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1.
Conversion of NO x with reducing agents H 2, CO and CH 4, with and without O 2, H 2O, and CO 2 were studied with catalysts based on MOR zeolite loaded with palladium and cerium. The catalysts reached high NO x to N 2 conversion with H 2 and CO (>90% conversion and N 2 selectivity) range under lean conditions. The formation of N 2O is absent in the presence of both H 2 and CO together with oxygen in the feed, which will be the case in lean engine exhaust. PdMOR shows synergic co-operation between H 2 and CO at 450–500 K. The positive effect of cerium is significant in the case of H 2 and CH 4 reducing agent but is less obvious with H 2/CO mixture and under lean conditions. Cerium lowers the reducibility of Pd species in the zeolite micropores. The catalysts showed excellent stability at temperatures up to 673 K in a feed with 2500 ppm CH 4, 500 ppm NO, 5% O 2, 10% H 2O (0–1% H 2), N 2 balance but deactivation is noticed at higher temperatures. Combining results of the present study with those of previous studies it shows that the PdMOR-based catalysts are good catalysts for NO x reduction with H 2, CO, hydrocarbons, alcohols and aldehydes under lean conditions at temperatures up to 673 K. 相似文献
2.
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
3.
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
4.
The influence of NO 2 on the selective catalytic reduction (SCR) of NO with ammonia was studied over Fe-ZSM5 coated on cordierite monolith. NO 2 in the feed drastically enhanced the NO x removal efficiency (DeNOx) up to 600 °C, whereas the promoting effect was most pronounced at the low temperature end. The maximum activity was found for NO 2/NO x = 50%, which is explained by the stoichiometry of the actual SCR reaction over Fe-ZSM5, requiring a NH 3:NO:NO 2 ratio of 2:1:1. In this context, it is a special feature of Fe-ZSM5 to keep this activity level almost up to NO 2/NO x = 100%. The addition of NO 2 to the feed gas was always accompanied by the production of N 2O at lower and intermediate temperatures. The absence of N 2O at the high temperature end is explained by the N 2O decomposition and N 2O-SCR reaction. Water and oxygen influence the SCR reaction indirectly. Oxygen enhances the oxidation of NO to NO 2 and water suppresses the oxidation of NO to NO 2, which is an essential preceding step of the actual SCR reaction for NO 2/NO x < 50%. DRIFT spectra of the catalyst under different pre-treatment and operating conditions suggest a common intermediate, from which the main product N 2 is formed with NO and the side-product N 2O by reaction with gas phase NO 2. 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 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
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.
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
9.
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
10.
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
11.
The selective catalytic reduction of nitrogen oxides (NO x) with ammonia over ZSM-5 catalysts was studied with and without water vapor. The activity of H-, Na- and Cu-ZSM-5 was compared and the result showed that the activity was greatly enhanced by the introduction of copper ions. A comparison between Cu-ZSM-5 of different silica to alumina ratios was also performed. The highest NO conversion was observed over the sample with the lowest silica to alumina ratio and the highest copper content. Further studies were performed with the Cu-ZSM-5-27 (silica/alumina = 27) sample to investigate the effect of changes in the feed gas. Oxygen improves the activity at temperatures below 250 °C, but at higher temperatures O 2 decreases the activity. The presence of water enhances the NO reduction, especially at high temperature. It is important to use about equal amounts of nitrogen oxides and ammonia at 175 °C to avoid ammonia slip and a blocking effect, but also to have high enough concentration to reduce the NO x. At high temperature higher NH 3 concentrations result in additional NO x reduction since more NH 3 becomes available for the NO reduction. At these higher temperatures ammonia oxidation increases so that there is no ammonia slip. Exposing the catalyst to equimolecular amounts of NO and NO 2 increases the conversion of NO x, but causes an increased formation of N 2O. 相似文献
12.
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
13.
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
14.
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
15.
