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1.
The effect of treatment with different mineral acids (H 2SO 4, H 3PO 4, HNO 3 and HCl) on the activity of monolithic CoO x/γ-Al 2O 3 catalysts in the reduction of nitric oxide with methane in the presence of oxygen (CH 4-SCR of NO x) was studied. Their behaviour in the methane oxidation reaction in both the presence and absence of NO x was determined in order to interpret the results in terms of intrinsic activity and competition between both processes. Depending on the nature of the acid used, significant differences were observed in the catalytic activities which were related to the textural states, surface acidities and the nature of the detected species. The best results were obtained after treatment with H 2SO 4, which increased the activity towards NO x elimination compared to the other catalysts. This behaviour was attributed not only to an increase in surface acidity but also to the stabilisation of the active Co 2+ species, thus avoiding the formation of Co 3O 4 spinel that is responsible for the strongly adsorbed NO x species that lead to NO 2 formation which increase the rate of the undesired methane oxidation reaction at high temperatures. 相似文献
2.
Reaction activities of several developed catalysts for NO oxidation and NO x (NO + NO 2) reduction have been determined in a fixed bed differential reactor. Among all the catalysts tested, Co 3O 4 based catalysts are the most active ones for both NO oxidation and NO x reduction reactions even at high space velocity (SV) and low temperature in the fast selective catalytic reduction (SCR) process. Over Co 3O 4 catalyst, the effects of calcination temperatures, SO 2 concentration, optimum SV for 50% conversion of NO to NO 2 were determined. Also, Co 3O 4 based catalysts (Co 3O 4-WO 3) exhibit significantly higher conversion than all the developed DeNO x catalysts (supported/unsupported) having maximum conversion of NO x even at lower temperature and higher SV since the mixed oxide Co-W nanocomposite is formed. In case of the fast SCR, N 2O formation over Co 3O 4-WO 3 catalyst is far less than that over the other catalysts but the standard SCR produces high concentration of N 2O over all the catalysts. The effect of SO 2 concentration on NO x reduction is found to be almost negligible may be due to the presence of WO 3 that resists SO 2 oxidation. 相似文献
3.
Several hexaaluminate-related materials were prepared via hydrolysis of alkoxide and powder mixing method for high temperature combustion of CH 4 and C 3H 8, in order to investigate the effect of the concentration of the fuels, O 2 and H 2O on NO x emission and combustion characteristics. Among the hexaaluminate catalysts, Sr 0.8La 0.2MnAl 11O 19− prepared by the alkoxide method exhibited the highest activity for methane combustion and low NO x emission capability. NO x emission at 1500 °C was increased linearly with O 2 concentration, whereas water vapor addition decreased NO x emission in CH 4 combustion over the Sr 0.8La 0.2MnAl 11O 19− catalyst. In the catalytic combustion of C 3H 8 over the Sr 0.8La 0.2MnAl 11O 19− catalyst, the amount of NO x emitted was raised in the temperature range between 1000 and 1500 °C when the C 3H 8 concentration increased from 1 to 2 vol.%. It was found that NO x emission in this temperature range was reduced effectively by adding water vapor. 相似文献
4.
The catalytic properties of transition metal oxides (Cr, Ce, and Co) supported on ZrO 2 synthesized by various methods, as well as the effect of rhodium on the performance of the M xO y/ZrO 2 oxide systems in NO reduction with hydrocarbons (methane, propane–butane mixture, and propene) were studied. Scanning electron microscopy, ammonia thermoprogrammed desorption (NH 3-TPD), XPS, and IR spectroscopy were used to study the physicochemical indices of rhodium-promoted M xO y/ZrO 2 oxide catalysts. The enhancement of the redox properties of the oxide catalysts upon the introduction of rhodium does not alter their bifunctional nature in SCR activity: these catalysts have both redox and strong acid Brønsted-sites. 相似文献
5.
