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1.
The applicability of a catalyst based on copper dispersed on γ-Al 2O 3 spheres (1 mm diameter) for fluidized bed catalytic combustion of methane has been assessed. Catalyst properties have been determined by physico-chemical characterization techniques and fixed bed activity tests revealing the presence of a surface CuAl 2O 4 spinel phase, still active and stable in methane combustion after repeated thermal ageing treatments at 800 °C. Methane catalytic combustion experiments have been performed in a 100 mm premixed fluidized bed reactor under lean conditions (0.15–3% inlet methane concentration), showing that complete CH 4 conversion can be attained below 700 °C in a fluidized bed of 1 mm solids with a gas superficial velocity about twice the incipient fluidization velocity. 相似文献
2.
In this work, the effect of niobium addition on textural, structural, acidic, and catalytic properties of Co/γ-Al 2O 3 catalysts for use in the total combustion of methane was studied. The catalysts were prepared by using the sol–gel technique and characterized by X-ray diffraction (XRD), infrared spectroscopy of adsorbed pyridine (IR), nitrogen adsorption (BET surface area), ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS), transmission electron microscopy in conjunction with energy-dispersive X-ray analysis (STEM–EDX), and activity in the total oxidation of methane. Results show that all cobalt-containing catalysts, regardless of the type of support, decrease the light off temperature of methane compared to pure γ-Al 2O 3. Therefore, the addition of 1.0% (by weight) of niobium to cobalt-containing alumina catalysts promoted a negative effect on the catalytic activity. Both cobalt-containing catalysts (4.5 and 9.3%, by weight, of Co) show a higher catalytic activity when compared to cobalt-containing niobia–alumina catalyst (6.3%, by weight, of Co). The lower activity of the niobia-containing catalysts may be associated to their acidic and textural properties. 相似文献
3.
Catalytic activity of ZrO 2 supported PdO catalysts for methane combustion has been investigated in comparison with Al 2O 3 supported PdO catalysts. It was found that the drop of catalytic activity owing to decomposition of PdO at a high temperature region (600–900°C) was suppressed by using ZrO 2 support. Temperature-programmed reduction (TPR) measurements of the catalyst with hydrogen revealed that the PdO of PdO/Al 2O 3 catalyst was reduced at the temperature less than 100°C, whereas in PdO/ZrO 2 catalyst the consumption of hydrogen was also observed at 200–300°C. This result indicates that the stable PdO species were present in the PdO/ZrO 2 catalyst. In order to confirm the formation of the solid solution of PdO and ZrO 2, X-ray diffraction (XRD) analyses of the mixtures of ZrO 2 and PdO calcined at 700–900°C in air were carried out. The lattice volume of ZrO 2 in the mixture was larger than that of ZrO 2. Furthermore, the Pd thin film on ZrO 2 substrate was prepared as a model catalyst and the depth profile of the elements in the Pd thin film was measured by Auger electron spectroscopy (AES). It was confirmed that Zr and O as well as Pd were present in the Pd thin film heated at 900°C in air. It was considered that the PdO on ZrO 2 support might be stabilized by the formation of the solid solution of PdO and ZrO 2. 相似文献
4.
The activity and excellent selectivity (>90%) of γ-Al 2O 3-supported Ni for the selective catalytic oxidation (SCO) of NH 3 to N 2 with excess O 2 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. N 2H 4 and NO have been used to model the intermediates of the SCO mechanism (direct formation of N 2 via the recombination of two NH x species) and of the in situ SCR mechanism (two-step formation of N 2 via the reduction of an in situ produced NO species by a NH x species), respectively. Two IR absorption bands appear during the TPO of NH 3 in the temperature range of N 2 formation and have been assigned to stable bidentate nitrate surface species. This represents strong evidence that under the present conditions, formation of N 2 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 NH 3 molecules fed to the catalyst. The fact that NO and NH 3 are able to react to give N 2 at low temperature (from 100°C) confirms that activation of NH 3 is the limiting step. In contrast, no evidence has been found to support the possibility of the SCO mechanism. 相似文献
5.
