinetics of stearic acid photomineralisation are zero order with an activation energy of ca. 2.5 kJ mol−1. All titania films tested, including those produced by APCVD, exhibit PSH. The light-induced fall, and dark recovery, in the water droplet contact angle made with titania paste films are similar in profile shape to those described by others for thin titania films produced by a traditional sol–gel route.

Visible light and Fe(III)-mediated degradation of Acid Orange 7 in the absence of H2O2

Abstract: The photodegradation of Acid Orange 7 (AO7) was successfully achieved in the presence of Fe(III) ions only under visible light (λ≥420 nm). Upon adding Fe(III) to AO7 solution, ferric ions formed complexes with AO7 mainly through the azo chromophoric group. This AO7–Fe(III) complex formation was highly pH-sensitive and maximized around pH 3.7. The visible light-induced degradation of AO7 was effective only when the complex formation was favored. The AO7 photodegradation accompanied the production of ferrous (Fe2+) ions and was not inhibited in the presence of excess OH radical scavenger (2-propanol), which indicated that OH radicals were not responsible for the dye degradation. The proposed mechanism of the dye degradation is the visible light-induced electron transfer from the azo chromophoric group to the iron center in the complex. Therefore, when the formation of AO7–Fe(III) complex was inhibited in the presence of excess interfering anions such as sulfites and sulfates, the photodegradation of the dye was also prevented. The photodegradation of AO7 under visible light produced o-phthalate and 4-hydroxybenzenesulfonate (4-HBS) as major products but did not reduce the total organic carbon (TOC) concentration. Since this process does not require the addition of hydrogen peroxide, it might be developed into an economically viable method to pretreat or decolorize azo-dye wastewaters using sunlight.

Heterogeneous photocatalytic degradation of nitrophenols

Abstract: The photocatalytic degradation of 2-, 3- and 4-nitrophenol has been investigated in oxygenated aqueous suspensions containing TiO2. The organic reaction intermediates have been determined by high performance liquid chromatography. The results indicate the complete mineralisation of the substrates and the formation of both nitrate and ammonium ions. The degradation pathways involve a rapid opening of the aromatic ring followed by a slower oxidation of the aliphatic compounds. The hydroxyl radicals are responsible for the primary attack of the nitrophenols with formation of dihydroxynitrobenzenes. The ring hydroxylation occurs in the positions activated by the contemporaneous presence of phenolic and nitro groups. The presence of ammonium ions confirms that reduction pathways are also operating.

Effects of organic hole scavengers on the photocatalytic reduction of selenium anions

Abstract: The photocatalytic reduction of selenium anions, selenate (Se(VI)) and selenite (Se(IV)) to elemental selenium (Se) over UV-illuminated TiO2 was performed using formic acid, acetic acid, methanol, ethanol, sucrose and salicylic acid as the organic hole scavengers. Photoreduction was only observed in the presence of formic acid, methanol or ethanol. The fastest rate of Se ions photoreduction was observed in the presence of formic acid followed by methanol and ethanol. This was attributed to the ability of formate ions to adsorb onto TiO2 in the presence of Se ions, its fast mineralisation rate and its ability to form reducing radicals quickly. For the methanol and ethanol systems, these two organic compounds could not compete with Se ions for the TiO2 surface and were not easily mineralised. The photocatalytic reduction of Se ions observed in the presence of these two organic compounds was attributed to their ability to form reducing radicals. When formic acid was used, optimum pH values at pH 3.5 and 4.0 was encountered for the Se(VI) and Se(IV) photoreduction, respectively. When methanol and ethanol were used as the hole scavenger in the pH range of 2.2–4.0, the greatest extent of Se ions photoreduction was encountered at pH 2.2. This suggests the different role of formic acid and methanol/ethanol in the photoreduction of Se ions.

