Gas-phase Ozone Research Articles

Ozone decomposition on Ag/SiO2 and Ag/clinoptilolite catalysts at ambient temperature

Silver modified zeolite (Bulgarian natural clinoptilolite) and Ag/silica catalysts were synthesized by ion exchange and incipient wet impregnation method respectively and characterized by different techniques. DC arc–AES was used for Ag detection. XRD spectra show that Ag is loaded over the surface of the SiO2 sample and that after the ion-exchange process the HEU type structure of clinoptilolite is retained. UV–VIS (specific reflection at 310nm) and IR (band at 695cm−1) spectroscopy analysis proved that silver is loaded as a T-atom into zeolite channels as Ag+, instead of Na+, Ca2+, or K+ ions, existing in the natural clinoptilolite form. The samples Ag/SiO2 and Ag-clinoptilolite were tested as catalysts for decomposition of gas phase ozone. Very high catalytic activity (up to 99%) was observed and at the same time the catalysts remained active over time at room temperature.

Prompt Formation of Organic Acids in Pulse Ozonation of Terpenes on Aqueous Surfaces

A major atmospheric process, the gas-phase ozonation of terpenes yields suites of products via a cascade of chemically activated intermediates that ranges from primary ozonides to dioxiranes. If a similar mechanism operated in water, as it is generally assumed, such intermediates would be deactivated within picoseconds and, henceforth, be unable to produce carboxylic acids in microseconds. Herein, we report the online electrospray mass spectrometric detection of (M + 2O − H+) and (M + 3O − H+) carboxylates on the surface of aqueous β-caryophyllene (C15H24, M = 204 Da) microjets exposed to a few ppmv of O3(g) for <10 μs. Since neither species is formed on dry solvent microjets and both incorporate deuterium from D2O, we infer that carboxylates ensue from the interaction of nascent intermediates with interfacial water via heretofore unreported processes. These interfacial events proceed much faster than those in bulk liquids saturated with ozone.

Light-induced heterogeneous ozone processing on organic coated particles: Kinetics and condensed-phase products

For the first time we investigated the effect of solar irradiation upon the heterogeneous ozonation of adsorbed 3,4,5-trimethoxybenzaldehyde on solid surface. Light-induced heterogeneous reactions between gas-phase ozone and 3,4,5-trimethoxybenzaldehyde adsorbed on silica particles were performed and the consecutive reaction products were identified. At an ozone mixing ratio of 250 ppb, the loss of 3,4,5-trimethoxybenzaldehyde ranged from 1.0 · 10 −6 s −1 in the dark to 2.9 · 10 −5 s −1 under light irradiation. Such large enhancement of 29 times clearly shows the importance of light ( λ > 300 nm) during the heterogeneous ozonolysis on organic coated particles. The reaction products identified in this study (3,4,5-trimethoxybenzoic acid, syringic acid, methyl 3,4,5-trimethoxybenzoate) absorb light in the spectral window ( λ > 300 nm) which implies that light-induced heterogeneous ozone processing can have an influence on the aerosol surfaces by changing their physico-chemical properties. The main identified product of the heterogeneous reactions between gas-phase ozone and 3,4,5-trimethoxybenzaldehyde under dark conditions and in presence of light was 3,4,5-trimethoxybenzoic acid. For this reason we estimated the carbon yield of 3,4,5-trimethoxybenzoic acid. Carbon yields of 3,4,5-trimethoxybenzoic acid decreased with increasing ozone mixing ratio; from 40% at 250 ppb to 15% at ≥2.5 ppm under dark conditions. At ozone mixing ratio (250 ppb–1 ppm), carbon yields of 3,4,5-trimethoxybenzaldehyde are relatively higher in the experiment under dark condition than under simulated solar light.

