Formation of the surface NO during N2O interaction at low temperature with iron-containing ZSM-5

Interaction of N2O at low temperatures (473-603 K) with Fe-ZSM-5 zeolites (Fe, 0.01-2.1 wt %) activated by steaming and/or thermal treatment in He at 1323 K was studied by the transient response method and temperature-programmed desorption (TPD). Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) of NO adsorbed at room temperature as a probe molecule indicated heterogeneity of surface Fe(II) sites. The most intensive bands were found at 1878 and 1891 cm(-1), characteristic of two types mononitrosyl species assigned to Fe2+(NO) involved in bi- and oligonuclear species. Fast loading of atomic oxygen from N2O on the surface and slower formation of adsorbed NO species were observed. The initial rate of adsorbed NO formation was linearly dependent on the concentration of active Fe sites assigned to bi- and oligonuclear species, evolving oxygen in the TPD at around 630-670 K. The maximal coverage of a zeolite surface by NO was estimated from the TPD of NO at approximately 700 K. This allowed the simulation of the dynamics of the adsorbed NO formation at 523 K, which was consistent with the experiments. The adsorbed NO facilitated the atomic oxygen recombination/desorption, the rate determining step during N2O decomposition to O2 and N2, taking place at temperatures > or =563 K.     

Effect of NO upon N2O decomposition over Fe/ZSM-5 with low iron loading

An Fe/ZSM-5 catalyst with a very high Si/Al ratio was prepared, and using it, the effect of NO upon the kinetics of N2O decomposition was studied. The addition of small, nonstoichiometric amounts of NO was observed to cause the rate to increase by more than an order of magnitude. The kinetics were well-fit by a rate expression that was first order in the partial pressure of N2O for the situation without added NO and separately for the situation where NO was added. The Arrhenius parameters of the rate coefficient differed for the two situations. The results are consistent with a mechanistic scheme wherein the reaction proceeds via an oxide-oxo redox cycle in the absence of NO. The results suggest that the NO-assisted decomposition of N2O does not require a second iron site adjacent to the active site and that NOx species adsorbed on the same cation site could serve as locations for oxygen storage if, in fact, the promotional effect of NO is related to such storage.     

聚集态的Fe氧化物在Fe/ZSM-5催化N_2O直接分解中的作用(英文)

研究了以离子交换法制备的Fe/ZSM-5上聚合态的Fe氧化物(FeOx)在催化N2O直接分解中的作用.Fe/ZSM-5催化剂利用600–900 ℃的高纯Ar处理,使催化剂中的FeOx发生聚合.利用一系列的表征手段(XRD,BET,DRIFTS,UV/vis-DRS,XAFS,N2O脉冲和O2-TPD)对催化剂进行了表征.结果发现,分子筛骨架外的FeOx在Fe/ZSM-5催化N2O分解中起重要作用;通过研究高温处理的Fe/ZSM-5催化剂中不同FeOx的比例与催化剂活性的关系,得到多核的FeOx是催化N2O分解的主要活性物种;并且,无定形态的FeOx中键长较长的Fe–O键((Fe–O)2)是参与反应的重要物种.     

Enhancement of alpha-oxygen formation and N2O decomposition on Fe/ZSM-5 catalysts by extraframework Al

The concentration of alpha-oxygen which can oxidize methane to methanol and benzene to phenol at RT, increases linearly with the amount of introduced extraframework Al on Fe/ZSM-5 catalysts prepared by solid-state exchange of FeCl3 and AlCl3 with H-ZSM-5.     

Reduction of NO with methane over Fe/ZSM-5 catalysts

<正>The catalytic activity of Fe/ZSM-5 for the selective reduction of NO to N_2 with methane in the presence of excess O_2 was studied.Fe/ZSM-5 catalysts with various Fe loadings were prepared by impregnation method.It is well known that methane is inactive when Fe/ZSM-5 as the catalyst for the selective catalytic reduction(SCR) of NO with methane.However,this paper shows that when the content of Fe was about 0.5%,Fe/ZSM-5 showed higher catalytic activity and selectivity of methane,and put forward measurable activation for CH_4 is an important factor for the reaction of removal of NOx with CH_4.     

