Showing papers in "Journal of Photochemistry and Photobiology A-chemistry in 2002"

Kinetics of photocatalytic degradation of reactive dyes in a TiO2 slurry reactor

Abstract: The textile industry consumes considerable amounts of water during the dyeing and finishing operations. Dyes are extensively used and hence wastewaters discharged in rivers or public sewage treatment plants are highly contaminated. In this work, a detailed investigation of the adsorption and photocatalytic degradation of the safira HEXL dye, an anionic azo dye of reactive class, is presented. H 2 O 2 and UV light have a negligible effect when they are used on their own. The adsorption of dye on the semiconductor shows a strong dependence on the pH and follows a Langmuir adsorption model. The photodegradation kinetics is discussed in terms of the Langmuir–Hinshelwood model. The effect of pH, amount of photocatalyst, UV light intensity and hydrogen peroxide concentration is discussed. The relative photonic efficiency of the system is reported using phenol as a standard organic compound.

Degradation of propanol diluted in water under visible light irradiation using metal ion-implanted titanium dioxide photocatalysts

Abstract: The metal ion-implantation of titanium dioxide (TiO 2 ) with metal ions (V + , Mn + , Fe + ) at high energy acceleration and the subsequent calcination of these metal ion-implanted TiO 2 in oxygen at around 725 K resulted in a definite shift in the absorption spectra toward visible light regions. These catalysts exhibited photocatalytic reactivity for degradation of 2-propanol diluted in water under visible light irradiation ( λ >450 nm). XAFS studies have revealed that the implanted metal ions are located at the lattice positions of Ti 4+ in TiO 2 after the calcination. These spectroscopic studies show that the substitution of Ti ions in TiO 2 lattice with implanted metal ions is important to modify TiO 2 to be able to adsorb visible light and operate as photocatalysts under visible light irradiation.

A web-based overview of semiconductor photochemistry-based current commercial applications

Abstract: The major current commercial applications of semiconductor photochemistry promoted on the world wide web are reviewed. The basic principles behind the different applications are discussed, including the use of semiconductor photochemistry to: photo-mineralise organics, photo-sterilise and photo-demist. The range of companies, and their products, which utilise semiconductor photochemistry are examined and typical examples listed. An analysis of the geographical distribution of current commercial activity in this area is made. The results indicate that commercial activity in this area is growing world-wide, but is especially strong in Japan. The number and geographical distribution of patents in semiconductor photocatalysis are also commented on. The trends in the numbers of US and Japanese patents over the last 6 years are discussed.

Nanostructured ZnO electrodes for dye-sensitized solar cell applications

Abstract: Dye-sensitized photoelectrochem. solar cells constitute a promising candidate in the search for cost-effective and environment-friendly solar cells. The most extensively studied, and to date the most efficient systems are based on titanium dioxide. In this paper, the possibilities to use nanostructured ZnO electrodes in photoelectrochem. solar cells are investigated. Various exptl. techniques (e.g. IR, photoelectron, femtosecond and nanosecond laser spectroscopies, laser flash induced photocurrent transient measurements, two- and three-electrode photoelectrochem. measurements) show that the thermodn., kinetics and charge transport properties are comparable for ZnO and TiO2. The prepn. techniques of ZnO provide more possibilities of varying the particle size and shape compared to TiO2. However, the dye-sensitization process is more complex in case of ZnO and care needs to be taken to achieve an optimal performance of the solar cell.

A new photocatalytic water splitting system under visible light irradiation mimicking a Z-scheme mechanism in photosynthesis

Abstract: We studied the water splitting into H2 and O2 using two different semiconductor photocatalysts and a redox mediator, mimicking the Z-scheme mechanism of the photosynthesis. It was found that the H2 evolution took place on a Pt–SrTiO3 (Cr–Ta-doped) photocatalyst using an I− electron donor under the visible light irradiation. The Pt–WO3 photocatalyst showed an excellent activity of the O2 evolution using an IO3− electron acceptor under visible light. Both H2 and O2 gases evolved in the stoichiometric ratio (H2/O2=2) for more than 250 h under visible light using a mixture of the Pt–WO3 and the Pt–SrTiO3 (Cr–Ta-doped) powders suspended in NaI aqueous solution. It is for the first time that the stoichiometric water splitting occurred over oxide semiconductor photocatalysts under the visible light irradiation. We proposed a two-step photo-excitation mechanism using a pair of I−/IO3− redox mediators. The quantum efficiency of the stoichiometric water splitting was ca. 0.1% at 420.7 nm.

