介质阻挡放电甲醇脱氢偶联一步合成乙二醇反应中氢气的催化作用

利用原位发射光谱表征和在线色谱分析,研究了甲醇介质阻挡放电脱氢偶联一步合成乙二醇反应中氢气的催化作用,考察了放电频率、甲醇和氢气进料量以及反应压力的影响.结果表明,在介质阻挡放电产生的非平衡等离子体中,H2不但能显著提高甲醇转化率,而且能显著提高乙二醇的选择性.在300°C,0.1 MPa,反应器注入功率为11 W,放电频率为12.0 k Hz,甲醇气体进料量为11.1 m L/min,氢气进料量为80–180 m L/min的条件下,甲醇转化率接近30%,乙二醇选择性大于75%.乙二醇收率与激发态氢原子的Hα谱线强度之间存在同增同减关系.由此推测,氢原子是起催化作用的活性氢物种.活性氢物种的生成途径是:基态氢分子通过与电子碰撞变成激发态,激发态氢分子通过第一激发态氢自动解离为基态氢原子.放电反应条件通过影响氢分子解离来影响氢气的催化作用.氢气在非平衡等离子体中显示的催化作用有可能为开辟新的化学合成途径提供重要机遇.     

等离子体引发CH3OH/NH3偶联反应合成腈类化合物

腈类化合物广泛用于医药和精细化学品合成。然而,许多腈类的生产过程产生大量污染物。本文采用介质阻挡放电(DBD)等离子体活化甲醇和氨气分子,发现等离子体引发的CH₃OH/NH₃偶联反应可合成二甲基氰胺、二甲基氨基乙腈和氨基乙腈等高附加值含N有机化合物。系统研究了反应器结构、放电条件、反应条件和填充材料对甲醇转化率和产物选择性的影响。实验结果表明,在最优条件下,甲醇的转化率达到51.5%,腈类化合物选择性达到22.1%。CH₃OH/NH₃等离子体发射光谱结果表明,C≡N自由基物种可能是生成腈类化合物的重要中间体。该CH₃OH/NH₃等离子体偶联反应为二甲基氰胺、二甲基氨基乙腈和氨基乙腈提供了一种绿色合成方法,也为甲醇和氨气精细化利用开辟了一种新途径。     

介质阻挡放电和CuZSM-5结合体系中等离子体对C2H4的作用

采用吸附、程序升温脱附及氧化和发射光谱等技术研究了介质阻挡放电对气相和催化剂表面吸附乙烯的作用. 实验表明, 介质阻挡放电等离子体能脱附催化剂表面吸附物种(如CO2和H2O等), 并引发表面化学反应生成新物种(如在等离子体作用下C2H4和O2生成CO2和H2O); 改变催化剂表面积碳化合物结构, 并降低其起燃点; 引发气相中乙烯发生反应生成中间物种或碎片(如CN和CH等). 在富氧体系NO/O2/N2中加入C2H4, 能使介质阻挡放电等离子体和CuZSM-5“一段法”结合体系产生协同效应, 提高NOx转化率. 该协同效应的产生与等离子体在气相及催化剂表面引发化学反应, 产生参与NOx还原反应的新稳态物种和短寿命高能活性物种有关.     

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.     

介质阻挡放电等离子体与吸附在CuZSM-5上的NO或NO/O2的相互作用

采用吸附和程序升温脱附(TPD)技术研究了介质阻挡放电等离子体对CuZSM-5催化剂上吸附的氮氧化物作用.实验表明,介质阻挡放电等离子体使催化剂表面吸附的NO及Cu活性位上吸附的NOx物种脱附,并引发表面化学反应生成新的氮氧化物.对于NO/N2体系,介质阻挡放电等离子体与吸附在CuZSM-5上NO作用,主要生成N2O和O2.在富氧体系NO/O2/N2,则生成较大量的N2O、NO2和NO.等离子体预处理活性下降的CuZSM-5,可明显提高其催化分解NO活性.对比有或无介质阻挡放电等离子体预处理NO或NO/O2饱和吸附的CuZSM-5上的NO-TPD结果表明,等离子体提高催化剂活性的原因与其使催化剂Cu活性位上吸附的NOx物种脱附有关.     

介质阻挡放电等离子体与吸附在CuZSM-5上的NO或NO/O2的相互作用

采用吸附和程序升温脱附(TPD)技术研究了介质阻挡放电等离子体对CuZSM-5催化剂上吸附的氮氧化物作用. 实验表明, 介质阻挡放电等离子体使催化剂表面吸附的NO及Cu活性位上吸附的NOx物种脱附, 并引发表面化学反应生成新的氮氧化物. 对于NO/N2体系, 介质阻挡放电等离子体与吸附在CuZSM-5上NO作用, 主要生成N2O和O2. 在富氧体系NO/O2/N2, 则生成较大量的N2O、NO2和NO. 等离子体预处理活性下降的CuZSM-5, 可明显提高其催化分解NO活性. 对比有或无介质阻挡放电等离子体预处理NO或NO/O2饱和吸附的CuZSM-5上的NO-TPD结果表明, 等离子体提高催化剂活性的原因与其使催化剂Cu活性位上吸附的NOx物种脱附有关.     

