Point and Multipoint to Plane Barrier Discharge Process for Removal of NOx from Engine Exhaust Gases: Understanding of the Reactional Mechanism by Isotopic Labeling

An experimental study on the removal of NO_x in a simulated vehicle exhaust gas has been carried out using point to plane and multipoint to plane DBD corona reactors. Hydrocarbon (C₃H₆) and NO_x by-products were systematically investigated with a Gas Chromatography coupled to a Mass Spectrometry (GC/MS). NO_x (NO and NO₂) and CO output were also monitored with a gas analyzer in order to complete the mass balance. ¹⁸O tracer technique analyzes is applied to investigate the mechanism of propylene decomposition. From the plasma chemical reaction pathway proposed, it is apparent that the oxygen activation is one of the important steps for initiating the oxidation processes and the R-NO_x formation. We present data for the reaction of the (N₂/O₂/C₃H₆/CO₂NO/H₂O system in the corona discharge reactors mentioned above. This system has been shown to generate a significant amount of aldehyde. CH₃NO₂ and CH₃ONO₂ are the main R-NO_x compounds produced. Reactant composition and discharge energy densities (controlled by a numerical oscilloscope) were the operating parameters under study in wet and dry air mixture. Water vapors played an important role in NO_x removal (especially in NO₂ removal) via the reaction forming HNO₃. Therefore, in wet-gas mixture supplied reactors the highest removal rates of NO_x were as high as 30%, while in dry-gas only 15%. Different dielectric materials such as Al₂O₃/SiO₂ and TiO₂ on Al₂O₃/SiO₂ support have been used.     

Oxidation and Reduction Processes During NO x Removal with Corona-Induced Nonthermal Plasma

In this paper, the NO-to-NO ₂ conversion in various gaseous mixtures is experimentally investigated. Streamer coronas are produced with a dc-superimposed high-frequency ac power supply (10–60 kHz). According to NO _x removal experiments in N ₂ +NO _x and N ₂ +O ₂ +NO _x gaseous mixtures, it is supposed that the reverse reaction NO ₂ +ONO+O ₂ may not only limit NO ₂ production in N ₂ +NO _x mixtures, but also increase the energy cost for NO removal. Oxygen could significantly suppress reduction reactions and enhance oxidation processes. The reduction reactions, such as N+NON ₂ +O, induce negligible NO removal provided the O ₂ concentration is larger than 3.6%. With adding H ₂ O into the reactor, the produced NO ₂ per unit removed NO can be significantly reduced due to NO ₂ oxidation. NH ₃ injection could also significantly decrease the produced NO ₂ via NH and NH ₂ - related reduction reactions. Almost 100% of NO ₂ can be removed in gaseous mixtures of N ₂ +O ₂ +H ₂ O+NO ₂ with negligible NO production.     

NO2对Cu/SAPO-34分子筛催化剂上NH3选择性催化还原NO性能的影响

主要考察了NO₂对Cu/SAPO-34 分子筛催化剂在整个温度范围内（100-500 ℃）NH₃选择性催化还原（SCR）NO性能的影响. 研究所使用样品为新鲜Cu/SAPO-34 催化剂在750 ℃下水热处理4 h 的稳定期样品.通过X射线衍射（XRD）和扫描电子显微镜（SEM）对样品的结构以及形貌进行表征,采用SCR活性评价、动力学实验以及原位漫反射傅里叶变换红外光谱（in situ-DRIFTS）表征催化剂的性能以及催化剂表面物种的变化. 活性评价实验结果表明,NO₂会抑制催化剂的低温（100-280 ℃）活性,但其存在会提高催化剂的高温（280 ℃以上）活性. 与此同时,随着反应物中NO/NO₂的摩尔比例减少,由于NH₄NO₃物种的分解,副产物（N₂2O）的浓度增大. 动力学结果表明,Cu/SAPO-34 催化剂上快速SCR反应的表观活化能（E_a=64.02 kJ·mol^-1）比标准SCR反应的表观活化能（E_a=48.00 kJ·mol^-1）更大. In situ-DRIFTS实验结果表明NO比NO₂更容易在催化剂表面形成硝酸盐,并且NO₂更容易与吸附在Brønsted 酸性位上的NH₃物种反应生成NH₄NO₃. 低温下,催化剂表面的NH₄NO₃物种会覆盖SCR反应的活性位,造成活性降低,但在高温时,形成的NH₄NO₃物种一部分会被NO还原为N2,而另一部分会直接热分解为N₂O,造成催化剂的选择性降低.     

