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1.
The interaction of sulfur dioxide with a commercial NO x storage-reduction catalyst (NSR) has been investigated using in situ IR and X-ray absorption spectroscopy. Two pathways of catalyst deactivation by SO 2 were identified. Under lean conditions (exposure to SO 2 and O 2) at 350 °C the storage component forms barium sulfates, which transform from surface to hardly reducible bulk sulfate species. The irreversible blocking of the Ba sites led to a decrease in NO x storage capacity. Under fuel rich conditions (SO 2/C 3H 6) at 350–500 °C evidence for the formation of sulfides on the oxidation/reduction component (Pt) of the catalyst was found, which blocks the metal surface and thus hinders the further reduction of the sulfides. 相似文献
2.
The simultaneous adsorption of SO 2 and NO x on Na-γ-alumina was studied by means of step experiments in a fixed bed plug flow reactor at 387 K and atmospheric pressure. Typically the molar composition of the feed gas was 1.5% SO 2, 1% O 2, 4000 ppm NO, 500 ppm NO 2, and Ar. First the adsorption behavior of the pure components was measured. SO 2 and NO 2 adsorb easily, whereas NO and O 2 do not adsorb. Moreover there is no influence of O 2 on the adsorption behavior of the pure components. NO and O2 adsorption require the simultaneous presence of SO2, NO, and O2. The NO and O2 adsorption rate is enhanced by an increasing SO2/NO ratio. The total amount of SO2 adsorbed is not affected by the simultaneous adsorption of NO and O2. However, NO2 adsorption increases the SO2 adsorption capacity. In the presence of NO2 most of the adsorbed NOx is released as NO. 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.
This study addresses the catalytic reaction of NO x and soot into N 2 and CO 2 under O 2-rich conditions. To elucidate the mechanism of the soot/NO x/O 2 reaction and particularly the role of the catalyst -Fe 2O 3 is used as model sample. Furthermore, a series of examinations is also made with pure soot for reference purposes. Temperature programmed oxidation and transient experiments in which the soot/O 2 and soot/NO reaction are temporally separated show that the NO reduction occurs on the soot surface without direct participation of the Fe 2O 3 catalyst. The first reaction step is the formation of CC(O) groups that is mainly associated with the attack of oxygen on the soot surface. The decomposition of these complexes leads to active carbon sites on which NO is adsorbed. Furthermore, the oxidation of soot by oxygen provides a specific configuration of active carbon sites with suitable atomic orbital orientation that enables the chemisorption and dissociation of NO as well as the recombination of two adjacent N atoms to evolve N 2. Moreover, carbothermal reaction, high resolution transmission electron microscopy and isotopic studies result in a mechanistic model that describes the role of the Fe 2O 3 catalyst. This model includes the dissociative adsorption of O 2 on the iron oxide, surface migration of the oxygen to the contact points of soot and catalyst and then final transfer of O to the soot. Moreover, our experimental data suggest that the contact between both solids is maintained up to high conversion levels thus resulting in continuous oxygen transfer from catalyst to soot. As no coordinative interaction of soot and Fe 2O 3 catalyst is evidenced by diffuse reflectance infrared Fourier transform spectroscopy a van der Waals type interaction is supposed. 相似文献
5.
The emissions of CO 2, NO x and SO 2 from the combustion of a high-volatile coal with N 2- and CO 2-based, high O 2 concentration (20, 50, 80, 100%) inlet gases were investigated in an electrically heated up-flow-tube furnace at elevated gas temperatures (1123–1573 K). The fuel equivalence ratio, φ, was varied in the range of 0.4–1.6. Results showed that CO 2 concentrations in flue gas were higher than 95% for the processes with O 2 and CO 2-based inlet gases. NO x emissions increased with φ under fuel-lean conditions, then declined dramatically after φ=0.8, and the peak values increased from about 1000 ppm for the air combustion process and 500 ppm for the O 2(20%)+CO 2(80%) inlet gas process to about 4500 ppm for the oxygen combustion process. When φ>1.4 the emissions decreased to the same level for different O 2 concentration inlet gas processes. On the other hand, NO x emission indexes decreased monotonically with φ under both fuel-lean and fuel-rich combustion. SO 2 emissions increased with φ under fuel-lean conditions, then declined slightly after φ>1.2. Temperature has a large effect on the NO x emission. Peak values of the NO x emission increased by 50–70% for the N 2-based inlet gas processes and by 30–50% for the CO 2-based inlet gas process from 1123 to 1573 K. However, there was only a small effect of temperature on the SO 2 emission. 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.
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
8.
The activity of several catalysts are studied in the soot combustion reaction using air and NO/air as oxidising agents. Over Al 2O 3-supported catalysts NO (g) is a promoter for the combustion reaction with the extent of promotion depending on the Na loading. Over these catalysts SO 42− poisons this promotion by preventing NO oxidation through a site blocking mechanism. SiO 2 is unable to adsorb NO or catalyse its oxidation and over SiO 2-supported Na catalysts NO (g) inhibits the combustion reaction. This is ascribed to a competition between NO and O 2. Over Fe-ZSM-5 catalysts the presence of a NO x trapping component does not increase the combustion of soot in the presence of NO (g) and it is proposed that this previously reported effect is only seen under continuous NO x trap operation as NO 2 is periodically released during regeneration and thus available for soot combustion. Experiments during which the [NO] (g) is varied show that CO, rather than an adsorbed carbonyl-like intermediate, is formed upon reaction between NO 2 (the proposed oxygen carrier) and soot. 相似文献
9.
