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1.
Catalytic methane combustion and CO oxidation were investigated over AFeO 3 (A=La, Nd, Sm) and LaFe 1−xMg xO 3 ( x=0.1, 0.2, 0.3, 0.4, 0.5) perovskites prepared by citrate method and calcined at 1073 K. The catalysts were characterized by X-ray diffraction (XRD). Redox properties and the content of Fe 4+ were derived from temperature programmed reduction (TPR). Specific surface areas (SA) of perovskites were in 2.3–9.7 m 2 g −1 range. XRD analysis showed that LaFeO 3, NdFeO 3, SmFeO 3 and LaFe 1−xMg xO 3 ( x·0.3) are single phase perovskite-type oxides. Traces of La 2O 3, in addition to the perovskite phase, were detected in the LaFe 1−xMg xO 3 catalysts with x=0.4 and 0.5. TPR gave evidence of the presence in AFeO 3 of a very small fraction of Fe 4+ which reduces to Fe 3+. The fraction of Fe 4+ in the LaFe 1−xMg xO 3 samples increased with increasing magnesium content up to x=0.2, then it remained nearly constant. Catalytic activity tests showed that all samples gave methane and CO complete conversion with 100% selectivity to CO 2 below 973 and 773 K, respectively. For the AFeO 3 materials the order of activity towards methane combustion is La>Nd>Sm, whereas the activity, per unit SA, of the LaFe 1−xMg xO 3 catalysts decreases with the amount of Mg at least for the catalysts showing a single perovskite phase ( x=0.3). Concerning the CO oxidation, the order of activity for the AFeO 3 materials is Nd>La>Sm, while the activity (per unit SA) of the LaFe 1−xMg xO 3 catalysts decreases at high magnesium content. 相似文献
2.
The catalytic oxidation of unsymmetrical dimethylhydrazine (UDMH) by air has been studied in a vibro-fluidized catalyst bed laboratory kinetic setup over catalysts Cu xMg 1−xCr 2O 4/γ-Al 2O 3, 32.9%Ir/γ-Al 2O 3 and β-Si 3N 4 in a temperature range 150–400 °C. The catalyst Cu xMg 1−xCr 2O 4/γ-Al 2O 3 was found to be optimal regarding high yields of CO 2 and low yields of NO x. A probable mechanism of UDMH heterogeneous catalytic oxidation is proposed. Catalyst Cu xMg 1−xCr 2O 4/γ-Al 2O 3 has been further used in the pilot plant specially designed for the destruction of UDMH. Results of testing the main fluidized bed catalytic reactor for UDMH oxidation and the reactor for selective catalytic reduction of NO x with NH 3 are presented. These results prove that the developed UDMH destruction technology is highly efficient and environmentally safe. 相似文献
3.
A series of the Ce 1−xCu xO 2−x/Al 2O 3/FeCrAl catalysts ( x = 0–1) were prepared. The structure of the catalysts was characterized using XRD, SEM and H 2-TPR. The catalytic activity of the catalysts for the combustion of methane was evaluated. The results indicated that in the Ce 1−xCu xO 2−x/Al 2O 3/FeCrAl catalysts the surface phase structure were the Ce 1−xCu xO 2−x solid solution, -Al 2O 3 and γ-Al 2O 3. The surface particle shape and size were different with the variety of the molar ratio of Ce to Cu in the Ce 1−xCu xO 2−x solid solution. The Cu component of the Ce 1−xCu xO 2−x/Al 2O 3/FeCrAl catalysts played an important role to the catalytic activity for the methane combustion. There were the stronger interaction among the Ce 1−xCu xO 2−x solid solution and the Al 2O 3 washcoats and the FeCrAl support. 相似文献
4.
