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1.
Chunjie Wang Yue Wang Wenzhi Huang Binglin Zou Zuhair Subhani Khan Yu Zhao Jili Yang Xueqiang Cao 《Ceramics International》2012,38(3):2087-2094
Nanopowders with cubic fluorite-type structure as well as uniform distribution in particle size were synthesized by hydrothermal method in the ternary oxide zirconia–yttria–ceria system with ceria content of 0–25 mol%. X-ray diffraction (XRD), thermogravimetric analysis/differential scanning calorimeter (TG/DSC), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy (Raman), specific surface area (SBET) and high resolution transmission electron microscopy (HRTEM) were applied to characterize the structure, thermal decomposition, morphological characteristic and crystal growth of the produced powders. Qualitative analyses indicate that the as-synthesized nanoparticles are single-phase crystallites with an average particle size of 4–9 nm. The specific surface area, lattice parameter and microstrain are closely related to Ce4+ concentration. Moreover, activation energy of crystal growth is significantly dependent on the dopant (CeO2) concentration. It firstly increased and then decreased with increasing dopant concentration, and the maximum value was observed at the dopant concentration of 5 mol%. 相似文献
2.
《Journal of the European Ceramic Society》2003,23(2):273-282
The local structure around Zr, Ce and dopant atoms (Fe and Ni) in the ZrO2–CeO2 system investigated by X-ray absorption spectroscopy (XAS) is reported to better understand the tetragonal phase stabilization process of zirconia. The first coordination shell around Zr atoms is not sensitive to the introduction of dopants or to an increase in the ceria content (from 12 to 20 mol%). Ce ions maintain the eight-fold coordination as in CeO2, but with an altered bond distance. The formation of vacancies resulting from reduction of Ce atoms can be discarded, because XANES spectra clearly show that Ce ions are preferentially in a tetravalent state. XANES and EXAFS experiments at the Fe K-edge evidence that the local order around Fe is quite different from that of the Fe2O3 oxide. On the one hand, ab initio EXAFS calculations show that iron atoms form a solid solution with tetragonal ZrO2. The EXAFS simulation of the first coordination shell around iron evidences that the substitution of zirconium atoms by iron ones generates oxygen vacancies into the tetragonal network. This is a driven force for the tetragonal phase stabilization process. For Ni doped samples, EXAFS results show that Ni–O mean bond length is similar to the distance found in the oxide material, i.e., NiO compound. Besides this result, no evidence of similar solid solution formation for Ni-doped systems has emerged from the EXAFS analysis. 相似文献
3.
Hydroxymethylation of anisole has been carried out over SnO2–CeO2 catalysts in the temperature range 623–723 K. Methoxybenzaldehyde (anisaldehyde) and condensation products were formed along
with minor quantities of methoxybenzyl alcohol, o‐cresol, phenol and 2,6‐xylenol. A maximum anisaldehyde selectivity of 64% was obtained at 623 K at an anisole conversion
of 46% under optimized conditions. Catalytic activity of these systems in the formation of aldehyde is ascribed to the presence
of weak acid sites and redox metal sites.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
4.
5.
The dehydration of 1,3-butanediol was investigated over CeO2–ZrO2 catalysts prepared by impregnation at temperatures of 325–375 °C. Pure CeO2 selectively catalyzed the dehydration of 1,3-butanediol to form 3-buten-2-ol and 2-buten-1-ol, while pure ZrO2, which was less active than pure CeO2, catalyzed the dehydration to 3-buten-1-ol. In the CeO2/ZrO2 catalyst in which CeO2 was supported on zirconia, the presence of a small amount of CeO2 suppressed the formation of 3-buten-1-ol and induced the dehydration of 1,3-butanediol to form 3-buten-2-ol and 2-buten-1-ol and the subsequent dehydrogenation of 3-buten-2-ol to form 3-buten-2-one and butanone. The activity would be related to the redox features of CeO2. The monoclinic phase of zirconia support decreased while the cubic CeO2 phase increased as CeO2 content was increased. In contrast, in the ZrO2/CeO2 catalyst in which ZrO2 was supported on cubic CeO2, only the cubic CeO2 phase was observed and ZrO2 species appeared in the form of a solid solution of CeO2–ZrO2 with fluorite structure. Regardless of zirconia loading, ZrO2 species did not affect the catalytic activity of ZrO2/CeO2, which was controlled by CeO2 species. 相似文献
6.
