Showing papers in "Journal of Photochemistry and Photobiology A-chemistry in 1998"

Influence of metallic silver and of platinum-silver bimetallic deposits on the photocatalytic activity of titania (anatase and rutile) in organic and aqueous media

Abstract: The influence of deposited silver upon the photocatalytic activity of titania in the rutile and anatase allotropic forms has been studied in three different reactions. The common feature found for these three reactions was an electron transfer from illuminated TiO 2 to silver particles. In the first reaction (platinum photodeposition). it has been shown that the initial presence of metallic silver orientates the localization of subsequent Pt photodeposits with, in particular, a 100% selectivity to Pt deposition on top of silver particles or agglomerates previously deposited on anatase. In the second reaction (2-propanol oxidation), Ag deposit was found beneficial for the activity of rutile and detrimental for that of anatase. For rutile which is less active, silver helps for the electron-hole pair dissociation. By contrast, for anatase, the negatively charged Ag particles preferentially attract photoholes and become recombination centers, thus decreasing the photocatalytic activity. For the third reaction (dehydrogenation of 2-propanol), Ag/TiO 2 catalysts were found very poorly active, as expected for a group I-B metal. However, in the additional presence of platinum deposits, Pt/Ag/TiO 2 catalysts are as active as their Pt/TiO 2 homologues, thus confirming that Pt is deposited on top of silver with good electronic contacts between both metals. The three photocatalytic reactions have common electronic processes based on the photoelectron generation on titania and the subsequent electron transfer to the metal (s).

Benzene and toluene gas-phase photocatalytic degradation over H2O and HCL pretreated TiO2: by-products and mechanisms

Abstract: Photocatalytic oxidations of toluene and benzene in air were carried out over water (TiO 2 /H 2 O) and HCl (TiO 2 /HCl) pretreated TiO 2 as this latter pretreatment enhances the toluene removal rate. The main purpose was to identify intermediate products. No gas-phase by-products were detected by direct GC/FID analysis under our conditions despite the high aromatic concentration (50 mg m −3 ) and the short contact time (ms); this result illustrates an attractive capability of this air purification method. Adsorbed intermediate products recovered by extraction from the used photocatalysts were the same over TiO 2 /H 2 O and TiO 2 /HCl; i.e. no chlorinated products were found in this latter case. Benzoic acid, benzaldehyde, and benzyl alcohol were three major toluene intermediate products identified; trace derivatives of these products and of toluene that were monohydroxylated on the ring were also detected in catalyst extract samples. The benzene major by-product was phenol which was accompanied by hydroquinone and 1,4-benzoquinone. Acetic and formic acids were also formed from both benzene and toluene. For both aromatics, a water extraction of the used photocatalysts permitted separation of a yellow viscous material that settled between water and TiO 2 after centrifugation. We have not identified the products contained in this material but believe them to be polymeric products which may be at the origin of the decreased photocatalytic activity of used TiO 2 with respect to fresh TiO 2 . Finally, we discuss the photocatalytic oxidation pathways.

An investigation on photocatalytic activities of mixed TiO2-rare earth oxides for the oxidation of acetone in air

Abstract: The photocatalytic activities of the mixtures of TiO2 (P25) with three rare earth oxides were investigated. The effects of the rare earth oxide contents and calcination temperature on the photocatalytic activities were studied. The results reveal that the mixtures of TiO2 with La2O3 (0.5 wt.%) or Y2O3 (0.5 wt.%) calcined at 700 °C or 650 °C exhibit higher photoactivity than pure TiO2 (P25) for the oxidation of acetone. On the other hand, the mixtures of TiO2 with CeO2 have lower photoactivity than pure TiO2. Experimental results of polycrystalline X-ray diffraction, photoexcited transient absorption decay and zeta potential measurements show that the presence of these rare earth oxides can inhibit anatase to rutile transformation at elevated temperatures. The lower photocatalytic activity of TiO2/CeO2 mixtures can be explained by the fast recombination rate of photogenerated electron-hole pairs and high isoelectric point in terms of pH value. The preparation method of these mixtures is also described.

