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.     

锰基催化剂协同等离子降解VOCs研究进展

等离子催化技术中，Mn基催化剂以其优异的催化降解VOCs和臭氧性能受到国内外研究者的广泛关注。本文从以下方面进行了综述：催化剂在等离子降解中的作用机理，包括改变放电状态、激发新的活性自由基、提供反应位点；常见的单金属及复合金属活性相类型；介绍了浸渍法、水热合成法、溶胶凝胶法和共沉淀法等主要制备方法；催化剂和等离子的协同方式；从统计角度分析文献报道中Mn基催化剂对甲苯的降解效率及抑制臭氧、NO_x的作用；最后对其研究前景进行了展望：Mn基催化剂将依然是该领域的研究热点；通过引入其他金属和非金属、增加活性相分散度、提升载体吸附性能等方法进一步提高催化剂活性和稳定性；利用原位技术探索等离子与活性相的作用机理。     

Improved oxidation of air pollutants in a non-thermal plasma

The performance of non-thermal plasma (NTP) for the removal of organic air pollutants (especially in low concentrations) is improved by the introduction of ferroelectric and catalytically active materials into the discharge zone of an NTP reactor. Experiments with model systems (various contaminants and packed-bed materials) have shown that such a modification of a homogeneous gas-phase plasma can overcome the most serious restrictions of the NTP technique at its present state of the art: the incomplete total oxidation (i.e. the low selectivity to CO₂) and the energetic inefficiency.

Placing a ferroelectric packed-bed material in the discharge zone was shown to result in a lowering of the energy input required. The main effects of plasma catalysis enabled by the introduction of a catalytically active material were an enhanced conversion of pollutants and a higher CO₂ selectivity. These improvements are based on the presence of short-lived oxidising species in the inner volume of porous catalysts. Additionally, the formation of a reservoir of adsorbed oxidants in the NTP zone could be shown. The combination of both modifications (ferroelectric packed-bed materials and plasma catalysis) is a promising method to support the NTP-initiated oxidation of air pollutants.     

Nonthermal plasma (NTP) activated metal–organic frameworks (MOFs) catalyst for catalytic CO2 hydrogenation

A systematic study of Ni supported on metal–organic frameworks (MOFs) catalyst (i.e., 15Ni/UiO-66) for catalytic CO₂ hydrogenation under nonthermal plasma (NTP) conditions was presented. The catalyst outperformed other catalysts based on conventional supports such as ZrO₂, representing highest CO₂ conversion and CH₄ selectivity at about 85 and 99%, respectively. We found that the turnover frequency of the NTP catalysis system (1.8 ± 0.02 s⁻¹) has a nearly two-fold improvement compared with the thermal catalysis (1.0 ± 0.06 s⁻¹). After 20 hr test, XPS and HRTEM characterizations confirmed the stability of the 15Ni/UiO-66 catalyst in the NTP-activated catalysis. The activation barrier for the NTP-activated catalysis was calculated as ~32 kJ mol⁻¹, being lower than the activation energy of the thermal catalysis (~70 kJ mol⁻¹). In situ DRIFTS characterization confirmed the formation of multiple carbonates and formates on catalyst surface activated by NTP, surpassing the control catalysts (e.g., 15Ni/α-Al₂O₃ and 15Ni/ZrO₂).     

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。     

Combination of non-thermal plasma and heterogeneous catalysis for oxidation of volatile organic compounds: Part 2. Ozone decomposition and deactivation of γ-Al2O3

The role of ozone was studied for two different configurations combining non-thermal plasma (NTP) and heterogeneous catalysis, namely the use of a gas phase plasma with subsequent exposure of the effluent to a catalyst in a packed-bed reactor (post-plasma treatment) and the placement of the catalyst directly in the discharge zone (in-plasma catalysis). Non-porous and porous alumina and silica were deployed as model catalysts. The oxidation of immobilised hydrocarbons, toluene as a volatile organic compound and CO as an inorganic pollutant were studied in both operational modes.

