柴油车用NH3-SCR铜基分子筛催化剂孤立态Cu2+研究进展

NH₃选择催化还原技术(NH₃-SCR)作为一种高效去除NO_x的手段,已广泛地应用到柴油车尾气脱硝过程当中。车用NH₃-SCR技术常采用铜基分子筛作为催化剂,尾气中的氮氧化物可在催化剂的作用下,与NH₃反应转化为N₂;但在实际应用过程中,N₂O的生成、催化剂的水热老化与尾气中的SO₂会影响铜基分子筛催化剂的脱硝性能。因此,本文以Cu-SSZ-13分子筛催化剂中孤立态Cu²⁺的研究进展为基础,总结了Cu-SSZ-13中两种孤立态Cu²⁺对NH₃-SCR反应与N₂O生成的影响;综述了两种孤立态Cu²⁺对水热老化与SO₂响应的差异;归纳了诱导Cu-2Z生成的手段。同时,本文以Cu-LTA分子筛催化剂的研究现状为例,简要回顾了Cu-LTA中孤立态Cu²⁺的研究进展,对C...     

单空缺石墨烯负载的Pd单原子催化剂上NO还原的密度泛函理论研究

采用密度泛函理论（DFT）方法对单空缺石墨烯负载的Pd单原子（Pd/SVG）催化剂上H₂还原NO的反应进行了研究,探究了Pd/SVG上NO还原生成N₂和NH₃的路径。在Pd/SVG上NO容易加氢形成HNO,需要的活化能为67.0 kJ·mol^-1,显示了极高的催化活性。N₂生成的有利路径为NO活化生成HNO后,HNO继续加氢生成中间体NH₂O和NH₂OH,然后NH₂OH解离生成NH₂和OH,生成的NH₂中间体结合NO形成NH₂NO,然后NH₂NO异构化形成的NHNOH再经解离生成N₂与H₂O,这个过程中的决速步骤为NH₂NO分子内氢转移生成NHNOH,能垒为144.3 kJ·mol^-1。对于NH₃的生成,从NO的活化到中间体NH₂的形成与N₂的形成过程相同,最后NH₂加氢即可形成NH₃,这个过程中的决速步骤为NH₂O加氢生成NH₂OH,能垒为86.4 kJ·mol^-1。比较生成N₂和NH₃的决速步能垒可见,Pd/SVG催化剂上NO经H₂还原更容易形成NH₃。本研究为石墨烯负载型Pd基催化剂上H₂还原NO的实验及工业应用提供理论参考。     

非热等离子体强化TiO2催化尿素分解副产物水解性能的研究

尿素-选择性催化还原技术低温下运行时尿素分解不彻底,易形成缩二脲、三聚氰酸和三聚氰胺等副产物。本研究将TiO₂催化剂与介质阻挡放电等离子体相结合,在程序升温条件下考察了载气中有无O₂时引入等离子体前后TiO₂催化尿素分解副产物水解的性能。结果表明：TiO₂表面缩二脲、三聚氰酸和三聚氰胺分别在43~261℃、217~300℃和199~300℃水解生成NH₃和CO₂,载气中有无O₂对催化水解过程几乎无影响。引入等离子体后缩二脲、三聚氰酸和三聚氰胺水解所需温度显著降低,载气中无O₂时引入等离子体NH₃产率变化不大,副产物仅有少量N₂O和NO,有O₂时NH₃产率显著降低,且生成较多N₂O、NO、NO₂及少量NH₄NO₂和NH₄NO₃。未来需从优化放电条件和催化剂组成等方面解决引入等离子体导致副产物形成等问题。     

重型柴油机主要含氮化合物的排放特性

采用傅里叶变换红外光谱(FTIR)技术,研究了加装选择性催化还原SCR装置的重型柴油机主要含氮化合物排放,重点探索了不同工况下主要含氮化合物NO、NO₂和N₂O的排放特性。结果表明:未加装SCR的原机,随负荷的增加,柴油机NO排放持续上升,NO₂排放先升后降,N₂O排放很少。加装SCR后该柴油机NO与NO₂排放均明显下降,标定转速下NO₂排放降幅较大,主要是其NO₂/NO值稍高导致快速SCR反应较多的原因。由于存在SCR副反应,与原机相比,柴油机N₂O排放比原机平均增加2倍以上,最大转矩转速下N₂O排放升幅更高。N₂O排放随负荷的增加而上升,主要是排温升高导致NH₃氧化生成N₂O反应速率增加的原因。加装SCR后,该机排气中的NO/NO_x值要明显低于原机状态,而外特性的NO₂/NO_x值和N₂O/NO_x值高达12.8%和20.7%,均远高于原机的3.0%和0.5%。     

