Kinetic study for photocatalytic degradation of volatile organic compounds in air using thin film TiO2 photocatalyst

The present paper examined the kinetics of photocatalytic degradation of volatile organic compounds (VOCs) including gaseous trichloroethylene (TCE), acetone, methanol and toluene. Variable parameters were initial concentration of VOCs, water vapor content and photon flux of ultra-violet (UV) light. A batch photo-reactor was specifically designed for this work. The photocatalytic degradation rate increased with increasing the initial concentration of VOCs, but maintained almost constant beyond a certain concentration. It matched well with the Langmuir–Hinshelwood (L–H) kinetic model. For the influence of water vapor in a gas phase photocatalytic degradation rate, there was an optimum concentration of water vapor in the degradation of TCE and methanol. And, water vapor enhanced the photocatalytic degradation rate of toluene, whereas it inhibited that of acetone. As for the effect of photon flux, it was found that photocatalytic degradation occurs in two regimes with respect to photon flux.     

The effect of water vapor and inert gases on the carbon-oxygen reaction

The interpretation of the effect of small quantities of water vapor on the kinetics of the gasification of carbon by oxygen is re-examined in the light of recent studies describing conditions under which an explosive reaction may occur in this mixture of reactant and products: oxygen, carbon monoxide, carbon dioxide and water vapor. The generation of water from residual hydrogen in the carbon is a key factor in the development of explosive conditions through a radical chain reaction in the gas phase. Some effects of inert gases on the rate of gasification are also explained by their influence on gas-phase reactions through stabilization of radicals and inhibition of diffusion of atoms to the walls. It is concluded that the possibility of gas-phase radical reactions in mixtures of carbon monoxide and oxygen, which depends critically on the presence of water vapor, should be considered in the interpretation of the kinetics of carbon gasification by oxygen.     

Effect of humidity on the photocatalytic degradation of trichloroethylene in gas Phase over TiO2 thin films treated by different conditions

The effect of humidity on the photocatalytic degradation reaction of trichloroethylene (TCE) in gas phase was investigated by using pretreated TiO₂ sol-gel films. It was observed that the photocatalytic activity of the TiO₂ films depended more greatly on the pretreatment conditions, for example, UV pre-illumination, than on the moisture content. When the relative humidity was over 50%, the reaction rates decreased regardless of treatment conditions of the photocatalyst. The photocatalytic decomposition rate of TCE increased with the increase of light intensity. However, the influence of humidity on the reaction rate was less significant under the increased light intensity. The intermediates and byproducts of the reaction were not changed in different humidity conditions.     

The kinetics of photocatalytic degradation of trichloroethylene in gas phase over TiO2 supported on glass bead

In this investigation, a packed bed filled with coated titanium dioxide glass beads to study the kinetics of photocatalytic degradation of trichloroethylene under irradiation of 365 nm UV light. In the range of 100–500 ml/min of flowrate, the reaction rate for 6 μM TCE increased with an increasing flowrate upto 300 ml/min, while was not affected by the flowrate at the values higher than 300 ml/min. For moisture in the range of 9.4–1222.2 μM, the reaction rate of TCE was decreased with an increasing humidity. The adsorbed water on the catalyst surface could compete with the adsorption of TCE on the sites. The reaction rate of 6 μM TCE increased as the light intensity increased, and was proportional to the 0.61 order of the light intensity. Among the three L–H bimolecular form models, the experimental data had the best fit for one of models:

     

Photocatalytic degradation of trace gaseous acetone and acetaldehyde using TiO2 supported on fiberglass cloth

The feasibility of photocatalytic degradation of trace gaseous acetone and acetaldehyde using TiO₂ supported on fiberglass cloth was studied. The results show that gaseous acetone and acetaldehyde can be completely photocatalytically degraded into CO₂ and H₂O, after 35 min illumination with a 375 W medium pressure mercury lamp. The effects of parameters such as the flow of reactant gases and the concentrations of oxygen and water vapor in the feed stream on the photocatalytic degradation were studied. TiO₂ supported on fiberglass cloth was not easily detached. After 150 h illumination there was no significant loss of the photocatalytic activity of TiO₂. © 1998 Society of Chemical Industry     

