Showing papers on "Photocatalysis published in 1990"

Effects of common inorganic anions on rates of photocatalytic oxidation of organic carbon over illuminated titanium dioxide

Abstract: Effects of pH and common inorganic anions on the rates of photocatalytic oxidation of salicylic acid, aniline, and ethanol to CO{sub 2} over glass-supported near-UV illuminated TiO{sub 2} have been investigated by using conductivity detection of evolved CO{sub 2}. In the presence of chlorides and the absence of organics, lowering the pH of the solution below 3.6 caused an increase in conductivity with increasing circulation time. At pH 4.0 {plus minus} 0.2, the rate of oxidation decreased with increasing concentration of chlorides. Perchlorates and nitrates have very little effect. Sulfates or phosphates even at millimolar concentrations are rapidly adsorbed by the catalyst and reduce the rates of oxidation by 20-70%. The catalyst can be readily regenerated by washing with 0.1 M NaHCO{sub 3} solution. The data are considered in terms of kinetic salt effects, surface active sites having discrete adsorption properties, and Langmuir-Hinshelwood kinetic models. The implications for water purification processes based on photocatalytic oxidation using titanium dioxide are discussed.

Influence of the preparation methods of titanium dioxide on the photocatalytic degradation of phenol in aqueous dispersion

Abstract: Several commercial and homemade TiO{sub 2} samples were used as photocatalysts to prove that the physicochemical features as determined by the origin and preparation methods affect the photocatalytic behavior, in addition to the semiconducting properties. The photodegradation of aqueous phenol solution in TiO{sub 2} dispersion, carried out in a batch reactor, was used as a test reaction. The results show a great variability of photocatalytic behavior, and in particular it has been observed that the rutile phase is active or inactive according to the preparation conditions.

Titanium dioxide photosensitized reactions studied by diffuse reflectance flash photolysis in aqueous suspensions of TiO2 powder

Abstract: Diffuse reflectance and transmission flash photolysis have been used to study the primary reaction intermediates in titanium dioxide (TiO{sub 2}) photosensitized reactions of potassium iodide, 2,4,5-trichlorophenol, methyl viologen dichloride, potassium tetrachloroplatinum(II), tris(1,10-phenanthroline)-iron(II) perchlorate, N,N,N{prime},N{prime}-tetramethyl-p-phenylenediamine, and thianthrene. In the cases where the product of OH radical mediated oxidation is different from that of direct electron transfer oxidation, only products of direct electron transfer oxidation are observed. The products of direct electron transfer and OH radical mediated oxidation of iodide and tetrachloroplatinum are identical. The transient decay kinetics in TiO{sub 2} powder systems appear to be first order with a Gaussian distribution of reaction rate constants reflecting the distribution of particle radii.

Kinetics of the gas-solid heterogeneous photocatalytic oxidation of trichloroethylene by near UV illuminated titanium dioxide

Abstract: Kinetics of the gas/solid heterogeneous photocatalytic oxidation of dilute trichloroethylene (TCE) vapors by ultraviolet-illuminated titanium dioxide have been determined using a fixed-bed dynamic photoreactor. Reaction rate dependences on inlet TCE, oxygen and water vapor concentrations were found to consist of both reactant sensitive and insensitive regions. In the reactant sensitive regions, measured limiting apparent reaction rate orders for TCE, oxygen and water vapor are 0.8, 1.7 and — 3, respectively. Water vapor in the reactant stream lowersinitial reaction rates relative to corresponding water free conditions, but is required to sustain photocatalytic activity for extended periods of time.

Photocatalytic oxidation of phenol in the presence of hydrogen peroxide and titanium dioxide powders

Abstract: The effect of hydrogen peroxide on the photocatalytic oxidation of phenol on illuminated TiO 2 has been investigated The experimental results indicate that transition metal ions, such as Fe 3+ and Cu 2+ , affect the photocatalytic oxidation of phenol In the absence of added H 2 O 2 the ferric ions induce the occurrence of the photo-Fenton-type reaction so that the phenol removal of an initial 1000 mg 1 −1 solution is enhanced from 23 to 33% within 8 h However, the cupric ions show a negative effect In the presence of added H 2 O 2 , both the ferric and the cupric ions enhance the phenol oxidation rate drastically A 1000 mg 1 −1 phenol solution can be completely decomposed within 1 h and the total organic carbon removal reaches 80% A reaction mechanism which involves the generation of hydroxyl radicals is proposed

Novel visible-light-driven photocatalyst: poly(p-phenylene)-catalyzed photoreductions of water, carbonyl compounds, and olefins

Abstract: The insoluble yellow powder of poly(p-phenylene) (PPP) prepared by nickel-catalyzed polycondensation of the Grignard reagent from 1,4-dibromobenzene shows photocatalytic activity under visible light toward water, carbonyl compounds, and olefins. Water is photoreduced to H{sub 2} in the presence of amines as sacrificial electron donors. The H{sub 2} evolution is enhanced 3-20 times by noble-metal deposition, in which Ru deposition is the most effective.

