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1.
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
2.
The binary systems ReS 2–TaS 2 and OsS 2-TaS 2 are studied. Mixed layer structure (MLS) phases are found in Re xTa 1−xS 2 with a composition range of 0.25 x0.5, as well as in the Os xTa 1−xS 2 with a composition range of 0.26 x0.33. The MLSs of both phases are constructed by a random and mixed stacking of the 2Hb-layers and 3R-layers. The magnetic susceptibilities of samples from both phases show a weak Pauli-paramagnetism. The paramagnetic moment and the electrical conductivity of both phases decrease as the composition x increases. The behaviour of the paramagnetic moment and the electrical conductivity of those phases offer us a good example of the number of conduction electrons and their effect. 相似文献
3.
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
4.
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
5.
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
6.
The structural and electronic properties of selected compositions of Sn xTi 1−xO 2 solid solutions ( x=0, 1/24, 1/16, 1/12, 1/8, 1/6, 1/4, 1/2, 3/4, 5/6, 7/8, 11/12, 15/16, 23/24 and 1) were investigated by means of periodic density functional theory (DFT) calculations at B3LYP level. The calculations show that the corresponding lattice parameters vary non-linearly with composition, supporting positive deviations from Vegard’s law in the Sn xTi 1−xO 2 system. Our results also account for the fact that chemical decomposition in Sn xTi 1−xO 2 system is dominated by composition fluctuations along [0 0 1] direction. A nearly continuous evolution of the direct band gap and the Fermi level with the growing value of x is predicted. Ti 3d states dominate the lower portion of the conduction band of Sn xTi 1−xO 2 solid solutions. Sn substitution for Ti in TiO 2 increases the oxidation–reduction potential of the oxide as well as it renders the lowest energy transition to be indirect. These two effects can be the key factors controlling the rate for the photogenerated electron–hole recombination. These theoretical results are capable to explain the enhancement of photoactivity in Sn xTi 1−xO 2 solid solutions. 相似文献
7.
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
8.
Electrical resistivity and Seebeck ( S) measurements were performed on (La 1−xSr x)MnO 3 (0.02 x0.50) and (La 1−xSr x)CoO 3 (0 x0.15) in air up to 1073 K. (La 1−xSr x)MnO 3 ( x0.35) showed a metal-to-semiconductor transition; the transition temperature almost linearly increased from 250 to 390 K with increasing Sr content. The semiconductor phase above the transition temperature showed negative values of S. (La 1−xSr x)CoO 3 (0 x0.10) showed a semiconductor-to-metal transition at 500 K. Dominant carriers were holes for the samples of x0.02 above room temperature. LaCoO 3 showed large negative values of S below ca. 400 K, indicative of the electron conduction in the semiconductor phase. 相似文献
9.
Ceramics with a composition close to BaZn 2Ti 4O 11 were synthesized according to various substitutional mechanisms in order to verify an existence of a homogeneity range in the vicinity of this composition. Structural and microstructural investigations showed that the crystal structure of BaZn 2Ti 4O 11 was formed in the homogeneity range corresponding to the formula BaZn 2 − xTi 4O 11 − x (0 < x < 0.1). Densely sintered BaZn 2 − xTi 4O 11 − x (0 < x < 0.1) ceramics exhibited a dielectric constant around 30, τf = −30 ppm/K and high Q × f values, which increased from 68,000 GHz at x = 0 to 83,000 GHz at x = 0.05. Structurally, the deficiency of Zn in BaZn 2 − xTi 4O 11 − x (0 < x < 0.1) resulted in a slight decrease in the unit-cell volume. The influence of secondary phases in the BaZn 2Ti 4O 11-based materials on the microwave dielectric properties was also investigated. A presence of small amounts of ZnO, BaTiO 3, hollandite-type solid solutions (Ba xZn xTi 8 − xO 16) and BaTi 4O 9 caused a decrease in Q × f values. 相似文献
10.
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
11.
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
12.
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
13.
The weight and plaque diameter change following trermal treatment, and the density and lattice constant of lithium-rich Li xNi 1−xO solid solutions obtained by heat treatment at 750°C of Li 2CO 3---Li yNi 1−yO ( y < x) mixtures were measured as a function of time. It was found that densification of the solid solutions depends on the method of obtaining the Li 2CO 3---Li yNi 1−yO mixtures. When a large amount of carbonate is put into the starting mixtures Ni---Li 2CO 3-binder, and the powders are milled together, sintering occurs; in contrast, when the carbonate is added after the formation of Li yNi 1−yO solid solution, so that the interstices of the plaque are filled with molten carbonate, the resulting mixtures exhibit no densification. For densification to occur, Li 2CO 3 must be in intimate contact with the lithium-doped NiO grains. 相似文献
14.
