Showing papers on "Photocatalysis published in 1994"

Photocatalytic oxidation of organic acids on quantum-sized semiconductor colloids.

Abstract: A detailed analysis of the reaction products and mechanisms of the photocatalytic oxidation of acetate in the presence of quantum-sized ZnO colloids (Dp ≈ 40 A) is presented. The principal oxidation products and reaction intermediates are determined to be CO_2, HCO_2^-, CHOCO_2^-, HCHO, CH_3OOH, CH_3COOOH, and H_2O_2. Formate and glyoxylate, which are found as intermediates in the photooxidation of acetate, also serve as effective electron donors on illuminated ZnO surfaces. The proposed relative reactivity of electron donors toward photooxidation is in the following order: CHOCO_2- > HCO_2^- > HCHO > CH_3CO_2^- ≥ H_2O_2 CH_3COOOH > CH_3OOH. Observed product distributions are discussed in terms of pathways involving direct oxidation of surface-bound acetate by valence band holes (or trapped holes) and the indirect oxidation of acetate by surface-bound hydroxyl radicals. The product distribution observed at low photon fluxes is not consistent with oxidation primarily by free hydroxyl radicals. A mechanism involving the reaction of an intermediate carbon-centered radical with > ZnOH surface sites is proposed. When electron donors are strongly adsorbed to semiconductor surfaces, surface-mediated reactions appear to play a dominant role in the determination of the time-dependent product distributions.

New Reactor Design for Photocatalytic Wastewater Treatment with TiO2 Immobilized on Fused-Silica Glass Fibers: Photomineralization of 4-Chlorophenol.

Abstract: A new reactor design for the use of titanium dioxide-coated fused-silica glass fibers for wastewater treatment is described. The manufacture of the coating of the fibers is explained in detail. 4-Chlorophenol was used as the test compound. The influences of temperature and irradiation wavelength and the effect of hydrogen peroxide addition were investigated. Activation energy for the initial attack of an OH radical on 4-chlorophenol was calculated to be 20.6 kJ/mol. Degradation rates and quantum yields obtained with this reactor were compared with results measured with Degussa P25 TiO[sub 2]-slurry treatment. With the present design, the degradation rate of 4-chlorophenol is 1.6 times higher and the destruction of the total organic carbon (TOC) is 2.8 times faster. 35 refs., 6 figs., 4 tabs.

Electrochemically Assisted Photocatalysis. 2. The Role of Oxygen and Reaction Intermediates in the Degradation of 4-Chlorophenol on Immobilized TiO2 Particulate Films

Abstract: The electrochemically-assisted photocatalytic degradation of 4-chlorophenol (4-CP) using immobilized TiO[sub 2] particulate films has been investigated by analyzing reaction intermediates under a variety of experimental conditions. The degradations were carried out in both nitrogen- and oxygen-saturated solutions to explore the role of reduced oxygen species and molecular oxygen in the formation of reaction intermediates and in the mineralization of 4-CP. The degradation rate can be greatly improved even in a nitrogen-saturated atmosphere by applying an anodic bias to the TiO[sub 2] film electrodes. 4-Chlorocatechol (4-CC) is the predominant intermediate observed in oxygen-saturated solutions, whereas hydroquinone (HQ) is the primary intermediate in nitrogen-saturated solutions. Molecular oxygen plays an important role in the enhancement of the electrochemically assisted photocatalytic decay rate of 4-CP and the subsequent degradation of reaction intermediates, viz., 4-CC and HQ. 37 refs., 7 figs., 1 tab.

