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Applied Catalysis A: General 458 (2013) 21– 27

Contents lists available at SciVerse ScienceDirect

Applied Catalysis A: General

j ourna l h om epage: www.elsev ier .com/ locate /apcata

onolithic honeycomb design applied to carbon materials foratalytic methane decomposition

osé M. Gatica, Diana M. Gómez, Sanae Harti, Hilario Vidal ∗

epartamento C.M., I.M. y Química Inorgánica, Universidad de Cádiz, Puerto Real 11510, Spain

a r t i c l e i n f o

rticle history:eceived 21 November 2012eceived in revised form 12 March 2013ccepted 13 March 2013vailable online xxx

a b s t r a c t

As is well known, carbonaceous materials are good catalysts for methane decomposition, a promisingalternative to methane reforming and water gas shift reactions, as the CO2-free production of hydrogencan be attained. Nevertheless, the continued absence of literature on the use of carbon-based honeycombmonoliths for this application is surprising, despite the monolithic honeycomb design offering obviousadvantages compared with conventional packed beds. Therefore, in this work we have prepared integral

eywords:arbononeycomb monolithsethane decomposition

atalysis2 production

carbon monoliths and tested them in catalytic CH4 decomposition. The reaction has been studied atdifferent temperatures (750–900 ◦C) and the deactivation with time has been also monitored. Specialattention was paid to the influence of the texture generated by different thermal treatments to which themonoliths are subjected after their preparation to eliminate the additives and activate the carbon. Forthis purpose a deep textural characterization has been carried out. Comparison with the same samplesbut in the form of powders was also performed.

. Introduction

Methane decomposition has been a very promising route forO2-free hydrogen production for over 20 years. It is also wellnown that carbon materials in general are good catalysts for thiseaction because, compared with metal catalysts, they are cheapernd show higher resistance against deactivation [1]. Literature onhis topic is really vast, going from the many studies publishedy Muradov [2–6], one of the pioneers in this research field, tohe most recent reviews in high level journals [7,8], and dealingith all kinds of carbon materials: carbon blacks, activated carbons,

raphite, carbon nanotubes, etc.Nevertheless, the absence of studies in the literature into

he application of honeycomb monoliths for CH4 decompositions surprising. This is so despite the fact that, as many authorsave pointed out [9–11], honeycomb monoliths offer obviousdvantages with regard to packed bed reactors. In particular theyan reduce pressure drop, which might be very important in CH4ecomposition due to the carbon deposits formed during theeaction.

Even most popular design, fluidized bed reactors, has several

imitations [12–15]. First, they normally incorporate a unit to sepa-ate the carbon particles carried out of the reactor by the high flowf operation. Second, they sometimes have to be stopped to remove

∗ Corresponding author. Tel.: +34 956 012744; fax: +34 956 016288.E-mail address: hilario.vidal@uca.es (H. Vidal).

926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apcata.2013.03.016

© 2013 Elsevier B.V. All rights reserved.

the carbon inside the reactor before it may literally block the flow.It is evident that the application of honeycomb monolith reactorsmight minimize or even avoid these operations, because they arevery open structures and they allow working at much lower flows,thus also reducing the fraction of carbon ashes carried out.

The pair formed by carbon and honeycomb monoliths is alsounusual, although some examples can be found in the open liter-ature [16] and also patents [17]. This is so because traditionallycarbon has been considered a non appropriate material for extru-sion. However, our group has demonstrated that carbon extrusionis quite simple using the right additives and procedure [18]. Infact, in the last few years we have prepared carbon-based honey-comb monoliths for different applications, such as VOC adsorption[19], selective catalytic reduction of NOx [20] and, more recently,adsorption of organic pollutants in the liquid phase [21].

With this framework, it seems interesting to combine thesethree usually separate concepts, carbon, honeycomb monolithsand methane decomposition. It is true that references from otherauthors on carbon-based honeycomb monoliths for environmen-tal applications are already available, but they either just depositthe carbon on a monolithic honeycomb substrate [22] or, if theyprepare integral monoliths, they either employ polymeric resinsinstead of a natural carbon as a starting material [23] or the carboncontent hardly exceeds 50% by weight of the extrudable paste [24].