Heterogeneous photocatalysis for water purification: Contaminant mineralization kinetics and elementary reactor analysis

Abstract: Heterogeneous photocatalysis is the third and newest of an emerging series of photolytic technologies for water treatment and purification. The earlier examples of ultraviolet ozonation and ultraviolet peroxidation involved direct photoactivation of the contaminant and/or dissolved oxidant and are dominated by solution phase chemistry. Photocatalysis, in contrast, involves photoactivation of the heterogeneous catalyst (an example of indirect photochemical processes) and is dominated by reaction events initiated at the catalyst surface. This paper summarizes (1) current rate equations for photocatalyzed simultaneous destruction of multiple oxidizable organic contaminants and (2) simplified convective transport equations appropriate to photoreactor design under various degrees of fluid mixing and simultaneous reaction contributions by heterogeneous (photocatalyzed) as well as homogeneous reactions.

Degradation of organic chemicals with titanium ceramic membranes

Abstract: Complex organic molecules, such as polychlorinated biphenyls can be degraded on porous titanium ceramic membranes by photocatalysis under ultraviolet light.

Photocatalytic activity of sodium hexatitanate, Na2Ti6O13, with a tunnel structure for decomposition of water

Abstract: Sodium hexatitanate, Na2Ti6O13, with a tunnel structure, combined with oxidized Ru, was found to be an efficient photocatalyst for the complete decomposition of water.

Photocatalytic treatment of water

Abstract: An ultraviolet driven photocatalytic post-treatment technique for the purification of waste water distillates, reverse osmosis permeates and spacecraft habitat atmospheric humidity condensates is described. Experimental results show that organic impurity carbon content of simulated reclamation waters at nominal 40 PPM level are reduced to, PPB using a recirculating batch reactor. The organic impurities common to reclaimed waste waters are completely oxidized employing minimum expendables (stoichiometric oxygen). This paper discusses test results and parameteric data obtained for design and fabrication of a bread-board system. The parametric testing includes UV light source evaluation, photolysis vs photocatalysis comparison, oxygen concentration dependence, temperature dependence, reactor mixing, disinfection features, photocatalyst loading, photocatalyst degradation studies and power consumption estimates. This novel post-treatment approach for waste water reclamation shows potential for integration with closed-loop life support systems.

Influence of the Density of Surface Hydroxyl Groups on TiO2 Photocatalytic Activities

Abstract: The photocatalytic activities of titanium dioxide for the reduction of silver ions and the oxidation of isopropanol were evaluated by using titanium dioxide with the various amount of surface hydroxyl groups per unit surface area. The photocatalytic activities increased with increasing concentration of the surface hydroxyl groups on the catalyst and then decreased from an inflection point. The mechanism was discussed from the stand point of the surface hydroxyl radical formation and their interaction.

Photo-oxidation of formic acid by oxygen in the presence of titanium dioxide and dissolved copper ions: oxygen transfer and reaction kinetics

Abstract: Photocatalytic activity of TiO2 in the oxidation by oxygen of formic acid is considerably enhanced by the presence of dissolved copper ions. The role of oxygen transfer between the gas phase and the slurry is examined. The kinetic results obtained in the absence of mass transfer limitations lead to a mechanism based upon a redox cycle between Cu2+ and Cu+ species adsorbed on the surface of TiO2.

Visible-light-induced two-electron-transfer photoreductions on cadmium sulfide: effects of morphology

Abstract: Freshly prepared CdS suspensions (CdS-O) consisting of quantized particles and their loose aggregation catalyze photoreductions of aromatic ketones and olefins in methanol under visible light irradiation using triethylamine as sacrificial electron donor, yielding alcohols and dihydro compounds, respectively, which are more selective than photocatalysis of commercially available crystalline CdS (Aldrich) (CdS-Ald)

Production of ultra fine SiC powder from SiC bulk by arc-plasma irradiation under different atmospheres and its application to photocatalysts

Abstract: The production of ultrafine SiC powder from a SiC bulk (containing 10wt% Si) was examined by the arc-plasma method under different atmospheres such as argon, argon and hydrogen, argon and helium, and argon and nitrogen. No melting of SiC bulk occurred during arcplasma irradiation, and even pure argon gas was effective for the production of the ultrafine powder. The SiC powder produced has the structure of β-SiC, compared to α-structure of the bulk SiC. The average particle size of the SiC powder produced decreased to one-half of the value when hydrogen or helium was added to argon. Smaller particles of SiC are formed by the arc-plasma gases with a large thermal conductivity. The present results confirm that sublimation is a major process in the production of ultrafine SiC powder. The SiC powder is found to be effective as a photocatalyst for the decomposition of water.

