Dr. José Angel Menéndez Díaz was born in Lugones, Asturias, Spain. He graduated from University of Oviedo, Spain, where received his M.Sc. in Chemistry and Ph.D. in Chemical Engineering in 1988 and 1994, respectively. He worked as research assistant at the Penn State University, USA, from 1995 to 1996. In 1997, he joined INCAR-CSIC, Spain, where he is currently working as a scientific researcher. His research activity is mainly focused in carbon materials and the use of microwave heating applied to carbon-related technological processes, leading various research projects on these fields. He is author and co-author of more than 100 scientific publications including various book chapters and patents. Member of the executive committee of the Spanish Carbon Group (GEC) from 2003 to 2011. Editor of the GEC bulletin. Cofounder of Xerolutions . Google Scholar citations
Abstract: Vivimos en una sociedad que demanda cantidades importantes de agua para su uso en numerosas actividades, por lo que la extracción de caudales de su ámbito natural de circulación es algo natural. En este libro se analizan los nuevos tratamientos y tecnologÃas que actualmente se llevan a cabo para el tratamiento en el ciclo del agua.
Abstract: The electrochemical behavior of carbon xerogels was studied with the aim of analyzing the performance of the materials used as electrochemical supercapacitors (SC) and to relate with physicochemical parameters. These materials have areas involving 1500e2000 m2/g measured with the BET equation and a range of pore size distributions. Conventional electrochemical techniques were used as cyclic voltammetry (CV), which allowed electrochemical characterization of different materials, and chronopotentiometry (CD), in order to determine the charge storage capacity of the xerogel at different currents or discharge rates Experimental results using electrochemical impedance spectroscopy (EIS) were interpreted with a physicochemical model that permitted identifying different parameters of the electrode, which explain the differences in the behavior of materials.
Abstract: The electrochemical behavior of carbon xerogel/multiwalled carbon nanotubes composite in a 6 M KOH solution has been investigated. Three different ixtures of teflonized carbons with varying nanotube content were prepared. The electrodes containing multiwalled carbon nanotubes were found to provide enhanced capacities compared with those prepared with only carbon xerogel. Cyclic voltammetry and chargeâdischarge experiments reveal the presence of a strong resistive component, which decreases as the amount of nanotubes increases. Electrochemical impedance spectroscopy results analyzed in terms of an adequate physicochemical model of the porous electrode, show that an increasing amount of nanotubes enhances both the effective solid-phase conductivity and the effective liquid-phase conductivity, linked to the porosity of the electrodes.
Abstract: The CO2 reforming of coke oven gases (COG) was carried out over a Ni/γAl2O3 catalyst in order to obtain a suitable syngas for methanol synthesis. The influence of different operating conditions, such as temperature and volumetric hourly space velocity (VHSV), was studied. It was found that the H2 present in the feed gas promotes the Reverse Water Gas Shift reaction (RWGS), which produces water. Nevertheless, the Ni/γAl2O3 catalyst showed a high selectivity to the CO2 reforming reaction and it was possible to avoid the RWGS under certain operating conditions. Moreover, a part of the reaction could take place via a different path (RWGS followed by the steam reforming of methane instead of the direct dry reforming of methane). The deactivation of the Ni/γAl2O3 catalyst was also studied. Both the methane and the carbon dioxide conversions remained steady for 50 hours without showing any sign of deactivation. However, the sinterization of the nickel particles and the presence of carbon deposits seemed to indicate that the catalyst would eventually loose catalytic activity.
Abstract: The purpose of this work was to explore the application of microwaves for the low temperature regeneration of activated carbons saturated with a pharmaceutical compound (promethazine). Contrary to expectations, microwave-assisted regeneration did not lead to better results than those obtained under conventional electric heating. At low temperatures the regeneration was incomplete either under microwave and conventional heating, being this attributed to the insufficient input energy. At mild temperatures, a fall in the adsorption capacity upon cycling was obtained in both devices, although this was much more pronounced for the microwave. These results contrast with previous studies on the benefits of microwaves for the regeneration of carbon materials. The fall in the adsorption capacity after regeneration was due to the thermal cracking of the adsorbed molecules inside the carbon porous network, although this effect applies to both devices. When microwaves are used, along with the thermal heating of the carbon bed, a fraction of the microwave energy seemed to be directly used in the decomposition of promethazine through the excitation of the molecular bonds by microwaves (microwave-lysis). These results point out the nature of the adsorbate and its ability to interact with microwave are key factors that control the application of microwaves for regeneration of exhausted activated carbons.
Abstract: Carbon xerogel spheres with millimeter-scale diameters were synthesized by a simple process using microwave radiation as the heating source. Using this form of heating it is possible to establish the gelation point of different resorcinol-formaldehyde solutions and stop the gelation of the material at exactly this point. Organic gel spheres can then be obtained directly by stirring in a silicone bath at 80 ºC. Finally, carbonization is performed to obtain carbon xerogels with a spherical shape. The size and porous texture of the spheres can be controlled by adjusting the synthesis conditions.
Abstract: An ultracapacitor (UCap) based on carbon xerogel-electrodes of different weights and an aqueous electrolyte was tested in the voltage range of 0 to 1.8 V. The UCap exhibited a good capacitance, a high specific energy and an excellent cycling stability. The large BET specific surface area of the carbon (c.a. 3100 m2/g) and the high working voltage, made it possible to achieve 17.5 Wh/kg of specific energy. This result is notably higher than that of similar capacitors based on aqueous electrolyte investigated in literature.
Abstract: Resorcinol-formaldehyde xerogels were chemically activated using potassium hydroxide under different activation conditions of time, temperature and heating mechanism. A study of the activation time showed that carbon xerogels with a well-developed porosity (SBET around 2200 m2 g-1) can be produced in the range of 6-30 minutes. It was found that by means of short exposures to microwaves it is possible to partially preserve the original mesoporosity of the precursor and to achieve a higher volume of narrower micropores. However, under conventional heating the mesoporosity created during the synthesis of the organic precursor was destroyed.
Abstract: The use of steel-making slag as catalysts for microwave-assisted dry reforming of CH4 was studied. Two carbon materials (an activated carbon and a metallurgical coke), mixtures of the carbon materials and Fe-rich slag, and mixtures of the carbon materials and Ni/Al2O3 were tested as catalysts. The mixtures of slag with carbons gave rise to higher and steadier conversions than those achieved over the carbon materials alone. In addition, the use of the metallurgical coke mixed with metal-rich catalysts gave rise to remarkable results. Thus, no CH4 and CO2 conversions were achieved when coke was used alone, whereas high conversions were obtained when it was mixed with the metal rich catalysts.
Abstract: The valorisation of renewable, widely abundant and cheap biomass and/or waste feedstocks has attracted a great deal of interest in recent times as alternative ways to convert low value starting materials to valuable products including chemicals, materials and energy (e.g. biofuels). Pyrolysis of biomass is one of such relevant technologies that have been extensively investigated and applied in past decades to valorise feedstocks ranging from organic residues to plastics and many others. This strategy involves heating of the feedstock to high temperatures in relatively short periods of time to disintegrate the original raw material turning it into various products including gases (e.g. CO,H2, CO2, CH4), a so-called pyrolysis oil containing mostly low volatile organic compounds (mixed with some water) and so-called biochar (a carbonaceous deposit). In this opinion article, a comparison between microwave-assisted pyrolysis versus conventional pyrolysis is given, with pros and cons of both in view of their potential of biomass and/or waste valorisation.
Abstract: In this work the microwave-assisted CO2 reforming of CH4 over carbon-based catalysts, carried out in a lab-scale microwave pilot plant, was investigated focusing on the conversions achieved and energy consumption. The activated carbon, FY5, and a heterogeneous mixture of FY5 and an in-lab prepared Ni/Al2O3 were selected as catalysts. Regardless of the catalyst used (FY5 or FY5 + Ni/Al2O3), high and steady CH4 and CO2 conversions were obtained. However, the catalytic activity of the heterogeneous mixture of FY5 and Ni/Al2O3 was found to be better. The energy consumption values of the scale-up microwave-assisted CO2 reforming of CH4 were estimated from the experimental data and a flow rate of 1 m3 h-1 of introduced CH4 was used as calculation basis. An energy consumption of 44.4 kWâ¢h per m3 of H2 produced, with FY5 as catalyst, and of 4.6 kWâ¢h per m3 of H2 produced, with FY5 + Ni/Al2O3, was estimated. A comparison of these estimations with the bibliographic values of energy consumption for steam methane reforming (1.2 kWâ¢h per m3 of H2 produced), showed the microwave-assisted CO2 reforming of CH4 over mixtures of activated carbon and metal-based catalyst to be a promising process able to rival the steam reforming process.
Abstract: The decomposition and CO2 reforming of methane, respectively, are promising alternatives to industrial steam methane reforming.
In recent years, research has been focused on the development of catalysts that can operate without getting deactivated by carbon deposition, where, in particular, carbon catalysts have shown positive results. In this work, the role of carbon materials in heterogeneous catalysis is assessed and publications on methane decomposition and CO2 reforming of methane over carbon materials are reviewed. The influence of textural properties (BET surface area and micropore volume, etc.) and oxygen surface groups on the catalytic activity of carbon materials are discussed. In addition, this review examines how activated carbon and carbon black catalysts, which are the most commonly used carbon catalysts, are deactivated. Characteristics of the carbon eposits from methane are discussed and the influence of the reactivity to CO2 of fresh carbon and carbonaceous deposits for high and steady conversion during CO2 reforming of CH4 are also considered.
Abstract: Resorcinolâformaldehyde carbon xerogels with several initial pH were synthesized using two different heating methods (conventional and microwave heating). The effect of the pH of the precursor solution and the method of synthesis employed on the textural and chemical properties of the final materials
was evaluated. It was found that both methods produce tailored carbon xerogels depending on the initial pH and that the pores of the carbon xerogels become larger as the initial pH decreases. High pHs result in exclusively microporous carbon xerogels, while a decrease in the amount of NaOH added, i.e. lower pH,
causes the materials to evolve firstly into microâmesoporous samples and then into microâmacroporous carbon xerogels. The main difference between the two heating methods studied, apart from the duration of the synthesis (i.e. approximately 5 h for the microwave-assisted synthesis as opposed to several days by conventional methods) lies in the mesoâmacroporosity of the resulting materials, since microwave radiation produces mainly mesoporous carbon xerogels with a specific mesopore size over a wider range of pH than conventional synthesis. For example, the pH range for mesoporous MW samples is 4.5â6.5 while equivalent samples that are conventionally synthesized require an initial pH of between 5.8 and 6.5. This work also illustrates a simple and precise method for determining the gelation point (tg) of different pH resorcinolâformaldehyde mixtures, based on varying the energy consumed by the microwave device during the synthesis of organic gels, without the need for other more complicated techniques.