In this sense, the approach of this paper is original because weprepare nearly pure carbon honeycombs starting from a naturalcoal. In fact, this is mixed with the minimum amount of additivesthat confer it appropriate rheological properties for extrusion. After

22 J.M. Gatica et al. / Applied Catalysis A: General 458 (2013) 21– 27

is wor

ofa

2

2

cmdpitoattmcnw1

iltlmwlbit

2

uaia

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reached in consecutive steps with a heating rate of 10 C min .A powdered sample subjected to the same treatment as the CAmonoliths was also studied as a reference. In this particular casethe experimental conditions were adjusted to allow comparison

PCA

CA

C

Pre-oxidation Carbonization Activation

Air, 25 0 ºC, 24h Ar, 84 0 ºC, 1h Ar/33 %v/v H2O, 860 ºC, w.l . 15%

Te

mp

.T

em

p.

Te

mp

.

PCA

CA

C

Pre-oxidation Carbonization Activation

Air, 25 0 ºC, 24h Ar, 84 0 ºC, 1h Ar/33 %v/v H2O, 860 ºC, w.l . 15%

Te

mp

.T

em

p.

Te

mp

.

Fig. 1. (a) Image of the carbon-based honeycomb monoliths obtained in th

btaining the monoliths they are just dried and subjected to dif-erent thermal treatments that both eliminate the additives andctivate the carbon.

. Experimental

.1. Materials and monoliths preparation

Casagrande’s method, developed many years ago to prepareeramic honeycomb monoliths, was used to prepare the carbononoliths. It has the virtue of allowing extrudibility to be pre-

icted by simply measuring two parameters, the liquid limit and thelasticity index of the paste to be extruded [18,25]. In this study,

t was applied to a medium volatile bituminous coal supplied byhe National Institute of Carbon (INCAR) in Spain. Small amountsf a natural clay (9.5%), glycerine (2.5%), methylcellulose (1.9%)nd aluminum phosphate (0.3%) were used as additives to obtainhe right rheological properties (weight percentages referred tohe extrudable paste excluding water). Employing a pneumatic

achine, square section carbon honeycomb monoliths (with 3 × 3onfiguration) were extruded, having 13.7 cells cm−2, a wall thick-ess of 0.08 cm and a geometric surface area of 10.4 cm2 cm−3

ith 49% open frontal area. The external dimensions were approx. cm × 1 cm for the section and 5 cm for the length (Fig. 1).

One of the main focuses of this study was to investigate thenfluence of the different thermal treatments to which the mono-iths can be subjected after their preparation. In this sense, threeypes of carbon monoliths were prepared: the so-called C mono-iths (obtained by simple carbonization under Ar at 840 ◦C), the CA

onoliths (which included activation after the carbonization usingater vapor as an activating agent at 860 ◦C), and the PCA mono-

iths (subjected to additional pre-oxidation until reaching a 15%urn-off degree) (Fig. 2). This last treatment was applied consider-

ng the bibliography, according to which this can further improvehe textural properties of a carbon material [26].

.2. Characterization techniques

The properties of the resulting monoliths were investigatedsing different techniques. First, we measured their resistance toxial fracture in a Shimadzu AG-IS universal machine for mechan-cal assays capable of working at a maximum pressure of 100 kN,ccording to the standard procedure UNE-EN ISO 604.

N2 physisorption at −196 ◦C was employed to study the specificurface area and the micro- and mesoporosity. This was performed

n a Micromeritics ASAP 2020 over both small pieces of crushed

onoliths and entire monoliths, in this case both before and afterhe catalytic tests. In all the cases the samples were first subjectedo a heating treatment under vacuum at 350 ◦C for 1 h.

k; (b) drawing with the geometric parameters of the monolithic structure.

As this textural study involved structured materials, it was com-plemented by investigating the macropore structure by means ofmercury porosimetry. This was carried out on monolith specimenswith a volume of around 1 cm3 using a Pascal porosimeter fromFisons Instruments.

The monoliths were also observed directly by scanning elec-tron microscopy (SEM). Surface images from the monoliths’ wallswere captured with a QUANTA-200 microscope from Phillips, witha nominal resolution of 3.5 nm and operating at 30 kV.

The chemical nature of the carbon surface was also studied byinfrared spectroscopy. Fourier transformed infrared (FTIR) spectrawere collected in a Bruker (Vertex 70) spectrometer in the formof pellets prepared by mixing 100 mg of KBr with 5 mg of crushedcarbon monoliths, and operating in transmission mode with a res-olution of 4 cm−1 and with 100 scans.

2.3. Catalytic activity tests

Finally, the monoliths were tested in the methane decompo-sition reaction using a Pfeiffer mass spectrometer (ThermostarQMS 200) for the analysis. A second electron multiplier detectorwas used to enhance analysis sensitivity. For these experimentsentire monolithic pieces (around 2.7 g of weight) were placed insidea quartz tubular reactor, the total gas flow of CH4(5%)/He was60 cm3 min−1 and the catalytic activity was evaluated for 60 minin each case at three different temperatures, 750, 800 and 900 ◦C,

◦ −1

TimeTime

Fig. 2. The three different treatments to which the carbon-based honeycomb mono-liths were subjected in this study.

