CO2 Capture in Different Carbon MaterialsVicente Jimenez,* Ana Ramírez-Lucas, Jose Antonio Díaz, Paula Sanchez, and Amaya Romero

Facultad de Ciencias Químicas/Escuela Tecnica Agrícola, Departamento de Ingeniería Química, Universidad de Castilla-La Mancha,13071 Ciudad Real, Spain

ABSTRACT: In this work, the CO2 capture capacity of different types ofcarbon nanofibers (platelet, fishbone, and ribbon) and amorphous carbonhave been measured at 26 °C as at different pressures. The results showedthat the more graphitic carbon materials adsorbed less CO2 than moreamorphous materials. Then, the aim was to improve the CO2 adsorptioncapacity of the carbon materials by increasing the porosity during thechemical activation process.After chemical activation process, the amorphous carbon and plateletCNFs increased the CO2 adsorption capacity 1.6 times, whereas fishboneand ribbon CNFs increased their CO2 adsorption capacity 1.1 and 8.2times, respectively. This increase of CO2 adsorption capacity afterchemical activation was due to an increase of BET surface area and porevolume in all carbon materials. Finally, the CO2 adsorption isothermsshowed that activated amorphous carbon exhibited the best CO2 capturecapacity with 72.0 wt % of CO2 at 26 °C and 8 bar.

1. INTRODUCTIONClimate change is considered to be one of the greatestenvironmental threats of our times.1 The atmosphericconcentration of greenhouse gases can roughly be dividedinto carbon dioxide, nitrous oxide, methane, fluorocarbons, andchlorofluorocarbons.2 In the last century, carbon dioxide (CO2)concentration in the atmosphere has constantly increased. Thisincrease was related with the increase in earth’s temperature.3

Thus, more and more attention has been paid toward reductionCO2 emissions as a global environmental challenge. There hasbeen increasing pressure from the public-opinion all over theworld to curb CO2 emissions, and over industries to developefficient carbon capture systems. Current or proposed methodsfor capturing CO2 from flue-gas have included absorption,adsorption, cryogenic distillation, and membrane separation.However, the commercial CO2 capture technology that existstoday is very expensive and energy intensive. Improving thetechnologies for CO2 capture is necessary in order to achievelow energy rewards.4 CO2 adsorption is typically used as a finalpolishing step in the hybrid CO2 capture system.5 Ideal CO2

adsorbents should have the following characteristics: a highspecific surface area, well-developed micro- and mesopores, andmany active sites on the surface such an amine functionalgroups and basic metal oxides.6−9 Research is currently directto identify and develop such sorbents, which include bothnaturally occurring materials such a coal and syntheticmaterials.5,10

Porous solid materials offer an attractive alternative forseparating CO2 from combustion flue-gas mixtures towardreducing greenhouse gas emissions. Examples of CO2-philicsorbent materials are carbons,11−14 zeolites,13,15,16 silicas,17−19

and metal−organic frameworks (MOFs).20−22 Porous carbona-

ceous materials are especially attractive because they arerelatively easy to regenerate,23 due to their moderate heats ofsorption, and nonexpensive preparation, not to mention thatthey are not as sensitive to water vapor as the other CO2-philicmaterials.Efforts have been made to improve the CO2 adsorption

capacity of carbon-based adsorbents by modifying the chemicalstructure of the carbon by means of impregnation withamines,24−26 heat treatment in the presence of ammonia gas27 or functionalization with amino groups by electrophilicaromatic substitution;28 other strategies were based on theintroduction of nitrogen-rich carbon precursor as carbonmatrix.29 Moreover, porous carbon can be modified byactivation process, which can lead to very large pore volumeand surface areas. Such activated carbons are widely used forpurification of water and air, and for separation of gasmixtures.23 In previous work,30−34 we developed highly porouscarbon materials by chemical activation method for hydrogenstorage.35−37 Several researchers have reported methods forobtaining porous carbons, including activation at high temper-ature with CO2 or a water steam and a chemical activationmethod.38−41

Most of the literature on carbons applied to CO2 capture isbased on maximum adsorption capacities determined from CO2

adsorption isotherms or on equilibrium CO2 uptakes at thedesired temperature. Thus, comparison of the results is notalways straightforward and inconsistencies between authors are

Received: December 24, 2011Revised: April 23, 2012Accepted: June 8, 2012Published: June 8, 2012

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often found depending on the conditions tested. Besides, littlework has been published on the influence of both textural andsurface chemistry factor on the CO2 adsorption capacity.Therefore, the search for an optimized carbon for CO2 capturebecomes highly empirical.42

The present contribution, based on the systematic study ofseries of modified carbon materials (amorphous carbon, anddifferent types of carbon nanofibers), provides more detailedunderstanding of CO2 capture by carbon materials. Therefore,carbon materials were chemically activated in order to increasethe specific surface area. Finally,the effect of the surface area ofcarbon material in the CO2 capture was studied.

