The role of NO2 in the reduction of NO by hydrocarbon over Cu-ZrO2 and Cu-ZSM-5 catalysts

The reduction of NO _x with propene or propane in the presence of 1 or 4% O₂ was studied at low conversions over a 7.4 wt% Cu-ZrO₂ and a 3.2 wt% Cu-ZSM-5 catalyst. The rates of N₂ production were compared in experiments using only NO or a mixture of NO and NO₂ in the feed. They were also compared with the rates of NO₂ reduction to NO under the same conditions, and of NO oxidation to NO₂ in the absence of hydrocarbon. It was found that the reduction of NO₂ to NO was very fast, consistent with literature data. The data were best explained by a reaction scheme in which the hydrocarbon was activated primarily by reaction with adsorbed NO₂ to form an adsorbed oxidized N-containing hydrocarbon intermediate, the reaction of which with NO was the principal route to produce N₂ under lean NO _x conditions.On leave from State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China.     

Cobalt catalysts in selective catalytic reduction of NO by methane

Impregnated cobalt-containing catalysts are studied in the selective catalytic reduction of NO by methane. The active component of all the impregnated cobalt-containing catalysts is Co₃O₄. The role of O₂ seems to maintain the surface stoichiometry of Co₃O₄. The main reason of decrease of catalytic activity of samples based on MgO, SiO₂, Al₂O₃ in selective catalytic reduction of NO is due to oxide-oxide interaction promoted by water. Catalysts based on montmorillonite (HMM) are stable in the presence of water. All the carriers can be placed by the capability for oxide–oxide interaction in the following order: SiO₂ ≫ MgO > Al₂O₃ ≫ HMM. This revised version was published online in August 2006 with corrections to the Cover Date.     

NO2 formation and its effect on the selective catalytic reduction of NO over Co/ZSM-5

Catalytic performance of Co/ZSM-5 with different metal loadings and of HZSM-5 was compared in the NO + O₂, C₃H₈ + O₂, and NO + C₃H₈ + O₂ reactions. It was found that Co/ZSM-5 catalysts containing only isolated cobalt ions in cationic positions are inactive in NO₂ formation. To achieve appreciable NO conversion in the SCR process over these catalysts higher reaction temperatures are required. These results make it possible to suggest that NO₂ formation is not a prerequisite for the SCR of NO with hydrocarbons over Co/ZSM-5. With increasing Co loading, however, Co/ZSM-5 begins to exhibit activity in NO₂ formation. This is explained by the formation of cobalt oxide particles on the zeolite carrier, which are active in the NO₂ formation. Increase in NO₂ formation strongly enhances catalytic activity in SCR of NO at lower reaction temperatures. Comparison of the C₃H₈ conversion in the C₃H₈ + O₂ and C₃H₈ + O₂ + NO reactions provides evidence that NO₂ activates hydrocarbon molecules resulting in the formation of the reaction intermediates of the SCR process.On leave from N.D. Zelinskii Institute of Organic Chemistry, Leninskii Pr. 47, Moscow, Russia.     

ZSM-5分子筛催化剂上氨选择性催化还原NO_x的研究进展

氨选择性催化还原(NH_3-SCR)是控制NO_x排放的有效手段,对Fe-ZSM-5、Cu-ZSM-5、Mn-ZSM-5及多金属负载的ZSM-5分子筛催化剂在NH_3-SCR脱除NO_x中的性能及影响因素进行总结,并展望ZSM-5分子筛在NO_x脱除中的发展方向。     

Role of organic nitro compounds in selective reduction of NOx with ethanol over different supported silver catalysts

The adsorption of organic nitro compounds such as nitromethane and nitroethane on different supported silver catalysts (Ag/Al₂O₃, Ag/TiO₂, Ag/SiO₂) has been studied using infrared spectroscopy. The adsorbed NCO species formation was strongly influenced by the catalyst support and therefore clearly detected on Ag/Al₂O₃ and Ag/TiO₂ catalysts by thermal decomposition of nitromethane and nitroethane at temperatures higher than 150°C. With the Ag/SiO₂ catalyst, very little NCO formation was observed at 350°C. On the other hand, the catalyst support was found to affect the N₂ formation in the selective reduction of NO_x on supported silver catalysts. On the basis of these findings, the role of adsorbed nitromethane, nitroethane and isocyanate species in the selective reduction of NO_x is discussed with respect to the catalyst support effect and the catalytic activity. This revised version was published online in July 2006 with corrections to the Cover Date.     