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
10.
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
11.
Free energy minimization calculations are used to determine the thermodynamic equilibrium concentrations of NO x and other species in stoichiometric and lean gas mixtures over a range of temperatures and compositions. Under lean (excess N 2 and O 2) conditions, the NO decomposition (NO↔(1/2)N 2+(1/2)O 2) and NO oxidation (NO+(1/2)O 2↔NO 2) equilibria impose lower bounds on the NO x concentrations achievable by thermodynamic equilibration or NO x decomposition, and these equilibrium NO x concentrations can be practically significant. Assuming a perfect isothermal catalyst acting on a representative diesel exhaust stream collected over the federal test procedure (FTP) cycle, equilibrium NO x levels exceed upcoming California Low Emission Vehicle II (LEV-II) and Tier II NO x emissions standards for automobiles and trucks at temperatures above approximately 800 K. Consideration of a perfect adiabatic catalyst acting on the same diesel exhaust shows that equilibrium NO x values can fall below NO x emissions standards at lower temperatures, but to achieve these low concentrations would require the catalyst to attain 100% approach to equilibrium at very low temperatures. It is concluded that NO x removal based on a thermodynamic equilibrating catalyst under lean exhaust conditions is not practically viable for automotive application, and that to achieve upcoming NO x standards will require selective NO x catalysts that vigorously promote NO x reactions with reductant and do not promote NO decomposition or oxidation. Finally, the ability of a selective NO x catalyst system to reduce NO x concentrations to or below thermodynamic equilibrium values is proposed as a useful measure for selective catalytic reduction (SCR) activity. 相似文献
12.
Catalytic NO x reduction by carbon supporting transition metals (Fe, Co, Ni, Cu) and potassium has been studied. The effect of oxygen on the catalytic properties of the metals has been analyzed. Temperature-programmed reactions and isothermal reactions have been conducted in a fixed bed flow reactor. Temperature-programmed reduction in hydrogen, XRD and XPS have been used to characterize the catalysts. All the metals studied catalyze the NO x reduction by carbon in the presence of oxygen, but also the O 2–carbon reaction. Metal catalytic activity is the result of two factors, the tendency of the metal to be oxidized by NO and the easiness of the resulting oxide to be reduced by carbon. Among the metals studied, nickel exhibits the highest selectivity for NO x reduction. The results of this study strengthen the possible benefit of the lack of a gaseous reducing agent (such as ammonia or hydrocarbons) since the reduction of NOx is performed by the carbon support itself. 相似文献
13.
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
14.
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
15.
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
16.
Pt-USY was used for the selective catalytic reduction of NO x with hydrocarbons in the presence of excess oxygen. The catalyst was prepared by an ion-exchange method and characterized by XRD, TEM, CO chemisorption, and Ar adsorption at 87 K. The platinum particle size distribution was found to be broad (2–20 nm), with no apparent sintering of the active phase during the HC-SCR process after 25 h time-on-stream. Generally, large metal clusters (>15 nm) are situated at the external surface of the zeolite, while the smaller ones are located in the pores of the support. Pt-USY shows an excellent activity in the deNO x reaction (molar NO x conversion 90% at 475 K) with propene as the reductant in 5 kPa O 2, as well as stable operation during time-on-stream. Propane only yields a low NO x conversion compared to propene. The presence of high oxygen contents (5–10 kPa O 2) slightly inhibits the reaction. No significant decrease in deNO x activity was observed at high space velocities (up to 100,000 h −1). The presence of SO 2 and H 2O in the feed stream did not significantly affect the deNO x activity. Pt-USY performs better under lean-burn conditions than other Pt-catalysts supported on e.g. ZSM-5, Al 2O 3, or SiO 2. The selectivity to N 2 was similar to the other Pt-based catalysts (30%), the other major product being N 2O. 相似文献
17.
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
18.
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
19.
In this work, we investigated the activity and stability of Ag–alumina catalysts for the SCR of NO with methane in gas streams with a high concentration of SO 2, typical of coal-fired power plant flue gases. Ag–alumina catalysts were prepared by coprecipitation–gelation, and dilute nitric-acid solutions were used to remove weakly bound silver species from the surface of the as prepared catalysts after calcination. SO 2 has a severe inhibitory effect, essentially quenching the CH 4-SCR reaction on this type catalysts at temperatures <600 °C. SO 2 adsorbs strongly on the surface forming aluminum and silver sulfates that are not active for CH 4-SCR of NO x. Above 600 °C, however, the reaction takes place without catalyst deactivation even in the presence of 1000 ppm SO 2. The reaction light-off coincides with the onset of silver sulfate decomposition, indicating the critical role of silver in the reaction mechanism. SO 2 is reversibly adsorbed on silver above 600 °C. While alumina sites remain sulfated, this does not hinder the reaction. Sulfation of alumina only decreases the extent of adsoption of NO x, but adsorption of NO x is not the limiting step. Methane activation is the limiting step, hence the presence of sulfur-free Ag–O–Al species is a requirement for the reaction. Strong adsorption of SO 2 on Ag–alumina decreases the rates of the reaction, and increases the activation energies of both the reduction of NO to N 2 and the oxidation of CH 4, the latter more than the former. Our results indicate partial contribution of gas phase reactions to the formation of N 2 above 600 °C. H 2O does not inhibit the reaction at 625 °C, and the effect of co-addition of H 2O and SO 2 is totally reversible. 相似文献
20.
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
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