Energy & Fuels 1996,9,704-706
704
Isotope Effects in Plasma Arcing Experiments with Various Carbon Anodes Louis S. K. Pang, Michael A. Wilson,* Robert Pallasser, and Luke Prochazka CSIRO Division of Petroleum Resources, P.O. Box 136, North Ryde, NSW 2113, Australia Received January 23, 1995. Revised Manuscript Received May 4, 1995@
Carbon stable isotope ratios in solid products from plasma arcing brown coal char, brown coal, and graphite electrodes have been measured. The results support the proposal that components of the soot (fullerene black) originate from weak bond cleavage in the anode, rather than that held by the traditional view, namely, that all products originate via CI routes.
Introduction When an arc is created across graphite electrodes in a helium atmosphere, several products are formed including soot containing fullerenes (fullerene black), microfilaments, and a deposit on the cathode, which has the shape of a pencil and contains nanotubes. We have found1that the soot is isotopically heavy compared with the original graphite and the pencil deposit is isotopically light. This result has recently been confirmed by another laboratory.2 Possible explanations for isotopic fractionation must take into account the fact that cationic species appear to play an important part in the growth process by which nanotubes and fullerenes are f ~ r m e d and ~ - ~also that isotopic distribution studies using carbon 13C enriched mixtures of powdered carbon demonstrate that isotopes are scrambled in product fullerenes.6-11 We interpreted isotope fractionation in terms of the lower diffusion time and higher diffusion velocity of 12Cover 13C species of C1 or equivalent species to the cathode because of the higher charge to mass ratio. The slower 13C species will have a greater chance of undergoing recombination reactions with other carbon species. Their products have a smaller charge to mass ratio and may escape the electric field of the plasma arc and form soot or fullerenes. Nevertheless, it is possible that many components of the soot arise directly from the anode. That is, they
* To whom correspondence should be addressed. @Abstractpublished in Advance ACS Abstracts, June 1, 1995. (1)Pallasser, R.; Pang, L. S. K.; Prochazka, L.; Rigby, D.; Wilson, M. A. J.A m . Chem. SOC.1993,115,11634. (2)Thomas, K. M.; Lewis, J. M.; Bottrell, S. H.; Dean, S. P.; Foulkes, J. Carbon 1994,32,991. (3) Bunshah, R. F.; Jou, S.; Prakash, S.; Doerr, H. J.; Issacs, L.; Wehosig, A,; Yeretzian, C.; Cynn, H.; and Diederich, F. J . Phys. Chem. 1992,96,6866. (4) Rubin, Y.; Kahr, M.; Knobler, C. B.; Diederich, F.; Wilkins, C. L. J . Am. Chem. SOC.1991,113,495. (5)McElvany, S. W.; Ross, M. M.; Goroff, N. C.; Diederich, F. Science 1993,259,1594. (6)Hawkins, J . M. Acc. Chem. Res. 1992,25,150. (7) Heath, J . R. In Large Carbon CZusters; Hammond, G. S., Kuck, V. J.. Eds.: ACS Symposium Series: American Chemical Society: Washington, DC, 1993.(8) Meijer, G.; Bethune, D. S. J . Phys. Chem. 1990,93,780. (9) McElvany, S. W.; Ross, M. R.; Callahan, J. H. Acc. Chem. Res. 1992,25,162. (10) Hawkins, J . M.; Meyer, A,; Loren, S.;Nunlist, R. J . Am. Chem. SOC.1992.113. 9394. (11)Ebbesen, J . W.; Tabuchi, J.; Tanagiki, K. Chem. Phys. Lett. 1992,191,336.
represent fragment molecules from the electrode, and
to investigate this possibility in this paper we have measured stable isotope ratios of products from plasma arcing electrodes composed from char from brown coal and brown coal and graphite mixtures. These materials contain weak bonds not present in highly ordered graphite and the bonds may be centers for scission into fragments much larger than C1. Data on graphite is also presented for comparative purposes.