Numerical modeling of the benzene reaction with ozone in gas phase using differential neural networks

In the present paper a mathematical model of a gas–gas reaction between ozone and benzene in a tubular reactor is considered. Usually, mathematical models of chemical process are governed by a set of ordinary differential equations assuming that the corresponding concentration dynamics depends only on time. On the other hand, the spatial distribution of the mass, energy and concentrations may be observed in the case of a more complex model structure that demands the use of models described by partial differential equations. The example of such complex model describing, the reaction between benzene and ozone in the gas phase, is considered here. The approach suggested in this study is based on the differential neural network (DNN) technique which permits to convert the task of mathematical modeling of a tubular reactor containing an uncertain (not well-defined) dynamics to a non-parametric identification problem. The asymptotic convergence of the obtained identification error to an ellipsoidal zone containing the origin is shown using the Lyapunov-like analysis. The coincidence between the benzene and ozone concentrations variation calculated by the suggested DNN-algorithm and those generated by a kinetic model is shown to be good enough.

Heterogeneous ozonation kinetics of 4-phenoxyphenol in the presence of photosensitizer

Abstract. In this work we have quantitatively measured the degradation of 4-phenoxyphenol adsorbed on silica particles following oxidative processing by gas-phase ozone. This was performed under dark conditions and in the presence of 4-carboxybenzophenone under simulated sunlight irradiation of the particles surface. At the mixing ratio of 60 ppb which corresponds to strongly polluted ozone areas, the first order of decay of 4-phenoxyphenol is k1=9.95×10−6 s−1. At a very high ozone mixing ratio of 6 ppm the first order rate constants for 4-phenoxyphenol degradation were the following: k1=2.86×10−5 s−1 under dark conditions and k1=5.58×10−5 s−1 in the presence of photosensitizer (4-carboxybenzophenone) under light illumination of the particles surface. In both cases, the experimental data follow the modified Langmuir-Hinshelwood equation for surface reactions. The Langmuir-Hinshelwood and Langmuir-Rideal mechanisms for bimolecular surface reactions are also discussed along with the experimental results. Most importantly, the quantities of the oligomers such as 2-(4-Phenoxyphenoxy)-4-phenoxyphenol and 4-[4-(4-Phenoxyphenoxy)phenoxy]phenol formed during the heterogeneous ozonolysis of adsorbed 4-phenoxyphenol were much higher under solar light irradiation of the surface in comparison to the dark conditions.

Kinetic studies of the heterogeneous oxidation of maleic and fumaric acid aerosols by ozone under conditions of high relative humidity

In this paper, a kinetic study of the oxidation of maleic and fumaric acid organic particles by gas-phase ozone at relative humidities ranging from 90 to 93% is reported. A flow of single component aqueous particles with average size diameters in the range 2.6-2.9 µm were exposed to a known concentration of ozone for a controlled period of time in an aerosol flow tube in which products were monitored by infrared spectroscopy. The results obtained are consistent with a Langmuir-Hinshelwood type mechanism for the heterogeneous oxidation of maleic/fumaric acid aerosol particles by gas-phase ozone, for which the following parameters were found: for the reaction of maleic acid aerosols, K(O(3)) = (9 ± 4) × 10(-15) cm(3) molecule(-1) and k = (0.21 ± 0.01) s(-1); for the reaction of fumaric acid aerosols, K(O(3)) = (5 ± 2) × 10(-15) cm(3) molecule(-1) and k = (0.19 ± 0.01) s(-1). From the pseudo-first-order coefficients, apparent uptake coefficient values were calculated for which a decreasing trend with increasing ozone concentrations was observed. Comparison with previous measurements of the same system under dry conditions reveals a direct effect of the presence of water on the mechanism of these reactions, in which the water is seen to increase the formation of CO(2) and formic acid (HCO(2)H) through increased levels of hydroxyacetyl hydroperoxide intermediate.

