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Energy & Fuels 1988,2, 438-445
Combustion Tube Soot from a Diesel Fuel/Air Mixture: Issues in Structure and Reactivity? Lawrence B. Ebert* and Joseph C . Scanlon Exxon Corporate Research Laboratories, Annandale, New Jersey 08801
Chris A. Clausen Department of Chemistry, University of Central Florida, Orlando, Florida 32816 Received August 31, 1987. Revised Manuscript Received January 11, 1988 To address current uncertainties in the molecular structure of soot, we have generated combustion tube soot from number two diesel fuel in a turbulent diffusion flame and we have characterized this soot by X-ray diffraction and by chemical reduction. The appearance of diffraction peaks at 351, 208,174, and 120 pm can be reconciled with a model involving stacked, planar benzenoid carbon arrays; the line widths of the diffraction peaks suggest correlation lengths on the order of 2 nm. This soot can be reduced by naphthalenide(-1) and then alkylated by methyl iodide. The soot does not intercalate KO at 110 “C, in contrast to graphite and calcined petroleum coke. These results are consistent with the traditional model of soot as collections of large polynuclear aromatic hydrocarbons and heterocycles. One does not need to invoke c60 clusters or open spiraling clusters as models for the majority of the carbon of the soot, and X-ray diffraction simulations suggest that such clusters cannot account for the details of the experimentally observed diffraction patterns.
Introduction Carbonaceous particulate matter from combustion-related processes can be both useful (carbon black for tires, phonograph records, and adsorbents) and detrimental (soot from diesel engines and combustors). Such materials have been long studied. The Chinese used lampblack to make ink thousands of years before Christ,’ and today the most advanced techniques are used to probe the nature of s00t.~-~ Surprisingly, the detailed molecular structure of soot is not known. To account for the spherical morphology of particles, older models have proposed that polynuclear aromatic molecules are arranged with their planes tangential to concentric spherical annuli? However, recently Zhang, O’Brien, Heath, Liu, Curl, Kroto, and Smalley (abbreviated herein as ZOHLCKS) have related soot to three-dimensional carbon clusters, suggesting that because of dehydrogenation reactions “the polycyclic aromatic molecules known to be present in high concentrations in sooting flames may therefore adopt pentagonal rings as they grow, so as to generate structures which maximize the number of C-C linkagesan6 While ZOHLCKS did not propose that soot would contain pure c 6 0 clusters (“buckminsterfullerene”), they suggested “the result of such a process would be a soot nucleus consisting of concentric, but slightly imperfect spheres.”6 There are a variety of papers giving experimental data on soot. The review “The Formation of Carbon from Gases” by Palmer and Cullis’ covers work through the early 1960s and “Soot Formation” by Haynes and Wagner6 covers through 1980. The latter points out that much information on soot structure comes from investigation of materials from carbon black processes, which are “not the same as combustion processes, but sufficiently closely related to be relevant in some cases.”6 Medalia and Riving compared a variety of soots and carbon blacks. Diesel soot was referred to as aciniform (defined as “clustered like ‘Presented a t the Symposium on Advances in Soot Chemistry, 194th National Meeting of the American Chemical Society, New Orleans, LA, August 30-September 4, 1987. 0887-0624 I88 12502-0438SO1.50 I O
grapes”), and a photograph showed spheroidal particles fused in clusters of size ca. 0.2 pm. Ban and HesslO used high-resolution (lattice image) phase contrast microscopy to obtain photographs of carbon blacks, in which one can see benzenoid crystallites tangential to the radii of the spherical “grapes”. Work such as this is the basis of the older structural models for soots and carbon blacks, which are reviewed in the paper by D ~ n n e t . ~ In this paper, we shall describe structural and chemical studies on a combustion tube soot made from diesel fuel, in part to address the ZOHLCKS proposal, and in part to generate data to compare soot to other carbonaceous materials. Divergences are observed between experimentally observed diffraction data and the pattern expected for truncated icosahedral clusters. The chemistry of soot is that expected for aromatic hydrocarbons and heterocycles. Thus, while it is possible that clusters may be involved in flame chemistry,” our data can easily be reconciled with a model of soot as groupings of aromatic molecules.