J.M. Gatica et al. / Applied Catalysis A: General 458 (2013) 21– 27 23

w11pt

3

3

bfta

c

Fr

Relative presure (P/P0)

0.0 0.2 0.4 0.6 0.8 1.0

Am

ountadsorb

ed (

cm

3/g

ST

P)

0

2

4

50

100

150

200

C

CA

PCA

Relative presure (P/P0)

0.0 0.2 0.4 0.6 0.8 1.0

Am

ountadsorb

ed (

cm

3/g

ST

P)

0

2

4

50

100

150

200

C

CA

PCA

icant modification of the textural properties with a strong decreaseof both the BET surface area and pore volume. This observation

Fig. 3. Schematic diagram of the experimental setup for the catalytic study.

ith the monoliths, using 0.6 g of sample and a total flow of3.4 cm3 min−1 to keep W/F constant. The GHSV was 1130 h−1 and020 h−1 for the experiments with powdered and monolithic sam-les respectively. Figs. 3 and 4 show the experimental setup andhe scheme of the experiments, respectively.

. Results and discussion

.1. Carbon monoliths characterization

The study of the mechanical resistance indicated that the car-on monoliths prepared had an axial crushing strength rangingrom 3 to 5 MPa. This result should be considered good enough for

he application investigated here, and greatly valued taking intoccount that we are not working with ceramics.

The N2 physisorption study revealed that both the PCA and CArushed monoliths showed type I isotherms, which are character-

ig. 4. Scheme of the experiments followed to study the methane decompositioneaction.

Fig. 5. Nitrogen adsorption (о)/desorption (•) isotherms of the crushed carbonmonoliths studied.

istic of microporous solids (Fig. 5). On the other hand, the C crushedmonoliths presented a type II isotherm, related to a macroporoussolid [27]. Mercury porosimetry also evidenced clear differencesbetween the samples in the macropore range (Fig. 6). Notice howin the C monoliths, pores with an average diameter between 10and 100 �m are abundant, while in the PCA, 1 �m sized pores aredominant. The processing of these curves led to the results shownin Table 1. It is evident that the different thermal treatments inducevery different textures. In particular, the C monoliths have almostnegligible surface area, while the PCA have more than 500 m2 g−1.Finally, the CA monoliths exhibit an intermediate texture regardingboth surface and porosity.

Taking into account that the catalytic study has been performedon entire monolithic pieces (see below), the N2 physisorption mea-surements have been also carried out over not crushed monoliths(results also included in Table 1). This study has evidenced a signif-

Pore diameter (

0.01 0.1 1 10 10 0 10 00

Rela

tive

volu

me

(mm

3/g

)

0

20

40

60

C

CAPCA

Pore diameter (µm)

0.01 0.1 1 10 10 0 10 00

Rela

tive

volu

me

(mm

3/g

)

0

20

40

60

C

CAPCA

Fig. 6. Pore size distribution curves as obtained from mercury porosimetry on thestudied monoliths.

24 J.M. Gatica et al. / Applied Catalysis A: General 458 (2013) 21– 27

Table 1Textural properties of the monolith samples.

Sample SBET(m2 g−1) Pore volume (cm3 g−1)

Microporesa Mesoporesb Macroporesc

C, crushed, fresh 2 0.001 0.003 –C, fresh 4 0.001 0.012 0.372C, spent 2 0.001 0.005 –CA, crushed, fresh 225 0.099 0.018 –CA, fresh 102 0.038 0.018 0.334CA, spent 3 0.001 0.008 –PCA, crushed, fresh 521 0.195 0.079 –PCA, fresh 145 0.060 0.007 0.337PCA, spent 4 0.000 0.006 –

data.ption

ia

dtmvcabtppibccafdam

mdP

a As estimated by means of t-plot (Harkins–Jura) analysis from N2 physisorptionb Cumulative pore volume obtained in the mesopore width range from the adsorc Data from mercury intrusion porosimetry technique.

s consistent with the fact that a fraction of the material is notccessible to the gas phase in an integral monolith.