The bifunctional reaction pathway and dual kinetic regimes in NOx SCR by methane over cobalt mordenite catalysts

The pathway for selective reduction of NO_x by methane over Co mordenite cataysts has been studied by comparing the rates of the individual reactions (NO oxidation, CH₄ oxidation, NO₂ reduction) with that of the combined reaction (NO + O₂ + CH₄). Co⁽⁺²⁾ was exchanged into H-MOR and Na-MOR to give catalysts with different metal loading and number of support protons. Additionally, exchanged Co⁽⁺²⁾ ions were precipitated with NaOH to produce dispersed cobalt oxide on Na-MOR. The NO oxidation rate is the same for ion exchanged Co⁽⁺²⁾ ions in H-MOR and Na-MOR, but the rate of Co⁽⁺²⁾ ions is much lower than that of cobalt oxide. NO oxidation equilibrium is obtained only for those catalysts with high metal loading, cobalt oxide or run at low GHSV. Under the conditions of selective catalytic reduction, methane oxidation by O₂ is low for all catalysts. The turnover frequency of Co on Na-MOR, however, is higher than that on H-MOR. The rate of NO₂ reduction to N₂ is directly proportional to the number of support acid sites and independent of the amount of Co. Comparison of the rates and selectivities for the individual reactions with the combined reaction of NO + O₂ + CH₄ indicates that there are two types of catalysts. For the first, the NO oxidation is in equilibrium and the rate determining step is reduction of NO₂. For these catalysts, the rate (and selectivity) for formation of N₂ is identical from NO + O₂ + CH₄ and NO₂ + CH₄. These catalysts have high metal loading and few acid sites. Nevertheless, the rate of N₂ formation increases with increasing number of protons. For the second type of catalyst, NO oxidation is not in equilibrium and is the rate limiting step. For these catalysts the rate of N₂ formation increases with increasing metal loading. Neither catalyst type, however, is optimized for the maximum formation of N₂. By using a mixture of catalysts, one with high NO oxidation activity and one with a large number of Brønsted acid sites, the rate of N₂ is greater than the weighted sum of the individual catalysts. The current results support the proposal that the pathway for selective catalytic reduction is bifunctional where metal sites affect NO oxidation, while support protons catalyze the formation of N₂.     

分子筛类催化剂上甲烷选择性催化还原NOx研究进展

选择性催化还原NOx是处理工业废气和稀燃汽车尾气NOx的有效方法。由于还原剂甲烷廉价易得,甲烷选择性催化还原NOx（简称CH4-SCR）已成为近年来的研究热点,而分子筛类催化剂因催化活性高而得到广泛研究。本文综述了CH4-SCR脱除NOx体系中不同金属负载的分子筛催化剂及反应机理方面的研究进展,包括Co系、Pd系、In系等分子筛催化剂在催化性能、反应机理及掺杂改性等方面的研究现状,并提出了分子筛类催化剂用于CH4-SCR的研究方向。     

NOx选择催化还原用Au/海泡石催化剂的研究

以天然海泡石为原料,对其进行酸改性后作为催化剂载体。采用浸渍法制备了用于CO选择催化还原NOx的负载型Au/海泡石催化剂。设计正交实验对海泡石酸改性条件进行了研究,对催化剂催化还原NOx的活性进行了评价。实验结果表明,该催化剂对于NOx催化还原反应有一定活性。对酸改性前后的海泡石及制得的催化剂进行了TGA、IR和XRD分析,证实海泡石是一种较好的催化剂载体。     

In situ high temperature FTIR studies of NOx reduction with propylene over Cu/ZSM-5 catalysts

High temperature in situ FTIR has been used to investigate the surface species present on Cu/ZSM-5 during the reduction of NO_x with propylene in a lean environment. Parallels have been observed between adsorbed surface species and catalytic activity for this reaction. Species detected at low temperatures are not representative of those detected at high temperatures where the catalyst is active. An oxidized nitrogen-containing species has been observed at 2580 cm^–1 on Cu during reaction conditions (400°C). In contrast, at low temperatures, where the catalyst is less active, coke and Cu⁺-CO predominated. The effects of Cu weight loading, C/NO ratio, reaction temperature, and catalyst deactivation by steaming have been investigated with IR.     

