Search

Department of Chemistry, University of Allahabad, Allahabad-211 002, Uttar Pradesh, India E-mail : mcc46@rediffmail.com, nnc2004200l@gmail.com Manuscript received 19 February 2010, revised 04 June 2010, accepted 21 June 2010 In this paper ultrafine powder of gadolinia doped ceria was prepared by a combustion synthesis method using metal nitrates, both as a precursor and oxidizer and glycine as fuel for combustion. The influence of glycine to metal molar ratio on the characteristics of the powders has been studied. Characterization of the powder samples was performed using, FTIR spectroscopy, TG/DTA analysis, X-ray diffractometry, Small angle X-ray scattcering (SAXS) technique and scanning electron microscopy. The ultrafine powders prepared by glycine to metal ratio 1.2 gives the best powder properties. The indexing or diffraction pattern shows that the gadolinia doped ceria powders have face centered cubic symmetry.

Electrical and dielectric properties, and their correlations were discussed in the nanostructured Ce0.8 Dy0.2 O2-? materials with average grain sizes of 15, 35, and 58 nm using the dielectric functions such as dielectric permittivity (??), loss tangent (tan ?), and electric modulus (M?). The ionic transport mechanism varies with the grain size of the materials such that the material with an average grain size of 35 nm exhibited a higher value of ionic conductivity above 440�C (with ?=8.96� 10-4 S cm-1 at 550�C), whereas the material with an average grain size of 15 nm shows higher value conductivity at lower temperatures. The oxygen vacancy in the material decreases with the decrease in grain size. The migration energy of oxygen ions also decreases with the decrease in grain size of the nanostructured Ce0.8 Dy 0.2 O2-? material. The material with an average grain size of 35 nm shows a very low value association energy (i.e., 0.05 eV) compared to the others. At temperatures below 420�C, the local motion of oxygen vacancies around Dy+3 controls the conductivity of all the materials, and at higher temperatures, the conductivity in the materials with a larger grain size is mainly due to the long-range motion of vacancies. � 2010 The Electrochemical Society.

A series of ceria oxides doped with 10 mol.% of Zr, Zn and Fe have been prepared by a pseudo sol-gel method throughout the thermal decomposition of the corresponding metallic propionates. With these supports, 1 wt.% gold catalysts were prepared by the deposition-precipitation method. All the solids were characterized by means of XRF, N2 adsorption, XRD, Raman spectroscopy and SEM techniques, and their catalytic activity toward preferential oxidation of CO (PROX) reaction tested. The results showed solid solution when doping with Zr and Fe and ZnO surface segregation in the case of Zn. We demonstrate that gold dispersion depends on not only the oxygen vacancy concentration but also the nature of the doping agent. Finally, the catalytic activity was highly promoted by gold in all cases, being the doped gold catalysts more active than Au/CeO2 at low temperature. © 2010 Elsevier Inc. All rights reserved.

Nanometric powders of ceria doped with 10, 15 and 20% at. of samarium were prepared by a sol-gel method. The powders were compacted in cylindrical pellets and sintered at different temperatures in order to obtain grain sizes ranging from 1 to 10 microns. The electrical conductivity was measured in air by electrochemical impedance spectroscopy in the 1166 - 473 K temperature range. The bulk and grain boundary contributions to the overall conductivity were estimated by equivalent circuit analysis and the application of the brick layer model. While bulk conductivity values were markedly independent of the grain size, a relevant decrease in the grain boundary specific conductivity was observed for the finer grain samples in the lower dopant grade. For increased samarium content, this decrease in specific grain boundary conductivity with grain size becomes less relevant.