2. EXPERIMENTAL SECTION2.1. Synthesis of Carbon Nanofibers (CNFs). Carbon

nanofibers were synthesized at atmospheric pressure in a fixed-bed reactor that consisted of a quartz tube of 9 cm diameterand 100 cm length located in a horizontal electric furnace (JHHornos) with an effective heating zone of 80 cm. Supportedcatalyst was placed in a quartz boat, which was kept inside theheating zone during the carbon nanofibers synthesis. In eachsynthesis run, 5 g of the prepared catalyst (10% wt. Ni/SiO2)was placed in the center of the reactor and activated by heating(10 °C min−1) in a flow of dry 20% v/v H2/He at the desiredreaction temperature (450, 600, and 850 °C). The reducedactivated catalyst was thoroughly flushed with dry He for 1 hbefore introducing the C2H4/H2 (4/1 v/v) feed. The growthtime was 1 h and the space velocity was 25 000 h−1. Silicasupports were subsequently separated from the carbon productby leaching the primary product in hydrofluoridic acid (48%)for 15 h with vigorous stirring followed by filtration andwashing with distilled water until neutral washings wereobtained. The resulting materials were dried at 110 °C for 12h in air to remove water prior to activation.43

2.2. Amorphous Carbon. Amorphous carbon was suppliedby PANREAC. For comparison purposes, this material alsoreceived the same HF treatment as CNFs.30

2.3. Chemical KOH Activation of the Carbon Materi-als. The experimental set up used for the preparation ofactivated carbon materials consisted of a horizontal quartzreactor tube located inside a conventional horizontal furnace.Carbon precursors were chemically activated with KOH.Carbon precursors were mixed with the activating agent (1:5wt:wt) and distilled water (5 mL water per 1 g activatingagent).30−34 The mixture was heated at 85 °C for 4 h withstirring and then dried at 110 °C for 12 h. The mixture wasplaced on a ceramic crucible located inside the horizontalquartz reactor. The heat treatment consisted of a heating rampfrom ambient temperature to the final heat treatmenttemperature (850 °C) at a heating rate of 5 °C min−1, followedby a 3 h plateau. The system was then cooled back to the initialtemperature. He was selected as the inert gas with a flow rate of700 mL min−1.30−34 The activated products were washed withhydrochloric acid (5 M) to remove the KOH and then washedwith distilled water until neutral washings were obtained. Theresulting materials were dried in air at 110 °C for 12 h toremove water prior to characterization.30−34

2.4. Characterization of Carbon Materials. Surface area/porosity measurements were carried out using a MicromeriticsASAP 2010 sorptometer apparatus with N2 at −196 °C as thesorbate. The samples were outgassed at 180 °C under vacuum(6.6 × 10−9 bar) for 16 h prior to analysis; specific surface areaswere determined by the multi point BET method, mesopore

volume and size distributions were evaluated using the standardBJH treatment and micropore volume and size distributionwere evaluated using the Horvath−Kawazoe (H−K) equa-tion.44−47

XRD analyses were carried out on a Philips X’Pertinstrument using nickel-filtered Cu−Kα radiation; the sampleswere scanned at a rate of 0.02° step−1 over the range 5° ≤ 2θ ≤90° (scan time = 2 s step−1). This technique was used toevaluate the graphitic nature of the carbon materials.Average diameter and morphology of the different CNFs

were probed by transmission electron microscopy (TEM) usinga Philips Tecnai 20T, operated at an acceleration voltage of 200keV. Suitable specimens were prepared by ultrasonic dispersionin acetone with a drop of the resultant suspension evaporatedonto a holey carbon supported grid. The average diameter wasmeasured by counting ∼200 CNFs on the TEM images. MeanCNF diameter is quoted in this paper as number averagediameter (d n):

=∑

dn d

nni i i

i (1)

where ni represents the number of particles with diameter di.Temperature-programmed oxidation (TPO) was used to

determine the qualitative crystallinity of the carbon materials.The analyses were performed on 50 mg samples using aMicromeritics AutoChem 2950 HP apparatus with a flow of 50mL min−1 of 20% (v/v) O2/He mixture and a heating rate of 5°C min−1 up to 1000 °C.The elemental composition of the carbon materials were

determined using a LECO elemental analyzer (model CHNS-932), which had an IR analyzer for carbon, hydrogen and sulfurand a TCD analyzer for nitrogen. Oxygen was assessed bydifference up to 100%.