Abstract: Microwave heating can be used to promote heterogeneous reactions with carbon materials, mainly because of the heating mechanism involved in microwave radiation and the phenomenon known as microplasmas. This work presents photographic evidence of plasma formation when different carbon materials are subjected to microwave heating. Two different kinds of plasmas were observed: ball lightning and arc discharge plasmas. The intensity of the plasmas in the less ordered carbons was significantly higher at the beginning of the process.
Abstract: Symmetric supercapacitors (SSC) and asymmetric supercapacitors (ASC) that use carbon xerogels with different porous textures as negative electrode and manganese oxide as positive electrode were investigated. The electrochemical performance of symmetric supercapacitors with carbon xerogel electrodes was mainly influenced by the textural characteristics of the carbon, pore size distribution being the property that has the strongest influence on the capacitance performance. The asymmetric supercapacitor showed an excellent capacitance performance (i.e. 213 F gâ1) when a chemical activated carbon xerogel with a high SBET (i.e. 2360 m2 gâ1) was used as negative electrode and high performing oxide-based manganese as positive electrode, thereby demonstrating that carbon xerogels and manganese oxide have potential applications in supercapacitor devices.
Abstract: The CO2 reforming of coke oven gas for the production of synthesis gas has been studied over an activated carbon, an in-lab prepared Ni/Al2O3 catalyst and physical mixtures of both materials in different proportions (AC þ Ni) at 800 C. It was found that there are two possible coexisting reaction pathways: the direct dry reforming of methane (decomposition of methane followed by gasification of the carbon deposits) and the reverse water gas shift reaction followed by the steam reforming of methane. If the process is carried out with the physical mixtures AC þ Ni, there is a synergetic effect between both materials. The experimental conversions are higher than the conversions predicted by the law of mixtures, whereas the production of water is lower, resulting in a higher selectivity. The mixtures also showed a lower loss of porosity than when the activated carbon and the inlab prepared Ni/Al2O3 were used individually. Therefore, the combination of these materials may produce catalysts that are more resistant to deactivation. The synthesis gas obtained was analyzed and it was found suitable for the production of methanol.
Abstract: A series of biomass wastes (sewage sludges, coffee hulls and glycerol) were subjected to pyrolysis experiments under conventional and microwave heating. The influence of the initial characteristics of the raw materials upon syngas production was studied. Glycerol yielded the highest concentration of syngas, but the lowest H2/CO ratio, whereas sewage sludges produced the lowest syngas production with the highest H2/CO molar ratio. Coffee hull displayed intermediate values for both parameters. Microwave heating produced greater gas yields with elevated syngas content than conventional pyrolysis. Moreover, microwave pyrolysis always achieved the desired effect with temperature increase upon the pyrolysis products, whatever biomass material was employed. This could be due to the hot spot phenomenon, which only occurs under microwave heating. In addition, a comparison of the energy consumption of the traditional and microwave-assisted pyrolysis is also presented. Results point at microwave system as less time and energy consuming in comparison to conventional system.
Abstract: In this work, the microwave-assisted CO2 reforming of CH4 over mixtures of carbonaceous materials and an in-lab prepared Ni/Al2O3 was studied. Ni/Al2O3 is not heated by microwave radiation, and for this reason, microwave receptors, such as carbonaceous materials, must be mixed with this catalyst. In order to evaluate the role of the carbonaceous component of the blend, two different carbonaceous materials were used: an activated carbon, FY5, and a metallurgical coke, CQ. The carbonaceous component acted not only as microwave receptor but also as catalyst and, consequently, it influenced the catalytic activity of the mixture. FY5 + Ni/Al2O3 was found to be a better catalyst than CQ + Ni/Al2O3, since FY5 on its own showed a better catalytic activity than CQ. Ni/FY5, which consists of Ni impregnated directly onto the microwave receptor, was also evaluated as a catalyst. It was found that the catalytic activity of the mixture FY5 + Ni/Al2O3 was better than that of Ni/FY5. Finally, the influence of the heating device on the catalytic activity of FY5 + Ni/Al2O3 was studied. Conversions over FY5 + Ni/Al2O3 and microwave heating were found to be similar to conversions over Ni/Al2O3 and conventional heating.
Abstract: The dry reforming of coke oven gases (COG) over an activated carbon used as catalyst has been studied in order to produce a syngas suitable for methanol synthesis. The primary aim of this work was to study the influence of the high amount of hydrogen present in the COG on the process of dry reforming, as well as the influence of other operation conditions, such us temperature and volumetric hourly space velocity (VHSV). It was found that the reverse water gas shift (RWGS) reaction takes place due to the hydrogen present in the COG, and that its influence on the process increases as the temperature decreases. This situation may give rise to the consumption of the hydrogen present in the COG, and the consequent formation of a syngas which is inappropriate for the synthesis of methanol. This reaction can be avoided by working at high temperatures (about 1000 º) in order to produce a syngas that is suitable for methanol synthesis. It was also found that the RWGS reaction is favoured by an increase in the VHSV. In addition, the active carbon FY5 was proven to be an adequate catalyst for the production of syngas from COG.
Abstract: A series of carbon-based nickel (Ni) catalystswas prepared in order to investigate the effect of the preparation method on the dispersion of Ni and its final catalytic activity in the dry reforming of methane, i.e. CH4+ CO2=2H2+2CO. Three parameterswere studied: (i) the influence of the surface chemistry of the carbon used as support; (ii) the method of drying (conventional vs. microwave drying); and, (iii) the temperature of the 21 reduction stage. In order to study the role of the surface chemistry of the commercial activated carbon used as support, the active carbon was tested as received and oxidized. Although a better Ni dispersion was achieved over the oxidized support, the conversions were much lower. It was also found that microwave drying offers various advantages over conventional drying, the main one being that less time is required to prepare the catalyst. Two reduction temperatureswere used (300 and 500 °C), being found that it is necessary to adjust this parameter to prevent the Ni particles from sintering.
Abstract: Carbon materials are, in general, very good absorbents of microwaves, i.e., they are easily heated by microwave radiation. This characteristic allows them to be transformed by microwave heating, giving rise to new carbons with tailored properties, to be used as microwave receptors, in order to heat other materials indirectly, or to act as a catalyst and microwave receptor in different heterogeneous reactions. In recent years, the number of processes that combine the use of carbons and microwave heating instead of other methods based on conventional heating has increased. In this paper some of the microwave-assisted processes in which carbon materials are produced, transformed or used in thermal treatments (generally, as microwave absorbers and catalysts) are reviewed and the main achievements of this technique are compared with those obtained by means of conventional (non microwave-assisted) methods in similar conditions.
Abstract: The spent catalyst from vinyl acetate synthesis contains large quantity of zinc. The present study attempts to leach zinc using a mixture of ammonia, ammonium carbonate and water solution, after microwave treatment. The effect of important parameters such as leaching time, liquid/solid ratio and the ammonia concentration was investigated and the process conditions were optimized using surface response methodology (RSM) based on central composite design (CCD). The optimum condition for leaching of zinc from spent catalyst was identified to be a leaching time of 2.50 h, a liquid/solid ratio of 6 and ammonia concentration 5.37 mol/L. A maximum of 97% of zinc was recovered under the optimum experimental conditions. The proposed model equation using RSM has shown good agreement with the experimental data, with a correlation coefficient (R2) of 0.95. The samples were characterized before and after leaching using X-ray diffraction (XRD), nitrogen adsorption and scanning electron microscope (SEM).
Abstract: A regenerated activated carbon used as catalyst support in the synthesis of vinyl acetate has been tested as a low-cost adsorbent for the removal of dyes. After a thorough textural characterization of the regenerated activated carbon, its adsorption isotherms and kinetics were determined using methylene blue as model compound at different initial concentrations. Both Langmuir and Freundlich isotherm models were developed and then compared. It was found that the equilibrium data were best represented by the Langmuir isotherm model. The kinetic data were fitted to pseudo-first-order, pseudo-second-order and intraparticle diffusion models, and it was found that the best fitting corresponded to the pseudo-second-order kinetic model. The results showed that this novel adsorbent had a high adsorption capacity, making it suitable for use in the treatment of methylene blue enriched wastewater
Abstract: In-lab synthesized Ni-doped carbon xerogels were used as a carbon support to study the possibility to improve the electrochemical behavior of AB5-type alloy electrodes for nickel/metal hydride rechargeable batteries. The carbons used for this work were synthesized by polymerization of resorcinol and formaldehyde. These carbon supports were mixed with an AB5-type alloy. For the electrochemical characterization of the electrodes, charge and discharge techniques, a high rate of discharge, and electrochemical impedance spectroscopy were used. Results show that this kind of carbon could be used as a support for hydrogen storage electrodes with good electrochemical behavior and kinetic response.
Abstract: Resorcinol-formaldehyde carbon xerogels were prepared by means of two different synthesis methods: conventional (C) and microwave heating (MW). The influence of the heating method and the pH of the precursor solution on the textural and chemical properties of the carbon xerogels was investigated. It was found that by modifying the initial pH, it is possible to control the porosity of carbon xerogels independent of the heating method used. The electrochemical performance of a selection of synthesized carbon xerogels as electrode materials in electric double-layer capacitors was studied by cyclic voltammetry and charge/ discharge experiments in an acidic medium (1MH2SO4). The electrochemical performance of the carbon xerogels was compared to that of an activated carbon commercialized for this application (Norit Super DLC-50), and it can be seen that the carbon xerogels display similar specific capacitances to those of the commercial carbon. Moreover, carbon xerogels have a good cycle durability after 18 000 galvanostatic cycles, with a drop in specific capacitance of around 10%. This excellent cycle durability, together with the attractive properties of carbon xerogels and the saving of time and energy achieved with microwave-assisted synthesis, would make resorcinol-formaldehyde carbon xerogels promising materials forapplications of an electric double-layer capacitor (EDLC).
