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1.
Mechanistic and kinetic aspects of the direct decomposition of N2O over steam-activated Fe-silicalite were investigated by transient experiments in vacuum (N2O peak pressure of ca. 10 Pa) using the temporal analysis of products (TAP) reactor in the temperature range of 773–848 K. The transient responses of N2O, N2, and O2 obtained upon N2O decomposition were fitted to different micro-kinetic models. Through model discrimination it was concluded that both free iron sites and iron sites with adsorbed mono-atomic oxygen (*O) species are active for N2O decomposition. Oxygen formation occurs via decomposition of bi-atomic (*O2) oxygen species adsorbed over the iron site. This bi-atomic oxygen species originates from another bi-atomic oxygen species (O*O), which is initially formed via interaction of N2O with iron site possessing mono-atomic oxygen species (*O). Based on our modeling, the recombination of two mono-atomic oxygen (*O) species or direct O2 formation via reaction of N2O with *O can be excluded as potential reaction pathways yielding gas-phase O2. The simulation results predict that the overall rate of N2O decomposition is controlled by regeneration of free iron sites via a multi-step oxygen formation at least below 700 K. 相似文献
2.
The reaction pathways of N2 and N2O formation in the direct decomposition and reduction of NO by NH3 were investigated over a polycrystalline Pt catalyst between 323 and 973 K by transient experiments using the temporal analysis of products (TAP-2) reactor. The interaction between nitric oxide and ammonia was studied in the sequential pulse mode applying 15NO. Differently labelled nitrogen and nitrous oxide molecules were detected. In both, direct NO decomposition and NH3–NO interaction, N2O formation was most marked between 573 and 673 K, whereas N2 formation dominated at higher temperatures. An unusual interruption of nitrogen formation in the 15NO pulse at 473 K was caused by an inhibiting effect of adsorbed NO species. The detailed analysis of the product distribution at this temperature clearly indicates different reaction pathways leading to the product formation. Nitrogen formation occurs via recombination of nitrogen atoms formed by dissociation of nitric oxide or/and complete dehydrogenation of ammonia. N2O is formed via recombination of adsorbed NO molecules. Additionally, both products are formed via interactions between adsorbed ammonia fragments and nitric oxide. 相似文献
3.
Catalytic partial oxidation of methane to synthesis gas over ZrO2 and yttrium-stabilized zirconia (YSZ) is studied using O2 and N2O as oxidants. ZrO2 is much more active than YSZ in oxidation of methane with N2O. In contrast, YSZ is significantly more active than ZrO2 when O2 is used as an oxidant. The presence of O2 does not influence the rate of N2O decomposition over ZrO2 and YSZ, while the presence of H2O in the system decreases N2O conversion significantly. O2 and N2O are activated at different active sites. Y-induced oxygen vacancies are active for O2 activation, whereas oxygen co-ordinatively unsaturated Zr cations (Zr-CUS) located at corners, edges, steps and kinks are responsible for N2O activation. These sites are also capable of dissociating H2O, resulting in competition between H2O and N2O. As compared with N2O, molecular O2 is easier to be activated over YSZ and ZrO2. 相似文献
4.
M.N. Debbagh C. Salinas Martínez de Lecea J. Prez-Ramírez 《Applied catalysis. B, Environmental》2007,70(1-4):335-341
The catalytic reduction of N2O by CH4, CO, and their mixtures has been comparatively investigated over steam-activated FeZSM-5 zeolite. The influence of the molar feed ratio between N2O and the reducing agents, the gas-hourly space velocity, and the presence of O2 on the catalytic performance were studied in the temperature range of 475–850 K. The CH4 is more efficient than CO for N2O reduction, achieving the same degree of conversion at significantly lower temperatures. The apparent activation energy for N2O reduction by CH4 was very similar to that of direct N2O decomposition (140 kJ mol−1), being much lower for the N2O 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 N2O + CH4 + CO system did not affect the N2O conversion with respect to the binary N2O + CH4 system, indicating that CO does not interfere in the low-temperature reduction of N2O by CH4. In the ternary system, CO contributed to N2O reduction when methane was the limiting reactant. The conversion and selectivity of the reactions of N2O with CH4, CO, and their mixtures were not altered upon adding excess O2 in the feed. 相似文献
5.
