Showing papers on "Photocatalysis published in 1993"

Photocatalytic properties of titanium dioxide (TiO2)

Abstract: This paper reviews the photocatalytic activity of TiO[sub 2] toward the degradation of organic compounds in aqueous solutions at low concentrations. The photocatalytic activity of both polycrystalline samples and thin films can be related to the method of preparation of the catalyst. The increase in the catalytic activity of TiO[sub 2] when certain metals such as silver, gold, and palladium are deposited on the surface is also discussed. These catalysts have potential use in removing harmful carcinogens from water as well as possible application in reducing the harmful effects of oil spills. 45 refs., 2 figs.

Electrochemically assisted photocatalysis: titania particulate film electrodes for photocatalytic degradation of 4-chlorophenol

Abstract: A commercially available TiO[sub 2] powder (Degussa P25) has been used to prepare thin particulate films on conducting glass plates. These semiconductor particulate films are photoelectrochemically active with properties similar to an n-type semiconductor. The recombination between the photogenerated charge carriers can be suppressed by applying an external anodic bias. Electron scavengers, such as oxygen, affect the photocurrent generation by competing for the photogenerated electrons. These particulate films provide a convenient method for accelerating the photocatalytic reaction by applying an external bias. For example, the rate of photocatalytic degradation of 4-chlorophenol greatly increases when the TiO[sub 2] particulate film electrode is maintained at an external anodic bias (0.6 V vs SCE) during the UV photolysis. 26 refs., 6 figs.

Inactivation of Escherichia coli by titanium dioxide photocatalytic oxidation.

Abstract: Titanium dioxide in the anatase crystalline form was used as a photocatalyst to generate hydroxyl radicals in a flowthrough water reactor. Experiments were performed on pure cultures of Escherichia coli in dechlorinated tap water and a surface water sample to evaluate the disinfection capabilities of the reactor. In water devoid of significant amounts of inorganic-radical scavengers, rapid cell death was observed with both pure cultures and members of the indigenous flora in a natural water sample.

An investigation of titanium dioxide photocatalysis for the treatment of water contaminated with metals and organic chemicals

Abstract: Laboratory experiments were performed to investigate TiO 2 photocatalysis for treating water contaminated with dissolved metals (Ag, Au, Cd, Cr, Cu, Hg, Ni, and Pt) and a variety of organics (e.g., methanol, formic acid, salicylic acid, EDTA, phenol, and nitrobenzene). It was found that only those metals with half-reaction standard reduction potentials more positive than 0.3 V (vs normal hydrogen electrode) can be treated using TiO 2 as the photocatalyst. Kinetic data illustrating the synergism between oxidation and reduction are presented. Experiments using singly substituted benzenes as electron donors show that the rate of reduction of Cr(VI) is correlated with Hammett a constants

Chemical Oxidation Technologies: Ultraviolet Light/Hydrogen Peroxide, Fenton's Reagent, and Titanium Dioxide-Assisted Photocatalysis

Abstract: The application status and potential of three chemical oxidation treatment methods which generate powerful oxidants (hydroxyl radicals): ultraviolet light (UV)/hydrogen peroxide (H2O2) process, Fenton's reagent treatment, and titanium dioxide (TiO2)-assisted photocatalytic degradation, are described and discussed. These oxidation methods are known to effectively degrade and, in several cases, mineralize contaminants ranging from inorganic compounds (such as cyanides) to chlorinated aliphatic compounds and complex aromatic compounds in reaction times on the order of a few minutes to a few hours. Of the three oxidation systems discussed, the technology for the UV/H2O2 process is the most advanced, with numerous successful full-scale treatment units already in existence. Applications of both the Fenton's reagent and TiO2-assisted photodegradation processes are currently being developed, with the concepts proven in numerous laboratory-scale studies for a wide range of contaminants. However, both of t...

