NH3-NO/NO2 SCR for diesel exhausts after treatment: mechanism and modelling of a catalytic converter

We review herein the key mechanistic and kinetic features of the reactions involved in the NH₃-NO/NO₂ SCR system investigated by dynamic reactive experiments over a V-based commercial powdered catalyst, eventually leading to the proposal of an original redox scheme which accounts for stoichiometry, selectivity and intrinsic kinetics of the global SCR process.     

Reactivity of NO/NO2–NH3 SCR system for diesel exhaust aftertreatment: Identification of the reaction network as a function of temperature and NO2 feed content

We present a systematic study of the NH₃-SCR reactivity over a commercial V₂O₅–WO₃/TiO₂ catalyst in a wide range of temperatures and NO/NO₂ feed ratios, which cover (and exceed) those of interest for industrial applications to the aftertreatment of exhaust gases from diesel vehicles. The experiments confirm that the best deNO_x efficiency is achieved with a 1/1 NO/NO₂ feed ratio. The main reactions prevailing at the different operating conditions have been identified, and an overall reaction scheme is herein proposed.

Particular attention has been paid to the role of ammonium nitrate, which forms rapidly at low temperatures and with excess NO₂, determining a lower N₂ selectivity of the deNO_x process. Data are presented which show that the chemistry of the NO/NO₂–NH₃ reacting system can be fully interpreted according to a mechanism which involves: (i) dimerization/disproportion of NO₂ and reaction with NH₃ and water to give ammonium nitrite and ammonium nitrate; (ii) reduction of ammonium nitrate by NO to ammonium nitrite; (iii) decomposition of ammonium nitrite to nitrogen. Such a scheme explains the peculiar deNO_x reactivity at low temperature in the presence of NO₂, the optimal stoichiometry (NO/NO₂ = 1/1), and the observed selectivities to all the major N-containing products (N₂, NH₄NO₃, HNO₃, N₂O). It also provides the basis for the development of a mechanistic kinetic model of the NO/NO₂–NH₃ SCR reacting system.     

Vanadium supported on carbon-coated monoliths for the SCR of NO at low temperature: effect of pore structure

Carbon-coated monoliths with different degrees of mesoporosity have been prepared. This has been accomplished by coating cordierite monoliths with a blend of two polymers, viz. Furan resin and polyethylene glycol (PEG), in different proportions. Upon carbonization at 973 K the former yields a carbon coating while the latter pyrolizes generating mesoporosity. Additionally the carbon-coated monoliths were activated with CO₂ to generate microporosity. Vanadium was impregnated in these carbon-coated monoliths by equilibrium adsorption using ammonium metavanadate as precursor and they were tested in the SCR of NO at low temperature. By increasing the amount of PEG, the mesopore volume increases in the range of narrow mesoporosity (2–5 nm). It was found that the more mesopore volume, the more oxygenated surface groups are formed. This turned out to be crucial for the deposition of vanadium in a dispersed fashion and also for the activity in the SCR of NO. On the contrary, the narrow microporosity (<0.7 nm) does not contribute to enhance the dispersion of the catalyst. The kinetic rate constants of the monolithic catalyst prepared are in the range of the most active catalyst reported in the literature for the SCR of NO at low temperature.     

Mn-Mg-O_x催化剂低温NH_3选择性催化还原NO的性能的研究

采用低温固相法制备了系列Mn-Mg-Ox催化剂,考察了催化剂的低温NH3选择性催化还原(NH3-SCR)NO的性能。结果表明,当Mn/Mg摩尔比为1,氨氮体积比为1,空速为30 000 h-1时,催化剂表现出优异的低温催化活性和抗硫性。在100~175℃温度范围内,无硫时,NO催化转化率能保持在99%以上;加入100μg/g SO2后,催化剂脱硝性能变化不大,仍可维持在95%以上。     

柴油车尾气四效催化净化技术研究进展

从四效催化转化器和四效催化剂2个方面概述了用于柴油车尾气(主要污染物包括烃类、一氧化碳、颗粒物和氮氧化物)处理的四效催化技术的国内外研究进展情况,并探讨了其主要难点与发展前景.     

