Showing papers on "Photocatalysis published in 1997"

Improved photocatalytic activity and characterization of mixed tio2/sio2 and tio2/al2o3 materials

Abstract: The characterization of mixed oxides of TiO2/SiO2 and TiO2/Al2O3 prepared by sol−gel methods is described. Application of the TiO2/Al2O3 material for the photocatalytic decomposition of salicylic acid gave improved activity relative to TiO2-only materials. The photocatalytic activity of TiO2/SiO2, TiO2/Al2O3, and TiO2 toward the photocatalytic decomposition of phenol demonstrated the enhancement of local TiO2 sites on TiO2/SiO2 materials relative to the other materials. The characterization of the catalysts indicates that the TiO2/SiO2 materials consist of matrix-isolated TiO2 quantum particles, while the TiO2/Al2O3 materials do not.

Kinetics of photocatalytic reactions under extremely low-intensity uv illumination on titanium dioxide thin films

Abstract: The photocatalytic decomposition efficiency of gaseous 2-propanol was studied using a titanium dioxide thin film under very weak UV light; the incident UV light intensity was 36 nW−45 μW·cm-2. Under such low-intensity UV illumination, the value of the quantum yield (QY) increased gradually with decreasing number of absorbed photons and finally saturated (28%) for a number of absorbed photons less than 4 × 1011 quanta·cm-2·s-1 for an initial 2-propanol concentration of 1000 ppmv. Thus, purely light-limited conditions were reached. For lower initial concentrations, the QY values decreased, but the same maximum QY value as that for 1000 ppmv was also approached with decreasing light intensity. We discuss these results in terms of the normalized absorbed photon number (Inorm/s-1), a parameter that we have defined as the ratio of the number of absorbed photons ([photon]ab) to the number of adsorbed 2-propanol molecules ([M]ad). When all of the experimental QY values were plotted as a function of Inorm, all of ...

Photocatalytic Reduction of CO2 with H2O on Titanium Oxides Anchored within Micropores of Zeolites: Effects of the Structure of the Active Sites and the Addition of Pt

Abstract: Titanium oxide species anchored within the Y-zeolite cavities by an ion-exchange method exhibit a high and unique photocatalytic reactivity for the reduction of CO2 with H2O at 328 K with a high se

Characterization and photocatalytic activity in aqueous medium of TiO2 and Ag-TiO2 coatings on quartz

Abstract: In the present study, TiO2 and Ag-TiO2 catalysts have been supported in the form of thin layers by a dip-coating procedure on quartz substrate. The resulting materials have been characterized by SEM/EDX, XRD, XPS and UV-vis absorption spectroscopy. The immobilized catalysts were tested in the photocatalytic degradation of malic acid. For this reaction, the presence of metallic silver does not produce an intrinsic increase in photocatalytic activity in comparison with pure titania. The apparent increase observed in activity is principally due to the increase in the exposed surface due to the textural characteristics of the Ag-TiO2 layer in comparison with TiO2. In addition, the presence of metallic silver always produces an increase in activity in comparison with oxidized Ag+ ions. This can be explained by the increase in the electron-hole pair-separation efficiency induced by trapping of electrons by metallic silver.

Photocatalysis on titanium oxide catalysts: Approaches in achieving highly efficient reactions and realizing the use of visible light

Abstract: The photocatalytic reactivities of various Ti-based photocatalysts have been investigated for various different types of reactions in order to achieve as highly efficient photocatalytic reactivities as possible. The reactivity of powdered TiO2 is dramatically enhanced by the addition of small amounts of Pt which initiates an effective charge separation of the photo-formed electrons and holes. The highly dispersed titanium oxide species prepared and encapsulated within the zeolite cavities as well as into the SiO 2 matrices exhibit high photocatalytic reactivities due to the high reactivities of their charge transfer excited states. With regard to the use of visible light, ion implantation of metal ions such as Cr or V into powdered TiO2 catalysts has been found to modify the electronic state of TiO 2 , resulting in the shift of the absorption band to longer wavelength regions, i.e., into the visible light region. The extent of the shift strongly depends on the type and concentration of the implanted metal. The present study focuses on the preparation of the photocatalysts, a detailed characterization of the active sites and their dynamics, the direct detection of the reaction intermediate species, as well as a clarification of the mechanisms behind the observed photocatalytic reactions at the molecular level. This work significantly contributes to advances in the design of photocatalysts which will be able to operate efficiently and effectively not only under UV irradiation but, most ideally, under visible light.

