- Email: [email protected]
On the identity of soot
912
Letters to the Editor
On the identity of soot (Received 24 May 1990; accepted in revised form 21 June 1990)
Key Words - Soot, buckminsterfullerene,
In the last few years, there has been discussion about non-planar networks of sp2-hybridized carbon, both in the context of molecular species such as (.5)-circulene (“corannulene” [l]) and 9, 18 diphenyltetrabenz [a,c,h,j] anthracene [2] and in the context of carbonaceous solids such as soot [3,4]. Much excitement has attended the proposal of carbon in the form of a truncated icosahedron, containing 60 carbon atoms and dubbed “buckminsterfullerene”, which has been suggested to “shed a totally new and revealing light on several important aspects of carbon’s chemical and physical properties that were quite unsuspected and others that weie not previoisly weli understood” [3]. Buckminsterfullerene. illustrated in Figure 1. has been the subject of several &dculations [S, 6,171,anh has been proposed to be of relevance not only to soot but also to interstellar carbon [3]. While the detailed study of cosmic carbon may be difficult, there is an abundance of soot available for investigation. Although models have been advanced to account for various properties of soot [8,9, lo], the idea that as mundane a material as soot might have an exciting cluster structure is captivating. Furthermore, there are arguments that can be advanced in favor of the cluster/spiralling cluster proposal. The icospiral nucleation process [3], in which the carbon cluster grows somewhat as a chambered nautilus, can geometrically account for the known spherical morphology of soot particles. Additionally, mass spectrometric experiments performed directly on a sooting flame found evidence for C& [ 111.
Fig. 1. Truncated icosahedron (buckminsterfullerene).
potassium
Do other experiments support this interpretation? What do we really know about the structure and the chemistry of soot? Herein, we discuss issues to refine our knowledge of soot. Experiments were done on a previously-described combustion tube soot [12-161, which we believe to be representative of, although not necessarily identical to, other soots. We believe that the results of these experiments are consistent with a conventional “soot as aromatic molecule” model, in which edge chemistry, involving elements other than carbon, plays a significant role in the identity of the soot. Electron microscopy shows particles of spherical morphology of approximate diameters 20 to 30 nm [ 141. X-ray diffraction shows broad peaks at d values typical of (002), (loo), (004), and (110) peaks of stacked benzenoid arrays [12], with linewidths indicative of correlation lengths of order 2 nm. Issues have appeared with respect to interpretation of both “d value” and linewidth. Because of the weak intensity of the soot peaks, we had previously used 1.5 mm deep sample holders, but suggestions have been made that the measured “d values” (351 pm for the (002)) and measured linewidths (ca. 4’ 28 = 0.07 radian for the (002)) might be distorted because of significant penet;ation of x-rays into the low atomic-number elements cca. 92-94 wt% C: 3-6 wt% 01 of soot. To address this, we have perf&med experiients over the region of the (002) peak in 0.5 mm deep sample holders using Brazilian quartz as a standard (which has peaks at 426 and 334 pm which bracket the (002) peak of the soot). As noted in Figure 2, within the limits of error, our “d value” and linewidth for the (002) are as previously reported, and the observation that d(O02) of soot (ca. 350 pm) is higher than the d(O02) of graphite (=335 pm) is correct. Soot is distinct from graphite in xray diffraction. The use of the nomenclature “graphitic” or “graphite-lie” to describe the more refractory portion of soot [ 17, 181 is not optimum. There are some corrections to linewidth which can be made which will slightly increase the inferred value of crystallite size: taking into account the limiting width imposed by the slit of the diffractometer (ca. 0.1’ 28) and considering broadening caused by Kal/Kaz components of the Cu x-radiation, which we estimate to be of order < 0.2” 28 (using an approach similar to that for delayed coke [ 141). However, these corrections do not exceed the f 20% uncertainty intrinsic to the Scherrer equation, and one will not find that inclusion of these corrections alters the crystallite size of soot from ca. 2 nm to the ;r 40 nm characteristic of graphite. In structure, soot is distinct from graphite in d values and crystallite sizes; the small crystallite sizes of soot, in which there is of the order of l-carbon atom in 10 at an edge, may have other implications in diffraction [ 191, and such small sizes suggest that the chemistry of soot could be different from the chemistry of graphite.
