共查询到20条相似文献,搜索用时 31 毫秒
1.
K. Hadjiivanov D. Klissurski G. Ramis G. Busca 《Applied catalysis. B, Environmental》1996,7(3-4):251-267
The adsorption and coadsorption of selective catalytic reduction (SCR) reactants and reaction products on CuZSM-5-37 containing 11 wt.-% CuO have been studied by FTIR spectroscopy. The catalyst surface is characterized by both weak acidity and weak basicity as revealed by testing with probe molecules (CO2, NH3, H2O). NO2 adsorption results in formation of different kinds of nitrates. The same species are formed when NO is coadsorbed with oxygen at 180°C. NO adsorption at ambient temperature also leads to formation of nitrates as well as of Cu2+NO species. In the presence of oxygen the latter are converted according to the scheme: NO → N2O3 → N2O4 → NO2 → NO3. It is concluded that the surface nitrates are important intermediates in the SCR process. They are thermally stable and resistant towards interaction with CO2, N2, O2, and are only slightly affected by H2O and NO. However, they posses a high oxidation ability and are fully reduced by propane at 180°C. It is concluded that one of the most important roles of oxygen in SCR by hydrocarbons is to convert NOx into highly active surface nitrates. 相似文献
2.
The activity and excellent selectivity (>90%) of γ-Al2O3-supported Ni for the selective catalytic oxidation (SCO) of NH3 to N2 with excess O2 has been shown by microreactor studies. Further studies of the mechanism involved in this reaction have been carried out using TPD, TPO, TPReaction as well as DRIFTS. N2H4 and NO have been used to model the intermediates of the SCO mechanism (direct formation of N2 via the recombination of two NHx species) and of the in situ SCR mechanism (two-step formation of N2 via the reduction of an in situ produced NO species by a NHx species), respectively. Two IR absorption bands appear during the TPO of NH3 in the temperature range of N2 formation and have been assigned to stable bidentate nitrate surface species. This represents strong evidence that under the present conditions, formation of N2 occurs via the in situ SCR mechanism. This also explains the sudden “NO jump” observed on various systems once the temperature is high enough to activate 50% of the NH3 molecules fed to the catalyst. The fact that NO and NH3 are able to react to give N2 at low temperature (from 100°C) confirms that activation of NH3 is the limiting step. In contrast, no evidence has been found to support the possibility of the SCO mechanism. 相似文献
3.
The mechanism of the selective reduction of nitrogen oxides by hydrocarbons on zeolite catalysts 总被引:3,自引:0,他引:3
The mechanism of the selective catalytic reduction (SCR) of nitrogen oxides over 3d transition metal zeolites has been investigated in a variety of ways. The initial step is the abstraction of hydrogen from the hydrocarbon by adsorbed NO2 species which is rate determining with methane but not with isobutane. The subsequent path appears to involve nitroso and/or nitro compounds. Comparative studies of the reactions of such compounds indicate that nitromethane is more likely to be an intermediate than nitrosomethane during the methane-SCR reaction over Co-MFI although the latter cannot be ruled out entirely. In both cases the predominant route to N2 is an initial decomposition to carbon oxides and ammonia followed by the NH3-SCR reaction. The isobutane-SCR reaction over Fe-MFI produces substantial amounts of hydrogen cyanide which disappears only at temperatures where all the hydrocarbon has been consumed. Hydrogen cyanide appears to arise from isobutyronitrile, the expected dehydration product if an initially formed nitroso compound undergoes tautomerism to an oxime. HCN is converted to N2 largely by reaction with NO2 which is fast well below 300°C in the absence of isobutane. The corresponding isobutane-SCR reaction over Cu-MFI gives rise to cyanogen (C2N2) rather than HCN. The general path is probably the same in the two systems with the difference arising from variation in the relative reactivity of HCN. The copper-containing catalyst is very effective at forming and dimerising adsorbed cyanide groups while the iron catalyst has higher activity for the oxidation of NO to the NO2 needed to convert adsorbed cyanide to N2. The difference between the apparent involvement of a nitro route in methane-SCR with Co-MFI, and a nitroso one with isobutane, is similarly explainable. The former reaction proceeds with simultaneous production of NO2 which can participate in the intermediate chemistry that follows. However, the NO2 concentration is low during the latter reaction over Cu-MFI and Fe-MFI as long as any hydrocarbon remains. This is due to the blocking of sites for NO oxidation by deposits and the recycling of NO2 back to NO during hydrocarbon oxidation. Thus only NO is available and the nitroso route prevails. The extent to which this picture applies with other catalysts and other hydrocarbons remains to be established. 相似文献
4.
Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) has been used to study NH3 and NO adsorption over a 15% w/w vanadia/titania catalyst. NH3 is adsorbed as coordinate NH3 and NH4+ species over the oxidised catalyst, leading to the reduction of the vanadia surface. At 300°C, adsorbed nitrosyls species are detected, suggesting that the oxidation of gaseous or adsorbed ammonia species takes place over the V=O sites. Coadsorption experiments show that NO is able to reoxidise about the 57% of the reduced V=O groups, resulting in N2, according to a NO+V□→1/2N2+V=O reaction. On the other hand, NO is only adsorbed over vanadia reduced surfaces. The measure of the area of the 2ν(V=O) bands results in an estimate of the oxidation state of vanadium. From this estimate it can be concluded that nitrosyls species are adsorbed on the catalyst surface for vanadium atoms having an oxidation state ranging from +4 to +3.1. 相似文献
5.
Mukundan Devadas Oliver Krcher Martin Elsener Alexander Wokaun Nicola Sger Marcus Pfeifer Yvonne Demel Lothar Mussmann 《Applied catalysis. B, Environmental》2006,67(3-4):187-196
The influence of NO2 on the selective catalytic reduction (SCR) of NO with ammonia was studied over Fe-ZSM5 coated on cordierite monolith. NO2 in the feed drastically enhanced the NOx 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 NO2/NOx = 50%, which is explained by the stoichiometry of the actual SCR reaction over Fe-ZSM5, requiring a NH3:NO:NO2 ratio of 2:1:1. In this context, it is a special feature of Fe-ZSM5 to keep this activity level almost up to NO2/NOx = 100%. The addition of NO2 to the feed gas was always accompanied by the production of N2O at lower and intermediate temperatures. The absence of N2O at the high temperature end is explained by the N2O decomposition and N2O-SCR reaction. Water and oxygen influence the SCR reaction indirectly. Oxygen enhances the oxidation of NO to NO2 and water suppresses the oxidation of NO to NO2, which is an essential preceding step of the actual SCR reaction for NO2/NOx < 50%. DRIFT spectra of the catalyst under different pre-treatment and operating conditions suggest a common intermediate, from which the main product N2 is formed with NO and the side-product N2O by reaction with gas phase NO2. 相似文献
6.
Tetsuya Nanba Shouichi Masukawa Atsushi Ogata Junko Uchisawa Akira Obuchi 《Applied catalysis. B, Environmental》2005,61(3-4):288-296
The catalytic decomposition of acrylonitrile (AN) over Cu-ZSM-5 prepared with various Cu loadings was investigated. AN conversion, during which the nitrogen atoms in AN were mainly converted to N2, increased as Cu loading increased. N2 selectivities as high as 90–95% were attained. X-ray diffraction measurements (XRD) and temperature-programmed reduction by H2 (H2-TPR) showed the existence of bulk CuO in Cu-ZSM-5 with a Cu loading of 6.4 wt% and the existence of highly dispersed CuO in Cu-ZSM-5 with a Cu loading of 3.3 wt%. Electron spin resonance measurements revealed that Cu-ZSM-5 contains three forms of isolated Cu2+ ions (square-planar, square-pyramidal, and distorted square-pyramidal). The H2-TPR results suggested that in Cu-ZSM-5 with a Cu loading of 2.9 wt% and below, Cu+ existed even after oxidizing pretreatment. The activity of AN decomposition over Cu/SiO2 suggested that CuO could form N2, but, independent of the CuO dispersion, nitrogen oxides (NOx) were formed above 350 °C. Cu+ and the square-pyramidal and distorted square-pyramidal forms of Cu2+ showed low activity for AN decomposition. Temperature-programmed desorption of NH3 suggested that N2 formation from NH3 proceeded on Cu2+, resulting in the formation of Cu+. The Cu+ ions were oxidized to Cu2+ at around 300 °C. Thus, high N2 selectivity over Cu-ZSM-5 with a wide range of temperature was probably attained by the reaction over the square-planar Cu2+, which can be reversibly reduced and oxidized. 相似文献
7.