Elucidation of the Surprising Role of NO in N2O Decomposition over FeZSM-5

The decomposition of N₂O is strongly promoted by NO over steam-activated FeZSM-5. The promoting effect of NO is catalytic, and in addition to NO₂, ₂ is formed much more extensively at lower temperatures than in the absence of NO. The promotion effect only requires low NO concentrations in the feed, with no significant improvements at molar NO/N₂O feed ratios higher than 0.25. No inhibition by NO was identified even at a molar NO/N₂O feed ratio of 10, suggesting different sites for NO adsorption and oxygen deposition by N₂O. The latter sites seem to be remote from each other. Transient experiments using in situ FT-IR/MS and Multitrack over FeZSM-5 further elucidate the mechanism of NO promotion. The release of oxygen from the catalyst surface during direct N₂O decomposition is a rate-determining step due to the slow oxygen recombination, which is favored by high reaction temperatures. NO addition promotes this oxygen desorption, acting as an oxygen transfer agent, probably via NO₂ species. Adsorbed NO may facilitate the migration of atomic oxygen to enhance their recombination. Less than 0.9% of Fe seems to participate in this promotion. A model is proposed to explain the phenomena observed in NO-assisted N₂O decomposition, including NO₂ decomposition.     

N2O decomposition over Fe/ZSM-5: reversible generation of highly active cationic Fe species

Fe-oxide species in Fe/ZSM-5 (prepared by chemical vapor deposition of FeCl3)--active in N2O decomposition--react with zeolite protons during high temperature calcination to give highly active cationic Fe species, this transformation being reversible upon exposure to water vapor at lower temperature.     

Direct catalytic decomposition of N2O over Co(x)/RPSA catalysts

Research on Chemical Intermediates - As one of the main greenhouse gases, reducing emissions of N2O is imperative, for which catalytic decomposition is one of the most effective methods. A series...     

铂族金属催化剂低温 CO氧化研究近期进展

CO氧化可能是多相催化领域最常见的反应,它不仅能作为探针反应研究催化剂结构、反应活性位等,而且在诸多实际过程如空气净化、汽车尾气污染物控制、燃料电池所用氢源净化等扮演重要角色.最早的 CO氧化催化剂为霍加拉特剂,其组分主要为 CuO与 MnO2混合氧化物,然而在实际应用过程中存在低温活性低、吸湿易失活等缺点.1987年, Haruta等发现湿化学法制备的氧化物负载 Au催化剂表现出非常高的低温 CO氧化活性及耐水稳定性,其 Au粒子以纳米尺度分散,进而引发了催化研究领域的“淘金热”及纳米催化研究热潮.而 CO氧化通常作为考察 Au催化剂结构性质的探针反应,也成为考核其它金属催化剂是否具有高活性的判据之一. Pt族金属上 CO氧化反应从 Langmuir等研究开始至今已有100多年,然而低温下该金属催化剂活性与 Au催化剂相比要低一个数量级.本质原因为 Pt族金属上 CO吸附较强, O2吸附与活化受到抑制,而该步骤被认为是 CO氧化的速控步,因而表现出较低的催化活性.通常 Pt族金属催化剂需要100oC以上 CO才能脱附, O2进而得以吸附.目前研究人员采取多种策略,其基本原则为削弱 Pt族金属上 CO吸附强度或者提供其它活性位供 O2吸附与活化.本综述将概括近十年来Pt族金属催化剂 CO氧化研究进展,主要总结室温甚至超低温条件下的研究成果.高活性 CO氧化催化剂主要是通过采用可还原氧化物为载体或助剂,或者改变催化剂表面性质如使表面富 OH基物种来形成. Au催化剂的研究发现,改变金属粒子尺寸极有可能获得不同寻常的催化性能,而常规的 Pt族金属催化剂研究主要是在纳米尺度.近期人们发现逐渐减小 Pt族金属粒子尺寸,从纳米到亚纳米甚至单原子时,其电荷状态逐渐呈正价形式,这有利于削弱其 CO吸附强度.此外,可通过增强金属载体间的相互作用,改变金属载体接触方式,如从核壳到交叉结联结构,构筑出更多的金属载体界面,使得 O2更容易吸附与活化或稳定更多的 OH基物种进而在此界面与吸附的 CO反应.伴随着表征技术的发展, CO氧化机理的认识也更加深入,这给催化剂的设计带来更多新的思路.(1)改变 CO吸附活化位,将 CO吸附活化位从金属转移到载体上,从而大大降低 CO吸附强度,活化的 CO物种在反应过程中容易溢流到金属载体界面处,这甚至有利于超低温度下(–100oC左右) CO氧化.(2)改变 O2活化形式. O2通常在 Pt族金属上容易以解离氧原子形式存在,通过改变载体、金属载体界面性质使得 O2以分子氧形式活化,如形成超氧或过氧物种,这有利于降低 CO氧化的活化能垒,进而提高其低温甚至超低温下 CO氧化活性.今后,设计并合成出在超低温度下能够氧化 CO的 Pt族金属催化剂将成为 CO氧化催化剂研究的重要方向之一.     

