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Laser microprobe analysis of soot precursor particles and carbonaceous soot
Laser Microprobe Analysis of Soot Precursor Particles and Carbonaceous Soot R. A. DOBBINS* Division of Engineering, Brown University, Providence, R1 02906
R. A. FLETCHER Chemical Science and Technology Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899
W. LU Division of Engineering, Brown University, Providence, RI 02906 Samples of soot precursor particles and carbonaceous soot from a nonsmoking ethene diffusion flame have been analyzed by means of laser microprobe mass spectrometry (LMMS). The mass spectra of soot precursor particles from the lower flame display many peaks in the 200-300-amu range that are characteristic of polycyclic aromatic hydrocarbons (PAHs). The most prominent of these are the masses 252, 276, and 300, which correspond to the isomers of C20H12, C22H12, and C24H12 , respectively. These masses are among those predicted by Stein and Fahr to be the most thermodynamically stable ("stabilomers") under typical hydrocarbon flame conditions, and they have been previously reported as components of soot collected from a variety of fuels and combustion configurations. Carbonaceous soot from the upper region of the flame yields mass spectra composed of carbon-hydrogen clusters, CxHy with x = 3 to 24 and y usually 0, 1 or 2. Smaller amounts of PAH-like species with masses in the 418 to 444 ainu range are also found. These results suggest that the carbonization process observed in this flame is the dehydrogenation of the PAH species formed in the lower flame unaccompanied by polymeric growth. The LMMS technique provides a lower bound value of the hydrogen mole fraction X H of 0.36 ( C / H = 1.8) for precursor particles from the lower diffusion flame and 0.15 ( C / H = 5.6) for carbonaceous soot aggregates.
INTRODUCTION--SOOT PRECURSOR PARTICLES AND CARBONACEOUS SOOT
The discovery, using thermophoretic sampling and transmission electron microscopy (TEM), of soot precursor particles in hydrocarbon diffusion flames has recently been reported [1, 2]. Similar findings result from experiments using laser sources to study the scattering and fluorescence in the lower regions of premixed flames [3, 4]. TEM micrographs (Fig. 1A), show that precursor particles are polydisperse, singlet particles that are more transparent to the electron beam than are the mature soot aggregates. The precursor particles coalesce upon collision to form new singlet particles. In contrast, carbonaceous soot (Fig. 1B) is highly * Corresponding author. Presented at the Twenty-Fifth Symposium (International) on Combustion, Irvine, California, 31 July-5 August 1994.
opaque, consists of clusters made up of monomers that are approximately both spherical and monodisperse. The discovery of precursor particles in both diffusion and premixed flames is consistent with earlier observations of soot formation formed by pyrolysis in flow reactors [5, 6], shock tubes [7], and with earlier studies of premixed acetylene flames at reduced pressure [8]. The discovery of soot shells surrounding burning droplets in microgravity [9] can also be explained as the accumulation of precursor particles at a radial position where the drag force due to Stefan flow is equal to the opposing thermophoretic force. Precursor particles are observed in diffusion flames on the fuel side of the flame front and undergo conversion to carbonaceous soot in the high temperature regions of the flame by the process of carbonization. The purpose of this work has been to seek information on the chemical nature of soot precursor particles and
COMBUSTION AND FLAME 100:301-309 (1995) Copyright © 1995 by The Combustion Institute Published by Elsevier Science Inc.
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(a)
Co) Fig. 1. TEM micrographs of particles sampled from two heights on the axis of the ethene diffusion flame, a. Soot precursor particles from Z = 20 mm. b. Carbonaceous soot from Z = 50 mm.
