Carbon Nanotube Synthesis Using Coal PyrolysisKapil Moothi,†,‡ Geoffrey S. Simate,† Rosemary Falcon,† Sunny E. Iyuke,*,†,‡ and M. Meyyappan§

†School of Chemical and Metallurgical Engineering and ‡DST/NRF Centre of Excellence in Strong Materials, University of theWitwatersrand, Johannesburg, 2050, South Africa§Center for Nanotechnology, NASA Ames Research Center, Moffett Field, California 94035, United States

*S Supporting Information

ABSTRACT: This study investigates carbon nanotube (CNT)production from coal pyrolysis wherein the output gases are used in achemical vapor deposition reactor. The carbon products are similar tothose using commercial coal gas as feedstock, but coal is a relativelycheaper feedstock compared to high purity source gases. A Gibbsminimization model has been developed to predict the volumepercentages of product gases from coal pyrolysis. Methane and carbonmonoxide were the largest carbon components of the product streamand thus formed the primary source for CNT synthesis. Both the modeland the observations showed that increasing the furnace temperature ledto a decrease in the absolute quantities of “useful” product gases, withthe optimal temperature between 400 and 500 °C. Based on theexperimental data, a kinetic rate law for CNT from coal pyrolysis wasderived as d[CNT]/dt = K([CO][CH4])

1/2, where K is a function ofseveral equilibrium constants representing various reactions in the CNT formation process.

1. INTRODUCTION

Carbon nanotubes (CNTs), due to their unique and interestingproperties such as high strength and good thermal and electricalconductivities, have been considered for a wide range ofapplications.1 This makes development of large-scale industrialproduction processes a worthwhile endeavor. A significantamount of time and effort has been devoted to CNT growthfrom high purity gases such as methane, ethane, acetylene, andothers as noted from the extensive CNT literature. Thesesource gases are expensive due to the fact that they are highpurity feedstock. Therefore, a strong need exists for producingCNTs from a relatively low cost feedstock such as coal in orderto make the production of CNTs viable on an industrial scale.2

This article focuses on the production of CNTs directly fromcoal pyrolysis wherein the gas output is used in a chemicalvapor deposition (CVD) reactor for producing nanotubes.Previous efforts have used coal directly in an arc dischargefurnace, and this has been reviewed in detail in ref 2. Onlycommercial coal gas has been used in CVD-based techniques todate.2 Here we provide CNT growth results from a coal-to-CNT CVD equipment at various pyrolysis temperatures. Thefirst step in the use of coal as a feedstock is the identification ofthe product gases and predicting their volume percentage.Therefore, we have also developed a model of the mainconstituent gases produced from the pyrolysis of coal to furtherenable a model for the CNT synthesis from such coal-derivedgases.

2. GIBBS MINIMIZATION METHOD

Existing pyrolysis models such as first-order kinetic model orthe distributed activation energy model (DAEM) are based ona kinetic rate law approach.3,4 These methods are commonlyused in the modeling of coal pyrolysis in order to determine theweight loss characteristics of the coal; however, they do notprovide much information regarding the composition of thepyrolysis gases that are produced. Here, we have implementedthe Gibbs free energy minimization method for modeling thecoal pyrolysis (in a coal-to-CNT equipment) as it predicts theproduct gases on a volume percentage basis while not being toocomputationally complex.The Gibbs free energy minimization technique is a method

commonly used in industry to model pyrolysis and gasification-type processes.5−7 This method is based on an equilibriumapproach and states, from the use of first-principles ofthermodynamics, that a system’s total Gibbs free energy(ΔG) with respect to a reference defined by the total enthalpy(25 °C and 1.013 bar here) will be minimized in order toestablish equilibrium at those conditions.8,9 This implies thatthere is a specific combination of products (solid, liquid, or gas)exiting the pyrolysis unit (or indeed any reactor) that willminimize the total free energy of the system. The total Gibbsfree energy of a system is given by the Gibbs formation values

Received: May 26, 2015Revised: July 28, 2015Published: August 11, 2015

Article

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© 2015 American Chemical Society 9464 DOI: 10.1021/acs.langmuir.5b01894Langmuir 2015, 31, 9464−9472

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as a function of temperature as well as a correction value for thefugacity of the mixture.8,9