While conversion and selectivity of hydrocarbon oxidation in the case of catalytic post-plasma treatment can be fully explained by the catalytic decomposition of O₃ on γ-Al₂O₃, the conversion processes for in-plasma catalysis are more complex and significant oxidation was also measured for the other three materials (-Al₂O₃, quartz and silica gel). It became obvious that additional synergetic effects can be utilised in the case of in-plasma catalysis due to short-lived species formed in the NTP.

The capability of porous alumina for ozone decomposition was found to be correlated with its activity for oxidation of carbon-containing agents. It could be clearly shown that the reaction product CO₂ poisons the catalytic sites at the γ-Al₂O₃ surface. The catalytic activity for O₃ decomposition can be partially re-established by NTP treatment. However, for practical purposes the additional reaction pathways provided by in-plasma catalytic processes are essential for satisfactory conversion and selectivity.     

挥发性有机化合物催化氧化技术

基于环境友好,对使用催化氧化法去除挥发性有机化合物(VOCs)的原理、特点以及催化剂、工艺流程等研究进行综述。挥发性有机化合物完全催化氧化机理分为:Mars-van Krevelen(MVK)模型、Langmuir-Hinshelwood(L-H)模型和Eley-Rideal(E-R)模型。复合金属氧化物催化剂是研究的热点,去除VOCs的核心是使反应温度降低,即具有低温和高活性的催化剂。延长催化剂寿命、提高去除效率也可以带来良好的节能效果和降低投资成本。对于低浓度和大体积VOCs排放,可通过吸附+催化混合法先进技术实现去除,且成功应用于实践。     

固定源废气处理的催化剂涂覆工艺研究进展

由工业固定源排放的NO_x和挥发性有机物（VOCs）是大气复合污染物的重要前体物质。整体式催化剂常被用于脱除NO_x和VOCs。涂覆法是可工业化大规模生产整体式催化剂的工艺之一，具有活性组分用量少、生产成本低的优点，此工艺被广泛用于固定源废气处理的催化剂制备领域。但是由于高通量烟气的冲刷会造成表面活性组分损失和寿命降低，限制其应用，所以在设计催化剂的过程中不但需要关注催化剂涂层的高活性，还需要重点考察其耐磨损性。本文综述了国内外用于固定源废气处理整体式催化剂涂覆工艺的研究进展，主要分析了预处理方法对蜂窝陶瓷载体物理化学性质的影响；重点综述了蜂窝陶瓷负载催化剂活性组分的常规涂覆方法，分为间接涂覆法和直接涂覆法，进一步对间接涂覆法的第二载体涂层进行分类探讨，简述了直接涂覆法的浆料配方及工艺参数对涂层性能的影响，然后对两种涂覆工艺的优缺点进行论述，最后总结了整体式催化剂在固定源废气（NO_x和VOCs）处理领域的相关应用。     

燃烧处理挥发性有机污染物的研究进展

随着环境问题的日益严重,治理作为PM_2.5前体的挥发性有机物（VOCs）越来越受到重视,燃烧法是目前常用的处理VOCs污染物技术之一。本文从燃烧的机理出发综述了燃烧法处理VOCs的研究进展,将燃烧法分为两大类,即非催化燃烧法和催化燃烧法。非催化燃烧法中从燃烧方式出发,总结了直接燃烧法、蓄热式热力燃烧法、多孔介质燃烧法的研究进展,并对燃烧影响因素进行了综述。在催化燃烧法中阐述了贵金属催化剂、非贵金属催化剂和复合金属氧化物催化剂的研究进展,探讨了催化剂的失活问题,分析了每种催化剂的优势与不足。贵金属催化剂活性高,但是价格昂贵、稳定性差;非贵金属催化剂价格低廉、寿命长,但是起燃温度高;复合金属氧化物催化剂活性高、抗毒性强,但是制备工艺复杂。最后基于目前的研究现状和不足,展望了未来燃烧法处理VOCs的研究方向为：结合实际应用的工艺条件和催化燃烧的机理,制备出活性高、价格低廉、抗毒性强和寿命长的催化剂用于蓄热式催化燃烧技术;将催化燃烧和多孔介质燃烧相结合,开发出高效、稳定、经济的燃烧技术处理VOCs污染物。     

