The Journal of
Physical Chemistry
0 Copyright 1994 by the American Chemical Society
VOLUME 98, NUMBER 15, APRIL 14,1994
LETTERS Oxidative Stability of Fullerenes Joseph C. Scanlon,t James M. Brown,t and Lawrence B. Ebert'a Exxon Corporate Research Laboratory, Annandale, New Jersey 08801, and School of Law, University of Chicago, Chicago, Illinois 60637 Received: November 29, 1993; In Final Form: January 25, 1994'
Mixtures of fullerenes are more susceptible to oxidation than the pure phases of fullerenes. For example, a mixture of 90 wt % Cao/10 wt 5% C ~ reacts O with air at 300 OC for 24 h to yield a product containing 95% of the initial mass but which manifests an atomic ratio of carbon to oxygen of 2.3; in contrast, there is I1 wt % loss and no detectible oxygen content in 99.5%C ~ under O the same conditions. Furthermore, toluene-extracted fullerene soot prepared via the Krltschmer synthesis is found to be reactive to oxygen at ambient and higher temperatures. Room temperature handling of a toluene-extracted soot prepared from graphite leads to a steady-state oxygen content of about 3.8 wt %and a C/O atomic ratio of 33. Moreover, the reaction of extracted fullerene soot with air for 24 h at 300 O C yields a product of 60% of the initial mass of soot which contains 16.70 wt % oxygen and which shows a strong photoacoustically detected carbonyl infrared band at 1771 cm-1 and a C,,2-O band at 1249 cm-l. With respect to changes in structure on heating in air, there is evidence for buckminsterfullerene (C60) enhancing the growth of crystallite size measured along the (002), (loo), and (1 10) directions of the soot relative to the case icwhich there is no buckminsterfullerene in the soot.
Introduction With the discovery of a synthetic pathway leading to large amounts of buckminsterfullerene (CSO),' research in the area of novel carbon structures has been intense.293 There has been some discussion of the stability to oxygen of such structures, both in the context of C6o itself4qsand of the toluene-insoluble component of fullerene soot.6 We report herein that 90/10 wt % CK,/C,O mixtures are less stable to oxidative attack than is 99.5%pure c60. Furthermore, we find that the toluene-insoluble component of fullerene soot prepared by the Kratschmer synthesis is unstable with respect to oxygen at ambient, and higher, temperatures. Finally, we report that the structure of the solid product formed by heating toluene-extracted fullerene soot in air depends upon whether or not c 6 0 is present, with the presence of CK,fostering f
Exxon Corporate Reserach Laboratory.
t University of Chicago. Current mailing address: Pennie & Edmonds, 1155 Avenue of the Americas, New York, N Y 10036. e Abstract published in Aduance ACS Absrructs, April 1, 1994.
0022-3654/94/2098-3921$04.50/0
crystallite sizes of the order of 60 nm, about 4 times the sizes obtained in the absence of Cm.
Experimental Details Pure CK,(99.5%) and a 90/10 wt % CK,/C,Omixture were obtained from Strem and showed the X-ray diffraction pattern Of C60. Toluene-extractedfullerenesoot was obtained from Strem. No buckminsterfullerene could be detected in the soot by X-ray diffraction, 13C nuclear magnetic resonance spectroscopy, or photoacoustic infrared spectroscopy. Analysis by Galbraith Laboratories of the soot material from the container, with no pretreatment, showed 3.92 wt % oxygen, 93.80 wt % carbon, and less than 0.5 wt % hydrogen, for an atomic C/O ratio of 31.9. The amount of oxygen which could be tied up as water with 0.5 wt % hydrogen (4.96 mmol/g) is 3.97 wt % (2.48 mmol/g). With vacuum drying of the soot at 100 OC for 24 h immediately before analysis, the respective numbers are 3.818, 94.23%. and less than OS%, suggesting that the presence of water in the fullerene soot is not a significant contributor to the oxygen content, in 0 1994 American Chemical Society
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The Journal of Physical Chemistry, Vol. 98, No. IS, 1994
TABLE 1: Mass Loss and Microanalysis of Heated Fullerenes temp time mass weight % weight % species ("C) (h) loss(%) 0 C soot
soot Soot Soot Cm-90% c~-90% cm-99.5% Cm-99.5% Cm-90% Cm-99.5% Soot C&3-99.5% C&3-99.5%
115 115 300 300 300 300 300 300 350 350 400 400 600
24 28 24 24 24 22 22 24 24 22 22 21 2
3 42
5 1 1 0.8 95 10 80 94 100
5.99 5.5 16.70 15.1 35.03 33.38 263 14.8
contrast to the situation with combustion soot, for which such vacuum drying under identical conditions produces changes in the microanalysis results.7 X-ray diffraction with Cu Ka radiation and a graphite monochromator utilized a Rigaku unit, and nuclear magnetic resonance employed a Bruker MSL operating at a carbon observation frequency of 90 MHz.