913
Letters to the Editor
1
0.5 mm deep holders
Diffraction, Cu. graphite monochromator
426
15
17
19
21
27 TWF
-
TETFI
31 CDEG2RSEES
II
33
3’5
Fig. 2. X-ray diffraction of soot with a Brazilian low quartz standard (top) and soot alone (bottom), both taken in 0.5 mm deep sample holders, with CuKa radiation, graphite monochromator, narrow slits (0.3, 0.3, 0.3, 0.018, O.lY), and a count time of 5 s per 0.02’ interval. The d value of the (002) line of soot is distinct from that of graphite.
In terms of elemental composition, soot is more than simply carbon, with other elements presumably With number 2 diesel decorating edges of crystallites. fuel as the source, our studied soot contained not only carbon but also hydrogen, oxygen, sulfur, and nitrogen. Furthermore, chemical derivativization studies of soot with using reductive alkylation (reduction K+naphthalene(- l)/alkylation with alkyl iodides) showed a multiplicity of carbon and oxygen environments [ 12, 151. Using tH nuclear magnetic resonance, we have found that there are two different kinds of hydrogen in soot. As illustrated in Figure 3, as-received soot shows a composite broad/narrow line (top) from which the narrow line can be removed by repeated helium flow/vacuum pumping (ca. 10 microns Hg) cycles (bottom). The values of Tt are different for the two lines, with the narrow line showing a value of ca. 270
Note: These spectra were with a repeat cycle of 2 s, which causes a relative anenuation of the broad line
Fig. 3. tH nuclear magnetic resonance at 300 MHz of as-received soot (top) and vacuum-dried soot (bottom).
ms, the broad line 2-5 s. The narrow line may be associated with water, and drying the soot at 110°C for 24 hours immediately before microanalysis was found to lower the wt%H from 1.11 to 0.76 in the extreme case. The broad line has a full width at half maximum of 25.879 kHz (=6.1 G), a value that is compatible with the presence of some protons in soot being coordinated as in phenacenes (e.g., the 4 and 5 protons of phenanthrene: the 1 and 12 protons of perylene). One argument advanced in favor of the “soot as aromatic compound” model is the ability of soot to undergo reductive alkylation chemistry, as happens with aromatic molecules and as does not happen with graphite [16]. We have performed an additional experiment under conditions less favorable to the success of reductive alkylation than previously reported [ 121. Soot (0.588 g) was added to a solution of performed K+naphthalene-dg(-I) (0.51 mmol in 10.76 g tetrahydrofuran with excess KO; presence of radical anion verified by ESR) and the solution was kept at room temperature, without stirring. During reaction, aliquots were removed for examination by ESR. This showed increased radical density, which could be removed by reaction with either methyl iodide or methanol, to again yield the ESR pattern of the initial soot [ 161. After 23 hours, excess potassium metal was removed, and a quench was made with 13C enriched (99 atom%) methyl iodide, with reaction proceeding for only 40 minutes. The filtered solid product was examined by 13C NMR at 50 MHz, with cross-polarization and magic angle spinning, and was found to contain methyl groups on carbon at 23 6 (1.6 atom% of final carbon) and methyl groups on oxygen at 57 6 (0.3 atom% of final carbon), corresponding to 1.9 methyl groups added per 100 carbon atoms. The ratio of methylation on carbon to methylation on oxygen is 4.9, comparable to the 4.4 previously reported [ 161, and definitely suggesting that the chemistry of oxygen must be considered in defining a model of soot. In summary, diffraction and chemical experiments suggest that soot can be viewed as a “poorly crystalline” arrangement of aromatic compounds, in which the existence of edges, involving both carbon and oxygen sites which are chemically labile, plays an
914
Letters to the Editor
important role. To more deeply probe the edge sites, isotopic exchange experiments may be of interest [ZO], and analogies to the chemistry of other poorly ordered carbon may be rewarding 1211.
8.
9. 10.
Exxon Research & Engineering Co. Route 22 East, Clinton Township Annandale. NJ 08801
L.B. EBERT 3.C. SCANLON A.R. GARCL4 C.F. PICTROSKI L.A. GEBHARD
REFERENCES 1.
2.
:: 5. 6. 7.
