Zirconia Mixed Oxides Research Articles

Gold versus platinum on ceria–zirconia mixed oxides in oxidation of ethanol and toluene

The Ce–Zr mixed oxides have been examined as supports for platinum and gold in oxidation of ethanol and toluene. A weight ratio of Ce/Zr in the synthesized supports was 1/1. Two supported noble metals with a different loading (2.5 and 0.3 wt.%) were compared to each other as well as to selected industrial catalysts (HHC-5557 and VOC-1544). Whatever the metal content was, Pt catalysts were always more active in oxidation of ethanol and toluene than Au catalysts. Better reductive capability of platinum was confirmed by H 2-TPR profiles and a higher activity of platinum may also be linked to changes in surface basic properties (a higher amount of basic centers) illustrated by CO 2-TPD profiles. Deposition of gold on Ce–Zr mixed oxide resulted in a formation of certain amounts of crystalline Au phase detected by X-ray diffraction while the deposition of platinum did not produce any crystalline Pt phase. As observed by FE-SEM, part of Au was well dispersed but significant amount of Au was present as inactive crystals. Platinum catalysts were comparable or even more active than industrial catalysts (50% conversion of ethanol was achieved with 2.5Pt/CeZr(1) at 99 °C). On the other hand, gold catalysts proved the best selectivity in oxidation of ethanol.

Effect of barium addition on CO oxidation activity of palladium catalysts

CO oxidation over Pd supported on barium fixed alumina (Pd/Ba/Al) and ceria–zirconia mixed oxide (Pd/Ba/CZ) catalysts was investigated by XPS, TPR and in situ DRIFT during light-off using a stoichiometric CO–O 2 mixture. The activity of Pd supported on alumina catalyst was improved by increasing barium loading, but the negative effect was observed for Pd on ceria–zirconia mixed oxide. The role of barium in improving the activity of CO oxidation is considered to weaken the adsorption strength of CO on palladium supported on alumina. On the other hand, barium suppresses the interaction of Pd particles with ceria–zirconia mixed oxide support.

Effect of NCO/OH ratio and Ce‐Zr nanoparticles on the thermo‐mechanical properties of hyperbranched polyurethane urea coatings

AbstractThe present study describes the effect of NCO/OH ratio and addition of Cerium (Ce)‐Zirconium (Zr) mixed oxide nanoparticles on the properties of Hyperbranched Polyurethane Urea (HBPUU) Coatings. Initially a hydroxyl terminated hyperbranched polymer (HTBP) was synthesized through A3 + CB2 approach. The HTBP and Ce‐Zr nanopowder dispersed HTBP, both were reacted with hexamethylene diisocyanate (HDI) separately; at various NCO/OH eq. ratios to get different NCO terminated HBPU and HBPU/Ce‐Zr hybrid prepolymers. These prepolymers were used for the preparation of HBPUU and HBPUU/Ce‐Zr hybrid coating films through moisture curing. The techniques such as 1H NMR, 13C NMR, FT‐IR, and XRD have been used for structural information while Dynamic mechanical and thermal analyzer (DMTA), Thermogravimetric analysis (TGA) and Universal testing machine (UTM) have been used for evaluation of thermo‐mechanical properties. The combined spectroscopic investigations results indicate the formation of HBPUU network with a degree of branching of 76% while FT‐IR deconvolution results indicates the formation of more hydrogen bonded structure with increasing NCO/OH ratio. The XRD and FT‐IR studies confirm the presence of Ce‐Zr mixed nanoparticles in the HBPUU hybrids. As per TGA and DMTA analysis the thermal stability, char residue, storage modulus (E', material stiffness) and glass transition temperature (Tg), increases with increasing NCO/OH ratio and Ce‐Zr nanoparticle loading in HBPUU coatings. In general, UTM data suggest that the tensile strength increases and per cent elongation at break decreases with increasing the NCO/OH ratio and addition level of nanoparticles in HBPUU coatings. Copyright © 2009 John Wiley & Sons, Ltd.