2.5. CO2 Captures Measurements. CO2 capturecapacities of carbon materials were measured using aMicromeritics ASAP 2050. The samples were outgassed at180 °C under vacuum (6.6 × 10−9 bar) for 16 h prior toanalysis. The isotherm adsorption was obtained up to 8 bar.The Micromeritics software recorded the CO2 adsorbed foreach pressure. The adsorption isotherms were determined bythe multi point BET method.36

3. RESULTS AND DISCUSSION3.1. Structural Characterization of Carbon Materials.

The morphological features of the nonactivated carbonmaterials are illustrated in Figure 1. Amorphous carbon is anallotrope of carbon that does not have any crystalline structureand, thus, some short-range order can be observed. On theother hand, the angle with respect to the fiber axis has atendency to decrease at higher CNF synthesis temperatures andit is possible to find angles in the range from 40 to 90° at 450°C (platelet CNFs), from 25 to 50° at 600 °C (fishboneCNFs), and from 0 to 20° at 850 °C (ribbon CNFs). Thisarrangement provided fibers with a large number of accessibleedge sites, particularly in platelet and fishbone CNFs. On theother hand, representative TEM micrographs of activatedcarbon materials are also shown in Figure 1. These images showthat, upon activation, the arrangement of the initial graphenelayers was maintained. However, a large number of latticedefects were present in comparison with the relatively smoothsurface of the respective initial CNFs. Comparable disruption ofthe lattice structure of the fibers, which leads to a ladder-like

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structure by local removal or broadening of the graphene layers,has been reported before.31 These morphological changes thatoccur on chemical activation resulted in a significantmodification of the physicochemical properties of the CNFs.Textural characterization results of the parent and activated

carbon materials are given in Table 1. As can be observed, BETsurface area of amorphous carbon was larger than that of thedifferent types of CNFs due to its inherent microporosity (notethe high ultramicropore volume). Moreover, it can be observedthat the BET surface areas of activated samples were, in allcases, significantly larger than those of the nonactivated ones −mainly due to an increase in the micropore volume − thusdemonstrating the important effect that the treatment had onthe porosity development of different carbon structures.47−49

Nitrogen adsorption/desorption isotherms measured at−196 °C for parent and activated carbon materials arepresented in Figure 2. Isotherms associated with differentcarbon materials can be attributed to a combination of type Iand IV (according to the IUPAC classification). Thus,isotherms corresponding to amorphous carbon showed a highN2 adsorption volume at low partial pressure values (P/P0 ≤0.03) which is consistent with a type I and indicative of amicroporous structure. The small hysteresis loop at higherpartial pressures was due to capillary condensation and matches

that observed for a type IV being associated with somemesoporosity.32 For CNFs, a moderate rise at very low relativepressure (Platelet > Fishbone > Ribbon) and a slow increase insorption values as the relative pressure of the adsorbateincreases was observed. However, in the pressure region closeto saturation, adsorption sharply increased, approaching thecurve asymptotically to P/P0 = 1. Also, a well-defined hysteresis(H3 type according to IUPAC classification) was observedindicating the existence of a substantial volume of mesoporeswhere irreversible capillary condensation occurred.50,51 Afteractivation, the shape of the isotherm was preserved, but asignificant increase in the N2 adsorbed at low partial pressurestook place due to the increase in micropore volume. Amoderate rise at very low relative pressure (platelet > fishbone> ribbon) and a gradual increase in sorption values onincreasing the relative pressure of the adsorbate was observedfor the nonactivated CNFs (see Figure 2b). Moreover, in thepressure region close to the saturation, the adsorption increasedsharply for both the nonactivated and activated CNFs,approaching the curve asymptotically to P/P0 = 1. All thesevalues can be confirmed from the information given in Table 1,in which, can be observed the high micropore volume value(mainly ultramicropore volume) of the amorphous carbon incomparison with that of CNFs. On the other hand, comparingthe different types of CNFs, it is clearly observed as the porestructure of the platelet type CNFs was more developed thanthat of the fishbone and ribbon types, which could be related toa high number of adsorption sites.52 Textural parameters, therange of CNF diameters and the average CNF diameter aresummarized in Table 1. It can be observed that all of theseparameters were smaller after the activation process as aconsequence of the changes produced on the material surface.53

