BOLETIN - Grupo Español del Carbón (GEC) · Grupo Español del Carbón Grupo Español del Carbón...

Primaryparticlenetwork

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Coloidal primary particles Clusters

Substituted resorcinolFormaldehyde

Resorcinol Addition Condensation

BOLETIN delGrupo Español del CarbónGrupo Español del Carbón

www.gecarbon.org

ISSN 2172 - 6094nº 26/Diciembre 2012

This issue of the Boletín del GrupoEspañol del Carbón (Spanish CarbonGroup Bulletin) is dedicated to carbongels. These are porous carbonmate r i a l s p repa red by t hecarbonization of organic gels obtainedby the sol-gel polymerization of certainorganic monomers, e.g., resorcinoland formaldehyde, in the presence of basic or acid catalysts. Sol-gelprocessing is a versatile method forsynthesizing solid materials and iscarried out in a liquid solution and atlow temperatures, generally below100 ºC.An important step in the preparationof these materials is the techniqueused to dry the wet gel or hydrogel,because it affects the surfaceproperties of the final carbon. Threemain methods have been developedfor this purpose, i.e., subcritical drying,supercritical drying and freeze-drying,which give rise to xerogels, aerogelsand cryogels, respectively.Research on carbon gels has grownrapidly since resorcinol-formaldehydegels were first prepared by R.W.Pekala in 1989 [1]. This is largely dueto their well-developed micro- andmesoporosity, whose size and volumecan be controlled independently, andto their large surface area. In addition,they can be readily doped with otherheteroatoms and obtained in varyingforms such as powders, microspheres,monoliths and thin films. Thanks tothese characteristics, they are usedin adsorption, heterogeneous catalysisand electrochemical energy storage,among other applications [2-4].Various authors have contributed tothis special issue. Prof. A. Celzardand coworkers from the Université deLorraine (France) show that carbongels can be prepared from phenolicresources of natural origin such as

tannin and lignin, significantly reducingthe costs of the preparation process.Prof. S.R. Mukai from the HokkaidoUniversity (Japan) describes theversatility of carbon gels to adoptdifferent morphologies, attributable totheir behavior as readily moldablefluids before their gelation. Prof. J.L.Figueiredo from the Universidade doPorto (Portugal) shows how thecharacteristics of carbon xerogels canbe modified to ensure their adequateperformance as metal-free catalystsfor a wide variety of reactions in eitherliquid or gas phase. Finally, Prof. N.Job from the University of Liège(Belgium) addresses mass transportin carbon gels with tuned porosity,which is an important issue in manyof the applications of these materials.I wish to express my gratitude to allauthors that have contributed withtheir time and effort to this specialissue of the Bulletin.

Carlos Moreno-CastillaGuest Editor

1 Pekala RW. Organic aerogels fromthe polycondensation of resorcinolwith formaldehyde. J Mat Sci 1989;V24(9):3221-3227.2 Fairén-Jiménez D, Carrasco-MarínF, Moreno-Castilla C. Adsorption ofbenzene, toluene and xylenes onmonolithic carbon aerogels from dryair flows. Langmuir 2007; 23(20):10095-101.3 Moreno-Castilla, C. Carbon gels incatalysis. In Serp P, Figueiredo JL,editors. Carbon materials for catalysis,New York: J. Wiley & Sons, 2009:Chapter 10.4 Zhang LL, Zhao XS. Carbon-basedmater ia ls as supercapac i to relectrodes. Chem Soc Rev 2009;38(9):2520-2531.

Editor Jefe:

J. Angel MenéndezINCAR-CSIC. Oviedo

Editores:

Ana ArenillasINCAR-CSIC. Oviedo

Jorge BediaUniversidad Autónoma. Madrid

M. Angeles Lillo-RódenasUniversidad de Alicante

Manuel Sanchez-PoloUniversidad de Granada

Isabel SuelvesICB-CSIC. Zaragoza

Indice PrefacePreface .........................................1Carbon gels derived from naturalresources ....................................2Controlling the Morphology ofCarbon Gels .................................8

Carbon xerogels for catalyticapplications ................................12

Mass transport in carbon gels withtuned porosity ............................18

Reseña. Xerolutions, a companyproducing carbon gels, gets theSodeco award ......................... 24

XII Reunión del GEC .................24

Sol-gel polymerization reaction of resorcinol and formaldehyde. Adapted from reference [1]

nº26/Diciembre 2012

A. Celzard*1,2, V. Fierro2, G. Amaral-Labat1,2, A. Szczurek1,2, F. Braghiroli1,2, J. Parmentier3, A. Pizzi1,4, L.I.Grishechko1,2,5, B.N. Kuznetsov5

1 Université de Lorraine, ENSTIB, 27 rue Ph. Seguin, BP 1041, 88051 Epinal, France2 Institut Jean Lamour, UMR CNRS 7198, ENSTIB, 27 rue Ph. Seguin, BP 1041, 88051 Epinal, France3 Institut de Science des Matériaux de Mulhouse, LRC CNRS 7228, Université de Haute Alsace, 15 rue Jean Starcky,BP 2488, 68057 Mulhouse Cedex, France4 LERMAB, EA 4370, ENSTIB, 27 rue Ph. Seguin, BP 1041, 88051 Epinal, France5 Institute of Chemistry and Chemical Technology, SB RAS, 42 K. Marx Street, Krasnoyarsk 660049, Russia* Corresponding author: [email protected]

AbstractMost carbon gels investigated so far and reportedin the literature were prepared from resorcinolcrosslinked with formaldehyde in water, and weregenerally dried with supercritical CO2 before beingpyrolysed. In the present paper, through someselected examples, we show how valuable carbongels can be derived from other phenolic resourceshaving a natural origin. Special emphasis is givento tannin and lignin, both derived from wood, aspotential precursors of carbon aero- and cryogels.However, natural compounds not obeying the usualconcepts of sol-gel chemistry may also be used forpreparing carbon gels, such as cellulose, and evenglucose. In the latter case, hydrothermal treatmentforces the phase separation to occur, and leads tomonoliths which can be advantageously convertedinto carbon aerogels by supercritical drying andsubsequent pyrolysis.

ResumenLa mayoría de los geles de carbón investigadoshasta ahora y citados en la literatura han sidopreparados a partir de resorcinol reticulado conformaldehído en agua, y generalmente secados conCO2 supercrítico antes de ser pirolizados. En estemanuscrito, hemos seleccionado algunos ejemplosy mostramos como se pueden preparar excelentesgeles de carbón a partir de compuestos fenólicosque tienen un origen natural. Especial menciónmerecen los taninos y ligninas, extraídos de lamadera, como potenciales precursores de aero- ycriogeles de carbón. Sin embargo, otros compuestosnaturales que no obedecen al concepto habitual dela química de sol-gel también pueden ser usadospara preparar geles de carbón, como la celulosa yla glucosa. En este último caso, el tratamientohidrotermal fuerza la separación de fase, y conducea monolitos que pueden ser convertidos en aerogelesde carbón por secado supercrítico y posterior pirólisis.

1. IntroductionGels are, among other possible definitions that canbe used for describing them, semisolid systemscomprising two phases, solid and liquid, embeddedin each other in such a way that the pores of thesolid, filled with solvent, are of colloidal dimensions[1]. Removal of the liquid phase leads to a highlyporous backbone, especially if supercritical of freezedrying has been carried out. The latter processesindeed prevent the shrinkage of the material (thoughnever completely) and, therefore, the collapse of thepores induced by capillary forces can be minimized.Getting carbon gels is thus possible, as it is alreadythe case for most carbonaceous solids, from thecontrolled pyrolysis of organic precursors, hereorganic gels, in non-oxidizing atmosphere.From the simple aforementioned assertions, it canbe concluded that using biopolymers or naturalresources in general for preparing carbon gels

involves several requirements. First, if the porousstructure has to be maintained, the precursors shouldnot melt when heated. Therefore, all chemical (i.e.crosslinked) gels might be used for that purpose,since chemical gels are insoluble, infusible, three-dimensional polymer networks. In contrast, jelliesbased on fruits and aspic based on gelatine, forexample, are physical (i.e. coagulated) gels, andmelt when heated. Then, the boiling and the pyrolysisof the resultant liquid lead to an ill-defined brittleresidue, which is porous due to the evolution ofgases, but which is certainly not a carbon gel.Nevertheless, infusible biopolymers which can bedissolved and next coagulated may also becomeprecursors of carbon gels, provided that the porousstructure made up by their entanglement can bemaintained all along the drying and heat-treatmentprocesses. This is the case of cellulose, whosecarbon yield, however, is rather low.Possible natural precursors for preparing carbongels should thus be prone to gelation or at least torigid entanglement, be infusible before or after beingcrosslinked, have a high carbon yield and be cheap.One advantage of using them is indeed, on top oftheir “greener” character, the correspondingly reducedcost of the resultant material. In the following,emphasis will be given to phenolic biopolymers suchas tannin and lignin, especially well matching theserequirements. They are described in section 2, andtheir use for preparing gels is developed in section3 and partly in section 4. However, natural moleculesnot obeying the aforementioned requirements may,in some cases, also lead to carbon gels using specialpreparation processes such as hydrothermaltreatment. Some examples are given in section 4.

2. Tannin and lignin as precursors of carbon gelsTannin and lignin are both derived from plants, andespecially from wood. There are fundamentaldifferences between them, as explained below, butboth share valuable characteristics such as veryaromatic character, hence leading to a high carbonyield, around 45%, and ability of being easilycrosslinked thanks to their number of reactivehydroxyl groups. Figure 1 shows the typical chemicalstructure of tannin and lignin, though that of lignincan just be a model, strongly depending on itsvegetable origin and on the severity of its extractionprocess.Tannins are extractible molecules, industriallyexploited from wattle, pine and quebracho trees, forexample. They are oligomers based on the repetitionof the unit shown in Figure 1a, whose exact structuredepends on the origin. For instance, wattle tanninhas OH groups in positions 7, 3’, 4’ and 5’, and thecorresponding flavonoid units are mainly 4,6-linked,leading to water-soluble phenolic compounds withtypical molecular weights ranging from 0.5 to 3.5kDa [2]. Lignin is also a phenolic molecule, but is athree-dimensional, crosslinked, polymer. It is the

Carbon gels derived from natural resourcesGeles de carbón de origen natural

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Bol. Grupo Español Carbón

most common natural polymer after cellulose, andits role in plant tissues is cementing cellulose fibres.It is based on three phenylpropanoid monomers, asshown in Figure 1b-d, connected with each othersthrough various inter-unit linkages, thus resulting ina complex macromolecular structure [3,4]. Lignin isgenerally considered as a waste material from thepulp and paper industry, and is most often used asfuel for the energy balance of pulping process.

Tannin and lignin can be suitably crosslinked byaldehydes, such as formaldehyde, and are thusnatural counterparts of resorcinol, from which mostcarbon gels have been prepared and investigated.However, tannin can be used either alone withformaldehyde for preparing organic gels and derivedcarbon gels [5], or in combination with other syntheticphenolic molecules such as phenol or resorcinol [6],whereas lignin can never be used alone. Whateverthe way it is crosslinked, and as far as the authorsknow, lignin alone could never lead to homogeneous,stable, organic gels from which carbon gels mightbe prepared. Instead, hard precipitates are obtained.Nice, reproducible, gels could only be obtained bycombining lignin with smaller phenolic moleculessuch as phenol [7], resorcinol [8] and tannin [9].

3. Carbon gels from tannin and lignin: selectedexamplesAs explained above, tannin easily gels in the presenceof formaldehyde. Unlike what is known for resorcinol,a broad range of pH can be successfully used withtannin, leading to materials having different porousstructures, as demonstrated below. The typical pHof a tannin solution is close to 4.5, at which thereactivity with formaldehyde is the lowest, the highest

reactivity being found at pH lower than 3 and higherthan 7. The pH can thus be advantageously increasedor decreased through the use of any base or acid,respectively, abusively but very frequently referredto as “catalysts”. As a consequence, the gelationtime is strongly modified, and the highest BET surfaceareas of the resulting carbon aerogels, above 700m2/g, are found at pH 4 – 5, whereas the mesoporevolumes increase with pH, up to 0.9 cm3/g [5].Carbon aerogels derived from tannin present a typicalnodular structure, see Figure 2, and the averagediameter of such nodules decreases when the pHincreases. Such features are very usual for all othergels derived from phenolic precursors. The materialsshown in Figure 2 were obtained by supercriticaldrying in acetone, which is as efficient as CO2 forthat purpose [10] but much cheaper. Combined withthe fact that a very inexpensive precursor was used,tannin, these resultant carbon aerogels are 5 timescheaper than their resorcinol-based counterparts[5].Instead of drying supercritically the monoliths beforepyrolysis, freeze-drying can be carried out. Theprocess is a bit longer, since it requires several stepsof solvent exchange, but is cheaper. Carbon cryogelsderived from mixtures of tannin and resorcinol weresuccessfully prepared, in which both proportions ofreactants and pH were changed. As a result, a muchbroader range of porous structures than those oftypical resorcinol – formaldehyde gels could beobtained. Interestingly, pore size distributions (PSD)could be tuned through the pH for both tannin –formaldehyde (TF) carbon aerogels and tannin –resorcinol – formaldehyde (TRF) carbon cryogels,either shifting the PSD at rather constant mesoporevolume (for TRF), or keeping the PSD centred onthe same pore width but changing the mesoporevolumes (for TF), see Figure 3 [5,6].The porous structure of cryogels is different fromthat of aerogels of identical composition for twopossible reasons. One is related to shrinkage duringdrying, which may be very different depending onthe process. Thus, diluted gels can be freeze-driedwith a much lower shrinkage than what can beobserved when using supercritical solvents. As aconsequence, extremely high porosities, butcorrespondingly low mechanical properties, areobtained. Such weak materials may then suffer a

Figure 1. Chemical structure of: (a) the flavonoid unit of condensedtannins; (b)-(d) the structural units of lignin: (b) p-coumaryl alcohol;(c) coniferyl alcohol (guaiacyl); (d) sinapyl alcohol (syringyl).Figura 1. Estructura química de: (a) la unidad flavonoide detaninos condensados; (b)-(d) las unidades estructurales de lalignina: (b) alcohol p-cumarílico; (c) alcohol coniferílico (guaiacilo);(d) alcohol sinapinílico (siringilo).