We have used a complex sol–gel process to synthesize a family of compounds LiNi xCo 1−xO 2 ( x = 0, 0.25, 0.5, 0.75, 1). These compounds are candidates for electrode materials in high-energy-density batteries. Starting sols were prepared from xNi 2+ + (1 − x) Co 2+ acetates/ascorbic acid aqueous solutions by alkalizing with LiOH and NH 3. With thermal treatment in air, nickel carbonates formed in quantities roughly proportional to Ni concentration. The carbonate impurities could not be fully removed by heating in air to high temperatures. Because formation of pure layered oxides was inhibited by the presence of the carbonates, we developed a new way to remove them from just-formed precursors by treating the intermediate phases (those formed after calcination at 750 °C) with concentrated HNO 3 and H 2O 2. All resulting powders were phase pure by X-ray diffraction and were easily friable. Various electrochemical properties of compacts prepared from these powders were measured. 相似文献
15.
This study focuses on the direct sulfur recovery process (DSRP), in which SO 2 can be directly converted into elemental sulfur using a variety of reducing agents over Ce 1−xZr xO 2 catalysts. Ce 1−xZr xO 2 catalysts (where x = 0.2, 0.5, and 0.8) were prepared by a citric complexation method. The experimental conditions used for SO 2 reduction were as follow: the space velocity (GHSV) was 30,000 ml/g -cat h and the ratio of [CO (or H 2, H 2 + CO)]/[SO 2] was 2.0. It was found that the catalyst and reducing agent providing the best performance were the Ce 0.5Zr 0.5O 2 catalyst and CO, respectively. In this case, the SO 2 conversion was about 92% and the sulfur yield was about 90% at 550 °C. Also, a higher efficiency of SO 2 removal and elemental sulfur recovery was achieved in the reduction of SO 2 with CO as a reducing agent than that with H 2. In the reduction of SO 2 by H 2 over the Ce 0.5Zr 0.5O 2 catalyst, SO 2 conversion and sulfur yield were about 92.7% and 73%, respectively, at 800 °C. Also, the reduction of SO 2 using synthetic gas with various [CO]/[H 2] molar ratios over the Ce 0.5Zr 0.5O 2 catalyst was performed, in order to investigate the possibility of using coal-derived gas as a reducing agent in the DSRP. It was found that the reactivity of the SO 2 reduction using the synthetic gas with various [CO]/[H 2] molar ratios was increased with increasing CO content of the synthetic gas. Therefore, it was found that the Ce 1−xZr xO 2 catalysts are applicable to the DSRP using coal-derived gas, which contains a larger percentage of CO than H 2. 相似文献
16.
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
17.
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
18.
Layered Li[Ni (1−x)/3Mn (1−x)/3Co (1−x)/3Cr x]O 2 materials with x = 0, 0.01, 0.02, 0.03, 0.05 are prepared by a solid-state pyrolysis method. The oxide compounds were calcined with various Cr-doped contents, which result in greater difference in morphological (shape, particle size and specific surface area) and the electrochemical (first charge profile, reversible capacity and rate capability) differences. The Li[Ni (1−x)/3Mn (1−x)/3Co (1−x)/3Cr x]O 2 powders were characterized by means of X-ray diffraction (XRD), charge/discharge cycling, cyclic voltammetry, and SEM. XRD experiment revealed that the Li[Ni (1−x)/3Mn (1−x)/3Co (1−x)/3Cr x]O 2 ( x = 0, 0.01, 0.02, 0.03, 0.05) were crystallized to well layered -NaFeO 2 structure. The first specific discharge capacity and coulombic efficiency of the electrode of Cr-doped materials were higher than that of pristine material. When x = 0.02, the sample showed the highest first discharge capacity of 241.9 mAh g −1 at a current density of 30 mA g −1 in the voltage range 2.3–4.6 V, and the Cr-doped samples exhibited higher discharge capacity and better cycleability under medium and high current densities at room temperature. 相似文献
19.
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
20.
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
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