Mohsan Nawaz Sidra Maghfoor Muhammad Waseem Ali Bahader Masroor Ahmad Bangesh Hameed Ullah 《Polymer-Plastics Technology and Engineering》2017,56(14):1487-1494
This work focuses on the flow behavior of the blend comprising polyvinyl pyrrolidone and cerium (IV) oxide (CeO2) particles in submicron size, under low shear rates. The polyvinyl pyrrolidone–CeO2 blends have been prepared and characterized by scanning electron microscopy, X-ray diffraction, and viscometry. The generation of core–shell morphology was verified from the scanning electron micrographs. Scanning electron microscopy shows that the blend formed is of porous nature. The particle size of CeO2 increases with the concentration of both CeO2 and polymer due to aggregation. The blend containing as high as 35?wt% of CeO2 was found to exhibit pseudo-plastic response under low shear rate. The reasons for the observed morphology and other properties along with mechanism were explained. The main factor, which governs the properties of the end product, was van der Waals attractive forces that exist among the constituents of the system prepared. 相似文献
7.
XUYanqing CHENKaixian 《中国耐火材料》1998,7(4):21-24
Dolomite refractory products with excellent hydration resistance have been produced by using CeO2-and CeO2/Fe2O3-bearing dolomite clinkers,Their physical properties as well as hydration resistance have been investigated,The addition of CeO2 has little harmful effect on the high temperature properties of dolomite refractory products such as hot MOR and slag resistance,And the shelf lives of the dolomite refractory products containing CeO2 and CeO2/Fe2O3 additions at the same condition are two times that of the common dolomite refractory produt.The dolomite refractory product containing CeO2/Fe2O3 combination possesses the best hydration resistance,but gives poor slag resistance. 相似文献
8.
W. H. Cassinelli L. S. F. Feio J. C. S. Araújo C. E. Hori F. B. Noronha C. M. P. Marques J. M. C. Bueno 《Catalysis Letters》2008,120(1-2):86-94
The effect of the addition of CeO2 or La2O3 on the surface properties and catalytic behaviors of Al2O3-supported Pd catalysts was studied in the steam reforming of methane. The FTIR spectroscopy of adsorbed CO and the Pd dispersion
suggest the partial coverage of Pd0 by ceria or lanthana species. This could lead to the formation of an adduct MPd
x
O (M = Ce or La) at the surface of the metal crystallites. The addition of ceria or lanthana resulted in an increase of the
turnover rate and specific rate for steam reforming of methane. One possible explanation if that the Pd0*Pdδ+O–M interfacial species (M = Ce or La) are oxidized by H2O or CO2, promoting the O* transfer to the metal surface. This could facilitate the removal of C* species from the metal surface,
resulting in the increase of specific reaction rate and increase of the accessibility of CH4 to metal active sites. 相似文献
9.
The interaction between CO2 and CeO2 and its role in the surface reactivity of alumina-supported cerium oxide has been studied by programmed thermodesorption
(TPD) of CO2 and FTIR spectroscopy. The performance of Ce/Al2O3 systems was then analyzed for the propane oxidation in presence of CO2. The results have shown that the catalytic activity decreased when carbonate species are formed at the surface of CeO2. This behavior was attributed to the presence of CO2 from three different sources: contamination before use, during the handling of the samples, contamination proceeding from
the reactants and from CO2 produced by the reaction itself.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
10.
《Journal of the European Ceramic Society》2003,23(1):99-106
A thermodynamic estimation of the ZrO2–CeO2 and ZrO2–CeO1.5 systems, as well as the cubic phase in the CeO1.5–CeO2 system has been developed and the complex relation between the nonstoichiometry, y, in CezO2–y and the oxygen partial pressure at different temperatures is evaluated. The behavior of the nonstoichiometry phase Zr1–zCezO2–x is described based on the thermodynamic estimation in the ZrO2–CeO2, CeO1.5–CeO2 and ZrO2–CeO1.5 systems. Additionally, the interdependence among miscellaneous factors, which can be used to describe the change in oxidation states of cerium such as the oxygen partial pressure, the CeO1.5 fraction in CeO1.5–CeO2 in the quasi-ternary system, the nonstoichiometry y and the difference between the activity of CeO2 and CeO1.5 are predicted. The calculated results are found to be very useful to explain the influence of pressureless sintering at different O2 partial pressures on the mechanical properties of CeO2-stabilised ZrO2 ceramics 相似文献
11.
