Microondas para aplicaciones tecnológicas

Cuadernillo didáctico y de divulgación científica presentado en la Semana de la Ciencia y la Tecnología.Peer reviewe

Simultaneous adjustment of the main chemical variables to fine-tune the porosity of carbon xerogels

The objective of this study was to assess the effect of the resorcinol-formaldehyde molar ratio of precursor solutions with pH values ranging from 5.0 to 7.0 and dilution ratios between 5 and 10 upon the final porous properties of carbon xerogels synthesized by microwave heating, using an optimal design of response surface methodology. It was found that when the resorcinol-formaldehyde molar ratio was decreased the microporosity was enhanced as the addition reaction was favored. Hence, the micropore volume of carbon xerogels not only depends on the conditions used during carbonization, but also on the synthesis conditions. Mesoporosity was also increased when the resorcinol-formaldehyde molar ratio decreased while macroporosity was only influenced by the initial pH of the precursor solutions. The simultaneous variations of the three chemical variables allowed an exhaustive control of the final porous properties of carbon xerogels, which exhibited pore sizes and pore volumes that could not be obtained by modifying just the pH and the dilution ratio. © 2014 Elsevier Ltd. All rights reserved.Financial support from the Ministerio de Economía y Competitividad of Spain MINECO (under Projects MAT2011-23733 and IPT-2012-0689-420000) is greatly acknowledged. NRR is also grateful to MINECO for her predoctoral research grant.Peer Reviewe

Optimization of the process variables in the microwave-induced synthesis of carbon xerogels

Carbon xerogels (CX) can be synthesized by microwave-assisted heating. The transfer of this technology to an industrial scale passes through the optimization of the variables that affect the process. The effect of the main operational variables, i.e., initial volume of the precursor, gelation and ageing time and temperature of the synthesis, on the final porous properties of CX has been evaluated. It was found that the development of porosity in the CX synthesised in the microwave oven is hardly influenced by the increase in the initial volume of the precursor solution. This suggests that it is feasible to scale up the production of these materials by means of microwave heating. Furthermore, the consumption of energy does not increase in proportion to the volume of xerogel synthesized. Thus, the process is energy efficient, saves a considerable amount of time and requires only a single device to carry it out. These advantages, along with the fact that a temperature variation of 10 C is not determinative in the xerogels' final properties, indicate that CX could be produced on a large scale in a cost effective way. © 2013 Springer Science+Business Media New York.Financial support from the Ministerio de Economía y Competitividad of Spain MINECO (under Projects MAT2011-23733 and IPT-2012-0689-420000) is greatly acknowledged. NRR is also grateful to MINECO for her predoctoral research grant.Peer Reviewe

A visual validation of the combined effect of pH and dilution on the porosity of carbon xerogels

The hypothesis about the formation of the porous structure of carbon xerogels when the pH and the dilution ratio of the precursor solution are simultaneously modified has been validated by means of scanning electron microscope analysis. The morphology of the carbon xerogels showed that the size and number of the clusters produced during the sol–gel reaction is not only influenced by the pH but also by the dilution ratio. As with a decrease in the pH value, an increase in the dilution ratio caused the formation of a small number of large clusters, leading to materials with large pores. However, depending on the values selected the effect of the pH and dilution ratio was enhanced or diminished by the effect of the other. Hence, an appropriate adjustment of these two variables allowed microporous, micro-mesoporous and micro-macroporous materials to be obtained with pore volumes that could not be achieved by modifying the pH or the dilution ratio separately.Financial support from the Ministerio de Economía y Competitividad of Spain MINECO (under Projects MAT2011-23733 and IPT-2012-0689-420000, (BES-2012-052676)) is greatly acknowledged. NRR is also grateful to MINECO for her predoctoral research grant.Peer reviewe

Hybrid direct carbon fuel cell anode processes investigated using a 3-electrode half-cell setup

