首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 62 毫秒
1.
Preliminary activity tests show a synergic effect on the yield of the N2O+CO reaction by the addition of small quantities of rhodium in a Ag/Al2O3 catalyst. An analytical comparative kinetic study over Rh/Al2O3, Ag/Al2O3 and Rh-Ag/Al2O3 was performed in order to explain this effect. The reaction kinetics seems to follow a L–H mechanism with competitive adsorption of N2O and CO over the rhodium catalyst, where a strong CO inhibition effect was obvious. On the two other catalysts (silver and mixed) a L–H mechanism with the reactants adsorbed in different active sites seems to be followed. The kinetic, adsorption and thermodynamic constants were calculated and compared. From the results it seems that the synergic effect is connected with an increase of the activity of rhodium since silver offers more active sites for CO adsorption and removes the inhibition effect, while rhodium offers more active sites for both reactants to be adsorbed. The above findings show that there are no strong interactions (e.g. alloying) between silver and rhodium for the catalyst prepared by the method and active constituents concentrations of this study.  相似文献   

2.
Rh(1%)@CexZr1−xO2–Al2O3 nanocomposites have been investigated as active and thermally stable catalysts for ethanol steam reforming. Preformed Rh nanoparticles have been efficiently protected from deactivation/sintering by a porous layer of nanocomposite oxides. Chemisorption and activity data confirm the good accessibility of the metal phase to the reaction mixture. No appreciable deactivation is observed after 160 h of reaction at 873 K. The ceria–zirconia mixed oxides favour reforming reactions, reduce coke formation and facilitate its removal. The alumina component is important to stabilize the ceria–zirconia mixed oxides, preventing their sintering.  相似文献   

3.
The effect of different reducing agents (H2, CO, C3H6 and C3H8) on the reduction of stored NOx over PM/BaO/Al2O3 catalysts (PM = Pt, Pd or Rh) at 350, 250 and 150 °C was studied by the use of both NO2-TPD and transient reactor experiments. With the aim of comparing the different reducing agents and precious metals, constant molar reduction capacity was used during the reduction period for samples with the same molar amount of precious metal. The results reveal that H2 and CO have a relatively high NOx reduction efficiency compared to C3H6 and especially C3H8 that does not show any NOx reduction ability except at 350 °C over Pd/BaO/Al2O3. The type of precious metals affects the NOx storage-reduction properties, where the Pd/BaO/Al2O3 catalyst shows both a high storage and a high reduction ability. The Rh/BaO/Al2O3 catalyst shows a high reduction ability but a relatively low NOx storage capacity.  相似文献   

4.
The study of catalytic decomposition of nitrous oxide to nitrogen and oxygen over Rh catalysts supported on various supports (USY, NaY, Al2O3, ZrO2, FSM-16, CeO2, La2O3) showed that the activities of Rh/Al2O3 and Rh/USY (ultrastable Y zeolite) catalysts were comparable to or higher than the other catalysts reported in the literatures. The catalytic activity of N2O decomposition was sensitive not only to the Rh dispersion but also to the preparation variables such as the Rh precursors and the supports used. A pulsed N2O experiment over a Rh/USY catalyst suggested that the catalytic N2O decomposition occurs on oxygen-covered surface and that O2 may be freed on collision of N2O molecules with the adsorbed oxygen atoms.  相似文献   

5.
Reforming of methane with carbon dioxide to synthesis gas (CO/H2) has been investigated over rhodium supported on SiO2, TiO2, γ-Al2O3, MgO, CeO2, and YSZ (ZrO2 (8 mol% Y2O3)) catalysts in the temperature range of 650–750°C at 1 bar total pressure. A strong carrier effect on the initial specific activity, deactivation rate, and carbon accumulation was found to exist. A strong dependence of the specific activity of the methane reforming reaction on rhodium particle size was observed over certain catalysts. Tracing experiments (using 13CH4) coupled with temperature-programmed oxidation (TPO) revealed that the carbon species accumulated on the surface of the Rh/Al2O3 catalyst during reforming reaction at 750°C are primarily derived from the CO2 molecular route. The amount of carbon present on the working catalyst surface which is derived from the CH4 molecular route is found to be very small.  相似文献   

