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1.
New metal/oxide (Co–Fe) catalysts (with no reduction or thermal pre-treatment) are efficient to produce light hydrocarbons with a low selectivity in CO 2 by the Fischer–Tropsch synthesis. The low selectivity in CO 2 is due to the occurrence of the CO 2/H 2 reaction. These materials are stable under reaction conditions, and only few carbides are formed during the Fischer–Tropsch reaction. X-ray analyses indicate that the most degraded phase is the (Co–Fe) alloy phase in CO/H 2 reaction and the spinel phase in the CO 2/H 2 reaction. It was demonstrated that these composites do not behave as the simple sum of a spinel phase and a (Co–Fe) alloy but have their own properties. 相似文献
2.
Monolithic structures made of cordierite, γ-Al 2O 3 and steel have been prepared as catalysts and tested for Fischer–Tropsch activity. The monoliths made of cordierite and steel were washcoated with a 20 wt.% Co–1 wt.% Re/γ-Al 2O 3 Fischer–Tropsch catalyst whereas the γ-Al 2O 3 monoliths were made by direct impregnation with an aqueous solution of the Co and Re salts resulting in a loading of 12 wt.% Co and 0.5 wt.% Re. The activity and selectivity of the different monoliths were compared with the corresponding powder catalysts. Higher washcoat loadings resulted in decreased C5+ selectivity and olefin/paraffin ratios due to increased transport limitations. The impregnated γ-Al2O3 monoliths also showed similar C5+ selectivities as powder catalysts of small particle size (38–53 μm). Lower activities were observed with the steel monoliths probably due to experimental problems. 相似文献
3.
Co/ZSM-5 catalysts were prepared by several methods, including wet ion exchange (WIE), its combination with impregnation (IMP), solid state ion exchange (SSI) and sublimation (SUB). FTIR results show that the zeolite protons in H-ZSM-5 are completely removed when CoCl 2 vapor is deposited. TPR shows peaks for Co 2+ ions at 695–705°C and for Co 3O 4 at 385–390°C, but a peak in the 220–250°C region appears to indicate Co 2+ oxo-ions. The catalysts have been tested for the selective reduction of NOx with iso-C4H10 under O2-rich conditions and in the absence of O2, both with dry and wet feeds. A bifunctional mechanism appears to operate at low temperature: oxo-ions or Co3O4 clusters first oxidize NO to NO2, which is chemisorbed as NOy (y≥2) and reduced. In this modus operandi catalyst SUB shows the highest N2 yield 90% near 390°C for dry and wet feeds. It is found to be quite stable in a 52 h run with a wet feed. In contrast, the WIE catalyst, which mainly contains isolated Co2+ ions and has poor activity below 400°C, excels at T>430°C. This and the observation that, at high temperature, NO is reduced in O2-free feeds over Co/MFI catalysts, suggest that NO can be reduced over Co2+ ions without intermediate formation of NO2. The bifunctional mechanism at low temperature is supported by the fact that a strongly enhanced performance is obtained by mixing WIE with Fe/FER, a catalyst known to promote NO2 formation. 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.
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
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.
Co and Co–P catalysts electroplated on Cu in sulfate based solution without or with an addition of H 2PO 2− ions were developed for hydrogen generation from alkaline NaBH 4 solution. The microstructures of the Co and Co–P catalysts and their hydrogen generation properties were analyzed as a function of cathodic current density and plating time during the electrodeposition. An amorphous Co–P electrodeposit with micro-cracks was formed by electroplating in the sulfate based solution containing H 2PO 2− ions. It was found that the amorphous Co–P catalyst formed at 0.01 A/cm 2 exhibited 18 times higher catalytic activity for hydrolysis of NaBH 4 than did the polycrystalline Co catalyst. The catalytic activity of the electrodeposited Co–P catalyst for hydrolysis of NaBH 4 was found to be a function of both cathodic current density and plating time, that is, parameters determining the concentration of P in the Co–P catalyst. Especially, Co–13 at.% P catalyst electroplated on Cu in the Co–P bath at a cathodic current density of 0.01 A/cm 2 for 1080 s showed the best hydrogen generation rate of 954 ml/min g-catalyst in 1 wt.% NaOH + 10 wt.% NaBH 4 solution at 30 °C. 相似文献
8.
