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1.
The H 2-TPR (temperature-programmed reduction) study was performed for supported copper oxide catalysts with low loading (0.5 wt% as copper). Among the various kinds of support materials (γ-Al 2O 3, TiO 2, ZrO 2, SiO 2, ZSM-5), alumina-supported copper oxide indicated a one-electron reduction behavior of Cu 2+ into Cu + ions in the presence of H 2. The reduction of the isolated Cu 2+ species in a tetragonally distorted octahedral symmetry into the low coordinated Cu + ions was identified by means of X-ray absorption spectroscopy (XANES and EXAFS). The isolated Cu + ions hosted by γ-Al 2O 3 surface were prevented from further reduction into metallic Cu 0 state under reducing condition with H 2 at 773 K. Less dispersed supported copper oxide species were easily reduced to Cu 0 metal particles with H 2 at 573 K regardless of the kinds of support materials. It is suggested that the one-electron redox behavior of the isolated copper oxide species over γ-Al 2O 3 promotes the catalytic reduction of NO with CO in the presence of oxygen on the basis of redox-type mechanism between Cu 2+ and Cu + in atomically dispersed state. 相似文献
2.
Ion-exchanged pillared clays (PILCs) were studied as catalysts for selective catalytic reduction (SCR) of NO by ethylene. Three most important pillared clays, Al 2O 3-PILC (or Al-PILC), ZrO 2-PILC (or Zr-PILC) and TiO 2-PILC (or Ti-PILC), were synthesized. Cation exchanges were performed to prepare the following catalysts: Cu–Ti-PILC, Cu–Al-PILC, Cu–Zr-PILC, Cu–Al–Laponite, Fe–Ti-PILC, Ce–Ti-PILC, Ce–Ti-PILC, Co–Ti-PILC, Ag–Ti-PILC and Ga–Ti-PILC. Cu–Ti-PILC showed the highest activities at temperatures below 370°C, while Cu–Al-PILC was most active at above 370°C, and both catalysts were substantially more active than Cu-ZSM-5. No detectable N 2O was formed by all of these catalysts. H 2O and SO 2 only slightly deactivated the SCR activity of Cu–Ti-PILC, whereas severe deactivation was observed for Cu-ZSM-5. The catalytic activity of Cu–Ti-PILC was found to depend on the method and amount of copper loading. The catalytic activity increased with copper content until it reached 245% ion-exchange. The doping of 0.5 wt% Ce 2O 3 on Cu–Ti-PILC increased the activities from 10% to 30% while 1.0 wt% of Ce 2O 3 decreased the activity of Cu–Ti-PILC due to pore plugging. Cu–Ti-PILC was found to be an excellent catalyst for NO SCR by NH 3, but inactive when CH 4 was used as the reducing agent. Subjecting the Cu–Ti-PILC catalyst to 5% H 20 and 50 ppm SO 2 at 700°C for 2 h only slightly decreased its activity. TPR results showed that the overexchanged (245%) PILC sample contained Cu 2+, Cu + and CuO. The TPR temperatures for the Cu–Ti-PILC were substantially lower than that for Cu-ZSM-5, indicating easier redox on the PILC catalyst and hence higher SCR activity. 相似文献
3.
采用等体积浸渍法将过渡金属Cu负载到ZSM-5分子筛上,并与其他金属(Fe、Co、Ag、Pd、Ce)共浸渍得到负载型催化剂,将其用于全组分丙烯腈废气的催化脱除过程。催化活性评价结果表明,丙烯腈在Cu/ZSM-5催化剂上320℃可以实现完全转化;掺杂质量分数2%Ce后,丙烯腈的完全转化温度降低到300℃,高选择生成N2的温度窗口也变宽,催化剂稳定性高。通过X射线衍射、氮气吸附-脱附、氢气-程序升温还原、氨气-程序升温脱附和X射线光电子能谱等对催化剂的物化性能进行表征,结果表明,催化剂催化氧化丙烯腈尾气的性能依赖于Cu^2+的还原能力、催化剂表面弱酸与中强酸含量以及表面Cu^2+丰度。 相似文献
4.