NO SCR reduction by hydrogen generated in line on perovskite-type catalysts for automotive diesel exhaust gas treatment

This paper deals with the preparation (by solution combustion synthesis, SCS), characterization (by XRD, BET, FESEM and TPD/R analyses), catalytic activity evaluation (in a temperature-programmed reaction—TPRe apparatus), and the assessment of the reaction mechanism of NO reduction by H₂ in the presence of oxygen on a series of perovskite-type catalysts belonging to the LaFeO₃ family (LaFeO₃, La_0.8Sr_0.2FeO₃, Pd/La_0.8Sr_0.2FeO₃, La_0.8Sr_0.2Fe_0.9Pd_0.1O₃, La_0.7Sr_0.2Ce_0.1FeO₃, Pd/La_0.7Sr_0.2Ce_0.1FeO₃, La_0.7Sr_0.2Ce_0.1Fe_0.9Pd_0.1O₃). The catalysts have been studied in the 25-350 °C temperature range. Significant catalytic activities were measured at 150-250 °C. Among the catalysts screened, La_0.8Sr_0.2Fe_0.9Pd_0.1O₃, showed the best performance. Hence, it was deposited directly over a ceramic honeycomb monolith by in situ SCS and then tested in a lab-scale test rig. A mechanistic analysis is presented concerning the relationship between the observed activity and the reducibility of the B site, determined from TPR experiments, as well as the correlation between the observed oxygen inhibition and the proposed NO_x reduction mechanism.Some final conclusions are drawn on the perspective of the practical application of the investigated after-treatment route for diesel exhaust gases.     

Alumina- and titania-based monolithic catalysts for low temperature selective catalytic reduction of nitrogen oxides

The selective catalytic reduction of NO+NO₂ (NO_x) at low temperature (180–230°C) with ammonia has been investigated with copper-nickel and vanadium oxides supported on titania and alumina monoliths. The influence of the operating temperature, as well as NH₃/NO_x and NO/NO₂ inlet ratios has been studied. High NO_x conversions were obtained at operating conditions similar to those used in industrial scale units with all the catalysts. Reaction temperature, ammonia and nitrogen dioxide inlet concentration increased the N₂O formation with the copper-nickel catalysts, while no increase was observed with the vanadium catalysts. The vanadium-titania catalyst exhibited the highest DeNO_x activity, with no detectable ammonia slip and a low N₂O formation when NH₃/NO_x inlet ratio was kept below 0.8. TPR results of this catalyst with NO/NH₃/O₂, NO₂/NH₃/O₂ and NO/NO₂/NH₃/O₂ feed mixtures indicated that the presence of NO₂ as the only nitrogen oxide increases the quantity of adsorbed species, which seem to be responsible for N₂O formation. When NO was also present, N₂O formation was not observed.     

Pt-loaded zeolites for reducing exhaust gas emissions at low temperatures and in lean conditions

In this study, pure and platinum-loaded zeolites, ZSM-5, Beta, zeolite Y and Ferrierite, were examined for the reduction of NO with propene in lean conditions and at low temperatures. The studies were carried out by utilising the FT-IR technique both in determination of surface species as well as concentrations in the gas flow at reactor outlet. The maximum in the intermediate formation can be observed at the light-off temperatures over all studied catalyst materials. The maximum conversions of NO were reached with 1 wt% Pt-loaded Beta and Y zeolites in excess oxygen. The lowest light-off temperatures of NO as well as propene can be detected also with Beta and Y zeolite catalysts.     

Measurements of diesel soot oxidation kinetics in an isothermal flow reactor–catalytic effects using Pt based coatings

Despite the significant progress in soot oxidation studies, there is still high uncertainty regarding the rate expressions to model the reactions in diesel particulate filters (DPF). This uncertainty arises from inherent difficulties in sampling and measuring the reaction rate in a realistic way, as well as different properties of the examined soot. In this context, the scope of this study is the development of a novel experimental set-up capable of overcoming existing experimental difficulties. The developed set-up allows for real diesel soot oxidation studies in an isothermal flow reactor. The reaction of soot with oxygen and NO₂ is studied with synthetic gas and with real diesel exhaust and the reaction kinetics are derived for both bare and Pt-based catalyzed substrates by combining experimental and model results.     

Selective catalytic reduction of NO and NO2 at low temperatures

The fast SCR reaction using equimolar amounts of NO and NO₂ is a powerful means to enhance the NO_x conversion over a given SCR catalyst. NO₂ fractions in excess of 50% of total NO_x should be avoided because the reaction with NO₂ only is slower than the standard SCR reaction.