The proportion of ultramicropores, supermicropores andmesopores and the pore volume distribution, of bothnonactivated and activated carbon materials are shown inFigures 3 and 4, respectively. The following classification of thepores was considered (IUPAC): micropore <2 nm (super-micropore: 0.7−2 nm and ultramicropore: < 0.7 nm); 2 nm <mesopore <50 nm and macropore >50 nm). As can beobserved, amorphous carbon presented a high amount ofultramicropores and so high micropore volume (Table 1), incomparison with other carbon materials. After activation, themicropore volume distribution of the activated samplesmarkedly increased, mainly over the ultramicropore region.On the other hand, the predominant mesopore size waspreserved after activation (3−5 nm), but the mesopore volumeconsiderably increased. Analyzing the mesopore volumedistribution after activation, a great increase in the adsorbedvolume in all the mesopore range, attributed to the creation ofsmaller mesopores, was observed. Fishbone type CNFmicroporosity was also substantially developed after activation(mainly ultramicropores but also supermicropores). Moreover,it is interesting to remark that the mesopore volume was alsoincreased in a meaningful way after activation, with the creationof new mesopores of small size (nonexisting in nonactivatedCNFs). Finally, micropore volume modification in platelet andribbon type CNF was similar as that of fishbone CNFs. Afteractivation, their micropore volume markedly increased over theentire ultramicropore region and, in a lesser extension, over thesupermicropore region. Moreover, after activation, themesopore size distribution of both type of CNFs remainedunchanged, whereas the mesopore volume being alsoincreased.48

Figure 1. Representative amplified TEM images of (a) platelet CNFs,(b) activated platelet CNFs, (c) fishbone CNFs, (d) activated fishboneCNFs, (e) ribbon CNFs, (f) activated ribbon CNFs, (g) amorphouscarbon, and (h) activated amorphous carbon. Activated conditions: 5:1ratio KOH:CNFs, 3 h activation time, 850 °C activation temperatureand 700 mL·min−1 helium flow rate.

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TPO analyses were used to study the graphitic character ofthe parent carbon materials. It is well established that the morestructured a carbon material is, the higher is the temperaturerequired for gasification during TPO.43,54 Table 1 shows theoxidation temperature range and the maximum oxidationtemperature for all samples. Logically, the oxidation temper-ature range and the maximum oxidation temperature ofamorphous carbon were lower than the obtained for thedifferent types of CNFs due to its amorphous nature.According to the maximum oxidation temperature sequenceobtained with CNFs (Platelet < Fishbone < Ribbon), could bedemonstrated as the graphite character of Ribbon type CNFswas the highest. Moreover, it can be seen that these parameterswere consistently shifted to lower oxidation temperatures forthe activated samples, indicating easier oxidation and, therefore,a lesser structural order. While TPO analyses provided anindirect assessment of carbon structural order, the inferredtrends were further supported by XRD analyses (see Table 1).The two main XRD analysis parameters, namely the averageinterlayer spacing (d002) and the average stacking height ofcarbon planes (Lc002), were adopted as a quantitativemeasurement of the graphitic character.48,55 It can be seenthat the graphitic character degree of amorphous carbon waslower than that of CNFs. Moreover, for CNFs the d002 valueswere consistent with a graphitic product. d002 values increasedupon activation of the carbon materials, indicating an increaseddeviation from an “ideal” graphitic material. Analogously, theaverage crystalline parameter (Lc) deduced from the half-widthof the (002) diffraction peak decreased after activation, which isindicative of a less efficient packing of the graphene layers.In order to explain the morphological changes suffered upon

chemical activation, it is necessary to attend to the chemicalproperties of the activating agents (KOH). During theactivation process, the corresponding alkaline metal (K) isformed through a series of redox reactions where carbon isoxidized to form carbonate and the hydroxide is reduced. Whenthe carbon material/activating agent mixture is heated, part of

Table 1. Physico-Chemical Properties of the Parent and Activated Carbon Materials

amorphouscarbon

activatedamorphouscarbon platelet CNFs

activatedplatelet CNFs fishbone CNFs

activatedfishboneCNFs ribbon CNFs

activatedribbon CNFs

BET surface area (m2/g)