Photocatalytic reduction of CO2 using surface-modified CdS photocatalysts in organic solvents

Abstract: The photocatalytic reduction of carbon dioxide on CdS particles with and without surface modification by several kinds of thiol compounds was investigated in various kinds of solvents. Formate and carbon monoxide were obtained as the major reduction products, and the ratio of the former to the latter was largely influenced by the kind of solvents when naked cadmium sulfide was used as the photocatalyst, and with increase of the dielectric constant of the solvent, the ratio increased. Even if CdS particles were modified by thiol compounds, the effect of the solvent was not eventually altered, but the ratio of formate to carbon monoxide became greater with increase of the surface modification of CdS. The observed effect of the surface modification of CdS is discussed in terms of involvement of Cd2 sites adjacent to surface vacancies of CdS photocatalysts in adsorption of anion radicals of CO2.

Photocatalytic reduction of high pressure carbon dioxide using TiO2 powders with a positive hole scavenger

Abstract: The photocatalytic reduction of high pressure CO2 using Ti02 powders with a positive hole scavenger has been reported. The Ti02 powders suspended in iso-propyl alcohol solution were irradiated with a Xe lamp. The main reduction product from CO2 by photocatalytic reduction was methane. Iso-propyl alcohol acted as the positive hole scavenger. Under the optimum experimental conditions, 1.3 μmol· (g-Ti)−1 of methane was obtained by the photochemical reduction of high pressure CO2, corresponding to 0.43 μmol · (g-Ti)−1 h−1 of the formation rate. The C02-reduction system developed may become of practical interest for the efficient C02-conversion and fixation system, storage of solar energy and the production of raw materials for the photochemical industry.

Intramolecular energy transfer mechanism between ligands in ternary rare earth complexes with aromatic carboxylic acids and 1,10-phenanthroline

Abstract: A series of binary and ternary rare earth (Gd, Eu, Tb) complexes with aromatic acids and 1,10-phenanthroline have been synthesized. The lowest triplet state energies of ligands have been obtained by measuring the phosphorescence spectra of binary gadolinium complexes. By comparing the phosphorescence spectra of binary complexes with those of ternary ones, it is found that there exists another intramolecular energy transfer process from the aromatic acids to 1,10-phenanthroline besides the intramolecular energy transfer process between the aromatic acids and the central rare earth ions. The intramolecular energy transfer efficiencies have been calculated by determining phosphorescence lifetimes of binary and ternary gadolinium complexes. The luminescence properties of corresponding europium and terbium complexes are in agreement with the prediction based on energy transfer mechanism.

Photocatalytic decomposition of triclopyr over aqueous semiconductor suspensions

Abstract: The photocatalytic degradation of 3,5,6-trichloro-2-pyridinyloxyacetic acid (Triclopyr), a pyridine analogue of the phenoxy herbicides, has been investigated in aqueous heterogeneous solutions containing TiO2 or ZnO as photocatalyst. The disappearance of the organic molecule follows approximately a pseudo-first kinetic order according to the Langmuir-Hinselwood model. As mineralization products, CO2 and Cl− ions have been identified, whereas the nitrogen atom forms only NH4+ ions. Various commercial photocatalysts were compared with respect to their overall efficiency, as well as the production of CO2. The effect of pH and H2O2 on the reaction rate was ascertained. The photooxidation of Triclopyr has also been monitored with FTIR spectroscopy.

Tautomerism of flavonol glucosides: relevance to plant UV protection and flower colour

