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1.
MnO x–CeO 2 mixed oxides with a Mn/(Mn + Ce) molar ratios of 0–1 were prepared by a modified coprecipitation method and investigated for the complete oxidation of formaldehyde. The MnO x–CeO 2 with Mn/(Mn + Ce) molar ratio of 0.5 exhibited the highest catalytic activity among the MnO x–CeO 2 mixed oxides. Structure analysis by X-ray powder diffraction and temperature-programmed reduction of hydrogen revealed that the formation of MnO x–CeO 2 solid solution greatly improved the low-temperature reducibility, resulting in a higher catalytic activity for the oxidation of formaldehyde. Promoting effect of Pt on the MnO x–CeO 2 mixed oxide indicated that both the Pt precursors and the reduction temperature greatly affected the catalytic performance. Pt/MnO x–CeO 2 catalyst prepared from chlorine-free precursor showed extremely high activity and stability after pretreatment with hydrogen at 473 K. 100% conversion of formaldehyde was achieved at ambient temperature and no deactivation was observed for 120 h time-on-stream. The promoting effect of Pt was ascribed to enhance the effective activation of oxygen molecule on the MnO x–CeO 2 support. 相似文献
2.
Metal oxide/active carbon/ceramic (MO x/AC/C) monolithic catalysts were prepared by impregnation method for selective catalytic reduction (SCR) of NO x with NH 3 at low-temperature, and they also had been characterized by elemental analysis, N 2-BET, XRD, SEM and NO-TPD. The adsorption capability of the monolithic catalyst was greatly enhanced due to the attached active carbon. An ultrasonic treatment was used to improve the impregnation process, and which can increase their catalytic activities. More than 90% NO x conversion could be achieved over the Mn-based monolithic catalysts at low-temperature, and which could be improved further by doping Ce, from 30% to 78% at 100 °C. Mn–Fe–Ce and Mn–V–Ce monolithic catalysts had better tolerance to SO 2 than Mn or Mn–Ce monolithic catalysts. 相似文献
3.
Preliminary studies on a series of nanocomposite BaO–Fe ZSM-5 materials have been carried out to determine the feasibility of combining NO x trapping and SCR-NH 3 reactions to develop a system that might be applicable to reducing NO x emissions from diesel-powered vehicles. The materials are analysed for SCR-NH 3 and SCR-urea reactivity, their NO x trapping and NH 3 trapping capacities are probed using temperature programmed desorption (TPD) and the activities of the catalysts for promoting the NH 3 ads + NO/O 2 → N 2 and NO x ads + NH 3 → N 2 reactions are studied using temperature programmed surface reaction (TPSR). 相似文献
4.
A catalytic deSoot–deNO x system, comprising Pt and Ce fuel additives, a Pt-impregnated wall-flow monolith soot filter and a vanadia-type monolithic NH 3-SCR catalyst, was tested with a two-cylinder DI diesel engine. The soot removal efficiency of the filter was 98–99 mass% with a balance temperature (stationary pressure drop) of 315 °C at an engine load of 55%. The NO x conversion ranged from 40 to 73%, at a NH 3/NO x molar ratio of 0.9. Both systems were measured at a GHSV of 52 000 l/(l h). The maximum NO x conversion was obtained at 400 °C. The reason for the moderate deNO x performance is discussed. No deactivation was observed after 380 h time on stream. The NO x emission at high engine loads is around 15% lower than that of engines running without fuel additives. 相似文献
5.
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
6.
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
7.
MnO x and Sm–Mn catalysts were prepared with the coprecipitation method, and they showed excellent activities and sulfur resistances for the selective catalytic reduction of NO x by NH 3 between 50 and 300 °C in the presence of excess oxygen. 0.10Sm–Mn catalyst indicated better catalytic activity and sulfur resistance. Additionally, the Sm doping led to multi-aspect impacts on the phases, morphology structures, gas adsorption, reactions process, and specific surface areas. Therefore, it significantly enhances the NO conversion, N 2 selectivity, and sulfur resistance. Based on various experimental characterization results, the reaction mechanism of catalysts and the effect of SO 2 on the reaction process about the catalysts were extensively explored. For 0.10Sm–Mn catalyst, manganese sulfate and sulfur ammonium cannot be generated broadly under the influence of SO 2 and the amount of surface adsorbed oxygen. The Bronsted acid sites strengthen significantly due to the addition of SO 2, enhancing the sulfur resistance of the 0.10Sm–Mn catalyst. 相似文献
8.
