Effect of Co addition on the lattice parameter, electrical conductivity and sintering of gadolinia-doped ceria

Co-sintering aid has been added to Ce1.9Gd0.1O1.95 (CGO) by treating a commercial powder with Co(NO3)(2) (COCGO), X-ray diffraction (XRD) measurements of lattice parameter indicated that the Co was located on the CGO particle surface after calcination at 650 degreesC. After heat treatment at temperatures above 650 degreesC, the room temperature lattice parameter of CGO was found to increase, indicating redistribution of the Gd. Compared to CGO, the lattice parameter of CGO + 2 cation% Co (2CoCGO) was lower for a given temperature (650-1100 degreesC), A.C. impedance revealed that the lattice conductivity of 2CoCGO was enhanced when densified at lower temperatures, Transmission electron microscopy (TEM) showed that, even after sintering for 4 h at 980 degreesC, most of the Co was located at grain boundaries. (C) 2002 Published by Elsevier Science B.V.

Pilas de combustible soportadas sobre el anodo de una sola camara basadas en electrolitos de ceria dopada con Gd

La utilización de electrolitos soportados en el ánodo es una estrategia muy útil para mejorar las propiedades eléctricas de las pilas de combustible de óxido sólido, debido a que permiten disminuir considerablemente el espesor de los electrolitos. Para este trabajo, se han preparado exitosamente pilas de combustible de óxido sólido con electrolitos de ceria dopada con Gd, Ce1-xGdxO2-y (CGO) soportados sobre un ánodo formado por un cermet de Ni/CGO. Dichas pilas se han instalado y caracterizado en un reactor de una sola cámara donde se ha hecho circular una mezcla de propano y aire. Para ello, se han preparado mezclas de polvos de NiO y de ceria dopada con gadolinia, con diferentes composiciones y tamaño de partículas, para obtener los ánodos con porosidades apropiadas y así utilizarlos como soporte del electrolito en las pilas. Los polvos de los electrolitos de CGO se han preparado por la técnica sol-gel y se han depositado por"dip coating" con diferentes espesores (15-30 ¿m) utilizando una tinta preparada a partir de partículas nanométricas dispersadas en una resina comercial. Los cátodos de La1-xSrxCoO3-S (LSCO) se han preparado también por la técnica sol-gel y se han depositado sobre la capa fina del electrolito. Finalmente, las propiedades eléctricas se han determinado en un reactor de una sola cámara dónde el propano se ha mezclado con aire sintético por encima del límite superior de inflamabilidad. En estas condiciones experimentales se han obtenido altas densidades de potencia estables, controlando las velocidades de flujo total de gas transportador y la presión parcial de propano.

Pilas de combustible de una sola camara, basadas en electrolitos de ceria dopada con gadolinia y operadas con metano y propano

La principal ventaja de las pilas de combustible de óxido sólido (SOFCs) de una sola cámara, frente a las bicamerales convencionales, es que permiten simplificar el diseño del dispositivo y operar con mezclas de hidrocarburos (metano, propano...) y aire, sin necesidad de separar ambos gases, por medio del uso de electrodos selectivos a la oxidación del combustible y reducción del oxidante. En el presente trabajo, se han fabricado monopilas soportadas sobre electrolitos de ceria dopada con gadolinia (GDC), de 200 ¿m de espesor, usando Ni-GDC como ánodo y LSC(La0.5Sr0.5CoO3-¿)-GDC-Ag2O como cátodo. Las propiedades eléctricas de la celda se determinaron en un reactor de una sola cámara, usando mezclas de metano + aire y propano + aire. Se investigó la influencia de la temperatura, de las presiones parciales del combustible y oxígeno, y de la velocidad de flujo total sobre el rendimiento de las pilas preparadas. Así, la densidad de potencia se incrementó fuertemente con el aumento de la temperatura, la velocidad de flujo total y la composición de hidrocarburo. Una vez se optimizaron los parámetros operacionales, la celda presentó unas densidades de potencia máximas de 70 y 320 mW/ cm2, operando con propano a 600ºC y con metano a 795ºC, respectivamente.

Reduced ceria nanofilms from structure prediction

Experimentally, Ce2O3 films are used to study cerium oxide in its fully or partially reduced state, as present in many applications. We have explored the space of low energy Ce2O3 nanofilms using structure prediction and density functional calculations, yielding more than 30 distinct nanofilm structures. First, our results help to rationalize the roles of thermodynamics and kinetics in the preparation of reduced ceria nanofilms with different bulk crystalline structures (e.g. A-type or bixbyite) depending on the support used. Second, we predict a novel, as yet experimentally unresolved, nanofilm which has a structure that does not correspond to any previously reported bulk A2B3 phase and which has an energetic stability between that of A-type and bixbyite. To assist identification and fabrication of this new Ce2O3 nanofilm we calculate some observable properties and propose supports for its epitaxial growth.