A comparative study on decomposition of gaseous toluene by O3/UV, TiO2/UV and O3/TiO2/UV

Abstract: The degradations of trace toluene (1.0–20 ppmv) in the gas-phase by O 3 /UV, TiO 2 /UV and O 3 /TiO 2 /UV were studied. The effects of the inlet concentration of toluene, flow rate (retention time), relative humidity and ultraviolet (UV) light wavelength on the conversion of toluene in the three processes were examined, respectively. The experimental results showed that the addition of ozone to the photocatalysis process could greatly increase the conversion of toluene. In particular, the deactivation of the photocatalyst at high inlet concentration of toluene was avoided in the presence of ozone. The O 3 /TiO 2 /UV process was more efficient than the O 3 /UV in decomposing toluene in most cases. And the residual ozone concentration in the O 3 /TiO 2 /UV process was much lower than that in the O 3 /UV process. Among them, combined photocatalysis and ozonation (O 3 /TiO 2 /UV) was the most prospective process for removing trace volatile organic compounds (VOCs) such as in indoor air.

Photoelectrocatalytic degradation of Remazol Brilliant Orange 3R on titanium dioxide thin-film electrodes

Abstract: Degradation of reactive dye Remazol Brilliant Orange 3R (RBO) has been performed using photoeletrocatalysis. A biased potential is applied across a titanium dioxide thin-film photoelectrode illuminated by UV light. It is suggested that charges photogenerated at the electrode surface give rise to chlorine generation and powerful oxidants (OH ) that causes the dye solution to decolorize. Rate constants calculated from color decay versus time reveal a first-order reaction up to 5.0×10 −5 mol l −1 in dye concentration. The best experimental conditions were found to be pH 6.0 and 1.0 mol l −1 NaCl when the photoelectrode was biased at +1 V (versus SCE). Almost complete mineralization of the dye content (70% TOC reduction) was achieved in a 3-h period using these conditions. Effects of other electrolytes, dye concentration and applied potentials also have been investigated and are discussed.

Performance and stability of TiO2/dye solar cells assembled with flexible electrodes and a polymer electrolyte

Abstract: Solid-state, flexible TiO2/dye solar cells were assembled using flexible electrodes, a polymer electrolyte with I−/I3− and a Pt coated counter-electrode. The efficiency of the cells was enhanced when the plastic electrodes coated with TiO2 were exposed to UV radiation, followed by heating at 140 °C in dry conditions. For comparison, a similar cell was prepared by the same procedure but using glass electrodes. The performance of these cells was investigated during a period of 50 days by current–potential and electrochemical impedance spectroscopy measurements. The flexible, solid-state TiO2/dye solar cells (1 cm2) presented an open circuit potential of 0.72 V, short-circuit current of 60 μA cm−2 and an efficiency of 0.32% under a light intensity of 10 mW cm−2. This efficiency was maintained until the fourth day after assembling and decayed to 0.17% on the 14th day, remaining constant until the 40th day and decreasing to 0.13% on the 50th day. Impedance spectroscopy revealed that the series resistance increased with time, lowering the cell efficiency. This effect was not so evident for cells assembled with glass electrodes. Therefore, the flexible electrode limits the preparation of the porous TiO2 photoelectrode and creates a large series resistance in the solar cell. However, these results are very promising for developing solar cells with lower costs and broader applicabilities.

A critical review of the absorption cross-sections of O3 and NO2 in the ultraviolet and visible

Abstract: The available laboratory measurements of UV-Vis (240–790 nm) absorption cross-sections of O3 and NO2 are critically reviewed taking into account the variation of the cross-sections with temperature (in the 200–300 K range) and with total pressure (

Photocatalytic oxidation of toluene on irradiated TiO2: comparison of degradation performance in humidified air, in water and in water containing a zwitterionic surfactant

