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1.
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
2.
Selective catalytic reduction of nitrogen oxide by propene in an oxidising atmosphere was studied on several CuMFI catalysts with different Si/Al ratios (11 Si/Al 100) and different copper loadings (between 0 and 5.5 wt.-%). From the results it was observed that the influence of zeolite Si/Al ratio on CuMFI catalytic activity for NO SCR by propene is dependent on the catalyst copper loading. Furthermore, the effect of catalyst copper loading on catalytic performance depended on the catalyst Si/Al ratio. The results also demonstrated that CuMFI catalysts with different Si/Al ratios and copper loadings, but with the same Cu/Al ratio and, therefore, the same copper exchange level have similar catalytic activity profiles for NO SCR. It was further observed that not all Cu cations exchanged into MFI catalysts have equivalent catalytic activity for NO SCR, which made the existence of different copper environments on CuMFI catalysts evident, isolated Cu2+ ions being the most active species for NO SCR by propene. Moreover, the results showed an improvement of the CuMFI catalytic activity at low temperatures by increasing the catalyst copper exchange level and, consequently, decreasing the number of Brönsted acid sites, which can be performed either by increasing the zeolite Si/Al ratio or copper loading. 相似文献
3.
The catalytic decomposition of acrylonitrile (AN) over Cu-ZSM-5 prepared with various Cu loadings was investigated. AN conversion, during which the nitrogen atoms in AN were mainly converted to N 2, increased as Cu loading increased. N 2 selectivities as high as 90–95% were attained. X-ray diffraction measurements (XRD) and temperature-programmed reduction by H 2 (H 2-TPR) showed the existence of bulk CuO in Cu-ZSM-5 with a Cu loading of 6.4 wt% and the existence of highly dispersed CuO in Cu-ZSM-5 with a Cu loading of 3.3 wt%. Electron spin resonance measurements revealed that Cu-ZSM-5 contains three forms of isolated Cu 2+ ions (square-planar, square-pyramidal, and distorted square-pyramidal). The H 2-TPR results suggested that in Cu-ZSM-5 with a Cu loading of 2.9 wt% and below, Cu + existed even after oxidizing pretreatment. The activity of AN decomposition over Cu/SiO 2 suggested that CuO could form N 2, but, independent of the CuO dispersion, nitrogen oxides (NO x) were formed above 350 °C. Cu + and the square-pyramidal and distorted square-pyramidal forms of Cu 2+ showed low activity for AN decomposition. Temperature-programmed desorption of NH 3 suggested that N 2 formation from NH 3 proceeded on Cu 2+, resulting in the formation of Cu +. The Cu + ions were oxidized to Cu 2+ at around 300 °C. Thus, high N 2 selectivity over Cu-ZSM-5 with a wide range of temperature was probably attained by the reaction over the square-planar Cu 2+, which can be reversibly reduced and oxidized. 相似文献
4.
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
5.
Fe-ZSM5 was prepared with high iron content by solid-state ion exchange and characterized by ICP-AES, BET surface measurements, TEM, UV–vis, EPR and DRIFT spectroscopy as well as supplementing catalytic tests in order to clear up its functionality in urea-SCR. Due to the over-exchange with iron small Fe 2O 3 particles were formed, identified by UV–vis, EPR and TEM measurements, which were proved to be not active for the SCR reaction. However, the oxidation of NO to NO 2 over Fe 3+ ions in the catalyst was realized to be a pre-requisite for the SCR reaction and the rate-determining step. DRIFT investigations under SCR conditions showed adsorbates on Fe 2+ up to 300 °C. The high SCR activity above 300 °C can be explained by the faster reoxidation of Fe 2+ to Fe 3+ sites at high temperatures. The observed inhibition of the SCR reaction by excess ammonia at low and intermediate temperatures can be explained in this context by the reducing properties of ammonia converting Fe 3+ to Fe 2+ or by preventing the reoxidation of Fe 2+. 相似文献
6.
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
7.
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
8.
The state of isolated copper ions in Cu-ZSM-5 containing additions of La, Ce, and Co was monitored in-situ by ESR under flow conditions. Treatment by steam at 630°C for 17 h or high-temperature dry calcination at 850°C induce an irreversible change in coordination for practically all square-planar Cu 2+ ions in mono-cationic Cu-ZSM-5 without agglomeration or encapsulation of the isolated ions. All Cu 2+ ions remain accessible to gas-phase molecules, but the catalytic reactivity of these altered copper sites decreases drastically. A stabilizing effect is noted for samples modified by a relatively large amount, ca. 5.0 wt.-%, of multivalent rare-earth ions La or Ce. Here a part of the copper ions (20–30%) preserves the parent square-planar Cu 2+ state even after calcination at 850°C for 0.5 h. The effect of ca. 1% La or Ce is much less pronounced. The catalytic activity in the complete oxidation of ethane correlates well with the number of square-planar cupric cations retained by the samples after different treatments. The introduction of cobalt sharply increases the ethane oxidation activity of samples calcined at 500–650°C. 相似文献
9.