Regarding the infrared spectroscopy measurements, this studyid not reveal any significant difference between the samples inves-igated (Fig. 7). All the spectra obtained were very similar and the

ost significant peaks detected were assigned either to C C bondibrations of the basal planes of carbon (1578 cm−1) [28] or to thelay used as an additive to extrude the carbon (1115 and 473 cm−1,nd those between them) [18]. On the other hand, the low intenseands located in the upper region (above 3000 cm−1) as well ashe minor peak at 1381 cm−1 should be reasonably associated withhenol groups [29]. However, according to their low intensity androfile, the presence of these species must be scarce and similar

n all the samples [30]. It is also noticeable the absence of the car-onyl peak appearing at 1690 cm−1 that has been reported for otherarbon materials [28]. These observations are consistent with thearbon bibliography according to which our thermal treatmentsre not the most appropriate to induce the formation of oxygenatedunctional groups on the carbon surface [31]. On the contrary, whenifferent activation treatments (acidic, ozonation, plasma, etc.) arepplied development of oxygenated groups is easily observed byeans of IR spectroscopy [32,33].

On the other hand, the SEM study led to results in good agree-

ent with those from the mercury porosimetry (Fig. 8). Notice theevelopment of very wide macropores in the C monoliths while theCA had a rougher surface with much smaller macropores, below

11600

PCA

CA

C

1578

Wavenumber (cm-1)

3250350037504000

Abso

rbance

(a.u

.)

0.00 5

PCA

CA

C 0.2

Fig. 7. FTIR spectra of the

branch of the N2 physisorption isotherm following the BJH method.

1 �m of size. The look of the CA monoliths is intermediate, betweenthe other two.

3.2. Methane decomposition study

One of the main singularities of the study performed was theway to follow the reaction. While other authors estimate the activ-ity by measuring the carbon formed during the reaction, in our casemass spectrometry (MS) allowed monitoring simultaneously bothmethane consumption, referred to the initial feed composition, andhydrogen production. This is illustrated by Fig. 9, in which curvesobtained for the CA monoliths are represented. Similar plots wereobtained for the rest of the monoliths. Other species such as ethaneand ethylene were analyzed not being detected under the experi-mental conditions followed in this study. This finding is consistentwith other authors’ observations who either reported the presenceof these by-products only after prolonged time-on-stream and inany case in very low concentrations [4,8] or did not find measur-able amounts of gas products other than hydrogen [34]. Moreover,mass balance from MS signals analysis revealed that the amountof methane disappeared fits well with that of hydrogen produced,suggesting that decomposition of methane occurs with high selec-

tivity toward pure hydrogen and elemental carbon.

Concerning the methane conversion, the CA monoliths showedthe best performance among the samples studied (Table 2), no cleartrend on the effect of temperature on the catalytic activity of each

Wavenumber (cm-1)

40060080010001200400

Abso

rbance

(a.u

.)

1381 1115 473

studied monoliths.

J.M. Gatica et al. / Applied Catalysis A: General 458 (2013) 21– 27 25

sardpd

Table 2Methane conversion (%) and H2 produced (mmol g−1 min−1) in the catalytic experi-ments performed over the carbon monoliths studied (average data obtained in theisothermal regime for 50 min).

Sample 750 ◦C 800 ◦C 900 ◦C

Conv. H2 Conv. H2 Conv. H2

C 23 0.022 16 0.015 21 0.020CA 31 0.031 33 0.033 25 0.025PCA 15 0.015 16 0.015 18 0.018

Fig. 8. SEM images of the studied monoliths.

ample being observed. On this regard, it is convenient to note thatccording to the thermodynamics of this reaction [2] we are not

eaching equilibrium conditions at any temperature, most likelyue to mass transfer limitations that in a monolithic reactor must bearticularly important. This obviously limits comparison betweenata obtained at different temperatures. It should be also noticed

CA powder 13 0.013 15 0.015 13 0.013

that all the monoliths prepared were competitive in comparisonwith the powder investigated regarding the methane conversion.

As expected, some deactivation with time (Fig. 9) was detectedin all cases at the three temperatures studied (35–50% of activ-ity loss, during 1 h of reaction at 750 ◦C; 50–67% 1 h at 800 ◦C;and 50–55% 1 h at 900 ◦C). Nevertheless, this effect was similar oreven smaller if compared with previous results reported for othercarbonaceous materials. In this sense, although any comparisonbetween catalytic activity data from different sources should becarried out with caution due to the influence of the experimentalconditions, some reference values of methane conversion decrease(%) are: 25–65% for activated carbons (850 ◦C, 1 h) [7], 63–80% forcarbon blacks and activated carbons (850–900 ◦C, 1 h) [3], 20–41%for activated carbons (850 ◦C, 1 h) [13], or 70% for chars (1000 ◦C,40 min) [35].