Transformation of carbon dioxide to diethyl carbonate over ceria and ceria-supported catalysts

Carbon dioxide is regarded, nowadays, as a primary anthropogenic greenhouse gas leading to global warming. Hence, chemical fixation of CO2 has attracted much attention as a possible way to manufacture useful chemicals. One of the most interesting approaches of CO2 transformations is the synthesis of organic carbonates. Since conventional production technologies of these compounds involve poisonous phosgene and carbon monoxide, there is a need to develop novel synthetic methods that would better match the principles of "Green Chemistry" towards protection of the environment and human health. Over the years, synthesis of dimethyl carbonate was under intensive investigation in the academia and industry. Therefore, this study was entirely directed towards equally important homologue of carbonic esters family namely diethyl carbonate (DEC). Novel synthesis method of DEC starting from ethanol and CO2 over heterogeneous catalysts based on ceria (CeO2) was studied in the batch reactor. However, the plausible drawback of the reaction is thermodynamic limitations. The calculated values revealed that the reaction is exothermic (ΔrHØ298K = ─ 16.6 J/ ) and does not occur spontaneously at rooms temperature (ΔrGØ 298K = 35.85 kJ/mol). Moreover, co-produced water easily shifts the reaction equilibrium towards reactants excluding achievement of high yields of the carbonate. Therefore, in-situ dehydration has been applied using butylene oxide as a chemical water trap. A 9-fold enhancement in the amount of DEC was observed upon introduction of butylene oxide to the reaction media in comparison to the synthetic method without any water removal. This result confirms that reaction equilibrium was shifted in favour of the desired product and thermodynamic boundaries of the reaction were suppressed by using butylene oxide as a water scavenger. In order to obtain insight into the reaction network, the kinetic experiments were performed over commercial cerium oxide. On the basis of the selectivity/conversion profile it could be concluded that the one-pot synthesis of diethyl carbonate from ethanol, CO2 and butylene oxide occurs via a consecutive route involving cyclic carbonate as an intermediate. Since commercial cerium oxide suffers from the deactivation problems already after first reaction cycle, in-house CeO2 was prepared applying room temperature precipitation technique. Variation of the synthesis parameters such as synthesis time, calcination temperature and pH of the reaction solution turned to have considerable influence on the physico-chemical and catalytic properties of CeO2. The increase of the synthesis time resulted in high specific surface area of cerium oxide and catalyst prepared within 50 h exhibited the highest amount of basic sites on its surface. Furthermore, synthesis under pH 11 yielded cerium oxide with the highest specific surface area, 139 m2/g, among all prepared catalysts. Moreover, CeO2─pH11 catalyst demonstrated the best catalytic activity and 2 mmol of DEC was produced at 180 oC and 9 MPa of the final reaction pressure. In addition, ceria-supported onto high specific surface area silicas MCM-41, SBA-15 and silica gel were synthesized and tested for the first time as catalysts in the synthesis of DEC. Deposition of cerium oxide on MCM-41 and SiO2 supports resulted in a substantial increase of the alkalinity of the carrier materials. Hexagonal SBA-15 modified with 20 wt % of ceria exhibited the second highest basicity in the series of supported catalysts. Evaluation of the catalytic activity of ceria-supported catalysts showed that reaction carried out over 20 wt % CeO2-SBA-15 generated the highest amount of DEC.

Acidity/basicity, electron donor properties and catalytic activity of sulphate modified ceria

The changes in surface acidity/basicity and catalytic activity of cerium oxide due to surface modification by sulphate ion have been investigated. Electron donor properties of both the modified and unmodified oxides have been studied using electron acceptors of various electron affinity values, viz. 7,7,8,8-tetracyanoquinodimethane, 2,3,5,6-tetrachloro--l, 4-benzoquinone. p-dinitrobenzene and m-dinitrobenzene in order to find out whether the increase in acidity on suphation is due to the generation of new acidic sites or they are formed at the expense of some of the basic sites. The surface acidity/basicity has been determined using a set of Hammett indicators. The data have been correlated with the catalytic activity of the oxides for esterification of acetic acid using l-butanol, reduction of cyclohexanone with 2- propanol and oxidation of cyclohexanol using benzophenone.

