Nature of Cu active sites in zeolite-based catalysts for selective catalytic oxidation of methane

Research on Chemical Intermediates - Though copper-containing zeolites (Cu–zeolites) have shown great potential in the direct conversion of methane to methanol under low temperature, the...     

碳化硅上生长Cu/SSZ-13分子筛及其NH3-SCR催化性能

汽车尾气和柴油不完全燃烧所释放的NOx严重污染了大气环境.为了降低对大气的污染,可将其催化还原成氮气.氨气选择性催化还原(NH3-SCR)是使用较广泛的机动车高效脱硝技术.用于催化脱硝反应的催化剂有多种类型,分子筛具有特殊的孔道结构和骨架结构及高比表面积,因而已广泛用作脱硝反应催化剂.与传统三效催化剂相比,分子筛催化剂总体表现出更好的脱硝催化活性,但在高温下不稳定,容易失活,不耐热冲击,水热稳定性差.SiC具有耐酸碱、耐腐蚀、抗氧化、耐磨及良好的热稳定性和导电性,因此作为催化剂载体近年来引起广泛关注.但是其本身也存在许多缺点,如比表面积低、表面性质不活泼、不利于金属物种分散等.因此,本文通过原位水热法将SSZ-13生长在SiC表面,制备出新型催化复合材料SSZ-13@SiC.采用X射线衍射(XRD)、扫描电子显微镜(SEM)和N2吸附-脱附等手段研究了不同碱量和晶化时间对SSZ-13在SiC表面生长的影响,负载Cu后获得Cu/SSZ-13@SiC作为催化剂,研究了SiC对Cu/SSZ-13中高温下脱硝活性的影响规律.结果表明,碱含量会影响SSZ-13在SiC表面的结晶程度.当SiO2/NaOH ≥ 0.1时,SSZ-13有杂相出现,并且结晶度都不高; 当SiO2/NaOH < 0.1时,SiC表面会生长成纯相的SSZ-13晶粒且具有较高的结晶度.晶化时间也会影响SSZ-13在SiC表面的覆盖程度: 反应1 d时,SiC表面会生长零星的SSZ-13晶粒; 2 d时,SSZ-13达到全面覆盖; 3 d后,SSZ-13在SiC上的生长达到饱和,其比表面积达到最大值,约为201.3 m2/g.通过离子交换将不同含量Cu离子交换到分子筛表面,其中Cu(1.71)/SSZ-13@SiC样品具有最佳的脱硝活性,接近200 °C时,NO转化率就达到90%以上,到高温500 °C时,NO转化率仍能保持在70%以上.相比于未负载的Cu/SSZ-13,Cu/SSZ-13@SiC催化剂在NH3-SCR测试中具有更高的高温催化活性,同时催化活性窗口明显拓宽.上述结果说明SiC对Cu/SSZ-13的高温催化活性具有一定的提高和稳定作用.     

Co-doped MnCeOx/ZrO2 catalysts for low temperature selective catalytic reduction of NO

Co-doped MnCeO_x/ZrO₂ catalysts were synthesized by impregnation method and their low temperature deNOx performance were evaluated. The physicochemical properties of the catalysts were studied. The results showed that the doped Co could promote the deNOx performance of MnCeO_x/ZrO₂ significantly, and the doped catalyst with the Co/Mn molar ratio of 1:2 possessed the best catalytic performance. Compared with pure MnCeO_x/ZrO₂ catalyst, the deNOx efficiency of the optimal 1Co₂MnCeO_x/ZrO₂ was higher to 93% at 100 °C, improved nearly by 17%. The complete removal of NO was achieved at the temperature range of 120–250 °C. The promoted catalytic performance of Co-doped MnCeO_x/ZrO₂ catalyst was mainly attributed to the improvement of the catalyst support structure and surface acidity by Co. The catalytic reaction of NO with NH₃ over 1Co2MnCeO_x/ZrO₂ catalyst follows both Eley–Rideal mechanism and Langmiur–Hinshelwood mechanism.

