Oxygen Storage Capacity Measurements Research Articles

Preparation of nanometric CeO 2–ZrO 2–Nd 2O 3 solid solution and its catalytic performances

A kind of nanometric CeO 2–ZrO 2–Nd 2O 3 (CZN) solid solution for a carrier in the automotive three-way catalysts was synthesized by a coprecipitation method and characterized by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), nitrogen adsorption–desorption (BET), scanning electron microscopy (SEM) and oxygen storage capacity (OSC). For the purpose of comparison, an unincorporated CeO 2–ZrO 2 (CZ) was also synthesized. The XRD measurements disclose the prepared CeO 2–ZrO 2–Nd 2O 3 have a face-centered cubic fluorite structure and nanoparticle sizes. According to the results of XPS, Nd 3+ ions can enter the CZ lattice and form a homogenous solid solution. Oxygen storage capacity measurements reveal that CeO 2–ZrO 2–Nd 2O 3 display high oxygen mobility at a low temperature. The results of the activity tests show that the catalyst exhibits good three-way catalytic activity and fairly wide range of air-to-fuel ratios.

Synthesis of monophasic Ce0.5Zr0.5O2 solid solution by microwave-induced combustion method

Nanocrystalline monophasic Ce0.5Zr0.5O2 solid solution (1:1 molar ratio) has been synthesized by microwave-induced combustion method in a modified domestic microwave oven (2.45 GHz, 700 W) in approximately 40 min from cerium nitrate and zirconium nitrate precursors using urea as ignition fuel. For the purpose of better comparison, a CexZr1 − xO2 solid solution (1:1 molar ratio) was also synthesized by a conventional co-precipitation method from nitrate precursors and subjected to different calcination temperatures. The synthesized powders of both methods were characterized by means of X-ray powder diffraction, thermogravimetry/differential thermal analysis, scanning electron microscopy, and BET surface area techniques. Oxygen storage capacity (OSC) measurements were performed to understand the usefulness of these materials for various applications. The characterization results reveal that the sample obtained by microwave-induced combustion-synthesis route exhibits homogeneous monophasic Ce0.5Zr0.5O2 solid solution whereas the co-precipitated sample displays compositional heterogeneity. The OSC measurements reveal that the materials synthesized by both methods exhibit comparable oxygen vacancy content (δ).

Comparison of the microstructure and oxygen storage capacity modification of Ce 0.67Zr 0.33O 2 from CaO and MgO doping

Doping effect of CaO and MgO on the microstructure and dynamic oxygen storage capacity (OSC) of Ce 0.67Zr 0.33O 2 was compared. XRD, Raman spectra and HRTEM observation confirmed that after MgO and CaO doping, more oxygen ion vacancies and lattice defect in the fluorite lattice were produced due to charge compensation and radius effect. On the other hand, improved phase thermal stability was obtained from doping. Results of dynamic OSC measurements showed that MgO and CaO doping enhance the dynamic OSC of Ce 0.67Zr 0.33O 2. But after redox treatment difference was found between the two dopants. The improved OSC of CaO doping was due to the proper ionic radius and effective modification of microstructure. The decreased OSC of MgO doped Ce 0.67Zr 0.33O 2 after redox treatment was due to the small solubility of Mg 2+ in ceria lattice and collapse of the defective structure of Mg–Ce–Zr–O system.

Surface Stabilized Nanosized CexZr1-xO2 Solid Solutions over SiO2: Characterization by XRD, Raman, and HREM Techniques

Ce(x)Zr(1)(-)(x)O(2) solid solutions deposited over silica surface were investigated by X-ray diffraction (XRD), Raman spectroscopy (RS), and high-resolution transmission electron microscopy (HREM) techniques in order to understand the role of silica support and the temperature stability of these composite oxides. For the purpose of comparison, an unsupported Ce(x)Zr(1)(-)(x)O(2) was also synthesized and subjected to characterization by various techniques. The Ce(x)Zr(1)(-)(x)O(2)/SiO(2) (CZ/S) (1:1:2 mole ratio based on oxides) was synthesized by depositing Ce(x)Zr(1)(-)(x)O(2) solid solution over a colloidal SiO(2) support by a deposition precipitation method and unsupported Ce(x)Zr(1)(-)(x)O(2) (CZ) (1:1 mole ratio based on oxides) was prepared by a coprecipitation procedure, and the obtained catalysts were subjected to thermal treatments from 773 to 1073 K. The XRD measurements disclose the presence of cubic phases with the composition Ce(0.75)Zr(0.25)O(2) and Ce(0.6)Zr(0.4)O(2) in CZ samples, while CZ/S samples possess Ce(0.75)Zr(0.25)O(2), Ce(0.6)Zr(0.4)O(2), and Ce(0.5)Zr(0.5)O(2) in different proportions. The crystallinity of these phases increased with increasing calcination temperature. The cell a parameter estimations indicate contraction of ceria lattice due to the incorporation of zirconium cations into the CeO(2) unit cell. Raman measurements indicate the presence of oxygen vacancies, lattice defects, and displacement of oxygen ions from their normal lattice positions in both the series of samples. The HREM results reveal, in the case of CZ/S samples, a well-dispersed nanosized Ce-Zr-oxides over the surface of amorphous SiO(2). The structural features of these crystals as determined by digital diffraction analysis of experimental images reveal that the Ce-Zr-oxides are mainly in the cubic geometry and exhibit high thermal stability. Oxygen storage capacity measurements by a thermogravimetric method reveal a substantial enhancement in the oxygen vacancy concentration of CZ/S sample over the unsupported CZ sample.