Abstract: A simple and precise method for determining the point of gelation in microwave-assisted synthesis of organic gels is reported. This method provide a relevant information in the carbon gels field, as it can be applied to stop the gelation process at the precise instant for using the material in further processes where it is necessary to have the material with a specific viscosity (i.e., point of gelation).
Abstract: After being subjected to conventional thermal treatment, a carbon-based spent catalyst from vinyl acetate synthesis has been proved to be a good precursor for the production of ZnO/activated carbon composites. Such composites can be used as photocatalysts for the removal of pollutants from wastewaters. Different operating temperatures were found to influence the properties of the final composites. As the temperature rises, the surface area of the activated carbon that is acting as catalyst support increases due to the decomposition of the organic deposits that are blocking the porous structure. XRD was used to evaluate the transformation of zinc acetate to zinc oxide after thermal treatment, showing higher crystal sizes with the rise in temperature. Both the adsorption capacity and the yield of the resulting composites were calculated and SEM-EDX analysis was used to evaluate the changes in surface morphology.
Abstract: The thermal valorization of glycerol to produce synthesis gas has been investigated under conventional and microwave heating systems. Different processes (pyrolysis, steam reforming and dry reforming) are compared, using a commercial activated carbon as catalyst. The reforming processes that employ oxidizing agents (CO2 or H2O) were found to promote higher glycerol conversions than mere thermal decomposition. Steam reforming generates the lowest gas fraction and the highest amounts of hydrogen and syngas, while the opposite occurs in the dry reforming experiments. Microwave processing produced higher gas yields with large syngas content than conventional heating processes in all cases. The use of carbon-based catalysts appears to be highly suitable for producing synthesis gas with a H2/CO ratio close to 1, minimum CO2 emissions being an additional advantage.
Abstract: In this work, various carbon materials were studied as catalysts/microwave receptors for the CO2 reforming of the CH4 reaction. Carbon materials with a different textural development (metallurgical coke, activated carbons, re-activated carbon) were selected as catalysts in order to determine the role of porosity and pore size in dry reforming. Microporosity was found to be necessary for a good performance of the carbon catalysts. An activated carbon and an oxidized activated carbon were compared in order to evaluate the influence of oxygen surface groups on the catalytic activity of carbons for the dry reforming reaction. Oxidized carbons were found to be bad catalysts, especially under microwave heating. The importance of CO2 reactivity for carbon materials to be able to act as acceptable catalysts was also established.
Abstract: In this work the catalytic activity of heterogeneous mixtures of an activated carbon, FY5, and an in-lab prepared Ni/Al2O3 in the CO2 reforming of CH4 reaction was investigated. Initially, the reaction was carried out over FY5 and Ni/Al2O3 separately. Under the operating conditions employed, the CO2 and CH4 conversions obtained over FY5 were negligible, whereas they were high and steady over Ni/Al2O3. In the latter case, it was found that the spinel NiAl2O4 was formed due to the interaction between the reduced Ni and the support during heating under a N2 atmosphere. The reaction was also carried out over different mixtures of FY5 + Ni/Al2O3. The experimental conversions thus attained were higher than the conversions calculated by adding the weighted individual conversions (mixtures law). Therefore, a synergetic effect exists between the carbonaceous fraction and the metal-based fraction. This effect was found to increase with temperature. Differences between the experimental and theoretical CO2 conversion were also observed to increase when the proportion of FY5 in the mixture was increased, whereas in the case of CH4 conversion, the increase depended on the proportion of Ni/Al2O3 added. It was also observed that, when a heterogeneous mixture FY5 + Ni/Al2O3 was used as catalyst for the CO2 reforming of methane, the decomposition of CH4 occurred preferentially over the Ni-based fraction and that CO2 tended to gasify both the initial carbonaceous fraction and the carbon deposits from methane.
Abstract: ctivated carbon xerogels, with selected characteristics, were doped with Ni, using different methods, and tested for hydrogen storage. The results obtained show that the amount of nickel incorporated, the Niâcarbon interaction and the nickel particle size distribution depend more on the doping method used than on the textural properties of the carbon support. The amount of nickel incorporated by strong electrostatic adsorption is lower than that incorporated by dry impregnation. However, the strong electrostatic adsorption method produces Ni-doped carbon previous xerogels with a high Niâcarbon interaction and a narrower Ni particle size distribution. The influence of Ni on H2 storage capacity depends on the operating conditions and the doping conditions used. Thus, at â196 °C and 40 bar, storage capacity seems to be mainly influenced by the textural properties of carbon support while, at 25 °C and 200 bar, the spillover effect plays a significant role, being the interaction between the support and Ni particles key factor in the storage process. The best Ni-doped carbon xerogels obtained in this work exhibit hydrogen storage capacities of 6 wt.% and 31.8 g lâ1 at â196 °C and 40 bar.
Abstract: The present work gathers a general review of the properties, advantages and disadvantages of the principal electrode materials used in electrochemical capacitors (alsocalled supercapacitors). The choice of these materials has an enormous influence on the final characteristics of the electrochemical device. Among the available electrode materials for their application in supercapacitors, carbonbased porous materials, transition metal oxides and conductive polymers are included. Each of these families of materials presents some advantages and drawbacks, so, in many occasions, a single material is not used for the preparation of the electrodes. The trend is directed to the use of hybrid electrodes, namely, those made of materials of different nature to combine the beneficial aspects and to offset the limitations of each material separately.
Abstract: Binary CuO/ZnO and ternary CuO/ZnO/Al2O3 catalysts have been rapidly synthesized in a multimode microwave oven by the homogeneous precipitation of aurichalcite and hydrotalcite-like precursors using urea hydrolysis. For purposes of comparison, the same catalysts were prepared under conventional heating. The corresponding metal nitrates were mixed with various amounts of urea to yield different urea/(ΣM+) molar ratios. The precipitation proceeded stepwise, copper being the first metal to be hydrolyzed. It was found that the higher the urea content, the higher the alkalinization of the solution, an effect which favored the precipitation of Zn (II) (the most pH dependent metal), and in turn, the synthesis of aurichalcite and hydrotalcite-like precursors.
Microwave-synthesized catalysts presented similar characteristics to those obtained under conventional heating, but in considerably reduced aging times. Microwave radiation proved to be more efficient than conventional heating under harsh conditions of precipitation, i.e., lower molar ratios of urea with respect to the metal cations in the solution.
Abstract: Sewage sludge was pyrolysed in a quartz reactor at 350, 450, 550 and 950 °C. The pyrolysis oils from the sewage sludge were characterized in detail by means of gas chromatographyâmass spectrometry (GCâMS). Changes in the composition of the oils related to the process conditions were assessed by normalizing the areas of the peaks. It was demonstrated that, as the temperature of pyrolysis increased from 350 to 950 °C, the concentration of mono-aromatic hydrocarbons in the oils also increased. Conversely, phenol and its alkyl derivatives showed a strong decrease in their concentration as temperature rose. Polycyclic aromatic hydrocarbons (PAHs) with two to three rings passed through a maximum at a pyrolysis temperature of 450 °C. PAHs with 4â5 rings also presented a major increase as temperature increased up to 450 °C, the concentration at 950 °C being slightly higher than that at 450 °C. Quantification of the main compounds showed that sewage sludge pyrolysis oils contain significant quantities of potentially high-value hydrocarbons such as mono-aromatic hydrocarbons and phenolic compounds. The oils also contain substantial concentrations of PAHs, even at the lowest temperature of 350 °C. The pathway to PAH formation is believed to be via the DielsâAlder reaction and also via secondary reactions of oxygenated compounds such as phenols.
Abstract: The pyrolysis of glycerol was carried out over carbonaceous catalysts to produce synthesis gas (up to 81 vol.%). The catalytic effect of two activated carbons was tested and compared with quartz glass chips as packing materials in a fixed-bed reactor. The main advantage of using a carbonaceous catalyst seems to be its higher selectivity toward hydrogen, resulting in a synthesis gas with a greater H-2/CO ratio. The influence of the heating method (electrical furnace and microwave oven) and temperature (from 400 to 900 degrees C) on the pyrolysis of glycerol over a selected activated carbon was also studied. As a result, an elevated gas fraction with a higher H-2 + CO Composition was obtained under microwave heating, even at low temperatures. This proves that microwave heating could be used to upgrade the industrial surplus generated from bio-diesel production. (c) 2009 Elsevier B.V. All rights reserved.
Abstract: The effect of nickel distribution and content in Ni-doped carbon nanospheres on hydrogen storage capacity under conditions of moderate temperature and pressure was studied. It was found that the nickel distribution, obtained by using different doping techniques and conditions, has a noticeable influence on hydrogen storage capacity. The samples with the most homogeneous nickel distribution, obtained by pre-oxidising the carbon nanospheres, displayed the highest storage capacity. In addition, storage capacity is influenced by the amount of nickel. it was found a higher storage capacity in samples containing 5 wt.% of Ni. This is due to the greater interactions between the nickel and the support that produce a higher activation of the solid through a spillover effect. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
Abstract: This work deals with the catalytic decomposition of carbon-containing gases (ethylene) over nickel catalysts to grow graphitic capsules at relatively low temperatures (853 K). The influence of different experimental conditions, such as catalyst size, reactant flow ratio (C2H4:O2), gas flow rate and temperature, is studied. The size of the nickel particles determines the type of structures; while nano-sized nickel particles favor the growth of nanofilaments, graphitic capsules occur over nickel microparticles. The use of reducing and oxidizing environments in the growth of graphitic structures over microparticles are compared, the latter being more appropriate for growing nanofilaments. Below 853 K, no graphitic structures are formed. Moreover, a new model is proposed for the encapsulation of the catalyst metal particles by graphite layers.
Abstract: The characteristics of carbonaceous materials deposited in fuel rich ethyleneâoxygen mixtures on three types of palladium: foil, sputtered film, and nanopowder, are reported. It was found that the form of palladium has a dramatic influence on the morphology of the deposited carbon. In particular, on sputtered film and powder, tight âweavesâ of sub-micron filaments formed quickly. In contrast, on foils under identical conditions, the dominant morphology is carbon thin films with basal planes oriented parallel to the substrate surface. Temperature, gas flow rate, reactant flow ratio (C2H4:O2), and residence time (position) were found to influence both growth rate and type for all three forms of Pd. X-ray diffraction, high resolution transmission electron microscopy, temperature-programmed oxidation, and Raman spectroscopy were used to assess the crystallinity of the as-deposited carbon, and it was determined that transmission electron microscopy and X-ray diffraction were the most reliable methods for determining crystallinity. The dependence of growth on reactor position, and the fact that no growth was observed in the absence of oxygen support the postulate that the carbon deposition proceeds by combustion generated radical species.