∑ ∑= Δ ° +

nG n T P n G T RTf

f( , , ) ( ) lni i f

i

i0i

(1)

where fi is the fugacity of component i in gas mixture and fi0 is

fugacity of pure component i. The fugacity of the mixture isgiven by

ϕ = f yPi i i (2)

Assuming that the individual component fugacity is equal tounity, which is reasonable for low pressure operations, eqs 1and 2 can be combined to form

∑ ∑∑ ∑ ϕ

= Δ ° +

+ +

nG n T P n G T n RT P

n RT y n RT

( , , ) ( ) ln

ln ln

i i f i

i i i i

i

(3)

Equation 3 is limited to the elemental balance for each of themajor elements found in coal, i.e., carbon, hydrogen, oxygen,nitrogen, and sulfur, as in

∑ ==

n a bi

N

i ji j1 (4)

Gibbs free energy of formation data are calculated usingenthalpy of formation and entropy of formation at a specificreference temperature and pressure according to eq 5.8,9 Thedata are sourced from the National Institute of Science andTechnology database.10

Δ ° = Δ ° − Δ °G H T S (5)

In order to determine the product gas composition on avolume percentage basis, a suitable equation of state (EOS)such as the Soave−Redlich−Kwong (SRK) should be used.The EOS is fairly accurate in describing the relationshipsbetween temperature, specific volume, and pressure for gases,particularly at high temperatures, and is also applicable tohydrocarbon gases.9 The SRK EOS was used to find the partialpressure of each species within the gas mixture according to

= −

− +

PRT

V ba T

V V b( )

( ) (6)

where the variables a and b are given by

α=a TR T

PT( ) 0.42748 ( )

2c

2

c (7)

=bRTP

0.08664 c

c (8)

α κ= + +⎛⎝⎜⎜

⎞⎠⎟⎟T

TT

( ) 1 1c (9)

κ ω ω= + −0.480 1.574 0.176 2 (10)

where ω is the acentric factor and Tc and Pc are criticaltemperatures and pressures.9

In order to calculate the specific volume for use in the SRKEOS, the molar and volumetric flow rates of various gases arerequired:

ρ =

=VVn

MW(11)

Thus, the following assumptions were made in the modeldevelopment: (i) The fugacity coefficient was approximated asunity due to the low operational system pressure (∼1 bar).8,9

(ii) The system operates at atmospheric pressure. (iii) The ashis assumed to be inert, as it consists of nonreactive silicates andaluminates among other nonreactive substances.11 (iv) Theviable gas products from the pyrolysis reactor9 were taken to becarbon monoxide (CO), hydrogen (H2), carbon dioxide(CO2), methane (CH4), nitrogen (N2), hydrogen sulfide(H2S), carbonyl sulfide (COS), water (H2O), ammonia(NH3), nitrogen dioxide (NO2), and sulfur dioxide (SO2).(v) In addition, char is considered as the solid carbonaceousproduct (coke and ash) exiting the reactor, the composition ofwhich is determined theoretically by elemental mass balance.The flow rate of nitrogen is also determined theoretically inorder to minimize the total Gibbs free energy.

3. EXPERIMENTAL WORKThe materials used to perform the experimental work were BankColliery-Witbank coal from South Africa, 99.0% purity grade Ar, N2,H2, and dry air (AFROX), and ferrocene for catalyst (Sigma-Aldrich).The coal samples were crushed and ground using a Retsch ZM 200centrifugal to increase the surface area. Characteristics of the coal suchas its chemical composition, ash content, etc., were determined usingvarious analytical methods listed in Table 1 along with the findings inthe Supporting Information.

The pyrolysis of coal for the synthesis of CNTs was conductedusing a coal-to-CNT equipment, whose operation is described below.The system consists of a horizontal tube for pyrolysis with a hopper tofeed coal, followed by a vertical reactor tube for the synthesis of CNTs.A weighed coal sample was loaded into the hopper, and the systemwas first flushed with N2 and then Ar while the O2 concentration in thesystem was monitored with an O2 sensor. The tube furnaces (both thehorizontal and vertical) as well as the heated transfer line were set tothe desired temperatures. Thereafter, the hopper and reactor feedcontrols were set to the desired feed rate of coal. Once the coalparticles were fed into the reactor and pyrolysis was initiated, the O2concentration and the temperature of the pyrolysis reactor (horizontalfurnace) were monitored to ensure that they did not rise too sharply.Once pyrolysis was transpiring, the catalyst vaporizer (containing thecatalyst precursor) at the base of the vertical furnace was switched on.After a run time of ∼45 min, the horizontal furnace and hopper feedmotor were turned off. However, the reactor screw motor was allowedto run so as to ensure that all solid products were moved out of thepyrolysis reactor. After completion of the CVD process for CNTsynthesis, Ar and then N2 were allowed to flow through the system tocool both the horizontal and vertical furnaces.