オゾン水中におけるエチレン‐プロピレン‐ジエン三元共重合体の劣化機構

The degradation of ethylene-propylene-diene terpolymer (EPDM) in ozone water has been investigated. The surface of EPDM specimen degraded rapidly in ozone water. It showed adherence after ozone water treatment, since the polymer near the surface decomposed. From Fourier transform infrared (FT-IR) spectroscopy, the aggregation structure near the surface of EPDM treated with ozone in aqueous solution was different from that treated with ozone in gas phase. This may be due to that the degradation of EPDM by ozone in aqueous solution was initiated by hydroxyl radical, which was generated by self-decomposition of ozone. Through nuclear magnetic resonance (NMR) spectroscopy, the detected structures from degraded EPDM after ozone water treatment agreed with the most probable structures, which may be produced by ozone water treatment. It was concluded that the degradation of EPDM proceeded in ozone water, regardless of its good ozone resistance in air, because hydroxyl radical caused the decomposition of EPDM, which was initiated from proton abstraction from saturated hydrogen carbon linkage of EPDM main chain.

Light induced multiphase chemistry of gas-phase ozone on aqueous pyruvic and oxalic acids

In this study for the first time it has been shown that pyruvic acid can affect the atmospheric multiphase reactions of ozone with oxalic acid due to its properties as a photosensitizer. To this end, the photochemical batch multiphase reactions of a mixture of pyruvic acid/oxalic acid (PA/OA) and gas-phase ozone under simulated sunlight were studied as a function of time using high pressure liquid chromatography equipped with a UV detector (HPLC-UV) and electrospray ionization mass spectrometry (ESI-MS) to investigate product formation. Following the simultaneous ozone and light irradiation the first peak for pyruvic and oxalic acids (retention time = 3.68 min) decreased to 67% of the initial intensity after a 12 h reaction while a broad and not well defined peak appeared at longer retention times. After prolonged exposure times this broad peak shifted to shorter retention times: from 14 min at 2 h reaction to 8 min at 12 h. The HPLC-UV analysis of the reaction mixture simultaneously exposed to ozone and irradiated by simulated sunlight for 6-12 h revealed the presence of high weight molecular mass products and formation at longer times of highly non-polar products. The results obtained from ESI-MS have clearly demonstrated that the distribution of high molecular weight products is consistent with an oligomer system. No evidence of oligomer formation was found after the sample (PA/OA) was exposed only to either ozone or irradiated with UV/Vis light using the same instrumental conditions.

Photoenhanced degradation of veratraldehyde upon the heterogeneous ozone reactions

Light-induced heterogeneous reactions between gas-phase ozone and veratraldehyde adsorbed on silica particles were performed. At an ozone mixing ratio of 250 ppb, the loss of veratraldehyde largely increased from 1.81 x 10(-6) s(-1) in the dark to 2.54 x 10(-5) s(-1) upon exposure to simulated sunlight (lambda > 300 nm). The observed rates of degradation exhibited linear dependence with the ozone in the dark ozonolysis experiments which change in the non-linear Langmuir-Hinshelwood dependence in the experiments with simultaneous ozone and light exposure of the coated particles. When the coated silica particles were exposed only to simulated sunlight in absence of ozone the loss of veratraldehyde was about three times higher i.e. 5.97 x 10(-6) s(-1) in comparison to the ozonolysis experiment under dark conditions at 250 ppb ozone mixing ratio, 1.81 x 10(-6) s(-1).These results clearly show that the most important loss of veratraldehyde occurs under simultaneous ozone and light exposure of the coated silica particles. The main identified product in the heterogeneous reactions between gaseous ozone and adsorbed veratraldehyde under dark conditions and in presence of light was veratric acid.Carbon yields of veratric acid were calculated and the obtained results indicated that at low ozone mixing ratio (250 ppb) the carbon yield obtained under dark conditions is 70% whereas the carbon yield obtained in the experiments with simultaneous ozone and light exposure of the coated particles is 40%. In both cases the carbon yield of veratric acid exponentially decayed leading to the plateau ( approximately 35% of carbon yield) at an ozone mixing ratio of 6 ppm. Two reaction products i.e. 3-hydroxy-4-methoxybenzoic acid and 4-hydroxy-3-methoxybenzoic acid were identified (confirmed with the standards) only in the experiments performed under simultaneous ozonolysis and light irradiation of the particles.

