Photodecomposition Rate Research Articles

Laboratory insights into the chemical and kinetic evolution of several organic molecules under simulated Mars surface UV radiation conditions

The search for organic carbon at the surface of Mars, as clues of past habitability or remnants of life, is a major science goal of Mars’ exploration. Understanding the chemical evolution of organic molecules under current martian environmental conditions is essential to support the analyses performed in situ. What molecule can be preserved? What is the timescale of organic evolution at the surface? This paper presents the results of laboratory investigations dedicated to monitor the evolution of organic molecules when submitted to simulated Mars surface ultraviolet radiation (190–400nm), mean temperature (218±2K) and pressure (6±1mbar) conditions. Experiments are done with the MOMIE simulation setup (for Mars Organic Molecules Irradiation and Evolution) allowing both a qualitative and quantitative characterization of the evolution the tested molecules undergo (Poch, O. et al. [2013]. Planet. Space Sci. 85, 188–197). The chemical structures of the solid products and the kinetic parameters of the photoreaction (photolysis rate, half-life and quantum efficiency of photodecomposition) are determined for glycine, urea, adenine and chrysene. Mellitic trianhydride is also studied in order to complete a previous study done with mellitic acid (Stalport, F., Coll, P., Szopa, C., Raulin, F. [2009]. Astrobiology 9, 543–549), by studying the evolution of mellitic trianhydride. The results show that solid layers of the studied molecules have half-lives of 10–103h at the surface of Mars, when exposed directly to martian UV radiation. However, organic layers having aromatic moieties and reactive chemical groups, as adenine and mellitic acid, lead to the formation of photoresistant solid residues, probably of macromolecular nature, which could exhibit a longer photostability. Such solid organic layers are found in micrometeorites or could have been formed endogenously on Mars. Finally, the quantum efficiencies of photodecomposition at wavelengths from 200 to 250nm, determined for each of the studied molecules, range from 10−2 to 10−6moleculephoton−1 and apply for isolated molecules exposed at the surface of Mars. These kinetic parameters provide essential inputs for numerical modeling of the evolution of Mars’ current reservoir of organic molecules. Organic molecules adsorbed on martian minerals may have different kinetic parameters and lead to different endproducts. The present study paves the way for the interpretation of more complex simulation experiments where organics will be mixed with martian mineral analogs.

Low temperature growth of ZnO nanowire arrays with enhanced high performance photocatalytic activity and reusability

Aligned ZnO nanowire arrays have been grown on the silicon substrates with ZnO seed layer by an aqueous chemical growth process at low reaction temperature. The volumes of ammonium hydroxide solution are not only controlled by the dimension and density of ZnO nanowires, but are also influenced by their optical emission property. The ZnO nanowire arrays exhibit very strong and broad green emission from defect in the cathodoluminescence spectrum. In addition, the ZnO nanowires grown on the silicon substrates can serve as effective and convenient recyclable photocatalysts. Only a very slight decrease in the photodecomposition rate was observed after ten cycles of the photocatalysis experiment.

Study on synthesis and photochemical behavior of carbamate photobase generators for fluorescence imaging

ABSTRACTA fluorescence imaging system is designed based on the photodecomposition of photobase generators embedded in polyvinyl butyral (PVB) matrix. These photobase generators were synthesized with propyl and phenethyl units served as leaving groups, and phthalimido, succinimido, di‐2‐thienyl ketoximino, and benzophenoneoximino units acted as photosensitive species, respectively. The influences of the two leaving groups and four photosensitive species on the photochemical behavior were investigated in detail. The results demonstrate that the photochemical properties of these photobase generators depend on the photosensitive species other than the leaving groups. And further study shows that succinimido propylcarbamate (SPrC) exhibits the best thermal stability and relatively quick rate of photodecomposition among all the photobase generators. Therefore, a PVB film embedded with SPrC is used as a fluorescence imaging material. The patterned fluorescence image obtained from the fluorescamine‐treated irradiated PVB film showed bright green fluorescence, which provides potential application for optical storage or color electronic displays. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 41058.

