Zirconia Mixed Oxides Research Articles

Synthesis, characterization and activity studies of vanadia catalysts supported on sol–gel derived Al 2O 3–ZrO 2 mixed oxide

A series of catalysts with vanadia contents varying between 4 and 18 wt.% were prepared using sol–gel derived alumina–zirconia mixed oxide as a support. The catalysts were characterized employing 51 V , 1 H solid-state MAS NMR, diffuse reflectance FT-IR and BET surface area measurements. The activities of the catalysts were tested for the partial oxidation of ethanol. 51 V NMR studies indicated the presence of tetrahedral vanadate species at all the loadings studied except in the catalyst with a V 2O 5 loading of 18 wt.%, in which six coordinated vanadia species was observed. Upon outgassing treatment of the calcined catalysts, a new tetrahedral species was observed, indicating the presence of water molecules or hydroxyl groups in the coordination sphere of vanadium. The 1 H MAS NMR studies showed the peaks corresponding to hydroxyl groups of alumina and zirconia. With increase in vanadia loading there was a decrease in the intensity of the basic hydroxyl groups of alumina showing the preferential reaction of vanadia with these groups. The DRIFT studies of the catalysts indicated the vibrations corresponding to agglomerates of V 2O 5 only in the sample with highest V 2O 5 content. Ethanol partial oxidation studies of the catalysts showed acetaldehyde as the major product with traces of ether, acetic acid, ethyl acetate, CO x , etc.

Oxygen Mobility in CeO2 and CexZr(1-x)O2 Compounds: Study by CO Transient Oxidation and 18O/16O Isotopic Exchange

Cerium−zirconium mixed oxides (CexZr(1-x)O2), precalcined at 900 °C in dry air, were supplied by Rhodia Terres Rares as monophasic solid solutions. Introduction of some zirconium atoms in the ceria lattice by isomorphous substitution clearly influences the final properties of these materials as long as the cubic structure of ceria is maintained. Modifications in oxygen storage capacity (OSC measurements), redox properties (CO TPR), and oxygen exchange processes (TPIE) were studied. Ce0.63Zr0.37O2 was shown to have the most promising properties with the largest OSC at 400 °C and the highest reactivity in O2 exchange. All mixed oxides are able to exchange very large amounts of oxygen compared to ceria, implying the participation of bulk oxygen. Furthermore, on CexZr(1-x)O2 samples, oxygen is predominantly exchanged via a multiple heteroexchange mechanism involving surface dioxygen species as superoxides or peroxides.

Effects of surface modification by chemical filing on the redox behavior of a ceria–zirconia mixed oxide

Abstract A ceria–zirconia mixed oxide was prepared by means of the thermal decomposition of cerium zirconyl oxalate in an argon flow and subsequent removal of the top surface by the formation of volatile complexes of AlCl 3 –CeCl 3 and AlCl 3 –ZrCl 4 , “chemical filing”. The obtained ceria-zirconia sample showed redox behavior at low temperatures even after redox cycles at 1273 K, while the conventional ceria–zirconia samples prepared via hydrogen or carbon reduction released a smaller amount of oxygen at higher temperatures after high temperature redox cycles. High redox activity at low temperatures and high thermal resistance are the requirements for the next generation of ceria-based materials, especially when applied in automotive exhaust cleaning catalysts.

Synthesis, characterization and catalytic activity in the reduction of NO by CO on alumina–zirconia sol–gel derived mixed oxides

The synthesis and characterization of Al 2O 3–ZrO 2 mixed oxides of 5, 10 and 20 wt% zirconia contents was carried out. Aluminum tri-sec-butoxide and zirconium n-butoxide were used as chemical precursors. Gelation of the alkoxides was obtained in basic (pH 9) medium using NH 4OH as hydrolysis catalyst. Zirconia–alumina mixed oxides were obtained as the final materials. The oxides showed higher specific surface area (283–354 m 2/g) and large pore size diameters (195–222 Å). FT-IR absorption bands assigned to Zr–O–Al bonds were observed in the mixed oxides. Acidity and basicity were determined by ammonia and CO 2 TPD thermodesorption. Low crystallinity was observed by XRD spectra, however, incipient monoclinic and tetragonal zirconia phases were identified. The solids were found active for the reduction of NO by CO. The formation of alumina–zirconia mixed oxides in which an important dilution of the alumina matrix by zirconia was inferred.

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.

