methane activated carbon spain

8/8/2019 Methane Activated Carbon Spain

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International Journal of Hydrogen Energy 32 (2007) 4792 4799

www.elsevier.com/locate/ijhydene

Microwave-assisted catalytic decomposition of methane over activatedcarbon for CO2-free hydrogen production

A. Domnguez, B. Fidalgo, Y. Fernndez, J.J. Pis, J.A. Menndez

Instituto Nacional del Carbon, CSIC, Apartado 73, 33080 Oviedo, Spain

Received 6 February 2007; received in revised form 14 June 2007; accepted 17 July 2007

Available online 10 September 2007

Abstract

The aim of this work was to combine microwave heating with the use of low-cost granular activated carbon as a catalyst for the production

of CO2-free hydrogen by methane decomposition in a fixed bed quartz-tube flow reactor. In order to compare the results achieved, conventional

heating was also applied to the catalytic decomposition reaction of methane over the activated carbon. It was found that methane conversions

were higher under microwave conditions than with conventional heating when the temperature measured was lower than or equal to 800 C.

However, when the temperature was increased, the difference between the conversions under microwave and conventional heating was reduced.

The influence of volumetric hourly space velocity (VHSV) on the conversion tests using both microwave and conventional heating was also

studied. In general, there is a substantial initial conversion, which declines sharply during the first stages of the reaction but tends to stabilise

with time. An increase in the VHSV has a negative effect on CH 4 conversion, and even more so in the case of microwave heating. Nevertheless,

the conversions obtained in the microwave device at the beginning of the experiments are, in general, better than the conversions reported in

other works which also use a carbonaceous-based catalyst. Additionally, the formation of carbon nanofibres in one of the microwave experiments

is also reported.

2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Keywords: Methane; Hydrogen; Microwave; Catalytic decomposition; Carbon nanofibres

1. Introduction

The alarm generated by climatic change has motivated a

search for alternative clean power sources and for a way to min-

imize the emission of greenhouse gases. Hydrogen is consid-

ered the ideal fuel of the future because its combustion product

is H2O. Methane is a good source of hydrogen due to its high

hydrogen-to-carbon ratio [13].

There are several catalytic processes for the production of

hydrogen from methane:

Steam reforming of methane: CH4 + H2O 3H2 + CO,

H298 = +206 kJ/mol.

Dry reforming of methane: CH4 + CO2 2H2 + 2CO,

H298 = +247 kJ/mol.

Corresponding author. Tel.: +34 985118972.

E-mail address: [email protected] (J.A. Menndez).

0360-3199/$- see front matter 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

doi:10.1016/j.ijhydene.2007.07.041

Partial oxidation: CH4 +12 O2 2H2 + CO, H298 =

8.5 kJ/mol.

Thermal decomposition of methane: CH4 C + 2H2,

H298 = +75kJ/mol.

Catalytic steam reforming is currently used for the production

of several chemicals via synthesis gas. A critical parameter for

the subsequent reaction of the syngas is the H2/CO ratio which

must be adjusted close to unity for some syntheses, implyingfurther post-reformer reactions [4]. These steps could be elim-

inated by carbon dioxide reforming, which is more endother-

mic than steam reforming but yields syngas with lower H2/CO

ratios.

From an environmental point of view, dry reforming [5]

would lead to a mitigation of CO2 emissions and contribute to

the consumption of CH4, both of which are undesirable green-

house gases.

Despite these advantages, there is still no commercially feasi-

ble process for CO2 reforming. The high temperatures required
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A. Domnguez et al. / International Journal of Hydrogen Energy 32 (2007) 47924799 4793

to attain high conversions favour carbon deposition which de-

activates the tested conventional catalysts [46]. In the steam

reforming reaction, this problem is avoided by increasing the

steam concentrations. This improves the selectivity towards hy-

drogen but also increases the energy consumed in the process.

An alternative process for the production of syngas which

avoids the disadvantages of the use of steam is partial oxidation.This gives a H2/CO ratio of 2; it is an exothermic reaction and

it may proceed at much moderate temperatures [7]. However,

the partial oxidation of methane to synthesis gas is very difficult

to achieve.

Recently, a process that has attracted considerable attention

is a combination of partial oxidation and steam reforming (au-

tothermal reforming) due to its higher energy efficiency com-

pared to other endothermic processes [7,8].

