共查询到20条相似文献,搜索用时 31 毫秒
1.
A series of bifunctional Ni-H 3PW 12O 40/SiO 2 catalysts for the hydrocracking of n-decane were designed and prepared. The evaluation results of the catalysts show that Ni-H 3PW 12O 40/SiO 2 catalysts possess a high activity for hydrocracking of n-decane and an excellent tolerance to the sulfur and nitrogen compounds in the feedstock. Under the reaction conditions: reaction temperature 300 °C; H 2/ n-decane volume ratio of 1500; total pressure of 2 Mpa and the LHSV 2 h −1, the conversion of n-decane over reduced 5%Ni-50%H 3PW 12O 40/SiO 2 catalysts is as high as 90%, the C 5+ selectivity equal to 70%. In order to reveal the structure and nature of the catalysts, a number of characterizations including XRD, Raman, H 2-TPD, NH 3-TPD, XPS and FT-IR of pyridine adsorption were carried out. The characteristic results show that the high activity of the catalysts and high C 5+ selectivity can be related to the unique structure of the H 3PW 12O 40 and its suitable acidity. 相似文献
2.
The inhibition effect of H 2O on V 2O 5/AC catalyst for NO reduction with NH 3 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 H 2O does not reduce NO and NH 3 adsorption on V 2O 5/AC catalyst surface, but promotes NH 3 adsorption due to increases in Brønsted acid sites. Many kinds of NH 3 forms present on the catalyst surface, but only NH 4+ on Brønsted acid sites and a small portion of NH 3 on Lewis acid sites are reactive with NO at 250 °C or below, and most of the NH 3 on Lewis acid sites does not react with NO, regardless the presence of H 2O in the feed gas. H 2O inhibits the SCR reaction between the NH 3 on the Lewis acid sites and NO, and the inhibition effect increases with increasing H 2O content. The inhibition effect is reversible and H 2O does not poison the V 2O 5/AC catalyst. 相似文献
3.
The fast SCR reaction using equimolar amounts of NO and NO 2 is a powerful means to enhance the NO x conversion over a given SCR catalyst. NO 2 fractions in excess of 50% of total NO x should be avoided because the reaction with NO 2 only is slower than the standard SCR reaction. At temperatures below 200 °C, due to its negative temperature coefficient, the ammonium nitrate reaction gets increasingly important. Half of each NH3 and NO2 react to form dinitrogen and water in analogy to a typical SCR reaction. The other half of NH3 and NO2 form ammonium nitrate in close analogy to a NOx storage-reduction catalyst. Ammonium nitrate tends to deposit in solid or liquid form in the pores of the catalyst and this will lead to its temporary deactivation. The various reactions have been studied experimentally in the temperature range 150–450 °C for various NO2/NOx ratios. The fate of the deposited ammonium nitrate during a later reheating of the catalyst has also been investigated. In the absence of NO, the thermal decomposition yields mainly ammonia and nitric acid. If NO is present, its reaction with nitric acid on the catalyst will cause the formation of NO2. 相似文献
4.
The kinetics of N 2O decomposition to gaseous nitrogen and oxygen over HZSM-5 catalysts with low content of iron (<400 ppm) under transient and steady-state conditions was investigated in the temperature range of 250–380 °C. The catalysts were prepared from the HZSM-5 with Fe in the framework upon steaming at 550 °C followed by thermal activation in He at 1050 °C. The N 2O decomposition began at 280 °C. The reaction kinetics was first order towards N 2O during the transient period, and of zero order under steady-state conditions. The increase of the reaction rate with time (autocatalytic behaviour) was observed up to the steady state. This increase was assigned to the catalysis by adsorbed NO formed slowly on the zeolite surface from N 2O. The formation of NO was confirmed by temperature-programmed desorption at temperatures >360 °C. The amount of surface NO during the transient increases with the reaction temperature, the reaction time, and the N 2O concentration in the gas phase up to a maximum value. The maximum amount of surface NO was found to be independent on the temperature and N 2O concentration in the gas phase. This leads to a first-order N 2O decomposition during the transient period, and to a zero-order under steady state. A kinetic model is proposed for the autocatalytic reaction. The simulated concentration–time profiles were consistent with the experimental data under transient as well as under steady-state conditions giving a proof for the kinetic model suggested in this study. 相似文献
5.