8.
Characterization and reactivity of pure TiO2–SO4 SCR catalyst: influence of SO4 content 总被引:1,自引:0,他引:1
The kinetics of the reaction of NO, N2O and CO2 with activated carbon without catalyst and impregnated with a precursor salt of vanadium (ammonium monovanadate) was investigated. The conversion of NO, N2O and CO2 was studied (450–900°C) using a TGA apparatus and a fixed bed reactor. The reactor effluents were analysed using a GC/MS on line. The addition of vanadium increased carbon reactivity and adsorption at lower temperatures. For NO and N2O conversion the main products obtained were N2, N2O, CO and CO2 but for CO2 conversion only CO was detected. In situ XRD was a useful tool for interpreting catalyst behaviour and identifying phases present during reaction conditions. The catalytic effect of vanadium can be explained by the occurrence of redox processes in which the catalyst is reduced to lower oxidation states such as V2O5/V6O13. 相似文献
9.
M.J. Lzaro M.E. Glvez C. Ruiz R. Juan R. Moliner 《Applied catalysis. B, Environmental》2006,68(3-4):130-138
Carbon-based SCR catalysts for the reduction of NO with NH3 at low temperatures have been prepared using activated carbons obtained from a local Spanish coal, doped with several vanadium compounds. Among them, the ashes of a petroleum coke (PCA) were also employed. Both the catalysts and the carbon supports have been characterized by means of N2 and CO2 physisorption, NH3 and O2 chemisorption and temperature programmed desorption (TPD). The activity of the catalysts has been tested in a laboratory-scale unit, measuring significant conversions of NO (above 50%) with almost 100% selectivity toward N2 at 150 °C. The feasibility of using the petroleum coke ashes as the active phase was confirmed comparing the activity of the catalysts doped with these residues, with the one measured for the catalysts prepared using model vanadium compounds. The physical–chemical features of the carbon support resulted of key importance for achieving a considerable catalytic activity. The values of apparent energy of activation calculated for the catalysts presented in this paper were very similar to other carbon-based catalysts and smaller than the ones corresponding to TiO2-supported systems. The gas residence time on the catalytic bed influences the catalytic activity to a great extent, thus being a determinant parameter for designing the SCR de-NOx unit. To avoid ammonia slip, inlet concentrations of NH3 has to be little under the stoichometric NH3/NO ratio (0.7). The catalysts stability was tested in terms of carbon support gasification followed by termogravimetric analysis and gas chromatography. The activity of the catalysts was maintained at least over 24 h of reaction. 相似文献
10.
Parvaneh Nakhostin Panahi Darush Salari Aligholi Niaei Seyed Mahdi Mousavi 《中国化学工程学报》2015,23(10):1647-1654
A series of different transition metals (V, Co, Cr, Mn, Fe, Ni, Cu and Zn) promoted H-ZSM-5 catalysts were prepared by impregnation method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The catalytic activity of these catalysts was evaluated for the selective catalytic reduction (SCR) of NO with NH3 as reductant in the presence of oxygen. The results revealed that the catalytic activity of Cu-ZSM-5 nanocatalyst for NO conversion to N2 was 80% at 300 ℃, which was the best among various promoted metals. Design of experiments (DOEs) with Taguchi method was employed to optimize NH3-SCR process parameters such as NH3/NO ratio, O2 concentration, and gas hourly space velocity (GHSV) over Cu-ZSM-5 nanocatalyst at 250 and 300 ℃. Results showed that the most important parameter in NH3-SCR of NO is O2 concentration; followed by NH3/NO ratio and GHSV has little importance. The NO conversion to N2 of 63.1% and 94.86%was observed at 250 ℃ and 300 ℃, respectively under the obtained optimum conditions. 相似文献
11.
Hanna Sjvall Louise Olsson Erik Fridell Richard J. Blint 《Applied catalysis. B, Environmental》2006,64(3-4):180-188
The selective catalytic reduction of nitrogen oxides (NOx) 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 O2 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 NOx. At high temperature higher NH3 concentrations result in additional NOx reduction since more NH3 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 NO2 increases the conversion of NOx, but causes an increased formation of N2O. 相似文献
12.