6.
The reduction of NOx by hydrogen under lean burn conditions over Pt/Al2O3 is strongly poisoned by carbon monoxide. This is due to the strong adsorption and subsequent high coverage of CO, which significantly increases the temperature required to initiate the reaction. Even relatively small concentrations of CO dramatically reduce the maximum NOx conversions achievable. In contrast, the presence of CO has a pronounced promoting influence in the case of Pd/Al2O3. In this case, although pure H2 and pure CO are ineffective for NOx reduction under lean burn conditions, H2/CO mixtures are very effective. With a realistic (1:3) H2:CO ratio, typical of actual exhaust gas, Pd/Al2O3 is significantly more active than Pt/Al2O3, delivering 45% NOx conversion at 160 °C, compared to >15% for Pt/Al2O3 under identical conditions. The nature of the support is also critically important, with Pd/Al2O3 being much more active than Pd/SiO2. Possible mechanisms for the improved performance of Pd/Al2O3 in the presence of H2+CO are discussed.  相似文献   

7.
The capability of flame-made Rh/Ce0.5Zr0.5O2 nanoparticles catalyzing the production of H2- and CO-rich syngas from butane was investigated for different Rh loadings (0–2.0 wt% Rh) and two different ceramic fibers (Al2O3/SiO2 and SiO2) as plugging material in a packed bed reactor for a temperature range from 225 to 750 °C. The main goal of this study was the efficient processing of butane at temperatures between 500 and 600 °C for a micro-intermediate-temperature SOFC system. Our results showed that Rh/Ce0.5Zr0.5O2 nanoparticles offer a very promising material for butane-to-syngas conversion with complete butane conversion and a hydrogen yield of 77% at 600 °C. The catalytic performance of packed beds strongly depended on the use of either Al2O3/SiO2 or SiO2 fiber plugs. This astonishing effect could be attributed to the interplay of homogeneous and heterogeneous chemical reactions during the high-temperatures within the reactor.  相似文献   

8.
Selective catalytic reduction of NOx by C3H6 in the presence of H2 over Ag/Al2O3 was investigated using in situ DRIFTS and GC–MS measurements. The addition of H2 promoted the partial oxidation of C3H6 to enolic species, the formation of –NCO and the reactions of enolic species and –NCO with NOx on Ag/Al2O3 surface at low temperatures. Based on the results, we proposed reaction mechanism to explain the promotional effect of H2 on the SCR of NOx by C3H6 over Ag/Al2O3 catalyst.  相似文献   

9.
For the first time, the coupling of fast transient kinetic switching and the use of an isotopically labelled reactant (15NO) has allowed detailed analysis of the evolution of all the products and reactants involved in the regeneration of a NOx storage reduction (NSR) material. Using realistic regeneration times (ca. 1 s) for Pt, Rh and Pt/Rh-containing Ba/Al2O3 catalysts we have revealed an unexpected double peak in the evolution of nitrogen. The first peak occurred immediately on switching from lean to rich conditions, while the second peak started at the point at which the gases switched from rich to lean. The first evolution of nitrogen occurs as a result of the fast reaction between H2 and/or CO and NO on reduced Rh and/or Pt sites. The second N2 peak which occurs upon removal of the rich phase can be explained by reaction of stored ammonia with stored NOx, gas phase NOx or O2. The ammonia can be formed either by hydrolysis of isocyanates or by direct reaction of NO and H2.

The study highlights the importance of the relative rates of regeneration and storage in determining the overall performance of the catalysts. The performance of the monometallic 1.1%Rh/Ba/Al2O3 catalyst at 250 and 350 °C was found to be dependent on the rate of NOx storage, since the rate of regeneration was sufficient to remove the NOx stored in the lean phase. In contrast, for the monometallic 1.6%Pt/Ba/Al2O3 catalyst at 250 °C, the rate of regeneration was the determining factor with the result that the amount of NOx stored on the catalyst deteriorated from cycle to cycle until the amount of NOx stored in the lean phase matched the NOx reduced in the rich phase. On the basis of the ratio of exposed metal surface atoms to total Ba content, the monometallic 1.6%Pt/Ba/Al2O3 catalyst outperformed the Rh-containing catalysts at 250 and 350 °C even when CO was used as a reductant.  相似文献   