Chunlei Shi Arden B. Walters M. Albert Vannice 《Applied catalysis. B, Environmental》1997,14(3-4):175-188
Dispersing La2O3 on δ- or γ-Al2O3 significantly enhances the rate of NO reduction by CH4 in 1% O2, compared to unsupported La2O3. Typically, no bend-over in activity occurs between 500° and 700°C, and the rate at 700°C is 60% higher than that with a Co/ZSM-5 catalyst. The final activity was dependent upon the La2O3 precursor used, the pretreatment, and the La2O3 loading. The most active family of catalysts consisted of La2O3 on γ-Al2O3 prepared with lanthanum acetate and calcined at 750°C for 10 h. A maximum in rate (mol/s/g) and specific activity (mol/s/m2) occurred between the addition of one and two theoretical monolayers of La2O3 on the γ-Al2O3 surface. The best catalyst, 40% La2O3/γ-Al2O3, had a turnover frequency at 700°C of 0.05 s−1, based on NO chemisorption at 25°C, which was 15 times higher than that for Co/ZSM-5. These La2O3/Al2O3 catalysts exhibited stable activity under high conversion conditions as well as high CH4 selectivity (CH4 + NO vs. CH4 + O2). The addition of Sr to a 20% La2O3/γ-Al2O3 sample increased activity, and a maximum rate enhancement of 45% was obtained at a SrO loading of 5%. In contrast, addition of SO=4 to the latter Sr-promoted La2O3/Al2O3 catalyst decreased activity although sulfate increased the activity of Sr-promoted La2O3. Dispersing La2O3 on SiO2 produced catalysts with extremely low specific activities, and rates were even lower than with pure La2O3. This is presumably due to water sensitivity and silicate formation. The La2O3/Al2O3 catalysts are anticipated to show sufficient hydrothermal stability to allow their use in certain high-temperature applications. 相似文献
6.
Xiankuan Zhang Arden B. Walters M. Albert Vannice 《Applied catalysis. B, Environmental》1996,7(3-4):321-336
Both NO decomposition and NO reduction by CH4 over 4%Sr/La2O3 in the absence and presence of O2 were examined between 773 and 973 K, and N2O decomposition was also studied. The presence of CH4 greatly increased the conversion of NO to N2 and this activity was further enhanced by co-fed O2. For example, at 773 K and 15 Torr NO the specific activities of NO decomposition, reduction by CH4 in the absence of O2, and reduction with 1% O2 in the feed were 8.3·10−4, 4.6·10−3, and 1.3·10−2 μmol N2/s m2, 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 N2 formation by decomposition was inhibited by O2 in the feed even though the reaction order in NO remained the same. The rate of NO reduction by CH4 continuously increased with temperature to 973 K with no bend-over in either the absence or the presence of O2 with equal activation energies of 26 kcal/mol. The addition of O2 increased the reaction order in CH4 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 O2 was 0.26 up to 0.5% O2 during which time the CH4 concentration was not decreased significantly. N2O decomposition occurs rapidly on this catalyst with a specific activity of 1.6·10−4 μmol N2/s m2 at 623 K and 1220 ppm N2O and an activation energy of 24 kcal/mol. The addition of CH4 inhibits this decomposition reaction. Finally, the use of either CO or H2 as the reductant (no O2) produced specific activities at 773 K that were almost 5 times greater than that with CH4 and gave activation energies of 21–26 kcal/mol, thus demonstrating the potential of using CO/H2 to reduce NO to N2 over these REO catalysts. 相似文献
7.
Li Xue Changbin Zhang Hong He Yasutake Teraoka 《Applied catalysis. B, Environmental》2007,75(3-4):167-174
A series of CeO2 promoted cobalt spinel catalysts were prepared by the co-precipitation method and tested for the decomposition of nitrous oxide (N2O). Addition of CeO2 to Co3O4 led to an improvement in the catalytic activity for N2O decomposition. The catalyst was most active when the molar ratio of Ce/Co was around 0.05. Complete N2O conversion could be attained over the CoCe0.05 catalyst below 400 °C even in the presence of O2, H2O or NO. Methods of XRD, FE-SEM, BET, XPS, H2-TPR and O2-TPD were used to characterize these catalysts. The analytical results indicated that the addition of CeO2 could increase the surface area of Co3O4, and then improve the reduction of Co3+ to Co2+ by facilitating the desorption of adsorbed oxygen species, which is the rate-determining step of the N2O decomposition over cobalt spinel catalyst. We conclude that these effects, caused by the addition of CeO2, are responsible for the enhancement of catalytic activity of Co3O4. 相似文献
8.