The nitric acid industry is a source of both NO x and N 2O. The simultaneous selective catalytic reduction of both compounds using propane as a reductant has been investigated. A stacked catalyst bed with first a Co-ZSM-5 catalyst and second a Pd/Fe-ZSM-5 catalyst gives >80% conversion of N 2O and NO x above 300 °C at atmospheric pressure. At 4 bar absolute pressure (bara) the Co-ZSM-5 DeNO x catalyst shows higher NO x and propane conversion. This leaves not enough propane for the Pd/Fe-ZSM-5 DeN 2O catalyst, which causes a ‘dip’ in N 2O conversion. Reducing the space velocity (SV) of the first catalyst bed secures high NO x and N 2O conversions from 300 °C and up at 4 bara. 相似文献
16.
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
17.
Effect of the loading amount of Fe over ion-exchanged Fe-MFI catalysts on the catalytic performance of N 2O reduction with NH 3 was investigated, and the results indicated that the turnover frequency (TOF) was almost constant in the Fe/Al range between 0.05 and 0.40. The activity of N 2O + NH 3 reaction was much lower than that of N 2O + CH 4 reaction over Fe-MFI (Fe/Al = 0.40), and the preadsorption of NH 3 decreased drastically the activity of N 2O + CH 4 reaction. The temperature-programmed desorption (TPD) of NH 3 showed the formation of stronger acid sites on Fe-MFI (Fe/Al = 0.24 and 0.40), and the amount of the acid sites agrees well with the desorption amount O 2 in O 2-TPD in the low temperature range. The acid sites gave a 3610 cm −1 peak (Brønsted acid) in FTIR observation. These results suggest that the acid sites were formed on the bridge oxide ions in binuclear Fe species. Adsorbed NH 3 on the strong acid sites inhibited N 2O dissociation, which can be related to the low activity of N 2O + NH 3 reaction over Fe-MFI with high Fe loading. 相似文献
18.
This paper is a report of angle-resolved product desorption measurements in the course of catalyzed NO and N 2O reduction on Pd(1 1 0). Surface-nitrogen removal processes show different angular distributions, i.e. normally directed N 2 desorption takes place in process (i) 2N(a) → N 2(g). Highly inclined N 2 desorption towards the [0 0 1] direction is induced in process (ii) N 2O(a) → N 2(g) + O(a). N 2O or NH 3 desorption follows the cosine distribution characterizing the desorption after the thermalization in process (iii) N 2O(a) → N 2O(g) or (iv) N(a) + 3H(a) → NH 3(a) → NH 3(g). Thus, a combination of the angular and velocity distributions provides the analysis of most of surface-nitrogen removal processes in the course of catalyzed NO reduction. At temperatures below 600 K, processes (ii) and (iii) dominate and process (iv) is enhanced at H2 pressures higher than NO. Process (i) contributes significantly above 600 K. Only three processes except for NH3 formation are operative when CO is used. Only process (ii) was observed in a steady-state N2O + CO (or H2) reaction. 相似文献
19.
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
20.
The selective reduction of NO x over H-mordenite (H-m) was studied using CH 3OH as reducing agent. Results are compared with those obtained with other conventional reducing agents (ethylene and methane), with gas-phase reactions, and with other metal-exchanged mordenites (Cu-mordenite (Cu-m) and Co-mordenite (Co-m)). H-m was found to be an effective catalyst for the SCR of NO x with CH 3OH. When different reducing agents were compared over H-m, CH 3OH > C 2H 4 > CH 4 was the order according to the maximum NO conversion obtained using 1% of oxygen in the feed. Instead, if selectivity is considered, the order results CH 4 > CH 3OH > C 2H 4. In reaction experiments, two distinct zones defined by two maxima with NO to N 2 conversion are obtained at two different temperatures. A correlation exists between the said zones and the CO : CO 2 ratio. At low temperatures, CO prevails whereas at high temperatures CO 2 prevails. These results indicate that there exist different reaction intermediates. Evidence from reaction experiments, FTIR results, and transient experiments suggest that the reaction mechanism involves formaldehyde and dimethyl ether (DME) as intermediates in the 200–500°C temperature range. The surface interaction between CH 3OH (or its decomposition products) and NO is negligible if compared with NO 2, indicating that the oxidation of NO to NO 2 on acid sites is a fundamental path in this system. Different from other non-oxygenated reductants (methane and ethylene), a gas-phase NO x initiation effect on hydrocarbon combustion was not observed. 相似文献
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