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Figure 2. SEM pictures of tannin-based carbon aerogels prepared at two different pH. The effect of the pH on the porous structureis clearly seen even at low magnification (after [5]).Figura 2. Fotografías MEB de aerogeles de carbón preparados a partir de tanino y a dos pH diferentes. El efecto del pH sobre laestructura porosa se ve claramente incluso al menor aumento (de [5]).

a) b) c) d)


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serious pore collapse during pyrolysis, leading tomaterials that are finally less porous than theiraerogel counterparts, whose mechanical propertieswere higher due to lower amount of very wide pores.This phenomenon was observed for diluted tannin– formaldehyde cryogels, presenting a minimum ofBET surface area (400 m2/g) at pH 4 – 5 [11], i.e.at the pH at which a maximum was observed fortannin – formaldehyde aerogels (see above). Surfaceareas and micropore volumes as high as 1400 m2/gand 0.55 cm3/g, respectively, were measured at pH7.5. The fact that NaOH used for adjusting the pHat this value, remaining inside the porosity and actingas an activating agent during the pyrolysis, thusleading to somewhat activated carbon gels, can notbe discarded. The second reason for differences ofporosity between aero- and cryogels is related tothe nature of the solvent that has been used forfreeze-drying. Tert-butanol is most frequently used,but is able to crystallise inside the porosity of thegels, leading to acicular crystallites and deeplymodifying the resultant porosity. Figure 4 shows theexample of carbon cryogels particles prepared fromtannin and formaldehyde using the emulsionpolymerization process formerly described byYamamoto et al. [12], and in which needle-like poresare easily seen. Materials freeze-dried in water,whose ice volume is much bigger than that of theliquid phase, comprise even bigger pores. As aconclusion, foreseeing the kind of porosity that acarbon gel will have after drying and pyrolysis istotally impossible.Preparing a chemical gel from any phenolic precursorassumes that the conditions of gelation are knownand have been optimized. As explained above, highinitial dilution of the precursor resin leads to high

pore volumes, but the solid backbone is so weakthat a significant part of the porosity is lost duringdrying and/or subsequent pyrolysis. Less dilutedgels should thus be preferred, having a lower initialporosity, but whose major part will be maintainedbecause they are stiffer, so they can finally presentmuch more developed porous structure. Mechanicalstrength is indeed a very important parameter, directlyrelated to porosity which can be controlled by initialdilution, pH and amount of crosslinker. There existsan optimum for each of these experimentalparameters. Additionally, some values that theseparameters may take can be in favour, or in contrasthinder and even prevent, gelation.Therefore, for better understanding phenolic systems,building a phase diagram may be of great interest.This has been recently done by our group in thecase of tannin – formaldehyde aqueous solutions,see below. The situation is more complicated whenmore than one phenolic molecule is present, suchas in lignin – tannin or lignin – phenol systems. Inthe latter cases, some parameters must be fixed foravoiding multi-dimensional diagrams that would beimpossible to represent. Figure 5 illustrates tannin– formaldehyde and tannin – lignin – formaldehydediagrams as a function of a limited number ofparameters. The existence of areas within which nogelation occurs, or within which reversible (physical)gels are obtained, is evidenced.The organic gels presented in Figure 5, especiallythe aerogels, have been investigated recently [7,9]but still deserve additional studies. All stable chemicalgels present the expected nodular structure, whateverthe composition, and so do their carbonaceousderivatives. Both packing and average diameter oftheir nodules depend on the amount of crosslinker,pH and dilution of the resin. Pyrolysis leads to nicecarbon aerogels, whose porous texture is still underinvestigation and will be published in the near future.The first results confirm the relevance of lignin asprecursor of carbon gels, leading to surface areashigher than 500 m2/g, just like for many other carbongels derived from synthetic resources.

4. Carbon gels prepared from alternative methodsor derived from other resourcesThe usual sol-gel chemistry used above is notnecessarily involved for preparing organic monolithsfrom which carbon gels can be finally obtained. Inother words, the classical scheme consisting indissolving a precursor into a solvent, and whose

Figure 3. Pore size distributions of tannin – resorcinol –formaldehyde carbon cryogels (top) and tannin – formaldehydecarbon aerogels (bottom), showing the significant impact of theinitial pH of the solution before gelation (after [5,6]).Figura 3. Distribuciones de tamaño de poro en criogeles decarbón preparados a partir de tanino - resorcinol - formaldehído(arriba) y en aerogeles de carbón preparados a partir de tanino- formaldehído (abajo), mostrando el impacto significativo del pHinicial de la solución antes de la gelificación (de [5,6]).

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Figure 4. Needle-like macroporosity exhibited by carbon cryogelparticles made by inverse emulsion polymerization of a tannin-formaldehyde solution, followed by freeze-drying in t-butanol andpyrolysis at 900°C.Figura 4. Macroporosidad acicular que muestran las partículasde criogel de carbón sintetizadas por polimerización en emulsióninversa de una solución de tanino - formaldehído, seguida deliofilización en ter-butanol y pirólisis a 900°C.


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crosslinking will lead first to a sol, i.e. to a very finesuspension of solid, and next to a gel through thegradual clustering of the suspended particles, canbe more or less modified, depending on both thesynthesis protocols and on the precursors.For example, the phase separation leading to theoccurrence of polymer particles and hence to a gelcan be forced when hydrothermal conditions areused. In this case, separation between gelation andprecipitation is not always easy, and someexperimental conditions lead to an overlapping ofthese two regimes. In a general way, it can beassumed that a gel is formed when the size of theconstitutive nodules is both “small” (say, a few nmto a few tens of nm) and homogeneous, unlikeprecipitates whose particles have all possibledimensions, and can be much bigger. Figure 6 thusshows the materials which could be derived fromwattle tannin dissolved in aqueous solutions ofdifferent pH and submitted to 180°C under 1MPa.Gels obtained this way are, just like their counterpartsprepared by conventional sol-gel chemistry, basedon rather spherical nodules. However, usinghydrothermal conditions, neither crosslinker norsupercritical drying was required. The porosity wasindeed fully maintained after normal (i.e. subcritical)drying in air and subsequent pyrolysis at 900°C.Thus, the sample shown at the left of Figure 6 hada BET surface area of 495 m2/g [13]. Additionally,and interestingly, if not pure water but concentratedammonia solution is used instead, directfunctionalization of the precursor can be achieved,leading to hydrothermal carbons having nitrogencontents as high as around 15 wt.%. However, evenif the term “hydrothermal carbon” has been widelyused in the literature, the materials obtained directlyfrom the autoclave should never be considered asauthentic carbons, since their elemental carboncontent is around 60 wt.% only. Therefore, anadditional pyrolysis step is required for increasingthe carbon content above 80%. Doing so, thematerials functionalized with nitrogen loose part ofthe latter, which is decreased to a value (a few %)

which depends on the final temperature.Hydrothermal treatment is also a very interestingprocess for getting carbons from precursors whosecarbon yield is intrinsically low when directly pyrolysedin standard conditions, and/or whose gelation doesnot occur according to the classical sol-gel chemistry.Hydrothermal process indeed partly prevents theevolution of gases based on light elements such asC and N, and forces the phase separation to occur.When a highly dispersed monolith, based on veryfine particles, is obtained, i.e. a true gel, its dryingand subsequent pyrolysis may lead to a carbon gelderived from a priori unexpected precursors. Thebest examples have been reported by the group ofAntonietti, using glucose as the main startingcompound. A rather fibrous carbon aerogel couldindeed be prepared from the hydrothermal treatmentof a mixture of glucose and ovalbumin, see Figure7. BET surface area and mesopore volume higherthan 300 m2/g and 0.5 cm3/g, respectively, weremeasured after pyrolysis at 900°C. Due to thepresence of protein among the precursors, thiscarbon aerogel was naturally doped with around 6wt.% of nitrogen [14].Such materials could also be easily doped by sulphurby simply incorporating a S source in the form oflow amounts of either S-(2-thienyl)-L cysteine or 2-thiophene carboxaldehyde, leading to similar carbonaerogels as above, after supercritical drying andpyrolysis, but presenting special catalytic propertiesrelated to their surface composition [15]. Finally,borax has been recently suggested as both catalystand structure-directing agent, allowing the preparationof monoliths from glucose treated in hydrothermalconditions. After supercritical drying and pyrolysis,carbon aerogels having different porous texturescould be obtained, depending on the amount ofborax. The highest BET surface area and mesoporevolume were around 360 m2/g and 0.4 cm3/g,respectively [16]. As found in many other worksbefore, the size of the constitutive nodules changedwith the pH, hence with the amount of borax in thisspecial case.

N - Not gelledPG - Physical gelationO - OpaqueT - TransparentST - Semi-transparentT(LB) - Transparent (light black)T(LB) - Transparent (dark black)

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Figure 5. Phase diagrams of tannin – formaldehyde (TF, top) and tannin – lignin – formaldehyde (TLF, bottom, reprinted from [9] withpermission) solutions, showing the kinds of materials obtained, depending on a limited number of experimental parameters (for TF:pH and dilution at constant T/F ratio = 1.35; for TLF: T/L proportions and amount of formaldehyde at fixed dilution = 26% and constantpH = 10). Supercritical, freeze-, or subcritical drying and pyrolysis of such gels leads to a very broad range of carbon gels, whosecomplete investigation is far from being accomplished.Figura 5. Diagramas de fase de las disoluciones tanino - formaldehído (TF, arriba) y tanino - lignina - formaldehído (TLF, abajo,reproducido de [9] con permiso), mostrando las clases de materiales obtenidos, según un número limitado de parámetros experimentales(para TF: pH y proporción de la dilución a T/F constante = 1.35; para TLF: Proporción de T/L y cantidad de formaldehído a diluciónfija = 26 % y pH constante = 10). El secado supercrítico, por liofilización, o subcrítico de esos geles y su posterior pirólisis conducea una muy amplia gama de geles de carbón, cuya investigación completa está lejos de haberse finalizado.


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Cellulose gels, aero- and cryogels, have beensuccessfully prepared according to a number oftechniques [17-21]. Cellulose could be used as suchor as an ester: cellulose acetate. The former leadsto physical (coagulated) gels, whereas the latter,through the use of isocyanate as crosslinker, leadsto chemical gels. Both kinds of materials could beconverted into carbon gels. Due to the fibrous natureof the precursor, the morphology of the resultantcarbons was found to be fibrillar, thus was differentfrom the traditional nodular structure most frequentlyreported so far. However, the characteristics of suchgels are fully comparable with those based onphenolic precursors. For example, carbon aerogelsderived from cellulose acetate (dried with supercriticalCO2) presented BET surface areas higher than 400m2/g [22] and mesopore volumes around 0.2 cm3/g[23]. Because of such valuable characteristics, thesematerials could be successfully doped with noblemetals and suggested as catalyst for PEM fuel cells[24]. From physical cellulose gels, fibrillar carboncryogels have been obtained. These cryogelspresented surface areas and mesopore volumeshigher than 500 m2/g and 0.6 cm3/g, respectively[20]. Due to the low carbon yield of cellulose, thesematerials were advantageously pyrolysed in HCl,making the carbon yield increase from 15 to 30%,see Figure 8.

