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1.
Mesoporous silicas (MCM-48, SBA-15, MCF), reflecting various porous structures, were modified with copper and iron oxides by two different methods. For a first series of the samples the molecular designed dispersion (MDD) method using acetylacetonate complexes of copper and iron was applied for the deposition of transition metal oxides on the silica supports. A second series of the catalysts was obtained by the incipient impregnation technique using aqueous solutions of the suitable metal nitrates. The modified materials were characterized with respect to the texture (BET), composition (electron microprobe analysis), coordination of the transition metals (UV–vis–DRS) and surface acidity (NH 3-TPD, FTIR). The mesoporous silica supports modified with transition metal oxides were tested as catalysts of the selective reduction of NO with ammonia. The catalytic performance of the studied samples depended on the method used for the deposition of transition metal oxide as well as the kind of mesoporous silica used as a catalytic support. In general, the Cu-containing mesoporous samples effectively operated at lower temperatures than silicas modified with iron. The samples obtained by the MDD method have been found to be more active and selective compared to the analogous samples prepared by the impregnation technique. An introduction of water vapor into the reaction mixture only slightly decreased the NO conversion and selectivity towards N 2 over the MCF mesoporous silica modified with copper or iron oxide. 相似文献
2.
The selective catalytic reduction (SCR) of NO with isobutane and with NH 3 was studied over Fe-MFI catalysts which differ strongly in Brønsted acidity but are similar in Fe content and structure of Fe sites, having shown similar activity in N 2O decomposition in related work. The catalysts were prepared by exchange of Na-ZSM-5 (Si/Al ca. 14) with Fe 2+ ions formed in situ by acidic dissolution of Fe powder and by steam extraction of framework iron from Fe-silicalite or from H-[Fe]-ZSM-5 (Si/Al ca. 30). The characterization of acidic properties by ammonia TPD and by IR of adsorbed pyridine at different temperatures revealed marked differences in acidity between exchanged and steam-activated samples, the latter being (almost) void of strong Brønsted sites. The structural similarity of the iron sites was confirmed by UV–Vis and EPR spectroscopic results. The weakly acidic samples were inferior both in isobutane-SCR and in ammonia-SCR. With isobutane, dramatic differences over the whole range of parameters studied imply a vital role of Brønsted acidity in the reaction mechanism (e.g. in isobutane activation). In NH 3-SCR, large reaction rates were achieved with non-acidic catalysts as well, but a promoting effect of acidity was noted for catalysts that contain the iron in the most favorable site structure (oligomeric Fe oxo clusters). This suggests that an acid-catalyzed step (e.g. the decomposition of NH 4NO 2) may be rate-limiting at low temperatures. 相似文献
3.
A series of catalysts of iron–manganese oxide supported on mesoporous silica (MPS) with different Mn/Fe ratio were studied for low-temperature selective catalytic reduction (SCR) of NO with ammonia in the presence of excess oxygen. Effects of amounts of iron–manganese oxide and calcination temperatures on NO conversion were also investigated. It was found that the Mn–Fe/MPS with Mn/Fe = 1 at the calcination temperature of 673 K showed the highest activity. The results showed that this catalyst yielded 99.1% NO conversion at 433 K at a space velocity of 20,000 h −1. H 2O has no adverse impact on the activity when the SCR reaction temperature is above 413 K. In addition, the SCR activity was suppressed gradually in the presence of SO 2 and H 2O, while such effect was reversible after heating treatment. 相似文献
4.