Ga改性ZSM-5分子筛催化丙烷芳构化性能研究

用离子交换法制备Ga改性ZSM-5双功能催化剂,并结合XRD、SEM、BET、NH₃-TPD、Py-IR及ICP、XPS等多种技术进行表征,考察分子筛硅铝比(Si/Al)和催化剂氧化还原预处理条件对分子筛的酸性质、Ga物种的存在状态及其丙烷芳构化催化性能的影响。研究表明,硅铝比不仅可以改变分子筛的酸性,也会影响分子筛中非骨架Ga物种与分子筛表面的相互作用程度,进而影响含Ga分子筛的丙烷芳构化性能。在质量空速1.0 h^-1、反应温度550℃ 下,Si/Al比为30的Ga-HZSM-5分子筛丙烷转化率和芳烃收率最高。Ga物种的引入可以提高丙烷的转化率和芳烃的选择性,并抑制烷烃、烯烃的裂解。H₂还原处理,将分子筛表面Ga₂O₃还原为低价的Ga⁺和GaH⁺₂物种,促进了Ga物种向分子筛微孔迁移;还原-氧化处理后,Ga⁺和GaH⁺₂物种氧化成GaO⁺,占据分子筛孔道离子交换位,显著提高了催化剂的芳构化活性。     

Mn-V-Ce/TiO_2低温催化还原NO性能研究

采用浸渍法制备了NH3选择性催化还原(SCR)NO新型催化剂Mn-V-Ce/TiO2,考察了操作条件对其活性的影响;通过NO、O2、NH3的暂态响应实验分析了在该催化剂上进行SCR反应的机理;对催化剂进行了FT-IR和TG分析,探讨了H2O、SO2对其活性的影响。结果表明,该催化剂具有很好的低温SCR活性,空速5 000 h-1、150℃下的NO转化率达99.2%。170℃下同时通硫、水后的350 min内,活性始终保持在98%;单独通入SO2时,中毒速率较快,但体积分数为8%的H2O对该催化剂的活性没有影响,转化率始终保持在98%以上。该催化剂能应用于不含SO2的燃气锅炉烟气和不含SO2的硝酸尾气等NOx工业废气的低温脱硝。     

N2O在Au/Co3O4和Au/ZnCo2O4催化剂上的分解反应

通过调变HAuCl₄溶液的pH值和Au负载量,用沉积-沉淀法制备了一系列Au/Co₃O₄催化剂,并采用AES、BET、XRD、SEM、XPS和H₂-TPR等技术对催化剂的结构和组成进行了表征,考察了制备条件对其在有氧气氛中催化N₂O分解反应性能的影响规律,得到了催化剂最佳制备条件:HAuCl₄溶液pH值为9,Au负载量为0.29%。催化测试结果表明:虽然ZnCo₂O₄的催化活性优于Co₃O₄,但0.31%Au/ZnCo₂O₄的活性和稳定性低于0.29%Au/Co₃O₄。500℃、在含氧气氛中连续反应10 h, 两者均可完全分解N₂O,但在含氧、含水气氛中0.29%Au/Co₃O₄和0.31%Au/ZnCo₂O₄上的N₂O转化率分别为92%和63%。究其原因,发现Au/Co₃O₄中Au和Co组分间存在协同效应,而Au/ZnCo₂O₄中Au和Co组分间则没有协同效应。     