The oxidation of benzene to phenol has been successfully carried out in air over Cu-ZSM-5 at moderate temperatures. Several parameters such as Cu loading, calcination temperature and co-exchanged metal ions influence the nature of the catalyst. At low Cu loadings, the catalyst is more selective to phenol while at high Cu loadings CO 2 is the major product. In situ H 2-TPR XAFS studies reveal that at low Cu loadings, Cu exists as isolated pentacoordinated ions, with 4 equatorial oxygens at 1.94 Å and a more distant axial oxygen at 2.34 Å. At higher loadings, monomeric as well as dimeric Cu species exist, with a Cu–Cu distance of 2.92 Å. This suggests that the isolated Cu sites are the active sites responsible for phenol formation. When the catalyst was calcined at 450 °C, the activity peaked in the first hour and then slowly deactivated, but when the calcination temperature was increased to 850 °C, the activity slowly increased until it reached a plateau. Analysis of the XANES region during in situ H 2-TPR shows that at lower calcination temperatures two reduction peaks are present, corresponding to Cu 2+ → Cu + and Cu + → Cu 0. At high calcination temperatures, only a small fraction of the Cu undergoes the two-step reduction and most of the Cu remains in the +2 state. Slow deactivation of the catalyst calcined at 450 °C is due to migration of the Cu ions to inaccessible sites in the zeolite; at high calcination temperatures the Cu is tightly bound to the framework and is unable to migrate. EXAFS analysis of the sample calcined at 850 °C reveals two Cu–Si(Al) scattering paths at 2.83 Å. Doping the catalyst with other metals, in particular Ag and Pd, further improves the activity and selectivity of the reaction. The addition of water to the reaction increases the selectivity of the reaction by displacing the product from the active site. 相似文献
5.
NH 3选择性催化还原NO x(NH 3-SCR)是目前最有应用前景的柴油车尾气净化技术,该技术的核心是开发具备优异催化性能的催化剂。以具有菱沸石(chabazite,CHA)结构的小孔分子筛SSZ-13为载体制备的Cu/SSZ-13催化剂,因具有优异的催化性能和水热稳定性能而受到广泛关注。制备了系列Cu x/SSZ-13催化剂,并通过CO原位漫反射红外光谱(DRIFT)和H 2-TPR等方法能够确定具有高催化活性的铜离子在SSZ-13分子筛上的落位和存在状态。CO红外吸附实验发现,采用Cu(NO 3) 2水溶液离子交换法制备的Cu/SSZ-13催化剂上存在多种落位的Cu +活性中心。在较低的Cu +交换度条件下,Cu +优先落位于SSZ-13分子筛的八元环位置,随着交换度的提高,Cu +开始落位于SSZ-13分子筛双六元环的位置。H 2-TPR结果表明Cu x/SSZ-13催化剂上也存在大量落位在八元环位置不稳定的Cu 2+,这些Cu 2+很容易被还原为Cu +。Cu x/SSZ-13催化剂经800℃水蒸气连续老化16 h,分子筛骨架崩塌程度随着Cu含量的增加而提高,骨架铝的脱除,导致Cu物种发生团聚,而第二金属Ce元素的引入能够在一定程度上提高Cu/SSZ-13的水热稳定性。催化剂构效关系研究表明,具有一定量稳定存在的Cu +,并且拥有大量不稳定存在Cu 2+的催化剂具有较宽温度范围的脱硝性能。 相似文献
6.
A series of silver modified Cu/SiO_2 catalysts were synthesized with ammonia-evaporation method and applied in vapor-phase hydrogenation of methyl acetate to ethanol. The influence of additive ‘Ag' on the structural evolution of catalyst was systematically studied by several characterization techniques, such as N_2 adsorption–desorption, N_2O titration, PXRD, FTIR, in-situ FTIR, H_2-TPR, H_2-TPD, XPS and TEM. Results showed that incorporation of a small amount of Ag could enhance the structural stability, and the strong interaction between Cu and Ag species was conducive to increase the dispersion of copper species and create a suitable Cu~(+)/(Cu~0+ Cu~(+)) ratio, which was proposed to be responsible for the improved catalytic activity. The maximum conversion of MA(94.1%)and selectivity of ethanol(91.3%) over optimized Cu-0.5 Ag/SiO_2 and 120 h on stream without deactivation under optimal conditions demonstrates its excellent stability. 相似文献
7.