Selective liquid phase hydrogenation of cinnamaldehyde is reported, for the first time, over CeO2, ZrO2, and CeO2–ZrO2-supported Pt catalysts. Cinnamyl alcohol is the selective product. These catalysts are highly active and selective even at 25 °C and found to be superior to most of the hitherto known supported Pt catalysts. Alkali addition (NaOH) has enhanced the performance of these catalysts. At an optimized reaction condition, 95.8% conversion of cinnamaldehyde and 93.4% selectivity of cinnamyl alcohol have been obtained. Acidity of the support (due to the presence of ZrO2 component) and higher electron density at Pt (due to CeO2 component) are attributed to be responsible for the superior catalytic activity of Pt supported on CeO2–ZrO2 composite material. 相似文献
12.
《Journal of the European Ceramic Society》2003,23(5):685-690
The difference of Gibbs free energy between tetragonal and monoclinic phases in ZrO2–CeO2–Y2O3 as a function of composition and temperature is thermodynamically calculated from the three related binary systems. In 8 mol% CeO2–0.5 mol% Y2O3–ZrO2, the equilibrium temperature between tetragonal and monoclinic phases, T0, is obtained as 832.5 K and the Ms temperature of this alloy with a mean grain size of 0.90 μm is calculated as 249.9 K using the approach derived by Hsu et al. [J. Mater. Sci., 18(1983)3206; 20(1985)23; Acta Metall., 37(1989)3091; Acta Metall. Mater., 39(1991)1045; Mater. Trans. JIM, 37(1996)1284], which is in good agreement with the experimental one of 253 K. 相似文献
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14.
《Catalysis communications》2004,5(5):231-235
Hydrogen production by (combined) steam reforming of methanol (CSRM) was investigated over CuO–CeO2 catalysts prepared via the urea-combustion method. The characteristics of the resulting oxides were strongly influenced by the autoignition time during synthesis and the sample prepared with near stoichiometric quantity of urea had less favorable catalytic properties. Catalysts prepared from urea-rich or lean mixtures were more active and selective and an optimum behavior was obtained with 75% excess of urea and Cu/(Cu + Ce) ratio equal to 0.15. The higher methanol conversion in the CSRM process may be attributed to more efficient heat transfer in the bed due to combustion of part of methanol. 相似文献
15.
M. Radović Z. Dohčević-Mitrović A. Golubović V. Fruth S. Preda M. Šćepanović Z.V. Popović 《Ceramics International》2013,39(5):4929-4936
Ce1?xFexO2?y (0≤x≤0.05) nanopowders were synthesized using hydrothermal method at low calcination temperature and low doping regime. Structural and morphological characterization has been carried out by the X-ray diffraction method and non-contact atomic force microscopy. Vibrational properties were investigated by Raman spectroscopy. It was observed that the content of oxygen vacancies increased significantly with Fe doping up to 3 mol%. For higher dopant concentration, phase separation was detected. The optical properties of pure and Fe3+-doped CeO2?y samples were investigated by spectroscopic ellipsometry. Several analytical models were applied to analyze the optical absorption onset of ceria defective structure. It was found that, Cody–Lorentz model most suitably described the sub-band gap region of CeO2?y nanopowders and consequently gave more accurate band gap values, which are closer to the direct band gap transitions than to the indirect ones. The increased content of localized defect states in the ceria gap and corresponding shift of the optical absorption edge towards visible range in Fe-doped samples can significantly improve the optical activity of nanocrystalline ceria. 相似文献
16.
