Catalytic properties of Ce x Zr1–x O2 prepared using a template in the oxidation of CO (original) (raw)
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Arabian Journal of Chemistry, 2015
A series of mixed oxide catalysts Ce x Zr 1Àx O 2 with different compositions (x = 0.93, 0.80, 0.75, 0.60, 0.55, and 0.50) were synthesized by co-precipitation technique using ammonia as the precipitant agent. Textural and structural properties of calcined ceria-zirconia oxides were characterized by nitrogen adsorption, X-ray diffraction and Raman spectroscopy. The redox properties were investigated by temperature-programmed reduction measurement using H 2. The procedure of preparation as well as the composition has a great influence on the structural and textural properties of mixed oxides. Fluorite type oxides were shown to be a very interesting support for the total oxidation, since these materials led to the complete decomposition of ethyl acetate at rather low temperatures. Of the synthesized materials, Ce 0.5 Zr 0.5 O 2 showed the best catalytic activity.
Journal of Physical Chemistry C, 2008
In this study, Ce 0.65 Zr 0.25 RE 0.1 O 2−δ (RE = Tb, Gd, Eu, Sm, Nd, Pr, and La) solid solutions were successfully prepared by the glycine-nitrate process and tested for CO oxidation activity. The X-ray diffraction results confirmed the formation of complete Ce 0.65 Zr 0.25 RE 0.1 O 2−δ solid solutions. The Raman spectroscopy measurements suggested the presence of oxygen vacancies due to defective structure formation and further evidenced the formation of solid solution. The high-resolution transmission electron microscopy observations showed the nanocrystalline nature of the solid solutions. From X-ray photoelectron spectroscopy analysis it was revealed that the cerium, terbium, and praseodymium are present in +3 and +4 oxidation states. The UV−vis diffuse reflectance spectroscopy indicated that the Pr 3+ ions in the Ce 0.65 Zr 0.25 Pr 0.1 O 2−δ system provoked a significant increase in the Ce 3+ fraction on the surface. H 2 temperature-programmed reduction measurements showed an enhanced surface reduction at much lower temperatures for Ce 0.65 Zr 0.25 Pr 0.1 O 2−δ sample compared to others, indicating increased oxygen mobility in these samples, which enable the enhanced oxygen diffusion at lower temperatures. Significantly high CO oxidation activity is exhibited by Ce 0.65 Zr 0.25 Pr 0.1 O 2 solid solution.
Catalysis Today, 2011
Nanosized ceria-zirconia, ceria-hafnia and ceria-terbia solid solutions were synthesized by a modified coprecipitation method and evaluated for oxygen storage/release capacity (OSC) and CO oxidation activity. The physicochemical characteristics were investigated by various techniques namely, XRD, Raman, XPS, ISS, UV-vis DRS, TEM and BET surface area. XRD and Raman results revealed formation of Ce 0.75 Zr 0.25 O 2 , Ce 0.8 Hf 0.2 O 2 and Ce 0.8 Tb 0.2 O 2−ı solid solutions typical of fluorite like cubic structure. Variation in the lattice parameter confirmed the successful incorporation of dopant cations into the ceria lattice. XPS measurements revealed that Ce is present in both 3+ and 4+ oxidation states in all samples. UV-vis DRS studies suggested the presence of Ce 3+ ← O 2− charge transfer transitions attributed to oxygen defects in all the samples. ISS measurements indicated surface enrichment of cerium in ceria-hafnia solid solutions. The ceria-hafnia solid solutions exhibited a high OSC and better CO oxidation activity among the investigated samples. Systematic investigation of these materials provided valid information about the key factors that control the catalytic efficiency of the solid solutions for OSC and CO oxidation.