Photocatalytic decolorization of wastewater dyes

Abstract: The photocatalytic decolorization of municipal wastewater contaminated with textile dyes was studied using a batch reactor. Degussa P25 titanium dioxide was used as the photocatalyst and proved to be effective for dye degradation when irradiated with UV light in the presence of air. In addition to removing the color from the wastewater, the photocatalytic reaction simultaneously reduced the COD which suggests that the dissolved organics had been oxidized. The activation energy for the photocatalytic decolorization reaction was only about 3 to 6 kJ/mole indicating a weak temperature dependence of the rate. These results suggest that the photocatalytic degradation of textile dyes may be a viable method for decolorizing and oxidizing organics in wastewater

Effect of Na2CO3 addition on photocatalytic decomposition of liquid water over various semiconductor catalysis

Abstract: Photocatalytic decomposition of liquid water into H2 and O2 over various semiconductor catalysts, such as TiO2, Ta2O5 and ZrO2, suspended in pure water, was studied with a special interest on the effect of Na2CO3 addition. It was found, for the first time, that both RuO2Ta2O5 and NiOxTa2O5 photocatalysts were able to decompose liquid water into H2 and O2 in pure water as well as with the addition of Na2CO3. It was speculated that a negative flat-band potential and wide band gap of the photocatalysts are very important for the stoichiometric decomposition of liquid water. Furthermore, the addition of Na2CO3 is very effective for the photocatalytic decomposition of water for PtNa2Ti6O13, PtK2Ti6O13 and Pt−K4Nb6O17 catalysts which did not show photocatalytic activity in pure water. The significant effect of Na2CO3 addition was observed commonly for both the TiO2 catalyst system and the Pt-loaded catalyst system.

Photoelectrocatalytic degradation of formic acid using a porous titanium dioxide thin-film electrode.

Abstract: The degradation of formic acid (HCOOH) using titanium dioxide (TiO[sub 2]) in photocatalytic and photoelectrocatalytic reactions was investigated in order to determine whether electrical biasing could improve the efficiency of photocatalytic reactions. This study addressed the effects of film thickness, biasing potential, presence of oxygen, and added inorganic electrolytes on the photocatalytic degradation of HCOOH. The results of these experiments showed that the degradation of HCOOH in this system was due only to the photocatalytic as opposed to homogeneous photolysis reactions. Degradation efficiency of the photocatalytic reaction was roughly proportional to the TiO[sub 2] film thickness. In the photoelectrocatalytic reaction, positive potentials (vs saturated calomel electrode, SCE) improved the degradation efficiency and +0.0 V (vs SCE) was enough to obtain a maximum efficiency. The supply of oxygen was essential in the photocatalytic reaction, while the photoelectrocatalytic reaction was not significantly affected by the removal of oxygen. The presence of inorganic electrolytes lowered the efficiency of the photocatalytic degradation of HCOOH. However, the efficiency of photoelectrocatalytic degradation was not affected by inorganic electrolytes. Overall, when used with the bias, the system showed efficient degradation over a wide range of conditions. 21 refs., 9 figs.

Effects of metal-ion dopants on the photocatalytic reactivity of quantum-sized TiO2 particles

Abstract: Oxidation and reduction rates are increased when the photocatalyst TiO_2 is doped with metal ions. The photochemical oxidation of CHCl_3 and the photochemical reduction of CCl_4 were examined in the presence of 21 different doped TiO_2 catalysts. Dopant metal ions with closed shells were found to have little effect on reactivity. The authors present their results in the form of a periodic chart.

Inactivation of Phage MS2 by Iron-Aided Titanium Dioxide Photocatalysis.

Abstract: Near-UV photocatalytic disinfection was accomplished in aqueous titanium dioxide suspensions. A level of inactivation of phage MS2 of 90% increased to 99.9% after 2 μM ferrous sulfate was added. Hydroxyl radical oxidation, with Fenton reaction enhancement, is suspected to be primarily responsible for the viral degradation observed.

Photocatalytic reduction of CO2 with H2O on TiO2 and Cu/TiO2 catalysts

Abstract: Photoinduced reduction of CO2 by H2O to produce CH4 and CH3OH has been investigated on wellcharacterized standard TiO2 catalysts and on a Cu2+ loaded TiO2 catalyst. The efficiency of this photoreaction depends strongly on the kind of catalyst and the ratio of H2O to CO2. Anatase TiO2, which has a large band gap and numerous surface OH groups, shows high efficiency for photocatalytic CH4 formation. Photogenerated Ti3+ ions, H and CH3 radicals are observed as reactive intermediates, by ESR at 77 K. Cu-loading of the small, powdered TiO2 catalyst (Cu/TiO2) brings about additional formation of CH3OH. XPS studies suggest that Cu+ plays a significant role in CH3OH formation.