We have stabilized the perovskite La 2/3TiO 3 by adding LaFeO 3 and shown that in general the stabilization mechanism for the (1 − x)La 2/3TiO 3–xLaFeO 3 mixture involves the formation of a solid solution for compositions with x ≥ 0.04. The crystal structure of the solid solution transforms from orthorhombic to tetragonal at x = 0.2, becomes cubic in the range 0.3 < x < 0.8, and transforms again into orthorhombic (typical for pure LaFeO 3) for values greater than 0.8. Detailed impedance-spectroscopy measurements for various compositions and conditions showed that the limiting step in the conduction mechanism was conduction across the grain boundaries. In the concentration range 0.04 < x < 0.25 the room temperature conductivity increases up to 0.0017 S cm −1, after which it decreases again. Part of the initial increase is probably due to the formation of free electrons in accordance with (Fe Ti)′ → (Fe Ti) x + n′. Other defect-formation mechanisms are also discussed, but are ruled out for a variety of reasons. Another interesting phenomenon that also affected the average conductivity was identified, i.e., the variation of the average particle size with composition. 相似文献
9.
This work reports a study of oxygen mobility in a variety of LaCo 1−xFe xO 3 perovskites. The methods used to evaluate oxygen reactivity were temperature programmed oxygen desorption and oxygen isotopic exchange. Three kinds of oxygen are distinguished, depending on their reactivities. First, surface oxygens were found to be the most active forms of oxygen. In oxygen desorption experiments, they are responsible for two desorption peaks, designated as 1- and 2-oxygens in this work. Grain boundary oxygen is distinguished from bulk oxygen, and is proposed to be responsible for β-oxygen desorption observed at high temperature (>750 °C). These two kinds of oxygen, surface and grain boundary oxygen, are however quickly exchanged with the gaseous oxygen during the isotopic exchange reaction. The third kind, and less reactive, oxygen species is proposed to be the bulk oxygen. The mobility of this oxygen in the bulk of LaCoO 3 is measured by the initial rate of isotopic exchange. Its mobility is related to the activation of the bulk Co 3+, and migration of some anionic vacancies. 相似文献
10.
Fischer–Tropsch synthesis was carried out in slurry phase over uniformly dispersed Co–SiO 2 catalysts prepared by the sol–gel method. When 0.01–1 wt.% of noble metals were added to the Co–SiO 2 catalysts, a high and stable catalytic activity was obtained over 60 h of the reaction at 503 K and 1 MPa. The addition of noble metals increased the reducibility of surface Co on the catalysts, without changing the particle size of Co metal significantly. High dispersion of metallic Co species stabilized on SiO 2 was responsible for stable activity. The uniform pore size of the catalysts was enlarged by varying the preparation conditions and by adding organic compounds such as N, N-dimethylformamide and formamide. Increased pore size resulted in decrease in CO conversion and selectivity for CO 2, a byproduct, and an increase in the olefin/paraffin ratio of the products. By modifying the surface of wide pore silica with Co–SiO 2 prepared by the sol–gel method, a bimodal pore structured catalyst was prepared. The bimodal catalyst showed high catalytic performance with reducing the amount of the expensive sol–gel Co–SiO 2. 相似文献
11.
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
12.
Bi 0.5(Na 1−x−yK xAg y) 0.5TiO 3 piezoelectric ceramics were prepared by conventional ceramic processes. X-ray diffraction patterns show a pure perovskite structure, indicating that the K + and Ag + ions substitute for the Na + ions in Bi 0.5Na 0.5TiO 3. The temperature dependence of the dielectric constant and dissipation factor shows all ceramics to experience two phase transitions: from ferroelectric to anti-ferroelectric and from anti-ferroelectric to paraelectric. The transition temperature from ferroelectric to anti-ferroelectric and the temperature at which the dielectric constant reaches its maximum value decrease with the increase of K + amount. At room temperature, the ceramics containing 17.5–20 mol% K + and 2 mol% Ag + exhibit high piezoelectric constant ( d33 = 180 pC/N) and high electromechanical coupling factor ( kp = 35%). 相似文献
13.
