氧气常压介质阻挡放电的发射光谱及能量传递机理

为研究氧气常压介质阻挡放电中的物理化学行为, 以纯氧作为放电体系, 用发射光谱(optical emission spectroscopy)诊断技术分析了等离子体中可能存在的化学活性物种. 利用在500-950 nm范围的氧原子发射光谱计算出等离子体中的电子温度为(1.02±0.03) eV; 观测了760 nm处的具有清晰转动结构的氧气A带(atmospheric band)O2(b1∑+g-X3∑-g), 并用其转动结构计算了转动温度(气体温度)为(650±20) K; 在500-700 nm范围观测了氧气的第一负带系(first negative system) O+2(b4∑-g-a4∏u), 在190-240 nm范围观测了微弱但特征清晰的氧气的Hopfield带系O+2(c4∑+u-b4∑-g). 研究发现, 在氧气常压介质阻挡放电等离子体中存在多种激发态氧原子、激发态氧气分子、基态和激发态氧气分子离子等反应活性物种, 这些活性物种的形成涉及氧气分子的激发、解离和电离等多种过程, 每个过程都包含多个能量传递步骤, 氧分子解离产生的氧原子是导致一系列高激发态氧原子生成和氧气电离激发的主要因素.     

A New Approach to Metal Surface Nitriding Using Dielectric Barrier Discharge at Atmospheric Pressure

A new approach to metal surface nitriding using dielectric barrier discharge (DBD) plasma at atmospheric pressure is presented in this paper. Results of the study show that the plasma nitriding at atmospheric pressure using DBDs is realizable. Harder and thicker compound layer and diffusion layer on the treated surface has been formed in shorter treatment time comparing with the conventional vacuum plasma nitriding, Increasing the applied voltage will facilitate the formation of a thicker nitrided layer using the DBD. The nitrided layer acquired by this new approach is mainly composed of ɛ phase and γ′ phase, and the crystal grains of the ɛ phase is fine and has high dislocations density.     

Chlorinated Organic Compounds Decomposition in a Dielectric Barrier Discharge

The decomposition of chlorinated volatile organic compounds by non-thermal plasma generated in a dielectric barrier discharge was investigated. As model compounds trichloroethylene (TCE) and 1,2-dichloroethane (DCE) were chosen. It was found that TCE removal exceeds 95% for input energy densities above 0.2 eV/molecule, regardless of the initial concentration of TCE, in the range 100–750 ppm. On the other hand, DCE was more difficult to decompose, the removal rate reached a maximum of 60% at the highest input energy used. For both investigated compounds the selectivity towards carbon dioxide was significantly influenced by their initial concentration, increasing when low concentrations were used. The gas flow rate had also an effect on CO₂ selectivity, which is higher at low flow rate, due to the higher residence time of the gas in the plasma. The best values obtained in these experiments were around 80%.     

Concentration Dependence of VOC Decomposition by Dielectric Barrier Discharges

A comparative study of the decomposition of Volatile Organic Compounds (VOCs: Vinyl Chloride (VC), Ethyl Acetate (EA), Toluene (T), Acetone (A)) in dielectric barrier discharges (DBD) in zero and humidified air at atmospheric pressure was performed. Small scale tubular-flow with pulse excitation and large scale planar-flow with sinusoidal excitation were used to determine the removal efficiency in dependence on inlet concentration S₀ and air humidity. According to the destruction law S=S₀ exp(–E/), where E is the plasma energy density, linear functions were found for the -parameters with respect to S₀ containing an absolute term ₀. By modeling the reaction kinetics it was possible to discriminate active species responsible for the decomposition. Ozone was confirmed to be involved in VC decomposition in zero air whereas OH radicals were best suited to explain the absolute efficiency of EA and toluene destruction in humid air. Their decomposition in zero air however, as well as the degradation of acetone cannot be explained in a similar way.     