Derivation of the optical constants of spin coated CeO 2–TiO 2–ZrO 2 thin films prepared by sol–gel route

Ternary thin films of cerium titanium zirconium mixed oxide were prepared by the sol–gel process and deposited by a spin coating technique at different spin speeds (1000–4000 rpm). Ceric ammonium nitrate, Ce(NO 3) 6(NH 4) 2, titanium butoxide, Ti[O(CH 2) 3CH 3] 4, and zirconium propoxide, Zr(OCH 2CH 2CH 3) 4, were used as starting materials. Differential calorimetric analysis (DSC) and thermogravimetric analysis (TGA) were carried out on the CeO 2–TiO 2–ZrO 2 gel to study the decomposition and phase transition of the gel. For molecular, structural, elemental, and morphological characterization of the films, Fourier Transform Infrared (FTIR) spectral analysis, X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), cross-sectional scanning electron microscopy (SEM), and atomic force microscopy (AFM) were carried out. All the ternary oxide thin films were amorphous. The optical constants (refractive index, extinction coefficient, band gap) and thickness of the films were determined in the 350–1000 nm wavelength range by using an nkd spectrophotometer. The refractive index, extinction coefficient, and thickness of the films were changed by varying the spin speed. The oscillator and dispersion energies were obtained using the Wemple–DiDomenico dispersion relationship. The optical band gap is independent of the spin speed and has a value of about E g ≈2.82±0.04 eV for indirect transition.

Organic modification of cerium–zirconium mixed oxide mesoporous materials

Cerium–zirconium mixed oxide mesoporous materials were synthesised by using block polymer Pluronic P123 as soft template under the hydrothermal conditions. To enhance the interfacial interaction of cerium–zirconium mixed oxide mesoporous materials in filled polymer composites, an organic grafting method was applied to modify mesoporous materials by treating with oleic acid or silane coupling agent and grafting polymethylmethacrylate (PMMA) onto the mesoporous particles. The structural and surface properties of CeO2–ZrO2 mesoporous particles before and after organic modification were characterised by different methods. The results showed that the desired polymer chains were covalently bonded to the surface of CeO2–ZrO2 mesoporous particles to increase lipophilic degree and reduce the aggregation of these particles. It was also found that the heat decomposition temperature of PMMA grafted CeO2–ZrO2 was higher than that of pure PMMA, which demonstrated that the CeO2–ZrO2 mesoporous materials were potential superior performance filler into thermoplastic polymers to improve polymer properties.

Preparation of a mesoporous ceria–zirconia supported Ni–Fe catalyst for the high temperature water–gas shift reaction

A Ni–Fe/ceria–zirconia catalyst with ordered mesostructure was prepared by the hard-template method employing mesoporous silica (KIT-6) as a template to impart its highly ordered structure to the ceria–zirconia mixed oxide support. Catalytic activities of the Ni–Fe/CeO2–ZrO2 catalyst for the water–gas shift reaction were superior to those of a commercial Fe–Cr-based catalyst. The ordered structure of Ni–Fe/CeO2–ZrO2 catalyst became more stable compared to one prepared without zirconia due to structural stabilization of the mixed oxide by added zirconia in the framework. Alloying of Ni and Fe and enhanced mobility of lattice oxygen in the oxide support may promote its catalytic activity and selectivity for the water–gas shift reaction.

Influence of the Catalyst Preparation Method, Surfactant Amount, and Steam on CO2Reforming of CH4over 5Ni/Ce0.6Zr0.4O2Catalysts

A series of ceria−zirconia mixed oxide supports with nominal composition “Ce0.6Zr0.4O2” were synthesized by two different routes, namely, a surfactant-assisted route and a coprecipitation route. Am...

