Oxidation Of M-xylene Research Articles

Impacts of inorganic anions and natural organic matter on thermally activated persulfate oxidation of BTEX in water

The present study investigated the impacts of water matrix constituents (CO32−, HCO3−, Cl−, Br−, PO43−, HPO42−, H2PO4−, NO3−, SO42− and natural organic matters (NOM) on the oxidation of a mixture of benzene, toluene, ethylbenzene, and xylenes (BTEX) by thermally activated persulfate (PS). In the absence of matrix constituents, the BTEX oxidation rates decreased in the following order: xylenes > toluene ≈ ethylbenzene > benzene. HCO3−/CO32− and NOM inhibited the BTEX oxidation and the inhibiting effects became more pronounced as the HCO3−/CO32−/NOM concentration increased. SO42−, NO3−, PO43− and H2PO4− did not affect the BTEX oxidation while HPO42− slightly inhibited the reaction. The impacts of Cl− and Br− were complex. Cl− inhibited the benzene oxidation while 100 mM and 500 mM of Cl− promoted the oxidation of m-xylene and p-xylene. Br− completely suppressed the benzene oxidation while 500 mM of Br− strongly promoted the oxidation of xylenes. Detailed explanations on the influence of each matrix constituent were discussed. In addition, various halogenated degradation byproducts were detected in the treatments containing Cl− and Br−. Overall, this study indicates that some matrix constituents such as NOM, HCO3−, CO32−, H2PO4−, Cl− and Br− may reduce the BTEX removal efficiency of sulfate radical-based advanced oxidation process (SR-AOP) and the presence of Cl− and Br− may even lead to the formation of toxic halogenated byproducts.

Chemical ignition delay of candidate drop-in replacement jet fuels under fuel-lean conditions: A shock tube study

Ignition delay times were measured behind reflected shock waves under fuel lean, high pressures conditions for conventional (JP-8) and alternative jet fuels — Fischer-Tropsch (F-T), alcohol to jet (ATJ), direct sugar to hydrocarbon (DSHC), and four bio-jet — and a biodiesel-like fuel. The primary goal of this study was to investigate the effect of fuel structure on ignition delay in order to assess differences in fuel ignition delay relative to JP-8. To support this effort, the ignition characteristics of a few hydrocarbons that are proposed as jet fuel surrogate components were also investigated: n-heptane, n-dodecane, m-xylene, n-dodecane/m-xylene blend (77%/23% by volume) and 2,2,4,6,6-pentamethylheptane (iso-dodecane). Two single-pulse shock tubes (heated and non-heated) were used to obtain the current data set. Using argon as the diluent (93% by volume), the experimental conditions covered a pre-ignition temperature range of approximately 980–1800K at a pressure of 16±0.8atm and an equivalence ratio of 0.5. Experimental results show negligible differences between the ignition delay times of JP-8, F-T, DSHC, biodiesel, and the four bio-jet fuels. They are very similar to those of n-heptane, n-dodecane, n-dodecane/m-xylene blend and iso-dodecane under similar conditions. However, the ignition delay times of the ATJ were slightly longer at higher temperatures, and similar to iso-dodecane. m-Xylene reported significantly longer ignition delay times than the other fuels tested under identical conditions. Kinetic modeling results show that light branching and carbon chain length of large n-paraffins do not alter ignition delay because the predominant reactions are oxidation of C1-C4 fuel fragments. They also show for the n-dodecane/m-xylene blend that a discernable slowing of ignition delay by fuel decomposition and oxidation of m-xylene occurs at m-xylene concentrations greater than 50% (by mol.). Further testing is required to understand this behavior at stoichiometric and fuel rich conditions. In addition, an empirical correlation was obtained between the global activation energy of the measured ignition delay times for the single paraffinic species and their derived cetane numbers. This contributes to recent efforts aimed to provide relationships between pre-vaporized, homogeneous combustion properties with existing combustion indicators.