Photocatalytic oxidation of organics in water using pure and silver-modified titanium dioxide particles

Abstract: The simultaneous photocatalytic degradation of organic compounds and reduction of silver ions in titanium dioxide suspensions at pH 3.0-3.5 has been studied. The organic compounds of interest were sucrose and salicylic acid. The presence of silver ions in TiO 2 suspensions was found to enhance the photooxidation of high loadings (2001) μgC) of sucrose. However, for low sucrose loadings (100 μg C), pure TiO 2 particles performed as well as modified TiO 2 particles. An optimum silver ion loading of 2.0 at.% Ag + was observed for the mineralisation of 2000 μg C sucrose. At this silver ion loading, the mineralisation of 2000 μg C sucrose was enhanced by a factor of approximately 4.0 (based on 90% overall oxidation rates). In contrast, the addition of silver ions to TiO 2 suspensions did not have any significant effect on the photocatalytic mineralisation of salicylic acid to carbon dioxide, for both low and high loadings of salicylic acid in the suspension. It was also observed that pure TiO 2 particles performed as well as silver-modified TiO 2 particles for the degradation of 2000 μg C salicylic acid. The higher activity of silver ion-modified titanium dioxide suspensions for sucrose mineralisation is predominantly due to the presence of small silver particles on the titania surface, rather than due to the trapping of electrons during the reduction of silver ions. Approximately 50% of the initial mass of silver ions added to TiO 2 suspensions were reduced to metallic silver deposits in the presence of sucrose and salicylic acid mineralisation at the 2.0 at.% Ag + loading. Nanosize silver deposits on TiO 2 particles act as sites of electron accumulation where the reduction of adsorbed species such as oxygen occur. The enhanced reduction of oxygen through better electron-hole separation in Ag/TiO 2 particles compared to pure TiO 2 particles increases the rate of sucrose mineralisation. Therefore, it is proposed that the rate-limiting step in the sucrose photooxidation reaction is the transfer of electrons to dissolved oxygen molecules, whereas in the case of salicylic acid degradation and mineralisation, the rate-limiting step is the attack of salicylic acid molecules and its degradation intermediate products by holes and hydroxyl radicals. Hence silver deposits on TiO 2 particles are not beneficial for the photocatalytic degradation and mineralisation of salicylic acid but are advantageous for the mineralisation of sucrose.

Preparation, characterization and photocatalytic activity of ZnO, Fe2O3 and ZnFe2O4

Abstract: ZnO, Fe 2 O 3 and ZnFe 2 O 4 were synthesized by precipitation or coprecipitation at constant pH After aging, washing and drying, the solids were calcined at 800 C The samples were characterized by XRD, TGA, N 2 physisorption, UV-Vis and TPR The photocatalytic degradation of phenol was studied in a batch reactor and TiO 2 (Degussa, P-25) was used as a reference catalyst Pure ZnO and Fe 2 O 3 were obtained at 800 C, whereas ZnFe 2 O 4 and α-Fe 2 O 3 (segregated) were detected in the zinc ferrite sample TPR experiments showed the reduction peaks in the range of 350-800 C only with Fe 2 O 3 and ZnFe 2 O 4 , attributed to Fe 3+ to Fe 2+ reduction and complete dehydroxylation The photodegradation of phenol (samples calcined at 800 C) showed small but significant variations that decreased in the order: TiO 2 > ZnFe 2 O 4 > ZnO > Fe 2 O 3 Subproducts such as hydroquinone, catechol and benzoquinone were mainly detected In particular, attention was focused on our prepared materials because after 24 h they did not dissolve as happened with TiO 2

