Synthesis of doped ceria-zirconia core-shell nanocomposites via sol-gel process

In order to introduce more conductive interfaces, the doped ceria-zirconia core-shell nanocomposites are synthesized via a simple and low-cost sol-gel process. Nitrates, citric acid and polyethylene glycol (PEG) are used as starting materials, and the compositions of the core and the shell are Ce_0.9Gd_0.1O_1.95 (GDC) and 8 mol% Sc₂O₃ doped ZrO₂ (ScDZ), respectively. The room-temperature ammonia co-precipitation method is used to prepare the core materials. X-ray diffraction (XRD) indicates that the average grain sizes of the core and shell materials are about 6 nm and 8 nm. The core-shell nanostructure with about 60 nm diameter GDC core (approximate) and about 20 nm thick ScDZ shell, is supported by the field-emission scanning electron microscopy (FESEM) and the transmission electron microscopy (TEM) results. The effects of PEG and the mechanisms during the process of forming the core-shell nanocomposites are discussed in detail.     

Noble metal catalysts supported on gadolinium doped ceria used for natural gas reforming in fuel cell applications

An attractive possibility to simplify a fuel cell system would be the use of a sulfur-tolerant reforming catalyst. In an effort to find such a catalyst, platinum, rhodium and ruthenium catalysts supported on ceria doped with 20% gadolinium and on pure ceria were synthesized and characterized. A temperature-programmed reduction study of the reduction behavior of the catalysts showed that the doping of ceria with gadolinium enhances the low temperature reduction, while the high temperature reduction is suppressed. The activity as well as the stability of the catalysts can be correlated with the reducibility of the materials. The most stable catalyst, rhodium supported on gadolinium doped ceria, shows promising sulfur-tolerance.     

An all-in-one flourite-based symmetrical solid oxide fuel cell

A novel concept of solid oxide fuel cell (SOFC), the symmetrical SOFC, that uses simultaneously the same material as both anode and cathode has been investigated. Common materials typically used as anode components such as a combination of YSZ and CeO₂ plus a noble metal may be considered good candidates for such a configuration at relatively high temperatures (i.e. above 900 °C). These symmetrical electrodes exhibit enhanced electrochemical properties under both reducing and oxidising conditions, in part due to the catalytic properties of the noble metal used. In air the polarisation values are improved by a factor of four compared to electrodes without CeO₂, whereas under reducing conditions an improvement of two–three orders of magnitude has been observed. The best results correspond to cermets containing 50–60% of CeO₂.     

Hydrogen production over Ni/ceria-supported catalysts by partial oxidation of methane

The catalytic performance of Ni dispersed on ceria-doped supports, (Ce_0.88La_0.12) O_2-x, (Ce_0.91Gd_0.09) O_2-x, (Ce_0.71Gd_0.29) O_2-x, (Ce_0.56Zr_0.44) O_2-x and pure ceria, was tested for the catalytic partial oxidation of Methane (CPOX). The catalysts were characterized by Brunauer Emmett Teller (BET), X-ray diffraction (XRD), temperature programmed reduction (TPR) and temperature programmed oxidation (TPO). Ni/ (Ce_0.56Zr_0.44) O_2-x showed higher Hydrogen production than the Ni/Gadolinium-doped catalysts, which may be due to its higher reducibility and surface area. By enhancing the support reducibility in Ni/doped-ceria catalysts, their catalytic activity is promoted because the availability of surface lattice oxygen is increased, which can participate in the formation of CO and H₂. It was also found that Ni/(Ce_0.56Zr_0.44) O_2-x showed higher catalytic performance after redox pretreatments. Similarly, a higher amount of H₂ or O₂ was consumed during hydrogenation and oxidation pretreatments, respectively. This may be correlated to re-dispersion of metallic particles and changes on the metal-support interface. In addition, it was observed that the ionic conductivity of Ni/(Ce_0.56Zr_0.44) O_2-x had an effect on the amount of carbon formed during the CPOX reaction at oxygen concentrations lower than the stoichiometric required, O/C ratios lower than 0.6. Its high oxygen mobility may have accelerated the surface oxidation reactions of carbon by reactive oxygen species, thus, inhibiting carbon growth on the catalyst surface.     

Catalytic oxidation of VOCs on Au/Ce-Ti-O

Ce_xTi_1−xO₂ oxides have been synthesised by sol–gel method with x varying from 0 to 0.3 and characterised by XRD and TPR. The structure of oxides changes with the Ce/Ti molar ratio. The presence of ceria in Ce-Ti oxides inhibits the phase transition from anatase to rutile. When x = 0.3 (Ce_0.3Ti_0.7O₂ sample), the solid presents an amorphous state. The TPR results indicate that the presence of Ti enhances the reducibility of cerium oxide species. Catalytic oxidation of propene is investigated on Ce-Ti oxides and the better conversion is obtained with Ce_0.3Ti_0.7O₂ but the CO₂ selectivity reaches 63% at 400 °C. Gold is then deposited on theses oxides to improve the catalytic activity. On the basis of characterisation data (H₂ TPR), it has been suggested that gold influences the reduction of the Ce-Ti oxide support and the catalytic activity to the propene oxidation. Thus, Au/Ce-Ti-O system catalysts are promising catalysts for propene oxidation.     