A series of vanadia doped TiO 2-pillared clay (TiO 2-PILC) catalysts with various amount of vanadia were studied for selective catalytic reduction (SCR) of NO by ammonia in the presence of excess oxygen. It was found that the V 2O 5/TiO 2-PILC catalysts were highly active for the SCR reaction. The catalysts showed a broad temperature window, and the maximum NO conversion was higher than that on V 2O 5/TiO 2 catalyst and was the same as the commercial V 2O 5 + WO 3/TiO 2 catalyst. The V 2O 5/TiO 2-PILC catalysts also had higher N 2/N 2O product selectivities as compared to V 2O 5 doped TiO 2 catalysts. In addition, H 2O + SO 2 slightly increased the activities at high temperatures (>350°C) for the V 2O 5/TiO 2-PILC catalysts. Addition of WO 3 to V 2O 5 further increased the activities of the PILC catalysts. These results indicate that TiO 2-PILC is a good support for vanadia catalysts for the SCR reaction. In situ FT–IR experiment indicated that both Brønsted acid sites and Lewis acid sites exist on the catalyst surface, but with a large proportion being Brønsted acid sites at low temperatures (e.g., 100°C). The reaction path for NO reduction by NH 3 on the V 2O 5/TiO 2-PILC is similar to that on V 2O 5/TiO 2 catalyst, i.e., N 2 originates from the reaction between gaseous NO and NH 3 adspecies. 相似文献
5.
采用正交实验设计和浸渍法制备Mn-Cu-Fe-Ce/REY催化剂。采用固定床微型反应器评价SO 2存在下催化剂在NH 3选择性催化还原NO反应中的活性,考察Mn、Cu、Fe和Ce各活性组分对催化剂活性的影响,并采用XRD、H 2-TPR和SEM等手段对催化剂进行表征。结果表明,Mn、Cu、Fe和Ce各活性组分对催化剂活性影响顺序为:Cu>Fe>Ce>Mn,催化剂的氧化还原性能影响催化剂活性。 相似文献
6.
The catalytic selective reduction of NO over Cu-exchanged natural zeolites (mordenite (MP) and clinoptilolite (HC)) from Cuba using NH 3 as reducing agent and in the presence of excess oxygen was studied. Cu(II)-exchanged zeolites are very active catalysts, with conversions of NO of 95%, a high selectivity to N 2 at low temperatures, and exhibiting good water tolerance. The chemical state of the Cu(II) in exchanged zeolites was characterized by H 2-TPR and XPS. Cu(II)-exchanged clinoptilolite underwent a severe deactivation in the presence of SO 2. However, Cu(II)-exchanged mordenite not only maintained its catalytic activity, but even showed a slight improvement after 20 h of reaction in the presence of 100 ppm of SO 2. 相似文献
7.
Selective catalytic reduction of nitric oxide with ammonia in synthetic low temperature flue gases has been investigated on a commercially available precious metal catalyst, NO xCAT 920 LT TM. It has been found that this catalyst is capable of achieving up to 90% conversion at temperatures below 300°C and low space velocities (12 000 h −1), even in the presence of 20 ppm sulfur dioxide. The ideal ammonia concentration to reduce slip and achieve maximum conversion seems to be a stoichiometric match between ammonia concentration and nitric oxide concentration. A dual site model is proposed to explain the selectivity dependence on the presence of water vapor or sulfur dioxide. 相似文献
8.
A global kinetic model which describes H 2‐assisted NH 3‐SCR over an Ag/Al 2O 3 monolith catalyst has been developed. The intention is that the model can be applied for dosing NH 3 and H 2 to an Ag/Al 2O 3 catalyst in a real automotive application as well as contribute to an increased understanding of the reaction mechanism for NH 3‐SCR. Therefore, the model needs to be simple and accurately predict the conversion of NO x. The reduction of NO is described by a global reaction, with a molar stoichiometry between NO, NH 3 and H 2 of 1:1:2. Further reactions included in the model are the oxidation of NH 3 to N 2 and NO, oxidation of H 2, and the adsorption and desorption of NH 3. The model was fitted to the results of an NH 3‐TPD experiment, an NH 3 oxidation experiment, and a series of H 2‐assisted NH 3‐SCR steady‐state experiments. The model predicts the conversion of NO x well even during transient experiments. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4325–4333, 2013 相似文献
9.
