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1.
Barium, lanthanum, and praseodymium perovskites were prepared by malic acid complexation. Surface areas of the La and Pr perovskites
were between 17.1 and 21.6 m 2 g −1. The moderate low surface areas (5.7 m 2 g −1) observed for corresponding barium perovskites were due to the high calcination temperatures. The calcination temperature
also affected the shapes and sizes of the perovskite particles. According to SEM images the nanoparticles of the La and Pr
perovskites were spherical, whereas those of barium perovskites were flakes. The conversion of methane increased in the order
of A-site metal Ba < Pr < La. The CH 4 conversion after SO 2 treatment correlated with size of the perovskite particles: the smaller the particles the better the activity. The highest
methane conversion after SO 2 treatment was achieved with lanthanum perovskite with B-site metal combination Mn 0.4Fe 0.6. 相似文献
2.
Background The effect of the B cation on the surface properties and catalytic activity in the total combustion of toluene over La 1?xCa xBO 3 (B = Fe,Ni) perovskite‐type oxides was studied. Result For the La 1?xCa xFeO 3 series, the perovskite structure was maintained in the range of substitution studied. A completely different behaviour was observed for the La 1?xCa xNiO 3 series. A Brownmillerite‐type structure (La 2Ni 2O 5) with a large degree of phase segregation as well dispersed mixed oxides was observed upon the substitution of La for Ca. In the Fe series, the catalytic activity in the total combustion of toluene showed that the insertion of calcium ions into the perovskite lattice resulted in higher activity relative to the unsubstituted LaFeO 3 perovskite. In contrast, for the Ni series, substitution results in solids with lower activity than the pure LaNiO 3 perovskite. Conclusion For the Fe series, higher activity and stability are attributed to a synergy between Fe 4+/Fe 3+ and the oxygen vacancies generated by the calcium substitution. For the Ni counterpart, the structural modification leads to a lower activity of substituted solids compared with the pure LaNiO 3 perovskite, indicating that Ni 3+ ions are the active sites for toluene oxidation. Copyright © 2011 Society of Chemical Industry 相似文献
3.
This paper presents results which were obtained for the flameless combustion of methane over the Pd(PdO)/Al 2O 3 catalyst by using the steady state isotopic transient kinetic analysis method. During the reaction switches between 16O 2/Ar/CH 4/He and 18O 2/CH 4/He were carried out. The obtained results indicate the presence of large amounts of oxygen as well as of intermediates leading to the formation of carbon dioxide on the surface of the palladium catalyst. Additionally, information was obtained proving that the complete oxidation of methane over Pd/Al 2O 3 catalyst proceeds according to the Mars and van Krevelen redox mechanism. With the increase of the reaction temperature there is an increase in the number of active centres on the Pd(PdO)/Al 2O 3 catalyst surface—a larger amount of oxygen from the lattice of the catalyst is accessible for the reaction of methane oxidation. 相似文献
4.
Catalytic CO oxidation and C 3H 6 combustion have been studied over La 1−xSr xCrO 3 ( x = 0.0–0.3) oxides prepared by solid-state reaction and characterised by X-ray diffraction (XRD), nitrogen adsorption (BET analysis) and X-ray photoelectron spectroscopy (XPS). The expected orthorhombic perovskite structure of the chromite is observed for all levels of substitution. However, surface segregation of strontium along with a chromium oxidation process, leading to formation of Cr 6+-containing phases, is produced upon increasing x and shown to be detrimental to the catalytic activity. Maximum activity is achieved for the catalyst with x = 0.1 in which mixed oxide formation upon substitution of lanthanum by strontium in the chromite becomes maximised. 相似文献
5.
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
6.
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
7.
Ce 4+ doped Ba 3 WO 6 complex oxides were used as catalysts for methane oxidative coupling (MOC), and characterized by XPS and O 2-TPD-MS techniques. The results indicate that the ratio of electrophilic oxygen species O – and O
2
–
to lattice oxygen on the surface is crucial for C 2 selectivity. By adjusting the relative amount of cations in Ba-W-Ce complex oxides with perovskite superstructure interstitial oxygen species can be created which benefits C 2 selectivity by raising the relative amount of (O – + O
2
–
) on the surface. 相似文献
8.
A series of A‐modified hexaaluminates, ANiAl 11O 19-δ (A = Ca, Sr, Ba and La) as new catalysts for carbon dioxide reforming of methane to synthesis gas, were prepared by decomposition
of nitrates and calcination at high temperature. Nickel ions as active component were inlayed in the hexaaluminate lattices
to substitute part of Al ions. The structure and properties of these samples were characterized using XRD, XPS, TPR and TGA
techniques. The series of hexaaluminates exhibited significantly catalytic activity and stability at high temperature, for
instance at 780°C for 18 h, the conversion of CH 4 and CO 2 was kept over 91.0 and 93.7%, respectively, meanwhile no Ni sintering, phase transformation and catalyst deactivation due
to carbon deposition were found. Besides, the modifier A in the mirror plane layer of the lattices showed different effects
on reducibility and catalytic activity of transition metal Ni in the hexaaluminate lattices.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
9.
