共查询到19条相似文献,搜索用时 390 毫秒
1.
N 2O和NH 3的排放主要来自于机动车尾气排放。本文总结了近十几年来轻型汽油车N 2O和NH 3排放的研究进展,阐述了两种气态污染副产物在三效催化剂中的形成机理,通过对影响N 2O和NH 3生成的贵金属种类和含量、载体材料、不同气体组成和浓度、老化条件、不同车辆及测试工况、反应温度等主要影响因素的综述,总结了各要素对N 2O和NH 3形成的影响,得出N 2O和NH 3主要在富燃条件下冷启动阶段生成,NO的解离在N 2O和NH 3的生成中起关键作用;影响N 2O和NH 3生成的各因素之间相互关联,相互影响;催化剂的老化增加N 2O和NH 3的排放;贵金属Rh比Pd和Pt更有利于N 2O和NH 3的分解等结论。发动机、后处理策略系统的升级、更合适测试循环的开发以及催化剂的优化可以进一步降低N 2O和NH 3的排放。 相似文献
2.
采用密度泛函理论(DFT)方法研究了NO和NH 3在完整和有缺陷的γ-Al 2O 3(110)表面吸附与SCR(选择催化还原)反应特性。研究表明,NO在完整的(110)表面的吸附作用较弱,而NH 3分子的吸附作用较强,NH 3分子在Al原子顶位可形成稳定吸附。反应路径研究结果表明完整的(110)表面上SCR反应的决速步为-NH 2NO基团的分解,反应的最大能垒为235.75 kJ·mol -1。对于产生氧空穴的有缺陷(110)表面,NO和NH 3均可稳定吸附,NH 3在吸附过程中可直接裂解成NH 2和H。另外,SCR反应在有缺陷(110)表面的最大能垒明显较低,说明氧空穴的存在促进了SCR脱硝反应的进行。 相似文献
3.
NH 3选择催化还原技术(NH 3-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NH 3-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NH 3反应转化为N 2;但在实际应用过程中,N 2O的生成、催化剂的水热老化与尾气中的SO 2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Cu 2+的研究进展为基础,总结了Cu-SSZ-13中两种孤立态Cu 2+对NH 3-SCR反应与N 2O生成的影响;综述了两种孤立态Cu 2+对水热老化与SO 2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Cu 2+的研究进展,对C... 相似文献
4.
简述了NH 3和NO在催化剂表面吸附、转化活化和反应历程及H 2O和SO 2对以上反应行为的影响。分析表明,NH 3氧化脱氢进而与NO反应是决定NH 3反应性和最终产物的关键。NO以气态(Eley-Rideal机理)或硝基类物质等吸附态(Langmuir-Hinshelwood机理)形式参与选择催化还原(SCR)反应。提高催化剂酸性和氧化还原循环性能,利于NH 3和NO吸附和转化及相互间反应。高温时,H 2O影响轻微,而SO 2增强催化剂酸性,提高脱硝活性。低温时,H 2O和SO 2抑制NO吸附和转化活化,导致硫铵盐累积和活性位转变为硫酸盐使催化剂失活。因此,提高抗H 2O、抗SO 2性能是低温脱硝催化剂研发的重要方向。而发展在线升温等再生工艺以解决硝酸盐或含硫化合物导致的失活问题,对保障低温脱硝系统长期稳定运行具有重要意义。 相似文献
5.
H 2和O 2直接合成H 2O 2过程绿色环保,反应具有原子经济性,是最有潜力的H 2O 2合成新方法之一。采用等量浸渍法,将Pd负载于羟基磷灰石(HAp)载体上,得到了高分散的Pd/HAp纳米催化剂,Pd平均粒径2.5 nm。运用幂指数模型,研究该催化剂在H 2O 2加氢及H 2和O 2直接合成反应中的动力学,计算得到H 2O 2加氢、H 2O 2和H 2O的生成反应的表观活化能及O 2、H 2表观反应级数。结果表明低温及高O 2分压有利于H 2O 2的生成,而高H 2分压则有利于H 2O的生成。 相似文献
6.