Ir/H-ZSM-5 Catalysts in the Selective Reduction of NOx with Hydrocarbons

Three different Ir catalysts supported on H-ZSM-5 were prepared and tested for the selective catalytic reduction of NO under net oxidizing conditions using propene as reducing agent. The preparation of highly active Ir catalysts and the elaboration of a procedure for enhancing activity by on stream conditioning was targeted. Structural changes of the catalyst during conditioning were investigated by means of XRD, TEM and activity measurements. Under reaction conditions Ir was present as Ir⁰ and IrO₂. The presence of Ir⁰ was essential for high DeNOx activity. The ratio of Ir⁰/Ir⁴⁺ was found to depend on the size of Ir-containing crystallites. Larger crystallites contained predominantly Ir⁰. Crystallite size and oxidation state of Ir have been identified to be crucial for the NO reduction behaviour of Ir/H-ZSM-5.     

柴油车NOx选择性催化还原技术研究进展

综述了NH<,3>-SCR技术的反应机理、国内外不同催化体系针对NOx脱除的研究现状及NH<,3>-SCR技术涉及的其他关键技术,并对选择性催化还原技术的发展方向进行了展望.     

过渡金属/分子筛催化剂上选择性催化还原氮氧化物的研究进展

综述了近十年来过渡金属/分子筛催化剂上氨和碳氢化合物选择性催化还原NOx方面的研究进展。在NH3-SCR体系,着重介绍了铜基和铁基分子筛催化剂的研究状况,探讨了分子筛催化剂在该体系中的失活原因;在HC-SCR体系,总结了不同过渡金属、分子筛类型、还原剂、H2O和SO2等对催化剂活性的影响,探讨了目前比较公认的碳氢化合物选择性催化还原NOx的反应机理。最后展望了分子筛催化剂在选择性催化还原NOx领域今后的研究方向。     

凹凸棒石低温SCR脱硝催化剂的研究进展

选择性催化还原（SCR）脱硝技术是目前主流的氮氧化物脱除技术,其核心是催化剂。凹凸棒石成本低廉,性能优越,适合用作SCR催化剂的载体,而且以凹凸棒石为载体的催化剂显示出良好的低温选择性和稳定性,具有很好的应用前景。本文总结了凹凸棒石低温SCR脱硝催化剂的研究进展,阐述了活性组分、制备方法、前体物种、活性组分负载量、煅烧温度、元素掺杂等因素对催化剂脱硝活性的影响,同时简要介绍了导致此类催化剂失活的原因以及失活催化剂的再生方法,并指出在凹凸棒石负载型低温脱硝催化剂上进行的SCR脱硝反应遵循E-R机理,最后指出此类催化剂的未来研究方向主要是进一步提高现有催化剂的低温催化活性和抗中毒能力,实现工业化应用。     

Marked difference in activity of alumina catalysts for selective catalytic reduction of nitrogen monoxide by ethene in excess oxygen

Alumina is well-known as an effective catalyst for the title reaction. The present study revealed that the activity of an alumina catalyst for the reaction largely depended on its type. A critical factor affecting the activity of an alumina was neither the surface area nor pore structure, but the purity; the higher the purity, the higher the activity. Active alumina catalysts showed high activity for both the oxidation of NO to NO₂ and the reduction of NO₂ by ethene. This revised version was published online in July 2006 with corrections to the Cover Date.     