Also significant, among the samples studied the CA monolithsexhibited not only the highest conversion but also the lowest rate ofdeactivation with time. This result suggests that the pre-oxidationtreatment is not necessary for this application and can be avoided,with the consequent energy saving. Also remarkable, our findingis in agreement with other authors’ observations that associatemicroporosity with a faster deactivation due to easier blockage ofthe pore mouth [36–39]. In addition, all the monoliths remainedintact after the catalytic tests, in good agreement with the mechan-ical resistance measurements above commented.

For a better understanding of the origin of deactivation, texturalmeasurements of the spent samples after their use at the highesttemperature studied (900 ◦C) have been also performed (Table 1).Comparison with data obtained for the fresh CA and PCA monolithsreveals that after the reaction the micropores volume becomesalmost negligible with a consequent strong decrease of the BETsurface area, so indicating that the internal surface is no moreaccessible, most likely due to carbon deposits. Recall that thecatalytic results indicated formation of elemental carbon (seeabove). In spite of this, it is worthy noting that all the samples keepshowing activity (Fig. 9) what suggests that active sites must bestill operating in the external surface, not discarding the creationof new sites during the reaction [4,7,8].

In terms of the amount of hydrogen produced (Table 2), ourresults are clearly lower than those that can be derived fromMuradov et al. research. This author found values of CH4 decompo-sition rates ranging from 0.2 to 0.5 mmol g−1 min−1 [3] and from 1.3to 1.8 mmol g−1min−1 [4] for different carbon blacks and activatedcarbons respectively, in both cases operating at 850 ◦C. Howeverin these investigations pure methane was employed, well apartfrom our CH4(5%)/He feed composition. In fact, the monoliths stud-ied here behaved similarly to other carbon materials reported inliterature if this parameter is considered: 0.1–0.35 mmol CH4 con-verted g−1 min−1 for activated carbons working with pure methaneat 750 ◦C [34] or 0.004–0.665 mmol H2 produced g−1 min−1 for car-bon materials of different nature under CH (10%)/Ar at 950 ◦C [1].

4Moreover, in all references available in literature powdered sam-ples were used in which mass diffusion problems must be obviously

26 J.M. Gatica et al. / Applied Catalysis A: General 458 (2013) 21– 27

Tem

pera

ture

(ºC

)0

200

400

600

800

1000

CH

4 % v/v

0

1

2

3

4

5

Time (min.)

0 50 10 0 15 0 20 0 25 0 30 0

H2 %

v/v

0

1

2

3

4

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ig. 9. Catalytic run versus time in the CA monoliths with isothermal steps at 750, 8o the 5% feed) and H2.

ower if compared with the integral carbon monoliths proposed inhis investigation.

Furthermore, in preliminary experiments in which the mono-iths were heated under methane from room temperature (resultsot shown), hydrogen production was detected starting at temper-tures as low as 600 ◦C. These results are particularly interestingnd illustrate the potential of the monoliths investigated for realpplication. On the other hand, the fact that the carbon samplesere studied were activated using steam can be considered anxtra advantage, as accordingly to literature [6] the same treat-ent allows regenerating the surface after deactivation without

ignificantly modifying the starting material.

. Conclusions

In this study, integral carbon honeycomb monoliths were pre-ared following a very simple method usually employed foreramics.

The monoliths showed acceptable mechanical resistance andere efficient at producing hydrogen by thermocatalytic methaneecomposition at relatively low temperatures.

Special attention was paid to the influence on the monoliths’atalytic behavior of the thermal treatments to which they wereubjected after being prepared. Like other authors, we did not find

clear correlation between the activity and the texture inducedy the treatments, as monoliths submitted only to activation with-ut pre-oxidation with an intermediate surface area and porosityxhibited the best performance in terms of both conversion andesistance to deactivation.

Having ruled out the existence of significant differencesetween the samples regarding their surface functional groups byeans of infrared spectroscopy, further research is in progress to

valuate the influence of other parameters such as the crystallinityf the samples, the defect concentration, or the nature of the carbonroduced in the course of the reaction.

cknowledgements

The authors thank the FEDER Program of the EU (ProjectAT2008-00889/NAN) and the Junta de Andalucía (FQM-110

[

[

d 900 ◦C (1 h). Mass spectrometer signals for CH4 (notice the difference with regard

group) for their financial support. We also acknowledge the SCCyTof the UCA for the use of their SEM facilities. Figs. 1, 5 and 6are reprinted from Applied Surface Science 256, J.M. Gatica et al.,Changing the adsorption capacity of coal-based honeycomb mono-liths for pollutant removal from liquid streams by controlling theirporosity, 7111–7117, Copyright (2010), with permission from Else-vier.

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