Chemical Imaging at Atomic Resolution as a Technique To Refine the Local Structure of Nanocrystals

The challenging problem of mapping the chemical composition of cation columns in individual nanocrystals at atomic resolution is addressed by using a method based on aberration-corrected electron microscopy, core-loss electron energyloss spectroscopy, and simulations. The potential of this novel approach to provide unique structural information, which is the key to rationalizing macroscopic behavior, is illustrated with the analysis of ceria–zirconia mixed oxides, which are nanomaterials with substantial technological impact. Metal nanoparticles supported on this family of oxides are currently materials of interest as catalysts in a variety of chemical transformations in the area of environmental protection, such as low-temperature water-gas shift, selective oxidation of CO in the presence of large amounts of hydrogen, or three-way catalysis. Strong variations in the chemistry of ceria–zirconia mixed oxide catalysts have been observed after they have undergone redox cycles involving reduction treatments at high temperatures ( 1173 K) then oxidation at mild temperatures ( 823 K). In particular their reducibility is significantly enhanced after such aging treatments. Scanning transmission electron microscopy (STEM) techniques have provided crucial information to account for these changes in the redox behavior. High-resolution electron microscopy (HREM) combined with high-angle annular dark-field (HAADF) imaging and tomography have revealed the occurrence of a disorder–order transformation in the cationic sublattice of these oxides, which tend to rearrange into a distribution characteristic of the so called pyrochlore phase. This phase is an archetype structure for A2B2O7 (A= + 3 cation, B=+ 4 cation) compounds and can be considered a fluorite superstructure. The structural transformation takes place during the reduction step of the cycle, in which the fully reduced mixed oxide with Ce/Zr molar ratio 1:1 adopts the Ce2Zr2O7 stoichiometry. Nevertheless, HAADF studies have clearly shown that, in the case of ceria–zirconia mixed oxides, this cation-ordered arrangement is preserved even after full reoxidation, that is, in the oxide with Ce2Zr2O8 stoichiometry, whenever the oxidation temperature does not exceed 823 K. Electron-microscopy studies have also revealed another remarkable feature of the ceria–zirconia aged oxides with the pyrochlore-type cation sublattice: the occurrence of compositional heterogeneities at the nanometer scale. Taking these observations into account and also considering that the disorder–order transition may not be completed in the time scale and under the temperature conditions used in the redox-cycling treatments, the important question arises whether these heterogeneities are in fact occurring on a finer scale, that is, at the atomic level. Such heterogeneities, compatible with the HREM and HAADF observations, will strongly influence the details of the counterpart oxygen sublattice and, consequently, the chemical and catalytic response of these oxides. To date, the atomic-column by atomic-column compositional analysis of the oxidized pyrochlore required to justify such a possibility has not been accomplished. Herein, using the capabilities of an aberration-corrected Nion UltraSTEM microscope (operated at 100 kV) we not only provide the first direct chemical evidence of the cationic order present in the Ce2Zr2O8 oxidized pyrochlore but we also show how atomicresolution electron energy-loss spectroscopy (EELS) mapping, based on core–shell ionization, can be combined with EELS image simulation to detect quite subtle local deviations in the cation sublattice from the completely ordered structure. This information provides a much more accurate structural description of the active catalyst nanocrystals, which must be considered to model both their oxygen-exchange capabilities and, eventually, their catalytic performance. [*] Dr. S. Trasobares, Dr. M. L pez-Haro, Dr. J. A. Perez-Omil, Dr. J. J. Calvino Departamento de Ciencia de los Materiales e Ingenier a Metalfflrgica y Qu mica Inorg nica Facultad de Ciencias, Universidad de C diz Campus Rio San Pedro, 11510-Puerto Real, C diz (Spain) Fax: (+34)956-016286 E-mail: susana.trasobares@uca.es Homepage: http://www.uca.es/tem-uca

New insights into the performance of ceria–zirconia mixed oxides as soot combustion catalysts. Identification of the role of “active oxygen” production

Soot combustion experiments with O 2 and with O 2 + NOx mixtures were performed with model and real soot samples and with a Pt-catalyst and several Ce–Zr catalysts. It was demonstrated that all the Ce–Zr catalysts tested are more effective for soot combustion (due to the important production of active oxygen), than the Pt-catalyst is, even though the noble metal yielded the highest NO 2 production level. A synergetic effect was observed between the NO 2-assisted and active oxygen-assisted soot combustion for the Ce–Zr catalysts studied. The contribution of the active oxygen generated by these mixed oxides to soot combustion, with regard to the NO 2-assisted combustion, becomes more important as soot combustion temperature increases (because of the decrease of soot reactivity). The properties of the Ce–Zr catalysts affect significantly their active oxygen production capacities.