Plasma Nitrogen Oxides Synthesis in a Milli-Scale Gliding Arc Reactor: Investigating the Electrical and Process Parameters

Nitrogen fixed in the form of nitrogen oxides is essential to produce fertilizers and many other chemical products, which is vital to sustain life. The performance of a milli-scale gliding arc reactor operated under atmospheric pressure has been studied for nitrogen oxides synthesis. In this work, the electrical and process parameters of the gliding arc reactor, such as frequency, pulse width, amplitude and feed ratio were investigated respectively. The experiments were performed at 1 L/min in a gliding arc discharge regime. The highest concentration of NO_x was found to be ~1 % at energy consumption of 10 kWh/kg of NO_x. Increase in frequency, pulse width and amplitude resulted in an increased specific energy input and NO_x concentration. The feed ratio (N₂/O₂) affected the amount of NO and NO₂ produced, which gives possibility to independently obtain the desired ratio of NO/NO₂ by tuning the electrical and process parameters.     

The reaction of ammonia with nitrogen dioxide in a flow reactor: Implications for the NH2 + NO2 reaction

The NH₃/NO₂ system has been investigated experimentally in an isothermal flow reactor in the temperature range 850–1350 K. The experimental data were interpreted in terms of a detailed reaction mechanism. The flow reactor results, supported by a theoretical analysis of the NH₂? NO₂ complex, suggest that the NH₂ + NO₂ reaction has two major product channels, both proceeding without activation barriers: Our findings indicate that the N₂O + H₂O channel is dominant at low temperatures while H₂NO + NO dominates at high temperatures. The rate constant for reaction (R21) is estimated to be 3.5 · 10¹² cm³/mol-s in the temperature range studied with an uncertainty of a factor of 3. © 1995 John Wiley & Sons, Inc.     

