共查询到20条相似文献,搜索用时 15 毫秒
1.
The sintering properties of La 1−xSr xFeO 3−δ ( x = 0.1, 0.25) mixed conductors have been investigated with particular emphasis on the effect of secondary phases due to cation non-stoichiometry (±5 mol% La excess and deficiency). Secondary phases, located at grain boundaries in cation non-stoichiometric materials, increased the sintering temperature compared to single-phase materials. Extensive swelling in final stage of sintering was observed in all materials, which resulted in micro-porous materials. The swelling was most pronounced in the phase pure and two-phase materials due to La-deficiency, while refractory secondary phases in La-excess materials inhibited both sintering, grain growth and swelling. In La-deficient materials, formation of molten secondary phases resulted in rapid swelling due to viscous flow. The present findings demonstrated the importance of controlling sintering temperature and time, as well as careful control of the cation stoichiometry of La 1−xSr xFeO 3−δ in order to achieve fully dense and homogenous La 1−xSr xFeO 3−δ ceramics. 相似文献
2.
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
3.
La 1−xSr xMnO 3 ( x=0, 0.1, 0.3, 0.5, 0.7) perovskite-type oxides (PTOs) were prepared by coprecipitation under various calcination temperature, and their performances for the NO reduction were evaluated under a simulated exhaust gas mixture. The X-ray diffraction (XRD) and thermogravimetric analysis were carried out to find the formation process of the perovskite. The NO reduction rate under different reaction temperature, the concentration of oxygen and the presence of hydrocarbon were observed by the input/output analysis. In the presence of 10% excess oxygen, the catalyst La 0.7Sr 0.3MnO 3 calcined at 900 °C showed a NO reduction rate of 61% at 380 °C. The study of the reaction curves showed that C 3H 8 could act as the reducer for the NO reduction below 400 °C. The NO reduction is highly affected by increasing the O 2 concentration from 0.5 to 10%, especially at high temperatures when oxygen becomes more competitive than NO on the oxidation of C 3H 8, leading to a decrease of the NO reduction from 100% to zero at 560 °C. 相似文献
4.
Nanometer perovskite-type oxides La 1−xSr xMO 3−δ (M = Co, Mn; x = 0, 0.4) have been prepared using the citric acid complexing-hydrothermal-coupled method and characterized by means of techniques, such as X-ray diffraction (XRD), BET, high-resolution scanning electron microscopy (HRSEM), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and temperature-programmed reduction (TPR). The catalytic performance of these nanoperovskites in the combustion of ethylacetate (EA) has also been evaluated. The XRD results indicate that all the samples possessed single-phase rhombohedral crystal structures. The surface areas of these nanomaterials ranged from 20 to 33 m 2 g −1, the achievement of such high surface areas are due to the uniform morphology with the typical particle size of 40–80 nm (as can be clearly seen in their HRSEM images) that were derived with the citric acid complexing-hydrothermally coupled strategy. The XPS results demonstrate the presence of Mn 4+ and Mn 3+ in La 1−xSr xMnO 3−δ and Co 3+ and Co 2+ in La 1−xSr xCoO 3−δ, Sr substitution induced the rises in Mn 4+ and Co 3+ concentrations; adsorbed oxygen species (O −, O 2−, or O 22−) were detected on the catalyst surfaces. The O 2-TPD profiles indicate that Sr doping increased desorption of the adsorbed oxygen and lattice oxygen species at low temperatures. The H 2-TPR results reveal that the nanoperovskite catalysts could be reduced at much lower temperatures (<240 °C) after Sr doping. It is observed that under the conditions of EA concentration = 1000 ppm, EA/oxygen molar ratio = 1/400, and space velocity = 20,000 h −1, the catalytic activity (as reflected by the temperature ( T100%) for EA complete conversion) increased in the order of LaCoO 2.91 ( T100% = 230 °C) ≈ LaMnO 3.12 ( T100% = 235 °C) < La 0.6Sr 0.4MnO 3.02 ( T100% = 190 °C) < La 0.6Sr 0.4CoO 2.78 ( T100% = 175 °C); furthermore, there were no formation of partially oxidized by-products over these catalysts. Based on the above results, we conclude that the excellent catalytic performance is associated with the high surface areas, good redox properties (derived from higher Mn 4+/Mn 3+ and Co 3+/Co 2+ ratios), and rich lattice defects of the nanostructured La 1−xSr xMO 3−δ materials. 相似文献
5.
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
6.
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
7.
Effect of substitution of CuO and WO 3 on the microwave dielectric properties of BiNbO 4 ceramics and the co-firing between ceramics and copper electrode were investigated. The (Bi 1−xCu x)(Nb 1−xW x)O 4 ( x = 0.005, 0.01, 0.015, 0.02) composition can be densified between 900 and 990 °C. The microwave dielectric constants lie between 36 and 45 and the pores in ceramics were found to be the main influence. The Q values changes between 1400 and 2900 with different x values and sintering temperatures while Qf values lie between 6000 and 16,000 GHz. The microwave dielectric losses, mainly affected by the grain size, pores, and the secondary phase, are discussed. The (Bi 1−xCu x)(Nb 1−xW x)O 4 ceramics and copper electrode was co-fired under N 2 atmosphere at 850 °C and the EDS analysis showed no reaction between the dielectrics and copper electrodes. This result presented the (Bi 1−xCu x)(Nb 1−xW x)O 4 dielectric materials to be good candidates for LTCC applications with copper electrode. 相似文献
8.