5. ConclusionProducing carbon gels from natural resources,whether they are aerogels, cryogels or xerogels,was reported to be perfectly feasible, provided thatthe precursors are suitably chosen. Given that meltingand extensive thermal decomposition should be

absolutely avoided, most of the porous, organic,precursors are chemical gels of phenolic nature.However, other systems have been reported, whosemain drawbacks, especially low carbon yield, couldbe got round through a preliminary hydrothermaltreatment or through a pyrolysis in unusualatmospheres. Nevertheless, such methods are notenough when the precursor can not resist thepyrolysis step, such as materials mainly based onproteins. For example, soy-tannin aerogels recentlyreported could never be converted into theircarbonaceous counterparts [25].When compared to carbon gels made from syntheticmolecules, biopolymer-derived carbon gels generallyhave very similar micropore volumes but possesshigher macropore volumes and much broadermesopore size distributions. This finding is certainlyrelated to the much higher molecular weight ofstructural units of bio-oligomers and biopolymers.Crosslinked phenolic biopolymers are indeedcharacterized by long polymer chains leading tosteric hindrance, hence preventing the formation ofvery narrow pores related to high surface areas.This is well known for tannin oligomers, which areessentially branched and rigid, preventing thecomplete reactivity of all possible sites withformaldehyde. Additionally, the reaction with thecrosslinker further provides a 3D-character to thepolymer network, e.g. in the case of lignin, thuspreventing the formation of very narrow pores. Incontrast, organic and carbon gels based on smallersynthetic molecules, such as resorcinol, may presenthigher surface areas, higher mesopore volumes andnarrower mesopore size distributions, but aresignificantly more expensive.

a few hundredsof m2/g

a few tens ofm2/g

± branched particlesGels spherical particles

Figure 6. Various materials obtained by hydrothermal treatment of wattle tannin followed by pyrolysis at 900°C, using various pHsand conditions of dissolution. Those at the left are true carbon gels (after [13]). Even wider range of materials can be obtained bychanging temperature, time and initial dilution.Figura 6. Varios materiales obtenidos por tratamiento hidrotermal de tanino de acacia seguido de pirólisis a 900°C, usando variospH y diluciones. A la izquierda, verdaderos geles de carbón (de [13]). Cambiando la temperatura, el tiempo y la dilución inicial sepuede obtener una gama de materiales más amplia incluso.

Figure 7. Fibrous carbon aerogels derived from the hydrothermaltreatment of a mixture of glucose and ovalbumin (after [14],reproduced by permission of the Royal Society of Chemistry).Figura 7. Aerogeles de carbón fibrosos preparados por tratamientohidrotermal de una mezcla de glucosa y albúmina de clara dehuevo (de [14], reproducido con el permiso de Royal Society ofChemistry).

Figure 8. Fibrillar carbon cryogels derived from cellulose andpyrolysed at 600°C in nitrogen (left) and HCl (right). Reproducedfrom [20] with permission.Figura 8. Criogeles de carbón fibrilares preparados a partir decelulosa y pirolizados a 600°C en nitrógeno (izquierda) y HCl(derecha). Reproducido de [20] con permiso.


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Anyway, and provided that their purity is high enough,carbon gels derived from natural resources areexpected to have, and in some cases were alreadyfound to have, the same applications as thosesuggested for their synthetic counterparts, i.e.,adsorbents, porous electrodes for electric doublelayer supercapacitors or secondary batteries, fillingmaterial for HPLC columns, catalyst supports, andheat insulators at high temperature.

6. AcknowledgementsThe authors gratefully acknowledge the financialsupport of the CPER 2007–2013 “Structuration duPôle de Compétit ivi té Fibres Grand’Est”(Competitiveness Fiber Cluster), through local(Conseil Général des Vosges), regional (RégionLorraine), national (DRRT and FNADT) andEuropean (FEDER) funds. We also thank the FrenchForeign Ministry (MAE) and the Région Lorraine,especially one of us (L.I.G) for her grant through theLorraine-Russia ARCUS cooperation program.

7. References1Brinker CJ, Scherer GW. Sol – gel science. Thephysics and chemistry of sol–gel processing.Academic Press, 1990.2Pasch H, Pizzi A, Rode K. MALDI-TOF massspectrometry of polyflavonoid tannins. Polymer2001;42(18):7531–7539.3Abreu HS, do Nascimento AM, Maria MA. Ligninstructure and wood properties. Wood Fiber Sci1999;31(4):426–433.4Adler E. Lignin chemistry: past, present and futures.Wood Sc iTechno l 1977;11(3) :169–218.5Szczurek A, Amaral-Labat G, Fierro V, Pizzi A,Masson E, Celzard A. The use of tannin to preparecarbon gels. Part I. Carbon aerogels. Carbon2011;49(8):2773–2784.6Szczurek A, Amaral-Labat G, Fierro V, Pizzi A,Celzard A. The use of tannin to prepare carbon gels.Part II. Carbon cryogels. Carbon 2011;49(8):2785–2794.7Grishechko LI, Amaral-Labat G, Szczurek A, FierroV, Kuznetsov BN, Celzard A. Lignin – phenol –formaldehyde aerogels and cryogels. MicroporMesopor Mater 2012; 2013;168: 19-29.8Chen F, Xu M, Wang L, Li J. Preparation andcharacterization of organic aerogels from a lignin –resorcinol – formaldehyde copolymer. Biores2011;6(2):1261-1272.9Grishechko LI, Amaral-Labat G, Szczurek A, FierroV, Kuznetsov BN, Pizzi A, Celzard A. New tannin –lignin aerogels. Ind Crops Prod 2013;41(1):347-355.10Amaral-Labat G, Szczurek A, Fierro V, Pizzi A,Masson E, Celzard A. Impact of depressurizing rateon the porosity of aerogels. Micropor Mesopor Mater2012;152:240–245.11Amaral-Labat G, Szczurek A, Fierro V, Stein N,Boulanger C, Pizzi A, Celzard A. Pore structure andelectrochemical performances of tannin-basedcarbon cryogels. Biomass Bioenergy 2012;39:274-282.

12Yamamoto T, Sugimoto T, Suzuki T, Mukai SR,Tamon H. Preparation and characterization of carboncryogel microspheres. Carbon 2002; 40(8):1345 –1351.13Braghiroli F, Fierro V, Izquierdo MT, ParmentierJ, Pizzi A, Celzard A. Nitrogen-doped carbonmaterials produced from hydrothermally treatedtannin. Carbon 2012; 50(15): 5411-5420.14White RJ, Yoshizawa N, Antonietti M, Titirici MM.A sustainable synthesis of nitrogen-doped carbonaerogels. Green Chem 2011;13(9):2428-2434.h t t p : / / d x . d o i . o r g / 1 0 . 1 0 3 9 / C 1 G C 1 5 3 4 9 H15Wohlgemuth SA, White RJ, Willinger MG, TitiriciMM, Antonietti M. A one-pot hydrothermal synthesisof sulfur and nitrogen doped carbon aerogels withenhanced electrocatalytic activity in the oxygenreduction reaction. Green Chem 2012;14(5):1515-1523.16Fellinger TP, White RJ, Titirici MM, Antonietti M.Borax-mediated formation of carbon aerogels fromglucose. Adv Funct Mater 2012;22(15):3254-3260.17Tan, C, Fung, M, Newman, JK, Vu C. Organicaerogels with very high impact strength. Adv Mater2001;13(9):644-646.18Fischer F, Rigacci A, Pirard R, Berthon-Fabry S,Achard P. Cellulose-based aerogels. Polymer2006;47(22):7636–7645.19Jin H, Nishiyama H, Wada M, Kuga S. Nanofibrillarcellulose aerogels. Colloids Surfaces A PhysicochemEng Aspects 2004;240(1):63-67.20Ishida O, Kim Dy, Kuga S, Nishiyama Y, MalcolBrown R. Microfibrillar carbon from native cellulose.Cellulose 2004;11(3,4):475-489.21Innerlohinger J, Weber Hk, Kraft G. Aerocellulose:Aerogels and aerogel-like materials made fromcellulose. Macromol Symp 2006;244:126-135.22Guilminot E, Fischer F, Chatenet M, Rigacci A,Berthon-Fabry S, Achard P, Chainet E. Use ofcellulose-based carbon aerogels as catalyst supportfor PEM fuel cell electrodes: Electrochemicalc h a r a c t e r i z a t i o n . J P o w e r S o u r c e s2007;166(1):104–111.23Grzyb B, Hildenbrand C, Berthon-Fabry S, BéginD, Job N, Rigacci A, Achard P. Functionalisation andchemical characterisation of cellulose-derived carbonaerogels. Carbon 2010;48(8):2297–2307.24Rooke J, De Matos Passos C, Chatenet M,Sescousse R, Budtova T, Berthon-Fabry S, MosdaleR, Maillard F. Synthesis and properties of platinumnanocatalyst supported on cellulose-based carbonaerogel for applications in PEMFCs. J ElectrochemSoc 2011;158(7):B779-B789.25Amaral-Labat G, Grishechko LI, Szczurek A, FierroV, Kuznetsov BN, Pizzi A, Celzard A. Highlymesoporous organic aerogels derived from soy andtannin. Green Chem 2012; 14(11): 3099-3106.


8

S. R. Mukai*

Div. of Chem. Process Engineering, Graduate School of Engineering, Hokkaido University, N13W8 Kita-ku, Sapporo060-8628, Japan* Corresponding author: [email protected]

AbstractCarbon gels are unique porous carbons, which aretypically obtained through the carbonization ofresorcinol-formaldehyde gels. This material ispractically an aggregate of nanometer-sized carbonparticles. Nanopores, mostly in the size range ofmesopores, exist between the particles. Smallerpores, micropores being the majority, also exist withinthe particles. Therefore, this material has ahierarchical pore system in which short microporesare directly connected to mesopores.The precursor of carbon gels can be obtained throughsol-gel transition. Therefore there is a high possibilitythat the morphology of the resulting carbon can beeasily controlled using various molding methods.We have actually challenged the controlling of themorphology of carbon gels, and have succeeded inobtaining them in the form of disks, microspheresand microhoneycombs. Details of such carbon gelswill be reported.

ResumenLos geles de carbón son materiales porosos únicosque se obtienen normalmente mediante lacarbonización de geles de resorcinol-formaldehído.Este material es prácticamente un agregado deparículas de carbono nanométricas. Los nanoporos,mayoritariamente en el intervalo de tamaño de losmesoporos, existen entre las partículas. Los porosmás pequeños, mayoritariamente microporos existentambién dentro de las partículas. Por tanto, estematerial tiene un sitema jerárquico de poros en elque los cortos microporos están directamenteconectados a los mesoporos.El precursor de los geles de carbon se puede obtenera través de una transición soil-gel. Por tanto hayalta posibilidad de que la morfología del carbónresultante se pueda controlar fácilmente usandodiferentes métodos de moldeo. De hecho, hemosafrontado el reto de controlar la morfología de losgeles de carbón y hemos tenido éxito al obtenerlosen forma de discos, microesferas y micro panalesde abejas. Los detalles de estos geles de carbónse presentarán en este trabajo.

1. IntroductionCarbon gels are unique porous carbons which wereintroduced in the late 1980s by Dr. Pekala and hiscoworkers[1-3]. Carbon gels are practically anaggregate of carbon nanoparticles. The voids formedbetween the nanoparticles are in the mesopore size-range, so this material is widely renowned as amesoporous carbon. Moreover, micropores existwithin the nanoparticles. Therefore it can be saidthat this material has a hierarchical pore system ofmicropores and mesopores[4-6].Carbon gels are usually synthesized through thefollowing method [1-3]: First resorcinol is mixed withformaldehyde, and with the aid of a base catalystsuch as sodium carbonate, substituted resorcinolsare formed. Through polycondensation, colloidalparticles are formed, and these particles coagulateand form a 3 dimensional network of particles which

is called a resorcinol-formaldehyde (RF) hydrogel. Carbon gels can be obtained by drying andcarbonizing this material. In the drying process,special drying methods such as supercritical dryingare typically used in order to suppress shrinkageand maintain the unique structure of RF hydrogels.We showed that a more economical drying method,freeze drying, can be used in place of supercriticaldrying [7, 8]. Microwave drying can also be used inlimited cases [9, 10].It is also well known that carbon gels have a highnanostructure controllability [5, 6]. For example, thesize of the mesopores of carbon gels depends onthe size of the nanoparticles which consist it, andthe size of such nanoparticles can be controlled bythe catalyst concentration of the starting solution,which means that the size of the mesopores canalso be controlled indirectly by adjusting thisconcentration. Moreover, the volume of microporescan be easily increased using conventional activationmethods without hardly affecting the size and volumeof mesopores. Therefore if a method to give carbongels a desired morphology without affecting its porousproperties can be established, unique porouscarbons, the micro-, meso- and macro-scalestructures of which can be independently controlledto best match their usages will be obtained. In thisreport, the attempts our group has made to obtainsuch carbons will be introduced.

2. DisksAs the precursor of carbon gels, RF gels, are usuallyobtained through sol-gel transition, we expected thatit would be quite easy to mold them into a simpledisk form without using any binders. To verify thispossibility, first starting solutions with typicalcompositions were prepared, and then they werepoured into teflon molds of different thicknesses.After sol-gel transition occurred, the resulting RFhydrogels were released from the molds, and weresubsequently freeze-dried and carbonized yieldingcarbon gel disks [11].

Fig. 1 shows a photograph of typical disks obtainedthrough this method. The disk on the left side is anRF hydrogel, the one in the middle an RF cryogel,and the one on the right, a carbon cryogel. Significantshrinkage occurred during drying and carbonization,but distorted disks or disks with cracks were hardlyobtained, which indicates that shrinkage occurredfairly isotropically. Fig. 2 shows how the shrinkageratio depends on the catalyst concentration. Isotropic

Controlling the Morphology of Carbon GelsControl de la morfología de los geles de carbón

Figure 1. Typical disks obtained in this work.Figura 1. Discos típicos obtenidos en este trabajo.


a) b) c) d)

e) f)

9

shrinkage can also be confirmed from this graph.Therefore it can be concluded that monolithic carbongel disks can easily be obtained through this method.