     

Selective catalytic reduction of NO with hydrocarbon on Cu2+-exchanged pillared clay: An IR study of the NO decomposition mechanism

In an earlier study, it has been found that Cu²⁺ ion-exchanged pillared clay (Cu-PILC) has a substantially higher activity for the selective catalytic reduction of NO by ethylene over Cu-ZSM-5. Moreover, it is not significantly deactivated by water vapor and SO₂. In this study, the activity for direct NO decomposition in the presence of O₂ on Cu-PILC was studied and an in situ IR study for the key intermediates and the reaction mechanism was made. The direct NO decomposition activities for Cu-PILC and Cu-ZSM-5 were similar. Under in situ NO and O₂ reaction conditions at temperatures up to 300°C, IR absorption bands at well-defined peak positions are identified. The band at 1699 cm⁻¹ is assigned to a dinitrosyl species on Cu⁺. The bands with peaks at 1609, 1530–1480 and in the region of 1440–1335 cm⁻¹ are assigned to bidentate nitrate, monodentate nitrate and nitro species bonded to Cu²⁺. A redox mechanism is proposed for NO decomposition. The limiting step is thought to be the N–N coupling between surface nitrate and gaseous nitric oxide to form nitrogen. The existence of substantial amounts of nitrate formed from NO alone indicates the important role of the large amount of lattice oxygen that is available on Cu-PILC. As a result, the role of external oxygen supply is only to replenish the consumed lattice oxygen. The proposed NO decomposition mechanism suggests that the redox property of Cu-PILC is crucial for this reaction.     

Cu/La2O3-ZrO2-Al2O3催化剂的制备及其对NO选择性还原的催化性能

采用将Al（NO3）3、La（NO3）3和ZrOCl2的混合液滴入沉淀剂（NH4）2CO3中的共沉淀法制备La2O3-ZrO2-Al2O3复合载体,然后负载上Cu2＋,制成Cu/La2O3-ZrO2-Al2O3催化剂。考察了该催化剂在富氧条件下对C3H6选择还原NO的催化性能,并借助扫描电子显微镜（SEM）、X射线衍射（XRD）、比表面积测定（BET）、吡啶吸附红外光谱（Py-IR）、程序升温还原（TPR）和热重分析（TG）等方法研究催化剂制备方法与结构、性能的关系。实验结果表明,采用将Al（NO3）3滴入（NH4）2CO3制得的γ-Al2O3能有效地增大催化剂的比表面积,加入La2O3能提高催化剂的热稳定性,加入ZrO2能大幅度增加催化剂表面L酸和B酸的酸量。因此,采用共沉淀法制备的La2O3-ZrO2-Al2O3复合载体能够使Cu/La2O3-ZrO2-Al2O3催化剂具有良好的催化性能,最佳催化活性温度为300℃,NO最大转化率高达88.9%,在有10%水蒸气存在的情况下,仍可达81.9%。     

Effect of SO2 on the selective catalytic reduction of NO by ammonia

An experimental study has shown that the addition of SO₂ may either hinder or accelerate the selective catalytic reduction of NO by ammonia on oxide catalysts. The nature of the effect of SO₂ depending on its concentration and temperature has been elucidated.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 29, No. 2, pp. 165–168, March–April, 1993.     

Fe-PILC在贫燃条件下催化丙烯选择性还原NO

采用羟基铁离子柱撑Na-Mont制备了铁柱撑黏土催化剂(Fe-PILC),铁物种作为柱撑成分,同时充当活性组分,研究其在贫燃条件下催化丙烯选择性还原NO的特性。通过XRD、N_2吸附-脱附、H_2-TPR、UV-vis、Py-FTIR等方法对催化剂进行物理化学性质表征,进一步研究其反应机理。研究表明,1.0Fe-PILC在450-600℃时NO的转化率超过98%,N_2的选择性可达97%以上,且抗水蒸气和SO_2的能力较强。XRD和N_2吸附-脱附研究表明,Fe-PILC中铁物种柱撑进入Na-Mont层间,使催化剂的比表面积和孔容增大。H_2-TPR研究表明,Fe-PILC在400℃左右还原能力较强,主要体现为Fe~(3+)→Fe~(2+)的还原。UVvis研究表明,Fe-PILC的脱硝活性与铁氧低聚物种FexOy呈正相关。Py-FTIR研究表明,Fe-PILC表面同时含有Lewis酸和Br■nsted酸性位,Lew is酸性位是C_3H_6与NO进行催化反应的主要活性中心。     

Metal oxide promoted TiO2 catalysts for photo-assisted selective catalytic reduction of NO with NH3

