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1.
The decomposition of N 2O, and the catalytic reduction by NH 3 of N 2O and N 2O + NO, have been studied on Fe-BEA, -ZSM-5 and -FER catalysts. These catalysts were prepared by classical ion exchange and characterized by TPR after various activation treatments. Fe-FER is the most active material in the catalytic decomposition because “oxo-species” reducible at low temperature, appearing upon interaction of Fe II-zeolite with N 2O (-oxygen), are formed in largest amounts with this material. The decomposition of N 2O is promoted by addition of NH 3, and even more with NH 3 + NO in the case of Fe-FER and -BEA. It is proposed that the NO-promoted reduction of N 2O originated from the fast surface reaction between -oxygen O * and NO * to yield NO 2*, which in turn reacts immediately with NH 3. 相似文献
2.
Various spinel-type catalysts AB 2O 4 (where A = Mg, Ca, Mn, Co, Ni, Cu, Cr, Fe, Zn and B = Cr, Fe, Co) were prepared and characterized by XRD, BET, TEM and FESEM-EDS. The performance of these catalysts towards the decomposition of N 2O to N 2 and O 2 was evaluated in a temperature programmed reaction (TPR) apparatus in the absence and the presence of oxygen. Spinel-type oxides containing Co at the B site were found to provide the best activity. The half conversion temperature of nitrous oxide over the MgCo 2O 4 catalyst was 440 °C and 470 °C in the absence and presence of oxygen, respectively (GHSV = 80,000 h −1). On the grounds of temperature programmed oxygen desorption (TPD) analyses as well as of reactive runs, the prevalent activity of the MgCo2O4 catalyst could be explained by its higher concentration of suprafacial, weakly chemisorbed oxygen species, whose related vacancies contribute actively to nitrous oxide catalytic decomposition. This indicates the way for the development of new, more active catalysts, possibly capable of delivering at low temperatures amounts of these oxygen species even higher than those characteristic of MgCo2O4. 相似文献
3.
Effect of the loading amount of Fe over ion-exchanged Fe-MFI catalysts on the catalytic performance of N 2O reduction with NH 3 was investigated, and the results indicated that the turnover frequency (TOF) was almost constant in the Fe/Al range between 0.05 and 0.40. The activity of N 2O + NH 3 reaction was much lower than that of N 2O + CH 4 reaction over Fe-MFI (Fe/Al = 0.40), and the preadsorption of NH 3 decreased drastically the activity of N 2O + CH 4 reaction. The temperature-programmed desorption (TPD) of NH 3 showed the formation of stronger acid sites on Fe-MFI (Fe/Al = 0.24 and 0.40), and the amount of the acid sites agrees well with the desorption amount O 2 in O 2-TPD in the low temperature range. The acid sites gave a 3610 cm −1 peak (Brønsted acid) in FTIR observation. These results suggest that the acid sites were formed on the bridge oxide ions in binuclear Fe species. Adsorbed NH 3 on the strong acid sites inhibited N 2O dissociation, which can be related to the low activity of N 2O + NH 3 reaction over Fe-MFI with high Fe loading. 相似文献
4.