WO3改性CeO2-TiO2催化剂的低温NH3-NO/NO2 SCR活性和机理研究

在铈钛基NH3-SCR催化材料中,改性元素对催化材料的酸性位和氧化还原性能的影响较大.本文采用过量浸渍法分别制备了CeO2-TiO2(CeTi)和CeO2/WO3-TiO2(CeWTi)催化剂,研究了CeWTi催化材料结构、酸性位及氧化还原性能对NH3-NO/NO2 SCR反应性能的影响.结果发现,CeTi和CeWTi样品均有较优异的NH3-NO/NO2 SCR催化性能,后者略高.WO3的加入增加了催化材料的表面酸性,对其氧化还原性能影响不大.通过对反应中间物种NH4NO3的研究,发现NH4NO3的分解主要与氧化还原性能相关,而NO还原NH4NO3的反应需要氧化还原能力和酸性位共同作用,即在氧化还原性能差异不大的条件下,酸性对该反应起到重要作用.而该反应也是NH3-NO/NO2 SCR的限速步骤,这是CeWTi催化材料活性高于CeTi催化材料的原因.同时,为了获得NH3-NO/NO2 SCR反应的高活性,NO2:NO比例宜为1:1.然而现实情况中,预氧化催化材料的氧化活性、NOx浓度、温度等变量使得准确控制NO2的比例较难,因此,深入了解NO2浓度对NH3–NO/NO2 SCR反应的影响至关重要.本文探讨NO2:NO的比例、O2浓度等对NH3-NO/NO2 SCR反应性能的影响;并研究了不同NO2含量条件下NH3-NO/NO2 SCR反应网络.通过分析CeWTi材料上NH3-NO/NO2 SCR反应网络可知,当NO与NO2比例为1:1时,NH3-SCR催化活性最高,并以快速SCR形式进行;当NO与NO2比例为1:1消耗完全之后,剩余的NO或NO2各自独立以标准或慢速SCR进行,不影响其本来的反应活性.催化材料的标准SCR、快速SCR和慢速SCR均取决于材料表面酸度和氧化还原性能,但快速SCR和慢速SCR对材料这两方面性能的要求相对较低.同时O2并不参与快速和慢速SCR,而NO2可以取代O2作为SCR反应中主要的氧化剂,氧化Ce4+为Ce3+,甚至比O2和NO再氧化活性位的能力更强,保持催化材料的高催化活性.低温条件时,慢速SCR和快速SCR反应均在材料表面生成硝酸铵中间物种,但由于慢速SCR气氛中缺乏NO将硝酸铵还原,进而引发快速SCR反应,因此材料表面快速SCR的NOx转化率要高于慢速SCR反应;高温条件下,由于硝酸铵容易热分解,导致硝酸铵的抑制效应不太明显.NH4NO3分解是NO2含量升高后N2O的形成的主要途径.     

The Effect of Oxygen and Water Vapor on Nitric Oxide Conversion with a Dielectric Barrier Discharge Reactor

The effect of O₂ and H₂O vapor on the Nitric oxide (NO) removal rate, the NO₂ generation rate and the discharge characteristics were investigated using the dielectric barrier discharge (DBD) reactor at 1 atm pressure and at room temperature (20°). The results showed that the O₂ present in the flue gas always hampered the removal of NO and the generation of N₂O, but that the O₂ could enhance the generation of NO₂ in the NO/N₂/O₂ mixtures. Furthermore, with the increase of oxygen, the average discharge current gradually decreases in the reactor. The H₂O present in N₂/NO hindered the removal of NO and the generation of NO₂ but had no impact on the average discharge current in the reactor in the NO/N₂/H₂O mixtures in which the HNO₂ and HNO₃ was detected. The energy efficiency of the DBD used to remove the NO from the flue gas was also estimated.     

酸性金属氧化物对锰铈氧化物催化剂脱硝温度窗口的调控作用

利用溶胶-凝胶法,采用三种酸性金属氧化物（氧化铌、氧化钨和氧化钼）对锰铈复合氧化物催化剂进行了改性. 测试了催化剂的氮氧化物选择性催化还原（SCR）活性,以筛选对应不同温度窗口的合适酸性氧化物改性剂. 同时评价了催化剂的NO氧化和NH₃氧化活性. 利用X射线衍射、BET比表面积测试、H₂程序升温还原、NH₃/NO_x程序升温脱附和NH₃/NO_x吸附红外光谱等手段对催化剂进行了表征. MnO_x-CeO₂催化剂表现出良好的低温（100-150 ℃）活性. 酸性金属氧化物的添加削弱了催化剂的氧化还原特性,从而抑制了NH₃的活化和NO₂辅助的快速SCR反应. 与此同时,相对高温（250-350 ℃）区NH₃的氧化也受到了抑制,B酸和L酸上的NH₃吸附得以增强. 因此,催化剂的SCR脱硝温度窗口向高温移动,改性效果Nb₂O₅ < WO₃ < MoO₃.     

