Carbon Nanotube Synthesis Using Coal Pyrolysis - Langmuir (ACS

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Carbon nanotube synthesis using coal pyrolysis Kapil Moothi, GEOFFREY SIMATE, Rosemary Falcon, Sunny Esayegbemu Iyuke, and M. Meyyappan Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b01894 • Publication Date (Web): 11 Aug 2015 Downloaded from http://pubs.acs.org on August 15, 2015

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Carbon nanotube synthesis using coal pyrolysis

Kapil Moothi1,2, Geoffrey S. Simate1, Rosemary Falcon1, Sunny E. Iyuke1,2, M. Meyyappan3 1

School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, Private Bag 3, WITS 2050, South Africa

2

3

DST/NRF Centre of Excellence in Strong Materials, University of the Witwatersrand

Center for Nanotechnology, NASA Ames Research Center, Moffett Field, CA 94035 USA

Abstract This study investigates carbon nanotube (CNT) production from coal pyrolysis wherein the output gases are used in a chemical vapor deposition reactor. The carbon products are similar to those using commercial coal gas as feedstock but coal is a relatively cheaper feedstock compared to high purity source gases. A Gibbs minimization model has been developed to predict the volume percentages of product gases from coal pyrolysis. Methane and carbon monoxide were the largest carbon components of the product stream and thus formed the primary source for CNT synthesis. Both the model and the observations showed that increasing the furnace temperature led to a decrease in the absolute quantities of ‘useful’ product gases, with the optimal temperature between 400°C and 500°C. Based on the experimental data, a kinetic rate law for CNT from coal pyrolysis was derived as: d[CNT]/dt = K*([CO][CH4])1/2, where K is a function of several equilibrium constants representing various reactions in the CNT formation process.

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1. Introduction Carbon nanotubes (CNTs), due to their unique and interesting properties such as high strength and good thermal and electrical conductivities, have been considered for a wide range of applications1. This makes development of large-scale industrial production processes a worthwhile endeavor. A significant amount of time and effort has been devoted to CNT growth from high purity gases such as methane, ethane, acetylene and others as noted from the extensive CNT literature. These source gases are expensive due to the fact that they are high purity feedstock. Therefore, a strong need exists for producing CNTs from a relatively low cost feedstock such as coal in order to make the production of CNTs viable on an industrial scale2. This article focuses on the production of CNTs directly from coal pyrolysis wherein the gas output is used in a chemical vapor deposition (CVD) reactor for producing nanotubes. Previous efforts have used coal directly in an arc discharge furnace and this has been reviewed in detail in ref. 2. Only commercial coal gas has been used in CVD based techniques to date2. Here we provide CNT growth results from a coal-to-CNT CVD equipment at various pyrolysis temperatures. The first step in the use of coal as a feedstock is the identification of the product gases and predicting their volume percentage. Therefore, we have also developed a model of the main constituent gases produced from the pyrolysis of coal to further enable a model for the CNT synthesis from such coal-derived gases.

2. Gibbs Minimization Method Existing pyrolysis models such as first order kinetic model or the Distributed Activation Energy Model (DAEM) are based on a kinetic rate law approach3, 4. These methods are commonly used in the modeling of coal pyrolysis in order to determine the weight loss characteristics of the coal; however, they do not provide much information regarding the composition of the pyrolysis gases that are produced. Here, we have implemented the Gibbs 2


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free energy minimization method for modeling the coal pyrolysis (in a coal-to-CNT equipment) as it predicts the product gases on a volume percentage basis while not being too computationally complex.

The Gibbs free energy minimization technique is a method commonly used in industry to model pyrolysis and gasification-type processes5-7. This method is based on an equilibrium approach and states, from the use of first principles of thermodynamics, that a system’s total Gibbs free energy (∆G) with respect to a reference defined by the total enthalpy (25°C and 1.013 bar here) will be minimized in order to establish equilibrium at those conditions8, 9. This implies that there is a specific combination of products (solid, liquid or gas) exiting the pyrolysis unit (or indeed any reactor) that will minimize the total free energy of the system. The total Gibbs free energy of a system is given by the Gibbs formation values as a function of temperature as well as a correction value for the fugacity of the mixture8, 9.