Experimental Section Anodes. The elemental composition of the Loy Yang coal used in these experiments is given e1sewhere,l2 as is the procedure for preparation of the anode electrodes.I2 In short, in experiments with char, Loy Yang coal was mixed with a small amount of binder and packed into a Swagelok type stainless steel tube of size 12 mm i.d. by 125 mm length and sealed. The tube was heated at 500 "C for 20 h to form a rod.I3 This rod was then further carbonized to 1200 "C for 5 h (well below the temperature for graphitisation on this time scale1*). In experiments with Loy Yang coal, a graphite rod was drilled out to form a 3 mm passage.I2 Powdered coal was inserted into the hole and the ends plugged with 2 mm x 3 mm graphite plugs. Arcing Procedure. Arcing procedures have also been described in detail e l ~ e w h e r e . ' ~Dc J ~ current was employed. Graphite was used as the cathode. Details of helium pressure, current, and voltage are given in Table 1. After reaction, the soot was collected from the outer perimeter of the reaction chamber and the microfilaments and pencil deposit were carefully collected from the cathode. Yields of the various products formed are given in Table 1. For stable isotope analysis, products were combusted and stable isotope ratios measured on a Finnigan MAT252 isotope ratio mass spectrometer. Isotope distributions are reported in the d notation relative to the international standard Peedee Belemnite (PDB).Isotope distributions are also listed in Table 1.
Results and Discussion Table 1shows that the different electrodes used have different isotopic values. The coal and graphite mixture (12) Pang, L. S. K.; Prochazka, L.; Quezada, R. A,; Wilson, M. A.; Pallasser, R.; Fisher, K. J.; Fitzgerald, J . D.; Taylor, G. H.; Willett, G. D.; Dance, I. G. Energy Fuels 1995,9, 38. (13) Pang, L. S. K.; Vassallo, A. M.; Wilson, M. A.Energy Fuels 1992, 6, 176-179. (14) Brooks, J. D.; Taylor, G. H. In Chemistry and Physics of Carbon; Walker, P. H. Jr., Ed.; Marcel Dekker: New York, 1968; p 282.
0887-0624/95/2509-0704$09.00/00 1995 American Chemical Society
Plasma Arcing Experiments with Various Carbon Anodes
Energy & Fuels, Vol. 9, No. 4, 1995 705
Table 1. Stable Isotope Experiments on Products from Arcing Carbon Anodes Prepared from Different Materials _ _ _ _ ~
1
2
anode material reaction conditions
Loy Yang char' Loy Yang char
current (A) voltage (VI helium pressure (kPa) time (h) isotope proportions anode 613c%G(i,) soot 613CCC shell 6l3C%0 filaments 6l3C%G core 613C%G yields fractional yield soot fractional yield shell fractional yield filaments fractional yield core fractional yield l3C soot 6l3C%G fractional yield l3C shell 613c%G fractional yield filaments 6 W % 0 fractional yield core 6l3C%G total 6l3C%Gproducts (&$pip) 613C%~ anode - 6% %G products (ia -
32 20 125 2.3
80 20 125 1.2
-10.9 f 2 -10.0 f .2 -11.5 f .2 -11.1 f .2 -13.2 f .2
-10.7 -11.4 -11.2 -12.1 -13.5
0.17 0.42 0.16 0.26 -1.67 -4.83 -1.78 -3.43 -11.7 0.8
0.15 0.59 0.07 0.19 -1.71 -6.61 -0.85 -2.57 -11.7 1.0
a
f2 f .2 f .2 f .2 f .2
3
5
4
6
Loy Yang char graphitel Loy Yang coal 60 55 25 20 133 140 0.7 1.7
graphitel graphite" Loy Yang coal 80 65 22 20 145 135 1.0 1.0
-10.9 -10.9 -12.5 -12.7 -13.6
-29.2 -13.9 -16.5 -16.0 -16.8
0.48 0.36 0.04 0.12 -5.23 -4.50 -0.51 -1.63 -11.9 1.0
i2 f .2 f .2 f .2 f .2
-29.2 -13.8 -17.2 -15.3 -16.8
f .2 f .2 f .2 f .2 f .2
0.33 0.41 0.02 0.24 -4.56 -7.05 -0.31 -4.03 -15.95 -13.3
f .2 f .2 f .2 f .2 f .2
0.33 0.45 0.02 0.19 -4.59 -7.56 -0.32 -3.14 -15.61 -13.6
-29.3 -27.8 -30.5 -29.9 -31.7
f .2 f .2 & .2 5 .2 f .2
0.42 0.44 0.01 0.12 -11.7 -13.4 -0.30 -3.80 -29.2 -0.1
From ref 1.
