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1.
Reaction mechanism of the reduction of nitrogen monoxide by methane in an oxygen excess atmosphere (NO–CH 4–O 2 reaction) catalyzed by Pd/H-ZSM-5 has been studied at 623–703 K in the absence of water vapor, in comparison with the mechanism for Co-ZSM-5. Kinetic isotope effect for the N 2 formation in NO–CH 4–O 2 vs. NO–CD 4–O 2 reactions was 1.65 at 673 K and decreased with a decrease in the reaction temperature. In addition, H–D isotopic exchange took place significantly in NO–(CH 4+CD 4)–O 2 reaction. These results are in marked contrast with the case of Co-ZSM-5, for which the C–H dissociation of methane is the only rate-determining step, and show that the C–H dissociation is slow but not the only rate-determining step in the case of Pd/H-ZSM-5. A reaction scheme was proposed, in which the relative rates of the three steps ((i)–(iii) below) vary depending on the reaction conditions. Further, in contrast to Co-ZSM-5, NO x–CH 4–O 2 reaction was much slower than CH 4–O 2 reaction for Pd/H-ZSM-5; the presence of NO x retards the reaction of CH 4 over the latter catalyst, while it accelerates the reaction over the former. It is suggested that CH 4 is activated directly by the Pd atoms in the case of Pd/H-ZSM-5, but by NO 2 strongly adsorbed on Co ion for Co-ZSM-5. The reaction order of the NO–CH 4–O 2 reaction with respect to NO pressure was consistent with this mechanism; 1.05 for Pd/H-ZSM-5 and 0.11 for Co-ZSM-5. 相似文献
2.
In order to meet the stringent regulatory norms of NO x and CO emitted by automobiles, reduction of these pollutants has become an intense field of research. Various catalysts like Pt, Rh, Ir, Cu, and Fe have been found to possess high activity for the reduction of NO. However, the available detailed surface reaction mechanisms are not satisfactory in clarifying all the aspects of the simultaneous reduction of NO and oxidation of CO. Here we have developed a quantitative surface reaction mechanism based on elementary steps, in order to comprehend the phenomena of catalytic reduction of NO by CO. Eleven elementary steps are proposed for the NO–CO and NO–CO–O 2 systems on Pt group catalysts. The elementary reaction mechanism is coupled with the continuously stirred tank reactor/packed bed reactor models and the simulation results are validated against literature experiments for the NO–CO reaction on Pt, and the NO–CO–O 2 reaction on Ir catalyst. Despite the simplicity, the CSTR model is able to capture the observed phenomena well on Pt and Ir catalysts. The effect of O 2 on the activity of CO for NO reduction is also analysed in detail through the simulations. 相似文献
3.
Catalytic activity of H- and FeH-ferrierite (FER) zeolites with iron content from 50 to 4000 ppm in NO–NO 2 equilibration and SCR of NO x by propane was measured, both in NO 2-poor and NO 2-rich streams. The activity of FeH-FER in SCR in NO 2-poor streams depends strongly on the Fe content; this relationship is valid down to traces of iron, while no such correlation was indicated in NO 2-rich streams. This was rationalized by realizing the negligible activity of zeolite protons for NO–NO 2 equilibration. Accordingly the SCR activity of H-FER in NO 2-poor streams necessitates presence of iron traces. In the NO 2–O 2–propane mixtures a process in absence of zeolite catalyst initiating propane oxidation and NO 2→NO conversion, but without N 2 formation, was evidenced at temperatures over 350 °C. It is suggested that such a radical process participate in characteristic narrow temperature window for NO x reduction by propane. 相似文献
4.
The effect of additives on Pt-ZSM-5 catalysts was studied for the selective NO reduction by H 2 in the presence of excess O 2 (NO–H 2–O 2 reaction) at 100 °C. The reaction of NO in a stream of 0.08% NO, 0.28% H 2, 10% O 2, and He balance yielded N 2 with less than 10% selectivity, which could not be increased by changing Pt loading or H 2 concentration in the gas feed. Co-impregnation of NaHCO 3 and Pt onto ZSM-5 decreased the BET surface area and the Pt dispersion. Nevertheless, the Na-loaded catalyst (Na-Pt-ZSM-5) exhibited the higher NO x conversion (>90%) and the N 2 selectivity (ca. 50%). Such a high catalytic activity even at high Na loadings (≥10 wt.%) is completely contrast to other Na-added Pt catalyst systems reported so far. Further improvement of N 2 selectivity was attained by the post-impregnation of NaHCO 3 onto Pt-ZSM-5. In situ DRIFT measurements suggested that the addition of Na promotes the adsorption of NO as NO 2−-type species, which would play a role of an intermediate to yield N 2. The introduction of Lewis base to the acidic supports including ZSM-5 would be applied to the catalyst design for selective NO–H 2–O 2 reaction at low temperatures. 相似文献
5.