, , = ∑ ∆ + ∑

(1)

where is the fugacity of component i in gas mixture and is fugacity of pure component i. The fugacity of the mixture is given by: = ∅

(2)

Assuming that the individual component fugacity is equal to unity, which is reasonable for low pressure operations, equations 1 and 2 can be combined to form , , = ∑ ∆ + ∑ + ∑ + ∑ ∅

(3)

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


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∑ =

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(4)

Gibbs free energy of formation data is calculated using enthalpy of formation and entropy of formation at a specific reference temperature and pressure according to Equation 58. 9. The data is sourced from the National Institute of Science and Technology database10. ∆ = ∆ − ∆!

(5)

In order to determine the product gas composition on a volume 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 relationships between temperature, specific volume and pressure for gases, particularly at high temperatures and is also applicable to hydrocarbon gases9. The SRK EOS was used to find the partial pressure of each species within the gas mixture according to "#

(#

= $%&' − $%$%)'

(6)

where the variables a and b are given by: = 0.42748 = 0.08664

" 0 #1 0 21

3

"#1

(7) (8)

21

#

53 = 1 + 7 81 + 9# :

(9)

7 = 0.480 + 1.574< − 0.176? = = A@

(11)

D

Thus, the following assumptions were made in the model development: -

The fugacity coefficient was approximated as unity due to the low operational system pressure (~ 1 bar)8, 9.

-

The system operates at atmospheric pressure.

-

The ash is assumed to be inert, as it consists of non-reactive silicates and aluminates among other non-reactive substances11.

-

The viable gas products from the pyrolysis reactor9 were taken to be carbon 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).

-

In addition, char is considered as the solid carbonaceous product (coke and ash) exiting the reactor, the composition of which is determined theoretically by elemental mass balance. The flow rate of nitrogen is also determined theoretically in order to minimize the total Gibbs free energy.

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

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

Three individual gas chromatographic (GC) analysis runs were carried out on each sample at intervals of 15 minutes. Once complete, the remaining coal sample was removed and allowed to cool, before being weighed to determine the mass lost during pyrolysis. Additionally, the optimum temperature for the production of the CNTs using ferrocene as the catalyst precursor was found to be around 900°C12. Therefore, this temperature setting was used for the CVD during operation of the coal-to-CNT system.

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The various exiting gas concentrations from the pyrolysis furnace were measured using the online gas chromatographer (GC), which has a thermal conductivity detector (TCD) and a flame ionization detector (FID). The GC was calibrated using a standard reference gas with the following 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) temperature levels (400, 450, 500, 550, 600, 650 and 700oC) was done in order to detect the effect of temperature on the amount and quality of gases produced during pyrolysis.

The

nanostructured products were analyzed using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy.

4. Results and discussion The GC analysis showed that the predominant carbon-containing gases produced by the pyrolysis were methane and carbon monoxide (see Table 2 under Supplementary Information). This was consistent over the entire range of tested temperatures. Integration of the peaks (see Supplementary Information for details) shows that methane is the largest carbon-containing component of the pyrolysis product stream, with carbon monoxide as the second largest. Increasing the temperature of the furnace resulted in decreasing peak heights, implying decreasing volumes of these gases at higher temperatures. Tar and liquor are also products in the temperature region from 400 to 750oC, in addition to gases13. Tar is the roomtemperature condensable species formed during pyrolysis14. The chemical processes occurring during coal pyrolysis are the decomposition of individual functional groups to form light gases, and the decomposition of macromolecular networks to produce smaller fragments, which can form tar11. Therefore, as the pyrolysis temperature increased, production of such additional products also increased. However, the relative gas amounts 7