All these conventional processes give rise to the simulta-

neous production of CO2, which needs to be reduced due to

the greenhouse effect. A viable alternative is the catalytic ther-

mal decomposition of methane, which produces hydrogen in

a single-step process without CO/CO2 emission, eliminating

the need for a water gas-shift reaction and the CO2 removal

stages [9]. In addition, CO2 emissions could be reduced if

the energy needed for methane decomposition was supplied

by burning 1520% of the hydrogen produced in the pro-

cess [2]. Moreover, the carbon obtained as a by-product from

methane decomposition is potentially more valuable than CO2in the current market, and could reduce the cost of the overall

process [10].

The main drawback of thermal decomposition as an operat-

ing process is the deactivation of the catalyst associated with

the carbon deposited on the catalyst surface. In order to re-

generate the original catalytic activity, the gasification or com-bustion of the carbon deposits has been proposed. These pro-

cesses, however, would result in the production of CO2 as a

by-product [11].

Muradov et al. [1114] have proposed the use of carbon-

based catalysts instead of metal catalysts due to their availabil-

ity, durability and low cost. These catalysts offer some advan-

tages over metal catalysts such as high temperature resistance

and tolerance to sulphur as well as other potentially harmful

impurities in the feedstock.

On the other hand, the operation of endothermic catalytic re-

actors is limited by the transfer of radial heat through the packed

bed. This results in a pronounced temperature profile, andaffects the rate of reaction [15]. The effect of microwave heat-

ing on several heterogeneous catalytic reactions has been stud-

ied since the late 1980s, with enhanced reaction rates, higher

yields and improved product selectivities. The reactions com-

pared above are among these tested systems. Zhang et al. [4]

studied the application of microwaves to the carbon dioxide

reforming of methane, comparing its performance with con-

ventional heating. The steam reforming of methane and water

gas-shift reactions were investigated by Wan et al. [16]. Even-

tually, Cooney et al. [17] compared microwave heating and

conventional heating applied to the production of hydrogen by

means of the decomposition of pure methane over a coal char

catalyst.

The microwave heating of a dielectric material, which oc-

curs through the conversion of electromagnetic energy into

heat within the irradiated material, offers a number of advan-

tages over conventional heating such as (i) noncontact heating;

(ii) energy transfer, no heat transfer; (iii) rapid heating; (iv)

selective material heating; (v) volumetric heating; (vi) quick

start-up and stopping; (vii) the heating starts from the interiorof the material body; (viii) a higher level of safety and automa-

tion and (ix) it can be transported from the source through a

hollow nonmagnetic metal tube [18,19].

The purpose of this work is to study the reaction of methane

decomposition when activated carbon is used as a catalyst and

conventional heating is replaced by microwave heating. The de-

composition is carried out at various temperatures and reaction

times, focusing on catalyst activity and methane conversion,

which differ depending on the heating system used.

2. Experimental

A commercial activated carbon (Filtracarb FY5), made from

coconut shell and activated with steam, was used as a catalyst in

the methane decomposition experiments. The surface area and

the bulk density of the FY5 were 826 m2/g and 0.49 g/cm3,

respectively, and the size of particles ranged within 0.52 mm.

Proximate analysis (moisture, ash and volatile matter content)

and elemental analysis (C, H, O, N, S) were carried out in a

LECO TGA-601 thermo-balance and in a LECO CNHS-932

apparatus, respectively; the oxygen content was calculated by

the difference. Table 1 shows the main chemical characteristics

and the ash composition of the activated carbon.

Table 2 shows the conditions employed for each test. The

CH4 decomposition reactions were carried out in a quartz re-actor (45cm length 2.2 cm i.d.) charged with a variable mass

of catalyst, previously dried overnight at 110 C, and heated in

an electrical furnace (EF) or a single mode microwave oven

(MW). Details of this equipment setup have been described pre-

viously [20]. Before the reactant gas was introduced, nitrogen

was used as inert gas at a flow rate of 82 mL/min for 20 min

after which the system was heated up to a pre-set temperature

under inert atmosphere. In order to determine the best operat-

ing temperature, runs from A to F were conducted. To study the

long-term behaviour of the catalyst and the influence of volu-

metric hourly space velocity (VHSV) on the conversion, tests

G to N were carried out. As can be seen in Table 2, some VHSVand mass of catalyst values for the tests carried out in the MW

differ slightly from the values obtained in the electric furnace.