Este trabalho investigou a condutividade elétrica e a microestrutura de amostras com composições Ce1-xGd xO2-δ (x = 0, 5, 10 e 15%em mol) preparadas com CeO2 com 99,9% pureza. A sinterização de corpos de prova obtidos por prensagem isostática foi realizada ao ar a 1450 ºC por 2 e 10 horas de patamar e a 1500 ºC por 8 horas de patamar. Amostras sinterizadas foram caracterizadas por microscopia eletrônica de varredura e espectroscopia de impedância entre 200-600ºC e na faixa de freqüência 5Hz-13MHz. A sinterização ocorreu via fase líquida e amostras sinterizadas a 1450 °C com 10 h apresentaram densidade superior a 97 %. A maior condutividade elétrica do grão foi obtida para amostra contendo 10 mol Gd2O3 e sinterizada a 1500 °C - 8 h. A condutividade elétrica do contorno de grão apresentou aumento sistemático com o aumento do teor de Gd2O3 sendo que a composição CeGdO15 (com 15% de Gd2O3) sinterizada a 1450 °C - 10 h apresentou a maior condutividade total. This study investigated the electrical conductivity and microstructure of samples with compositions Ce1-xGd xO2-δ (x = 0, 5, 10 and 15% in mol) prepared with CeO2 with 99.9% purity. The sintering of the samples obtained by isostatic pressing was performed in air at 1450 for 2 and 10 hours soaking time and 1500 ºC for 8 hours of soaking time. Sintered samples were characterized by scanning electron microscopy and impedance spectroscopy between 200-600 ºC and the frequency range was 5Hz-13MHz. The sintering occurred via liquid phase. The 1450 °C with 10 h sintered samples showed more than 97% density. The highest electrical conductivity of the grain was obtained for sample containing 10 mol Gd2O3 and sintered at 1500 °C - 8 h. The electrical conductivity of the grain boundaries has increased systematically with the Gd2O3 amount increasing. The composition CeGdO15 (15 mol % of Gd2O3) sintered at 1450 °C - 10 h had the highest total conductivity.

The reduction of NO by CO was studied over gold catalysts supported on ceria modified by Me3+ ions (Me = Al, La, Sm, Gd and Yb). The ceria supports were prepared by mechanochemical activation. The samples were characterized using XRD, TPR, XPS and Raman spectroscopy. According to the XPS data the concentration of the oxidized gold species was higher than that of metallic gold in the fresh samples modified by lanthanides. On the fresh samples modified by Al only a small part of metallic gold existed in oxidized state. After the catalytic test, only metallic gold was found on the lanthanide-containing catalysts while on the Al-modified catalyst a small amount of oxidized Au species in addition to metallic Au was detected. No substantial differences in the average particle sizes of gold, the lattice parameters and the average size of ceria particles were observed. The nature of the modifier and the applied method of ceria supports preparation and gold deposition determined most likely the differences observed in the Raman and TPR data, as well as the catalytic activity results. The catalytic tests were performed under two different conditions: (i) in the presence of H2 in the gas feed and (ii) adding also water to the gas feed. The lowest activity was observed over the Al-containing catalyst under dry feed, which correlates with the TPR results. The addition of water to the feed led to a significant improvement of the NO and CO conversions over all of the samples studied. At 200 °C, Yb-containing gold catalyst exhibited the highest NO and CO conversions. Very promising results for the selectivity toward N2 were achieved using the lanthanides as dopants. In contrast to the gold supported on Al-doped ceria, no NH3 formation was observed within the whole temperature interval up to 400 °C over gold catalysts supported on ceria modified by La, Sm, Gd or Yb. © 2009 Elsevier B.V. All rights reserved.