Catalytic combustion of natural gas, for applications such as gas turbines, can reduce NO x emissions. Palladium-on-stabilised alumina has been found to be the most efficient catalyst for the complete oxidation of methane to carbon dioxide and water. However, its poor durability is considered to be an obstruction for the development of catalytic combustion. This work was aimed at identifying the origin of this deactivation: metal sintering, support sintering, transformation
or coking. Catalytic combustion of methane was studied in a 15 mm i.d. and 50 mm length lab reactor and in a 25 mm i.d. pilot test rig on monolithic honeycomb substrates. Experiments were performed at GHSV of 50 000 h−1 in lab test and 500 000 h−1 in pilot test. The catalysts used were palladium on different supports on cordierite substrate. The catalysts were characterised by XRD, STEM, ATG and XPS. In steady-state conditions, deactivation has been found to be dependent on the air/methane ratio, the palladium content on the washcoat and the amount of washcoat on the substrate. An oscillating behaviour of the methane conversion was even observed under specific conditions, due to the reducibility of palladium oxide PdO to Pd. The influence of the nature of the support on the catalyst deactivation was also investigated. It has been shown that some supports can surprisingly eliminate this oscillating behaviour. However, in pilot test, deactivation was found to be very rapid, even with stabilised alumina supports. Furthermore, successive tests performed on the same catalyst revealed that the activity (light-off temperature, conversion) falls strongly from one test to another. Then, the stabilised alumina support was calcined at 1230°C for 16 h prior to its impregnation by palladium, in order to rule out its sintering. Experiments carried out on precalcined catalysts point out that deactivation is mostly correlated to the metal transformation under reaction conditions: activity decreases gradually as PdO sinters, but it dropped much more steeply in relation to appearance of metallic palladium. 相似文献
6.
Well crystallised aluminium borate Al 18B 4O 33 has been synthesised from alumina and boric acid with a BET area of 18 m 2/g after calcination at 1100 °C. Afterwards, 2 wt.% Pd/Al 18B 4O 33 was prepared by conventional impregnation of Pd(NO 3) 2 aqueous solution and calcination in air at 500 °C. The catalytic activity of Pd/Al 18B 4O 33 in the complete oxidation of methane was measured between 300 and 900 °C and compared with that of Pd/Al 2O 3. Pd/Al 18B 4O 33 exhibited a much lower activity than Pd/Al 2O 3 when treated in hydrogen at 500 °C or aged in O 2/H 2O (90:10) at 800 °C prior to catalytic testing. Surprisingly, a catalytic reaction run up to 900 °C in the reaction mixture induced a steep increase of the catalytic activity of Pd/Al 18B 4O 33 which became as active as Pd/Al 2O 3. Moreover, the decrease of the catalytic activity observed around 750 °C for Pd/Al 2O 3 and attributed to PdO decomposition into metallic Pd was significantly shifted to higher temperatures (820 °C) in the case of Pd/Al 18B 4O 33. The existence of two distinct types of PdO species formed on Al 18B 4O 33 and being, respectively, responsible for the improvement of the activity at low and high temperature was proposed on the basis of diffuse reflectance spectroscopy and temperature-programmed desorption of O 2. 相似文献
7.
The influence of the Ti-grafting of γ-Al 2O 3, SiO 2 and SnO 2 over Pd-supported catalysts and the presence of CO 2 as co-feeding, in the catalytic combustion of methane, were investigated. Important modifications in the catalytic performances due to grafting of supports were observed. The grafting method leads to formation of titania nanoparticles on the support surface. The interaction between Ti and support, changes in the size of Pd particles, changes in the acidity of supports could explain the modifications in catalytic performances due to grafting. The catalytic performances depend on the nature of the support and are different when CO 2 is introduced in the feed. CO 2 could play an important role, increasing or inhibiting the catalytic performance. 相似文献
8.
The performance of different Ce-modified PdO/ZrO 2 catalysts for methane oxidation in lean mixtures (5000 ppm of CH 4) in presence of external water has been studied in this work. Deactivation experiments carried out in presence of 20,000 ppm of external water showed that water reversibly inhibits the reaction. However, it was observed that these catalysts can increase their activity in presence of water at low temperature (350 °C).In order to explain this behaviour, different samples of this catalyst were treated with wet air (20,000 ppm of H 2O for 30 h). After this pre-treatment, their activity and stability for methane combustion were studied by recording light-off curves for the fresh catalysts and the catalyst after 50 h on stream for the oxidation of methane at 500 °C. As general trend, the hydro-ageing at the lowest temperature (300 °C) leaded to a very active catalyst (similar activity than the parent one), but it was more markedly deactivated. Hydro-ageing of the catalyst at higher temperatures enhanced its thermal stability. 相似文献
9.
