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2.
Sharp NO and O 2 desorption peaks, which were caused by the decomposition of nitro and nitrate species over Fe species, were observed in the range of 520–673 K in temperature-programmed desorption (TPD) from Fe-MFI after H 2 treatment at 773 K or high-temperature (HT) treatment at 1073 K followed by N 2O treatment. The amounts of O 2 and NO desorption were dependent on the pretreatment pressure of N 2O in the H 2 and N 2O treatment. The adsorbed species could be regenerated by the H 2 and N 2O treatment after TPD, and might be considered to be active oxygen species in selective catalytic reduction (SCR) of N 2O with CH 4. However, the reaction rate of CH 4 activation by the adsorbed species formed after the H 2 and N 2O or the HT and N 2O treatment was not so high as that of the CH 4 + N 2O reaction over the catalyst after O 2 treatment. The simultaneous presence of CH 4 and N 2O is essential for the high activity of the reaction, which suggests that nascent oxygen species formed by N 2O dissociation can activate CH 4 in the SCR of N 2O with CH 4. 相似文献
3.
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
4.
The catalytic reduction of N 2O by CH 4, CO, and their mixtures has been comparatively investigated over steam-activated FeZSM-5 zeolite. The influence of the molar feed ratio between N 2O and the reducing agents, the gas-hourly space velocity, and the presence of O 2 on the catalytic performance were studied in the temperature range of 475–850 K. The CH 4 is more efficient than CO for N 2O reduction, achieving the same degree of conversion at significantly lower temperatures. The apparent activation energy for N 2O reduction by CH 4 was very similar to that of direct N 2O decomposition (140 kJ mol −1), being much lower for the N 2O reduction by CO (60 kJ mol −1). This suggests that the reactions have a markedly different mechanism. Addition of CO using equimolar mixtures in the ternary N 2O + CH 4 + CO system did not affect the N 2O conversion with respect to the binary N 2O + CH 4 system, indicating that CO does not interfere in the low-temperature reduction of N 2O by CH 4. In the ternary system, CO contributed to N 2O reduction when methane was the limiting reactant. The conversion and selectivity of the reactions of N 2O with CH 4, CO, and their mixtures were not altered upon adding excess O 2 in the feed. 相似文献
5.
A multi-component NO x-trap catalyst consisting of Pt and K supported on γ-Al 2O 3 was studied at 250 °C to determine the roles of the individual catalyst components, to identify the adsorbing species during the lean capture cycle, and to assess the effects of H 2O and CO 2 on NO x storage. The Al 2O 3 support was shown to have NO x trapping capability with and without Pt present (at 250 °C Pt/Al 2O 3 adsorbs 2.3 μmols NO x/m 2). NO x is primarily trapped on Al 2O 3 in the form of nitrates with monodentate, chelating and bridged forms apparent in Diffuse Reflectance mid-Infrared Fourier Transform Spectroscopy (DRIFTS) analysis. The addition of K to the catalyst increases the adsorption capacity to 6.2 μmols NO x/m 2, and the primary storage form on K is a free nitrate ion. Quantitative DRIFTS analysis shows that 12% of the nitrates on a Pt/K/Al 2O 3 catalyst are coordinated on the Al 2O 3 support at saturation. When 5% CO2 was included in a feed stream with 300 ppm NO and 12% O2, the amount of K-based nitrate storage decreased by 45% after 1 h on stream due to the competition of adsorbed free nitrates with carboxylates for adsorption sites. When 5% H2O was included in a feed stream with 300 ppm NO and 12% O2, the amount of K-based nitrate storage decreased by only 16% after 1 h, but the Al2O3-based nitrates decreased by 92%. Interestingly, with both 5% CO2 and 5% H2O in the feed, the total storage only decreased by 11%, as the hydroxyl groups generated on Al2O3 destabilized the K–CO2 bond; specifically, H2O mitigates the NOx storage capacity losses associated with carboxylate competition. 相似文献
6.
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
7.
In this paper, the effect of CO 2 and H 2O on NO x storage and reduction over a Pt–Ba/γ-Al 2O 3 (1 wt.% Pt and 30 wt.% Ba) catalyst is shown. The experimental results reveal that in the presence of CO 2 and H 2O, NO x is stored on BaCO 3 sites only. Moreover, H 2O inhibits the NO oxidation capability of the catalyst and no NO 2 formation is observed. Only 16% of the total barium is utilized in NO storage. The rich phase shows 95% selectivity towards N 2 as well as complete regeneration of stored NO. In the presence of CO 2, NO is oxidized into NO 2 and more NO x is stored as in the presence of H 2O, resulting in 30% barium utilization. Bulk barium sites are inactive in NO x trapping in the presence of CO 2·NH 3 formation is seen in the rich phase and the selectivity towards N 2 is 83%. Ba(NO 3) 2 is always completely regenerated during the subsequent rich phase. In the absence of CO 2 and H 2O, both surface and bulk barium sites are active in NO x storage. As lean/rich cycling proceeds, the selectivity towards N 2 in the rich phase decreases from 82% to 47% and the N balance for successive lean/rich cycles shows incomplete regeneration of the catalyst. This incomplete regeneration along with a 40% decrease in the Pt dispersion and BET surface area, explains the observed decrease in NO x storage. 相似文献
8.
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
9.
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
10.
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
11.
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
12.
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
13.