Fonte de Energia Alternativa, Renovável e Limpa é um tema cada vez mais presente no dia a dia da atual sociedade mundial. Pilhas a combustível surgem como uma opção real para tornar as fontes alternativas um dos principais modos de geração de energia em futuro próximo. Zircônia estabilizada com 8% mol de ítria (ZEI8) é o material mais utilizado como eletrólito em pilha a combustível de óxido sólido (PaCOS), por apresentar condutividade iônica e estabilidade química em atmosferas oxidante e redutora que permitem à pilha apresentar bom rendimento quando operando em torno de 1000ºC. Céria dopada com terras raras tem recebido atenção especial devido a sua elevada condutividade elétrica, o que possibilita uma temperatura de operação de PaCOS entre 500 e 800 °C. Neste trabalho, a condutividade elétrica de céria dopada com ítria e gadolínia é discutida em função das microestruturas obtidas em diferentes condições de sinterização. Resultados de zircônia estabilizada com 8 %mol de ítria, também, são apresentados com o objetivo de usá-los como uma referência. A condutividade elétrica do grão de céria dopada, individualmente, com 10 %mol Y2O3 e 12,5 %mol Gd2O3 é maior que da ZrO2 dopada com 8 %mol Y2O3. Contudo, a condutividade do contorno de grão de céria dopada é muito sensível às condições de sinterização, especificamente, à temperatura e ao tempo de patamar, podendo ser mais do que uma ordem de grandeza inferior à condutividade do grão. A elevada sensibilidade da condutividade do contorno de grão de céria dopada com as condições de sinterização está relacionada com a formação e distribuição de segunda fase no contorno. A formação dessa fase depende do tipo de dopante sendo favorecida pela presença de Y2O3. Clean renewable alternative energy source is a great issue of work nowadays. In a near future fuel cells will be the most promising answer of this issue. 8 mol% Yttria stabilized zirconia (8YSZ) is the most used material as electrolyte because of its ionic conduction and chemical stability under reducing and oxidant atmosphere, which allows that this device shows good efficiency at operating temperatures around 1000 ºC. Rare earth doped ceria has received special attention due to its high electrical conductivity that permits SOFC's to operate into a range of 500 to 800ºC. In this work Y doped ceria and Gd doped ceria electrical conduction are discussed as function of its microstructures obtained in different sintering conditions. 8YSZ results are also presented as reference. The grain electrical conductivity of rare earth doped ceria is bigger than 8YSZ. However, the grain boundary conductivity is mostly affected by the sintering conditions, specially sintering temperature and soaking time. It can be more than 1 order of magnitude smaller than grain conductivity. The grain boundary sensibility is related to the second phase formation along the grain boundary during sintering. The formation of this phase depends on the kind of dopants and it is clearly favored by the presence of Y2O3.

High surface area ceria was doped with lanthanum ions and calcined at 900 K under various partial pressures of oxygen and water vapour. The kinetic rates of crystallite size increase were observed to vary as P − 1 6 O 2 . By addition of 0.5 and 1% of lanthanum ions, the rates were lowered, and then significantly decreased for 10% (cat.). A model of this process involved association between lanthanum ions and oxygen vacancies in order to explain the better stability of doped ceria.

Strontium and magnesium doped ceria solid solutions (Ce 0.99 Sr 0.01 O 1.995 and Ce 0.99 M 0.01 O 1.995 ) were synthesized by a cost effective solid state reaction. The doped and un-doped CeO 2 samples were sintered at 1200 °C, 1300 °C and 1400 °C to investigate the effect of sintering temperature and doping on density, structural and morphological properties. The density was measured by Archimedes’ method. It is observed that the density increases with increasing sintering temperature and with doping of strontium. The crystal structure and surface morphology have been characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD and SEM reveals that the synthesized samples are single phase with a cubic fluorite structure, and the grains formed are of different sizes. The grain size depends on sintering temperature and type of doping. The lattice parameter increases with sintering temperature and substitution of Sr in ceria. The Grain size of Sr-doped ceria decreases, whereas that of Mg-doped ceria increases. EDS spectra show that the samples are free of contaminants. The Ce 0.99 Sr 0.01 O 1.995 shows a more open structure than un-doped ceria.

