Ce Zr Mixed Oxides Research Articles

Influence of thermal sintering on the activity for CO–O2 and CO–O2–NO stoichiometric reactions over Pd/(Ce, Zr)Ox/Al2O3 catalysts

A Pd/(Ce, Zr)Ox/Al2O3 catalyst has been studied to determine how thermal aging under air at 1273K influences its structural characteristics and catalytic properties for CO oxidation reactions under stoichiometric conditions, employing either O2 or an NO–O2 mixture as oxidants. XRD and TEM-EDS show that significant sintering of both Pd and Ce–Zr mixed oxide active components is produced upon aging. As a consequence, lower CO oxidation or NO reduction activities were observed in catalytic tests. In situ DRIFTS studies for both reactions indicate that the lower activity of the aged systems is not only due to the sintering of active components but also due to a consequent loss of (Ce, Zr)Ox-induced promoting effects on Pd activity.

Effects of Copper on the Catalytic Properties of Bimetallic Pd–Cu/(Ce,Zr)Ox/Al2O3 and Pd–Cu/(Ce,Zr)Ox Catalysts for CO and NO Elimination

A series of Pd–Cu bimetallic catalysts supported on Ce–Zr mixed oxide or on (Ce,Zr)Ox/Al2O3 mixed supports (with loadings of 10 and 33 wt% Ce–Zr mixed oxide) have been examined and compared to reference monometallic catalysts to determine the influence of copper on the catalytic activity for CO oxidation and NO reduction using a mixture of either O2 or O2–NO as oxidant under stoichiometric conditions. The samples were characterized by X-ray diffraction, transmission electron microscopy–X-ray energy-dispersive spectrometry, and electron paramagnetic resonance, employing in situ diffuse reflectance Fourier transform spectroscopy and X-ray absorption near-edge spectroscopy techniques to characterize physicochemical processes taking place during the course of the reactions. All the catalysts show Ce–Zr mixed oxide nanoparticles with pseudocubic phases and a Ce/Zr atomic ratio close to 1. The main differences between the samples are attributed to changes in the distributions of the two metallic components over the supports. The beneficial effects of copper on the catalytic activity of alumina-containing samples are related to the formation of an active Pd–Cu alloy in contact with the Ce–Zr mixed oxide component. A preferential interaction between copper and alumina in these samples is proposed to optimize the properties of the alloy phase by decreasing the copper concentration in the latter. However, the destruction of the alloy under reaction conditions at high temperatures, required for NO activation, removes the copper-promoting effects for NO reduction. The catalytic behavior of the Pd–Cu/(Ce,Zr)Ox sample is governed by the Cu–(Ce,Zr)Ox character of its active sites, which induces both beneficial and detrimental effects, on its catalytic properties, depending on the type of reaction.

Ce–Zr mixed oxides prepared in molten nitrates

Dispersed Ce 0.75Zr 0.25O 2 and Zr 0.84Ce 0.16O 2 cerium(IV)–zirconium(IV) mixed oxides were prepared by the flux method, from the reaction of hydrated Ce(NO 3) 3 and ZrOCl 2 in molten NaNO 3 at 450–500 °C in the presence of ammonium fluoride. The temperature and the stoichiometry of reactions were determined by mass spectrometry. Powder X-ray diffraction, specific surface area measurements (BET), and scanning electron microscopy were used to study the morphology and particle size distribution of the solid products. Preparation conditions were optimized to obtain pure oxides. Reaction at 550 °C in the presence of 1% wt. of ammonium fluoride gave the best result.

Effect of Thermal Sintering on Light-Off Performance of Pd/(Ce,Zr)Ox/Al2O3 Three-Way Catalysts: Model Gas and Engine Tests

The effect of thermal degradation on the catalytic performance in light-off tests of Pd three-way catalysts supported on alumina and promoted with Ce–Zr mixed oxides has been investigated either under synthetic gas mixtures (for catalysts in powder form) or in engine tests (for catalyst monoliths). Accelerated aging of the systems by oven heating at 1273 K produces a significant sintering of both Pd and Ce–Zr mixed oxide active components, indicated by X-ray diffraction, transmission electron microscopy, and diffuse reflectance Fourier transform infrared (DRIFTS) techniques. In spite of these effects, the small differences between the light-off performances of fresh and aged systems show the relatively low relevance (particularly in real conditions under engine emissions) of the thermal degradation on the overall performance of the system. In situ DRIFTS results suggest that this is the result of the relatively low structural sensitivity of the desorption of hydrocarbon fragments adsorbed on the active sites, which act mainly as self-poisons for the catalytic reactions.

Effect of Ce–Zr mixed oxides on the chemical state of Rh in alumina supported automotive exhaust catalysts studied by XPS and XRD

Several γ-Al 2O 3 supported Rh catalysts containing Ce–Zr mixed oxides in various proportions were prepared by different manufacturing methods. Both fresh and aged catalysts were investigated, as well as Rh/Ce–Zr mixed oxide powders. Aging was performed in air at 1000°C for 3 h. Disposition of the Rh-oxide formed in the aging towards reduction was investigated by exposing the catalysts to 400 mbar of H 2 at 300°C for 30 min. Rhodium supported on pure alumina was found to sinter easily and react irreversibly with the alumina support at elevated temperatures to form an irreducible oxide phase. On the contrary, rhodium supported on Ce–Zr mixed oxide was observed to remain in a reducible form also in high temperature aging. It was also found that Ce–Zr mixed oxide is capable of stabilizing Rh particles against sintering at elevated temperatures. However, incorporation of Ce–Zr mixed oxide to γ-Al 2O 3 supported catalysts was found to be ineffective in preventing the reaction between rhodium and alumina. After 3 h aging in air at 1000°C an irreducible rhodium oxide, which is likely RhO 2 in intimate contact with alumina surface, was formed regardless of the type of Ce–Zr mixed oxide additive and manufacturing method of the catalyst.