The decomposition of N2O, and the catalytic reduction by NH3 of N2O and N2O + NO, have been studied on Fe-BEA, -ZSM-5 and -FER catalysts. These catalysts were prepared by classical ion exchange and characterized by TPR after various activation treatments. Fe-FER is the most active material in the catalytic decomposition because “oxo-species” reducible at low temperature, appearing upon interaction of FeII-zeolite with N2O (-oxygen), are formed in largest amounts with this material. The decomposition of N2O is promoted by addition of NH3, and even more with NH3 + NO in the case of Fe-FER and -BEA. It is proposed that the NO-promoted reduction of N2O originated from the fast surface reaction between -oxygen O* and NO* to yield NO2*, which in turn reacts immediately with NH3. 相似文献
9.
M. Santhosh Kumar J. Prez-Ramírez M.N. Debbagh B. Smarsly U. Bentrup A. Brückner 《Applied catalysis. B, Environmental》2006,62(3-4):244-254
Fe-silicalite and Fe-SBA-15 with similar iron content have been characterized by N2 adsorption, small angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS), scanning electron microscopy (SEM), high-resolution transmission electron micrographs (HRTEM), UV–vis–DRS and EPR, and tested in direct N2O decomposition, N2O reduction by CO and N2O-mediated propane oxidative dehydrogenation. Both catalysts contain almost exclusively isolated Fe3+ sites of similar concentration and structure, which are, however, stabilized in markedly different pore geometries (intersecting channels of ca. 0.55 nm diameter in Fe-silicalite versus parallel linear pores of ca. 7.5 nm diameter in Fe-SBA-15). This aspect is of vital importance in order to exclusively ascribe different catalytic performances to the environment where the iron species are stabilized. Fe-silicalite revealed to be much more active than Fe-SBA-15 in all reactions studied. This clearly illustrates that the confinement of the iron species in pores of suitable geometry (structure and size) is essential to originate their remarkable catalytic properties. The large pores in ordered mesoporous materials apparently do not generate the required intimate contact between potentially active Fe sites and reactant molecules. 相似文献
10.
The oxidation of CH4 over Pt–NiO/δ-Al2O3 has been studied in a fluidised bed reactor as part of a major project on an autothermal (combined oxidation–steam reforming) system for CH4 conversion. The kinetic data were collected between 773 and 893 K and 101 kPa total pressure using CH4 and O2 compositions of 10–35% and 8–30%, respectively. Rate–temperature data were also obtained over alumina-supported monometallic catalysts, Pt and NiO. The bimetallic Pt–NiO system has a lower activation energy (80.8 kJ mol−1) than either Pt (86.45 kJ mol−1) and NiO (103.73 kJ mol−1). The superior performance of the bimetallic catalyst was attributed to chemical synergy. The reaction rate over the Pt–NiO catalyst increased monotonically with CH4 partial pressure but was inhibited by O2. At low partial pressures (<30 kPa), H2O has a detrimental effect on CH4 conversion, whilst above 30 kPa, the rate increased dramatically with water content. 相似文献
11.
Coupled semiconductor (CS) Cu/CdS–TiO2/SiO2 photocatalyst was prepared using a mutli-step impregnation method. Its optical property was characterized by UV–vis spectra. BET, XRD, Raman and IR were used to study the structure of the photocatalyst. Fine CdS was found dispersed over the surface of anatase TiO2/SiO2 substrate. Chemisorption and IR analysis showed methane absorbed in the molecular state interacted weakly with the surface of catalyst, and the interaction of CO2 with CS produced various forms of absorbed CO2 species that were primarily present in the form of formate, bidentate and linear absorption species. Photocatalytic direct conversion of CH4 and CO2 was performed under the operation conditions: 373 K, 1:1 of CO2/CH4, 1 atm, space velocity of 200 h−1 and UV intensity of 20.0 mW/cm2. The conversion was 1.47% for CH4 and 0.74% for CO2 with a selectivity of acetone up to 92.3%. The reaction mechanisms were proposed based on the experimental observations. 相似文献
12.
Kinetics of the simultaneous reduction N2O and NO by CO on CuCo2O4 has been studied. The reactants are adsorbed onto the coordination-unsaturated cations of the catalyst. The studies showed that the reactions of N2O and CO and of NO and CO occur between the adsorbed reactants on the catalyst surface; the catalyst surface is partially reduced during both these reactions. It was found that NO inhibits the reaction between N2O and CO, because N2O and NO compete for the active surface sites. The adsorption capacity of the catalyst is significantly higher for NO than for N2O and hence NO displaces N2Oads from the surface. The inhibition occurs on strongly localized sites and does not affect on the behaviour of the remaining free sites. At such blockage, the N2O reduction rate decreases in direct proportion to the amount of adsorbed NO. 相似文献
13.