Surface and bulk properties of silica–zirconia mixed-oxides: acid vs base catalysed condensation

The influence of acid and alkali catalysed condensation on the nature of silica–zirconia mixed oxides derived from sol–gel preparations has been compared. The bulk and surface properties have been characterised and where appropriate compared with results for a sample of Aerosil 200 where an equivalent zirconia loading was deposited by precipitation. Results indicate that the sol–gel preparations generate solids with similar surface areas, but only alkali catalysed condensation leads to solids with large mesopore volumes. Both solids give similar acid site densities although solids prepared by alkali catalysed condensation contained a higher proportion of strong Brønsted acid sites. An increased acid site density was not created by increasing the degree of mixing of the two components.

Ceria films on zirconia substrates: models for understanding oxygen-storage properties

The oxygen-storage properties of ceria in three-way automotive catalysts are promoted and stabilized by mixing with zirconia. In the present study, this promotion was investigated using model catalysts in which ceria films were vapor deposited onto α-Al 2O 3, polycrystalline ZrO 2, polycrystalline Y 2O 3-stabilized ZrO 2 (YSZ), and YSZ(1 0 0), (1 1 1), and (1 1 0) single crystals. Following deposition of Pd, both TPD of CO and steady-state CO-oxidation kinetics suggest that the ceria films on the zirconia-based substrates were much more easily reduced than films on α-Al 2O 3. Polycrystalline zirconia and YSZ and the YSZ single crystals were equally effective in promoting ceria reducibility. Structural studies of ceria on YSZ(1 0 0), using both TEM and EDSXD (energy-dispersive, surface X-ray diffraction), demonstrate that ceria forms ordered overlayers on YSZ(1 0 0), oriented with respect to the YSZ surface. The lattice parameter for ceria is decreased by only 0.6% compared to bulk CeO 2, but the coherence length suggests that the overlayer may have a high defect density. It is suggested that the structure-directing properties of zirconia are responsible for the enhanced properties of ceria–zirconia mixed oxides.

Structural and Optical Properties of Silver-Doped Zirconia and Mixed Zirconia−Silica Matrices Obtained by Sol−Gel Processing

The annealing behavior of zirconia and mixed zirconia−silica matrices (with different ZrO2/SiO2 molar ratios) containing silver nanoclusters, synthesized by the sol−gel technique has been investigated. The film samples of the general formula (SiO2)x(ZrO2)(0.9-x)Ag0.1 (x = 0, 0.225, 0.45, 0.675, and 0.9) were treated in different atmospheres (air, N2, and 5%H2−95%N2) at temperatures ranging from 200 to 900 °C. The ZrO2 matrix undergoes a structural phase transition from amorphous to cubic-tetragonal polycrystalline between 400 and 500 °C and on further raising the temperature near 600−700 °C the monoclinic phase also develops. On the contrary, the ZrO2:SiO2 system remains amorphous up to ∼700 °C. Ag nanoclusters are formed in all the systems. It has been observed that the change in the ZrO2/SiO2 molar ratio is an effective way to control not only the overall refractive index of the matrix but also the Ag cluster size. The shifting of the surface plasma resonance position of Ag nanoclusters is observed as a function of the refractive indices of the matrix following the Mie theory. Samples where characterized by transmission electron microscopy, Rutherford backscattering spectrometry, and optical spectrometry to correlate the cluster structure to their optical properties.

XPS Study of Pt/CexZr1−xO2/Si Composite Systems

ABSTRACTThis work describes the study of the surface reduction of ceria zirconia mixed oxides (CeZrO) as either thin films or powders, both with and without Pt present. XPS was used to measure the composition of the surface and the oxidation states of all metals contained within the material. The thin films of CeZrO showed little reactivity towards the reducing conditions used. Grazing incidence angle XRD showed the presence of Ce0.75Zr0.25O2. The thin films prepared with Pt showed that surface reduction of Ce4+ occurred under reducing conditions. The size of the Pt clusters was also determined from the data. The Pt was found to always exist in the metallic state. The Zr4+ was not seen to change during all treatments. For the powder samples the Ce4+ was readily reduced to approximately 60%. Pt was found to be initially oxidised with the % of metallic Pt increasing with reduction temperature. Again no change in the Zr was observed.