Cu-ion-exchanged iron-pillared interlayer clays (Fe-PILCs) were prepared under different pH conditions to analyze the influence on the distribution of the copper species over their structure, and on the catalytic performance for the selective catalytic reduction (SCR) of NO x by propene. It was observed that for those samples prepared without pH control, the copper was as isolated Cu 2+ ions. When the samples were prepared under acid pH, the catalytic activity decreased and an appreciable CO production was observed, likely due to the low amount of Cu 2+ cations in those catalysts. Finally, for the samples prepared under alkaline pH, the copper was as Cu 2+ ions and CuO clusters. Their catalytic tests showed the best results for the SCR of NO x. The presence of CuO species led to an improvement in NO x yield to N 2. With the catalytic tests and a study by in situ FTIR of SCR of NO, a reaction mechanism has been proposed, where the reaction intermediates are mainly acetates, organic nitro compounds and nitrous oxide species. 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.
The redox behavior and states of Cu ions in Cu ion-exchanged MFI (Cu( n)-MFI, n: exchange level) have been investigated by means of temperature-programmed desorption (TPD) of oxygen, diffuse reflectance (DR) UV–VIS spectroscopy and Cu + photoluminescence (PL) spectroscopy. TPD chromatograms of oxygen from Cu(n)-MFI were characterized by the appearance of three desorption peaks: (below 200°C), β (300–500°C) and γ (above 500°C). It has been suggested that and β oxygen are extra-lattice oxygen adsorbed on Cu ions, while γ oxygen is lattice oxygen coordinated to Cu ions. The Cu + emission was tremendously reduced once the catalyst contacted with O 2 and NO at elevated temperatures such as 500°C, and it was almost invisible under the working state of the catalyst, suggesting that PL-active Cu + ions are not real active sites under the working state. The desorption of β oxygen was intimately related to the creation of active centers for the NO decomposition reaction. DR measurements showed that the desorption of β oxygen caused tetragonal Cu 2+ to decrease and trigonal Cu 2+ to increase simultaneously. It has been proposed that both Cu 2+ and Cu + are involved in the NO decomposition catalysis over Cu-MFI under the working state. 相似文献
12.
NO TPD, H 2-TPR and XRD have been used to characterise copper-exchanged mordenites with different Si/Al ratios, copper contents and cocations. The results showed that copper is mainly in the form of isolated Cu 2+ ions in CuMOR catalysts with copper exchange ≤20%, whereas at higher copper exchange CuO species are also present. These results were obtained with H and Na as cocation and were achieved by changing either the catalyst Si/Al ratio or the copper content. The data also indicate that the cocation mainly affects copper location and that copper is more easily reduced in sodium form catalysts than in protonic form. It was found that the isolated Cu2+ ions are the most effective species for NO adsorption and the most active species for NO SCR. 相似文献
13.
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
14.
The selective catalytic reduction (SCR) of NO by hydrocarbon is an efficient way to remove NO emission from lean-burn gasoline and diesel exhaust. In this paper, a thermally and hydrothermally stable Al–Ce-pillared clay (Al–Ce-PILC) was synthesized and then modified by SO 42−, whose surface area and average pore diameter calcined at 773 K were 161 m 2/g and 12.15 nm, respectively. Copper-impregnated Al–Ce-pillared clay catalyst (Cu/SO 42−/Al–Ce-PILC) was applied for the SCR of NO by C 3H 6 in the presence of oxygen. The catalyst 2 wt% Cu/SO 42−/Al–Ce-PILC showed good performance over a broad range of temperature, its maximum conversion of NO was 56% at 623 K and remained as high as 22% at 973 K. Furthermore, the presence of 10% water slightly decreased its activity, and this effect was reversible following the removal of water from the feed. Py-IR results showed SO 42− modification greatly enhanced the number and strength of Brönsted acidity on the surface of Cu/SO 42−/Al–Ce-PILC, which played a vital role in the improvement of NO conversion. TPR and XPS results indicated that both Cu + and isolated Cu 2+ species existed on the optimal catalyst, mainly Cu +, as Cu content increased to 5 wt%, another species CuO aggregates which facilitated the combustion of C 3H 6 were formed. 相似文献
15.