Three individual gas chromatographic (GC) analysis runs werecarried out on each sample at intervals of 15 min. Once complete, theremaining coal sample was removed and allowed to cool, before beingweighed to determine the mass lost during pyrolysis. Additionally, theoptimum temperature for the production of the CNTs using ferroceneas the catalyst precursor was found to be around 900 °C.12 Therefore,this temperature setting was used for the CVD during operation of thecoal-to-CNT system.

The various exiting gas concentrations from the pyrolysis furnacewere measured using the online gas chromatographer (GC), which hasa thermal conductivity detector (TCD) and a flame ionization detector(FID). The GC was calibrated using a standard reference gas with thefollowing composition (molar percentage basis): acetylene (1.03%),carbon dioxide (1.0%), carbon monoxide (1.0%), ethane (1.01%),ethylene (0.999%), methane (1.0%), and nitrogen (93.961%).Assessing different pyrolysis reactor (horizontal furnace) temperaturelevels (400, 450, 500, 550, 600, 650, and 700 °C) was done in order to

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detect the effect of temperature on the amount and quality of gasesproduced during pyrolysis. The nanostructured products wereanalyzed using scanning electron microscopy (SEM), transmissionelectron microscopy (TEM), and Raman spectroscopy.

4. RESULTS AND DISCUSSIONThe GC analysis showed that the predominant carbon-containing gases produced by the pyrolysis were methane andcarbon monoxide (see Table 2 in the Supporting Information).This was consistent over the entire range of tested temper-atures. Integration of the peaks (see Supporting Information fordetails) shows that methane is the largest carbon-containingcomponent of the pyrolysis product stream, with carbonmonoxide as the second largest. Increasing the temperature ofthe furnace resulted in decreasing peak heights, implyingdecreasing volumes of these gases at higher temperatures. Tarand liquor are also products in the temperature region from400 to 750 °C, in addition to gases.13 Tar is the room-temperature condensable species formed during pyrolysis.14

The chemical processes occurring during coal pyrolysis are thedecomposition of individual functional groups to form lightgases and the decomposition of macromolecular networks toproduce smaller fragments, which can form tar.11 Therefore, asthe pyrolysis temperature increased, production of suchadditional products also increased. However, the relative gasamounts remained constant, with methane forming the largestproportion and carbon monoxide the second largest amount.The optimum temperature was determined to be ∼400 °C, andno peaks were observed at lower temperatures, indicating that400 °C is the temperature at which pyrolysis begins to occur.4.1. Comparison of Experimental and Model Results.

The absolute quantities of the primary gases from theexperimental pyrolysis runs are compared to the theoreticalabsolute quantities predicted by the Gibbs free energyminimization model in Figure 1. The model results and theexperimental data compare favorably for both methane andcarbon monoxide at all temperatures (400−700 °C). Thevolume percentages decrease rapidly with increasing temper-ature, and the trend is similar from the model and the data.These results indicate that the Gibbs minimization model maybe used to predict the relative quantities of gas, provided thepresence and quantity of at least two (preferably more) gasescan be determined in order to predict the remaining gascomposition.Figure 2 shows the product gas composition as predicted by

the Gibbs minimization model at temperatures of 400−700 °C.The reason for the methane trend could be explained bymethane reforming according to

+ ↔ +CH CO 2CO 2H4 2 2 (12)

Methane production appears to be favored by low temperaturesand high pressures in the case of gasification in inertatmospheres;15 the same, however, cannot be said for carbonmonoxide production as this quantity usually increases withincreasing temperature. It is possible that the carbon monoxideproduced is consumed at higher temperatures by means of thewater-gas shift reaction:

+ ↔ +CO H O CO H2 (g) 2 2 (13)

Reaction 13, however, is unlikely to occur because it is the feedcomposition, not the system condition, that affects the kineticsof the aforementioned reaction. In addition, carbon dioxide wasnot detected by the GC at higher furnace temperatures. An

alternative reason for the observed CO trend may be attributedto the production of methanol (or other alcohols) by thehydrogen emanating from the coal itself.15