Alumina- and titania-based monolithic catalysts for low temperature selective catalytic reduction of nitrogen oxides 总被引:3,自引:0,他引:3
Jesús Blanco Pedro Avila Silvia Surez Juan Antonio Martín Carlos Knapp 《Applied catalysis. B, Environmental》2000,28(3-4):235-244
The selective catalytic reduction of NO+NO2 (NOx) 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 NH3/NOx and NO/NO2 inlet ratios has been studied. High NOx 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 N2O formation with the copper-nickel catalysts, while no increase was observed with the vanadium catalysts. The vanadium-titania catalyst exhibited the highest DeNOx activity, with no detectable ammonia slip and a low N2O formation when NH3/NOx inlet ratio was kept below 0.8. TPR results of this catalyst with NO/NH3/O2, NO2/NH3/O2 and NO/NO2/NH3/O2 feed mixtures indicated that the presence of NO2 as the only nitrogen oxide increases the quantity of adsorbed species, which seem to be responsible for N2O formation. When NO was also present, N2O formation was not observed. 相似文献
13.
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
14.
A vanadium pentoxide supported activated coke (V2O5/AC) catalyst-sorbent has been reported to be very active for simultaneous removal of SO2 and NO under dry conditions at temperatures of 200 °C and below. Regeneration of the SO2-captured catalyst-sorbent is a key step in operation of such a process, which influences the catalyst-sorbent's SO2 and NO removal activities, lifetime, as well as recovery of sulfur. Due to limited information in this regard, this paper studies thermal regeneration of a V2O5/AC catalyst-sorbent with emphases on the effect of atmosphere. The optimum regeneration temperature is found to be 380 °C in an Ar stream and 300 °C in a 5% NH3/Ar stream. Compared to the fresh V2O5/AC, the V2O5/ACs regenerated in Ar show lower SO2 adsorption capacities and higher NO removal activities, while the regenerated V2O5/ACs by 5% NH3/Ar show higher and stable SO2 adsorption capacities and higher NO removal activities. Two types of reactions occur during the regeneration: reduction of the adsorbed sulfur species by carbon to SO2 and CO2, and oxidation of carbon by oxygen in the V2O5/AC to CO2. The carbon consumption of the latter is much more than that of the former in an Ar atmosphere, but fully suppressed by the presence of 5% NH3. Detailed analysis and characterization of the V2O5/AC subjected to the regenerations are presented. 相似文献
15.
采用密度泛函理论(DFT)方法对单空缺石墨烯负载的Pd单原子(Pd/SVG)催化剂上H2还原NO的反应进行了研究,探究了Pd/SVG上NO还原生成N2和NH3的路径。在Pd/SVG上NO容易加氢形成HNO,需要的活化能为67.0 kJ·mol-1,显示了极高的催化活性。N2生成的有利路径为NO活化生成HNO后,HNO继续加氢生成中间体NH2O和NH2OH,然后NH2OH解离生成NH2和OH,生成的NH2中间体结合NO形成NH2NO,然后NH2NO异构化形成的NHNOH再经解离生成N2与H2O,这个过程中的决速步骤为NH2NO分子内氢转移生成NHNOH,能垒为144.3 kJ·mol-1。对于NH3的生成,从NO的活化到中间体NH2的形成与N2的形成过程相同,最后NH2加氢即可形成NH3,这个过程中的决速步骤为NH2O加氢生成NH2OH,能垒为86.4 kJ·mol-1。比较生成N2和NH3的决速步能垒可见,Pd/SVG催化剂上NO经H2还原更容易形成NH3。本研究为石墨烯负载型Pd基催化剂上H2还原NO的实验及工业应用提供理论参考。 相似文献
16.
The conversion of fuel-nitrogen to HCN and NH3 and to nitrogen oxides was studied with nitrogen-containing model compounds, chosen to represent the main nitrogen and oxygen functionalities in fossil fuels. Two kinds of experiments were performed in an entrained-flow reactor at 800 °C. The conversion of model-compound-N to HCN and NH3 was determined under inert conditions, and the formation of NO, N2O and NO2 was determined under oxidizing conditions. In inert atmosphere, oxygen-containing functional groups had an important effect on the ratio of HCN to NH3. In particular, OH groups bound directly in the ring structure increased the conversion of nitrogen to NH3. In oxidizing atmosphere, the conversions of model-compound-N to N2O were high, but the substituent groups had no well-defined effect on the ratio of N2O to NO. The formation of NO2 was insignificant. 相似文献
17.