Ceria–zirconia mixed oxides as thermal resistant catalysts for the decomposition of nitrous oxide at high temperature

This present study deals with the development of thermally stable catalysts for the decomposition of nitrous oxide. There is actually a growing interest to minimise the emissions of nitrous oxide (N 2O) as side-product from nitric acid plants due to a global warming potential of approximately 300 times higher than that of CO 2. Different strategies have already been implemented at industrial scale but they are still suffering from significant drawbacks essentially associated to a poor selectivity and sometimes a short lifetime in particular when the catalytic process is inserted downstream the ammonia burner. In those critical conditions, with high residual temperature of ∼900 °C, significant surface and bulk reconstructions take place and then lead to drastic losses of specific surface area and related catalytic performances in the decomposition of N 2O to N 2. Previous investigations performed in our laboratory revealed the potentiality of zirconia particularly after yttria incorporation [Appl. Catal. B 62 (2005) 236]. Novel developments of more stable ceria–zirconia based catalysts are reported in this study. Further stabilisation and enhancement in activity have been discussed on the basis of oxygen mobility related to structural properties.

Advanced Electron Microscopy Investigation of Ceria–Zirconia‐Based Catalysts

AbstractThe potentials of advanced transmission and scanning transmission electron microscopy in nanostructural studies of ceria–zirconia mixed oxides are overviewed. The crystallographic criteria that allow us to discriminate the different CeO2–ZrO2 polymorphs and the nanocrystal size range within which they can be applied are discussed. The combined use of high resolution electron microscopy (HREM) and high‐angle annular dark field scanning transmission electron microscopy (HAADF‐STEM) to detect disorder–order transitions in the cation sublattice of this family of oxides and the size limit down to which each of them can be used for that purpose are also analyzed. Criteria to discriminate, on the basis of HREM images, the oxygen arrangement of the so called κ‐Ce2Zr2O8 phase from that of an oxidized pyrochlore Ce2Zr2O8 phase are presented and applied to the interpretation of experimental HREM images. HAADF‐STEM tomography studies and detailed analysis of HAADF‐STEM images establish both the crystallographic and compositional features of the surfaces of the mixed oxides that give enhanced redox activity. These findings allow deeper understanding of the influence of different thermal ageing pretreatments on the redox behavior of this family of mixed oxides. Novel characterization data do evidence that this model does not only fruitfully apply to binary ceria–zirconia oxides but also to more complex ternary oxides containing terbium.

Novel nontoxic nanopigments based on zirconia and rare earth mixed oxides

AbstractThis paper embodies the result of the investigations carried out on the synthesis of zirconia‐ rare earth mixed oxide pigments: ZrCe1‐xYxO4‐δ and ZrCe1‐xPrxO4‐δ by the solution combustion method and their characterization by X‐ray powder diffraction, transmission electron microscopy, energy dispersive spectrometric analysis, thermal analysis, reflectance spectral data and surface area measurements. The synthesized nano pigments possess low a and high b and exhibit yellow color when doped with Y and changes from yellow to orange‐red color when doped with Pr, with increase in x values. These pigments show maximum reflectance of 90% when the wavelength is above 550 nm. Due to the homogeneity of the phase formed and resistance to water, acids and alkali, the rare earth mixed oxides developed are found to be promising candidates as nontoxic pigments. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of Zr and Ti doping on the dielectric response of [formula omitted]: A comparative first-principles study

Zr doping in ceria (CeO 2) results in enhanced static dielectric response compared to pure ceria. On the other hand, Ti doping in ceria keeps its dielectric constant unchanged. We use first-principles density functional theory calculations based on pseudopotentials and a plane wave basis to determine electronic properties and dielectric response of Zr/Ti-doped and oxygen-vacancy-introduced ceria. Softening of phonon modes is responsible for the enhancement in dielectric response of Zr-doped ceria compared to that of pure ceria. The ceria–zirconia mixed oxides should have potential use as high- k materials in the semiconductor industry.