NH 3 stored on zeolites in the form of NH 4+ ions easily reacts with NO to N 2 in the presence of O 2 at temperatures <373 K under dry conditions. Wet conditions require a modification of the catalyst system. It is shown that MnO 2 deposited on the external surface of zeolite Y by precipitation considerably enhances the NO x conversion by zeolite fixed NH 4+ ions in the presence of water at 400–430 K. Particle-size analysis, temperature-programmed reduction, textural characterization, chemical analysis, ESR and XRD gave a subtle picture of the MnO 2 phase structure. The MnO 2 is a non-stoichiometric, amorphous phase that contains minor amounts of Mn 2+ ions. It loses O 2 upon inert heating up to 873 K, but does not crystallize or sinter. The phase is reducible by H 2 in two stages via intermediate formation of Mn 3O 4. The manufacture of extrudates preserving stored NH 4+ ions for NO x reduction is described. It was found that MnO 2 can oxidize NO by bulk oxygen. This enables the reduction of NO to N 2 by the zeolitic NH 4+ ions without gas-phase oxygen for limited time periods. The composite catalyst retains storage capacity for both, oxygen and NH 4+ ions despite the presence of moisture and allows short-term reduction of NO without gaseous O 2 or additional reductants. The catalyst is likewise suitable for steady-state DeNO x operation at higher space velocities if gaseous NH 3 is permanently supplied. 相似文献
14.
NO x sorption and reduction capacities of 12-tungstophosphoric acid hexahydrate (H 3PW 12O 40·6H 2O, HPW) were measured under representative alternating conditions of lean and rich exhaust-type gas mixture. Under lean conditions, the sorption of NO x is large and is equivalent to 37 mg of NO x/g HPW. Although a part of these NO x remains unreduced, HPW is able to reduce some of the NO x to produce N 2 by a reaction between the sorbed NO 2 and hydrocarbon (HC), but this process is slow. The addition of 1% Pt affects strongly the chemical behaviour occurring during the course of a rich operation. The NO desorption observed at the beginning of the rich phase is strongly accelerated. The direct correlation between NO 2 consumption and CO 2 production shows that the principal pathway is the reaction CO+NO 2→CO 2+NO. In a mixture of reducing gas (CO, HC, H 2), the competition is strongly in favour of CO though in its absence the reaction observed was the hydrogenation of propene to propane. 相似文献
15.
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
16.
The release and reduction of NO x in a NO x storage-reduction (NSR) catalyst were studied with a transient reaction analysis in the millisecond range, which was made possible by the combination of pulsed injection of gases and time resolved time-of-flight mass spectrometry. After an O 2 pulse and a subsequent NO pulse were injected into a pellet of the Pt/Ba/Al 2O 3 catalyst, the time profiles of several gas products, NO, N 2, NH 3 and H 2O, were obtained as a result of the release and reduction of NO x caused by H 2 injection. Comparing the time profiles in another analysis, which were obtained using a model catalyst consisting of a flat 5 nmPt/Ba(NO 3) 2/cordierite plate, the release and reduction of NO x on Pt/Ba/Al 2O 3 catalyst that stored NO x took the following two steps; in the first step NO molecules were released from Ba and in the second step the released NO was reduced into N 2 by H 2 pulse injection. When this H 2 pulse was injected in a large amount, NO was reduced to NH 3 instead of N 2. A only small amount of H2O was detected because of the strong affinity for alumina support. We can analyze the NOx regeneration process to separate two steps of the NOx release and reduction by a detailed analysis of the time profiles using a two-step reaction model. From the result of the analysis, it is found that the rate constant for NOx release increased as temperature increase. 相似文献
17.
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
18.
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
19.
The nature of the adsorbed species on Cu-ZSM-5 (Cu-Z), Cu-Mordenite (Cu-M), and Cu-Y-zeolite (Cu-Y) was investigated by means of temperature programmed desorption (TPD). When dinitrogen monoxide (N 2O) came into contact with Cu-zeolites above 573 K, the decomposition of N 2O occurred accompanied by the formation of adsorbed oxygen species and adsorbed nitrogen oxide species. In the TPD runs, three O 2 desorption peaks appeared at temperatures of 623, 673, and 753 K and were named -, β-, and γ-peaks, respectively. The O 2 desorption at the - and γ-peaks became quickly saturated after contacting N 2O at 598 K, while the amount of O 2 desorbed at the β-peak increased with time, not reaching a constant level until 120 min of exposure. The activity for the decomposition of N 2O decreased with the accumulation of β-oxygen over the catalyst. The rate of N 2O decomposition depended upon the nature and amount of the copper zeolite catalysts available, as determined by the formation of - and/or β-oxygen. 相似文献
20.
Transient behaviour of catalytic monolith converter with NO x storage is studied under conditions typical for automobiles with lean-burn engines (i.e., diesel and advanced gasoline ones). Periodical alternation of inlet concentrations is applied—NO x are adsorbed on the catalyst surface during a long reductant-lean phase (2–3 min) and then reduced to N 2 within a short reductant-rich phase (2–6 s). Samples of industrial NO x storage and reduction catalyst of NM/Ba/CeO 2/γ-Al 2O 3 type (NM = noble metal), washcoated on 400 cpsi cordierite substrate, are used in the study. Effects of the rich-phase length and composition on the overall NO x conversions are examined experimentally. Reduction of NO x by CO, H 2 and unburned hydrocarbons (represented by C 3H 6) in the presence of CO 2 and H 2O is considered. Effective, spatially 1D, heterogeneous mathematical model of catalytic monolith with NOx and oxygen storage capacity is described. The minimum set of experiments needed for the evaluation of relevant reaction kinetic parameters is discussed: (i) CO, H2 and HC oxidation light-off under both lean and rich conditions, including inhibition effects, (ii) NO/NO2 transformation, (iii) NOx storage, including temperature dependence of effective NOx storage capacity, (iv) water gas shift and steam reforming under rich conditions, i.e., in situ production of hydrogen, (v) oxygen storage and reduction, including temperature dependence of effective oxygen storage capacity, and (vi) NOx desorption and reduction under rich conditions. The experimental data are compared with the simulation results. 相似文献
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