共查询到20条相似文献,搜索用时 15 毫秒
1.
In the present work, we describe the use of fluorite-structured solid solutions, based on mixtures of CeO 2/La 2O 3/Pr 2O 3, and similar systems, i.e. CeO 2/Ln 2O 3/Pr 2O 3 where Ln=Tb, Sm, and Gd as effective catalysts for methane combustion. Mixed lanthanide oxide doped ceria solid solutions were prepared using an evaporation method from nitrate precursors and were subsequently calcined to 1073 K. Sample characterisation was achieved using powder X-ray diffraction (PXRD) and temperature programmed reduction (TPR). PXRDs obtained subsequent to a 12 h calcination at 1073 K for 12 h all show peaks corresponding to a fluorite-based structure. There are no additional features suggestive of large-scale segregation or ordered superstructure formation. This was confirmed by Rietveld analysis which showed that the lattice possesses cubic nature with a= b= c=5.50 Å. Methane combustion tests show that the most effective catalysts are those lanthanide oxide doped ceria catalysts prepared with 5% Pr molar metal ratios. TPR experiments suggest PrO 2 is an essential component and both surface and bulk chemistries have important roles in catalytic activity. 相似文献
2.
Yttria-doped ceria (YDC) and pure ceria (CeO 2), respectively, were deposited on γ-alumina (γ-Al 2O 3) using the impregnation method; then, copper oxide was also supported on them by employing the impregnation method. For comparison, CuO/γ-Al 2O 3 catalysts were prepared in this work. The catalysts were characterized by temperature-programmed reduction (TPR) and X-ray diffraction (XRD). For CuO/γ-Al 2O 3 catalysts, two TPR peaks, namely β and γ, were observed. These have been attributed to the reduction of highly dispersed copper oxide species and bulk-like copper oxide, respectively. For CuO/CeO 2/γ-Al 2O 3 and CuO/YDC/γ-Al 2O 3 catalysts, four TPR peaks, namely 1, 2, β′ and γ′, could be observed. The peaks with lower peak temperatures as compared to those of β′ and γ′ peaks have been attributed to the reduction of interface-boundary copper oxide species that contact closely and interact strongly with the supported ceria or YDC. Crystal sizes calculated from XRD measurements confirmed that yttria (Y 2O 3) addition could lead to crystal growth of ceria and correspondingly enhance the dispersion of the supported copper oxide due to the partition of YDC crystallite. Hence, this work shows that supported YDC and ceria can act bi-functionally as a textural promoter as well as a structural promoter. 相似文献
3.
Over the past several years, cerium oxide and CeO 2-containing materials have come under intense scrutiny as catalysts and as structural and electronic promoters of heterogeneous catalytic reactions. Recent developments regarding the characterization of ceria and CeO 2-containing catalysts are critically reviewed with a special focus towards catalyst interaction with small molecules such as hydrogen, carbon monoxide, oxygen, and nitric oxide. Relevant catalytic and technological applications such as the use of ceria in automotive exhaust emission control and in the formulation of SO x reduction catalysts is described. A survey of the use of CeO 2-containing materials as oxidation and reduction catalysts is also presented. 相似文献
4.
氯化氢催化氧化技术在副产氯化氢的处理和利用方面有着巨大的应用价值和良好的工业前景。CeO 2基催化剂因其廉价、不易烧结及抗氯化性能优良等诸多优点,被认为是目前最具有应用前景的氯化氢氧化催化剂之一。本文简要地介绍了氯化氢氧化制氯循环工艺,评述了近年来用于氯化氢氧化反应的CeO 2基催化材料,包括CeO 2催化剂、掺杂改性CeO 2催化剂以及CeO 2作助催化剂等。同时在文中还对CeO 2基材料催化HCl氧化的表面反应机理进行阐述。最后结合目前的研究现状,对如何进一步提升CeO 2基催化HCl氧化的反应活性进行了展望。 相似文献
5.