Chromium oxide catalysts supported on TiO 2 and Al 2O 3 were examined in a fixed-bed flow reactor system for the removal of PCE (perchloroethylene), a simulant of 2,3,7,8-TCDD (2,3,7,8-tetrachlorodibenzo- p-dioxin), and in a pilot plant employing actual flue gas from a sintering plant for the removal of PCDDs/PCDFs (poly-chlorinated dibenzo-dioxin/poly-chlorinated dibenzo-furan). The 12.5 wt.% chromium oxides supported on TiO 2 and Al 2O 3 revealed excellent stability and performance of PCE removal in the feed gas stream containing water vapor. In a pilot plant study, the catalysts washcoated on the honeycomb reactor revealed 93–95% of PCDDs/PCDFs removal activity over CrO x/Al 2O 3-HC20 (CrO x/Al 2O 3 catalyst washcoated on 20 cell-honeycomb), and more than 99% of the decomposition activity over CrO x/TiO 2-HC20 (CrO x/TiO 2 catalyst washcoated on 20 cell-honeycomb) at 325 °C and 5000 h −1 of reactor space velocity without the de novo synthesis of PCDDs/PCDFs. In particular, CrO x/TiO 2-HC20 showed 94% of PCDDs/PCDFs decomposition activity even at 280 °C reaction temperature. The catalyst also exhibited significant NO removal activity. The chromium oxide seems to be a promising catalyst for the removal of PCDDs/PCDFs and NO x contained in the flue gas. 相似文献
6.
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
7.
The lean selective catalytic reduction of NO x by methane over protonic palladium loaded ZSM-5, FER and MOR, as well as, on bimetallic Pd–Pt-HMOR was examined. Special emphasis was paid on the combined effects of water and SO 2 in the feed stream. Under dry conditions and in the absence of SO 2, the degree of NO x conversion at 450°C decreases as follows: Pd-HZSM-5>Pd-HMOR>Pd-HFER. Sulfur dioxide alone has no apparent effect on the activity for NO x reduction, but the coexistence of water and SO 2 inhibits both NO x and methane conversions. The extent of inhibition by water and SO 2 on NO x reduction is Pd-HFER>Pd-HZSM-5>Pd-HMOR. Acid mordenite doped with low levels of Pt and Pd leads to an active catalyst that is more tolerant to the presence of either water or SO 2 than the corresponding monometallic Pt- and Pd-HMOR. Nevertheless, NO x reduction is also inhibited at temperatures below 450°C when SO 2 and water are both present. TPD experiments of water over calcined samples of protonic Pd supported pentasil zeolites, Pd/γ-Al 2O 3 and Pt–Pd-HMOR with and without pretreatment in SO 2+O 2 indicate that sulfation of the surface increases water chemisorption by the support. Therefore, the observed decrease of NO x reduction on Pd-loaded zeolite catalysts when SO 2 and H 2O coexist in the feed stream may be due to enhanced water inhibition and presumably active site poisoning. 相似文献
8.
Novel NO x storage-reduction (NO xSR) catalysts prepared by Pt and/or Cu impregnation of Mg–Al (60:40) hydrotalcite (HT)-type compounds show better performances in NO x storage than Pt–Ba/Al 2O 3 Toyota-type NO xSR catalysts at reaction temperatures lower than 250 °C. The presence of Pt or Cu considerably enhances the activity, with the former more active. The nature of the HT source, however, also influences performance. The co-presence of Pt and Cu slightly worsens the low temperature activity, but considerably promotes the resistance to deactivation after severe hydrothermal treatment and in the presence of SO 2. This effect is attributed to both the possibility of formation of a Pt–Cu alloy after reduction, and the modification of the HT induced during the deposition of Cu. The overall Pt–Cu/HT performances are thus superior to those of the Pt–Ba/Al 2O 3 Toyota-type NO xSR catalysts. 相似文献
9.
A series of calcium-modified alumina-supported cobalt catalysts were prepared with a two-step impregnation method, and the effect of calcium on the catalytic performances of the catalysts for the partial oxidation of methane to syngas (CO and H 2) was investigated at 750 °C. Also, the catalysts were characterized by XRD, TEM, TPR and ( in situ) Raman. At 6 wt.% of cobalt loading, the unmodified alumina-supported cobalt catalyst showed a very low activity and a rapid deactivation, while the calcium-modified catalyst presented a good performance for this process with the CH 4 conversion of 88%, CO selectivity of 94% and undetectable carbon deposition during a long-time running. Characterization results showed that the calcium modification can effectively increase the dispersion and reducibility of Co 3O 4, decrease the Co metal particle size, and suppress the reoxidation of cobalt as well as the phase transformation to form CoAl 2O 4 spinel phases under the reaction conditions. These could be related to the excellent catalytic performances of Co/Ca/Al 2O 3 catalysts. 相似文献
10.