Abstract: The aim of this work was to study the microwave-assisted pyrolysis of CH4 over an activated carbon, which acted as both microwave receptor and catalyst, and the influence of using different CH4/N-2 ratios on the conversion of CH4 to H-2. In order to compare the results obtained in the microwave oven, the pyrolysis was also carried out under conventional heating (electric furnace, EF). The effects of N-2, which enhanced significantly CH4 conversion, differed depending on the heating device used. Under EF heating, N-2 seemed to have an effect similar to distribute the CH4 molecules within the activated carbon bed. Under microwave heating (MW), the N-2, as well as distributing the CH4 molecules, favoured the generation of energetic microplasmas, leading to higher conversions. The prevalence of one role over the other depended on the CH4/N-2 ratio, the appearance of energetic microplasmas being favoured with high percentages of N-2. Consequently, CH4 conversion was higher at low CH4/N-2 ratios. Additionally, the formation of carbon nanofibres in experiments where a combination of N-2 and MW heating was used is also reported. (C) 2008 Elsevier B.V. All rights reserved.
Abstract: Textural properties of carbon gels can be controlled by varying the synthesis and drying process conditions. In this work, the influence of the initial pH and the drying method on the final properties of carbon gels, synthesized using methanol as a solvent, was evaluated. Furthermore, the use of microwaves as a drying method for the synthesis of carbon xerogels was assessed. In the light of the results obtained, in order to synthesize monolithic and microporous carbon gels in a short period of time, the use of a multimode microwave oven is proposed. The use of pH 7 also leads to shorter gelation times and more consistent monoliths. Furthermore, the multimode microwave drying can produce homogeneous microporosity and surface areas of up to 1341 M-2 g(-1), in a very short time (i.e., only 6 min is required for the drying step). (C) 2007 Elsevier B.V. All rights reserved.
Abstract: The aim of this work was to study the growth of nanofilaments on two carbon-based materials (i.e. an activated carbon and carbon fibres) from microwave-assisted decomposition of CH4. It was found that the nature of grown nanofilaments differed depending on the carbon support, probably due to the presence of diverse metals which catalyze the nanofilaments growth. Besides, it was also studied the influence of the composition of the in-flow gases on the growth of nanofilaments. Thus, tests with pure CH4 and different CH4/N-2 mixtures were carried out, the yield in nanofilaments being higher when mixtures were fed in. No nanofilaments were found after analogous tests carried out under conventional heating, therefore it can be stated that microwave heating is a promising technology for growing nanofilaments. (C) 2007 Elsevier B.V. All rights reserved.
Abstract: This work deals with the conventional and microwave-assisted pyrolysis of coffee hulls in order to determine the influence of the pyrolysis method on the characteristics of the pyrolysis products. The high syngas content obtained in microwave heating (73 vol %) is thought to be a result of the fact that self-gasification of the char with CO2 and the catalytic decomposition of CH. are favoured in microwave. Individual reactions were also studied in both heating methods. The conversions were greater for microwave than for conventional heating. In the case of methane decomposition, CH. conversion experiences a significant drop due to the formation of coke deposits which reduce catalytic activity. However, when a CH4/CO2 mixture was used, this problem was minimized since the CO2 partly removed the carbon deposits formed, providing an "in situ" route for catalyst regeneration. Results reported in this paper have serve as starting point for the development of a new process for conversion of biogas into syngas.
Abstract: Microwave drying was used to prepare resorcinol-formaldehyde aqueous gels, without performing any pretreatment, and to see whether it was possible to use this drying option to obtain porous carbon xerogels with controlled textural properties. By using microwave drying, the process for obtaining carbon gels is greatly simplified, textural properties are controlled likewise with other drying methods, but the time involved in the process is significantly reduced and no pretreatment is necessary. Therefore, microwave drying could help to simplify the carbon xerogels synthesis and reduce the associated costs. (C) 2008 Elsevier B.V. All rights reserved.
Abstract: This paper assesses the feasibility of producing syngas from sewage sludge via two pyrolysis processes: microwave-induced pyrolysis (MWP) and conventional pyrolysis (CP). The changes in the composition of the produced gas as a function of the pyrolysis treatment and the initial moisture content of the sludge were evaluated. It was found that MWP produced a gas with a higher concentration of syngas than CP, reaching values of up to 94vol%. Moreover, this gas showed a CO2 and CH4 concentration around 50% and 70%, respectively, lower than that obtained in the gas from CP. With respect to the effect of moisture on gas composition, this was more pronounced in CP than in MWP. Thus, the presence of moisture increases the concentration of H2 and CO2 and decreases that of CO, especially when CP was used. In order to elucidate the behaviour of CO2 during the pyrolysis, the CO2 gasification kinetics of the char obtained from the pyrolysis were investigated. It was established that in microwave heating the gasification reaction is much more favoured than in conventional heating. Therefore, the low concentration of CO2 and the high concentration of CO in the microwave pyrolysis gas could be due to the self-gasification of the residue by the CO2 produced during the devolatilization of the sewage sludge in the pyrolysis process.
Abstract: The microwave-assisted dry reforming of methane over an activated carbon, which acted as catalyst and microwave receptor, was investigated. As a preliminary study, the CO2 reforming of CH4 was carried out using conventional heating and microwave heating in order to compare both heating devices. Higher conversions of CH4 and CO2 were achieved by microwave heating. Under microwave heating, various operating variables were studied in order to determine the best conditions for performing dry reforming with high conversions and the most suitable H-2/CO ratio. Thus, the dry reforming reaction was studied at different temperatures. An optimum range of working temperatures (between 700 degrees C and 800 degrees C) was established. In this range of temperatures, the dry reforming reaction is believed to take place as a combination of CH4 decomposition and CO2 gasification. Carbonaceous deposits from CH4 decomposition are gasified by CO2 and, as a result, active centres for the dry reforming reaction are constantly regenerated. The effect of the proportion of CO2 fed in on the CH4 and CO2 conversions was also investigated. Small increases in the percentage of CO2 fed in gave rise to large increases in both conversions, but especially in the case of CH4. The volumetric hourly space velocity was also studied. It was found that the lower the space velocity, the higher the conversions obtained. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
Abstract: This paper describes the conventional and microwave-assisted pyrolysis of coffee hulls at 500, 800 and 1000 degrees C. The influence of the pyrolysis method and temperature on the product yields and on the characteristics of the pyrolysis products is discussed. It was found that the pyrolysis of this particular residue gives rise to a larger yield of the gas fraction compared to the other fractions, even at relatively low temperatures. A comparison of microwave-assisted pyrolysis and conventional pyrolysis showed that microwave treatment produces more gas and less oil than conventional pyrolysis. In addition, the gas from the microwave has much higher H-2 and syngas (H-2 + CO) contents (up to 40 and 72 vol.%, respectively) than those obtained by conventional pyrolysis (up to 30 and 53 vol.%, respectively), in an electric furnace, at similar temperatures. From the pyrolysis fraction yields and their higher heating values it was found that the energy distribution in the pyrolysis products decreases as follows: gas > solid > oil. Moreover, the energy accumulated in the gas increases with the pyrolysis temperature. By contrast, the energy accumulated in the char decreases with the temperature. This effect is enhanced when microwave pyrolysis is used. (C) 2006 Elsevier B.V. All fights reserved.
Abstract: Pellets of coffee hulls were pyrolyzed at different temperatures using microwave and electrical heating. A comparison of the gas composition obtained by both methods suggests that the different mechanisms of heating that take place in the microwave, in comparison to conventional heating, give rise to the formation of "microplasmas", which induce self-gasification of the char that is being formed. This hypothesis was corroborated by subjecting the char to reaction with CO2 at different temperatures using both methods of heating. The results showed that, whereas the transition in the reaction mechanism controlling the Boudouard reaction (i.e., chemical or diffusional control) takes place at about 800 degrees C in conventional heating, in the case of microwave heating the temperature is much lower and the reaction never proceeds under pure chemical control, the differences between microwave and conventional heating being quite significant even at low temperatures.
Abstract: The aim of this paper is to study the reforming of CH4 with CO2 using as a catalyst a rich potassium char obtained from biomass pyrolysis. The reaction was carried out at 800 degrees C by means of two different methods of heating, microwave and electrical heating (MWH and EH, respectively). In addition, the individual reactions proposed for the dry reforming of methane, that is, (i) the decomposition of CH4 to form hydrogen and carbon and (ii) the dissociative adsorption of CO2 followed by reduction to give CO, were also studied with both heating methods at the same temperature. The results showed that MWH produces hot spots (microplasmas located inside the catalyst bed) that favor heterogeneous catalytic reactions. Thus, it was found that the conversion of CH4 and CO2 in the individual reactions was greater for MWH than for EH. An examination of the CH4 decomposition reaction proved the formation of coke deposits, which reduced catalytic activity and CH4 conversion. When a CH4/CO2 mixture was used, this problem was minimized since the CO2 partly removed the carbon deposits formed, thereby prolonging the activity of the catalyst. This gasification reaction, catalyzed by the high level of K contained in the char, provides an "in situ" route for catalyst regeneration. The results indicated that the presence of CO2 increased the conversion of CH4 to H-2, the values being higher in MWH than in EH. Both heating methods produced an outlet gas composed mainly of syngas (CO + H-2) and practically free of CO2 and CH4 (especially in the case of MWH). In addition, the study of the exhausted catalysts by scanning electron microscopy revealed the presence of significant amounts of carbon nanofibers on the char surface, but only in the case of MWH, these nanofibers being more abundant in the dry reforming reaction than in the single CH4 decomposition.
