共查询到20条相似文献,搜索用时 15 毫秒
2.
Low loaded alumina supported manganese oxides exhibit a high activity and selectivity for the selective catalytic reduction (SCR) of NO in the temperature range 383–623 K. The impact of low concentrations of SO 2 on the activity of these catalysts has been investigated. Upon SO 2 addition to the flue gas, the catalysts lose their high initial activity in a few hours due to stoichiometric SO 2 uptake. Analysis of the deactivated samples by mercury porosimetry, FTIR, TPR and TPD shows that the deactivation is not due to the formation of (bulk or surface) Al 2(SO 4) 3 or deposition of ammonium sulphates. Comparison of the results with unsupported Mn 2O 3 and MnO 2 provides evidence that formation of surface MnSO 4 is the main deactivation route. This process is independent of the oxidation state of the manganese and the presence of oxygen in the gas stream. The formed sulphates decompose at 1020 K and are reduced by H 2 at temperatures above 810 K. This means that regeneration of the catalysts is not very feasible. The results restrict practical application of these catalysts to sulphur free conditions. 相似文献
3.
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
4.
Catalytic properties of supported gallium oxides have been examined for the selective reduction of NO by CH 4 in excess oxygen. The activity was greatly affected by the support; Ga 2O 3/Al 2O 3 (Al 2O 3 supported Ga 2O 3) and Ga 2O 3–Al 2O 3 mixed oxide exhibited high activity and selectivity as comparable to Ga-ZSM-5, while unsupported Ga 2O 3 and the other supported Ga 2O 3 were ineffective. For Ga 2O 3/Al 2O 3, the activity changed with Ga 2O 3 content, and was highest at about 30 wt% Ga 2O 3, which corresponds to a theoretical monolayer coverage. Gallium oxide highly dispersed on Al 2O 3 is considered to be responsible for the high activity and selectivity. The reaction characteristics of Ga 2O 3/Al 2O 3 were studied and compared with Ga-ZSM-5 and Co-ZSM-5. Ga 2O 3/Al 2O 3 exhibited the highest activity and selectivity at high temperature. In addition, Ga 2O 3/Al 2O 3 showed higher tolerance against water than Ga-ZSM-5. C 3H 8 and C 3H 6 were also evaluated as reducing agents, and Ga 2O 3/Al 2O 3 showed higher activity than Ga-ZSM-5 above 723 K achieving almost complete reduction of NO to N 2. 相似文献
5.
A series of CoO x/Al 2O 3 catalysts was prepared, characterized, and applied for the selective catalytic reduction (SCR) of NO by C 3H 8. The results of XRD, UV–vis, IR, Far-IR and ESR characterizations of the catalysts suggest that the predominant oxidation state of cobalt species is +2 for the catalysts with low cobalt loading (≤2 mol%) and for the catalysts with 4 mol% cobalt loading prepared by sol–gel and co-precipitation. Co 3O 4 crystallites or agglomerates are the predominant species in the catalysts with high cobalt loading prepared by incipient wetness impregnation and solid dispersion. An optimized CoO x/Al 2O 3 catalyst shows high activity in SCR of NO by C 3H 8 (100% conversion of NO at 723 K, GHSV: 10,000 h −1). The activity of the selective catalytic reduction of NO by C 3H 8 increases with the increase of cobalt–alumina interactions in the catalysts. The influences of cobalt loading and catalyst preparation method on the catalytic performance suggest that tiny CoAl 2O 4 crystallites highly dispersed on alumina are responsible for the efficient catalytic reduction of NO, whereas Co 3O 4 crystallites catalyze the combustion of C 3H 8 only. 相似文献
6.
The nitric acid industry is a source of both NO x and N 2O. The simultaneous selective catalytic reduction of both compounds using propane as a reductant has been investigated. A stacked catalyst bed with first a Co-ZSM-5 catalyst and second a Pd/Fe-ZSM-5 catalyst gives >80% conversion of N 2O and NO x above 300 °C at atmospheric pressure. At 4 bar absolute pressure (bara) the Co-ZSM-5 DeNO x catalyst shows higher NO x and propane conversion. This leaves not enough propane for the Pd/Fe-ZSM-5 DeN 2O catalyst, which causes a ‘dip’ in N 2O conversion. Reducing the space velocity (SV) of the first catalyst bed secures high NO x and N 2O conversions from 300 °C and up at 4 bara. 相似文献
7.