Glancing-angle Raman spectroscopic probe for reaction kinetics at water surfaces

We report glancing-angle Raman spectra acquired at the surface of aqueous dimethyl sulfoxide solutions and demonstrate that this technique can be used to measure the surface concentration of solutes. The presence of some solute molecules at the surface suppresses the intensity of the OH-stretching band of water there. We used this phenomenon to study the interfacial reaction of gas-phase ozone with aqueous NaX solutions (X = Br, I) by monitoring the decrease in intensity of the OH-stretching band of water over time. UV-VIS analysis of the product solutions indicates that X(3)(-), formed from X(2) evolved in the ozonation reaction, is the species most likely responsible for the decrease in OH-stretching intensity at the surface. The dependence of the rate of OH-Raman signal loss at the water surface on the bulk halide concentration is well described by a Langmuir-Hinshelwood kinetic model. The Langmuir-Hinshelwood parameters indicated that iodide has a approximately 50 times greater propensity for the surface compared to bromide.

Impact of the Knudsen number and mass-transfer expression on multi-phase kinetic modeling

Three different mass-transfer expressions are employed within the Model of Aerosol, Gas, and Interfacial Chemistry (MAGIC) to study gas-phase molecular chlorine and bromine production from NaCl and NaBr aerosols, respectively. Simulations of chamber experiments are performed in which NaCl aerosols react with gas-phase ozone in the presence of UV light, in order to identify the importance of the Knudsen number and mass-transfer expression in systems with varying contributions from gas-phase, aqueous-phase, and interfacial chemistry. In the case of NaBr aerosols, simulations are performed of both dark and photolytic conditions. A range of Knudsen numbers spanning the continuum, transition and free-molecular regimes is studied. Particle size is varied over three orders of magnitude, and particle concentration is changed to keep either (a) total aerosol volume or (b) total aerosol surface area constant. When total aerosol volume is constant, the total amount of surface area available for interfacial reaction increases linearly with Knudsen number. Consequently peak gas-phase Cl 2 and Br 2 concentrations increase by two orders of magnitude from the continuum regime to the free-molecular regime. When total aerosol surface area is constant, total aerosol volume is inversely proportional to Knudsen number, with lesser volume being available at higher Knudsen numbers. Consequently Cl − depletion in the kinetic regime leads to most gas-phase Cl 2 being produced in the transition regime. Gas-phase Br 2 concentration trends are determined by aqueous-phase reaction mechanisms, leading to a monotonic decrease in production with Knudsen number. At all Knudsen numbers, more gas-phase bromine is produced in the photolytic case than in the dark case, the difference being significant in the transition regime. Results of this study suggest that halogen production is insensitive to the mass-transfer expression used in the simulations.

Solid redox polymer electrolyte-based amperometric sensors for the direct monitoring of ozone in gas phase

The response of solid redox polymer electrolytes of the type polyethylene oxide (PEO)/XI/I 2 (X: Li, Rb) towards O 3 in gas phase is reported for the first time. The proposed approach is based on the current recorded at a redox polymer electrolyte-modified two-gold-surface electrode assembly poised at +300 mV. Ozone reacts with the iodide species in the bulk electrolyte producing triiodides. This results in the increase of the diffusion rate of I 3 - to the cathode where I 3 - are reduced to iodides, giving thus rise to the production of a faradic response. Various experimental parameters were optimized and the best results were obtained for the PEO/RbI/I 2 redox polymer, which is successfully used for the on-line monitoring of O 3 in synthetic air samples in ambient conditions.

Effect of Ozone and Relative Humidity on the Heterogeneous Uptake of Octamethylcyclotetrasiloxane and Decamethylcyclopentasiloxane on Model Mineral Dust Aerosol Components