Both NO decomposition and NO reduction by CH 4 over 4%Sr/La 2O 3 in the absence and presence of O 2 were examined between 773 and 973 K, and N 2O decomposition was also studied. The presence of CH 4 greatly increased the conversion of NO to N 2 and this activity was further enhanced by co-fed O 2. For example, at 773 K and 15 Torr NO the specific activities of NO decomposition, reduction by CH 4 in the absence of O 2, and reduction with 1% O 2 in the feed were 8.3·10 −4, 4.6·10 −3, and 1.3·10 −2 μmol N 2/s m 2, respectively. This oxygen-enhanced activity for NO reduction is attributed to the formation of methyl (and/or methylene) species on the oxide surface. NO decomposition on this catalyst occurred with an activation energy of 28 kcal/mol and the reaction order at 923 K with respect to NO was 1.1. The rate of N 2 formation by decomposition was inhibited by O 2 in the feed even though the reaction order in NO remained the same. The rate of NO reduction by CH 4 continuously increased with temperature to 973 K with no bend-over in either the absence or the presence of O 2 with equal activation energies of 26 kcal/mol. The addition of O 2 increased the reaction order in CH 4 at 923 K from 0.19 to 0.87, while it decreased the reaction order in NO from 0.73 to 0.55. The reaction order in O 2 was 0.26 up to 0.5% O 2 during which time the CH 4 concentration was not decreased significantly. N 2O decomposition occurs rapidly on this catalyst with a specific activity of 1.6·10 −4 μmol N 2/s m 2 at 623 K and 1220 ppm N 2O and an activation energy of 24 kcal/mol. The addition of CH 4 inhibits this decomposition reaction. Finally, the use of either CO or H 2 as the reductant (no O 2) produced specific activities at 773 K that were almost 5 times greater than that with CH 4 and gave activation energies of 21–26 kcal/mol, thus demonstrating the potential of using CO/H 2 to reduce NO to N 2 over these REO catalysts. 相似文献
5.
The effect of the nature and distribution of VO x species over amorphous and well-ordered (MCM-41) SiO 2 as well as over γ-Al 2O 3 on their performance in the oxidative dehydrogenation of propane with O 2 and N 2O was studied using in situ UV–vis, ex situ XRD and H 2-TPR analysis in combination with steady-state catalytic tests. As compared to the alumina support, differently structured SiO 2 supports stabilise highly dispersed surface VO x species at higher vanadium loading. These species are more selective over the latter materials than over V/γ-Al 2O 3 catalysts. This finding was explained by the difference in acidic properties of silica- and alumina-based supports. C 3H 6 selectivity over V/γ-Al 2O 3 materials is improved by covering the support fully with well-dispersed VO x species. Additionally, C 3H 6 selectivity over all materials studied can be tuned by using an alternative oxidising agent (N 2O). The improving effect of N 2O on C 3H 6 selectivity is related to the lower ability of N 2O for catalyst reoxidation resulting in an increase in the degree of catalyst reduction, i.e. spatial separation of active lattice oxygen in surface VO x species. Such separation favours selective oxidation over CO x formation. 相似文献
6.
Transient isotopic studies in the temporal analysis of products (TAP) reactor evidenced the importance of the lifetime of oxygen species generated upon N 2O decomposition on extraframework iron sites of Fe-silicalite for methane oxidation at 723 K. Fe-silicalite effectively activates CH 4 when N 2O and CH 4 are pulsed together in the reactor. However, these oxygen species gradually become inactive for methane oxidation as the time delay between the N 2O and CH 4 pulses is increased from 0 to 2 s. 相似文献
7.
Nitric oxide and nitric dioxide compounds (NO x) present in stack gases from nitric acid plants are usually eliminated by selective catalytic reduction (SCR) with ammonia. In this process, small quantities of nitrous oxide (N 2O) are produced. This undesirable molecule has a high greenhouse gas potential and a long lifetime in the atmosphere, where it can contribute to stratospheric ozone depletion. The influence of catalyst composition and some operating variables were evaluated in terms of N 2O formation, using V 2O 5/TiO 2 catalysts. High vanadia catalyst loading, nitric oxide inlet concentration and reaction temperature increase the generation of this undesirable compound. The results suggest that adsorbed ammonia not only reacts with NO via SCR, but also with small quantities of oxygen activated by the presence of NO. The mechanism proposed for N 2O generation at low temperature is based on the formation of surface V–ON species which may be produced by the partial oxidation of dissociatively adsorbed ammonia species with NO + O 2 (eventually NO 2). When these active sites are in close proximity they can interact to form an N 2O molecule. This mechanism seems to be affected by changes in the active site density produced by increasing the catalyst vanadia loading. 相似文献
8.
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
9.