Surface Behavior of Supported 12-Heteropoly Acid As Revealed by Nuclear Magnetic Resonance, X-ray Photoelectron Spectroscopy, and Fourier Transform Infrared Techniques

Bulk, silica-, and zirconia−silica-supported 12-molybdophosphoric acid (HPMo) catalysts have been characterized by 31P and 1H-magic angle spinning NMR spectroscopy, X-ray photoelectron spectroscopy and Fourier transform infrared (FTIR) spectroscopy of pyridine adsorption. Two species, bound to the support surface and acid-free components, were found in both types of supported catalysts. The relative amount of these species varied depending on the type of the support and on the degree of hydration of the samples. Thermally stable molybdophosphate species up to higher temperature (723 K) were formed on silica support compared to bulk. The mixed zirconia−silica support causes the destruction of HPMo at significantly lower temperature, due to a stronger interaction of the zirconia with supported molybdophosphates. The FTIR of pyridine adsorption showed two types of acidity on the supported samples: Lewis and Brönsted. A correlation between the change in the intensities of the Brönsted acidity band and 1H NMR signal of acidic protons with temperature was found for supported HPMo: high intenstity of 1H NMR signal for HPMo on the mixed zirconia−silica support corresponds to high Brönsted acidity band caused from the residual protons involved in bridging OH groups.

The preparation of high surface area CeO 2–ZrO 2 mixed oxides by a surfactant-assisted approach

The study investigates the preparation of high surface area, three-way catalysts (TWC)-like, ceria–zirconia mixed oxide. It is shown that under basic conditions cationic surfactants effectively incorporate into hydrous oxides of cerium and zirconium. The presence of cerium inhibits the action of alkyl-trimethyl-ammonium salts as true templating agents and there is no formation of a regular pore structure. The elimination of surfactants upon calcination gives rise to the formation of high surface area, fluorite-structured CeO 2–ZrO 2 solid solution characterized by a fairly good compositional homogeneity. Surface areas in excess of 230 m 2/g were obtained after calcination at 723 K, which drop to ca. 40 m 2/g following treatment at 1173 K.

Role of cerium–zirconium mixed oxides as catalysts for car pollution: A short review

This article focuses on the role of mixed oxides in the Ce–Zr–O system, which is important as an excellent promoter for oxygen storage capacity in automotive three-way catalysts. In a model of Pt–alumina catalysts, there is a clear effect of CeO2–ZrO2 solid solution on NO and CO removal activities under dynamic air/fuel conditions. It is suggested that the design and development of automotive catalyst can be realized through research on complex oxides in Ce–Zr–O and related systems.

The electronic structure of oxide-supported tungsten oxide catalysts as studied by UV spectroscopy

The UV spectra of tungsten oxide catalysts supported on alumina, titania and zirconia, and on titania–alumina and titania–zirconia mixed oxides are reported and discussed. Evidence is provided for the different electronic structure of supports and catalysts, which could affect the behavior of the tungsten oxide centres in the different cases. On alumina, tungsten oxide centres are “isolated” by the insulating support, while on titania-based materials they are likely in electronic contact with each other and with Ti centres through the support conduction band. In the case of WOx2013;ZrO2, the 5d levels of tungsten ions fall just below of the lower energy limit of the support conduction band.

Microstructure of a Pd/ceria–zirconia catalyst after high-temperature aging

A model automotive exhaust catalyst, consisting of 2.25 wt% Pd supported on initially high-surface-area (88 m2/g) cerium–zirconium mixed oxide (Ce0.5Zr0.5O2), was studied by X-ray diffraction (XRD), thermogravimetric analysis (TGA), and transmission electron microscopy (TEM), following high-temperature (1150°C) redox aging. Spherical Pd particles with size varying from 30 to 80 nm in diameter were found to be embedded both within single grains and at interfaces between multiple grains of the mixed oxide. X-ray energy-dispersive spectroscopy (EDS) and microbeam electron diffraction show that these particles are pure Pd, with face-centeredcubic structure. These results, together with elemental distribution maps obtained by EDS imaging, provide detailed visualization of an encapsulation phenomenon previously reported on the basis of XRD and TGA measurements alone.

Inorganic sol gel synthesis of zirconium titanate fibres

This note describes the preparation of zirconium titanate gel fibres from an aqueous mixed zirconia titania sol. The fibres are well formed and free of shot. Heat treatment to 750 °C produces a crystalline porous zirconium titanate fibre which shows a good (1%) strain to break. Further firing to 1100 °C causes a deterioration in properties, with retention of some porosity and a reduced strain to break. Recommendations for improving the material are suggested.