This is because the volume of the catalyst was used as a refer-

ence instead of the catalyst mass since microwave heating is a

volumetric heating process [21].

The outflow gas was collected in 0.5, 1 or 3 L Tedlar sam-

ple bags. During the first 30 min, each bag was filled with the

outflow gases for 3 min. After the first half an hour, each sam-

ple bag was filled with the gases for 10 min. The gases were

analysed in a gas-chromatograph Perkin-Elmer Sigma 15 fitted

with a TCD detector. A Teknokroma 10FT Porapak N, 60/80,

and a Teknokroma 3FT Molecular Sieve 13X, 80/100, columns

were also used. The oven temperature was pre-set at 50 C and
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4794 A. Domnguez et al. / International Journal of Hydrogen Energy 32 (2007) 47924799

Table 1

Main chemical characteristics of the activated carbon used as a catalyst for methane decomposition

Chemical characteristics

Proximate analysis (wt%) Ultimate analysis (wt%)a

Moisture Asha Volatile mattera C (%) H (%) N (%) S (%) O (%)b H/C

6.7 2.8 3.0 95.7 0.5 0.5 0.2 0.3 0.068

Inorganic composition of the ashes (expressed as wt% of metal oxides) a

SiO2 K2O Al2O3 Fe2O2 CaO Na2O SO3 MgO TiO2 Ni Co

39.79 25.40 9.06 9.04 6.40 3.01 2.77 2.71 1.18 n.d.c n.d.c

aDry base.bCalculated by difference.cn.d., non detected.

Table 2

Test conditions

Run Temperature (C) Heating device VHSVa (L/g h) Space timeb (s) CH4 flow rate (mL/min) Mass of catalyst (g) Time (min)

A 900 MW 0.16 46 36 13.5 30

B 900 EF 0.16 46 36 13.5 30

C 800 MW 0.16 46 36 13.4 30

D 800 EF 0.16 46 36 13.5 30

E 700 MW 0.16 46 36 13.6 30

F 700 EF 0.16 46 36 13.5 30

G 800 MW 0.16 46 36 13.6 120

H 800 EF 0.16 46 36 13.5 120

I 800 MW 0.31 24 65 12.6 120

J 800 EF 0.31 24 65 12.5 120

K 800 MW 0.31 24 44 8.6 120

L 800 EF 0.31 24 36 6.9 120

M 800 MW 0.72 10 98 8.2 120

N 800 EF 0.71 10 98 8.3 120

aVolumetric hourly space velocity, defined as VHSV = flow rate of CH4 (L h1)/mass of catalyst (g).

bSpace time is defined as the ratio between the volume of catalyst (L) and the flow rate of CH4 (L s1), i.e., space time = 3600/[VHSV (L g1 h1)* bulk

density (g L1)].

the carrier gas flow rate (He) was 20 mL/min. The injector and

detector temperatures were 100 and 150 C, respectively. The

TCD was calibrated with a standard gas mixture at periodic

intervals.

The conversion of methane was calculated employing the

concentrations of hydrogen and methane in the effluent gas,

which were determined by chromatography:

CH4 conversion, %= 100 [(H2)out/2]/[(CH4)out + (H2)out/2],

where (CH4)out and (H2)out are the methane and hydrogen con-

centrations in the effluent gas (% by volume), respectively.

Textural characterisation of fresh and some used samples

of catalyst was carried out by CO2 adsorptiondesorption

isotherms at 0 C and N2 adsorptiondesorption isotherms at

196 C in the pressure range of 01 bar in a Micromeritics

Tristar 3000 apparatus. The DubininRaduskevich method was

applied to the CO2 and N2 adsorption isotherms to determine

the volume of micropores, while the BET equation was ap-

plied in the case of the N2 adsorption isotherms. The results

for each sample are shown in Table 3.

Table 3

Textural properties of fresh and selected used catalysts

Sample SBET (m2/g) a Vtotal (cm

3/g)a,c Vmic (cm3/g)a Vmic (cm

3/g)b

FY5 826 0.341 0.320 0.255

C 530 0.224 0.206 0.205

D 748 0.308 0.289 0.231

G 227 0.096 0.089 0.100

H 527 0.222 0.209 0.224

a Calculated from isotherms of nitrogen at 77 K.bCalculated from isotherms of carbon dioxide at 273K.cCalculated by means of the Gurvitsh rule [29].