W.E. Barth and R.G. Lawton, J. Am. Chem. Sot., 93, 1730 (1971). R.A. Pascal, W.D. McMillan, D.Van Engen, and R.G. Eason, J. Am. Chem. Sot., 109, 4660
(1987). H. Kroto, Science, 242, 1139 (1988). W.C. Hemdon, in “Polynuclear Aromatic Compounds”, (L.B. Ebert, ed.), American
Chemical Society, Washington, 1988, pp. 1-12. B.A. Hess and L.J. Schaad, J. Org. Chem.. 51, 3902 (1986).
W. Weltner and R.J. Van Zee, Chem. Rev., 89, 1713 (1989). P.W. ‘Fowier, P. Lazzeretti, and R. Zanasi, Chem. Phys. &tr., 165, 79 (1990).
“+
H.B. Palmer and C.F. Cullis, in “Chemisrry and Physics of Carbon” , Vol. 1, Dekker, New York, 1965, pp. 265-325. B.S. Haynes and H.G. Wagner, Frog. Energy Combust. Sci.. 7. 229 (1981). J.B. Donnet, d&bon, iO,26& (1982). P. Gerhardt, S. Loffler, and K.H. Homann, Chem. Phys. K&f., 137, 306 (1987).
12’ 13. 14.
L.B. Ebert, J.C. Scanlon, and C.A. Clausen, Energy & Fuels, 2,438 (1988). M. Frenklach and L.B. Ebert, J. Phys. Chem., 92,561
(1988).
L.B. Ebert, R.V. Kastrup, J.C. Scanion. and R.D. Sherwood, in Nineteenth Biennial
Conference on Carbon, Extended Abstracts and Program, American Carbon Society, 1989,396-
Z‘
17: :;: 2. 21
.
397. J.C. ScanIon and L.B. Ebert, Ibid., pp. 144-145. L.B. Ebert, Science, 247, 1468 (1990). H. Rosen, A.D.A. Hansen, R.L. Dod, and T. Novokov, Science, 208,741 (1980). C. Wong, Carbon., 26.723 (1988). B.K. Nath, G.D. Nigam, and B.K. Samantaray, Indian J. Phys., 49, 841 (1975). N.H. Werstiuk and G. Timmins, Can. J. Chem., 59, 3218 (1981). J,H. Knox, B. Kauer, and G.R. Mi~Iw~, f. Chromat., 352, 3 (1986).
Letters to the Editor
On the identity of soot (Received 24 May 1990; accepted in revised form 21 June 1990)
Key Words - Soot, buckminsterfullerene,
In the last few years, there has been discussion about non-planar networks of sp2-hybridized carbon, both in the context of molecular species such as (.5)-circulene (“corannulene” [l]) and 9, 18 diphenyltetrabenz [a,c,h,j] anthracene [2] and in the context of carbonaceous solids such as soot [3,4]. Much excitement has attended the proposal of carbon in the form of a truncated icosahedron, containing 60 carbon atoms and dubbed “buckminsterfullerene”, which has been suggested to “shed a totally new and revealing light on several important aspects of carbon’s chemical and physical properties that were quite unsuspected and others that weie not previoisly weli understood” [3]. Buckminsterfullerene. illustrated in Figure 1. has been the subject of several &dculations [S, 6,171,anh has been proposed to be of relevance not only to soot but also to interstellar carbon [3]. While the detailed study of cosmic carbon may be difficult, there is an abundance of soot available for investigation. Although models have been advanced to account for various properties of soot [8,9, lo], the idea that as mundane a material as soot might have an exciting cluster structure is captivating. Furthermore, there are arguments that can be advanced in favor of the cluster/spiralling cluster proposal. The icospiral nucleation process [3], in which the carbon cluster grows somewhat as a chambered nautilus, can geometrically account for the known spherical morphology of soot particles. Additionally, mass spectrometric experiments performed directly on a sooting flame found evidence for C& [ 111.