Catalytic combustion of methane has been investigated over AMnO 3 (A = La, Nd, Sm) and Sm 1−xSr xMnO 3 ( x = 0.1, 0.3, 0.5) perovskites prepared by citrate method. The catalysts were characterized by chemical analysis, XRD and TPR techniques. Catalytic activity measurements were carried out with a fixed bed reactor at T = 623–1023 K, space velocity = 40 000 N cm 3 g −1 h −1, CH 4 concentration = 0.4% v/v, O 2 concentration = 10% v/v. Specific surface areas of perovskites were in the range 13–20 m2 g−1. XRD analysis showed that LaMnO3, NdMnO3, SmMnO3 and Sm1−xSrxMnO3 (x = 0.1) are single phase perovskite type oxides. Traces of Sm2O3 besides the perovskite phase were detected in the Sm1−xSrxMnO3 catalysts for x = 0.3, 0.5. Chemical analysis gave evidence of the presence of a significant fraction of Mn(IV) in AMnO3. The fraction of Mn(IV) in the Sm1−xSrxMnO3 samples increased with x. TPR measurements on AMnO3 showed that the perovskites were reduced in two steps at low and high temperature, related to Mn(IV) → Mn(III) and Mn(III) → Mn(II) reductions, respectively. The onset temperatures were in the order LaMnO3 > NdMnO3 > SmMnO3. In Sm1−xSrxMnO3 the Sr substitution for Sm caused the formation of Mn(IV) easily reducible to Mn(II) even at low temperature. Catalytic activity tests showed that all samples gave methane complete conversion with 100% selectivity to CO2 below 1023 K. The activation energies of the AMnO3 perovskites varied in the same order as the onset temperatures in TPR experiments suggesting that the catalytic activity is affected by the reducibility of manganese. Sr substitution for Sm in SmMnO3 perovskites resulted in a reduction of activity with respect to the unsubstituted perovskite. This behaviour was related to the reduction of Mn(IV) to Mn(II), occurring under reaction conditions, hindering the redox mechanism. 相似文献
5.
Structural, redox and catalytic deep oxidation properties of LaAl 1−xMn xO 3 ( x=0.0, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0) solid solutions prepared by the citrate method and calcined at 1073 K were investigated. XRD analysis showed that all the LaAl 1−xMn xO 3 samples are single phase perovskite-type solid solutions. Particle sizes and surface areas (SA) are in the 280–1180 Å and 4–33 m 2 g −1 ranges, respectively. Redox properties and the content of Mn 4+ were derived from temperature programmed reduction (TPR) with H 2. Two reduction steps are observed by TPR for pure LaMnO 3, the first attributed to the reduction of Mn 4+ to Mn 3+ and the second due to complete reduction of Mn 3+ to Mn 2+. The presence of Al in the LaAl 1−xMn xO 3 solid solutions produces a strong promoting effect on the Mn 4+→Mn 3+ reducibility and inhibits the further reduction to Mn 2+. Both for methane combustion and CO oxidation all Mn-containing perovskites are much more active than LaAlO 3, so pointing to the essential role of the transition metal ion in developing highly active catalysts. Partial dilution with Al appears to enhance the specific activity of Mn sites for methane combustion. 相似文献
6.
A series of palladium-substituted La 2CuO 4, corresponding to the formula La 2Cu 1 −xPd xO 4 ( x = 0−0.2) were prepared by metal nitrate decomposition in a polyacrylamide gel. This method allows an easy incorporation of palladium in the mixed-oxides, which are formed at moderate temperature with rather high specific areas (13–17 m 2/g). The partial substitution of copper for palladium allows a strong improvement of the three-way catalytic activity, in particular for NO reduction. The light-off temperatures for the conversions of CO, NO and C 3H 6 decreased markedly when increasing the palladium content, the activity of catalysts La 2Cu 0.9Pd 0.1O 4 and La 2Cu 0.8Pd 0.2O 4 being comparable to that of a Pt-Rh/CeO 2–Al 2O 3 catalyst for NO reduction, and higher for CO and C 3H 6 oxidation. All the La2Cu1 − x PdxO4 catalysts are activated under reacting conditions. This activation corresponds to the destruction of the mixed-oxide structure, with formation of reduced Pd0 ions atomically dispersed, surrounded by Cu+ and Cu2+ species on a lanthanum oxycarbonate matrix. This high dispersion state of the two transition metals in various oxidation states is supposed to originate from the initial La2Cu1 −xPdxO4 structure. 相似文献
7.
