Xerogeles de carbono como soporte de elecrocatalizadores para pilas de combustible

Los xerogeles de carbono (CXGs) son materiales porosos con una estructura reticular formada por partículas interconectadas compuestas principalmente de carbono, de forma esferoidal y tamaño nanométrico. La versatilidad de sus propiedades en cuanto a estructura, forma y textura favorece su aplicación en muy diversos campos: catálisis, conversión y almacenamiento de energía, etc. El trabajo presentado en esta memoria describe el estudio de la influencia de condiciones de síntesis de los CXGs en sus propiedades. Se analiza el efecto de la composición de la mezcla de precursores del gel de carbono y el pH sobre la estructura porosa, el ordenamiento del carbono, la química superficial así como la conductividad eléctrica. Determinadas condiciones favorecen la obtención de CXGs con una elevada porosidad y un elevado grado de entrecruzamiento. Se ha modificado la composición de los CXG con distintos heteroátomos: O, N y S, mediante oxidación, dopado y vulcanización, respectivamente. Dichos tratamientos han permitido enlazar fuertemente estos heteroátomos a la superficie del carbón, modificando propiedades como su grado de ordenación, química superficial, conductividad eléctrica y porosidad. El estudio se centra posteriormente en el uso de los CXG como soporte de electro-catalizadores basados en Pt para pilas de combustible de metanol directo. Este tipo de dispositivos convierten eficientemente la energía química de un combustible directamente en energía eléctrica. Las características del soporte determinan las propiedades de los electro-catalizadores. El uso de CXGs con elevada porosidad como soporte electro-catalítico resulta ventajoso debido a la mejora de la difusión de reactivos y productos tanto para la reacción de oxidación de metanol como para la reacción de reducción de oxígeno (ORR). La modificación de la composición de los CXG, particularmente cuando se introduce S y N en el CXG aumenta la actividad electrocatalítica particularmente para la ORR. Tanto los CXGs como los catalizadores se han sometido a procesos de degradación acelerada, observándose que es preferible emplear CXGs con porosidades intermedias para obtener catalizadores más resistentes a la degradación, a costa de una ligera menor actividad

Catalizadores para pilas de combustible de alcohol directo

El hidrógeno es pronosticado por muchos expertos como el portador de energía del futuro, debido principalmente a que es renovable y prácticamente no contaminante, en comparación con otros portadores de energía como la gasolina o el diesel. La producción actual de transportadores de hidrógeno utiliza combustibles fósiles como el gas natural, aunque los portadores de hidrógeno pueden ser producidos a partir de biomasa sin alterar el balance de CO2. Los portadores líquidos de hidrógeno se pueden introducir y almacenar con mucha más facilidad que el hidrógeno gas, y pueden incluso hasta cierto punto, utilizar la infraestructura existente. Compuestos orgánicos como el metanol o el ácido fórmico, presentan la estructura más simple de todos los combustibles orgánicos posibles y por lo tanto deberían tener los mecanismos de reacción más directa. Sin embargo, la baja densidad de intercambio vigente en la mayoría de los electrodos hace que sean mucho menos activos que el hidrógeno y hace necesaria la utilización de un catalizador de metal precioso como el platino para obtener rendimientos razonables de reacción. Por desgracia, los catalizadores de platino son fácilmente envenenados por los intermediarios de reacción de la oxidación de combustibles orgánicos, lo que limita significativamente su rendimiento. Uno de los objetivos dentro del campo de investigación de las pilas de combustible es reducir la cantidad de metal utilizado en el electrocatalizador, para así reducir el coste de la pila. Para lograr este objetivo se propone el uso de nuevos materiales de carbono con propiedades texturales y química superficial controlables como soporte de los electrocatalizadores. El catalizador más utilizado en este tipo de pilas está basado en platino soportado sobre negros de carbono, siendo el Vulcan XC-72(R) el soporte más utilizado. La optimización de las propiedades de los soportes carbonosos es muy importante en el desarrollo de este tipo de pilas de combustible, debido a que las misma influyen en distintas propiedades de los catalizadores como el tamaño de partícula, la morfología, la estabilidad y la dispersión. En este contexto, la investigación llevada a cabo en la presente tesis doctoral se ha basado en la síntesis y la optimización de las propiedades de las nanoespirales de carbono con el fin de maximizar la utilización del metal y la actividad del catalizador. Dichos materiales se han sometido a diferentes tratamientos de oxidación para modificar su química superficial y su morfología y así, determinar su influencia sobre las propiedades de los catalizadores. Posteriormente, se depositaron metales y aleaciones (Pt, Pd, Pt-Ru y nanopartículas de Au-Pd con estructura core-shell) a través de diferentes rutas químicas sobre los materiales carbonosos. Se evaluaron las propiedades estructurales de los electrocatalizadores sintetizados, en comparación con materiales comercialmente disponibles. Se investigó la función oxidativa de los diversos electrocatalizadores para metanol, etanol y ácido fórmico. También se estudió la oxidación de monóxido de carbono, ya que el mismo es conocido por ser un veneno cuando es producido como intermedio de reacción en sistemas de pilas de combustible