3. MicrospheresThe next morphology to be explained about ismicrospheres. As the initial source of carbon gels isan aqueous solution, we assumed that they couldbe molded into microspheres through inverseemulsion polymerization. In order to verify thispossibility, first a typical starting solution was preparedand was kept at a constant temperature Tgel, andjust before the solution was about to transform to agel, it was dispersed into a mixture of cyclohexaneand SPAN80, a surfactant which was maintained atthe same temperature Tgel. After agitating the resultingemulsion for 5 to 10 h, RF hydrogel microsphereswere formed. Carbon gel microspheres were obtainedby freeze drying and carbonizing these microspheres[12, 13].Fig.3 shows SEM micrographs of typicalmicrospheres obtained through this method. It canbe seen that microspheres with diameters in therange of 5 to about 30 micrometers were obtained.The inner parts of the microspheres were found tobe formed by nanoparticles, like typical carbon gels,but interestingly, macropores or large mesoporescould not be found at the surface of the microspheres.So next we analyzed the surface structure of themicrospheres through the so-called molecular probemethod. Fig. 4 shows the results where the microporevolume accessible to the probe molecule is plottedas a function of the minimum dimension of the probemolecule. It was confirmed that when Tgel was lowand the carbonization temperature (Tpyro) was high,the surfaces of the microspheres tended to become

microporous, and in certain cases, microspheres inwhich even carbon dioxide cannot penetrate intocould be obtained. This indicates that microsphereshaving mesoporous inner parts and surfaces withunique properties can be obtained through thismethod. Therefore, this method not only enables thesynthesis of carbon gel microspheres, but also allowsthe controlling of the nanostructure of the surface ofthe resulting microspheres.

4. MicrohoneycombsPorous carbons are usually synthesized in the formof particles, therefore the size of the particledetermines the length of the diffusion paths withinit. This means that the particle size must be smallenough to ensure a high accessibilty to their innerparts. However as the voids formed between theparticles provide paths or macropores to the material,the size of the particles also determines the resistancethey cause when fluids are passed through them.So the particle size must be large to avoid significantresistance to flows. This means that a highaccessibility to their inner part is a complete trade-off with a low resistance to flows. Indeed, there aremany cases in which large particles are used just toavoid severe pressure drops. This dilemma can beavoided by changing the morphology of the material.A microhoneycomb, which is a monolith havingstraight and aligned channels which sizes are in themicrometer range, and which channels are formedby nanoporous walls which thicknesses are also inthe micrometer range is thought to be one example

Carbonization

Diameter

Thickness

ThicknessDrying

0 100 200 6005004003004050

90

80

70

60

100

110

R/C

Shr

inka

ge R

atio

/ %

Figure 2. Shrinkage ratio of the obtained disks.Figura 2. Relación de encogimiento de los discos obtenidos.

Figure 3. SEM micrograph of typical microsphere samples.Figura 3. Microfotografía SEM de muestras de microesferas típicas.

a) Microspheres b) Cross-Section c) Surface

Minimum Dimension of Probe / nm

Mic

ropo

re V

olum

e A

cces

sibl

eto

Pro

be /

cm3

g-1

0.3 0.4 0.5

0.1

0.2

0.3

0

CO2 C2H6 i-C4H10

Tgel = 333K

Tgel = 298KTpyro =1023K

1573K 1273K

Figure 4. Estimation of inlet size of the surface pores of typicalcarbon gel microspheres.Figura 4. Estimación del tamaño de entrada de los porossuperficiales de microesferas típicas de geles de carbón.


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of an ideal morphology. As the thickness of the wallsis in the micrometer range, the diffusion paths withinthem are extremely short, and as the channels arestraight and aligned, the resistance when fluids arepassed through this monolith can be minimized. Itwas difficult, if not impossible to synthesize suchmicrohoneycombs using conventional methods, butwe found that such microhoneycombs can besynthesized through the sol-gel method using icecrystals as the template [14-16].In the developed method, first a hydrogel precursoris prepared, and then this hydrogel is dipped at aconstant rate into a cold bath which temperaturewas maintained below the freezing temperature ofthe gel. This freezing process is called unidirectionalfreezing. During this freezing process, an array ofneedle shaped ice crystals appears within thehydrogel and elongates in the freezing direction.This causes the hydrogel to be condensed betweenthe growing ice crystals. So, a monolithicmicrohoneycomb as shown here could be obtainedby thawing and drying the completely frozen material.As ice crystals act as the template in this method,we named it the ice templating method [14-16]. Thismethod was originally developed using a silicahydrogel, but considering the similarity of hydrogels,we thought that this method will also be applicableto resorcinol-formaldehyde hydrogels.We actually tested this possibility [17, 18]. First atypical starting solution was prepared, and throughsol-gel polycondensation, rod type RF hydrogelswere formed. As the catalyst was thought to disturbthe growth of ice crystals, the hydrogels werethoroughly washed with distilled water. Then thesamples were dipped at a constant rate into a coldbath. The obtained samples were aged, freeze-driedand carbonized yielding monolithic carbon gels.

Figure 5 shows a SEM micrograph of the crosssection of a typical sample obtained in this work. Itcan be noticed that the RF gel was successfullymolded into the form of a microhoneycomb.Nanoparticles which are the origin of mesoporositycan be clearly distinguished in the enlargedmicrograph of the honeycomb wall which indicatesthat the nanostructure of the gels is not altered byfreezing. This was also confirmed through adsorptionexperiments. Therefore a mesoporous RF gel was

successfully obtained.

Next, we attempted to carbonize the RF gelmicrohoneycommb. Figure 6 shows a cross sectionalSEM micrograph of a typical sample aftercarbonization. It can be noticed that although slightshrinkage occurs, the sample maintains its uniquemorphology. Therefore, it can be concluded thatcarbon gels having a unique microhoneycombmorphology can be obtained through ice templating.

5. Concluding RemarksAs was introduced in this article, we found that it isvery easy to control the morphology of carbon gels.Therefore, not only the micro- and meso-structures,but also the macro-structure of this unique materialcan be tuned to best match its application. We arenow investigating applications which can make fulluse of such carbon gels with controlled morphologies,and hope to report the results in the near future.

6. References1Pekala RW. Organic aerogels from thepolycondensation of resorcinol with formaldehyde,J. Mater. Sci. 1989; 24: 3221-3227.2 Pekala RW, Kong FM. A synthetic route to organicaerogels - mechanism, structure and properties. J.de Physique Coll. Suppl. 1989; 50: C4-33-40.3 Pekala RW. Synthetic control of molecular structurein organic aerogels. Mater. Res. Soc. Proc. 1990;171: 285-292.4 Ruben GC, Pekala RW, Tillotson TM, Hrubesh LW.Imaging aerogels at the molecular-level. J. Mater.Sci. 1992; 27(16): 4341-4349.5 Yamamoto T, Yoshida T, Suzuki T, Mukai SR,Tamon H. Dynamic and static light scattering studyon the sol-gel transition of resorcinol-formaldehydeaqueous solution. J. Colloid Interface Sci.2002;245(2): 391-396.6 Yamamoto T, Mukai SR, Endo A, Nakaiwa M,Tamon H. Interpretation of structure formation duringthe sol-gel transition of a resorcinol-formaldehydesolution by population balance. J. Colloid InterfaceSci. 2003; 264(2): 532-537.

Figure 5. Cross sectional SEM image of a typical monolithic RFgel microhoneycomb.Figura 5. Imagen SEM de la sección transversal de un típico gelRF monolítico en panal de abeja.

Figure 6. Cross sectional SEM image of a typical monolithiccarbon gel microhoneycomb microhoneycomb.Figura 6. Imagen SEM de la sección transversal de un típicogel monolítico de carbón en panal de abeja.


11

7 Tamon H, Ishizaka H, Yamamoto T, Suzuki T.Freeze drying for preparation of aerogel-like carbon.Dry. Technol. 2001; 19(2): 313-324.8 Yamamoto T, Nishimura T, Suzuki T, Tamon H.Control of mesoporosity of carbon gels prepared bysol-gel polycondensation and freeze drying. J. Non-Cryst. Solids 2001; 288(1-3): 46-55.9 Tamon H, Mukai SR, Nishihara H, Yoshida T,Yamamoto T, Tonanon N, Tanthapanichakoon W.Synthesis of Mesoporous Carbon Gels usingMicrowave Drying. Carbon 2003 (Oviedo).10 Tonanon N, Wareenin Y, Siyasukh A,Tanthapanichakoon W, Nishihara H, Mukai SR,Tamon H. Preparation of Resorcinol Formaldehyde(RF) Carbon Gels: Use of Ultrasonic IrradiationFollowed by Microwave Drying. J.Non-Cryst. Solids2006; 352(52-54): 5683-5686.11 Mukai SR, Kambara H, Hasegawa T, Tamon H.Binder-free Synthesis of Carbon Cryogel Tabletsand their Performance as Electrodes for ElectricDouble Layer Capacitors. Carbon 2005 (Gyeongju).12 Yamamoto T, Sugimoto T, Suzuki T, Mukai SR,Tamon H. Preparation and characterization of carboncryogel microspheres. Carbon 2002; 40(8): 1345-1351.

13 Yamamoto T, Endo A, Ohmori T, Nakaiwa M,Mukai SR, Tamon H. Effect of drying method on gasadsorption characteristics of carbon gel microspheres.Dry. Technol. 2005; 23(9-11): 2119-2129.14 Mukai SR, Nishihara H, Tamon H. Porousproperties of silica gels with controlled morphologysynthesized by unidirectional freeze-gelation.Micropor. Mesopor. Mat. 2003; 63(1-3) 43-51.15 Mukai SR, Nishihara H, Tamon H. Formation ofmonolithic silica gel microhoneycombs (SMHs) usingpseudosteady state growth of microstructural icecrystals. Chem. Commun. 2004; (7): 874-875.16 Nishihara H, Mukai SR, Yamashita D, Tamon H.Ordered macroporous silica by ice templating. Chem.Mater. 2005; 17(3): 683-689.17 Nishihara H, Mukai SR, Tamon H. Preparationof resorcinol-formaldehyde carbon cryogelmicrohoneycombs. Carbon 2004; 42(4): 899-901.18 Mukai SR, Nishihara H, Yoshida T, Taniguchi K-I, Tamon H. Morphology of resorcinol-formaldehydegels obtained through ice-templating. Carbon 2005;43(7): 1563-1565.


ResorcinolFormaldehyde

WaterSodium carbonate(sodium hydroxide)

Carbonxerogelcatalyst

Carbon xerogel

RF Xerogel

Condensation

PolymerizationGelling

CuringSubcritical

Drying

Activation

Functionalization

Sol

Wet gel

&

Car

boni

zatio

n

12

J. L. Figueiredo

Laboratório de Catálise e Materiais (LCM), Laboratório Associado LSRE/LCM, Departamento de Engenharia Química,Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal.* Corresponding author: [email protected]

AbstractThe synthesis and properties of carbon xerogels arebriefly described, emphasizing the methods usedfor tuning of their surface chemistry and texturalproperties, in order to design catalysts suitable forspecific applications.

ResumenLa síntesis y las propiedades de los xerogeles decarbón han sido descritas brevemente, incidiendoen los métodos de preparación usados para lamodificación de las propiedades texturales y laquímica superficial, con el objetivo de diseñarcatalizadores adecuados para aplicacionesespecíficas.

1. IntroductionThe versatility of carbon materials in catalysis is welldocumented in the literature, and has been thesubject of a recent review [1]. Activated carbons andcarbon blacks are the carbon materials which havetraditionally been used in catalysis, either as catalystcarriers or as catalysts on their own. One of thedrawbacks of activated carbons is that they areessentially microporous materials, while mesoporousmaterials would be advantageous for catalysis inorder to minimize diffusion limitations and catalystdeact ivat ion. However, various types ofnanostructured carbon materials have beendeveloped in recent years which offer quite interestingproperties for catalytic applications. In particular,carbon gels can be produced by carbonization oforganic gels obtained by the sol–gel polycondensationof organic monomers such as resorcinol andformaldehyde [2]; they are designated as aerogels,xerogels or cryogels according to the method usedfor drying the aqueous organic gels (supercriticaldrying, conventional drying and freeze drying,respectively). Carbon gels are materials of highporosity and surface area, controllable pore size,and can be shaped in various forms for practicalapplications (such as monoliths, thin films and pellets).By adequate tuning of their surface chemistry andtextural properties, it is possible to design catalystssuitable for specific applications. Previous reviewsfocused mainly on metal-doped carbon gels [3,4],while here we will concentrate on the developmentof carbon xerogels as catalysts on their own,highlighting some of the most recent results reported.

2. Synthesis and propertiesSimilarly to any other solid catalyst, the catalyticperformance of a carbon xerogel is determined bythe nature, concentration and accessibility of itsactive sites, and these factors depend both on thephysical and the chemical properties of the material.Figure 1 represents schematically the different stepsin the synthesis of a carbon xerogel. The texturalproperties of the materials obtained are largelydetermined by the synthesis conditions, and variousparameters can be modified in order to optimize theprocess, including the formaldehyde/resorcinol ratio,type and amount of catalyst, curing temperature,addition of other monomers (for instance, cresol),

and drying procedure.