The NO-H 2-O 2 reaction was studied over supported bimetallic catalysts, Pt-Mo and Pt-W, which were prepared by coexchange of hydrotalcite-like Mg-Al double layered hydroxides by Pt(NO 2) 42−, MoO 42−, and/or WO 42− and subsequent heating at 600 °C in H 2. The Pt–Mo interaction could obviously be seen when the catalyst after reduction treatment was exposed to a mixture of NO and H 2 in the absence of O 2. The Pt-HT catalyst showed the almost complete NO conversion at 70 °C, whereas the Pt-Mo-HT showed a negligible conversion. Upon exposure to O 2, however, Pt-Mo-HT exhibited the NO conversion at the lowest temperature of ≥30 °C, compared to ≥60 °C required for Pt-HT. EXAFS/XANES, XPS and IR results suggested that the role of Mo is very sensitive to the oxidation state, i.e., oxidized Mo species residing in Pt particles are postulated to retard the oxidative adsorption of NO as NO 3 and promote the catalytic conversion of NO to N 2O at low temperatures. 相似文献
6.
In order to improve a “Three Function Catalysts Model”, the present paper deals with alumina based catalysts containing cobalt and palladium for the NO reduction by methane. The deNOx temperature window was estimated by adsorption and subsequent desorption of NO in lean conditions. Two NOx desorption peaks were detected for both catalysts. For Pd(0.63)Co(0.58)/Al2O3, the two desorption peaks appeared at 205 and 423 °C, whereas for Pd(0.14)Co(0.57)/Al2O3, the maxima desorption temperature peaks were at 205 and 487 °C. In addition, NO oxidation was also studied to evaluate the catalyst first function. It was found that, the oxidation begins on Co–Pd/Al2O3 around 250 °C. On Pd(0.63)Co(0.58)/Al2O3, 8% of deNOx were found in the range of the second NOx desorption peak temperature (410 °C). During TPSR, CxHyOz species such as formaldehyde were detected. These oxygenate species are the reactive intermediate for deNOx by methane. 相似文献
7.
A highly desirable selective catalytic reduction (SCR) of NO with real life diesel fuel over Pt supported zeolites with different topologies (Pt-ZSM-5, Pt-FER, Pt-MOR and Pt-BEA) is studied under simulated exhaust conditions. The catalysts are characterized by CO chemisorption, NH 3-TPD and TGA. The NO conversion ability of these catalysts has been correlated with zeolite structure and acidity. Pt-MOR is found to be the most active catalyst, 90% NO conversion at 300 °C, however Pt-FER showed highly desirable low temperature window, 77% NO conversion below 260 °C. Over ZSM-5, BEA and Y with three dimensional pore structures extensive carbonaceous deposits are observed by TGA which are detrimental to NO conversion. On the other hand, FER zeolite having one dimensional pore structure did not allow extensive coke formation resulting in a highly desired low temperature NO conversion. The results suggest that, NO reduction mainly take place near the zeolite pore opening, which is in reasonable agreement considering the long and bulky molecules in diesel fuel. 相似文献
8.
Cu 2+ ion-exchanged pillared clays are substantially more active than Cu 2+-ZSM-5 for selective catalytic reduction (SCR) of NO by hydrocarbons. More importantly, H 2O (or SO 2) has only mild effects on their activities. First results on Cu 2+-exchanged TiO 2-pillared montmorillonite were reported by this laboratory (Yang and Li, Ref. [1]), that showed overall activities two to four times higher than Cu 2+-ZSM-5. A delaminated pillared clay was subjected to Cu2+ ion-exchange and studied for SCR by C2H4 in this work. The Cu2+ ion-exchanged delaminated Al2O3-pillared clay yielded substantially higher SCR rates than both Cu2+-exchanged TiO2-pillared clay and Cu2+-ZSM-5 at temperatures above 400°C. The peak NO conversion was 90% at 550°C and at a space velocity of 15,000 h−1 (with O2 = 2%). The peak temperature decreased as the concentration of O2 was increased. The macroporosity in the delaminated pillared clay was partially responsible for its higher peak temperatures (than that for laminated pillared clays). At 1000 ppm each for NO and C2H4, the NO conversion peaked at 2% O2 for all temperatures. H2O and SO2 caused only mild deactivation, likely due to competitive adsorption (of SO2 on Cu2+ sites and H2O on acid sites). The high activity of Cu2+-exchanged Al2O3-pillared clay was due to a unique combination of the redox property of the Cu2+ sites and the strong Lewis acidity of the pillared clay. The suggested mechanism involved NO chemisorption (in the presence of O2) on Cu2+OAl3+-on the pillars, and C2H4 activation on the Lewis acid sites to form an oxygenated species. 相似文献
9.