Abstract: The fluorescence emission spectra of the naturally occurring flavonoids, quercetin-7-glucoside, a 3′,4′-dihydroxyflavonol, and kaempferol-7-glucoside, a 4′-hydroxyflavonol, have been determined as a function of concentration in aqueous solutions. These spectra indicate that the extent of keto–enol phototautomerism in both flavonoids is greatest at high concentrations: a situation which favours molecules aggregation/dimerization. Such behaviour is consistent with phototautomerism being facilitated by a concerted, intermolecular transfer of protons between the partners in the flavonoid dimer. This excited state tautomerism dissipates absorbed energy harmlessly and as such provides a possible mechanism by which these molecules may function in the protection of plants from damaging UV radiation. The fluorescence excitation spectra of both kaempferol and quercetin-7-glucosides at high concentrations in aqueous solutions indicate the presence of significant amounts of the enolic tautomeric form in the ground-state. At lower concentrations only the kaempferol glucoside spectrum shows this. When kaempferol- and quercetin-7-glucosides were deposited on a cellulosic support, their reflectance and fluorescence excitation spectra could be resolved into contributions from keto and enol ground-state chromophores. The absorption of the enolic tautomer is at longer wavelengths (ca. 450 nm) than that of the keto tautomer (ca. 370 nm) and as it extends into the blue spectral region, would account for the yellow appearance of these flavonols in aggregation on cellulose and in concentrated solution in petal vacuoles.

Photocatalytic degradation of organic dyes in aqueous solution with TiO2 nanoparticles immobilized on foamed polyethylene sheet

Abstract: Degussa TiO2 nanoparticles have been immobilized on a foamed polyethylene by thermal bonding to produce a stable catalyst sheet containing 07 mg TiO2 cm(-2) and retaining 40-50% of active surface area of the particles On such a catalyst sheet, exposed to radiation of a 125 W mercury vapour lamp, decomposition of about 03 mg of methylene blue (MB) is obtained per cm(2) in 1 h at ambient temperature from an aqueous solution of 200 ppm MB The rate data fit well to classical Langmuir-Hinshelwood (L-H) rate form This rate form also results from mechanisms based on the assumption of hydroxyl radical formation on the irradiated catalyst and a reaction between the hydroxyl radical and the organic dye molecule, either or none of them being adsorbed on the catalyst surface An activation energy of 145 kcal mol(-1) is obtained for the photocatalytic decomposition of methylene blue following the L-H rate laws

Photodegradation of Rhodamine B catalyzed by TiO2 thin films

Abstract: Transparent TiO2 thin films were prepared on glass substrates by a sol-gel method. They were confirmed to be anatase crystallites by X-ray diffraction (XRD) and Raman spectra analyses. The photocatalytic properties of the sol-gel derived films were assessed by measuring the photodegradative oxidation of Rhodamine A in aqueous solution and compared with Degussa P-25 coated films. These SG-TiO2 films showed much higher photoactivity than P-25 coated films at the initial time of the reaction. Results of this study also indicate that the photodegradative mechanisms assisted by the two kinds of films are obviously different: highly efficient and fast N-de-ethylation accompanied the degradation of Rhodamine B catalyzed by a SG-TiO2 thin film, whereas only degradative reaction took place when P-25 coated film existed.

Photomineralisation of 4-chlorophenol sensitised by TiO2 thin films

Abstract: The results of a study of the oxidative mineralisation of 4-CP by oxygen, sensitised by thin films of Degussa P25 TiO 2 , are reported. The films are used under conditions in which the kinetics of photomineralisation are independent of mass transfer effects and stable towards repeated irradiation. Using a TiO 2 film, the process goes through the same mechanism as a TiO 2 dispersion, generating the same intermediates, namely: 4-chlorocatechol and hydroquinone. The kinetics of photomineralisation show clear differences between a TiO 2 film and a dispersion. With TiO 2 films the initial rate of photomineralisation is strongly dependent upon photocatalyst loading, (units; g dm −3 ) reaching a distinct maximum, which appears to be associated with the formation of a monolayer of aggregated particles – the diameter of the aggregated particles is estimated as 0.44 μm. A simple 2D model is presented to help illustrate the features of such a system. With TiO 2 dispersions the rate usually reaches a plateau at ca. 0.5 g dm −3 of TiO 2 . For TiO 2 films the initial rate depends directly upon the incident light intensity, implying that the photocatalytically active particles are under low illumination conditions, partially shielded by the other particles making up each aggregated particle. In contrast, with TiO 2 dispersions R i depends upon I 0.64 , implying that the different light intensities used spanned both the high ( R i ∝I 1/2 ) and low ( R i ∝I ) intensity kinetic regions. The kinetics of photomineralisation of 4-CP, sensitised by TiO 2 films obey the same Langmuir–Hinshelwood expressions as found in most semiconductor photocatalyst work conducted with TiO 2 dispersions. However, in a study of the variation R i as a function of [4-CP] and [O 2 ] the values for the maximum rates were larger, and those for the apparent Langmuir adsorption coefficients were smaller, than those found for TiO 2 dispersions.