The carbon deposits formed on the activated carbon surface

were studied by a Scanning Electron Microscope DSM 942

from Zeiss, equipped with an EDX detector (OXFORD LINK-

ISIS).

3. Results and discussion

Before the methane decomposition was studied using acti-

vated carbon as a catalyst, blank tests were carried out in the


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0

20

40

60

80

100

0 5 10 15 20 25 30

time (min)

CH4conversion(%) blank (a)

blank (b)

blank (c)

0

20

40

60

80

100

CH4 H2 C2H4gas

composition(vol%)

blank (a)

blank (b)

Fig. 1. Conversion of methane for blank tests carried out at different conditions: (a) 900 C with a thermocouple; (b) 900 C without a thermocouple and

(c) 800 C with a thermocouple.

electric furnace in order to find out if thermal decomposition of

methane was possible at the operating temperature. These tests

consisted in passing a 36 mL/min flow rate of CH4 through aquartz wool bed at 800 and 900 C, using an experimental pro-

cedure similar to the one described previously to evaluate the

conversion of CH4. Fig. 1 shows the results obtained for these

experiments. Interestingly, at 800 C there was no CH4 con-

version at all, while at 900 C about 7% of CH4 was mainly

converted to H2, a small proportion of ethene (C2H4) also be-

ing found. Moreover, the results of this experiment were repro-

duced both with and without a thermocouple, which rules out

any possible catalytic effect from the thermocouple. In sum, a

small fraction of CH4 can be thermally decomposed at 900C

but not at 800 C.

In order to study the performance of the activated carbonas a catalyst under different temperatures and to compare mi-

crowave and electrical heating, experiments from A to F (see

Table 2) were carried out. It was found that, unlike thermal de-

composition, C2H4 was not produced in the presence of acti-

vated carbon. Thus, the gases detected by gas chromatography

were H2, N2 and CH4 as well as some small (< 1%) amounts

of CO and CO2 in the first bags collected (< 9 min). The pres-

ence of N2 is due to remnants of the inert gas used during the

warming up of the catalyst, whereas CO and CO2 are attributed

both to the decomposition of oxygen-containing surface groups

present on the surface of the activated carbon and to the air

occluded in micropores, since these carbon samples were not

degasified.

The results of these experiments are plotted in Fig. 2. In gen-

eral it can be seen that, as expected, the conversion increases

with the temperature. On the other hand, with the exception

of experiment F, the conversion decreases with time, which

points to a loss of catalytic activity due to blockage of the ac-

tive centres by the carbon deposits produced in the decompo-

sition of CH4 (chemical vapour deposition, CVD). In addition,

as can be inferred from the textural parameters summarised in

Table 3, the catalysts used in runs C and D undergo a significant

decrease in their BET surface areas and micropore volumes.

This agrees with the fact that carbon deposits block part of the

porosity, preventing the CH4 molecules from gaining access to

0

20

40

60

80

0 5 10 15 20 25 30

A C E

B D F

CH4conversion(%) 100

time (min)

Fig. 2. Effect of the reaction temperature on methane conversion, under

microwave heating (solid symbol) or conventional heating (empty symbol).

Runs A and B: 900 C; runs C and D: 800 C; and runs E and F: 700 C.

VHSV = 0.160L/g h.

this part of the surface area of the carbon. As a result catalytic

activity is reduced. Moreover, when the micropore volumes ob-

tained from the N2 and CO2 isotherms are compared, it can

be observed that, while the differences between these two pa-

rameters are relatively important in the case of the fresh cata-

lyst (FY5), indicating a broad micropore size distribution, these

differences are very small in the samples obtained from runs

C and D, pointing to a narrower micropore size distribution

[22]. Thus, it seems that carbonaceous deposits which modifythe pore structure of the carbon are formed from CH4 decom-

position, i.e., these carbon deposits appear to be preferentially

located at the pore mouths narrowing them (in a similar way

than that used to obtain carbon molecular sieves by CVD). The

decrease in the textural parameters is greater in the case of the

sample from run C, which was performed in the microwave, in

accordance with the results ofFig. 2. Thus, microwave heating

was more effective in decomposing CH4 to H2 than the electric

furnace (run D) and gave rise to a greater number of carbon

deposits.