室内空气中三氯乙烯在TiO_2/AC上的光催化氧化反应动力学

采用具有多层结构的蜂窝状整体式光催化净化块为催化材料 ,设计了室内空气净化光催化反应器 ,以三氯乙烯 (TCE)为模拟污染物 ,在 2 5℃、常压下 ,进行水、TCE的吸附等温线的测试和 TCE的光催化降解反应动力学研究。水与 TCE的平衡吸附采用 L angmuir吸附等温线表示 ;TCE在 Ti O2 表面的光催化反应采用简单的 L- H速率方程来表征 ;并进行了模型值与实验值的比较 ,结果令人满意     

PHOTOCATALYTIC DEGRADATION OF GASEOUS AMMONIA AND TRICHLOROETHYLENE OVER TiO 2 ULTRAFINE POWDERS DEPOSITED ON ACTIVATED CARBON PARTICLES

Two kinds of activated carbon (AC) particles, which possess large adsorption capacity for gaseous ammonia and trichloroethylene (TCE), were selected as the carrier of TiO 2 ultrafine powders. Commercially available TiO 2 sol solutions were used as the source of TiO 2 powders. For the sake of comparison, porous glass (PG) particles and alumina (AL) particles were also selected as the carrier. The photocatalytic activity of TiO 2 /AC, TiO 2 /PG, and TiO 2 /AL was examined using a circulation reaction method. The photocatalytic degradation of gaseous ammonia and TCE was found apparently to obey the first-order reaction with respect to ammonia or TCE. The first-order reaction rate constant over TiO 2 /AC was shown apparently to be higher than those over TiO 2 /PG and TiO 2 /AL. The observed relationship between the apparent first-order rate constant and the TiO 2 catalyst loading could be interpreted satisfactorily in terms of a reaction model based on the premise that the degradation rate constant is proportional to the illumination intensity, whose decay obeys the Lambert-Beer law; this is an extension of the reaction model in a three-phase fluidized-bed reactor under illumination proposed in our preceding paper.     

Photocatalytic degradation of gaseous trichloroethene using immobilized ZnO/SnO2 coupled oxide in a flow‐through photocatalytic reactor

The photocatalytic degradation of gaseous trichloroethene (TCE) was investigated on immobilized ZnO/SnO₂ coupled oxide in a flow‐through photocatalytic reactor. It was found that gaseous photocatalysis is an efficient method for volatile organic compounds' abatement and air purification. Degradation of ～100% was found for TCE at the concentrations examined, up to 400 ppmv, in a flow‐through dry synthetic gas stream. In our tested conditions, the flow rate had little influence on the photocatalytic degradation efficiencies of TCE, while the relative humidity had a significant influence on the photocatalytic degradation of TCE. The photocatalytic degradation efficiencies of TCE increased slowly below 20% relative humidity and then decreased as the relative humidity increased further. The deactivation of used immobilized photocatalyst was not observed within the 200 h testing period in the present experiment, although the surface of the photocatalyst changed greatly during the use of the photocatalyst. Copyright © 2004 Society of Chemical Industry     

Studies on the catalytic dechlorination and abatement of chlorided VOC: the cases of 2-chloropropane, 1,2-dichloropropane and trichloroethylene

The conversion of monochloropropanes and dichloropropanes over acid catalysts has been investigated in the presence of oxygen. In the temperature range of 450–550 K, dehydrochlorination of monochloropropanes to propene and HCl occurs selectively over silica–alumina, while significant formation of chlorinated by-products is observed over ZSM5 zeolite catalyst even at higher temperatures. Dichloropropanes conversion over silica–alumina catalyst gives rise mainly to chloropropenes in the temperature range 500–700 K. CO_x are predominant products only at the highest reaction temperatures (just above 700 K). Water vapor in the feed only slightly affects conversions and selectivities. Deactivation processes occur upon dichloropropane conversion, mainly due to coke deposition.