Surface catalyzed electron transfer from polycyclic aromatic hydrocarbons (PAH) to methyl viologen dication: evidence for ground-state charge transfer complex formation on silica gel

Abstract: Porous silica surfaces are shown to slowly catalyze the oxidation of adsorbed polycyclic aromatic hydrocarbons (PAH) to the corresponding radical cation via Lewis acid sites present on the surface. When a good electron acceptor such as methyl viologen dication (MV ++ ) is co-adsorbed on silica surface, a red-shifted structureless absorption band characteristic of a ground-state charge transfer (CT) complex formed between the PAH and MV ++ is observed. Oxygen efficiently competes with MV ++ for the trapped electrons on the active sites of silica surface causing a significant decrease in the concentration of ground-state CT complex. The rate of this electron transfer process is enhanced dramatically at the solid/liquid interface when solution of PAH in a non-polar solvent is added to dry silica containing adsorbed MV ++ . Room temperature electron paramagnetic resonance (EPR) spectra of PAHs adsorbed on silica show a broad unresolved signal ( g =2.0031–2.0045) due to PAH ⋅+ radical cation which disappears in the presence of air but can be restored upon evacuation of the sample. The EPR measurements of mixed samples containing PAH and MV ++ co-adsorbed on silica show a composite signal with hyperfine structure that may be due to presence of two paramagnetic species corresponding to MV ⋅+ and possibly PAH radical cation.

Photocatalytic reduction of nitrate ions on TiO2 by oxalic acid

Abstract: Using oxalic acid as a hole scavenger, the photocatalytic reduction of nitrate in aqueous dispersions of TiO 2 has been investigated. Oxalic acid accelerates the reaction considerably compared to the reaction in the absence of oxalic acid. In the presence of nitrate, the oxidation of oxalate is also accelerated. The reduction product of the nitrate is mainly ammonia. The effects of concentrations of nitrate, of oxalate, and of pH were studied. The adsorption of the reactants on TiO 2 is an important factor for accelerating the reaction.

Photocatalytic activity and reaction mechanism of Pt-intercalated K4Nb6O17 catalyst on the water splitting in carbonate salt aqueous solution

Abstract: Both the structure of Pt-intercalated K4Nb6O7 catalyst and the photocatalytic activity on the water splitting in aqueous carbonate salt solution were studied using various kinds of analytical measurements such as FT-IR, EXAFS, TEM, SEM and TPR. K4Nb6O17 was the layered semiconductor which have two kinds of interlayer spaces (interlayers I and II), and it was found by FT-IR that the [ Pt(NH3)4]2+ cation was preferentially intercalated only into the interlayer I of K4Nb6O17. Fine Pt metal particles less than 6 A (1 A = 0.l nm) were prepared homogeneously in the interlayer space I by the photoreduction method. By H2 reduction, some Pt particles grew more than 15 A and contributed to the destruction of the layered sheet. The photocatalytic water splitting reaction occurred by the addition of carbonate salt in spite of the exposure of Pt on the outer surface of catalyst. The photocatalytic activity of the Pt/K4Nb6O17 prepared by the photcreduction was higher than that prepared by the H2 reduction. The deactivation of the catalyst was not observed in the carbonate aqueous solution for long time. The reaction mechanism that the Pt panicles in the interlayer I acted as H2 evolution site and the O2 evolved in the interlayer II was proposed.