The effect of oxygen concentration on the pulse and steady-state selective catalytic reduction (SCR) of NO with C 3H 6 over CuO/γ-Al 2O 3 has been studied by infrared spectroscopy (IR) coupled with mass spectroscopy studies. IR studies revealed that the pulse SCR occurred via (i) the oxidation of Cu 0/Cu + to Cu 2+ by NO and O 2, (ii) the co-adsorption of NO/NO 2/O 2 to produce Cu 2+(NO 3−) 2, and (iii) the reaction of Cu 2+(NO 3−) 2 with C 3H 6 to produce N 2, CO 2, and H 2O. Increasing the O 2/NO ratio from 25.0 to 83.4 promotes the formation of NO 2 from gas phase oxidation of NO, resulting in a reactant mixture of NO/NO 2/O 2. This reactant mixture allows the formation of Cu 2+(NO 3−) 2 and its reaction with the C 3H 6 to occur at a higher rate with a higher selectivity toward N 2 than the low O 2/NO flow. Both the high and low O 2/NO steady-state SCR reactions follow the same pathway, proceeding via adsorbed C 3H 7---NO 2, C 3H 7---ONO, CH 3COO −, Cu 0---CN, and Cu +---NCO intermediates toward N 2, CO 2, and H 2O products. High O 2 concentration in the high O 2/NO SCR accelerates both the formation and destruction of adsorbates, resulting in their intensities similar to the low O 2/NO SCR at 523–698 K. High O 2 concentration in the reactant mixture resulted in a higher rate of destruction of the intermediates than low O 2 concentration at temperatures above 723 K. 相似文献
8.
Decomposition of NO was studied on Cu-ZSM-5 catalysts prepared by solid state ion exchange using CuCl 2 (I), CuO (II) and by conventional liquid phase ion exchange with copper acetate (III). There was no difference in the catalytic activity among samples (I), (II) and (III) using the same copper loading. Treatment of the samples in argon, in air or in NO/Ar mixture at 700°C was necessary to develop optimum catalytic activity. Transient kinetic experiments using NO carried out under isothermal conditions, showed overshoots in the N 2 and O 2 concentration at the front and tail edge, respectively. Fourier transform-infrared studies indicated the formation of oxidized copper sites and adsorbed NO 2 species during the NO decomposition. In a proposed mechanism Cu 2+(O)(NO)(NO 2) intermediate was suggested to play a key role in the NO decomposition. 相似文献
9.
ZSM-5 zeolites modified with Cu + ions were prepared either by the high-temperature chemical reaction of hydrogen form with CuCl vapour or by the wet ion exchange with subsequent reduction of the modified samples in CO at 873 K. Adsorption of H 2, N 2 or C 2H 6 by Cu + ions was studied by DRIFTS and by volumetric technique. The conclusions about the structure of adsorption complexes were supported by the DFT cluster quantum chemical calculations. The obtained results indicated that in addition to the previously reported strong adsorption of nitrogen, the univalent copper also unusually strongly adsorbs molecular hydrogen and ethane. Adsorption of hydrogen is the most amazing since the observed low-frequency shifts of the HH stretching vibrations were as high as about 1000 cm −1. This is quite different from much weaker H 2 perturbation by Cu 2+ cations. Adsorption of ethane by Cu + ions also resulted in the low-frequency shifts of some of CH IR stretching bands up to 400 cm −1. The DFT cluster modelling indicated that both adsorption of hydrogen and ethane could be explained by interaction with the isolated Cu + ions localized at the sites of the ZSM-5 framework. Quantum chemical calculations indicated the important role in the bonding of adsorbed hydrogen and ethane of electron back donation from d π-orbitals of Cu + ions to the σ *HH or CH orbitals. The overall yield of Cu + sites of the strong H 2 or N 2 adsorption is about twice lower than the total copper content. 相似文献
10.
Copper-exchanged zeolites with different structures (CuMFI, CuMOR and CuY) used as catalysts on the selective catalytic reduction (SCR) of NO by propene have been studied. Different types of Cu species were identified (Cu 2+, Cu +, and CuO) by H 2-TPR and NO TPD. The structure of each zeolite determines the nature and concentration of those species and the catalytic behavior for SCR of NO by propene in the presence of oxygen. A correlation was observed between the catalytic activity, and the presence of isolated Cu 2+ species, which is enhanced by MFI structure. 相似文献
11.