The microwave dielectric properties of (Ba xMg 1−x)(A 0.05Ti 0.95)TiO 3 (A=Zr, Sn) ceramics were investigated with regard to substitution of Ba for Mg of A-site. The microwave dielectric properties were correlated with the Ba content. With an increase in Ba content from 0.01 to 0.1, the dielectric constant and the τf value increased, but the Q× f value decreased. The sintered (Ba xMg 1−x)(Zr 0.05Ti 0.95)TiO 3 (called B xMZT) ceramics had a permittivity in the range of 19.1−20.6, quality factor from 180,000 to 25,000 GHz, and variation in temperature coefficient of resonant frequency from −35 to −39 ppm/°C with increasing composition x. For sintered (Ba xMg 1−x)(Sn 0.05Ti 0.95)TiO 3 (called B xMST) ceramics, the dielectric constant increased from 19 to 20.5, Q× f value increased from 120,000 to 37,000 (GHz), and the τf value increased from −50 to −3.3 ppm/°C as the x increased from 0.01 to 0.1. When A=Sn and x=0.1, (Ba 0.1Mg 0.9)(Sn 0.05Ti 0.95)TiO 3 ceramics exhibited dielectric constant of 20.5, Q× f value of 37,000 (GHz), and a near-zero τf value of −3.3 ppm/°C sintered at 1210 °C for 4 h. 相似文献
10.
Isotopic transient techniques were applied to study oxidative coupling of methane over lanthanum oxide and strontium promoted La 2O 3 catalysts. Results of the 18O 2/ 16O 2 isotopic exchange experiments indicate that Sr promotion increases oxygen uptake from the lattice of the catalyst. Oxygen self diffusion coefficients, which were determined for the series of lanthana catalysts, reach a maximum for the 1% Sr/La 2O 3. Steps of 18O 2 in the presence of a steady flow of methane over Sr/La 2O 3 catalysts, indicate that surface and bulk oxygen appear in the reaction products before gas-phase 18O 2. Steps of CO 2 over catalysts in which lattice oxygen has been exchanged with 18O 2, show that gas/solid exchange involves over 50% of the lattice oxygen. Under reaction conditions, methane pulses with no gas-phase oxygen yield negligible amounts of products which indicates that methane interacts with lattice oxygen only in the presence of the gas-phase oxygen. 相似文献
11.
A series of high-entropy oxides (La 0.25Sm 0.25Gd 0.25Yb 0.25) 2Ce 2+xO 7+2x were synthesised adopting a improved sol-gel technique and fritting method. The crystal-lattice, microstructure, elemental constitution, and thermal-physical performances were studied. The results showed that the synthesised high-entropy oxides have a single-fluorite lattice structure. The bulk specimen exhibits a compact microstructure, and clear grain boundaries. The thermal conductivities of the obtained high-entropy oxides are lower than those of CeO 2 and 7YSZ due to lattice strains and numerous oxygen vacancies. The obtained high-entropy oxides have greater thermal expansion coefficients than 7YSZ. The thermal conductivity and expansion coefficient are elevated because of the addition of excess CeO 2. The synthesised high-entropy oxides also exhibit outstanding lattice steadiness up to 1200 °C. 相似文献
12.
Addition of promoters, such as Li 2O, Na 2O, PbO, La 2O 3, MgCl 2 and CaCl 2, to MgO causes a large increase in its surface basicity (particularly strong basic sites) and catalytic activity/selectivity in oxidative coupling of methane, but the correlation between the basicity and C 2-yield is poor, indicating that factors other than basicity are also important in deciding catalytic performance. 相似文献
13.
Zirconia supported on alumina was prepared and characterized by BET surface area, X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), temperature programmed desorption (TPD), and pulse reaction. 0.2% Pd/ZrO 2/Al 2O 3 catalyst were prepared by incipient wetness impregnation of supports with aqueous solution of Pd(NO 3) 2. The effects of support properties on catalytic activity for methane combustion and CO oxidation were investigated. The results show that ZrO 2 is highly dispersed on the surface of Al 2O 3 up to 10 wt.% ZrO 2, beyond this value tetragonal ZrO 2 is formed. The presence of a small amount of ZrO 2 can increase the surface area, pore volume and acidity of support. CO–TPD results show that the increase of CO adsorption capacity and the activation of CO bond after the presence of ZrO 2 lead to the increase of catalytic activity of Pd catalyst for CO oxidation. CO pulse reaction results indicate that the lattice oxygen of support can be activated at lower temperature following the presence of ZrO 2, but it does not accelerate the activity of 0.2% Pd/ZrO 2/Al 2O 3 for methane combustion. 0.2% Pd/ZrO 2/Al 2O 3 dried at 120 °C shows highest activity for CH 4 combustion, and the activity can be further enhanced following the repeat run. The increase of treatment temperature and pre-reduction can decrease the activity of catalyst for CH 4 combustion. 相似文献
14.