The method of drying the aqueous organic gel isone of the key factors for the synthesis of amesoporous carbon. Indeed, it is necessary toprevent the collapse of the 3D network which mayoccur as result of the strong capillary forces of water,and this may be accomplished by solvent exchange.One recipe which is commonly used to producecarbon xerogels in our Laboratory is the following:formaldehyde/resorcinol ratio = 2.0; gelling andcuring step performed over a period of three days(at 30, 50, and 75 ºC, one day each); water removedby sequential exchange with acetone (initiallycontaining 5% acetic acid) and cyclohexane, followedby drying overnight at 80 ºC; carbonisation at 800ºC (ramp @ 3 ºC min-1) under nitrogen flow (100cm3 min-1, 6 h). But in particular, the texturalproperties are most sensitive to the pH used in thesol-gel processing [5]. As an example, Figure 2presents data for two carbon xerogels with quitedifferent textural properties which were synthesizedunder similar conditions, except for the pH (adjustedwith NaOH solution) which was 5.6 (sample 37CXUA)or 6.0 (sample 39CXUA). Figure 2a) shows thenitrogen adsorption isotherms, and Figure 2b) thecorresponding mesopore size distributions. The BETsurface areas of these samples are very similar (653m2/g for sample 37CXUA and 645 m2/g for sample39CXUA), but the average mesopore diameters arequite different: 16 nm for sample 37CXUA and 4 nmfor sample 39CXUA.Carbon xerogels (CX) have a graphitic structure,and so the active sites are expected to be foundmainly at the edges of the graphene layers, wherethe unsaturated carbon atoms may chemisorboxygen, water, or other compounds like ammonia,originating surface groups such as those representedschematically in Figure 3. Similar surface groupscan be formed on basal plane defects. In addition,the π electron system of the basal planes contributesto the carbon basicity, affecting its adsorption andcatalytic properties [6,7].Both gas phase and liquid phase treatments can beused to generate oxygen groups on the surface of

Carbon xerogels for catalytic applicationsXerogeles de carbón para aplicaciones catalíticas

Figure 1. Steps in the synthesis of a carbon xerogel catalyst.Figura 1. Etapas de síntesis de un catalizador de xerogel decarbón.


13

carbon xerogels [8,9], while heating under inertatmosphere may be used to selectively remove someof these functions. Typical gas-phase activationconsists of heating the CX sample up to atemperature in the range 350-450 ºC using 5% O2(diluted in N2) with adequate duration of theisothermal heating period in order to achieve thedesired burn-off (BO).

Both the mesopore surface area and the microporevolume are expected to increase upon oxidation, asa result of the widening of existing pores and/orcreation of new ones by selective gasification ofstructural components, or by the opening of someof the previously inaccessible pores [8]. On the otherhand, liquid phase oxidation with nitric acid can beperformed on a Soxhlet, with different acidconcentrations and duration of the treatment, followedby washing with distilled water, and then drying at110 ºC [9]. Alternatively, a recently developedhydrothermal procedure can be used, allowing fora finer control of the extent of surface oxidation [10].The textural changes are negligible when using lowconcentrations of nitric acid, but can be of the same

order of magnitude as those induced by gas phaseoxidation when using higher concentrations.The nature and concentration of the surface oxygengroups can be assessed by temperature-programmeddesorption with mass spectrometry (TPD-MS), amethod which has been improved in our group[11,12]. Upon heating, carboxylic acids and lactonesrelease CO2, while phenol and carbonyl groupsrelease CO; carboxylic anhydrides decompose byreleasing both CO and CO2. As an example, Figure4 shows the TPD spectra obtained with a carbonxerogel (sample 28CX-UA, average mesoporediameter = 28 nm) oxidised by two different methods,namely: treatment with HNO3 7M in a Soxhlet, underreflux, for 3 hours (sample 28CX-NA), and oxidationin the gas phase with a mixture of 5% O2 in N2, at415ºC for 30 hours, to a burn-off of 17% (sample28CX-AR) [9]. The TPD profiles of nitric acid and airactivated samples are quite different, but both showlarge increases in the amounts of evolved CO andCO2 in comparison to the original sample.Deconvolution of the CO and CO2 spectra canprovide reliable estimates of the amounts of eachtype of oxygen group [11,12]. Activation by air createsmainly phenol and carbonyl groups. Nitric acidactivation is most suitable to introduce carboxylicgroups, but large amounts of phenol and carbonylgroups are also generated.Nitrogen doping can be accomplished by theintroduction of a nitrogen-containing precursor in theorganic xerogel, such as urea and melamine [13,14],or by post-treatments with nitrogen compounds, suchas ammonia, urea or amines. Different types ofsurface nitrogen groups can be obtained, as shownin Figure 3, which can be determined bydeconvolution of XPS spectra [13-15]. The additionalelectrons provided by nitrogen increase the surfacebasicity. Moreover, the presence of nitrogen atomsin the carbon matrix, particularly in the case ofpyridinic (N6) and pyrrolic (N5) groups, has beenshown to enhance the catalytic activity of carbonmaterials in oxidation reactions [16,17].

3. Catalysis with carbon xerogelsHeterogeneous catalysts are generally classifiedinto three groups, namely metals, non-stoichiometricmetal oxides, and acids, each group showing catalyticactivity for a defined set of reactions [18]. Interestingly,carbon materials can be active catalysts for reactionswhich are typical of all three classes, such asdehydrogenations, oxidations and reductions, andalkylation and dehydration, a fact that was recognized

Figure 2. Textural properties of two carbon xerogels synthesized at different pH: a) nitrogen adsorption isotherms at 77 K; b) mesoporesize distributions.Figura 2. Propiedades texturales de dos xerogeles de carbón sintetizados a distintos valores de pH: a) isotermas de adsorción deN2 a 77 K, b) distribución de tamaño de mesoporos.

Figure 3. Nitrogen and oxygen surface groups on carbon.Reprinted from reference [7] with permission from Elsevier.Figura 3. Grupos funcionales de nitrógeno y oxígeno en materialesde carbón. Reproducido de [7] con permiso de Elsevier.


14

Figure 4. CO and CO2 TPD spectra of activated and original carbon xerogel samples: original sample (28CX-UA); sample treated innitric acid (28CX-NA); and sample treated in oxygen (28CX-AR). Adapted from reference [9]..Figura 4. Perfiles de desorción térmica programada de CO y CO2 para las muestras de xerogeles de carbón activados y originales:muestra original (28CX-UA); muestra tratada con ácido nítrico (28CX-NA); y muestra tratada con oxígeno (28CX-AR). Adaptada de[9].

long time ago [19]. The versatility of carbon as acatalyst [7, 20] stems from the rich variety of surfacefunctional groups (Figure 3) which can act as activesites for different types of reactions. Selectedexamples are reviewed in the next sections,highlighting some of the recent results obtained inour group.

3.1 Carbon xerogels as acid catalystsAcidic surface oxygen groups include carboxylicacids and anhydrides, lactones or lactols, and phenols(Figure 3). Previous reports with oxidized activatedcarbons showed that carboxylic acid groups areresponsible for their catalytic activity in thedehydration of alcohols [21, 22]. But, in general,stronger active acid sites are required to catalyzeother reactions, such as alkylations, esterifications,acetalizations and hydrolyses. Efficient acid catalystsfor these reactions can be obtained byfunctionalization with sulfonic acid groups [23]. Forinstance, the activity of carbon xerogels treated withnitric acid and with sulfuric acid was recently assessedin the ring opening reaction of epoxides by alcoholsand amines [24]. The introduction of sulfonic acidgroups (CX treated with H2SO4) provided for themost active catalyst, the conversion of styrene oxidewith ethanol reaching nearly 100 % at roomtemperature after 30 minutes, with a selectivity of97 % towards 2-ethoxy-2-phenylethanol. However,similar performances were observed with both carbonxerogels in the reaction of styrene oxide with aniline[24].Preliminary results for the esterification of acetic acidwith ethanol were obtained in our group [25], usingtwo different carbon xerogel catalysts: CX-HNO3(treated with HNO3 5M; acidity = 1.58 mmol g-1) andCX-H2SO4 (treated with concentrated H2SO4; acidity= 1.91 mmol g-1). As expected, the samplefunctionalized with sulphuric acid was the mostactive, with a yield of ethyl acetate of 52 % after 12h, while only 13 % was obtained with sample CX-HNO3 (T=70 °C; molar ratio ethanol/acetic acid=10;0.2 g of catalyst). In order to establish a correlationwith the concentration of the active sites, threesamples were prepared with different amounts ofsulfonic acid groups. Two samples werefunctionalized with concentrated H2SO4 at 150 ºCfor 6 hours, but with different ratios of acid to solid(150 mL and 20 mL H2SO4 per gram of CX). Theconcentrations of sulfonic acid groups, determined

by XPS, were 994 and 591 µmol g-1, respectively.A third sample was obtained from the first one byheat treatment in helium at 250 ºC ([SO3H] = 78µmol g-1). Figure 5 shows that the catalytic activitycorrelates nicely with the concentration of surfacesulfonic acid groups.

3.2 Carbon xerogels as oxidation catalystsThe oxidative dehydrogenation of hydrocarbons(ODH) is one of the most interesting applications ofcarbon materials as catalysts. It has been establishedthat the reaction involves the quinone-hydroquinonegroups [26], as shown in Figure 6.The active sites are the quinone groups (pairs ofcarbonyl groups at the edges of the carbon layersconnected by a resonance structure), which areconverted into hydroquinones, and regenerated backto the original sites by oxygen. This reaction hasbeen intensively studied using activated carbonsand carbon nanotubes as catalysts [7,20,26-28]. Wehave recently started to investigate the use of carbonxerogels as catalysts for the ODH, but no resultshave been reported so far.Another reaction which has been carried out withcarbon xerogel catalysts is the oxidation of NO [29].This is an interesting route for the control of NOemissions, since the NO2 formed can besubsequently removed by absorption in water. TheNO conversions are quite high, showing that carbonxerogels are efficient catalysts for NO oxidation. A

[SO3H] (µmol g-1)

EtA

c Fo

rmat

ion

(mm

ol m

in-1

g-1)

0

50

1200800400

100

150

200

250

300

Figure 5. Correlation between the catalytic activity of carbonxerogels for the esterification of acetic acid with ethanol and theconcentration of sulfonic acid groups.Figura 5. Correlación entre la actividad catalítica de los xerogelesde carbón para la esterificación de ácido acético con etanol y laconcentración de grupos sulfónicos.


15

conversion of 98 % was obtained at room temperaturewith a concentration of NO of 1000 ppm and 10 %of O2. It is assumed that oxygen is first chemisorbedon the active sites, reacting then with NO to formNO2, as schematically shown in Figure 7.

The incorporation of nitrogen was found to improvesignificantly the catalytic activity of the carbonxerogels [29], the rate of NO oxidation increasingwith the concentration of pyridine and pyrrole groups,as shown in Figure 8, in agreement with theoreticalpredictions [16].

3.3 Carbon xerogels as catalysts for AdvancedOxidation ProcessesAdvanced oxidation processes (AOPs) is thecollective designation of a set of chemical treatmentsdesigned to remove (mainly) organic pollutants fromwater and waste water by oxidation via highly reactivehydroxyl radicals. Among others, oxygen (or air),hydrogen peroxide or ozone can be used as oxidizingagents, the processes being called wet air oxidation(WAO), wet peroxide oxidation (WPO) and ozonation,respectively. Various types of catalysts have been

used in these processes, but it has been shownrecently that carbon materials without any supportedmetal or oxide phase can be highly active for thecomplete mineralization of the organic pollutants ortheir intermediate oxidation products into CO2 andinorganic ions [7,20]. In this way, the costs associatedwith the use of expensive metal or oxide catalysts,and the danger of leaching of these catalytic phasesinto the effluents, are avoided.We have already reported on the performance ofcarbon xerogel catalysts for the wet air oxidation ofaniline [30] and nitro-aromatic compounds [31], forthe wet peroxide oxidation of dyes [32], and for theozonation of different types of dyes and also of oxalicacid, a common end-product of the oxidation of dyeswhich is refractory to non-catalytic ozonation [33].Figure 9 presents data for the total organic carbon(TOC) removal during the ozonation of a dye (C.I.Acid Blue 113) catalyzed by carbon xerogels [33].Three carbon xerogels with an average mesoporediameter of 37 nm were used as catalysts, namelythe original material (XC0, Smeso = 329 m2 g-1, pHpzc= 7.8), a HNO3 oxidized sample (XC1, Smeso = 440m2 g-1, pHpzc = 3.1), and a thermally treated sample(XC2, Smeso = 252 m2 g-1, pHpzc = 8.7). The resultsclearly show that the best performance is achievedwith the basic carbons: XC2 > XC0 > XC1.