The selective catalytic reduction of NO+NO 2 (NO x) at low temperature (180–230°C) with ammonia has been investigated with copper-nickel and vanadium oxides supported on titania and alumina monoliths. The influence of the operating temperature, as well as NH 3/NO x and NO/NO 2 inlet ratios has been studied. High NO x conversions were obtained at operating conditions similar to those used in industrial scale units with all the catalysts. Reaction temperature, ammonia and nitrogen dioxide inlet concentration increased the N 2O formation with the copper-nickel catalysts, while no increase was observed with the vanadium catalysts. The vanadium-titania catalyst exhibited the highest DeNO x activity, with no detectable ammonia slip and a low N 2O formation when NH 3/NO x inlet ratio was kept below 0.8. TPR results of this catalyst with NO/NH 3/O 2, NO 2/NH 3/O 2 and NO/NO 2/NH 3/O 2 feed mixtures indicated that the presence of NO 2 as the only nitrogen oxide increases the quantity of adsorbed species, which seem to be responsible for N 2O formation. When NO was also present, N 2O formation was not observed. 相似文献
9.
Two series of FeZSM-5 catalysts prepared from Na + and NH 4+ ZSM-5 precursors are studied in the selective reduction of NO x using NH 3 and urea as reducing agents. All Fe-containing catalysts are active for NO x reduction in the SCR-NH 3 reaction with ex-NH 4+ catalysts being more active than ex-Na + materials and the activity depending (to a minor extent within each series of catalysts) upon [Fe]. Catalysts with Bronstead acid sites also show a small transient deNO x activity at low temperatures. All catalysts are less active for the SCR-urea reaction but the ex-Na + catalysts retain far more deNO x activity than the ex-NH 4+ materials. NH 3 TPD shows that strongly binding Bronstead acid sites are present on the ex-NH 4+ materials and H +-treated parent zeolites while Urea TPD shows that the mode of decomposition of urea differs as a function of initial zeolite counter-ion. Urea TPSR shows that the reaction between adsorbed urea and gaseous NO/O 2 is related to [Fe]. It is proposed that the decreased activity of the ex-NH 4+ catalysts in the SCR-urea reaction is due to a less favourable mode of decomposition over these catalysts. Furthermore it is suggested that the Bronstead acidity plays some part in this less favoured decomposition. 相似文献
10.
采用柠檬酸络合浸渍法,通过对A位离子进行掺杂或取代和调节Ce的掺杂量,制备了一系列的负载型催化剂La 1-xA xMnO 3/赤铁矿(A=Ce、Co, x=0/0.1/0.2/0.3),并将催化剂应用于固定床NH 3-SCR脱硝。采用XRD、BET、XRF、H 2-TPR、NH 3-TPD等表征手段对催化剂进行分析和表征。结果表明,Ce的掺杂提高了催化剂LaMnO 3/赤铁矿的低温脱硝效率和拓宽了催化剂的活性温度窗口,如La 0.8Ce 0.2MnO 3/赤铁矿催化剂,在180℃时,脱硝效率就高达98%,且在150~250℃温度区间的脱硝效率均在90%以上。结合表征分析证明,稀土元素Ce的适量掺杂,明显提高了LaMnO 3/赤铁矿催化剂的氧化还原能力和催化剂表面NH 3分子的吸附能力;稀土元素Ce的适量掺杂也改善了催化剂比表面积,也使活性物质高度分散在载体表面上。 相似文献
11.
