CeO2 Al2O3 Research Articles

Transparent two-phase composite oxide thin films with high conductivity

New types of transparent thin films based on CeO2–Al2O3 and SiO2 and ion-doped CeO2 composite oxides have been developed by sol-gel process. The films have two phases: a host phase; CeO2 or ion-doped CeO2; and a guest phase Al2O3 or SiO2, so called binary composites. This is the unique character to distinguish from other transparent films, e.g. CeO2–SnO2, TiO2 and ZrO2 single phase (solid solution), for electrochromic applications. Film conductivity was determined by impedance analysis and electrochemical property was investigated by cyclic voltammetry. The conductivity of the composite films is in the range of 10−2–0.2 S/cm in the temperature range of 500–700°C. Binary films containing 30% alumina and 50% silica with thickness 35 and 30 nm have the highest amount of charge exchange for 1.0 Li/Ce and 0.7 Li/Ce, respectively. After 380 cycles the charge capacity of the CeO2Al2O3 reached about the same value as started in the beginning. The films show a high transmittance for visible light. Unusual properties for these composite thin films have been implemented due to the special thin film nano-structure and two-phase regions and interfaces. The ionic transport and insertion/extraction mechanisms in these new composite film materials are discussed based on a proposed physical model. These composite films have a broad application in various electrochemical devices, such as electrochromic and fuel cell devices.

Sol–Gel Pd Exhaust Catalysts and N2O Production

Al2O3, CeO2–Al2O3, CeO2–Tb4O7–Al2O3 and ZrO2–Al2O3 supported Pd samples have been prepared by sol–gel methods. Extents and mechanisms of N2O production in CO–NO and CO–NO–O2 reactions on these have been considered. This occurs most selectively under oxidising (lean-burn) conditions or in the presence of CeO2 and CeO2–Tb4O7 promoters near the CO–NO light off temperature. Over Pd/ZrO2–Al2O3 the CO–NO reaction at 573 K has CO and NO conversions that are second order with respect to p CO and p NO. Over this catalyst NO conversion is faster than that of CO until O2(g) is added, causing CO conversion and N2O production at 573 K to rise simultaneously. CeO2 or CeO2–Tb4O7 incorporation into a Pd/Al2O3 catalyst enhances N2O production near the CO–NO light-off temperature in the absence of added O2 without CO conversion being raised. There is current attention on pollution control opportunities through lean-burn conditions, Pd catalysts and oxygen storage capacity enhancement. The present work suggests that their role in N2O production may need to be better understood and controlled. For the moment N2O formation provides a window on mechanisms of TWC operation.

Effect of synthesis conditions on the phase composition of ZrO2-CeO2-Al2O3 sol-gel powders

The morphology and phase composition of ultrafine ZrO2–CeO2–Al2O3 powders prepared by heat-treating coprecipitated and successively precipitated hydroxides were shown to depend on the precipitation procedure. The relative amounts of tetragonal and monoclinic zirconia in the sol–gel powders can be controlled by adjusting the precipitation conditions.

Oxygen Titration of Spill-Over Hydrogen in Ceria and Ceria–Alumina Supported Platinum–Rhodium Catalysts: Application to the Determination of the Ceria Surface in Contact with Metal

Successive O2/H2/O2 chemisorption measurements were performed at room temperature over Pt/CeO2 catalysts of different BET surface areas after reduction at 573 K under H2. It has been determined that about 4 μatom of oxygen per m2 of ceria is required to fill the oxygen vacancies created by the reduction in agreement with the value deduced from the TPR profiles. The same value was also obtained from the H2/O2 titration, which shows that oxygen is able to titrate the hydrogen which has spilled over the ceria surface via the metallic particles. Thus, the consistency of the three techniques demonstrates the validity of this method of measuring both the total ceria surface area and the ceria surface in contact with the precious metals. The application of this methodology to the case of Pt and Pt–Rh /CeO2–Al2O3 model three-way catalysts leads to a set of ceria surface areas deduced from O2 chemisorption and O2 titration much lower than those obtained by TPR. This behaviour has been attributed to the presence of chlorine ions originating from the precursor and to the formation of stabilised Ce3+ ions at the CeO2/Al2O3 interface during reduction. After an ageing treatment at 1273 K under N2 + 10% H2O, an even greater difference was observed between the TPR ceria surface areas and those deduced from the chemisorption method. The results indicate an increased fraction of ceria without metal due to sintering and to the segregation between ceria and the metal particles.