酸预处理对于 Fe-ZSM-5和 Fe-beta分子筛催化消除 N2O的影响

作为六大温室气体之一, N2O的增温潜能是 CO2的310倍,甲烷的21倍,目前仍然以0.80 ppb/年的速度增长,但是减排成本很低,因此对 N2O的消除具有重要意义.在工业中金属修饰的微孔分子筛因其优良的催化活性,高水热稳定性,低成本等优点而成为研究重点.但是微孔分子筛狭窄的微孔孔道限制了金属前驱体的进入,导致活性金属含量低,进而限制了活性的提高.因此采用一定的改性手段减小分子筛颗粒尺寸,缩短微孔孔道长度或者扩大微孔孔道来增加活性铁物种的含量进而提高分子筛催化活性.
　　本文选用商用的 ZSM-5和 beta分子筛作为母板分子筛,按照3 g ：50 mL比例将分子筛母板与1.0 mol/L的 HNO3在室温下混合,分别搅拌0,2和24 h,然后采用液相离子交换法负载金属铁制备得到 Fe-ZSM-5和 Fe-beta.通过 X射线衍射、N2物理吸吸脱附、电感耦合等离子原子发射光谱仪、扫描电镜、透射电镜、NH3程序升温脱附及紫外漫反射(UV-vis)等手段对不同时间处理的分子筛的形貌、酸性和铁物种等物理化学性质进行表征.对两种催化剂催化消除 N2O的反应性能进行了测试.结果显示,温和的酸处理下分子筛脱除了部分 Al,其中, ZSM-5分子筛的表现为由外向内逐层刻蚀,颗粒尺寸减小,孔道长度缩短,但是由于 MFI型分子筛较高的稳定性,酸处理对分子筛孔道大小的改变并不明显,而对于 beta分子筛,首先是其中大量无定形物种的去除,然后对孔道进行修饰,使之略微扩大,但是对颗粒尺寸的影响不大.
　　 ICP结果显示,商用 ZSM-5和 beta分子筛经过温和的酸处理改性后, Si/Al比增大,负载 Fe的含量明显增加,各催化剂催化消除 N2O的活性也出现了不同程度的提高. Fe-ZSM-5和 Fe-beta分子筛上 N2O完全转化温度分别向低温段移动了10–15和30°C. UV-vis谱图显示,分子筛中存在着不同种类的铁物种,通过分峰计算发现,孤立的 Fe3+铁离子和低聚态的 Fex3+Oy均是催化活性铁物种,其含量的增加部分也解释了活性提高的原因.     

碱金属助剂对N2O直接分解催化剂NiO活性的促进作用

考察了一系列碱金属促进和未促进的NiO催化剂上N2O直接分解反应性能,并采用X射线衍射, X射线光电子能谱(XPS)和前场扫描电镜对这些催化剂进行了表征。 XPS结果表明,催化剂表面的Ni以金属颗粒, NiO和Ni(OH)2三种形式存在。研究发现,在催化剂表面主要是Ni0原子与N2O分子发生相互作用。未促进的NiO催化剂表现出非常低的催化N2O分解活性;碱金属的加入使得NiO催化剂活性大大提高。     

N_2O在Mg-Fe复合氧化物上的催化分解(英文)

用共沉淀法制备了一组Mg-Fe复合氧化物催化剂,用于N2O催化分解,考察了催化剂组成、焙烧温度、负载助剂钾等参数对其催化活性的影响。采用N2物理吸附、X射线衍射、H2-程序升温还原等技术对催化剂进行了结构表征。结果表明,500℃焙烧的Mg0.6Fe0.4Fe2O4催化剂对于N2O分解反应有较高活性,而K改性的催化剂活性均有所降低,且K的负载量越高,改性催化剂的活性越低,这是由于负载的K粒子抑制了表面铁物种的还原,降低了催化剂的比表面积。在有氧无水、水氧共存条件下连续反应10h,Mg0.6Fe0.4Fe2O4的催化活性和稳定性均显著高于FeOx催化剂。     