C.M.L. Scholz V.R. Gangwal M.H.J.M. de Croon J.C. Schouten 《Applied catalysis. B, Environmental》2007,71(3-4):143-150
In this paper, the effect of CO2 and H2O on NOx storage and reduction over a Pt–Ba/γ-Al2O3 (1 wt.% Pt and 30 wt.% Ba) catalyst is shown. The experimental results reveal that in the presence of CO2 and H2O, NOx is stored on BaCO3 sites only. Moreover, H2O inhibits the NO oxidation capability of the catalyst and no NO2 formation is observed. Only 16% of the total barium is utilized in NO storage. The rich phase shows 95% selectivity towards N2 as well as complete regeneration of stored NO. In the presence of CO2, NO is oxidized into NO2 and more NOx is stored as in the presence of H2O, resulting in 30% barium utilization. Bulk barium sites are inactive in NOx trapping in the presence of CO2·NH3 formation is seen in the rich phase and the selectivity towards N2 is 83%. Ba(NO3)2 is always completely regenerated during the subsequent rich phase. In the absence of CO2 and H2O, both surface and bulk barium sites are active in NOx storage. As lean/rich cycling proceeds, the selectivity towards N2 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 NOx storage. 相似文献
18.
尿素-选择性催化还原技术低温下运行时尿素分解不彻底,易形成缩二脲、三聚氰酸和三聚氰胺等副产物。本研究将TiO2催化剂与介质阻挡放电等离子体相结合,在程序升温条件下考察了载气中有无O2时引入等离子体前后TiO2催化尿素分解副产物水解的性能。结果表明:TiO2表面缩二脲、三聚氰酸和三聚氰胺分别在43~261℃、217~300℃和199~300℃水解生成NH3和CO2,载气中有无O2对催化水解过程几乎无影响。引入等离子体后缩二脲、三聚氰酸和三聚氰胺水解所需温度显著降低,载气中无O2时引入等离子体NH3产率变化不大,副产物仅有少量N2O和NO,有O2时NH3产率显著降低,且生成较多N2O、NO、NO2及少量NH4NO2和NH4NO3。未来需从优化放电条件和催化剂组成等方面解决引入等离子体导致副产物形成等问题。 相似文献
19.
Zhanggen Huang Zhenyu Liu Xianlong Zhang Qingya Liu 《Applied catalysis. B, Environmental》2006,63(3-4):260-265
The inhibition effect of H2O on V2O5/AC catalyst for NO reduction with NH3 is studied at temperatures up to 250 °C through TPD, elemental analyses, temperature-programmed surface reaction (TPSR) and FT-IR analyses. The results show that H2O does not reduce NO and NH3 adsorption on V2O5/AC catalyst surface, but promotes NH3 adsorption due to increases in Brønsted acid sites. Many kinds of NH3 forms present on the catalyst surface, but only NH4+ on Brønsted acid sites and a small portion of NH3 on Lewis acid sites are reactive with NO at 250 °C or below, and most of the NH3 on Lewis acid sites does not react with NO, regardless the presence of H2O in the feed gas. H2O inhibits the SCR reaction between the NH3 on the Lewis acid sites and NO, and the inhibition effect increases with increasing H2O content. The inhibition effect is reversible and H2O does not poison the V2O5/AC catalyst. 相似文献
20.
The selective catalytic reduction with aqueous solutions of urea is currently seen having the highest potential to reduce NOx and particulate emissions for commercial diesel powered vehicles. Ammonia as the actual reduction medium is formed from urea in two consecutive reactions, i.e. via the thermolysis of urea to isocyanic acid and NH3 and the catalyzed hydrolysis of HNCO over TiO2 to NH3 and CO2. A kinetic model for the hydrolysis reaction was derived for a reaction scheme comprising a set of elementary steps. To minimize the number of unknown variables in the kinetic model for the overall rate, the equilibrium constants for both reactants (HNCO and H2O) and products (NH3 and CO2) were determined from adsorption isotherms using Langmuir and multilayer adsorption models. A data set consisting of 49 data points for the rate determined at varying reactant concentrations was fitted with the kinetic model using a non-linear least mean squares regression analysis. 相似文献