Results Heating fullerene materials in air produces both a mass loss and an increase in oxygen content. Pertinent results are summarized in Table 1. All samples show less than 0.5 wt % H except the 90% Cm/lO% C70 residues at 300 OC, which showed 1.64 wt % H (24 h) and 0.69 wt % H (22 h), and one 300 "C residue (that with 15.1%0),which showed 0.54 wt % H. Blank entries indicate that the measurement was not made. The results on the 300 OC residue of the 90% Cm/lO% C ~ O mixture show little mass loss to volatile species as CO and C 0 2 yet considerable oxygen content in the residue. A solid-state species approaching CmO30 is consistent with the data, and this possibility will be explored elsewhere. Our focus here emphasizes the oxidative instability of Cw at 300 OC. Photoacoustic infrared spectroscopy performed on the heated products suggests the presence of some carbonyl functionality. Although the initial fullerenesoot gave a spectrum of poor signalto-noise, the 300 OC soot residue showed strong peaks near 1771 cm-1 (indicative of carbonyls) and near 1249 cm-l, suggestive of sp2-hybridized carbon singly bonded to oxygen.* There is no evidence of a peak near 2318 cm-l, as has been reported el~ewhere.~fj The 1600-cm-1 aromatic ring stretching vibration, which was present in the initial soot, is enhanced in intensity in the 300 OC soot residue. The residue of the 99.5% Cm oxidation at 300 OC showed carbonyl bands at 1772and 1749cm-l, a broad C-0 envelope near 1200 cm-1, a C02 peak near 23 18 cm-1, and a CO peak near 2128 cm-l. Additionally, the 99% Cm residue shows peaks at 1426, 1177, 578, and 520 cm-1, associated with pure C60,l and peaks at 1610 and 1539 cm-1. 13Cnuclear magnetic resonance, at 90 MHz without sample spinning, was performed on the initial fullerene soot and the 300 OC thermal residue. The most intensepeakof thestarting material was at 191 6, presumably the in-plane component of an aromatic chemical shift tensor. The 300 OC thermal residueof soot showed not only this peak but also a strong peak at 157 6, which was not visible in the initial material. This is in the region known for acid anhydrides and esters;g for example, the oxygen-bonded carbon of perylene tetracarboxy dianhydride is at 161 Thus, the NMR result supports the inferences obtained from infrared spectroscopy. With a C/O atomic ratio of 6.5, the 300 OC soot thermal residue has a level of oxygen comparable to that of highly oxidized planar molecular species such as anthraquinone (7/ 1) or perylene tetracarboxy dianhydride (4/ 1). This suggests the presence of
Letters a high level of reactive edge sites. To further analyze "molecular" size, we turned to line width analysis of peaks in X-ray diffraction. The only X-ray diffraction peaks observable for the initial tolueneextracted fullerene soot are those associated with lamellar benzenoid lattices, such as that of graphite itself ((002) at 335 pm, (004) at 167 pm, (100) at 213 pm, (110) at 123 pm, (112) at 115 pm). On heating in air, the toluene-extracted soot does not manifest significant changes in either the d value or the crystallite size for the above-noted diffraction peaks. The anticipated decreases in crystallite size because of oxidation are difficult to measure, because some crystallites retain their initial size. Specifically, on heating to 600 OC for 40 min, the crystallite size in the (002) direction goes from 25 to 23 nm, that in the (1 10) direction goes from 14 to 15 nm, and there is no observable change in the (100) direction (approximately constant at 5 nm). However the addition of 5 wt % Cm with heating produces dramatic changes in the crystallitesizes measured in all directions. In the (1 10) direction, on heating at 600 OC for 40 min, the crystallite size increases by a factor of 4 from 15 to 60 nm. Separately, the crystallite size measured along the (100) direction increases by about a factor of 4 to over 20 nm. The crystallite size in the (002) direction increases by a factor of about 1.5 to 39 nm. Because the crystallite size along (1 10) is relatively large in this fullerene soot, we have used the (1 10) diffraction peak line width to screen the interactionof Cm with carbonaceoussubstrates other than fullerene soot. The enhancement in (1 10) crystallite size does not occur for all substrates. Using a poorly ordered benzenoid material (a Flexicoker coke of (1 10) crystallite size less than 2 nm), a calcined delayed coke of (1 10) crystallite size of about 5 nm, and a highly crystalline benzenoid material (SP-1 graphite of (1 10) crystallite size 60 nm), the addition of 5 wt % Cm had no effect on the (1 10) crystallite size of the substrate after placing the mixture under air at 550 "C for 40 min. Additionally, the addition of 5 wt % Cm to a petroleum asphaltene, followed by heating in air at 500 OC for 40 min, had no effect on the (1 10) crystallite size of the petroleum asphaltene. Discussion