Humidity Effect on Toluene Decomposition in a Wire-plate Dielectric Barrier Discharge Reactor

Laboratory-scale experiments were performed to evaluate the humidity effect on toluene decomposition by using a wire-plate dielectric barrier discharge (DBD) reactor at room temperature and atmospheric pressure. The toluene decomposition efficiency as well as the carbon dioxide selectivity with/without water in a gas stream of N₂ with 5% O₂ was investigated. Under the optimal humidity of 0.2% the characteristics of toluene decomposition in various background gas, including air, N₂ with 500 ppm O₂, and N₂ with 5% O₂ were observed. In addition, the influence of a catalyst on the decomposition was studied at selected humidities. It was found that the optimum toluene removal efficiency was achieved by the gas stream containing 0.2% H₂O, since the presence of water enhanced the CO₂ selectivity. In addition, the toluene removal efficiency increased significantly in a dry gas stream but decreased with an increase in the humidity when the Co₃O₄/Al₂O₃/nickel foam catalyst was introduced into the discharge area.     

Destruction of Styrene in an Air Stream by Packed Dielectric Barrier Discharge Reactors

This study presents the decomposition rates of styrene vapors with non-packed and packed bed dielectric barrier discharge reactors. The concentrations of intermediate byproducts at various plasma operation conditions were evaluated. The results showed that although styrene vapors could be almost completely removed at low styrene inlet concentration of 132 ppm, the selectivity of CO₂ as the major product was rather low in a non-packed bed reactor. It was found that solid carbon containing compound was the major byproduct. An increase in the styrene inlet concentration tended to reduce the styrene removal efficiency, it also led to increase in the solid byproduct. The reactors that packed with glass, Al₂O₃ or Pt–Pd /Al₂O₃ pellets could improve the styrene decomposition efficiency and reduce the formation of intermediate products, of which the best oxidation of styrene to CO₂ could be achieved with a Pt–Pd /Al₂O₃ packed bed reactor. The carbon byproducts could also be reduced if the rector length was increased. The concentrations of ozone formed during the plasma process were also evaluated for the non-packed and packed bed reactors. The plasma reactor that packed with Pt–Pd /Al₂O₃ pellets was proved to have the lowest O₃ concentration.     

Sterilization of Polymer Foils with Dielectric Barrier Discharges at Atmospheric Pressure

The emission of UV light as well as chemical reaction in plasmas allow them to be used for decontamination of food packaging. Sterilization efficiency of different dielectric barrier discharge (DBD) setups at atmospheric pressure was investigated for spores of B. subtilis and A. niger sprayed onto PET foils. In normal DBDs the efficiency of spore reduction in different gases (nitrogen, argon, synthetic air) can be related to the UV spectra of these gases in the discharge. With special so-called cascaded dielectric barrier discharges (CDBDs) a fast reduction of viable cells by more than four orders of magnitude is possible within few seconds, even for UV resistant cells. The sealing properties of commonly used PE-PET-laminate can be maintained in CDBD which is not observed for single-gap DBD.     

A Brief Catalyst Study on Direct Methane Conversion Using a Dielectric Barrier Discharge

A series of metal catalysts was used for methane conversion to higher hydrocarbons and hydrogen in a dielectric barrier discharge. The main goal of this study is to identify the metal catalyst components which can influence the reactions in room‐temperature plasma conditions. The catalysts supported by γ‐Al₂O₃ and zeolite (ZSM 5x) were prepared by the incipient wetness method with solutions containing the metal ions of the second component. Among the catalysts tested, only Pt and Fe catalysts showed a unique result of catalytic reaction in a reactor bed packed with glass beads.     

Comparative Actions of NiO and TiO<Subscript>2</Subscript> Catalysts on the Destruction of Phenol and its Derivatives in a Dielectric Barrier Discharge

This study examined processes of decomposing phenol and its derivatives (resorcin, pyrocatechol and hydroquinone) in aqueous solutions under the action of an atmospheric pressure oxygen dielectric barrier discharge in the presence or absence of catalysts in the plasma zone. Two types of catalysts were tested, NiO and TiO₂. It was found that both materials exhibited catalytic properties. The action of NiO accelerated the step of phenol destruction while the action of TiO₂ catalyst resulted in a more preferable composition of decomposition products and provided a higher degree of carboxylic acid conversion into carbon dioxide than the NiO catalyst.     