Highly Efficient TiO2 Film Photocatalyst. Degradation of Gaseous Acetaldehyde

Abstract: A glass-supported pure anatase film with an extremely high photocatalytic activity has been prepared. Photocatalytic properties of the film were investigated by measuring the degradation of gaseous acetaldehyde under relatively strong (black fluorescent light) and weak (white fluorescent light) UV light irradiations. It was found that the photocatalytic efficiency was higher than that of Degussa P25, one of the most efficient commercial TiO2 powders.

A photocatalytic approach for the reductive decolorization of textile azo dyes in colloidal semiconductor suspensions

Abstract: Two representative commercially used textile azo dyes, Acid Orange 7 and Direct Blue 1, have been decolorized using colloidal TiO[sub 2] and WO[sub 3] photocatalytic systems. Under UV irradiation, these dyes undergo rapid decolorization as they are reduced at the semiconductor surface by the trapped electrons. The trapping of electrons in irradiated semiconductor colloids and their participation in the dye reduction process have been probed by steady-state and laser flash photolysis techniques. The quantum efficiency for such a reductive process has been determined to be 4.7%. The rate constant for the electron transfer between the excited semiconductor colloid and the dye is of the order of 10[sup 8] M[sup [minus]1] s[sup [minus]1]. This photocatalytic decolorization approach has potential applications in the treatment of textile dye wastes. 28 refs., 8 figs.

Photocatalysts with tunnel structures for decomposition of water. Part 1.—BaTi4O9, a pentagonal prism tunnel structure, and its combination with various promoters

Abstract: Photocatalysts have been prepared by impregnating barium tetratitanate, BaTi4O9, which has a pentagonalprism tunnel structure, with aqueous solutions of CoCl2, Ni(NO3)2, In(NO3)3, RuCl3, H2IrCl6 or H2PtCl6, and then activating with either reduction or oxidation. For decomposition of water under irradiation with light from a Xe lamp, reduction caused low photocatalytic activities. Oxidation of RuCl3-impregnated BaTi4O9 at 848 K led to active photocatalysts which produce H2 and O2 in the correct stoichiometric ratio. The photocatalytic activity increased with an increase in the amount of loaded Ru until it reached 1 wt. %, and remained nearly constant up to 3 wt. %. The X-ray photoelectron spectra showed that the active Ru was in a tetravalent state, forming RuO2 on the BaTi4O9 surface. UV diffuse reflectance spectra showed that BaTi4O9 had a threshold of absorption of light at around 410 nm which reached a maximum at 320 nm. From high-resolution transmission electron microscopy and microanalysis the spherical RuO2 particles of 1.4–3.0 nm in diameter were found to be dispersed uniformly on the regular lattice of BaTi4O9. It is concluded that the pentagonal-prism tunnel structure of BaTi4O9 has a ‘nest’ effect and is responsible for the high dispersion of RuO2 particles, which leads to the high photocatalytic activity.

Comparison of the photocatalytic efficiency of TiO2, iron oxides and mixed Ti(IV)Fe(III) oxides: photodegradation of oligocarboxylic acids

Abstract: Comparative photo-oxidations of three oligocarboxylic acids, i.e. oxalic acid, EDTA and malonic acid, were performed with different catalysts, i.e. TiO 2 (Degussa P-25), α-Fe 2 O 3 , γ-Fe 3 O 4 and two samples of mixed Fe(III)Ti(IV) oxides containing 0.5 wt.% and 5 wt.% Fe respectively. Different irradiation wavelengths were used. It was found that the photocatalytic efficiency depends mainly on the physicochemical properties of the catalyst, the irradiation wavelength, and the redox potential and adsorption ability of the substrate. The effect of surface complexes formed between the oxide and the organic compound was analysed. TiO 2 was found to be the best catalyst. The relative utility and stability of iron oxides and mixed FeTi oxides is discussed.