Kinetics of N 2O decomposition over catalyst prepared by calcination of Co–Mn hydrotalcite was examined in integral fixed-bed reactor ( ) at various N 2O and O 2 initial partial pressure at temperature range of 330–450 °C. Kinetic data were evaluated by linear and non-linear regression method, 15 kinetic expressions were tested. Based on the obtained results a redox model of N 2O decomposition was proposed. At low pressures of O 2, adsorbed oxygen is formed by the N 2O decomposition; the N 2O chemisorption is considered as the rate-determining step. On the contrary, at high O 2 pressure it could be assumed that adsorbed oxygen species appear as a result of O 2 adsorption and the Eley–Rideal mechanism is the rate determining. N 2O decomposition is well described by the 1st rate law at N 2O and O 2 concentrations typical for waste gases. 相似文献
10.
The catalytic properties of cobalt containing ZSM-5 zeolites prepared by various methods were compared. TPR, XRD, N 2-BET, XPS, FTIR and UV–vis spectroscopy were used for characterizing the samples. Well-dispersed cobalt oxide-like species and isolated Co 2+ ions in charge compensation positions were found in the zeolite. Catalysts prepared using a single step cation exchange method showed high activity for N 2O decomposition in a temperature range 300–550°C, in the presence of 0–5% O 2, and high stability in the presence of 10% H 2O to the feed. UV–vis spectra and TPR experiments indicated the presence of some cobalt oxides, not detected by DRX, in a Co-ZSM-5 catalyst containing 3.76 wt% Co, prepared by a solid-state reaction procedure. The N 2O conversion over this catalyst was strongly affected by addition of both O 2 and H 2O to the feed. 相似文献
11.
以等体积浸渍法制备γ-Al_2O_3负载的Co、Cu、Ce和Fe氧化物催化剂,利用正交试验设计实验条件,采用XRD、BET和H_2-TPR等对催化剂进行表征,并考察活性组分对催化剂催化分解N_2O活性的影响。结果表明,催化剂具有尖晶石结构,其BET比表面积随着金属氧化物负载量增加而降低。催化剂中铜的氧化物可以降低还原峰温度,进而明显提高催化活性,Co和Fe的加入对活性有一定的提高,Ce对催化活性没有明显影响。 相似文献
12.
A mechanistic scheme of N 2O and N 2 formation in the selective catalytic reduction of NO with NH 3 over a Ag/Al 2O 3 catalyst in the presence and absence of H 2 and O 2 was developed by applying a combination of different techniques: transient experiments with isotopic tracers in the temporal analysis of products reactor, HRTEM, in situ UV/vis and in situ FTIR spectroscopy. Based on the results of transient isotopic analysis and in situ IR experiments, it is suggested that N 2 and N 2O are formed via direct or oxygen-induced decomposition of surface NH 2NO species. These intermediates originate from NO and surface NH 2 fragments. The latter NH 2 species are formed upon stripping of hydrogen from ammonia by adsorbed oxygen species, which are produced over reduced silver species from NO, N 2O and O 2. The latter is the dominant supplier of active oxygen species. Lattice oxygen in oxidized AgO x particles is less active than adsorbed oxygen species particularly below 623 K. The previously reported significant diminishing of N 2O production in the presence of H 2 is ascribed to hydrogen-induced generation of metallic silver sites, which are responsible for N 2O decomposition. 相似文献
13.