Experimental Section Soot was generated in a turbulent diffusion flame b y t h e combination of preheated air (600 “C) with preheated number two diesel fuel (350 “C) in a combustion tube of inner diameter (1) Schwob, Y. In Chemistry and Physics of Carbon; Dekker: New York, 1980; Vol. 15, pp 109-219. (2) Shaw, R. W. Sa.Am. 1987,257, 96-101. (3) Smyth, K. C.; Miller, J. H. Science (Washington, D.C.)1987,236, 1540-1546. (4) Schlogl, R.; Indlekofer, G.; Oefhafen, P. Angew. Chem., Int. Ed. Engl. 1987,26, 309-319. (5) Donnet, J. B. Carbon 1982, 20, 266-282. (6) Zhang, Q. L.; O’Brien, S. C.; Heath, J. R.; Liu, Y.; Curl, R. F.; Kroto, H. W.; Smalley, R. E. J.Phys. Chem. 1986,90, 525-528. (7) Palmer, H. B.; Cullis, C. F. In Chemistry and Physics of Carbon; Dekker: New York, 1965; Vol. 1, pp 265-325. (8) Haynes, B. S.; Wagner, H. Gg. Prog. Energy Combust. Sci. 1981, 7, 229-273. (9) Medalia, A. I.; Rivin, D. Carbon 1982, 20, 481-492. (10) Ban, L. L.; Hess, W. M. In Petroleum Derived Carbons; ACS Symposium Series 21; American Chemical Society: Washington, DC, 1976; pp 358-377. (11) Gerhardt, Ph.; Loffler, S.; Homann, K. H. Chem. Phys. Lett. 1987, 137, 306-310. Q 1988 A m e r i c a n C h e m i c a l Societv
Combustion Tube Soot
Energy & Fuels, Vol. 2, No. 4,1988 439
aromaticity = 19%
.. .____
u
Figure 1. High-resolution 13C nuclear magnetic resonance spectrum at 22.49 MHz of number two diesel fuel.
9.8 cm. The mixture ignited spontaneously. The air to fuel ratio on a mass basis was 5:1, and the combustion process proceeded at 1 atm pressure. The temperature as measured by a pyrometer was 1350 "C and as measured by downstream thermocouples was ca. 900 OC. This sample is designated as sample 2, and this was used in most of our studies. Sample 3 was made in a similar way, except the air was heated to 800-850 OC and the pyrometer temperature was 1500-1600 "C. Sample 1 was made in the same apparatus as sample 2. However, the air and fuel were preheated only to 75 "C, and injected into the combustion zone heated to 1150 "C. The solution-phase carbon-13 nuclear magnetic resonance spectrum of the diesel fuel, taken at 22.49 MHz with C$+ doping, showed an aromaticity of 19%, as illustrated in Figure 1. Other analyses of the fuel are available in ref 12. Four microanalyses for soot sample 2 were performed by Galbraith for carbon and hydrogen by using a Pragl method with extended combustion times. The results are as follows:
%C 92.19 92.00 92.01 92.02 av 92.06 =F 0.08
%H 1.08 1.25 0.91
1.19
1.11 7 0.13 The atomic H/C ratio is 0.14. Other analyses were for % S (0.48, 0.44; average 0.46), % N (0.30), and % 0 (via Carlo-Erba at lo00 OC; 6.11). Our average mass balance is 100.04%. A sample of soot sample 2 was mixed with methylene chloride (1 g/35 g) and filtered through a fine frit. We recovered 96% of the solid, which showed a microanalysisof 92.01% C and 1.01% H. The low solubility of this soot is indicative of the presence of less unmodified fuel in the solid (relative to an engine soot) because of combustion from prevaporized fuel, rather than liquid droplets, as would be found in the combustion of fuel in an atomizing burner. The low solubility of the soot in methylene chloride led us to perform our experiments on the whole soot, rather than the insoluble portion. Soot sample 3 appeared to have a higher atomic H/C ratio than did soot sample 2. Analyses on % C (88.41,90.23, 89.00,88.47; average = 89.03 =F 00.73) and on % H (2.19,1.88,1.89,2.05;average = 2.00 F 0.13) yielded an average atomic H/C ratio of 0.27. Other analyses were for % S (0.58, 0.56, 0.54; average = 0.56 7 0.02), % N (0.43,0.32,0.00;average = 0.25 7 0.18),and % 0 (6.88,8.31, 7.59,9.70; average = 8.12 =F 1.04). Our average mass balance is 99.96%. The poor precision in % N and 90 0 and initial difficulties in closing mass balance on sample 2 (in which H/C atomic appeared to be 0.23) suggest that caution be exercised in the use (12) Jenkins, R. A.; Holmberg, R. W.; Wike, J. S.; Moneyhun, J. H.; Brazell, R. S. In "Chemicaland Physical Characterizationof Diesel Fuel Smoke";Oak Ridge National Laboratory Report ONRL/TM-9196; Oak Ridge National Laboratory: Oak Ridge, TN, 1983. Available through NTIS.