Both Cml0and the benzenoid phase of fullerene soot6J1are less stable to oxygen than is graphite. The present investigation shows that the instability to oxygen extends to temperatures lower than 500 OC. Our workisconsistent with previous worksuggesting the oxidative instability of Cm.sJ0912J3These observations are of relevance to the proposal Of C60 in ordinary soot, originating from combustion procesess from turbulent flames formed from complex fuel mixtures at ambient, and higher, pressures. Oxidation of both Cm and fullerene soot below 500 OC can lead to mass loss, presumably as CO and C02. Oxygen is also found in the residual solid. The form of the oxidized species of both fullerene soot and Cm involves carbonyl. This may be as either esters or anhydrides. While the relative symmetry of the carbonyl infrared band might argue for esters, the fact that only one new 13C NMR peak was created on oxidation, and that peak is at a position known for anhydrides, might argue for anhydrides. Furthermore, in the absence of hydrogen, one might envision six-membered-ring anhydrides forming from CO2 attached to adjacent positions of an acene ring structure (separated by 245 Pm). Generally, the oxidation of the fullerene soot can be viewed in terms of planar models for the soot entities, consistent with our previous discussion.11 The increase in intensity in the 1600-cm-l band of the fullerene soot on oxidation arises from a breakdown in symmetry caused by oxidation. For symmetric ring systems such as graphite, the 1600-cm-1 vibration is infrared inactive. The addition of oxygen to the ring breaks the symmetry and results in a significant enhancement in the ring-stretching intensity, as is observed in phenols and phenolic esters. It is
Let tcrs
The Journal of Physical Chemistry, Vol. 98, No. 15, 1994 3923
notable differences. As discussed, combustion soot readily picks tempting tospeculatethat the 1610-cm-l peakobservedinoxidized up water,7 but, based upon microanalysis, fullerenesoot does not. 99.5% Cw arises from a planar aromatic decomposition product. Although fullerene soot is facilely studied by photoacoustic A peak near 1600 cm-1 had previously been observed in the infrared spectroscopy, combustion soot, both as formed and as oxidation of 85% pure Cm14, but that conceivably could have arisen from an initial phase other than Cm. chemicallyderivatized,shows no features in photoacoustic infrared spectroscopy.26 Combustion soot typically has greater than 1 wt The chemistry of c 6 0 in air can be influenced by other carbonaceous phases. Although 99.5% Cm is not readily attacked % hydrogen,26and fullerene soot has less than 0.5 wt % hydrogen, the limit of detection in our reported measurements. In having by air oxygen at 300 OC,90% Cw/lO% C70 is facilely converted to a solid phase with an atomic C/O ratio of approximately 2.3. edges of relatively large benzenoid crystallitesdecorated primarily by oxygen, fullerene soot may manifest thermal chemistry shown This disparity may be suggested by the different fluorination kinetics reported for less pure CmI5 and higher purity C60.l~ neither by graphite nor by combustion soot. The growth of crystallite size of fullerene soot, on exposure to air in the presence of Cm, may involve oxidative attack on c 6 0 References and Notes moderated by groups on the fullerene soot. Although Cm by (1) Krfitschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. itself is relatively kinetically stable to oxidative attack up to 500 Nature 1990, 347, 354. O C , 4 5 both fullerene soot and C ~ lower O this stability. This (2) Krfitschmer, W.; Huffman, D. R. Carbon 1992, 30, 1143. (3) Endo, M.; Kroto, H. W. J. Phys. Chem. 1992, 96, 6941. crystallite growth is significant. First, a given crystallite