The current work is devoted to study of CO interaction with PdO/Al 2O 3–(Ce x–Zr 1−x)O 2 catalysts. Ceria–zirconia–alumina supports with different Ce/Zr ratio were prepared by sol–gel technique. The FT-IR characterization of CO adsorbed at −120 and 25 °C on oxidized and reduced samples revealed that Ce/Zr ratio modifies the surface properties of support and oxidation state of palladium. The catalyst with Ce/Zr molar ratio 0.5/0.5 was characterized with the highest ability to stabilize palladium in oxide state and the highest activity to oxidize CO. Redox treatment of catalysts improves their catalytic activity. 相似文献
12.
The Co–Mn–Al mixed oxide catalysts were prepared by thermal decomposition of hydrotalcite-like precursors with Co/(Mn + Al) molar ratio of 2 and Mn/Al molar ratio varying from 0 to 2. The obtained catalysts were characterized by powder XRD, XPS, BET surface area and TPR measurements and tested in N 2O decomposition. The most active Co4MnAl catalyst exhibited both the optimum Mn/Al molar ratio and the optimum amount of components reducible in the temperature region in which the catalytic reaction proceeds (350–450 °C). 相似文献
13.
The reaction between hydrogen and NO was studied over 1 wt.% Pd supported on NO x-sorbing material, MnO x–CeO 2, at low temperatures. The result of pulse mode reactions suggest that NO x adsorbed as nitrate and/or nitrite on MnO x–CeO 2 was reduced by hydrogen, which was spilt-over from Pd catalyst. The NO x storage and reduction (NSR) cycles were carried out over Pd/MnO x–CeO 2 in a conventional flow reactor at 150 °C. In a storage step, NO was removed by the oxidative adsorption from a stream of 0.04–0.08% NO, 5–10% O 2, and He balance. This was followed by a reducing step, where a stream of 1% H 2/He was supplied to ensure the conversion of nitrate/nitrite to N 2 and thus restore the adsorbability. It was revealed that the NSR cycle is much more suitable for the H 2–deNO x process in excess O 2, compared to a conventional steady state reaction mode. 相似文献
14.
The application of non-thermal electrical discharges instead of thermal energy has been shown to be a suitable alternative for the treatment of exhaust gases, especially for low concentrations (<100 ppmv) of contaminants. In the present paper, we describe the synergetic application of plasma and catalytic treatment for the oxidative removal of volatile organic compounds (VOCs) and the low-temperature conversion of NO x to N 2 in excess oxygen. The catalytic oxidation of butyl acetate and dichloroethene (DCE) as typical VOC has been studied at 50–200°C in combination with pulse corona or dielectric barrier discharges (DBDs). The best results with a significant synergetic effect were obtained with mixed transition metal oxide catalysts, which are able to decompose the ozone at low (<120°C) temperature. With respect to treatment by the plasma alone, the combination shifts the process towards total oxidation. The amount of reaction by-products such as formyl chloride (in case of chlorocarbons), CO, nitric oxides and ozone is lowered. The synergetic effect has been verified at technical conditions in the plasmacatalytic removal of VOC from stripper air in a groundwater cleaning plant. The NH3-SCR of NOx in excess oxygen using a NH4-loaded zeolite at 100°C can be significantly accelerated if combined with a plasma discharge. The mechanism of the reaction has been verified by labeling NO with 15NO and by a double-labeled experiment using 15NO and 15NH4-zeolite. Experiments with a diesel engine have shown that the plasmacatalytic conversion of NOx to N2 is also effective at real technical conditions. 相似文献
15.
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
16.
A series of Pt and Pt,Cu supported catalysts were prepared by wet impregnation of Mg–Al supports obtained from hydrotalcite-type (HT) precursor compounds. These novel NO x storage-reduction (NO xSR) catalysts show improved performances in NO x storage than Pt,Ba/alumina NO xSR catalysts at reaction temperatures lower than 200 °C. These catalysts show also improved resistance to deactivation by SO 2. The effect is attributed to the formation of well dispersed Mg(Al)O particles which show good NO x storage properties. The promoted low temperature activity is explained by the lower basicity of the Mg(Al)O mixed oxide in comparison to BaO, which induces on one hand a lower inhibition on Pt activity (NO to NO 2 oxidation and/or hydrocarbon oxidation) due to electronic effect, and on the other hand a lower thermal stability of the stored NO x. The presence of Cu slightly inhibits activity at low temperature, although improves activity and resistance to deactivation at 300 °C. On these catalysts FT-IR characterization evidences the formation of a Pt–Cu alloy after reduction. 相似文献
17.
