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1.
Co 3O 4/CeO 2 composite oxides with different cobalt loading (5, 15, 30, 50, 70 wt.% as Co 3O 4) were prepared by co-precipitation method and investigated for the oxidation of methane under stoichiometric conditions. Pure oxides, Co 3O 4 and CeO 2 were used as reference. Characterization studies by X-ray diffraction (XRD), BET, temperature programmed reduction/oxidation (TPR/TPO) and X-ray photoelectron spectroscopy (XPS) were carried out. An improvement of the catalytic activity and thermal stability of the composite oxides was observed with respect to pure Co3O4 in correspondence of Co3O4–CeO2 containing 30% by weight of Co3O4. The combined effect of cobalt oxide and ceria, at this composition, strongly influences the morphological and redox properties of the composite oxides, by dispersing the Co3O4 phase and promoting the efficiency of the Co3+–Co2+ redox couple. The presence in the sample Co3O4(30 wt.%)–CeO2 of a high relative amount of Ce3+/(Ce4+ + Ce3+) as detected by XPS confirms the enhanced oxygen mobility. The catalysts stability under reaction conditions was investigated by XRD and XPS analysis of the used samples, paying particular attention to the Co3O4 phase decomposition. Methane oxidation tests were performed over fresh (as prepared) and thermal aged samples (after ageing at 750 °C for 7 h, in furnace). The resistance to water vapour poisoning was evaluated for pure Co3O4 and Co3O4(30 wt.%)–CeO2, performing the tests in the presence of 5 vol.% H2O. A methane oxidation test upon hydrothermal ageing (flowing at 600 °C for 16 h a mixture 5 vol.% H2O + 5 vol.%O2 in He) of the Co3O4(30 wt.%)–CeO2 sample was also performed. All the results confirm the superiority of this composite oxide. 相似文献
2.
CO and CH 4 combined oxidation tests were performed over a Pd (70 g/ft 3)/Co 3O 4 monolithic catalyst in conditions of GHSV = 100,000 h −1 and feed composition close to that of emission from bi-fuel vehicles. The effect of SO 2 (5 ppm) on CO and CH 4 oxidation activity under lean condition ( λ = 2) was investigated. The presence of sulphur strongly deactivated the catalyst towards methane oxidation, while the poisoning effect was less drastic in the oxidation of CO. Saturation of the Pd/Co 3O 4 catalytic sites via chemisorbed SO 3 and/or sulphates occurred upon exposure to SO 2. A treatment of regeneration to remove sulphate species was attempted by performing a heating/cooling cycle up to 900 °C in oxidizing atmosphere. Decomposition of PdO and Co 3O 4 phases at high temperature, above 750 °C, was observed. Moreover, sintering of Pd 0 and PdO particles along with of CoO crystallites takes place. 相似文献
3.
In this paper, Co 3O 4/CeO 2 catalysts for steam reforming of ethanol (SRE) were prepared by co-precipitation and impregnation methods. The catalysts prepared by co-precipitation were very active and selective for SRE. Over 10%Co 3O 4/CeO 2 catalyst, ethanol conversion was close to 100% and hydrogen selectivity was about 70% at 450 °C. The catalysts were characterized by X-ray diffraction, temperature-programmed reduction (TPR) and BET surface area measurements. The preparation method influenced the interaction between cobalt and CeO 2 evidently. The incorporation of Co ions into CeO 2 crystal lattice resulted in weaker interaction between cobalt and ceria on catalyst surface. In comparison with catalysts prepared by impregnation, more cobalt ions entered into CeO 2 lattice, and resulted in weaker interaction between active phase and ceria on surface of Co 3O 4/CeO 2 prepared by co-precipitation. Thus, cobalt oxides was easier to be reduced to metal cobalt which was the key active component for SRE. Meanwhile, the incorporation of Co ions into CeO 2 crystal lattice was beneficial for resistance to carbon deposition. 相似文献
4.