A comparison study was carried out on non-thermal plasma (NTP)-assisted selective catalytic reduction (SCR) of NO x by propene over Ag/USY and Ag/Al 2O 3 catalysts. Ag/USY was almost inactive in thermal SCR while it showed obvious activities in NTP-assisted SCR at 100 °C–200 °C. Although the NO x conversion over Ag/Al 2O 3 was also enhanced at 300 °C–400 °C by the assistance of NTP, it was ineffective below 250 °C. The intermediates over Ag/USY and Ag/Al 2O 3 were investigated using in situ DRIFTS method. It was found that key intermediates in HC-SCR, such as NCO, CN, oxygenates and some N-containing organic species were enriched after the assistance of NTP. The differences in the behaviors of above intermediates were not found between these two kinds of catalysts. However, some evidences suggested that different properties of the absorbed NO x species resulted in the distinction of SCR reactions over Ag/USY and Ag/Al 2O 3. TPD profiles of Ag/Al 2O 3 showed that nitrates formed over the catalyst were quite stable at low temperatures, which might occupy the active sites and were unfavorable to SCR reactions. The nitrates over Ag/USY were unstable, among which the unidentate nitrate species is probably contributed to the SCR reactions at low temperatures. 相似文献
10.
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
11.
Through electron spin resonance (e.s.r.) studies, three different kinds of free radicals have been identified in pyrite samples containing coal impurities. Changes in the e.s.r. parameters ( g-value, linewidths, and concentration n) of free radicals have been monitored as the coal-derived pyrites are heated in different atmospheres (vacuum, N 2 and H 2) from room temperature to 500°C in 50°C steps for a residence time of 30 min at each temperature. Changes in n (concentration/g) and other parameters begin to occur at 300°C in vacuum, at 200°C in N 2, and at 100°C in H 2 amtospheres. At these temperatures conversion of pyrite to pyrrhotite is also observed to occur. It is argued that the enhancement of n in these samples is related to the conversion mechanism of pyrite to pyrrhotites. The maximum relative increase in n follows the sequence n (vacuum)> n(N 2)> n(H 2). At high temperature n decreases due to recombination of free radicals formed so that at 500°C, all three types of radicals have small but equal concentrations. 相似文献
12.
As part of a fundamental and applied work on the development of an unsteady mathematical model of the NH 3-selective catalytic reduction (SCR) process for design and control of integrated after-treatment systems of heavy-duty engines, we present herein a transient kinetic analysis of the standard SCR NO + NH 3 system which provides new insight in the catalytic kinetics and mechanism prevailing at low temperatures. Based on kinetic runs performed over a commercial powdered V 2O 5–WO 3–TiO 2 catalyst in the 175–450 °C T-range feeding NH 3 and NO (1000 ppm) in the presence of H 2O (1–10%, v/v) and O 2 (2–6%, v/v), an original dual-site modified redox rate law is derived which effectively accounts for NH 3 inhibition effects observed during transient reactive experiments at T < 250 °C. We also demonstrate that implementation of the novel modified redox kinetics into a fully predictive 1D + 1D model of SCR monolith reactors can significantly improve simulations of SCR transient runs at different scales, including engine test bench experiments over full-scale SCR honeycomb catalysts. 相似文献
13.
航空煤油实际燃烧过程中往往存在高化学当量比( Φ)的贫氧条件,导致航空煤油着火困难、燃烧效率较低。本文以航空煤油一元替代燃料正癸烷为燃料,实验研究了贫氧条件下( Φ=2~4)微圆管内Pt/ZSM-5催化剂和石英砂填充床中正癸烷的贫氧催化/无催化燃烧特性,分析了当量比( Φ=2~4)、温度(300~450℃)和催化剂对正癸烷转化率、燃烧效率以及气相产物分布特性的影响。结果表明:Pt/ZSM-5催化剂对正癸烷燃烧反应的促进作用明显,存在温度激增现象,当量比 Φ从2增大到3.5时,动态着火点从196℃上升到271℃,而无催化则没有明显的着火点。贫氧催化条件下正癸烷的转化率始终低于无催化条件,但燃烧效率明显高于无催化。正癸烷催化燃烧的主要气相产物为CO 2,无催化的主要气相产物则为CO和烯烃。 相似文献
14.