The photo-oxidative degradation of sodium dodecyl sulphate in aerated aqueous TiO2 suspension

Abstract: This paper reports the photocatalytic degradation of sodium dodecyl sulphate (SDS) in a bubble column reactor containing suspended irradiated (λ = 250–310 nm) titania particles. The effects of six major process variables were investigated. Reaction rate increased linearly with light intensity and the variation in temperature revealed an activation energy of about 41 kJ mol−1. Although an increase in solution pH enhanced the hydrolysis of SDS and hence an increase in free dodecyl sulphate anion for adsorption, a downturn in rate was observed beyond pH = 7.5 probably due to reduced surface concentration of the dodecyl sulphate species on the predominantly negatively charged titania surface at pH values higher than the isoelectric point, IEP = 6. The optimum seen in the rate vs catalyst loading curve was also attributed to the compromise between increase in number of sites (particles) at higher catalyst concentration and the occurrence of light scattering and hence, reduced light-harvesting efficiency at these higher values. The data on the role of SDS concentration and oxygen partial pressure similarly suggest nonlinear dependency of rate on these reactants. The empirical models derived were consistent with the proposition of a dual-site (photogenerated positive and electron-rich sites) Langmuir–Hinshelwood mechanism where the rate-controlling step is the surface interaction between the adsorbed dodecyl sulphate and peroxy radicals. Highly reactive peroxy radicals were presumably formed from the protonation of surface superoxide anion, O2−, produced via adsorption of oxygen on the electron-rich site.

Adsorption and photo-induced reduction of Cr(VI) ion in Cr(VI)-4CP (4-chlorophenol) aqueous system in the presence of TiO2 as photocatalyst

Abstract: Dark adsorption of Cr(VI) ion on TiO2 is mainly dependent on Cr(VI) concentration and the system acidity. 4-Chlorophenol (4CP) has no interference on Cr(VI) adsorption. Cr(VI) can easily be photocatalytically reduced under UV irradiation with TiO2 as catalyst. The reduction is in accord with the Langmuir-Hinshelwood (L-H) kinetic equation. Under the UV irradiation, photo-induced Cr(VI) reduction is observed in either Cr(VI)-4CP or Cr(VI)-4CP-TiO2 system. Light intensity and system acidity are two main roles influencing both reactions. The linear decrease in Cr(VI) concentration reveals the zero order of Cr(VI) reduction rate in both systems. Two different reactions are involved in Cr(VI)-4CP-TiO2 system under the experimental UV irradiation: the photo-induced homogeneous reaction between Cr(VI) and 4CP, and the photocatalytic reaction on TiO2 semiconductor photocatalyst. In the condition of excluding homogeneous reactions between Cr(VI) and 4CP, the promotive effect of 4CP on photocatalytic reduction of Cr(VI) in Cr(VI)-4CP-TiO2 system is distinguished and confirmed. Under the sunlight irradiation, no reaction is observed in Cr(VI)-4CP system. Photocatalytic reduction efficiency of Cr(VI) ion on TiO2 in Cr(VI)-4CP-TiO2 system under the sunlight irradiation is much larger than that in Cr(VI)-TiO2 system.

Photocatalytic oxidation of water on TiO2-coated WO3 particles by visible light using Iron(III) ions as electron acceptor

Abstract: Photocatalytic oxidation of water on TiO2-coated WO3 particles was studied using iron(III) ions as the electron acceptor with the aim of constructing a photochemical energy conversion system. Although WO3 photocatalysts can utilize part of visible light, the reaction was decelerated as the concentration of iron(II) ions in solution increased. This was a marked contrast with the reaction using TiO2 photocatalysts, whose photocatalytic activity is scarcely affected by iron(II) ions in solution. In order to modify the surface of WO3 particles, they were coated with a thin TiO2 layer. Using such photocatalysts, the harmful effect by iron(II) ions on the WO3 photocatalyst was restrained to some extent, and the efficiency of photooxidation of water by visible light was improved.

Photocatalytic decomposition of DMMP on titania

Abstract: The photocatalytic decomposition of gas-phase dimethyl methylphosphonate (DMMP) on TiO 2 -coated glass substrate was studied using a single-pass reactor. The disappearance of DMMP and the appearance of products were monitored for several DMMP concentrations between 1 and 90 ppmv in air. Intrinsic reaction rates were determined, products were identified and reaction chemistry has been postulated. The main reaction products identified were carbon dioxide and carbon monoxide in the gas-phase, and methylphosphonic acid and phosphate on the catalyst. It was found that due to build-up of surface phosphorus-containing species, the catalyst de-activated relatively rapidly. One of the interesting results of this study is that the catalyst activity was completely recovered by washing with water and partially recovered by exposure to UV (UVA; ∼ 352 nm) light.