A series of CeO 2 promoted cobalt spinel catalysts were prepared by the co-precipitation method and tested for the decomposition of nitrous oxide (N 2O). Addition of CeO 2 to Co 3O 4 led to an improvement in the catalytic activity for N 2O decomposition. The catalyst was most active when the molar ratio of Ce/Co was around 0.05. Complete N 2O conversion could be attained over the CoCe0.05 catalyst below 400 °C even in the presence of O 2, H 2O or NO. Methods of XRD, FE-SEM, BET, XPS, H 2-TPR and O 2-TPD were used to characterize these catalysts. The analytical results indicated that the addition of CeO 2 could increase the surface area of Co 3O 4, and then improve the reduction of Co 3+ to Co 2+ by facilitating the desorption of adsorbed oxygen species, which is the rate-determining step of the N 2O decomposition over cobalt spinel catalyst. We conclude that these effects, caused by the addition of CeO 2, are responsible for the enhancement of catalytic activity of Co 3O 4. 相似文献
5.
The NiSO 4 supported on Fe 2O 3-promoted ZrO 2 catalysts were prepared by the impregnation method. Fe 2O 3-promoted ZrO 2 was prepared by the coprecipitation method using a mixed aqueous solution of zirconium oxychloride and iron nitrate solution followed by adding an aqueous ammonia solution. No diffraction line of nickel sulfate was observed up to 20 wt.%, indicating good dispersion of nickel sulfate on the surface of Fe 2O 3–ZrO 2. The addition of nickel sulfate (or Fe 2O 3) to ZrO 2 shifted the phase transition of ZrO 2 (from amorphous to tetragonal) to higher temperatures because of the interaction between nickel sulfate (or Fe 2O 3) and ZrO 2. 15-NiSO 4/5-Fe 2O 3–ZrO 2 containing 15 wt.% NiSO 4 and 5 mol% Fe 2O 3, and calcined at 500 °C exhibited a maximum catalytic activity for ethylene dimerization. NiSO 4/Fe 2O 3–ZrO 2 catalysts was very effective for ethylene dimerization even at room temperature, but Fe 2O 3–ZrO 2 without NiSO 4 did not exhibit any catalytic activity at all. The catalytic activities were correlated with the acidity of catalysts measured by the ammonia chemisorption method. The addition of Fe 2O 3 up to 5 mol% enhanced the acidity, surface area, thermal property, and catalytic activities of catalysts gradually, due to the interaction between Fe 2O 3 and ZrO 2 and due to consequent formation of Fe–O–Zr bond. 相似文献
7.
A suspension stabilizer-coating technique was employed to prepare x mol% Yb 2O 3 ( x = 1.0, 2.0, 3.0 and 4.0) and 1.0 mol% Y 2O 3 co-doped ZrO 2 powder. A systematic study was conducted on the sintering behaviour, phase assemblage, microstructural development and mechanical properties of Yb 2O 3 and Y 2O 3 co-doped zirconia ceramics. Fully dense ZrO 2 ceramics were obtained by means of pressureless sintering in air for 1 h at 1450 °C. The phase composition of the ceramics could be controlled by tuning the Yb 2O 3 content and the sintering parameters. Polycrystalline tetragonal ZrO 2 (TZP) and fully stabilised cubic ZrO 2 (FSZ) were achieved in the 1.0 mol% Y 2O 3 stabilised ceramic, co-doped with 1.0 mol% Yb 2O 3 and 4.0 mol% Yb 2O 3, respectively. The amount of stabilizer needed to form cubic ZrO 2 phase in the Yb 2O 3 and Y 2O 3 co-doped ZrO 2 ceramics was lower than that of single phase Y 2O 3-doped materials. The indentation fracture toughness could be tailored up to 8.5 MPa m 1/2 in combination with a hardness of 12 GPa by sintering a 1.0 mol% Yb 2O 3 and 1.0 mol% Y 2O 3 ceramic at 1450 °C for 1 h. 相似文献
8.
