Environmental plasma-catalysis for the energy-efficient treatment of volatile organic compounds

Nonthermal plasma (NTP) coupled with catalysis is a promising technique for the abatement of dilute volatile organic compounds (VOCs), because it is operable under mild reaction conditions, i.e., low temperature and atmospheric pressure. This review addresses the mechanistic aspects of catalyst activation by NTP, such as the generation and fixation of reactive species, facilitation of redox cycles, photocatalysis, and local heating, to clarify the combined effects of plasma and catalysis. The plasma-catalytic removal of VOCs preferentially requires the catalyst to have a large specific surface area, high surface oxygen storage capacity, and to be highly reducible. The energy consumption and deactivation of catalysts are considered by comparing continuous and cyclic operations in terms of specific input energy, VOC removal and energy efficiencies, and byproduct formation. Based on the information in the literature, a plasma-catalytic system operating in cyclic adsorption-oxidation mode is recommended for the treatment of air contaminated by dilute VOCs. Finally, the effects of NTP on the regeneration of deactivated catalysts are also discussed.     

Catalysis in VOC Abatement

Volatile organic compounds (VOCs) are harmful to environment and human health. Catalytic oxidation has been used in VOC abatement for over 60 years, and it has proven to be an effective technology. A large variety of VOCs set high demands for the treatment, and therefore catalytic oxidation needs still to be developed further. This paper reviews current aspects and future research needs related to VOCs and catalytic VOC treatment concentrating on solvent-based, chlorinated and sulphur-containing VOCs.     

工业挥发性有机污染物控制与资源化利用

综述了多种不同的工业挥发性有机污染物(VOCs)控制技术,从工程应用角度分析了其适用范围、优缺点、应用现状和研究方向等。在此基础上,针对目前工业VOCs控制过程存在的低浓度大风量VOCs处理问题、复杂VOCs废气体系处理问题及单一控制方法治理效率低等问题,提出对现有工艺技术设备的优化建议、不同处理方式集成组合、源头治理等改进手段,从宏观角度探讨了VOCs资源化的社会发展需求及解决方案。     

Removal of VOCs from gas streams via plasma and catalysis

Emission of volatile organic compounds (VOCs) has resulted in various environmental issues. Therefore, development of effective VOC removal technology is essential for reducing the adverse effects associated. This work provides a systematic review on VOC removal from gas stream via catalytic oxidation, plasma degradation, and plasma catalysis. For catalytic oxidation of VOCs, possible reaction mechanisms and how physicochemical properties of catalyst influences catalytic performance are presented and discussed, followed by plasma removal of VOCs, VOC degradation, and byproduct formation mechanisms. Next, interactions between plasma and catalyst are interpreted for comprehensive understanding. Last, perspectives are provided for further development of VOC removal technology.     

挥发性有机化合物(VOCs)的低温等离子体-催化协同净化

低温等离子体-催化协同净化技术是一种理想的环境污染治理技术。催化剂的加入可提高等离子体反应中污染物的脱除效率和二氧化碳的选择性，减少副产物的产生，并进一步降低能耗。分析了低温等离子体-催化协同净化挥发性有机化合物的效果与净化原理，并从影响污染物降解率的因素、产物分析和反应动力学等机理性研究方面概括了目前国内外在应用该技术去除挥发性有机污染物方面取得的成果，最后提出了该项技术在环境保护领域的应用前景以及研究方向。     

Hydroxylation of polymethylsiloxane surfaces by oxidizing plasmas

The reaction of a radiofrequency-excited oxygen plasma with the surfaces of cured and uncured polymethylsiloxane produces intense hydroxylation of the surface region as followed by FMIR spectroscopy. Characteristic infrared features indicative of intraor intermolecular hydrogen bonding are evident. Plasma oxidation differs markedly from thermal oxidation processes. Reactions of polymethylsiloxane with nitrogen and air plasmas were also investigated and compared to corona reactions of oxygen, nitrogen, and air. In air corona, nitrogen moieties appear to be introduced. The behavior of polymethylsiloxane surfaces in oxidizing acids is also described.     