10.
In this study, a novel bifunctional catalyst IrFe/Al2O3, which is very active and selective for preferential oxidation of CO under H2-rich atmosphere, has been developed. When the molar ratio of Fe/Ir was 5/1, the IrFe/Al2O3 catalyst performed best, with CO conversion of 68% and oxygen selectivity towards CO2 formation of 86.8% attained at 100 °C. It has also been found that the impregnation sequence of Ir and Fe species on the Al2O3 support had a remarkable effect on the catalytic performance; the activity decreased following the order of IrFe/Al2O3 > co-IrFe/Al2O3 > FeIr/Al2O3. The three catalysts were characterized by XRD, H2-TPR, FT-IR and microcalorimetry. The results demonstrated that when Ir was supported on the pre-formed Fe/Al2O3, the resulting structure (IrFe/Al2O3) allowed more metallic Ir sites exposed on the surface and accessible for CO adsorption, while did not interfere with the O2 activation on the FeOx species. Thus, a bifunctional catalytic mechanism has been proposed where CO adsorbed on Ir sites and O2 adsorbed on FeOx sites; the reaction may take place at the interface of Ir and FeOx or via a spill-over process.  相似文献   

11.
A multi-component NOx-trap catalyst consisting of Pt and K supported on γ-Al2O3 was studied at 250 °C to determine the roles of the individual catalyst components, to identify the adsorbing species during the lean capture cycle, and to assess the effects of H2O and CO2 on NOx storage. The Al2O3 support was shown to have NOx trapping capability with and without Pt present (at 250 °C Pt/Al2O3 adsorbs 2.3 μmols NOx/m2). NOx is primarily trapped on Al2O3 in the form of nitrates with monodentate, chelating and bridged forms apparent in Diffuse Reflectance mid-Infrared Fourier Transform Spectroscopy (DRIFTS) analysis. The addition of K to the catalyst increases the adsorption capacity to 6.2 μmols NOx/m2, and the primary storage form on K is a free nitrate ion. Quantitative DRIFTS analysis shows that 12% of the nitrates on a Pt/K/Al2O3 catalyst are coordinated on the Al2O3 support at saturation.

When 5% CO2 was included in a feed stream with 300 ppm NO and 12% O2, the amount of K-based nitrate storage decreased by 45% after 1 h on stream due to the competition of adsorbed free nitrates with carboxylates for adsorption sites. When 5% H2O was included in a feed stream with 300 ppm NO and 12% O2, the amount of K-based nitrate storage decreased by only 16% after 1 h, but the Al2O3-based nitrates decreased by 92%. Interestingly, with both 5% CO2 and 5% H2O in the feed, the total storage only decreased by 11%, as the hydroxyl groups generated on Al2O3 destabilized the K–CO2 bond; specifically, H2O mitigates the NOx storage capacity losses associated with carboxylate competition.  相似文献   


12.
The effect of the Pd addition method into the fresh Pd/(OSC + Al2O3) and (Pd + OSC)/Al2O3 catalysts (OSC material = CexZr1−xO2 mixed oxides) was investigated in this study. The CO + NO and CO + NO + O2 model reactions were studied over fresh and aged catalysts. The differences in the fresh catalysts were insignificant compared to the aged catalysts. During the CO + NO reaction, only small differences were observed in the behaviour of the fresh catalysts. The light-off temperature of CO was about 20 °C lower for the fresh Pd/(OSC + Al2O3) catalyst than for the fresh (Pd + OSC)/Al2O3 catalyst during the CO + NO + O2 reaction. For the aged catalysts lower NO reduction and CO oxidation activities were observed, as expected. Pd on OSC-containing alumina was more active than Pd on OSC material after the agings. The activity decline is due to a decrease in the number of active sites on the surface, which was observed as a larger Pd particle size for aged catalysts than for fresh catalysts. In addition, the oxygen storage capacity of the aged Pd/(OSC + Al2O3) catalyst was higher than that of the (Pd + OSC)/Al2O3 catalyst.  相似文献   

