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1.
The reduction of nitrogen oxides by propene in the presence of air under net oxidising conditions has been studied for two Cu/alumina catalysts of low (1%) and high (5%) copper loadings in a flow microreactor and by DRIFT. The reaction was studied in the range 473–773 K using mixtures of 2.5% NO, 1% C 3H 6 and 10% O 2 with a balance of nitrogen or helium, using samples which were pretreated in air at 673 K and also over samples which had been pre-exposed to SO 2 at 473 K. The surface species present under reaction conditions have been identified and the sensitivity of their adsorption sites in the two different loaded catalysts to SO 2 pre-treatment has been investigated. SO 2 adsorption enhanced NO adsorption at 473 K in the absence of oxygen and, in reaction, enhanced formation of NCO species leading to increased levels of adsorbed CO as a decomposition product. 相似文献
2.
The reduction of nitrogen monoxide by propene on V 2O 5/ZrO 2 doped with or without calcium has been studied by FTIR spectroscopy as well as by analysis of the reaction products. Considerable promoting effect of calcium doping on the reduction of nitrogen monoxide by propene was observed on the V 2O 5/ZrO 2 catalysts. For the reaction of a mixture of NO+C 3H 6, carbonyl and carboxylate species were observed above 373 K, although nitrate species formed at room temperature on V 2O 5/ZrO 2 doped with calcium. No bands due to a compound including both carbon and nitrogen atoms were observed. Thus, the redox mechanism, i.e. propene reduces the catalyst and nitrogen monoxide oxidizes the catalyst, is confirmed on V 2O 5/ZrO 2 catalysts doped with or without calcium. The analysis of the V=O band in the region of 1100–900 cm −1 indicates that this promotion is mainly due to new V=O species formed by the addition of calcium onto the catalyst. This species is easily reproduced in comparison with the other V=O species on the surface in the reoxidation process of the catalyst. 相似文献
3.
Perovskite-type oxide La 0.6Ce 0.4CoO 3 and its doped Ag catalysts were prepared and their catalytic performances were evaluated for the direct decomposition of NO and the selective reduction of NO with propene in the presence of oxygen. A noticeable enhancement in activity was achieved by doping Ag and the optimum Ag loading was 1%. The effects of H 2O, SO 2, CO 2 and O 2 on the performances of Ag/La 0.6Ce 0.4CoO 3 catalysts for NO decomposition were also investigated. The resistance against H 2O and SO 2 appears satisfactory. The inhibition by CO 2 is strong, although it is reversible. Oxygen did not inhibit the NO decomposition reaction but significantly promoted it. Compared with other perovskite-type oxides reported previously, higher conversions were obtained over the present catalysts for the NO reduction by propene. We speculate that the decomposition of NO is the predominant process even in the presence of propene. The catalysts were characterized by N 2-adsorption, XRD, XPS and NO-TPD and some explanations were put forward. 相似文献
4.
Molybdenum impregnated HZSM-5 zeolite catalysts with MoO 3 loading from 1 to 8 wt.% were studied in detail for the selective catalytic reduction (C 2H 2-SCR) of NO by acetylene. A 83.9% of NO could be removed by the reductant at 350 °C under 1600 ppm of NO, 800 ppm of C 2H 2 and 9.95% of O 2 in He over 2%MoO 3/HZSM-5 catalyst with a specific activity of in NO elimination and the competitiveness factor (c.f.) of 33.6% for the reductant. The NO elimination level and the c.f. value were ca. 3–4 times as high as those using methane or propene as reductant over the catalyst in the same reaction condition. About same reaction rate was estimated in NO oxidation as that in the NO reduction over each xMoO 3/HZSM-5 ( x = 0–8%) catalyst, which confirms that NO 2 is a crucial intermediate for the aimed reaction over the catalysts. Appropriate amount of Mo incorporation to HZSM-5 considerably enhanced the title reaction, both by accelerating the intermediate formation and by strengthening the adsorption NO x on the catalyst surface under the reaction conditions. Rather lower adsorption tendency of acetylene compared with propene on the catalysts explains the catalyst's steady performance in the C 2H 2-SCR of NO and rapid deactivation in the C 3H 6-SCR of NO. 相似文献
5.