In order to improve a “Three Function Catalysts Model”, the present paper deals with alumina based catalysts containing cobalt and palladium for the NO reduction by methane. The deNOx temperature window was estimated by adsorption and subsequent desorption of NO in lean conditions. Two NOx desorption peaks were detected for both catalysts. For Pd(0.63)Co(0.58)/Al2O3, the two desorption peaks appeared at 205 and 423 °C, whereas for Pd(0.14)Co(0.57)/Al2O3, the maxima desorption temperature peaks were at 205 and 487 °C. In addition, NO oxidation was also studied to evaluate the catalyst first function. It was found that, the oxidation begins on Co–Pd/Al2O3 around 250 °C. On Pd(0.63)Co(0.58)/Al2O3, 8% of deNOx were found in the range of the second NOx desorption peak temperature (410 °C). During TPSR, CxHyOz species such as formaldehyde were detected. These oxygenate species are the reactive intermediate for deNOx by methane. 相似文献
11.
Novel Rh–La(Mn,Co)O 3 structured catalysts were developed for the partial oxidation of methane to syngas intended as a preliminary conversion step in combustion systems such as power turbines and utility burners employing a fuel-rich fuel-lean approach to reduce NO x formation. Active components were impregnated on La–γ-Al 2O 3 washcoated honeycomb monoliths and the catalysts were characterised by BET, SEM/EDS, H 2-TPR, and in situ FT-IR under reaction conditions. Catalytic partial oxidation of methane was tested under both pseudo-isothermal and pseudo-adiabatic conditions showing that the process can be conducted with high yield and selectivity: improved and stable performances were found especially in the case of Rh–LaMnO 3 catalyst, due to the synergism between active sites and to the stabilization of the noble metal. 相似文献
12.
In this work, a kinetic model is constructed to simulate sulfur deactivation of the NO x storage performance of BaO/Al 2O 3 and Pt/BaO/Al 2O 3 catalysts. The model is based on a previous model for NO x storage under sulfur-free conditions. In the present model the storage of NO x is allowed on two storage sites, one for complete NO x uptake and one for a slower NO x sorption. The adsorption of SO x is allowed on both of these NO x storage sites and on one additional site which represent bulk storage. The present model is built-up of six sub-models: (i) NO x storage under sulfur-free conditions; (ii) SO 2 storage on NO x storage sites; (iii) SO 2 oxidation; (iv) SO 3 storage on bulk sites; (v) SO 2 interaction with platinum in the presence of H 2; (vi) oxidation of accumulated sulfur compounds on platinum by NO 2. Data from flow reactor experiments are used in the implementation of the model. The model is tested for simulation of experiments for NO x storage before exposure to sulfur and after pre-treatments either with SO 2 + O 2 or SO 2 + H 2. The simulations show that the model is able to describe the main features observed experimentally. 相似文献
13.
Performance of NO x traps after high-temperature treatments in different redox environments was studied. Two types of treatments were considered: aging and pretreatment. Lean and rich agings were examined for a model NO x trap, Pt–Ba/Al 2O 3. These were done at 950 °C for 3 h, in air and in 1% H 2/N 2, respectively. Lean aging had a severe impact on NO x trap performance, including HC and CO oxidation, and NH 3 and N 2O formation. Rich aging had minimal impact on performance, compared to fresh/degreened performance. Deactivation from lean aging was essentially irreversible due to Pt sintering, but Pt remained dispersed with the rich aging. Pretreatments were examined for a commercially feasible fully formulated NO x trap and two model NO x traps, Pt–Ba/Al 2O 3 and Pt–Ba–Ce/Al 2O 3. Pretreatments were done at 600 °C for 10 min, and used feed gas that simulated diesel exhaust under several conditions. Lean pretreatment severely suppressed NO x, HC, CO, NH 3 and N 2O activities for the ceria-containing NO x traps, but had no impact on Pt–Ba/Al 2O 3. Subsequently, a relatively mild rich pretreatment reversed this deactivation, which appears to be due to a form of Pt–ceria interaction, an effect that is well known from early work on three-way catalysts. Practical applications of results of this work are discussed with respect to NO x traps for light-duty diesel vehicles. 相似文献
14.