The preparation of high-surface area, thermally-stable, metal-oxide catalysts and supports by a cellulose templating approach

A simple and convenient templating approach for the preparation of metal-oxide catalysts and supports has been studied. The method, based on the absorption of an aqueous solution of metal salts by cellulose material, followed by drying and combustion of the organic matrix, leads to the formation of high-surface area mesoporous materials with unusually high thermal stability. Examples include Ce–Zr mixed oxides (BET surface areas of 90–130 m 2/g after calcination at 800°C for 2 h; 21–30 m 2/g after 12 h at 1050°C) and La-stabilized alumina (BET surface areas of 275–320 m 2/g after calcination at 800°C for 2 h, 88–141 m 2/g after 12 h at 1050°C). The pore-size distribution, morphology, and the effect of preparation parameters on surface area are discussed.

Structural Characteristics and Redox Behavior of CeO2–ZrO2/Al2O3 Supports

Two alumina-supported Ce–Zr mixed oxide samples (with 10 and 33 wt% loads) are prepared by a microemulsion method, and their structural and redox properties are investigated using Electron Paramagnetic Resonance (EPR) and Raman spectroscopies, X-ray diffraction, temperature-programmed reduction, and transmission electron microscopy. The results show that the preparation method produces highly dispersed Ce–Zr materials which crystallize with an average particle size around 2 nm, the latter depending only weakly on the loading in the range analyzed. The Ce–Zr mixed oxide particles formed have Ce : Zr atomic ratios close to 1 with a slight enrichment in Ce for the 33 wt% CeO2–ZrO2/Al2O3 sample, and present a t″ phase type tetragonal symmetry. In both samples, EPR results show that a certain amount of the mixed oxide is present on the alumina surface as 2D (two-dimensional) patches which have redox sites with properties different from those existing in 3D, bulk-like mixed oxide particles; the characteristics of these latter also differ from those observable in unsupported Ce–Zr mixed oxide (possibly due to an epitaxial relationship with the alumina surface), as evidenced by the absence of formation of associated oxygen vacancies on the samples subjected to outgassing at 773 K. Redox cycling of these materials shows a decrease in reduction onset temperature, similar to the behavior known for unsupported Zr–Ce oxides.

Optical and electrochemical properties of cerium–zirconium mixed oxide thin films deposited by sol–gel and r.f. sputtering

Films of Ce–Zr mixed oxide were produced by sol–gel and r.f. sputtering. These films can be used as `passive' counter-electrodes in electrochromic smart windows because they retain their full transparency in both the oxidised and reduced state. Li intercalation was accomplished electrochemically using a liquid electrolyte. Electrochemical behaviour of the samples was found to be dependent on the heat treatment (sol–gel deposited film) and crystallite orientation (sputter deposited films). XRD analysis on sputter deposited films showed that the films are crystalline and grow following the orientation of the underlying tin doped indium oxide (ITO) film. Films of Ce–Zr mixed oxide lacking in (111) crystallite orientation show continuous evolution of the voltammograms and reach a maximum value for the cycled charge only after a large number of cycles. The lithium diffusion coefficient, calculated from GITT measurements, is in the range 10 −12–10 −14 cm 2 s −1 for sputter deposited films and becomes as low as 10 −15 cm 2 s −1 for sol–gel deposited films. Optical constants of the thin films were calculated from reflectance and transmittance spectra. Refractive index values are in the range of 2.15–2.30 at λ=633 mn depending on the deposition method. A sharp absorption edge at about 320 nm is seen in accordance with CeO 2 optical properties.

Textural and phase stability of Ce xZr 1− xO 2 mixed oxides under high temperature oxidising conditions

The textural and structural changes in several Ce–Zr mixed oxides were studied. Changes in the crystal size and specific surface area upon calcination were followed at 1223 K over time. Different gas compositions (O 2, CO 2 and H 2O) were employed during calcination in order to better understand the sintering process in the Ce x Zr 1− x O 2 system. From S BET and mean crystal size ( D) curves, we have tried to obtain the kinetic parameters. In order to do that, all curves were fitted using a mathematical expression which allows the derivative and then the kinetic rates for S BET decrease and D increase with calcination time to be obtained. It was also possible to correlate these rates with the Ce content of the mixed oxide, and to obtain information about the influence of composition on the sintering process at the surface and the crystalline structure of the mixed oxides. Complementary HREM/TEM data are also presented. With the help of this technique, it has been possible to elucidate the morphological and crystallographic changes in the crystallites upon calcination of the samples under oxidising atmospheres.

Optically passive cerium containing counter-electrodes for electrochromic devices

Electrochemical and optical behavior of sputter deposited CeO2, Ce-Zr mixed oxide and Ce-V mixed oxide thin films are reported. The films were deposited starting from a target prepared by mixing and weakly pressing the oxide powders in the desired molar ratio. Li intercalation was accomplished from a liquid electrolyte. CeO2 and Ce-Zr mixed oxide thin films are transparent in the visible range and behave as passive material upon oxidation/ reduction. During the cyclic voltammetries performed on the thin films a charge of few mC was reversibly cycled. The diffusion coefficient was evaluated from galvanostatic intermittent titration technique (GITT) experiments.