Abstract: The aim of this work was to combine microwave heating with the use of low-cost granular activated carbon as a catalyst for the production Of CO2-free hydrogen by methane decomposition in a fixed bed quartz-tube flow reactor. In order to compare the results achieved, conventional heating was also applied to the catalytic decomposition reaction of methane over the activated carbon. It was found that methane conversions were higher under microwave conditions than with conventional heating when the temperature measured was lower than or equal to 800 degrees C. However, when the temperature was increased, the difference between the conversions under microwave and conventional heating was reduced. The influence of volumetric hourly space velocity (VHSV) on the conversion tests using both microwave and conventional heating was also studied. In general, there is a substantial initial conversion, which declines sharply during the first stages of the reaction but tends to stabilise with time. An increase in the VHSV has a negative effect on CH4 conversion, and even more so in the case of microwave heating. Nevertheless, the conversions obtained in the microwave device at the beginning of the experiments are, in general, better than the conversions reported in other works which also use a carbonaceous-based catalyst. Additionally, the formation of carbon nanofibres in one of the microwave experiments is also reported. (c) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
Abstract: The purpose of this work was to explore the application of microwaves for the regeneration of activated carbons spent with salicylic acid, a metabolite of a common analgesic frequently found in wastewater from the pharmaceutical industry. The exhausted carbon was treated in a quartz reactor by microwave irradiation at 2450 MHz at different temperatures and atmospheres, the regeneration efficiency being highly dependent on the operating conditions. Quantitative desorption of the pollutant was achieved at high temperature and oxidizing atmosphere, with regeneration efficiencies as high as 99% after six cycles. The stripping efficiency was superior to 95% at high temperatures and decreased at 450 degrees C. The incomplete desorption of the adsorbate at low temperature was further confirmed by the changes in the porosity observed by N2 and CO2 adsorption isotherms. Hence, micropores remain blocked which results in a reduction in loading capacities in successive cycles.
Abstract: The pyrolysis of sewage sludge was investigated using microwave and electrical ovens as the sources of heat, and graphite and char as microwave absorbers. The main objective of this work was to maximize the gas yield and to assess its quality as a fuel and as a source of hydrogen or syngas (H2 + CO). Both gases were produced in a higher proportion by microwave pyrolysis than by conventional pyrolysis, with a maximum value of 38% for H2 and 66% for H2 + CO. The oils obtained were also characterized using FTIR and GC-MS. The use of conventional electrical heating in the pyrolysis of sewage sludge produced an oil that could have a significant environmental and toxicological impact. Conversely, microwave pyrolysis still preserved some of the functional groups of the initial sludge such as aliphatic and oxygenated compounds, whereas no heavy PACs were detected.
Abstract: Carbon nanofibres, obtained at industrial scale by chemical vapour deposition, were subjected to oxidative treatments under CO2 and air at different temperatures. The experiments were performed in electric and microwave furnaces, both at bench scale. The effect of the different treatments on the fibre content (i.e., depuration degree), surface chemistry, porous structure and graphitic properties, of the product obtained in the industrial reactor, were evaluated. The results obtained show that microwave treatments were less selective regarding the depuration process, but this process presents advantages like the low time consuming, besides the increase of the oxidation degree and the graphitic properties of the final sample.
Abstract: The modification of activated carbon fibres prepared from a commercial textile acrylic fibre into materials with monolithic shape using phenolic resin as binder was studied. The molecular sieving properties for the gas separations CO2/CH4 and O-2/N-2 were evaluated from the gas uptake volume and selectivity at 100 s contact time taken from the kinetic adsorption curves of the individual gases. The pseudo-first order rate constant was also determined by the application of the LDF model. The samples produced show high CO, and 02 rates of adsorption, in the range 3-35 x 10(-3) s(-1), and in most cases null or very low adsorption of CH4 and N-2 which make them very promising samples to use in PSA systems, or similar. Although the selectivity was very high, the adsorption capacity was low in certain cases. However, the gas uptake in two samples reached 23 cm(3) g(-1) for CO2 and 5 cm(3) g(-1) for O-2, which can be considered very good. The materials were heat-treated using a microwave furnace, which is a novel and more economic method, when compared with conventional furnaces, to improve the molecular sieves properties. (c) 2005 Elsevier Ltd. All rights reserved.
Abstract: The present work explores an alternative method to the one generally employed for the pyrolysis of sewage sludge. This method consists in carrying out the pyrolysis of a wet sewage sludge as it is produced in the water treatment plant, i.e. with a typical moisture content of between 70 and 80 wt%, without subjecting it to any previous thermal drying process. In order to maximize gas production, the treatment is carried out at elevated temperatures (i.e. 1000 degrees C) using long gas residence times and high heating rates. Under these conditions the natural moisture of the sludge is converted during the process into steam, which gives rise to the partial gasification of the sludge and the reforming of the organic vapours at an early stage. In addition, homogeneous reactions between non-condensable gases are also favoured, especially in the water gas shift reaction. In order to establish the most favourable conditions for the production of a hydrogen rich fuel gas, the influence of the moisture content, heating rate and soaking time on the process was studied. It was found that subjecting the vapours to long residence times at high temperature combined with high heating rates and, above all, the presence of water in the raw sewage sludge, give rise to a process that combines both pyrolysis and gasification. As a result of this process, not only is a higher gaseous fraction produced but also gases of a much higher hydrogen content than in the pyrolysis of dry sludge. (C) 2006 Elsevier B.V. All rights reserved.
Abstract: GC-MS was used to determine the main components of high temperature oils obtained from the microwave pyrolysis of sewage sludge under different conditions. The effect of a multimode and a singlemode microwave oven and graphite and char as microwave absorbers on the pyrolysis process was investigated. The pyrolysis of sewage sludge was rapid with both absorbers, temperatures of up to 1000 T being reached within a few minutes. Although the qualitative composition of the pyrolysis oils was the same for both microwave ovens and absorbers, certain quantitative differences were observed. For example, the use of graphite instead of char produced more cracking in the large aliphatic chains, a higher proportion of 1-alkenes than alkanes and an increase in the proportion of monoaromatics. The multimode microwave oven also favoured cracking and dehydrogenation reactions to a greater extent than the singlemode microwave oven. Compared with the electrical furnace, microwave-assisted heating required shorter times for pyrolysis. Moreover, the microwave pyrolysis oils were more aliphatic and oxygenated and did not contain environmentally harmful compounds such as heavy PAHs. Conversely, the pyrolysis of the sludge at high temperatures using conventional methods gave rise to an oil rich in PAHs including compounds such as benzo[e] and benzo[a]pyrene and benzo[ghi]perylene with 5 or 6 aromatic rings. (c) 2004 Elsevier B.V All rights reserved.
Abstract: A novel method for sewage sludge treatment based on subjecting the wet sludge to high temperature thermal treatment in a microwave is being investigated at INCAR. Under the appropriate operating conditions, drying, pyrolysis and gasification of the sewage sludge take place, giving rise to a gas with a high CO and H-2 content and an oil with a low PAH content. Moreover, due to the high temperatures that can be attained during the process it is possible to obtain a solid residue which is partially vitrified. Unlike other methods aimed at maximizing the porous texture of the solid residue in order to produce adsorbents, the aim of the method proposed in this work is to obtain a solid residue with minimal porous textural development, where the heavy metals present in the residue are occluded in a glassy-like matrix. The advantages of this technique are a substantial volume reduction with respect to the initial sludge and a solid residue that is more resistant to the leaching of organic substances and heavy metals than the char obtained by conventional pyrolysis. (C) 2005 Elsevier B.V All rights reserved.
Abstract: The effect of different heating mechanisms on the porous network of activated carbons (AC) previously exhausted with phenol has been studied. To this end, thermal treatment of the exhausted AC was performed using two experimental devices: a single mode microwave device operating at 2450 MHz and a conventional electric furnace. By employing microwave energy, the regeneration time was considerably shortened compared to conventional thermal heating. Moreover, microwave heating preserved the porous structure of the regenerated AC more efficiently than treatment in a conventional device. In both cases successive regeneration cycles reduced the microporosity of the activated carbons. However, conventional heating shifted the micropore size distribution to pores of narrower sizes. The apparent BET surface areas were also reduced significantly over the regeneration cycles. A loss of the adsorptive capacity of the carbon material was observed after six adsorption-desorption cycles in both systems. The phenol adsorption capacities decreased to a greater extent in the samples regenerated in the electric furnace. (c) 2005 Elsevier Inc. All rights reserved.
Abstract: Bituminous coal-based activated carbon was modified by impregnation with melamine and heat treatment at 850 degreesC. Another sample was impregnated with melamine and urea and heat treated at 650 and 850 degreesC. Chemical and physical properties of the materials were determined using Boehm titration, thermal analysis, sorption of nitrogen and SEM with EDX. Then the H2S breakthrough capacity tests were carried out and the sorption capacity was calculated. The results revealed that carbons modified with nitrogen-containing species and heat-treated at 850 degreesC have a hydrogen sulfide removal capacity exceeding more then 10 times the capacity of unmodified sample. H2S on the surface of these materials is oxidized to sulfuric acid and elemental sulfur and stored in the pore system. New carbons are hypothesized to act as catalytic reactors promoting two different pathways of hydrogen sulfide oxidation in two different locations. In small micropores, where water is present, hydrogen sulfide dissociate to HS- ions and those ions are oxidized to sulfur radicals and sulfur dioxide leading to the formation of sulfuric acid. In larger pores with incorporated nitrogen, basic sites promote dissociation and formation of sulfur polymers, which are resistant to further oxidation. (C) 2003 Elsevier Ltd. All rights reserved.
Abstract: Four wet sewage sludges from different waste water treatment plants were dried and pyrolyzed (in a single process) at laboratory scale, using a multimode-microwave oven. The gases obtained from these pyrolysis experiments were analyzed and compared with those from a more conventional pyrolysis employing an electrical furnace. The results help to explain the complex mechanisms that take place during the pyrolysis of sewage sludge. This process produces a considerable amount of gases with potential value as fuels due to the fact that they have relatively high calorific values. Upon comparing the two pyrolysis processes, it was found that microwave pyrolysis takes a much shorter time than when using the electrical furnace. It was also found, that while microwave pyrolysis gives rise to a gas with a high content (up to 62%) of CO and H-2 (synthesis gas), conventional pyrolysis generates a gas with an elevated proportion (ca. 25%) of hydrocarbons of high calorific value. (C) 2003 Elsevier B.V. All rights reserved.
Abstract: The nature of basic sites on carbon surfaces is a controversial issue within Carbon Science. Traditionally, oxygen-containing groups like: T pyrone-type, chromene, diketone or quinone groups, on the one hand, and delocalized pi-electrons of the basal planes, on the other, are assumed to have a basic nature. However, there is no consensus about neither the strength as bases of these sites, nor the extent of their contribution to the overall carbon basicity. Researchers are divided between those that support basic-oxygen-containing groups as main responsible of carbon basicity and those that attribute the overall basicity of a carbon to the delocalized pi-electrons of the basal planes. In this work we will discuss the virtues and drawbacks of the different hypotheses about basic carbon sites in view of experimental and theoretical results. (C) 2004 Elsevier Ltd. All rights reserved.