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.     

介质阻挡放电等离子体中·OH和HO2·自由基的数值模拟计算

在介质阻挡放电等离子体N2/O2/H2O/HCHO体系中通过解Boitzmann方程,得到电子能量分布函数,利用得到的电子能量分布函数计算电子-分子碰撞反应速率常数.然后把有关的反应速率常数带入速率方程,计算得到该体系在介质阻挡放电时,·OH、HO2·和电子的浓度随时间的演变以及·OH、HO2·浓度随H2O、O2摩尔分数的变化,并将模拟结果与实验值进行了对比,两者符合得较好.     

低温等离子体转化NO/O2/N2气氛中NO的实验研究

通过建立低温等离子体实验系统, 研究了介质阻挡放电型低温等离子体反应器作用于NO/O2/N2混合气体系时, NO, O2初始浓度对NO的转化效率的影响以及NOx, O3浓度随能量密度的变化关系. 低温等离子体作用于NO/O2/N2混合气体系时, NO同时发生氧化还原反应, 氧化反应占主导地位, 大部分NO转化为NO2; NO转化率随O2, NO初始浓度增大而降低, 能量密度在450～600 J/L时转化率较高; 产生的O3浓度随能量密度的增大呈先增后减的趋势.     

介质阻挡放电等离子体中·OH和HO2·自由基的数值模拟计算

在介质阻挡放电等离子体N2/O2/H2O/HCHO体系中通过解Boltzmann方程, 得到电子能量分布函数, 利用得到的电子能量分布函数计算电子-分子碰撞反应速率常数. 然后把有关的反应速率常数带入速率方程, 计算得到该体系在介质阻挡放电时,·OH、HO2·和电子的浓度随时间的演变以及·OH、HO2·浓度随H2O、O2摩尔分数的变化, 并将模拟结果与实验值进行了对比, 两者符合得较好.     

Study of SO2 Removal Using Non-thermal Plasma Induced by Dielectric Barrier Discharge (DBD)

Dielectric Barrier Discharge (DBD) non-thermal plasma reactors built with three different dielectric materials for SO₂ removal were studied. The discharge characteristics of the three dielectrics, namely glass, Teflon, and glass fiber-based epoxy resin, were analyzed using Lissajous figures. From the Lissajous figures, the transition charge and energy deposition for each dielectric material were determined. When both the discharge characteristics and mechanical processability were considered, glass fiber-based epoxy resin was regarded as the best dielectric barrier among the three for DBD plasma reactors. A multi-cell DBD reactor built with glass fiber-based epoxy resin was used for treating air stream containing SO₂. SO₂ % removal decreased with increasing initial SO₂ concentration in a biphasic fashion. SO₂ removal was greatly improved by adding NH₃ into the air stream. Raising the relative humidity of the air stream also helped SO₂ removal. A SEM (scanning electron microscope) test illustrated some changes in surface morphology of Teflon and glass fiber-based epoxy resin.     

Dielectric Barrier Discharge for Ammonia Production

The leading after-treatment technology for NOx removal process in Diesel engines for stationary and mobile applications is the selective catalytic reduction of oxides of nitrogen [NOx] by ammonia [NH₃]. A novel non-thermal plasma electrode with a needle array in a dielectric barrier discharge reactor, powered by a high frequency neon transformer, is used for the thermal decomposition of solid urea [(NH₂)CO(NH₂)] to produce ammonia. The thermolysis of urea produces iso-cyanic acid [HNCO] as a byproduct, besides ammonia, which can react with water in the gas phase, thus giving carbon dioxide and more ammonia. The presence of water fed before and/or after the plasma reactor was studied to assess its effect on the amount of produced ammonia. Results clearly showed that water fed to the entrance of the reactor can efficiently promote the reaction of iso-cyanic acid to produce ammonia and this result can be improved when air is used as carrier gas for 115 V of input voltage to a neon transformer and with a gas flow rate of 4 L/min.     

A Study of Sulfonol Decomposition in Water Solutions Under the Action of Dielectric Barrier Discharge in the Presence of Different Heterogeneous Catalysts

The decomposition kinetics of sulfonol (surfactant) in water solutions as well as the formation kinetics of decomposition products under the action of an oxygen dielectric barrier discharge (DBD) at atmospheric pressure in the presence or absence of TiO₂, NiO and Ag₂O catalysts in the plasma zone was studied. The DBD discharge was shown to have high decomposition efficiency (phenol-up to 98%, sulfonol-up to 80%). In a plasma-catalytic hybrid process, the efficiency of organic substances decomposition was higher than efficiency for the DBD treatment without catalyst. The catalysts application resulted in a change in the ratio and yield of decomposition products.     

