Combustion synthesis of copper catalysts for selective CO oxidation

Copper catalysts supported on ceria, zirconia and niobia were prepared by combustion method with urea, containing a CuO loading of 6 wt.%, and tested on selective oxidation of CO. The characterization of the samples by X-ray diffraction (XRD) presented the formation of solid solution on CuO–CeO₂ catalyst and a change in crystalline structure of the support with copper insertion on ZrO₂ and Nb₂O₅ catalysts. The analysis of temperature-programmed reduction (TPR) revealed different interaction degrees of copper with the supports, with reduction peaks between 222 and 390 °C. The temperature-programmed desorption of CO (TPD-CO) profiles showed formation of CO₂ and H₂ only for the ceria and zirconia catalysts. In relation to the catalytic tests, the CuO–CeO₂ catalyst presented the best performance, with CO conversion of 95% at 150 °C up to 45 h on stream, and CO₂ selectivity of 55%.     

Electrical and microstructural characterization of two-step sintered ceria-based electrolytes

Gadolinium-doped ceria-based materials with and without Ga-additions were prepared following several firing schedules including one peak sintering temperature (up to 1300 °C) with or without subsequent dwell at lower temperature (at 1150 °C). Sintered disks with submicrometric grain size and densifications in the order of 92% or higher, were obtained in this manner, with the final result depending slightly on the sintering profile and presence of Ga as dopant. All materials were characterized by scanning electron microscopy, X-ray diffraction and impedance spectroscopy in air, in the temperature range 200–800 °C. The grain boundary arcs were found slightly dependent on grain size and porosity but significantly on Ga-doping, due to the likely presence of large concentrations of Ga along the grain boundary region.     

汽车尾气净化催化剂用Ce_xZr_（1-x）O_2固溶体的研究进展

综述了CeO2和CexZr1-xZr1-xO2固溶体在汽车尾气净化催化剂重要作用、铈锆氧化物的体相结构及影响铈锆氧化物固溶体储养能力（OSC）和织构热稳定性的因素,并对其在催化剂中的应用作了简要陈述。     

Mixtures of room temperature ionic liquid/ethanol solutions as electrolytic media for cerium oxide thin layer electrodeposition

A cerium oxide thin layer was electrodeposited onto stainless steel, using mixed room temperature ionic liquid (the 1-methyl-3-butylimidazolium bis(trifluoromethyl sulfonyl)imide)/ethanol solutions, as electrolytic medium. The hydrophobic ionic liquid content is one of the main parameters in the morphology control influencing the ceria growth rate and crystallinity. Micro-nano structural properties and electrical behaviour are presented, using XRD, SEM/EDS and impedance spectroscopy, as a function of electrodeposition conditions.     

Aging effects of nanoscale ceria in ceria–platinum composite electrodes for direct alcohol electro-oxidation

Nanocrystalline Pt/CeO₂ composite electrodes were fabricated to study the electrochemical oxidation of methanol and ethanol. The performance of the electrodes was tested as the ceria solutions aged over time. It was observed that the performance oscillated with time, suggesting that the catalytic behavior towards alcohol oxidation was greatly dependent on the aging of the particles. These results point to a great dependence of the catalytic effect on the redox state of the ceria particles.     

Electrical conductivity of metastable κ-CeZrO4 phase possessing ordered arrangement of Ce and Zr ions

Electrical conductivity σ_t of metastable κ-CeZrO₄ possessing ordered arrangement of Ce and Zr ions in a manner similar to a pyrochlore-type was measured as a function of temperature and time, and compared with tetragonal metastable t′-(Ce_0.5Zr_0.5)O₂ and t′_meta-(Ce_0.5Zr_0.5)O₂ phases possessing random arrangement of the cations. The κ disk was prepared by reoxidizing a pyrochlore-type precursor in O₂ gas at 873 K. The σ_t as measured for the κ-CeZrO₄ was reproducible as a function of temperatures between 957 and 1190 K. At increasing temperatures above 1233 K, the σ_t decreased gradually with time due to the phase transition: κ→t′, and became consistent with the t′. Although the κ-CeZrO₄ phase is thermodynamically less stable than the t′-(Ce_0.5Zr_0.5)O₂, it was virtually stable up to around 1233 K. It was found from the change in the σ_t due to the phase transition that the σ_t for the metastable κ-CeZrO₄ was an order of magnitude higher than that for the t′ and was similar to that for the t′_meta. It was previously reported that a phase transition, t′_meta→t′, occurred above 1143 K. The phase transition, κ→t′, accompanied by redistribution of the cations appeared to occur at higher temperatures than that for t′_meta→t′ leaving random cation arrangement.     