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remained constant, with methane forming the largest proportion and carbon monoxide the second largest amount. The optimum temperature was determined to be ~400 °C and no peaks were observed at lower temperatures, indicating that 400°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 the experimental pyrolysis runs are compared to the theoretical absolute quantities predicted by the Gibbs free energy minimization model in Fig. 1. The model results and the experimental data compare favorably for both methane and carbon monoxide at all temperatures (400 – 700°C). The volume percentages decrease rapidly with increasing temperature and the trend is similar from the model and the data. These results indicate that the Gibbs minimization model may be used to predict the relative quantities of gas, provided the presence and quantity of at least two (preferably more) gases can be determined in order to predict the remaining gas composition.

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 by methane reforming according to: EF + EG= ↔ 2EG + 2=

(12)

Methane production appears to be favored by low temperatures and high pressures in the case of gasification in inert atmospheres15; the same, however, cannot be said for carbon monoxide production as this quantity usually increases with increasing temperature. It is

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possible that the carbon monoxide produced is consumed at higher temperatures by means of the water gas shift reaction: EG + = GI ↔ EG= + =

(13)

Reaction 13, however, is unlikely to occur because it is the feed composition, not the system condition, that affects the kinetics of the aforementioned reaction. In addition, carbon dioxide was not detected by the GC at higher furnace temperatures. An alternative reason for the observed CO trend may be attributed to the production of methanol (or other alcohols) by the hydrogen emanating from the coal itself15. EG + 2= ↔ EJ G

(14)

This is a well-known fuel conversion route in the synthetic fuel industry and optimal selectivity of methanol requires a catalyst; however, the conversion does occur to a certain extent without the presence of a catalyst15. This reaction is exothermic thus justifying the trend favoring the forward reaction at higher temperatures. The results show reasonable agreement that pyrolysis of coal for the synthesis of CNTs should occur at lower temperatures to enable production of large quantities of carbon-containing gases such as CH4 and CO.

Figure 2 also shows an increasing amount of CO2 with increasing temperature, thus reinforcing the water gas shift reaction proposed by reaction 13. The CO2 was not detected by the GC due to the relatively small quantities present even at higher temperatures. Additionally, undesirable gases (such as H2S, NH3, COS, etc.) display a similar trend of increasing with an increase in system temperature. This implies that the system should be

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operated at lower temperatures5, 15. The undesirable gases are removed by a gas purification system prior to entering the CVD reactor.

4.2 Identification of synthesized CNT products CNT product collected from the CVD reactor at the end of the experimental runs was identified by TEM as shown in Figures 3 and 4. Since coal has a wide range of compositions and purity levels, higher purity and an increase in the ratio of carbon to hydrogen content resulting in more carbon ions are important for CNT growth16, 17. Therefore, we synthesized CNTs at all the tested temperatures even though the quantity of CH4 and CO produced decreased as the coal pyrolysis temperature increased. At 400oC, 6% CO and 31% CH4 were attained and the CNT outer diameter ranged from 20 to 40 nm. Qiu et al.17 successfully produced CNTs and carbon nanocapsules using commercially available coal-gas with a composition of 15.1% CH4, 13.8% CO and 5.4 % CO2. The gas scrubbing system (at this scale of the coal-to-CNT apparatus) worked well for the removal of acid gases, ammonia and water. However, a possible mitigation technique in the case of industrial scale CNT production is to use a flue gas scrubber such as those employed in industry on large smoke stacks or else to hydro-treat the gas using high pressures18.