低温燃烧下NH3的氧化特性

NH₃的气相氧化是低温燃烧过程中NO_x（NO和NO₂）与N₂O的重要来源,为了深入认识其反应规律,在管式流动反应器系统中进行了实验研究。重点考察了挥发分中的可燃气（CO、CH₄或H₂）和NO对NH₃氧化及氮氧化物排放的影响规律,并根据化学反应机理对实验结果进行了分析。研究结果表明,低温氧化性气氛下微量的可燃气就能够显著促进NH₃的氧化,并使NO_x和N₂O的生成量大幅度升高。当可燃气体浓度相同时,H₂对NH₃氧化的影响最大,CO的影响最小,CH₄对NH₃氧化的影响略大于CO。随着可燃气体浓度的升高,其对NH₃氧化与氮氧化物生成的影响先逐渐增加,然后趋于稳定。反应初始气体中存在NO时,也会加速NH₃的氧化。     

新型双流化床炉内NOx生成特性数值模拟

运用煤燃烧及NO_x生成的详细化学反应机理,通过搭建一维化学反应器网络（1D-CRN）,对一个新型双流化床（DCFB）内燃料型N转化为NO_x的基元化学反应进行了敏感性分析并讨论了反应温度、过量空气系数以及一、二次风配比对燃料型NO_x生成的影响。研究发现,在相同条件下,循环流化床炉膛出口的NO_x排放值为224.48 mg·m^-3,而双流化床炉膛出口的NO_x排放值为97.29 mg·m^-3,双流化床对于燃料型NO_x的减排幅度达到了56.66%。此外,促进NO_x生成的基元反应主要有R398（NH₂+O?HNO+H）、R1-N-1（N-Vol?NH₃+HCN）、R569（NCO+O₂?NO+CO₂）、R17（H+O₂?O+OH）等反应,而抑制NO_x生成的反应包括R411（NH₂+NO?N₂+H₂O）、R412（NH₂+NO?NNH+OH）、R570（NCO+NO?N₂O+CO）、R571（NCO+NO?N₂+CO₂）以及R5（Char+NO?Char+N₂+O₂）和R6（Soot+NO?n Soot+N₂+CO）等反应。这说明反应区域氧气浓度是影响NO_x生成的关键,低氧浓度可抑制燃料N向NO_x转化。另外,NO_x生成值随着反应温度的升高而降低,但随着过量空气系数和一次风所占比例的增大而增加。     

二维AuP2材料电催化固氮性能的理论研究

通过电化学反应将氮气（N₂）和水（H₂O）在常温常压的条件下转化为氨气（NH₃）是一种绿色环保的合成氨方法。但由于N₂具有非常高的化学惰性,必须借助电催化剂来加速反应的动力学过程。通过密度泛函理论计算揭示出新型二维无机材料AuP₂对N₂电化学还原制NH₃具有很好的催化活性。在二维AuP₂材料中,Au与P之间由于电负性差异发生显著的电荷转移,使带有正电荷的P可作为活性位点促进氮还原。计算表明整个反应的速控步是N₂生成^*NNH的过程,限制电压为1.2 V,催化活性可以跟部分金属催化剂相媲美。为设计高效氮还原电催化剂提供了新的思路。     

CeO2含量对柴油机商用稀土SCR催化剂脱硝性能的影响

考察了不同CeO₂含量对柴油机商用稀土选择性催化还原（SCR）催化剂NH₃-SCR性能的影响。通过X射线衍射、N₂吸附-脱附、X射线光电子能谱、H₂-程序升温还原和NH₃-程序升温脱附等表征手段对催化剂的结构及性能进行了研究。研究发现,CeO₂含量对催化剂的晶体结构没有影响,添加后促进了稀土催化剂表面SiO₂的分散,从而对催化剂的脱硝性能及水热老化性能产生影响;CeO₂含量影响催化剂的织构性质,较大的孔体积和适当的比表面有利于催化剂水热老化后催化剂的脱硝性能的提高;随着CeO₂含量从14%增加到20%,其表面Ce含量呈先降低后升高的趋势,其中Ce⁴⁺含量与催化剂的水热老化后催化剂的脱硝性能成正相关;CeO₂含量的增加会阻碍稀土催化剂中其他组分的还原或增加其热稳定性,从而影响催化剂的还原能力和酸性位数量;通过脱硝性能测试,稀土催化剂中CeO₂的最佳含量为16%。     