The conversion of highly chlorinated compounds, such as trichloroethylene (TCE) has been tested over silica–alumina and over HY zeolite in the presence of water vapor in the so-called “steam reforming” conditions (HVOC:water=1:2). With diluted feed (1200 ppm) on HY, reaction occurs above 800 K and formation of chlorinated by-products is minimized, CO_x being the main reaction products. At higher HVOC concentrations conversion is obtained at even lower temperature (600 K), but no more negligible by-products formation has been detected. In our conditions zeolite catalyst is more effective in TCE total conversion than silica–alumina.     

Photocatalytic Degradation of Metoprolol Tartrate

The advanced oxidation of high purity metoprolol tartrate, extracted from a commercial medicament, with TiO₂ and UV light (365 nm) was investigated to determine the effect of initial reactant concentration on the reaction rate and the role of direct photolysis on the photocatalytic process. Analysis of the reaction samples by UV–Vis spectroscopy indicated that metoprolol tartrate is efficiently degraded by photocatalysis via hydroxylation of the aromatic ring. Kinetic studies indicated that the photocatalytic degradation of metoprolol tartrate follows a Langmuir, Hinshelwood, Hougen and Watson (LHHW) mechanism where the reaction order shifts from zero order to first order as the reactant concentration drops. Additional experiments showed that direct photolysis plays a minor role on the photocatalytic oxidation of metoprolol tartrate. Total organic carbon (TOC) studies demonstrated that metoprolol tartrate is transformed to other organic intermediate reaction products before complete mineralization to CO₂. The fraction of reactant transformed into intermediate organic products was evaluated by a material balance using the results of analysis of the reaction samples by high performance liquid chromatography and TOC.     

微乳法制备棒状ZnO材料及其光催化性能

采用浊度法绘制十六烷基三甲基溴化铵(CTAB)/正丁醇/环己烷/硝酸锌或草酸铵水溶液体系的拟三元相图,确定微乳液稳定区域最大时CTAB与正丁醇的质量比为1∶1。在此条件下,选择m(CTAB+正丁醇)∶m(环己烷)=3∶7,以硝酸锌和草酸铵为原料,采用微乳法制备ZnO的前驱体二水合草酸锌,然后通过煅烧得到ZnO样品。考察水与CTAB的摩尔比(R)和反应物浓度对ZnO材料结构、形貌、光学性质和光催化性能的影响。实验结果表明:ZnO样品为纯的六方纤锌矿结构晶体,形貌均为棒状,且样品的尺寸随R值和反应物浓度的不同而不同。在300 W汞灯紫外光照射下,ZnO样品对亚甲基蓝溶液均具有较好的光催化性能,光催化反应过程符合准一级反应动力学。其中,在反应物浓度为0.15 mol/L,R=15条件下制备的ZnO样品在光照90 min时可使亚甲基蓝的降解率达到97.0%,且降解反应速率常数k最大,其光催化性能最好。     

Removal of Refractory Organics from Water by Aeration. I. Methyl Chloroform

Abstract

A mathematical model for the removal of volatile organics from water by aeration and by solvent sublation into an organic phase is presented. The model includes the effect of the finite rate of solute mass transfer from the aqueous into the vapor phase. Results are calculated for the removal of 1,1,1-trichloroethane (TCE) chloroform, and benzene from water with 1-octanol, anisole, or kerosene as the organic phase. Experimental data on the aeration of TCE and on its solvent sublation into I-octanol are in good agreement with the model. Small bubbles and long water columns greatly reduce the inefficiency of the process caused by mass transfer rate limitations.     

Molecular Dynamics Study of a Model SN1 Dissociation Reaction at Liquid/Liquid Interfaces: Effect of Liquid Polarity