Palladium-based catalysts as electrodes for direct methanol fuel cells: a last ten years review

Platinum-based materials are accepted as the suitable electrocatalysts for anodes and cathodes in direct methanol fuel cells (DMFCs). Nonetheless, the increased demand and scarce world reserves of Pt, as well as some technical problems associated with its use, have motivated a wide research focused to design Pd-based catalysts, considering the similar properties between this metal and Pt. In this review, we present the most recent advancements about Pd-based catalysts, considering Pd, Pd alloys with different transition metals and non-carbon supported nanoparticles, as possible electrodes in DMFCs. In the case of the anode, different reported works have highlighted the capacity of these new materials for overcoming the CO poisoning and promote the oxidation of other intermediates generated during the methanol oxidation. Regarding the cathode, the studies have showed more positive onset potentials, as fundamental parameter for determining the mechanism of the oxygen reduction reaction (ORR) and thus, making them able for achieving high efficiencies, with less production of hydrogen peroxide as collateral product. This revision suggests that it is possible to replace the conventional Pt catalysts by Pd-based materials, although several efforts must be made in order to improve their performance in DMFCs.The authors want to thank the Spanish Ministry of Economy and Competitiveness and FEDER for financial support under the projects ENE2014-52518-C2-1-R. We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI)

Catalytic decomposition of biogas to produce H2-rich fuel gas and carbon nanofibers. Parametric study and characterization

El pdf del artículo es la versión post-print.- Available online December 2, 2011One of the main problems that our society must deal with in a near future is the progressive substitution of traditional fossil fuels by different energy sources, such as renewable energies. In this context, biogas will play a vital role in the future. Nowadays, one of the most important uses of biogas is the production of heat and electricity from its direct combustion in co-generation plants. An interesting alternative consists on its direct valorisation to produce a syn-gas that can be further processed to produce chemicals, liquid fuels, or hydrogen. Results showed in this work evidenced that catalytic decomposition of biogas (CH4/CO2 mixtures) can be carried out with a Ni/Al2O3 catalyst obtaining simultaneously a syn-gas with high H2 content together with carbonaceous nanostructured materials with high added value. The parametric study revealed that temperature, WHSV (Weight Hourly Space Velocity, defined here as the total flow rate at normal conditions per gram of catalyst initially loaded) and CH4:CO2 feed ratio influence directly in CH4 and CO2 conversion, H2:CO ratio and carbon generation (gC/gcat). It was also evidenced that carbon structure depends on temperature. At 600ºC, fishbone like nanofibers with no hollow core are obtained while at 700ºC a mixture of fishbone and ribbon like nanofibers with a clear hollow core are formed.The authors acknowledge the Spanish Science and Innovation Ministry for the financial support of the Project ENE2008-06516.Peer reviewe