ESR investigation into the effects of heat treatment and crystal structure on radicals produced over irradiated TiO2 powder

Abstract: Electron spin resonance measurements were carried out at 77 K under irradiation for anatase TiO 2 powders treated by heating at various temperatures in the air. For the untreated powder photoproduced holes were trapped at the surface forming Ti 4+ O − Ti 4+ OH − radicals, while, for the heated powder, they were trapped as Ti 4+ O 2− Ti 4+ O − radicals at the surface. Photoproduced electrons were trapped as Ti 3+ at the desorption of surface hydroxyl groups and the change in the surface accompanied by the crystalline growth. No photoproduced radicals were detected for rutile TiO 2 powder, which may explain the low photocatalytic activity of this crystalline structure.

Role of platinum deposits on titanium(IV) oxide particles : structural and kinetic analyses of photocatalytic reaction in aqueous alcohol and amino acid solutions

Abstract: Photocatalytic reaction at 298 K by platinum-loaded titanium(IV) oxide (TiO2−Pt) particles suspended in deaerated aqueous solutions of 2-propanol or (S)-lysine (Lys) was investigated. The TiO2 catalysts with various amounts of Pt loadings were prepared by impregnation from aqueous chloroplatinic acid solution onto a commercial TiO2 (Degussa P-25) followed by hydrogen reduction at 753 K. The physical properties of deposited Pt, e.g., particle size, surface area, and electronic state, were studied respectively by transmission electron microscopy, volumetric gas adsorption measurement, and X-ray photoelectron spectroscopy as well as infrared spectroscopy of adsorbed carbon monoxide. The increase in Pt amount mainly resulted in an increase of the number of Pt deposits, not of their size. The catalysts were suspended in the aqueous solutions and photoirradiated at a wavelength >300 nm under an argon (Ar) atmosphere. The overall rate of photocatalytic reactions for both 2-propanol and Lys, corresponding to the ...

Fabrication of titanium dioxide thin flakes and their porous aggregate

Abstract: Thin flakes of titanium dioxide have been synthesized through a novel route via exfoliation of a layered precursor. A protonic oxide of HxTi2-x/4□ x/4O4·H2O (x ∼ 0.7; □, vacancy) was delaminated into colloidal single sheets (thickness 0.75 nm) by being interacted with a bulky organic amine, (C4H9)4NOH. The resulting titania sol was freeze-dried to produce a gel in a thin filmlike texture. The gelation took place by reassembling 10−20 titanate sheets and consequently yielding lamellar aggregates. Upon heating above 400 °C, the gel was transformed into titanium dioxide (anatase) in thin flaky morphology (20−30 nm in thickness versus ∼μm in the lateral dimension). Intermediates at various stages of the synthetic process as well as the final product were examined by applying various characterization techniques such as X-ray diffraction (XRD), scanning and transmission electron microscopes (SEM, TEM), FT-IR and Raman spectroscopies, thermogravimetry, and elemental analysis. The flaky particulates were aggregat...

Probing the TiO2 Photocatalytic Mechanisms in Water Purification by Use of Quinoline, Photo-Fenton Generated OH• Radicals and Superoxide Dismutase†

Abstract: In an attempt to improve our understanding of the basic mechanisms of the degradation of aromatic pollutants in water by TiO2 photocatalysis, quinoline (benzo[b]pyridine) was selected as a molecular probe, principally because of the difference in electron density over its two rings. This study was based on the identification and quantification of the primary products or principal secondary products of quinoline degradation either by TiO2 photocatalysis at pH 3 and 6 or by OH• radicals generated via the photo-Fenton reaction (Fe(II/III)−H2O2−UV) at pH 3. In this latter case, the three major products were those expected from the preferential electrophilic attack of OH• radicals on the electron-richer benzene moiety, viz., 5-, and 8-hydroxyquinolines and quinoline-5,8-dione derived from them. TiO2 photocatalysis did not yield this dione, and at the same percentages of degraded quinoline, the amounts of 5-hydroxyquinoline were lower by a factor of ca. 2 at pH 3 and ca. 10 at pH 6 (those of the 8-isomer were a...

A highly active photocatalyst for overall water splitting with a hydrated layered perovskite structure

Abstract: Photocatalytic decomposition of H2O into H2 and O2 over a novel photocatalyst, K2La2Ti3O10, was accomplished. K2La2Ti3O10, a layered perovskite-type compound with a hydrated interlayer space, exhibited a high activity for overall water splitting with Ni-loading. The highest activity was obtained over Ni(3.0 wt%)–K2La2Ti3O10 when the reaction was carried out in aqueous KOH solution (0.1 M, pH=12.8). By comparison with other Ni-loaded photocatalysts reported previously, the reaction mechanism of Ni–K2La2Ti3O10 was discussed. © 1997 Elsevier Science S.A.