The photo-assisted selective catalytic reduction (SCR) of NO with NH₃ (Photo-SCR) was performed over TiO₂ modified by supporting 1 wt% of various transition metal (V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Ta or W) oxides aiming at the improvement of the photo-SCR activity. The addition of Nb, Mo or W oxide to TiO₂ was found to enhance the photo-SCR activity. We have reported that the amount of acid sites on TiO₂ is one of the key factors to the photo-SCR activity. The increase in the activity depends on the enhancement of acidity of catalyst by the addition of Nb, Mo or W oxide. In contrast, the addition of V, Cr, Mn, Fe, Co, Ni or Cu oxide to TiO₂ lowered the photo-SCR activity, although addition of metal cations also changed the acidity of TiO₂. We guess that the reduction of the activity was caused by two reasons; the first is that the sites newly formed on these transition metal oxides is not photoactive and the second is that TiO₂ supporting V, Cr, Mn, Fe, Co, Ni or Cu oxides had low stabilities under the reaction conditions, i.e., the chemical state of the cations changed during the reaction. Therefore, we concluded that the increase in the acid sites that are active sites for photo-SCR and the stability of the catalysts are important for the photo-SCR.     

钛纳米管负载锰催化剂的低温选择性催化还原脱硝性能

采用水热法合成了钛纳米管（TiNT）,以400℃焙烧后得到的TiO₂纳米管为载体,采用浸渍法制备了MnO_x/TiNT催化剂,用于低温NH₃选择性催化还原脱NO反应（SCR）。BET、TEM、XRD及TG测试表明,经过400℃焙烧之后的钛纳米管主要成分为锐钛矿型TiO₂,所制备的催化剂活性组分分散性较好。在模拟烟气条件下,考察了锰负载量、空速、O₂含量、氨氮比及进口NO浓度对MnO_x/TiNT的SCR催化性能的影响。在150℃、\[NH₃\]/\[NO\]比为1.2、O₂浓度为3%、NO浓度为0.06%、空速GHSV为23613.8h^-1、Mn的负载量为5%~15%的条件下,NO的转化率达到95%以上。反应气氛中单独通入水会造成催化剂的活性下降;切断H₂O,催化剂的活性可以恢复至初始水平。温度越高,催化剂抗水性能越好,而且水存在情况下的抗硫性能优于其单独抗硫性能。再次切断H₂O和SO₂,催化剂的活性逐渐上升,但不能恢复到初始水平。     

Li改性MnO_2及LiMn_2O_4催化NH_3-SCR反应性能研究

采用高温固相反应法、Pechini合成方法和柠檬酸配位法,制备了系列锂锰复合氧化物LiMn2O4催化剂,应用于NH3-SCR反应,并与固相反应法合成的MnO2进行了比较。采用N2吸附-脱附、扫描电镜、X射线衍射、H2程序升温还原、NH3程序升温脱附、NO程序升温脱附和X射线光电子能谱对LiMn2O4催化剂进行表征。结果表明,引入Li有利于提高锰基催化剂的SCR活性和抗硫性。Pechini法制备LiMn2O4的NO转化率可在130~260℃达到90%以上;固相反应法制备LiMn2O4的NO转化率大于90%的温度为90~310℃;MnO2的温度窗口则仅为140~280℃。与MnO2相比,引入Li可形成LiMn2O4结构,因此,催化剂中更多的锰离子保持在相对较低的价态Mn3+,并调整表面活性氧含量;同时,Li的存在调变了LiMn2O4表面的酸位,从而减少高温下MnO2表面容易发生的NH3非选择性氧化,改善其催化NH3-SCR反应的温度窗口,也增强了抗硫性。     

Production from ilmenite of TiO2-supported catalysts for selective catalytic reduction of NO with NH3

Economic production of titanium dioxide (yield >98 %) from ilmenite has been achieved by use of a modified sulfate reduction process. A series of samples were prepared by varying the concentration of titanium dioxide nuclei (0.2, 0.3, and 0.6 %) and further impregnation with antimony and vanadia. The structural and acidic properties of the samples were comprehensively studied by X-ray diffraction (XRD), transmission electron microscopy, BJH pore size distribution, and temperature-programmed desorption of NH₃. The XRD results revealed the presence of intense peaks from anatase titanium dioxide. Enhancement of surface area was observed for second-time filtered samples, possibly because of loss of iron from the bulk. As a result, formation of additional micropores was apparent from N₂ adsorption and desorption isotherms. Among all the antimony and vanadia-doped samples, the first-time filtered sample with the low concentration of nuclei (0.2 %) had the highest catalytic activity at low temperatures, owing to its larger pore size and abundant acidic species.     