13.
Catalytic performance of Sn/Al2O3 catalysts prepared by impregnation (IM) and sol–gel (SG) method for selective catalytic reduction of NOx by propene under lean burn condition were investigated. The physical properties of catalyst were characterized by BET, XRD, XPS and TPD. The results showed that NO2 had higher reactivity than NO to nitrogen, the maximum NO conversion was 82% on the 5% Sn/Al2O3 (SG) catalyst, and the maximum NO2 conversion reached nearly 100% around 425 °C. Such a temperature of maximum NO conversion was in accordance with those of NOx desorption accompanied with O2 around 450 °C. The activity of NO reduction was enhanced remarkably by the presence of H2O and SO2 at low temperature, and the temperature window was also broadened in the presence of H2O and SO2, however the NOx desorption and NO conversion decreased sharply on the 300 ppm SO2 treated catalyst, the catalytic activity was inhibited by the presence of SO2 due to formation of sulfate species (SO42−) on the catalysts. The presence of oxygen played an essential role in NO reduction, and the activity of the 5% Sn/Al2O3 (SG) was not decreased in the presence of large oxygen.  相似文献   

14.
The water-gas shift (WGS) activity of platinum catalysts dispersed on a variety of single metal oxides as well as on composite MOx/Al2O3 and MOx/TiO2 supports (M = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, La, Ce, Nd, Sm, Eu, Gd, Ho, Er, Tm) has been investigated in the temperature range of 150–500 °C, using a feed composition consisting of 3% CO an 10% H2O. For Pt catalysts supported on single metal oxides, it has been found that both the apparent activation energy of the reaction and the intrinsic rate depend strongly on the nature of the support. In particular, specific activity of Pt at 250 °C is 1–2 orders of magnitude higher when supported on “reducible” compared to “irreducible” metal oxides. For composite Pt/MOx/Al2O3 and Pt/MOx/TiO2 catalysts, it is shown that the presence of MOx results in a shift of the CO conversion curve toward lower reaction temperatures, compared to that obtained for Pt/Al2O3 or Pt/TiO2, respectively. The specific reaction rate is in most cases higher for composite catalysts and varies in a manner which depends on the nature, loading, and primary crystallite size of dispersed MOx. Results are explained by considering that reducibility of small oxide particles increases with decreasing crystallite size, thereby resulting in enhanced WGS activity. Therefore, evidence is provided that the metal oxide support is directly involved in the WGS reaction mechanism and determines to a significant extent the catalytic performance of supported noble metal catalysts. Results of catalytic performance tests obtained under realistic feed composition, consisting of 3% CO, 10% H2O, 20% H2 and 6% CO2, showed that certain composite Pt/MOx/Al2O3 and Pt/MOx/TiO2 catalysts are promising candidates for the development of active WGS catalysts suitable for fuel cell applications.  相似文献   

15.
Junhua Li  Rui Ke  Wei Li  Jiming Hao 《Catalysis Today》2007,126(3-4):272-278
A comparison study was carried out on non-thermal plasma (NTP)-assisted selective catalytic reduction (SCR) of NOx by propene over Ag/USY and Ag/Al2O3 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 NOx conversion over Ag/Al2O3 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/Al2O3 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 NOx species resulted in the distinction of SCR reactions over Ag/USY and Ag/Al2O3. TPD profiles of Ag/Al2O3 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   

16.
Effect of additives, In2O3, SnO2, CoO, CuO and Ag, on the catalytic performance of Ga2O3–Al2O3 prepared by sol–gel method for the selective reduction of NO with propene in the presence of oxygen was studied. As for the reaction in the absence of H2O, CoO, CuO and Ag showed good additive effect. When H2O was added to the reaction gas, the activity of CoO-, CuO- and Ag-doped Ga2O3–Al2O3 was depressed considerably, while an intensifying effect of H2O was observed for In2O3- and SnO2-doped Ga2O3–Al2O3. Of several metal oxide additives, In2O3-doped Ga2O3–Al2O3 showed the highest activity for NO reduction by propene in the presence of H2O. Kinetic studies on NO reduction over In2O3–Ga2O3–Al2O3 revealed that the rate-determining step in the absence of H2O is the reaction of NO2 formed on Ga2O3–Al2O3 with C3H6-derived species, whereas that in the presence of H2O is the formation of C3H6-derived species. We presumed the reason for the promotional effect of H2O as follows: the rate for the formation of C3H6-derived species in the presence of H2O is sufficiently fast compared with that for the reaction of NO2 with C3H6-derived species in the absence of H2O. Although the retarding effect of SO2 on the activity was observed for all of the catalysts, SnO2–Ga2O3–Al2O3 showed still relatively high activity in the lower temperature region.  相似文献   