Figure 3 shows CNTs produced at 900oC in the CVD reactor with temperatures ranging from 400-550oC in the coal pyrolysis reactor. These CNTs are 60 to 130 nm in diameter and range between 250 and 550 nm in length. The TEM images in Figures 3 and 4 show black dots on the inside of CNTs, which consist of carbonaceous nanoparticles, amorphous carbon and catalyst metal particles. Catalyst particles can be seen within the hollow tube of the CNT produced at 450°C (Figure 3 (b)). Figure 4 shows CNTs produced at 900oC in the CVD 10


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reactor with temperatures ranging from 600 - 700oC in the coal pyrolysis reactor. At 600oC, a CNT with an outer diameter of ~75 nm is visible whereas CNTs grown at 650oC exhibit outer diameters ranging from 24 – 30 nm and inner diameters of 6 to 10 nm. Figure 4 (c) shows a CNT with a length of ~ 440 nm. As the coal was beneficiated, the risk of catalyst poisoning due to the impurities in the coal was minimized. However, previous studies have shown that the presence of some species such as CO and H2 in the feedstock is beneficial in preventing catalyst poisoning by preventing the oxidation of nano-sized catalyst active sites and carbon species19. The carbon products were analyzed by Raman spectroscopy at nominally room temperature and the power at the sample was kept low (~1.2 MW) to minimize local heating. The argon laser line was at an excitation wavelength of 514.5 nm. The Raman spectra (Figure S1) show the characteristic D (around 1350 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 is proportional to the amount of carbon impurities and defects in the nanotube sample. The width of the D-band may be used as a relative measure of the amount of carbon impurities20-21. The spectra in Figure S1 (a) and (b) are similar, while those in Figure S1 (c) and (d) are different. In particular, Figure S1 (c) shows a spectrum that is quite dissimilar, indicating the possibility of carbon nanofibers (CNFs) or amorphous carbon being formed in addition to CNTs, as confirmed in the scanning electron microscopy (SEM) images (Figures S2 and S3). The absence of the radial breathing mode (RBM) in the region of 100400cm-1 in the Raman spectra implies that the CNTs produced are multiwalled22. The surface morphology of the growth products (CNTs and CNFs) was observed using SEM in order to examine the nanoscale crystal structure of these carbon products. The nanofibers in Figures

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S2 and S3 typically have an outer diameter of 70 to 210 nm, a hollow core of 40-100 nm, and a length in the order of 100 to 150 microns. The average production of the CNTs varied from 4.0±0.07 to 6.1±0.42 g/hr at 95% confidence interval. The highest production rate was obtained at a coal pyrolysis temperature of ~400°C. Previous bulk CNT production studies, regardless of coal or other sources, do not provide a production rate2 and therefore, a comparison is not possible. However, in comparison with our previous base case study of using only ferrocene as the carbon feedstock (~ 2 g)12, the amount of CNTs produced is higher when using coal-derived hydrocarbon products here. Production of CNTs was at a minimum at the pyrolysis temperature of 650700oC.

4.3 Kinetic model development Once the pyrolysis gases pass into the CVD reactor, reformation of methane transpires, which is strongly endothermic and is, therefore, favored by the high temperatures within the CVD reactor23. Methane reforming takes place in a temperature range of 600-1000°C24 in the presence of an iron catalyst. The ferrocene here provides catalyst for both the methane reformation reaction(s) and in the CNT synthesis. The details of methane reforming for the CNT model are discussed below.

The following reaction typically proceeds due to lower activation energy than the dry reforming reaction itself given in reaction (14). EG= + 2= ↔ E + 2= G

(15)

12


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However, the side reaction of reforming also occurs via the intermediate reaction steps shown below24: EF ↔ E(KL + 2=

(16)

E(KL + = G ↔ EG + =

(17)

The first reaction among these two is the rate limiting24, implying that the overall rate is governed by the concentration of methane and adsorbed carbon in the forward and reverse directions, respectively. MN(MK = O PEF Q

(18)

MRSRMLR = O& TE(KL U

(19)

Introducing an equilibrium constant K1 results in: W

PZ[\ Q

V = W X = PZ YX

(20)

]^_ Q

In addition to the methane acting as a carbon source for the formation of CNTs, the CO produced during coal pyrolysis also serves this function in a secondary manner. However, the procedure by which CO forms the adsorbed carbon suitable for CNT formation is more complex than that of methane, and the kinetics of this approach has been discussed previously25. It has been deduced that CO will split in one of the two possible reactions as shown below: 2EG → E + EG=