The activities of perovskites depend on compositions and preparation methods. Various perovskites, La 1−xM xMnO 3 (M=Ag, Sr, Ce, La), have been prepared by two different methods (co-precipitation and spray decomposition). The new preparation method, spray decomposition, produced perovskites of a high surface area of over 10 m 2/g. The catalytic activities for CH 4 and CO oxidation have been studied on a series of catalysts, La 1−xM xMnO 3. The perovskite-type oxide, La 0.7Ag 0.3MnO 3, shows the highest catalytic activity: the complete conversion of CO and CH 4 at 370 and 825 K, respectively. 相似文献
9.
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
10.
This paper presents an investigation on the NO oxidation properties of perovskite oxides. La 1−xCe xCoO 3 ( x = 0, 0.05, 0.1, 0.2, 0.3, 0.4) perovskite-type oxides were synthesized through a citrate method and characterized by XRD, BET and XPS. The catalytic activities were enhanced significantly with Ce substitution, and achieved the best when x was 0.2, but decreased at higher x values. The performed characterizations reveal that the adsorbed oxygen on the surface plays an important role in the oxidation of NO into NO 2. The surface compounds after the co-adsorption of NO and O 2 at room temperature, were investigated by DRIFTS and TPD experiments. Three species: the bridging nitrate, the hyponitrites and the monodentate nitrate, were formed on the surface. The order of thermal stabilities was as follows: monodentate > hyponitrite > bridging. Among them, only the monodentate nitrate which decomposed at above 300 °C, would desorb NO 2 into the gas phase. When Ce was added, the temperature of monodentate nitrate desorption became low and the adsorption of the other two species decreased. This might be related to the oxidation state of Co on the surface. Analysis by synthesizing the characterization results and catalytic activity data shows that large amounts of adsorbed oxygen, small amount of inactive compounds on the surface and low NO 2 desorption temperature are favorable for the oxidation of NO. 相似文献
11.
The (Ba 1 − xSr x) (Nd 1/2Nb 1/2) O 3 ceramics have been prepared by the conventional ceramic route for different values of x. Addition of a small amount of CeO 2 (1 wt%) as a sintering aid increased the density of the samples. The structure and microstructure of the sintered samples are studied by X-ray diffraction and SEM methods. The dielectric properties of the samples are measured in the microwave frequency region as a function of composition. The dielectric constant decreases as x increases. The coefficient of thermal variation of resonant frequency decreases as the Sr content increases and goes to the negative side. The dielectric properties of (Ba 1 − xSr x) (Nd 1/2Nb 1/2) O 3 are in the range suitable for application as dielectric resonators in microwave circuits. 相似文献
12.
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
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.
Attrition and ball milling are used as mechanical means to reduce grain size of optimized fast oxide-ion conductors La 2−xR xMo 2−yW yO 9 (R: rare earths). Dilatometry is used to determine the optimal sintering conditions in order to obtain high density samples (greater than 96% of relative density) with help of scanning electron microscopy to characterize their microstructure. The optimal sintering temperatures are highly dependent on the chemical composition, and therefore identical annealing temperatures do not warrant similar relative densities. Complex impedance spectroscopy show that above the transition temperature of La 2Mo 2O 9 at 580 °C, the conductivity of all the studied compounds is lower than that of the parent compound, whereas just below the transition, in most cases the stabilization of the cubic phase increases conductivity. An interesting result is that tungsten substitution, which stabilizes La 2Mo 2O 9 against reduction, does not affect significantly the oxide ion conduction. 相似文献
15.
Mixed oxides of the general formula La 0.5Sr xCe yFeO z were prepared by using the nitrate method and characterized by XRD and Mössbauer techniques. The crystal phases detected were perovskites LaFeO 3 and SrFeO 3−x and oxides -Fe 2O 3 and CeO 2 depending on x and y values. The low surface area ceramic materials have been tested for the NO+CO and NO+CH 4+O 2 (“lean-NO x”) reactions in the temperature range 250–550°C. A noticeable enhancement in NO conversion was achieved by the substitution of La 3+ cation at A-site with divalent Sr +2 and tetravalent Ce +4 cations. Comparison of the activity of the present and other perovskite-type materials has pointed out that the ability of the La 0.5Sr xCe yFeO z materials to reduce NO by CO or by CH 4 under “lean-NO x” conditions is very satisfying. In particular, for the NO+CO reaction estimation of turnover frequencies (TOFs, s −1) at 300°C (based on NO chemisorption) revealed values comparable to Rh/-Al 2O 3 catalyst. This is an important result considering the current tendency for replacing the very active but expensive Rh and Pt metals. It was found that there is a direct correlation between the percentage of crystal phases containing iron in La 0.5Sr xCe yFeO z solids and their catalytic activity. O 2 TPD (temperature-programmed desorption) and NO TPD studies confirmed that the catalytic activity for both tested reactions is related to the defect positions in the lattice of the catalysts (e.g., oxygen vacancies, cationic defects). Additionally, a remarkable oscillatory behavior during O 2 TPD studies was observed for the La 0.5Sr 0.2Ce 0.3FeO z and La 0.5Sr 0.5FeO z solids. 相似文献
16.