挥发性有机化合物的净化处理技术

有机废气中大多含有低浓度的苯、甲苯、苯乙烯、多环芳烃等挥发性有机化合物（VOCs）。治理VOCs污染是大气污染治理的重要组成部分。叙述了吸收法、吸附法、生物法和低温等离子体4种废气净化技术的原理和国内外研究进展情况,并对其发展前景和研究方向进行了探讨。最后认为有机废气的联合协同处理方法是今后的一个重要研究方向。     

On Vehicle VOC and NOx Emissions from the Standpoint of the Tropospheric Ozone Problem

Ozone is formed in and downwind of urban areas from urban emissions of NO_x and volatile organic compounds (VOCs) in the presence of sunlight. The main sources of VOCs in polluted air are motor vehicles, industrial solvents, processes in the petroleum and chemical industries, and vegetation. The main NO_x sources are stationary-source fuel combustion (mainly electric utilities) and motor vehicles. Recent studies have demonstrated that VOC emissions from motor vehicles have been seriously underestimated, and this may well explain why ambient O₃ has not responded well to control efforts. This review presents an overview of the sources, formation, and potential abatement strategies for O₃ pollution in the troposphere, with particular emphasis on the mobile source contribution to O₃ formation.     

Application of the monoliths in DeNOx catalysis

Air pollution abatement catalysis refers to catalytic technologies and processes for reducing emissions of environmentally unacceptable compounds. Major problems related to these catalytic clean-up technologies are mobile emission control, removal of nitrogen oxides (NO_x), sulphur compounds, volatile organic compounds (VOCs) and other pollutants generated by industry or by other stationary sources. Application of the monolith catalysts and/or reactors is a key solution to these problems. This overview describes basic features of the monolithic structures and discusses their development and application prospects focusing on DeNO_x catalysis. The status and ongoing modeling of the monolithic reactors are outlined. Particular emphasis is put on experimental validation and practical applications of the mathematical models of a monolithic reactor.     

Oxygen partial pressure-dependent behavior of various catalysts for the total oxidation of VOCs using cycled system of adsorption and oxygen plasma

In this work, comprehensive investigation was done on the oxygen partial pressure-dependent behavior of the various catalysts using a flow-type plasma-driven catalyst (PDC) reactor. These data provide a useful guideline for the optimization of the cycled system using adsorption and the O₂ plasma-driven catalysis of adsorbed volatile organic compounds (VOCs). The potentials of the tested catalysts for the cycled system were evaluated based on the enhancement factor and the adsorption capability. All the tested materials (TiO₂, γ-Al₂O₃, zeolites) exhibited positive enhancement factor, while negative values with the dielectric-barrier discharge (DBD) plasma alone. TiO₂ catalysts showed the highest enhancement factor of about 100 regardless of the type of metal catalysts and their supporting amount. Based on the experimental findings in this study and the literature information, a plausible mechanism of plasma-driven catalysis of VOCs was suggested.     

低温等离子体处理挥发性有机物的研究进展

低温等离子体技术在处理挥发性有机物(VOCs),特别是在对空气中低含量的VOCs的处理中,具有独特的作用.概述了低温等离子体降解VOCs的基本原理及等离子体反应器结构;综述了低温等离子体技术以及和催化剂联合作用在处理VOCs中的应用;并讨论了低温等离子体处理VOCs的进一步研究方向及其应用前景.     

