共查询到20条相似文献,搜索用时 15 毫秒
1.
The influence of catalyst pre-treatment temperature (650 and 750 °C) and oxygen concentration ( λ = 8 and 1) on the light-off temperature of methane combustion has been investigated over two composite oxides, Co 3O 4/CeO 2 and Co 3O 4/CeO 2–ZrO 2 containing 30 wt.% of Co 3O 4. The catalytic materials prepared by the co-precipitation method were calcined at 650 °C for 5 h (fresh samples); a portion of them was further treated at 750 °C for 7 h, in a furnace in static air (aged samples). Tests of methane combustion were carried out on fresh and aged catalysts at two different WHSV values (12 000 and 60 000 mL g−1 h−1). The catalytic performance of Co3O4/CeO2 and Co3O4/CeO2–ZrO2 were compared with those of two pure Co3O4 oxides, a sample obtained by the precipitation method and a commercial reference. Characterization studies by X-ray diffraction (XRD), BET and temperature-programmed reduction (TPR) show that the catalytic activity is related to the dispersion of crystalline phases, Co3O4/CeO2 and Co3O4/CeO2–ZrO2 as well as to their reducibility. Particular attention was paid to the thermal stability of the Co3O4 phase in the temperature range of 750–800 °C, in both static (in a furnace) and dynamic conditions (continuous flow). The results indicate that the thermal stability of the phase Co3O4 heated up to 800 °C depends on the size of the cobalt oxide crystallites (fresh or aged samples) and on the oxygen content (excess λ = 8, stoichiometric λ = 1) in the reaction mixture. A stabilizing effect due to the presence of ceria or ceria–zirconia against Co3O4 decomposition into CoO was observed. Moreover, the role of ceria and ceria–zirconia is to maintain a good combustion activity of the cobalt composite oxides by dispersing the active phase Co3O4 and by promoting the reduction at low temperature. 相似文献
2.
The catalytic activity study of cobalt oxides dispersed on different supports evidenced first the highest performances of zirconia based catalysts in the reaction of toluene oxidation. The influence of the presence of ethylenediamine (en) during the preparation of Co/ZrO 2 and the ZrO 2 support modification by Y 2O 3 were then studied and compared with reference catalyst prepared conventionally by impregnation of ZrO 2 with an aqueous solution of Co(NO 3) 2. Addition of an aqueous solution of ethylenediamine to a cobalt nitrate solution led to a strong increase on the catalytic activity of the activated solids in the toluene deep oxidation as compared with the reference catalyst. The best catalytic results were explained in terms of cobalt oxides dispersion but also in terms of Co–support interaction. The generated cobalt species were reducible at much lower temperatures and then were more active in the toluene total oxidation. Finally an efficient catalyst for VOC oxidation was produced combining the modifications of ZrO 2 by yttrium and of the precursor. 相似文献
3.
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
4.
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
5.
The promotive effects of cerium oxide on commercial three-way catalysts (TWCs) for purification of motor exhaust gases have been widely investigated in recent years. This work shows the cooperative effects of CeO 2–Pd on the kinetics of CO oxidation over Pd/CeO 2–ZrO 2. Under reducing-to-moderately oxidizing conditions, a zero-order O 2 pressure dependence is found which can be interpreted on the basis of a mechanism involving a reaction between CO adsorbed on Pd and surface oxygen from the support. The high oxygen-exchange capability of the CeO 2–ZrO 2 support, as determined from temperature-programmed reduction/oxygen uptake measurements is suggested as being responsible for such a catalytic behavior. 相似文献
6.
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
7.
Ce 0.5Zr 0.5O 2, Ce 0.5Zr 0.2Mn 0.3O 2 and Ce 0.5Mn 0.5O 2 were prepared by citric acid sol–gel method. The effect of manganese on the structural and redox properties of ceria-based mixed oxides was investigated by means of powder X-ray diffraction, X-ray photoelectron spectroscopy, Brunauer–Emmett–Teller analyses, temperature-programmed reduction and catalytic activity evaluation in the presence of excess O 2. The results showed that some Mn cations could enter into the ceria lattice to form solid solutions. Mn 3O 4 appeared due to the instability of the mixed oxides with increment of the Mn doping ratio while another oxide Mn 2O 3 is detected in the physical mixture of ceria and manganese oxide. These Mn-doped mixed oxides, especially Ce 0.5Mn 0.5O 2, presented better catalytic activities than Ce 0.5Zr 0.5O 2 and even Pt-loaded catalyst for total oxidation of C 3H 8 and oxidative sorption of NO in the presence of excess oxygen. The oxidation ability of Mn and the strong interaction between Mn and Ce were suggested to promote the oxygen storage/transport capacity of the mixed oxides as well as reactive adsorption of nitric oxide and hydrocarbons. 相似文献
8.