A simple strategy of Cu modification was proposed to broaden the operation temperature window for NbCe catalyst. The best catalyst Cu 0.010/Nb 1Ce 3 presented over 90% NO conversion in a wide temperature range of 200-400 ℃ and exhibited an excellent H 2O or/and SO 2 resistance at 275 ℃. To understand the promotional mechanism of Cu modification, the correlation among the “activity-structure-property” were tried to establish systematically. Cu species highly dispersed on NbCe catalyst to serve as the active component. The strong interaction among Cu, Nb and Ce promoted the emergence of NbO 4 and induced more Brønsted acid sites. And Cu modification obviously enhanced the redox behavior of the NbCe catalyst. Besides, EPR probed the Cu species exited in the form of monomeric and dimeric Cu 2+, the isolated Cu 2+ acted as catalytic active sites to promote the reaction: Cu 2+-NO 3-+NO(g) → Cu 2+-NO 2-+NO 2(g). Then the generated NO 2 would accelerate the fast-SCR reaction process and thus facilitated the low-temperature deNO efficiency. Moreover, surface nitrates became unstable and easy to decompose after Cu modification, thus providing additional adsorption and activation sites for NH 3, and ensuring the improvement of catalytic activity at high temperature. Since the NH 3-SCR reaction followed by E-R reaction pathway efficaciously over Cu 0.010/Nb 1Ce 3 catalyst, the excellent H 2O and SO 2 resistance was as expected. 相似文献
12.
This paper deals with the redox properties of Cu ions implanted in ZSM-5 and supported on Al 2O 3, catalysts active in the selective reduction of NO by hydrocarbons such as propane. Data on the reducibility of the Cu systems in various atmospheres (vacuum, CO, H 2, O 2) and on their DeNO x activity are presented. The methods used to obtain informations on the surface and bulk transformations (and their link with catalytic behaviour) are complementary: UV–visible diffuse reflectance spectroscopy being useful to detect the presence of Cu 2+ and Cu 0, while Cu + is detected indirectly by the analysis of the IR spectrum of CO bound selectively to this cation. The main contributions to the previous knowledge are the following: it is possible to distinguish CO bound to isolated and non-isolated Cu+ ions; the isolated Cu2+ ions are reducible under vacuum without participation of organic impurities; the more active solids for the NO reduction into N2 are characterized by the presence of isolated Cun+ ions beside the additional influence of the zeolitic framework; after the formation of Cu+ ions the redox cycles are reversible but, after the formation of Cu0, the reversibility or irreversibility of the redox cycles and the restoration of the SCR activity are function of the copper content; the activity decreases after agglomeration into bulk oxides; there is no formation of bulk CuO during the reaction and, with reducing and moderate oxidizing mixtures, part of the copper remains as cuprous ions. 相似文献
13.
为提升三氧化二铁(Fe 2O 3)催化剂的脱硝性能,扩展催化剂的活性温度窗口,采用共沉淀法引入助剂钇(Y)元素对Fe 2O 3催化剂进行改性。通过X射线衍射(XRD)、氮气等温吸-脱附(N 2-BET)、X射线光电子能谱(XPS)、氨气程序升温脱附(NH 3-TPD)、氢气程序升温还原(H 2-TPR)等表征方法对样品进行了表征分析。XRD和N 2-BET结果表明,Y的掺杂使催化剂结构发生变化,比表面积增加、孔径减小。XPS和NH 3-TPD结果证明,Y掺杂Fe 2O 3具有更多的表面吸附氧(O Ⅰ)、Fe 2+以及更多的酸量。H 2-TPR结果表明,Y的掺杂使催化剂的氧化还原能力略有下降。测试了不同含量Y掺杂的Fe 2O 3催化剂在150~400℃的脱硝性能,其中Fe<... 相似文献
14.