Reaction of13N atoms formed in propionic acid-d6 irradiated in a nuclear reactor

The chemical behaviour of¹³N formed by the¹²C/d,n/¹³N reaction by recoil deuterons was studied in propionic acid-d₆ irradiated at 195 to 295 K for 10 min in a reactor. The yields of¹³NH₃ and H¹³NO_x showed remarkable change with the phase change of the media from solid to liquid, i.e., the yield of¹³NH₃ increased from 49.1±2.6 to 57.3±1.5% and that of H¹³NO_x decreased from 28.9±1.8 to 20.4±1.9%. Based on the results, reaction mechanisms involved were argued as: /A/ formation of primary radicals, NH and HNO or NO, and /B/ subsequent reactions converting primary products to the final forms.     

焙烧温度对MnOx-CeO2-WO3-ZrO2催化剂上NH3选择性还原NO的影响

采用共沉淀法制备了MnO_x-CeO₂-WO₃-ZrO₂催化剂,考察了催化剂焙烧温度对O₂和H₂O存在下NH₃选择性催化还原（NH₃-SCR） NO的影响,并利用低温N₂吸附、X射线衍射（XRD）、透射电镜（TEM）、X射线光电子能谱（XPS）、NH₃程序升温脱附（NH₃-TPD）和CO脉冲反应对催化剂进行了表征. 结果表明在NH₃-SCR反应中,催化剂的低温活性随焙烧温度的提高而降低,这是由于催化剂表面化学吸附氧和酸性位减少引起的;催化剂的高温活性随焙烧温度的提高先增加后减小,这与催化剂表面最易释放氧数量的变化趋势相反. 700 ℃焙烧的催化剂具有良好的低温活性和最宽的反应温度窗口,在空速为90000 h^-1的条件下,该催化剂的起燃温度（50% NO转化率）为189 ℃,且反应温度在218-431 ℃范围内,NO转化率可达到80%-100%.     

Reactivity of NO with NH3 in the Presence of O2 over Ce-ZSM5 with and without Moisture

Cerium based ZSM5 catalysts are used to study NO reduction with NH₃ in the presence of oxygen with and without moisture. The Ce-ZSM5 was prepared by wet impregnation method and characterized by X-ray diffraction technique, BET surface and SEM. Ce-ZSM5 showed better NO_x reduction than H-ZSM5 which is a poor catalyst for NO_x reduction with NH₃. The metal incorporation in H-ZSM5 has increased the catalytic activity. The catalytic activity showed significant difference in NO_x conversion with and without moisture. The disperse Ce species are the active centers for the reduction of NO with NH₃ in the presence of oxygen. This revised version was published online in June 2006 with corrections to the Cover Date.     

NOx-Sorbing Metal Oxides, MnOx–CeO2. Oxidative NO Adsorption and NOx–H2 Reaction

(n)MnO_x–(1–n)CeO₂ binary oxides have been studied for the sorptive NO removal and subsequent reduction of NO_x sorbed to N₂ at low temperatures (150 °C). The solid solution with a fluorite-type structure was found to be effective for oxidative NO adsorption, which yielded nitrate (NO^– ₃) and/or nitrite (NO^– ₂) species on the surface depending on temperature, O₂ concentration in the gas feed, and composition of the binary oxide (n). A surface reaction model was derived on the basis of XPS, TPD, and DRIFTS analyses. Redox of Mn accompanied by simultaneous oxygen equilibration between the surface and the gas phase promoted the oxidative NO adsorption. The reactivity of the adsorbed NO_x toward H₂ was examined for MnO_x–CeO₂ impregnated with Pd, which is known as a nonselective catalyst toward NO–H₂ reaction in the presence of excess oxygen. The Pd/MnO_x–CeO₂ catalyst after saturated by the NO uptake could be regenerated by micropulse injections of H₂ at 150 °C. Evidence was presented to show that the role of Pd is to generate reactive hydrogen atoms, which spillover onto the MnO_x–CeO₂ surface and reduce nitrite/nitrate adsorbing thereon. Because of the lower reducibility of nitrate and the competitive H₂–O₂ combustion, H₂–NO reaction was suppressed to a certain extent in the presence of O₂. Nevertheless, Pd/MnO_x–CeO₂ attained 65% NO-conversion in a steady stream of 0.08% NO, 2% H₂, and 6% O₂ in He at as low as 150 °C, compared to ca. 30% conversion for Pd/–Al₂O₃ at the same temperature. The combination of NO_x-sorbing materials and H₂-activation catalysts is expected to pave the way to development of novel NO_x-sorbing catalysts for selective deNO_x at very low temperatures.     

