Synthesis of nano-structured carbon by microwave cold plasma cracking of methane Mi Tian a , Congxiao Shang a* , Simon Batty b 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: [email protected].
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Carbon Structure Produced by Microwave Cold Plasma Cracking of Natural Gas 12102011
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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
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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