V.A. Kondratenko U. Bentrup M. Richter T.W. Hansen E.V. Kondratenko 《Applied catalysis. B, Environmental》2008,84(3-4):497-504
A mechanistic scheme of N2O and N2 formation in the selective catalytic reduction of NO with NH3 over a Ag/Al2O3 catalyst in the presence and absence of H2 and O2 was developed by applying a combination of different techniques: transient experiments with isotopic tracers in the temporal analysis of products reactor, HRTEM, in situ UV/vis and in situ FTIR spectroscopy. Based on the results of transient isotopic analysis and in situ IR experiments, it is suggested that N2 and N2O are formed via direct or oxygen-induced decomposition of surface NH2NO species. These intermediates originate from NO and surface NH2 fragments. The latter NH2 species are formed upon stripping of hydrogen from ammonia by adsorbed oxygen species, which are produced over reduced silver species from NO, N2O and O2. The latter is the dominant supplier of active oxygen species. Lattice oxygen in oxidized AgOx particles is less active than adsorbed oxygen species particularly below 623 K. The previously reported significant diminishing of N2O production in the presence of H2 is ascribed to hydrogen-induced generation of metallic silver sites, which are responsible for N2O decomposition. 相似文献
14.
CO_2吸附强化CH_4/H_2O重整制氢是提供低成本高纯氢气和实现CO_2减排的方法之一。其中,催化剂和吸附剂是该工艺的重要组成部分,其活性与选择性制约了反应速率和产率,寿命长短关系到生产成本。综述了CO_2吸附强化CH_4/H_2O重整制氢催化剂和吸附剂的研究现状及存在的问题,机械混合的催化剂与吸附剂在反应过程中存在吸附产物包覆催化活性位点的问题,导致催化剂活性迅速下降。针对该问题,进一步探讨了不同结构双功能复合催化剂的结构特性、研究现状及其在循环-再生过程中存在的问题,核壳型双功能催化剂具有吸附组分与催化剂组分相对独立、催化组分分散分布和比表面积大等优点,在吸附强化制氢中有进一步研究的潜力。利用双功能催化剂的结构特点,实现反复循环再生过程中催化与脱碳反应的匹配,是推动CO_2吸附强化CH_4/H_2O重整制氢技术工业化发展的关键。 相似文献
15.
The effect of acidic treatments on N2O reduction over Ni catalysts supported on activated carbon was systematically studied. The catalysts were characterized by N2 adsorption, mass titration, temperature-programmed desorption (TPD), and X-ray photoelectron spectrometry (XPS). It is found that surface chemistry plays an important role in N2O-carbon reaction catalyzed by Ni catalyst. HNO3 treatment produces more active acidic surface groups such as carboxyl and lactone, resulting in a more uniform catalyst dispersion and higher catalytic activity. However, HCl treatment decreases active acidic groups and increases the inactive groups, playing an opposite role in the catalyst dispersion and catalytic activity. A thorough discussion of the mechanism of the N2O catalytic reduction is made based upon results from isothermal reactions, temperature-programmed reactions (TPR) and characterization of catalysts. The effect of acidic treatment on pore structure is also discussed. 相似文献
16.
Kimihiro Asano Chie Ohnishi Shinji Iwamoto Yasushi Shioya Masashi Inoue 《Applied catalysis. B, Environmental》2008,78(3-4):242-249
Direct decomposition of nitrous oxide (N2O) on K-doped Co3O4 catalysts was examined. The K-doped Co3O4 catalyst showed a high activity even in the presence of water. In the durability test of the K-doped Co3O4 catalyst, the activity was maintained at least for 12 h. It was found that the activity of the K-doped Co3O4 catalyst strongly depended on the amount of K in the catalyst. In order to reveal the role of the K component on the catalytic activity, the catalyst was characterized by XRD, XPS, TPR and TPD. The results suggested that regeneration of the Co2+ species from the Co3+ species formed by oxidation of Co2+ with the oxygen atoms formed by N2O decomposition was promoted by the addition of K to the Co3O4 catalyst. 相似文献
17.