A series of CeO 2 promoted cobalt spinel catalysts were prepared by the co-precipitation method and tested for the decomposition of nitrous oxide (N 2O). Addition of CeO 2 to Co 3O 4 led to an improvement in the catalytic activity for N 2O decomposition. The catalyst was most active when the molar ratio of Ce/Co was around 0.05. Complete N 2O conversion could be attained over the CoCe0.05 catalyst below 400 °C even in the presence of O 2, H 2O or NO. Methods of XRD, FE-SEM, BET, XPS, H 2-TPR and O 2-TPD were used to characterize these catalysts. The analytical results indicated that the addition of CeO 2 could increase the surface area of Co 3O 4, and then improve the reduction of Co 3+ to Co 2+ by facilitating the desorption of adsorbed oxygen species, which is the rate-determining step of the N 2O decomposition over cobalt spinel catalyst. We conclude that these effects, caused by the addition of CeO 2, are responsible for the enhancement of catalytic activity of Co 3O 4. 相似文献
15.
Cu-ZSM-5 and Cu-AlTS-1 catalysts were prepared by solid state ion exchange and studied in DeNO x reactions. A NO 3 type surface complex was found to be an active intermediate in the decomposition of NO and N 2O. Copper was oxidized to Cu 2+ in the decomposition reactions. Oscillations at full N 2O conversion were observed in the gas phase O 2 concentration, without any change in the N 2 concentration. The oscillation was synchronized by gas phase NO formed from the NO 3 complex. The same complex seems to be an active intermediate also in NO selective catalytic reduction (SCR) by methane, whereas carbonaceous deposits play a role in NO SCR by propane. TPD reveals that only 10–20% of the total copper in the zeolites participates in the catalytic cycles. 相似文献
16.
催化剂CuO/HZSM-5在氧化亚氮(N 2O)催化分解领域有着重要地位,完成对催化剂成型条件的探索是其工业实际应用的重要步骤。在CuO/HZSM-5成型过程中,胶溶剂的用量对成型催化剂的机械强度,催化活性以及其他物化性质有重要的影响。通过机械强度测定、活性评价、X射线衍射分析(XRD)、孔结构分析、氨气程序升温吸附脱附分析(NH 3-TPD)、氢气程序升温还原分析(H 2-TPR)等手段对催化剂性能进行分析。机械强度测定与NH 3-TPD分析结果显示,胶溶剂用量对成型催化剂机械强度和酸量影响较大,胶溶剂用量为体积分数2%时,机械强度最高为135.8 N·cm -1,而催化剂酸量在胶溶剂用量为体积分数7%时最高。XRD与孔结构分析结果显示胶溶剂用量由0增长至体积分数7%,催化剂的孔容积改变幅度较小,但比表面积由277 cm 2·g -1增至293 cm 2·g -1,HZSM-5的特征峰变化较小。活性评价与H 2-TPR分析结果显示,胶溶剂用量为0的催化剂与添加胶溶剂后的成型催化剂,N 2O完全分解的温度提高幅度最大为20 ℃,活性下降,且H 2-TPR中还原峰提高幅度最大为27 ℃,可还原性变差。而胶溶剂用量由体积分数2%提高至7%,成型催化剂CuO/HZSM-5的催化活性以及可还原性差异较小。 相似文献
17.
The chemical changes that occurred in a Cu-ZSM-5 catalyst during the selective reduction of NO with i-C 4H 10 in the presence and absence of O 2 were catalogued. In the presence of excess O 2 complete conversion of the NO to N 2 and the hydrocarbon to CO 2 and H 2O occurred and the Cu 2+ concentration estimated from the integrated intensity of the electron paramagnetic resonance (EPR) signal was not significantly changed from its initial value. When the oxygen concentration was lowered below the point of stoichiometry, however, both of these conversions decreased modestly, but when O 2 was eliminated from the feed both conversions fell precipitously and the acid catalyzed decomposition products of isobutane appeared in the products instead of CO 2 and H 2O. These changes were accompanied by corresponding changes in the EPR data. Lowering the O 2 below the point of stoichiometry effected a loss of from 30% to 50% of the intensity of the Cu 2+ signal. Eliminating O 2 reduced the signal by several orders of magnitude. Remarkably, these reduced catalysts could be restored to their initial oxidation states by adding excess O 2 into the feed stream, even when there was evidence that Cu 0 was present. Dealumination accompanied selective reduction even in excess O 2, particularly above 623 K. This was probably caused by steaming of the catalyst by the H 2O produced in the reaction. 相似文献
18.