The active site in ZSM-5 zeolite-supported palladium, which shows the catalytic activity for NO reduction with methane as a reducing agent, has been investigated qualitatively and quantitatively by means of NO chemisorption and NaCl titration, comparing with PdO supported on silica. Palladium species in 0.4 wt.% Pd loaded H-ZSM-5 can adsorb NO equimolarly after calcination at 773 K, and almost all the NO was desorbed at around 673 K, while the palladium species on PdO/SiO 2 hardly adsorbed NO. The palladium species in Pd(0.4)/H-ZSM-5 are ion-exchangeable with Na + in NaCl solution, indicating that they exist in a cationic state of an isolated Pd 2+. This method for quantitative analysis of the isolated Pd 2+ cations is named as ‘NaCl titration’. The amount of the isolated Pd 2+ cationic species increased with increasing palladium content on Pd/H-ZSM-5, and PdO co-existed above 1 wt.%. The amount of the isolated Pd 2+ cation was unchanged after the reaction of NO 2–CH 4, NO 2–CH 4–O 2, or CH 4–O 2 at 673 K, while the adsorbed amount of NO per the Pd 2+ as determined by NO-TPD decreased after the NO 2–CH 4–O 2 reaction. It was found by NaCl titration that the catalytic activity of Pd/H-ZSM-5 for NO 2–CH 4–O 2 reaction increased with increasing amount of the isolated Pd 2+ cationic species up to 0.7 wt.%, while the increase in the amount of PdO led to decrease in selectivity towards NO 2 reduction. The palladium species that are active and selective for NO reduction with CH 4 will be proposed. 相似文献
6.
Catalytic performances of ZSM-5 based catalysts containing indium or palladium were examined for NO reduction with CH 4 and NO x chemisorption. The amounts of NO x chemisorbed on In/H-ZSM-5 were well proportional to the catalytic activities for NO x reduction. Pd/H-ZSM-5, on the other hand, hardly chemisorbed NO 2, while the catalytic activity for NO 2 reduction with CH 4 is very high. Furthermore, Pd loaded on SiO 2 showed comparably high catalytic activity for NO 2 reduction with CH 4 at 400°C in the absence of oxygen as Pd/H-ZSM-5. CH 4 combustion during NO x reduction with CH 4 in the presence of oxygen significantly occurred over PdO on SiO 2, while less over Pd/H-ZSM-5. The role of zeolite might be slightly different between In/H-ZSM-5 and Pd/H-ZSM-5: the zeolitic porous structure is needed for In/H-ZSM-5 in order to concentrate NO 2 adspecies on InO + sites, which is important for NO reduction with CH 4 on In/H-ZSM-5 based catalysts, while the ion-exchangeable ability of zeolite is needed for Pd/H-ZSM-5 in order to make Pd 2+ located in a highly dispersed state, on which NO is strongly chemisorbed. 相似文献
7.
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
8.
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
9.
A series of sulfated zirconia supported Pd/Co catalysts was synthesized by the sol–gel method and examined for NO x reduction by methane. The NO conversion increased up to a Co/S ratio of 0.43, and then decreased at a higher Co loading (Co/S = 0.95). Sulfate content was also essential for obtaining high selectivity to molecular nitrogen. A catalyst loaded with 0.06 wt.% Pd, 2.1 wt.% Co and 2.1 wt.% S (Pd/Co-SZ-2) exhibited remarkable performance under lean conditions and displayed stability in a long-term durability test using a synthetic reaction mixture containing 10% water vapor. This catalyst exhibited the highest sulfur retention most probably as cobalt sulfide. Besides, the catalytic oxidation of NO to NO y groups was confirmed by FT-IR, in agreement with the general mechanism for the SCR of NO by hydrocarbons. In the absence of oxygen in the feed stream, the catalyst was highly active for NO reduction with methane. IR stretching bands assigned to N 2O and adsorbed nitro groups were identified upon adsorbing NO on Pd/Co-SZ-2. This indicates that under rich conditions disproportionation of NO to N 2O and NO 2 occurs and confirms that the formation of NO 2 species is an essential step for NO reduction by CH 4. 相似文献
10.