A series of Fe2O3–CeO2 composite catalysts were synthesized by coprecipitation and characterized by X-ray diffraction (XRD), BET surface area measurement, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Their catalytic activities in CO oxidation were also tested. The Fe2O3–CeO2 composites with an Fe molar percentage below 0.3 form solid solutions with the CeO2 cubic fluorite structure, in which the doped Fe3+ initially substitutes Ce4+ in fluorite cubic CeO2, but then mostly locate in the interstitial sites after a critical concentration of doped Fe3+. With an Fe molar percentage between 0.3 and 0.95, the Fe2O3–CeO2 composites are mixed oxides of the cubic fluorite CeO2 solid solution and the hematite Fe2O3. XPS results indicate that CeO2 is enriched in the surface region of Fe2O3–CeO2 composites. The Fe2O3–CeO2 composites have much higher catalytic activities in CO oxidation than the individual pure CeO2 and Fe2O3, and the Fe0.1Ce0.9 composite shows the best catalytic performance. The structure-activity relation of the Fe2O3–CeO2 composites in CO oxidation is discussed in terms of the formation of solid solution and surface oxygen vacancies. Our results demonstrate a proportional relation between the catalytic activity of cubic CeO2-like solid solutions and their density of oxygen vacancies, which directly proves the formation of oxygen vacancies as the key step in CO oxidation over oxide catalysts. 相似文献
17.
Acetic acid (HAc) aqueous was used as solvent in wetness impregnation to prepare CeO2-modified γ-Al2O3 support. With the help of HAc, the dispersion of CeO2 on γ-Al2O3 is significantly improved and the size of CeO2 nanoparticles can be controlled through tuning the concentration of HAc aqueous. XPS analysis shows that the percentages of Ce3 + in CeO2 nanoparticles will vary with the size. Then we load CuO on the as-prepared CeO2-modified γ-Al2O3 support and choose NO reduction with CO as a probe reaction to investigate the influences of impregnation solvent on the catalytic properties. The results demonstrate that the CuO/CeO2/γ-Al2O3 prepared in the solvent with volume ratio of 20:1 (H2O:HAc) has the highest activity in NO + CO reaction. Combing the structural characterizations and catalytic performances, we think that the size of the CeO2 nanoparticles should be a key factor that affects the activities of CuO/CeO2/γ-Al2O3. Furthermore, CuO dispersed on CeO2 nanoparticles with an average size of ca. 5 nm should be the highest active sites for NO + CO reaction. 相似文献
18.
Lei Li Yingying Zhan Qi Zheng Yuanhui Zheng Chongqi Chen Yusheng She Xingyi Lin Kemei Wei 《Catalysis Letters》2009,130(3-4):532-540
A series of CuO/CeO2 catalysts were prepared through a two-step process: (1) CeO2 supports were firstly prepared by precipitation (P), hydrothermal (HT) and sol-gel (SG) methods, respectively; and (2) CuO was deposited on the above CeO2 supports by deposition-precipitation method. The as-synthesized CeO2 supports and CuO/CeO2 catalysts were characterized by N2-physisorption, XRD, XPS, Raman, and H2-TPR. The CuO/CeO2 catalysts were examined with respect to their catalytic activity for the water–gas shift reaction, and their catalytic activities are ranked as: CuO/CeO2-P > CuO/CeO2-HT > CuO/CeO2-SG. The results suggest that the CeO2 prepared by precipitation (i.e., CeO2-P-300) has the best thermal stability and the most amounts of surface oxygen vacancies, which make the corresponding CuO/CeO2-P catalyst present the largest pore volume, the smallest crystal size of CuO, the highest microstrain (i.e., the highest surface energy) and the most amounts of active sites (i.e., the moderate copper oxide (crystalline) interacted with surface oxygen vacancies of ceria). Therefore, the catalytic activity of CuO/CeO2 catalysts, in nature, depends on the thermal stability and the number of surface oxygen vacancies of the CeO2 supports previously prepared by different methods. 相似文献
19.
Nanocrystalline TiO2 with average crystallite sizes 7 and 15 nm were synthesized by the solvothermal method using titanium n-butoxide and 1,4-butanediol as titanium precursor and solvent, respectively. Four transition metals (Ag, Co, Ni, Pt) were supported on the TiO2 supports by incipient wetness impregnation with metal loadings 1 and 25 atomic%. For any metal type, metal dispersion and CO oxidation activities were higher when the catalysts were prepared on the larger crystallite size TiO2 for both high and low metal loadings, despite their lower surface areas. The presence of higher amount of Ti3+ on the larger crystallite size TiO2 probably stabilized small metal particles during impregnation, calcination, and reduction steps via stronger metal-support interaction; hence higher CO oxidation activities and lower light-off temperature were obtained. 相似文献