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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.
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
3.
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
4.
LaMn 1−xMg xO 3 perovskite catalysts ( x=0–0.5) were synthesised by the so-called “citrates method”, characterised (chemical analysis, TEM, BET, XRD, temperature-programmed desorption of oxygen) and tested for their activity towards the catalytic combustion of methane. The role of MgO as a textural promoter, which hinders the sintering of the catalyst crystals by geometrical interposition, has also been assessed. Finally, a kinetics study was performed on the most promising catalysts prepared (LaMnO 3 and LaMn 0.8Mg 0.2O 3). The major results obtained are: (i) Mg substitution in the basic LaMnO 3 perovskite has a positive effect on the catalytic activity only at low x values ( x≤0.2); (ii) as opposed to the results of previous studies on the LaCr 1−xMg xO 3 system, the role of MgO as a textural promoter is not always significant and depends strongly on the calcination temperature of the samples (800–1200°C) and on the value of x; (iii) an Eley–Rideal mechanism could satisfactorily fit the experimental kinetics results for both catalysts, even though, as opposed to LaMnO 3, the catalytic combustion over LaMn 0.8Mg 0.2O 3 seems to involve two different types of adsorbed oxygen species, depending on the operating temperature. 相似文献
5.
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
6.
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
7.
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
8.
To clarify the effect of substitutional electron doping on the thermoelectric figure of merit ( ZT = S2σTκ−1) of Ruddlesden–Popper phase SrO(SrTiO 3) n (or Sr n+1Ti nO 3n+1), measurements were conducted for several thermoelectric parameters, e.g. electrical conductivity ( σ), Seebeck coefficient ( S) and thermal conductivity ( κ), of (Sr 1−xREx) n+1Ti nO 3n+1 ( n = 1 or 2, RE (rare earth): La or Nd, x = 0.05 and 0.1) dense ceramics prepared by a conventional solid-state reaction and hot-pressing technique. Crystal structures of the resultant ceramics were represented as (Sr 1−xREx) n+1 Ti nO 3n+1 evaluated by powder X-ray diffraction followed by the Rietveld analysis. All the ceramics exhibited electrical conductivity and the σ values simply depended on the dopant concentration, indicating that both La 3+ and Nd 3+ ions act as electron donors. The | S| values increased with temperature due to decrease in the chemical potential. Significant reduction of the κ values was observed as compared to cubic-perovskite SrTiO 3. The ZT value increased with temperature and reached 0.15 at 1000 K for (Sr 0.95La 0.05) 3Ti 2O 7. 相似文献
9.
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
10.
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
11.
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
12.
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
13.
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
14.
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
15.
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
16.
A series of La(Co, Mn, Fe) 1−x(Cu, Pd) xO 3 perovskites having high specific surface areas and nanosized crystal domains was prepared by reactive grinding. The solids were characterized by N 2 adsorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), temperature programmed desorption (TPD) of O 2, NO + O 2, C 3H 6, in the absence or presence of 5% H 2O, Fourier transform infrared (FTIR) spectroscopy, as well as activity tests towards NO reduction by propene under the conditions of 3000 ppm NO, 3000 ppm C 3H 6, 1% O 2, 0 or 10% H 2O, and 50,000 h −1 space velocity. The objective was to investigate the influence of H 2O addition on catalytic behavior. A good performance (100% NO conversion, 77% N 2 yield, and 90% C 3H 6 conversion) was achieved at 600 °C over LaFe 0.8Cu 0.2O 3 under a dry feed stream. With the exposure of LaFe 0.8Cu 0.2O 3 to a humid atmosphere containing 10% water vapor, the catalytic activity was slightly decreased yielding 91% NO conversion, 51% N 2 yield, and 86% C 3H 6 conversion. A competitive adsorption between H 2O vapor with O 2 and NO molecules at anion vacancies over LaFe 0.8Cu 0.2O 3 was found by means of TPD studies here. A deactivation mechanism was therefore proposed involving the occupation of available active sites by water vapor, resulting in an inhibition of catalytic activity in C 3H 6 + NO + O 2 reaction. This H 2O deactivation was also verified to be strictly reversible by removing steam from the feed. 相似文献
17.