The behavior of V=O band over V 2O 5 crystallite during NH 3 adsorption and SCR reaction was characterized by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and the results are correlated with the reactivity in NH 3 oxidation and SCR reaction. It is found that the decrease of V=O band intensity is due either to the reduction of V 2O 5 surface and/or to the adsorption of ammonia. The 70% intensity of original V=O band is preserved up to 573 K under the conditions of SCR reaction. The vanadium oxidation state is about +4.4. When the temperature reached 673 K, almost all the V=O band was recovered. From these results, it can be suggested that the decrease of the apparent SCR activity due to the increase of NO amount through NH 3 oxidation above 673 K be attributed to the increase of two neighboring V=O sites, which favor the NO formation in ammonia oxidation. 相似文献
10.
This paper investigates the selective catalytic reduction (SCR) of NO with NH 3 using fly ash catalyst. The catalytically active elements investigated here included Fe, Cu, Ni and V. The results indicated that fly ash, after pre-treatment, can be reasonably used as the SCR catalyst support to remove NO from flue gas. Cu gave the highest catalytic activity and NO conversion, compared with Fe, Ni and V. In the pre-treatment process, the nitric acid treatment and drying temperatures for the fly ash particles had little effect on the NO conversion. However, the calcination temperature had an important effect on the catalyst preparation process. 相似文献
11.
Co–Siβ and Fe–Siβ catalysts prepared by a two-step post-synthesis method were characterised by EPR, diffuse reflectance UV–vis, XRD and N 2-physisorption. Iron and cobalt ions are present as isolated lattice tetrahedral Co II and Fe III species for low metal contents (0.7 and 0.9 wt.%, respectively). For higher iron content, FeO x oligomers appear. Zeolites with isolated Co II and Fe III species are active in selective catalytic reduction of NO with ethanol. On FeO x oligomers the oxidation of NO to NO 2 starts to dominate in the selective catalytic reduction of NO with ethanol at the temperatures higher than 700 K. 相似文献
12.
The fast SCR reaction using equimolar amounts of NO and NO 2 is a powerful means to enhance the NO x conversion over a given SCR catalyst. NO 2 fractions in excess of 50% of total NO x should be avoided because the reaction with NO 2 only is slower than the standard SCR reaction. At temperatures below 200 °C, due to its negative temperature coefficient, the ammonium nitrate reaction gets increasingly important. Half of each NH3 and NO2 react to form dinitrogen and water in analogy to a typical SCR reaction. The other half of NH3 and NO2 form ammonium nitrate in close analogy to a NOx storage-reduction catalyst. Ammonium nitrate tends to deposit in solid or liquid form in the pores of the catalyst and this will lead to its temporary deactivation. The various reactions have been studied experimentally in the temperature range 150–450 °C for various NO2/NOx ratios. The fate of the deposited ammonium nitrate during a later reheating of the catalyst has also been investigated. In the absence of NO, the thermal decomposition yields mainly ammonia and nitric acid. If NO is present, its reaction with nitric acid on the catalyst will cause the formation of NO2. 相似文献
13.
考察了不同负载量的Ni/H-USY(ultra stable Y zeolite)催化剂在氧气含量5%情况下甲烷选择性催化还原氮氧化物性能,同时考察了添加微量Pd和Pt对15%(质量分数)Ni催化剂性能的影响.结果表明,催化剂的催化活性与活性组分的含量间存在密切关系,具有一个最佳的负载量.Pd、Pt的添加提高了氮氧化物的去除率,拓宽了反应活化温度的窗口,同时具有较强的抑制N 2O生成的能力.此外采用XRD、TPR等技术对Ni/H-USY体系催化剂的物相结构及氧化还原性能进行了研究,初步探讨了催化剂的催化活性与活性中心的大小与分布之间的关系. 相似文献
14.