Photocatalytic degradation of wastewater pollutants. Titanium-dioxide-mediated oxidation of polynuclear aromatic hydrocarbons

Abstract: Polynuclear aromatic hydrocarbons, such as acenaphthene, anthracene, fluorene and naphthalene, were found to undergo efficient photocatalytic oxidation in aqueous suspensions of TiO2 on irradiation with artificial light as well as sunlight. The rate of photo-oxidation was monitored by following the loss of fluorescence and oxygen uptake in these systems. 2-Formyl-3-hydroxycinnamaldehyde (7) and 5,8-dihydroxy-2,4-naphthaquinone (13) from naphthalene and anthraquinone (17) from anthracene were isolated during photo-oxidation. A mechanism involving hydroxyl and superoxide radicals has been proposed for the formation of the different intermediates in these cases.

Surface properties of iron-titania photocatalysts employed for 4-nitrophenol photodegradation in aqueous TiO2 dispersion

Abstract: Iron(III) doped specimens (0.01–5% atomic Fe ∶ Ti) have been prepared by impregnating polycrystalline “home prepared” TiO2 (mainly anatase) and have been studied for photocatalytic degradation of 4-nitrophenol in aqueous dispersions. Some structural and surface properties have been studied by X-ray diffraction, specific surface area and porosity measurements and FTIR monitoring of pyridine, ammonia and methanoic acid adsorption for surface acidity and basicity. Their surface properties were compared with a corresponding series of photocatalysts prepared by a coprecipitation method. Adsorption of pyridine and ammonia indicates the presence of surface acid Lewis sites, a low concentration of surface Bronsted sites being present only in the samples prepared by impregnation. The photocatalytic experiments indicated that the presence of iron ions does not influence or is detrimental for the occurrence of the 4-nitrophenol photodegradation.

Metal Oxides as Photocatalysts for Environmental Detoxification

Abstract: Heterogeneous photocatalysis occurs over semiconductor particulates irradiated with light of proper wavelengths, which generates charge carriers (electrons and holes). These e /h+ pairs are ultimately positioned at the particle surface to undergo a variety of events (recombination, trapping) in competition with other processes (redox chemistry). These charge carriers may be implicated in photocatalytic transformations of inorganic anions (cyanides, sulfides) and reduction of precious and/or toxic metals. When organic compounds are present in aerated suspensions, the photocatalytic oxidation leads to total mineralization to carbon dioxide and inorganic anions (for example, chloride from chlorinated organics). The present Comment focuses on the fundamental and primary events occurring in the heterogeneous photocatalysis on semiconductor metal oxides, with particular reference to TiO2 and reports on the mechanism of some redox transformations concerning inorganic and organic species. Actual research...

Performance of Nonconcentrating Solar Photocatalytic Oxidation Reactors: Part I—Flat-Plate Configuration

Abstract: A nonconcentrating flat-plate photoreactor has been fabricated and tested in an outdoor solar photocatalytic oxidation facility. It has been demonstrated that 4-chlorophenol (4CP) is successfully oxidized when operating under either sunny or cloudy atmospheric conditions. Tests have been conducted with the photocatalyst, titanium dioxide (TiO[sub 2]), either mixed into the 4CP solution to form a slurry or adhered to a fiberglass mesh in a fixed configuration. The reaction rate constant for the slurry mode is typically two to five times greater than that for the fixed catalyst mesh tested at similar ultraviolet (UV) insolation conditions. In addition, the reaction rate constant appears to vary linearly with the UV insolation, and it shows no dependence on liquid film thickness in the slurry mode, but appears to vary linearly with the inverse of film thickness in the fixed catalyst mode. All tests were performed in the laminar flow regime. Design recommendations for industrial development are presented.

Photocatalytic oxidation of phenol on TiO2 powders. A Fourier transform infrared study

Abstract: In the present paper a detailed investigation of a well known photoreaction, i.e. phenol photodegradation in the presence of TiO2, has been carried out. The Fourier transform infrared (FT-IR) technique has been used to characterize the catalyst surface and to follow the photoprocess by “in situ” measurements in a gas—solid regime, simulating the aqueous liquid—solid regime in which this photodegradation reaction has been usually carried out. The influence on the photoprocess of the surface hydroxylation of the catalyst has been investigated and the presence of some reaction intermediates has been revealed by FT-IR. The experimental results have demonstrated that the chosen gas—solid system in the presence of water vapour is an acceptable simulation of the aqueous liquid—solid regime.