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
9.
Ni catalysts supported on various solid solutions of ZrO 2 with alkaline earth oxide and/or rare earth oxide were synthesized. The catalytic activities were compared for partial oxidation of methane and autothermal reforming of methane. For partial oxidation of methane, the Ni catalyst supported on a CaO–ZrO 2 solid solution showed a high activity. Incorporation of CaO in the ZrO 2 matrix was effective for increasing the reduction rate of the NiO particles and for decreasing the coke formation. On the other hand, the Ni particles supported on the CaO–CeO 2–ZrO 2 solid solution had a strong interaction with the support, and the Ni particles showed high activity and stability for autothermal reforming of methane. 相似文献
10.
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
11.
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
12.
Graphene nanosheet (GNS)/Co 3O 4 composite has been rapidly synthesized by microwave-assisted method. Field emission scanning electron microscopy and transmission electron microscopy observation reveals the homogeneous distribution of Co 3O 4 nanoparticles (3-5 nm in size) on graphene sheets. Electrochemical properties are characterized by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy. A maximum specific capacitance of 243.2 F g −1 has been obtained at a scan rate of 10 mV s −1 in 6 M KOH aqueous solution for GNS/Co 3O 4 composite. Furthermore, the composite exhibits excellent long cycle life along with ∼95.6% specific capacitance retained after 2000 cycle tests. 相似文献
14.
Modification of cobaltic oxide (obtained from the reduction of high-valence cobalt oxide and assigned as R230, SBET = 100 m 2 g −1) with different loading of ceria was proceeded using the impregnation method (assigned as Ce X/R230, X = 4, 12, 20, 35 and 50 wt%). The Ce X/R230 catalysts were characterized by X-ray diffraction (XRD), nitrogen adsorption at −196 °C, temperature-programmed reduction (TPR) and transmission electron microscopy (TEM). Their catalytic activities towards the CO oxidation were studied in a continuous flow micro-reactor. The results revealed that the optimal modification, i.e., Ce20/R230, can increase the surface area ( SBET = 109 m 2 g −1) of cobaltic oxide, further weaken the bond strength of CoO and lower the activation of CO oxidation among Ce X/R230 catalysts due to the combined effect of cobaltic oxide and ceria. The Ce20/R230 catalyst exhibited the best catalytic activity in CO oxidation with T50 (temperature for 50% CO conversion) at 88 °C. 相似文献
15.
This work aims at exploring the thermal ageing mechanism of Pt on ceria-based mixed oxides and the corresponding effect on the oxygen storage capacity (OSC) performance of the support material. Pt was supported on low-surface-area CeO 2–ZrO 2–La 2O 3 mixed oxides (CK) by impregnation method and subsequently calcined in static air at 500, 700 and 900 °C, respectively. The evolutions of textural, microstructural and redox properties of catalysts after the thermal treatments were identified by means of X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR) and high-resolution transmission electron microscope (HRTEM). The results reveal that, besides the sintering of Pt, encapsulation of metal by the mixed oxides occurs at the calcination temperature of 700 °C and above. The burial of Pt crystallites by support particles is proposed as a potential mechanism for the encapsulation. Further, the HRTEM images show that the distortion of the mixed oxides lattice and other crystal defects are distributed at the metal/oxides interface, probably indicating the interdiffusion/interaction between the metal and mixed oxide. In this way, encapsulation of Pt is capable to promote the formation of Ce 3+ or oxygen vacancy on the surface and in the bulk of support. The OSC results show that the reducibility and oxygen release behavior of catalysts are related to both the metal dispersion and metal/oxides interface, and the latter seems to be more crucial for those supported on low-surface-area mixed oxides. Judging by the dynamic oxygen storage capacity (DOSC), oxygen storage capacity complete (OSCC) and oxygen releasing rate, the catalyst calcined at 700 °C shows the best OSC performance. This evident promotion of OSC performance is believed to benefit from the partial encapsulation of Pt species, which leads to the increment of Ce 3+ or oxygen vacancies both on the surface and in the bulk of oxides despite a loss of chemisorption sites on the surface of metal particles. 相似文献
16.