Concurrent catalytic removal of typical volatile organic compound mixtures over Au-Pd/α-MnO2 nanotubes

α-MnO2 nanotubes and their supported Au-Pd alloy nanocatalysts were prepared using hydrothermal and polyvinyl alcohol-protected reduction methods, respectively. Their catalytic activity for the oxidation of toluene/m-xylene, acetone/ethyl acetate, acetone/m-xylene and ethyl acetate/m-xylene mixtures was evaluated. It was found that the interaction between Au-Pd alloy nanoparticles and α-MnO2 nanotubes significantly improved the reactivity of lattice oxygen, and the 0.91wt.% Au0.48Pd/α-MnO2 nanotube catalyst outperformed the α-MnO2 nanotube catalyst in the oxidation of toluene, m-xylene, ethyl acetate and acetone. Over the 0.91wt.% Au0.48Pd/α-MnO2 nanotube catalyst, (i) toluene oxidation was greatly inhibited in the toluene/m-xylene mixture, while m-xylene oxidation was not influenced; (ii) acetone and ethyl acetate oxidation suffered a minor impact in the acetone/ethyl acetate mixture; and (iii) m-xylene oxidation was enhanced whereas the oxidation of the oxygenated VOCs (volatile organic compounds) was suppressed in the acetone/m-xylene or ethyl acetate/m-xylene mixtures. The competitive adsorption of these typical VOCs on the catalyst surface induced an inhibitive effect on their oxidation, and increasing the temperature favored the oxidation of the VOCs. The mixed VOCs could be completely oxidized into CO2 and H2O below 320°C at a space velocity of 40,000mL/(g·hr). The 0.91wt.% Au0.48Pd/α-MnO2 nanotube catalyst exhibited high catalytic stability as well as good tolerance to water vapor and CO2 in the oxidation of the VOC mixtures. Thus, the α-MnO2 nanotube-supported noble metal alloy catalysts hold promise for the efficient elimination of VOC mixtures.

NGHIÊN CỨU CHẾ TẠO VÀ KHẢ NĂNG OXI HÓA m-XYLEN CỦA VẬT LIỆU XÚC TÁC Bi2Zr2O7

NGHIÊN CỨU CHẾ TẠO VÀ KHẢ NĂNG OXI HÓA m-XYLEN CỦA VẬT LIỆU XÚC TÁC Bi2Zr2O7

OH-Initiated Oxidation of m-Xylene on Black Carbon Aging.

Laboratory experiments are conducted to investigate aging of size-classified black carbon (BC) particles from OH-initiated oxidation of m-xylene. The variations in the particle size, mass, effective density, morphology, optical properties, hygroscopicity, and activation as cloud condensation nuclei (CCN) are simultaneously measured by a suite of aerosol instruments, when BC particles are exposed to the oxidation products of the OH-m-xylene reactions. The BC aging is governed by the coating thickness (Δrve), which is correlated to the reaction time and initial concentrations of m-xylene and NOx. For an initial diameter of 100 nm and Δrve = 44 nm, the particle size and mass increase by a factor of 1.5 and 10.4, respectively, and the effective density increases from 0.43 to 1.45 g cm(-3) due to organic coating and collapsing of the BC core. The BC particles are fully converted from a highly fractal to nearly spherical morphology for Δrve = 30 nm. The scattering, absorption, and single scattering albedo of BC particles are enhanced accordingly with organic coating. The critical supersaturation for CCN activation is reduced to 0.1% with Δrve = 44 nm. The results imply that the oxidation of m-xylene exhibits larger impacts in modifying the BC particle properties than those for the OH-initiated oxidation of isoprene and toluene.

Development of aroCACM/MPMPO 1.0: a model to simulate secondary organic aerosol from aromatic precursors in regional models