Perovskite oxides La 0.8R 0.2Fe 0.5Cr 0.5O 3 (R=La, Nd, Gd and Dy) have been synthesized by solid state reaction method. The Rietveld analysis of the XRD data showed that the samples crystallize with orthorhombic structure in the space group Pbnm. The results of resistivity measurements revealed that all the samples show insulating behaviour throughout the temperature range studied. The electrical conduction in these polycrystalline samples can be described by the adiabatic small polaron hoping model, with an apparent increase of polaron activation energy with the substitution of heavier rare earth ion. 相似文献
15.
合成了稀土高氯酸盐与咪唑、DL α 丙氨酸的4种配合物晶体。经傅里叶变换红外光谱(FTIR)、元素分析和化学分析测定后确定其组成为[Ln(C3H7NO2)x(C3H4N2)(H2O)](ClO4)3(Ln=La,x=3;Ln=Pr,Nd,Sm,x=2)。用差示扫描量热法(DSC)测定了4种配合物的热分解数据,4个配合物开始熔化分解的温度分别为502 5K,532 4K,495 7K,516 7K,配合物有较高的热稳定性。 相似文献
16.
(La, Ce, Pr, Nd) 2MgNi 9 hydrogen storage alloys were prepared through induction melting followed by a long annealing treatment. The structure and electrochemical properties of annealed alloys have been investigated by orthogonal design experiments. Both the individual effects of each substituting element and their interaction in alloys were studied systemically. It has been shown that the structure of main phase in alloys belongs to PuNi 3-type with a space group R-3 m. Substituting rare-earth elements have a significant effect on both the phase structure of alloys and microstructure. The anisotropic change in the crystal structure of alloys can cause the acceleration of pulverization of alloy particles and result in the deterioration of the cyclic stability of alloy electrodes. Misch metals can raise the plateau pressure of hydrogen absorption/desorption. The discharge capacity of alloy ranges from 342.97 to 380.68 mAh g −1 depending on the sort and content of substituting elements. Both cerium and neodymium can obviously reduce the discharge capacity of alloy electrodes. When compared to the La 2MgNi 9 alloy electrode, mish metals can significantly improve the high rate dischargeability of alloy electrodes. The improvement of the kinetic characteristic of alloy electrodes mainly results from the increase of the hydrogen diffusion rate in alloy bulk. 相似文献
17.
Effect on the ionic conductivity of B-site ion (M) substitution in (Li 3xLa 2/3−x) 1+y/2M yTi 1−yO 3 (M=Al, Cr) has been investigated. It has been found that partial substitution of smaller Al 3+ for Ti 4+ is effective to enhance the ionic conductivity of Li 3xLa 2/3−xTiO 3. At 300 K, the maximum bulk conductivity of (1.58±0.01)×10 −3 S cm −1 is observed from the composition of (Li 0.39La 0.54) 1−y/2Al yTi 1−yO 3 with y=0.02 ( x=0.13), that is the highest yet reported for known perovskite solutions at room temperature. The conductivity enhancement is interpreted as being due to the substitution-induced bond-strength change rather than due to bottleneck size change for Li migration, TiO 6-octahedron tilting or A-site cation ordering. 相似文献
18.
Gd 3+-ESR spectroscopy can be used as a sensitive method for the study of lanthanide additives in catalysts. Here we present the results of a comparative study of Gd/SiO 2-Al 2O 3 and Gd/HZSM-5. ESR gives evidence of rigid bonding of isolated Gd 3+ ions into both amorphous silica-alumina and into HZSM-5. In addition, the zeolitic matrix stabilizes very small Gd 3+-clusters (containing only a few ions) capable of interacting with water molecules. Excess Gd is present as non-dispersed, particulate oxide. Strong bonding of PO 33- anionic ligands irreversibly changes the local environment and reactivity towards H 2O of the Gd 3+-clusters in HZSM-5. The Gd 3+ ions do not block the cationic positions of HZSM-5 from further interaction with paramagnetic Cu 2+ or Rh 2+ cations. This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
19.
The present study deals with the mechanism of the conversion of NO
x
and soot into N 2 and CO 2 on Fe 2O 3 catalyst. The results of TPO, TRM, DRIFTS and HRTEM examinations suggest a mechanism, in which NO is reduced by dissociation
on active carbon sites leading to the formation of N 2 and surface oxygen groups. The role of the catalyst lies in the activation of the soot by transferring oxygen from Fe 2O 3 to soot surface. 相似文献
20.
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
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