It should be stressed here that the removal of colour,that is, the degradation of the dye, is very fast evenwithout a catalyst; thus, complete decolourisationwas achieved by ozone at room temperature in lessthan 15 minutes [33]. However, single ozonationcannot remove most of the TOC in solution, showingthat some of the dye degradation products still remainin solution. Mineralization degrees close to 100%require the presence of a catalyst, and carbonxerogels with a basic surface are capable ofapproaching this performance.The AOPs reaction mechanisms are complex, andmay involve a combination of homogeneous and

Alkane Alkene

H2O 1/2 O2

O O OH OH

Figure 6. Catalytic cycle proposed for the oxidativedehydrogenation of hydrocarbons on carbon materials. Adaptedfrom [7].Figura 6. Mecanismo catalítico propuesto para la deshidrogenaciónoxidativa de hidrocarburos en materiales de carbón. Adaptadade [7].

CCO2(g)

CC

OO2NO(g)

CC

NO2 NO22NO2(g)

CC

Figure 7. Proposed steps in the oxidation of NO on carbon xerogels.Figura 7. Etapas propuestas en la oxidación de NO en xerogeles de carbón.

Figure 9. Normalized TOC removal in the ozonation of C.I. AcidBlue 113 at room temperature with carbon xerogel catalysts ofdifferent surface chemistry. Adapted from [33].Figura 9. Eliminación de COT mediante la ozonización de C.I.Azul ácido 113 con xerogeles de carbón con distinta químicasuperficial a temperatura ambiente. Adaptada de [33].

N6÷N5 [w t.%]

F NO

/S m

eso [

mol

min

-1 m

-2]

0.000000.00002

0.00010

0.000080.00006

0.00004

0.00012

0.00016

0.00014

0 642 8 10

Figure 8. Rate of NO oxidation versus the concentration ofpyridine and pyrrole groups.Figura 8. Velocidad de oxidación de NO en función de laconcentración de los grupos piridina y pirrol.


16

heterogeneous steps. In the case of ozonation, it isaccepted that the carbon catalyst accelerates thedecomposition of ozone. The delocalized π-electronsystem, or the surface basic groups (pyrone,chromene, pyrrole, cf. Figure 3) have been identifiedas the active sites for decomposition of ozone,leading to the formation of free radical species insolution, such as HO·, and also of surface freeradicals (O-·, O2-·, O3-·). Thus, both pathways(homogeneous and heterogeneous) can beresponsible for the oxidation of the organiccompounds [34].

4. ConclusionThe physical and chemical properties of carbonxerogels can be easily modified by selecting thesynthesis conditions and/or by suitable post-treatments, so that these materials can performadequately as metal-free catalysts for a large varietyof reactions, both in the liquid or in the gas phase.Selected examples from recent reports were reviewedin this paper, highlighting the performance of carbonxerogels as solid acid catalysts and also as oxidationcatalysts. Further advances in the field require theproper identification and quantification of the activesites involved in each application.

AcknowledgmentsThe work reported here was partially supported byprojects PEst-C/EQB/LA0020/2011 and PTDC/EQU-ERQ/101456/2008, financed by FEDER throughCOMPETE - Programa Operacional Factores deCompetitividade and by FCT - Fundação para aCiência e a Tecnologia. The author gratefullyacknowledges a sabbatical leave from Faculdadede Engenharia, Universidade do Porto.

References1 Serp P, Figueiredo JL (eds.). Carbon Materials forCatalysis. John Wiley & Sons, 2009.2 Pekala RW. Organic aerogels from thepolycondensation of resorcinol with formaldehyde.J. Mater. Sci.1989; 24: 3221-3227.3 Moreno-Castilla C, Maldonado-Hódar FJ. Carbonaerogels for catalysis applications: An overview.Carbon 2005; 43: 455–465.4 Moreno-Castilla C. Carbon Gels in Catalysis. In:Philippe Serp and José Luís Figueiredo Eds. CarbonMaterials for Catalysis. John Wiley & Sons, 2009 p.373-399.5 Job N, Pirard R, Marien J, Pirard JP. Porous carbonxerogels with texture tailored by pH control duringsol–gel process. Carbon 2004; 42: 619-628.6 Boehm HP. Some aspects of the surface chemistryof carbon blacks and other carbons. Carbon 1994;32: 759-769.7 Figueiredo JL, Pereira MFR. The role of surfacechemistry in catalysis with carbons. Catal. Today2010; 150: 2-7.8 Samant PV, Gonçalves F, Freitas MMA, PereiraMFR, Figueiredo JL. Surface activation of a polymerbased carbon. Carbon 2004; 42: 1321-1325.9 Mahata N, Pereira MFR, Suárez-García F,

Martínez-Alonso A, Tascón JMD, Figueiredo JL.Tuning of texture and surface chemistry of carbonxerogels. J. Colloid Interface Sci. 2008; 324: 150–155.10 Silva AMT, Machado BF, Figueiredo JL, Faria JL.Controlling the surface chemistry of carbon xerogelsusing HNO3 hydrothermal activation, Carbon 2009;47: 1670-1679.11 Figueiredo JL, Pereira MFR, Freitas MMA, ÓrfãoJJM. Modification of the surface chemistry of activatedcarbons. Carbon 1999; 37: 1379-1389.12 Figueiredo JL, Pereira MFR, Freitas MMA, ÓrfãoJJM. Characterization of active sites on carboncatalysts. Ind. Eng. Chem. Res. 2007; 46: 4110-4115.13 Gorgulho HF, Gonçalves F, Pereira MFR,Figueiredo JL. Synthesis and characterization ofnitrogen-doped carbon xerogels. Carbon 2009; 47:2032-2039.14 Pérez-Cadenas M, Moreno-Castilla C, Carrasco-Marín F, Pérez-Cadenas AF. Surface Chemistry,Porous Texture, and Morphology of N-Doped CarbonXerogels. Langmuir 2009; 25: 466-470.15 Lahaye J. The chemistry of carbon surfaces. Fuel1998; 77: 543-548.16 Strelko VV, Kuts VS, Thrower PA. On themechanism of possible influence of heteroatoms ofnitrogen, boron and phosphorus in a carbon matrixon the catalytic activity of carbons in electron transferreactions. Carbon 2000; 38: 1499-1503.17 Stöhr B, Boehm H, Schlögl R. Enhancement ofthe catalytic activity of activated carbons in oxidationreactions by thermal treatment with ammonia orhydrogen cyanide and observation of a superoxidespecies as a possible intermediate. Carbon 1991;29: 707-720.18 Bond GC. Catalysis by Metals. Academic Press,NY. 1962.19 Coughlin RW. Carbon as adsorbent and catalyst.Ind. Eng. Chem. Prod. Res. Dev. 1969; 8: 12-23.20 Figueiredo JL, Pereira MFR. Carbon as Catalyst,In: Philippe Serp and José Luís Figueiredo Eds.Carbon Materials for Catalysis. John Wiley & Sons,2009 p. 177-217.21 Moreno-Castilla C, Carrasco-Marin F, Parejo-Perez C, Ramon MVL. Dehydration of methanol todimethyl ether catalyzed by oxidized activatedcarbons with varying surface acidic character. Carbon2001; 39: 869-875.22 Szymanski GS, Karpinski Z, Biniak S, SwiatkowskiA. The effect of the gradual termal decompositionof surface oxygen species on the chemical andcatalytic properties of oxidized activated carbon.Carbon 2002; 40: 2627-2639.23 Budarin VL, Clark JH, Luque R, Macquarrie DJ.Versatile mesoporous carbonaceous materials foracid catalysis. Chem. Commun. 2007; 634-636.24 Matos I, Neves PD, Castanheiro JE, Perez-Mayoral E, Martin-Aranda R, Duran-Valle C, Vital J,


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Rego AMB, Fonseca IM. Mesoporous carbon as anefficient catalyst for alcoholysis and aminolysis ofepoxides. Appl. Catal. A: General 2012; 439-440:24-30.25 Figueiredo JL, Rocha RP, Pereira MFR. Carbon,a versatile catalyst. Carbon2012, The Annual WorldConference on Carbon, June 17-22, 2012. Krakow,Poland.26 Pereira MFR, Órfão JJM, Figueiredo JL. Oxidativedehydrogenation of ethylbenzene on activated carboncatalysts: 1- Influence of surface chemical groups.Appl.Catal.A: General 1999; 184: 153-160.27 Zhang J, Liu X, Blume R, Zhang A, Schlögl R,Su DS. Surface-modified carbon nanotubes catalyzeoxidative dehydrogenation of n-butane. Science2008; 322: 73-77.28 Zhang J, Su DS, Zhang A, Wang D, Schlögl R,Hébert C. Nanocarbon as robust catalyst: Mechanisticinsight into carbon-mediated catalysis. Angew. Chem.Int. Ed. 2007; 46: 7319-7323.29 Sousa JPS, Pereira MFR, Figueiredo JL. NOoxidation over nitrogen doped carbon xerogels.Applied Catalysis B: Environmental 2012; 125: 398–408.30 Gomes HT, Machado BF, Ribeiro A, Moreira I,

Rosário M, Silva AMT, Figueiredo JL, Faria JL.Catalytic properties of carbon materials for wetoxidation of aniline. Journal of Hazardous Materials2008; 159: 420-426.31 Apolinário AC, Silva AMT, Machado BF, GomesHT, Araújo PP, Figueiredo JL, Faria JL. Wet AirOxidation of Nitro-Aromatic Compounds: Reactivityon Single- and Multi-Component Systems andSurface Chemistry Studies with a Carbon Xerogel.Appl. Catal. B: Environmental 2008; 84: 75-86.32 Ribeiro RS, Fathy NA, Attia AA, Silva AMT, FariaJL, Gomes HT. Materiais de Carbono comoCatalisadores no Processo de Oxidação Catalíticacom Peróxido de Hidrogénio. XXIII CongresoIberoamericano de Catálisis, Santa Fé, Argentina,2-7 septiembre, 2012.33 Orge CA, Sousa JPS, Gonçalves F, Freire C,Órfão JJM, Pereira MFR. Development of NovelMesoporous Carbon Materials for the CatalyticOzonation of Organic Pollutants. Catal. Lett. 2009;132: 1-9.34 Faria PCC, Órfão JJM, Pereira MFR. Activatedcarbon and ceria catalysts applied to the catalyticozonation of dyes and textile effluents. Appl. Catal.B: Environ. 2009; 88: 341-350.


AbstractDiffusional limitations in carbon gel-supportedcatalysts are often encountered despite their openstructure. However, the analysis of mass transportis rarely taken into account in studies dealing withcatalyst preparation and test using thesenanostructured carbons as supports. Any catalyticsystem should be first subject to mass transportanalysis before any conclusion can be drawn aboutrelationships between the physico-chemicalproperties and the measured activity of the catalyst.

ResumenPese a su estructura abierta, a menudo se observanlimitaciones difusionales en catalizadores soportadosen geles de carbono. Sin embargo, en los diversosestudios de preparación y testeo de catalizadoressoportados en estos materiales nanoestructurados,pocas veces se lleva a cabo un análisis detransferencia de masa. Así, antes de obtenercualquier conclusión sobre la relación entre laspropiedades fisicoquímicas y la actividad de uncatalizador, el sistema debería someterse a unanálisis de trasporte de masa.

1. IntroductionPorous carbon materials are widely used in manyapplications: adsorption in liquid or gas phase, gasseparation, supports for catalysts, electrocatalysis,materials for batteries, etc. In all these processes,the pore texture of the chosen carbon material playsa major role. Indeed, whatever the above-mentionedapplication, species which can be adsorbates,reactants or ions must circulate within the poretexture. The control of the carbon pore texture isthus key to an efficient process.However, carbons used in industrial applications orin electrochemical devices, such as activated carbonsor carbon blacks, most often display quiteinappropriate pore textures with regard to masstransport. The texture of activated carbons, forinstance, is generally microporous, with lowmacropore or mesopore volumes, which ofteninduces diffusional limitations during catalytic and

adsorption processes. Carbon blacks, which arecomposed of microporous near-spherical particlesof colloidal sizes (~ 20 – 60 nm), coalesced togetheras aggregates (1 – 100 µm), remain the electedmaterial for Proton Exchange Membrane fuel cellelectrodes [1]; however, the pore texture of theelectrodes is quite dependant on the electrodeprocessing because it is to a large extent definedby the packing of the carbon aggregates, whichdepends on the electrode manufacture technique.These drawbacks call for the development of carbonmaterials with controllable and tunable pore texture.This is why so many recent works were dedicatedto the preparation of synthetic porous carbonmaterials, with special attention to the control of thetextural properties. Several routes were investigatedin the last 20 years and, among the numerous newcarbon materials developed, carbon gels, i.e. carbonxerogels, aerogels and cryogels, have beensuccessfully used at laboratory scale as alternativeto commercial carbons in several processes. Theadvantage of carbon gels is that their pore texture,i.e. pore size and pore volume, can be very accuratelytuned within a wide range. As a result, one can adaptthe pore texture of the used carbon with regard tothe application in order to get rid of diffusionallimitations encountered with mainly microporousmaterials. However, the elimination of mass transportlimitations is not straightforward since it does notonly depend on the pore size, but also on the voidfraction, on the tortuosity of the pores and on thecharacteristic dimension of the catalyst pellet orparticle. The aim of the present paper is to highlightthe advantages of these materials in terms of mass-transport control, and to underline the precautionsto be taken when using these a priori “diffusionallimitation-free” materials.