(21)

EG → E + 1a2 G=

(22)

13


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Reaction 22 is unlikely to take place due to the extremely high temperatures required25 and therefore, only the Boudouard reaction (21) is considered here. It is well known that the Boudouard reaction produces CNTs and has, therefore, resulted in CO becoming one of the most common sources for CNT production in the literature. However, this reaction cannot be represented by elementary kinetics as it is a catalytic reaction25. Before CNT synthesis can take place, a number of intermediate surface reactions must take place in order for elemental carbon to be deposited on the catalyst surface. These include adsorption of the produced CO2 gas onto the catalyst surface, followed by the dissociation of CO2 into adsorbed CO and O: EG=I(L ↔ EG=(KL

(23)

EG=(KL ↔ EG(KL + G(KL

(24)

The adsorbed CO then dissociates into adsorbed carbon and oxygen. The oxygen then reacts with remaining CO to form CO2, which desorbs to form gaseous CO2. This process is shown by the following reactions: EG(KL ↔ E(KL + G(KL

(25)

EG(KL + G(KL ↔ EG=(KL

(26)

EG=(KL ↔ EG=I(L

(27)

Reactions 25 and 26 can be combined to form a single overall equation defining the production of adsorbed C: 2EG(KL ↔ E(KL + EG=(KL

(28)

14


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The forward and reverse rate equations for reaction 28 are used to define an equilibrium constant K2: MN(MK = O= PEG(KL Q= MRSRMLR = O&= TE(KL UPEG=(KL Q Accordingly, the equilibrium constant K2 can be defined by: V= =

W0

WY0

=

PZb]^_ Q0

TZ]^_ UPZb0]^_ Q

(29)

In order to have a single expression for the concentration of adsorbed carbon, the product of equations 20 and 29 gives: V= V =

PZb]^_ Q0 PZ[\ Q 0

TZ]^_ U PZb0]^_ Q

(30)

The adsorbed carbon produced by reaction 25 is predicted to nucleate on the catalyst particle and form CNT on the surface of the catalyst particle. E(KL → Ec

(31)

The rate of CNT growth can therefore be given by: = OJ TE(KL U

(32)

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

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KPZ#Q

= OJ TE(KL U

Kd

Page 16 of 31

(33)

Substituting equation 30 into equation 33 gives:

KPZ#Q Kd

X

PZb]^_ Q0 PZ[\ Q 0

= OJ e f

0 fX PZb0]^_ Q

g

(34)

Equation 24 can be used to further eliminate CO2 from the rate law, by defining the reaction rate (35) and a new equilibrium constant, K4. EG(KL + G(KL ↔ EG=(KL

(35)

MN(MK = OF TEG(KL UPG(KL Q MRSRMLR = O&F PEG=(KL Q At equilibrium, VF =

TZb]^_ UPb]^_ Q

(36)

PZb0 ]^_ Q

Combining equations 34 and 36 gives:

KPZ#Q Kd

= OJ e

X

f\ 0 PZb0 ]^_ QPZ[\ h]_ Q 0

g

f0 fX Pb]^_ Q0

(37)

which may be further simplified to:

KPZ#Q Kd

= OJ

X

f\ iPZb0 ]^_ QPZ[\ Qj0 5f0 fX Pb]^_ Q

(38)

Substitution of 27 into 38 gives: 16


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KPZ#Q Kd

= OJ

X

f\ PZb0 QPZ[\ Q 0

(39)

5f0 fX Pb]^_ Q

Defining an overall equilibrium value (K), the rate of CNT production can be given as a function of CO2 (g) and CH4 (g) concentration by: KPZ#Q Kd

where V =

X

= VPEG= QPEF Q 0

(40)

Wk f\

5f0 fX Pb(KL Q

Equation 35 can be given as a function of the CO concentration in terms of the water gas shift reaction defined by: EGI + = GI ↔ EG= I + = I

(41)