沥青基球状活性炭担载尿素用于NO2的低温选择性催化还原行为研究

将作为还原剂的尿素担载于沥青基球状活性炭(PSAC)上研究了NO₂的低温选择性催化还原(SCR)反应。结果表明,PSAC上尿素担载量的提高可以增大NO₂与尿素的反应几率;当尿素担载量由8%提高到30%时,SCR反应脱硝活性显著提高,脱硝时间相应延长。在30~90℃,升高反应温度会减小NO₂在PSAC表面的吸附量,从而导致NO_x的脱除量减小。增加反应气氛中NO₂和O₂的浓度均有利于脱硝活性的提高,但当O₂进料浓度大于9%时,继续增加O₂进料浓度对脱硝活性的改善作用变得微弱。降低空速有利于提高脱硝活性和延长脱硝时间。当反应温度为30℃、空速为2000h^-1、NO₂和O₂的进料浓度分别为0.05%和21%时,尿素担载量为8%的PSAC可在49h内实现85%以上的NO_x转化率。     

A nanoporous metal-organic framework with accessible Cu2+ sites for the catalytic Henry reaction

A novel 3D porous metal-organic framework containing 1D nanoscale opening channels was constructed from two kinds of pyridine carboxylates and copper(II) nodes with reachable copper sites located in the channel walls, which can be subsequently used to prompt the Henry reaction of benzaldehydes and nitroalkanes with remarkable catalytic activity compared with homogeneous copper cations.     

Fe-Mn/Al2O3催化剂低温 NH3选择性催化还原 NO的性能

本文制备了一系列 Fe-Mn/Al2O3催化剂,并在固定床上考察了其 NH3低温选择性催化还原 NO的性能.首先考察了不同 Fe负载量制备的催化剂的脱硝性能,优选出最佳的 Fe负载量;在此基础上,研究了 Mn负载量对催化剂脱硝效率的影响;最后,对优选催化剂的抗 H2O和抗 SO2性能进行了实验研究;同时,对催化剂由于 SO2所造成的失活机制进行了考察.采用 N2吸附-脱附、X射线衍射、透射电镜、能量弥散 X射线谱、程序升温还原、程序升温脱附、X射线光电子能谱、热重和傅里叶变换红外光谱等方法对催化剂进行了表征.结果表明,最佳的 Fe和 Mn负载量均为8%,所制的8Fe-8Mn/Al2O3催化剂在150°C的脱硝效率可达近99%;同时,在整个低温测试区间(90–210°C)的脱硝效率均超过了92.6%. Fe在催化剂表面主要以 Fe3+形态存在,而 Mn主要包括 Mn4+和 Mn3+; Mn的添加提高了 Fe在催化剂表面的积累,促进了催化剂比表面积增大和活性物种分散,改善了催化剂氧化还原性能和对 NH3的吸附能力.催化剂的高活性主要是由于其具有较大的比表面积、高度分散的活性物种、增加的还原特性和表面酸性、较低的结合能、较高的 Mn4+/Mn3+和增强的表面吸附氧.此外,8Fe-8Mn/Al2O3的催化性能受 H2O和 SO2影响较小,抗 H2O和 SO2能力较强.同时,反应温度对催化剂的抗硫性有重要影响,在较低的反应温度下,催化剂抗硫性更好; SO2造成催化剂活性降低主要是由于催化剂表面硫酸盐物种的生成.一方面,表面硫酸铵盐的生成造成催化剂孔道堵塞和比表面积降低,减少了反应中的气固接触从而导致活性降低;另一方面,催化剂表面的活性物种被硫酸化,造成反应中的有效活性位减少,从而降低了催化剂活性.     