Abstract. The atmospheric oxidation of aromatic compounds is an important source of secondary organic aerosol (SOA) in urban areas. The oxidation of aromatics depends strongly on the levels of nitrogen oxides (NOx). However, details of the mechanisms by which oxidation occurs have only recently been elucidated. Xu et al. (2015) developed an updated version of the gas-phase Caltech Atmospheric Chemistry Mechanism (CACM) designed to simulate toluene and m-xylene oxidation in chamber experiments over a range of NOx conditions. The output from such a mechanism can be used in thermodynamic predictions of gas–particle partitioning leading to SOA. The current work reports the development of a model for SOA formation that combines the gas-phase mechanism of Xu et al. (2015) with an updated lumped SOA-partitioning scheme (Model to Predict the Multi-phase Partitioning of Organics, MPMPO) that allows partitioning to multiple aerosol phases and that is designed for use in larger-scale three-dimensional models. The resulting model is termed aroCACM/MPMPO 1.0. The model is integrated into the University of California, Irvine – California Institute of Technology (UCI-CIT) Airshed Model, which simulates the South Coast Air Basin (SoCAB) of California. Simulations using 2012 emissions indicate that “low-NOx” pathways to SOA formation from aromatic oxidation play an important role, even in regions that typically exhibit high-NOx concentrations.

Waste-Reducing Catalytic Oxidation of m-Xylene to m-Toluic Acid

Catalytic performances of soluble Co, Co/Mn, Co/Mn/Br, and Co/Mn/Ce catalysts for m-xylene (MX) oxidation without acetic acid (HAc) solvent were studied, comparing with Co/HAc catalysts. In m-toluic acid (MTA) production, a Ce-salt promoter could effectively substitute for bromide salts and the recycle alcohol/aldehyde liquid mixture acts as a more efficient promotor than either peroxides or MTA itself. The Co/Mn/Ce system, free of acetic acid and Br−, was a superior catalyst in terms of activity and selectivity to MTA. Just as for p-xylene oxidation using Co/Mn/Br catalysts, both the MX reaction rate and the MTA selectivity increased with temperature when using Co/Mn/Ce catalysts. Unlike p-xylene oxidation however, MX oxidation resulted in more coupling products. For MX oxidation catalyzed by a supported Co-imide complex in supercritical CO2, the activity was lower by more than an order of magnitude, but the MTA selectivity was greater. The selectivity behavior of the catalyst systems indicate that the Co/Mn/Ce system, free of acetic acid solvent and corrosive Br−, was a superior catalyst, both in terms of overall activity and selectivity for MTA.

Reactive uptake of ammonia to secondary organic aerosols: kinetics of organonitrogen formation

Abstract. As a class of brown carbon, organonitrogen compounds originating from the heterogeneous uptake of NH3 by secondary organic aerosol (SOA) have received significant attention recently. In the current work, particulate organonitrogen formation during the ozonolysis of α-pinene and the OH oxidation of m-xylene in the presence of ammonia (34–125 ppb) was studied in a smog chamber equipped with a high resolution time-of-flight aerosol mass spectrometer and a quantum cascade laser instrument. A large diversity of nitrogen-containing organic (NOC) fragments was observed which were consistent with the reactions between ammonia and carbonyl-containing SOA. Ammonia uptake coefficients onto SOA which led to organonitrogen compounds were reported for the first time, and were in the range of ∼ 10-3–10−2, decreasing significantly to < 10-5 after 6 h of reaction. At the end of experiments (~ 6 h) the NOC mass contributed 8.9 ± 1.7 and 31.5 ± 4.4 wt % to the total α-pinene- and m-xylene-derived SOA, respectively, and 4–15 wt % of the total nitrogen in the system. Uptake coefficients were also found to be positively correlated with particle acidity and negatively correlated with NH3 concentration, indicating that heterogeneous reactions were responsible for the observed NOC mass, possibly limited by liquid phase diffusion. Under these conditions, the data also indicate that the formation of NOC can compete kinetically with inorganic acid neutralization. The formation of NOC in this study suggests that a significant portion of the ambient particle associated N may be derived from NH3 heterogeneous reactions with SOA. NOC from such a mechanism may be an important and unaccounted for source of PM associated nitrogen. This mechanism may also contribute to the medium or long-range transport and wet/dry deposition of atmospheric nitrogen.

Reaction Mechanism for m-Xylene Oxidation in the Claus Process by Sulfur Dioxide.