carbonaceous soot, and ultimately on the nature of the carbonization process. The particles examined in this work were taken by the thermophoretic sampling technique [10, 11] from the nonsmoking laminar
diffusion flame. Although this technique is of relatively recent origin, it has been used by several groups [12-18] to obtain detailed information on the carbonaceous and refractory particles formed in flames. In the present work,
ANALYSIS OF SOOT PARTICLES
303
thermophoretic sampling is employed as part of a procedure for securing information about the chemical composition of soot particles. The coannular burner previously described [19] has been used in this work. The burner tube of 11.1 mm i.d. is surrounded by a ceramic grid of 102 mm o.d. through which the coannular air stream flows. The fuel and air flow rates were 3.85 m L / s and 715 mL/s, respectively, which results in a nonsmoking diffusion flame of 88 mm height that profusely emits the continuum radiation characteristic of soot. The measured axial velocity points vs. height for this flame [20] are well fitted by a curve of constant acceleration equal to 32.1 m / s 2 from which the height-time conversion is made. Temperatures were measured by the rapid insertion technique [20, 21], and they agree with previously published values [20] for this flame. Particles were extracted from the axis of the coannular flame at two heights, Z = 20 and Z = 50 mm above burner mouth (see sketch, Fig. 2). At Z = 20 mm the micrographs, Fig. 1A, reveal the particles to be singlet precursors at an early stage of development. At this point the flame temperature was measured to be 1428 K and the time elapsed from Z = 0 is 35 ms. In contrast, at Z = 50 mm the particles, Fig. 1B, are clusters of various chain lengths characteristic of mature soot particles, whose monomer units have a volume mean diameter of 29.7 nm. At this height the measured temVisible Height 88 mm
P°"
II
50
mm
20 turn
Fig. 2. Sketch of coannular burner showing TEM grid on the flame axis.
perature is 1570 K and the elapsed time is 56 ms.
LASER MICROPROBE MASS SPECTROMETRY
While various microanalytical techniques can provide detailed information on chemical composition, many require microgram size sampies. Laser microprobe mass spectrometry (LMMS) has the capability of providing qualitative and semiquantitative information [22, 23] on samples as small a s 10 -12 to 10 -18 g. This technique in combination with the thermophoretic sampling procedure provides the basis for the study of the evolution of soot particle composition in flames. A great advantage of the LMMS method lies in its ability to detect the presence of a wide range of cluster or fragment masses in microscopic samples. The application of the LMMS technique to the analysis of PAHs has been described in the literature [24-29]. The method has been used to explain the ultrafine particle emissions from a nonsmoking acetylene flame [30]. In the LMMS technique the output of the Nd:YAG laser is frequency quadrupled to achieve the wavelength of 265 nni in a pulsed mode. Microscope optics can be used to focus the beam to a circular pattern as small as 1 /xm diameter. Energy levels of 1-100 /xJ can thus produce an irradiance of 10 +l° W / c m 2 for pulse durations of 10 ns. These levels of radiation cause the sample to be ablated and singly ionized. The LAMMA-5001 (LeyboldHeraeus, Cologne, Germany) [22] employs a collinear He-Ne search laser with an optical microscope to permit the definition of the analytical region of the sample which is mounted on a movable x - y stage. The sample is mounted in a vacuum (10 4 Pa) on an uncoated copper electron microscope grid or on a quartz cover glass that has been cleaned to remove foreign materials. The ionized ablation products are
1 Commercial equipment, instruments, materials, or software are identified in this report to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement of these items by the NIST, nor does it imply that they are the best available for the purpose.