低温等离子体结合催化去除VOCs的研究进展

等离子体技术因其工艺简单、处理流程短及适用范围广的优点被用于VOCs的去除,而近年来兴起的低温等离子体结合催化技术,能进一步的提高去除率、降低能耗、减少二次污染,为有效去除VOCs指引了一个新的发展方向.文章综合概述了国内外近几年对此技术的作用机理、影响去除率的因素及尝试去除VOCs有机物的研究进展,最后对此技术进行了展望.     

Combination of non-thermal plasma and heterogeneous catalysis for oxidation of volatile organic compounds: Part 3. Electron paramagnetic resonance (EPR) studies of plasma-treated porous alumina

The effect of plasma processes inside the intra-particle volume of porous materials (especially Al₂O₃) was studied in order to evaluate the potential of the combination of non-thermal plasma (NTP) and in situ heterogeneous catalysis (plasma catalysis), for the improvement of efficiency and selectivity towards total oxidation of organic pollutants in gas cleaning applications. Electron paramagnetic resonance (EPR) spectroscopy was applied as an appropriate method to detect both the formation of radical species by the NTP as well as the initiation of structural changes to the catalyst.

The presence of paramagnetic oxygen or hydroxyl species (O⁻, O₂⁻ or OH) could not be detected by EPR spectroscopy. The observed signal was not significantly influenced either by the type of atmosphere present during NTP treatment or by applying reducing agents to the sample after plasma treatment.

However, by using non-porous and porous alumina (- and γ-Al₂O₃) as model catalysts, the effect of NTP modifying the surface structure in the interior of a porous material could be clearly demonstrated. A paramagnetic species probably related to an AlOO aluminium peroxyl group was formed by NTP processes independently of the oxygen content of the gas atmosphere. It was not formed when the alumina sample was positioned in the off-gas flow of a plasma reactor, i.e. used in the post-treatment mode.

The structure of the paramagnetic site was investigated by employing several spectroscopic tools (X- and Q-band EPR, electron spin echo envelope modulation [ESEEM] and EPR measurements after pre-deuteration).     

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.     

负载贵金属催化剂在挥发性有机物氧化反应中的应用

挥发性有机物(VOCs)是大气污染的主要来源,危害人体健康。催化氧化法是消除挥发性有机物的有效手段,其核心是高效催化剂,新型、高活性、高稳定性催化剂的研发具有重要意义。简要综述近年来负载Au、Pd和Pt贵金属催化剂对VOCs氧化消除的催化性能,分析VOCs氧化在典型催化剂表面形成的活性物种及其对催化活性的影响,并展望VOCs催化氧化的未来发展趋势。     

催化型低温等离子体反应器净化废气研究进展

催化型低温等离子体反应器可有效地提高废气治理的能量效率和净化效果.现有数据表明,在一定能量密度下,催化型低温等离子体反应器比传统低温等离子体反应器能量效率有1.1～12倍的提高,这和污染物种类,反应器构型及催化剂参数有关.本文介绍了反应机理、反应器构型及催化剂参数选择等对反应器性能的影响,并指出今后研究的发展方向.     

Recent advances in non-thermal plasma(NTP) catalysis towards C1 chemistry

C1 chemistry mainly involves the catalytic transformation of C1 molecules(i.e., CO, CO_2, CH_4 and CH_3OH), which usually encounters thermodynamic and/or kinetic limitations. To address these limitations, non-thermal plasma(NTP) activated heterogeneous catalysis offers a number of advantages, such as relatively mild reaction conditions and energy efficiency, in comparison to the conventional thermal catalysis. This review presents the state-of-the-art for the application of NTP-catalysis towards C1 chemistry, including the CO_2 hydrogenation,reforming of CH_4 and CH_3OH, and water-gas shift(WGS) reaction. In the hybrid NTP-catalyst system, the plasma-catalyst interactions are multifaceted. Accordingly, this review also includes a brief discussion on the fundamental research into the mechanisms of NTP activated catalytic C1 chemistry, such as the advanced characterisation methods(e.g., in situ diffuse reflectance infrared Fourier transform spectroscopy, DRIFTS), temperatureprogrammed plasma surface reaction(TPPSR), kinetic studies. Finally, prospects for the future research on the development of tailor-made catalysts for NTP-catalysis systems(which will enable the further understanding of its mechanism) and the translation of the hybrid technique to practical applications of catalytic C1 chemistry are discussed.     