WO 3/Nb 2O 5-supported samples prepared by impregnation are characterised by X-ray diffraction (XRD), Raman spectroscopy and X-ray absorption spectroscopy (XAS) at the W–L 3 absorption edge, as well as temperature programmed reduction (TPR) and FT-IR monitoring of pyridine adsorption. Results are compared with those obtained for WO 3/Al 2O 3 samples prepared in the same conditions, showing that niobia is able to disperse tungsta better than alumina does. Formation of a crystalline WO 3 needs larger tungsten contents on niobia than on alumina, since tungsten solution into niobia is easier than into alumina. Raman and XAS spectra recorded under ambient conditions suggest that similar WO x species are formed on both supports at tungsten contents 0.5–1 theoretical monolayers; however, TPR results for the low tungsten loaded samples indicate that, when reduction starts (always at temperatures higher than 700 K under H 2/Ar flow) there is a larger concentration of tetrahedral [WO 4] species on alumina, than on niobia. Samples with low tungsten loading have been tested in isopropanol decomposition and ethylene oxidation, following both processes by FT-IR of adsorbed species up to 673 K. Results show that adsorption of ethylene on WO 3/Nb 2O 5 yields acetaldehyde and acetate at 473 K, while this adsorption is non-reactive either on the supports or on WO 3/Al 2O 3. Isopropanol adsorbs dissociatively on both supports, leading to acetone and propene formation on tungsta–niobia, but only propene on tungsta–alumina, probably due to the larger reducibility of the tungsten-containing phases. 相似文献
6.
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
7.
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
8.
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
9.
The reaction mechanism of the reduction of NO by propene over Pd-based catalysts was studied by FTIR spectroscopy. It was observed that the reaction between NO and propene most probably goes via isocyanate (2256–2230 cm −1), nitrate (1310–1250 cm −1) and acetate (1560 and 1460 cm −1) intermediates formation. Other possible intermediates such as partially oxidized hydrocarbons, NO 2, and formates were also detected. The reaction between nitrates and acetates or carbonates reduced nitrates to N 2 and oxidized carbon compounds to CO 2. In situ DRIFT provides quick and rather easily elucidated data from adsorbed compounds and reaction intermediates on the catalyst surface. The activity experiments were carried out to find out the possible reaction mechanism and furthermore the kinetic equation for NO reduction by propene. 相似文献
10.
NO removal using CH 4 as a reductant in a dual-bed system has been investigated with Co-NaX and Ag-NaX catalysts, which were prepared by Co 2+-, Ag +-ion exchange into zeolite NaX, respectively, and activation for 5 h at 500 °C. The experimental result has been compared with that of a Co-NaX-CO catalyst, additionally pre-treated under CO flow for the Co-NaX catalyst. The cobalt crystal structure of a Co-NaX-CO catalyst is Co 3O 4, which promotes NO oxidation to NO 2 by excess O 2 at a low temperature (523 K). The mechanical mixture of Co-NaX-CO and Ag-NaX catalysts shows a synergy effect on NO reduction to N 2 by CH 4 in the presence of excess O 2 and H 2O, but the NO reduction decreases quickly as time passes. However, the NO reduction to N 2 in a deNO bed at 523 K and a deNO 2 bed at 423 K, which are relatively lower than the reaction temperatures for common SCR systems, still remained at 67% even in a H 2O 10% gas mixture after 160 min. 相似文献
11.