We have carried out kinetic and reaction yield studies to determine the effect of O(3) on the heterogeneous reaction of two cyclic volatile methylsiloxanes (cVMS), octamethylcyclotetrasiloxane (D(4)) and decamethylcyclopentasiloxane (D(5)), with model mineral dust aerosol in order to obtain a better understanding of the atmospheric fate of cVMS. The heterogeneous chemistry was studied in an environmental reaction chamber using FT-IR spectroscopy to monitor the reaction progress. The uptake kinetics and the reaction extent for D(4) and D(5) in the presence of O(3) were quantified for two components of mineral dust aerosol, hematite and kaolinite. Some experiments with a carbonaceous particulate, carbon black, were also performed for D(5). The relative humidity (RH) inside the chamber was varied to investigate the influence of surface adsorbed water on the heterogeneous chemistry of the dust samples. With the dust samples, but not carbon black, the coadsorption of O(3) introduced a new reaction pathway, characterized by a linear, zero-order, decay of both gas phase cVMS and ozone. The new pathway does not saturate on the time scale of our experiments. Elevated RH was observed to decrease the total uptake of cVMS and ozone by the end of the experiment, but the characteristic linear decay was still present. The atmospheric loss of cVMS due to heterogeneous uptake is enhanced due to O(3), even at higher RH values, but the overall loss rate is reduced at RH values typical of the troposphere.

Infrared spectroscopic studies of the heterogeneous reaction of ozone with dry maleic and fumaric acid aerosol particles

Dicarboxylic acids, either directly emitted or formed in chemical processes, are found to be a significant component of tropospheric aerosols. To assess any potential chemical transformation of short unsaturated dicarboxylic acids in tropospheric heterogeneous chemistry, maleic and fumaric acid were selected as surrogates in this study. A novel aerosol flow tube apparatus is employed to perform kinetic studies of the oxidation of these organic compounds by gas-phase ozone. The system consists of a particle generation system, a vertically oriented glass flow tube and an infrared observation White cell with a Fourier transform infrared (FTIR) spectrometer for the detection system. A flow of single component organic aerosols with mean diameters ranging between 0.7 and 1.1 microm is introduced in a flow tube, in which the particles are subsequently exposed to a known concentration of ozone for a controlled period of time. A band assignment of infrared vibrational frequencies for dry maleic and fumaric acid aerosol spectra is presented. These studies are complemented with off-line analysis on the reaction products. The reaction exhibited pseudo-first-order kinetics on gas product formation, and the pseudo-first-order rate coefficients displayed a Langmuir-Hinshelwood dependence on gas-phase ozone concentration for both materials. By assuming a Langmuir-Hinshelwood behaviour, the following parameters were obtained: for the reaction of maleic acid aerosols, K(O3) = (3.3 + 0.5) x 10(-16) cm3 molecule(-1) and k(I)(max) = (0.038 + 0.004) s(-1); for the reaction of fumaric acid aerosols, K(O3) = (1.6 + 0.5) x 10(-16) cm3 molecule(-1) and k(I)(max) = (0.048 + 0.007) s(-1), where K(O3) is a parameter that describes the partitioning of ozone to the particle surface and k is the maximum pseudo-first-order coefficient at high ozone concentrations. Apparent reactive uptake coefficients were estimated from the pseudo-first-order rate coefficient and a trend of decreasing uptake coefficients with increasing ozone concentrations was observed, in good agreement with literature values.

Preparation of oxygen-containing organic products from bed-oxidized brown coal by ozonation

The possibility of modifying the functional composition of humic acids by gas-phase ozonation of bed-oxidized brown coal was examined. About 90% of the organic matter of brown coal was converted to low-molecular-weight soluble oxygen-containing products by stepwise liquid-phase ozonation (in chloroform and acetic acid).

Photoenhanced ozone loss on solid pyrene films

This work presents the results of two complementary studies of the heterogeneous reaction of gas-phase ozone with solid pyrene films. In the first study, ozone uptake by the pyrene film was determined using a coated-wall flow tube system. In the second, pyrene loss within the film upon exposure to ozone was monitored using a laser-induced fluorescence technique. The dependence of the reactive loss rate on ozone concentration observed in both methods suggests that the reaction proceeds via a Langmuir-Hinshelwood-type surface mechanism. At a mixing ratio of 50 ppb, the steady-state reactive uptake coefficient of ozone by pyrene films increased from 5.0x10(-6) in the dark to 3.7x10(-5) upon exposure to near-UV radiation (300-420 nm). The uptake coefficient increased linearly as a function of UV-A spectral irradiance and decreased markedly with increasing relative humidity. The loss of surface pyrene upon exposure to ozone also displayed a light enhancement: analysis of Langmuir-Hinshelwood plots for the light and dark reactions revealed a small increase in the two-dimensional reaction rate in the presence of light (lambda>or=295 nm). This modest enhancement, however, was less significant than the corresponding enhancement in the loss of gas-phase ozone. In order to explain these observations, we present an integrated mechanism whereby the light-enhanced ozone uptake arises from the reaction of ozone with O2(1Sigmag+) formed via energy transfer from excited-state pyrene and the enhanced pyrene loss occurs via the formation of a charge-transfer complex between excited-state pyrene and adsorbed ozone. The disparity between surface- and gas-phase results underscores the important role that multifaceted strategies can play in elucidating the mechanisms of heterogeneous atmospheric reactions.