Effect of Y-doping on the catalytic properties of CuO/CeO2 catalysts for water-gas shift reaction

CuO/ceria and CuO/Y-doped ceria catalysts were synthesized. The Y-modified supports (1.0, 2.5 and 5.0 wt% Y₂O₃) were prepared by coprecipitation. CuO (3 wt% Cu) was loaded by deposition-precipitation. Having in mind the known effect of Y³⁺ modification for the generation of oxygen vacancies in ceria, its positive role on the water-gas shift (WGS) performance was expected. However, the catalytic test showed a trend of decreased WGS activity by increasing the Y-dopant amount, nevertheless that the differences were not very substantial. On the basis of XRD, XPS, EPR, Raman spectroscopy and H₂-TPR results the explanation related to the key role of the oxygen mobility influenced by Y-doping could be proposed. The reason of the inferior WGS performance with increasing Y-content would be the higher amount of surface oxygen vacancies around Y³⁺ ions which disturbed the Cu–O_vac–Ce active sites for WGS reaction. Though, during the long run catalytic tests in WGS reaction a positive effect of Y-doping for improved stability of CuO/ceria catalysts was evidenced.     

The role of SiO2 and sintering temperature on the grain boundary properties of Ce0.8Sm0.2O2−δ

The solid solution Ce_0.8Sm_0.2O_2−δ (20CSO) was synthesized by freeze-drying precursor procedure. Well-crystallized powders with nanometric grain sizes were obtained after calcining the precursor at 375 °C for 4 h. The effect of SiO₂-addition and sintering temperature on the properties of the bulk and grain boundary processes were studied. For this purpose, 20CSO-SiO₂ samples were prepared by the addition of 0.05 or 0.5 mol% SiO₂ to Ce_0.8Sm_0.2O_2−δ, in the form of tetraethyl orthosilicate (TEOS). Also, 2 mol% Co was added to some of the precalcined compositions with and without silica-addition. Cobalt free samples were sintered at 1400, 1500 and 1600 °C and cobalt-added samples were sintered 1150 °C, for 10 h to obtain dense pellets. The electrical behaviour of the bulk was revealed to be nearly independent on sintering temperature and/or on the addition of impurities of SiO₂ and Co to the grain boundaries. This was explained by the low solubility of impurities in the grain fluorite structure. However, the grain boundary resistance showed important differences as function of sintering temperature and with the presence of impurities. The analysis of grain boundary properties suggests that segregated impurities affect the microstructure and also segregation of Sm at the space charge layer, thus changing both the specific grain boundary conductivity and microstructural parameters.     

Polishing behaviors of single crystalline ceria abrasives on silicon dioxide and silicon nitride CMP

The effects of single crystalline ceria (CeO₂) abrasives in chemical mechanical polishing (CMP) slurries were investigated for silicon dioxide (SiO₂) and silicon nitride (Si₃N₄) CMP process. The size of ceria abrasives was controlled by varying hydrothermal reaction conditions. Polishing removal rate was measured with four slurries, with different mean primary particle size of 62, 116, 163 and 232 nm. The polishing results showed that the single crystalline ceria abrasives were not easily broken-down by mechanical force during CMP process. It was found that the removal rate of oxide and nitride film strongly depend upon abrasive size, whereas the surface uniformity deteriorates as abrasive size increases. The observed polishing results confirmed that there exists an optimum abrasive size (163 nm) for maximum removal selectivity between oxide and nitride films. The polishing behavior of the single crystalline ceria abrasives was discussed in terms of morphological properties of the abrasive particle.     

Electrostatic Spray Deposition of Doped Ceria Films

Dense and thin electrolyte films are desirable for solid oxide fuel cells (SOFCs) because of their low gas leakage and low ohmic resistances. This work aims at the preparation of thin dense Gd‐doped ceria (CGO) electrolyte films using a cost‐effective deposition method in ambient atmosphere–electrostatic spray deposition (ESD). The deposition parameters such as deposition temperature, concentration and flow rate of precursor solution were changed systematically to examine their effects on film morphology and hence electrochemical performance. While the film morphology was examined by a scanning electron microscope, the electrochemical performance was revealed by measuring open circuit voltages (OCVs) of NiO‐CGO/CGO/Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3‐δ (BSCF) cells in 500–700 °C with humidified hydrogen as fuel and air as oxidant. The results show that a CGO film of 25 μm thick obtained at a deposition temperature of 400 °C, a precursor solution flow rate of 6 ml h^–1 and a precursor concentration of 0.3 M was dense with very few isolated pores and the OCV of the associated cell was 0.915 V at 500 °C. This implies that the CGO film has negligible gas leakage and ESD is a promising method for preparing thin dense electrolyte films for SOFCs.