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
18.
Supported LaCoO 3 perovskites with 10 and 20 wt.% loading were obtained by wet impregnation of different Ce 1−xZr xO 2 ( x = 0–0.3) supports with a solution prepared from La and Co nitrates, and citric acid. Supports were also prepared using the “citrate method”. All materials were calcined at 700 °C for 6 h and investigated by N 2 adsorption at −196 °C, XRD and XPS. XRD patterns and XPS measurements evidenced the formation of a pure perovskite phase, preferentially accumulated at the outer surface. These materials were comparatively tested in benzene and toluene total oxidation in the temperature range 100–500 °C. All catalysts showed a lower T50 than the corresponding Ce 1−xZr xO 2 supports. Twenty weight percent LaCoO 3 catalysts presented lower T50 than bulk LaCoO 3. In terms of reaction rates per mass unit of perovskite calculated at 300 °C, two facts should be noted (i) the activity order is more than 10 times higher for toluene and (ii) the reverse variation with the loading as a function of the reactant, a better activity being observed for low loadings in the case of benzene. For the same loading, the support composition influences drastically the oxidative abilities of LaCoO 3 by the surface area and the oxygen mobility. 相似文献
19.
Microwave dielectric properties of (Zn 1/3Nb 2/3) 0.40(Ti 1−xSn x) 0.60O 2 ceramics were investigated as a function of SnO 2 content (0.15 ≤ x ≤ 0.30). A single phase with tetragonal rutile structure was obtained through the entire composition. The unit-cell volume of the specimens was increased with SnO 2 content, due to the larger ionic radius of Sn 4+ (0.69 Å) than that of Ti 4+ (0.605 Å) for octahedral site. Dielectric constant ( K) of the sintered specimens was affected by the dielectric polarizability. Quality factor ( Qf) was dependent on the degree of reduction of Ti 4+ ion. With an increase of SnO 2 content, the temperature coefficient of resonant frequency ( TCF) of the specimens decreased due to the decrease of the octahedral distortion of rutile structure. 相似文献
20.
Supported Au catalysts Au-Au +-Cl x/Fe(OH) y ( x < 4, y ≤ 3) and Au-Cl x/Fe 2O 3 prepared with co-precipitation without any washing to remove Cl − and without calcining or calcined at 400 °C were studied. It was found that the presence of Cl − had little impact on the activity over the unwashed and uncalcined catalysts; however, the activity for CO oxidation would be greatly reduced only after Au-Au +-Cl x/Fe(OH) y was further calcined at elevated temperatures, such as 400 °C. XPS investigation showed that Au in catalyst without calcining was composed of Au and Au +, while after calcined at 400 °C it reduced to Au 0 completely. It also showed that catalysts precipitated at 70 °C could form more Au + species than that precipitated at room temperatures. Results of XRD and TEM characterizations indicated that without calcining not only the Au nano-particles but also the supports were highly dispersed, while calcined at 400 °C, the Au nano-particles aggregated and the supports changed to lump sinter. Results of UV–vis observation showed that the Fe(NO 3) 3 and HAuCl 4 hydrolyzed partially to form Fe(OH) 3 and [AuCl x(OH) 4−x] − ( x = 1–3), respectively, at 70 °C, and such pre-partially hydrolyzed iron and gold species and the possible interaction between them during the hydrolysis may be favorable for the formation of more active precursor and to avoid the formation of Au–Cl bonds. Results of computer simulation showed that the reaction molecular of CO or O 2 were more easily adsorbed on Au + and Au 0, but was very difficultly absorbed on Au −. It also indicated that when Cl − was adsorbed on Au 0, the Au atom would mostly take a negative electric charge, which would restrain the adsorption of the reaction molecular severely and restrain the subsequent reactions while when Cl − was adsorbed on Au + there only a little of the Au atom take negative electric charge, which resulting a little impact on the activity. 相似文献
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