无烟煤化学链燃烧过程燃料氮转化释放特性

基于赤铁矿石载氧体,在小型单流化床反应器上,开展煤挥发分和焦炭的化学链燃烧研究,探讨挥发分氮和焦氮在化学链燃烧过程中的转化特性。研究表明:燃料氮释放的中间产物HCN和NH₃与铁矿石载氧体具有较高的化学反应亲和性,易于被载氧体氧化生成N₂和NO。淮北无烟煤挥发分氮转化过程中,NO是唯一的氮氧化物,反应器出口中间产物NH₃的释放份额略高于HCN。在煤焦化学链燃烧还原过程中,部分燃料氮释放的中间产物HCN和NH₃被铁矿石氧化导致少量NO的生成,还原过程中无N₂O的释放;较高的还原反应温度加速了NO的生成。减少进入载氧体氧化再生过程的焦炭量可减少空气反应器NO和N₂O的生成。     

杂多酸改性V-Mo/Ti-W催化剂的宽温SCR脱硝性能

采用等体积浸渍法制备了一系列Keggin型杂多酸改性的V-Mo/Ti-W催化剂,并通过X射线衍射（XRD）、BET比表面分析（BET）、NH₃程序升温脱附（NH₃-TPD）、H₂程序升温还原（H₂-TPR）、X射线光电子能谱（XPS）和傅里叶变换红外光谱（FTIR）等表征方法对催化剂物化性质进行了表征分析。各项表征分析结果表明,杂多酸改性后的HPAs-V-Mo/Ti-W催化剂相比于改性前的催化剂,平均孔径尺寸更大,可抑制硫酸氢铵沉积,以提高催化剂抗硫性能;与V-Mo/Ti-W样品相比,改性后的催化剂具有更强的NH₃吸附能力、更好的还原性能和更多的化学吸附氧物种,从而表现出更优异的脱硝性能和N₂选择性。杂多酸改性后的催化剂在200~380℃温度范围内、150000h^-1高空速、高浓度SO₂和H₂O存在的模拟烟气中表现出高 NH₃-SCR活性、优异的抗SO₂性能,并保持几乎100%的N₂选择性,可作为燃煤电厂深度调峰的宽温催化剂,具备良好的工业应用价值。     

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.     

Mechanistic insights into the formation of N2O and N2 in NO reduction by NH3 over a polycrystalline platinum catalyst

The reaction pathways of N₂ and N₂O formation in the direct decomposition and reduction of NO by NH₃ were investigated over a polycrystalline Pt catalyst between 323 and 973 K by transient experiments using the temporal analysis of products (TAP-2) reactor. The interaction between nitric oxide and ammonia was studied in the sequential pulse mode applying ¹⁵NO. Differently labelled nitrogen and nitrous oxide molecules were detected. In both, direct NO decomposition and NH₃–NO interaction, N₂O formation was most marked between 573 and 673 K, whereas N₂ formation dominated at higher temperatures. An unusual interruption of nitrogen formation in the ¹⁵NO pulse at 473 K was caused by an inhibiting effect of adsorbed NO species. The detailed analysis of the product distribution at this temperature clearly indicates different reaction pathways leading to the product formation. Nitrogen formation occurs via recombination of nitrogen atoms formed by dissociation of nitric oxide or/and complete dehydrogenation of ammonia. N₂O is formed via recombination of adsorbed NO molecules. Additionally, both products are formed via interactions between adsorbed ammonia fragments and nitric oxide.     

REDUCTION OF NO WITH NH3 ON Fe-Cr OXIDES CATALYST

A catalyst consisting of a mixture of Fe₂O₃ and Cr₂O₃ (5/1 atomic ratio of Fe:Cr) supported on alumina pellets was examined for the complete reduction of NO with NH₃ to N₂ in an isothermal flow reactor. The reduction of NO was determined at various inlet concentrations of NO (approximating utility boiler emissions) and NH₃, and catalyst bed temperatures. In the absence of O₂, NO reduced completely to N₂ without N₂O formation. Correlation of the kinetic data considering both external and internal reactant mass transfer provided the following rate expression for the reduction of NO, r = 7.71 × 10⁴ exp. ( - 12,300/RT) P^0.9_NO P^0.1_NH3, moles / g.cat-sec atm.     