Application of Dielectric Barrier Discharge for Waste Water Purification

This study considers treatment of real city rain sewage under the action of an oxygen dielectric barrier discharge (DBD) at atmospheric pressure in the presence or absence of TiO₂ catalyst in the plasma zone. The DBD discharge has been shown to have high decomposition efficiency (up to 98%) for oil hydrocarbons, phenols and synthetic surfactants. The discharge action resulted in the decrease of heavy metal (Pb, Cd, Fe, Mn) content as well. In a plasma-catalytic hybrid process, the efficiency of organic substances decomposition was higher than efficiency for the DBD treatment without catalyst.     

Preparation and Application of Ca0.8 Sr0.2 TiO3 for Plasma Activation of CO2

Despite a large interest in plasma-assisted catalytic technology (PACT), very little has been reported about the catalytic effects of different dielectric barriers on a dielectric barrier discharge (DBD) reaction. In the present study, Ca_0.8Sr_0.2TiO₃, that possesses a high permittivity, was prepared by liquid phase sintering and used as a dielectric barrier in a DBD reactor to break CO₂. The mechanical and dielectric properties of Ca_0.8Sr_0.2TiO₃ were greatly enhanced by adding 0.5 wt.% Li₂Si₂O₅. A DBD plasma was successfully generated by using this Ca_0.8Sr_0.2TiO₃ as a dielectric barrier and 18.8% CO₂ conversion was achieved with the residence time of 0.17 s at the frequency of 8 kHz, which was much higher than with those using an alumina or a silica glass barrier. It was found that the plasma power increased with the increasing of the permittivity, and finally very dense and strong microdischarges were initiated to decompose CO₂.     

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.     

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.     

Ethylene Epoxidation in Low-Temperature AC Dielectric Barrier Discharge: Effect of Electrode Geometry

In this work, the epoxidation of ethylene under a cylindrical dielectric barrier discharge (DBD) reactor and a parallel DBD reactor was comparatively studied. The effects of important operating parameters—feed O₂/C₂H₄ molar ratio, applied voltage, input frequency, and residence time—were investigated on the reaction performance in terms of reactant conversions, product selectivities, product yields, and power consumptions per molecule of ethylene converted and per molecule of ethylene oxide produced. The optimum conditions obtained from the operating parameter investigation were used for a comparative performance evaluation of both DBD reactor systems. It was found that under the optimum conditions of each system, the cylindrical DBD system exhibited superior epoxidation performance for ethylene oxide production compared to the parallel DBD system, indicating that the electrode geometry (electrode edge length-to-electrode surface area ratio) plays a significant role in the ethylene epoxidation.     

H2S and NH3 Removal by Silent Discharge Plasma and Ozone Combo-System

Treatment of H₂S and NH₃ using the non-thermal plasma (NTP) methods was investigated. Two NTP systems were used in this study, one consisting of a multi-cell plate-to-wire reactor (PTW), and the other consisting of an ozonization chamber and the multi-cell PTW reactor. Each cell of the PTW reactor had a sheet of copper foil embedded in dielectric layers as its high voltage electrode and a wired rack as its gounded electrode. Use of the wired rack type electrode allowed large flow throughput, and promoted intense local electric fields. The experiments showed that under constant energy input, the decomposition efficiency of H₂S or NH₃ decreased with increasing initial concentration of the gas, and increased with increasing injected ozone and relative humidity. Injection of NH₃ into H₂S stream did not improve the H2S decomposition efficiency but was necessary for removal of sulfite-containing compounds in the discharge air.     

Product analysis and mechanism of toluene degradation by low temperature plasma with single dielectric barrier discharge

A single dielectric barrier discharge (DBD) low-temperature plasma reactor was set up, and toluene was selected as the representative substance for volatile organic compounds (VOCs), to study the reaction products and degradation mechanism of VOCs degradation by low-temperature plasma. Different parameters effect on the concentration of O₃ and NO_x during the degradation of toluene were studied. The exhaust in the process of toluene degradation was continuously detected and analyzed, and the degradation mechanism of toluene was explored. The results showed that the concentration of O₃ increased with the increase of the power density and discharge voltage of the plasma device. However, as the initial concentration of toluene increased, the concentration of O₃ basically keep steady. The concentration of NO_x in the by-products increased with the discharge voltage, power density, and initial concentration of toluene in the plasma device, and the concentration of NO₂ was much higher than the concentration of NO. The degradation process of toluene was detected and analyzed. The results showed that the degradation mechanism of toluene by plasma includes high energy electron bombardment reaction, active radical reaction and ion molecule reaction. Among them, the effect of high-energy electrons on toluene degradation is the largest, followed by the effect of free radicals, in which oxygen radicals participated in the reaction mainly through the formation of C–O bond, CO bond, (CO)–O– bond and –OH radical, while nitrogen radicals participate in the reaction mainly through the formation of C–NH₂, (CNH)- bond, CN bond and C–NO₂ bond. The results can provide some data supports for the study of low-temperature plasma degradation of VOCs.