Unique Effects of Iron(III) Ions on Photocatalytic and Photoelectrochemical Properties of Titanium Dioxide

Abstract: Photocatalytic oxidation of water on TiO2 (rutile) powder proceeded with a fairly high efficiency (about 9%) when iron(III) ions were used as the electron acceptor. The reaction continued until all iron(III) ions added to the solution were reduced into iron(II) ions. This behavior was in marked contrast to other reversible photocatalytic reactions, whose reaction rates decelerate as the result of the back reactions of the products on the photocatalysts. The efficient oxidation of water in the presence of iron(III) ions was attributed to preferential adsorption of iron(III) ions on TiO2 over iron(II) ions, which enabled efficient oxidation of water, although this reaction was thermodynamically less favorable than oxidation of iron(II) ions. Furthermore, from the measurements of photocurrents at crystalline TiO2 electrodes, iron(III) ions were concluded to have a catalytic function for the oxidation of water on photoirradiated TiO2.

Synthesis, characterization and photocatalytic properties of iron-doped TiO2 catalysts

Abstract: Fe-doped TiO2 catalysts were prepared by coprecipitation and the sol-gel method. The anatase to rutile phase transformation is dependent on the nature of the precursor used. Sol-gel-derived iron-doped catalysts show the presence of rutile and pseudobrookite phases, whereas coprecipitated catalysts show only the anatase phase. The role of the dopant ion is primarily to improve the charge separation of the photoproduced electron-hole pairs via a permanent electric field. The photocatalytic activity of the iron-doped catalysts can be explained in terms of the heterojunction formed between the Fe/TiO2 and α-Fe2O3 phases for the sol-gel-derived catalyst.

Electrochemical production of hydroxyl radical at polycrystalline Nb-doped TiO2 electrodes and estimation of the partitioning between hydroxyl radical and direct hole oxidation pathways

Abstract: The use of TiO_2 as a photocatalyst for the destruction of organic chemical pollutants in aqueous systems has been extensively studied. One obstacle to the effective utilization of these systems is the relatively inefficient use of the solar spectrum by the photocatalyst. In addition, light delivery to the photocatalyst can be impeded by UV-absorbing components in mixed effluent streams. We present a novel use of TiO_2 as a catalyst for the oxidative degradation of organic compounds in water that uses a potential source instead of light to generate reactive oxidants. Application of an anodic bias of >+2 V vs NHE to titanium electrodes coated with niobium-doped, polycrystalline TiO_2 particles electrochemically generates hydroxyl radicals at the TiO_2 surface. This process has been demonstrated to efficiently degrade a variety of environmentally important pollutants. In addition, these electrodes offer a unique opportunity to probe mechanistic questions in TiO_2 catalysis. By comparing substrate degradation rates with increases in current density upon substrate addition, the extent of degradation due to direct oxidation and •OH oxidation can be quantified. The branching ratio for these two pathways depends on the nature of the organic substrate. Formate is shown to degrade primarily via a hydroxyl radical mechanism at these electrodes, whereas the current increase data for compounds such as 4-chlorocatechol indicate that a higher percentage of their degradation may occur through direct oxidation. In addition, the direct oxidation pathway is shown to be more important for 4-chlorocatechol, a strongly adsorbing substrate, than for 4-chlorophenol, which does not adsorb strongly to TiO_2.

Application of Chemiluminescent Probe to Monitoring Superoxide Radicals and Hydrogen-Peroxide in TiO2 Photocatalysis

Abstract: A chemiluminescent probe, luminol, was successfully applied to monitoring superoxide ions (O2•-) and hydrogen peroxide (H2O2) produced on photocatalytic reaction in aqueous TiO2 suspension. Two chemiluminescent reactions were distinguished from the decay profile after the end of the irradiation, and the reaction mechanism was analyzed. The fast decay component gives information about O2•- and the slow one provides the amount of H2O2. The rate constant for the reaction of O2•- with luminol was found to be 1 × 104 M-1 s-1. The amount of O2•- during the irradiation on TiO2 in alkaline solution could be estimated to be on the order of 10-13 M. Detection of H2O2 in concentrations as small as 10-9 M was demonstrated in the photocatalytic water oxidation.

Oxidation of Metal−EDTA Complexes by TiO2 Photocatalysis

Abstract: Ethylenediaminetetraacetic acid (EDTA), a common industrial agent for complexing metal ions in water, frequently inhibits conventional metals-removal technologies used in water treatment. This study investigated the use of TiO2 photocatalysis for the aqueous-phase oxidation of EDTA and several metal complexes of EDTA. Reactions were performed at 0.1 wt % loading of Degussa P-25 TiO2, a solute concentration of 0.8 mM and at a constant pH. The different metal−EDTA complexes exhibited widely different photocatalytic oxidation rates under equivalent conditions of pH = 4 ± 0.1 in an aerobic system: Cu(II)−EDTA > Pb(II)−EDTA >> EDTA > Ni(II)−EDTA ≈ Cd(II)−EDTA ≈ Zn(II)−EDTA >>> Cr(III)−EDTA. Photoefficiency based on the Cu(II)−EDTA initial rate is nearly 60%. The rates of total organic carbon (TOC) removal and formaldehyde generation during photocatalytic EDTA oxidation indicate similarities to electrochemical oxidations of EDTA. Several means were explored to enhance the oxidation of Ni(II)−EDTA, whose behavi...