A nickel-based catalytic filter material for the use in integrated high temperature removal of tars and particles from biomass gasification gas was tested in a broad range of parameters allowing the identification of the operational region of such a filter. Small-scale porous alumina filter discs, loaded with approximately 2.5 wt% Al 2O 3, 1.0 wt% Ni and 0.5 wt% MgO were tested with a particle free synthetic gasification gas with 50 vol% N 2, 12 vol% CO, 10 vol% H 2, 11 vol% CO 2, 12 vol% H 2O, 5 vol% CH 4 and 0–200 ppm H 2S, and the selected model tar compounds: naphthalene and benzene. At a typical face velocity of 2.5 cm/s, in the presence of H 2S and at 900 °C, the conversion of naphthalene is almost complete and a 1000-fold reduction in tar content is obtained. Technically, it would be better to run the filter close to the exit temperature of the gasifier around 800–850 °C. At 850 °C, conversions of 99.0% could be achieved in typical conditions, but as expected, only 77% reduction in tars was achieved at 800 °C. Conversion data can be reasonably well described with first order kinetics and a dominant adsorption inhibition of the Ni sites by H2S. The apparent activation energies obtained are similar to those reported by other investigators: 177 kJ/mol for benzene and 92 kJ/mol for naphthalene. The estimated heat of adsorption of H2S is 71 kJ/mol in the benzene experiments and 182 kJ/mol in the naphthalene experiments, which points at very strong adsorption of H2S. Good operation of the present material can hence only be guaranteed at temperatures above 830 °C mainly due to the strong deactivation by H2S at lower temperatures. 相似文献
15.
The hydroisomerization and hydrocracking of n-hexadecane, n-octacosane and n-hexatriacontane on a 0.3% platinum/amorphous silica–alumina (MSA/E) catalyst was investigated in a stirred microautoclave at 345, 360 and 380°C and between 2 and 13.1 MPa hydrogen pressure. For each n-paraffin, the reaction pathway and the kinetic parameters were determined. The results were used to elucidate the effect of chain length and operating conditions on isomerization and cracking selectivity. The conversion of the n-paraffins lead to the formation of a mixture of the respective isomers, as the main product, together with cracking products. At every temperature, the iso-alkane/ n-alkane ratio of cracking products increased considerably with increasing conversion degree. At the same conversion level, higher reaction temperatures lead to cracking products characterized by a lower iso-alkane/ n-alkane ratio. The conversion rate constants showed a considerable increase between n-C 16 and n-C 28, whereas a slight decrease between n-C 28 and n-C 36 was observed. The hydroisomerization selectivities showed a decrease as a function of chain length and with increasing conversion levels. The increase in reaction temperature leads to a small decrease in the isomerization selectivities only at low-medium conversion degrees and at the highest temperature investigated, while the effect of this parameter on the maximum yields achievable in iso-C16, iso-C28 and iso-C36 was negligible. The results indicate that the conversion of the n-paraffins follows a first-order kinetic in hydrocarbon while the order in hydrogen pressure was −1.1 ± 0.21 for n-C 16 and −0.66 ± 0.15 for n-C 28. Furthermore, an increase in hydroisomerization selectivity at higher hydrogen pressure for n-C 28 conversion was observed. 相似文献
16.
Porous BaTiO 3 ( n-BaTiO 3) ceramics doped donor were fabricated by the addition of polyethylene glycol (PEG) into the n-BaTiO 3 powder. The effects of PEG on the microstructure and PTCR characteristics of the porous n-BaTiO 3 ceramics have been investigated. An endotherm was found at 60 °C, with strong exotherm at 262 °C, weight loss commenced at 165 °C and was virtually complete by 265 °C from the differential thermal analysis (DTA) and thermogravimetric analysis (TGA) of the PEG. It was also found that the porosity increased and the grain size decreased with increasing PEG. The crystalline structure of n-BaTiO 3 ceramics was independent on the PEG content and the n-BaTiO 3 ceramics containing PEG showed the presence of (Ba, Sr)TiO 3 peaks only from the XRD results. The n-BaTiO 3 ceramics containing PEG showed higher PTCR characteristics than that of the n-BaTiO 3 ceramics without PEG. 相似文献
17.