The photocatalytic removal of bacterial pollutants from drinking water

Abstract: Pathogens in drinking water supplies can be removed by sand filtration followed by chlorine or ozone disinfection. These processes reduce the possibility of any pathogens entering the drinking water distribution network. However, there is doubt about the ability of these methods to remove chlorine resistant microorganisms including protozoan oocysts. Concern has also been raised about the production of disinfection by-products following the chlorination process. Titanium dioxide (TiO 2 ) photocatalysis is a possible alternative/complementary drinking water treatment method. TiO 2 electrodes were prepared by the electrophoretic immobilisation of TiO 2 powder (Aldrich and Degussa P25). These electrodes were tested for their photocatalytic bactericidal efficiency. E. coli K12 was used as a model test organism. The rate of disinfection was greater for the P25 electrode compared to the Aldrich electrode under open circuit conditions. The application of an electrical bias to the working electrode increased the rate of disinfection by ∼40% for the P25 electrode and ∼80% for the Aldrich electrode. The effect of applied potential was more pronounced under conditions of high initial bacterial cell loading and high light intensities. Bacterial recovery did not occur up to 48 h after disinfection.

Photocatalytic degradation of methyl orange as a model compound

Abstract: Titanium dioxide (TiO 2 ) was used as a photocatalyst for the detoxification of water containing methyl orange (MO), which was used as a model compound. Solar radiation was used as an irradiation source. It was found that there was no degradation for the MO in the dark and in the presence of TiO 2 . Also no degradation was observed for MO when the solution placed under solar radiation but without TiO 2 . Several experiments were used to optimize the experimental parameters. In the first set of experiments variable amounts of TiO 2 were used with a fixed concentration of MO. It was found that 0.4% of TiO 2 gave the highest degradation rate constant, 0.619 h -1 . In the second set of experiments TiO 2 concentration was fixed at 0.4% and the MO concentration was varied, the highest rate constants was obtained when the concentration of MO was 4 x 10 -5 M and it was found to be 0.639h - The degradation became negligible in the presence of high concentrations of MO. The highest degradation rate was obtained at pH = 3 with a rate constant K = 2.6683h -1 , followed by that at pH = 9 where the calculated K = 0.7585 h -1 .

Novel TiO2 CVD films for semiconductor photocatalysis

Abstract: A novel CVD film of titanium(IV) oxide has been prepared on glass, via the reaction of titanium(IV) chloride and ethyl acetate, using a CVD technique. The film is clear, very robust mechanically and comprised of a thin (24 nm) layer of nanocrystalline anatase titania that absorbs light of λ

Photocatalytic degradation of 4-nitrophenol in aqueous TiO2 suspensions: Theoretical prediction of the intermediates

Abstract: The kinetics of the photocatalytic degradation of 4-nitrophenol (4-NP) in the presence of TiO2 has been investigated experimentally and theoretically. The effects of the catalyst loading, the initial concentration of 4-NP, H2O2 and the added Cu2+ ions on the degradation rate have been examined. A pseudo-first order kinetic model has been used to describe the results. A linear dependence of the rate constant upon the reciprocal of the initial 4-NP concentration has been obtained. The addition of H2O2 increases the reaction rate while Cu2+ ions has a detrimental effect. With the intention of predicting the primary intermediates, geometry optimizations of the reactants, the products and the transition state complexes have been performed with the semi-empirical PM3 method. The molecular orbital calculations have been carried out by an SCF method using RHF or UHF formalisms. Based on the results of the quantum mechanical calculations, the rate constants of the two possible reaction paths have been calculated by means of the transition state theory, and 1,2-dihydroxy-4-nitro-cyclohexadienyl radical which then forms 4-nitrocatechol has been determined as the most probable primary intermediate by the application of three different theoretical shortcut methods.

Enhancement of the photocatalytic activity of various TiO2 materials by platinisation

Abstract: In the present work, three commercially available TiO2-catalysts, namely Degussa P25, Sachtleben Hombikat UV100 and Millennium TiONA PC50, were platinised by a photochemical impregnation method in two ratios of platinum deposits (0.5 and 1 wt.%). The physical characterisation of the new synthesised catalysts was carried out by measurements of the BET-surface area, the light absorption properties and the adsorption of the model compounds. The photocatalytic activities of these samples were determined using three different model compounds: EDTA, 4-chlorophenol (4-CP), and dichloroacetic acid (DCA). While in the case of EDTA its disappearance was studied, total mineralisation was measured for 4-CP and DCA. In all cases, the photocatalytical activity was found to increase with rising amounts of Pt, e.g., the photonic efficiency for DCA degradation increased from 12.2% for pure Hombikat UV100 to 32.1% for Hombikat UV100/0.5 wt.% Pt and to 42.7% for Hombikat UV100/1 wt.% Pt. Promising results were also achieved for the total mineralisation of 4-CP. The photonic efficiency rised from 0.82% using unmodified PC50 to 1.14% with PC50/0.5 wt.% Pt (zero-order kinetics assumed in all cases). Similar results were obtained with the other new synthesised catalyst samples and for the model compound EDTA. No immediate relationship between the photocatalytic activity of the catalyst samples and their physical properties (surface area, adsorption of pollutants, absorption of light) could be observed.