In fact, it can be observed that at the same temperature,

all the experiments performed in the microwave resulted in a

higher CH4 conversion than those carried out in the electric


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furnace. These differences are particularly significant at lower

temperatures, becoming smaller as the temperature of the ex-

periment increases. Similar results were reported by Zhang

et al. [4] when comparing the CO2 reforming of CH4 using

microwave and conventional heating. These results can be at-

tributed to the different way the materials are heated in the

microwave. Thus, in the case of MW heating, the catalyst(activated carbon) is heated directly by the action of the mi-

crowaves, and so is at a much higher temperature than the

surrounding atmosphere, whereas in the EF the heat trans-

fer proceeds through the surface of the catalyst, resulting in

a totally different temperature gradient. Moreover, during mi-

crowave heating numerous small sparks can be observed prac-

tically throughout the entire experiment. These sparks are hot

spots which, in a way, can be considered as micro-plasmas,

both from the point of view of time (since they only last for a

fraction of a second) and space (since they are located in a very

tiny spot). Therefore, whereas the overall temperature corre-

sponds to the one measured by the optical pyrometer, the tem-

perature in these micro-plasmas must be much higher. Such

a situation can be expected to favour heterogeneous reactions

between the solid catalyst and the gaseous CH4, enhancing

CH4 decomposition.

From Fig. 2 it can be seen that at 700 C the differences be-

tween the two methods (EF and MW) are quite large but the

conversion of CH4 is relatively small in both cases. At 900C,

however, both conversions are quite high, whereas the differ-

ences between the two methods employed are relatively small.

Thus, 800 C was selected as the most appropriate temperature

to study the long-term behaviour of the catalyst and the influ-

ence of VHSV on conversion (Fig. 3). In order to check the

repeatability of the experients, runs G and H have been plottedalong with runs C and D in Fig. 3a. As can be seen, the ex-

periments are completely reproducible in the EF, whereas the

repeatability of the experiments in the MW is worse, although

still reasonably good. This could be due to the hot spots referred

to above, which are generated in a random way for each sam-

ple, thereby adding uncertainty to the temperature values given

by the pyrometer. If the 120 min-long experiments (G and H)

are compared, it can be observed that, although the conversion

is higher in the MW at the beginning of the experiment, the de-

crease is also more pronounced than in the EF, so that after 2 h

both conversions are practically the same. In fact, the decom-

position of CH4 to H2 by MW treatment is more efficient thanin the EF, although the unavoidable carbon deposition results in

a greater loss of catalytic activity. Again the textural properties

summarised in Table 3 illustrate that there is a substantial de-

crease in the textural properties after 120 min-long treatments,

this decrease being much more significant in the MW than in

the EF. In sum, MW treatments favour CH4 conversion, but at

the same time CVD is also favoured so that, although the cat-

alyst works more efficiently, it is for a shorter time than in the

case of the EF treatments.

Fig. 3b shows the conversion obtained in runs I to L. In these

experiments, the flow rate of CH4 and the mass of catalyst were

varied in such a way that the VHSV remained the same (see

Table 2). In runs I to K (MW) and runs J to L (EF), a good

repeatability was achieved (although once again the result was

better in the EF than in the MW). This indicates that, when

the rest of the operating conditions are kept constant, it is the

ratio between these two parameters (VHSV) that controls the

CH4 conversion rather than the flow or the amount of catalyst

evaluated as individual variables.

On the other hand, a comparison of Figs. 3ac suggests thatan increase in the VHSV leads to a smaller conversion of CH 4.

In fact, an increase in the VHSV reduces the capacity of the cat-

alyst to process the feeding of CH4. In other words, it reduces

the contact time between the CH4 molecules and the active cen-

tres present on the catalyst, thereby impairing the conversion.

However, it should be noted that the MW treatments seem to

be much more affected by the increase in VHSV than the EF

treatments. Thus, although at the beginning of the experiments

the conversion is always higher in the MW, the decay in this

conversion is also greater, leading to an earlier crossover be-

tween the conversion curves of the MW and EF.

The results collected in this work can be compared with oth-

ers found in the bibliography. Various studies on CH4 decom-

position have been carried out over metal-based catalysts, such

as Ni or Cu [2326]. It has been found that CH4 initial conver-

sions are often higher when activated carbons are used as cata-

lyst, but it is the metallic catalyst that shows the best long-term

behaviour, high conversions being achieved even after 510 h.

