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1.
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
2.
The steady-and unsteady-state catalytic behaviour of Cu-MFI in the conversion of propane and NO in the presence of O 2 is reported, showing how the chemisorption and transformation of reactants may influence the surface reactivity. Various effects were observed: (i) a change in the surface reactivity and kinetics in going from low to high concentrations of NO or propane, (ii) the transformation of NO to N 2 and N 2O promoted at low temperature (250°C) by oxygen in the absence of hydrocarbon, (iii) the influence of NO over the surface reactivity of the catalyst in the conversion of propane and (iv) the influence of surface precoverage with oxidized nitrogen oxides (N xO y) or carboxylate species on the catalyst transient reactivity in the reduction of NO to N 2. In particular, Cu-MFI is initially more active when oxidized nitrogen oxides are present, suggesting that the active intermediate in the reduction of NO with propane is a complex formed by the reaction of nitrate with activated hydrocarbon. It is shown, however, that strongly bound oxidized nitrogen oxides may have also additional effects on the surface reactivity: (i) can promote the conversion of NO to N 2 and N 2O in transient conditions and (ii) can give a partial inhibition of the surface reactivity blocking copper ions due to their strong chemisorption. Furthermore, it is shown that NO reacts faster with oxygen than hydrocarbon forming N xO y species which are then the oxidizing agent for the hydrocarbon. It is thus suggested that the surface reactivity of Cu-MFI in the reduction of NO with propane/oxygen depends on the surface population of nitrogen oxide adspecies which influence not only the surface reactivity, but also the pathway of hydrocarbon oxidation. 相似文献
3.
SO 2 and NO emitted from coal-fired power plants have caused serious air pollution in China. In this study, a test system for NO oxidation using O 3 is established. The basic characteristics of NO oxidation and products forms are studied. A separate test system for the combined removal of SO 2 and NO x is also established, and the absorption characteristics of NO x are studied. The characteristics of NO oxidation and NO x absorption were verified in a 35 t·h -1 industrial boiler wet combined desulfurization and denitrification project. The operating economy of ozone oxidation wet denitrification technology is analyzed. The results show that O 3 has a high rate and strong selectivity for NO oxidation. When O 3 is insufficient, the primary oxidation product is NO 2. When O 3 is present in excess, NO 2 continues to get oxidized to N 2O 5 or NO 3. The removal efficiency of NO 2 in alkaline absorption system is low (only about 15%). NO x removal efficiency can be improved by oxidizing NO x to N 2O 5 or NO 3 by increasing ozone ratio. When the molar ratio of O 3/NO is 1.77, the NO x removal efficiency reaches 90.3%, while the operating cost of removing NO x per kilogram is 6.06 USD (NO 2). 相似文献
4.
采用厌氧/缺氧/好氧运行的序批式生物反应器(An/A/O-SBR),经不同NO 3-浓度(10,20,30和40 mg/L,以氮计)长期驯化,考察了不同NO 3-条件下An/A/O-SBR脱氮除磷及N 2O释放特性,基于不同微生物降解特性分析,确定了不同NO 3-浓度下SBR系统内反硝化聚磷菌(denitrifying phosphorus accumulating organisms,DPAOs)和聚糖菌(glycogen-accumulating organisms, GAOs)竞争关系。结果表明:随NO 3-浓度增加,总氮(TN)去除率由90%以上降至41.3%,TP去除率呈先增高后降低的趋势,N 2O产率(N 2O emission/NO x-removal)分别为1.68%、4.17%、8.92%和14.28%。An/A/O-SBR内微生物呈PAOs和GAOs共存的污染物降解特性,高浓度NO 3-缺氧吸磷过程出现NO 2-积累,抑制DPAOs活性,GAOs碳源竞争能力增强,NO 3--N由10 mg/L增至40 mg/L,厌氧阶段PAOs的COD耗量比例由33.5%降至25.1%,相应GAOs的COD耗量由59.3%增至74.1%。DPAOs-GAOs共生体系内,反硝化过程NO 2-/HNO 2积累耦合反硝化聚糖菌比例增加,加剧了高NO 3-下An/A/O-SBR内N 2O释放。 相似文献
5.