Decomposition of Toluene and Acetone in Packed Dielectric Barrier Discharge Reactors

The influences of TiO₂ catalytic material and glass pellet packing on the decomposition efficiency of toluene and acetone in air by dielectric barrier discharge (DBD) reactors were experimentally investigated in this study. The effects of both packing materials on the formation of byproducts such as CO and CO₂ were also evaluated. Experimental results indicate that the introduction of glass materials into the plasma zone of a wire-tube reactor would improve the decomposition efficiency of toluene and acetone compared to a nonpacked reactor. The apparent decomposition rate constant of a glass packed-bed reactor was 4.5–4.8 times greater than that of a nonpacked reactor. The results also indicate that the decomposition rate constant of toluene was approximately 2.6 times higher than that of acetone no matter which type reactor was utilized. The application of TiO₂ coated pellets in DBD reactors will enforce the hydrocarbon byproducts to further be oxidized to CO₂, notwithstanding, it will not significantly improve the performance of the reactors in the decomposition of toluene and acetone, and in the formation of CO. The results show that the best selectivity of CO₂ for acetone decomposition in a TiO₂ coated pellets packed-bed reactor was approximately 40% higher than that in a glass packed-bed reactor.     

Spectroscopic Characterization of CH4 + CO2 Plasmas Excited by a Dielectric Barrier Discharge at Atmospheric Pressure

Plasma processing of a (CH ₄ +CO ₂ ) mixture can lead to the formation of synthesis gas (CO+H ₂ ). The use of a nonthermal plasma for this type of process seems very promising. We report here an electric and spectroscopic characteristic of plasma created in a (CH ₄ +CO ₂ ) mixture by a high-voltage, steep front-voltage (>10 ¹² V/s), very-short-pulse triggered dielectric barrier discharge in a tubular cell. Particular attention was payed to the determination of the rotational temperature for C ₂ . Time resolved investigation of the Swan band leads to an estimated value around 3000 K.     

单电极介质阻挡放电离子源研究

介质阻挡放电离子源是一种常压敞开式离子源,由于免试剂、适用范围广、易于小型化等特点而备受关注.该类离子源多采用表面双电极或针-环电极设计方式.表面双电极的接地电极会减弱氦气在强电场中电离形成的流注崩头能量,缩短等离子束喷射距离.针-环电极的电场主要集中在针电极尖端,流注崩头能量小,等离子束喷射距离比表面双电极还短.本研究对放电影响因素进行分析,通过改变电极形状和增加绝缘介质部件进行电场调整,使强电场区域集中于电极一侧,解决了单电极回流放电问题,从而获得稳定、高效的等离子束,其最大长度可达8 cm以上.基于电场调整技术,研制出单电极介质阻挡放电离子源,它主要由惰性载气、高压电极、绝缘介质管、气控以及温控部分组成.使用新型离子源对咖啡因液态样品和扑热息痛固体药片进行了质谱分析,前者的定量曲线R2值为99.66％,100 μg/L的信噪比为23;后者的主要成分对乙酰氨基酚可在质谱中快速检出,响应强度为1.26×106.上述结果表明,新型离子源可以实现样品的定量和快速原位分析.     