A fluorescent sensor for Cu2+ at the sub-ppm level

Abstract: Two 4-(aminoalkyl)aminonaphthalimide compounds which exhibit fluorescence enhancement and associated spectral changes in the presence of Cu2+, Mn2+ and Ni2+ are reported. The compounds are believed to undergo intramolecular quenching by the distal nitrogen on the 4-substituent. When metal ions are present, this quenching mechanism is partially or wholly removed due to complexation of the metal ions with the 4-substituent. The two compounds are particularly sensitive to Cu2+ such that concentrations in the 0–1 ppm range are sufficient to effect complete restoration of the naphthalimide fluorescence.

Photocatalytic degradation of chlorobenzoic isomers in aqueous suspensions of neat and modified titania

Abstract: The three isomers of chlorobenzoic acid (CBA) have been degraded using photocatalysis. The three pollutants disappeared from water in the following order: 3-CBA < 2-CBA < 4-CBA, the para position being the most reactive. The zeroth kinetic order was interpreted by a Langmuir-Hinshelwood mechanism involving a saturation of the adsorption sites. The chemisorption of CBA molecules involves the carboxylic group linked to the surface with the aromatic ring possibly oriented perpendicularly to the surface. The initial step of CBA disappearance was the decarboxylation of the molecule (photo-Kolbe reaction) with an initial rate of CO2 formation equal to that of the disappearance of CBA. The mass balance could be established for all the final products. All the intermediate products found correspond to successive hydroxylations of the corresponding chlorophenol obtained after the initial photo-Kolbe reaction. They all disappeared within less than 2 h of UV irradiation. In 2-CBA degradation, a transient condensation product (4′-chloro,3,4-dihydroxy-(1,1′)biphenyl) could be identified before its degradation. Modification of titania by doping with Cr3+ ions or by depositing 1 wt.% Pt was detrimental for the activity, the best catalyst remaining neat titania. The present study illustrates the difference in the kinetics of photodegradation of various isomers for one compound.

Absorption and electrochemical properties of ruthenium(II) dyes, studied by semiempirical quantum chemical calculations

Abstract: The electronic properties of Ru-polypyridine dyes were investigated by means of semiempirical calculations. Intermediate neglect of differential overlap (INDO)/S including configuration interaction (CI) was used to calculate electronic density of state (D

Sensitization of nano-porous films of TiO2 with santalin (red sandalwood pigment) and construction of dye-sensitized solid-state photovoltaic cells

Abstract: Santalin, a pigment extracted from red sandalwood strongly surface complex to TiO2, sensitizing nano-porous film of TiO2 to the visible spectrum. Because of the stability of such films, dye-sensitized solid state photovoltaic cells of sandwich structure (nano-porous n-TiO2/santalin/p-type semiconductor) can be constructed by depositing p-CuI or p-CuCNS on santalin coated nano-porous films of TiO2. High stability of santalin complexed to TiO2 enables deposition of the p-type semiconductor without destroying the pigment layer. Light absorbed by the dye-molecules injects electrons into TiO2 and holes into CuI (or CuCNS) generating short-circuit photocurrent ∼6 mA/cm2 and photovoltage ∼450 mV at 800 W/m2 simulated sunlight.

Transient absorption of symmetrical carbocyanines

Abstract: Transient absorption in several symmetrical cyanine and carbncyanine dyes is measured by time-resolved spectroscopy with subpicosecond white-light continuum in a wide spectral range (0.35–1 μm). The results are compared with data available in the literature on ultrashort and long-lived transients. The linear shift law observed here for the S1Sn absorption bands in the wavelength scale allows one to predict the main induced absorption bands for members of several carbocyanine families (vinylogous series) which have not yet been studied. In the case of the shortest methine chains (cyanines and monocarbocyanines) a tew transient absorption bands are assigned to the formation of photoisomers. The absolute values of the excited-state or photoisomer cross sections of polymethine carbocyanines are evaluated by global spectral analysis, from the measured differential optical density spectra. Several cases of large values (higher than 10) of the absorption cross section ratio (excited/ground state) are found at new wavelengths, which allows applications of photo-induced absorption to photonics and laser technologies over a broad spectral range.