Optimization of dye-sensitized solar cells prepared by compression method

Abstract: Nanostructured TiO2 films have been deposited onto conducting glass and on flexible conducting plastic substrates using a compression technique. Dye-sensitized solar cells prepd. from Degussa P25 TiO2 powder, red dye (Ru(dcbpy)2(SCN)2)(NBu4)2 (dcbpy = 2,2'-bipyridine-4,4'-dicarboxylate) or black dye (Ru(tcterpy)(SCN)3) (tcterpy = 4,4',4''-tricarboxy-2,2':6',2''-terpyridine) and an electrolyte contg. LiI and I2 in 3-methoxypropionitrile were tested using std. photoelectrochem. techniques. The av. overall efficiency of small open cells sensitized with the red dye on plastic substrates was 4.5% (100 W-m-2). In a direct comparison, red and black dye gave about the same efficiencies. For both dyes, addn. of 4-tert-butylpyridine to the electrolyte resulted in a decreased IPCE response in the whole spectral range, with a specific decrease in the red region, which is attributed to a shift in the conduction band edge of the nanostructured TiO2.

Photocatalytic degradation of a cationic azo dye by TiO2/bentonite nanocomposite

Abstract: Titanium dioxide/bentonite clay nanocomposite prepared by acid-catalyzed sol–gel method was used as photocatalyst in the reaction of cationic azo dye decomposition in water. The incorporation of TiO2 was confirmed by powder X-ray diffraction (XRD) and X-ray photoelectron spectrometer (XPS). The photocatalytic activity of those nanocomposite photocatalysts was much higher than that of the pure titanium dioxides. The nanocomposite created a kinetic synergy effect in Cationic Red GTL (GTL) disappearance with an increase of the rate constant by a factor of 2.57 for neat TiO2 (P-25). The photo-activities were greatly dependent on the solution pH, and it was more effective for GTL to be degraded under alkaline condition. That was likely to contribute for the acid–base equilibria on the surface of the nanocomposite. Results also indicated that the proper addition of hydrogen peroxide could improve the decolorization rate, but the excess hydrogen peroxide could quench the formation of OH.

Synthesis of size-controlled silica-supported TiO2 photocatalysts

Abstract: Supported nanocrystalline titanium dioxide has been prepared by a sol–gel method through the use of mesoporous silica materials. The work has been addressed to study the effect of the different type of silica support on the mean titania particle size and their effect in the photocatalytic activity. The prepared samples have been characterised by powder X-ray diffraction (XRD), nitrogen adsorption, UV–VIS spectroscopy, EDS, and TEM and their photocatalytic activity has been checked for cyanide oxidation in liquid phase. A correlation between the textural properties of the chosen silica support and the diameter of the obtained titania particles is observed. The use of mesostructured SBA-15 silica support with a pore diameter around 7 nm has shown to be an effective method to obtain photocatalysts with controlled TiO2 particle size even at high semiconductor contents (up to 60 wt.%).

Preparation, characterization and photocatalytic activity of nanocrystalline thin film TiO2 catalysts towards 3,5-dichlorophenol degradation

Abstract: Both opaque and transparent TiO2 nanocrystalline thin films were developed on glass substrates by applying dip coating and doctor-blade deposition techniques, using titanium(IV) butoxide and Degussa P25 TiO2 powder as precursor and starting material, respectively. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) evaluated the surface characteristics of the films. Results on their structure and crystallinity were obtained by means of X-ray diffraction and Raman spectroscopy. The catalytic activity of the films towards photodegradation of 3,5-dichlorophenol (3,5-DCP) pollutant was examined and their efficiency was compared to that of the TiO2 powder (slurry) suspensions. Pseudo-first-order photodegradation kinetics were observed and the reaction constants were determined. It has been shown that the film photocatalysts can efficiently decompose the pollutant, although relatively higher decomposition rates were observed with the commercial starting powder. Differences in the film efficiencies can be attributed to differences in their grain size, surface roughness and fractal parameters. No altering on the doctor-blade films surface characteristics was observed for several hours of cyclic operation during which their photocatalytic efficiency remained remarkably stable.