Titanium dioxide aerogels for photocatalytic decontamination of aquatic environments

Abstract: We describe synthesis and applications of nonsupported semiconducting porous structures that can be used as photocatalysts for photodegradation of organic pollutants in aqueous environments. The photocatalysts, TiO 2 aerogels, are prepared through the sol-gel method and supercritical point drying. These materials have a porosity of 85%, a BET surface area of 600 m 2 /g, a and a bulk density of 0.5 g/cm 3 . X-ray diffraction show the anatase crystalline phase with a crystallite size of 50 A

Photosensitization and photocatalysis using inorganic and organometallic compounds

Abstract: Preface. Evolution of Inorganic and Organometallic Photochemistry. Historical Perspectives and Commentary A.W. Adamson. Intermolecular Energy and Electron Transfer Processes V. Balzani, M. Maestri. Photocatalysis: Definitions and Classifications H. Hennig, R. Billing, H. Knoll. Charge Transfer Excitation of Coordination Compounds. Generation of Reactive Intermediates A. Vogler, H. Kunkely. Photoredox and Sensitization Processes involving Transition Metal Polypyridine Complexes K. Kalyanasundaram. Intramolecular Energy and Electron Transfer in Polynuclear Complexes F. Scandola, C.A. Bignozzi, M.T. Indelli. Photocatalysis: Reduction of CO2 and Water-Gas Shift Reaction catalysed by 2,2'-bipyridine or 1,10-phenanthroline Co(II), Ru(II), Re(I) and Ir(III) complexes R. Ziessel. Metal Complexes as Photosensitizers in Photoelectrochemical Cells M. Gratzel, K. Kalyanasundaram. Photosensitization by (Metallo)porphyrins: Formation and Photophysical Properties of Porphyrin Assemblies A. Harriman. Photocatalytic and Photoredox Properties of Polyoxometalate Systems C.L. Hill, C.M. Prosser-McCartha. Photocatalytic Transformations of Organic Substrates using INorganic Salts and Complexes U. Kolle. Photocatalytic Reactions involving Metal Carbonyls P.C. Ford, W. Boese, B. Lee, K.L. MacFarlane. Photocatalysis - C-H Activation R.H. Crabtree. Electron Donor-Acceptor Interactions and Photoinduced Electron Transfer of Organometallic Compounds T.M. Bockman, J.K. Kochi. Index.

Photocatalytic reduction of carbon dioxide to methane and acetic acid by an aqueous suspension of metal-deposited TiO2

Abstract: Deposition of a metal on TiO2 considerably accelerated photocatalytic reduction of carbon dioxide to methane and/ or acetic acid, and product distribution was dependent on the kind of metal on the surface of TiO2.

Ultrasmall Metal Oxide Particles: Preparation, Photophysical Characterization, and Photocatalytic Properties

Abstract: This paper gives an overview of the author's activities in the research of extremely small metal oxide particles in recent years. In particular, the synthesis of transparent colloidal solutions of extremely small zinc oxide, titanium dioxide, hematite, and titanium/iron mixed oxide particles (2 nm < d 20 nm) in water, ethanol, and 2-propanol is described. Quantum (Q)-size effects are observed during particle growth and at me final stages of synthesis. A simple molecular orbital (MO) picture is presented for the qualitative interpretation of these effects, while quantitative calculations have been carried out using a quantum mechanical model developed by Brus. The photophysical properties of the particles have also been investigated extensively. Fluorescence spectra of the ZnO sols suggest that adsorbed electron relays are necessary to shuttle electrons from the conduction band to lower-lying traps. Excess negative charge on the particles, resulting from either deprotonated surface hydroxyl groups or from photogenerated or externally injected charge carriers, causes a blue-shift in the electronic absorption spectrum, which is explained by electrostatic and MO models. The zero point of charge (pHzpc) of the aqueous colloidal suspensions has been determined by several independent methods. While zinc oxide, titanium dioxide, and titanium/iron mixed oxide particles exhibit considerable photocatalytic activity (as illustrated for the reduction of molecular oxygen and the oxidation of various halogenated carboxyl acids), hematite particles are only found to oxidize S(IV) under bandgap illumination to a reasonable extent (ϕ < 0.3). A mechanism involving surface-bound molecules and free radical intermediates is presented to explain these differences in reactivity.