Photocatalytic oxidation of 2,4-dichlorophenoxyacetic acid with titania photocatalyst. Comparison of supported and suspended TiO2

Abstract: The photodegradation of 2,4-dichlorophenoxyacetic acid using illuminated titania particles was studied in buffered water solutions at pH 9.2. A simplified phenomenological model which enables comparison of activity of suspended and supported photocatalyst on the basis of utilisation of light is proposed. The model shows saturation of photodegradation rate with increase of amount of photocatalyst on support or in suspension. The features of the model were verified by studies of the photocatalytic activity of TiO2 suspended and supported on flat glass plate and on floating particles.

Spectrokinetic study of 2,2-diphenyl-5,6-benzo(2H)chromene: a thermoreversible and photoreversible photochromic system

Abstract: In this paper the photochromic reaction of the 2,2-diphenyl-5,6-benzo(2H)chromene is investigated by spectrophotometric methods using monochromatic irradiation wavelengths in the UV and visible ranges. Two coloured species are produced upon irradiation, one is thermoreversible, the other is photoreversible. The colour-forming kinetics were followed under steady irradiation using UV light (357 nm). The kinetic parameters (rate coefficient and activation energy) of the thermal ring-closure reaction were determined. The photochemical bleaching was induced by exciting with visible light (422 nm). To determine reaction quantum yields and molar absorption coefficients of the metastable species, both photostationary and photokinetic methods were employed. Based on the results obtained, a plausible reaction mechanism is proposed

Oxidative photocatalytic degradation of benzene vapor over TiO2

Abstract: The variables included in a parametric study of the photocatalytic oxidative degradation (PCO) of benzene in a closed reactor over films of Degussa P-25 TiO2 under fluorescent black-light radiation centered at 367 nm were: concentrations of benzene, O2 and water vapor; photons/sec incident on the catalyst; and temperature. Concentrations of benzene and CO2 were measured. All measurements of photocatalytic behavior were performed with freshly deposited films of P-25 which had been preconditioned by irradiating for 15 h while flowing 80 ml min−1 of dry air through the reactor. It was found that water vapor promoted the dark-evolution of CO2 and inhibited the dark-adsorption of benzene by the preconditioned catalyst. Under irradiation, water vapor promoted the formation of CO2 and the initial removal of benzene. Dark-adsorption of benzene increased linearly with concentration of benzene vapor. The initial rate of photocatalytic removal of benzene was first order in concentration of benzene vapor but the higher the initial concentration of benzene the more the rate of its removal diminished as reaction proceeded. Photocatalytic production of CO2 lagged far behind removal of benzene. The initial rates of photocatalytic removal of benzene vapor and generation of CO2 were, respectively, independent of concentration of O2 from 800 ppmv to 100% and approximately one-third order in O2. The initial kinetic orders in number of photons/sec entering the reactor of removal of vaporized benzene and generation of CO, over the range (7–42) × 10−8 Einstein s−1 were, respectively, 1.85 ± .03 and 1.60 ± .07 in air into which 0.44 mmol of water had been injected and 1.59 ± .07 and 1.41 ± .05 in dry air. Over the range 15–70 °C, the rate of removal of benzene did not vary significantly with temperature in the presence or absence of water vapor while the rate of generation of CO2 varied in a way corresponding, respectively, to EA = 3.9 ± 0.4 and 9.4 ± 0.9 kJ mol −1 in the presence and absence of water vapor. Mechanistic implications of the data are discussed.