Biodegradability enhancement of textile dyes and textile wastewater by VUV photolysis

Abstract: Photo-degradation and biodegradability have been studied for three different families of non-biodegradable textile dyes (Intracron reactive dyes, Direct dyes and Nylanthrene acid dyes) and a textile wastewater, using VUV photolysis. Ninety percent of color removal of dye solutions and wastewater is achieved within 7 min of irradiation. Biological oxygen demand (BOD) was found to increase during discoloration process while chemical oxygen demand (COD) decreased. The biodegradability index (BOD5/COD) increases up to 0.40 for most of the dye solutions when total discoloration is obtained. It implies that VUV photolysis tends to enhance the biodegradability of dye containing solutions. Thus, this technique could be used as a pre-treatment step for conventional biological wastewater treatment.

Enhancement of photocatalytic activity by semiconductor heterojunctions: α-Fe2O3, WO3 and CdS deposited on ZnO

Abstract: α-Fe2O3, WO3 and CdS deposited on ZnO have been prepared and their photocatalytic activity was determined by photooxidation of dichloro acetic acid (DCA) in aqueous solution illuminated under medium pressure mercury lamp. These particulates were examined spectroscopically and by surface analytical methods such as surface area, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The effects of the α-Fe2O3, WO3 and CdS contents on the photocatalytic activities were studied. The experiments were carried out under three different conditions: (1) under continuous purging with molecular oxygen, (2) in the presence of naturally dissolved oxygen and (3) in the absence of oxygen. The results reveal that the mixtures of ZnO with α-Fe2O3 (0.5 wt.%), WO3 (0.02 wt.%) and CdS (0.2 wt.%) exhibit higher photocatalytic activity than pure ZnO.

Heterogeneous photocatalytic reactions comparing tio2 and pt/tio2

Abstract: Two commercial samples, namely Degussa P-25 and Sachtleben Hombikat UV 100, and platinized modifications of these samples have been tested comparatively as photocatalysts for EDTA oxidation and for Cr(VI) reduction in the absence and in the presence of EDTA. Platinized photocatalysts were prepared by photoimpregnation of H 2 PtCl 6 on TiO 2 by prolonged irradiation in the presence of methanol. Only slight differences in efficiency were found between both pure commercial forms, except for the Cr(VI) reduction in the presence of EDTA, where Hombikat UV 100 samples were more active. Platinized Hombikat UV 100 was found to be less active for Cr(VI) reduction in water. The concentration of dissolved molecular oxygen was crucial for EDTA oxidation and had no influence on Cr(VI) reduction. From kinetic profiles, initial photonic efficiencies ( ζ ) and conversion degrees have been calculated. Platinization did not improve Cr(VI) reduction but increased the initial photonic efficiency for EDTA oxidation. However, after prolonged irradiation, the conversion of EDTA was lower employing platinized samples instead of pure samples. The effect of platinum on titania was analyzed on the basis of proposed mechanisms.

Simultaneous photocatalytic Cr(VI) reduction and dye oxidation in a TiO2 slurry reactor

Abstract: The semiconductor photocatalytic reduction is a relatively new technique for the removal of dissolved metal ions, such as Cr(VI) and organic compounds in wastewater. Many studies concern only the catalytic activity of TiO 2 with single substrates. However, for practical applications it is necessary to evaluate the photocatalytic simultaneous reduction and oxidation reactions. In this paper, we report the results of a study on the simultaneous Cr(VI) reduction and the Luranzol S Kong dye (from the tannery industry) oxidation by photocatalysis. The experiments were performed in a UV reactor system with a lamp of 150 W, using TiO 2 as a catalyst. We evaluated the effect of pH and concentration of pollutants on the kinetics of the reactions. Isolated tests showed that both Cr(VI) and Luranzol S Kong are degraded in acidic pH, according to the Langmuir–Hinshelwood kinetic model. In neutral pH, the Cr(VI) is not reduced, but this might be explained by the fact that Cr(VI) is not adsorbed on TiO 2 surface. The dye is almost completely degraded by both acidic and neutral pH. Under UV irradiation, photo-induced Cr(VI) reduction is observed in both Cr(VI)–TiO 2 and dye–Cr(VI)–TiO 2 , and dye oxidation is observed in both dye–TiO 2 and dye–Cr(VI)–TiO 2 . The concentration of Cr(VI) and dye, and the pH are the main parameters influencing both reactions. In the simultaneous reduction–oxidation process, the reduction of Cr(VI) proceeds quicker than it does in the single process, showing synergism between the oxidation and reduction reactions.