Pt/Al 2O 3 catalysts with Pt loadings ranging from 0.5 to 11 wt.% were synthesized by supercritical carbon dioxide (scCO 2) deposition method. Transmission electron microscopy (TEM) images showed that the synthesized catalysts contained small Pt nanoparticles (1–4 nm in diameter) with a narrow size distribution, no observable agglomeration, and uniformly dispersed on the alumina support. The catalysts were found to be active for hydrodesulfurization of dibenzothiophene (DBT) dissolved in n-hexadecane ( n-HD) without sulfiding the metal phase. The reaction proceeded only via the direct hydrogenolysis route in the temperature range 310–400 °C and at atmospheric pressure. The activity increased with increasing the metal loading. Increasing [H 2] 0/[DBT] 0 by either increasing [H 2] 0 or decreasing [DBT] 0, increased the DBT conversion. At a fixed weight hourly space velocity and feed concentration, conversion did not increase with increasing temperature beyond 330 °C. The presence of toluene inhibited the catalyst activity presumably due to competitive adsorption between DBT and toluene. Under the operating conditions, the reaction was far from equilibrium. 相似文献
18.
Activity and stability of an industrial Cr-free iron-based catalyst (NBC-1) for high-temperature water gas shift (WGS) reaction were studied in a fixed-bed reactor under 350 °C, 1 atm, H 2O:gas = 1:1 and 3000 h −1 (dry-gas basis). Physical properties of the NBC-1 catalyst before and after the WGS reaction, the desorption behavior of H 2O, CO, CO 2 and H 2, and surface reaction over the catalyst were characterized by BET, X-ray diffraction (XRD), Mössbauer emission spectroscopy (MES), temperature programmed desorption (TPD) and temperature programmed surface reaction (TPSR). The NBC-1 catalyst is active and has excellent thermo-stability even after pretreatment at a high temperature of 530 °C. Its activity and thermo-stability are comparable to those of an UCI commercial Fe-Cr catalyst, C12-4. XRD and MES studies show that iron in the fresh NBC-1 catalyst is present as γ-Fe 2O 3, most of which is converted to Fe 3O 4 during reduction and reaction. Results of TPD demonstrate that adsorbed CO 2 and CO cannot exist on the NBC-1 surface beyond the temperature of 300 °C while higher temperatures (>400 °C) are required to completely desorb H 2O. A redox mechanism of WGS on the NBC-1 surface is proposed based on the TPD and TPSR observations. 相似文献
19.
Fe-ZSM5 was prepared with high iron content by solid-state ion exchange and characterized by ICP-AES, BET surface measurements, TEM, UV–vis, EPR and DRIFT spectroscopy as well as supplementing catalytic tests in order to clear up its functionality in urea-SCR. Due to the over-exchange with iron small Fe 2O 3 particles were formed, identified by UV–vis, EPR and TEM measurements, which were proved to be not active for the SCR reaction. However, the oxidation of NO to NO 2 over Fe 3+ ions in the catalyst was realized to be a pre-requisite for the SCR reaction and the rate-determining step. DRIFT investigations under SCR conditions showed adsorbates on Fe 2+ up to 300 °C. The high SCR activity above 300 °C can be explained by the faster reoxidation of Fe 2+ to Fe 3+ sites at high temperatures. The observed inhibition of the SCR reaction by excess ammonia at low and intermediate temperatures can be explained in this context by the reducing properties of ammonia converting Fe 3+ to Fe 2+ or by preventing the reoxidation of Fe 2+. 相似文献
20.
Pd-zirconia-based monolithic catalysts were prepared with various commercial zirconia raw materials and a natural magnesium silicate binder, sepiolite, for the selective catalytic reduction (SCR) of NO with CH 4 in oxygen excess. The different textural properties, metastable tetragonal zirconia phase stability, surface acidity, Pd dispersion and catalytic properties of these monoliths were compared to select the most suitable structured catalyst for NO x control in natural gas-fired power plants. The influence of operating temperature in the two reactions, NO reduction and CH 4 combustion, with the monolithic catalysts was determined. A 0.4 wt.% Pd-zirconia catalyst, manufactured from a sulphated zirconium hydroxide raw material, was selected as the most appropriate in the reaction under study, reaching a maximum NO conversion at 400 °C. 相似文献
|