A 3-electrode half-cell setup consisting of a yttria-stabilized zirconia (YSZ) electrolyte support was employed to investigate the chemical and electrochemical processes occurring in the vicinity of a model hybrid direct carbon fuel cell (HDCFC) anode (Ni-YSZ) in contact with a molten carbon-alkali carbonate slurry. Electrochemical testing, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), with and without the Ni-YSZ layer highlighted the promotional effect of the Ni-YSZ anode layer, and revealed the contributions of Ni/NiO, and potentially K/K2O, redox couple(s). Treated anthracite and bituminous coals, as well as carbon black, were tested, revealing similar open circuit potential and activation energies in mixed 96–4 vol% N2–CO2 and 50–50 vol% CO–CO2 environments between 700 and 800 °C. Bituminous coal showed the highest activity, likely associated to a high O/C ratio and hydrogen content. Based on acquired data, a reaction scheme was proposed for processes at the working electrode, including the role of bubble formation in the vicinity of the electrochemically active solid/molten medium interface.This work was funded in part by the European Commission Research Fund for Coal and Steel, as the Efficient Conversion of Coal to Electricity – Direct Coal Fuel Cells project (RFC-PR-10007), in collaboration with the University of St. Andrews, University of Western Macedonia, and the Spanish Instituto Nacional del Carbón (INCAR). Additional funding was supplied by the Department of Energy Conversion and Storage at the Danish Technical University (DTU)-Risoe Campus. We extend our thanks to M. Nielsen, A. Petersen and F. Vico, as well as to Drs. C. Graves, P. Holtappels, D. Ippolito, M. Mogensen, and S. Veltzé at the DTU Department of Energy Conversion and Storage for all assistance.Peer reviewe

Comparison between the reactivity of coal and synthetic coal models

A mixture of carbon compounds was pyrolysed under an inert atmosphere at different temperatures in a fixed bed reactor. The resultant chars were characterised in terms of texture and thermal behaviour. Textural characterisation of the chars was carried out by N2 and CO2 adsorption isotherms at −196 and 0 °C, respectively. Char isothermal reactivity in air at 500 °C, and in CO2 at 1000 °C, was performed in a thermogravimetric analyser (TGA). Temperature-programmed combustion tests under 20% oxygen in argon were also performed in the TGA linked to a mass spectrometer (TGA/MS). The results showed that char textural properties do not always relate well to their reactivity. Not only do physical properties (e.g. surface area, porosity) but also chemical properties (e.g. active sites concentration and distribution) play an important role in the reaction of carbonaceous material and oxidant. On the other hand, in terms of chemical composition the chars obtained from the mixture of carbon compounds were very similar to the chars produced under the same experimental conditions by a high volatile bituminous coal. The fact that carbon compounds are well known makes it easier to obtain knowledge about the functional groups present in synthetic char, and to study the mechanisms of heterogeneous reactions such as the reduction of NO with carbon.Peer reviewe

Advances in tailoring the porosity of tannin-based carbon xerogels

Usually, carbon xerogels are obtained from resorcinol–formaldehyde organic gels. However, more cost-effective and eco-friendly carbon xerogels can be synthesised by using tannins instead of resorcinol, provided that a suitable surfactant is added to prevent the collapse of the structure. The use of tannin, a natural phenolic compound derived from wood, allows obtaining carbon xerogels with controlled porosity, as the porous properties of these materials can be tailored by an appropriate choice of the synthesis conditions. In this work, tannin–formaldehyde xerogels containing different amounts of surfactant and formaldehyde were synthesised in order to evaluate their effect on the porous structure and chemical composition. It was found that porosity and density depend greatly on the amount of surfactant. The lowest density and highest porosity values −0.34 g/cm3 and 78%, respectively-were obtained by adding 10 wt.% of surfactant. It was also found that S-doped carbon xerogels can be easily synthesized due to the strong affinity between the carbon in the structure and the sulphur from the surfactant. Furthermore, statistical analysis showed that there is interdependence between the effect of formaldehyde and the surfactant, especially in the case of volume and pore size. Hence, the choice of the appropriate surfactant-formaldehyde concentration is essential for controlling the formation of the porous polymeric structure.Financial support from the Ministerio de Economía y Competitividad of Spain MINECO (under Projects MAT2011-23733, IPT-2012-0689-420000 and CTQ2013-49433-EXP) is greatly acknowledged. NRR is also grateful to MINECO for her predoctoral research grant. The French authors also gratefully acknowledge the financial support of the CPER 2007–2013 “Structuration du Pô le de Compé titivité Fibres Grand'Est” (Competitiveness Fibre Cluster), through local (Conseil Général des Vosges), regional (Région Lorraine), national (DRRT and FNADT) and European (FEDER) funds.Peer reviewe

