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1.
A reliable method to continuously monitor NH 3 in a gas stream containing CO—NO—O 2 and H 2O has been developed. The method is based on a quantitative oxidation of NH 3 to NO on a Pt catalyst. The extent of this reaction is affected by temperature, excess oxygen present, and space-velocity. There is a significant effect of inlet O 2 concentration on extent of various reactions in the CO—NO—O 2—H 2O system on a Pt/γAl 2O 3 catalyst. At fixed space-velocity and catalyst temperature, and for fixed reactor inlet concentrations of CO and NO. there is negligible CO—NO reaction either in the absence of oxygen or in the presence of excess oxygen. However, short of the stoichiometric amount of O 2 required for CO oxidation, there is appreciable CO—NO (and possibly also CO—NO—H 2O) reaction whose extent increases with increasing oxygen concentration. This increase is especially dramatic in a narrow window of O 2: concentrations near the stoichiometric point. Interestingly enough, near the stoichiometric point, self-sustained isothermal oscillations in the outlet CO and NO concentrations are also observed (Subramaniam and Varma. submitted for publication) 相似文献
2.
The reduction of NO with CO in the presence of excess oxygen was investigated over different noble metal catalysts for probing the relationship between catalytic properties and adsorption behaviors. Among the four precious metal catalysts investigated, Ir/ZSM-5 was found to be the only active one for NO reduction with CO under lean conditions. With the decreasing of the Ir content, higher NO conversion and CO selectivity was obtained. Temperature-programmed reaction (TPR) studies of NO/H 2/O 2 and NO/CO/O 2 showed that the Pt/ZSM-5 was active when H 2 was used as the reductant, whereas, the Ir/ZSM-5 was active when CO was the reducing agent. This difference is due to the different mechanisms of the two reactions. Temperature-programmed desorption (TPD) of NO, CO and O 2 showed that NO could dissociate more easily over the Ir/ZSM-5 than on the Pt/ZSM-5, while the oxidation of CO by O 2 proceeded more rapidly on the Pt/ZSM-5 than on the Ir/ZSM-5. The presence of excess O 2 inhibited drastically the dissociation of NO, which is considered as the key step for the NO–CO reaction. The high dissociation rate of NO over the Ir/ZSM-5 is visualized as the key factor for its superior high activity in NO reduction with CO under lean conditions. 相似文献
3.
The selective catalytic reduction of NO x by methane on noble metal-loaded sulfated zirconia (SZ) catalysts was studied. Ru, Rh, Pd, Ag, Ir, Pt, and Au-loaded sulfated zirconia catalysts were compared with the intact sulfated zirconia. For the NO–CH 4–O 2 reaction, Ru, Rh, Pd, Ir, and Pt showed promotion effect on NO x reduction, while for the NO 2–CH 4–O 2 reaction, only Rh and Pd showed promotion effect. Over intact and Rh, Pd, Ag, and Au-loaded sulfated zirconia, NO x conversion in NO 2–CH 4–O 2 reaction was significantly higher than that in NO–CH 4–O 2 reaction, while clear difference was not observed over Ru, Ir, and Pt-loaded sulfated zirconia. Comparison of [NO 2]/([NO]+[NO 2]) in the effluent gases in NO–O 2 and NO 2–O 2 reactions showed that Ru, Ir, and Pt has high activity for NO oxidation under the reaction conditions. These facts suggest that effects of these metals toward NO x reduction by methane can be categorized into the following three groups: (i) low activity for NO oxidation to NO 2, and high activity for NO 2 reduction to N 2 (Pd, Rh); (ii) high activity for NO oxidation to NO 2, and low activity for NO 2 reduction to N 2 (Ru, Ir, Pt); (iii) low activity for both reactions (Ag, Au). To confirm these suggestions, combination of these metals were investigated on binary or physically-mixed catalysts. The combination of Pd or Rh with Pt or Ru gave high activity for the selective reduction of NO x by methane. 相似文献
4.
Self-sustained, isothermal oscillations in outlet species concentrations were observed under certain steady inlet conditions in the case of the CO—NO—O 2—H 2O reaction system on Pt/γAl 2O 3 catalyst in a fixed-bed tubuiar reactor. The oscillations were mostly aperiodic, and for fixed inlet concentrations of the other species, they occurred in a rather narrow window of inlet oxygen concentrations near the stoichiometric point. The amplitude and frequency of these oscillations were affected by temperature and by the inlet concenlrations of CO and NO. Systemalic experiments, conducted to understand the cause of these oscillations, revealed that the catalyst aging procedure and the presence of water vapor induced the complex dynamic behavior observed in the CO—NO—O 2—H 2O system. The oscillations are explained qualitatively in terms of the competition among the various reactants for adsorption and subsequent reaction on the catalyst surface. 相似文献
5.