+ ↔CO 2H CH OH2 3 (14)

This is a well-known fuel conversion route in the synthetic fuelindustry, and optimal selectivity of methanol requires a catalyst;however, the conversion does occur to a certain extent withoutthe presence of a catalyst.15 This reaction is exothermic, thusjustifying the trend favoring the forward reaction at highertemperatures. The results show reasonable agreement thatpyrolysis of coal for the synthesis of CNTs should occur atlower temperatures to enable production of large quantities ofcarbon-containing gases such as CH4 and CO.Figure 2 also shows an increasing amount of CO2 with

increasing temperature, thus reinforcing the water-gas shiftreaction proposed by reaction 13. The CO2 was not detectedby the GC due to the relatively small quantities present even at

Figure 1. (a) CO and (b) CH4 vol % obtained during coal pyrolysis atdifferent temperatures.

Figure 2. Gas quantities predicted by the Gibbs minimization modelfrom 400 to 700 °C.

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higher temperatures. Additionally, undesirable gases (such asH2S, NH3, COS, etc.) display a similar trend of increasing withan increase in system temperature. This implies that the systemshould be operated at lower temperatures.5,15 The undesirablegases are removed by a gas purification system prior to enteringthe CVD reactor.4.2. Identification of Synthesized CNT Products. CNT

product collected from the CVD reactor at the end of theexperimental runs was identified by TEM as shown in Figures 3and 4. Since coal has a wide range of compositions and puritylevels, higher purity and an increase in the ratio of carbon tohydrogen content resulting in more carbon ions are importantfor CNT growth.16,17 Therefore, we synthesized CNTs at allthe tested temperatures even though the quantity of CH4 andCO produced decreased as the coal pyrolysis temperatureincreased. At 400 °C, 6% CO and 31% CH4 were attained, andthe CNT outer diameter ranged from 20 to 40 nm. Qiu et al.17

successfully produced CNTs and carbon nanocapsules usingcommercially available coal gas with a composition of 15.1%CH4, 13.8% CO, and 5.4% CO2. The gas scrubbing system (atthis scale of the coal-to-CNT apparatus) worked well for theremoval of acid gases, ammonia, and water. However, a possiblemitigation technique in the case of industrial scale CNT

production is to use a flue gas scrubber such as those employedin industry on large smoke stacks or else to hydrotreat the gasusing high pressures.18

Figure 3 shows CNTs produced at 900 °C in the CVDreactor with temperatures ranging from 400 to 550 °C in thecoal pyrolysis reactor. These CNTs are 60−130 nm in diameterand range between 250 and 550 nm in length. The TEMimages in Figures 3 and 4 show black dots on the inside ofCNTs, which consist of carbonaceous nanoparticles, amor-phous carbon, and catalyst metal particles. Catalyst particles canbe seen within the hollow tube of the CNT produced at 450 °C(Figure 3b). Figure 4 shows CNTs produced at 900 °C in theCVD reactor with temperatures ranging from 600 to 700 °C inthe coal pyrolysis reactor. At 600 °C, a CNT with an outerdiameter of ∼75 nm is visible whereas CNTs grown at 650 °Cexhibit outer diameters ranging from 24 to 30 nm and innerdiameters of 6−10 nm. Figure 4c shows a CNT with a length of∼440 nm. As the coal was beneficiated, the risk of catalystpoisoning due to the impurities in the coal was minimized.However, previous studies have shown that the presence ofsome species such as CO and H2 in the feedstock is beneficialin preventing catalyst poisoning by preventing the oxidation ofnanosized catalyst active sites and carbon species.19

Figure 3. TEM images of CNTs produced at a temperature of 900 °C in the CVD reactor with temperatures of (a) 400, (b) 450, (c) 500, and (d)550 °C in the coal pyrolysis reactor.