以硅胶改性堇青石蜂窝陶瓷为载体,分别制备了以Cu-O、Cu-Ce-O和Cu-Ce-Mn-O为活性组分的催化剂。以CO(NH 2) 2为还原剂,在固定床反应器中进行选择性催化还原NO的研究。采用XRD、SEM和BET等测试方法对催化剂进行表征。结果表明,在温度(300~500) ℃,催化剂Cu-Ce-Mn-O/SiO 2/堇青石的活性优于催化剂Cu-O/SiO 2/堇青石和Cu-Ce-O/SiO 2/堇青石,反应温度为450 ℃、空速为8 000 h -1时,Cu-Ce-Mn-O/SiO 2/堇青石催化剂催化还原NO的转化率可达到88%。 相似文献
15.
The effect of Co content on the catalytic activity of CoSiBEA zeolites in the selective catalytic reduction (SCR) of NO with ethanol is investigated. The Co xSiBEA zeolites ( x = 0.3, 0.7, 3.6 and 6.75 Co wt.%) are prepared by a two-step postsynthesis method which allows to control the introduction of cobalt into zeolite and thus to obtain catalysts with specific Co sites. The nature of the active sites is characterized by XRD, diffuse reflectance UV–vis, H 2-TPR and XPS. The catalytic activity of CoxSiBEA strongly depends on the nature and environment of Co species. Zeolites with isolated lattice tetrahedral Co(II) (Co0.3SiBEA and Co0.7SiBEA samples) are active in SCR of NO with ethanol with selectivity toward N2 exceeding 85% for NO conversion from 20 to 70%. When additional isolated extra-lattice octahedral Co(II) species appear (Co3.6SiBEA sample), the full oxidation of ethanol by dioxygen becomes a very important reaction pathway. In presence of additional cobalt oxides (Co6.75SiBEA sample), the activity and selectivity toward N2 substantially change and full oxidation of ethanol to CO2 is the main reaction pathway and full NO oxidation also takes place in the temperature range 550–775 K. The lack of correlation between the activity in SCR of NO with ethanol and NO oxidation to NO2 suggests that the two reactions are more competitive than sequential. 相似文献
16.
Manganese–cerium mixed oxide catalysts with different molar ratio Mn/(Mn + Ce) (0, 0.25, 0.50, 0.75, 1) were prepared by citric acid method and investigated concerning their adsorption behavior, redox properties and behavior in the selective catalytic reduction of NO x by NH 3. The studies based on pulse thermal analysis combined with mass spectroscopy and FT-IR spectroscopy uncovered a clear correlation between the dependence of these properties and the mixed oxide composition. Highest activity to nitrogen formation was found for catalysts with a molar ratio Mn/(Mn + Ce) of 0.25, whereas the activity was much lower for the pure constituent oxides. Measurements of adsorption uptake of reactants, NO x (NO, NO 2) and NH 3, and reducibility showed similar dependence on the mixed oxide composition indicating a clear correlation of these properties with catalytic activity. The adsorption studies indicated that NO x and NH 3 are adsorbed on separate sites. Consecutive adsorption measurements of the reactants showed similar uptakes as separate measurements indicating that there was no interference between adsorbed reactants. Mechanistic investigations by changing the sequence of admittance of reactants (NO x, NH 3) indicated that at 100–150 °C nitrogen formation follows an Eley–Rideal type mechanism, where adsorbed ammonia reacts with NO x in the gas phase, whereas adsorbed NO x showed no significant reactivity under conditions used. 相似文献
17.
CuO X-carbon nanotubes (CNTs), CuO X-activated carbon (AC) and CuO X-graphite catalysts were prepared by wet impregnation method and their De-NO X property were studied. SEM, EDX, TPD, TPR, AAS and XPS were employed to probe the effect of carbonaceous material supports and the nature of CuO X species appeared on catalyst surface. The existence of Cu +, good dispersion and strong acid sites were found to contribute to the low temperature catalytic activity of CuO X-CNTs with a NO removal efficiency of 67% at 250 °C. Moreover, the regeneration of deactivated CuO X-CNTs catalyst caused by SO 2 could be achieved by 300 °C heating the catalyst in N 2. 相似文献
18.