MN(MK = Ol PEGQ MRSRMLR = O&l PEG= Q

At equilibrium, hydrogen and water (also produced according to the water gas shift reaction, 41) are assumed to be non-limiting. W

Vl = W m = Ym

PZb0 Q

(42)

PZbQ

Substituting 42 into 41 gives: KPZ#Q Kd

X

= VVl PEGQPEF Q 0

(43)

Finally, the rate of CNT formation as a function of the concentration of CO and CH4 is postulated to be represented by: 17


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KPZ#Q Kd

X

= V ∗ PEGQPEF Q 0

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(44)

where V ∗ = V 5Vl

The overall rate law given by equation 44 shows that the rate of CNT growth is dependent on the concentrations of methane and carbon monoxide produced by coal pyrolysis, which act as the carbon sources for the CNT synthesis. The rate of diffusion of adsorbed carbon through the catalyst particle and the precipitation of the carbon nanostructures has been previously shown to be the rate-limiting step25. The growth rate of the CNTs is, therefore, controlled by reaction (31) while the intermediate product reactions are considered to occur much faster. The rate of diffusion into the catalyst particle by the adsorbed carbon is the overall rate limiting step and the rate constant K* is proportional to the diffusion coefficient of carbon into the iron catalyst. The model above is not complete as it does not account for catalyst deactivation, for example and future work is needed to build upon the above foundation.

Using the concentrations of methane and carbon monoxide predicted by the Gibbs minimization model, the growth rate of CNTs can be modeled as a function of temperature, as shown in Fig. 5. The effect of temperature on the rate law can be determined by the use of the adjusted Arrhenius equation where the activation energy is the diffusion energy of carbon into the iron catalyst26. &t

V ∗≈ p = 0.02qrs i "#]j

(45)

Figure 5 shows that the CNT growth rate increases exponentially with increasing temperature. This would imply that CNT formation should occur at the maximum possible temperature; however, the growth rate of CNTs is known to be limited kinetically below 18


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750°C and to be limited thermodynamically above 850°C26. The growth rate varies over a factor of 1014 over the temperature range of 400 - 700°C (this is the range at which concentrations were measured here), showing that temperature plays a large role in the rate of CNT synthesis.

Since the present study is the first of its kind combining coal pyrolysis and CVD, there is no direct comparison possible with previous literature in terms of growth rate or even product stream out of the pyrolysis chamber. He et al. investigated a mechanism of coal gasification in an air and steam medium under arc plasma conditions27 which are different from the present work. In their study, the yields of CO, CO2 and O2 increased while the yield of H2 and the peak intensities of OH radicals, C atoms, H atoms and CH radicals decreased with an increase in the flowrate of air. The H2 and CO content in the product gases could reach 75.0 vol% with the CO2 content being less than 3.0 vol%. Co-conversion of CO2 with coal (bituminous coal from the Taiji coal mine of China) under plasma conditions conducted28 by He et al. showed that an increase 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 could reach 88.6%, and the content of H2 and CO (synthesis gas) could reach 87.4% in volume while CO2 concentration was no more than 4.0 vol%. Furthermore, coal conversion was in the range 54.7 – 68.7%. There is no information on temperature effects as we have in Figures 1 and 2 here.

5. Conclusion A sustainable and cost effective process for large scale production of CNTs is highly desirable due to their increased industrial importance. One of the challenges is an abundant, cheap source of carbon for CNT production. This paper has focused on the synthesis of CNTs 19


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from direct pyrolysis of coal using a scalable technique as well as development of a kinetic model of CNT growth using coal-derived gases as the carbon source. Experimental results obtained through the pyrolysis of coal demonstrated that the Gibbs minimization model is a fairly accurate method of predicting the products of the coal pyrolysis. Since CH4 and CO formed the primary source of carbon for the synthesis of CNTs within the CVD reactor, it was found that increasing the temperature of the pyrolysis furnace led to a decrease in the absolute quantities of these product gases. The optimal pyrolysis temperature range was established to be between 400 and 500°C. A kinetic rate law proposed for CNT growth is found to be dependent on the concentrations of CH4 and CO produced by the pyrolysis of coal. Coal-derived hydrocarbon products are shown to be a viable and abundant source of carbon for CNT synthesis.