The ionic dissociation step of the nucleophilic substitution reaction: t-BuCl → t-Bu⁺ + Cl⁻ is studied at the water/dichloroethane (DCE) interface using molecular dynamics computer simulations. The t-BuCl is modeled using an empirical valence bond method where two diabatic states, covalent and ionic, are coupled in the electronically adiabatic limit. Umbrella sampling is used to determine the potential of mean force (PMF) along the reaction coordinate R (defined as the t-Bu to Cl distance) in several interfacial regions of varying distances from the Gibbs dividing surface. The results at the water/DCE interface are compared to previous molecular dynamics calculations of t-BuCl at the water liquid/vapor and water/carbon tetrachloride interfaces. As in the other systems, the transition state shifts to larger R values, and the activation barrier and ΔG_rxn increase with decreasing solvent polarity. In contrast with the water/carbon tetrachloride interface, a well-defined transition state exists at the water/DCE interface and persists even as the solute is moved 3 to 6 Å into the DCE phase. Dynamical flux correlation calculations reveal larger deviation of the rate from TST than in bulk water due to slower vibrational relaxation of the product ions. However, the increased density at the water/DCE interface increases the rate of dissociation relative to the water liquid/vapor interface. The transmission coefficient at the water/DCE interface was found to be 25% of the TST rate prediction, or about twice the rate at the water liquid/vapor interface.     

An Unreacted Shrinking Core Model for Calcination and Similar Solid-to-Gas Reactions

A variation on the unreacted shrinking core model has been developed for calcination and similar non-catalytic solid-to-gas decomposition reactions in which no gaseous reactant is involved and the reaction rate decreases with increasing product gas concentration. The numerical solution of the model has been validated against an analytical solution for the isothermal case. The model parameters have been tuned using literature data for the thermal dehydration (calcination) of gibbsite to alumina over a wide range of temperatures, from 490 to 923 K. The model results for gibbsite conversion agreed well with the published experimental data. A reaction order with respect to water vapor concentration of n = ?1 was found to give a good fit to the data and yield activation energies consistent with literature values. Predictions of the non-isothermal unreacted shrinking core model compare well with a more complex distributed model developed previously by the authors.     

液相合成纳米氧化锌及其光催化性能探讨

以硝酸锌和氢氧化钠为原料,分别以水和醇作为溶剂,采用液相直接沉淀法合成纳米氧化锌。在以水为反应溶剂条件下,研究了反应时间、反应温度、反应物浓度和反应物配比对纳米氧化锌中位径和形貌的影响,得到制备较小中位径及合适长径比纳米氧化锌的最优反应条件,即:反应时间为3 h、反应温度为70 ℃、锌离子浓度为0.5 mol/L、锌离子与氢氧根浓度比为1:2。在上述最优反应条件下,研究了不同醇溶剂对纳米氧化锌中位径、形貌及其光催化性能的影响。研究发现,随着溶剂碳链长度的增长,纳米氧化锌中位径呈现上下波动,无明显变化规律,形貌由球状向柱状发展,光催化性能总体不断减弱。     

Gas-phase photocatalytic degradation of trichloroethylene on pretreated TiO2

This paper reports the effects of preillumination, prechlorination and prehydroxylation of TiO₂ glass fiber cloth (TiO₂-GFC) on the photocatalytic degradation (PCD) reaction of trichloroethylene (TCE) in gas–solid regime. The reaction was monitored in situ by FT-IR spectroscopy at room temperature (298 K). Product analysis by gas chromatography–mass spectrometry (GC–MS) revealed the formation of a new by-product, 1,1-dichloroethane (1,1-DCE) in significant amount along with other known by-products. The photocatalytic activities of TiO₂-GFC and the mineralization of TCE were dependent on preillumination, prehydroxylation and prechlorination while the product yield was significantly influenced by prehydroxylation and prechlorination of TiO₂-GFC. Prechlorination increased the yields of phosgene (COCl₂) and pentachloroethane (C₂HCl₅) while prehydroxylation decreased the yield of COCl₂ with corresponding increases in the yield of oxalyl chloride (COClCOCl) and the mineralization of TCE, suggesting a possible surface-mediated hydrolysis of phosgene to COClCOCl in the latter case. Reaction schemes have been proposed to account for the formation of 1,1-DCE and COClCOCl. The photocatalytic activity of TiO₂-GFC and the mineralization of TCE have been found to correlate with the concentration of HCl employed for the prechlorination of TiO₂-GFC.     