At temperatures below 200 °C, due to its negative temperature coefficient, the ammonium nitrate reaction gets increasingly important. Half of each NH₃ and NO₂ react to form dinitrogen and water in analogy to a typical SCR reaction. The other half of NH₃ and NO₂ form ammonium nitrate in close analogy to a NO_x storage-reduction catalyst. Ammonium nitrate tends to deposit in solid or liquid form in the pores of the catalyst and this will lead to its temporary deactivation.

The various reactions have been studied experimentally in the temperature range 150–450 °C for various NO₂/NO_x ratios. The fate of the deposited ammonium nitrate during a later reheating of the catalyst has also been investigated. In the absence of NO, the thermal decomposition yields mainly ammonia and nitric acid. If NO is present, its reaction with nitric acid on the catalyst will cause the formation of NO₂.     

具有低温活性的高效脱硝催化体系研究进展

NH3选择性催化还原技术在烟气脱硝中应用广泛,但由于传统的氧化钒催化剂起活温度偏高,因此开发具有低温活性的高效脱硝体系成为当前国内外研究热点.介绍了该领域目前已经开发出的贵金属、分子筛、金属氧化物等低温脱硝体系和处于研究前沿的固体杂多酸催化脱硝新体系的最新研究进展.指出固体杂多酸催化体系具有耐硫、抗水抑制等优点,具有很好的研究和应用前景.     

低温选择催化还原NO催化剂的制备及其性能研究

采用等体积浸渍法制备Mn-Ti-PILC、Co-Ti-PILC、Fe-Ti-PILC、Cu-Ti-PILC和V-Ti-PILC催化剂,并应用于丙烯选择催化还原NO(C3H6-SCR)体系中。结果显示,N2最高收率由高到低的顺序依次为Cu-Ti-PILC>Fe-Ti-PILC>V-Ti-PILC>Mn-Ti-PILC>Co-Ti-PILC,N2最高收率所对应的反应温度由低到高的顺序依次为Mn-Ti-PILC相似文献   

低温选择催化还原NO催化剂的制备及其性能研究

采用等体积浸渍法制备Mn-Ti-PILC、Co-Ti-PILC、Fe-Ti-PILC、Cu-Ti-PILC和V-Ti-PILC催化剂,并应用于丙烯选择催化还原NO(C3H6-SCR)体系中。结果显示,N2最高收率由高到低的顺序依次为Cu-Ti-PILCFe-Ti-PILCV-Ti-PILCMn-Ti-PILCCo-Ti-PILC,N2最高收率所对应的反应温度由低到高的顺序依次为Mn-Ti-PILCCu-TiPILCFe-Ti-PILC=Co-Ti-PILCV-Ti-PILC。     

Combined use of a mass-spectrometer and a UV analyzer in the dynamic study of NH3-SCR for diesel exhaust aftertreatment

A mass spectrometer (MS) and a novel UV analyzer were coupled in the experimental study of the transient Selective Catalytic Reduction (SCR) reactivity, aimed at the development of a dynamic numerical model of SCR converters for the selective reduction of NO _x in Diesel exhausts. Their parallel use revealed an effective method for the understanding of key elements in the SCR reaction mechanism under real operating conditions.     

AA-MA型柴油低温流动性能改进剂的研制

用丙烯酸高碳醇酯和马来酸酐的共聚物经高碳数脂肪酸的胺化合成了AA MA型柴油低温流动改进剂 ,分析了该降凝剂的结构和降凝机理 ,结果表明它对大港油田炼制的 5# 和 0 # 柴油有良好的降凝效果。     

复合氧化物的制备及其在碳黑颗粒物氧化中的应用

采用柠檬酸络合燃烧法合成钾铈和钾铈镧复合氧化物催化剂。利用XRD对复合氧化物进行了表征。利用程序升温反应（TPR）方法研究了其对碳黑颗粒物的催化氧化性能,考查了焙烧温度、焙烧时间、前驱体中K、Ce的物质的量比等制备条件对催化剂活性的影响及稳定性。结果表明：钾铈镧复合氧化物对碳黑颗粒物具有较高的催化活性和稳定性。     

