Synthesis of nano-structured carbon by microwave cold plasma cracking of methane

Mi Tiana, Congxiao Shanga*, Simon Battyb

a School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK

b EnPlas Ltd, Dev Farm, University of East Anglia, Norwich, NR4 7TJ, UK

* Corresponding Author: Tel: + 44-1603-593123; Fax: +44-1603-591327. E-mail: c.shang@uea.ac.uk.

Abstract

One of the attractive methods for production of hydrogen and carbon is reforming of

hydrocarbons using a high energy plasma source. In this work, nano-structured carbon

was produced by cracking the chemical bonds of methane to form carbon and hydrogen

in a relative low-energy cold plasma reactor. The carbon produced was collected at

different positions in the reactor. The surface area and microstructure of the materials are

characterized by BET method (Brunauer, Emmett and Teller, 1938), X-ray diffraction

(XRD) and high resolution transmission electron microscopy (HRTEM). The flow rate,

temperature and power were recorded to identify the effect of various parameters on

carbon structure formation and changes. The results showed that temperature differences

at the same sample collection position with and without methane were an important

factor with regards of characteristics of surface area and pore volume. The highest

surface area achieved from this study was 111 m2/g at the collecting position of 17 cm

from the plasma source with the power of 1500W. The HRTEM shows the mixture of

amorphous structures and highly crystalline graphite sheets.

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Table of Contents

Abstract................................................................................................................................2

1 Introduction..................................................................................................................4

2 Experimental methods..................................................................................................6

3 Results and discussion..................................................................................................8

3.1 XRD results of plasma carbon..............................................................................8

3.2 TEM of plasma carbon..........................................................................................9

3.3 BET surface area and Porosity............................................................................11

4 Conclusion..................................................................................................................16

Acknowledgements............................................................................................................17

References..........................................................................................................................18

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1 Introduction

Porous carbon materials, such as activated carbons, carbon black and carbon nanotubes,

have been widely used for gas separation, water purification, catalyst supports, and

electrodes for batteries and fuel cells[1, 2]. This is due to their stable physicochemical

properties, good conductivity, low cost, and high surface area or porosity [3]. Recently,

porous graphitic carbons with high crystalline structure and large surface area were used

as catalyst supports and have attracted extensive attention. Porous graphitic carbons are

usually prepared by two methods: 1) utilising a conventional method requiring high

temperature (>2500 °C) to form carbons with well-developed graphitic order[4]; 2)

carbon containing graphitic structures can be also prepared at a relatively low

temperature (<1000 °C) by means of heterogeneous graphitization with the aid of

catalysts e.g. Fe, Co, Ni, etc. [2, 5]. However, although such materials exhibit graphitic

structure, they have low surface areas[6]. In addition, the synthetic procedures employed

to produce these materials are quite complicated: the operational temperatures are

between 500 and 1800 °C; the corresponding aerogels used in the process need to be

prepared by the sol−gel method from polymerization of resorcinol with formaldehyde[7].

Recently the development of high energy plasma systems has become an attractive

method for reforming hydrocarbons into hydrogen and carbon. This is a single-stage and

non-catalytic reaction, Cn Hm → nC+ m2

H2 [8-11]. The plasma contains reactive radicals,

ions and high-energetic electrons. It is reported that plasma reforming overcomes many

limitations of conventional techniques in terms of cost and deterioration of the catalysts,

slow reaction rate, and restrictions on hydrogen production from heavy hydrocarbons

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[12-14]. Microwave plasma systems operating at atmospheric pressure seem to have a

high potential for the production of hydrogen and carbon materials[8]. This plasma

technique providing an electron temperatures of 4000 – 10000 K, and the heavy particle

temperatures of 2000 – 6000 K[10]. Such properties of plasma are the appropriate

conditions for methane reforming into carbon and hydrogen. The different plasma

technologies can be generally distinguished into two main categories: thermal plasma and

non-thermal plasma. Thermal plasma technologies are characterized by the

thermodynamic equilibrium. For non-thermal plasma, the different species are not at

thermodynamic equilibrium. The temperature of heavy particles (neutrals, ions) can be

much lower than those of electrons [15]. In non-thermal plasma, electrons have very high

temperatures on the order of 104-105 K, but the overall gas temperature can remain as low

as a room temperature[16]. The highly energised electrons fill the system, and collide

with other particles (molecules, ions, etc.). The energy transferred from electrons to these

particles retains potential energy instead of kinetic energy. Therefore, instead of wasting

energy to increase the gas temperature (kinetic movement of the gas particles), the energy

is more efficiently and specifically utilized in chemical processes, such as ionization and

bond dissociation[17].