Combustion of CO, ethyl acetate and ethanol was studied over CuO x/Al 2O 3, CuO x–CeO 2/Al 2O 3, CuMn 2O 4/Al 2O 3 and Mn 2O 3/Al 2O 3 catalysts. It was found that modification of the alumina with ceria before subsequent copper oxide deposition increases the activity for combustion of CO substantially, but the effect of ceria was small on the combustion of ethyl acetate and ethanol. The activity increases with the CuO x loading until crystalline CuO particles are formed, which contribute little to the total active surface. The CuO x–CeO 2/Al 2O 3 catalyst is more active than the CuMn 2O 4/Al 2O 3 catalyst for the oxidation of CO but the CuMn 2O 4/Al 2O 3 catalyst is more active for the combustion of ethyl acetate and ethanol. Thermal ageing and water vapour in the feed caused a modest decrease in activity and did not affect the CuOx–CeO2/Al2O3 and CuMn2O4/Al2O3 catalysts differently. In addition, no difference in intermediates formed over the two catalysts was observed. Characterisation with XRD, FT-Raman and TPR indicates that the copper oxide is present as a copper aluminate surface phase on alumina at low loading. At high loading, bulk CuO crystallites are present as well. Modification of the alumina with ceria before the copper oxide deposition gives well dispersed copper oxide species and bulk CuO crystallites associated to the ceria, in addition to the two copper oxide species on the bare alumina. The distribution of copper species depends on the ceria and copper oxide loading. The alumina supported copper manganese oxide and manganese oxide catalysts consist mainly of crystalline CuMn2O4 and Mn2O3, respectively, on Al2O3. 相似文献
6.
New gold catalytic system prepared on ceria-modified mesoporous titania (CeMTi) used as water-gas shift (WGS) reaction catalyst is reported. Mesoporous titania (MTi) was synthesized using surfactant templating method through a neutral [C 13(EO) 6–Ti(OC 3H 7) 4] assembly pathway. Ceria modifying additive was deposited on MTi by deposition precipitation (DP) method. Gold-based catalysts with different gold content (1–5 wt.%) were synthesized by DP of gold hydroxide on mixed metal oxide support. The supports and the catalysts were characterized by powder X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), N 2 adsorption analysis and temperature-programmed reduction (TPR). The catalytic behavior of the gold-based catalysts was evaluated in WGS reaction in a wide temperature range (140–300 °C) and at different space velocities and H 2O/CO ratios. The influence of gold content and particle size on the catalytic performance was investigated. The WGS activity of the new gold/ceria-modified mesoporous titania catalysts was compared with that of gold catalysts supported on simple oxides CeO 2 and mesoporous TiO 2, as well as gold/ceria-modified titania and reference catalyst Au/TiO 2 type A (World Gold Council). A high degree of synergistic interaction between ceria and mesoporous titania and a positive modification of structural and catalytic properties by ceria has been achieved. It is clearly revealed that the ceria-modified mesoporous titania is of much interest as potential support for gold-based catalyst. The Au/ceria-modified mesoporous titania catalytic system is found to be efficient catalyst for WGSR. 相似文献
7.