Solar Water Detoxification: Novel TiO2 Powders as Highly Active Photocatalysts

Abstract: The photocatalytic activity of a novel anatase titanium dioxide powder, namely Sachtoeben Hombikat UV 100, has been investigated and compared with that of a well-studied material (i.e., Degussa P25). Laboratory experiments using dichloroacetate (DCA) as the model pollutant have been carried out showing a very high photonic efficiency ({zeta}) reaching almost unity at low-light intensities. The effect of pH, substrate concentration, catalyst concentration, and addition of various inorganic salts has been thoroughly investigated. The inhibiting effect of anions such as chloride, nitrate, sulphate, and phosphate was completely reversible upon washing the catalyst powder with pure water or diluted base, respectively. The overall photonic efficiency of the DCA degradation was found to decrease with increasing illumination intensity. Various kinetic and mechanistic models are proposed to explain this effect as well as the results of the other experiments. Implications of these laboratory results for real world applications, e.g., for the design of appropriate photoreactors, are discussed.

Novel synthesis of microcrystalline titanium(IV) oxide having high thermal stability and ultra-high photocatalytic activity: thermal decomposition of titanium(IV) alkoxide in organic solvents

Abstract: Thermal decomposition of titanium(IV) tetra-tert-butoxide (TTB) in inert organic solvents at 573 K yielded microcrystalline anatase (titanium(IV) oxide, TiO2) powders with a crystallite size of ca. 9 nm and a surface area of <100 m2 g-1. Primary and secondary alkoxides of titanium(IV), however, were not decomposed under similar conditions, indicating that the thermal stability of C-O bonds in the alkoxides was a decisive factor for their decomposition. The TiO2 prepared from TTB by this manner was thermally stable upon calcination in air and retained high surface area of ca. 100 m2 g-1 even after calcination at 823 K. The as-prepared TiO2 powders, without calcination, exhibited much higher rate of carbon dioxide formation than any other active photocatalysts such as Degussa P-25 and Ishihara ST-01 in the photocatalytic mineralization of acetic acid in aerated aqueous solutions. The higher activity of the present TiO2 photocatalysts is attributed to both high crystallinity and large surface of the present product. The calcination of the as-prepared TiO2 in air reduced the photocatalytic activity, but it was still higher than the other commercially available TiO2's.

Photocatalytic Disinfection of Indoor Air

Abstract: The present study demonstrated the antibacterial effect of photocatalytic oxidation in indoor air using titanium dioxide as the catalyst. Through a series of experiments, it was determined that titanium dioxide did enhance the inactivation rate of the microorganisms under certain conditions. In these experiments the air velocity, relative humidity, UV intensity were varied. It was found that higher velocities retarded destruction rate, due to the low retention time in the reactor. It was also determined that TiO{sub 2} did not accelerate the reaction at low humidities (30%). At a relative humidity of 50%, there was complete inactivation of the organisms, but at higher humidities (85%), 10% of the organisms were still viable. The experiments showed that at higher UV intensities, most of the inactivation was being done by the UV photons. However, the photons were not able to completely inactivate the microorganisms. In the photocatalysis experiments, there was complete inactivation of the bacteria.

Photocatalytic Degradation of 2-Chlorophenol in TiO2 Aqueous Suspension: Modeling of Reaction Rate

Abstract: The photoassisted oxidation of a dilute aqueous solution of 2-chlorophenol (2-CP) was investigated, over a heterogeneous catalyst of TiO2 (anatase), in an annular photocatalytic reactor. The effects of some physical and chemical parameters such as 2-CP concentration, catalyst concentration, dissolved oxygen concentration, pH, temperature, and absorbed light intensity were studied in order to optimize the process. The experiment, carried out in the presence of electron scavengers such as metallic ions, shows that the reaction rate is significantly higher than that obtained when oxygen is used alone. The results obtained in this study have led us, on the basis of experimentally determined adsorption, to propose a kinetic approach in which the rate-determining step is the reaction of OH• radicals, identified by a spin trapping technique (EPR), with adsorbed 2-CP. A kinetic model proposed was based on a Langmuir type adsorption involving a competition between solvent and substrate with a supplementary assumpt...