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.     

Catalytically Promoted NOx Sorption by ZrO2-Based Oxides

Metal promoted zirconia-based oxide sorbents, such as Pt–ZrO₂/Al₂O₃ for NO _x have been investigated. To clarify the role of the catalyst component, sorption of NO and NO₂ was compared using the samples with and without Pt. The catalytic oxidation of NO to NO₂ and successively to nitrate ions is an important role for the Pt catalyst. The experimental results indicate that a high-temperature calcination is essential to remove residual Cl from Pt–ZrO₂–Al₂O₃ prepared from H₂PtCl₆ in order to provide more active NO _x sorption sites. Of M–ZrO₂–Al₂O₃ samples investigated, ruthenium as well as Pt demonstrated relatively good performance as a catalyst component in the sorbent. The FT-IR spectra after sorption of NO and NO₂ demonstrated a strong band attributed to stored nitrate ions. The Pt catalyst was more resistant to sulfur poisoning than a base metal catalyst. However, the NO _x sorptive capacities of the Pt–ZrO₂/Al₂O₃ sorbents were expected to be deteriorated in dilute SO₂ as far as observed from FT-IR spectra.     

Catalytic Performance of Spherical MCM-41 Modified with Copper and Iron as Catalysts of NH3-SCR Process

Spherical MCM-41 with various copper and iron loadings was prepared by surfactant directed co-condensation method. The obtained samples were characterized with respect to their structure (X-ray diffraction, XRD), texture (N₂ sorption), morphology (scanning electron microscopy, SEM), chemical composition (inductively coupled plasma optical emission spectrometry, ICP-OES), surface acidity (temperature programmed desorption of ammonia, NH₃-TPD), form, and aggregation of iron and copper species (diffuse reflectance UV-Vis spectroscopy, UV-Vis DRS) as well as their reducibility (temperature programmed reduction with hydrogen, H₂-TPR). The spherical MCM-41 samples modified with transition metals were tested as catalysts of selective catalytic reduction of NO with ammonia (NH₃-SCR). Copper containing catalysts presented high catalytic activity at low-temperature NH₃-SCR with a very high selectivity to nitrogen, which is desired reaction products. Similar results were obtained for iron containing catalysts, however in this case the loadings and forms of iron incorporated into silica samples very strongly influenced catalytic performance of the studied samples. The efficiency of the NH₃-SCR process at higher temperatures was significantly limited by the side reaction of direct ammonia oxidation. The reactivity of ammonia molecules chemisorbed on the catalysts surface in NO reduction (NH₃-SCR) and their selective oxidation (NH₃-SCO) was verified by temperature-programmed surface reactions.     