Taking into account that these tests were run at lower tem-

peratures, it can be inferred that, in general, activated carbon

displays a worse performance as a catalyst than metal-based

catalysts. This could be due to either a higher concentration

of active centres on the metallic catalysts or a greater facility

for carbon deposition on the active centres of the carbon cat-

alysts. Other works which used carbonaceous-based catalystsfor CH4 decomposition were also consulted but the results can-

not be directly compared with the ones of this work because

the VHSV chosen are not similar (see Table 4). Even so, a de-

creasing trend in CH4 conversion with time can be observed,

except when carbon blacks are used as catalysts [3]. In fact,

after 90120 min, there is usually no CH4 conversion, the high-

est conversion being no higher than 10%. Thus, the operating

conditions chosen for the tests carried out in this work provide

better conversion results (see Table 4). Moliner et al. [28] stud-

ied the decomposition of methane at 850 C over several com-

mercial activated carbons, whose BET surface was larger than

the BET surface of FY5, using a space time of 25 s, as in runsI to L in this work (see Table 2). Their initial H2 production

was in the range of 6535 vol% and around 4530 vol% after

120 min. Hydrogen production for runs I to L was in the range

of 6559 vol% for the first few minutes and about 4027 vol%

after 120 min, which implies that their results were similar to

ours, although in the present work the temperature was lower

and the surface area of the FY5 was not so large.

The nature and characteristics of the carbon deposits formed

on the catalyst surface after CH4 decomposition also need to

be discussed. SEM microphotographs reveal that the nature of

these carbon deposits is very heterogeneous. Both for MW and

EF heating, most of the carbon is deposited on the surface

of the active carbon where it forms discrete deposits, which
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0

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100

0 10 20 30 40 50 60 70 80 90 100 110 120 130

time (min)

CH4conversion(%) G

H

C

D

0

20

40

60

80

100

0 10 20 30 40 50 60 70 80 90 100 110 120 130

time (min)

CH4conversion(%)

I

J

K

L

0

20

40

60

80

100

0 10 20 30 40 50 60 70 80 90 100 110 120 130

time (min)

CH4conversion(%)

M

N

Fig. 3. Effect of the VHSV on methane conversion, under microwave heating (solid symbol) conventional heating (empty symbol): (a) VHSV = 0.16 L/g h;

(b) VHSV = 0.31 L/gh and (c) VHSV = 0.72 L/g h. T = 800 C.

can be seen in Fig. 4a. Eventually, these deposits coalesce and

develop a uniform layer of carbon, which cannot be seen by

SEM. However, in the specific case of run M, in addition to

the discrete carbon deposits, the formation of several skeins

of nanofibres with heterogeneous shapes (some nanofibres are

relatively straight, whereas others are twisted into spirals) and

sizes ranging from a few nanometres to about 200 nm was ob-

served. Fig. 4b shows an example of these skeins, which are

found scattered over the surface of the catalysts. The nanofibres

are formed under conditions unfavourable for the CH4 conver-

sion, i.e., the largest VHSV value, which implies a low H2/CH4

ratio in the outflow gases. This result seems to contradict the

general trend found in the literature, where it is reported that a

higher proportion of nanotubes vs. amorphous carbon deposits

is favoured by the presence of H2. On the other hand, these

nanofibres are only formed under MW heating, suggesting that

micro-plasmas (hot spots which do not appear in EF heating)

may play a decisive role in their formation. Some authors have

studied the synthesis of nanostructures (nanotubes and nanofi-

bres) under microwave heating, using different operating con-

ditions, i.e., acetylene as a hydrocarbon source and Co, Ni and

Fe supported over carbon black (a microwave absorber) or Co


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Table 4

Summary of CH4 conversions using carbonaceous-based catalysts at different conditions

Reference Catalyst Temperature range (C) VHSV (L/g h) XCH4 rangea (%) XCH4 range

b (%)

Present work Activated carbon 800 0.160.72 10049 297

Kim et al. [27] Activated carbon 750900 15 142 30

Bai et al. [1] Activated carbon 750900 3 355 92.5

Muradov [14] Activated carbon 850 10 2721 20.5Carbon black 61 3.50.5

Lee et al. [3] Carbon black 7501050 15 500 602

Bai et al. [2] Coal char 750900 3 345 60

a CH4 initial conversion evaluated during the first 5 min.bCH4 final conversion evaluated after 90 min.