Characteristics of MnO y–ZrO 2 and Pt–ZrO 2–Al 2O 3 as reversible sorbents of NO x were investigated under dynamic changes in atmosphere. These sorbents can be used reversibly with a change of C 3H 8 concentration in the reaction gases. Catalytic reduction of NO occurred in the presence of propane, which was more pronounced on Pt–ZrO 2–Al 2O 3 than on MnO y-ZrO 2 due to high activity of Pt surface for this reaction on MnO y in MnO y–ZrO 2. The sorption was observed as soon as the atmosphere changed from a reducing to an oxidizing one. This implies that a high equilibrium partial pressure of O 2 is necessary for NO uptake since the sorbed NO−3 species becomes stable. The beginning of NO x desorption atmospheres was somewhat dependent on the amount of stored NO x. The presence of propane in the gas phase strongly affected the characteristic sorption and desorption properties of MnO y–ZrO 2 and Pt–ZrO 2–Al 2O 3. The sorption and desorption properties are different for MnO y–ZrO 2 and Pt–ZrO 2–Al 2O 3, since the noble metal or metal oxide possesses unique activity for the NO reaction with C 3H 8 and the amount of oxygen available for oxidative sorption of NO. 相似文献
11.
The reaction condition for high yield of methanol in a gaseous reaction between methane and oxygen in the presence of NO at atmospheric pressure was explored. Methane partial oxidation without NO (CH 4–O 2) gave only 1% conversion of methane at 966 K. The addition of NO led to a remarkable increase in methane conversion and to high selectivity to C 1-oxygenates. The conversion of methane attained 10% at 808 K in the presence of NO (0.5%) where the selectivities to methanol and formaldehyde were 22.1 and 24.1%, respectively. Nitromethane and carbon oxides were also observed in the product gas. The amount of nitromethane was almost equal and/or near to that of initial NO. The carbon monoxide produced was several times higher than carbon dioxide. Influences of NO concentration, ratio of methane to oxygen, water vapor, and dilution with helium gas on product distribution were measured. Low concentration of NO (0.35–0.55%) was favorable for methanol formation. High selectivity to methanol was obtained at low value of the ratio of methane to oxygen (2.0–3.0) or low concentration of dilution gas (<16%). The NO 2 added promoted methane partial oxidation and selectivity to methanol. Therefore, it was assured that NO x promoted the formation of CH 3√ and CH 3O√ in the gas phase reaction for CH 4–O 2–NO. 相似文献
12.
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
13.
The interactions NO—CO and O 2—NO—CO have been studied onCuCo 2O 4γ-Al 2O 3 and on γ-Al 2O 3- and CuCo 2O 4γ-Al 2O 3-supported Pt, Rh and Pt—Rh catalysts. The deposition of noble metals (Pt, Rh and Pt—Rh) on CuCo 2O 4γ-Al 2O 3 instead of γ-Al 2O 3 is beneficial in: lowering the temperature at which maximum N 2O is formed and decreasing the maximum N 2O concentration attained; lowering the onset temperature of NO to N 2 reduction, and increasing the N 2 selectivity; preserving the activity towards NO to N 2 reduction on a higher level following the concentration step NO + COO 2+ NO + CO and changing the conditions from stoichiometric to oxidizing (50% excess of oxidants). The reason for this behaviour of the CuCo 2O 4γ-Al 2O 3-based noble metal catalysts is the formation (reversible) of a reduced surface layer on the CuCo 2O 4 supported spinel under the conditions of a stoichiometric NO + CO mixture. 相似文献
14.
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
15.
Mixed oxides of the general formula La 0.5Sr xCe yFeO z were prepared by using the nitrate method and characterized by XRD and Mössbauer techniques. The crystal phases detected were perovskites LaFeO 3 and SrFeO 3−x and oxides -Fe 2O 3 and CeO 2 depending on x and y values. The low surface area ceramic materials have been tested for the NO+CO and NO+CH 4+O 2 (“lean-NO x”) reactions in the temperature range 250–550°C. A noticeable enhancement in NO conversion was achieved by the substitution of La 3+ cation at A-site with divalent Sr +2 and tetravalent Ce +4 cations. Comparison of the activity of the present and other perovskite-type materials has pointed out that the ability of the La 0.5Sr xCe yFeO z materials to reduce NO by CO or by CH 4 under “lean-NO x” conditions is very satisfying. In particular, for the NO+CO reaction estimation of turnover frequencies (TOFs, s −1) at 300°C (based on NO chemisorption) revealed values comparable to Rh/-Al 2O 3 catalyst. This is an important result considering the current tendency for replacing the very active but expensive Rh and Pt metals. It was found that there is a direct correlation between the percentage of crystal phases containing iron in La 0.5Sr xCe yFeO z solids and their catalytic activity. O 2 TPD (temperature-programmed desorption) and NO TPD studies confirmed that the catalytic activity for both tested reactions is related to the defect positions in the lattice of the catalysts (e.g., oxygen vacancies, cationic defects). Additionally, a remarkable oscillatory behavior during O 2 TPD studies was observed for the La 0.5Sr 0.2Ce 0.3FeO z and La 0.5Sr 0.5FeO z solids. 相似文献
16.