低温等离子体协同催化降解挥发性有机物的研究进展

低温等离子体协同催化技术在挥发性有机物（VOCs）治理中因具有反应高效、反应条件温和、设备简易等优点而受到广泛的研究和应用。文章介绍了低温等离子体协同催化降解VOCs的基本原理、技术研究进展,简述了低温等离子体的高反应活性在与催化剂的高反应选择性结合后所产生的协同作用,二者的结合不但提高了VOCs的降解效率、减少有害副产物生成,还弥补了单一使用低温等离子体技术的高能耗、副产物多的缺陷。此外,分析了低温等离子体与催化剂的联合方式及特点、低温等离子体与催化剂之间的相互作用和影响以及低温等离子体联合不同类型催化剂的协同原理。指出了研究中对完整机理分析的欠缺以及应用过程中对中间过程监测分析的困难,这也是低温等离子体协同催化降解挥发性有机物研究中的重要内容。     

Nanonickel Particles Supported on Silica. Morphology Effects on Their Surface and Hydrogenating Properties

The combustion of light hydrocarbons finds an important application in volatile organic compounds (VOCs) abatement. Catalytic combustion is interesting in this domain, because it may be carried out at relatively low temperatures and under large air excess. For this purpose, precious metal-based catalysts are very performing, but the quest for a lower-cost alternative solution is necessary. In this field, conducting mixed oxides such as perovskites are good candidates, especially when they are electrochemically promoted. The present work shows that electrochemical promotion of catalytic activity (NEMCA effect) in propene total combustion can be carried out with La_0.8Sr_0.2Co_0.8Fe_0.2O₃, an electron-conducting perovskite-type oxide, deposited on YSZ.     

Organic nitrogen compounds in gas oil blends, their hydrotreated products and the importance to hydrotreatment

In the pretreatment of feeds for catalytic cracking and for HDA, the primary objective is to reduce the amount of organic sulfur and nitrogen compounds in the feedstock [Catal. Rev.-Sci. Eng. 36 (1994) 75] [1]. Organic nitrogen compounds have a significantly negative kinetic effect on hydrotreating reactions. The distribution of the organic nitrogen compounds in feed and hydrotreated products is discussed. Alkyl-substituted carbazoles are found to be the dominant and most refractive organic nitrogen compound in the feed. Our results show that indoles and quinolines are very reactive as compared with carbazoles. From the characterization of the pyrrole benzologues, it is concluded that the more the substituents, the lesser the reactivity. It is well known that conversion of organic sulfur occurs via two different mechanistic routes: the direct and the hydrogenation route [J. Catal. 61 (1981) 523; AIChE J. 27 (1981) 663; J. Catal. 97 (1986) 52; Catal. Today, in press; Polyhedron 16 (1997) 3213] [2, 3, 4, 5 and 6]. The hydrogenation route converts the most refractive S-molecules and plays a very important role in the conversion of N-compounds. N-containing molecules often show a very low reactivity as compared with the analogous sulfur compounds. Several studies using model feedstocks show that nitrogen-containing molecules, and in particular, basic organic nitrogen compounds inhibit the HDS reaction [Appl. Catal. A 170 (1998) 1] [7]. In this study, real feed experiments have demonstrated that even though carbazoles are slow to react and are among the predominant N-compounds, it is the basic N-compounds that are the major inhibiting species in diesel fuels.     

Modeling of Drying of Gaseous Mixtures in TSA System with Fixed Bed of Solid Desiccants

A nonisothermal, nonequilibrium mathematical model was developed to theoretically analyze adsorptive drying of gaseous mixtures containing water and volatile organic compounds (VOCs). The analysis concerns a four-bed cyclic temperature swing adsorption (TSA) system. The two fixed beds are formed of silica gel primarily as the water vapor adsorbent. The other two consist of activated carbon as the adsorbent of the organic component (e.g., benzene, isopropanol). In the model, possible insignificant interactions among the VOCs and water during adsorption and desorption were neglected. The following parameters were considered to study their effect on the process efficiency: relative humidity of the inlet gas, temperature of the purge gas, and height of the adsorbent beds. Simulation results showed that both the shape of the adsorption isotherm and heat effects played an important role in the breakthrough behavior of water vapor adsorption on silica gel. The model accurately simulated experimental data taken from literature.     