Fig. 4. SEM images of the FY5 obtained after the reaction at 800 C and 0.72 L/g h for 2 h: (a) carbon deposits on the surface active carbon, obtained after

CH4 decomposition in the EF ; (b) detail of the nanofibres obtained after CH 4 decomposition in the MW.

on a SiO2 support (a microwave insulator) [30], directly heat-

ing samples of graphite and sucrose, calcined sucrose or boric

acid and collecting the condensed material on a fused silica tar-

get [31], or decomposing ferrocene molecules on the surfaceof Fe nanoparticles [32]. Interestingly, among these metals that

catalyse the growth of nanofibres, Fe is present in the ashes of

the activated carbon used in this work. These issues, however,

require a more complete study and they will be addressed in

more detail in a future study.

4. Conclusions

In this work it has been pointed out that the thermal decom-

position of CH4 to H2 and C2H4, with a very low but notice-

able CH4 conversion, takes place at 900C, while below this

temperature CH4 does not decompose. However, when an ac-tivated carbon is used as catalyst, CH4 decomposes selectively

to H2 with much higher conversions. It was found that mi-

crowave heating gives rise to higher conversions than electric

heating (conventional heating), probably due to the formation of

hot spots (micro-plasmas) inside the catalyst bed, which favour

CH4 decomposition. Experiments conducted for 30 min have

established that the differences between these heating methods

are much greater at lower temperatures. In addition, although

an increase in the temperature has a positive effect on the CH 4conversion for both methods, at temperatures around 900 C

these differences become smaller. Experiments conducted for

2 h at 800 C showed that there is a decrease in conversion with

time, associated with a drop in the activity of the catalyst due

to carbon deposits entering the pores, and reducing the surface

area and pore volume. Thus, the more efficient the process is,

the more carbon is deposited, this effect being even more detri-

mental in the case of microwave heating. Therefore, althoughfor a given temperature the conversion in the microwave at

the beginning is higher than in the electric furnace, with time

they both become similar, and microwave conversion eventu-

ally falls below the one obtained by conventional heating. An

increase in the volumetric hourly space velocity (VHSV) has

a negative effect on CH4 conversion, which is more noticeable

for microwave heating. Comparison of these results with those

of other works revealed that, for a similar space time, the per-

formance of the activated catalyst used as a catalyst for CH4decomposition is similar to activated carbons which have larger

surface areas and are heated to a higher temperature. Under

certain experimental conditions, microwave heating gave rise

to carbon nanofibres on the activated carbon surface.

Acknowledgements

B. Fidalgo and Y. Fernndez are grateful to CSIC of Spain

and the European Social Fund (ESF) for financial support under

thesis Grant I3P-BDP-2006.

References

[1] Bai Z, Chen H, Li B, Li W. Catalytic decomposition of methane over

activated carbon. J Anal Appl Pyrolysis 2005;73:33541.

[2] Bai Z, Chen H, Li B, Li W. Hydrogen production by methane

decomposition over char coal. Int J Hydrogen Energy 2006;31:899905.


8/8


[3] Lee EK, Lee SY, Han GY, Lee BK, Lee T-J, Jun JH. et al. Catalytic

decomposition of methane over carbon blacks for CO2-free hydrogen

production. Carbon 2004;42:26418.

[4] Zhang X, Lee CS-M, Mingos DMP, Hayward DO. Carbon dioxide

reforming of methane with Pt catalysts using microwave dielectric

heating. Catal Lett 2003;88(34):12939.

[5] Juan-Juan J, Romn-Martnez MC, Illn-Gmez MJ. Catalytic activity

and characterization of Ni/Al2O3 and NiK/Al2O3 catalysts for CO2methane reforming. Appl Catal A Gen 2004;264(2):16974.

[6] Bradford MCJ, Vannice MA. CO2 reforming of CH4. Catal Rev Sci

Eng 1999;41(1):142.

[7] Bharadwaj SS, Schmidt LD. Catalytic partial oxidation of natural gas

to syngas. Fuel Process Technol. 1995;42(23):10927.

[8] Cheekatamarla PK, Finnerty CM. Reforming catalysts for hydrogen

generation in fuel cell applications. J Power Sources 2006;160(1):4909.

[9] Balasubramanian B, Ortiz AL, Kaytakoglu S, Harrison DP. Hydrogen

from methane in a single-step process. Chem Eng Sci 1999;54(1516):

354352.

[10] Steinberg M. Fossil fuel decarbonisation technology for mitigating global

warming. Int J Hydrogen Energy 1999;24(8):7717.