The development of a catalytically active filter element for combined particle separation and NO x removal or VOC total oxidation, respectively, is presented. For NO x removal by selective catalytic reduction (SCR) a catalytic coating based on a TiO 2–V 2O 5–WO 3 catalyst system was developed on a ceramic filter element. Different TiO 2 sols of tailor-made mean particle size between 40 and 190 nm were prepared by the sol–gel process and used for the impregnation of filter element cylinders by the incipient wetness technique. The obtained TiO 2-impregnated sintered filter element cylinders exhibit BET surface areas in the range between 0.5 and 1.3 m 2/g. Selected TiO 2-impregnated filter element cylinders of high BET surface area were catalytically activated by impregnation with a V 2O 5 and WO 3 precursor solution. The obtained catalytic filter element cylinders show high SCR activity leading to 96% NO conversion at 300 °C, a filtration velocity of 2 cm/s and an NO inlet concentration of 500 vol.-ppm. The corresponding differential pressures fulfill the requirements for typical hot gas filtration applications. For VOC total oxidation, a TiO 2-impregnated filter element support was catalytically activated with a Pt/V 2O 5 system. Complete oxidation of propene with 100% selectivity to CO 2 was achieved at 300 °C, a filtration velocity of 2 cm/s and a propene inlet concentration of 300 vol.-ppm. 相似文献
12.
A detailed temperature-programmed desorption (TPD) study on NO and O 2 saturated Cu-ZSM-5 at different temperatures (300–723 K) has been performed. In the temperature range 373–723 K, the evolution of O 2 and NO 2 accompanying the desorption of NO from NO saturated Cu-ZSM-5 suggested the formation of nitrite/nitrate species. The amount of O 2 absorbed was very much lower than that of NO. The desorption profile of O 2 after contacting Cu-ZSM-5 with O 2 at 623 K showed a low temperature peak (369K) confirming the spontaneous ability of O 2 desorption from copper zeolite. Moreover, successive saturation cycles of NO followed by O 2 and vice versa have been performed at various temperatures (298–623 K) to understand the modifications which the adsorption sites undergo when the two molecules NO and O 2 are available together for adsorption on the catalyst sites. After each saturation cycle, a TPD profile was recorded following the evolution of NO, O 2 and other NO x species. The competitive adsorption experiments revealed that, at 623 K, NO was not able to successfully compete with O 2 for the adsorption sites, therefore the adsorption of NO at 623 K on O 2 saturated catalyst was not completely restored. On the basis of the experimental work, an overall adsorption reaction scheme of NO on Cu-ZSM-5 was proposed 相似文献
13.
In this work the catalytic behaviour of pure zinc manganite, ZnMn 2O 4, and cobalt–zinc manganites for the reduction of NO by propane and propene is reported. The NO and N 2O decomposition as well as the reduction of N 2O by propane and propene were also investigated. The catalysts are prepared starting from carbonate monophasic precursors that are decomposed in air at 973 K for 24 h. In all cases a spinel-like phase is obtained. Pure zinc manganite is an efficient catalyst for the NO reduction with both propane and propene and the selectivity to N 2 and CO 2 was almost one. However the presence of cobalt in the catalyst enhances the catalytic activity, in particular when propene is used as reducing agent of NO. All catalysts are stable up to 873 K upon contacting with the propane containing reactant stream whereas in the case of propene they preserve the original spinel structure up to about 773 K. In fact with propene the catalysts start to lose their stability as the reaction temperature increases above 773 K and disaggregate, by reduction of the spinel framework Mn 3+ cations to Mn 2+, forming a complex mixture of ZnO and MnO oxides. Despite the collapsing of the spinel phase, the disaggregated polyphasic catalysts still show a good activity and selectivity. An hypothesis for explaining this unusual behaviour is formulated. Finally, the reaction mechanisms presented in literature are consequently revisited on the basis of the results found in this work. 相似文献
14.
An In 2O 3/Al 2O 3 catalyst shows high activity for the selective catalytic reduction of NO with propene in the presence of oxygen. The presence of SO 2 in feed gas suppressed the catalytic activity dramatically at high temperatures; however it was enhanced in the low temperature range of 473–573 K. In TPD and FT-IR studies, the formation of sulfate species on the surface of the catalyst caused an inhibition of NO X adsorption sites, and the absorbance ability of NO was suppressed by the presence of SO 2, and the amount of ad-NO 3− species decreased obviously. This leads to a decrease of catalytic activity at higher temperatures. However, addition of SO 2 enhanced the formation of carboxylate and formate species, which can explain the promotional effect of SO 2 at low temperature, because active C 3H 6 (partially oxidized C 3H 6) is crucial at low temperature. 相似文献
15.