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
6.
The adsorption of CO at 130 K has been studied on Co-H-FER, Co-H-MFI and Co-H-MOR, as well as on Co-silica–alumina and on Co-containing mesoporous materials. Over Co-H-MFI also the adsorption of NO and of ammonia and the coadsorption of ortho-toluonitrile and CO have been investigated. The data show that on all samples Lewis acidic isolated Co 2+ species are predominant. However, small amounts of oxidizing sites, possibly Co 3+ and/or cobalt oxide particles also exist. This is shown by the oxidation of part of CO to CO 2 and of NO to NO + at very low temperature as well as by the formation of Co 3+ mononitrosyls upon adsorption of NO. These sites, although difficult to be evidenced by direct spectroscopic measurements, are likley the active sites for CH 4-SCR, where NO is activated as an adsorbed N xO y species able to react selectively with methane. 相似文献
7.
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
8.
以Al 2O 3、SiO 2和TiO 2为载体,采用凝胶-溶胶法和浸渍法制备铁基堇青石整体式催化剂,并对其丙烯选择性催化还原NO性能进行了研究。通过N 2物理吸附/脱附、XRD、SEM、H 2-TPR、Py-FTIR和原位DRIFTS技术对催化剂进行了表征。不同载体对催化剂的表面酸性、氧化还原性能、比表面积和表面形貌有显著影响,从而导致丙烯还原NO的催化活性明显差异。C 3H 6-SCR的催化活性按Fe/Al 2O 3/CM > Fe/SiO 2/CM > Fe/TiO 2/CM依次降低。在450℃的有氧条件下,在Fe/Al 2O 3/CM上催化C 3H 6还原NO效率可达到100%,这主要是因为较好的氧化还原性能和丰富的Lewis酸性位。基于原位的DRIFTS研究表明,Lewis酸性位的增加有助于促进形成NO 2/NO 3-物种,从而提高了催化性能。 相似文献
9.
The behavior of the selective catalytic reduction of nitrogen oxides (NO x) assisted by a dielectric barrier discharge was investigated. The principal function of the dielectric barrier discharge in the present system is to generate ozone, which is continuously fed to a chamber where the ozone and NO-rich exhaust gas (NO forms the large majority of NO x) are mixed. In the ozonization chamber, a part of NO contained in the exhaust gas is oxidized to NO 2, and then the mixture of NO and NO 2 enters the catalytic reactor. The ozonization method proposed in this study was found to be more energy-efficient for the oxidation of NO to NO 2 than the typical nonthermal plasma process. The degree of NO oxidation was approximately equal to the amount of ozone added to the exhaust gas, implying that the decomposition of ozone into molecular oxygen was relatively slow, compared to its reaction with NO. When the exhaust gas was first treated by ozone to produce a mixture of NO and NO 2, a remarkable enhancement in the catalytic reduction of nitrogen oxides was observed. Neither NO 3 nor N 2O 5 was formed in the present system, but small amounts of ozone and N 2O (less than 5 ppm) were detected in the outlet gas. 相似文献
10.