Ceria (CeO 2) and rare-earth modified ceria (CeReO x with Re = La, Pr, Sm, Y) catalysts are prepared by nitrate precursor calcination and are characterised by BET surface area, XRD, H 2-TPR, and Raman spectroscopy. Potential of the catalysts in the soot oxidation is evaluated in TGA with a feed gas containing O 2. Seven hundred degree Celsius calcination leads to a decrease in the surface area of the rare-earth modified CeO 2 compared with CeO 2. However, an increase in the meso/macro pore volume, an important parameter for the soot oxidation with O 2, is observed. Rare-earth ion doping led to the stabilisation of the CeO 2 surface area when calcined at 1000 °C. XRD, H 2-TPR, and Raman characterisation show a solid solution formation in most of the mixed oxide catalysts. Surface segregation of dopant and even separate phases, in CeSmO x and CeYO x catalysts, are, however, observed. CePrO x and CeLaO x catalysts show superior soot oxidation activity (100% soot oxidation below 550 °C) compared with CeSmO x, CeYO x, and CeO 2. The improved soot oxidation activity of rare-earth doped CeO 2 catalysts with O 2 can be correlated with the increased meso/micro pore volume and stabilisation of external surface area. The segregation of the phases and the enrichment of the catalyst surface with unreducible dopant decrease the intrinsic soot oxidation activity of the potential CeO 2 catalytic sites. Doping CeO 2 with a reducible ion such as Pr 4+/3+ shows an increase in the soot oxidation. However, the ease of catalyst reduction and the bulk oxygen-storage capacity is not a critical parameter in the determination of the soot oxidation activity. During the soot oxidation with O 2, the function of the catalyst is to increase the ‘active oxygen’ transfer to the soot surface, but it does not change the rate-determining step, as evident from the unchanged apparent activation energy (around 150 kJ mol −1), for the catalysed and un-catalysed soot oxidation. Spill over of oxygen on the soot surface and its subsequent adsorption at the active carbon sites is an important intermediate step in the soot oxidation mechanism. 相似文献
8.
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
9.
Steam reforming of methanol was carried out over a series of doped CuO–CeO 2 catalysts prepared via the urea–nitrate combustion method. XRD analysis showed that at least part of the dopant cations enter the ceria lattice. The addition of various metal oxide dopants in the catalyst composition affected in a different way the catalytic performance towards H 2 production. Small amounts of oxides of Sm and Zn improved the performance of CuO–CeO 2, while further addition of these oxides caused a decrease in catalyst activity. XPS analysis of Zn- and Sm-doped catalysts showed that increase of dopant loading leads to surface segregation of the dopant and decrease of copper oxide dispersion. The addition of oxides of La, Zr, Mg, Gd, Y or Ca lowered or had no effect on catalytic activity, but led to less CO in the reaction products. Noble-metal modified catalysts had slightly higher activity, but the CO selectivity was also significantly higher. 相似文献
10.
The present work reports on the characterisation of several ceria supported rhodium catalysts. The selected reduction temperature were: 623 K and 773 K; the metal loading: 1% and 2.5%. The surface area of the two starting ceria samples used as support were: 20 m 2.g −1 (LS) and 110 m 2.g −1 (HS), respectively. The apparent H.Rh and CO/Rh were found to be quite different, which according to the results of the TPD study can be interpreted as due to the occurrence of strong adsorption of hydrogen on the support. Upon reduction at 773 K the adsorption capabilities are notably reduced. The effect is much stronger in the case of the phases based on HS ceria. For Rh/CeO2 (LS) catalysts, the metal dispersions determined by HRTEM agree fairly well with those estimated from CO adsorption. By contrast, over Rh/CeO2 (HS) catalysts, the values determined by TEM were found to be larger than those deduced from chemisorption. Taking into account that the reduction treatment leading to the Rh/CeO2 (HS) induces strong sintering phenomena on the support and that this effect is negligible in the case of Rh/CeO2 (LS), the results above have been interpreted assuming that the inhibition of the chemisorptive capacity of Rh/CeO2 (HS) reduced at 773 K is primarily due to the encapsulation of the metal particles associated to the sintering occurred at the support. Reoxidation of the phases reduced at 773 K did not allow to recover the chemisorptive properties of the catalysts, which confirms that no classic SMSI occurs in the present case. 相似文献
11.