MnO_x-CeO_2催化剂在低温下催化氧化低浓度甲醛的性能研究

采用修饰共沉淀法制得铈锰氧化物MnO_x-CeO_2催化剂,采用环境舱法,模拟太阳光在贴近真实夏季车内环境的低温低浓度工况下,考察太阳辐射强度、甲醛初始浓度对MnO_x-CeO_2热催化甲醛的性能影响,并进行催化动力学探究。结果表明,该催化剂孔道结构丰富,光热转化能力优秀,与单独的铈锰氧化物相比氧化能力有明显提升。在太阳辐射强度为450～650 W/m²时,初始浓度为0.5 mg/m2时,初始浓度为0.5 mg/m³的甲醛降解率可达76.2%～82.1%,且E-R模型更贴近该实验数据。在太阳辐射强度为550 W/m3的甲醛降解率可达76.2%～82.1%,且E-R模型更贴近该实验数据。在太阳辐射强度为550 W/m²,初始浓度分别为0.2,0.5,1.0 mg/m2,初始浓度分别为0.2,0.5,1.0 mg/m³时,降解率分别为63%,70.6%,78.3%,降解效果明显。此外,5种催化剂(MnO_x-CeO_2、MnO_x-CeO_2-TiO_2、TiO_2、Bi-V-O、CuO-MnO_2)在相同实验条件下的甲醛催化效果表明,MnO_x-CeO_2降解率最高,为78.3%。该催化剂具备良好的稳定性。表明该催化剂在车内等低温低浓度环境下的甲醛催化氧化有着广泛的应用前景。     

Pore structure analysis of an SCR catalyst using a new method for interpreting nitrogen sorption hysteresis

Nitrogen sorption hysteresis of a selective catalytic reduction (SCR) catalyst in its “fresh” and “used” form [Catal. Today 56 (2000) 335] was analysed by employing the Corrugated Pore Structure Model (CPSM-nitrogen) reported elsewhere [Ind. Eng. Chem. Res. 39 (2000) 3747 and Ind. Eng. Chem. Res., 39 (2000) 3764]. The BET surface area and intrinsic pore volume, and surface area distributions were deduced via the CPSM fitting over the relevant hysteresis loop and compared to those obtained by applying various conventional methods. CPSM predictions of the total specific pore surface area, are in perfect agreement with BET estimates, while those calculated by the Roberts method, irrespective of the hysteresis loop branch chosen, deviate substantially. CPSM prediction of tortuosity factors (CPSM-tortuosity [Ind. Eng. Chem. Res., in press]) for the catalysts under consideration were found to be: τ_CPSM,fresh=4.6 and 4.5. These values are typical of porous catalysts and approach those determined experimentally (i.e. τ=5–6 [Appl. Catal. 10 (1996) 299]). The pore volume distribution of the latter catalyst obtained from mercury porosimetry measurements approaches that deduced by the CPSM method.     

钛基铜系低温SCR催化剂的制备及其脱硝性能研究

采用浸渍法制备M-Cu/Ti-PILC(M=Mn、Ce、Co、Fe、Zr和Ag)催化剂,并应用于丙烯选择催化还原NO体系(C3H6-SCR),考察第二种活性物种M对Cu/Ti-PILC催化性能的影响。结果表明,Mn物种会降低Cu/Ti-PILC催化剂的催化活性和反应温度;Ce物种能拓宽Cu/Ti-PILC催化剂的反应窗口;Co对Cu/Ti-PILC催化剂的催化活性及反应温度无影响;Fe、Zr、Ag会提高Cu/Ti-PILC催化剂的催化活性,并且Fe、Ag物种不影响Cu/Ti-PILC催化剂的反应温度,而Zr会提高Cu/Ti-PILC催化剂的反应温度。     

钛基铜系低温SCR催化剂的制备及其脱硝性能研究

采用浸渍法制备M-Cu/Ti-PILC(M=Mn、Ce、Co、Fe、Zr和Ag)催化剂,并应用于丙烯选择催化还原NO体系(C3H6-SCR),考察第二种活性物种M对Cu/Ti-PILC催化性能的影响。结果表明,Mn物种会降低Cu/Ti-PILC催化剂的催化活性和反应温度;Ce物种能拓宽Cu/Ti-PILC催化剂的反应窗口;Co对Cu/Ti-PILC催化剂的催化活性及反应温度无影响;Fe、Zr、Ag会提高Cu/Ti-PILC催化剂的催化活性,并且Fe、Ag物种不影响Cu/Ti-PILC催化剂的反应温度,而Zr会提高Cu/Ti-PILC催化剂的反应温度。