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The carbon products were analyzed by Raman spectroscopyat nominally room temperature, and the power at the samplewas kept low (∼1.2 MW) to minimize local heating. The argonlaser line was at an excitation wavelength of 514.5 nm. TheRaman spectra (Figure S1) show the characteristic D (around1350 cm−1) and G (around 1580 cm−1) bands of sp2 carbon(indicative of CNTs) as well as the second-order 2D band(around 2700 cm−1). The relative intensity of the D-band isproportional to the amount of carbon impurities and defects inthe nanotube sample. The width of the D-band may be used asa relative measure of the amount of carbon impurities.20,21 Thespectra in Figure S1a,b are similar, while those in Figure S1c,dare different. In particular, Figure S1c shows a spectrum that isquite dissimilar, indicating the possibility of carbon nanofibers(CNFs) or amorphous carbon being formed in addition toCNTs, as confirmed in the scanning electron microscopy(SEM) images (Figures S2 and S3). The absence of the radialbreathing mode (RBM) in the region of 100−400 cm−1 in theRaman spectra implies that the CNTs produced are multi-walled.22 The surface morphology of the growth products(CNTs and CNFs) was observed using SEM in order toexamine the nanoscale crystal structure of these carbonproducts. The nanofibers in Figures S2 and S3 typically have

an outer diameter of 70−210 nm, a hollow core of 40−100 nm,and a length in the order of 100−150 μm.The average production of the CNTs varied from 4.0 ± 0.07

to 6.1 ± 0.42 g/h at 95% confidence interval. The highestproduction rate was obtained at a coal pyrolysis temperature of∼400 °C. Previous bulk CNT production studies, regardless ofcoal or other sources, do not provide a production rate,2 andtherefore a comparison is not possible. However, in comparisonwith our previous base case study of using only ferrocene as thecarbon feedstock (∼2 g),12 the amount of CNTs produced ishigher when using coal-derived hydrocarbon products here.Production of CNTs was at a minimum at the pyrolysistemperature of 650−700 °C.

4.3. Kinetic Model Development. Once the pyrolysisgases pass into the CVD reactor, reformation of methanetranspires, which is strongly endothermic and is thereforefavored by the high temperatures within the CVD reactor.23

Methane reforming takes place in a temperature range of 600−1000 °C24 in the presence of an iron catalyst. The ferrocenehere provides catalyst for both the methane reformationreaction(s) and in the CNT synthesis. The details of methanereforming for the CNT model are discussed below.

Figure 4. TEM images of CNTs produced at a temperature of 900 °C in the CVD reactor with temperatures of (a) 600, (b) 650, and (c) 700 °C inthe coal pyrolysis reactor.

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The following reaction typically proceeds due to loweractivation energy than the dry reforming reaction itself given inreaction 14.

+ ↔ +CO 2H C 2H O2 2 2 (15)

However, the side reaction of reforming also occurs via theintermediate reaction steps shown below:24

↔ +CH C 2H4 (ads) 2 (16)

+ ↔ +C H O CO H(ads) 2 2 (17)

The first reaction among these two is the rate-limiting,24

implying that the overall rate is governed by the concentrationof methane and adsorbed carbon in the forward and reversedirections, respectively.

=R k [CH ]forward 1 4 (18)

= −R k [C ]reverse 1 (ads) (19)

Introducing an equilibrium constant K1 results in

= =−

Kk

k[CH ][C ]1

1

1

4

(ads) (20)

In addition to the methane acting as a carbon source for theformation of CNTs, the CO produced during coal pyrolysisalso serves this function in a secondary manner. However, theprocedure by which CO forms the adsorbed carbon suitable forCNT formation is more complex than that of methane, and thekinetics of this approach has been discussed previously.25 It hasbeen deduced that CO will split in one of the two possiblereactions as shown below:

→ +2CO C CO2 (21)

→ +CO C12

O2 (22)

Reaction 22 is unlikely to take place due to the extremely hightemperatures required,25 and therefore only the Boudouardreaction 21 is considered here. It is well-known that theBoudouard reaction produces CNTs and has therefore resultedin CO becoming one of the most common sources for CNTproduction in the literature. However, this reaction cannot berepresented by elementary kinetics as it is a catalytic reaction.25

Before CNT synthesis can take place, a number of intermediatesurface reactions must take place in order for elemental carbonto be deposited on the catalyst surface. These includeadsorption of the produced CO2 gas onto the catalyst surface,followed by the dissociation of CO2 into adsorbed CO and O:

↔CO CO2(gas) 2(ads) (23)

↔ +CO CO O2(ads) (ads) (ads) (24)

The adsorbed CO then dissociates into adsorbed carbon andoxygen. The oxygen then reacts with remaining CO to formCO2, which desorbs to form gaseous CO2. This process isshown by the following reactions:

↔ +CO C O(ads) (ads) (ads) (25)

+ ↔CO O CO(ads) (ads) 2(ads) (26)

↔CO CO2(ads) 2(gas) (27)

Reactions 25 and 26 can be combined to form a single overallequation defining the production of adsorbed C:

↔ +2CO C CO(ads) (ads) 2(ads) (28)

The forward and reverse rate equations for reaction 28 are usedto define an equilibrium constant K2:

=R k [CO ]forward 2 (ads)2

= −R k [C ][CO ]reverse 2 (ads) 2(ads)

Accordingly, the equilibrium constant K2 can be defined by

= =−

Kk

k

[CO ]

[C ][CO ]22

2

(ads)2

(ads) 2(ads) (29)

In order to have a single expression for the concentration ofadsorbed carbon, the product of eqs 20 and 29 gives

=K K[CO ] [CH ]

[C ] [CO ]2 1(ads)

24

(ads)2

2(ads) (30)

The adsorbed carbon produced by reaction 25 is predicted tonucleate on the catalyst particle and form CNT on the surfaceof the catalyst particle.

→C CNT(ads) (31)

The rate of CNT growth can therefore be given by

=R k [C ]3 (ads) (32)

The equilibrium constants K1 and K2 can be used to developthe rate law for CNT growth. The rate law governing thegrowth of CNTs was previously shown to be proportional tothe quantity of adsorbed carbon on the catalyst particle asshown by eq 16.

=t

kd[CNT]

d[C ]3 (ads) (33)

Substituting eq 30 into eq 33 gives

=⎡⎣⎢⎢

⎤⎦⎥⎥t

kK K

d[CNT]d

[CO ] [CH ]

[CO ]3(ads)

24

2 1 2(ads)

1/2

(34)

Equation 24 can be used to further eliminate CO2 from the ratelaw, by defining the reaction rate (35) and a new equilibriumconstant, K4.

+ ↔CO O CO(ads) (ads) 2(ads) (35)

=R k [CO ][O ]forward 4 (ads) (ads)

= −R k [CO ]reverse 4 2(ads)

At equilibrium

=K[CO ][O ]

[CO ]4(ads) (ads)

2(ads) (36)

Combining eqs 34 and 36 gives

=⎡⎣⎢⎢

⎤⎦⎥⎥t

kK

K Kd[CNT]

d

[CO ][CH ]

[O ]34

22(ads) 4(gas)

2 1 (ads)2

1/2

(37)

which may be further simplified to

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=t

kK

K Kd[CNT]

d

([CO ][CH ])

[O ]34 2(ads) 4

1/2

2 1 (ads) (38)

Substitution of (27) into (38) gives

=t

kK

K K Od[CNT]

d([CO ][CH ])

[ ]34 2 4

1/2

2 1 (ads) (39)

Defining an overall equilibrium value (K), the rate of CNTproduction can be given as a function of CO2 (g) and CH4 (g)concentration by

=t

Kd[CNT]

d([CO ][CH ])2 4

1/2(40)

where

=Kk K

K K [O(ads)]3 4

2 1

Equation 35 can be given as a function of the CO concentrationin terms of the water-gas shift reaction defined by

+ ↔ +CO H O CO Hg( ) 2 (g) 2(g) 2(g) (41)

=R k [CO]forward 5

= −R k [CO ]reverse 5 2

At equilibrium, hydrogen and water (also produced accordingto the water-gas shift reaction, 41) are assumed to benonlimiting.

= =−

Kk

k[CO ][CO]5

5

5

2

(42)

Substituting (42) into (41) gives

=t

K Kd[CNT]

d( [CO][CH ])5 4

1/2(43)

Finally, the rate of CNT formation as a function of theconcentration of CO and CH4 is postulated to be representedby

= *t

Kd[CNT]

d([CO][CH ])4

1/2(44)

where

* =K K K5

The overall rate law given by eq 44 shows that the rate of CNTgrowth is dependent on the concentrations of methane andcarbon monoxide produced by coal pyrolysis, which act as thecarbon sources for the CNT synthesis. The rate of diffusion ofadsorbed carbon through the catalyst particle and theprecipitation of the carbon nanostructures has been previouslyshown to be the rate-limiting step.25 The growth rate of theCNTs is therefore controlled by reaction 31 while theintermediate product reactions are considered to occur muchfaster. The rate of diffusion into the catalyst particle by theadsorbed carbon is the overall rate-limiting step and the rateconstant K* is proportional to the diffusion coefficient ofcarbon into the iron catalyst. The model above is not completeas it does not account for catalyst deactivation, for example, andfuture work is needed to build upon the above foundation.