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
6.
The kinetics of CO oxidation and NO reduction reactions over alumina and alumina-ceria supported Pt, Rh and bimetallic Pt/Rh catalysts coated on metallic monoliths were investigated using the step response technique at atmospheric pressure and at temperatures 30–350°C. The feed step change experiments from an inert flow to a flow of a reagent (O 2, CO, NO and H 2) showed that the ceria promoted catalysts had higher adsorption capacities, higher reaction rates and promoting effects by preventing the inhibitory effects of reactants, than the alumina supported noble metal catalysts. The effect of ceria was explained with adsorbate spillover from the noble metal sites to ceria. The step change experiments CO/O 2 and O 2/CO also revealed the enhancing effect of ceria. The step change experiments NO/H 2 and H 2/NO gave nitrogen as a main reduction product and N 2O as a by-product. Preadsorption of NO on the catalyst surface decreased the catalyst activity in the reduction of NO with H 2. The CO oxidation transients were modeled with a mechanism which consistent of CO and O 2 adsorption and a surface reaction step. The NO reduction experiments with H 2 revealed the role of N 2O as a surface intermediate in the formation of N 2. The formation of NN bonding was assumed to take place prior to, partly prior to or totally following to the NO bond breakage. High NO coverage favors N 2O formation. Pt was shown to be more efficient than Rh for NO reduction by H 2. 相似文献
7.
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
8.
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
9.
We have investigated the catalytic behavior of Pt encapsulated TiO 2 nanotubes for the water gas shift reaction as well as the hydrogenation of CO. Pt–TiO 2 nanotube catalysts were prepared by employing fine fiber shaped crystals of [Pt(NH 3) 4](HCO 3) 2 complex as a structure determining template material. The turnover frequencies (TOF) of these nanotube catalysts were more than one order of magnitude larger than conventional impregnation Pt/TiO 2 catalysts, and the selectivity for methanol in CO–H 2 reaction was extraordinary high compared to the impregnation catalysts. The XPS and XRD analyses of the nanotubes revealed characteristic electronic state of reduced TiO 2 (Ti 3+ in rutile structure) with zerovalent Pt even after the calcination at 773 K. In WGS reaction, electron rich Ti 3+ on the nanotube wall may play an important role to activate water molecules for the oxidation of CO. In CO–H 2 reaction, similar promotion effect of Ti 3+ species may be operating for selective methanol formation by supplying active OH(a). 相似文献
10.
CO广泛存在于燃煤烟气及汽车尾气中,利用未完全燃烧的CO催化还原NO可同时脱除NO和CO,过程中催化剂起着决定性作用。本文对近年来含氧条件下CO催化还原NO的研究成果进行了系统梳理,重点关注了Pd系、Ir系、Cu系、其他贵金属及金属氧化物催化剂的研究进展,分析了催化剂制备方法、掺杂改性及反应条件对催化性能的影响,同时考察了O 2浓度、H 2O以及SO 2对催化反应的影响,总结并对比了不同体系催化剂的活性位点及其催化机理,指明了O 2在催化还原过程中的抑制机理,得出了几种体系催化剂催化CO还原NO的活性顺序。最后,针对富氧条件下CO催化还原NO所存在的问题和难点,提出深入研究O 2抑制机理、降低贵金属用量、添加活性助剂是今后的研究方向。 相似文献
11.
Steady-state activity of Pt-ZSM-5 catalysts has been investigated experimentally for the NO + C 2H 4 + O 2 reaction under highly oxidizing conditions, typical of lean-burn gasoline engine exhaust. Effects of temperature, space velocity, feed concentration, Pt loading and water vapor on the catalytic activity have been examined using a packed-bed laboratory reactor. The catalytic activity of Pt-ZSM-5 is discussed in comparison with that of Cu-ZSM-5 and Pt/Al 2O 3. Results show that Pt-ZSM-5 catalysts are much more active than Cu-ZSM-5 catalysts for lean-NO x reduction at low temperatures, while the kinetic behavior of Pt/Al 2O 3 is very similar to that of Pt-ZSM-5. Conversion of both NO and C 2H 4 during the NO + C 2H 4 + O 2 reaction over Pt-ZSM-5 around the reaction lightoff temperature is strongly inhibited by the presence of NO. The NO/C 2H 4 ratio in the feedstream is an important factor determining the NO reduction activity of the catalyst, and there exists an optimum value of this ratio for a maximum conversion of NO. Based on the steady-state NO conversion data, a correlation between the reactor performance and the feed concentration has been developed, and the feasibility of Pt-based catalysts for lean-NO x reduction is discussed in terms of their activity, selectivity and durability. 相似文献
12.