Methane Conversion Using Dielectric Barrier Discharge：Comparison with Thermal Process and Catalyst Effects

The direct conversion of methane using a dielectric barrier discharge has been experimentally studied. Experiments with different values of flow rates and discharge voltages have been performed to investigate the effects on the conversion and reaction products both qualitatively and quantitatively. Experimental results indicate that the maximum conversion of methane has been 80% at an input flow rate of 5 ml/min and a discharge voltage of 4 kV. Experimental results also show that the optimum condition has occurred at a high discharge voltage and higher input flow rate. In terms of product distribution, a higher flow rate or shorter residence time can increase the selectivity for higher hydrocarbons. No hydrocarbon product was detected using the thermal method, except hydrogen and carbon. Increasing selectivity for ethane was found when Pt and Ru catalysts presented in the plasma reaction. Hydrogenation of acetylene in the catalyst surface could have been the reason for this phenomenon as the selectivity for acetylene in the products was decreasing.     

Gas-Phase Removal of Acetaldehyde via Packed-Bed Dielectric Barrier Discharge Reactor

The effectiveness of applying packed-bed dielectric barrier discharge(PBDBD) technology for removing acetaldehyde from gas streams wasinvestigated. Operating parameters examined in this study include appliedvoltage, oxygen content, and gas-flow rate. Experimental results indicatethat the destruction efficiency of acetaldehyde predominantly depends onthe applied voltage. Removal of 99% of acetaldehyde has been achieved forgas streams containing 1000 ppmv acetaldehyde, 5% oxygen, with nitrogen asthe carrier gas. The oxygen content in the gas stream plays an importantrole in removing acetaldehyde within PBDBD. A higher CH3CHO removalefficiency is achieved for the gas stream containing less oxygen, since itwill dissipate energy due to its electronegative property. Carbon dioxideis the major end product, which is less hazardous to the environment and tohuman health. However, undesirable products, e.g., NO₂ and N₂O,CH₃OC₂H₅, CH₃COOH, CH₃NO₂,HCN, CH₃NO₃, and CH₃OH, are detected as well.     

高频介质阻挡放电烟气脱硫研究

采用高频介质阻挡放电的方式产生低温等离子体,研究在不加入NH₃的情况下流动态SO₂的去除情况.实验表明,在输入电压为12 kV,SO₂浓度为5400 mg/m³,气体流量为0.36 m³/h,相对湿度为55%时,脱硫率可达到70%以上;水气的存在对SO₂的去除有较大的促进作用,升高电压和增加O₂量对脱硫率的促进作用有限.对实验结果进行了解释,并提出了反应机理.     

介质阻挡放电等离子体脱除氮氧化物的发射光谱研究

在大气压下, NO/N2体系中, 利用发射光谱技术对50 Hz和5 kHz交流介质阻挡放电等离子体在200～900 nm范围内进行了诊断. 在632、674.5、715.5和742 nm等处测得了N原子的谱线. 利用化学发光法NOx分析仪, 模块式红外吸收气体分析检测仪, 大气压下直连质谱多种检测手段对放电前后的稳定物种进行了分析, 观察到O2的生成. 初步讨论了无氧条件下介质阻挡放电等离子体中NO脱除的反应机制.     

介质阻挡放电中OH自由基对甲醛脱除的影响

对管线式介质阻挡放电中的甲醛脱除进行了实验研究, 测量了介质阻挡放电产生的OH (A²Σ→X ²Π, 0-0)自由基发射光谱. 研究了在一个大气压下不同放电峰值电压、放电频率、添加氩气和氧气时甲醛脱除率与OH自由基发射光谱强度的变化关系. 实验结果表明: 在氮气含甲醛体系中, 提高放电峰值电压、放电频率和增大氩气含量时, 甲醛脱除率随OH (A²Σ→X ²Π, 0-0)自由基发射光谱强度的增强而提高; 当在氮气含甲醛体系中增大氧气含量时, 甲醛脱除率随OH (A²Σ→X ²Π, 0-0)自由基发射光谱强度的减弱而降低. 在11.5 kV放电峰值电压和9 kHz放电频率下, 氮气含甲醛体系中甲醛脱除率达93.8%.     