The influence of NO 2 on the selective catalytic reduction (SCR) of NO with ammonia was studied over Fe-ZSM5 coated on cordierite monolith. NO 2 in the feed drastically enhanced the NO x removal efficiency (DeNOx) up to 600 °C, whereas the promoting effect was most pronounced at the low temperature end. The maximum activity was found for NO 2/NO x = 50%, which is explained by the stoichiometry of the actual SCR reaction over Fe-ZSM5, requiring a NH 3:NO:NO 2 ratio of 2:1:1. In this context, it is a special feature of Fe-ZSM5 to keep this activity level almost up to NO 2/NO x = 100%. The addition of NO 2 to the feed gas was always accompanied by the production of N 2O at lower and intermediate temperatures. The absence of N 2O at the high temperature end is explained by the N 2O decomposition and N 2O-SCR reaction. Water and oxygen influence the SCR reaction indirectly. Oxygen enhances the oxidation of NO to NO 2 and water suppresses the oxidation of NO to NO 2, which is an essential preceding step of the actual SCR reaction for NO 2/NO x < 50%. DRIFT spectra of the catalyst under different pre-treatment and operating conditions suggest a common intermediate, from which the main product N 2 is formed with NO and the side-product N 2O by reaction with gas phase NO 2. 相似文献
19.
Mesoporous and conventional Fe-containing ZSM-5 and ZSM-12 catalysts (0.5–8 wt% Fe) were prepared using a simple impregnation method and tested in the selective catalytic reduction (SCR) of NO with NH 3. It was found that for both Fe/HZSM-5 and Fe/HZSM-12 catalysts with similar Fe contents, the activity of the mesoporous samples in NO SCR with NH 3 is significantly higher than for conventional samples. Such a difference in the activity is probably related with the better diffusion of reactants and products in the mesopores and better dispersion of the iron particles in the mesoporous zeolite as was confirmed by SEM analysis. Moreover, the maximum activity for the mesoporous zeolites is found at higher Fe concentrations than for the conventional zeolites. This also illustrates that the mesoporous zeolites allow a better dispersion of the metal component than the conventional zeolites. Finally, the influence of different pretreatment conditions on the catalytic activity was studied and interestingly, it was found that it is possible to increase the SCR performance significantly by preactivation of the catalysts in a 1% NH 3/N 2 mixture at 500 °C for 5 h. After preactivation, the activity of mesoporous 6 wt% Fe/HZSM-5 and 6 wt% Fe/HZSM-12 catalyst is comparable with that of traditional 3 wt% V 2O 5/TiO 2 catalyst used as a reference at temperatures below 400 °C and even more active at higher temperatures. 相似文献
20.
The adsorption and transformation of ammonia over V 2O 5, V 2O 5/TiO 2, V 2O 5-WO 3/TiO 2 and CuO/TiO 2 systems has been investigated by FT-IR spectroscopy. In all cases ammonia is first coordinated over Lewis acid sites and later undergoes hydrogen abstraction giving rise either to NH 2 amide species or to its dimeric form N 2H 4, hydrazine. Other species, tentatively identified as imide NH, nitroxyl HNO, nitrogen anions N 2− and azide anions N 3− are further observed over CuO/TiO 2. The comparison of the infrared spectra of the species arising from both NH 3 and N 2H 4 adsorbed over CuO/TiO 2 strongly suggest that N 2H 4 is an intermediate in NH 3 oxidation over this active selective catalytic reduction (SCR) and selective catalytic oxidation (SCO) catalysts. This implies that ammonia is activated in the form of NH 2 species for both SCR and SCO, and it can later dimerize. Ammonia protonation to ammonium ion is detected over V 2O 5-based systems, but not over CuO/TiO 2, in spite of the high SCR and SCO activity of this catalyst. Consequently Brönsted acidity is not necessary for the SCR activity. 相似文献
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