A series of sulfated zirconia supported Pd/Co catalysts was synthesized by the sol–gel method and examined for NO x reduction by methane. The NO conversion increased up to a Co/S ratio of 0.43, and then decreased at a higher Co loading (Co/S = 0.95). Sulfate content was also essential for obtaining high selectivity to molecular nitrogen. A catalyst loaded with 0.06 wt.% Pd, 2.1 wt.% Co and 2.1 wt.% S (Pd/Co-SZ-2) exhibited remarkable performance under lean conditions and displayed stability in a long-term durability test using a synthetic reaction mixture containing 10% water vapor. This catalyst exhibited the highest sulfur retention most probably as cobalt sulfide. Besides, the catalytic oxidation of NO to NO y groups was confirmed by FT-IR, in agreement with the general mechanism for the SCR of NO by hydrocarbons. In the absence of oxygen in the feed stream, the catalyst was highly active for NO reduction with methane. IR stretching bands assigned to N 2O and adsorbed nitro groups were identified upon adsorbing NO on Pd/Co-SZ-2. This indicates that under rich conditions disproportionation of NO to N 2O and NO 2 occurs and confirms that the formation of NO 2 species is an essential step for NO reduction by CH 4. 相似文献
14.
Fe 2O 3 catalysts supported on Al 2O 3 were used to remove nitrous oxide from the nitric acid plant simulated process stream (containing O 2, NO and H 2O). Catalysts were prepared by the coprecipitation method and were characterized for their physico-chemical properties by BET, XRD, AFM and TPR analysis. A strong influence of the post-preparation heating conditions on the structural and catalytic properties of the catalysts has been evidenced. Laboratory tests revealed 95% conversion of N 2O at temperature 750 °C and a slight decrease in activity in the presence of H 2O and NO. The catalysts were inert towards decomposition of NO. The pilot-plant reactor and real plant studies (up to 3300 h time-on-stream) confirmed high activity and very good mechanical stability of the catalysts as well as no decomposition of nitric oxide. 相似文献
15.
The development of an activated carbon supported bimetallic catalyst for the simultaneous reduction of NO and N 2O is described. Base metal catalysts were found to deactivate due to the oxidation of the metallic phase, suggesting the use of a noble metal. Platinum catalysts were very active for NO reduction, but they lacked selectivity towards N 2, since N 2O was produced. The association of Pt and K, a good N 2O reduction catalyst, was capable of achieving the complete conversion of both gases at 350 °C, the catalyst being stable over extended periods of time (up to 17 h). A synergistic effect between Pt and K was observed, similar to the effect previously reported for Ni and K. 相似文献
16.
The N 2O decomposition activity of Fe-ZSM-5 strongly depends on the iron content and the preparation methods, including wet (WIE) and solid state ion exchanges (SSIE). The state of Fe species formed on the surface of a series of Fe-ZSM-5 catalysts containing a variety of Fe contents with respect to the preparation method and their role for N 2O decomposition activity have been systematically examined. The general trend for the decomposition activity of Fe-ZSM-5-SSIE is higher than that of Fe-ZSM-5-WIE, indicating the formation of a distinctive local structure of Fe on the catalyst surface during the course of the ion-exchange procedure. Based upon the Fourier transformed Fe K-edge EXAFS spectra for the series of Fe-ZSM-5-SSIE and -WIE catalysts, most of the Fe species on the surface of Fe-ZSM-5-SSIE with high Fe loading are well dispersed in the form of oxygen-bridged binuclear Fe species. The turnover frequency (TOF) for N 2O decomposition under dry and wet conditions has been confirmed assuming that Fe-ZSM-5-SSIE samples with Fe/Al = 0.20 and Fe/Al = 0.65 only contain mononuclear and binuclear Fe species, respectively, as active reaction species on their surface. The high performance of Fe-ZSM-5-SSIE may be mainly due to the formation of the binuclear Fe species onto its surface during the preparation of the catalyst. 相似文献
17.