In the Claus process, the presence of aromatic contaminants such benzene, toluene, and xylenes (BTX), in the H2S feed stream has a detrimental effect on catalytic reactors, where BTX form soot particles and clog and deactivate the catalysts. Among BTX, xylenes are proven to be most damaging contaminant for catalysts. BTX oxidation in the Claus furnace, before they enter catalyst beds, provides a solution to this problem. A reaction kinetics study on m-xylene oxidation by SO2, an oxidant present in Claus furnace, is presented. The density functional theory is used to study the formation of m-xylene radicals (3-methylbenzyl, 2,6-dimethylphenyl, 2,4-dimethylphenyl, and 3,5-dimethylphenyl) through H-abstraction and their oxidation by SO2. The mechanism begins with SO2 addition on the radicals through an O-atom rather than the S-atom with the release of 180.0-183.1 kJ/mol of reaction energies. This exothermic reaction involves energy barriers in the range 3.9-5.2 kJ/mol for several m-xylene radicals. Thereafter, O-S bond scission takes place to release SO, and the O-atom remaining on aromatics leads to CO formation. Among four m-xylene radicals, the resonantly stabilized 3-methylbenzyl exhibited the lowest SO2 addition and SO elimination rates. The reaction rate constants are provided to facilitate Claus process simulations to find conditions suitable for BTX oxidation.

A Study on the Production of Isophthalic Acid from M-xylene under the Catalysis of Cobalt and H3PW12O40/Carbon Modified by HNO3 Solution

Abstract The oxidation of m-xylene(MX) to isophthalic acid(IPA) catalyzed by phosphotungstic acid(HPW) supported on modified activated carbon was investigated. The activated carbon loading with HPW is modified by HNO3 solution to ameliorate its catalytic capability in the oxidation of MX to IPA. Experiments have been carried out to study the effects of modification conditions, such as HNO3 concentration, impregnation time, impregnation temperature, activation temperature and activation time, on the catalytic performance of activated carbon. The experimental results demonstrate that the carbon sample impregnated in 10%(vol) HNO3 solution at 45°C for 8 h followed by calcined at 700°C for 4 h has the best catalytic capability. The characterization results imply that the specific surface area and micropores of the carbon samples decrease after being treated with HNO3 solution. But the acidic functional groups on the activated carbon surface increase, which play a vital role in improving the catalytic ability of the HPW/C catalyst in the oxidization of m-xylene to IPA.

Experimental Study and Modeling of Homogenous Catalytic Oxidation of m-Xylene to Isophthalic Acid

The homogeneous oxidation of m-xylene (MX) to isophthalic acid (IPA) with acetic acid as solvent catalyzed by Co–Mn–Br catalyst was investigated at temperatures from 448.2 to 466.2 K by semicontinuous and continuous experiments. Based on the free radical chain reaction mechanism of hydrocarbon, one simplified kinetic model of MX oxidation with six parameters was developed involving MX, IPA, and important intermediates. The kinetic model fitted experimental data well, in which there was only one adjustable parameter, i.e., chain initiation rate constant corresponding to various temperatures, while other parameters were kept constant. It was found that the oxidation rate constants concerning the methyl on the benzene ring of MX was well consistent with the predictions by the Hammett structure–reactivity relationship. Through reasonable assumptions, the well-mixed reactor model of MX oxidation was further developed, by which successful predictions of the continuous MX oxidation process under different operating conditions were performed. Hopefully the kinetic experiments and related models may bring fundamental data and some valuable insights to industrial MX oxidation.

Fabrication of three dimensional (3D) hierarchical Ag/WO3 flower-like catalyst materials for the selective oxidation of m-xylene to isophthalic acid

A three dimensional (3D) hierarchical silver supported tungsten oxide flower-like microsphere catalyst has been fabricated using a cationic surfactant CTAB. It was found that the crystal-splitting mechanism plays a key role in the formation of this flower-like structure. This catalyst was proved to be highly effective in the liquid phase selective oxidation of m-xylene to isophthalic acid.