The phase diagram of the Al 2O 3–ZrO 2–Nd 2O 3 system was constructed in the temperature range 1250–2800 °C. The liquidus surface of the phase diagram reflects the preferentially eutectic interaction in the system. Two new ternary and one new binary eutectics were found. The minimum melting temperature is 1675 °C and it corresponds to the ternary eutectic Nd 2O 3·11Al 2O 3 + F-ZrO 2 + NdAlO 3. The solidus surface projection and the schematic of the alloy crystallization path confirm the preferentially congruent character of phase interaction in the ternary system. The polythermal sections present the complete phase diagram of the Al 2O 3–ZrO 2–Nd 2O 3 system. No ternary compounds or regions of remarkable solid solution were found in the components or binaries in this ternary system. 相似文献
9.
Mixed oxides of alumina and zirconia having a relative composition of 50, 80 and 100% Zr 2O were synthesized by means of sol–gel methods. The catalysts were sulfated with H 2SO 4 1N, and were loaded with 0.3% Pt metal using the incipient wetness technique. The characterization of the physicochemical properties was carried out using XRD, N 2-adsorption at 78 K, and SEM. The catalytic properties of the Al 2O 3–ZrO 2 series were studied by means of dehydration of 2-propanol at 180°C and isomerization of n-hexane at 250°C, 1 atm. The sulfated solids presented a high surface acidity and a limited crystallinity, together with high activity for alcohol dehydration (i.e. 2-propanol). On the other hand, the Al 2O 3–ZrO 2 solid solutions (i.e. those having a 20–80% composition) turned out to be the most active ones for the isomerization of n-hexane. 相似文献
10.
Ni catalysts supported on γ-Al 2O 3, CeO 2 and CeO 2–Al 2O 3 systems were tested for catalytic CO 2 reforming of methane into synthesis gas. Ni/CeO 2–Al 2O 3 catalysts showed much better catalytic performance than either CeO 2- or γ-Al 2O 3-supported Ni catalysts. CeO 2 as a support for Ni catalysts produced a strong metal–support interaction (SMSI), which reduced the catalytic activity and carbon deposition. However, CeO 2 had positive effect on catalytic activity, stability, and carbon suppression when used as a promoter in Ni/γ-Al 2O 3 catalysts for this reaction. A weight loading of 1–5 wt% CeO 2 was found to be the optimum. Ni catalysts with CeO 2 promoters reduced the chemical interaction between nickel and support, resulting in an increase in reducibility and stronger dispersion of nickel. The stability and less coking on CeO 2-promoted catalysts are attributed to the oxidative properties of CeO 2. 相似文献
11.
The introduction of trivalent cation — Y 3+ or La 3+ — into the lattice of CeO 2–ZrO 2 solid solutions allows to stabilise a cubic structure at low ceria content (30 mol%). The reducibility of the samples has been compared in the experiments of temperature-programmed reduction (TPR). The introduction of lanthanum cations decreases the amount of hydrogen consumed during TPR, while the introduction of yttrium ones increases this value. At the same time, the value of temperature of the maximum speed of reduction ( Tmax) is independent on the trivalent dopant. The reducibility of these solid solutions did not change during repeated red–ox treatments at temperature below 1220 K. It is connected with the high thermostability of all systems in this temperature interval. TPR up to 1470 K causes a significant shift of Tmax value to higher temperature and a slight decrease of hydrogen consumption in two to three cycles. It is suggested that this alterations are connected with the sharp decrease of the specific surface area of all samples and partially phase decomposition of CeO 2–ZrO 2 and Y 2O 3–CeO 2–ZrO 2 solid solutions. Raman characterisation of the oxygen sublattice of the fresh samples and of the samples after TPR has been carried out. 相似文献
12.
The direct decomposition of nitric oxide (NO) over barium catalysts supported on various metal oxides was examined in the absence and presence of O 2. Among the Ba catalysts supported on single-component metal oxides, Ba/Co 3O 4 and Ba/CeO 2 showed high NO decomposition activities, while Ba/Al 2O 3, Ba/SiO 2, and Ba/TiO 2 exhibited quite low activities. The effect of an addition of second components to Co and Ce oxides was further examined, and it was found that the activities were significantly enhanced using Ce–Mn mixed oxides as support materials. XRD results indicated the formation of CeO 2–MnO x solid solutions with the cubic fluorite structure. O 2-TPD of the CeO 2–MnO x solid solutions showed a large desorption peak in a range of relatively low temperature. The BET surface areas of the CeO 2–MnO x solid solutions were larger than those of pure CeO 2 and Mn 2O 3. These effects caused by the addition of Mn are responsible for the enhanced activities of the Ba catalysts supported on Ce–Mn mixed oxides. 相似文献
13.