Synergistic photocatalytic activity of LnFeO3 (Ln=Pr, Y) perovskites under visible-light illumination

Uniform LnFeO3 (Ln=Pr, Y) nano-/submicro-particles were prepared by a sol–gel method with the assistance of glycol at 800°C for 4h. The as-synthesized LnFeO3 was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and UV–vis diffuse reflectance spectra. The photocatalytic behaviors of LnFeO3 nanoparticles were evaluated by the photodegradation of rhodamine-B under visible light irradiation. The results indicate that the as-prepared PrFeO3 nanoparticles exhibit remarkable visible-light photocatalytic activity through the synergism of semiconductor-photocatalyzed oxidation and heterogeneous Fenton-like reaction. Compared with the semiconductor photocatalysis, the photodecomposition rate of RhB was improved about 5 times when the simultaneous presence of the nanoparticles and H2O2, and a possible degradation mechanism was proposed.

Photodecomposition of Chloroform Catalyzed by Unmodified MCM‐41 Mesoporous Silica

Unactivated MCM-41 mesoporous silica catalyzes the photodecomposition of chloroform to phosgene and hydrogen chloride under near-UV (λ > 360 nm) irradiation. The rate of photodecomposition increases toward an asymptotic limit as the O(2) partial pressure is increased. Deuterochloroform does not decompose under the same experimental conditions. Low concentrations of both cyclohexane and ethanol quench the photodecomposition, whereas water, up to its solubility limit, does not. Dissolved tetraalkylammonium salts suppress photodecomposition. The data are consistent with a mechanism in which light absorption by an SiO(2) defect yields an electron-deficient oxygen atom, which then abstracts hydrogen from chloroform. The resulting CCl(3) radicals react with oxygen to form a peroxy radical that decomposes, eventually yielding phosgene and hydrogen chloride.

Efficiency of Cu/TiO2 to remove salicylic acid by photocatalytic decomposition: kinetic modelling

Titanium dioxide has waste applications as photocatalyst, as antimicrobial material and in biomedical field. By modifying the surface of TiO2 with noble metals its photoactivity will be enhanced. This paper aims to explain the role of the copper particles deposited on titania surface during the photodecomposition of salicylic acid and to provide evidence about which kinetic model describes best the photocatalysis. The research explains why the photodecomposition rate of salicylic acid is higher in Cu/TiO2 composite as compared to in TiO2. It consists in kinetic approach of the photocatalytic behaviour of Cu–TiO2 and TiO2, thus being possible to discuss the photodecomposition mechanism. This research will allow improvement of the techniques to remove salicylic acid from synthetic solution. This could be extended to other organic pollutants.

Efficient Photocatalytic Degradation of Rhodamine B Dye by Aligned Arrays of Self‐Assembled Hydrogen Titanate Nanotubes

We show that an aligned array of hydrothermally grown, multiwalled hydrogen titanate (H2Ti3O7) nanotubes—anchored to both faces of a metallic Ti foil—acts as an efficient photocatalyst. We studied the degradation of rhodamine B dye in the presence of the nanostructured photocatalyst under UV irradiation, by monitoring the optical absorption of the dye. Rhodamine B was chosen as a representative—and particularly harmful—industrial pollutant dye. The inner and outer diameters of the H2Ti3O7 nanotubes were 5 nm and 10 nm, respectively. The nanotube array catalyst is recyclable and structurally stable. Most importantly, it shows comparable or higher photodecomposition rate constant than those of both H2Ti3O7 nanotube powder and P‐25 (Degussa). The enhanced photocatalytic performance may be ascribed to the nanotube array having a superhydrophilic surface with a high accessible surface area.