La xSr 2−xMnO 4 (0 ≤ x ≤ 0.8) oxides were synthesized and single-phase K 2NiF 4-type oxides were obtained in the range of 0.1 ≤ x < 0.5. The catalytic activity of La xSr 2−xMnO 4 for NO–CO reaction increased with increasing x in the range of solubility limit of La. La 0.5Sr 1.5MnO 4 showed the highest activity among La xSr 2−xMnO 4 prepared in this study, but its activity was inferior to perovskite-type La 0.5Sr 0.5MnO 3. Among the Pd-loaded catalysts, however, Pd/La 0.8Sr 1.2MnO 4 showed the higher activity and the selectivity to N 2 than Pd/La 0.5Sr 0.5MnO 3 and Pd/γ-Al 2O 3. The excellent catalytic performance of Pd/La 0.2Sr 1.2MnO 4 could be ascribable to the formation of SrPd 3O 4 which was detected by XRD in the catalyst but not in the other two catalysts. 相似文献
8.
The aim of the present work is to obtain ceramic materials with a hexagonal structure and high density, hardness and mechanical strength at lower synthesis temperature. Ceramic samples with nominal composition La 1−xCa xAl 11−y−zMg yTi zO 18 ( x=0–1; y=0–3; z=0–3,5) are prepared. The samples are sintered at temperature 1500 °C by one-stage and two-stage ceramic technology. By X-ray diffraction and scanning electron microscopy, predominant phase LaAl 11O 18 and second phases LaAlO 3 and -Al 2O 3 are identified. Ceramic materials are characterized with high physico-mechanical properties and may be find application for production of mill bodies and materials for immobilization of nuclear waste. 相似文献
9.
Perovskites of different La 1−xSr xAl 1−y−y′Fe yMg y′O 3−δ compositions ( x=0, 0.1, 0.15, 0.2 and y=0.1, 0.3, 0.5, 0.8) were prepared from a reactive precursor slurry of hydrated oxides. Each sample was aged between 16 and 26 h up to 1473 K. Activity in methane combustion (1%/air) was determined in a plug-flow reactor, with 1 g catalyst and 24 l/h flowrate. Gradual decrease in activity due to thermal aging was observed, the degree of activity loss being composition dependent. Nevertheless, activity of samples aged at 1370 K was nearly independent of composition. The best thermal stability showed LaAl 0.65Fe 0.15Mg 0.2O 3 perovskite. None of the magnesium substituted perovskites performed better than a La 0.85Sr 0.15Al 0.87Fe 0.13O 3 reference sample. 相似文献
10.
The electronic states of LaMn 1−xCu xO 3+λ ( x=0–0.4) have been studied with X-ray photoelectron spectroscopy (XPS). The valence states of substituted copper ions were Cu 2+ and the manganese ions were a highly mixed state of Mn 3+ and Mn 4+. The nonstoichiometry and electronic state of lattice oxygen have been studied. The samples at x=0 and 0.1 had an excess of lattice oxygen but those at x=0.2–0.4 had lattice oxygen deficiency. A modified Auger parameter (Δ ′) was used to evaluate the electronic states of oxygen ions. The Δ ′ of lattice oxygen increased with increasing substitute quantity. This increase of Δ ′ reflected the decrease of ionic bond character of lattice oxygen. The adsorbed oxygen species on LaMn 1−xCu xO 3+λ was assigned mainly as O − from the peak positions of spectra for the O 1s and O KLL levels, and the Δ ′ of this O − decreased with x. This decrease, i.e., the increase of ionic bond character of adsorbed oxygen was correlated well with the value of nonstoichiometry of lattice oxygen. The rate of CO oxidation at 448 K was increased by the substitution till x=0.4. We consider that this enhancement of reactivity comes from the change of electronic state of adsorbed oxygen, O− itself, i.e., a weak interaction between O− and low coordinated metal site brings about a high reactivity. 相似文献
11.
Stoichiometric polycrystalline samples of Mn xMg 1−xFe 2O 4 (0·5 ≤ x ≤ 0·66) have been synthesized by following a novel route using stabilized MnO and Fe 2O 3 at high temperatures. This route precludes the formation of large amounts of Mn 3+ and Fe 2+ and precipitation of MgO and -Fe 2O 3 which are generally observed during the usual route of preparation by conventional ceramic techniques. These samples have been characterized for their structural and magnetic properties using X-ray diffraction, Fe 57 Mössbauer spectroscopy and bulk magnetic properties such as initial permeability, loss factor, ferromagnetic transition temperature, remanance and coercivity. For X = 0·62, these ferrites exhibit the highest remanance ratio 0·96, suitable for square loop applications. 相似文献
12.