2. Preparation of carbon gels with tuned poretextureThe preparation of carbon gels and the final poretexture obtained in function of the preparation recipeused is very well referenced in the literature [2].Although alternatives can be found, most studiesreport the mixing of resorcinol and formaldehyde

Mass transport in carbon gels with tuned porosityTransferencia de masa en geles de carbón con porosidad a medida

N. Job*

University of Liège, Laboratory of Chemical Engineering B6a, Sart-Tilman, B-4000 Liège, Belgium.* Corresponding author: [email protected]

18

Figure 1. (a) Scanning electron micrographs of three carbon xerogels prepared at the same dilution ratio (D = 5.7) and various pHconditions. (b) Pore size distribution of carbon xerogels (D = 5.7) prepared at pH = 5.25 (•), pH = 5.75 (p) and pH = 6.25 (n). (c)Carbon gel modeling: covalently bonded microporous spherical-like nodules separated by meso/macropores.Figura 1. Micrografías electrónicas de barrido de tres xerogeles de carbón preparados con la misma relación de dilución (D = 5.7) ydiferentes pHs. (b) Distribución del tamaño de poros de xerogeles de carbón (D = 5.7) preparados a pH = 5.25 (•), pH = 5.75 (p)and pH = 6.25 (n). (c) Modelo de gel de carbón: nódulos esferoidales microporosos enlazados covalentemente separados pormeso/macroporos.


19

(reagents) in water (solvent) and use of sodiumcarbonate as basification agent (also called ‘basecatalyst’). After gelation and ageing, the gel iscomposed of interconnected spherical nodulesdelimiting voids filled with the solvent. After dryingand pyrolysis, the so-called ‘string-of-pearl’-likestructure remains (Fig. 1a) whatever the dryingtechnique, although large variations in the final porevolume can be observed depending on the methodused (evaporation, supercritical drying or freeze-drying). Pyrolysis induces the development ofmicroporosity within the nodules. As a result, carbongels display a bimodal pore texture (Fig. 1b):meso/macropores are delimited by the spacingbetween microporous nodules (Fig. 1c).In order to change (tune) the pore texture of the finalmaterial, one can play on three main variables [3]:(i) the pH of the precursor solution, (ii) the dilutionratio, i.e. the water/reactants molar ratio and (iii) thedrying technique. In a global way, the nodule size ismainly fixed by the pH of the precursor solution whilethe pore volume and pore size are mostly related tothe dilution ratio and drying technique. However, therelationship between the synthesis/drying variablesand the pore texture parameters is notstraightforward. The dilution ratio, for instance, alsoimpacts the nodule size, although to a less extentthan the pH of the solution. The nature andconcentration of ions also modifies the texture: forinstance, adjusting two gels at the same pH withNaOH or Mg(OH)2 leads to two different materials[4]. The drying technique impacts strongly the poretexture when the pores are small and when thedilution ratio is high. Indeed, in this case, evaporationleads to a shrunk material with low pore volumecompared to materials obtained by supercriticaldrying or freeze-drying. However, as the solid fractionand pore size increases, capillary tensions inducedby the curved liquid-vapor interface duringevaporation decreases, and no shrinkage is observedwhatever the drying technique [3]. Finally, the poresize and pore volume of carbon gels can be tunedindependently within a wide range: from a few nmto a few µm, and from 0 to 5-6 cm³/g, respectively.This leaves plenty of room for adjustment with regardto the final application.

3. Use in mass-transport dependant processes3.1. Heterogeneous catalysisThe deposition of active species on carbon gels isnot more difficult than in the case of other carbonsupports, but one of the advantages is that thesurface chemistry can also be tuned by post-treatments [5], which further eases the impregnationwith metal precursors for instance, or even thegrafting of metal complexes. Many studies havereported the preparation of metal, alloy or oxidecatalytic nanoparticles on such supports and theiruse in various catalytic processes [6]. However, fewof them include the analysis of mass transportlimitations in their discussion of results and, due tothe lack of appropriate data, it is often difficult todetermine whether the measured activity of thecatalyst is diffusion-free or not. The danger here isto attribute reaction rate constancy or modificationsto the physico-chemical characteristics of the catalyst(e.g. composition, surface chemistry, dispersion,etc.) while the system is in fact dominated by masstransport effects: in this case, the measured reactionrate is not the specific reaction rate, i.e. related tothe physico-chemical properties of the catalytic sites,but an apparent reaction rate, somewhat falsified bymass-transport limitations.

This problem is quite classical in heterogeneouscatalysis (Fig. 2) [7]: to reach the catalytic sites,reactants have first to diffuse through an externalstagnant film of fluid outside the catalyst pellet andmust then circulate within the pores of the support;products must then go the reverse way. In the casethe reaction is intrinsically slow and the masstransport is fast enough, the concentration ofreactants and products is quite homogeneouseverywhere (Fig. 2a: chemical regime). However,when the kinetics of the reaction is fast and thediffusion rate is slow, a concentration gradientappears in the catalyst pellet and/or in the externalstagnant film outside the pellet (Fig. 2b: diffusionalregime). As a result, the reactant concentration atthe catalytic site, C, which generally affects thekinetics of the reaction, is not that measured outsidethe catalyst pellet, Ce: the reaction rate measuredby the experimenter is a mix between various reactionrates because each catalytic site actually ‘sees’ adifferent concentration of reactants (and products).Another consequence is that catalytic sites locatedat the pellet centre are almost useless because theyare not reached by the reactants. The ratio betweenthis apparent reaction rate, ra, and the specificreaction rate, rs, is called the effectiveness factor ofthe catalyst, h. This factor quantifies the gap betweenthe true reaction kinetics over the catalytic sites andthe reaction kinetics observed, which depends onthe diffusion rate of the reactants and products inthe external layer and within the pore texture of thesupport.

3.2. Determination of the external and internallimitationsIn the case of carbon gels, like for any other porousmaterial, selecting supports with ‘large pores’ doesnot necessarily mean that no diffusional problemswill occur. As an example, Fig.2b shows theeffectiveness factor of Pd-Ag catalysts supportedon carbon xerogels with various meso/macropore

Pellet size (µm)

Effe

ctiv

enes

s fa

ctor

, η(-)

0 300 600 900 12000

0.25

0.5

0.75

1

(c)

Catalystpellet

Flowingfluid

Flowingfluid

Ce

Ce

Cs

Cs

(a)

(b)

z

C

Figure 2. Mass transport in a catalyst pellet. (a) Reactantconcentration profile in the case of chemical regime. (b) Reactantconcentration profile in diffusional regime. Ce and Cs representthe concentration of a reactant in the flowing fluid and at thesurface of the pellet, respectively. (c) Hydrodechlorination of 1,2-dichloroethane into ethylene on Pd-Ag/carbon xerogel catalysts,T = 573K. Effectiveness factor, h, as a function of the pellet size.Average pore size of the support: (p) ~10 nm, (n) ~30 nm, (u)~70 nm. Data adapted from [8].Figura 2. Transferencia de masa en una partícula de catalizador.(a) Perfil de concentración del reactivo en el caso de régimenquímico. (b) Perfil de concentración del reactivo en régimendifusional. Ce and Cs representan la concentración del reactivoen el flujo y en la superficie de la partícula, respectivamente. (c)Hidrodecloración de 1,2-dicloroetano en etileno sobre catalizadoresde Pd-Ag/xerogel de carbón, T = 573K. Factor de efectividad, h,en funcion del tamaño de partícula. Tamaño medio de poro delsuporte: (p) ~10 nm, (n) ~30 nm, (u) ~70 nm. Datos adaptadosde [8].


20

size in the case of hydrodechlorination of 1,2-dichloroethane into ethylene performed in gas phaseat 573K [8]. One can observe that supports withsmall pore size lead to a rapid decrease of h., evenwith quite small pellets (500 µm). Larger pore sizesallow keeping the effectiveness factor equal to 1 forpellets of larger size (up to 7 mm by calculation inthe case of 70 nm pores). Since too small pelletspiled up in a reactor would lead to prohibitive pressuredrop in the device, the possibility to use large pelletswithout loss of catalyst performance is obviously anadvantage brought by an adequate pore texture ofthe support. However, increasing the temperatureat 623K leads to internal diffusion limitations forpellets around 4 mm [8].As a matter of fact, it is impossible to tell without anycalculations or additional measurements whetherkinetics is observed in chemical regime or not. So,before any discussion of the catalytic properties ofcarbon gel-supported catalysts, one must check thatthe obtained kinetics data are mass transport-free.In a practical way, external limitations can be detectedby modifying the fluid flow rate (in the case of acontinuous tubular reactor) or by changing the stirringspeed (in the case of a continuous or discontinuousmixed reactor): these variables affect the thicknessof the limit layer outside the catalyst pellet. Internallimitations can be detected by changing the pelletsize: indeed, using smaller pellets obtained bygrinding of larger ones would lead to a shorteraverage distance between the pellet surface andthe catalytic sites and thus to a less severe internalconcentration gradient. Note however that modifyingthe pellet size also decreases the thickness of theexternal film: so, the detection of internal limitationsmust be performed in the absence of externallimitations.If the catalytic properties of a sample are notindependent of the flow rate or mixing and pelletsize, the obtained data are not specific to the physico-chemical properties of the catalyst itself but arerelevant to fluid dynamics. It is thus completelyillusory to link any of the catalyst properties to theobserved reaction rate. Additional measurementsare not always easy to perform, but the presence ofexternal or internal limitations can be deduced fromcalculation. The external diffusional limitations canbe quantified by the fraction of external resistancefe [7]:

(1)

where ra is the apparent specific reaction rate pervolume unit of catalyst, Lp is the characteristicdimension of the catalyst pellet (i.e., the volume tosurface ratio, equal to dp/6 in the case of a sphere,dp being the pellet diameter), Ce and Cs are theconcentration of the limiting reactant in the bulk ofthe fluid phase and at the external surface of thecatalytic pellet, respectively, and kd is the externaltransfer coefficient. kd can be estimated fromcorrelations between the Sherwood (Sh), Reynolds(Re) and Schmidt (Sc) dimensionless numbers [7].Indeed:

(2)

where Sh0 is the Sherwood number in the absenceof forced convection, Dm is the molecular diffusivity,dp is the particle diameter, r, m and u are respectivelythe density, the viscosity and the linear velocity ofthe fluid phase and B is a constant. In the case ofa stirred reactor, the velocity of the fluid phase iscalculated from the stirring speed of the reactor [7].Specific correlations of the Eq. (2) form are availablefor many systems and under various conditions. Allthe parameters of these equations are characteristicsof the fluid phase, reactor and catalyst pellet, whichare normally accessible to the experimenter. Thecalculation of Sh and thus of fe should not be aproblem for usual reactors. Finally, if fe is close to 1,one can conclude that no external limitations occur.A quantitative approach to evaluate the influence ofinternal diffusion on the overall catalytic processconsists in the calculation of the dimensionlessWeisz modulus, f, which is defined as the ratiobetween the apparent specific reaction rate and thediffusion rate of the reactant in the pore texture ofthe catalyst particle. For f larger than 1, the internaldiffusion limitations become rate-determining, andthe observed reaction rate is falsified by masstransport within the pore texture. The Weisz moduluscan be written as [7]:

(3)

where ra is the apparent reaction rate per volumeunit of catalyst, Lp is the characteristic dimension ofthe catalyst pellet, Cs is the concentration of thelimiting reactant at the external particle surface, andDe is the effective diffusivity through the catalystpores. Cs is not easily determined unless the catalyticmeasurements are performed in the absence ofexternal limitations, which is the usual way to proceed:in this case Cs = Ce. The effective diffusivity, De, isthe diffusivity in the pores, D, corrected by theaccessible void fraction, ε, and the pore tortuosity,ε [7]:

(4)

In liquid phase, D is the molecular diffusivity, Dm,available in tables. In gas phase and when the poresare small (typically < 100 nm), D is a combinationof the both the molecular and the Knudsen diffusivities[9], the latter mechanism being related to diffusionin which collisions with the pore walls are predominantwith regard to collisions between molecules. As aconsequence, in Knudsen-type regime, the diffusivitychanges with the pore size, which is not the case inmolecular-type diffusion. Note also that the tortuosityof the catalyst support,t, is often taken equal to 1/e.