Criteria for selecting fluorescent dye tracers for soil hydrological applications using Uranine as an example

Abstract Calibrating and verifying 2-D and 3-D vadose zone flow and transport models requires detailed information on water and solute redistribution. Among the different water flow and mass transfer determination methods, staining tracers have the best spatial resolution allowing visualization and quantification of fluid flow including preferential flow paths. Staining techniques have been used successfully for several decades; however, the hydrological community is still searching for an “ideal” vadose zone tracer regarding flow path visualization. To date, most research using staining dyes is carried out with Brilliant Blue FCF. Fluorescent dyes such as Uranine, however, have significant advantages over nonfluorescents which makes them a promising alternative. This paper presents the first analysis of key properties any fluorescent substance must possess to qualify as a staining fluorescent tracer in vadose zone hydrological applications. First, we summarize the main physico-chemical properties of Uranine and evaluate its staining tracer potential with conventional suitability indicators and visibility testing in a soil profile. Based on numerical analysis using the theory of fluorescence, we show that a low molar absorption coefficient is a crucial parameter to quantify concentration accurately. In addition, excitation of a tracer on wavelengths different from the maximum excitation wavelength can extend the linear range of the concentration-fluorescence relationship significantly. Finally, we develop criteria for evaluating the suitability of any potential fluorescent soil staining compound for soil hydrological applications: 1) high quantum yield, 2) low molar absorption coefficient, 3) fluorescence independent of temperature, 4) low photodecomposition rates, and 5) fluorescence stable across a wide range of pH values.

Hydrolysis and Photolysis of Herbicide Clomazone in Aqueous Solutions and Natural Water Under Abiotic Conditions

The hydrolysis and photolysis of clomazone in aqueous solutions and natural water were assessed under natural and controlled conditions. Kinetics of hydrolysis and photolysis of clomazone were determined by HPLC-DAD. Photoproducts were identified by HPLC-MS. No noticeable hydrolysis occurred in aqueous buffer solutions ((25±2)°C, pH (4.5±0.1), pH (7.4±0.1), pH (9.0±0.1); (50±2)°C, pH (4.5±0.1), pH (7.4±0.1)) or in natural water up to 90 d. At pH (9.0±0.1) and (50±2)°C the half-life of clomazone was 50.2 d. Clomazone photodecomposition rate in aqueous solutions under UV radiation and natural sunlight followed first-order kinetics. Degradation rates were faster under UV light (half-life of 51–59 min) compared to sunlight (half-life of 87–136 d). Under UV light, four major photoproducts were detected and tentatively identified according to HPLC-MS spectral information such as 2-chlorobenzamide, N-hydroxy-(2-benzyl)-2-methylpropan-amide, 2-[2-phenol]-4,4-dimethyl-3-isoxazolidinone and 2-[(4,6-dihydroxyl-2-chlorine phenol)]-4,4-dimethyl-3-isoxazolidinone. These results suggested that clomazone photodegradation proceeds via several reaction pathways: 1) dehalogenation; 2) substitution of chlorine group by hydroxyl; 3) cleavage of the side chain. Photosensitizers, such as H2O2 and riboflavin, could enhance photolysis of clomazone in natural sunlight. In summary, we found that photoreaction is an important dissipation pathway of clomazone in natural water systems.

Photocatalytic Decomposition of Methyl Formate over TiO2-Supported Pt Metals

The photoinduced vapor-phase decomposition of methyl formate was investigated on pure and on Pt-metals-promoted TiO2. Infrared spectroscopic studies revealed that illumination induced the dissociation of molecularly adsorbed methyl formate to formate on pure TiO2 even at ∼186 K. This process is indicated by the appearance of the absorption band at 1580–1590 cm–1 due to asymmetric stretch of formate species. The extent of the dissociation is increased with the time of irradiation. Deposition of Pt metals on TiO2 only slightly influenced this process. The photocatalytic decomposition of methyl formate vapor on pure TiO2 occurred to only a limited extent at 300 K, but the deposition of Pt metals on TiO2 appreciably enhanced the extent and the rate of photodecomposition. Nevertheless the photocatalytic reaction of methyl formate proceeded more slowly compared to that of formic acid on the same catalysts, and produced more CO. Addition of H2O to the methyl formate decreased the CO/H2 ratio by a factor of 4. Wh...