Fig. 1. Truncated icosahedron (buckminsterfullerene).
potassium
Do other experiments support this interpretation? What do we really know about the structure and the chemistry of soot? Herein, we discuss issues to refine our knowledge of soot. Experiments were done on a previously-described combustion tube soot [12-161, which we believe to be representative of, although not necessarily identical to, other soots. We believe that the results of these experiments are consistent with a conventional “soot as aromatic molecule” model, in which edge chemistry, involving elements other than carbon, plays a significant role in the identity of the soot. Electron microscopy shows particles of spherical morphology of approximate diameters 20 to 30 nm [ 141. X-ray diffraction shows broad peaks at d values typical of (002), (loo), (004), and (110) peaks of stacked benzenoid arrays [12], with linewidths indicative of correlation lengths of order 2 nm. Issues have appeared with respect to interpretation of both “d value” and linewidth. Because of the weak intensity of the soot peaks, we had previously used 1.5 mm deep sample holders, but suggestions have been made that the measured “d values” (351 pm for the (002)) and measured linewidths (ca. 4’ 28 = 0.07 radian for the (002)) might be distorted because of significant penet;ation of x-rays into the low atomic-number elements cca. 92-94 wt% C: 3-6 wt% 01 of soot. To address this, we have perf&med experiients over the region of the (002) peak in 0.5 mm deep sample holders using Brazilian quartz as a standard (which has peaks at 426 and 334 pm which bracket the (002) peak of the soot). As noted in Figure 2, within the limits of error, our “d value” and linewidth for the (002) are as previously reported, and the observation that d(O02) of soot (ca. 350 pm) is higher than the d(O02) of graphite (=335 pm) is correct. Soot is distinct from graphite in xray diffraction. The use of the nomenclature “graphitic” or “graphite-lie” to describe the more refractory portion of soot [ 17, 181 is not optimum. There are some corrections to linewidth which can be made which will slightly increase the inferred value of crystallite size: taking into account the limiting width imposed by the slit of the diffractometer (ca. 0.1’ 28) and considering broadening caused by Kal/Kaz components of the Cu x-radiation, which we estimate to be of order < 0.2” 28 (using an approach similar to that for delayed coke [ 141). However, these corrections do not exceed the f 20% uncertainty intrinsic to the Scherrer equation, and one will not find that inclusion of these corrections alters the crystallite size of soot from ca. 2 nm to the ;r 40 nm characteristic of graphite. In structure, soot is distinct from graphite in d values and crystallite sizes; the small crystallite sizes of soot, in which there is of the order of l-carbon atom in 10 at an edge, may have other implications in diffraction [ 191, and such small sizes suggest that the chemistry of soot could be different from the chemistry of graphite.
913
Letters to the Editor
1
0.5 mm deep holders
Diffraction, Cu. graphite monochromator
426
15
17
19
21
27 TWF
-
TETFI
31 CDEG2RSEES
II
33
3’5
Fig. 2. X-ray diffraction of soot with a Brazilian low quartz standard (top) and soot alone (bottom), both taken in 0.5 mm deep sample holders, with CuKa radiation, graphite monochromator, narrow slits (0.3, 0.3, 0.3, 0.018, O.lY), and a count time of 5 s per 0.02’ interval. The d value of the (002) line of soot is distinct from that of graphite.
In terms of elemental composition, soot is more than simply carbon, with other elements presumably With number 2 diesel decorating edges of crystallites. fuel as the source, our studied soot contained not only carbon but also hydrogen, oxygen, sulfur, and nitrogen. Furthermore, chemical derivativization studies of soot with using reductive alkylation (reduction K+naphthalene(- l)/alkylation with alkyl iodides) showed a multiplicity of carbon and oxygen environments [ 12, 151. Using tH nuclear magnetic resonance, we have found that there are two different kinds of hydrogen in soot. As illustrated in Figure 3, as-received soot shows a composite broad/narrow line (top) from which the narrow line can be removed by repeated helium flow/vacuum pumping (ca. 10 microns Hg) cycles (bottom). The values of Tt are different for the two lines, with the narrow line showing a value of ca. 270
Note: These spectra were with a repeat cycle of 2 s, which causes a relative anenuation of the broad line
Fig. 3. tH nuclear magnetic resonance at 300 MHz of as-received soot (top) and vacuum-dried soot (bottom).