Structural and catalytic investigation of vanadia supported on ceria promoted with high surface area rice husk silica

Rice husk silica was utilized as the promoter of ceria for preparing supported vanadia catalysts. Effect of vanadium content was investigated with 2–10 wt.% V2O5 loading over the support. Structural characterization of the catalysts was done by various techniques like energy dispersive X-ray (EDX), X-ray diffraction (XRD), BET surface area, thermal analysis (TGA/DTA), FT-infrared spectroscopy (FT-IR), UV–vis diffused reflectance spectroscopy (DR UV–vis), electron paramagnetic spectroscopy (EPR) and solid state magnetic resonance spectroscopies (29Si and 51V MASNMR). Catalytic activity was studied towards liquid-phase oxidation of benzene. Surface area of ceria enhanced upon rice husk silica promotion, thus makes dispersion of the active sites of vanadia easier. Highly dispersed vanadia was found for low V2O5 loading and formation of cerium orthovanadate (CeVO4) occurs as the loading increases. Spectroscopic investigation clearly confirms the formation of CeVO4 phase at higher loadings of V2O5. The oxidation activity increases with vanadia loading up to 8 wt.% V2O5, and further increase reduces the conversion rate. Selective formation of phenol can be attributed to the presence of highly dispersed active sites of vanadia over the support.

Influence of surface and acid properties of vanadia supported on ceria promoted with rice husk silica on cyclohexanol decomposition

Cyclohexanol decomposition activity of supported vanadia catalysts is ascribed to the high surface area, total acidity and interaction between supported vanadia and the amorphous support. Among the supported catalysts, the effect of vanadia over various wt% V2O5 (2–10) loading indicates that the catalyst comprising of 6 wt% V2O5 exhibits higher acidity and decomposition activity. Structural characterization of the catalysts has been done by techniques like energy dispersive X-ray analysis, X-ray diffraction and BET surface area. Acidity of the catalysts has been measured by temperature programmed desorption using ammonia as a probe molecule and the results have been correlated with the activity of catalysts.

Vanadia supported on ceria: Characterization and activity in liquid-phase oxidation of ethylbenzene

Vanadia/ceria catalysts (2–10 wt% of V2O5) were prepared by wet impregnation of ammonium metavanadate in oxalic acid solution. Structural characterization was done with energy dispersive X-ray analysis (EDX), powder X-ray diffraction (XRD), BET surface area measurements, FT-IR spectroscopy and nuclear magnetic spectral analysis (51V MASNMR). XRD and 51V MASNMR results show highly dispersed vanadia species at lower loadings and the formation of CeVO4 phase at higher V2O5 loading. The catalytic activity of catalysts was conducted in liquid phase oxidation of ethylbenzene with H2O2 as oxidant. The oxidation activity is increased with loading up to 8 wt% V2O5 and then decreased with further increase in V2O5 content to 10 wt%. Different vanadia species evidenced by various techniques were found to be selective towards ethylbenzene oxidation. The CeVO4 formation associated with increased concentration of vanadia on ceria results the production of acetophenone along with 2-hydroxyacetophenone.

Catalysis by Transition Metal Modified Ceria and Ceria-Zirconia Mixed Oxides Prepared via Sol-Gel Route

The aim of catalysis research is to apply the catalyst successfully in economically important reactions in an environmentally friendly way. The present work focuses on the modification of structural and surface properties of ceria and ceria-zirconia catalysts by the incorporation of transition metals. The applications of these catalysts in industrially important reactions like ethylbenzene oxidation, alkylation of aromatics are also investigated.Sol-gel method is effective for the preparation of transition metal modified ceria and ceria-zirconia mixed oxide since it produces catalyst with highly dispersed incorporated metal. Unlike that of impregnation method plugging of pores is not prominent for sol-gel derived catalyst materials. This prevents loss of surface area on metal modification as evident for BET surface area measurements.The powder X-ray diffraction analysis confirms the cubic structure of transition metal modified ceria and ceria-zirconia catalysts. The thermal stability is evident from TGA/DTA analysis. DR UV-vis spectra provide information on the coordination environment of the incorporated metal. EPR analysis ofCr, Mn and Cu modified ceria and a ceria-zirconia catalyst reveals the presence of different oxidation states of incorporated metal.Temperature programmed desorption of ammonia and thermogravimetric desorption of 2,6-dimethyl pyridine confirms the enhancement of acidity on metal incorporation. High a-methyl styrene selectivity in cumene cracking reaction implies the presence of comparatively more number of Lewis acid sites with some amount of Bronsted acid sites. The formation of cyclohexanone during cyclohexanol decomposition confirms the presence of basic sites on the catalyst surface.Mn and Cr modified catalysts show better activity towards ethylbenzene oxidation. A redox mechanism through oxometal pathway is suggested.All the catalysts were found to be active towards benzylation of toluene and a-xylene. The selectivity towards monoalkylated products remains almost 100%. The catalytic activity is correlated with the Lewis acidity of the prepared systems.The activity of the catalysts towards methylation of phenols depends on the strength acid sites as well as the redox properties of the catalysts. A strong dependence of methylation activity on the total acidity is illustrated.