Calcined and reduced catalysts Pd/LaBO 3 (B = Co, Fe, Mn, Ni) were used for the total oxidation of toluene. Easiness of toluene destruction was found to follow the sequence based on the T50 values (temperature at which 50% of toluene is converted): Pd/LaFeO 3 > Pd/LaMnO 3+δ > Pd/LaCoO 3 > Pd/LaNiO 3. In order to investigate the activation process (calcination and reduction) in detail, the reducibility of the samples was evaluated by H 2-TPR on the calcined catalysts. Additionally, characterization of the Pd/LaBO 3 (B = Co, Fe) surface was carried out by X-ray photoelectron spectroscopy (XPS) at each stage of the global process, namely after calcination, reduction and under catalytic reaction at either 150 or 200 °C for Pd/LaFeO 3 and either 200 or 250 °C for LaCoO 3. The different results showed that palladium oxidized entities were totally reduced after pre-reduction at 200 °C for 2 h (2 L/h, 1 °C/min). As LaFeO 3 was unaffected by such a treatment, for the other perovskites, the cations B are partially reduced as B 3+ (B = Mn) or B 2+ even to B 0 (B = Co, Ni). In the reactive stream (0.1% toluene in air), Pd 0 reoxidized partially, more rapidly over Co than Fe based catalysts, to give a Pd 2+/Pd 4+ and Pd 0/Pd 2+/Pd 4+ surface redox states, respectively. Noticeably, reduced cobalt species are progressively oxidized on stream into Co 3+ in a distorted environment. By contrast, only the lines characteristic of the initial perovskite lattice were detected by XRD studies on the used catalysts. The higher activity performance of Pd/LaFeO 3 for the total oxidation of toluene was attributed here to a low temperature of calcination and to a remarkable high stability of the perovskite lattice whatever the nature of the stream which allowed to keep a same palladium dispersion at the different stages of the process and to resist to the oxidizing experimental conditions. On the contrary, phase transformations for the other perovskite lattices along the process were believed to increase the palladium particle size responsible of a lower activity. 相似文献
14.
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
15.
We examined the effect of the activation process on the structural and morphological characteristics of a cobalt-based catalyst for Fischer–Tropsch synthesis. A 10 wt.% Co/SiO 2 catalyst prepared by wet impregnation was separately activated under H 2, CO or a H 2/CO mixture. The structural changes during activation from 298 to 773 K were studied by in situ X-ray diffraction. Catalysts were examined by SEM, TEM, XPS and in situ DRIFT-MS. The H 2/CO activation produced redispersion of cobalt particles and simultaneous carbon nanostructures formation. The catalyst showed the highest performance in the Fischer–Tropsch synthesis after the H 2/CO activation. 相似文献
16.
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
17.
Herein, we report the microstructure and properties of the newly developed near monophasic S-Sialon ceramic, based on the composition of Ba 2Si 12−xAl xO 2+xN 16−x ( x = 20.2). Appropriate amount of the precursor powders (BaCO 3, -Si 3N 4, AlN, Al 2O 3) with a targeted composition of BaAlSi 5O 2N 7 was ball milled and hot pressed to full density in the temperature range of 1600–1750 °C for 2 h in nitrogen atmosphere. Extensive transmission electron microscopy (TEM) study has been conducted to understand the microstructure development and characterise the various morphological features in hot pressed S-Sialon. The sintering mechanism is based on the liquid phase sintering route, which involves the formation of a Ba–Al silicate liquid (<5%) with dissolved nitrogen at intergranular pockets. The experimental observation suggests that the S-phase crystallises in elongated platelet morphology with preferred growth parallel to the orthorhombic ‘ c’ axis and primary facet planes parallel to (1 0 0) and (0 1 0). The Ba-S-phase ceramic has an acoustically measured Young modulus of 210–230 GPa, a hardness of 13 GPa and a fracture toughness of 4 MPa m 1/2, little lower than typical of a ceramic with morphologically anisotropic grains contributing to bridging and pullout mechanisms. 相似文献
18.
An evaluative investigation of the Fischer–Tropsch performance of two catalysts (20%Co/Al 2O 3 and 10%Co:10%Mo/Al 2O 3) has been carried out in a slurry reactor at 2 MPa and 220–260 °C. The addition of Mo to the Co-catalyst significantly increased the acid-site strength suggesting strong electron withdrawing character in the Co-Mo catalyst. Analysis of steady-state rate data however, indicates that the FT reaction proceeds via a similar mechanism on both catalysts (carbide mechanism with hydrogenation of surface precursors as the rate-determining step). Although chain growth, , on both catalysts were comparable ( 0.6), stronger CH 2 adsorption on the Co-Mo catalyst and lower surface concentration of hydrogen adatoms as a result of increased acid-site strength was responsible for the lower individual hydrocarbons production rate compared to the Co catalyst. The activation energy, E, for Co (96.6 kJ mol −1), is also smaller than the estimate for the Co-Mo catalyst (112 kJ mol −1). Transient hydrocarbon rate profiles on each catalyst are indicative of first-order processes, however the associated surface time constants are higher for alkanes than alkenes on individual catalysts. Even so, for each homologous class, surface time constants for paraffins are greater for Co-Mo than Co, indicative that the adsorption of CH 2 species on the Co-Mo surface is stronger than on the monometallic Co catalyst. 相似文献
19.