304 mass analyzed by a time-of-flight (TOF) mass spectrometer whose output is stored in a computer memory for subsequent analysis. The mass spectrometer achieves a mass resolution m/Am of ~ 500 at m/z values of 200 and, using 30,720 channels, covers the mass range of 0 to 5500 u. Data are recorded and stored in a 100-mHz transient recorder and are analyzed using software developed at the University of Antwerp. In the present work, a defocused beam was used to produce a nominal spot diameter of 50 to 100 micrometers resulting in irradiance levels of 10 +8 W / c m 2. The sample material was ablated from the grid surface that faces the laser source, and the resulting ions passed through the grid openings on their way to the spectrometer. Reproducibility of the LMMS technique is much poorer than other analytical methods [22] as a result of (a) variations of the sample composition on the microscopic scale, (b) imperfect control of the laser pulse resulting in variable irradiance levels, and (c) variations of the sample thickness. In view of these factors, it is necessary to compile the results of many LMMS spectra to obtain reliable data. LMMS ANALYSIS OF HYDROCARBON SAMPLES
In this work the positive ion mass peaks are + identified as consisting of CxHy with certain exceptions. The copper of the mounting grid will produce counts at 63 and 65 u. Lower levels of sodium and potassium contamination mass peaks at 23, 39, and 41 u. Counts at these masses are disregarded in the data reduction. If a quartz substrate is used, then mass numbers corresponding to Si +, SiO2+, and SiO3 + (28, 60, and 76) are discounted. All remaining + masses peaks are identified as CxHy . Since a mass with x carbon atoms plus 12 hydrogen atoms cannot be distinguished from the carbon cluster Cx+ a with no combined hydrogen, the information conveyed by the surrounding mass peaks was used to guide the interpretation. Thus, the 252 mass in the sequence 248, 250, 252 (in the absence of major peaks at 253, 254) is interpreted as Cz0H~2 while 252 in the sequence 252, 253, 254 (with no 248 or 250
R. A. DOBBINS ET AL. present) is interpreted as C21. These judgments were invariably reinforced by the presence of homologous sequences at many mass numbers in the same spectra. Mass peaks that are integer multiples of 12, i.e., peaks of CxHy with y = 0, represents ~2C, the abundant isotope of carbon. The 1.10% natural abundance of the ~3C isotope would produce 12x + 1 peaks even if hydrogen were not present. Since the ion counts corresponding to y = 1 and 2 that were substantially higher than is accounted for by the presence of ~3C, it is concluded that these peaks are predominantly carbon-hydrogen clusters. While the mass 300 does correspond solely to one species, namely, coronene C24H~2, many other PAHs have one or more isomers and an identification of the particular species is not possible. For example, no fewer than 16 isomers are given in one list of PAH species for the atomic mass of 252. Chromatographic methods have lead to the identification of five specific 252 isomers that have been found in hydrocarbon flames. The reliability of the above described interpretation procedure was tested using both a single known PAH and a mixture of PAH species analyzed with the LAMMA-500. Tests on perylene, C20H12 , showed that the parent 252 produced a strong signal and that the fragments C20H1~, and C20H10 were also produced in lesser quantities by the laser microprobe action. Many small carbon fragments, typically with y = 0, 1, and 2 were also produced from PAHs as previously noted [22-25]. The NIST Standard Reference Material 1491 (SRM) consisting of 24 PAHs of 14 different molecular masses ranging from 128 to 278 u was also tested. This mixture is provided in a hexane/toluene solvent with a complete gravimetric analysis. The mass spectrum of the SRM is shown in Fig. 3. The 11 lightest PAHs, 128 to 170 u, were not detected by the LAMMA-500 presumably because they evaporated in the test chamber vacuum. The 13 heaviest PAII were detected long with significant amounts of ion masses 250 and 274 that were not present in the PAH mixture. This result bears significantly on the purposes of this study as noted below. No large species of mass greater than 278 u were detected when the standard PAH
ANALYSIS OF SOOT PARTICLES 300
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mixture was tested. The ability of the LAMMA-500 to give semiquantitative results was tested on the standard PAHs mixture. An approximate correlation between the known mole fraction and the LAMMA-500 count fraction was obtained provided the runs with heavy conversion to small CxHy fragments were not included. For five prominent PAHs the rms deviation of count fraction from the known mole fraction was 52% over a mole fraction range of 6.5. The mass spectra shown in Figs. 4a and 4b show the contrasting results yielded by precursor particles and carbonaceous soot. In the former case there is a large population of masses throughout the 200-300 u range with mass numbers that correspond to PAH species. The mass spectra for carbonaceous soot consists mainly of carbon clusters with 0, 1, or 2 hydrogen atoms.