Reversible photoreductive deposition and oxidative dissolution of copper ions in titanium dioxide aqueous suspensions

Abstract: Particulate titanium dioxide was used to remove and concentrate Cu(II) ions in aqueous solutions through a cyclic process of photodeposition, separation, and oxidation. Illuminated, nitrogen-purged solutions containing copper sulfate, excess sodium formate (pH 3.6), and titanium dioxide formed a purple Cu-TiO[sub 2] species. Cu(II) concentrations in the supernatant were driven from 51 to [le] 0.018 [mu]g/mL. Upon purging with oxygen, this purple Cu-TiO[sub 2] system reverted back to white along with a corresponding increase in the Cu(II) supernatant concentration. The photodeposition step and the air oxidation step were utilized to demonstrate a volume reduction process. Eighty-six percent of the Cu(II) in a synthetic waste stream was concentrated to an organic-free solution having 7% of the initial volume. The remaining waste solution contained only 1% of the initial Cu(II), and in a subsequent step, the remaining formate ion was destroyed using conventional TiO[sub 2] photocatalytic oxidation. The overall process demonstrated the ability to separate copper ions from organics using only light and air. The reversible photoreduction deposition of Cu(II) from solution was observed in the pH range 1.84-6.60. The reversible photoreduction deposition of copper (II) was dependent on the organic used to scavenge holes and independent of the copper salt used.more » 33 refs., 4 figs., 4 tabs.« less

Heterogeneous photocatalytic oxidation of hazardous organic contaminants in water

Abstract: Photoactive catalysts, when illuminated with UV-light, generate highly reactive radicals that can oxidize the organic contaminants in water. One method to increase the efficiency of the process, and thereby reduce the light energy requirements, is by developing more active catalysts. Several catalysts that were obtained commercially and/or prepared in the laboratory were examined for their photoactivity, and they are: TiO 2 , Pt-TiO 2 with platinum loading varying from 0.5% to 10% by weight, SrTiO 3 , and 1.5% NiO-SrTiO 3 . The organic compounds used to identify the best catalyst were trichloroethylene (TCE), toluene, methyl ethyl ketone (MEK), salicylic acid, and 2,4-dichlorophenol, with initial concentration varying from 0.1 to 10.0 mg/L

Factors affecting the photocatalytic degradation of dichlorvos over titanium dioxide supported on glass

Abstract: The photocatalytic degradation of organophosphorus insecticide, dichlorvos, on glass-supported titanium dioxide was investigated. The photoreactor coated with titanium dioxide was illuminated with a 20 W black-light UV fluorescent tube. The aqueous solution containing dichlorvos was continuously pumped through the photoreactor. Several factors, such as the initial dichlorvos concentration, dissolved oxygen, electrolytes, flow rate and temperature, affecting the oxidation rate of dichlorvos were studied. The results indicate that photocatalysis can be an effective process for the degradation of dichlorvos. The activated energy for the photocatalytic degradation of dichlorvos is 28.4 kJ mol−1. The initial quantum yield for the destruction of dichlorvos is 2.67%. Increasing the flow rate and initial dichlorvos concentration increases the dichlorvos degradation rate. Total mineralization requires a much longer illumination time than the disappearance of dichlorvos. Phosphate-containing intermediates are more stable than chloride-containing intermediates. It is shown that the photocatalytic oxidation of dichlorvos follows the Langmuir—Hinshelwood-type behavior, and reaction byproducts display an inhibiting effect on degradation rate.

Photocatalytic degradation of pollutant over TiO2 in different crystal structures

Abstract: TiO2 samples of different crystal forms and crystal sizes were prepared by calcining the hydrolysis product of titanium tetraisopropoxide at different temperatures. The photocatalytic degradation rate of several compounds over TiO2 was either dependent on the calcination temperature (group I) or independent of it up to 500°C (group II). Group I compounds are sparsely water-soluble, whereas group II compounds are water-soluble. For group I compounds there was a proportional relationship between the degradation rate and crystal size of anatase. A hypothesis explaining the mechanism of the photocatalytic processes of the two groups was proposed on the basis of their different adsorption modes. The addition of H2O2 accelerated the photocatalytic degradation. This effect was more pronounced for rutile than anatase.