采用两种不同的简化煤焦模型,利用量子化学密度泛函理论研究了煤焦异相还原N 2O的反应机理。通过计算反应物、中间体以及过渡态的结构和能量明确了反应的过程,并通过热力学分析和动力学分析深入分析煤焦异相还原N 2O的反应机理。研究结果表明:单个碳原子无法体现N 2O分子在煤焦表面的吸附和脱附过程,不适于作为煤焦模型研究煤焦异相还原N 2O的反应,六环苯环簇碳基模型可以成功地研究煤焦异相还原N 2O的反应。煤焦异相还原N 2O的反应共经历三个过渡态和两个中间体将N 2O还原成N 2,N 2O分子在煤焦表面的吸附反应的活化能为51.01 kJ·mol -1,煤焦表面吸附N 2O的过程容易进行。煤焦异相还原N 2O的反应在所研究的温度范围(298.15~1500 K)内为放热反应,可以自发发生,反应平衡常数大于10 5,可以完全进行,认为是单向反应。煤焦异相还原N 2O的反应在所研究的温度范围(298.15~1500 K)内反应速率较快,反应活化能为43.55 kJ·mol -1,Arrhenius表达式为1.24×10 10exp(-5238.15/T)。 相似文献
7.
对国内某1000MW燃煤发电机组失活选择性催化还原(SCR)催化剂进行CeO 2改性再生。对再生前后样品进行N 2吸附-脱附、扫描电子显微镜(SEM)、X射线荧光光谱(XRF)、傅里叶变换红外光谱(FTIR)对比表征分析。在自制固定床反应系统上对CeO 2改性再生催化剂(CeReCat)进行Hg 0氧化性能测试,同时研究了SO 2、H 2O、NO和NH 3对Hg 0氧化性能的影响。结果表明,CeO 2改性再生方法可有效清洗失活SCR催化剂表面杂质,恢复催化剂表面活性位点和孔隙结构,可使Ce、V两种活性元素得到有效负载。CeO 2改性后的样品Hg 0氧化性能显著提升,3.0 CeReCat具有最佳Hg 0的氧化效率。此外,烟气中加入600μL/L SO 2后,3.0 CeReCat仍具有高达74.4%的Hg 0氧化效率,抗SO 2性能较好。烟气中的NO可轻微促进Hg 0的氧化。由于竞争吸附作用,烟气中的H 2O和NH 3会抑制Hg 0的氧化。CeO 2改性再生催化剂置于SCR系统下层时,由于烟气NH 3浓度较低而具有较高Hg 0氧化效率,具有良好的应用前景。 相似文献
8.
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(NH 2+O?HNO+H)、R1-N-1(N-Vol?NH 3+HCN)、R569(NCO+O 2?NO+CO 2)、R17(H+O 2?O+OH)等反应,而抑制NO x生成的反应包括R411(NH 2+NO?N 2+H 2O)、R412(NH 2+NO?NNH+OH)、R570(NCO+NO?N 2O+CO)、R571(NCO+NO?N 2+CO 2)以及R5(Char+NO?Char+N 2+O 2)和R6(Soot+NO?n Soot+N 2+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。 相似文献
9.
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TiO 2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O 2时引入等离子体前后TiO 2催化尿素分解副产物水解的性能。结果表明:TiO 2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NH 3和CO 2,载气中有无O 2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O 2时引入等离子体NH 3产率变化不大,副产物仅有少量N 2O和NO,有O 2时NH 3产率显著降低,且生成较多N 2O、NO、NO 2及少量NH 4NO 2和NH 4NO 3。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。 相似文献
10.
通过电化学反应将氮气(N 2)和水(H 2O)在常温常压的条件下转化为氨气(NH 3)是一种绿色环保的合成氨方法。但由于N 2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AuP 2对N 2电化学还原制NH 3具有很好的催化活性。在二维AuP 2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N 2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。 相似文献
11.
This paper is a report of angle-resolved product desorption measurements in the course of catalyzed NO and N 2O reduction on Pd(1 1 0). Surface-nitrogen removal processes show different angular distributions, i.e. normally directed N 2 desorption takes place in process (i) 2N(a) → N 2(g). Highly inclined N 2 desorption towards the [0 0 1] direction is induced in process (ii) N 2O(a) → N 2(g) + O(a). N 2O or NH 3 desorption follows the cosine distribution characterizing the desorption after the thermalization in process (iii) N 2O(a) → N 2O(g) or (iv) N(a) + 3H(a) → NH 3(a) → NH 3(g). Thus, a combination of the angular and velocity distributions provides the analysis of most of surface-nitrogen removal processes in the course of catalyzed NO reduction. At temperatures below 600 K, processes (ii) and (iii) dominate and process (iv) is enhanced at H2 pressures higher than NO. Process (i) contributes significantly above 600 K. Only three processes except for NH3 formation are operative when CO is used. Only process (ii) was observed in a steady-state N2O + CO (or H2) reaction. 相似文献
12.