Photodecomposition of dodecyltrimethylammonium bromide in aqueous solutions in presence of hydrogen peroxide

It is established that the cationic surfactant (dodecyltrimethylammonium bromide, DTAB) efficiently decomposes in an aqueous solution in the presence of H2O2 upon exposure to UV radiation. The presence of bicarbonate ions decreases the rate of photodecomposition, which is due to the acceptance of OH radicals generated in the photochemical reaction. The rate constant of OH radicals and DTAB reaction equals ∼109 L mol−1 s−1. The kinetics of DTAB decomposition are described by the first-order reaction equation, and the effective rate constant is proportional to the H2O2 content in the solution.

Hydrolysis, aqueous photolysis and soil degradation of fluroxypyr

The fate of fluroxypyr in the aqueous buffer solutions, natural water and three types of soil was examined under laboratory conditions. The hydrolysis of fluroxypyr was studied in aqueous buffer solutions of pH 4.5 ± 0.1, 7.4 ± 0.1 and 9.0 ± 0.1 at different temperatures (25 ± 2 and 50 ± 2 °C). Fluroxypyr was stable under pH 4.5 ± 0.1 acid condition and its half-lives in pH 7.4 ± 0.1 and 9.0 ± 0.1 buffer solutions at 25 ± 2 °C were 14.9 and 12.7 days, respectively. The chemical hydrolysis of fluroxypyr was both pH and temperature-dependent and accelerated by alkaline conditions and higher temperatures. The photodecomposition rate of fluroxypyr under UV light in aqueous buffer solutions and natural water was much higher than that under sunlight with the half-lives of 0.102–0.705 hour. The half-lives of direct photolysis in pH 7.4 ± 0.1 and 9.0 ± 0.1 buffer solutions were 7.14 and 5.34 days respectively and no significant variations of concentrations could be observed in natural water, distilled water and pH 4.5 ± 0.1 buffer solution samples. Photosensitisers such as H2O2 or riboflavin could strongly enhance photolysis of fluroxypyr in natural sunlight with half-lives ranging from 0.36 to 43.6 hours in water and buffer solutions. 4-amino-3, 5-dichloro-6-fluorop-yridin-2-ol and 4-amino-3, 5-dichloro-6-hydroxy-2-pyridyloxyacetic acid were identified as degradation products by LC-TOF-MS. The degradation of fluroxypyr in three nonsterilised Chinese soil samples followed first-order kinetics, with half-lives ranging from 27.2 to 43.1 days. No degradation of fluroxypyr occurred in sterilised soil samples up to 70 days. The data generated from this study could be helpful for risk assessment studies of the pesticide in the environment.

The influence of polychromatic light on methomyl degradation in TiO2 and ZnO aqueous suspension

The photocatalytic degradation of the insecticide methomyl in water using TiO2 Merck Eusolex® T (anatase) under Osram ultra-vitalux® lamp light (315–400 nm, 300 W) was studied. The effect of the operational parameters, such as initial concentration of catalyst, initial methomyl concentration, initial salt concentration (NaCl), and pH, was studied. The optimal concentration of catalyst was found to be 2.0 g/L. Using the Langmuir–Hinshelwood mechanism as a base, a pseudo-first-order kinetic model was illustrated and the adsorption equilibrium constant and the rate constant of the surface reaction were calculated (KMT = 0.079 L/mg and kC = 0.134 mg/L min, respectively). The photodegradation rate was higher in acidic than in alkaline conditions. The presence of Cl– ions significantly affected the photodegradation of the pollutant. The rate of photodecomposition of methomyl was measured using UV spectroscopy and high performance liquid chromatography, while its mineralization was followed using ion chromatography and total organic carbon analysis. In addition, several photocatalysts were used: ZnO (Merck), TiO2 P-25 (Degussa), TiO2 Merck (anatase), and TiO2 Merck R-706 (rutile), and were compared with TiO2 Merck Eusolex® T (anatase).