Beneficial effects of homogeneous photo-Fenton pretreatment upon the biodegradation of anthraquinone sulfonate in waste water treatment

Abstract: Homogeneous photo-oxidation of anthraquinone-2-sulfonic acid sodium salt (ASS) with H2O2 and Fe3+ was investigated. The continuous photoproduction of Fenton reagent can be achieved in this way. The behaviour of Fe3+ was similar to Fe2+ under the same experimental conditions and the photoactivity was favourably influenced in the presence of oxygen. The photo-Fenton reactions greatly accelerated ASS degradation in comparison with direct photolysis using Pyrex flasks (λ > 290 nm). The influence of Fe3+, Cu2+, H2O2 concentration, pH, temperature and gas atmosphere was systematically studied. A solution containing ASS 3·10−3 M was degraded up to ca. 90% under light (AM 1) at 60°C in 3 h and in ca. 5 h at 35°C. Complete oxidation was achieved after 15 h. About 90% of the total organic carbon was degraded to carbon dioxide when dearomatization was completed. Photo-Fenton systems could be used for initial treatment of waste waters containing ASS to obtain aliphatic and oxidized compounds susceptible to being more easily degraded in biological waste water treatment plants. Reaction products were characterized by total organic carbon (TOC); nuclear magnetic resonance (NMR); gas chromatography (GC) and spectroscopy (OD).

Photocatalytic air treatment process under room light

Abstract: A process for photocatalytically treating a hospital room contaminated by bacteria or an interior environment bearing airborne malodorous substances. A thin film of photocatalyst made of a semiconductor such as titanium dioxide is disposed on the inner wall of the hospital room or living space. The photocatalytic thin film is irradiated by a light source suitable for general lighting applications, such as a fluorescent lamp, and is photoexcited by the small amount of ultraviolet radiation included in the light of fluorescent lamp. The wattage of the fluorescent lamp as well as the distance between the photocatalytic thin film and the lamp are selected such that the intensity of those UV wavelengths which have a higher energy than the band gap energy of the photo-catalyst is 0.001-1 mW/cm2, preferably, 0.01-0.1 mW/cm2. The bacteria and chemical compounds deposited on the photoexcited thin film are photodecomposed.

Photocatalytic reaction of neat alcohols by metal-loaded titanium(IV) oxide particles

Abstract: The photocatalytic reaction (excitation wavelength, greater than 300 nm) of neat alcohols (ethanol or 2-propanol) by metal-loaded titanium(IV) oxide (TiO2) was conducted under argon at room temperature. The dehydrogenation products hydrogen (H2) and acetaldehyde (or acetone in the case of 2-propanol) were found in the absence of additives. The corresponding acetals, ethers, and alkanes were obtained by the addition of concentrated hydrochloric acid (HCl). The measurement of the product yields during the post-irradiation dark reaction in the presence of photocatalyst (TiO2) loaded with platinum(IV) oxide (PtO2) revealed that the production of ether and ethane and the regeneration of ethanol proceed by the hydrogenation of acetal over loaded metal, presumably via a hemiacetal intermediate. X-ray diffraction (XRD) analyses showed that the loaded PtO2 is reduced to platinum metal during the course of the photocatalytic reaction. For the production of ether from HCl-acidified ethanol, Pd-loaded catalysts, especially prepared by precipitation—reduction, exhibit the highest activity of several metal-loaded TiO2 catalysts. From XRD and X-ray photoelectron spectroscopy studies of Pd-loaded TiO2 particles, it was shown that the smaller the size of the Pd particles, the higher the selectivity for ether production from the acidified ethanol suspension. The application of this photocatalytic O-alkylation to the synthesis of cyclic ethers was demonstrated.