In this paper, the effect of CO 2 and H 2O on NO x storage and reduction over a Pt–Ba/γ-Al 2O 3 (1 wt.% Pt and 30 wt.% Ba) catalyst is shown. The experimental results reveal that in the presence of CO 2 and H 2O, NO x is stored on BaCO 3 sites only. Moreover, H 2O inhibits the NO oxidation capability of the catalyst and no NO 2 formation is observed. Only 16% of the total barium is utilized in NO storage. The rich phase shows 95% selectivity towards N 2 as well as complete regeneration of stored NO. In the presence of CO 2, NO is oxidized into NO 2 and more NO x is stored as in the presence of H 2O, resulting in 30% barium utilization. Bulk barium sites are inactive in NO x trapping in the presence of CO 2·NH 3 formation is seen in the rich phase and the selectivity towards N 2 is 83%. Ba(NO 3) 2 is always completely regenerated during the subsequent rich phase. In the absence of CO 2 and H 2O, both surface and bulk barium sites are active in NO x storage. As lean/rich cycling proceeds, the selectivity towards N 2 in the rich phase decreases from 82% to 47% and the N balance for successive lean/rich cycles shows incomplete regeneration of the catalyst. This incomplete regeneration along with a 40% decrease in the Pt dispersion and BET surface area, explains the observed decrease in NO x storage. 相似文献
13.
This paper reports a kinetic investigation of the global reduction of NO by H 2 which has been considered as a probe reaction for characterising the adsorption properties of supported palladium based catalysts. A particular attention has been paid towards the influence of the support on the catalytic properties of Pd, particularly towards the production of undesirable by-products such as nitrous oxide (N 2O) and ammonia (NH 3). It has been found that the kinetics of the overall NO + H 2 reaction on Pd/Al 2O 3 can be correctly depicted according to a Langmuir–Hinshelwood mechanism involving the dissociation of nitrosyl species assisted by chemisorbed hydrogen atoms. On the other hand, Pd/LaCoO 3 exhibits a different kinetic behaviour towards the adsorption of hydrogen depending on the pre-activation thermal treatment. In that case, different mechanisms may occur. 相似文献
14.
The role of La 2O 3 loading in Pd/Al 2O 3-La 2O 3 prepared by sol–gel on the catalytic properties in the NO reduction with H 2 was studied. The catalysts were characterized by N 2 physisorption, temperature-programmed reduction, differential thermal analysis, temperature-programmed oxidation and temperature-programmed desorption of NO. The physicochemical properties of Pd catalysts as well as the catalytic activity and selectivity are modified by La2O3 inclusion. The selectivity depends on the NO/H2 molar ratio (GHSV = 72,000 h−1) and the extent of interaction between Pd and La2O3. At NO/H2 = 0.5, the catalysts show high N2 selectivity (60–75%) at temperatures lower than 250 °C. For NO/H2 = 1, the N2 selectivity is almost 100% mainly for high temperatures, and even in the presence of 10% H2O vapor. The high N2 selectivity indicates a high capability of the catalysts to dissociate NO upon adsorption. This property is attributed to the creation of new adsorption sites through the formation of a surface PdOx phase interacting with La2O3. The formation of this phase is favored by the spreading of PdO promoted by La2O3. DTA shows that the phase transformation takes place at temperatures of 280–350 °C, while TPO indicates that this phase transformation is related to the oxidation process of PdO: in the case of Pd/Al2O3 the O2 uptake is consistent with the oxidation of PdO to PdO2, and when La2O3 is present the O2 uptake exceeds that amount (1.5 times). La2O3 in Pd catalysts promotes also the oxidation of Pd and dissociative adsorption of NO mainly at low temperatures (<250 °C) favoring the formation of N2. 相似文献
15.
The interactions between Pd/TiO 2 catalyst and the reactants and potential reaction intermediates present during aqueous nitrate reduction, including NO 3−, NO 2− and NO in the presence of H 2 and H 2O were studied by infrared spectroscopy. Adsorbed forms of NO, nitrite and nitrate could all be detected in the presence of water. In the presence of water/H 2, nitrate was the most stable surface species followed by nitrite and then highly reactive NO, suggesting that the reduction of nitrate to nitrite is the rate-limiting step. High concentrations of adsorbed nitrite appear to be linked to the detection of gaseous N 2O while the formation of ammonia is related to reactions on the Pd surface and the extent of formation is linked to high levels of adsorbed NO in addition to the surface hydrogen availability and the presence of water. 相似文献
16.