Mixed oxides with compositions SrTi 1−x−yZr xMn yO 3, with 0 ≤ x ≤ 1 and 0 ≤ y ≤ 0.2 have been prepared with a conventional coprecipitation method. Some of them are constituted by very pure perovskite-type solid solution phases, with tetravalent Zr and Mn substituting for Ti in the B site. The addition of Zr to SrTiO 3 tends to increase the surface areas, while the insertion of Mn tends to decrease it. Mn-containing materials are active in the catalytic combustion of 1% methane in air at temperatures higher than 700 K and can be competitive with pure manganite perovskites like LaMnO 3 in spite of the lower Mn content. Pyridine adsorption experiments show that medium strength Lewis acid sites are located at the surface of these materials, and could be involved in the hydrocarbon CH bond activation. 相似文献
13.
The aim of this work was to study the effect of cation-substitution on the reducibility of the perovskite, as well as the effect on the catalytic activity for the CH 4 oxidation reaction. Six perovskites (LaCoO 3, LaMnO 3, La 1−xSr xMnO 3 ( x = 0.2, 0.4), and La 1−xCe xMnO 3 ( x = 0.05, 0.1)) were synthesized by reactive grinding. The reducibility of the perovskite was studied by means of the oxygen storage capacity (OSC) measurement. OSC was performed at different temperatures on LaCoO 3 and LaMnO 3, in order to elucidate the different mechanisms of reduction involved at each temperature. The substituted samples showed that reduction profile is modified at high-substitution degrees; however, no differences were observed on the OSC values (amount of most active oxygen, calculated after one pulse of CO) between the pure lanthanum sample and the substituted ones. Tested in the CH4 oxidation reaction, the LaCoO3 sample was found to present a little higher activity than LaMnO3, even if the cobalt-based sample presented a smaller specific surface area. Moreover, all the substituted samples presented very slightly higher activities than the pure LaMnO3 solid. Because of the supposed redox oxidation mechanism (Mars-Van-Krevelen), this agrees well with the OSC results obtained for the reducibility of the manganese on these samples, by which it was observed that substitution does not clearly affect the immediate reduction of the manganese. 相似文献
14.
New catalytic systems, synthesised by a variant of the citrate route, are proposed for the partial oxidation of methane. They consist of solid solutions – barium, zirconium, rhodium and oxygen – with a perovskite structure of formula BaZr (1−x)Rh xO 3. Detailed analysis of the XRD diffractograms and the TGA cycles show that Rh is randomly distributed as Rh IV among the B sites of the perovskite, together with Zr. The activities of the catalysts have been tested for the catalytic partial oxidation of methane at short contact times to evaluate the potential of materials giving promising results in terms of syngas yield at low Rh loading. 相似文献
15.
Performances of BaTi 1 − xNi xO 3 perovskites, prepared using sol–gel method, as catalysts for partial oxidation of methane to syngas have been studied. The catalysts were characterized by XRD, BET and TEM. The experimental studies showed the calcination temperature and Ni content exhibited a significant influence on catalytic activity. Among catalysts tested, the catalyst BaTi 0.8Ni 0.2O 3 exhibited the best activity and excellent stability. 相似文献
16.
Mixed LaNi xFe (1−x)O 3 perovskite oxides (0≤ x≤1) have been prepared by a sol–gel related method, characterised by X-ray diffraction (XRD), specific surface area measurements, transmission electron microscopy (TEM) coupled to an energy dispersive X-ray spectrometer (EDS). These systems are the precursors of highly efficient catalysts in partial oxidation of methane to synthesis gas. Studies on the state of these systems after test show the stabilisation of active nickel by increasing the amount of iron. These systems permit to control the reversible migration of nickel from the structure to the surface. The best mixed perovskite for the partial oxidation of methane is LaNi 0.3Fe 0.7O 3. 相似文献
17.