Simulated impact of NOx on SOA formation from oxidation of toluene and m-xylene

Despite the crucial role of aromatic-derived secondary organic aerosol (SOA) in deteriorating air quality, its formation mechanism is not well understood, and the dependence of aromatic SOA formation on nitrogen oxides (NOx) is not captured fully by most SOA formation models. In this study, NOx-dependent mechanisms of toluene and m-xylene SOA formation are updated using the gas-phase Caltech Atmospheric Chemistry Mechanism (CACM) coupled to a gas/aerosol partitioning model. The updated models were optimized by comparison to eighteen chamber experiments performed under both high- and low-NOx conditions at the University of California – Riverside. Correction factors for vapor pressures imply uncharacterized association chemistry, likely in the aerosol phase. Difficulty of the presented model lies in predicting O3 production well, with overprediction (typically 20–40%), particularly under low NOx conditions (up to one fold). Mean normalized biases for other species (NO2 and SOA) and conditions are smaller (up to ±50%). The newly developed model can predict strong decreases of m-xylene SOA yield with increasing NOx. Simulated SOA speciation implies the importance of ring-opening products in governing SOA formation (up to ∼40–60% for both aromatics). Speciation distributions under varied NOx levels imply that competition between hydroperoxide radical and NO for reaction with a bicyclic peroxide radical as the most commonly accepted mechanism may not be the only factor influencing SOA formation. The reaction of aromatic peroxy radicals with NO competing with self-cyclization (a new mechanism updated in this study) might also affect NOx-dependence of SOA formation. Different simulated importance of a phenolic route in governing SOA formation between toluene and m-xylene suggests that the number of methyl groups on the aromatic ring is important in SOA formation chemistry.

Synthesis of nanometric LaCr0.5Mn0.5O3 perovskite at low temperature by the polyvinyl alcohol gel combustion method

Nanosized LaCr0.5Mn0.5O3 perovskite was synthesised with relatively high surface area (15.5 m2/g) at low temperature (650 °C) by the combustion method using polyvinyl alcohol (PVA) and corresponding metal nitrates. The perovskite was characterised by X-ray diffraction, thermogravimetric and differential thermal analysis, field emission scanning electron microscopy and Brunauer–Emmett–Teller technique. The optimal preparative conditions were PVA/metals 3:1 by mole, pH = 3–4 and 80 °C for gel formation. Perovskite exhibits a good catalytic activity in total oxidation of m-xylene at the low temperature of reaction (250 °C).

Production of isophthalic acid from m-xylene oxidation under the catalysis of the H3PW12O40/carbon and cobalt catalytic system

Isophthalic acid (IPA) is commercially produced from m-xylene oxidation with the catalysis of the homogeneous Co–Mn–Br catalyst system. In this study, a catalytic system consisting of HPW/C and Co(II) has been put forward to oxidize m-xylene (MX) to IPA. The experimental results prove that the HPW/C and Co catalytic system is capable of catalyzing the oxidation of MX to IPA, which can obtain a higher MX conversion and IPA concentration than the homogeneous H3PW12O40/Co(OAc)2/Mn(OAc)2 catalytic system. The heterogeneous catalytic system is also advantageous over the homogeneous catalytic system in the inhibition of the oxidation of acetic acid and IPA. The optimal amount of phosphotungstic acid supported on carbon is 7.5% (wt). The best dosage of HPW/C is 15gl−1. The optimum Co(II) concentration in the catalytic system for IPA production is 0.064% (wt). The best HPW/C activation temperature is 220°C.

Photocatalytic oxidation of benzene, toluene, ethylbenzene and m-xylene in the gas-phase over TiO2-based catalysts