CO2 capture by adsorption with nitrogen enriched carbons

9 pages, 7 figures, 3 tables.-- Printed version published Sep 2007.-- Issue title: The 6th European Conference on Coal Research and its Applications.The success of CO2 capture with solid sorbents is dependent on the development of a low cost sorbent with high CO2 selectivity and adsorption capacity. Immobilised amines are expected to offer the benefits of liquid amines in the typical absorption process, with the added advantages that solids are easy to handle and that they do not give rise to corrosion problems. In this work, different alkylamines were evaluated as a potential source of basic sites for CO2 capture, and a commercial activated carbon was used as a preliminary support in order to study the effect of the impregnation. The amine coating increased the basicity and nitrogen content of the carbon. However, it drastically reduced the microporous volume of the activated carbon, which is chiefly responsible for CO2 physisorption, thus decreasing the capacity of raw carbon at room temperature.This work was carried out with financial support from the Spanish MEC (Project CTM2005-03075/TECNO). M.G.P. and C.P. acknowledge the support from the CSIC I3P Program co-financed by the European Social Fund.Peer reviewe

Towards a feasible and scalable production of bio-xerogels

© 2015 Elsevier Inc. Hypothesis: The synthesis process of carbon xerogels is limited, mainly due to two drawbacks that prevent their introduction onto the market: (i) the long time required for producing the material and (ii) the reagents used for the synthesis, which are costly and harmful to the environment. Microwave radiation is expected to produce a reduction in time of more than 90%, while the use of tannin instead of resorcinol will probably result in a cost-effective carbonaceous material. Experiments: Resorcinol-tannin-formaldehyde xerogels containing different amounts of tannin, either with or without a surfactant (sodium dodecyl sulphate), were synthesized by means of two different heating methods: conventional and microwave heating. The effects of the surfactant, the heating method and the addition of tannin upon the porous structure and the chemical composition of the final materials were evaluated. Findings: It was found that the addition of surfactant is essential for obtaining highly porous xerogels when using tannins. The heating method also plays an important role, as conventionally synthesized samples display a greater volume of large pores. However, tannins are less sensitive to microwave radiation and their use results in tannin-formaldehyde xerogels that have a porous structure and chemical composition similar to those of resorcinol-formaldehyde xerogels.Financial support from the Ministerio de Economía y Competitividad of Spain MINECO (under Projects MAT2011-23733, IPT-2012-0689-420000 and CTQ2013-49433-EXP) is greatly acknowledged. NRR is also grateful to MINECO for her predoctoral research grant. The French authors also gratefully acknowledge the financial support of the CPER 2007–2013 “Structuration du Pôle de Compétitivité Fibres Grand’Est” (Competitiveness Fibre Cluster), through local (Conseil Général des Vosges), regional (Région Lorraine), national (DRRT and FNADT) and European (FEDER) funds.Peer Reviewe

Effect of fuel thermal pretreament on the electrochemical performance of a direct lignite coal fuel cell

Proceedings of the 20th International Conference on Solid State Ionics SSI-20The impact of fuel heat pretreatment on the performance of a direct carbon fuel cell (DCFC) is investigated by utilizing lignite (LG) coal as feedstock in a solid oxide fuel cell of the type: lignite | Co–CeO2/YSZ/Ag | air. Four LG samples are employed as feedstock: (i) pristine lignite (LG), and differently heat treated LG samples under inert (He) atmosphere at (ii) 200 °C overnight (LG200), (iii) 500 °C for 1 h (LG500) and (iv) 800 °C for 1 h (LG800). The impact of several process parameters, related to cell temperature (700–800 °C), carrier gas type (He or CO2), and molten carbonate infusion into the feedstock on the DCFC performance is additionally explored. The proximate and ultimate analysis of the original and pretreated lignite samples show that upon increasing the heat treatment temperature the carbon content is monotonically increased, whereas the volatile matter, moisture, sulfur and oxygen contents are decreased. In addition, although volatiles are eliminated upon increasing the treatment temperature and as a consequence more ordered carbonaceous structure remained, the heat treatment increases the reactivity of lignite with CO2 due mainly to the increased carbon content. These modifications are reflected on the achieved DCFC performance, which is clearly improved upon increasing the treatment temperature. An inferior cell performance is demonstrated by utilizing inert He instead of reactive CO2 atmosphere, as purging gas in the anode compartment, while carbonate infusion always results in ca. 70–100% increase in power output (15.1 mW cm− 2 at 800 °C). The obtained findings are discussed based also on AC impedance spectroscopy measurements, which revealed the impact of LG physicochemical characteristics and DCFC operating parameters on both ohmic and electrode resistances.The authors would like to acknowledge financial support from the European project “Efficient Conversion of Coal to Electricity — Direct Coal Fuel Cells”, which is funded by the Research Fund for Carbon & Steel (RFCR CT-2011-00004).Peer reviewe