Abstract: Thermal regeneration of activated carbons (AC) was carried out at 1123 K and under N-2 atmosphere. Experiments have been carried out using a single mode microwave device operating at 2450 MHz and a conventional electric furnace (EF) so as to compare the effect of the different heating mechanisms on the adsorptive capacities of the regenerated AC. The adsorbents were saturated with phenol in columns. Adsorptive capacities after subsequent regeneration cycles were evaluated from the breakthrough curves. Additionally, a complete textural and chemical characterization of the regenerated samples was also carried out in order to study the influence of the subsequent regeneration cycles on the texture and the adsorptive capacities of the AC. Textural characterization of the regenerated samples was carried out by means of N-2 adsorption isotherms at 77 K. (C) 2004 Elsevier Ltd. All rights reserved.
Abstract: This paper explores the use of microwave energy for sewage sludge drying. Four sewage sludges and two microwave devices -a multimode microwave cavity oven (MC) and a single-mode microwave cavity oven (SMC)were used. On comparing the results of microwave drying with those of conventional drying with hot air, it was found that the use of microwaves saved considerable time and energy. Thus, with the MC microwave oven only 5 minutes was needed to achieve less than 1 wt% moisture content whereas 8 min was required with the SMC and 55 min with conventional drying. The reduction in volume obtained during the drying process was also assessed, the values reached being between 65-90 %. The composition of the sludge was studied by IRTF and elemental analysis. The dried sludges showed a high oxygen content distributed among compounds with C=O and C-O-C groups and a high aliphatic hydrogen content.
Abstract: Thermal treatment of activated carbon fibres (ACF) has been carried out using a microwave device, instead of a conventional furnace. The results show that microwave treatment affects the porosity of the ACFs, causing a reduction in micropore volume and micropore size. More importantly, the results also show that microwave treatment is a very effective method for modifying the surface chemistry of the ACFs with the production of pyrone groups, detected by FTIR. As a result very basic carbons, with points of zero charge approximately equal to 11, are readily obtained. (C) 2004 Elsevier Ltd. All rights reserved.
Abstract: Vapour grown carbon nanofibres (VGCNFs) produced at industrial scale were subjected to different treatments in N-2, air and CO2 atmospheres. The effect of these treatments on the homogeneity of the industrial product was investigated by means of TG analysis. Temperature programmed oxidation (TPO) experiments were performed to obtain DTG profiles. The DTG curves were deconvoluted into a mixture of Gaussian-Lorentzian curves, so that the contribution of each peak could be evaluated and assigned to phases of different reactivity. It was found that treatments in air are more selective in removing the most reactive phase (which has a higher content of amorphous carbon) at low burn-off, while at an elevated burn-off degree CO2 treatments result in a greater enrichment of the product in the less reactive phase (which has a higher content of fibre). This behaviour is attributed to the presence of a certain amount of iron in the industrially produced materials and its catalytic effect on the oxidative reactions. The effect of the treatments on the porous texture, surface chemistry and graphite-like character of the samples was also investigated. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: The pyrolysis of sewage sludge was studied in a microwave oven using graphite as microwave absorber. The pyrolysis temperature ranged from 800 to 1000 degreesC depending on the type of sewage sludge. A conventional electrical furnace was also employed in order to compare the results obtained with both methods. The pyrolysis oils were trapped in a series of condensers and their characteristics such as elemental analysis and calorific value were determined and compared with those of the initial sludge. The oil composition was analyzed by GC-MS. The oils from the microwave oven had n-alkanes and 1-alkenes, aromatic compounds, ranging from benzene derivatives to polycyclic aromatic hydrocarbons (PAHs), nitrogenated compounds, long chain aliphatic carboxylic acids, ketones and esters and also monoterpenes and steroids. The oil from the electric oven was composed basically of PAHs such as naphthalene, acenapthylene, phenanthrene, fluoranthene, benzo[a]anthracene, benzofluoranthenes, benzopyrenes, indenepyrene, benzo[ghi]perylene, and anthanthrene. In contrast, these compounds were not produced in the case of microwave-assisted pyrolysis. (C) 2003 Elsevier B.V. All rights reserved.
Abstract: Herein we report results from quantum chemical calculations on a large series of pyrone-like model compounds which are relevant for carbon basicity. In consonance with previous work (Carbon 1999, 37, 1002), pyrone-like functionalities at the edge of graphene layers are predicted to exhibit a broad range of pK(a) values (4-13) depending on the relative position of the ketone and etheric rings. The thermodynamic stability of pyrones is discussed in terms of reaction energies calculated for selected isodesmic processes which give insight into the role of the adjacent basal plane, the relative abundance of polycyclic pyrone-like structures, and so forth. In addition, we show that hypothetical redox processes involving pyrone-like structures have estimated electrochemical potentials which lie in the range of those observed in many carbons. Finally, the ability of pyrone-like model compounds to provide a global view of the chemical and electrochemical properties of basic carbon surfaces is discussed.
Abstract: The present work reports a theoretical study of the infrared spectra of chemical structures that are suitable to the description of the surface chemistry of carbon materials. Prior to any consideration, the computational approach was tested and adapted by comparing the predicted IR spectra to those obtained experimentally for various reference compounds. Several models were considered, subsequently accounting for the most relevant functional groups that have been postulated to decorate the edges of graphene layers on carbon materials (i.e., anhydrides, carboxyls, lactones, phenolic, quinones, and pyrones). For each of the previous functional groups, different structures involving a different number of fused rings were considered. This strategy allowed us to establish the effect of conjugation on the shift of the IR frequencies corresponding to a given functional group. Cooperative effects between different functional groups (phenol-carboxyl, phenol-lactone, and so on) were another aspect that revealed itself to be an interesting issue when assigning frequencies in the IR spectra of highly oxidized carbon materials. Thus, it was found that the frequencies of the C=O bonds present in acid functional groups were systematically lowered when phenolic groups were close enough to establish hydrogen bonds. Special attention was also paid to the elucidation of the origin of the 1600-cm(-1) band of carbons. It was found that, in the case of acid carbons, this band can be assigned to C=C stretching of carbon rings decorated mainly with phenolic groups. Cyclic ethers in basic carbons would also promote absorption in the 1600-cm(-1) region of the IR spectrum. Finally, the predicted assignments are employed to interpret the IR spectra obtained experimentally for several activated carbons.
Abstract: The pyrolysis of a sewage sludge, produced by a Spanish urban wastewater treatment plant, was carried out in a laboratory furnace. Pyrolysis conditions, like heating rate and final pyrolysis temperature, were varied so that their influence on the characteristics of the resulting gases, liquids and solid residues could be studied. It was found that increasing the pyrolysis temperature decreases the solid fraction yield and increases the gas fraction yield while that of the liquid fraction remains almost constant. Furthermore, the effect of the heating rate was found to be important only at low final pyrolysis temperatures. Independently of the pyrolysis conditions, all the solid products obtained were of a basic nature and highly macroporous, the meso- and micro-pore volumes being relatively low. Both oils and gases produced in the pyrolysis showed relatively high overall heating values, comparable to some conventional fuels, revealing the potential application of these products as fuel. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: This paper describes a new method for pyrolyzing sewage sludge using a microwave furnace. It was found that if just the raw wet sludge is treated in the microwave, only drying of the sample takes place. However, if the sludge is mixed with a small amount of a suitable microwave absorber (such as the char produced in the pyrolysis itself) temperatures of up to 900 degrees C can be achieved, so that pyrolysis takes place rather than drying. Microwave treatments were also compared with those carried out in a conventional electric furnace, as well as the characteristics of their respective carbonaceous solid residues.
Abstract: A laboratory-scale study was performed in order to investigate the effects that the addition of petroleum coke has on the texture and reactivity of the resultant metallurgical cokes. It was observed that the addition of petroleum coke to coking blends causes a reduction in the micropore volume and reactivity of metallurgical cokes. A relation between these two parameters was found, suggesting that the decrease in reactivity is a consequence of the decrease in surface area undergone by the metallurgical cokes obtained with petroleum coke addition. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: Pyrolysis is currently being considered as an alternative method of treating sewage sludge. It yields residual oils and gases, which can be used as fuels, and a solid which can either be burned or physically activated with air or CO2. The aim of this work was to study the influence of different pyrolysis conditions (e.g. temperature and heating rate) on the reactivity in air and in CO2 of carbonaceous materials obtained from these types of residues. An anaerobic sewage sludge produced in a Spanish urban waste water treatment plant, containing 5 wt.% moisture after air-drying, was pyrolyzed in an electrical laboratory furnace under different pyrolysis conditions. Non-isothermal reactivities (up to 1100 degreesC) in air and in CO2 of the carbonaceous materials obtained after pyrolysis were performed in a thermobalance. The TG and DTG curves obtained from these experiments are discussed. (C) 2001 Elsevier Science B.V. All rights reserved.
Abstract: Thermal treatment of activated carbon fibres (ACF) in a flow of N-2 gas has been carried out using a microwave device operating at 2450 MHz and with a power input of 1000 W, instead of a conventional furnace, and the samples were analysed by means of low temperature N2 adsorption, elemental analysis and determination of points of zero charge. The results show that microwave treatment for periods between 5 and 30 min affects the porosity of the ACF, causing a reduction in micropore volume and micropore size. More importantly, the results also show that microwave treatment is a very effective method for modifying the surface chemistry of the ACF. During microwave treatment surface groups are completely eliminated, whereas oxygen and nitrogen atoms bonded within the pseudo-graphitic layer planes are retained. On re-exposure to air the surface groups only reform to a very limited extent and as a result very basic carbons, with points of zero charge approximately equal to 11, are readily obtained. (C) 2001 Elsevier Science B.V. All rights reserved.