Reticular oxygen of Al 2O 3 or CeO x supported on Al 2O 3 was used for the epoxidation of propene without any double bond cleavage. In batch reaction, Al 2O 3 alone was able to convert propene into propene oxide (PO) with 100% selectivity and 2% conversion of propene with a close to 3:1 ratio with respect to the number of Al(III) reduced to elemental Al. When Ce 2O 3/Al 2O 3 or CeO 2/Al 2O 3 was used, Al remained in its +3 oxidation state, while the Ce oxide was the oxidant as demonstrated by XPS analyses. CeO x/Al 2O 3 was more active (propene conversion yield of 4–5%) but the selectivity was lower (70%) as PO was isomerized into acetone and propionaldehyde. Interestingly the use of reticular oxygen very much improves the selectivity with respect to the use of pure O2. In fact, while propene was more efficiently oxidized (10%) with O2 in presence of Al2O3 or CeOx/Al2O3, the selectivity was as low as 40% because C1 and C2 products were formed. However, the use of reticular oxygen represents a selective two-step technique for the use of molecular oxygen as oxidant of propene. The used oxides can be re-oxidized and the whole process can be further improved towards higher yields. PO is quantitatively converted into propene carbonate by reaction with CO2 in presence of Nb2O5. 相似文献
16.
The influences of calcination temperatures and additives for 10 wt.% Cu/γ-Al 2O 3 catalysts on the surface properties and reactivity for NO reduction by C 3H 6 in the presence of excess oxygen were investigated. The results of XRD and XPS show that the 10 wt.% Cu/γ-Al 2O 3 catalysts calcined below 973 K possess highly dispersed surface and bulk CuO phases. The 10 wt.% Cu/γ-Al 2O 3 and 10 wt.% Mn–10 wt.% Cu/γ-Al 2O 3 catalysts calcined at 1073 K possess a CuAl 2O 4 phase with a spinel-type structure. In addition, the 10 wt.% La–10 wt.% Cu/γ-Al 2O 3 catalyst calcined at 1073 K possesses a bulk CuO phase. The result of NO reduction by C 3H 6 shows that the CuAl 2O 4 is a more active phase than the highly dispersed and bulk CuO phase. However, the 10 wt.% Mn–10 wt.% Cu/γ-Al 2O 3 catalyst calcined at 1073 K possesses significantly lower reactivity for NO reduction than the 10 wt.% Cu/γ-Al 2O 3 catalyst calcined at 1073 K, although these catalysts possess the same CuAl 2O 4 phase. The low reactivity for NO reduction for 10 wt.% Mn–10 wt.% Cu/γ-Al 2O 3 catalyst calcined at 1073 K is attributed to the formation of less active CuAl 2O 4 phase with high aggregation and preferential promotion of C 3H 6 combustion to CO x by MnO 2. The engine dynamometer test for NO reduction shows that the C 3H 6 is a more effective reducing agent for NO reduction than the C 2H 5OH. The maximum reactivity for NO reduction by C 3H 6 is reached when the NO/C 3H 6 ratio is one. 相似文献
17.
The monooxides copper, manganese, molybdenum and chromium catalysts supported on MgF 2 were tested in NO decomposition and reduction by propene. The effect of the oxides content, time on stream and O 2 concentration in reaction mixture during NO reduction on their catalytic activity was investigated. All the catalysts showed the optimum active phase concentration corresponding to 2–4 wt.% of the metal. For the best copper catalyst an effect of introduction of another oxide (manganese or chromium oxide) on the catalytic performance was studied. The double copper-manganese oxide sample containing 2 wt.% Cu and 4 wt.% Mn was proved to ensure the best catalytic performance. 相似文献
18.