Effects of Ni-Doping of Ceria-Based Materials on Their Micro-Structures and Dynamic Oxygen Storage and Release Behaviors

The structural, textural, and dynamic oxygen storage/release behaviors of Ni0.1Ce0.9Ox and Ni0.1Ce0.6Zr0.3Ox (prepared by sol–gel method) are investigated by X-ray diffraction (XRD), Raman, H2-TPR, X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and CO/O2 pulses with mass spectrometry studies. The dynamic oxygen storage capacity and rate are largely promoted by Ni doing, and the thermal stability of the sample is also enhanced. XRD and Raman results indicate that Ni0.1Ce0.9Ox and Ni0.1Ce0.6Zr0.3Ox have the fluorite-type cubic structure similar to CeO2. TPR, EPR, and XPS analyses reveal that the introduction Ni into ceria and ceria–zirconia mixed oxides strongly modifies the structural and textural properties, which influence the kinetics of bulk oxygen diffusion. The dynamic oxygen storage capacity (DOSC) of Ni doped samples (Ni0.1Ce0.9Ox and Ni0.1Ce0.6Zr0.3Ox) and reference samples (CeO2 and Ce0.67Zr0.33O2) were investigated with CO/O2 transient pulses. The results show that Ce-based materials containing Ni oxides with variable valences contribute to the majority of DOSC and DOSR performances, and the stability of DOSC is obviously enhanced by Ni and Zr codoping.

Effects of sulfation on the activity of Ce0.67Zr0.33O2 supported Pt catalyst for propane oxidation

Abstract The sulfated Ce0.67Zr0.33O2 support was prepared by impregnated with sulfuric acid solution, and the influence of sulfation on propane oxidation activity of the corresponding Pt supported catalysts was evaluated. Sulfation of the ceria–zirconia mixed oxides inhibits its intrinsic activity severely but promoted the Pt supported catalysts significantly. It is evidenced by CO adsorption that the interaction between sulfates and platinum makes the latter in more deficient states (Ptδ+). These metastable Ptδ+ species are liable to the atmosphere and act as active sites for propane oxidation.

Microstructural stability of zirconia–alumina composite coatings during hot corrosion test at 1050 °C

In the present work hot corrosion behavior of plasma sprayed zirconia–alumina coatings on Ni-base, IN-738, super alloy substrate was studied compared with normal zirconia. Hot corrosion resistance of the coatings was measured at 1050 °C using an atmospheric electrical furnace and a fused mixture of vanadium pentoxide and sodium sulfate salt. The hot corrosion test duration was 4 h in each cycle, while the specimens were cooled in the furnace. The general and peripheral conditions of the specimens were inspected. If there were any cracks or spallation in coating wedge the test was stopped, the time was recorded and coating microstructure was studied. Composite coatings of zirconia–alumina having alumina as a top coat or a mixed zirconia–alumina layer, showed better resistance in hot corrosion tests. It was concluded that alumina overlay on zirconia has promoted the hot corrosion resistance of the coatings.

Effects of Ce/Zr ratio on the reducibility, adsorption and catalytic activity of CuO/CexZr1−xO2/γ-Al2O3 catalysts for NO reduction by CO

Effects of Ce/Zr ratio on the physicochemical properties of CuO/CexZr1−xO2/γ-Al2O3 catalysts were investigated by BET, XRD, Raman and H2-TPR. The catalytic activity and the interaction between the reactants with these catalysts were compared by NO+CO model reaction and in situ FT-IR. The results suggested that the addition of ceria–zirconia mixed oxides significantly improved NO conversion and N2 yield due to dispersed copper species in proximity to ceria–zirconia. Especially, the ceria-rich catalysts displayed better performance in activity and reducibility than others, which would be resulted from the strong interaction among copper, ceria–zirconia and support. The IR results suggested NO reduction activity was correlated with the presence of Cu+ carbonyl species, and the catalysts with variable Ce/Zr ratios had no distinction at the adsorption type and rate of NO/CO at room temperature. However, on heating treatment would give distinct difference in CO2 intensity and the wavenumber of adsorbed nitrates. Simultaneously, the stability of these N- and C-containing intermediates contacted with alumina was influenced by the modified Ce/Zr ratio.