Abstract In the present work, the photocatalytic oxidation of benzene, toluene, ethylbenzene and m-xylene (BTEX) in the gas phase over various UV-irradiated TiO 2 -based catalysts was studied. Specifically, five catalysts were tested: four based on P25 from Degussa adding 0.25% (w/w) Pt, Fe or Ce (P25, P25/Pt, P25/Fe, P25/Ce) and one prepared according to the isopropoxide method. Inlet BTEX concentrations ranged in 0.5–21 ppmv, whereas oxygen concentration was fixed (21%, v/v) at all experiments and the residence time was adjusted to 11.5 s. Ce addition to P25 catalyst led to the highest photocatalytic oxidation rates for benzene and ethylbenzene, whereas P25 proved to be the most active catalyst on toluene photo-oxidation. Regarding m-xylene, the choice of the most effective catalyst depended on m-xylene reactor concentration. Taking all experimental results into account, P25/Ce was the most effective catalyst due to its superiority in the case of benzene and ethylbenzene and its comparable performance to the base catalyst for toluene and m-xylene photocatalytic oxidation. In general, all P25 based catalysts were more active than the isopropoxide catalyst. All conversions showed great dependence on the inlet concentration of the target compound. Water addition to the reactor during ethylbenzene photo-oxidation enhanced the rates achieved for all catalysts tested. It has to be noted that the differences in the reaction rates achieved from catalyst to catalyst decreased with water vapours addition to the reactor. Finally, a Langmuir–Hinshelwood kinetic model has been applied to the experimental data obtained. The kinetic data obtained confirmed the strong beneficial effect of Ce addition to P25 catalyst on benzene and ethylbenzene photocatalytic oxidation.

Chemical aging of <i>m</i>-xylene secondary organic aerosol: laboratory chamber study

Abstract. Secondary organic aerosol (SOA) can reside in the atmosphere for a week or more. While its initial formation from the gas-phase oxidation of volatile organic compounds tends to take place in the first few hours after emission, SOA can continue to evolve chemically over its atmospheric lifetime. Simulating this chemical aging over an extended time in the laboratory has proven to be challenging. We present here a procedure for studying SOA aging in laboratory chambers that is applied to achieve 36 h of oxidation. The formation and evolution of SOA from the photooxidation of m-xylene under low-NOx conditions and in the presence of either neutral or acidic seed particles is studied. In SOA aging, increasing molecular functionalization leads to less volatile products and an increase in SOA mass, whereas gas- or particle-phase fragmentation chemistry results in more volatile products and a loss of SOA. The challenge is to discern from measured chamber variables the extent to which these processes are important for a given SOA system. In the experiments conducted, m-xylene SOA mass, calculated under the assumption of size-invariant particle composition, increased over the initial 12–13 h of photooxidation and decreased beyond that time, suggesting the existence of fragmentation chemistry. The oxidation of the SOA, as manifested in the O:C elemental ratio and fraction of organic ion detected at m/z 44 measured by the Aerodyne aerosol mass spectrometer, increased continuously starting after 5 h of irradiation until the 36 h termination. This behavior is consistent with an initial period in which, as the mass of SOA increases, products of higher volatility partition to the aerosol phase, followed by an aging period in which gas- and particle-phase reaction products become increasingly more oxidized. When irradiation is stopped 12.4 h into one experiment, and OH generation ceases, minimal loss of SOA is observed, indicating that the loss of SOA is either light- or OH-induced. Chemical ionization mass spectrometry measurements of low-volatility m-xylene oxidation products exhibit behavior indicative of continuous photooxidation chemistry. A condensed chemical mechanism of m-xylene oxidation under low-NOx conditions is capable of reproducing the general behavior of gas-phase evolution observed here. Moreover, order of magnitude analysis of the mechanism suggests that gas-phase OH reaction of low volatility SOA precursors is the dominant pathway of aging in the m-xylene system although OH reaction with particle surfaces cannot be ruled out. Finally, the effect of size-dependent particle composition and size-dependent particle wall loss rates on different particle wall loss correction methods is discussed.

Environmentally Friendly Efficient One-Pot Electrochemical Synthesis of 2,4-Dimethylanisole by Selective Oxidation of m-Xylene in Methanol with Metals Promoted Sulfated Zirconia