Figure 2. X-ray diffraction pattern of combustion tube soot sample 2 (Cu Ka,28 = 15-105", Bakelite sample holder).
of the microanalytical data. However, it is evident that the soot is significantly different from that from the initial number two diesel fuel and, on the basis of the low H/C ratio, is presumably more highly aromatic in nature. X-ray diffraction was carried out with copper K a radiation using both a Siemens D-500 instrument with a graphite monochromator and a Siemens 8-8 instrument with a nickel filter. Data points were taken every 0.02O in 28 with count times ranging from 2 to 30 s. In part, because of the low bulk density of the soot, X-radiation could penetrate through 1.5 mm deep samples and cause diffraction from the aluminum at the bottom of aluminum sample holders. To obtain accurate measurements, we used Bakelite holders (which show no peaks above 30°) to measure benzenoid (100) and (110) peaks, and we used aluminum holders to measure the (002) peak of aromatic stacking,to avoid confusion with the diffuse low-angle peak of Bakelite. Simulation of diffraction patterns was performed by using the formalism of Debye internal interference, as has been described pre~iously.~~ The ~ ' ~output of these simulations is scattered intensity as a linear function of the reciprocal space variable k (k = 2n/d = 4a(sin 8)/A). One can make inferences about peak maxima and line width from these simulations, but for direct, point by point comparison to X-ray diffraction data, one must correct for experimental factors such as polarization and Compton scattering. We follow current convention in displaying the x axis units as reciprocal angstroms. 13Cnuclear magnetic resonance of soot sample 2 was measured with a JEOL FX-6OQ spectrometer at room temperature. The observation frequency for carbon is 15 MHz, and magic-angle spinning is at 2300 Hz. The contact time between 'H and 13C for the experiment was 2 ms, and 207 869 transients were accumulated. 2H nuclear magnetic resonance of solids was measured on a Bruker MSL spectrometer at 55.283 MHz at room temperature. The figures in this paper come from power spectrum transforms of free induction decays. More accurate first-order quadrupolar splittings were measured from both solid echo sequences and from wide-line NMR data. Thermogravimetric analysis was performed on a Perkin-Elmer System 4 instrument with a TGS-2 thermogravimetric analyzer. Runs were made under nitrogen at 10 OC/min. Reactions involving potassium metal were carried out in a Vacuum Atmospheres glovebox under helium as has been described previously.16
Results Diffraction. Figure 2 gives t h e X-ray diffraction pattern of sample 2 over t h e range 28 = 15-105'. As noted, (13) Ebert, L. B.; Scanlon, J. C.; Mills, D. R. Prep-Am. Chem. Soc., Diu. Pet. Chem. 1983,28(5),1353-1366. (14) Ebert, L. B.; Scanlon, J. C.; Mills, D. R. Liq. Fuels Technol. 1984, 2, 257-286. (15) Ebert, L. B.; Mills, D. R.; Scanlon, J. C. Mater. Res. Bull. 1982, 17, 1319-1328.
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there are four peaks in the pattern, which correspond in ”d value” to the (002), (loo), (004), and (110) of stacked benzenoid arrays. The specific d values are as follows: (002), 351 pm; (loo),208.4 7 3.0 pm; (004), 174 pm; and (110), 120.3 7 1.2 pm. For reference, in graphite itself, d(002) = 335 pm, d(004) = 0.5d(002), d(100) = 1.5(sp2 bond length) 212.3 pm, and d(110) = d(100)/31/2 122.6 pm.lS As one goes to carbonaceous materials less ordered than graphite, one typically observes increases in d(002) that may be due to interstitial carbon.16J7 One also can observe decreases in d(100) as one goes from the 11/3 bonds of the graphite C-C bond (141.5 pm) to the 11/2bonds of the benzene C-C bond (139 pm). If change in bond length were the only fador determining the d value of the (100) peak, one would see a shift from 212.3 to 208.5 pm. The line width of a diffraction peak gives an indication of the correlation length of order in the direction associated with that diffraction peak. The general form of the equation is
-
truncated Icosahedron
l h d-4.87A . 2 9 A
-
k-2.20A-’
I
0.850 # o f Points
-
500
I
3.345
2.098
4.593
K,A-l Increment 0.01
-
# o f Atoms
- EO
Diffraction simulatlon: Icosahedron with 1.376 and 1.465A bonds
-
0.65A
crystallite size (correlation length) = k,X/(cos 0 A(28)) with the parameters defined as follows: k, = Scherrer constant, typically 0.89 to 1.84; X = wavelength of radiation; 6 = Bragg angle; A(28) = full width at half maximum of diffraction peak, in radians. The line width of the (002) peak of sample 2 is 0.0698 radian (4.00O); with a Scherrer constant of 0.9; this corresponds to a crystallite size in the direction of stacking (c axis) of 2.0 nm (20 A). The line width of the (100) peak at 208.4 pm is 0.109 radian (6.25O),and the line width of the (110) peak at 120.3 pm is 0.168 radian (9.6O). If one used a Scherrer constant of 1.84, one would predict crystallite sizes of 2.8 and 2.2 nm. However, as pointed out by Warren and Bodensteinla and by Ergun,lg the use of the 1.84 constant can give unreliable results for in-plane crystallite sizes of