Nitrous oxide reduction with ammonia over Co---MgO catalyst and the influence of excess oxygen

N₂O decomposition on Co---MgO was studied under high (6.67 kPa) and low (75 Pa) N₂O pressure and the effect of reductant (C₂H₆, NH₃) was studied. The activity decays because of the strong adsorption of oxygen produced, while the reductant removed the adsorbed oxygen giving a steady catalysis. The reaction between NH₃ and excess amount of O₂ produced N₂O as a main product. Although N₂O gives the same intermediate (O) as O₂ does, the former decomposition seems to proceed faster than the latter on Co---MgO. The reaction mechanism studied here was compared with the SCR (NO---NH₃ in O₂) reaction on V₂O₂-TiO₂. Since NH₃, N₂O and O₂ gives only N₂ and water, Co---MgO can be a possible catalyst used in the boiler exhaust to reduce N₂O concentration by adding ammonia.     

Highly efficient subnanometer Ru-based catalyst for ammonia synthesis via an associative mechanism

The industrial manufacture of ammonia (NH₃) using Fe-based catalyst works under rigorous conditions. For the goal of carbon-neutrality, it is highly desired to develop advanced catalyst for NH₃ synthesis at mild conditions to reduce energy consumption and CO₂ emissions. However, the main challenge of NH₃ synthesis at mild conditions lies in the dissociation of steady NN triple bond. In this work, we report the design of subnanometer Ru clusters (0.8 nm) anchored on the hollow N-doped carbon spheres catalyst (Ru-SNCs), which effectively promotes the NH₃ synthesis at mild conditions via an associative route. The NH₃ synthesis rate over Ru-SNCs (0.49% (mass) Ru) reaches up to 11.7 mmol NH₃·(g cat)^-1·h^-1 at 400 ℃ and 3 MPa, which is superior to that of 8.3 mmol NH₃·(g cat)^-1·h^-1 over Ru nanoparticle catalyst (1.20% (mass) Ru). Various characterizations show that the N₂H₄ species are the main intermediates for NH₃ synthesis on Ru-SNCs catalyst. It demonstrates that Ru-SNCs catalyst can follow an associative route for N₂ activation, which circumvents the direct dissociation of N₂ and results in highly efficient NH₃ synthesis at mild conditions.     

Formation of nitrogen oxides from fuel-N through HCN and NH3: a model-compound study

The conversion of fuel-nitrogen to HCN and NH₃ and to nitrogen oxides was studied with nitrogen-containing model compounds, chosen to represent the main nitrogen and oxygen functionalities in fossil fuels. Two kinds of experiments were performed in an entrained-flow reactor at 800 °C. The conversion of model-compound-N to HCN and NH₃ was determined under inert conditions, and the formation of NO, N₂O and NO₂ was determined under oxidizing conditions. In inert atmosphere, oxygen-containing functional groups had an important effect on the ratio of HCN to NH₃. In particular, OH groups bound directly in the ring structure increased the conversion of nitrogen to NH₃. In oxidizing atmosphere, the conversions of model-compound-N to N₂O were high, but the substituent groups had no well-defined effect on the ratio of N₂O to NO. The formation of NO₂ was insignificant.     

Catalytic decomposition of N2O and catalytic reduction of N2O and N2O + NO by NH3 in the presence of O2 over Fe-zeolite

The decomposition of N₂O, and the catalytic reduction by NH₃ of N₂O and N₂O + NO, have been studied on Fe-BEA, -ZSM-5 and -FER catalysts. These catalysts were prepared by classical ion exchange and characterized by TPR after various activation treatments. Fe-FER is the most active material in the catalytic decomposition because “oxo-species” reducible at low temperature, appearing upon interaction of Fe^II-zeolite with N₂O (-oxygen), are formed in largest amounts with this material. The decomposition of N₂O is promoted by addition of NH₃, and even more with NH₃ + NO in the case of Fe-FER and -BEA. It is proposed that the NO-promoted reduction of N₂O originated from the fast surface reaction between -oxygen O^* and NO^* to yield NO₂^*, which in turn reacts immediately with NH₃.     

Fe-SAPO-44分子筛制备及催化脱硝性能

运用N2保护下的离子交换法制备Fe-SAPO-44分子筛催化剂,通过X射线粉末衍射(XRD)、N2吸附-脱附、电感耦合等离子体发射光谱仪(ICP-AES)、扫描电子显微镜(SEM)和氨气程序升温脱附(NH3-TPD)等研究Fe-SAPO-44催化剂的物化性质。对Fe-SAPO-44催化剂进行脱硝活性测试,考察其抗水热老化性能。结果表明,制备的Fe-SAPO-44催化剂的脱硝活性良好,并具有较好的抗水热老化能力。