We present a systematic study of the NH 3-SCR reactivity over a commercial V 2O 5–WO 3/TiO 2 catalyst in a wide range of temperatures and NO/NO 2 feed ratios, which cover (and exceed) those of interest for industrial applications to the aftertreatment of exhaust gases from diesel vehicles. The experiments confirm that the best deNO x efficiency is achieved with a 1/1 NO/NO 2 feed ratio. The main reactions prevailing at the different operating conditions have been identified, and an overall reaction scheme is herein proposed. Particular attention has been paid to the role of ammonium nitrate, which forms rapidly at low temperatures and with excess NO2, determining a lower N2 selectivity of the deNOx process. Data are presented which show that the chemistry of the NO/NO2–NH3 reacting system can be fully interpreted according to a mechanism which involves: (i) dimerization/disproportion of NO2 and reaction with NH3 and water to give ammonium nitrite and ammonium nitrate; (ii) reduction of ammonium nitrate by NO to ammonium nitrite; (iii) decomposition of ammonium nitrite to nitrogen. Such a scheme explains the peculiar deNOx reactivity at low temperature in the presence of NO2, the optimal stoichiometry (NO/NO2 = 1/1), and the observed selectivities to all the major N-containing products (N2, NH4NO3, HNO3, N2O). It also provides the basis for the development of a mechanistic kinetic model of the NO/NO2–NH3 SCR reacting system. 相似文献
11.
NO conversion to N 2 in the presence of methane and oxygen over 0.03 at.%Rh/Al 2O 3, 0.51 at.%Pt/Al 2O 3 and 0.34 at.%Pt–0.03 at.%Rh/Al 2O 3 catalysts was investigated. δ-Alumina and precious metal–aluminum alloy phases were revealed by XRD and HRTEM in the catalysts. The results of the catalytic activity investigations, with temperature-programmed as well as steady-state methods, showed that NO decomposition occurs at a reasonable rate on the alloy surfaces at temperatures up to 623 K whereas some CH4 deNOx takes place on δ-alumina above this temperature. A mechanism for the NO decomposition is proposed herein. It is based on NO adsorption on the precious metal atoms followed by the transfer of electrons from alloy to antibonding π orbitals of NO(ads.) molecules. The CH4 deNOx was shown to occur according to an earlier proposed mechanism, via methane oxidation by NO2(ads.) to oxygenates and then NO reduction by oxygenates to N2. 相似文献
12.
The selective catalytic reduction (SCR) of NO x assisted by propene is investigated on Pd/Ce 0.68Zr 0.32O 2 catalysts (Pd/CZ), and is compared, under identical experimental conditions, with that found on a Pd/SiO 2 reference catalyst. Physico-chemical characterisation of the studied catalysts along with their catalytic properties indicate that Pd is not fully reduced to metallic Pd for the Pd/CZ catalysts. This study shows that the incorporation of Pd to CZ greatly promotes the reduction of NO in the presence of C 3H 6. These catalysts display very stable deNO x activity even in the presence of 1.7% water, the addition of which induces a reversible deactivation of about 10%. The much higher N 2 selectivity obtained on Pd/CZ suggests that the lean deNO x mechanism occurring on these catalysts is different from that occurring on Pd 0/SiO 2. A detailed mechanism is proposed for which CZ achieves both NO oxidation to NO 2 and NO decomposition to N 2, whereas PdO x activates C 3H 6 via ad-NO 2 species, intermediately producing R-NO x compounds that further decompose to NO and C xH yO z. The role of the latter oxygenates is to reduce CZ to provide the catalytic sites responsible for NO decomposition. The proposed C 3H 6-assisted NO decomposition mechanism stresses the key role of NO 2, R-NO x and C xH yO z as intermediates of the SCR of NO x by hydrocarbons. 相似文献
13.
Photocatalysis of a hollandite compound K xGa xSn 8−xO 16 ( x = ca. 1.8) was examined for the reduction of nitrate ion with a reducing agent of methanol in water under UV irradiation. Hollandites have a characteristic one-dimensional tunnel structure. The hollandite powder, which was prepared by the sol–gel method and unloaded with any additives like metals, was used as the photocatalyst and its photocatalytic reaction was analyzed quantitatively by using ion chromatography and on-line mass spectrometry, and its reaction mechanism was analyzed by in-situ FT-IR. The hollandite photocatalyst showed a significant activity for the formation of N 2 from NO 3−. The nitrate was reduced to N 2 and NO 2−, while the reducing agent methanol was partly oxidized to change to formic acid. The conversion of NO 3−was proportional to the yields of N 2, NO 2−, and HCOO −. The present photocatalyzed decomposition of NO 3− to N 2 would be a useful photocatalysis for the environmental protection of water. 相似文献
14.