REVIEW OF VOLATILE ORGANIC COMPOUNDS DERIVED FROM POLYETHYLENE

The formation of volatile organic compounds (VOCs) in polyethylene (PE) is a topic of concern to industries involved in the packaging of items such as foodstuffs and pharmaceuticals that are sensitive to organoleptic contamination. This article reviews the available literature on VOCs that originate from PE during its manufacture, processing, storage, and service life. The package–product interactions that may occur between PE and packaged foodstuffs are also considered together with the wide range of methods for the analysis of VOCs. The following analytical methods are discussed: (i) sensory evaluation, (ii) chromatographic techniques and their associated sampling techniques, including the “hot-jar” method and dynamic headspace sampling, (iii) gas chromatography–olfactory sensing, and (iv) artificial olfaction or “electronic nose” technology.     

Vapor Adsorption of Volatile Organic Compounds Using Organically Modified Clay

Abstract

Organically modified clay was used to adsorb volatile organic compounds from a gaseous phase. The organoclay was prepared by adsorbing hexadecyltrimethylammonium (HDTMA) on the surface of montmorillonite particles. Two volatile organic compounds (VOCs), chlorobenzene and trichloroethylene, were adsorbed to the organoclay using a fixed adsorption bed. The adsorption was carried out at various inlet concentrations of gaseous VOCs in a carrier gas (nitrogen). The adsorption behavior of VOCs was investigated using natural clay and two types of organoclays, which had different HDTMA loadings. Adsorption breakthrough curves were obtained, and the adsorption data were modeled with two adsorption isotherms. Desorption of VOCs was also conducted using pure nitrogen, and the desorption profiles were fitted with two different theoretical models. It was found that the organoclay possesses significant adsorption capacity towards VOCs and the uptake depends on the degree of HDTMA loading on clay surface.     

介质阻挡放电技术处理挥发性有机物的研究进展

介质阻挡放电技术在处理低浓度挥发性有机物（VOCs）过程中具有反应快速、工艺简单及适应范围广等优点而受到广泛关注。本文从介质阻挡放电单独使用和介质阻挡放电协同催化两方面进行了概括总结。首先,简述了介质阻挡放电处理VOCs所用的驱动电源和等离子体发生器的研究现状及气体性质对VOCs降解性能的影响;其次,介绍了介质阻挡放电协同催化的两种方式（内置式和后置式）及各自情况下采用不同催化剂强化VOCs去除性能、提高能量效率、抑制副产物生成的过程机理;最后,分析了介质阻挡放电技术处理低浓度VOCs过程中存在的关键问题,并提出了未来的重要研究方向为：等离子体催化体系中VOCs的界面反应机理;催化剂的抗积碳性能的提高;适用于多组分VOCs的高效催化剂的开发。     

A review of prospects and challenges of photocatalytic decomposition of volatile organic compounds (VOCs) under humid environment

Volatile organic compounds (VOCs) are harmful for humans and the surrounding ecosystem. Emissions from these pollutants have caused a significant reduction in air quality, which has an effect on people's health. Alkanes, alkenes, alcohols, aromatics, and other VOC pollutants have all been broken down by TiO₂ photocatalytic processes. Due to several operating inefficiencies and deactivation issues in humid environments, the practical application of photocatalysis has not been realized on a broader scale. The effectiveness of photo-oxidation of VOCs is impacted by a variety of environmental conditions. In the photocatalytic oxidation of the VOCs, relative humidity (RH) is critical. Therefore, it is important to review the recent findings on how humidity affects the photocatalytic breakdown of VOCs in air. To satisfy this need, this work provides a critical review of the related literature with focus on the fundamentals of photocatalysis, photocatalytic degradation of air pollutants, and the influence of humidity on the photocatalytic process degradation for selected air pollutants. It also highlights the kinetic models and typical photocatalytic reactor and supports for VOC removal.