The effect of SO 2 on the NO x storage capacity and oxidation and reduction activities of a model Pt/Rh/BaO/Al 2O 3 NO x storage catalyst was investigated. Addition of 2.5, 7.5 or 25 vol. ppm SO 2 to a synthetic lean exhaust gas caused deactivation of the NO x storage function, the oxidation activity and the reduction activity of the catalyst. The degree of deactivation of the NO x storage capacity was found to be proportional to the total SO 2 dose that the catalyst had been exposed to. SO 2 was found to be accumulated in the catalyst as sulphate. 相似文献
8.
ZrO 2–TiO 2 mixed oxides, prepared using the sol–gel method, were used as supports for platinum catalysts. The effects of catalyst pre-reduction and surface acidity on the performance of Pt/ZT catalysts for the reduction of NO with CH 4 were studied. The diffuse reflectance infrared Fourier transformed (DRIFT) spectra of CO adsorbed on the Pt/ZT catalysts, and also on the Pt/T and Pt/Z references, pre-reduced at 773 K in hydrogen, revealed that an SMSI state is developed in the Ti-rich oxide-supported platinum catalysts. However, no shift in the binding energy of Pt 4f 7/2 level for Pt/T and Pt deposited on Ti-rich support counterparts pre-reduced at 773 K was found by photoelectron spectroscopy. The DRIFT spectra of the catalysts under the NO+O 2 co-adsorption revealed the appearance of nitrite/nitrate species on the surface of the Zr-containing catalysts, which displayed acidic properties, but were almost absent in the Pt/T catalyst. The intensity of these bands reached a maximum for the Pt/ZT(1:1) catalyst, which in turn exhibited a larger specific area. In the absence of oxygen in the feed stream, the NO+CH 4 reaction showed DRIFT spectra assigned to surface isocyano species. Since the intensity of this band is higher for the Pt/ZT (9:1) catalyst, it seems that such species are developed at the Pt–support interface. 相似文献
9.
NO x sorption and reduction capacities of 12-tungstophosphoric acid hexahydrate (H 3PW 12O 40·6H 2O, HPW) were measured under representative alternating conditions of lean and rich exhaust-type gas mixture. Under lean conditions, the sorption of NO x is large and is equivalent to 37 mg of NO x/g HPW. Although a part of these NO x remains unreduced, HPW is able to reduce some of the NO x to produce N 2 by a reaction between the sorbed NO 2 and hydrocarbon (HC), but this process is slow. The addition of 1% Pt affects strongly the chemical behaviour occurring during the course of a rich operation. The NO desorption observed at the beginning of the rich phase is strongly accelerated. The direct correlation between NO 2 consumption and CO 2 production shows that the principal pathway is the reaction CO+NO 2→CO 2+NO. In a mixture of reducing gas (CO, HC, H 2), the competition is strongly in favour of CO though in its absence the reaction observed was the hydrogenation of propene to propane. 相似文献
10.
The selective catalytic reduction of nitrogen oxides (NO x) with ammonia over ZSM-5 catalysts was studied with and without water vapor. The activity of H-, Na- and Cu-ZSM-5 was compared and the result showed that the activity was greatly enhanced by the introduction of copper ions. A comparison between Cu-ZSM-5 of different silica to alumina ratios was also performed. The highest NO conversion was observed over the sample with the lowest silica to alumina ratio and the highest copper content. Further studies were performed with the Cu-ZSM-5-27 (silica/alumina = 27) sample to investigate the effect of changes in the feed gas. Oxygen improves the activity at temperatures below 250 °C, but at higher temperatures O 2 decreases the activity. The presence of water enhances the NO reduction, especially at high temperature. It is important to use about equal amounts of nitrogen oxides and ammonia at 175 °C to avoid ammonia slip and a blocking effect, but also to have high enough concentration to reduce the NO x. At high temperature higher NH 3 concentrations result in additional NO x reduction since more NH 3 becomes available for the NO reduction. At these higher temperatures ammonia oxidation increases so that there is no ammonia slip. Exposing the catalyst to equimolecular amounts of NO and NO 2 increases the conversion of NO x, but causes an increased formation of N 2O. 相似文献
11.
Catalytic reduction of NO by propene in the presence of oxygen was studied over SnO 2-doped Ga 2O 3–Al 2O 3 prepared by sol–gel method. Although SnO 2-doped Ga 2O 3–Al 2O 3 gave lower NO conversion than Ga 2O 3–Al 2O 3 in the absence of H 2O, the activity was enhanced considerably by the presence of H 2O and much higher than that of Ga 2O 3–Al 2O 3. The presence of SnO 2 and Ga 2O 3–Al 2O 3 species having intimate Ga–O–Al bondings was found to be essential for the promotional effect of H 2O. The promotional effect of H 2O was interpreted by the following two reasons. The first one is the removal of carbonaceous materials deposited on the catalyst surface by H 2O. The other is the selective inhibition by H 2O of the reaction steps resulting in propene oxidation to CO x without reducing NO. 相似文献
12.