The temperature and pressure dependence of the absorption cross-sections of NO2 in the 250–800 nm region measured by Fourier-transform spectroscopy

Abstract: The absorption cross-sections of NO2 at atmospheric temperatures (223–293 K) and pressures (100 and 1000 mbar) were measured in the 250–800 nm (12500–40000 cm −1 ) region using Fourier-transform spectroscopy, at spectral resolutions of 0.5 cm −1 above 435 nm and 1.0 cm −1 below 435 nm (corresponding to about 8 and 16 pm at this wavelength). The wavenumber accuracy of the new cross-sections is better than 0.1 cm −1 (about 0.5 pm at 250 nm and about 6.4 pm at 800 nm), validated by recording of I2 absorption spectra in the visible using the same experimental set-up (light source, beam splitter, interferometer optics). The NO 2 absorption spectra were recorded at five different sample temperatures between 223 and 293 K, and at each temperature at two total pressures (100 and 1000 mbar) using pure N2 as buffer gas. Despite the weakness of this effect compared to the density of the NO2 absorption structures, pressure-broadening was clearly observed at all temperatures. The pressure-broadening was partially modeled using a convolution of the low-pressure NO2 absorption spectra with a Lorentzian lineshape. The pressure-broadening coefficient increases significantly with decreasing temperature, as already observed in the mid- and near-infrared vibration–rotation spectra of NO2. This effect is of importance for high-resolution spectroscopy of the earth’s atmosphere in the UV–visible region. © 2002 Elsevier Science B.V. All rights reserved.

Energy transfer in solution of lanthanide complexes

Abstract: The lanthanides with their well-defined energy levels provide an excellent basis to study different Ln(III)-specific energy transfer processes in a variety of chemical environments. The studies concerning intramolecular and intermolecular energy transfer processes with participation of Ln(III) ions and a variety of ligand groups in solution are reviewed. Phenomena of energy transfer from ligands to Ln(III) ions, resulting consequently in a great enhancement of the Ln(III) ion luminescence (ligand sensitized luminescence), as well as from Ln(III) to other species and between Ln(III) ions are presented.

Enhancing effects of water content and ultrasonic irradiation on the photocatalytic activity of nano-sized TiO2 powders

Abstract: A simple method for preparing highly photoactive nano-sized TiO2 photocatalyst with anatase and brookite phase was developed by hydrolysis of titanium tetraisopropoxide in pure water or the EtOH/H2O mixed solution under ultrasonic irradiation. The prepared TiO2 powders were characterized by differential thermal analysis (DTA), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and BET surface areas. The photocatalytic activity was evaluated by the photocatalytic oxidation of acetone in air. The results showed that the photocatalytic activity of TiO 2 powders prepared by this method from pure water or the EtOH/H2O mixed solutions with the molar ratio of EtOH/H2O = 1 exceeded that of Degussa P-25. The molar ratios of EtOH/H2O obviously influenced the crystallization, crystallite size, BET surface areas and photocatalytic activity of the prepared TiO 2 powders. Ultrasonic irradiation obviously enhanced the photocatalytic activity of TiO 2 powders whether the solvent is pure water or the EtOH/H2O mixed solutions. This may be ascribed to the fact that ultrasonic irradiation enhances hydrolysis of titanium alkoxide and crystallization of TiO2 gel. © 2002 Elsevier Science B.V. All rights reserved.