On an anodic alumina supported silver catalyst with a low Ag loading (1.68 wt.%), NO x (NO/He, NO/O 2/He, NO 2/He) adsorption measurements and NO x-temperature programmed decomposition (TPD)/temperature programmed surface-reaction (TPSR) measurements in different gas streams (He, C 3H 6/He, C 3H 6/O 2/He) were conducted to investigate the formation, consumption and reactivity of surface adsorbed NO x species. During NO adsorption, no noticeable uptake of NO was detected. Introducing oxygen greatly improved the formation of ads-NOx species. A greater quantity of surface nitrate species was found after NO2 adsorption, accompanied with gaseous NO release. The result of TPSR demonstrates the surface nitrate species can be effectively and preferentially reduced by propene. When introducing oxygen into the propene gas stream of TPSR test, the significantly increased amount of reacted nitrate undoubtedly shows the importance of oxygen in activating propene. The pathway for the selective reduction of NOx in the presence of excess oxygen is proposed to pass through the selective reduction of the adsorbed nitrate species with the activated propene. The enhanced NOx conversion when replacing NO with NO2 was attributed to the stronger NOx adsorption capacity and oxidation ability of NO2, than those for NO. With increasing oxygen concentration, the difference between NO and NO2 would gradually decrease, and finally disappear in a high excess of oxygen. 相似文献
19.
基于构建的Na-K-C-H-O-N-Cl化学反应机理模型,采用Chemkin动力学模拟软件,研究Na/K添加剂(NaOH、Na 2CO 3、NaCl、KOH、K 2CO 3和KCl)对选择性非催化还原(SNCR)脱硝性能影响,通过敏感性分析和产率分析,探讨Na/K添加剂对SNCR过程中NO还原的促进机理和路径。模拟结果表明,在温度为700~800℃且无Na/K添加剂条件下,SNCR脱硝效率几乎为零;Na/K添加剂能够显著提高低温区(小于800℃)SNCR脱硝效率,而其对高温区(大于900℃)SNCR脱硝的促进作用不明显。在温度为700℃和Na/K添加剂参与条件下SNCR脱硝效率可达43.86%~60.76%。不同Na/K添加剂对NO还原促进顺序为NaOH≈Na 2CO 3 > KOH≈K 2CO 3 > KCl > NaCl,但同一种Na/K添加剂的浓度变化(6.25~25.0 μmol·mol -1)对SNCR脱硝效率影响较小。Na/K添加剂通过不同的循环路径产生OH基,进而通过NH2基团促进NO的还原,其中碱金属氢氧化物(MOH)对SNCR脱硝的促进路径为NaOH→NaO 2→Na→NaO→NaOH,碱金属氯化物(MCl)则主要通过MCl→M→MCl削弱Na/K添加剂的促进作用。 相似文献
20.
The effect of tungsten and barium on the thermal stability of V 2O 5/TiO 2 catalyst for NO reduction by NH 3 was examined over a fixed bed flow reactor system. The activity of V 2O 5/sulfated TiO 2 catalyst gradually decreased with respect to the thermal aging time at 600 °C. The addition of tungsten to the catalyst surface significantly enhanced the thermal stability of V 2O 5 catalyst supported on sulfated TiO 2. On the basis of Raman and XRD measurements, the tungsten on the catalyst surface was identified as suppressing the progressive transformation of monomeric vanadyl species into crystalline V 2O 5 and of anatase into rutile phase of TiO 2. However, the NO removal activity of V 2O 5/sulfated TiO 2 catalyst including barium markedly decreased after a short aging time, 6 h at 600 °C. This may be due to the transformation of vanadium species to inactive V–O–Ba compound by the interaction with BaO which was formed by the decomposition of BaSO 4 on the catalyst surface at high reaction temperature of 600 °C. The addition of SO 2 to the feed gas stream could partly restore the NO removal activity of thermally aged V 2O 5/sulfated TiO 2 catalyst containing barium. 相似文献
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