The selective catalytic reduction (SCR) of NO x (NO + NO 2) by NH 3 in O 2 rich atmosphere has been studied on Cu-FAU catalysts with Cu nominal exchange degree from 25 to 195%. NO 2 promotes the NO conversion at NO/NO 2 = 1 and low Cu content. This is in agreement with next-nearest-neighbor (NNN) Cu ions as the most active sites and with N xO y adsorbed species formed between NO and NO 2 as a key intermediate. Special attention was paid to the origin of N 2O formation. CuO aggregates form 40–50% of N 2O at ca. 550 K and become inactive for the SCR above 650 K. NNN Cu ions located within the sodalite cages are active for N 2O formation above 600 K. This formation is greatly enhanced when NO 2 is present in the feed, and originated from the interaction between NO (or NO 2) and NH 3. The introduction of selected co-cations, e.g. Ba, reduces very significantly this N 2O formation. 相似文献
13.
The pathway for selective reduction of NO x by methane over Co mordenite cataysts has been studied by comparing the rates of the individual reactions (NO oxidation, CH 4 oxidation, NO 2 reduction) with that of the combined reaction (NO + O 2 + CH 4). Co (+2) was exchanged into H-MOR and Na-MOR to give catalysts with different metal loading and number of support protons. Additionally, exchanged Co (+2) ions were precipitated with NaOH to produce dispersed cobalt oxide on Na-MOR. The NO oxidation rate is the same for ion exchanged Co (+2) ions in H-MOR and Na-MOR, but the rate of Co (+2) ions is much lower than that of cobalt oxide. NO oxidation equilibrium is obtained only for those catalysts with high metal loading, cobalt oxide or run at low GHSV. Under the conditions of selective catalytic reduction, methane oxidation by O 2 is low for all catalysts. The turnover frequency of Co on Na-MOR, however, is higher than that on H-MOR. The rate of NO 2 reduction to N 2 is directly proportional to the number of support acid sites and independent of the amount of Co. Comparison of the rates and selectivities for the individual reactions with the combined reaction of NO + O 2 + CH 4 indicates that there are two types of catalysts. For the first, the NO oxidation is in equilibrium and the rate determining step is reduction of NO 2. For these catalysts, the rate (and selectivity) for formation of N 2 is identical from NO + O 2 + CH 4 and NO 2 + CH 4. These catalysts have high metal loading and few acid sites. Nevertheless, the rate of N 2 formation increases with increasing number of protons. For the second type of catalyst, NO oxidation is not in equilibrium and is the rate limiting step. For these catalysts the rate of N 2 formation increases with increasing metal loading. Neither catalyst type, however, is optimized for the maximum formation of N 2. By using a mixture of catalysts, one with high NO oxidation activity and one with a large number of Brønsted acid sites, the rate of N 2 is greater than the weighted sum of the individual catalysts. The current results support the proposal that the pathway for selective catalytic reduction is bifunctional where metal sites affect NO oxidation, while support protons catalyze the formation of N 2. 相似文献
14.
Pillared clays can be tailored to give high surface areas and porosities with different degrees of acidity. These materials often show better hydrothermal stability than zeolites. Al-PILC based Rh catalysts were tested in the selective catalytic reduction of NO by methane in presence of excess oxygen. The catalysts were prepared by two methods: incipient wetness impregnation (IMP) and wet ion exchange (IEX). The Rh 3+ ion exchanged PILCs showed better performances than their homologues prepared by IMP. On IEX samples the effect of temperature and O 2 concentration in SCR-CH 4 activity was analyzed. Different activation degrees detected in the catalysts after 25 vol.% O 2 and 450 °C reaction conditions, were related to the various Rh species present in the catalysts as a consequence of the exchange degree of the support in contact with the active phase solutions. From the results, a reaction mechanism is proposed by which exchanged Rh 3+ in the Al-PILC matrix and oxidized Rh 2O x in combination with the supports acid sites contribute to form oxygenated species as the process-determining step. The results from XPS and O 2 adsorption isotherms were used to support the proposed mechanism. 相似文献
15.