Copper ion-exchanged zeolites ZSM5 with SiO 2:Al 2O 3 molar ratios 33 and 53 have been subjected to activity tests for direct decomposition of NO (2000 ppm, GHSV 560–5400 h −1). In situ infrared measurements were used to follow the reaction and surface and gas phase compositions. IR studies were also done in excess oxygen with rapid NO 2 formation in the gas phase. A high level of overexchange of copper in the zeolite in combination with a low concentration of acid sites, concurrent with a high SiO2:Al2O3 ratio, enhances the conversion of NO. A vibrational band at 1631 cm−1 is observed below the light-off temperature and interpreted as a bridged nitrato group bound to Cu2+–O–Cu2+ dimers. This band disappears above the light-off temperature but the intensity below this temperature correlates with the catalytic activity. We interpret that these bridge bound nitrato groups act as siteblockers on the active sites for NO conversion and that a tentative reaction intermediate, N2O3, also binds in a bridge configuration to the same Cu2+–O–Cu2+ dimers. A second nitrato group with unidentate coordination and vibrational bands at 1598/1575 cm−1 probes isolated copper ions. A third infrared band at 2130 cm−1 confirms previous observations of
-ions bound to the zeolite. We conclude that these species are coordinated to deprotonated and negatively charged sites on the zeolite and that these sites for
adsorption are blocked by Cu2+ ion-exchange. The 2130 cm−1 species appear to have no role in direct NO decomposition but the adsorption sites are crucial for the stability of the zeolite and intimately related to ion mobility in the lattice. Prolonged immersion of the zeolite in dilute solutions of copper ions improves the catalyst performance by copper hydroxylation leading to enhanced formation of the above dimers. A high SiO2:Al2O3 ratio leads to more stable catalysts, particularly in combination with a modest overexchange of copper ions. Excessive amounts of copper escalates the deactivation of the Cu-ZSM5 catalyst through the migration and sintering of cupric oxide crystallites. 相似文献
16.
A combined Fourier transform IR (FT-IR) and electron paramagnetic resonance (EPR) study shows that copper in ‘excessively exchanged’ Cu/ZSM-5 is initially present as OH bridged Cu 2+ dimers, besides isolated Cu 2+ ions. Upon heating, the dimers lose water and become oxygen bridged [Cu---O---Cu] 2+ complexes. These are ‘EPR-silent’, presumably as a consequence of antiferromagnetic coupling of the unpaired electrons in each Cu 2+; they are, however, detectable by their perturbation of the lattice vibrations, detected by a FT-IR band at 918–923 cm −1. Reduction by hydrogen or carbon monoxide converts the [Cu---O---Cu] 2+ complexes to pairs of Cu + ions, while the color changes from green to grey. Reductive adsorption of nitrogen monoxide on Cu 2+ results in the formation of Cu +---NO +. Destructive thermal desorption of nitrogen monoxide at 100°C not only restores the Cu 2+ ions, but also appears to regenerate the [Cu---O---Cu] 2+ complex. The results suggest that pairs of copper ions are instrumental in the catalytic decomposition of nitrogen monoxide. 相似文献
17.
Among various Cu/ZnO/ZrO 2 catalysts with the Cu/Zn ratio of 3/7, the one with 15 wt.% of ZrO 2 obtains the best activity for methanol synthesis by hydrogenation of CO. The TPR, TPO and XPS analyses reveal that a new copper oxide phase is formed in the calcined Cu/ZnO/ZrO 2 catalysts by the dissolution of zirconium ions in copper oxide. In addition, the Cu/ZnO/ZrO 2 catalyst with 15 wt.% of ZrO 2 turns out to contain the largest amount of the new copper oxide phase. When the Cu/ZnO/ZrO 2 catalysts is reduced, the Cu 2+ species present in the ZrO 2 lattice is transformed to Cu + species. This leads to the speculation that the addition of ZrO 2 to Cu/ZnO catalysts gives rise to the formation of Cu + species, which is related to the methanol synthesis activity of Cu/ZnO/ZrO 2 catalyst in addition to Cu metal particles. Consequently, the ratio of Cu +/Cu 0 is an important factor for the specific activity of Cu/ZnO/ZrO 2 catalyst for methanol synthesis. 相似文献
18.
In this work, the catalytic nature of Mn loaded sulfated zirconia (SZ) catalysts for the selective catalytic reduction (SCR) of NO with methane was investigated by a combination of reactions and characterizations such as FT-IR spectroscopy, H 2-TPR, UV–vis diffuse reflectance spectroscopy (DRS) and NO-TPD. It was found from the results of reactions and FT-IR spectra that the strong Brønsted and Lewis acid sites in the Mn/SZ catalysts were essential for the SCR of NO with methane. The loading of Mn increased the number of strong Lewis acid sites on the surface of SZ catalyst, which is one reason for its promoting effect. On the other hand, FT-IR spectra, H 2-TPR and UV–vis DRS of the catalysts demonstrated that the presence of the SO 42− species occupied the terminal OH sites on the surface of ZrO 2 support and thereby restrained the formation of more oxidative and nonstoichiometrically dispersed MnO x (1.5 < x < 2) phase. Such an effect of SO 42− suppressed the combustion reaction of CH 4 by O 2 and increased the selectivity towards NO reduction. The NO-TPD showed that the loading of Mn increased the adsorption of NO over SZ catalyst, which is another reason for the promoting effect of Mn. 相似文献
19.
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
20.
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
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