铜系催化剂选择性催化还原脱除NOx研究

以硅胶改性堇青石蜂窝陶瓷为载体,分别制备了以Cu-O、Cu-Ce-O和Cu-Ce-Mn-O为活性组分的催化剂。以CO(NH₂)₂为还原剂,在固定床反应器中进行选择性催化还原NO的研究。采用XRD、SEM和BET等测试方法对催化剂进行表征。结果表明,在温度(300～500) ℃,催化剂Cu-Ce-Mn-O/SiO₂/堇青石的活性优于催化剂Cu-O/SiO₂/堇青石和Cu-Ce-O/SiO₂/堇青石,反应温度为450 ℃、空速为8 000 h^-1时,Cu-Ce-Mn-O/SiO₂/堇青石催化剂催化还原NO的转化率可达到88%。     

NOx低温选择性催化还原催化剂研究进展

催化剂是选择性催化还原脱硝技术的核心,其催化性能直接关系到脱硝效果的好坏。本文介绍了该领域新开发的贵金属、金属氧化物、分子筛、碳基催化剂等低温脱硝体系及其最新研究进展。对碳纳米材料催化剂和新型杂多酸催化剂等新成果作了介绍,并且对今后的研究方向作了展望。     

Wire-mesh honeycomb catalysts for selective catalytic reduction of NO with NH3

Both flat and corrugated wire mesh sheets were coated with aluminum powder by using electrophoretic deposition (EPD) method. Controlled thermal sintering of coated samples yielded uniform porous aluminum layer with a thickness of 100 μm that was attached firmly on the wire meshes. Subsequent controlled calcination formed a finite thickness of Al₂O₃ layer on the outer surface of each deposited aluminum particles, which resulted in the formation of Al₂O₃/Al double-layered composite particles that were attached firmly on the wire surface to form a certain thickness of porous layer. A rectangular-shaped wire-mesh honeycomb (WMH) module with triangular-shaped channels was manufactured by packing alternately the flat sheet and corrugated sheet of the Al₂O₃/Al-coated wire meshes. This WMH was further coated with V₂O₅-MoO₃-WO₃ catalyst by wash-coating method to be applied for the selective catalytic reduction (SCR) of NO with NH₃. With an optimized catalyst loading of 16 wt%, WMH catalyst module shows more than 90% NO conversion at 240 °C and almost complete NO conversion at temperatures higher than 300 °C at GHSV 5,000 h⁻¹. When compared with conventional ceramic honeycomb catalyst, WMH catalyst gives NO conversion higher by 20% due to reduced mass transfer resistance by the existence of three dimensional opening holes in WMH.     

CO含量对烟气选择性非催化还原反应的影响

分别以氨水和尿素溶液作为还原剂,在管式反应器上详细研究了CO含量对烟气SNCR（选择性非催化还原）脱硝性能的影响。结果表明,CO含量增加导致：（1）SNCR脱硝温度窗口和最佳脱硝温度向低温方向移动,同时脱硝温度窗口宽度变窄、最大脱硝效率降低,尤其是以氨水作为还原剂时更为突出;（2）N₂O排放的峰值升高;当以尿素溶液作还原剂时,N₂O排放温度窗口主要向低温方向扩展;当以氨水作还原剂时,N₂O排放温度窗口向低温和高温两个方向扩展;（3）NH₃及HNCO残留量曲线向低温方向移动。维持相同的φ（CO）/φ（urea）,氨氮比增加将导致最佳脱硝温度略微降低。氨氮比或氧含量的变化对NO_x、N₂O、NH₃及HNCO的排放趋势没有影响。     

硫酸化NOx选择性催化还原脱硝催化剂的研究进

新型的硫酸化NOx选择性催化还原(SCR)催化剂，由于对催化性能具有良好的促进作用以及强抗碱金属中毒特性等日益受到关注，成为SCR研究的热点。概述了硫酸化SCR催化剂的研究现状，详细介绍其制备方法、物化性能、催化特性以及反应机理，并对今后的研究方向作了展望。     

CO2 effect on activity and stability of NOx storage reduction catalysts

A catalyst for NO_x storage/reduction was prepared to improve the activity of Ba–Pt/γ-Al₂O₃ by replacing Ba with a mixture of Ba and Mg. The catalyst was prepared by impregnating Pt and then co-impregnating Ba and Mg (Mg:Ba molar ratio = 1) on commercial γ-Al₂O₃. The tests have been carried out in the presence of CO₂ at temperatures between 200 and 400 °C in order to understand the role of both the feed and various alkaline-earth metals. The storage capacity of the two catalysts was different like the mechanism in the reduction process.