Characterization of CeO2–Al2O3–SiO2 glasses by infrared and x-ray absorption near edge structure spectroscopies

Structural characteristics of CeO2–Al2O3–SiO2 glasses have been studied by infrared and x-ray absorption near edge structure (XANES) spectroscopies, and the Kramers-Kronig analysis of IR reflection data was performed. Results of IR spectra show that ceria is the network modifier and favors the formation of [SiO4] tetrahedron with nonbridging oxygens. The high frequency Si–O–Si stretching vibrational peak near 1100 cm−1 shifts toward the lower frequency as ceria content increases and alumina content is constant. XANES data of the cerium LIII edge show that the majority of cerium ions in cerium aluminosilicate glasses are Ce3+ ions. The ratio of Ce4+/Ce3+ increases with increasing the ratio of Ce/(Al + Si). However, the ratio of Ce4+/Ce3+ decreases with increasing the ratio of Al/(Al + Si).

Structure and Properties of CeO2–Al2O3–SiO2 Glasses

The density, molar volume, refractive index, molar refractivity, and infrared and XANES spectra of CeO2–Al2O3 –SiO2 glasses prepared at 1600° C for 3 h in N2 have been investigated. The IR reflection data are treated by Kramers-Kronig transformation. Results of IR spectra show that ceria is the network modifier and favors the formation of the [ SiO4] tetrahedron with nonbridging oxygen. The XANES data of the cerium LIII edge show that the majority of cerium ions in cerium aluminosilicate glasses are the Ce3+ ions. The density and refractive index of the glass increase with increasing alumina or ceria content.

Surface characterization of modified aluminas. Part 4.—Surface hydration and Lewis acidity of CeO2–Al2O3systems

In catalysis, it has been suggested that the addition of CeO2 and other rare-earth metal oxides to alumina can stabilize the surface area and the spinel transition phases of the alumina support used in automotive catalysis. As this structural stabilizing effect by CeO2 was recently observed to occur only to a very limited extent, it was thought that the beneficial effect of CeO2 addition to Al2O3 supports would be useful for surface chemistry rather than structurally and morphologically. The surface characterization of several CeO2–Al2O3 systems has been carried out mainly by in situ FTIR spectroscopy and adsorption microcalorimetry. Specific OH groups bound to Ce surface cations are difficult to identify, but the usual OH profile of aluminas is modified by the presence of CeO2 to an extent that depends on CeO2 content and on activation temperature. Using CO as a surface probe molecule the surface Lewis acidity of the CeO2–Al2O3 mixed systems was characterized and it was revealed that CeO2 accumulates preferably on the flat patches of low-index crystal planes of the spinel structure, and that the presence of Ce cations stabilizes, also at high temperatures, the most acidic Lewis centres. The latter are constituted by coordinatively unsaturated Al ions with (incomplete) tetrahedral coordination and are probably located in crystallographically defective situations. Moreover, the presence of CeO2 brings about the formation of a family of Lewis-acid sites of medium strength, which are not present at the surface of bare (transition) aluminas.

Surface characterization of modified aluminas. Part 5.—Surface acidity and basicity of CeO2–Al2O3systems

The addition of CeO2 to Al2O3 supports and Al2O3-based catalysts leads to improved performance, that can be explained by surface chemical rather than structural morphological effects. Surface characterization of CeO2–Al2O3 systems has been carried out by in situ FTIR spectroscopy. The adsorption–desorption of pyridine as a surface probe allowed us to test for Bronsted acidity, never found, and the total Lewis acidity, which is higher for CeO2–Al2O3 systems than for pure Al2O3 supports. Several types of surface centres of different acidity could be identified, mainly by testing the competitive adsorptivity towards CO after stepwise desorption of pyridine at various temperatures. The adsorption of CO2 was used to test the basicity of CeO2–Al2O3 systems. The presence of CeO2, a basic oxide, does not modify appreciably the basic properties of Al2O3, on which a low concentration of rather weak basic centres is present, whereas the presence of Al2O3 tends to inhibit the high basicity typical of CeO2.

Microstructural and oxygen-handling characteristics of CeO2with M3+promoters. Part 1.—Characterization of calcined powders by XRD and oxygen-isotope exchange

Comparisons of the oxygen-handling properties of calcined CeO2, CeO2–Al2O3, RhOx–CeO2, RhOx–CeO2/Al2O3 and La2O3–CeO2 materials are made on the basis of their experimentally observed abilities to bring about R0-type homophase oxygen isotope equilibration of an equimolar (16O2+18O2) mixture at room temperature, and/or to induce low onsettemperatures (ca. 550 K) for pairwise heterophase oxygen isotope-exchange between gas-phase 18O2 and oxygen-16 species from the oxide. Those oxygen equilibration/exchange processes were enhanced over precalcined RhOx–CeO2 materials prepared from chloride-free precursors, but not over La2O3–CeO2. That difference contrasted with expectations for similar concentrations of dopant-related, anion vacancy creation in each material, based upon XRD observations of the incorporation of each dopant into a slightly expanded CeO2(fluorite) lattice. Consideration is given not only to conventional contribution to oxygen equilibration/exchange by residual RhOX species at the RhOX–CeO2 surfaces, but also to ways in which bulk-incorporated, variable-valency rhodium ions can indirectly facilitate those processes.