Photodegradation of parathion in aqueous titanium dioxide and zero valent iron solutions in the presence of hydrogen peroxide

Abstract: The photocatalytic degradation of parathion using UV radiation in combination with hydrogen peroxide and titanium dioxide or iron compounds was studied. Apparent first-order rate constants (kobs) and initial rate constants were used to describe the degradation behavior of parathion under different conditions. The addition of TiO2 or iron compounds in combination with hydrogen peroxide was effective in the detoxification of parathon. The kobs and initial rate constants increased with the increase of TiO2 loading from 0.1 g l−1 to 1 g l−1. However, the degradation rates of parathion decreased when the TiO2 dose was increased to 2 g l−1. No significant difference between buffered and unbuffered solutions was observed. The addition of oxyanion oxidant can enhance the degradation rate of parathion in the order of ClO2− > IO3− > BrO3− > ClO3−. The existence of oxygen increased the initial rate of parathion in a TiO2-amended system, but has little effect on kobs. Also, the addition of iron compounds is a promising technique for the photodegradation of parathion. The rate constants for parathion ranged from 0.029 to 0.033 min−1 in the iron-amended system, which corresponds to 3-fold increase relative to UV/H2O2 system. Diethylphosphoric acid, p-nitrophenol, diethylmonothiophosphoric acid, O,O-ethyl p-nitrophenyl monothiophosphoric acid and oxalate were identified as intermediates and shown to be oxidized further. Moreover, oxalate and 4-nitrophenol were found to be the major intermediates in the degradation of parathion.

Photophysical process of MEH-PPV solution

Abstract: The solvent, temperature and concentration-dependent fluorescences of MEH-PPV solutions were examined. The aliphatic and aromatic solvents have different effects on the emission spectra of MEH-PPV solutions because of the different interaction mechanisms between MEH-PPV and different solvents at excited states. With the increase of the polarity of the solvents, the emission spectra show blue and red shift at 77 K and room temperature, respectively. With the decrease of the temperature, the emission spectra of MEH-PPV in xylene show red-shift and the fluorescence intensities increase. The concentration-dependent emission spectra remarkably changed with the increase of the concentration because of the existence of the aggregate states and the interchain interactions in concentrated solutions.

Photophysical studies on the interaction of two water-soluble porphyrins with bovine serum albumin. Effects upon the porphyrin triplet state characteristics

Abstract: The interaction of two metal-free water soluble porphyrins (PPh): meso -tetrakis ( p -sulfo-natophenyl)porphyrin (TPPS 4 ) and meso -tetrakis(4- N -methyl-pyridiniumyl)porphyrin (TMPyP), with bovine serum albumin (BSA) was investigated in the pH range from 4.0 to 8.5 using the flash-photolysis technique in comparison with results obtained by optical absorption, fluorescence and resonance light scattering. It was found that in the presence of BSA, TPPS 4 can exist in aqueous solutions as free monomers, aggregates and/or monomers bound to a single BSA molecule and monomers inside the BSA aggregates, while TMPyP does not form aggregates at binding. Binding to BSA transforms the profile of the triplet decay curve from monoexponential to biexponential form with the component lifetimes and relative amplitudes depending on binding characteristics. The triplet lifetime of a bound porphyrin monomer is longer than that of a free one. The aggregation of TPPS 4 observed at [TPPS 4 ]/[BSA] > I reduces the T-T absorption since the lifetimes of the aggregate excited states are very short and/or quantum yield of the aggregate triplet state is very low. The porphyrin binding to BSA reduces the quenching constants of the porphyrin triplet states by molecular oxygen due to obstacles produced by binding. This effect is especially pronounced for the porphyrin molecules located inside BSA aggregates formed around the porphyrin molecules in excess BSA.

Factors affecting the photoelectrochemical fixation of carbon dioxide with semiconductor colloids

Abstract: Carbon dioxide was reduced photocatalytically using aqueous CdS or ZnS colloids containing tetramethylammonium chloride to give the dimeric and tetrameric products namely, oxalate, glyoxylate, glycolate and tartrate. A model is presented to explain the role of the tetramethylammonium ions. Studies were also performed using ZnO, SiC, BaTiO 3 and Sr TiO 3 , which in the absence of tetramethylammonium ions produced formate and formaldehyde. The relative quantum efficiencies of the six semiconductors were related to their band gaps and conduction band potentials. The role and effectiveness of several `hole acceptor' (electron donor) compounds in this process is shown to be related to their redox potentials.