“Catalysis by Nanocrystalline Ceria Modified with Transition Metals”

The objective of the present work is to improve the textural and structural properties of cerium oxide by the incorporation of transition metals as well as sulphate ions. We have incorporated tungsten, molybdenum and chromium oxide into pure as well as sulphated cerium oxide and the catalytic systems thus prepared were characterised using various techniques. lndustrially important reactions such as acetalization and deacetalization, oxidative dehydrogenation of ethylbenzene, MTBE synthesis and Beckmann rearrangement of cinnamaldoxime and salicylaldoxime have been selected for the measurement of the catalytic activity of the systems. The work is presented in eight chapters

Synthesis, Characterization and Catalytic Activity of Nanocrystalline Ceria Modified With Zirconia

The use of catalysts in chemical and refining processes has increased rapidly since 1945, when oil began to replace coal as the most important industrial raw material. Catalysis has a major impact on the quality of human life as well as economic development. The demand for catalysts is still increasing since catalysis is looked up as a solution to eliminate or replace polluting processes. Metal oxides represent one of the most important and widely employed classes of solid catalysts. Much effort has been spent in the preparation, characterization and application of metal oxides. Recently, great interest has been devoted to the cerium dioxide (CeO2) containing materials due to their broad range of applications in various fields, ranging from catalysis to ceramics, fuel cell technologies, gas sensors, solid state electrolytes, ceramic biomaterials, etc., in addition to the classical application of CeO2 as an additive in the so-called three way catalysts (TWC) for automotive exhaust treatment. Moreover, it can promote water gas shift and steam reforming reactions, favours catalytic activity at the interfacial metal-support sites. The solid solutions of ceria with Group IV transitional-metals deserve particular attention for their applicability in various technologically important catalytic processes. Mesoporous CeO2−ZrO2 solid solutions have been reported to be employed in various reactions which include CO oxidation, soot oxidation, water-gas shift reaction, and so on. Inspired by the unique and promising characteristics of ceria based mixed oxides and solid solutions for various applications, we have selected ceria-zirconia oxides for our studies. The focus of the work is the synthesis and investigation of the structural and catalytic properties of modified and pure ceria-zirconia mixed oxide.

Comparison of new microemulsion prepared "Pt-in-Ceria" catalyst with conventional "Pt-on-Ceria" catalyst for water-gas shift reaction

New "Pt-in-CeO2" catalyst prepared by microemulsion method is shown to give higher activity for a water-gas shift reaction but with no formation of CH4, the side product from hydrogenation of carbon oxides using a hydrogen-rich reformate as compared to conventional "Pt-on-CeO2" catalysts. Detailed characterization by DRIFT analysis and temperature programmed reduction presented in this work clearly suggest the ceria coverage on Pt inhibits the metal from forming a strong CO adsorption.

Engineering Pt in ceria for a maximum metal-support interaction in catalysis

Conventional supported metal catalysts are metal nanoparticles deposited on high surface area oxide supports with a poorly defined metal−support interface. Typically, the traditionally prepared Pt/ceria catalyzes both methanation (H2/CO to CH4) and water−gas shift (CO/H2O to CO2/H2) reactions. By using simple nanochemistry techniques, we show for the first time that Pt or PtAu metal can be created inside each CeO2 particle with tailored dimensions. The encapsulated metal is shown to interact with the thin CeO2 overlayer in each single particle in an optimum geometry to create a unique interface, giving high activity and excellent selectivity for the water−gas shift reaction, but is totally inert for methanation. Thus, this work clearly demonstrates the significance of nanoengineering of a single catalyst particle by a bottom-up construction approach in modern catalyst design which could enable exploitation of catalyst site differentiation, leading to new catalytic properties.