Abstract: Photocatalytic degradation of toluene was carried out both in gas–solid and in liquid–solid regime by using polycrystalline samples of TiO2 Merck and TiO2 Degussa P25. For the gas–solid regime two types of continuous photoreactor were used, a fixed bed one of cylindrical shape and a Carberry type photoreactor, both irradiated by near-UV light. The inlet reacting mixture consisted of air containing toluene and water vapours. The influence of the gas flow rate and the presence of water vapour on the photocatalytic process was investigated. CO2 and benzaldehyde were the toluene degradation products detected in the gas phase by using TiO2 Merck. In the presence of water vapour this catalyst exhibited a stable activity, which greatly decreased in the absence of water vapour. On the contrary, TiO2 Degussa P25 produced CO2 and traces of benzaldehyde but it continuously deactivated even in the presence of water vapour. For the liquid–solid regime a batch photoreactor with immersed lamp was used. In order to increase the reaction rate, a zwitterionic surfactant, i.e. tetradecyldimethylamino-oxide, was added to the reacting mixture. A complete photo-oxidation of toluene was achieved after few hours of irradiation in the presence of both types of photocatalysts; longer irradiation times produced the photodegradation of surfactant. The main intermediates of toluene degradation were p-cresol and benzaldehyde while traces of pyrogallol and benzyl alcohol were also found. Benzoic acid, hydroquinone and trans, trans-muconic acid were detected only with TiO2 Merck. The reaction rate was higher in the presence of the surfactant suggesting that this compound acts as a sequestration agent. An FTIR study gave information on the role played by superficial hydroxyl groups both on the onset of activity and on the deactivation process. On the basis of photoreactivity results and of FTIR investigation the differences of activity and distribution and nature of toluene degradation products are critically discussed for the three reacting systems used.

Kinetic investigation on UV and UV/H2O2 degradations of pharmaceutical intermediates in aqueous solution

Abstract: The degradation kinetics of two pharmaceutical intermediates (5-methyl-1,3,4-thiadiazole-2-methylthio (MMTD-Me) and 5-methyl-1,3,4-thiadiazole-2-thiol (MMTD)) have been studied in order to assess the effectiveness and the feasibility of UV processes for the decontamination of water polluted by such intermediates Experiments were carried out, at 25 °C, treating, in a batch reactor, aqueous solutions (1 and 100 mg/l) of both compounds by UV radiations (254 nm) in the presence or absence of hydrogen peroxide For both substrates, the results showed that: (i) no degradation occurred when H2O2 alone was used; (ii) UV and UV/H2O2 processes were both effective for degrading the substrates; (iii) substrates degradation by photo-oxidation was always faster than by direct photolysis; (iv) during direct photolysis, a lower substrate initial concentration lead to a faster and more efficient degradation The quantum yields of the photolytic process were experimentally measured for both substrates resulting 141±15 and 120±07 mmol einstein−1 for MMTD-Me and MMTD, respectively Carrying out photo-oxidation experiments using excess of peroxide (ie, initial substrate concentration of 1 mg/l and H2O2/substrate molar ratios of 50/1, 42/1, 34/1 and 23/1), first- and second-order rate constants for MMTD-Me and MMTD degradation were calculated In particular, the values of these latter resulted (83±08)×108 and (16±05)×1010 M−1 s−1, respectively Our results show that to remove 99% of a few μg/l of the pharmaceutical intermediates with a H2O2 dose of 1 mg/l, 55 and 27 min for MMTD-Me and MMTD are necessary, respectively

Effects of surface fluorination of TiO2 on the photocatalytic degradation of tetramethylammonium