低于300℃时两种氧化铈材料对稀燃阶段NOx存储性能的影响

研究了低于300 ℃时两种氧化铈对稀燃阶段NO_x存储性能的影响,催化剂由2%（w）Pt/Al₂O₃（PA）与CeO₂-X（X=S,I）机械混合制备. X射线衍射（XRD）,BET表面积和扫描电子显微镜（SEM）用于表征材料的物理结构. X射线光电子能谱（XPS）和H₂程序升温还原（H₂-TPR）用于表面Ce³⁺和活性氧定量. 原位漫反射傅里叶变换红外光谱（in-situ DRIFTS）用于分析表面NO_x吸附物种. 相比于CeO₂-I,CeO₂-S 具有优良的物理化学性能,包括高比表面积、丰富的空隙结构、较高的抗老化能力及表面Ce³⁺浓度. 因而,Pt/Al₂O₃+CeO₂-S 表现出优异的NO_x存储能力. 此外,PA+CeO₂-X（X=S,I）上存在Pt 与CeO₂之间的相互作用,可提高表面氧物种的活性进而促进NO氧化及NO_x存储. PA+CeO₂-S上的这种相互作用要强于PA+CeO₂-I. 研究表明,表面Ce³⁺浓度和活性氧含量对NO_x存储起到重要作用. 然而经过水热处理后,Pt 与老化的氧化铈（ACS,ACI）之间的相互作用降低,并且两种氧化铈NO_x存储性能显著下降. 另外,与PA+ACS（ACI）相比,PA+PACS（PACI）样品NO_x存储能力得到改善,这归因于表面氧物种活性增加能促进硝酸盐的形成.     

CuFe-SAPO-34分子筛的结构和催化性能研究

采用一步水热合成法,通过调变初始凝胶中的硅铁比,制备系列铜铁含量不同的CuFe-SAPO-34催化剂用于NH₃选择性催化还原NO_x反应（NH₃-SCR）,并采用ICP、XRD、SEM、BET、H₂-TPR等方法对其结构进行表征.结果表明,CuFe-SAPO-34催化剂具有典型的CHA结构,Cu和Fe均处于分子筛载体的离子交换位.当初始凝胶SiO₂/Fe₂O₃=10时,Cu_2.5Fe_3.1-SAPO-34催化剂具有最大的比表面积和孔容.掺杂适量的Fe,可提高活性物种Cu²⁺的比例及其氧化还原性能,显著降低Cu物种的聚集程度.NH₃-SCR反应结果表明,Cu_2.5Fe_3.1-SAPO-34催化剂具有最宽的反应温度窗口.与Cu-SAPO-34相比,Fe的掺杂显著提高了其高温段的催化活性和低温抗水能力,提高了Cu-CHA催化剂在实际应用中的稳定性.     

Studies on Gas Purification by a Pulsed Microwave Discharge at 2.46 GHz in Mixtures of N2/NO/O2 at Atmospheric Pressure

Pulsed microwave discharges operated at atmospheric pressure in gas mixtures containing N₂, O₂, and NO are investigated experimentally and theoretically for various gas mixture constituents and operating conditions with respect to the ability of exhaust gas purification. The rotational gas temperature and the vibrational temperature of N₂ are derived from CARS measurements. The composition of the exhaust gas after treatment is monitored using FTIR spectroscopy. The processes of the chemical, electronic, and vibrational kinetics are described by a model that has been developed to calculate the species densities. The results obtained show that in N₂/NO gas mixtures an overall reduction of NO_x takes place. In the case of N₂/O₂/NO gas mixtures, no net reduction of NO_x is achieved for a pulsed microwave power below 3600 W, a pulse length of 50 s, and a typical repetition frequency of 2 kHz.     

Removal of SO2 and the simultaneous removal of SO2 and NO from simulated flue gas streams using dielectric barrier discharge plasmas

A gas-phase oxidation method using dielectric barrier discharges (DBDs) has been developed to remove SO₂ and to simultaneously remove SO₂ and NO from gas streams that are similar to gas streams generated by the combustion of fossil fuels. SO₂ and NO removal efficiencies are evaluated as a function of applied voltage, temperature, and concentrations of SO₂, NO, H₂O_(g), and NH3. With constant H₂O_(g) concentration, both SO₂ and NO removal efficiencies increase with increasing temperature from 100 to 160°C. At 160°C with 15% by volume H₂0_(g), more than 95% of the NO and 32% of the S0₂ are simultaneously removed from the gas stream. Injection of NH₃ into the gas stream caused an increase in S0₂ removal efficiency to essentially 100%. These results indicate that DBD plasmas have the potential to simultaneously remove SO₂ and NO from gas streams generated by large-scale fossil fuel combustors.