Electrochemical Investigation of Photocatalytic Oxidation of NO 2 − at TiO2 (Anatase) in the Presence of O 2

Abstract: Photocatalytic oxidation of was studied in connection with nitric acid formation in the environmental air. The oxidation of by proceeds in zero order reaction with respect to in aqueous (anatase) suspension under illumination. Electrochemical measurements with particle‐coated electrode revealed that the is oxidized in an electrochemical mechanism. The faradaic efficiencies of oxidation at catalyst‐coated electrode were less than 50%, reaching saturation at high concentrations under constant current conditions. did not promote oxidation of with and without electrode. The mechanism is discussed in terms of oxidation of by OH radical intermediately formed in anodic oxidation of at the catalyst surface under illumination.

Oxidation of 2-furoic acid via singlet oxygen generated photochemically

Abstract: UV irradiation of 2-furoic acid was carried out in the presence of TiO2 and hypochlorite ions (and methanol) and in the absence of oxygen. An endoperoxide compound was photogenerated; this endoperoxide underwent subsequent transformations. The significance of the introduction of oxygen into organic substrates using chlorinated TiO2 surfaces as photocatalysts is discussed.

Enhancement of processes for solar photocatalytic detoxification of water

Abstract: A solar-driven photocatalytic process is being developed to destroy low levels of toxic organics in water. Parabolic troughs with a glass pipe reactor and heliostats (large tracking mirrors) with a falling-film reactor were used to conduct engineering-scale solar detoxification of water experiments. We have assessed the effect of catalyst (titanium dioxide) loading and hydrogen peroxide concentration on the destruction of a model organic compound, salicylic acid. We found the optimal catalyst loading to be 0.1% for the conditions of 30 ppM salicylic acid and 300 ppM hydrogen peroxide. Hydrogen peroxide affected the reaction rates significantly, increasing the reaction rate over 4 times for stoichiometric amounts and more than 19 times for 10 times the stoichiometric amount. Destruction rates appear to be linearly proportional to the ultraviolet light intensity, though more data are needed to fully establish the relation. Initial tests with an actual water pollutant, trichloroethylene, demonstrated destruction from 1.2 ppM to less than 50 ppB in less than 5 minutes of exposure with a trough system. 15 refs., 6 figs.

Photocatalytic Reaction on Layered Cs–Nb–Ti Complex Oxide

Abstract: Ion-exchangeable layered titano-niobates, CsNbTi2O7 and CsNbTiO5, were prepared and the photocatalytic properties of those oxides as well as of the H+-exchanged forms were investigated. H2 or O2 evolution reaction from an aqueous methanol solution or from an aqueous silver nitrate solution was carried out under a high pressure mercury lamp irradiation. For H2 evolution, Pt (0.1 wt%)-loaded H+-exchanged forms showed remarkable activity both for CsNbTi2O7 and CsNbTiO5, while higher rates of O2 evolution were obtained for H+-exchanged forms than those for the original forms.

Dynamic Studies of Photocatalytic Oxidation of C3H6 WITH O2 On Vanadium Oxide Supported On Porous Vycor Glass

Abstract: The photocatalytic oxidation of C3H6 with O2 on highly dispersed supported vanadium oxides has been studied by dynamic photoluminescence, ESR and analysis of the reaction products These studies indicate that both O2 and C3H6 are activated through interaction with the charge transfer excited complexes of vanadyl species, (V4+—O−)* of the catalyst The reaction of these activated species results in the oxidation of C3H6 to produce CH3CHO, C2H5CHO, and CH2=CH—CHO

Photoinduced Electron-Transfer Reactions of Poly(4-trimethylsilylmethylstyrene) in the Presence of Polycyanobenzenes

Abstract: The photoreactions of poly(4-trimethylsilylmethylstyrene) (2) with o-, m-, and p-dicyanobenzenes in benzene-acetonitrile (3:2) afforded insoluble polymers in high yields. On the other hand, the photoreaction of 2 with 1,2,4,5-tetracyanobenzene gave a soluble polymer. In the latter photoreaction, the trimethylsilyl group of 2 was efficiently replaced by 2,4,5-tricyanophenyl group. The mechanistic features for the formation of insoluble polymers were also described.