Aluminium-doped mesoporous monolithic silica possessing fine mesopores has been prepared via the direct liquid crystal templating pathway using a non-ionic surfactant template and has been used as a support for cobalt-, copper-, and iron-based formulations in the selective catalytic reduction (SCR) of NO with ammonia in the presence of oxygen at low temperatures in the range of 373–723 K. The monolithic support was characterised by N 2 gas adsorption at 77 K, powder X-ray diffraction (XRD) and NH 3 adsorption at 373 K. Surface area, pore structure and surface acidity of the catalysts before and after being subjected to catalytic testing were determined, and good stability of pore structure and surface properties under SCR conditions was indicated. The NO conversions on aluminosilicate monolith-supported catalysts were compared with those observed on the reference catalyst, EUROCAT powder. The Co-functionalised catalysts appeared less relevant to DeNO x purposes. Two impregnated and two ion-exchanged catalysts containing copper and iron showed catalytic performance comparable to that of the reference catalyst. They produced only small amounts of undesired product N 2O, the Fe-containing formulations being even more selective than EUROCAT. The nature of metal species in these catalysts was investigated with the aid of Cu 2p X-ray photoelectron spectroscopy (XPS) and 57Fe Mössbauer spectroscopy. 相似文献
18.
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
19.
Cu–Mn mixed oxides were prepared by a co-precipitation method and applied for low temperature NO reduction with NH 3 in the presence of excess oxygen. Effects of [Cu]/[Mn] ratio and calcination temperatures on NOx conversions were investigated. Cu–Mn oxide catalysts containing small amounts of copper showed the complete NOx conversion in a wide range of reaction temperature from 323 to 473 K. This catalyst showed a reversible deactivation due to the presence of water vapor and SO 2. Different catalytic activities of Cu–Mn mixed oxides could be attributed mainly to surface areas and the crystalline nature. 相似文献
20.
The selective catalytic reduction (SCR) of nitrogen oxides (NO x) by propane in the presence of H 2 on sol–gel prepared Ag/Al 2O 3 catalysts (0.5–5 wt.% Ag) was investigated. It was confirmed that hydrocarbon-assisted SCR of NO x is remarkably enhanced by co-feeding hydrogen to a lean exhaust gas mixture (λ>1), attaining considerable activity within a wide temperature window (470–825 K). The samples had marginal activity at 575 K without co-fed H 2, but achieved up to 60% NO x conversion in the presence of H 2 at a space velocity of 30,000 h −1. NO 2 as NO x feed component is not converted to N 2 by C 3H 8 to a substantial extent under lean conditions. This points to an activation route of NO through direct conversion to adsorbed nitrite/nitrate or to a dissociation of NO over Ag 0, formed through short-term reduction by H 2. The nature of Ag species was characterized by X-ray diffraction, temperature-programmed reduction, pulse thermoanalytical measurements, electron microscopy and FTIR spectroscopy. It could be shown that Ag 2O nano-sized clusters are predominantly present on all samples, whereas formation of silver aluminate could not be confirmed. Nano-sized Ag 2O clusters can reversibly be reduced/reoxidized by H 2. A silver loading higher than 2 wt.% leads to a part of Ag 2O particles, which are thermally decomposed during calcination at 800 K or higher. The catalytic role of this metallic silver is still unclear. Formal kinetic analysis of catalytic data revealed that the activation energy of the overall reaction is significantly lowered in the presence of H 2. The presence of water does not change the activation energy. It is concluded that hydrogen reduces the nano-sized Ag 2O clusters to Ag 0 on a short-term scale. Zero-valent silver promotes a dissociation pathway of NO x conversion. The fact that more oxidized ad-species (nitrite/nitrate) are observed in the presence of H 2 is attributed to a dissociative activation of gas-phase oxygen on Ag 0. 相似文献
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