Industrial & Engineering Chemistry Research, 2009
A study has been undertaken on the thermal stability and dispersion behavior of nanostructured Ce x Zr 1-x O 2 mixed oxides over anatase-TiO 2 support, synthesized by a deposition coprecipitation method and subjected to heat treatments at different temperatures. An unsupported Ce x Zr 1-x O 2 mixed oxide was also synthesized by coprecipitation method for the purpose of comparison. The structural/morphological characterization was performed using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HREM), X-ray photoelectron spectroscopy (XPS), UV-visible diffuse reflectance spectroscopy (UV-vis DRS), and temperature-programmed reduction (TPR) techniques. XRD measurements revealed the presence of cubic Ce 0.75 Zr 0.25 O 2 phase at 773 K and incorporation of more zirconium into the ceria lattice at higher temperatures. HREM analysis conformed highly dispersed Ce-Zr nano-oxides over anatase-TiO 2 support having the sizes in the range of 3-5 nm at 773 K. At higher temperatures, a slight increase in the crystallite size (5-10 nm) has been observed. The XPS measurements revealed the stabilization of Ce 3+ at higher temperatures and presence of Zr and Ti in 4+ oxidation states. UV-vis DRS studies showed that oxygen to metal (Ce 4+ /Ce 3+ ) charge-transfer transitions occur at lower wavelengths in the case of Ce x Zr 1-x O 2 /TiO 2 compared to pure Ce x Zr 1-x O 2 . TPR experiments revealed that the surface reduction of Ce x Zr 1-x O 2 /TiO 2 takes place at lower temperatures, which facilitates better CO oxidation activity in comparison to the unsupported Ce x Zr 1-x O 2 .
J. Chem. Sci., 2015
In this work, nanosized Ce 0.7 M 0.3 O 2−δ (M = Mn, Fe, Co) solid solutions were prepared by a facile coprecipitation method and evaluated for CO oxidation. The physicochemical properties of the synthesized samples were investigated by various characterization techniques, namely, XRD, ICP-OES, BET surface area, SEM-EDX, TEM and HRTEM, Raman, XPS, and H 2-TPR. XRD studies confirmed the formation of nanocrystalline single phase Ce 0.7 M 0.3 O 2−δ solid solutions. ICP-OES analysis confirmed actual amount of metal loadings in the respective catalysts. The BET surface area of Ce 0.7 M 0.3 O 2−δ samples significantly enhanced after the incorporation of dopants. TEM studies confirmed nanosized nature of the samples and the average particle sizes of Ce 0.7 M 0.3 O 2−δ were found to be in the range of ∼8-16 nm. Raman studies indicated that the incorporation of dopant ions into the CeO 2 lattice promote the formation of more oxygen vacancies. The existence of oxygen vacancies and different oxidation states (Ce 3+ /Ce 4+ and Mn 2+ /Mn 3+ ,Fe 2+ /Fe 3+ , and Co 2+ /Co 3+) in the doped CeO 2 samples were further confirmed from XPS investigation. TPR measurements revealed an enhanced reducibility of ceria after the incorporation of dopants. The catalytic activity results indicated that the doped CeO 2 samples show excellent CO oxidation activity and the order of activity was found to be Ce 0.7 Mn 0.3 O 2−δ > Ce 0.7 Fe 0.3 O 2−δ > Ce 0.7 Co 0.3 O 2−δ > CeO 2. The superior CO oxidation performance of CeO 2-MnO x has been attributed to a unique Ce-Mn synergistic interaction, which facilitates materials with promoted redox properties and improved oxidation activity.
Design of transition and rare earth metal doped ceria nanocomposite oxides for CO oxidation
International Journal of Advances in Engineering Sciences and Applied Mathematics, 2013
Nanocrystalline Ce x TM 1-x O 2 (TM = Zr 4? and Hf 4? ) and Ce x RE 1-x O 2-d (RE = La 3? , Pr 3?/4? and Tb 3?/4? ) solid solutions were prepared by modified coprecipitation method and thermally treated at two different temperatures, 773 and 1073 K in order to determine the thermal behaviour. The synthesized samples were studied in detail by means of N 2 physisorption, XRD, Raman, X-ray photoelectron spectroscopy (XPS), and TEM/HREM techniques. The catalytic efficiency was performed towards oxygen storage/release capacity (OSC) and CO oxidation activity. XRD results indicated the formation of transition and rare earth metal doped ceria solid solutions with typical fluorite like cubic structure. Raman spectra displayed the existence of oxygen vacancies. XPS measurements revealed that Ce, Pr and Tb are present in both 3? and 4? oxidation states, La in 3? state, and Zr and Hf in 4? state. Among the investigated samples, the catalytic measurements revealed that ceria-hafnia solid solution exhibits superior catalytic activity towards CO oxidation, which could be due to a high OSC and other favourable physicochemical properties.
Journal of Industrial and Engineering Chemistry, 2013
The influence of Hf, Pr and Tb dopant cations on structural and catalytic properties of nanosized Ce x Zr 1Àx O 2 solid solutions has been investigated. A wide range of analytical techniques are utilized to characterize the synthesized materials, and the catalytic activity is evaluated for CO oxidation. XRD, Raman, SEM and TEM results suggested formation of dopant cation incorporated ceria-zirconia solid solutions with highly homogeneous morphology and lattice defects. The CO-TPR measurements revealed an enhanced reducibility of Ce x Zr 1Àx O 2 which is reflected in their better catalytic activity. The role of Tb 4+ /Tb 3+ and Pr 4+ /Pr 3+ redox couples in facilitating a higher activity has been addressed.