Carbon nanofibres coated with Ni decorated MoS2 nanosheets as catalyst for vacuum residue hydroprocessing

9 págs.,7 figures, 3 tablesCatalysts based on functionalised carbon nanofibers (FCNF) coated with Ni-decorated MoS2 nanosheets were obtained by direct decomposition of ammonium thiomolybdate and nickel nitrate impregnated on the FCNF under controlled temperature conditions in inert atmosphere. The catalysts were characterised by X-ray diffraction (XRD), N2 adsorption, Raman spectroscopy, temperature programmed reduction of sulfur species (TPR-S), NH3 temperature programmed desorption (NH3-TPD) and transmission electron microscopy (TEM). Decomposition temperature was found to have a paramount importance in the formation of uniform MoS2 slabs, as revealed by the TEM study: at 600°C, non-uniform covering of the carbon nanofiber (CNF) was observed together with the presence of small round-shaped metal particles (ca. 20nm). On the other hand, at 450°C CNF appeared homogeneously covered by amorphous MoS2 slabs decorated with Ni, resulting in higher amount of coordinated unsaturated sites (CUS), as determined by TPR-S. Catalysts were tested in the hydroprocessing of a vacuum residue and the results were compared against a benchmark alumina-supported NiMo catalyst. Higher asphaltene conversions were obtained for the CNF-supported catalysts prepared at 450°C, which overperformed the Al2O3-supported benchmark catalyst. However, the catalytic performance in hydrodesulfurisation and hydrodemetallisation of the CNF-based catalysts was slightly lower than that of the benchmark catalyst.J.L.P. thanks the Spanish MEC for a personal grant (Spanish Scientists Mobility Program, ref. EX2009-0822). H.P., D.T and S.d.LL. thank CSIC for the funding of the short stays at ICB-CSIC or Imperial College (project I-LINK ref.0439). H.P. and S.d.LL. thank the support of CONACYT Mexico and DGA (Spain), respectively, for the award of their PhD grant. DT thanks the support of FEDER and Spanish Ministery of Economy and Competitiveness for the award of his PhD grant under the frame of the research project ENE2011-28318-C03-01.Peer Reviewe

Catalizadores de hierro para la producción simultánea de hidrógeno y nanofilamentos de carbono mediante descomposición catalítica de metano

Catalizadores de hierro para la producción simultánea de hidrógeno y nanofilamentos de carbono mediante descomposición catalítica de metano. Catalizador de hierro para la producción de hidrógeno y nanofliamentos (nanofibras/nanotubos) de carbono mediante descomposición catalítica de metano (DCM). El catalizador es de fórmula Fe:X, donde X representa un óxido de aluminio o de magnesio.Peer reviewedConsejo Superior de Investigaciones Científicas (España)A1 Solicitud de patente con informe sobre el estado de la técnic

Optimizing the synthesis of carbon nanofiber based electrocatalysts for fuel cells

7 páginas, 2 tablas, 7 figuras.This work deals with an optimization of the platinum dispersion on low surface area carbon nanofibers (CNFs) by using different synthesis procedures and its electrocatalytic activity towards oxygen reduction. The selected CNFs were characterized by a BET surface area of ca. 100 m2 g-1 and were in-house synthesized by the decomposition of CH4 at 700ºC. Pt nanoparticles were deposited by using four different synthesis routes. A metal concentration of 20 wt% was confirmed by EDX and TGA. Two classical impregnation routes were employed, one using NaBH4 as reducing agent at 15ºC and the second one using formic acid at 80ºC. Two alternative processes consisted in a microemulsion procedure followed by reduction with NaBH4 and a colloidal route by using the sulphite complex method followed by reduction with hydrogen. The main differences regarded the platinum crystal size varying from 2.5 nm for the colloidal route to 8.1 nm for the impregnation route (formic acid). The classical impregnation procedures did not result appropriate to obtain a small particle size in the presence of this support, whereas microemulsion and colloidal methods fit the requirements for the cathodic oxygen reduction reaction in polymer electrolyte fuel cells, despite the low surface area of CNFs. The catalysts were subjected to an accelerated degradation test by continuous potential cycling. Although the initial activity was the highest for the microemulsion based catalyst, after the accelerated degradation test the colloidal based catalyst experienced a relatively lower loss of performance.The authors wish to thank FEDER and the Spanish MEC for financial support to project CTQ2011-28913-C02-01. The authors also acknowledge the support of bilateral CNR (Italy) -CSIC (Spain) joint agreement 2011-2012 (project Baglio/Lazaro 2010IT0026).Peer reviewe