637 2157 286 786 202 570 68 310

micropore area (m2/g)a

450 (71%) 1874 (87%) 57 (20%) 481 (61%) 25 (13%) 305 (54%) 2 (3%) 206 (66%)

differential microporevolume (cm3/g-Å)b

15.692 (0.58) 33.630 (0.52) 1.194 (0.62) 6.773 (0.58) 0.546 (0.62) 3.431 (0.60) 0.052 (0.64) 1.867 (0.56)

mesopore volume(cm3/g)c

0.244 (4.62) 0.805 (3.50) 0.409 (8.71) 0.792 (6.70) 0.480 (10.15) 1.149 (8.02) 0.246 (11.40) 0.268 (6.89)

differential porevolume between 0.85and 0.95 nm (cm3/g-Å)d

0.20 2.59 0.19 0.90 0.15 0.62 0.02 0.20

d002 (Å) 3.52 3.83 3.46 3.55 3.44 3.55 3.42 3.44Lc002 (Å) 5.8 5.1 26.8 7.0 29.4 23.6 42.1 39.1TPO temperaturerange (°C)e

392−512 (460) 300−468 (393) 435−580(494)

336−495(431)

430−618 (546) 290−490(390)

506−622 (586) 383−525(495)

range CNFs diameter(nm)f

15−105 (37) 15−55 (27) 5−135 (44) 5−75 (27) 35−130 (65) 10−85 (46)

aIn brackets: percentage of micropore area with respect to the total surface area. bCumulative differential pore volume obtained using the Horvath−Kawazoe method. In brackets: average micropore size (nm). cCumulative pore volume obtained using BJH method. In brackets: average mesoporesize (nm). dIn the range 0.85−0.95 nm: cumulative differential pore volume obtained using the Horvath−Kawazoe method. eIn brackets:temperatures at which the maximum of the TPO peaks appeared in the analysis. fRange of CNF diameters determined by counting ∼200 CNFs onthe TEM images. In brackets: average CNF diameters.

Figure 2. N2 adsorption−desorption isotherms of activated andnonactivated carbon materials: (a) amorphous carbon and (b)different types of carbon nanofibers. Activated conditions: 5:1 ratioKOH:CNFs, 3 h activation time, 850 °C activation temperature and700 mL·min−1 helium flow rate. (AC: activated).

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the metal is vaporized, but a portion remains intercalated withinthe carbon structure, producing the pore opening of the startingcarbon, since part of the structure is destroyed. Additionally,due to the oxygen coming from the hydroxide, part of thecarbon on the surface is burnt off, generating CO and CO2.

31

The following reactions take place:

+ ↔ + +6KOH 2C 2K 3H 2K CO2 2 3 (1)

↔ +K CO K O CO2 3 2 2 (2)

+ ↔ +K CO C K O 2CO2 3 2 (3)

+ ↔ +K O C 2K CO2 (4)

+ ↔ +2K CO K O CO2 2 (5)

Figure 3. Proportion of ultramicropores, supermicropores andmesopores of activated and nonactivated carbon materials. Activatedconditions: 5:1 ratio KOH:CNFs, 3 h activation time, 850 °Cactivation temperature and 700 mL·min−1 helium flow rate. (AC:activated).

Figure 4. Micropore/mesopore distribution of activated and non-activated carbon materials: (a) amorphous carbon, (b) Platelet CNFs,(c) Fishbone CNFs, and (d) Ribbon CNFs. Activated conditions: 5:1ratio KOH:CNFs, 3 h activation time, 850 °C activation temperatureand 700 mL·min−1 helium flow rate. (AC: activated).

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3.2. CO2 Capture on Different Carbon Materials. Thevariation in CO2 uptake (mmol CO2/g) at 26 °C and up to 8bar for the different parent and activated carbon materials arerepresented in Figure 5 and Table 2. It can be seen that, for the

parent carbon materials, the different types of CNFs presented

lower CO2 capture than amorphous carbon. In general, the

higher BET surface area and pore volume, the higher the CO2

capture capacity was observed.