The pulse corona plasma has been used as an activation method for reaction of methane and carbon dioxide, the product was C 2 hydrocarbons and by-products were CO and H 2. Methane conversion and the yield of C 2 hydrocarbons were affected by the carbon dioxide concentration in the feed. The conversion of methane increased with increasing carbon dioxide concentration in the feed whereas the yield of C 2 hydrocarbons decreased. The synergism of La 2O 3/γ-Al 2O 3 and plasma gave methane conversion of 24.9% and C 2 hydrocarbons yield of 18.1% were obtained at the power input of plasma was 30 W. The distribution of C 2 hydrocarbons changed by using Pd-La 2O 3/γ-Al 2O 3 catalyst, the major C 2 product was ethylene. 相似文献
10.
The catalytic combustion of chlorobenzene on a 2 wt.% Pt/γ-Al 2O 3 catalyst in binary mixtures with various hydrocarbons (toluene, benzene, cyclohexane, cyclohexene, 1,4-cyclohexadiene, 2-butene, and ethene) and with carbon monoxide has been explored. For all binary mixtures used the (excess of) added hydrocarbon increased the rate of conversion of chlorobenzene. With 2-butene, T50% and T100% for chlorobenzene were reduced by 100 and 200°C, respectively. Toluene and ethene were almost equally efficient as 2-butene. Co-feeding benzene or carbon monoxide resulted in a much smaller decrease of the T50%. The additional heat and water production in hydrocarbon combustion may contribute to some extent to the observed rate acceleration, but removal of Cl from the surface due to the hydrocarbon appears to be the major factor. The co-feeding of hydrocarbons invariably reduced the output of polychlorinated benzenes, which are formed as byproducts in the combustion of chlorobenzene on Pt/γ-Al2O3. Again, especially toluene, ethene, and 2-butene were very efficient. Benzene — as well as cyclohexane, cyclohexene, and 1,4-cyclohexadiene, which were converted in situ into benzene — was much less effective, due to chlorination of the aromatic nucleus. In chlorobenzene–CO mixtures the levels of polychlorinated benzenes were almost as high as with chlorobenzene per se. Removal of Cl from the surface (mainly in the form of HCl) by (non-aromatic) hydrocarbons is responsible for reducing the formation of byproducts. 相似文献
11.
For thermodynamic reasons, CO 2 has always been considered as inert at mild reaction temperatures (300 °C). In this study, we show that CO 2 may be used as a valuable compound for the catalytic combustion of methane (CCM), if ceria-based materials are used as support for the palladium active phase. Adding CO 2 in the feed significantly improves performances of ceria-zirconia supported catalysts. On the contrary, catalytic performances are inhibited on Pd/γ-Al 2O 3. Inhibition can be avoided by mixing the Pd/γ-Al 2O 3 catalyst with some CeO 2 evidencing cooperation phenomena between both catalysts. In situ DRIFTS experiments show that the inhibition of the alumina-supported catalyst is not due to formation of carbonates species. After an in situ reducing pre-treatment, pure CO 2 is able to rapidly oxidize reduced Pd/Ce 0.21Zr 0.79O 2 catalyst at 300 °C. Dissociation of CO 2 on Ce 0.21Zr 0.79O 2 would be responsible for the oxidation process. Thus, CO 2 helps in replenishing the O reservoir (OSC) of the Ce-Zr-O support which is normally consumed by reductants such as CH 4, H 2 or other HC's. XPS experiments show enrichment in oxygen species bound to Ce (Low BE O1s) on the surface of ceria-zirconia when working in the presence of CO 2. Implications of these results on the behavior of ceria-containing catalysts can be important for practical applications, e.g., in automotive exhaust catalysis. 相似文献
12.
The adsorption properties of a commercial Pd/Al 2O 3 catalyst were studied and compared with those of the Al 2O 3 support of the same specific surface area. Inverse gas chromatography (IGC) was used to determine the adsorption isotherms of five n-alkanes (C 8–C 12) in the 200–230 °C temperature range. Moreover, heats of adsorption, solubility coefficients and free energy of adsorption, are also reported. Interaction parameters of polar molecules with the stationary phase have also been determined and compared with those for the n-alkanes. Experiments with both the reduced and oxidized catalyst have been carried out by IGC and the results compared with those obtained by temperature programmed reduction (TPR) experiments. 相似文献
13.