Palladium (Pd) supported on CeO 2-promoted γ-Al 2O 3 with various CeO 2 (ceria) crystallinities, were used as catalysts in the methane steam reforming reaction. X-ray diffraction (XRD) analysis, FTIR spectroscopy of adsorbed CO, and X-ray photoelectron spectroscopy (XPS) were employed to characterize the samples in terms of Pd and CeO 2 structure and dispersion on the γ-Al 2O 3 support. These results were correlated with the observed catalytic activity and deactivation process. Arrhenius plots at steady-state conditions are presented as a function of CeO 2 structure. Pd is present on the oxidized CeO 2-promoted catalysts as Pd 0, Pd + and Pd 2+, at ratios strongly dependent on CeO 2 structure. XRD measurements indicated that Pd is well dispersed (particles <2 nm) on crystalline CeO 2 and is agglomerated as large clusters (particles in 10–20 nm range) on amorphous CeO 2. FTIR spectra of adsorbed CO revealed that after pre-treatment under H 2 or in the presence of amorphous CeO 2, partial encapsulation of Pd particles occurs. CeO 2 structure influences the CH 4 steam reforming reaction rates. Crystalline CeO 2 and dispersed Pd favor high reaction rates (low activation energy). The presence of CeO 2 as a promoter conferred high catalytic activity to the alumina-supported Pd catalysts. The catalytic activity is significantly lower on Pd/γ-Al 2O 3 or on amorphous (reduced) CeO 2/Al 2O 3 catalysts. The reaction rates are two orders of magnitude higher on Pd/CeO 2/γ-Al 2O 3 than on Pd/γ-Al 2O 3, which is attributed to a catalytic synergism between Pd and CeO 2. The low rates on the reduced Pd/CeO 2/Al 2O 3 catalysts can be correlated with the loss of Pd sites through encapsulation or particle agglomeration, a process found mostly irreversible after catalyst regeneration. 相似文献
12.
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
13.
Ceria has been widely explored as an additive in alumina-supported precious metal catalysts due to a number of unique properties. The success of ceria and ceria-based materials is mainly attributed to the unique combination of an elevated oxygen transport capacity coupled with the ability to shift easily between reduced and oxidised sates. In this study the influence of CeO 2 addition to a Pt/Al 2O 3 catalyst for low temperature (<540 °C) methane oxidation in an oxidising environment has been investigated. The resistance to H 2S-poisoning and influence on catalyst regeneration by oxidation or reductive treatments has been studied. The addition of CeO 2 to the support creates an increase in the level of activity based primarily on the oxygen storage capacity offered by the cerium oxide, causing an increase in oxygen activation. The ceria–alumina-supported catalyst showed a greater shift to poorer activity upon exposure to H 2S. It appears sulphur compounds react with the oxygen storage component causing a decrease in oxygen transfer, removing any benefit offered by the ceria. However, the level of Pt-agglomeration and support changes were reduced with the incorporation of ceria, emphasising the stabilising effect and promotion of metal particle dispersion associated with ceria. In order to obtain the maximum benefit of ceria addition to the support structure in terms of activity a reductive pretreatment is required. Upon exposure to a reducing atmosphere, it appears a Pt–CeO 2 interaction generates greater levels of activity. 相似文献
14.
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
15.
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
16.
This work investigates the effect of treatments under different CH 4-containing atmospheres on the reactivity of fresh and S-poisoned 2% w/w Pd/Al 2O 3/CeO 2 catalysts for methane combustion. Over the fresh catalyst the decomposition/reformation processes of PdO occurring during cycles of CH4-reducing/lean combustion pulses allowed the complete recovery of activity losses possibly associated with H2O poisoning which were observed during prolonged exposure under lean combustion conditions. The presence of CeO2 markedly enhances both the activity losses under lean combustion conditions and the rate of PdO reoxidation/reactivation upon Pd redox cycle. Under lean combustion conditions, regeneration of catalyst deactivated by exposure to SO2-containing atmosphere required very high temperatures (above 750 °C) in order to decompose stable sulphate species adsorbed on the support. Treatments consisting of alternate CH4-reducing/lean combustion pulses allowed a complete recovery of activity at much lower temperatures (550–600 °C) due to the reduction of sulphates by CH4 activated on the surface of Pd metal. A protecting role of CeO2 on Pd poisoning due either to exposure to SO2-containing atmosphere or to spill-back of support sulphates species was also evidenced. 相似文献
17.