Selective catalytic reduction (SCR) of NO with methane in the presence of excess oxygen has been investigated over a series of Mn-loaded sulfated zirconia (SZ) catalysts. It was found that the Mn/SZ with a metal loading of 2–3 wt.% exhibited high activity for the NO reduction, and the maximum NO conversion over the Mn/SZ catalyst was higher than that over Mn/HZSM-5. NH 3–TPD results of the catalysts showed that the sulfation process of the supports resulted in the generation of strong acid sites, which is essential for the SCR of NO with methane. On the other hand, the N 2 adsorption and the H 2–TPR of the catalysts demonstrated that the presence of the SO 42− species promoted the dispersion of the metal species and made the Mn species less reducible. Such an increased dispersion of metal species suppressed the combustion reaction of CH 4 by O 2 and increased the selectivity towards NO. The Mn/SZ catalysts prepared by different methods exhibited similar activities in the SCR of NO with methane, indicating the importance of SO 42−. The most attractive feature of the Mn/SZ catalysts was that they were more tolerant to water and SO 2 poisoning than Mn/HZSM-5 catalysts and exhibited higher reversibility after removal of SO 2. 相似文献
18.
Isothermal oscillations developed during N 2O decomposition over Co-ZSM-5 catalysts with different Si/Al ratios have been investigated. Spontaneous oscillations were observed between 350 and 450 °C. The maximum amplitude has been obtained for the catalysts having Si/Al of 40 and 50. The activation energies of the obtained oscillations were calculated in respect to cobalt concentration. The results showed that the Ea values increase linearly with an increasing Si/Al ratio of the zeolite. For Co-ZSM-5 catalyst (Si/Al = 25), increasing cobalt content in the catalyst led to a decrease in the frequency as well as the amplitude of the oscillations. Meanwhile, the increase in the Ea values was observed. The calculated reaction rate was found to be first order with respect to nitrous oxide concentration. Moreover, the developed oscillations were found to be sensitive to inlet N 2O concentration, catalyst weight and milling time duration. Decreasing the N 2O inlet concentration as well as the catalyst weight and increasing the milling time would lead to a quenching of the developed oscillations. 相似文献
19.
Nitrous oxide (N 2O) is a greenhouse gas and damages the ozone layer. Nitrous oxide from nitric acid plants is currently the major industrial N 2O source. Many catalysts are active for pure N 2O decomposition, but few are effective under realistic conditions, due to the relatively low N 2O concentration compared to those of inhibitors present in the exhaust gases, i.e. H 2O, O 2 and NO x. In the present work, Si-SBA-15 and Al-SBA-15 were used to prepare Fe, Ru or Rh catalysts by ion-exchange, impregnation or precipitation method. The effect of steam treatment was also studied. Their activities were evaluated in an automated six-flow reactor system using a model tail-gas under conditions representative for nitric acid plants. The performance depends on the metal species and preparation method. When the carrier, preparation method and metal loading are the same, the activity decreases as follows: Rh > Ru > Fe. Steam-treatment has little influence on catalytic activity unlike for FeZSM-5 catalysts. Impregnated Rh on Al-grafted SBA-15 catalyst shows the best performance per weight of catalyst. However, per mol of active metal (Rh, Ru and Fe) species, catalysts by ion exchange are better than those by impregnation. It is concluded that the improved performance of Rh on Al-grafted SBA-15 is caused by a better dispersion and an enhanced stability due to Al incorporation, but not due to an enhanced acidity thereof. 相似文献
20.
The selective catalytic reduction (SCR) of NO by methane in the presence of excess oxygen has been studied on a series of Pd catalysts supported on sulfated zirconia (SZ). This support is not as sensitive to structural damage by steaming as the acidic zeolites, such as H-ZSM-5 and H-Mor. In previous studies, it was shown that this type of acidic zeolites are able to stabilize Pd 2+ ions and promote high SCR activity and selectivity, which are typically not seen in Pd catalysts. In this contribution, it has been demonstrated that SZ is able to promote the NO reduction activity in a similar way to the acidic zeolites, by stabilizing Pd 2+ ions that is selective for NO reduction. As in the case of acidic zeolites, the stabilization of Pd 2+ ions can occur through a transfer of Pd species from particle to particle. One of the attractive features of Pd/SZ catalysts is that they are less sensitive to water and SO 2 poisoning than Pd/H-ZSM-5 catalyst and exhibit higher reversibility after removal of water or SO 2. 相似文献
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