Abstract: Photocatalytic degradation (PCD) of tetramethylammonium (TMA, (CH3)4N+) ions in water was studied using both naked-TiO2 and fluorinated-TiO2 (F–TiO2) in order to investigate how the modification in TiO2 surface functional groups affects the PCD reaction. A comparison between the naked-TiO2 and F–TiO2 systems shows that their relative photoreactivities strongly depend on pH. At pH 3, the addition of fluoride decreases the PCD rate whereas higher degradation rates are obtained at pH 5 and 7 with F–TiO2. Little fluoride effect on the TMA degradation rate is observed at pH 9. The addition of fluoride affects not only the PCD rate but also the mechanistic pathways of TMA degradation and subsequently the intermediates and product distribution. The modeling result of TiO2 surface speciation shows that the fluoride addition at pH 3 shifts the dominant surface species from Ti–OH2+ and Ti–OH to Ti–F (to near completion). This reduces the surface positive charge of TiO2 (at pH 3) upon adding fluoride and consequently lowers the electrostatic repulsion between the TMA cations and TiO2 surface. Accordingly, ATR-FTIR spectroscopic measurements show that the TMA concentration at the water/TiO2 interface is higher on F–TiO2 than naked-TiO2 film at pH 3. However, the PCD of TMA on F–TiO2 at pH 3 is reduced on the contrary, which is ascribed to the depletion of surface OH groups that are the site of surface OH radical formation. At pH 5 and 7, the surface OH sites are not completely diminished even in the presence of fluoride and the presence of surface Ti–F species in fact increases the TMA degradation rate. The fluoride-induced enhancement of PCD is yet to be understood although some speculative arguments are presented.

Photocatalytic oxidation of oxalic acid enhanced by silver deposition on a TiO2 surface

Abstract: Photooxidation of oxalic acid over TiO2 surface in the presence of silver ion was studied. Simultaneous deposition of silver and oxidation of oxalic acid were followed under irradiation of aqueous suspensions of pH 2.5–3.5. The rate of the reduction of Ag+ was increased by the progress of silver deposition until a considerable depletion of the silver ion in the liquid phase was achieved. Then it was slowed down when the transport of Ag+ from the bulk of the liquid phase to the surface of the semiconductor particle became the rate-determining process. It has been demonstrated that, after the completion of silver deposition, the small metal particles on TiO2 surface enhance the efficiency of the semiconductor by a factor of 5 for the photooxidation of oxalic acid.

Photoswitchable molecular receptors

Abstract: The review describes photoswitchable molecular receptors based on crown-ether systems. The important feature of the systems is reversible change in the capacity of a photochromic host molecule for association with guests upon irradiation. From the other side, the exploration of the complex formation process in crown-ether systems leads to a novel approach to the modification of photochromic behavior. The results obtained show that the introduction of crown-ether moieties into dye molecules affords compounds that can change their spectral and photochromic properties upon complex formation. The researchers believe that the novel photochromic systems can now be regarded as being promising for traditional applications (photochromic ophthalmic lenses or camera filters, reversible holographic systems and cosmetics) and molecular electronics, biomimetic chemistry, and optical information storage.

H2 evolution from aqueous sulfite solutions under visible-light irradiation over Pb and halogen-codoped ZnS photocatalysts

Abstract: Pb-doped ZnS showed a photocatalytic activity for H 2 evolution from an aqueous K 2 SO 3 solution under visible-light irradiation ( λ >420 nm) even without a Pt cocatalyst. Codoping of a halogen ion with Pb improved the photocatalytic activity. The visible-light response was due to the transition from a 6s orbital of Pb 2+ to the conduction band of ZnS of the host material.

Photocatalysis of sprayed nitrogen-containing Fe2O3–ZnO and WO3–ZnO composite powders in gas-phase acetaldehyde decomposition

Abstract: Two series of nano-sized N-containing MOx–ZnO (M: Fe, W) composite powders were synthesized by spray pyrolysis. The nitrogen content was controlled from 500 to 2100 ppm by changing the powder ingredients and spray pyrolysis temperature. Ultraviolet-visible (UV-Vis) spectra indicated that the N-containing MOx–ZnO powders absorbed not only ultraviolet light (λ<387 nm) like pure ZnO powder, but also part of visible light (λ<650 nm). Acetaldehyde decomposition was used as a probe reaction to evaluate the photocatalysis of these composite powders. The photocatalytic activity of the N-containing ZnO powder was pronouncedly enhanced by the WO3 addition under both UV and visible-light irradiation. However, that of the N-containing ZnO powder was suppressed by the Fe2O3 addition. A model of the semiconductor energy-band structure is proposed to explain the enhancement and suppression of photocatalytic activity.