The selective catalytic reduction of NO+NO 2 (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 NH 3/NO x and NO/NO 2 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 N 2O 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 N 2O formation when NH 3/NO x inlet ratio was kept below 0.8. TPR results of this catalyst with NO/NH 3/O 2, NO 2/NH 3/O 2 and NO/NO 2/NH 3/O 2 feed mixtures indicated that the presence of NO 2 as the only nitrogen oxide increases the quantity of adsorbed species, which seem to be responsible for N 2O formation. When NO was also present, N 2O formation was not observed. 相似文献
17.
The release and reduction of NO x in a NO x storage-reduction (NSR) catalyst were studied with a transient reaction analysis in the millisecond range, which was made possible by the combination of pulsed injection of gases and time resolved time-of-flight mass spectrometry. After an O 2 pulse and a subsequent NO pulse were injected into a pellet of the Pt/Ba/Al 2O 3 catalyst, the time profiles of several gas products, NO, N 2, NH 3 and H 2O, were obtained as a result of the release and reduction of NO x caused by H 2 injection. Comparing the time profiles in another analysis, which were obtained using a model catalyst consisting of a flat 5 nmPt/Ba(NO 3) 2/cordierite plate, the release and reduction of NO x on Pt/Ba/Al 2O 3 catalyst that stored NO x took the following two steps; in the first step NO molecules were released from Ba and in the second step the released NO was reduced into N 2 by H 2 pulse injection. When this H 2 pulse was injected in a large amount, NO was reduced to NH 3 instead of N 2. A only small amount of H2O was detected because of the strong affinity for alumina support. We can analyze the NOx regeneration process to separate two steps of the NOx release and reduction by a detailed analysis of the time profiles using a two-step reaction model. From the result of the analysis, it is found that the rate constant for NOx release increased as temperature increase. 相似文献
18.
The inhibition effect of H 2O on V 2O 5/AC catalyst for NO reduction with NH 3 is studied at temperatures up to 250 °C through TPD, elemental analyses, temperature-programmed surface reaction (TPSR) and FT-IR analyses. The results show that H 2O does not reduce NO and NH 3 adsorption on V 2O 5/AC catalyst surface, but promotes NH 3 adsorption due to increases in Brønsted acid sites. Many kinds of NH 3 forms present on the catalyst surface, but only NH 4+ on Brønsted acid sites and a small portion of NH 3 on Lewis acid sites are reactive with NO at 250 °C or below, and most of the NH 3 on Lewis acid sites does not react with NO, regardless the presence of H 2O in the feed gas. H 2O inhibits the SCR reaction between the NH 3 on the Lewis acid sites and NO, and the inhibition effect increases with increasing H 2O content. The inhibition effect is reversible and H 2O does not poison the V 2O 5/AC catalyst. 相似文献
19.
The kinetics of CO oxidation and NO reduction reactions over alumina and alumina-ceria supported Pt, Rh and bimetallic Pt/Rh catalysts coated on metallic monoliths were investigated using the step response technique at atmospheric pressure and at temperatures 30–350°C. The feed step change experiments from an inert flow to a flow of a reagent (O 2, CO, NO and H 2) showed that the ceria promoted catalysts had higher adsorption capacities, higher reaction rates and promoting effects by preventing the inhibitory effects of reactants, than the alumina supported noble metal catalysts. The effect of ceria was explained with adsorbate spillover from the noble metal sites to ceria. The step change experiments CO/O 2 and O 2/CO also revealed the enhancing effect of ceria. The step change experiments NO/H 2 and H 2/NO gave nitrogen as a main reduction product and N 2O as a by-product. Preadsorption of NO on the catalyst surface decreased the catalyst activity in the reduction of NO with H 2. The CO oxidation transients were modeled with a mechanism which consistent of CO and O 2 adsorption and a surface reaction step. The NO reduction experiments with H 2 revealed the role of N 2O as a surface intermediate in the formation of N 2. The formation of NN bonding was assumed to take place prior to, partly prior to or totally following to the NO bond breakage. High NO coverage favors N 2O formation. Pt was shown to be more efficient than Rh for NO reduction by H 2. 相似文献
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