Visible light induced photodegradation of organic pollutants on dye adsorbed TiO2 surface

Abstract: Visible light assisted degradation of aromatics, viz phenol, chlorophenol, 1,2-dichloroethane, trichloroethylene and surfactants, viz cetyl pyridinium chloride (CPC; cationic), sodium dodecylbenzene sulfonate (DBS; anionic) and neutral Triton-X 100 in air-equilibrated aqueous mixtures has been achieved on the surface of TiO2 semiconductor modified with methylene blue (MB) and rhodamine B (RB). Under 5 h of irradiation with a 150 W xenon lamp, over 40–75% degradation of pollutants has been observed. The failure of pollutants to degrade on non-conducting Al2O3 surface essentially suggests the role of semiconducting TiO2 photocatalyst in the photochemical process. A working mechanism involving excitation of surface adsorbed dye, followed by charge injection into the TiO2 conduction band and formation of reactive O2−/ HO2 radicals is proposed for the degradation of organics to carbon dioxide.

Photonic efficiency and quantum yield of formaldehyde formation from methanol in the presence of various TiO2 photocatalysts

Abstract: UV illumination of aqueous TiO2 suspensions yields hydroxyl radicals which can be trapped by methanol yielding formaldehyde (HCHO). The photonic efficiency of HCHO formation (345±35 nm cw illumination) in aqueous, oxygenated TiO2 suspensions (2.4 nm diameter TiO2 particles, Degussa P25 and Sachtleben Hombikat UV 100) containing methanol at pH ca. 1–12 has been determined as well as the respective quantum yield in case of 2.4 nm TiO2. Differences in the activity of the three photocatalysts have been found and are discussed. The photonic efficiency in the presence of P25 and Hombikat UV 100 depends on the catalyst loading (g l−1) and the pH. Below 2.5 g l−1 the photonic efficiency is higher for P25 than for Hombikat UV 100 and vice versa at above 2.5 g l−1. Optimum pH values for P25 and Hombikat UV 100 resulting in maximum photonic efficiencies (ca. 0.13 for P25 and 0.07 for Hombikat UV 100) are 7.7 and 10.4, respectively. Other than with P25 and Hombikat UV 100, which scatter light strongly, the quantum yield of HCHO formation in the colloidal 2.4 nm TiO2 suspension varies but little with the pH and virtually does not change with the photocatalyst loading (0.1–1.0 g/l). Employing these colloidal particles as photocatalysts the quantum yield varies as the inverse square root of light intensity. It increases from 0.02 to 0.08 when the absorbed photon flux decreases from 8.1×10−7 to 4.9×10−8 Einstein l−1 s−1. A simple model is presented to explain this observation.

Light-induced super-hydrophilicity and photocatalytic activity of mesoporous TiO2 thin films

Abstract: Transparent mesoporous TiO2 (MTiO2) and TiO2 nanometer thin films were prepared on fused quartz via the modified reverse micellar and sol–gel methods. The MTiO2 and TiO2 films were characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), X-ray diffraction (XRD), BET surface area and UV–Vis spectrophotometry. The photoinduced super-hydrophilicity and photocatalytic activity of the films were evaluated by the water contact angle measurement and photocatalytic oxidation of acetone in air, respectively. It was found that MTiO2 thin films at 500 °C showed higher photocatalytic activity and better light-induced hydrophilicity than the TiO2 thin films. This is attributed to the following reasons: (1) MTiO2 thin films are composed of smaller monodisperse spherical particles about 15 nm diameter, and possess more mesopores between spherical particles and higher surface areas and surface roughness, (2) the monodispersity of MTiO2 particles was beneficial to the transfer and separation of photo-generated electrons and holes inside of and on the surface of TiO2 particles, and reduced the recombination of photo-generated electrons and holes. At 900 °C, the MTiO2 and TiO2 films appeared to be in the rutile phase, but showed photoinduced hydrophilicity and no photocatalytic activity. This confirmed that the mechanism of photoinduced super-hydrophilicity of the films is different from that of photocatalytic oxidation.

Photo-Fenton treatment of a biorecalcitrant wastewater generated in textile activities: biodegradability of the photo-treated solution

Abstract: Photo-Fenton process was explored as photochemical pre-treatment to improve the biodegradability of a wastewater coming from a textile industry located in the south of France, which was characterized as very biorecalcitrant by means of Zahn–Wellens biodegradability test. The effect of H2O2, Fe(III) and temperature on the photo-mineralization processes have been studied and the optimal conditions were found. Experiments were made to obtain information concerning the evolution of the biodegradability of the treated effluent after 40 and 70% of photo-mineralization. The photo-treated effluent is not biocompatible and its complete mineralization cannot be performed by biological means. UV–VIS and high-performance liquid chromatography (HPLC) analyses show that aromatic intermediates remain in the effluent after the photo-treatment, which have been identified as the principal reason of the biorecalcitrance.