The influence of water on the reduction and reoxidation of ceria

Two different samples of ceria have been characterized by temperature programmed reduction in carbon monoxide (CO-TPR), surface area determination and transmission electron microscopy (TEM). The much higher reducibility at temperatures below 550 °C of a sample prepared by decomposition of cerium carbonate, compared to a commercial ceria, was attributed to the much lower crystallinity. When this ceria, in the partly reduced state, was brought in contact with water vapour, it was readily reoxidised with evolution of hydrogen. The oxygen storage capacity of this sample, determined in cycling experiments, was considerably lowered by the presence of water.     

Stuey of Thermodynamic Properties of Nonstoichiometric Phase 〈Zr1—zCezO2—x〉 with Compound Energy Model

Using compound energy model (CEM), the thermodynamic properties of and were evaluated. The evaluation was based on the optimization of ZrO2-CeO2 and ZrO2-CeO1.5 systems, as well as the miscibility gap in CeO1.5-CeO2 system. Except the cubic fluorite structure phase assessed with compound energy model, all the other solution phases were assessed with subsitutional solution model. The model parameters were evaluated through fitting the selected experimental data by means of thermodynamic optimization. A set of parameters with thermodynamics self-consistency was obtained and satisfactorily described the complex relation between y in and the partial pressure of oxygen at different temperatures, also the interdependence among miscellaneous factors such as temperature, oxygen partial pressure, the reduction amount of CeO2 as well as the nonstoichiometry in cubic phase . The calculated results seem to be reasonable when put into the explanation of pressureless sintering of CeO2-stabilized ZrO2 powder compacts at a controlled oxygen partial pressure.     

The effect of multiple doping on electrical conductivity of gadolinia-doped ceria electrolyte

Combinations of two or four trivalent oxides were substituted for gadolinia up to 5 mol% in the 20 mol% gadolinia-doped ceria electrolyte as additional dopants. Most of them increased the electrical conductivity of the electrolyte but some did not. Among them (YLa)_0.01Gd_0.19Ce_0.8O_1.9 and (YSm)_0.03Gd_0.17Ce_0.8O_1.9 showed the highest electrical conductivities, respectively, at 800 ‡C and 600 ‡C. These experimental results were analyzed in terms of activation enthalpy and entropy for ionic conduction. Among the various possible contributions to the activation entropy, only the configurational entropy was taken into account for the explanation.     

X-ray absorption fine structure analysis of local structure of CeO2–ZrO2 mixed oxides with the same composition ratio (Ce/Zr=1)

Three types of CeO₂–ZrO₂ (Ce:Zr=1:1 molar ratio) compounds with different oxygen storage/release capacities (OSCs) were characterized by means of the Ce K-edge and Zr K-edge X-ray absorption fine structure (XAFS). In order to investigate the relationship between the OSC and local structure, the quantitative EXAFS curve-fitting analysis was applied. By enhancing the homogeneity of the Ce and Zr atoms in the CeO₂–ZrO₂ solid solution, the OSC performance increased. Especially, the atomically homogeneous Ce_0.5Zr_0.5O₂ solid solution exhibited the highest OSC among these CeO₂–ZrO₂ samples. Additionally, the local oxygen environment around Ce and Zr was remarkably modified by enhancing the homogeneity of the CeO₂–ZrO₂ solid solution. It was postulated that the enhancement of the homogeneity of the CeO₂–ZrO₂ solid solution and the modification of the oxygen environment would be the source for the OSC improvement.