光热协同催化净化挥发性有机物的研究进展及展望

挥发性有机物（VOCs）是一类重要的空气污染物。催化氧化技术可以将VOCs转化为无毒的CO₂和H₂O,是有效的治理方式之一。针对传统的热催化氧化技术的高能耗和光催化净化VOCs技术的低效率问题,光催化耦合强化热催化的光热协同催化净化VOCs技术近些年来受到广泛关注,并表现出比传统热催化或光催化净化技术更优异的净化性能。总结了近年国内外研究者在光热催化净化VOCs领域所取得的主要研究进展,重点讨论了光热协同作用机制的认知和光热协同催化材料的设计理念,包括贵金属型和金属氧化物型光热协同催化材料,并对光热协同催化净化技术的未来发展方向进行了展望。     

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。     

The activity and mechanism of uranium oxide catalysts for the oxidative destruction of volatile organic compounds

Uranium oxide based catalysts have been investigated for the oxidative destruction of volatile organic compounds (VOCs) to carbon oxides and water. The catalysts have been tested for the destruction of a range of organic compounds at space velocities up to 70 000 h⁻¹. Destruction efficiencies greater than 99% can be achieved over the appropriate uranium based catalyst in the temperature range 300–450°C. Volatile organic compounds investigated include benzene, butylacetate, cyclohexanone, toluene, methanol, acetylene, butane, chlorobutane and chlorobenzene. The catalysts are thermally stable, destroy low concentrations and mixtures of VOCs and lifetime studies indicate that deactivation during oxidation of chlorinated VOCs did not occur. A temporal analysis of products (TAPs) reactor is used to investigate the mechanism of oxidation of VOCs by uranium oxide catalysts. Studies indicated that VOCs were oxidised directly to carbon oxides on the catalyst surface. A combination of TAP pulse experiments with oxygen present and absent in the gas phase has indicated that the lattice oxygen from the catalyst is responsible for the total oxidation activity. This has been confirmed by studies using isotopically labelled oxygen which indicates that the catalyst operates by a redox mechanism.     

Pd/Pt promoted Co3O4 catalysts for VOCs combustion: Preparation of active catalyst on metallic carrier

Emission control of volatile organic compounds (VOCs) is one of the priorities for environmental catalysis. Metallic microstructural short-channel reactors of various geometries are regarded as an alternative to ceramic monoliths. The paper presents the results on the catalyst preparation and optimisation. Chromium-aluminium (CrAl) steel was surveyed in terms of its applicability for carrier manufacturing and catalyst depositing. Alumina washcoat and cobalt catalyst were deposited as organic precursors using Langmuir–Blodgett method (LB) onto precalcined CrAl sheets. Additional noble metal promoters (Pd and Pt) were deposited by chemisorption. The LB method occurred useful for the preparation of nanocomposite catalyst since it enabled controlling the quantity and even distribution of the deposited material. The catalyst surface at various stages of preparation was examined using SEM, XPS and AFM methods. Catalytic tests showed that small amount of Co₃O₄ spinel, highly dispersed on Al₂O₃ layer, is active in combustion of diluted n-hexane. The apparent activation energy for the obtained cobalt catalyst (about 52 kJ/mol) was twice as low as for a standard Pt/Al₂O₃ catalyst. The promoting effect observed for Pd-containing cobalt catalysts by the decrease in the activation energy to around 15 kJ/mol, was correlated to the high surface concentration and dispersion of the cobalt catalyst containing PdO.