ZrO 2–TiO 2 mixed oxides, prepared using the sol–gel method, were used as supports for platinum catalysts. The effects of catalyst pre-reduction and surface acidity on the performance of Pt/ZT catalysts for the reduction of NO with CH 4 were studied. The diffuse reflectance infrared Fourier transformed (DRIFT) spectra of CO adsorbed on the Pt/ZT catalysts, and also on the Pt/T and Pt/Z references, pre-reduced at 773 K in hydrogen, revealed that an SMSI state is developed in the Ti-rich oxide-supported platinum catalysts. However, no shift in the binding energy of Pt 4f 7/2 level for Pt/T and Pt deposited on Ti-rich support counterparts pre-reduced at 773 K was found by photoelectron spectroscopy. The DRIFT spectra of the catalysts under the NO+O 2 co-adsorption revealed the appearance of nitrite/nitrate species on the surface of the Zr-containing catalysts, which displayed acidic properties, but were almost absent in the Pt/T catalyst. The intensity of these bands reached a maximum for the Pt/ZT(1:1) catalyst, which in turn exhibited a larger specific area. In the absence of oxygen in the feed stream, the NO+CH 4 reaction showed DRIFT spectra assigned to surface isocyano species. Since the intensity of this band is higher for the Pt/ZT (9:1) catalyst, it seems that such species are developed at the Pt–support interface. 相似文献
13.
The C 3H 6 + NO + O 2 reaction has been studied in a wide range of temperatures (ca. 250–400 °C) and oxygen concentrations (0–5% O 2) over potassium-modified Ir surfaces. The in situ electrochemical controlled concept of catalysts promotion was used by interfacing a polycrystalline Ir thin film with a potassium β″-Al 2O 3 solid electrolyte disc, a K + conductor. At low oxygen concentrations (i.e., at reducing conditions), the effect of potassium on the Ir activity and selectivity is negligible. However, at higher oxygen concentrations (oxidizing conditions), strong K-induced poisoning on both propene and NO turnover consumption rates, as high as 85% and 65%, respectively, were recorded. Significant reduction on the system selectivity towards N 2 was also recorded under these conditions (from 100% over K-free Ir surface to 70% on K-modified Ir surfaces). The performance of Ir under alkali promotion is dramatically different to that reported in the literature for Pt or Pd under similar conditions, where strong promotional effects have been found. This very different behaviour may be understood in terms of the electronic influence of co-adsorbed potassium on the adsorption strengths of the neighbor reactants on the Ir surface. 相似文献
14.
We have studied the activity and selectivity of Pd/γ-Al 2O 3, VO x/γ-Al 2O 3 and Pd–VO x/γ-Al 2O 3 catalysts for the decomposition of NO and the reduction of NO with CO. Pd–VO x/γ-Al 2O 3 catalysts were prepared by anchoring Pd(AcAc) 2 on VO x/γ-Al 2O 3. Characterization of the binary samples by hydrogen chemisorption and TPR measurements indicated that the reduction of VO x is enhanced by a close contact with palladium and that partially reduced vanadia decorate noble metal particles. This palladium–vanadium interaction alters the catalytic properties of palladium: the activity for NO decomposition is higher for the binary sample and, for the NO–CO reaction, both the activity and the selectivity to N 2 increase when vanadium is in contact with palladium. 相似文献
15.
The selective catalytic reduction (SCR) of nitric oxide by propene over Ir/Al 2O 3 under lean-burn conditions (1000 vpm NO, 2000 vpm C 3H 6, 500 vpm CO, 10 vol.% O 2) was studied. The activity was shown to be strongly enhanced after exposure of the catalyst at 600°C under the reaction mixture, irrespective of the oxidising or reducing pre-treatment. Simultaneously, the Ir dispersion decreased from 78 to 10%. The influence of each component of the reaction mixture on the activation process was examined. The presence of both CO and O 2 was found to be necessary to activate Ir/Al 2O 3 while NO would not be. In situ FT-IR results revealed that initially fully oxidised Ir particles partially reduced in the feed to form Ir 0 reduced surface sites ( νCO at 2060 cm −1) which adsorbed CO up to 350–400°C. The activation under reactants was related to the formation of these sites. The presence of reduced (or partially reduced) Ir sites, possibly siting at the surface of IrO 2 particles and stabilised by CO adsorption, was proposed to be responsible for the SCR activity. 相似文献
16.