通过焙烧猪骨和鸡骨获得羟磷灰石(nHAP)载体,并采用浸渍法制备Co3O4/nHAP催化剂。采用XRD、N2物理吸附-脱附、FT-IR和H2-TPR等对催化剂进行表征,在连续流动微反装置上考察催化剂催化分解N2O的性能。结果表明,相比于鸡骨源Co3O4/nHAP催化剂,以猪骨源HAP为载体的催化剂因其较大的比表面积以及较小的Co3O4粒径尺寸,提供了更多的活性位点。特别是猪骨源Co3O4/nHAP催化剂中适量的K、Na等元素促进了Co^3+到Co^2+的还原,削弱了Co-O键,使催化剂的催化活性显著提高。 相似文献
18.
L.F. Liotta G. Di Carlo G. Pantaleo A.M. Venezia G. Deganello E. Merlone Borla M. Pidria 《Applied catalysis. B, Environmental》2007,75(3-4):182-188
CO and CH4 combined oxidation tests were performed over a Pd (70 g/ft3)/Co3O4 monolithic catalyst in conditions of GHSV = 100,000 h−1 and feed composition close to that of emission from bi-fuel vehicles. The effect of SO2 (5 ppm) on CO and CH4 oxidation activity under lean condition (λ = 2) was investigated. The presence of sulphur strongly deactivated the catalyst towards methane oxidation, while the poisoning effect was less drastic in the oxidation of CO. Saturation of the Pd/Co3O4 catalytic sites via chemisorbed SO3 and/or sulphates occurred upon exposure to SO2. A treatment of regeneration to remove sulphate species was attempted by performing a heating/cooling cycle up to 900 °C in oxidizing atmosphere. Decomposition of PdO and Co3O4 phases at high temperature, above 750 °C, was observed. Moreover, sintering of Pd0 and PdO particles along with of CoO crystallites takes place. 相似文献
19.
The catalytic behavior in N2O reduction by propane in the presence of O2, H2O and SO2 of Fe/ZSM-5 catalysts prepared by ion exchange and chemical vapour deposition (CVD) is reported. The catalyst prepared by CVD shows a lower dependence of the rate of selective N2O reduction on the decrease in C3H8 to N2O ratio in the feed and a higher resistance to deactivation by SO2 in accelerated durability tests with high SO2 concentration (500 ppm). This catalyst shows stable catalytic behavior in the presence of SO2 for more than 600 h of time-on-stream. Characterization of the catalysts by UV–VIS–NIR diffuse reflectance indicates that the poor performances of the sample prepared by ion exchange could be related to the presence of highly clustered Fe3+ species, in this catalyst. On the other hand, Fe2O3 particles are not present in the sample prepared by CVD while mainly isolated Fe3+ ions and iron-oxide nanoclusters are present. 相似文献
20.
In this study, we examine the interaction of N2O with TiO2(1 1 0) in an effort to better understand the conversion of NOx species to N2 over TiO2-based catalysts. The TiO2(1 1 0) surface was chosen as a model system because this material is commonly used as a support and because oxygen vacancies on this surface are perhaps the best available models for the role of electronic defects in catalysis. Annealing TiO2(1 1 0) in vacuum at high temperature (above about 800 K) generates oxygen vacancy sites that are associated with reduced surface cations (Ti3+ sites) and that are easily quantified using temperature programmed desorption (TPD) of water. Using TPD, X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS), we found that the majority of N2O molecules adsorbed at 90 K on TiO2(1 1 0) are weakly held and desorb from the surface at 130 K. However, a small fraction of the N2O molecules exposed to TiO2(1 1 0) at 90 K decompose to N2 via one of two channels, both of which are vacancy-mediated. One channel occurs at 90 K, and results in N2 ejection from the surface and vacancy oxidation. We propose that this channel involves N2O molecules bound at vacancies with the O-end of the molecule in the vacancy. The second channel results from an adsorbed state of N2O that decomposes at 170 K to liberate N2 in the gas phase and deposit oxygen adatoms at non-defect Ti4+ sites. The presence of these O adatoms is clearly evident in subsequent water TPD measurements. We propose that this channel involves N2O molecules that are bound at vacancies with the N-end of the molecule in the vacancy, which permits the O-end of the molecule to interact with an adjacent Ti4+ site. The partitioning between these two channels is roughly 1:1 for adsorption at 90 K, but neither is observed to occur for moderate N2O exposures at temperatures above 200 K. EELS data indicate that vacancies readily transfer charge to N2O at 90 K, and this charge transfer facilitates N2O decomposition. Based on these results, it appears that the decomposition of N2O to N2 requires trapping of the molecule at vacancies and that the lifetime of the N2O–vacancy interaction may be key to the conversion of N2O to N2. 相似文献