The differences observed in the CO2 capture capacity of thedifferent carbon materials studied could be related to their porevolume. Generally, in large-porosity adsorbents, like meso-porous silicas, the CO2 kept increase due to condensation inthe available space within the mesopores.56,57 In this study, theintense and narrow micropore volume distribution and thebroad mesopore distribution led to a high CO2 adsorptioncapacity on amorphous carbon. Comparing the differentadsorbents based in carbon materials, it seems clear thatcarbon adsorbents with a strong ultramicropore nature are themost efficient for CO2 adsorption under ambient conditions,according to Tables 1 and 2 and Figure 5.7,9 On the other hand,according to different authors,9 those materials with a highvolume associated to pores with sizes <1 nm (mainly between0.85 and 0.95 nm) maximize the interaction potential betweenthe CO2 molecules and the carbon surface due to the overlapover potential fields from both sides of the pores which lead tothe pore filling and so, a major CO2 adsorption is reached atlow pressures.9,36 In the present study, it was not clear the CO2adsorption capacity dependence with the small pores with sizesaround 0.85−0.95 nm being nevertheless, totally clear thedependence with those pores with sizes in all the microporerange (0.7−2 nm, IUPAC classification).On the other hand, it could be also observed that, for all

carbon materials the CO2 adsorption at 26 °C was completelyreversible with almost no hysteresis phenomena and that allisotherms had a very similar shape. The isotherm can thereforebe fitted to a Langmuir-type equation (type I isotherm),indicating that saturation occurs with the formation of carbondioxide monolayer, as expected for micropores surfaces.36,58 Itcould be seen that the CO2 capture was almost a linear functionof pressure (mainly, adsorbents based in CNFs), which can beexplained attending to Henry’s law. Moreover, the saturationpoint was not reached at 8 bar, and consequently the CO2capture capacity could be increased by increasing thepressure.11

As observed in the characterization section, the activationprocess resulted in an important increase in both the surfacearea and micropore volume (including the volume of thosepores with sizes in the 0.85−0.95 nm region). As consequence,the adsorption capacity of the activated materials wasconsiderably higher than that of the nonactivated ones, ascould be also observed in Figure 5b) 7 (i.e., amorphous carbonadsorbed 46.8 wt % whereas the activated amorphous carbonadsorbed 72.0 wt % of CO2 at 26 °C and 8 bar).It is interesting to note that fishbone and platelet CNFs and

activated ribbon CNFs presented similar micropore volume inthe range 0.85−0.95 nm than amorphous carbon (Table 1).Nevertheless, their CO2 capture capacity was lower. To explainthese results, it is necessary to take account the total porevolume of the different carbon materials. Thus, the total porevolume of amorphous carbon is higher than fishbone andplatelet CNFs or activated ribbon CNFs, indicating theimportant role of porosity in the adsorption process. Thus, asthe molecular diameter of carbon dioxide is 0.33 nm, it can beexpected to pass thorough even the smallest micropores in acarbon sample whose dimensions are always greater than 0.33nm.7,59

Obtained results have demonstrated that carbon materials aregood enough in order to capture CO2. At the same time, theCO2 adsorption could be increased by a chemical activationprocess of the carbonaceous material.

Figure 5. CO2 adsorption−desorption isotherms at 8 bar and 26 °Cfor the carbon materials: (a) parent carbon materials and (b) activatedcarbon materials. Activated conditions: 5:1 ratio KOH:CNFs, 3 hactivation time, 850 °C activation temperature and 700 mL·min−1

helium flow rate. (AC: activated).

Table 2. CO2 Capture Capacity Values

temperature: 26 °C pressure: 8 bar

material mmol CO2/g CO2 wt.%

amorphous carbon 10.63 46.8AC-amorphous carbon 16.53 72.0fishbone CNFs 6.60 29.0AC-fishbone CNFs 7.15 31.5platelet CNFs 7.23 31.8AC-platelet CNFs 11.41 50.2ribbon CNFs 0.46 2.0AC-ribbon CNFs 3.77 16.6

aAC: activated.

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Finally, in Table 3 are compared the BET surface area andCO2 adsorption capacity of the activated amorphous carbon

with different MOF materials.60 It can be observed that theactivated amorphous carbon presented a lower BET surfacearea but it showed a higher CO2 uptake at 8 bar.

■ AUTHOR INFORMATIONCorresponding Author*Phone: +34-926295300; fax: +34-926295318; e-mail: vicente.jimenez@uclm.es.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe gratefully acknowledge financial support from Consejeriade Ciencia y Tecnologia de la Junta de Comunidades deCastilla-La Mancha, Spain (Projects PBI-05-038 and PCI 08-0020-1239).

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Table 3. CO2 uptake between activated amorphous carbonand MOF materials

samples BET surface area (m2/g) mmol CO2/g

MOF-200 4530 54.5a

MOF-205 4460 38.1a

MOF-210 6240 54.5a

activated amorphous carbon 2157 16.5b

aExperiments performed at 26 °C and 50 bar. bExperiment performedat 26 °C and 8 bar.

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