[11] Muradov N, Smith F, T-Raissi A. Catalytic activity of carbons for

methane decomposition reaction. Catal Today 2005;102103:22533.

[12] Muradov N. How to produce hydrogen from fossil-fuel without CO2emission. Int J Hydrogen Energy 1993;18(3):2115.

[13] Muradov N, Smith F, Huang C, T-Raissi A. Autothermal catalytic

pyrolysis of methane as a new route to hydrogen production with reduced

CO2 emissions. Catal Today 2006;116:2818.

[14] Muradov V. Catalysis of methane decomposition over elemental carbon.

Catal Commun 2001;2:8994.

[15] Perry WL, Dayte AK, Prinja AK, Brown LF, Katz JD. Microwave

heating of endothermic catalytic reaction: reforming of methanol. AIChE

J 2002;48(4):82031.

[16] Wan JKS, Wolf K, Heyding RD. In: Kaliaguine S, Mahay A, editors.

Catalysis on the energy scene. Amsterdam: Elsevier; 1984.

[17] Cooney DO, Xi Z. Production of hydrogen from methane and

methane/steam in a microwave irradiated char-loaded reactor. Fuel Sci

Technol Int 1996;14(8):111141.

[18] Haque KE. Microwave energy for mineral treatment processes: a briefreview. Int J Miner Process 1999;57(1):124.

[19] Conde LD, Marn C, Suib SL, Fathi Z. Frequency effects in the

catalytic oligomerization of methane via microwave heating. J Catal

2001;204(2):32432.

[20] Domnguez A, Menndez JA, Inguanzo M, Pis JJ. Investigations into

the characteristics of oils produced from microwave pyrolysis of sewage

sludge. Fuel Process Technol 2005;86(9):100720.

[21] Zlotorzynski A. The application of microwave radiation to analytical and

environmental chemistry. Crit Rev Anal Chem 1995;25(1):4376.

[22] Garrido J, Linares-Solano A, Martn-Martnez JM, Molina-Sabio M,

Rodrguez-Reinoso F, Torregrosa R. Use of N2 vs CO2 in the

characterization of activated carbons. Langmuir 1987;3(1):7681.[23] Suelves I, Lzaro MJ, Moliner R, Corbella BM, Palacios JM. Hydrogen

production by thermocatalytic decomposition of methane on Ni-based

catalysts: influence of operating conditions on catalyst deactivation and

carbon characteristics. Int J Hydrogen Energy 2005;30:155567.

[24] Nakagawa K, Nishitani-Gamo M, Ando T. Hydrogen production from

methane for fuel cell using oxidized diamond-supported catalysts. Int J

Hydrogen Energy 2005;30:2017.

[25] de Almeida RM, Fajardo HV, Mezalira DZ, Nuernberg GB, Noda LK,

Probst LFD. et al. Preparation and evaluation of porous nickelalumina

spheres as catalyst in the production of hydrogen from decomposition

of methane. J Mol Catal A Chem 2006;259:32835.

[26] Chen J, Li Y, Li Z, Zhang X. Production of COx -free hydrogen

and nanocarbon by direct decomposition of undiluted methane on

NiCualumina catalysts. Appl Catal A Gen 2004;269:17986.

[27] Kim MH, Lee EK, Jun JH, Kong SJ, Han GY, Lee BK. et al. Hydrogenproduction by catalytic decomposition of methane over activated carbons:

kinetic study. Int J Hydrogen Energy 2004;29:18793.

[28] Moliner R, Suelves I, Lzaro MJ, Moreno O. Thermocatalytic

decomposition of methane over activated carbons: influence of textural

properties and surface chemistry. Int J Hydrogen Energy 2005;30:

293300.

[29] Rouquerol F, Rouquerol J, Sing K. Adsorption by powders & porous

solids: principles, methodology and applications. London: Academic

Press; 1999 pp. 197198.

[30] Hong EH, Lee KH, Oh SH, Park CG. Synthesis of carbon nanotubes

using microwave radiation. Adv Funct Mater 2003;13:9616.

[31] Ortiz Mndez U, Kharissova OV, Rodrguez M. Synthesis and

morphology of nanostructures via microwave heating. Rev Adv Mater

Sci 2003;5:398402.

[32] Kharissova OV. Vertically aligned carbon nanotubes fabricated bymicrowaves. Rev Adv Mater Sci 2004;7:504.

methane activated carbon spain

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