采用浸渍法制备Cu/Al_2O_3和Cu-Ni/Al_2O_3催化剂,研究载体、CuO负载量和助剂Ni元素加入对催化剂的影响,采用XRD、BET、H_2-TPR和NH_3-TPD等对催化剂进行表征,考察反应温度、氨醇物质的量比和空速对催化剂性能的影响。结果表明,在反应温度295℃、氨醇物质的量比5. 5和乙醇空速0. 6 h~(-1)条件下,Cu1Ni1/Al_2O_3催化剂催化性能最佳,且运行550 h,催化活性未明显降低。 相似文献
19.
The selective catalytic reduction of NO by H 2 under strongly oxidizing conditions (H 2-SCR) in the low-temperature range of 100–200 °C has been studied over Pt supported on a series of metal oxides (e.g., La 2O 3, MgO, Y 2O 3, CaO, CeO 2, TiO 2, SiO 2 and MgO-CeO 2). The Pt/MgO and Pt/CeO 2 solids showed the best catalytic behavior with respect to N 2 yield and the widest temperature window of operation compared with the other single metal oxide-supported Pt solids. An optimum 50 wt% MgO-50wt% CeO 2 support composition and 0.3 wt% Pt loading (in the 0.1–2.0 wt% range) were found in terms of specific reaction rate of N 2 production (mols N 2/g cat s). High NO conversions (70–95%) and N 2 selectivities (80–85%) were also obtained in the 100–200 °C range at a GHSV of 80,000 h −1 with the lowest 0.1 wt% Pt loading and using a feed stream of 0.25 vol% NO, 1 vol% H 2, 5 vol% O 2 and He as balance gas. Addition of 5 vol% H 2O in the latter feed stream had a positive influence on the catalytic performance and practically no effect on the stability of the 0.1 wt% Pt/MgO-CeO 2 during 24 h on reaction stream. Moreover, the latter catalytic system exhibited a high stability in the presence of 25–40 ppm SO 2 in the feed stream following a given support pretreatment. N 2 selectivity values in the 80–85% range were obtained over the 0.1 wt% Pt/MgO-CeO 2 catalyst in the 100–200 °C range in the presence of water and SO 2 in the feed stream. The above-mentioned results led to the obtainment of patents for the commercial exploitation of Pt/MgO-CeO 2 catalyst towards a new NO x control technology in the low-temperature range of 100–200 °C using H 2 as reducing agent. Temperature-programmed desorption (TPD) of NO, and transient titration of the adsorbed surface intermediate NO x species with H 2 experiments, following reaction, have revealed important information towards the understanding of basic mechanistic issues of the present catalytic system (e.g., surface coverage, number and location of active NO x intermediate species, NO x spillover). 相似文献
20.
Cu/Mg/Al mixed oxides (CuO = 4.0–12.5 wt%), obtained by calcination of hydrotalcite-type (HT) anionic clays, were investigated in the selective catalytic reduction (SCR) of NO by NH 3, either in the absence or presence of oxygen, and their behaviours were compared with that of a CuO-supported catalyst (CuO = 10.0 wt%), prepared by incipient wetness impregnation of a Mg/Al mixed oxide also obtained by calcination of an HT precursor. XRD analysis, UV-visible-NIR diffuse reflectance spectra and temperature-programmed reduction analyses showed the formation, in the mixed oxide catalysts obtained from HT precursors, mainly of octahedrally coordinated Cu 2+ ions, more strongly stabilized than Cu-containing species in the supported catalyst, although the latter showed a lower percentage of reduction. The presence of well dispersed Cu 2+ ions improved the catalytic performances, although similar behaviours were observed for all catalysts in the absence of oxygen. On the contrary, when the mixture with excess oxygen was fed, very interesting catalytic performances were obtained for the catalyst richest in copper (CuO = 12.5 wt%). This catalyst exhibited a behaviour comparable to that of a commercial V 2O 5–WO 3TiO 2 catalyst, without any deactivation phenomena after four consecutive cycles and following 8 h of time-on-stream at 653 K. Decreasing the copper content or increasing the calcination time and temperature led to considerably poorer performances and catalytic behaviours similar to that of the CuO-supported catalyst, due to the side-reaction of NH 3 combustion on the free Mg/Al mixed oxide surface. 相似文献
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