In this work, a non-thermal microwave plasma process is presented to synthesis nano-

structured carbon materials. The relationship between carbon collection position and

surface area was studied. The effect of the other production conditions, e.g. temperature

and power source on surface area was clarified. X-ray diffraction (XRD), high resolution

transmission electron microscopy (HRTEM) and BET method (Brunauer, Emmett and

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Teller, 1938) were used to characterize the microstructure of the produced carbon

materials.

2 Experimental methods

The experimental apparatus for the microwave plasma system is schematically shown in

Figure 1. An electrodeless atmospheric microwave plasma torch system was assembled

using a commercially available magnetron (2) with a maximum power of 6 kW produced

by the power supply (1). The microwave radiation passed through a three stub tuner (3),

and was then fed into a waveguide (4), which is connected laterally to a cavity. The other

end of the waveguide is terminated with an adjustable plunger (6) used for impedance

matching. A quartz tube (9) with the diameter of 3.4 cm connected with a sample

collector (10) at the end was inserted into the nozzle (5) and intersected with the

waveguide.

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Figure 1 Sketch of the plasma reactor

The flow rates of CH4 or natural gas and N2, supplied from compressed gas cylinders (8),

were adjusted using a mass flow controller (7). The mixture of CH4 and N2 was fed into

the discharge zone by the nozzle. Tangential injection of the working gas enabled good

mixing, plasma stabilisation, and helped avoid quartz burning. The applied microwave

power was set up from 1.2 kW to 2.0 kW; N2 was the buffer gas. Carbon products were

collected by a sample collector (10) at the end of different length of quartz tubes of 15,

17, 22, 25, 35, 45 and 55cm, named as POS-15, POS-17, POS-22, POS-25, POS-35,

POS-45, and POS-55 respectively. Temperatures were detected at the collecting positions

at the end of the quartz tube with and without methane, which were termed as reaction

temperature and buffer gas temperature respectively.

The produced powder sample was dispersed in acetone and cleaned with an ultrasonator

to remove organic impurities from carbon particles, filtered, and then dried in vacuum

oven at 160 °C for 2 h.

The morphology of the powder mixtures was characterized by a High Resolution

Transmission electron microscope (HRTEM), TECNAI, FEI Company. The particle sizes

of the powder mixtures were calculated, using the Image Tool v.3.00 software, from the

TEM images as the equivalent circle diameter,ECD=12( 4 A

π), where A represents the

projected particle area. X-ray diffraction (XRD) was performed using an XTra

manufactured by Thermo ARL (US) diffractometer with Cu Kα radiation.

The BET surface area and porosity of each sample were determined from N2 adsorption

at 77 K using AUTOSORB-1 produced by Quantachrome Instruments and TRISTAR II

3020 Instrument. The cleaned carbon samples were degassed for 8 h at 573 K to remove

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any moisture or adsorbed contaminants that may be presented on their surfaces. The

manufacturer’s software can provide BET surface area of the carbons by applying the

BET equation to the adsorption data. The pore size distribution were evaluated by density

functional theory (DFT) method[18, 19].

3 Results and discussion3.1 XRD of plasma carbon

Figure 2 XRD patterns of plasma carbon samples

The XRD patterns of these plasma carbon samples at different sample collection

positions are shown in Figure 2. The XRD peaks at around 2θ = 26°and 43° are assigned

to the (002) and (100) diffractions of the graphitic framework, respectively[1], indicating

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the formation of the graphitic structures by the plasma system. Moreover, it can be seen

that the (002) peaks of all samples have a very low intensity that is superimposed over a

broad profile. This suggests the certain amount of amorphous carbon or low level of

crystallinity.