整体式催化剂性能高效、应用广泛,而制备方法是影响其性能的关键因素,因此探究简单高效的制备方法对整体式催化剂的工业应用至关重要。本研究以碳纤维布为载体,采用电沉积方法制备了一系列钴锰整体式催化剂,在固定床反应装置上考察了其催化氧化NO性能,并借助拉曼光谱仪、X射线衍射仪和扫描电镜等分析技术对催化剂进行表征。结果表明,采用电沉积法制备的钴锰二元催化剂比相同方法制备的氧化钴或氧化锰一元催化剂的粒径小,且钴锰比例对催化剂粒径影响很大,最小粒径为3~4nm。不同摩尔比例的钴锰二元催化剂焙烧之后物相差别较大,Mn/Co较高的催化剂的主要组成为Mn 3O 4,Mn/Co较低则趋向Co 3O 4,其中Mn/Co=2时,催化剂为(CoMn)(CoMn) 2O 4。电沉积法制备的钴锰整体式催化剂催化氧化NO的性能要远优于相同方法制备的氧化锰催化剂,在50℃下转化率均接近100%。 相似文献
15.
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
16.
Pt-based catalysts have been prepared using supports of different nature (γ-Al 2O 3, ZSM-5, USY, and activated carbon (ROXN)) for the C 3H 6-SCR of NO x in the presence of excess oxygen. Nitrogen adsorption at 77 K, pH measurements, temperature-programmed desorption of propene, and H 2 chemisorption were used for the characterization of the different supports and catalysts. The performance of these catalysts has been compared in terms of de-NO x activity, hydrocarbon adsorption and combustion at low temperature, and selectivity to N 2. Maximum NO x conversions for all the catalysts were achieved in the temperature range of 200–250°C. The order of activity was, Pt-USY>Pt/ROXNPt-ZSM-5Pt/Al 2O 3. At temperatures above 300°C only Pt/ROXN maintains a high activity caused by the consumption of the support, while the other catalysts present a strong deactivation. Propene combustion starts at the same temperature for all the catalytic systems (160°C). Complete hydrocarbon combustion is directly related to the acidity of the support, thus determining the temperature of the maximum NO x reduction. The support play an important role in the reaction mechanism through the hydrocarbon activation. N 2O formation was observed for all the catalysts. N 2 selectivity ranges from 15 to 30% with the order, Pt/ROXN>Pt-USYPt/Al 2O 3>Pt-ZSM-5. The catalytic systems exhibit a stable operation under isothermal conditions during time-on-stream experiments. 相似文献
17.
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
18.
The NO x storage behavior of a series of Pt-Ba/Al 2O 3 catalysts, prepared by wet impregnation of Pt/Al 2O 3 with Ba(Ac) 2, has been investigated. The catalysts with Ba loadings in the range 4.5–28 wt.% were calcined at 500 °C in air and subsequently exposed to NO pulses in 5 vol.% O 2/He atmosphere. Catalysts were characterized by means of thermogravimetry (TG) combined with mass spectroscopy (MS) and XRD before and after exposure to NO pulses. Characterization of the calcined catalysts corroborated the existence of three Ba-containing phases which are discernible based on their different thermal stability: BaO, LT-BaCO 3 and HT-BaCO 3. Characterization after NO x exposure showed that the different Ba-containing phases present in the catalysts possess different reactivity for barium nitrate formation, depending on their interfacial contact. The different Ba(NO 3) 2 species produced upon NO x exposure could be distinguished based on their thermal stability. The study revealed that during the NO x storage process a new thermally instable BaCO 3 phase formed by reaction of evolved CO 2 with active BaO. The fraction of Ba-containing species that were active in NO x storage depended on the Ba loading, showing a maximum at a Ba loading of about 17 wt.%. Lower and higher Ba loading resulted in a significant loss of the overall efficiency of the Ba-containing species in the storage process. The loss in efficiency observed at higher loading is attributed to the lower reactivity of the HT-BaCO 3, which becomes dominant at higher loading, and the increased mass transfer resistance. 相似文献
19.