3.3. Heterogeneous catalysis on carbon gelsThe case of carbon gels is more complicated sinceone clearly identifies two pore levels (Fig. 1b-c).Indeed, internal limitations could occur either at themeso/macropore level or at the micropore level ifthe catalytic species are located in the microporosity.With this particular double structure of granular pelletitself composed of porous spheres, one can imagineto perform the internal mass transport analysis atboth levels, i.e. by calculating the Weisz modulus atthe pellet (fp) and at the nodule (fn) level [8]. Allparameters related to the solid must then be thoserelated either to the pellet or to the nodule. For

fe = 1 -Ce

Cs=

ra Lp

kd Ce

mSho+BRe m Sc 3= kd dp

Dm= = Sho+B

113pudp m

pDm( () )

m

Sh

f = ra Lp

De Cs

2

De =eDt


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instance, the characteristic dimension Lp and thevoid fraction ε are different when considering eachlevel. In particular, the void fraction of the nodulesis almost constant (~0.35) because the structure ofthe nodules and thus their microporosity is quiteindependent on the synthesis conditions of thepristine gel, unless the carbon gel is further activatedto develop its specific surface area [10]; on thecontrary, the void fraction of carbon gel monolithsstrongly depends on the synthesis and dryingpathway (see §2). As an example, the Weisz moduluswas calculated at the pellet and nodule levels forPd-Ag catalysts supported on carbon xerogels ofvarious pore textures [8], and it was clearlydemonstrated that, when present, the diffusionallimitations occur at the meso/macropore level andnot in the micropores. This can be explained by thedifference of distance to be covered by the moleculesin the meso/macropores (~ 500 µm for 1 mm pellets)compared to micropores ( ~ 10 – 100 nm) to reachthe centre of either the pellet or the nodule; thisclearly demonstrates that the pore size is not theonly factor to be taken into account.Another source of complication is the fact that thecatalytic species may be not homogeneouslydispersed in the porosity of the support. For instance,the active particles can be located in an outer layerof the pellet (core-shell configuration). In this case,the characteristic dimension, Lp, to be used in Eq.(1) and (3) is the thickness of this layer. Pirard et al.[11] studied the oxidation of D-glucose into D-gluconicacid on Pd-Bi/carbon xerogel catalysts in aqueousmedia; the catalytic species (Bi and Pd) wereconcentrated in an external layer of the catalystpel lets, as shown by physico-chemicalcharacterization. Mass transport analysis wasperformed using the appropriate characteristicdimension; it showed that the kinetic measurementshad been performed under diffusional regime andthe authors concluded that measuring the true kineticreaction rate implied to choose the experimentalconditions within a very small range of values. Thisfinally confirms that the measurement of the specificcatalytic activity of catalysts supported onnanostructured carbons such as carbon gels is notan easy task; the choice of the experimentalconditions should be subject to high caution.Note that one could apply the same principles toadsorption; in this case, the reaction rate is replacedby the adsorption rate. Indeed, recent results haveshown that the pore texture of carbon gels has thesame impact on adsorption processes: the adsorptionrate can be increased by selecting a support withappropriate pore texture [12], adsorption sites beingmore easily reached by the adsorbate molecules.This is quite interesting for purification systems orgas masks, for instance, where the transientadsorption of molecules occurs.

3.2. Electrocatalytic processesThe advantages of carbon gels in heterogeneouscatalysis can be transferred to electrochemicaldevices such as Proton Exchange Membrane fuelcells. The catalytic layer of a PEM fuel cell iscomposed of Pt/carbon catalyst particles, interparticlevoids and ionomer (Nafion®, usually), hot-pressedbetween a proton-exchange membrane and a gasdiffusion layer (GDL) usually made of hydrophobiccarbon felt [1]. A catalytic layer of a PEM fuel cellcan be seen as a microreactor of heterogeneouscatalysis where mass transports are complicated by(i) the presence of two fluid phases (gas and

condensed water) and (ii) the proton transport viathe ionomer network. Indeed, to be active, the metal(Pt) catalyst particles must be in contact with thecarbon support and connected to the membrane viathe ionomer. In addition, reactants (H2 or O2, protonsand electrons) and water must circulate easily throughthe catalytic layers; in particular, the catalyst mustbe reached by the gas through the porous structureof the electrode. Usually, carbon blacks are used assupport; however, the packing of the carbon blackaggregates, and therefore the pore structure of thecatalytic layers, depends on the carbon black natureand on the electrode processing [1]. Typically, at theair-fed cathode, where oxygen, proton and watertransports hamper the oxygen reduction reaction,high potential losses due to diffusional limitationsoffset the cell performance [13]. This is compensatedby the use of catalytic layers with high Pt loading,which increases the electrode cost.The study of mass transport in such systems is noteasy because some parameters of the above-writtenequations are not readily available. For instance,due to the presence of ionomer within the poretexture, the pore size and pore volume is differentto that of the pristine carbon gel; this effect isreinforced by the fact that Nafion® swells whenhumidified, and that some water can condense withinthe pore texture, reducing further the available porevolume. As a first consequence, the true void fractionaccessible to reactants, ε, is difficult to estimate.Concentrations Ce and Cs evolve with the positionof the catalyst particles in the MEA, and correlationsfor the external transport, though available [14],strongly depend on the system geometry. At themoment, diffusional limitations are most oftenanalyzed through the voltage loss analysis of thecell. The total voltage loss of the cell with regard tothe reversible H2/O2 cell voltage can be decomposedinto several components [15]: (i) the kineticoverpotential, hORR, due to the slowness of the O2reduction; (ii) the ohmic losses, hOhm, due to theresistance of the proton migration through themembrane and the electronic contact resistances;(iii) the mass-transport losses, or ‘diffusionoverpotential’, hdiff. In such systems, kinetic andmass transport losses of the anode (H2 side) canbe neglected [15]. One can write:

(5) Ecell = Erev - hORR - hOhm - hdiff

where Erev is the reversible H2/O2 cell voltage underthe gas pressure and temperature of the cell. Thefirst two contributions to the voltage loss can bemeasured. Briefly, hORR is obtained frommeasurements at low current densities, i.e. fromdata obtained in the near-absence of mass transportlimitations and ohmic resistance (Tafel equation)[15]; hOhm is obtained by measurement of the ohmicresistance of the cell by impedance spectroscopy,performed in situ. Finally, all the other terms of Eq.(5) being known, hdiff can be deduced by difference.However, the overpotential attributed to masstransport, hdiff, remains a black box that actuallyencloses all the limitation sources that are notattributed to kinetic and ohmic phenomena throughdirect measurement. In particular it is difficult todetermine where mass transport limitations occur inthe catalytic layer. This question is still underinvestigation and remains of high importance in viewof electrode optimization.


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In search for new catalytic layer structures with lowdiffusion-induced potential losses, carbon gelsconstitute an interesting alternative to carbon blacksbecause their meso/macropore texture is totallyindependent on the electrode processing [16];besides, the high purity of carbon gels ensures theabsence of pollutants inherent to the support originand detrimental to the electrocatalytic activity. In theelectrode structure, the carbon black particleagglomerates are replaced by micromonoliths ofcarbon gel, which preserves the pores located in-between (Fig. 3a). These materials were usedrecently to prepare Membrane-Electrode Assemblies(MEAs) for air/H2 [16-18] or air/methanol PEM fuelcells [19], and were mainly tested at the cathode ofa monocell device. In both cases, the pore textureof the carbon gel was found to influence the cellperformances: it was possible to decrease thediffusion-induced voltage losses by selecting carbongels of appropriate pore texture (Fig. 3b). However,many variables influence the final electrodearchitecture and performance, and the optimizationof the pore texture of the carbon support is still anopen question.

4. ConclusionsDue to their pore texture versatility, carbon gels areattractive materials as (electro)catalyst supportsbecause an accurate choice of the pore texture candecrease mass transport limitations encountered inoperating catalytic processes. However, while fewstudies pay attention to this problem, the eliminationof diffusional limitations is never obvious. As a result,any catalytic system should be first subject to masstransport analysis before any conclusion can bedrawn about relationships between the physico-chemical properties and the measured activity of thecatalyst. Despite their complex structure, carbon gel-supported catalysts can be studied following classicalmethods provided that their double-porosity structureis taken into account.

5. AcknowledgementsNJ thanks the F.R.S.-FNRS (Belgium), the Fondsde Recherche Fondamentale Collective and theMinistère de la Région Wallonne (project INNOPEMn°1117490) for funding.

6. References1 Maillard F, Simonov P, Savinova ER. Carbonmaterials as supports for fuel cell electrocatalysts.In: Carbon Materials for Catalysis, Philippe Serp andJosé Luis Figueiredo Eds. Wiley, New York, 2009,p. 429-481.

2 Elkhatat AM, Al-Muhtaseb S. Adv Mater2011;23:2887-2903.

3 Job N, Théry A, Pirard R, Marien J, Kocon L,Rouzaud JN, Béguin F, Pirard JP. Carbon2005;43:2481-2494.

4 Job N, Gommes CJ, Pirard R, Pirard JP. J Non-Cryst Solids 2008;354:4698-4701.

5 Mahata N, Pereira MFR, Suárez-García F, Martínez-Alonso A, Tascón JMD, Figueiredo JL. J Coll InterfSci 2008;324:150-155.

6 Moreno-Castilla C. Carbon gels in catalysis. In:Carbon Materials for Catalysis, Philippe Serp andJosé Luis Figueiredo Eds. Wiley, New York, 2009,p. 373-400.

7 Froment GF, Bischoff KB. Chemical ReactorAnalysis and Design, 2nd ed., Wiley, New York,1990.

8 Job N, Heinrichs B, Lambert S, Pirard JP, ColomerJF, Vertruyen B, Marien J. AIChE J 2006;52:2663-2676.

Figure 3. (a) Catalytic layer of a PEM fuel cell with Pt/carbon gel as catalyst; the micromonoliths are made of carbon gel that retainsthe pore texture of the pristine material, despite grinding. (b) Diffusion overpotential as a function of the current density calculatedfrom measurements in PEM monocell with cathodes prepared from Pt/carbon xerogels of various average meso/macropore size andvolume: (p) ~40 nm and 1.0 cm³/g, (n) ~85 nm and 1.8 cm³/g, (u) ~300 nm and 1.9 cm³/g. Adapted from [16].Figura 3. (a) Capa catalítica de una pila de combustible PEM con Pt/gel de carbón como catalizador; los micromonolitos están hechosde gel de carbón que retiene la textura porosa del material original a pesar de la molienda. (b) Sobrepotencial de difusión en funciónde la densidad de corriente calculado de las medidas en una monopila PEM con cátodos preparados a partir de Pt/xerogeles decarbón de diferentes tamaño medio y volumen de meso/macroporos: (p) ~40 nm and 1.0 cm³/g, (n) ~85 nm and 1.8 cm³/g, (u) ~300nm and 1.9 cm³/g. Adaptado de [16].


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9 Satterfield CN. Mass Transfer in HeterogeneousCatalysis. Cambridge, MA: MIT Press; 1970.

10 Contreras M, Páez C, Zubizarreta L, Léonard A,Olivera C, Blacher S, Pirard JP, Job N. Carbon2010;48:3157-3168.

11 Pirard SL, Diverchy C, Hermans S, Devillers M,Pirard JP, Job N. Catal Comm 2011;12:441-445.

12 Almazán-Almazán MC, López-Domingo FJ,Domingo-García M, Léonard A, Pérez-Mendoza M,Pirard JP, López-Garzón FJ, Blacher S. Chem EngJ 2011;173:19-28.

13 Gasteiger HA, Gu W, Makharia R, Mathias MF,Sompalli B. In: Wolf Vielstich, Arnold. Lamm andHubert A. Gasteiger (Eds), Handbook of Fuel Cells– Fundamentals, Technology and Applications, Wiley,Chichester (UK), 2003, Vol. 3, p. 593.

14 Li PW, Zhang T, Wang QM, Schaefer L, ChyuMK. J Power Sources 2003;114:63-69.

15 Gasteiger HA, Kocha SS, Sompalli B, WagnerFT. Appl Catal B 2005;56:9-35.

16 Marie J, Berthon-Fabry S, Chatenet M, ChainetE, Pirard R, Cornet N, Achard P. J Appl Electrochem2007;37:147-153.

17 Marie J, Chenitz R, Chatenet M, Berthon-FabryS, Cornet N, Achard P. J Power Sources2009;190:423-434.

18 Job N, Marie J, Lambert S, Berthon-Fabry S,Achard P. Energ Convers Manage 2008;49:2461-2470.

19 Arbizzani C, Beninati S, Manferrari E, Soavi F,Mastragostino M. J Power Sources 2007;172:578-586.


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Last month of November, Xerolutions S.L. (http://www.xerolutions.com), a company that produces carbonxerogels, was awarded with the Sodeco 2012 award for business projects. The production of these materialsis based on an innovative technology developed and patented by the promoter group, Ana Arenillas andJ. Angel Menéndez, both from the INCAR-CSIC and members of the GEC. The material can be obtainedwith custom-designed properties for a particular application. The award panel took into account "the innovativenature of the project, which brings to market a new product, developed and patented by the promoter group,with multiple applications and in particular the production of electrodes for supercapacitors." The prize isworth 25,000 € for investment in the project.

Socios protectores del Grupo Español del Carbón

Reseña. Xerolutions, a company producing carbon gels, getsthe Sodeco award

David López, Almudena Trigo, Ana Arenillas and J. Angel Menéndez (1st, 2nd, 5th and 8th from the left, respectively) promoters ofXerolutions, receiving the Sodeco award from organizers and authorities.

La ETSI Minas de la Universidad Politécnica de Madrid y la SD deIngeniería Química de la Universidad Autónoma de Madrid tienen elplacer de invitarles a la próxima Reunión del Grupo Español delCarbón que tendrá lugar en el Edificio Histórico de la ETSI de Minasde Madrid, los días 20 a 23 de Octubre de 2013.Más información en http://www.minas.upm.es/gec2013.htmlEsperamos contar con su asistencia.Cordialmente, el Comité Organizador.

Anuncio. XII Reunión del GEC

http://www.nalonchem.com/es/

http://www.tecnovac.es/

http://www.grontal.es/

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