Co 3O 4–CeO 2 type mixed oxide catalyst compositions have been prepared by using co-precipitation method and, their catalytic activity towards diesel particulate matter (PM)/carbon oxidation has been evaluated under both loose and tight contact conditions. These catalysts show excellent catalytic activity for PM/carbon oxidation, despite their low surface area. The activation energy observed for non-catalyzed and catalyzed reactions are 163 kJ/mol and 140 kJ/mol, respectively, which also confirm the catalytic activity of catalyst for carbon/soot oxidation. The promotional effects of an optimum amount of cobalt oxide incorporation in ceria and presence of a small amount of potassium appears to be responsible for the excellent soot oxidation activity of this mixed oxide type material. The catalytic materials show good thermal stability, while their low cost will also add to their potential for practical applications. 相似文献
14.
CeO 2–ZrO 2–La 2O 3 (CZL) mixed oxides were prepared by citric acid sol–gel method. The as-received gel was calcined at 500, 700, 900 and 1050 °C to obtain the so-called C5, C7, C9 and CK, respectively. The C5, C7 and C9 powders were impregnated with H 2PtCl 6 and then calcined at 500 °C to prepare P5C5, P5C7 and P5C9, respectively. The impregnated CK powders were calcined at 500, 700 and 900 °C to prepare P5CK, P7CK and P9CK, respectively. The XRD and XPS analyses show that the surface distribution of Pt is evidently influenced by the structural and textural properties of the support. The CO adsorption followed by FTIR reveals that the dispersion and the chemisorption sites of Pt are reduced as the calcination temperature of CZL support increases. The chemisorption ability of the CK samples is even completely deactivated. The encapsulation mechanism, which has been applied to explain the so-called strong metal–support interaction (SMSI) after reductive treatment, is introduced here to demonstrate the abnormal observations though the samples were prepared in oxidative atmosphere. The HRTEM results also confirm this explanation. The effects of oxygen vacancies, the chemisorption sites on the Pt surface and Pt/Ce interfacial sites on the three-way catalytic activities are discussed. 相似文献
15.
The sintering of lime doped with up to 2 mol% La 2O 3 as well as CeO 2 was followed on firing at 1400, 1500 and 1600°C. X-ray diffraction and reflected-light microscopy techniques have been used to investigate the phase composition and texture of the fired samples. The load bearing capacity and the rate of hydration of the obtained dense lime samples have also been determined to assess its quality. It is found that doping lime with 0·5–1·0 mol% of either La 2O 3 or CeO 2 and firing up to 1600°C leads to dense and hydration-resistant lime with uniform texture. At this level of the dopant oxides, normal grain growth occurred with the increase in the degree of direct bonding. Doping with less than 0·5 mol% La 2O 3 or CeO 2 is insufficient for inhibiting the discontinuous grain growth of the lime grains. On the other hand, increase in the dopant content above 1·0 mol% showed finer grain sizes at the expense of the degree of direct bonding. 相似文献
16.
A novel CeO 2–Y 2O 3 (CY) washcoat on cordierite honeycomb was prepared by an impregnation method, which was used as a support to prepare a Pd catalyst. A model reaction of the complete combustion of toluene was conducted to evaluate the performance of the developed Pd/CY catalyst. The CY washcoat support and the Pd/CY catalyst were characterized by XRD, Raman spectroscopy, H 2-TPR and SEM techniques. The results show that compared with conventional washcoat the CY washcoat has better adhesion and higher vibration- and heat-resistance. The CY washcoat can anchor well Pd onto the cordierite honeycomb substrate. The formation of a CeO 2–Y 2O 3 solid solution and the steady present of PdO occur at high calcination temperatures, resulting in a better thermal stability. On a Pd/CY catalyst calcined at 500 °C, a 99% of toluene conversion was obtained at 210 °C, and it was stable for reaction time up to 30 h. 相似文献
17.