3.2 TEM of plasma carbon

The TEM micrographs obtained from the same plasma carbon sample are shown in

Figure 3. Both Figure 3b and Figure 3c are the magnified images of Figure 3a at different

areas of B and C. From the HRTEM image in Figure 3a, the fine, spherical particles can

be observed in nanometer size of about 40.75±8.72 nm in diameter. And the thin graphite

layers also exist in the same image. Figure 3b represents the microstructure of area B,

showing carbon nanocoils consisting of graphite sheets. It reveals that the lattice spacing

of two adjacent graphite sheets is ~ 0.34 nm, which is consistent with the separation of

(002) planes of hexagonal graphite[20]. Figure 3c shows the magnified image of

spherical particle in Figure 3a, which presents a certain level of crystallinity from nearly

amorphous carbon structure to being very crystalline, in agreement with the XRD results.

Both Figure 3b and Figure 3c indicate that there are large amount of defects appeared in

the graphite structure.

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0.34 nm

Figure 3 a) TEM image of a plasma carbon sample, b) HRTEM image of area B from Figure 3a, c) HRTEM image of area C from Figure 3a

* Corresponding Author: Tel: + 44-1603-593123; Fax: +44-1603-591327. E-mail: c.shang@uea.ac.uk.

3.3 BET surface area and Porosity

The BET specific surface area and porosity of the carbon powders were determined from

the nitrogen adsorption/desorption isotherm (Figure 4). The N2 isotherms shown in Figure

4 exhibits type III characteristics according to IUPAC classification of adsorption

isotherms, which is characteristic of weak adsorbate-adsorbent interactions[21] and is

most commonly associated with the porosity from non-pore to macro-pore. The weak

interactions between the adsorbate and the adsorbent lead to low uptakes at low relative

pressures. However, the adsorbate-adsorbent interaction becomes much stronger at higher

relative pressure, which resulting in accelerated uptakes [22]. Figure 5 shows the pore

size distribution of plasma carbon collected from different sample collection position

evaluated by density function theory (DFT). The samples had a wide pore size

distribution, which is mainly consisted of mesopores and macropores. The pore

distribution in the wide range of up to 240 nm is presented because the carbon structures

consist of disordered carbon and graphitic carbon layers. Due to the existence of the

nanosized graphitic sheets (part B in Figure 3), the pore size distribution is partially made

up of stacked graphitic sheets. The two dominant peaks at 17 nm and 37 nm are observed

in all the curves. The sample collected at 17 cm has maximum surface area of 111 m 2/g

and much higher pore volume than others, which means the sample collected at this point

has better porosity corresponding to higher surface area. The BET surface area, pore

volumes and pore sizes are summarized in table 1.

* Corresponding Author: Tel: + 44-1603-593123; Fax: +44-1603-591327. E-mail: c.shang@uea.ac.uk.

Figure 4 N2 isotherms at 77K of plasma carbon collected from the different length of the quartz tube, black for adsorption and red for desorption.

Figure 5 Pore-size distributions calculated from N2 adsorption isotherms given in Figure 4, using the DFT model

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Table 1 Physical properties of plasma carbons

Sample SBET (m2/g) Vp (cm3/g)a Pore Size (nm)b

POS-15 74 0.1245 6.7 POS-17 111.74 0.2055 7.4 POS-22 94.7 0.1786 7.5 POS-25 83.9 0.1953 9.3 POS-35 74.5 0.1696 9.1 POS-45 75.7 0.1448 7.6 POS-55 76.8 0.1422 7.4

a Single point adsorption total pore volume of pores less than 50 nm width at p/p0 =0.96. b Adsorption average pore width (4V/A by BET).