17.
Oxidation of propene and propane to CO2 and H2O has been studied over Au/Al2O3 and two different Au/CuO/Al2O3 (4 wt.% Au and 7.4 wt.% Au) catalysts and compared with the catalytic behaviour of Au/Co3O4/Al2O3 (4.1 wt.% Au) and Pt/Al2O3 (4.8 wt.% Pt) catalysts. The various characterization techniques employed (XRD, HRTEM, TPR and DR-UV–vis) revealed the presence of metallic gold, along with a highly dispersed CuO (6 wt.% CuO), or more crystalline CuO phase (12 wt.% CuO).

A higher CuO loading does not significantly influence the catalytic performance of the catalyst in propene oxidation, the gold loading appears to be more important. Moreover, it was found that 7.4Au/CuO/Al2O3 is almost as active as Pt/Al2O3, whereas Au/Co3O4/Al2O3 performs less than any of the CuO-containing gold-based catalysts.

The light-off temperature for C3H8 oxidation is significantly higher than for C3H6. For this reaction the particle size effect appears to prevail over the effect of gold loading. The most active catalysts are 4Au/CuO/Al2O3 (gold particles less than 3 nm) and 4Au/Co3O4/Al2O3 (gold particles less than 5 nm).  相似文献   


18.
Catalytic reduction of NO by propene in the presence of oxygen was studied over SnO2-doped Ga2O3–Al2O3 prepared by sol–gel method. Although SnO2-doped Ga2O3–Al2O3 gave lower NO conversion than Ga2O3–Al2O3 in the absence of H2O, the activity was enhanced considerably by the presence of H2O and much higher than that of Ga2O3–Al2O3. The presence of SnO2 and Ga2O3–Al2O3 species having intimate Ga–O–Al bondings was found to be essential for the promotional effect of H2O. The promotional effect of H2O was interpreted by the following two reasons. The first one is the removal of carbonaceous materials deposited on the catalyst surface by H2O. The other is the selective inhibition by H2O of the reaction steps resulting in propene oxidation to COx without reducing NO.  相似文献   

19.
The extent of Rh–niobia interaction in niobia-supported Rh (Rh/Nb2O5), niobia-promoted Rh/SiO2 (Nb2O5–Rh/SiO2) and RhNbO4/SiO2 catalyst after H2 reduction has been investigated by H2 and CO chemisorption measurements. These catalysts have been applied to selective CO oxidation in H2 (CO+H2+O2) and CO hydrogenation (CO+H2), and the results are compared with those of unpromoted Rh/SiO2 catalysts. It has been found that niobia (NbOx) increases the activity and selectivity for both the reactions.  相似文献   

20.
Water–gas shift reaction was studied over two nanostructured CuxCe1−xO2−y catalysts: a Cu0.1Ce0.9O2−y catalyst prepared by a sol–gel method and a Cu0.2Ce0.8O2−y catalyst prepared by co-precipitation method. A commercial low temperature water–gas shift CuO–ZnO–Al2O3 catalyst was used as reference. The kinetics was studied in a plug flow micro reactor at an atmospheric pressure in the temperature interval between 298 and 673 K at two different space velocities: 5.000 and 30.000 h−1, respectively. Experimentally estimated activation energy, Eaf, of the forward water–gas shift reaction at CO/H2O = 1/3 was 51 kJ/mol over the Cu0.1Ce0.9O2−y, 34 kJ/mol over the Cu0.2Ce0.8O2−y and 47 kJ/mol over the CuO–ZnO–Al2O3 catalyst. A simple rate expression approximating the water–gas shift process as a single reversible surface reaction was used to fit the experimental data in order to evaluate the rate constants of the forward and backward reactions and of the activation energy for the backward reaction.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号