Abstract: This study was designed to determine the value of microcalorimetry as a probe of acid/base sites on carbon surfaces. Calorimetric studies of ammonia adsorption on acid and ammonia pretreated activated carbon (BDH) samples demonstrated that the technique does titrate acid sites (at equilibrium) according to their relative strengths. However, only in conjunction with other techniques, including Boehm titration, point of zero charge (PZC) and temperature programmed desorption (TPD) is it possible to determine the probable identity of the acid sites present on a given carbon. Collective consideration of the data from all techniques suggests that ammonia pretreatments create a surface whose chemistry is completely different from that of the original carbon. Not only are new acidic sites created, but a high concentration of basic sites are introduced as well. Thus, the final surface is clearly amphoteric, to an extent which is largely dictated by the pretreatment temperature. (C) 2000 Elsevier Science Ltd. All rights reserved.
Abstract: Certain aspects of the Nippon Steel Corp. (NSC) based test for measuring metallurgical coke reactivity were studied at the Spanish National Coal Institute (INCAR), It was found that some highly reactive cokes do not follow the correlation between the coke reactivity index (CRI) and the coke strength after reaction (CSR) observed in most metallurgical cokes. It was proved that the temperature of the fu mace at the beginning of the NSC test is irrelevant to the subsequent determination of CRI and CSR. By starting the test with a hot furnace, the total operation time and energy consumption can be reduced. In addition, a good correlation was found between the NSC test and a simpler and more economical test: the ECE-INCAR reactivity test.
Abstract: Two commercial activated carbons were subjected to thermal treatment in a N-2 atmosphere using a microwave multimode resonant cavity and a conventional electric tube furnace as heat sources. The temperature of the carbon bed during the microwave treatment was monitored using an infrared pyrometer and an Inconel sheltered type-K thermocouple. A comparison between both methods of measuring temperature was made. When similar treatment temperatures are used, both techniques produce similar changes in the textural and chemical properties of the activated carbons. However, microwave treatment is much less time-consuming than conventional heating. Microwave treatment in an inert environment seems to be an efficient and attractive way of removing oxygenated functionalities from carbon surfaces and of increasing the hydrophobicity and basicity of carbons.
Abstract: At present the primary difficulty with employing carbon for the purposes of NOx removal is the lack of selectivity. Carbon generally reacts as readily with oxygen as with NO. Thus, carbons in lean-burn exhausts (O-2/NO > 100) generally "burn" rather than selectively removing NO. Here we report on microcalorimetric studies which show that high temperature (950 degrees C) hydrogen-treated carbons will adsorb NO at room temperature, but will not adsorb significant quantities of oxygen. In contrast, the same activated carbon treated at high temperature in nitrogen will strongly adsorb both species. The selective nature of the hydrogen-treated material and the less-selective nature of the nitrogen-treated material is fury consistent with an earlier model of carbon surface chemistry which highlights the contrast in the character and concentration of active surface species created by each of these treatments.
Abstract: The surface chemistry of two active carbons was modified using a microwave device as a heat source. When microwave treatments are conducted in a nitrogen how, most oxygen-containing groups are removed from the surface of the carbons. This in turn gives rise to a significant increase in the pH of the carbons. Microwave treatment is less time-consuming than conventional heating. Only a few minutes are required to transform an acidic carbon into a basic carbon with a relatively low oxygen content. On the other hand, microwave-treated carbons undergo re-oxidation, to a greater or lesser extent, upon atmospheric exposure. It was observed that atmospheric re-oxidation of microwave-treated carbons tends to restore more acidic groups when the treatment is carried out on an acidic precursor than when the carbon is of a basic character. Nevertheless, carbons exposed to the atmosphere for up to 2 months still showed higher pHs than those of the corresponding precursors. (C) 1999 Elsevier Science Ltd. All rights reserved.
Abstract: The contribution to carbon basicity of pyrone-type structures is examined in this work by carrying out ab initio calculations on various cluster models. The different basic sites in a bicyclic pyrone structure are studied at the MP2/6-311+G(2d,2p)//MP2/6-31G(d) level of theory, rendering a reaction energy of -84.2 kcal/mol when a proton transfer takes place from H3O+ to the carbonylic oxygen in pyrones. The interpretation of the theoretical results confirms that resonance stabilization is a crucial factor controlling the basic character of pyrone-type structures. Various effects,like the modification of the etheric position, the relative position of non-neighboring oxygen atoms, and the presence of an adjacent basal plane, are also taken into account. Due to the increase of resonance stabilization observed in large cluster models, a broad spectrum of base strength covered by pyrone-type structures is predicted. The reaction energies of these pyrone-type structures with H3O+ are a few kilocalories per mole more exoergic than that of quinoline, an illustrative organic base. These results support the outstanding role played by pyrone-type structures in carbon basicity.
Abstract: Microcalorimetry was employed to study the surface chemistry of activated carbon. The chemistry of active sites was found to be a function of the pretreatment procedure. Following treatment at high temperature (950 degrees C) in nitrogen, the carbon adsorbed a great deal of oxygen at 25 degrees C with a remarkably high heat of adsorption (approx. 125 kcal/gmol O-2), whereas after treatment at high temperature in hydrogen the same carbon adsorbed virtually no oxygen at 25 degrees C. It was found, however, that the hydrogen treated carbon did adsorb oxygen at an elevated temperature (150 degrees C). Changes in the character of oxygen interaction with the carbon surface can be related to the nature of unsaturated sites on the carbon surface. Specifically, after treatment in nitrogen the surface contains a high concentration of highly unsaturated ('dangling') carbon atoms which interact rapidly and strongly with oxygen. All these sites are removed by the hydrogen treatment leaving only partially unsaturated sites, which only interact with oxygen at elevated temperature. (C) 1998 Elsevier Science B.V.
Abstract: One calcined and five green petroleum cokes were subjected, in a thermobalance, to controlled thermal treatment up to 1000 degrees C in an N-2 atmosphere and then reacted with CO2 at this temperature. Based on the DTG curves, the volatile matter released during the pyrolysis stage was divided into five intervals (<150, 150-400, 400-500, 500-750 and 750-1000 degrees C). Relevant information can be obtained from the amount of volatile matter released in these intervals. In particular, it was found that the amount of volatile matter released in the 400-500 degrees C interval can be related to the modifications observed in the fluidity of coal/PC blends which in turn affects the properties of metallurgical cokes obtained from these blends. Petroleum coke reactivity parameters were also obtained. A good correlation was found between the reactivity at 20% of conversion (R-20) and the optical texture index (OTT) calculated from the quantitative analysis of anisotropic components in the cokes. In contrast with the relevance of the information obtained from the volatile matter released during the thermal treatment of the petroleum cokes, the reactivity of petroleum cokes was found to have little influence on the reactivity of metallurgical cokes prepared from coal/PC blends. (C) 1998 Elsevier Science B.V. All rights reserved.
Abstract: In this paper, the use of calorimetric techniques to characterize different carbon materials is reviewed. The focus of the review is on the use of calorimetric techniques to assess chemical properties of carbons (e.g. nature of surface groups, hydrophobic/hydrophilic character, acidic/basic behavior, etc.), and a small section is dedicated to the application of calorimetry in determining the physical properties of carbons (e.g. surface areas, pore size distribution, etc.). The following techniques are described: immersion and flow-adsorption calorimetry and gas-adsorption microcalorimetry. Several representative examples of the use of calorimetry techniques to evaluate both physical and chemical properties are presented. It is demonstrated that calorimetry has provided unique insights into the structure and chemistry of active sites on the surface of various carbons. (C) 1998 Elsevier Science B.V.
Abstract: The contribution to carbon basicity of pi-cation interactions in aromatic systems has been investigated in this work by carrying out ab initio calculations on various cluster models. According to various levels of theory, the mono-, bi-, and tridentate H3O+-benzene complexes present a binding enthalpy in the gas phase of about -28 kcal/mol showing intermolecular contact through unconventional hydrogen bonding between H3O+ and the pi-cloud of benzene. The interaction energies calculated for other cluster models (pyrene-benzene-H3O+, coronene-H3O+, and a C54H18-H3O+ cluster) indicate that the size of the basal plane has a slight influence on the strength of this pi-cation interaction, whereas the presence of the pi-pi contacts reinforces the electrostatic interaction with the H3O+ cation. These theoretical results support experimental data on the ability of the basal planes to contribute to carbon basicity and suggest that pi-cation interactions may play an important role in the surface chemistry of carbon materials.
Abstract: The addition of petroleum coke to a typical industrial coal blend used in the production of metallurgical coke was studied. Cokes were produced at semi-industrial scale at the INCAR coking plant, using petroleum coke of different particle size distribution as an additive. Special attention was paid to changes caused in the textural properties (porosity, pore size distribution, fissures at the interface between metallurgical coke and petroleum coke) which have been found to be responsible for variations in the metallurgical coke quality parameters (e.g., mechanical strength and reactivity towards CO2). Variation in porosity was found to depend on particle size and the proportion of the additive. The decrease in the microporosity (i.e., pore radius < 3.7 nm) of the metallurgical cokes observed when petroleum coke is added to the coal blend, is postulated to be one of the main factors responsible for the decrease in the reactivity of these cokes. The variation of the mechanical strength indices can be explained by the changes in porosity and the quality of the interfaces between petroleum coke and metallurgical coke. (C) 1998 Elsevier Science B,V. All rights reserved.
Abstract: Activated carbon was modified by thermal treatment in order to obtain carbon samples with a very different surface chemistry and similar textural properties. The distribution of oxygen-containing surface groups in these carbons was assessed. The difference between the point of zero charge and the isoelectric point (pH(PZC) - pH(IEP)) showed the same trend as the difference between the oxygen content on the external surface, determined by XPS, and that in the bulk, obtained by chemical analysis (%O-surface - %O-bulk). Thus, pH(PZC) - pH(IEP) can be used as a parameter for indicating the radial distribution of oxygen-containing groups in carbons. Carbon samples were loaded with molybdenum, using the procedure of incipient-wetness impregnation. Hydrophilicity conferred by the oxygen functionalities to the carbon substrates promotes the formation of small, well dispersed crystals of the metal precursor on the support. Results seem to indicate that oxygen-containing surface groups work as anchorage centres for molybdenum complexes. The activated catalyst prepared with the most acidic carbon showed a higher amount of molybdenum than in the case of the other carbon supports. This may be due to the presence of metal sulphides that are bound to the oxygen groups of the support. (C) 1998 Elsevier Science BN. All rights reserved.