Acknowledgements The authors acknowledge the financial support from the National Research Foundation (NRF) under South Africa Focus Area, NRF Nanotechnology flagship programme, Department of Science and Technology (DST)-funded Chair of Clean Coal Technology grant and DST/NRF Centre of Excellence. The student bursaries provided by the University of the Witwatersrand are acknowledged. Special thanks are due to S. Lekeletsan, D. Moor and L. Del for their help with laboratory testing.

Competing interests: None

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Author Contributions: KM designed and performed the experiments and analyzed the results; GS helped with the experiments. RF, SI and MM contributed to the analysis and all authors contributed to the manuscript preparation.

References (1) Meyyappan, M. Carbon Nanotubes: Science and Applications. CRC Press, Boca Raton, FL 2004. (2) Moothi, K.; Iyuke, S.; Meyyappan, M.; Falcon, R. Coal as a source for carbon nanotube synthesis. Carbon 2012, 50, 2679-2690. (3) Miura, K. A new and simple method to estimate f(E) and k0(E) in the distributed activation energy model from three sets of experimental data. Energy and Fuels 1995, 9, 302307. (4) Scott, S. A.; Dennis, J. S.; Davidson, J. F.; Hayhurst, A. N. An algorithm for determining the kinetics of devolatilisation of complex solid fuels from thermogravimetric experiments. Chem. Eng. Sci. 2006, 61, 2339-2348. (5) Watkinson, A. P.; Lucas, J. P.; Lim, C. J. A prediction of performance of commercial coal gasifiers. Fuel, 1991, 70, 519-527. (6) Li, X.; Grace, J. R.; Watkinson, A. P.; Lim, C. J.; Erguedenler, A. Equilibrium modeling of gasification: a free energy minimization approach and its application to a circulating fluidized bed coal gasifier. Fuel, 2001, 80, 195-207. 21


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(7) Jarungthammachote, S.; Dutta, A. Equilibrium modeling of gasification: Gibbs free energy minimization approach and its application to spouted bed and spout-fluid bed gasifiers. Energy Conversion and Management 2008, 49, 1345-1356. (8) Lwin, L. Chemical equilibrium by Gibbs energy minimization on spreadsheets. Int. J. Eng. Edu. 2000, 16, 335-339. (9) Sandler, S. I. Chemical, Biochemical and Engineering Thermodynamics. 4th Ed. New York, John Wiley and Sons Inc. 2006. (10) Linstrom, P. J.; Mallard, W. G. (Eds), NIST Chemistry WebBook, NIST Standard Reference Database Number 69, National Institute of Standards and Technology, Gaithersburg MD, 20899, INTERNET. http://webbook.nist.gov/ Cited August 2013. (11) Arenillas, A.; Rubiera, F.; Pevida, C.; Pis, J. J. A comparison of different methods for predicting coal devolatilisation kinetics. J. Anal. Appl. Pyrolysis 2002, 58-59, 685-701. (12) Yah, C. S.; Simate, G. S.; Moothi, K.; Maphutha, K. S.; Iyuke, S. E. Synthesis of large carbon nanotubes from ferrocene: the chemical vapour deposition technique. Trends Appl. Sci. Res. 2011, 6, 1270-1279. (13) Ladner, W. R. The products of coal pyrolysis: properties, conversion and reactivity. Fuel Process Technol. 1988, 20, 207-222. (14) Migliavacca, G.; Parodi, E.; Bonfanti, L.; Faravelli, T.; Pierucci, S.; Ranzi, E. A general mathematical model of solid fuels pyrolysis. Energy 2005, 30, 1453-1468. (15) Probstein, R. F.; Hicks, R. E. Synthetic Fuels: Gas from Coal. New York, Dover Publications Inc. 2006. (16) Qiu, J.; An, Y.; Zhao, Z.; Li, Y.; Zhou, Y. Catalytic synthesis of single-walled carbon nanotubes from coal gas by chemical vapor deposition method. Fuel Process Technol. 2004, 85, 913-920.