Dry reforming of methane has been investigated on two series of catalysts either prepared by co-precipitation: n(Ni xMg y)/Al, Ni xMg y and Ni xAl y or prepared by impregnation: Ni/MgO (mol% Ni = 5, 10). The catalysts, calcined at 600–900 °C, were characterized by different techniques: BET, H 2-TPR, TPO, XRD, IR, and TEM-EDX analysis. The surface BET (30–182 m 2 g −1) decreased with increasing the temperature of calcination, after reduction and in the presence of Mg element. The XRD analysis showed, for n(Ni xMg y)/Al catalysts, the presence of NiAl 2O 4 and NiO–MgO solid solutions. The catalyst reducibility decreased with increasing the temperature of pretreatment. The n(Ni xMg y)/Al catalysts were active for dry reforming of methane with a good resistance to coke formation. The bimetallic catalyst Ni 0.05Mg 0.95 (calcined at 750 °C and tested at 800 °C) presents a poor activity. In contrast, the 5% Ni/MgO catalyst, having the same composition but prepared by impregnation, presents a high activity for the same calcination and reaction conditions. For all the catalysts the activity decreased with increasing the temperature of calcination and a previous H 2-reduction of the catalyst improves the performances. The TPO profiles and TEM-EDX analysis showed mainly four types of coke: CH x species, surface carbon, nickel carbide and carbon nanotubes. 相似文献
18.
Novel Ir-substituted hexaaluminate catalysts were developed for the first time and used for catalytic decomposition of high concentration of N 2O. The catalysts were prepared by one-pot precipitation and characterized by X-ray diffraction (XRD), N 2-adsorption, scanning electronic microscopy (SEM) and temperature-programmed reduction (H 2-TPR). The XRD results showed that only a limited amount of iridium was incorporated into the hexaaluminate lattice by substituting Al 3+ to form BaIr xFe 1−xAl 11O 19 after being calcined at 1200 °C, while the other part of iridium existed as IrO 2 phase. The activity tests for high concentration (30%, v/v) of N 2O decomposition demonstrated that the BaIr xFe 1−xAl 11O 19 hexaaluminates exhibited much higher activities and stabilities than the Ir/Al 2O 3-1200, and the pre-reduction with H 2 was essential for activating the catalysts. By comparing BaIr xFe 1−xAl 11O 19 with BaIr xAl 12−xO 19 ( x = 0–0.8), it was found that iridium was the active component in the N 2O decomposition and the framework iridium was more active than the large IrO 2 particles. On the other hand, Fe facilitated the formation of hexaaluminate as well as the incorporation of iridium into the framework. 相似文献
19.
The structure evolution, and microwave dielectric properties of Nd (2−x)/3Li xTiO 3 ceramics (0 ≤ x ≤ 0.5) were investigated in this paper. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results show that samples with x = 0.2–0.4 exhibit single phase. Multi-phases of Nd 2Ti 2O 7, Nd 2/3TiO 3 and Nd 2Ti 4O 11 were observed when x = 0 and 0.1. The concentration and ordering degree of A-site decrease with the increase of x value. The dielectric constant increases up to x = 0.2 and then decreases with the further increase of x value. The Qf value decreases with the increase of x value. The temperature coefficient of resonant frequency exhibits negative value and the absolute value decreases greatly with the decrease of x value. 相似文献
20.
This paper reports the application of mixed conducting perovskite membranes for simultaneous in situ O 2 separation and catalytic oxidation of ammonia to nitric oxide. Calcium and strontium-substituted lanthanum ferrite perovskites (La 1−xA xFeO 3−δ, with A = Ca, Sr and x = 0.1–0.2) were prepared by a conventional wet complexation route with citric acid. Dense membrane disks were obtained by uniaxial pressing of the calcined powders followed by sintering. The oxidation of ammonia over these catalytic membranes in the temperature range of 1000–1333 K leads to NO selectivities up to 98% with no N 2O production, showing stable performance during several days on stream. This novel process demonstrates the great potential for intensification of nitric acid manufacture into miniaturized plants and is considered to be particularly attractive for on-site production of this important bulk chemical. 相似文献
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