为了提高硼氢化钠水解催化剂的活性与稳定性,本文采用化学还原和焙烧后处理制备了一系列CoB/CeO 2负载型催化剂,探究了该催化剂在NaBH 4液相释氢中的催化性能。通过扫描电镜、X射线衍射、X射线光电子能谱等表征手段分析了催化剂结构和组成,并结合催化剂在NaBH 4水解制氢中的催化活性,研究了不同焙烧后处理方式对CoB/CeO 2负载型催化剂体系的作用规律。结果表明CoB负载到CeO 2载体表面后催化剂仅呈现了CeO 2载体的萤石相结构,但针对活性组分对比分析表明在高温下(空气和氮气)仍然出现了CoB的显著晶化。CoB/CeO 2在空气和氮气焙烧气氛下都有利于提高CoB活性组分在CeO 2载体表面的固载强度,但是二者呈现出不同的微观形貌。在NaBH 4液相释氢中,CoB/CeO 2-air和CoB/CeO 2-N 2展示了显著不同的催化活性,并且随着焙烧温度的提高,二者催化性能差异更加明显,温度升高后CoB/CeO 2-air催化活性逐步降低,并在500℃左右完全失活,而CoB/CeO 2-N 2则呈现了趋于稳定的催化活性。 相似文献
18.
The effect of CeO 2 loading (1–20 wt.%) on the properties and catalytic behaviors of CeO 2–Al 2O 3-supported Pt catalysts on the partial oxidation of methane was studied. The catalysts were characterized by SBET, X-ray diffraction (XRD), temperature-programmed reduction (TPR) and oxygen storage capacity (OSC). XRD and TPR results showed that the pretreatment temperature of the support influences on the amount of CeO 2 with fluorite structure. The pretreatment temperature of the support and CeO 2 loading influenced the morphology of Pt. OSC analysis showed a significant increase in the oxygen storage capacity per weight of CeO 2 for samples with high CeO 2 loading (12 and 20 wt.%). TPR analyses showed that the addition of Pt promotes the reduction of CeO 2. This effect was more significant for the catalysts with high CeO 2 loading (≥12 wt.%). The dispersion of Pt, measured by the rate of cyclohexane dehydrogenation, increases with increasing of the pretreatment temperature of the support. It was shown that the kind of the support is very important for obtaining of catalysts resistant to carbon formation. The catalysts with high CeO 2 loading (≥12 wt.%) showed the highest catalytic activity and stability in the reaction of partial oxidation of methane due to a higher Pt–CeO 2 interface. 相似文献
19.
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
20.
The objective of this paper was to study a preferential oxidation (PROX) of carbon monoxide over monometallic catalysts including Pt, Au and Pt–Au bimetallic catalyst supported on ceria in hydrogen-rich reformate. Single step sol–gel method (SSG) and impregnation on sol–gel method (ISG) were chosen for the preparation of the catalysts. The characteristics of these catalysts were investigated by X-ray diffractometer (XRD), Brunauer–Emmet–Teller (BET) method, transmission electron microscope (TEM), scanning electron microscope (SEM) and temperature-programmed reduction (TPR). The XRD patterns of the catalysts showed only the peaks of ceria crystallite and no metal peak appeared. From TEM images, the active components were seen to be dispersed throughout the ceria support. The TPR patterns of PtAu/CeO 2 catalyst prepared by SSG showed the reduction peaks were within a low temperature range and therefore, the catalysts prepared by SSG exhibited excellent catalytic activity for preferential oxidation of CO. Bimetallic Pt–Au catalyst improved the activity (90% conversion and 50% selectivity at 90 °C) because of the formation of a new phase. When the metal content of (1:1) PtAu/CeO 2 catalyst prepared by SSG was increased, the CO conversion did not change much while the selectivity decreased in the low temperature range (50–90 °C). The CO conversion increased with increasing W/F ratio. The presence of CO 2 and H 2O had a negative effect on CO conversion and selectivity due to blocking of carbonate and water on active sites. 相似文献
|