Perovskite type catalysts La 0.7Sr 0.3Cr 1−xRu xO 3 (0.025 ≤ x ≤ 0.100) were synthesized by annealing a mixture of metal oxides and carbonates gradually up to 1000 °C in air, and characterized by XRPD, XPS, TPD, SEM-EDS and the van der Pauw method. The CO oxidation activity was investigated in a differential recycle reactor. According to the XRPD results, all samples achieved a perovskite structure, with a small presence of SrCrO 4 phase. The XPS results revealed that the surface composition of all samples differed considerably from the stoichiometric value with an important segregation of strontium and mainly ruthenium with regard to chromium at the surface of the catalysts. The sharp decrease of resistivity with increasing surface concentration of ruthenium and the independence of the resistivity on temperature for the sample with x = 0.100 imply the possible presence of SrRuO 3, La–Ru–O and highly dispersed RuO 2 (invisible by XRPD), known as good electric conductors, at the surface. The CO oxidation activity increases with increasing the degree of substitution ( x). The surface concentrations of ruthenium are almost the same in the samples with x = 0.075 and 0.100. Those samples showed the similar values of resistivity in whole investigated temperature range and very close CO oxidation activity, which indicates that the concentration of Ru 4+ in the surface region and its stability are determining factors for the CO oxidation activity. The main results of this study are that ruthenium perovskites have a high thermal stability and CO oxidation activity. 相似文献
20.
Relatively inexpensive heterogeneous catalysts for two reactions of great importance in air pollution control, NO reduction and VOC combustion, were prepared and characterized. Apart from their common practical goal and the frequent need for simultaneous removal of air pollutants, these reactions share a similar redox mechanism, in which the formulation of more effective catalysts requires an enhancement of oxygen transfer. For NO reduction, supported catalysts were prepared by adding a metal (Cu, Co, K) using ion exchange (IE) and incipient wetness impregnation (IWI) to chars obtained from pyrolysis of a subbituminous coal. The effects of pyrolysis temperature, between 550 and 1000 °C, on selected catalyst characteristics (e.g., BET surface area, XRD spectrum, support reactivity in O2) are reported. For IE catalysts, the surface area increased in the presence of the metals while the opposite occurred for IWI catalysts. For the Co-IE catalysts, the highest surface area was obtained at 700 °C. The XRD results showed that, except for Cu (which exhibited sharp Cu0 peaks), the catalysts may be highly dispersed (or amorphous) on the carbon surface. For the C–O2 reaction the order of (re)activity was K Co > Cu for IE catalysts and K > Cu > Co for IWI catalysts. For NO reduction the orders were K > Co > Cu (IE catalysts) and Cu > Co > K (IWI catalysts). In all cases the catalytic (re)activity for NO reduction was lower than that exhibited for the C–O2 reaction. The K-IE and Cu-IWI catalysts appeared to be the most promising ones, although further improvements in catalytic activity will be desirable. Some surprising results regarding CO and CO2 selectivity are also reported, especially for Co catalysts. In VOC combustion, the effect of the nature of ion B (Fe and Ni) on the partial substitution of ion A (Ca for La) in ABO3 perovskites (e.g., LaFeO3 and LaNiO3) and on their catalytic activity was studied. The perovskite-type oxides were characterized by means of surface area measurements, XRD, temperature-programmed desorption (TPD) and temperature-programmed reduction (TPR). The effect of partial substitution of La3+ by Ca2+ was more significant for the La1−xCaxFeO3 perovskites. In this case, the electronic perturbation is compensated by an oxidation state increase of part of Fe3+ to Fe4+. The TPD results revealed that, at higher substitution levels, oxygen vacancies are also formed to preserve electroneutrality. For the La1−xCaxNiO3 perovskites, the characterization results showed no evidence of large differences in electronic properties as calcium substitution increases. The La1−xCaxNiO3 perovskites exhibited lower activity than the simple LaNiO3 perovskite, whereas for the La1−xCaxFeO3 substituted perovskites the most active catalyst (exhibiting the lowest ignition temperature) was obtained at the highest substitution level, La0.6Ca0.4FeO3. The performance of both groups of catalysts is briefly discussed in terms of redox processes, in which the interplay between oxygen transfer and electron transfer requires further elucidation for the improvement of catalytic activity. 相似文献
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