Ceria-Zirconia Nanostructured Materials for Catalytic Applications: Textural Characteristics and Redox Properties

Highly dispersed solid solutions of ceria-zirconia with nominal composition Ce0.6Zr0.4O2 were prepared by sol-gel technique and the influence of the gel aging time (1 day–3 weeks) on the morphological and textural properties of the materials was investigated. The samples were calcined at 923 K for 8 h, then characterized by XRD, BET surface area and pore size measurements by gas adsorption and by mercury porosimetry. DTA, TEM and SEM analyses were also carried out. The aging of the gel seems to have a remarkable influence on the morphology of the materials, whose surface area and porosity increase by increasing aging time. The phase composition of the solid solutions of ceria-zirconia, as determined by XRD analysis, seems to be independent from the synthetic procedure; in any case a cubic phase (space group Fm-3m) was identified as the main component. XRD and TEM characterizations show the presence of highly dispersed crystallites, with diameters in the range 60–80 A. The redox properties and hydrogen chemisorption capacity have been studied by TPR/TPD/TPO experiments. Oxygen storage capacity measurements were carried out by a pulse technique, to determine the overall reducibility of the solid, OSCC (Oxygen storage capacity complete). OBC (Oxygen buffering capacity) measurement were also performed to determine the ability of the ceria-zirconia oxides to attenuate fast oscillations (0.1 Hz) of oxygen partial pressure. The correlation between textural, structural and redox properties of the oxides is discussed.

Oxygen storage capacity measurements of three-way catalysts under transient conditions

Oxygen storage capacity (OSC) measurements at 450°C were carried out on 10 different alumina and ceria-alumina supported Pd, Pt, Rh, PdRh and PtRh catalysts. Experiments were performed under both transient (0.5 and 1 Hz) and stationary reaction conditions using a mass spectrometer for the gas phase analysis every 100 ms. OSC was shown to be strongly dependent on the metal nature — oxidized or reduced. Rh appeared to be the best OSC promoter. Furthermore, ceria addition to the alumina support strongly enhances the OSC. In fact looking at the OSC values, a factor of 3 was evidenced between Pt/Al 2O 3 and Rh/Al 2O 3 and an additional factor of about 12 between Rh/Al 2O 3 and Rh/CeO 2-Al 2O 3. The metal oxides relative stability as well as the metal electronic configuration were shown to be strongly influent on the overall reactivity of the catalyst. Additional kinetic studies showed that the oxide reduction rate under CO is slower than the oxide oxidation rate under O 2. Finally a general scheme for the oxygen storage process is proposed and discussed.

Oxygen storage and mobility on model three-way catalysts

Oxygen storage capacity (OSC) of TW catalysts has a strong impact on their performances under oscillatory conditions. This paper reviews the different methods developed for OSC measurements, the effects of catalyst composition on OSC as well as the impact of OSC on TW catalysis, concentrating on the results obtained in our laboratory. Use of OSC measurements for catalysts control (on-board diagnostic) is also examined. The role of superoxide species as intermediates in the oxygen storage process will be emphasized. Finally, we shall review the recent developments on CeZr-based oxides.

Infrared Study of Oxygen Adsorption and Activation on Cerium–Zirconium Mixed Oxides

Oxygen adsorption, activation, and exchange over a wide range of CexZr(1−x)O2 (x=0, 0.15, 0.5, 0.63, and 1) mixed oxides were monitored by FT-IR spectroscopy. Oxygen adsorption at room temperature on preoxidized samples results in the formation of surface superoxides (O2−) that are well characterized by a sharp band at 1126 cm−1. The variation along the oxide composition of the relative amount of superoxide species present at the surface, deduced from the integrated area of this νO–O vibration band, appeared to correlate with the oxygen storage capacity (OSC) of the oxide measured at 400°C. Furthermore, 16O2− superoxide species adsorbed on ceria–zirconia samples were shown to be very reactive and selectively exchangeable to 18O2− species via the so-called multiple exchange mechanism. The key role of superoxides as “initiators” and/or “intermediates” in the activation of oxygen prior to further diffusion on ceria-containing solids is proposed to explain the similarities observed between previous OSC measurements, 16O/18O isotopic exchange experiments, and the present FT-IR study results.

Interactions of CO with Pt/ceria catalysts

The interaction between CO and Pt/ceria catalysts was investigated by oxygen storage capacity measurements, CO and CO 2 isotope exchange and FT-IR measurements. A Pt/alumina sample was also investigated for comparison. The capacity to store and release oxygen as a function of ceria surface-area and Pt–ceria contact was estimated. The presence of chlorine from the Pt pre-cursor was found not to decrease the oxygen storage capacity. However, FT-IR measurements showed that chlorine hindered the carbonate formation during CO exposure, which was found to be the main form of carbon storage on the pre-oxidized sample. On a strongly reduced sample, CO disproportionation was found to occur to an increasing extent with increasing degree of reduction. CO and CO 2 isotope exchange showed that these molecules exchange their oxygen with Pt/ceria and ceria quite easily at 400°C. CO chemisorption at −78°C as a method to determine the metal dispersion was also investigated. At this temperature, the CO uptake on ceria was strongly suppressed, especially on the Cl-free sample, but not completely hindered.