Kinetics and Catalysis, 2017
The effect of potassium and calcium additives on the catalytic activity of the Ce 0.8 Zr 0.2 O 2 system in the reaction of CO oxidation was studied. With the use of X-ray diffraction analysis, it was found that the Ce 0.8 Zr 0.2 O 2 and Ce 0.8 Zr 0.2 O 2-Ca,K samples contained a mixed oxide of cerium and zirconium; the presence of the independent phases of potassium and calcium compounds in the modified system was not detected. With the use of the low-temperature adsorption-desorption of nitrogen, X-ray photoelectron spectroscopy, and temperature-programmed reduction, it was established that the Ce 0.8 Zr 0.2 O 2-Ca,K system (in spite of the fact that its specific surface area was lower than that of Ce 0.8 Zr 0.2 O 2) contained more active oxygen on the surface; peroxide and superoxide complexes formed upon the chemisorption of O 2 can act as active oxygen species. This can be the reason for a higher efficiency of the Ce 0.8 Zr 0.2 O 2-Ca,K system in comparison with that of the unmodified oxide. The results obtained indicate that the ash impurities of Ca and K can increase the catalytic activity of the biomorphic mixed oxides Ce 0.8 Zr 0.2 O 2 prepared with the use of sawdust as a template.
A study on nanosized certium oxides systems for environmental catalysis
2003
This thesis concerns a study on the synthesis and characterisation of nanostructured ceria composites aiming at the enhancement of performance and thermal stability. These features are highly demanded in environmental catalysis, an area of great importance. While the worldwide motor vehicle production has been increasing dramatically during the last decades, environment has been contaminated with the exhaust gases of the automobiles, the most common of which are HCs, CO and NO x. With the advent of the three way catalytic converters, these detrimental gases can be simultaneously converted into harmless H 2 O, N 2 and less harmful CO 2. Such a conversion should be accomplished in as wide as possible range of air/fuel ratio, the so-called 'lambda (λ) window'. This λ-window is mainly provided by ceria (CeO 2), one of the main washcoat constitutes of these catalytic converters, due to its specific property of oxygen storage capacity. The performance properties of ceria can be enhanced via doping, i.e. partial substitution by other cations in its crystal lattice to create vacancies, and via nanostructuring to create surface crystal defects for more active sites. However, the performance of ceria deteriorates due to particles growth after extended use of the catalytic converters at high temperature. Therefore, there is a strong demand for an enhanced thermal stability of these materials. In the present study, this has been achieved by coating Al 2 O 3 on the nanosized particles of doped ceria, Ce 1-x Me x O 2-δ. A new method has been developed for the synthesis of nanosized particles of Zrand/or Ca-doped ceria, which are coated with Al 2 O 3. The synthesis was accomplished in three major steps: (1) co-precipitation of cerium oxalate and other compounds of doping components, e.g. zirconium and/or calcium, (2) sequential precipitation of Al(OH) 3 over the former particles, and (3) calcination of the precipitated precursors to the oxides. Several compositions have been synthesised and their physicochemical properties are compared with commercial state-of-the-art material. In the case of Ca doping, a procedure of simultaneous co-precipitation of all doping components including Al(OH) 3 was also tested for comparison. It has been found that the sequential precipitation of Al(OH) 3 is advantageous in the formation of Al 2 O 3-coating on the nanosized particles of doped ceria. The Al 2 O 3 coated layer can hinder the particles growth during the heat treatment at high temperatures, thus maintaining the materials with large specific surface area, high oxygen storage capacity and activity. Moreover, nanosized Ce-Cu-O powders have been synthesised via coprecipitation approach. Detailed XRD study does not show the formation of solid solution, as in the cases of Zr and Ca doping, but the composites consist of two separate oxides. X-ray absorption spectroscopy study revealed a variation of the existence of Cu and Ce in several oxidation states. Copper oxide seems to exist evenly distributed along the surfaces and boundaries of the crystal grains of CeO 2. The temperature-programmed reduction indicated that the Ce-Cu-O composite has an enhanced oxygen storage capacity, as well as its increased reactivity at much lower temperature compared to the pure CeO 2 alone.