has a value (613C -29.2%0)close to that of graphite since the value for coal (613C -25%0) is similar t o that of graphite (613C -29.3%0). However, the value for char and hence the char anode is much higher (-10.9%0) because during char formation light material escapes as gas due to the preferential breakage of 12C-12Cbonds over 12C-13C bonds. Table 1 illustrates results obtained with Loy Yang char, Loy Yang coal, and graphite. The products obtained have been divided into four groups, namely, soot (containing fullerenes), microfilaments (which grow on the side of the cathode pencil and have micron rather than nanotube morphology),15 and pencil which has been further subdivided into core and shell since it is known that the shell from arcing graphite electrodes is mainly pyrolytic carbon and contains few nanotubes.16 Table 1 also shows that like the graphite experiment, the pencil (shell plus core) is the lightest product from brown codgraphite mixture and char arc pyrolysis. The isotopic distributions between core and shell differ in the graphite experiment, and similarly, in the char experiment the core is lighter than the shell. However, with Loy Yang coaygraphite mixture the shell is almost isotopically indistinguishable from the core. Elsewhere12 we have shown that the amount of pyrolytic carbon in core is promoted by gases such as hydrogen and this pyrolytic carbon is similar in isotopic composition t o the anode electrode. The coaygraphite electrode will contain more hydrogen than the other electrodes. Hence the similar isotope distributions between core and shell probably reflects the greater concentration of hydrogen in the coal experiment compared with the char and graphite experiments, and this hydrogen is involved in promoting pyrolytic carbon formation. The difference in isotopic distribution between anode and core is of the order of +2.3%0(Table 1, experiment 1) for the char experiment. This is not unexpected in ~~
~
(15) Fitzgerald, J. D.; Brunckhorst, L.; Taylor, G. H.; Pang, L. S. K.; Wilson, M. A. Carbon 1993, 31, 234. (16)Taylor, G. H.; Fitzgerald, J. Pang, L. S. K.; Wilson, M. A. J. Crystal Growth 1994, 135, 157.
view of the difference observed for the graphite experiment (+2.4%0).However, for the coaygraphite mixture experiment this difference is large (-12.4%0,Table 1, experiment 4). This is definitive evidence that core carbon, and hence nanotube carbon, can arise from coal in the coaygraphite experiments and not just graphite. Moreover, since the core is much heavier than the core obtained from graphite above (experiment 61, during the process the coal must become isotopically heavy during pyrolysis. Elsewhere12we have shown that isotopically light methane is generated from coal. Since the core is isotopically heavy with respect t o the anode, the core is generated from the coal and not from the methane. Furthermore, methane must be liberated before the coal reaches arcing temperature, -2000 "C, because at this temperature 12C-12C and 13C-12C bonds would be broken at almost similar rates, and no isotopic fractionation would occur. The model is therefore one of slow pyrolysis of the coal yielding methane and char followed by rapid arc pyrolysis. This result is supported by isotope mass balance calculations. We have reported that the isotope ratio for the graphite anode (ia) is equal to the sum of the isotope ratios of the products (i,) (soot, microfilaments plus core plus shell) corrected for fractional yield cfp). That is
i , = 5 p i p or ia - 5 p i p= o P
P
Table 1also shows that t o a first approximation, this is also true for the char experiments since
i, - 5 p i p5 1 P
However, for the coal experiments there is a discrepancy of around - 13.5%0because isotopically light methane is produced. A number of groups12J7-22including ours have shown that if reactive gases, e.g., hydrogen, methane, and (17)Wilson, M. A,; Pang, L. S. K.; Quezada, R. A,; Fisher, K. J.; Dance, I. G. Carbon 1993,31, 393.