Herein, we reported the efficiently catalytic synthesis of 2,4-dimethylanisole with electrochemically selective oxidation of m-xylene in methanol under mild conditions. The catalysts of SO42–/M–ZrO2 (M = Mo, Cu, Fe, Co, and Cr), prepared by coprecipitation and postsynthesized sulfation, were characterized by means of Fourier transform infrared (FT-IR) spectroscopy and powder X-ray diffraction (XRD), which indicated that the modification of sulfates and the incorporation of metal ions would not influence the structure of zirconia and the doped metal ions were well dispersed. The electrochemical reaction process was monitored by UV–vis spectroscopy, and the distribution of the products was analyzed by gas chromatography–mass spectrometry (GC–MS). All catalysts exhibited excellent activity for the electrochemical reaction assisted by a pair of porous graphite plane electrodes. X-ray photoelectron spectroscopy (XPS) showed a mixture of Zr2+ and Zr4+ in the catalysts after the reaction, demonstrating the participation of catalyst in this redox reaction. The results revealed that a maximum yield of about 62.1% was achieved for 2,4-dimethylanisole in the SO42–/Mo–ZrO2 reaction system in 3 h. This method provides a new environmentally benign and simple way to synthesize 2,4-dimethylanisole.

The effect of water presence on the photocatalytic oxidation of benzene, toluene, ethylbenzene and m-xylene in the gas-phase

In the present work, the gas–solid heterogeneous photocatalytic oxidation of benzene, toluene, ethylbenzene and m-xylene (BTEX) over UV-irradiated titanium dioxide was studied in an annular reactor operated in the CSTR (continuous stirred-tank reactor) mode. GC-FID and GC-MS were used for analysing reactor inlet and outlet streams. Initial BTEX concentrations were in the low parts per million (ppmv) range, whereas the water concentration was in the range of 0–35,230 ppmv and the residence time varied from 50 to 210 s. The effect of water addition on the photocatalytic process showed strong dependence on the type of the BTEX and the water vapour concentration. The increase in residence time resulted in a considerable increase in the conversion achieved for all compounds and experimental conditions. There was a clear interaction between residence time and water presence regarding the effect on conversions achieved. It was established that conversions over 95% could be achieved by adjusting appropriately the experimental conditions and especially the water concentration in the reactor. In all cases, no by-products were detected above the detection limit and carbon dioxide was the only compound detected. Finally, various Langmuir-Hinshelwood kinetic models have been tested in the analysis of the experimental data obtained. The kinetic data obtained confirmed that water had an active participation in the photocatalytic reactions of benzene, toluene, ethylbenzene and m-xylene since the model involving reaction of BTEX and water adsorbed on different active sites yielded the most successful fitting to the experimental results for the first three compounds, whereas the kinetic model based on the assumption that reaction between VOC and water dissociatively adsorbed on the photocatalyst takes place was the most appropriate in the case of m-xylene.

Plasma Surface Modified TiO2 Nanoparticles: Improved Photocatalytic Oxidation of Gaseous m-Xylene

Titanium dioxide (TiO(2)) is a preferred catalyst for photocatalytic oxidation of many air pollutants. In an effort to enhance its photocatalytic activity, TiO(2) was modified by pulsed plasma treatment. In this work, TiO(2) nanoparticles, coated on a glass plate, were treated with a plasma discharge of hexafluoropropylene oxide (HFPO) gas. By appropriate adjustment of discharge conditions, it was discovered that the TiO(2) particles can be either directly fluorinated (Ti-F) or coated with thin perfluorocarbon films (C-F). Specifically, under relatively high power input, the plasma deposition process favored direct surface fluorination. The extent of Ti-F formation increased with increasing power input. In contrast, at lower average power inputs, perfluorocarbon films are deposited on the surface of the TiO(2) particles. The plasma surface modified TiO(2) nanoparticles were subsequently employed as catalysts in the photocatalytic oxidation of m-xylene in air, as carried out inside a batch reactor with closed loop constant gas circulation. Both types of modified TiO(2) were significantly more catalytically active than that of the unmodified particles. For example, the rate constant of m-xylene degradation was increased from 0.012 min(-1) with untreated TiO(2) to 0.074 min(-1) with fluorinated TiO(2). Although it is not possible to provide unequivocal reasons for this increased photocatalytic activity, it is noted that the plasma surface treatment converted the TiO(2) from hydrophilic to highly hydrophobic, which would provide more facile catalyst adsorption of the xylene from the flowing air. Also, based on literature reports, the use of fluorinated TiO(2) reduces electron-hole recombination rates, thus increasing the photocatalytic activity.