Adsorption of Strontium from Acidic Waste Solution by Mn–Zr Mixed Hydrous Oxide Prepared by Co-Precipitation

A manganese–zirconium mixed hydrous oxide with Mn to Zr mole ratio of 1:1 was prepared to study the sorption behavior and the removal of strontium from acidic radioactive waste solutions. Mixed hydrous oxide was identified and characterized by DTA/TGA, XRD, and surface analysis. The parameters affecting strontium adsorption, such as initial pH, contact time, initial metal ion concentration, temperature, adsorbent dose, and selectivity towards competing ions were investigated. Sorption data have been interpreted in terms of Freundlich and Langmuir equations. Thermodynamic parameters such as standard enthalpy (ΔH°), entropy (ΔS°), and free energy (ΔG°) were calculated from the slope and intercept of the plots of ln K D versus 1/T. The results indicated that the sorption of strontium onto manganese–zirconium mixed oxide is endothermic and spontaneous in nature.

Nanoparticulate ceria–zirconia anode materials for intermediate temperature solid oxide fuel cells using hydrocarbon fuels

Solid Oxide Fuel Cells (SOFCs) represent an attractive technology for the conversion of chemical to electrical energy because of their high efficiencies and low environmental impact, and because of the useful, high grade heat also generated. Direct utilisation of hydrocarbons in SOFCs would contribute to the more sparing utilisation of remaining fossil fuel reserves. In the longer term, this technology could be extended to work with more sustainable biofuel and waste-derived feedstocks. In this work, nanoparticulate Ceria–Zirconia mixed oxides, Ce1−xZrxO2 (x = 0.1, 0.25, 0.5, 0.75 and 0.9), were studied with a view to their application as anode materials in intermediate temperature (IT) SOFCs using hydrocarbon fuels. Impedance spectra were recorded in symmetrical cells under reducing conditions using gadolinium-doped ceria (GDC) as the electrolyte material. The spectra were analysed in terms of a double fractal finite length Gerischer impedance model. The model parameters were found to have monotonic dependences on temperature and more complex relationships with respect to Zr content. Diffusion-related processes and the electrochemical reaction were fastest for intermediate Zr contents while the chemical exchange reaction rate increased with decreasing Zr content. As a result, anode catalysts with 10 and 25 mol% Zr showed the lowest polarisation resistances of only 0.17 and 4.52 Ω cm2 at 700 °C in humidified 5% H2 and humidified 5% CH4, respectively. These values represented an approximately two-fold improvement on the pure ceria electrode. This performance compares very favourably with the currently most promising candidate anode materials applied in SOFCs for use with hydrocarbon fuels.

Alkyl chain grafting on silica–zirconia mixed oxides: preparation and characterization

Silica–zirconia mixed oxide substrates were prepared and grafted with C18 alkyl chains using n-octadecyltrichlorosilane. Si : Zr molar ratios in the mixed oxide samples before and after alkyl chain modification were determined by X-ray photoelectron spectroscopy (XPS). Nitrogen sorption measurements provided the porous features of these materials. By means of solid-state 29Si nuclear magnetic resonance (NMR) the amounts of the silanol groups present in the as-prepared, humidified and grafted mixed oxide systems as well as the degree of cross-linking of the silanes were obtained. 13C NMR spectroscopy was used to examine the conformational order of the n-octadecyl chains grafted on the silica–zirconia mixed oxide substrates. Similar information was available from variable temperature Fourier transform infrared (FTIR) investigations. Scanning electron microscopy (SEM) measurements and energy dispersive X-ray (EDX) analyses yielded the morphological features of the solid substrates and information about the distribution of the alkyl chains on the oxide surfaces. The annealing temperature for the preparation of the mixed oxides and the elemental composition of the samples, i.e. relative amount of the oxide components, were found to determine the surface properties and in turn the alkyl chain assembly on the mixed oxide surfaces. FTIR and solid-state 13C NMR data gave evidence for an unusual high conformational order of the grafted C18 alkyl chains in all samples, irrespective of the low surface coverage, which exceeds comparable C18 and C30 grafted silica and metal oxide systems. This finding was attributed to the formation of island structures with aggregated octadecyl chains of high conformational order due to strong intermolecular interactions. Differences in the conformational order of the various samples most likely arise from vertical polymerization which varies with the actual sample.