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(17) Qiu, J.; Li, Q.; Wang, Z.; Sun, Y.; Zhang, H. CVD synthesis of coal gas- derived carbon nanotubes and nanocapsules containing magnetic iron carbide and oxide. Carbon 2006, 44, 2565-2568. (18) Gary, H. J.; Handwerk, E. G. Petroleum Refining: Technology and Economics. CRC Press, New York, Marcel Dekker Inc. 2001. (19) Iyuke, S. E.; Simate, G. S. Synthesis of carbon nanomaterials in a swirled floating catalytic chemical vapor deposition reactor for continuous and large scale production. In: Carbon Nanotubes-Growth and Application. Croatia 2011, 35-58. (20) Feng, Y.; Zhang, H.; Hou, Y.; McNicholas, T. P.; Yuan, D.; Yang, S. et al. Room temperature purification of few-walled carbon nanotubes with high yield. ACS Nano 2008, 2, 1634-1638. (21) Hurst, K. E.; Dillon, A. C.; Yang, S.; Lehman, J. H. Purification of single-wall carbon nanotubes as a function of UV wavelength, atmosphere and temperature. J. Phys. Chem. C 2008, 112, 16296-16300. (22) Osswald, S.; Havel, M.; Gogotsi, Y. J. Monitoring oxidation of multi-walled carbon nanotubes by Raman spectroscopy. Raman Spectroscopy 2007, 38, 728-736. (23) Gallego, G.; Mondragon, F.; Tatibouet, J.; Barrault, J.; Batiot-Dupeyrat, C. Carbon dioxide reforming of methane over La2NiO4 as catalyst precursor-characterisation of carbon deposition. Catalysis Today 2008, 135, 200-209. (24) Munster, P.; Grabke, H. J. Kinetics of the steam reforming of methane with iron, nickel and iron-nickel alloys as catalysts. J. Catal. 1981, 72, 279-287. (25) Simate, G. S.; Moothi, K.; Meyyappan, M.; Iyuke, S. E.; Ndlovu, S.; Falcon, R. et al. Kinetic model of carbon nanotube production from carbon dioxide in a floating catalytic chemical vapour deposition reactor. RSC Adv. 2014, 4, 9564-9572. (26) Wert, C. A. Diffusion coefficient of C in alpha-iron. Phys. Rev. Lett. 1950, 79, 601. 23


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27. He, X., Ma, T., Qiu, J., Sun, T., Zhao, Z., Zhou, Y., Zhang, J. Mechanism of coal gasification in a steam medium under arc plasma conditions. Plasma Sources Science and Technology, 2004, 13, 446-453. 28. He, X., Zheng, M., Qiu, J., Zhao, Z., Ma, T. The formation mechanism of CO2 and its conversion in the process of coal gasification under arc plasma conditions. Plasma Sources Science and Technology, 2006, 15,246-252.

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Figure Captions 1. (a) CO and (b) CH4 volume % obtained during coal pyrolysis at different temperatures. 2. Gas quantities predicted by the Gibbs minimization model from 400 to 700oC 3. TEM images of CNTs produced at a temperature of 900oC in the CVD reactor with temperatures of (a) 400oC, (b) 450oC, (c) 500oC and (d) 550oC in the coal pyrolysis reactor. 4. TEM images of CNTs produced at a temperature of 900oC in the CVD reactor with temperatures of (a) 600oC, (b) 650oC and (c) 700oC in the coal pyrolysis reactor. 5. CNT growth rate vs. temperature (circles represent experimental data and solid line according to model i.e. Equation 44).

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Carbon Nanotube Synthesis Using Coal Pyrolysis - Langmuir (ACS

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