The adsorption of HCN on, its catalytic oxidation with 6% O 2 over 0.5% Pt/Al 2O 3, and the subsequent oxidation of strongly bound chemisorbed species upon heating were investigated. The observed N-containing products were N 2O, NO and NO 2, and some residual adsorbed N-containing species were oxidized to NO and NO 2 during subsequent temperature programmed oxidation. Because N-atom balance could not be obtained after accounting for the quantities of each of these product species, we propose that N 2 and was formed. Both the HCN conversion and the selectivity towards different N-containing products depend strongly on the reaction temperature and the composition of the reactant gas mixture. In particular, total HCN conversion reaches 95% above 250 °C. Furthermore, the temperature of maximum HCN conversion to N 2O is located between 200 and 250 °C, while raising the reaction temperature increases the proportion of NO x in the products. The co-feeding of H 2O and C 3H 6 had little, if any effect on the total HCN conversion, but C 3H 6 addition did increase the conversion to NO and decrease the conversion to NO 2, perhaps due to the competing presence of adsorbed fragments of reductive C 3H 6. Evidence is also presented that introduction of NO and NO 2 into the reactant gas mixture resulted in additional reaction pathways between these NO x species and HCN that provide for lean-NO x reduction coincident with HCN oxidation. 相似文献
15.
Structural (XRD) and spectroscopic (EPR, IR and Raman) investigations were performed to elucidate the influence of CeO 2 content on the phase composition and surface chemistry of Ce xZr 1−xO 2 solid solutions ( x = 0.10–0.85), interacting with NO and NO 2 in the absence and presence of oxygen. Strong influence of ceria loading on the adsorption modes of both nitrogen oxides and the nature of the resultant surface species was revealed. Adsorption of NO led to formation of mononitrosyl complexes, dimers and N 2O, whereas interaction of NO 2 with the ceria–zirconia catalyst resulted in the adsorbate disproportionation or coupling, depending on the sample composition. 相似文献
16.
A systematic reactivity study of N 2O, NO, and NO 2 on highly dispersed CuO phases over modified silica supports (SiO 2–Al 2O 3, SiO 2–TiO 2, and SiO 2–ZrO 2) has been performed. Different reaction paths for the nitrogen oxide species abatement were studied: from direct decomposition (N 2O) to selective reductions by hydrocarbons (N 2O, NO, and NO 2) and oxidation (NO to NO 2). The oxygen concentration, temperature, and contact time, were varied within suitable ranges in order to investigate the activity and in particular the selectivity in the different reactions studied. The support deeply influenced the catalytic properties of the active copper phase. The most acidic supports, SiO 2–Al 2O 3 and SiO 2–ZrO 2, led to a better activity and selectivity of CuO for the reactions of N 2O, NO, and NO 2 reductions and N 2O decomposition than SiO 2–TiO 2. The catalytic results are discussed in terms of actual turnover frequencies starting from the knowledge of the copper dispersion values. 相似文献
17.