Figure 6 Surface area and temperature change vs. power source at 22cm distance from power source with a gas mixture of 12 L/min N2 and 0.75 L/min CH4

Figure 6 shows the relationship between power, surface area and temperature difference

between the buffer gas temperature and reaction temperature with a gas mixture of 12

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L/min of N2 and 0.75 L/min of CH4 at a collecting position of 22 cm. The buffer gas

temperature was measured without introducing methane. The reaction temperature was

then measured when CH4 was introduced into the plasma. It is shown that the surface area

increases with the increase of the power from 1200 W to 2000 W, which is coincident

with the trend of temperature differences. The temperature difference between buffer gas

temperature and reaction temperature is around 400 to 500 °C. The black curve shows the

relationship between power and temperature difference. From this curve, the temperature

difference increases as the power increases. It is also interesting to note that the highest

surface area of 125 m2/g is produced by 2000 W power, which gives the maximum

difference of reaction temperature and buffer gas temperature, 496 °C. It could be

concluded that the BET surface area increases with an increase of temperature

differences.

The BET surface area of the sample collected at different distance from the plasma

source at 1500 W with a gas mixture 12 L/min of N2 and 0.75 L/min of CH4 are shown in

Figure 7. It is indicated that the sample collecting at about 17 cm from plasma source has

highest surface area of 111 m2/g, which is associated with highest temperature difference

between buffer gas temperature and reaction temperature. The sample collected at greater

distance from the plasma source has lower BET surface area. The BET surface area

shows the same trend with the temperature difference that is in good coincidence with the

results in Figure 6. Thus temperature difference between reaction temperature and buffer

gas temperature is an important factor that influences the surface area. Moreover the

result could be explained by thermal energy which is generated and measured by heat of

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any kind[23]. The thermal energy input is the amount by which the thermal energy

changes, ΔEt[23].

Δ E t=m∙ C ∙ ΔT Equation 1

m is mass, C is specific heat capacity and ΔT is the change in temperature during the

energy-input process. According to the Equation 1, the thermal energy input increases

with the increase of temperature differences since m and C are constants of corresponding

substance. The thermal energy change ΔE involved in the reaction is the result of

cracking the C-H bond and reforming the new bonds which influence the carbon

structures and surface area.

Figure 7 BET surface area and temperature vs. sample collection position with the power of 1500 W and a gas mixture of 12 L/min N2 and 0.75 L/min of CH4

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CH 4+75.6 kJ /mol →C(s)+2 H 2 Equation 2

The cracking of methane to form carbon and hydrogen is overall an endothermic reaction

with a reaction enthalpy of 75.6 kJ/mol [24]. The breaking of bonds is an endothermic

process and the formation of bonds is an exothermic process. C-H bonds are broken into

ions/radicals and electrons as methane passes though the electromagnetic field where the

plasma is produced. When these species pass out of the plasma zone, they are no longer

excited and start to recombine in what is called the afterglow region [25]. One observable

effect is a decrease in temperature when CH4 is introduced into plasma due to the

endothermic nature of the process. Then the carbon starts bonding with each other in the

afterglow region along the quarts tube, which releases certain amount of thermal energy.

It should be noticed, that along the quarts tube from the plasma source, high-energy

charged particle/atoms/molecules still exist and gradually reduce with the distance from

the plasma. The existence of high-energy charged particle/atoms/molecules causes bonds

recombination and re-breaking till the system energy drops down to minimum potential

energy. The endothermic and exothermic reactions coexist at the same time. Thus the

temperature difference is becoming smaller with the increase of distance from the plasma.

4 Conclusion

A synthetic method for successfully obtaining carbon materials is illustrated. The

methodology employed to obtain these carbon powders is based on the cold plasma

reactor. TEM images reveal that the formed carbon structures involve various degrees of

amorphous phase, crystallinity and graphite sheets. The sample collected at 17 cm

distance from the plasma source has maximum surface area of 111 m2/g and much higher

pore volume than others, which means the sample collected at 17 cm has better porosity.

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The surface area increases with the increase of temperature differences between reaction

temperature and buffer gas temperature.

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Acknowledgements

The research was supported by Council of Norway through the project ‘Gassmaks’. We

gratefully acknowledge the financial support of the Research Council of Norway and the

technical assistance of GasPLas team.

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