size can (4) Vassallo, A. M.; Pang, L. S. K.; Cole-Clark, P. A.; Wilson, M. A. be reached at much lower temperatures in the presence of c 6 0 J . Am. Chem. Soc. 1991,113,7821. than in the absence of Cm. Second, there is a preferred direction ( 5 ) Saxby, J. D.; Chatfield, S. P.; Palmisano, A. J.; Vassallo, A. M.; to thegrowth, withtheratioof [crystallitesizein(llO) direction]/ Wilson, M. A.; Pang, L. S. K. J. Phys. Chem. 1992, 96, 17. [crystallite size in (100) direction] attaining values higher than (6) Werner, H.; Herein, D.; Blocker, J.; Henschke, B.; Tegtmeyer, U.; Schedcl-Niedrig, Th.; Keil, M.; Bradshaw, a. M.; SchlOgl, R. Chem. Phys. those found in graphite itself." Lett. 1992, 914, 62. Ultimately, the growth of crystallite size of fullerene soot may (7) Ebcrt,L. B.; Scanlon, J. C.; Garcia, A. R.; Pictroski, C. F.; Gebhard, involve chemistry analogous to that of ladder polymers derived L. A. Carbon, 1990, 28,912. from anhydrides.17 Additionally, such chemistry might explain (8) Ebert, L.B.;Davis, W. H.; Mills,D. R.; Dennerlein,J. D. In Chemistry ofEngine CombustionDeposits; Ebcrt, L. B., Ed.; Plenum Press: New York, why buckytubes have greater apparent oxidative stability than 1985; pp 71-99. do buckyballs, even though buckytubes, but not buckyballs, have (9) Ebert, L. B.; Rose, K. D.; Melchior, M. T. In Chemistry of Engine exposed edge sites.'* If one postulates that the exposed edge of Combustion Deposits; Ebert, L. B., Ed.; Plenum Press: New York 1985; pp 119-144. the buckytube is an acene edge, for which anhydride formation (10) McKee, D. W. Carbon 1991, 29, 1057. is facile, the resultant ladder polymer-like chemistry might lead (11) Scanlon, J. C.; Ebcrt,L. B. J. Phys. Chem. 1993,97, 7138. to buckytube growth under conditions for which buckyballs would (12) Malhotra, R.; Lorcnts, D. C.; Bae, Y. K.; Becker, C. H.; Tse, D. S.; decompose. A simplemolecular model for anhydride functionality Jusinski, L. E.;Wachsman, E. D. ACS Symp. Ser. 1992, 481, 127. on an acene edge is the molecule 3,4,9,lO-perylenetetracarboxylic (13) Ismail, I. M. K.; Rodgers, S. L. Curbon 1992, 30, 229. (14) Chibante,L.P. F.;Pan,C.;Pierson,M.L.;Haufler,R.E.;Heymann, dianhydride (author-supplied CAS registry number 128-69-8). D. Carbon 1993,34, 185. Our results are of relevance to the discussion of the presence (15) Selig, H.; Lifshitz, C.;Peres, T.; Fischer, J. E.; McGhie, A. R.; of significant amounts of c 6 0 in ordinary soot.19920.21 Although Romanow, W. J.; McCauley, J. P., Jr.; Smith 111, A. B. J. Am. Chem. Soc. it is true that Cw is observed in laminar, helium-diluted, low1991, I 13, 5475. pressure flames formed from the combustion of b e n ~ e n e ,c~6 0~ . ~ ~ (16) Kniaz, K.; Fischer, J. E.; Selig, H.; Vaughan, G. E. M.; Romanow, W. J.; Cox, D. M.; Chowdhury, s. K.; McCauley, J. P.; Strongin, R. M.; Smith has not been observed in soot from conventional fuels under 111, A. B. J. Am. Chem. Soc. 1993,115,6060. conventional condition^.^^ A conventional diesel fuel contains (17) Iqbol, Z.; Ivoery, D. M.; Eckhardt, H. Mol. Cryst. Liq. Cryst. 1988, primarily sp3-hybridizedcarbon, such as paraffins. Conventional 158B, 337 and references 1-5 therein. (18) Pang, L. S. K.; Saxby, J. D.; Chatfield, S. P. J. Phys. Chem. 1993, conditions include turbulent flames undiluted by helium at high 97, 6941. pressures. In addition to the issue of oxidative instability of pure Kleppner, D. Physics T d u y 1991 (Dec.), 44, 9. Cmat about 500 OC, our results show that C ~can O be even less Herschbach, D., quoted in Lubkin, G. B. Physics Toduy 1992 (Nov.), stable to oxygen in the presence of other species, such as c 7 0 . This cooperative behavior counsels against the presence of significant Hoffman, R. Scientific American 1993 (Feb.), 268, 66, 72. Baum, Th.; Loffler, S.; Loffler, Ph.; Weilmunster, P.; Homann, K.amounts of Cm in ordinary soot, which is formed from a variety Bunsenges. Phys. Chem. 1992, 96, 841 and references therein. of aromatic species at temperatures in excess of 1300 OC in Gerhardt. Ph.; Homann, K. H. J. Phys. Chem. 1990, 94, 5381. turbulent flames.25 Malhotra, R.; Ross, D. S. J. Phys. Chem. 1991, 95, 4599. Although fullerene soot and ordinary combustion soot share Ebcrt, L. B. Curbon 1993, 31,999. Ebcrt, L. B. Science 1990, 247, 1468. a common name and a common spherical morphology, there are