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
17.
In this study, the role of lanthanide elements (Ce, Gd, La, and Yb) on Pd/TiO 2 catalysts in the catalytic reduction of NO with methane was investigated. Steady-state reaction experiments in the presence of oxygen showed that the addition of lanthanide elements increases the oxygen resistance of the catalyst. The post-reaction XPS characterization results revealed that majority of the Pd sites remained in the zero oxidation state in the presence of Ce or Gd. The effect of SO 2 (145 ppm) and H 2O (0–6.6%) in NO–CH 4–O 2 reaction over supported Pd and Gd–Pd catalysts was also investigated. Over the Gd–Pd catalyst with the presence of SO 2, more than 70% NO conversion was obtained for over 6 h while the Pd only catalyst showed a sharper drop in NO conversion. Over the Gd–Pd catalyst, the presence of H 2O showed no effect on NO conversion activity (>99% conversion) during the 18 h the catalyst was kept on stream. Among the lanthanide elements tested, Gd is the most effective, allowing the use of above stoichiometric oxygen concentration. 相似文献
18.
Ir-based additives, developed to reduce NO and CO emitted during the regeneration of spent fluid catalytic cracking (FCC) catalysts were characterized to correlate physicochemical properties with catalytic performance. Support, metal loading and the state of the metal significantly affected the catalytic performance. Increasing the Ir loading or using a Ce-promoted γ-alumina (CPBase) support results in the formation of larger Ir particles. Local reduction of iridium oxide surface in such particles leads to coexisting Ir and Ir 2O phases being very beneficial for the catalytic activity. NO reduction and CO oxidation take place thermally at 700 °C. Increasing the O2 concentration in the feed favors CO oxidation at the expense of NO reduction. With 500ppmIr/CPBase and 1000ppmIr/CPBase additives, complete NO reduction and CO oxidation is achieved in the presence of 40% excess oxygen. Higher oxygen excess, however, reduces or eliminates the NOx reduction activity of these materials. IR studies suggest that NO reduction by CO proceeds on Ir/alumina additives via the dissociative adsorption of NO, the formation of NCO species on Ir and their migration to the alumina support, where N2 and CO2 are formed. IR spectroscopy indicates that Ce modifies the Ir surface enhancing the CO oxidation and enabling NO reduction via the NO2 formation. 相似文献
19.
A transient kinetic model was developed for the CO oxidation by O 2 over a Pt/Rh/CeO 2/γ-Al 2O 3 three-way catalyst. The experiments which were modelled consisted of periodically switching between a feed stream containing 0.5 mol% CO in helium and a feed stream containing 0.5 mol% O 2 in helium, with a frequency from 0.1 to 0.25 Hz, in the temperature range 393–433 K. These temperatures are representative for cold start conditions. The transient experiments yield information about the reaction mechanism. A transient kinetic model based on elementary reaction steps was developed which describes the experimental data in the above mentioned range of experimental conditions adequately. The kinetic model consists of two monofunctional and one bifunctional contribution. The first monofunctional reaction path comprises competitive adsorption of CO and O 2 on the noble metal surface followed by a surface reaction. The second monofunctional reaction path consists of CO adsorption on an oxygen atom adsorbed on the noble metal surface, followed by a reaction to CO 2. The bifunctional reaction path involves a reaction between CO adsorbed on the noble metal surface and oxygen from ceria at the noble metal/ceria interface. Also, reversible adsorption of carbon dioxide on the support is taken into account. The kinetic parameters, i.e. preexponential factors and activation energies for the different elementary reaction steps, and the oxygen storage capacity were estimated using multi-response non-linear regression analysis of the oxygen, carbon monoxide and carbon dioxide outlet concentrations. 相似文献
20.
Catalytic lean NO reduction system in periodic two steps, lean/rich operations has been investigated over Rh-based catalysts. The investigation was done using a pulse reaction. In the reaction, after H 2 or CO was pulse-injected for a moment to achieve reducing conditions, the highly dispersed Rh catalyst could catalyze NO reduction at 200–400 °C in lean conditions for 1 min. Furthermore, NO was effectively reduced over the highly dispersed Rh/β-zeolite after exposure to the gas composed of 3% of O 2, 40 ppm of SO 2 and balance of He at 300 and 400 °C for 24 h. 相似文献
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