The reaction condition for high yield of methanol in a gaseous reaction between methane and oxygen in the presence of NO at atmospheric pressure was explored. Methane partial oxidation without NO (CH 4–O 2) gave only 1% conversion of methane at 966 K. The addition of NO led to a remarkable increase in methane conversion and to high selectivity to C 1-oxygenates. The conversion of methane attained 10% at 808 K in the presence of NO (0.5%) where the selectivities to methanol and formaldehyde were 22.1 and 24.1%, respectively. Nitromethane and carbon oxides were also observed in the product gas. The amount of nitromethane was almost equal and/or near to that of initial NO. The carbon monoxide produced was several times higher than carbon dioxide. Influences of NO concentration, ratio of methane to oxygen, water vapor, and dilution with helium gas on product distribution were measured. Low concentration of NO (0.35–0.55%) was favorable for methanol formation. High selectivity to methanol was obtained at low value of the ratio of methane to oxygen (2.0–3.0) or low concentration of dilution gas (<16%). The NO 2 added promoted methane partial oxidation and selectivity to methanol. Therefore, it was assured that NO x promoted the formation of CH 3√ and CH 3O√ in the gas phase reaction for CH 4–O 2–NO. 相似文献
17.
Au/Co 3O 4 catalysts with different morphologies (nanorods, nanopolyhedra and nanocubes) were successfully synthesized and evaluated for ethylene complete oxidation. We found that support morphology has a significant effect on catalytic activity, which is related to the exposed planes of different morphological Co 3O 4. HRTEM revealed the Co 3O 4-nanorods predominantly exposes {110} planes, while the dominant exposed planes of Co 3O 4-nanopolyhedra and -nanocubes are {011} and {001} planes, respectively. Compared with {011} and {001} planes, {110} planes exhibit the maximum amount of oxygen vacancies, which play a major role in ethylene oxidation. Therefore, Au/Co 3O 4-nanorods exhibits extraordinary catalytic activity, yielding 93.7% ethylene conversion at 0 °C. 相似文献
18.
The effect of La 2O 3 content on the structural properties and catalytic behavior of Pt/ xLa 2O 3–Al 2O 3 catalysts in steam reforming of methane and partial oxidation of methane was investigated. There was a decrease in the density of Pt sites with the increase of La 2O 3 loadings according to Fourier transform infrared spectroscopy of adsorbed CO and to dehydrogenation of cyclohexane results. However, transmission electron microscopy data indicates an opposite trend. This apparent disagreement could be due to the partial coverage of Pt sites by LaO x species. CH 4 turnover rates and specific rates of steam reforming of methane increased for higher La 2O 3 loadings. The Pt/Al 2O 3 catalyst was strongly deactivated during partial oxidation of methane, while La 2O 3-containing catalysts exhibited higher stability. The increase of activity observed during the reactions was ascribed to the ability of the [LaPt xO]Pt 0-like species to promote the gasification of coke. This cleaning mechanism led to higher accessibility of the active sites to CH 4. 相似文献
19.
High surface area CeO 2–ZrO 2 mixed oxides were treated at 900–950°C either under wet air or under successive reducing and oxidizing atmospheres in order to study the evolution of the oxygen storage capacity (OSC) of these solids after different aging treatments. Several complementary methods were used to characterize the redox behavior: temperature programmed reduction (TPR) by H 2, TPO, magnetic susceptibility measurements to obtain the Ce 3+ content, FT-IR spectroscopy of adsorbed methanol and a method to compare the oxygen buffering capacity (OBC) of the oxides. All the results confirm that the mixed oxides exhibit better redox properties than pure ceria, particularly after aging. The enhancement in the OSC at moderate temperature has to be related to a deeper penetration of the reduction process from the surface into the under-layers. Redox cycling aging promotes the reduction at low temperature of all the mixed oxides, the improvement being much more important for low surface area aged samples. The magnitude of this effect does not depend on the BET surface areas which have similar values after cycling. This underlines the critical influence that the preparation and activation procedure have on the final OSC behaviors of the ceria–zirconia mixed oxides. 相似文献
20.
The effect of noble metal addition on the catalytic properties of Co/Al 2O 3 was evaluated for the steam reforming of methane. Co/Al 2O 3 catalysts were prepared with addition of different noble metals (Pt, Pd, Ru and Ir 0.3 wt.%) by a wetness impregnation method and characterized by UV–vis spectroscopy, temperature programmed reduction (TPR) and temperature programmed oxidation (TPO) of the reduced catalysts. The UV–vis spectra of the samples indicate that, most likely, large amounts of the supported cobalt form Co species in which cobalt is in octahedral and tetrahedral symmetries. No peaks assigned to cobalt species from aluminate were found for the promoted and unpromoted cobalt catalysts. TPO analyses showed that the addition of the noble metals on the Co/Al 2O 3 catalyst leads to a more stable metallic state and less susceptible to the deactivation process during the reforming reaction. The Co/Al 2O 3 promoted with Pt showed higher stability and selectivity for H 2production during the methane steam reforming. 相似文献
|