Photocatalytic reaction of H2O+CO2 over pure and doped Rh/TiO2

Abstract: In the photocatalytic reduction of carbon dioxide to formic acid, formaldehyde and methanol in aqueous suspensions of TiO2 and Rh/TiO2, the effects of doping the TiO2 with W6+ were investigated.

The effect of the “Inert” support on relative photocatalytic activity in the oxidative decomposition of alcohols on irradiated titanium dioxide composites

Abstract: Reduced quantum efficiencies for the photocatalytic degradation of cyclohexanol, cyclododecanol, 2-hexanol, and benzyl alcohol are observed on TiO2 particles included within small pore zeolitic supports suspended in acetonitrile. A modest reduction in photocatalytic activity observed with TiO2 included within large pore zeolites and pillared clays is attributed to reflective and refractive losses of incident light. No appreciable substrate size selectivity could be observed in this series of supported TiO2 composites. TS-1, a titanium-containing zeolite with Ti in tetrahedral lattice sites, was photoactive, although ETS-10, a titanium-containing zeolite with Ti in octahedral lattice sites, was less so. The low quantum yield residual photoactivity observed with these alcoholic substrates on unmodified zeolites may derive from surface charge transfer complexation, as suggested by diffuse reflectance absorption measurements.

Photocatalytic Processes with Polyoxotungstates: Oxidation of Cyclohexylamine

Abstract: Photochemical excitation of (nBu 4 N) 4 W 10 O 32 in the presence of cyclohexylamine leads to the monoelectronic reduction of the decatungstate (O=8×10 -2 ) with the formation of cyclohexylamine radical species which can be detected by ESR spin-trapping investigations. Using this technique we could also reveal the presence of OH radicals coordinated to the polytungstate «surface». In the presence of oxygen, catalytic cycles are realized with thec formation of the following cyclohexylamine oxidation products

Laboratory- and pilot-plant-scale photodegradation of chloroaliphatics in aqueous solution by photocatalytic membranes immobilizing titanium dioxide☆

Abstract: The TiO2-medicated photodegradation of chloroaliphatics (dichloromethane, trichloroethene and mono-, di- and trichloroethanoic acids) was studied at 308±2 K (with the ratio between the hydrogen peroxide added and the stoichiometric amount (N) in the range 0–30), using PHOTOPERMTMCPP/313 membranes containing immobilized 30%±3% TiO2, at laboratory scale (radiant power in the absorption range, 145 W) and in a pilot plant (radiant power in the absorption range, 31 W). In addition to this semiconductor, some proprietary photocatalytic systems, including stabilized preparations containing CoIII, VV and FeIII organometallic compounds, were immobilized in the photocatalytic membranes. The initial rate of photodegradation was studied as a function of the initial concentration of the substrates (5.0×10-2–5.0×10-7M) using the linearized form of the Langmuir-Hinshelwood equation, which was well fitted by the membranes over the whole range of concentration, and from which the rate constants k and equilibrium adsorption constants K were evaluated. The contributions of processes (a)-(d) ((a) photolysis of micropollutant, independent of the presence of membranes and oxidizing agent; (b) photo- degradation due to UV and hydrogen peroxide; (c) photodegradation due to the semiconductor immobilized in the membrane; (d) the same as (c), but in the presence of a promoting photocatalytic system), which occur simultaneously in our experimental conditions, were measured. The contribution of process (d), particularly for a synergistic mixture of tri-(tert-butyl) and tri-(isopropyl) vanadate(V) or iron (III) potassium oxalate as photocatalysts, is greater than that of all other processes, while UV degradation in the presence of hydrogen peroxide (b) represents one-tenth or less of the whole. The rationalization of k values for this integrated membrane process is discussed on the basis of their dependence on the membrane surface and on the square root of the radiation intensity.