Destruction of 2,4 Dichlorophenol in an Atmospheric Pressure Dielectric Barrier Discharge in Oxygen

The processes of degradation of 2,4-dichlorophenol (2,4-DCP) under the action of atmospheric pressure of dielectric barrier discharge (DBD) in oxygen were studied. It was shown that the degradation of 2,4-DCP proceeds efficiently. Degree of decomposition reaches 90%. The degradation kinetics of 2,4-DCP obeys the formal first-order kinetic law on concentration of 2,4-DCP. The effective rate constants depend weakly on the experimental conditions and are equal to ~0.2 s^?1. Based on experimental data, the energy efficiency of decomposition of 2,4-DCP was determined. Depending on the conditions, the energy efficiency was in the range of (8–90) × 10^?3 molecules per 100 eV. The composition of the products was studied by gas chromatography (GC), gas chromatography–mass spectrometry (GC–MS), energy-dispersive X-ray spectroscopy (EDX), attenuated total reflection-fourier transform infrared (ATR-FTIR) spectroscopy, electron spin resonance (ESR) spectroscopy and UV/Visible spectroscopy. It was shown that about ~20% of 2,4-DCP is converted to CO₂, while the other part forms an organic film on the reactor wall. The substance formed is close to the carboxylic acids in chemical composition and exhibits electrical conductivity and paramagnetic properties. Almost all of the chlorine contained in the 2,4-DCP is released into the gas phase. The active species of the afterglow react with liquid hexane, forming the products of its oxidation. Some assumptions regarding the pathway of the process are discussed.     

The Role of Dielectric Barrier Discharge Atmosphere and Physics on Polypropylene Surface Treatment

Dielectric barrier discharge (DBD) is the discharge involved in corona treatment, widely used in industry to increase the wettability or the adhesion of polymer films or fibers. Usually DBD's are filamentary discharges but recently a homogeneous glow DBD has been obtained. The aim of this paper is to compare polypropylene surface transformations realized with filamentary and glow DBD in different atmospheres (He, N₂, N₂ + O₂ mixtures) and to determine the relative influence of both the discharge regime and the gas nature, on the polypropylene surface transformations. From wettability and XPS results it is shown that the discharge regime can have a significant effect on the surface transformations, because it changes both the ratio of electrons to gas metastables, and the space distribution of the plasma active species. This last parameter is important at atmospheric pressure because the mean free paths are short (m). These two points explain why in He, polypropylene wettability increase is greater by a glow DBD than by a filamentary DBD. In N₂, no significant effect of the discharge regime is observed because electrons and metastables lead to the same active species throughout the gas bulk. The specificity of a DBD in N₂ atmosphere compared to an atmosphere containing oxygen is that it allows very extensive surface transformations and a greater increase of the polypropylene surface wettability. Indeed, even in low concentration and independently of the discharge regime, when O₂ is present in the plasma gas, it controls the surface chemistry and degradation occurs.     

甲烷参与下催化剂填充型介质阻挡放电等离子体脱除NOx

 将In/HZSM-5催化剂填充于介质阻挡放电反应器中,考察了甲烷参与下NOx的脱除及其脱除产物. 结果表明,在200～350 ℃间,等离子体与催化剂共同作用时NOx的转化率明显高于等离子体或催化剂单独作用时NOx的转化率. 在0.03%NO-0.05%CH4-2%O2-97.92%N2,空速7200 h-1和300 ℃的条件下,单纯等离子体、单纯催化剂和二者共同作用下NOx的转化率分别为24%,25%和65%. 甲烷参与下等离子体和催化剂共同作用时,在催化剂表面没有硝酸盐或亚硝酸盐生成,仅有少量副产物N2O生成. 由此可以推断,NOx脱除的主要产物为N2. 低于300 ℃时,NOx的脱除以分解途径为主,甲烷的作用主要是抑制放电条件下NOx生成的副反应; 在300～350 ℃间,甲烷作为还原剂被等离子体和催化剂协同活化,NOx的脱除以还原途径为主. 采用催化剂填充型介质阻挡放电反应器,可在非常宽的温度区间实现NOx的脱除.