Oxidation of CO and methanol on Pd-Ni catalysts supported on different chemically-treated carbon nanofibers

In this work, palladium-nickel nanoparticles supported on carbon nanofibers were synthesized, with metal contents close to 25 wt % and Pd:Ni atomic ratios near to 1:2. These catalysts were previously studied in order to determine their activity toward the oxygen reduction reaction. Before the deposition of metals, the carbon nanofibers were chemically treated in order to generate oxygen and nitrogen groups on their surface. Transmission electron microscopy analysis (TEM) images revealed particle diameters between 3 and 4 nm, overcoming the sizes observed for the nanoparticles supported on carbon black (catalyst Pd-Ni CB 1:2). From the CO oxidation at different temperatures, the activation energy Eact for this reaction was determined. These values indicated a high tolerance of the catalysts toward the CO poisoning, especially in the case of the catalysts supported on the non-chemically treated carbon nanofibers. On the other hand, apparent activation energy Eap for the methanol oxidation was also determined finding—as a rate determining step—the COads diffusion to the OHads for the catalysts supported on carbon nanofibers. The results here presented showed that the surface functional groups only play a role in the obtaining of lower particle sizes, which is an important factor in the obtaining of low CO oxidation activation energies.The authors want to thank the Spanish Ministry of Economy and Competitiveness and FEDER for financial support under the projects ENE2014-52518-C2-1-R and CTQ2011-28913-C02-01. We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI)

Preparation of polymer composites using nanostructured carbon produced at large scale by catalytic decomposition of methane

7 págs, 7 figures, 1 Table.Polymer-based composites were prepared using different concentrations of nanostructured carbons (NCs), produced by catalytic decomposition of methane (CDM). Four carbonaceous nanostructures were produced using different catalysts (with Ni and Fe as active phases) in a rotary bed reactor capable of producing up to 20 g of carbon per hour. The effect of nanostructured carbon on the thermal and electrical behaviour of epoxy-based composites is studied. An increase in the thermal stability and the decrease of electrical resistivity were observed for the composites at carbon contents as low as 1 wt%. The highest reduction of the electrical resistivity was obtained using multi-walled carbon nanotubes obtained with the Fe based catalysts. This effect could be related to the high degree of structural order of these materials. The results were compared with those obtained using a commercial carbon nanofibre, showing that the use of carbon nanostructures from CDM can be a valid alternative to the commercial nanofibres.The authors acknowledge the Spanish Science and Innovation Ministry for the financial support of Project ENE2008-06516-C03-01 Spanish Economy and Competitiveness Ministry under project ENE2011-28318-C03-01.Peer Reviewe

Oportunidades de investigación en dispositivos de almacenamiento y generación de electricidad para el vehículo híbrido enchufable

47 diapositivas.-- Presentación de diapositivas en la X Reunión del Grupo Español del Carbón (GEC), Gerona (España), del 9-12 de mayo de 2010.Introducción: Descripción Vehículo Híbrido Enchufable.-- Dispositivos de Almacenamiento de Energía: Baterías Ión-Lítio -– Supercondensadores.-- Dispositivos de Generación de Energía Eléctrica: Pilas de Combustible PEM/H2 -- Almacenamiento de Hidrógeno -- Producción de Hidrógen