ms, the broad line 2-5 s. The narrow line may be associated with water, and drying the soot at 110°C for 24 hours immediately before microanalysis was found to lower the wt%H from 1.11 to 0.76 in the extreme case. The broad line has a full width at half maximum of 25.879 kHz (=6.1 G), a value that is compatible with the presence of some protons in soot being coordinated as in phenacenes (e.g., the 4 and 5 protons of phenanthrene: the 1 and 12 protons of perylene). One argument advanced in favor of the “soot as aromatic compound” model is the ability of soot to undergo reductive alkylation chemistry, as happens with aromatic molecules and as does not happen with graphite [16]. We have performed an additional experiment under conditions less favorable to the success of reductive alkylation than previously reported [ 121. Soot (0.588 g) was added to a solution of performed K+naphthalene-dg(-I) (0.51 mmol in 10.76 g tetrahydrofuran with excess KO; presence of radical anion verified by ESR) and the solution was kept at room temperature, without stirring. During reaction, aliquots were removed for examination by ESR. This showed increased radical density, which could be removed by reaction with either methyl iodide or methanol, to again yield the ESR pattern of the initial soot [ 161. After 23 hours, excess potassium metal was removed, and a quench was made with 13C enriched (99 atom%) methyl iodide, with reaction proceeding for only 40 minutes. The filtered solid product was examined by 13C NMR at 50 MHz, with cross-polarization and magic angle spinning, and was found to contain methyl groups on carbon at 23 6 (1.6 atom% of final carbon) and methyl groups on oxygen at 57 6 (0.3 atom% of final carbon), corresponding to 1.9 methyl groups added per 100 carbon atoms. The ratio of methylation on carbon to methylation on oxygen is 4.9, comparable to the 4.4 previously reported [ 161, and definitely suggesting that the chemistry of oxygen must be considered in defining a model of soot. In summary, diffraction and chemical experiments suggest that soot can be viewed as a “poorly crystalline” arrangement of aromatic compounds, in which the existence of edges, involving both carbon and oxygen sites which are chemically labile, plays an
914
Letters to the Editor
important role. To more deeply probe the edge sites, isotopic exchange experiments may be of interest [ZO], and analogies to the chemistry of other poorly ordered carbon may be rewarding 1211.
8.
9. 10.
Exxon Research & Engineering Co. Route 22 East, Clinton Township Annandale. NJ 08801
L.B. EBERT 3.C. SCANLON A.R. GARCL4 C.F. PICTROSKI L.A. GEBHARD
REFERENCES 1.
2.
:: 5. 6. 7.
W.E. Barth and R.G. Lawton, J. Am. Chem. Sot., 93, 1730 (1971). R.A. Pascal, W.D. McMillan, D.Van Engen, and R.G. Eason, J. Am. Chem. Sot., 109, 4660
(1987). H. Kroto, Science, 242, 1139 (1988). W.C. Hemdon, in “Polynuclear Aromatic Compounds”, (L.B. Ebert, ed.), American
Chemical Society, Washington, 1988, pp. 1-12. B.A. Hess and L.J. Schaad, J. Org. Chem.. 51, 3902 (1986).
W. Weltner and R.J. Van Zee, Chem. Rev., 89, 1713 (1989). P.W. ‘Fowier, P. Lazzeretti, and R. Zanasi, Chem. Phys. &tr., 165, 79 (1990).
“+
H.B. Palmer and C.F. Cullis, in “Chemisrry and Physics of Carbon” , Vol. 1, Dekker, New York, 1965, pp. 265-325. B.S. Haynes and H.G. Wagner, Frog. Energy Combust. Sci.. 7. 229 (1981). J.B. Donnet, d&bon, iO,26& (1982). P. Gerhardt, S. Loffler, and K.H. Homann, Chem. Phys. K&f., 137, 306 (1987).
12’ 13. 14.
L.B. Ebert, J.C. Scanlon, and C.A. Clausen, Energy & Fuels, 2,438 (1988). M. Frenklach and L.B. Ebert, J. Phys. Chem., 92,561
(1988).
L.B. Ebert, R.V. Kastrup, J.C. Scanion. and R.D. Sherwood, in Nineteenth Biennial
Conference on Carbon, Extended Abstracts and Program, American Carbon Society, 1989,396-
Z‘
17: :;: 2. 21
.
397. J.C. ScanIon and L.B. Ebert, Ibid., pp. 144-145. L.B. Ebert, Science, 247, 1468 (1990). H. Rosen, A.D.A. Hansen, R.L. Dod, and T. Novokov, Science, 208,741 (1980). C. Wong, Carbon., 26.723 (1988). B.K. Nath, G.D. Nigam, and B.K. Samantaray, Indian J. Phys., 49, 841 (1975). N.H. Werstiuk and G. Timmins, Can. J. Chem., 59, 3218 (1981). J,H. Knox, B. Kauer, and G.R. Mi~Iw~, f. Chromat., 352, 3 (1986).