The influence of the reaction parameters including temperature, oxygen concentration, and of in situ hydrogen reduction on the Pd catalyst activity towards complete methane oxidation is studied experimentally.Zero porosity α-alumina plates are used as a support for Pd catalyst. This lowers the influence of metal–support interaction on the catalyst state as confirmed by UV–visible spectroscopy. A plug flow reactor with a high linear gas velocity is used to measure the reaction rate. Overall conversion is kept low for most of the experiments so that the reaction is in the kinetically limited regime. The oxidation state of the catalyst before and after the reaction is determined using UV–visible reflectance spectroscopy of the plate surface. Changes in the catalyst activity with time are monitored after stepwise changes in the reaction parameters.Activity was found to decrease with time at low temperatures and high oxygen concentrations (condition when PdO phase is stable) and to increase with time at high temperatures and low oxygen concentrations (conditions when Pd is stable). A sharp increase in conversion was observed after the in situ hydrogen reduction of the sample.The experimental data is consistent with the reduced Pd form of the catalyst being more active towards methane oxidation than the oxidized PdO form at high temperatures. Possible particle size and morphology effects are discussed. 相似文献
14.
Addition of Na to Rh/γ-Al 2O 3 and Pd/γ-Al 2O 3 catalysts operated under identical simulated TWC conditions has dramatically different effects in the two cases (although the two metals respond similarly to Na promotion of NO x reduction in the absence of gaseous oxygen). Na addition to rhodium has a detrimental effect as manifested by severe poisoning and decreased nitrogen selectivity over the greater part of the temperature range studied. In contrast, Na promotion significantly improves the overall performance of Pd/γ-Al 2O 3 catalysts under simulated TWC conditions. This is manifested by a considerable widening of the gas composition window over which palladium delivers high NO x conversion, whilst at the same time exhibiting markedly improved selectivity towards N 2 formation. The very different behaviour of the two metals may be understood in terms of a single underlying effect, namely, the electronic influence of sodium ions on the adsorption strength (and hence relative coverages) of the various reactants on the metal surface. 相似文献
15.
In order to develop a catalyst with high activity and stability for catalytic wet air oxidation (CWAO) process at room temperature and atmospheric pressure, we prepared Fe 2O 3-CeO 2-TiO 2/γ-Al 2O 3 by consecutive impregnation, and determined its properties using BET, SEM, XRF, XPS and chemical analysis techniques. The degradation of an azo dye, methyl orange, in CWAO process with Fe 2O 3-CeO 2-TiO 2/γ-Al 2O 3 used as catalyst at room temperature and atmospheric pressure was also investigated, and the results show that the catalyst has an excellent catalytic activity in treating synthetic wastewater containing 500 mg/L methyl orange, and 98.09% of color and 96.08% of total organic carbon (TOC) can be removed in 2.5 h. The degradation pathway of methyl orange was analyzed by UV–vis and FT-IR spectra. The result of leaching tests shows the catalyst has an excellent stability with negligible leaching ions, and the leaching of Ce is effectively controlled by adding Ti, because Ce and Ti in the catalyst take the form of compound oxides, and the deactivation of the catalyst in successive runs is caused by the adsorption of intermediates on the surface and coverage of the active sites. The catalytic activity of the deactivated catalyst can be generally restored by rinsing it in hydrochloric acid followed by calcination. 相似文献
16.
The stability of methane conversion was studied over a Pd/Al 2O 3 catalyst and bimetallic Pd–Pt/Al 2O 3 catalysts. The activity of methane combustion over Pd/Al 2O 3 gradually decreased with time, whereas the methane conversion over bimetallic Pd–Pt catalysts was significantly more stable. The differences in combustion behavior were further investigated by activity tests where additional water vapor was periodically added to the feed stream. From these tests it was concluded that water speeds up the degradation process of the Pd/Al 2O 3 catalyst, whereas the catalyst containing Pt was not affected to the same extent. DRIFTS studies in a mixture of oxygen and methane revealed that both catalysts produce surface hydroxyls during combustion, although the steady state concentration on the pure Pd catalyst is higher for a fixed temperature and water partial pressure. The structure of the bimetallic catalyst grains with a PdO domain and a Pd–Pt alloy domain may be the reason for the higher stability, as the PdO domain appears to be more affected by the water generated in the combustion reaction than the alloy. Not all fuels that produce water during combustion will have stability issues. It appears that less strong binding in the fuel molecule will compensate for the degradation. 相似文献
17.