Evidence of critical illumination and dark recovery times for increasing the photoefficiency of aqueous heterogeneous photocatalysis

Abstract: A hypothesis is proposed and tested for increasing the photoefficiency of heterogeneous photocatalytic reactions on TiO 2 particles. Increasing the photoefficiency is vital for the economic scale-up of photocatalysis for decomposing organic compounds in water. An open-channel reactor was designed to test the following rationale. There is a critical illumination time during which the absorbed UV photons generate the oxidizing species on the particle surface. These species react further to oxidize the organic material in the water. This latter step can take place in the dark. After a critical dark recovery time the particle is illuminated again. Using this light—dark cycling, the photoefficiency for formate decomposition increased by 500% when the illumination time was 72 ms and the dark recovery time was 1.45 s.

Comparative catalytic activity of selected metal oxides and sulfides for the photo-oxidation of cyanide

Abstract: The photocatalytic oxidation of cyanide by TiO 2 (anatase and rutile), ZnS, ZnO, CdS, V 2 O 5 , SiO 2 and Fe 2 O 3 was investigated and compared with the photo-oxidation in the absence of catalyst. Only TiO 2 and ZnO are satisfactory catalysts. Cyanide is first oxidized to cyanate and then via nitrite to nitrate. In most systems cyanide is also volatilized as HCN. The amount of HCN volatilized is a function of the pH of the solution and the surface area of the catalyst and generally increases with decreasing pH and increasing surface area.

Photomineralisation of 4-chlorophenol sensitised by titanium dioxide: a study of the effect of annealing the photocatalyst at different temperatures

Abstract: The results of a study of the variation in photocatalytic activity of TiO 2 , as measured by its ability to photomineralise 4-chlorophenol, as a function of temperature used to anneal the TiO 2 , are reported. Heat treatment of the TiO 2 leads to a marked decrease in its photocatalytic activity at annealing temperatures above 600°C. This decrease is associated with a concomitant drop in the specific surface area of the TiO 2 , owing to particle sintering, rather than the anatase to rutile transformation, which occurs largely at temperatures above 700°C. There is a reasonable correlation between photocatalytic activity and the surface area of the aggregate particles in the dispersions of the different heat-treated TiO 2 samples.

Semiconductor photocatalysis: effect of light intensity on nanoscale cadmium sulfide-catalyzed photolysis of organic substrates

Abstract: The relationship between light intensity and product distribution in semiconductor photocatalysis was investigated by using nanoscale CdS microcrystallites (CdS-0) as photocatalysts, triethylamine (TEA) as the electron donor, and either aromatic ketones, electron-deficient alkenes, or 1-benzylnicotinamide (BNA + ) as substrates. In the case of the ketones and BNA + , the yield of their respective one-electron reduction products, pinacols and the dimer, (BNA) 2 , increased with decreasing light intensity. When alkenes are employed in the CdS-0 system, cis-trans photoisomerization always occur regardless of the light intensity

Photoactivation of silver loaded on titanium(IV) oxide for room-temperature decomposition of ozone

Abstract: Dark and photoinduced decomposition of ozone (O 3 , approximately 80 ppm) in air over silver (Ag) or silver(I) oxide (Ag 2 O) loaded onto commercially available titanium(IV) oxide (TiO 2 ) powder was studied using a flow-type reactor (1.5 dm 3 min −1 ) at room temperature. The relatively lower catalytic activity of bare TiO 2 in the dark was significantly improved by the loading of Ag or Ag 2 O via photodeposition or precipitation from an alkaline solution respectively. The enhanced activity was attributed to the thermal reaction with O 3 to yield silver(II) oxide (AgO). Therefore, in the dark, the catalytic activity decreased gradually with the formation of AgO, as was indicated by the simultaneous bleaching of the catalyst surface. During prolonged dark reaction, the total molar amount of O 3 decomposed over TiO 2 Ag 2 O catalyst was much larger than that estimated on the assumption of a stoichiometric equimolar reaction of O 3 with Ag 2 O to yield AgO, suggesting the slow thermal decomposition of AgO at room temperature. Photoirradiation of the deactivated AgO catalyst recovered the activity via regeneration of purple-grey Ag deposits. The recovered high activity was kept almost constant by photoirradiation. The mechanism of these O 3 decomposition processes is discussed.