Photocatalytic oxidation of dibenzothiophenes in acetonitrile using TiO2: effect of hydrogen peroxide and ultrasound irradiation

Abstract: Photocatalytic oxidation of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6DMDBT) in acetonitrile using titanium dioxide (TiO 2 ) was studied. Results obtained here can be used as the reference for evaluating reactions in hydrocarbons, which aims at the development of an oxidative desulfurization process for oils. A 200 W Hg–Xe lamp (>290 nm, 19 mW/cm 2 ) was used as a light source, and the catalytic performances of four commercially available TiO 2 photocatalysts (PC-1, PC-2, PC-3 and P25) were compared. DBT was more stable than 4,6DMDBT under light and in the presence of photocatalyst in acetonitrile. Of the photocatalysts used, P25 showed the highest rate of photooxidation. However with P25, the decrease in initial concentration of DBT or 4,6DMDBT was less than 40% after 10 h of irradiation. To accelerate the reaction, the effect of addition of hydrogen peroxide (H 2 O 2 ) and/or ultrasound irradiation on the TiO 2 -containing system was studied. Although the addition of hydrogen peroxide (3%) and/or ultrasound irradiation more than doubled the rate of photooxidation, this efficiency was obtainable with H 2 O 2 alone. Tentatively identified reaction products from DBT and 4,6DMDBT included the 5-oxide (sulfoxide) and the 5,5-dioxide (sulfone) of each substrate. Oxidation of the methyl group mainly occurred when 4,6DMDBT was reacted using only TiO 2 . This result led to a finding that the acceleration of oxidation for the methyl group occurs in non-polar hydrocarbons.

Synthesis of a novel magnetic photocatalyst by direct deposition of nanosized TiO2 crystals onto a magnetic core

Abstract: This paper presents the preliminary results of a study in which nanocrystalline titanium dioxide particles were directly coated onto a magnetic core. A modified hydrolysed alkoxide process, in which the hydrolysis and condensation of a titanium alkoxide (titanium isopropoxide) was carried out under acidic conditions, using a large water:alkoxide ratio, allowed for the synthesis of crystalline titanium dioxide at relatively low temperatures (90 °C). This method is attractive since it eliminated a high-temperature heat treatment step required in a typical sol–gel method, which is needed to transform the amorphous titanium dioxide formed into a photoactive crystalline phase. The prepared particles were photoactive as well as being easy to separate from a slurry-type photoreactor under the application of an external magnetic field. The application of the sol–gel process for the deposition of a crystalline coating of titanium dioxide onto fine particles has never been attempted before. Thus while this approach is envisaged to be quite challenging, its success will provide new fundamental knowledge and tremendous opportunities for the synthesis of tailored particles.

A solvent-free composite polymer/inorganic oxide electrolyte for high efficiency solid-state dye-sensitized solar cells

Abstract: A new solvent-free composite polymer electrolyte consisting of high-molecular mass polyethylene oxide (PEO) filled with titanium oxide and containing LiI and I2 was developed. The introduction of the inorganic filler (TiO2 Degussa P25) into the polymer matrix produces dramatic morphological changes to the host polymer structure. Upon addition of the inorganic oxide, the surface roughness increases, with respect to the original polymer and in parallel, the fractal dimension decreases. Both the thermograms and the atomic force microscope (AFM) pictures confirm the amorphicity of the composite electrolyte. The polymer sub-units are held together in a parallel orientation, forming straight long chains of about 500 nm in width, along which TiO2 spherical particles of about 20–25 nm in diameter are distributed. The polymer chains separated by the titania particles are arranged in a three-dimensional, mechanically stable network, that creates free space and voids into which the iodide/triodide anions can easily migrate. All solid-state dye-sensitized solar cells fabricated using this composite electrolyte present high efficiencies (typical maximum incident photon to current efficiency (IPCE) as high as 40% at 520 nm and overall conversion efficiency (η) of 0.96% (Voc=0.67 V, Jsc=2.050 mA/cm2, FF=39%)) under direct solar irradiation. Further improvement of the photovoltaic performance is expected by optimization of the electrolyte parameters and of the cell assembly.