Novel ultrasonic-modified MnOx/TiO2 for low-temperature selective catalytic reduction (SCR) of NO with ammonia

A novel ultrasonic-modified MnO(x)/TiO(2) catalyst was prepared and compared with two different kinds of MnO(x)/TiO(2) catalysts in the process of low-temperature selective catalytic reduction of NO with NH(3). The physicochemical properties of the catalysts were studied by using various characterization techniques, such as Brunauer-Emmett-Teller (BET) surface measurement, X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM), and in situ Fourier transform infrared spectroscopy (in situ FT-IR). The ultrasonic-modified process introduced ultrasound in the solution impregnation step of traditional impregnation method for MnO(x)/TiO(2) catalyst preparation. In this study, ultrasonic process significantly improved the dispersion behavior and surface acid property of manganese oxide on TiO(2) as well as the catalytic activity, especially at temperature below 120°C. The NO conversion could reach 90% at 100°C. For the novel ultrasonic-modified catalyst, the combination analysis of XRD and HRTEM confirmed that manganese oxide was in a highly dispersed state and Ti and Mn had strong interaction. Furthermore, in situ FT-IR studies revealed that there were significant amounts of Lewis acidity and high Mn atom concentration on the surface of the novel catalysts.     

State of the copper-containing component and the catalytic properties of Cu/ZSM-5 in selective NO reduction with propane

Supported Cu/ZSM-5 catalysts have been synthesized by ion exchange and impregnation using aqueous ammonia solutions of copper nitrate containing orbital-ordered copper ions. The state of the copper-containing component in the pore space of the catalysts and copper sorption in the catalysts have been investigated by a complex of physicochemical methods. The catalytic properties of Cu/ZSM-5 in the selective catalytic reduction of NO with propane are reported. The catalytic properties depend on the copper precursor structure and deposition method, which determine the size of the copper oxide clusters on the outer surface of zeolite crystals.     

铁铈复合氧化物催化剂SCR脱硝的改性研究

利用共沉淀法制备了铁铈催化剂,考察添加钛、锆、钨和钼对其SCR脱硝的改性规律。结果表明,钨和钼的添加提高了铁铈催化剂高温脱硝性能,却使其低温活性有所降低;钛的添加对铁铈催化剂脱硝性能具有促进作用,尤其提高了其低温活性,并拓宽了其完全转化温度窗口,为最佳改性物。当钛的物质的量比逐渐由0.10增至0.40,铁铈钛催化剂低温脱硝效率先增大后减小,但其高温脱硝效率逐渐增大至100%,钛的最佳物质的量比为0.15。XRD和N₂吸附分析结果表明,钛能优化铁铈催化剂的孔隙结构,增大其比表面积和比孔容,细化其孔径,并与催化剂中铁、铈氧化物形成良好的固溶体,从而提高了铁铈催化剂的SCR脱硝性能。Fe_0.8Ce_0.05Ti_0.15O_z催化剂在150～400℃取得了高于90%的NO_x转化率。     

V2O5-CeO2/TiO2催化剂上低温氨选择性催化还原NO的性能

考察了V2O5-CeO2/TiO2催化剂中V、Ce活性组分的担载量和焙烧温度对催化剂低温催化还原NO活性的影响及其在单独SO2、H2O和两者共存气氛下的抗毒化性能。结果表明,焙烧温度400℃下制备的5V30Ce/TiO2催化剂具有良好的低温催化还原NO活性,空速为10000h-1,165℃时NO转化率达99.2%;500℃以下低焙烧温度时,添加的Ce不与V相互作用,在催化剂表面主要以CeO2形式存在,有利于增大催化剂比表面积,增强V2O5在催化剂上的分散度,提高催化活性。而在500℃以上较高焙烧温度下,Ce与V会形成CeVO4,对活性提高不利。催化剂具有良好的低温抗水中毒性能,但受SO2毒化作用明显,其在SO2、H2O共存气氛下中毒程度较单独SO2下浅。     

不同WO3含量对CeO2-ZrO2-WO3催化剂在NH3选择性催化还原NO中的影响

采用水热法制备了CeO₂-ZrO₂-WO₃(CZW)催化剂,考察了WO₃含量对CZW催化剂上NH₃选择性催化还原NO_x性能的影响,并利用X射线衍射、N₂吸附-脱附、H₂程序升温还原、NH₃和NO程序升温脱附等方法对其进行了表征。结果表明,WO₃以无定形的形式存在于催化剂中,添加WO₃后显著提高了催化剂的表面酸性,并且在CZW催化剂上出现了强吸附的NO物种,从而有利于提高催化剂的活性。另外,适量的WO₃引入将增大催化剂的比表面积,促进催化剂的氧化还原性能,这将有利于提高SCR的催化活性。和CeO₂-ZrO₂催化剂相比,当WO₃的含量为20%时,CZW催化剂表现出良好的抗硫性能。此外,当空速为60 000 h^-1时,在200~463 ℃,该催化剂显示出了大于90% NO_x转化率。