The NO, NO/O 2, and NO/O 2/H 2O adsorption on MnO 2/NaY (5 and 15 wt.% MnO 2) composite catalyst and NaY has been studied by means of in situ FTIR and EPR spectroscopy at elevated temperatures and during heating under reaction-like conditions. NO adsorption and co-adsorption of NO and O 2 on NaY and MnO 2/NaY proceeds via oxidation of NO forming NO 2− and NO 3− species. Whereas the manganese dioxide preferably acts as oxidising agent, the zeolite stores the NO x species as nitrite and nitrate ions in the solid. In the presence of oxygen, the nitrate formation is enhanced due to additional oxidation of NO through gaseous oxygen leading to NO 2. Dimerisation of NO 2 to N 2O 4 and following disproportionation of the latter causes the formation of NO + and NO 3− species which are associated with nucleophilic zeolitic oxygen and especially alkali cations of the zeolite, respectively. The presence of oxygen facilitates reoxidation of Mn 2+ which keeps more Mn ions in the active state. Pre-adsorbed water and higher amounts of water vapour in the feed hinder the NO adsorption by blocking the adsorption sites and shift the nitrate formation to higher temperatures. The quantities and thermal stability of the nitrates formed during NO and NO/O 2 adsorption differs which points to a different mechanism of nitrate formation. In the absence of gaseous oxygen, nitrates are formed by participation of only lattice oxygen. In the presence of oxygen, nitrate formation by dimerisation and disproportionation reactions of NO 2 dominates. The manganese component of the composite catalyst supports the oxidation of NO to nitrite and subsequently to nitrate. During this process Mn 4+ is reduced to Mn 2+ as evidenced by in situ EPR measurements. 相似文献
18.
NO x reduction with NO 2 as the NO x gas in the absence of plasma was compared to plasma treated lean NO x exhaust where NO is converted to NO 2 in the plasma. Product nitrogen was measured to prove true chemical reduction of NO x to N 2. With plasma treatment, NO as the NO x gas, and a NaY catalyst, the maximum conversion to nitrogen was 50% between 180 and 230 °C. The activity decreased at higher and lower temperatures. At 130 °C a complete nitrogen balance could be obtained, however between 164 and 227 °C less than 20% of the NO x is converted to a nitrogen-containing compound or compounds not readily detected by gas chromatograph (GC) or Fourier transform infrared spectrometer (FT-IR) analysis. With plasma treatment, NO 2 as the NO x gas, and a NaY catalyst, a complete nitrogen balance is obtained with a maximum conversion to nitrogen of 55% at 225 °C. For γ-alumina, with plasma treatment and NO2 as the NOx gas, 59% of the NOx is converted to nitrogen at 340 °C. A complete nitrogen balance was obtained at these conditions. As high as 80% NOx removal over γ-alumina was measured by a chemiluminescent NOx meter with plasma treatment and NO as the NOx gas. When NO is replaced with NO2 and the simulated exhaust gases are not plasma treated, the maximum NOx reduction activity of NaY and γ-alumina decreases to 26 and 10%, respectively. This is a large reduction in activity compared to similar conditions where the simulated exhaust was plasma treated. Therefore, in addition to NO2, other plasma-generated species are required to maximize NOx reduction. 相似文献
19.
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
20.
In the off-gases of internal combustion engines running with oxygen excess, non-thermal plasmas (NTPs) have an oxidative potential, which results in an effective conversion of NO to NO 2. In combination with appropriate catalysts and ammonia (NH 3-SCR) or hydrocarbons (HC-SCR) as a reducing agent, this can be utilized to reduce nitric oxides (NO and NO 2) synergistically to molecular nitrogen. The combination of SCR and cold plasma enhanced the overall reaction rate and allowed an effective removal of NOX at low temperatures. Using NH3 as a reducing agent, NOX was converted to N2 on zeolites or NH3-SCR catalysts like V2O5–WO3/TiO2 at temperatures as low as 100–200 °C. Significant synergetic effects of plasma and catalyst treatment were observed both for NH3 stored by ion exchange on the zeolite and for continuous NH3 supply. Certain modifications of Al2O3 and ZrO2 have been found to be effective as catalysts in the plasma-assisted HC-SCR in oxygen excess. With an energy supply of about 30 eV/NO-molecule, 500 ppm NO was reduced by more than half at a temperature of 300 °C and a space velocity of 20 000 h−1 at the catalyst. The synergistic combinations of NTP and both NH3- and HC-SCR have been verified under real diesel engine exhaust conditions. 相似文献
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