Methane combustion over Pd/Al 2O 3 catalysts with and without added Pt and CeO 2 in both oxygen-rich and methane-rich mixtures at temperatures in the range 250–520°C has been investigated using a temperature-programmed reaction procedure with on-line gas analysis (FTIR). During the temperature loop under oxygen-rich conditions, there was an appreciable hysteresis in the activity of unmodified Pd/Al 2O 3, which was greatly enhanced over Pd–Pt/Al 2O 3. Over both catalysts the hysteresis was reversed under slightly methane-rich atmospheres, and as temperature was reduced, a sudden collapse or fluctuations in activity were shown respectively over Pd–Pt/Al 2O 3 and Pd/Al 2O 3. Such non-steady behaviour was almost eliminated over Pd/Al 2O 3–CeO 2. Under a very narrow range of conditions and over a Pd/Al 2O 3 packed bed, oscillation of methane combustion was observed. 相似文献
18.
The sintering behavior and dielectric properties of the monoclinic zirconolite-like structure compound Bi 2(Zn 1/3Nb 2/3) 2O 7 (BZN) and Bi 2(Zn 1/3Nb 2/3−xV x) 2O 7 (BZNV, x = 0.001) sintered under air and N 2 atmosphere were investigated. The pure phase were obtained between 810 and 990 °C both for BZN and BZNV ceramics. The substitution of V 2O 5 and N 2 atmosphere accelerated the densification of ceramics slightly. The influences on microwave dielectric properties from different atmosphere were discussed in this work. The best microwave properties of BZN ceramics were obtained at 900 °C under N 2 atmosphere with r = 76.1, Q = 850 and Qf = 3260 GHz while the best properties of BZNV ceramics were got at 930 °C under air atmosphere with r = 76.7, Q = 890 and Qf = 3580 GHz. The temperature coefficient of resonant frequency τf was not obviously influenced by the different atmospheres. For BZN ceramics the τf was −79.8 ppm/°C while τf is −87.5 ppm/°C for BZNV ceramics. 相似文献
19.
Three types of CeO 2–ZrO 2 (Ce:Zr=1:1 molar ratio) compounds with different oxygen storage/release capacities (OSCs) were characterized by means of the Ce K-edge and Zr K-edge X-ray absorption fine structure (XAFS). In order to investigate the relationship between the OSC and local structure, the quantitative EXAFS curve-fitting analysis was applied. By enhancing the homogeneity of the Ce and Zr atoms in the CeO 2–ZrO 2 solid solution, the OSC performance increased. Especially, the atomically homogeneous Ce 0.5Zr 0.5O 2 solid solution exhibited the highest OSC among these CeO 2–ZrO 2 samples. Additionally, the local oxygen environment around Ce and Zr was remarkably modified by enhancing the homogeneity of the CeO 2–ZrO 2 solid solution. It was postulated that the enhancement of the homogeneity of the CeO 2–ZrO 2 solid solution and the modification of the oxygen environment would be the source for the OSC improvement. 相似文献
20.
The one-step highly selective oxidation of cyclohexane into cyclohexanone and cyclohexanol as the essential intermediates of nylon-6 and nylon-66 is considerably challenging. Therefore, an efficient and low-cost catalyst must be urgently developed to improve the efficiency of this process. In this study, a Co 3O 4–CeO 2 composite oxide catalyst was successfully prepared through ultrasound-assisted co-precipitation. This catalyst exhibited a higher selectivity to KA-oil, which was benefited from the synergistic effects between Co 3+/Co 2+ and Ce 4+/Ce 3+ redox pairs, than bulk CeO 2 and/or Co 3O 4. Under the optimum reaction conditions, 89.6% selectivity to KA-oil with a cyclohexane conversion of 5.8% was achieved over Co 3O 4–CeO 2. Its catalytic performance remained unchanged after five runs. Using the synergistic effects between the redox pairs of different transition metals, this study provides a feasible strategy to design high-performance catalysts for the selective oxidation of alkanes. 相似文献
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