Heterogeneous Loss of Gas-Phase Ozone on n-Hexane Soot Surfaces: Similar Kinetics to Loss on Other Chemically Unsaturated Solid Surfaces

The heterogeneous loss of ozone on n-hexane soot surfaces has been studied in a coated-wall flow tube connected to a mass spectrometer for gas-phase analysis. Uptake measurements confirm earlier studies that the initial uptake is primarily noncatalytic and that the number of reactive surface sites is close to that of a full monolayer. The initial uptake kinetics exhibit an inverse dependence on ozone gas-phase concentration, as expected in the surface-saturated limit if the reaction proceeds via a Langmuir−Hinshelwood mechanism. Support for this reaction’s not being an Eley−Rideal process comes from the lack of temperature dependence of the initial uptake coefficient from 260 to 360 K and that a saturated surface coverage of adsorbed dodecane does not affect the kinetics. It is demonstrated that there is a strong similarity between the initial uptake kinetics for ozone loss on a wide variety of surfaces, including soot; 1-hexadecene (as studied in this work); metal oxides, including atmospheric mineral dust; and PAHs adsorbed on a variety of surfaces. This suggests that the ozone loss may proceed through a common reaction pathway on such surfaces.

Multiphase Chemistry of Ozone on Fulvic Acids Solutions

By means of a wetted-wall flow tube, we studied the multiphase chemistry of ozone on aqueous solutions containing fulvic acids (FA), taken as proxies for atmospheric "humic like substances", so-called HULIS. In these experiments, the loss of gaseous O3 was monitored by UV-visible absorption spectroscopy at the reactor outlet (i.e., after contact between the gaseous and liquid phases). Measurements are reported in terms of dimensionless uptake coefficients (gamma) in the range from 1.6 x 10(-7) to 1.3 x 10(-5) depending on ozone gas phase concentration (in the range from 6.6 to 34.4 x 10(11) molecules cm(-3)) and fulvic acid aqueous concentration (in the range from 0.25 to 2.5 mg L(-1)) and pH (in the range from 2.5 to 9.2). The measured kinetics were observed to follow a Langmuir-Hinshelwood type mechanism, in which O3 first adsorbs on the liquid surface and then reacts with the Fulvic Acid molecules. The reported uptake coefficients are greatly increased over those measured on pure water, demonstrating that the presence in solution of fulvic acids does greatly enhance the uptake kinetics. Accordingly, the chemical interactions of fulvic acids (or HULIS) may be a driving force for the uptake of ozone on liquid organic aerosols and can also represent an important mechanism for the O3 deposition to the rivers and lakes.

Photoenhanced Reaction of Ozone with Chlorophyll at the Seawater Surface

The kinetics of the heterogeneous reaction between gas-phase ozone and chlorophyll present at the air−salt water interface are altered in the presence of actinic radiation. Under dark conditions, a...

Study of the Ozonation of a Dye Using Kinetic Information Reconstruction

In this study, the kinetic information reconstruction method is applied to measure the degradation of a dye in an ozonation gas-liquid reactor. The application of this method combined with the study of the ozone gas phase concentration has made possible a deeper study of the fast reaction between ozone and blue indigo trisulfonate. For this kinetic study different rigorous mathematical models based on Film Layer Theory, Surface Renewal Model and Penetration Theory have been used.