Selective catalytic reduction of NO x by C 3H 6 in the presence of H 2 over Ag/Al 2O 3 was investigated using in situ DRIFTS and GC–MS measurements. The addition of H 2 promoted the partial oxidation of C 3H 6 to enolic species, the formation of –NCO and the reactions of enolic species and –NCO with NO x on Ag/Al 2O 3 surface at low temperatures. Based on the results, we proposed reaction mechanism to explain the promotional effect of H 2 on the SCR of NO x by C 3H 6 over Ag/Al 2O 3 catalyst. 相似文献
16.
Silver–aluminum mixed oxide catalyst (Ag–Al 2O 3) prepared by the sol–gel method was studied for the selective reduction of NO by various alkanes in the presence of water vapor. As the carbon number of alkanes increases, the de-NO x activity and water tolerance were markedly increased. In the case of n-octane as a reductant, the presence of water vapor markedly promoted NO reduction. The results of reaction studies and in situ IR experiment showed that the possible reasons for the promoting effect by water vapor are the inhibition of the n-octane oxidation by O 2 and the suppression of the poisoning effect caused by carboxylate and carbonate species. Among various alumina-supported transition metal catalysts, Ag–Al 2O 3 showed the highest activity for SCR by n-octane. Ag–Al 2O 3 showed higher NO conversion to N 2 and selectivity than alumina-supported Pt and Cu-ZSM-5 catalysts for the selective reduction of NO by n-octane and i-octane. 相似文献
18.
SCR-deNO x reaction and SO 2–SO 3 oxidation tests were carried out by different research groups over fresh and used EUROCAT oxide samples in order to characterize the reactivity of the catalysts and to compare data obtained in several laboratories (Politecnico of Milan, Università of Salerno, ENEL of Milan, Boreskov Insitute of Catalysis). Data are presented which indicate that the used EUROCAT catalyst is slightly more active both in the deNOx reaction and SO2–SO3 oxidation than the fresh sample. An analyses of data collected over honeycomb catalysts by means of a 2D, single-channel model of the SCR monolith reactor has been performed to evaluate the intrinsic kinetic constant of the deNOx reaction; a satisfactory comparison has been obtained between estimation of the intrinsic kinetic constant and estimation of the intrinsic catalyst activity from data collected over powdered catalysts. A good agreement has been found in the experimental results collected in the different labs, both for the deNOx reaction and SO2–SO3 oxidation. 相似文献
19.
Catalytic performances of ZSM-5 based catalysts containing indium or palladium were examined for NO reduction with CH 4 and NO x chemisorption. The amounts of NO x chemisorbed on In/H-ZSM-5 were well proportional to the catalytic activities for NO x reduction. Pd/H-ZSM-5, on the other hand, hardly chemisorbed NO 2, while the catalytic activity for NO 2 reduction with CH 4 is very high. Furthermore, Pd loaded on SiO 2 showed comparably high catalytic activity for NO 2 reduction with CH 4 at 400°C in the absence of oxygen as Pd/H-ZSM-5. CH 4 combustion during NO x reduction with CH 4 in the presence of oxygen significantly occurred over PdO on SiO 2, while less over Pd/H-ZSM-5. The role of zeolite might be slightly different between In/H-ZSM-5 and Pd/H-ZSM-5: the zeolitic porous structure is needed for In/H-ZSM-5 in order to concentrate NO 2 adspecies on InO + sites, which is important for NO reduction with CH 4 on In/H-ZSM-5 based catalysts, while the ion-exchangeable ability of zeolite is needed for Pd/H-ZSM-5 in order to make Pd 2+ located in a highly dispersed state, on which NO is strongly chemisorbed. 相似文献
20.
Catalytic performance of Sn/Al 2O 3 catalysts prepared by impregnation (IM) and sol–gel (SG) method for selective catalytic reduction of NO x by propene under lean burn condition were investigated. The physical properties of catalyst were characterized by BET, XRD, XPS and TPD. The results showed that NO 2 had higher reactivity than NO to nitrogen, the maximum NO conversion was 82% on the 5% Sn/Al 2O 3 (SG) catalyst, and the maximum NO 2 conversion reached nearly 100% around 425 °C. Such a temperature of maximum NO conversion was in accordance with those of NO x desorption accompanied with O 2 around 450 °C. The activity of NO reduction was enhanced remarkably by the presence of H 2O and SO 2 at low temperature, and the temperature window was also broadened in the presence of H 2O and SO 2, however the NO x desorption and NO conversion decreased sharply on the 300 ppm SO 2 treated catalyst, the catalytic activity was inhibited by the presence of SO 2 due to formation of sulfate species (SO 42−) on the catalysts. The presence of oxygen played an essential role in NO reduction, and the activity of the 5% Sn/Al 2O 3 (SG) was not decreased in the presence of large oxygen. 相似文献
|