Ce、Zr双组分改性对Cu/ZSM-5催化剂催化分解NO性能的影响

采用Ce、Zr双组分对Cu/ZSM-5催化剂进行改性,利用O₂-TPD和TG等手段对催化剂进行表征,考察了离子交换顺序、铈锆摩尔比以及反应条件对催化剂催化分解NO性能的影响。结果表明,铜物种是Zr-Cu-Ce/ZSM-5催化剂中催化分解NO的活性中心,是必须的催化活性组分,Ce、Zr双组分改性能显著提高催化剂在富氧条件下催化分解NO的活性,降低达到最高活性所需要的反应温度。铈锆摩尔比为1:1时,同时进行离子交换制备的Zr-Cu-Ce/ZSM-5催化剂催化分解NO的活性最好。     

DSC characterization of ZSM-5/MgO/Al2O3 catalysts

The physico-chemical characterization of magnesium-modified ZSM-5 zeolite catalysts has been performed by differential scanning calorimetry. Evidence has been found of the formation of magnesium oxide and the magnesium spinel phase in alumina-bonded ZSM-5 catalysts. DSC proved a suitable technique for characterization of these systems.

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Zusammenfassung Mg-modifizierte ZSM-5-Zeolithkatalysatoren wurden durch DSC charakterisiert. Es wurden Beweise für die Bildung von Magnesiumoxid und Magnesiumspinell in ZSM-5/Aluminiumoxid-Katalysatoren gefunden. DSC hat sich zur Characterisierung dieser Systeme als geeignet erwiesen.

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- 3CM-5, . - - . .

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This work was carried out within the Progetto finalizzato Energetica 2 of the National Research Council of Italy.     

Ab initio calculations of nitrogen oxide reactions: formation of N2O2, N2O3, N2O4, N2O5, and N4O2 from NO, NO2, NO3, and N2O

Ab initio computational methods were used to obtain Delta(r)H(o), Delta(r)G(o), and Delta(r)S(o) for the reactions 2 NO <=> N(2)O(2) (I), NO+NO(2) <=> N(2)O(3) (II), 2 NO(2) <=> N(2)O(4) (III), NO(2)+NO(3) <=> N(2)O(5) (IV), and 2 N(2)O <=> N(4)O(2) (V) at 298.15 K. Optimized geometries and frequencies were obtained at the CCSD(T) level for all molecules except for NO, NO(2), and NO(3), for which UCCSD(T) was used. In all cases the aug-cc-pVDZ (avdz) basis set was employed. The electronic energies of all species were obtained from complete basis set extrapolations (to aug-cc-pV5Z) using five different extrapolation methods. The [U]CCSD(T)/avdz geometries and frequencies of the N(x)O(y) compounds are compared with literature values, and problems associated with the values and assignments of low-frequency modes are discussed. The standard entropies are compared with values cited in the NIST/JANAF tables [NIST-JANAF Thermochemical Tables, J. Phys. Chem. Ref. Data Monograph No. 9, 4th ed. edited by M. W. Chase, Jr. (American Chemical Society and American Institute of Physics, Woodbury, NY, 1988)]. With the exception of I, in which the dimer is weakly bound, and V, for which thermodynamic data appears to be lacking, the calculated standard thermodynamic functions of reaction are in good agreement with literature values obtained both from statistical mechanical and various equilibrium methods. A multireference-configuration interaction calculation (MRCI+Q) for I provides a D(e) value that is consistent with previous calculations. The combined uncertainties of the NIST/JANAF values for Delta(r)H(o), Delta(r)G(o), and Delta(r)S(o) of II, III, and IV are discussed. The potential surface for the dissociation of N(2)O(4) was explored using multireference methods. No evidence of a barrier to dissociation was found.     

Kinetics of no decomposition over Cu/ZSM-5 catalyst

The kinetics of catalytic decomposition of NO over Cu/ZSM-5 catalyst has been studied in an integral flow reactor at atmospheric pressure. Kinetic analysis is based on the assumption that the surface reaction represents the rate-determining step. On the basis of theoretical considerations of different interactions between reactants and catalyst, and experimental evidences, three different mechanistic kinetic models were chosen. Also a power-law model was tested. The best agreement has been achieved with the model