Synthesis, structure, and properties of poly(dimethylsiloxane) networks reinforced by in situ‐precipitated silica–titania, silica–zirconia, and silica–alumina mixed oxides

AbstractSeveral novel fillers, specifically silica–titania, silica–zirconia, and silica–alumina mixed oxides, have been successfully precipitated into poly(dimethylsiloxane) (PDMS) networks using a sol‐gel approach. The resulting filled networks were found to have very good mechanical properties. In comparison with networks filled only with silica, these materials had good extensibilities as well as high strengths. Filler particle diameters were generally several hundred angstroms, but were found to decrease with increase in crosslink density of the networks. The distributions of particle size were relatively narrow, and there was very little particle aggregation. The presence of in situ silica–titania and silica–zirconia mixed oxides can also increase the onset temperature for degradation of the PDMS. © 1995 John Wiley & Sons, Inc.

Metal oxide analogue of metal alloy catalysts

Because of ion size, valence and oxide structure, hafnium and zirconium mixed oxide catalysts are ideal for study as a metal alloy analogue. Furthermore, hafnium oxide and zirconium oxide exhibit quite different selectivity for the conversion of 2-alcohols to alkene products. The alkene selectivity changed abruptly from that of pure HfO 2 to that of pure ZrO 2 at a composition with ca. 90 mol% Zr. Characterization of the catalysts by ESCA, Auger, and ISS spectroscopy show that the surface composition is very similar to the bulk composition and, thus, changes uniformly throughout the composition range rather than abruptly as does the alkene selectivity. Pure ZrO 2 impregnated with Hf to produce catalysts with the full range of Hf-Zr surface compositions showed a catalytic selectivity that paralleled the selectivity of the bulk composition catalysts. Zr or Th impregnated onto alumina serve as catalyst poisons; however, these two metals alter the selectivity in a quite different manner. The data indicate that the catalytic selectivity of these mixed oxide catalysts is determined by ligand, rather than bulk electronic, effects.

Metal oxide analogue of metal alloy catalysts

Because of ion size, valence and oxide structure, hafnium and zirconium mixed oxide catalysts are ideal for study as a metal alloy analogue. Furthermore, hafnium oxide and zirconium oxide exhibit quite different selectivity for the conversion of 2-alcohols to alkene products. The alkene selectivity changed abruptly from that of pure HfO2 to that of pure ZrO2 at a composition with ca. 90 mol% Zr. Characterization of the catalysts by ESCA, Auger, and ISS spectroscopy show that the surface composition is very similar to the bulk composition and, thus, changes uniformly throughout the composition range rather than abruptly as does the alkene selectivity. Pure ZrO2 impregnated with Hf to produce catalysts with the full range of Hf-Zr surface compositions showed a catalytic selectivity that paralleled the selectivity of the bulk composition catalysts. Zr or Th impregnated onto alumina serve as catalyst poisons; however, these two metals alter the selectivity in a quite different manner. The data indicate that the catalytic selectivity of these mixed oxide catalysts is determined by ligand, rather than bulk electronic, effects.

PROPENE OLIGOMERIZATION OVER TiO2–ZrO2 CATALYSTS

Abstract Titania–zirconia mixed oxides may be prepared in high specific area form, and with or without nickel are active as catalysts for the oligomerization of propene. The presence of nickel favors the formation of oligomers containing more than ten carbon atoms per molecule.

Pulmonary Vasculitis

We propose in this manuscript a new peroxo-mediated procedure for preparing magnesia–zirconia mixed oxides, with Mg/Zr molar ratio between 1 and 3, with enhanced distribution of basic sites. The mixed magnesia–zirconia oxides have been prepared from the gelled complex by Pechini-type method. The MgO–ZrO2 materials have been characterized and used as catalysts for acetone aldol condensation. The proposed preparation method provides a high degree of molecular homogeneity and favours the formation of magnesia-stabilized zirconia phase. Acetone gas-phase self-condensation was carried out over these catalysts as model reaction requiring the presence of basic sites. The condensation yields diacetone alcohol and mesityl oxide as mean C6 products, and phorones, isophorones and mesitylene as C9 products. In comparison to Mg–Zr oxide prepared by co-precipitation, these new materials present better conversions and higher selectivity to linear dimers and trimers (as mesitylene), whereas the selectivity for isophorones is significantly lower.