Nanoparticles of Ce xZr 1−xO 2 ( x = 0.75, 0.62) were prepared by the oxidation-coprecipitation method using H 2O 2 as an oxidant, and characterized by N 2 adsorption, XRD and H 2-TPR. Ce xZr 1−xO 2 prepared had single fluorite cubic structure, good thermal stability and reduction property. With the increasing of Ce/Zr ratio, the surface area of Ce xZr 1−xO 2 increased, but thermal stability of Ce xZr 1−xO 2 decreased. The surface area of Ce 0.62Zr 0.38O 2 was 41.2 m 2/g after calcination in air at 900 °C for 6 h. TPR results showed the formation of solid solution promoted the reduction of CeO 2, and the reduction properties of Ce xZr 1−xO 2 were enhanced by the cycle of TPR-reoxidation. The Pd-only three-way catalysts (TWC) were prepared by the impregnation method, in which Ce 0.75Zr 0.25O 2 was used as the active washcoat and Pd loading was 0.7 g/L. In the test of Air/Fuel, the conversion of C 3H 8 was close to 100% and NO was completely converted at λ < 1.025. The high conversion of C 3H 8 was induced by the steam reform and dissociation adsorption reaction of C 3H 8. Pd-only catalyst using Ce 0.75Zr 0.25O 2 as active washcoat showed high light off activity, the reaction temperatures ( T50) of 50% conversion of CO, C 3H 8 and NO were 180, 200 and 205 °C, respectively. However, the conversions of C 3H 8 and NO showed oscillation with continuously increasing the reaction temperature. The presence of La 2O 3 in washcoat decreased the light off activity and suppressed the oscillation of C 3H 8 and NO conversion. After being aged at 900 °C for 4 h, the operation windows of catalysts shifted slightly to rich burn. The presence of La 2O 3 in active washcoat can enhance the thermal stability of catalyst significantly. 相似文献
17.
Layered -titanate materials, Na xM x/2Ti 1−x/2O 2 (M=Co, Ni and Fe, x=0.2–0.4), were synthesized by flux reactions, and electrical properties of polycrystalline products were measured at 300–800 °C. After sintering at 1250 °C in Ar, all products show n-type thermoelectric behavior. The values of both d.c. conductivity and Seebeck coefficient of polycrystalline Na 0.4Ni 0.2Ti 0.8O 2 were ca. 7×10 3 S/m and ca. −193 μV/K around 700 °C, respectively. The measured thermal conductivity of layered -titanate materials has lower value than conductive oxide materials. It was ca. 1.5 Wm −1 K −1 at 800 °C. The estimated thermoelectric figure-of-merit, Z, of Na 0.4Ni 0.2Ti 0.8O 2 and Na 0.4Co 0.2Ti 0.8O 2 was about 1.9×10 −4 and 1.2×10 −4 K −1 around 700 °C, respectively. 相似文献
18.
Ultrafine lithium ion conducting La 2/3−xLi 3xTiO 3 ( x = 0.11, LLT) powder was synthesized by a simple polymerizable complex method based on the Pechini-type process. The formation mechanism, homogeneity and microstructure of the samples were investigated by thermal analysis (TG/DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). XRD analysis indicated the formation of pure perovskite-type phase. The powder synthesized at a temperature as low as 900 °C in a much shorter time than solid-state reaction method was well crystallized. The lithium ion conductivity of the LLT ceramics sintered at 1200 °C was found to be 9 × 10 −4 S/cm at room temperature. 相似文献
19.
It is known that Curie temperature of barium titanate system can be altered by the substitution of dopants into either A- or B-site. Dopants could pinch transition temperature, lower Curie temperature, and raise the rhombohedral–orthorhombic and orthorhombic–tetragonal phase transition close to room temperature. This isovalent substitution could improve the ferroelectric properties of the BaTiO 3-based system. In this study, barium zirconate titanate Ba(Zr xTi 1−x)O 3 (BZT; x = 0, 0.02, 0.05 and 0.08) ceramics were prepared by conventionally mixed-oxide method. The ferroelectric properties of BZT ceramics were investigated. Increasing Zr content in the BaTiO 3-based compositions caused a decrease in Curie temperature ( Tc). At Tc, the highest relative permittivity of BZT with an addition of 0.08 mol% of Zr was 12,780. The BZT specimens with the additions of 0.05 mol% and 0.08 mol% of Zr presented the remanent polarization at 25 μC/cm 2 and 30 μC/cm 2, respectively. 相似文献
20.
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
|