Abstract: Textural properties are usually considered as the most important properties of activated carbons, however many applications of these materials are conditioned by their chemical characteristics. The understanding of surface chemistry of activated carbons is being of increasing interest among manufactures, consumers and researchers of activated carbons. Despite of that, some aspects concerning surface chemistry of carbons are not well known. This work gives a global view about the importance, the characterization and the methods of modification of surface chemistry of activated carbons.
Abstract: Further insight into important aspects of the chemical surface properties of activated carbons is provided by rationalizing the differences in O-2 adsorption and desorption behavior of hydrogen- and nitrogen-treated activated carbons. The effects of heat treatment on the electrochemical behavior of these carbons were also instrumental in elucidating the nature and the distribution of carbon active sites, Activated carbon surfaces stabilized with hydrogen at 950 degrees C adsorb very little O-2 L at room temperature but the graphene layers are terminated with many free carbon active sites because significant O-2 adsorption does take place at 150 degrees C. Furthermore, the low-coverage differential heats of adsorption on sites accessible at 150 degrees C were lower than those on sites accessible at 25 degrees C. The role of these free carbon sites in determining the basicity of activated carbons was also addressed. On the basis of the important finding that the point of zero charge exhibits a maximum at intermediate heat-treatment temperatures, it is proposed that-in addition to the delocalized basal-plane pi electrons-the localized pi electrons at graphene edges (e.g., inplane divalent sigma pairs) act as Lewis bases that interact with protons in aqueous solution.
Abstract: The adsorption of model aromatic compounds (aniline and nitrobenzene) on chemically tailored activated carbons has been systematically investigated. Adsorption experiments at controlled solution pH conditions confirmed that both electrostatic and dispersive adsorbate/adsorbent interactions can have a significant influence on the equilibrium uptakes of ionic and nonionic adsorbate species. For aniline (a weak electrolyte), maximum uptakes were found on oxidized carbon surfaces at solution pH near the adsorbate's point of zero charge (pH(PZC)). In contrast, nondissociating nitrobenzene uptakes were enhanced on heat-treated surfaces with graphene layers unperturbed by electron-withdrawing Functional groups, particularly at solution pH similar to pH(PZC). A theoretical model that can successfully account for the observed trends is hereby proposed as a much needed predictor of the experimental conditions and adsorbent surface chemical properties that will maximize the uptake of aromatic compounds by activated carbons. (C) 1997 Elsevier Science Ltd.
Abstract: Activated carbons with various degrees of oxidation were prepared by treatment with concentrated nitric acid at various temperatures. Selective removal of the oxygen complexes by heating under nitrogen flow at 573, 773 and 1073 K was also considered. The samples were characterised through temperature-programmed desorption (TPD), Fourier transformed infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) techniques, FTIR and XPS techniques were found to be very useful for relating the shape of CO2 desorption profiles during pyrolysis with the nature of the functional groups desorbed. Three steps in TPD profiles were observed: (i) the desorption of carboxylic acid groups in the form of CO2 in the lower temperature range, 400-623 K; (ii) the decomposition of lactone groups created during HNO3 treatment and, to some extent, originated during heating rearrangements, in the temperature range 623-823 K; and (iii) the elimination of the more stable ether groups upon treatment at temperatures above 823 K. The combination of these three techniques (TPD, FTIR and XPS) gives a reasonable picture of the surface chemistry of the oxidised activated carbons. (C) 1997 Elsevier Science B.V.
Abstract: The relevance of petroleum coke activity during the plastic stage in cocarbonization with a low-volatile bituminous coal was assessed by establishing differences in lowering of Gieseler maximum fluidity of the blend (bituminous coal and petroleum coke) and in metallurgical coke quality. Differences in fluidity of the blend were well correlated with parameters used in petroleum coke characterization such as hydrogen donor ability, the volatile matter released between 400 and 500 degrees C, and the ratio of methyl and methylene groups in aromatic clusters, The metallurgical cokes were produced in a laboratory oven from blends of petroleum coke-bituminous coal and, then, characterized in terms of abrasion strength and reactivity to CO2. There is a clear relation between the chemical activity of petroleum coke in cocarbonization systems with coal and improvements in metallurgical coke quality. The ability of petroleum coke to interact with coal during the plastic stage contributes to good bonding between components as evidenced by the quantitative study of interfaces by optical microscopy.
Abstract: The effects of coal preheating on the quality, textural and surface properties of the coke were assessed in comparison with the conventional wet charging system using a Spanish hvb coal. Coal was flash dried and preheated in a 2 t h(-1) Precarbon pilot plant. Carbonization tests on the wet and preheated coals were carried out in a semi-industrial scale oven of 6 tonne capacity. Because textural and surface properties have an important effect on coke quality, an exhaustive characterization of the resultant cokes was made. Cokes were characterized by using microscopic image analysis, mercury porosimetry, density measurements, optical and scanning electron microscopy (SEM) of surfaces. Mechanical strength and reactivity towards CO2 were also studied. Surface areas were determined by N-2 adsorption at 77 K using the Brunauer-Emmett-Teller (BET) equation and CO2 adsorption at 273 K using the Dubinin-Radushkevich (DR) equation. Coal preheating improved coke mechanical strength considerably but at the same time increased the reactivity of the cokes. The higher reactivity could be due to the increase in microporosity and CO2 surface area.
Abstract: Differences between the surface chemical properties of hydrogen- and nitrogen-treated samples of an activated carbon were quantified using several complementary techniques. Calorimetric studies conducted at 303 K revealed that the sample treated in N-2 at 1223 K adsorbs a great deal of oxygen with unusually high differential heats. In fact, both the quantity and the heat of adsorption increased when the treatment temperature was raised from 773 to 1223 K. In contrast, samples treated in H-2 adsorbed less and less O-2 as the temperature of treatment was raised; after treatment at 1223 K, virtually no O-2 adsorption occurred. At the same time the H/C ratio in the H-2-treated samples decreased with increasing treatment temperature. Point of zero charge measurements revealed that only H-2 treatments at high temperature (> 1073 K) create basic (hydrophobic) surfaces which are stable after prolonged air exposure. These findings are consistent with the notion that the removal of oxygen in the form of CO and CO2 during high-temperature N-2 treatment leaves unsaturated carbon atoms at crystallite edges; these sites are very active for subsequent oxygen adsorption. In contrast, high-temperature H-2 treatment accomplishes three tasks: (a) it also removes surface oxygen; (b) it stabilizes some of the (re)active sites by forming stable C-H bonds; (c) it gasifies the most reactive unsaturated carbon atoms. The relative contributions of these three effects depend on the temperature of H-2 treatment. The carbon surface resulting from high-temperature H-2 treatment is stable against subsequent O-2 adsorption in ambient conditions.
Abstract: It is demonstrated that stable basic carbons, which will not adsorb oxygen in ambient laboratory conditions, can be created via a relatively low-temperature process. These highly basic carbons are created by treating mixtures of carbons and platinum (in the form of particles supported on a high surface area material) in hydrogen at 500 degrees C, or even at lower temperatures in some cases. In the absence of platinum, creation of highly basic and stable surfaces with the same starting material requires hydrogen treatments at far higher temperatures (ca. 900 degrees C). Evidence is presented to support the hypothesis that the role played by platinum (or any noble metal) is to produce atomic hydrogen, which spills over onto the carbon surface. This atomic hydrogen hydrogasifies the most reactive, unsaturated carbon atoms al far lower temperatures than molecular hydrogen, thus leading to surface stabilization at relatively low temperatures.
Abstract: It is often assumed that green petroleum coke behaves as an inert material in cocarbonization with coking coal blends and has no active behavior on the important thermoplastic properties of the coal blend. This paper investigates that assumption. The objective of this study is to clarify effects arising when different petroleum cokes are added to a single coal or an industrial blend. The effects studied include changes during the pyrolysis stages of the cocarbonization, using a bituminous coal. This was done to study if petroleum coke is totally inert at the plastic stage of a given coal or there is an influence at the plastic stage. A further aim is to show how conventional and nonconventional techniques for petroleum coke characterization relate to its activity with the plastic stage of coal. A range of six petroleum cokes was used. The petroleum cokes were studied in terms of (a) optical texture, (b) FTIR spectroscopy, (c) hydrogen donor ability, (d) thermogravimetric analysis of the pyrolysis stage, (e) free-swelling index, and (f) thermoplastic properties of blends made up of a bituminous coal and petroleum coke. Evidence for a significant activity of some petroleum cokes was assessed using the above techniques, which can be considered as nonconventional in petroleum coke characterization. A good correlation among the parameters obtained from the above techniques/methods was found, indicating that the presence of unreacted and partially carbonized material, the hydrogen donor ability, the relative proportion of methyl and methylene groups, the amount of volatile matter released at a temperature range between 400 and 500 degrees C, the temperature of maximum volatile matter evolution and, finally, the agglomeration degree of petroleum cokes can be considered as important factors in the plastic properties of cocarbonization systems with coking coals.
Abstract: The effects of a green petroleum coke addition on coke metallurgical quality in terms of mechanical strength (IRSID test), reactivity to carbon dioxide (GEE and NSC tests) and texture (porosity and pore size distribution) are described. Petroleum coke addition, generally decreases porosity, both to semiindustrial and laboratory scale mainly due to the microporosity, which helps to explain the observed decrease in the reactivity to CO2. A laboratory-scale methodology to assess the main properties of metallurgical cokes was developed. Laboratory results were backed by tests to semiindustrial scale.
Abstract: The relationship between the textural properties and reactivity in metallurgical cokes, obtained from different rank coals using wet and preheating techniques, was studied. Cokes from preheated coals exhibit a higher reactivity than those obtained from wet ones. This may be related to the higher microporosity and CO2 surface area.
Abstract: The influence of coal preheating on the properties of the resultant cokes has been studied using a wide range of coking coals which were carbonized, wet and after preheating, at the INCAR Coking Test Plant operating on a semi-industrial scale. Coke mechanical strength improved by using the preheating process rather than wet charging, except for some cokes produced from coals with a content of volatile matter between 19.5 and 25 wt% db. The most significant differences between cokes from preheated and wet coals were observed in terms of microporosity. Increase in microporosity as a consequence of preheating is accompanied by an increase in coke reactivity. As regards the optical texture of cokes, a slight increase in the total anisotropy and the size of mosaic features was observed.
Abstract: the MW pyrolysis as an original thermochemical process of materials is presented. This chapter comprises a general overview of the thermochemical and quantifying aspects of the pyrolysis process, including current application togethe with a compilation of the most frequently used materials.