The objective of this work was to study the promotional effect of Pt on Co-zeolite (viz. mordenite, ferrierite, ZSM-5 and Y-zeolite) and Co/Al 2O 3 on the selective catalytic reduction (SCR) of NO x with CH 4 under dry and wet reaction stream. After being reduced in H 2 at 350°C, the PtCo bimetallic zeolites showed higher NO to N 2 conversion and selectivity than the monometallic samples, as well as a combination of the latter samples such as mechanical mixtures or two-stage catalysts. After the same pretreatment, under wet reaction stream, the bimetallic samples were also more active. Among the other catalysts studied with 5% of water in the feed, (NO = CH 4 = 1000 ppm, O 2 = 2%), the NO conversion dropped to zero over Co 2.0Mor at 500°C and GHSV = 30,000 h −1, whereas it is 20% in Pt 0.5Co 2.0Mor. In Pt/Co/Al 2O 3 the NO x conversion dropped below 5% with only 2% of water under the same reaction conditions. The specific activity given as molecules of NO converted per total metal atom per second were 16.5 × 10 −4 s −1 for Pt 0.5Co 2.0Fer, 13 × 10 −4 s −1 for Pt 0.5Co 2.0Mor, 4.33 × 10 −4 s −1 for Pt 0.5Co 2.0ZSM-5 and 0.5 × 10 −4 s −1 for Pt/Co/Al 2O 3. The Y-zeolite-based samples were inactive in both mono and bimetallic samples. The species initially present in the solid were Pt° and Co°, together with Co 2+ and Pt 2+ at exchange positions. Co° seems not to participate as an active site in the SCR of NO x. Those species remained after the reaction but some reorganization occurred. A synergetic effect among the different species that enhances both the NO to NO 2 reaction, the activation of CH 4 and also the ability of the catalyst to adsorb NO, could be responsible for the high activity and selectivity of the bimetallic zeolites. 相似文献
20.
Co 3O 4/CeO 2 composite oxides with different cobalt loading (5, 15, 30, 50, 70 wt.% as Co 3O 4) were prepared by co-precipitation method and investigated for the oxidation of methane under stoichiometric conditions. Pure oxides, Co 3O 4 and CeO 2 were used as reference. Characterization studies by X-ray diffraction (XRD), BET, temperature programmed reduction/oxidation (TPR/TPO) and X-ray photoelectron spectroscopy (XPS) were carried out. An improvement of the catalytic activity and thermal stability of the composite oxides was observed with respect to pure Co3O4 in correspondence of Co3O4–CeO2 containing 30% by weight of Co3O4. The combined effect of cobalt oxide and ceria, at this composition, strongly influences the morphological and redox properties of the composite oxides, by dispersing the Co3O4 phase and promoting the efficiency of the Co3+–Co2+ redox couple. The presence in the sample Co3O4(30 wt.%)–CeO2 of a high relative amount of Ce3+/(Ce4+ + Ce3+) as detected by XPS confirms the enhanced oxygen mobility. The catalysts stability under reaction conditions was investigated by XRD and XPS analysis of the used samples, paying particular attention to the Co3O4 phase decomposition. Methane oxidation tests were performed over fresh (as prepared) and thermal aged samples (after ageing at 750 °C for 7 h, in furnace). The resistance to water vapour poisoning was evaluated for pure Co3O4 and Co3O4(30 wt.%)–CeO2, performing the tests in the presence of 5 vol.% H2O. A methane oxidation test upon hydrothermal ageing (flowing at 600 °C for 16 h a mixture 5 vol.% H2O + 5 vol.%O2 in He) of the Co3O4(30 wt.%)–CeO2 sample was also performed. All the results confirm the superiority of this composite oxide. 相似文献
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