Preparation of Mesoporous Silica Monoliths Doped with Titanium Clusters

Abstract Hierarchical nanoporous structures were fabricated from mesoporous silica and organic monolith with the aim of fabricating a high-efficiency catalyst carrier. The hierarchical structures were modified with photocatalytic titanium clusters. The hierarchical structures showed higher photodecomposition rate of isopropyl alcohol than mesoporous silica structures or monolith structure did, even though the surface area of the hierarchical structure was the smallest. In addition, the CH4 generation from CO2 photoreduction on the hierarchical mesoporous structure was also confirmed. These results suggest the possibilities of highly efficient mass-transport property and the ease of chemical modification of our hierarchical structure.

Dependence of Photocatalytic Reaction Yields on Catalyst Mass in Solid-Liquid Suspensions

When the yield of a heterogeneously photocatalyzed reaction is seen to pass through maximum as the mass of catalyst is increased in a suspension, this behavior may be attributable to a regime in which back-reflection most often occurs after a single photon-particle encounter, while absorption requires multiple internal reflections. This was modeled with a double exponential equation, which predicts that the yield will pass through a maximum with increasing catalyst mass when the catalyst has low absorptivity, but approach an asymptotic limit when the absorptivity is high. The rate of photodecomposition of chloroform, catalyzed by the chloride form of a polystyrene anion exchange resin (low absorptivity), was found to pass through a maximum as the amount of resin was increased, while catalysis by the CuCl4 2- form of the resin (higher absorptivity) caused an increase in the rate of decomposition with catalyst mass to an asymptotic value. Keywords: Heterogeneous photocatalysis, suspensions, chloroform photodegradation, reflectance, catalyst mass

Optical and photocatalytic properties of TiO2 films modified with oxides and gold

AbstractFilms consisting of TiO2/ZrO2/SiO2 undoped and doped with Au nanoparticles were synthesized by the sol‐gel method. The films were characterized with X‐ray diffraction (XRD), scanning electron microscopy (SEM), spectroscopic ellipsometry (SE) and optical absorption (OA) methods. The XRD study demonstrates the formation of TiO2 anatase nanocrystals and Au nanoparticles. SE and OA measurements of the undoped films showed no significant absorption in the spectral range of 400–1000 nm. Doping with Au nanoparticles resulted in the appearance of a surface plasmon resonance (SPR) absorption band at 600 nm as well as weaker absorption bands in the 300–500 nm region. The observed changes in optical properties agreed with the enhanced photocatalytic activity of Au‐doped TiO2/ZrO2/SiO2 films with gold compared to undoped films. The photocatalytic activity was evaluated by the photodegradation of the organic dye Rhodamine B. Under optimized conditions, the photodecomposition rate of the doped films was several times larger than that of the undoped films. Films of TiO2, TiO2/ZrO2/SiO2 and ternary oxides were investigated.

Degradation of TiO2 and/or SiO2 hybrid films doped with different cationic dyes

Hybrid thin films, silica–titanium oxides and silica–aluminum oxides, designed based on the sol–gel process are evaluated as catalysts in the photo-degradation of the cationic dyes. Silica matrices from different precursors with various organic functional groups and cross-linked with titanium or aluminum agents (tetraisopropyl orthotitanate and aluminum sec-butoxide) allow the surface property tailoring related to the high capacity of the dye adsorption respective, high photo-degradation activity. The cationic dyes (methylene blue, rhodamine B, crystal violet, malachite green) embedded on the hybrid silica matrix, under ultraviolet light, have a first order kinetics of photodegradation. The cross-linking agents play a key role in the photocatalytic degradation and silica matrix as dye absorbent. The photo-degradation rate for the binary system derived from methyltriethoxysilane/vinyltriethoxysilane precursors with both cross linkers showed a significant improvement by comparison with other hybrid materials. The significant increasing in the photodecomposition rate is related to the capacity to generate additional oxidizing species by each silica hybrid compounds.