Using the concentrations of methane and carbon monoxidepredicted by the Gibbs minimization model, the growth rate ofCNTs can be modeled as a function of temperature, as shownin Figure 5. The effect of temperature on the rate law can be

determined by the use of the adjusted Arrhenius equationwhere the activation energy is the diffusion energy of carboninto the iron catalyst.26

* ≈ =−⎜ ⎟⎛

⎝⎞⎠K D

ERT

0.02 exp a

(45)

Figure 5 shows that the CNT growth rate increasesexponentially with increasing temperature. This would implythat CNT formation should occur at the maximum possibletemperature; however, the growth rate of CNTs is known to belimited kinetically below 750 °C and to be limitedthermodynamically above 850 °C.26 The growth rate variesover a factor of 1014 over the temperature range of 400−700°C (this is the range at which concentrations were measuredhere), showing that temperature plays a large role in the rate ofCNT synthesis.Since the present study is the first of its kind combining coal

pyrolysis and CVD, there is no direct comparison possible withprevious literature in terms of growth rate or even productstream out of the pyrolysis chamber. He et al. investigated amechanism of coal gasification in an air and steam mediumunder arc plasma conditions27 which are different from thepresent work. In their study, the yields of CO, CO2, and O2increased while the yield of H2 and the peak intensities of OHradicals, C atoms, H atoms, and CH radicals decreased with anincrease in the flow rate of air. The H2 and CO content in theproduct gases could reach 75.0 vol % with the CO2 contentbeing less than 3.0 vol %. Co-conversion of CO2 with coal(bituminous coal from the Taiji coal mine of China) underplasma conditions conducted28 by He et al. showed that anincrease in CO2 flow rate increased the yields of H2, CO, CO2,and O2 in the gaseous products. The outcomes from the co-conversion process demonstrated that CO2 conversion couldreach 88.6%, and the content of H2 and CO (synthesis gas)could reach 87.4 vol % while CO2 concentration was no morethan 4.0 vol %. Furthermore, coal conversion was in the range54.7−68.7%. There is no information on temperature effects aswe have in Figures 1 and 2 here.

Figure 5. CNT growth rate vs temperature (circles representexperimental data and solid line according to model, i.e., eq 44).

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5. CONCLUSIONA sustainable and cost-effective process for large-scaleproduction of CNTs is highly desirable due to their increasedindustrial importance. One of the challenges is an abundant,cheap source of carbon for CNT production. This paper hasfocused on the synthesis of CNTs from direct pyrolysis of coalusing a scalable technique as well as development of a kineticmodel of CNT growth using coal-derived gases as the carbonsource. Experimental results obtained through the pyrolysis ofcoal demonstrated that the Gibbs minimization model is a fairlyaccurate method of predicting the products of the coalpyrolysis. Since CH4 and CO formed the primary source ofcarbon for the synthesis of CNTs within the CVD reactor, itwas found that increasing the temperature of the pyrolysisfurnace led to a decrease in the absolute quantities of theseproduct gases. The optimal pyrolysis temperature range wasestablished to be between 400 and 500 °C. A kinetic rate lawproposed for CNT growth is found to be dependent on theconcentrations of CH4 and CO produced by the pyrolysis ofcoal. Coal-derived hydrocarbon products are shown to be aviable and abundant source of carbon for CNT synthesis.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.lang-muir.5b01894.

Tables 1 and 2; Figures S1−S3 (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: Sunny.Iyuke@wits.ac.za (S.I.).Author ContributionsK.M. designed and performed the experiments and analyzedthe results; G.S. helped with the experiments. R.F., S.I., andM.M. contributed to the analysis and all authors contributed tothe manuscript preparation.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors acknowledge the financial support from theNational Research Foundation (NRF) under South AfricaFocus Area, NRF Nanotechnology flagship programme,Department of Science and Technology (DST)-funded Chairof Clean Coal Technology grant, and DST/NRF Centre ofExcellence. The student bursaries provided by the University ofthe Witwatersrand are acknowledged. Special thanks are due toS. Lekeletsan, D. Moor, and L. Del for their help withlaboratory testing.

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