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
17.
Selective catalytic reduction of NO x (SCR-NO x) with decane, and for comparison with propane and propene over Cu-ZSM-5 zeolite (Cu/Al 0.49, Si/Al 13.2) was investigated under presence and absence of water vapor. Decane behaves in SCR-NO x like propene, i.e. the Cu-zeolite activity increased under increasing concentration of water vapor, as demonstrated by a shift of the NO x–N 2 conversion to lower temperatures, in contrast to propane, where the NO x–N 2 conversion is highly suppressed. In situ FTIR spectra of sorbed intermediates revealed similar spectral features for C 10H 22– and C 3H 6–SCR-NO x, where –CH x, R–NO 2, –NO 3−, Cu +–CO, –CN, –NCO and –NH species were found. On contrary, with propane –CH x, R–NO 2, NO 3−, Cu +–CO represented prevailing species. A comparison of the in situ FTIR spectra (T–O–T and intermediate vibrations) recorded at pulses of propene and propane, moreover, under presence and absence of water vapor in the reaction mixture, revealed that the Cu 2+–Cu + redox cycle operates with the C 3H 6–SCR-NO x reactions in both presence/absence of water vapor, while with C 3H 8–SCR-NO x, the redox cycle is suppressed by water vapor. It is concluded that decane cracks to low-chain olefins and paraffins, the former ones, more reactive, preferably take part in SCR-NO x. It is concluded that formation of olefinic compounds at C 10H 22–SCR-NO x is decisive for the high activity in the presence of water vapor, while water molecules block propane activation. The increase in NO x–N 2 conversion due to water vapor in C 10H 22–SCR-NO x should be connected with the increased reactivity of intermediates. These are suggested to pass from R–NO x → –CN → –NCO → NH 3; the latter reacts with another activated NO x molecule to molecular nitrogen. The positive effect of water vapor on the NO x–N 2 conversion is attributed to increased hydrolysis of –NCO intermediates. 相似文献
18.
The role of a multifunctional catalyst for de-NO x process has been investigated. The NO x storage capacity of H 3PW 12O 40·6H 2O (HPW) was improved by the presence of a noble metal (Pt, Rh or Pd). Both HPW and noble metal were deposited on a specific support (based on Zr–Ce or Zr–Ti). The presence of noble metal in several oxidation states, as evidenced by TPR and IR, involves the possibility of forming different catalytic sites: (i) M 0 (zero-valent metal) and perhaps (ii) (metal–H) δ+ from specific interactions between noble metal and the HPW proton. Supports were also able to adsorb and activate NO x and to generate cationic catalytic sites (M x+). These cationic sites seem to be the clue for their important activity toward NO x reduction. This catalyst presents an outstanding resistance to SO 2 poisoning which can be related to NO and NO 2 absorption mechanism in HPW. The use of alternating short cycles of lean/rich mixtures allows us optimising the performance of this catalytic system in terms of both NO x reduction capacity and NO x storage efficiency: up to 48 and 84%, respectively (with a 2% CO + 1% H 2 mixture for reducing). Experimental results sustain two hypotheses: first, HPW-metal-support catalyst includes several (independent) catalytic functions required for a de-NO x process to occur and second, the formation of oxygenate active species must be indispensable for NO x reduction into nitrogen. 相似文献
19.
Noble metal (Rh, Pt, Pd, Ir, Ru, and Ag) and Ni catalysts supported on CeO 2–Al 2O 3 were investigated for water gas shift reaction at ultrahigh temperatures. Pt/CeO 2–Al 2O 3 and Ru/CeO 2–Al 2O 3 demonstrated as the best catalysts in terms of activity, hydrogen yield and hydrogen selectivity. At 700 °C and steam to CO ratio of 5.2:1, Pt/CeO 2–Al 2O 3 converted 76.3% of CO with 94.7% of hydrogen selectivity. At the same conditions, the activity and hydrogen selectivity for Ru/CeO 2–Al 2O 3 were 63.9% and 85.6%, respectively. Both catalysts showed a good stability over 9 h of continuous operation. However, both catalysts showed slight deactivation during the test period. The study revealed that Pt/CeO 2–Al 2O 3 and Ru/CeO 2–Al 2O 3 were excellent ultrahigh temperature water gas shift catalysts, which can be coupled with biomass gasification in a downstream reactor. 相似文献
20.
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
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