Influence of post-treatment temperature of TNTa photoelectrodes on photoelectrochemical properties and photocatalytic degradation of 4-nonylphenol

TiO2/Ti Nanotube array (TNTa) photoelectrodes were prepared by galvanostatic and potentiostatic anodization technique, and annealed at different temperature. The morphology and structure were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectra. Optical properties and photoelectrochemical (PECH) properties were investigated by surface photovoltage (SPV), ultra UV–vis diffuse reflectance spectra (UV/vis/DRS), open-circuit potential (OCP) and transient photocurrent. Photodecomposition performances were evaluated by the yield of •OH radicals and photocatalytic (PC) degradation rate of 4-nonylphenol (4-NP) under xenon light. The results showed that with the increase of post-treatment temperature, PECH and PC properties increased gradually and then decreased. The photocurrent densities, yield of ⁎OH radicals and photodecomposition rates of 4-NP were following the trend of TNT-823>TNT-873>TNT-773>TNT-673>TNT-973>TNT-298. When the annealing temperature was 823K, the photoelectrodes were composed of mixed crystal structure and ordered nanotube array, which exhibited superior PECH properties. When the annealed temperature arrived to 973K, the nanotube collapsed and the PECH and PC properties of TNTa photoelectrodes decreased. For TNT-823 photoelectrodes, after the assistant potential was applied, the degradation rate of 4-NP increased significantly from 77% to 97% after 120min illumination.

Photocatalytic degradation of N-heterocyclic aromatics—effects of number and position of nitrogen atoms in the ring

This study demonstrates the influences of position, number of nitrogen (N) atoms and -C-N- or -N=N- linkage present in the six membered heterocyclic compounds such as pyridine, pyrazine, and pyridazine on their photocatalytic degradation by Au, Ag, and Fe(+2) deposited TiO(2) photocatalyst. The photodegradation rate of these heterocyclic compounds follow the order pyridine > pyrazine > pyridazine due to the different extent of hydroxylation and difference in position and number of N atoms in the aromatic moiety. The Au photodeposition significantly improved the TiO(2) photoactivity as compared to Ag and Fe(+2) loading. The presence of two N atoms in pyrazine and pyridazine as compared to one N atom in pyridine hamper the nucleophilc attack of OH radicals in comparison to easy hydroxylation of pyridine ring. There is 1 N atom, 4C-C, 1C-N and 1C=N bond in pyridine, 2 N atoms in the 1 and 4 positions, 2C-C, 2C-N bonds and 2C=N bonds in pyrazine, and pyridazine ring contains 2 N atoms in the 1 and 2 positions, 3C-C, 1N-N bond and 2C=N bonds. The bond strength/energy decreases gradually as: C=N- (615 KJ/mol) > -N=N- (418 KJ/mol) > -C-C- (347 KJ/mol) > -C-N- (305 KJ/mol) > -N-N- (163 KJ/mol). As pyridine has 1C-N, 1C=N, and no N-N bond, it photodegrades easily as compared to 1 N-N and 2C=N bonds of pyridazine of lowest photodecomposition rate. The improved photoactivity of Au-TiO(2) is explained on the basis of its favorable redox potential, work function, and electron-capturing capacity, etc.

Photocatalysis of Chloroform Decomposition by the Hexachlororuthenate(IV) Ion

Dissolved hexachlororuthenate(IV) effectively catalyzes the photodecomposition of chloroform to hydrogen chloride and phosgene under near-UV (λ>345nm) irradiation, whereby RuCl6(2-) is not itself photocatalytically active, but is photochemically transformed into a species that is active, possibly RuCl5 (CHCl3 )(-) . Conversion to a photoactive species during irradiation is consistent with the acceleration of the decomposition rate during the early stages and with the apparent inverse dependence of the decomposition rate on the initial concentration of RuCl6(2-) . The displacement of Cl(-) by CHCl3 in the coordination sphere to create the photoactive species is consistent with the retardation of photodecomposition by both Cl(-) and H2 O. The much smaller photodecomposition rate in CDCl3 suggests that C-H bond dissociation occurs during the primary photochemical event, which is also consistent with the presence of a CHCl3 molecule in the first coordination sphere.