706 Energy & Fuels, Vol. 9, No. 4, 1995
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chlorine, are added to helium, polynuclear aromatics or polyynes are formed in the soot and the types and amounts of these depend on the starting material. This is evidence that components of the soot and possibly fullerenes do not all arise through a pathway from C1 units. Moreover, some papers demonstrate the formaand these reactions tion of fullerenes without may also proceed via paths not involving C1. The difference in isotopic distributions between graphite anode and soot observed here is -1.5%0, but the difference between soot and char anode is much smaller when similar arcing conditions of current, voltage, and pressure are used (compare experiments 3 and 6, Table 1) and the applied current appears t o affect this fractionation. This is strong evidence that much of the soot from the char and coal experiments is directly derived from anode and not solely from C1 units. The larger
isotopic difference between anode and soot observed for graphite is due to the absence of weak bonds which ensures that more of the soot forming process goes via C1 or equivalent species. Finally, it should be made clear that the soot formed in these experiments is not conventional soot. Whether it is formed from coal, graphite, or char, it contains some unusual products not present in conventional soot such as b u c k y c ~ n d o m and s ~ ~has ~ ~unique ~ r e a ~ t i v i t y . ~For ~-~l example, when heated under carbon dioxide the soot converts to a material with unusual pore properties which is highly adsorbent t o methane.30 Correct thermal processing can even produce more f ~ l l e r e n e s ~ * , ~ ~ and n a n o t ~ b e s .Thus ~ ~ the arcing process is unique and quite different from conventional thermal pyrolysis of coal.
(18)Heath, R. J.; Zhang, Q.; OBrien, S. C.; Curl, R. L.; Kroto, H. W.; Smalley, R. E. J . Am. Chem. SOC.1987,109,359. (19)Rohlfung, E.A. J . Chem. Phys. 1990,93,285. (20)Broyer, M.;Goeres, A,; Pellarin, M.; Sedimayr, E.; Vialle, J. L.; Woste, L. Chem. Phys. Lett. 1992,198,128. (21)Grosser, T.; Hirch, A. Angew. Chem., Int. Ed. Engl. 1993,32, 1340. (22)Chang, T. M.;Naim, A,; Ahmed, S. N.; Goodloe, G.; Sherlin, P. B. J . A m . Chem. SOC.1992,114,7603. (23)Taylor, R.; Hare, J. P.; Abdul-Sada, A. K.; Kroto, H. W. J.Chem. SOC.,Chem. Commun. 1990,1423. (24)von Helden. G.: Gotts. N. G.: Browers. M. J. J . Am. Chem. SOC. 1993.115.4363. (25)Taylor, R.;Langley, G. J.; Kroto, H. W.; Walton, D. R. M. Nature 1993,366,728.
EF950015W (26)Pang, L. S. K.; Wilson, M. A,; Taylor, G. H.; Fitzgerald, J.; Brunckhorst, L. Carbon 1992,30,1130. (27)Wilson, M. A.;Pang, L. S. K.; Quezada, R.; Prochazka, L.; Pallasser, R.; Rigby, D.; Taylor, G. H.; Fitzgerald, J. D. Proc. 6th Aust. Coal Sci. Conf. Aust. Inst. Energy, Newcastle, Aust. 1994,463-469. (28)Hopwood, F.; Fisher, K. J.; Dance, I. G.; Willett, G. D.; Pang, L. S. K.; Wilson, M. A. Org. Mass Spectrom. 1992,27,1006. (29)Ugarte, D.Carbon 1994,32,1245. (30)Tsang, S.C.; Harris, P. J. F.; Claridge, J. B.; Green, M. L. H. J. Chem. SOC.,Chem. Commun. 1993,1519. (31)Man, N.;Nagano, Y.; Kiyobayashi, T.; Sakiyama, M. J . Phys. Chem. 1995,99,2254.