The effect of solid oxide fuel cell cathode microstructure modification on its electrochemical activity is investigated. Inkjet printing infiltration was used to develop a nano-decoration pattern on the composite cathode scaffolds. Two types of composite La$_{0.6}$Sr$_{0.4}$Co$_{0.2}$Fe$_{0.8}$O$_{3−δ}$:Ce$_{0.9}$Gd$_{0.1}$O$_{1.9}$ cathodes with different volume ratios (60:40 and 40:60 vol%) were fabricated using inkjet printing of suspension inks. The electrodes were altered by single-step inkjet printing infiltration of ethanol-based Ce$_{0.9}$Gd$_{0.1}$O$_{1.9}$ ink. After heat treatments in air at 550 °C the cathodes’ surfaces were shown to be nano-decorated with Ce$_{0.9}$Gd$_{0.1}$O$_{1.9}$ particles (~20–120 nm in size) dispersed uniformly onto the electrode scaffold. The nano-engineered microstructure enhanced the active triple phase boundary of the electrode and promoted the surface exchange reaction of oxygen. Electrochemical impedance tests conducted on symmetrical cells showed a reduction in the polarization resistance of between 1.3 and 2.9 times. The effect was found to be more pronounced in the 60:40 vol% composite cathodes. Ageing of infiltrated electrodes up to 60 h in air revealed enhanced stability of gadolinium doped ceria nanoparticles decorated electrodes ascribed to the suppression of SrO surface segregation. This work demonstrated that single-step inkjet printing infiltration can produce reproducible performance enhancements and thus offers a cost-effective route for commercial solid oxide fuel cell infiltration processing.

Nanometric powders of ceria doped with 10, 15 and 20% at. of samarium were prepared by a sol-gel method. The powders were compacted in cylindrical pellets and sintered at different temperatures in order to obtain grain sizes ranging from 1 to 10 microns. The electrical conductivity was measured in air by electrochemical impedance spectroscopy in the 1166 - 473 K temperature range. The bulk and grain boundary contributions to the overall conductivity were estimated by equivalent circuit analysis and the application of the brick layer model. While bulk conductivity values were markedly independent of the grain size, a relevant decrease in the grain boundary specific conductivity was observed for the finer grain samples in the lower dopant grade. For increased samarium content, this decrease in specific grain boundary conductivity with grain size becomes less relevant.En el presente trabajo de investigación se han obtenido satisfactoriamente, muestras policristalinas de ceria dopada con 10, 15 y 20% at. de samario sintetizadas por el método sol-gel. Cada una de las muestras, previamente calcinadas, fueron compactadas y posteriormente sinterizadas a diferentes temperaturas con el fin de obtener distintos tamaños de grano que, como posteriormente se ha observado, oscilan en un rango de 1 a 10 micrómetros. La conductividad eléctrica de las pastillas densas se midió en atmósfera de aire, mediante la técnica de espectroscopía de impedancia electroquímica en el rango de temperatura de 1166-473 K. Las contribuciones del interior y del límite de grano a la conductividad total se estimaron mediante un análisis/ajuste por circuitos equivalentes aplicando el “brick layer model”. Es interesante señalar que los valores de conductividad del interior del grano son independientes del tamaño del mismo; por otro lado, en las muestras ligeramente dopadas con samario puede observarse una notoria disminución del valor de conductividad específica de límite de grano. No obstante, para mayores contenidos en samario, esta disminución es menos notoria.

Herein, the preferential oxidation of CO in excess hydrogen (PROX reaction) was studied over Au catalysts supported on ceria and Y or Nb doped ceria. Both supports and catalysts have been extensively characterized by a number of advanced techniques; XRD, N2-adsortion, Raman spectroscopy, XPS, and H2-TPR. The catalytic results showed that when an ideal mixture of H2 and CO is used for the PROX reaction the gold supported on pure ceria behaves better than the others samples. However, when a typical reformate gas composition containing CO2 and H2O is used, the gold supported on Nb doped sample behaves better than gold supported in pure ceria. It is suggested that niobium hampers the strong adsorption of CO2 and H2O in the active sites, thus improving the catalytic performance in real reformate gas. Financial support from Generalitat Valenciana and Ministerio de Economía y Competitividad (Spain) through projects PROMETEO/2009/02-FEDER and MAT2010-21147 is gratefully acknowledged. EOJ also thanks the CNPq – Brazil for her grant. EVRF gratefully acknowledge the Ministerio de Economía y Competitividad (Spain) for his Ramon y Cajal grant (RYC-2012-11427).