Characterization of TiO2 photocatalysts used in trichloroethene oxidation

Kinetic studies show deactivation of TiO₂ catalysts during aqueous-phase and gas-phase photooxidation of trichloroethene (TCE). Temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) were used to examine adsorbed species on TiO₂ photocatalyst surfaces after reaction, and TPD was used to determine how reactants and products adsorb on the TiO₂ surface. Used and deactivated catalysts were analyzed after participating in either aqueous-phase or gas-phase photooxidation of TCE. The XPS spectra showed little difference between the surface composition of fresh TiO₂ and that of a deactivated catalyst from the aqueous-phase photoreactor. Chlorine was observed only on catalysts used in the gas-phase photocatalytic decomposition of TCE. Differences due to photoreaction were observed in TPD spectra of water, carbon monoxide, and carbon dioxide. Both the total amount desorbed and the temperature of desorption of carbon monoxide and carbon dioxide were quite different for used and deactivated catalysts from the two photoreactions. Apparently strongly bound species, such as carbonates, accumulated on the surface and formed carbon monoxide upon high-temperature decomposition. Small amounts of chlorinated compounds desorbed from the used and deactivated catalysts following gas-phase photoreaction. Dichloroacetyl chloride (DCAC), a reaction intermediate, can adsorb strongly on TiO₂ and readily displaces TCE. Thermally decomposed DCAC reduces the number of available adsorption sites for DCAC and TCE. An interesting low-temperature oxygen desorption peak was observed from catalysts treated with H₂O₂, which improves catalytic activity. This feature indicates that H₂O₂ is stable on TiO₂ at room temperature and decomposes at 420 K.     

Investigation of thermochemical conversion of biomass in supercritical water using a batch reactor

This study focused on gasification of biomass and a biomass model compound. Data are presented that show the presence of supercritical water enhances gasification efficiency, as it participates as both a solvent and a reactant. It is established that biomass gasification efficiencies are in the same range for all types of biomass. The thermodynamic changes of state are functions of elemental composition, not biomass species. The oxidation state of carbon atom of biomass is a key variable in determining the changes in enthalpy during both conventional combustion and supercritical water gasification. The oxidation state of the feed (together with the reaction conditions that influence the degree to which water participates as a reactant) also determines the vapor product composition.Decomposition reactions to vapor products are rapid and complete at high temperature (?550 °C), catalytic mediation is not required. Temperature and residence time are important operating parameters for SCW gasification. Less important are the pressure of gasification (in the range of 40-67 MPa) and the presence of catalyst. The vapor yield, gas composition, the carbon and hydrogen balance of SCW gasification are functions of gasification temperature, residence time and biomass load (concentration).     

The nature of low temperature deactivation of CoCr2O4 and CrOx/γ-Al2O3 catalysts for the oxidative decomposition of trichloroethylene

The CoCr₂O₄ and CrO_x/γ-Al₂O₃ catalysts were used for the oxidative decomposition of trichloroethylene (TCE). Both catalysts showed an initial deactivation at low temperatures around 280 °C, mainly due to the dissociative adsorption of reactant TCE. This was confirmed by the temperature programmed oxidation of TCE where the carbon oxides were formed up to a temperature below 300 °C. Possible changes in the oxidation state of chromium species were observed with XANES and ESR. During the oxidation reaction at low temperatures, the Cr(VI) species were reduced to Cr(III) species, which seemed to be coupled with TCE adsorption. At higher temperatures, however, the Cr(VI) species appeared again and the catalytic activity was completely recovered.     

Photocatalytic Degradation of Phenol with Fe-Titania Catalysts

Iron titanate (FeTiO₃) obtained from rich titanium and iron mineral presented some absorption of UV light and photocatalytic activity for the degradation of phenol in aqueous solution. However, most of the reactant is converted to carboxylic acids that remain in the reaction mixture because natural ilmenite do not produce enough HO^? radicals to completely mineralize the original reactant at the same rate as pure titania (Degussa P25). The XRD analysis of Fe-titania catalysts prepared by simple mixing of FeTiO₃ and TiO₂ showed that the structure of TiO₂ is not modified when small amounts of FeTiO₃, in the order of 10% weight, are well mixed with pure titania. Therefore, the photocatalytic activity of these catalysts is not affected by the presence of FeTiO₃. But, heavy ilmenite particles induce the separation of the catalysts particles after the photocatalytic degradation processes.