The oxidation of carbon monoxide in the presence of various concentrations of molecular hydrogen has been studied over a Au/TiO 2 reference catalyst by combining diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectrometry. It is shown for the first time that H 2 enhances the CO oxidation rate on Au/TiO 2 without leading to any major loss of selectivity. Increasing the H 2 pressure induces higher CO and H 2 oxidation rates. Under H 2-free conditions, the surface species detected are Au δ+–CO, Ti 4+–CO, carbon dioxide and carbonates. Upon the addition of H 2, Au 0–CO, water and hydroxyl groups become the main surface species. The occurrence of a preferential CO oxidation mechanism involving H xO y species under the present experimental conditions is proposed. 相似文献
18.
This study focuses on the loading of catalytic materials, e.g., palladium on the surface of supporting materials, with the aim to obtain catalysts with high activity for methane combustion. The catalyst PdO/CeO 2-Al 2O 3 was prepared by impregnation under ultrasonic condition. The effect of different activation methods on the activity of catalysts for methane catalytic combustion was tested. The properties of reaction and adsorption of oxygen species on catalyst surface were characterized by H 2-temperature programmed reduction (H 2-TPR), and O 2-temperature programmed desorption (O 2-TPD). Furthermore, the sulfur tolerance and sulfur poisoning mode were investigated. The results indicate that the catalyst PdO/CeO 2-Al 2O 3 activated with rapid activation shows higher activity for methane combustion and better sulfur tolerance. The result of sulfur content analysis shows that there is a large number of sulfur species on the catalyst’s surface after reactivation at high temperature. It proves that the activity of catalysts cannot be fully restored by high-temperature reactivation. 相似文献
19.
Catalysts with 20% LaMnO 3 supported on MgO have been prepared via CTAB-1-butanol- iso-octane-nitrate salt microemulsion. The preparation method was successfully varied in order to obtain different degrees of interaction between LaMnO 3 and MgO as shown by TPR and activity tests after calcination at 900 °C. Activity was tested on structured catalysts with 1.5% CH 4 in air as test gas giving a GHSV of 100,000 h −1. The activity was greatly enhanced by supporting LaMnO 3 on MgO compared with the bulk LaMnO 3. After calcination at 1100 °C both the surface area and TPR profiles were similar, indicating that the preparation method is of little importance at this high temperature due to interaction between the phases. Pure LaMnO 3 and MgO were prepared using the same microemulsion method for comparison purposes. Pure MgO showed an impressive thermal stability with a BET surface area exceeding 30 m 2/g after calcination at 1300 °C. The method used to prepare pure LaMnO 3 appeared not to be suitable since the surface area dropped to 1.1 m 2/g already after calcination in 900 °C. 相似文献
20.
Pulse reaction method and in situ IR spectroscopy were used to characterize the active oxygen species for oxidative coupling of methane (OCM) over SrF 2/Nd 2O 3 catalyst. It was found that OCM activity of the catalyst was very low in the absence of gas phase oxygen, which indicated that lattice oxygen species contributed little to the yield of C 2 hydrocarbons. IR band of superoxide species (O 2−) was detected on the O 2-preadsorbed SrF 2/Nd 2O 3. The substitution of 18O 2 isotope for 16O 2 caused the IR band of O 2− at 1128 cm −1 to shift to lower wavenumbers (1094 and 1062 cm −1), consistent with the assignment of the spectra to the O 2− species. A good correlation between the rate of disappearance of surface O 2− and the rate of formation of gas phase C 2H 4 was observed upon interaction of CH 4 with O 2-preadsorbed catalyst at 700 °C. The O 2− species was also observed on the catalyst under working condition. These results suggest that O 2− species is the active oxygen species for OCM reaction on SrF 2/Nd 2O 3 catalyst. 相似文献
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