Precursor Particles Seven LMMS spectra of precursor particles were analyzed in detail and the results are presented in Table 1. The total mass counts (actually count peak area which is proportional to ion count) for each species in all seven runs were calculated from which the count fraction for each mass could be found. The count fractions of the various masses given in Table 1 are considered to be approximately correlated with the respective mole fractions. Three prominent masses, 252, 276, and 300, listed therein consti-
I O0
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m/z
Fig. 4. Mass spectra of particles sampled from the ethene diffusion flame, a. Soot precursor particles from Z = 20 mm. b. Carbonaceous soot aggregates from Z = 50 mm.
TABLE1 Principal Mass Numbers Found Among Precursor Particles from Low in the Ethene Diffusion Flame, Z = 20 mmg' b Mass (/x)
Formula
Count Fraction
276 252 274 250 300 263 248 240 296 288 283 261 226 264 290 298
C22 Hlz C2oHlz C 22 H 1o C2oHlo C 24H I2 C21 H 11 C20 H 8 C 19H 12 C24 H 8 C 23H 12 C23 H 7 C21H 9 C 1~H 1~ C 21H 12 C 2 3 H 14 C24Hio
0.201 0.163 0.0958 0.0911 0.0896 0.0886 0.0673 0.0289 0.0286 0.0274 0.0260 0.0231 0.0213 0.0200 0.0148 0.0128
Count fraction is based on the total counts of all mass numbers given for seven LMMS spectra. b Small counts for ion masses 202, 237, 239, 275, and 285 are also observed.
306
R . A . DOBBINS E T AL.
tute 45% of the total count population and are noteworthy in that they correspond to the molecular masses of the particular species found to be the most stable by Stein and Fahr [31], who studied the thermodynamic stability of PAHs under flame conditions. Most of the even numbered PAHs enumerated in Table 1 are found i~n the chart of "stabilomers" [31] which is reproduced in part in Table 2. The numbers shown in parentheses represent the count fraction of the various masses based on the total ion count for the 16 most prominent masses. Many of the m / z values with even mass numbers correspond to PAHs that have been reported in various studies of PAl-Is or soot formed by hydrocarbon fuels. While the PAHs identified in many previous studies were reported as gaseous species, the ion masses given in Table 1 specifically apply to soot precursor particles. The occurrence of the mass increment of 24 that is displayed by the data suggests that growth of the PAH species can be accounted for by the addition of an acetylene molecule with the release of hydrogen. The presence of the 252-276-300 u PAIl triad as the dominant P A H components of the precursor particle material is consistent with other reports of the chemical composition of flame generated soot from v.arious gaseous and liquid fuels in diverse flame configurations [32-37]. In all of these studies, the 252276-300 u species were found to coexist along with other P A H and a more carbonaceous component in relatively large soot samples captured on a cold surface or by a filter medium TABLE 2
Masses of PAH Stabilomers and Count Fraction Found in Soot Precursor Particlesa 14 Number 8 of Hydrogen 10 Atoms 12 14
176 (nd) 178 (nd)
Number of Carbon Atoms 16 18 20 22
24
200 (nd) 202 226 250 274 298 (t) (0.021) (0.091) (0.096) (0.013) 204 228 252 276 300 (rid) (nd) (0.16) (0.20) (0.090) 302 (t)
and, not detected, t, trace. In parenthesis are count traction data.
for subsequent analysis by chromatography. The frequent occurrence of these species is evidence that the route to the formation of carbonaceous soot is through the intermediate 200-300 u P A H stabilimers irrespective of the fuel composition or the nature of the combustion process. Masses 250, 274, and 298 that are found in the LMMS spectra also correspond to stabilimers, and they are expected on thermodynamic grounds. However, the tendency of the LAMMA-500 to form 250 and 274 from 252 and 276, respectively, has been noted in the above test of the SRM and also noted by others [27, 28]. This action may bias the counts shown in Tables 1 and 2. Odd-numbered masses listed in Table 1, viz., 261,263, and 283, represent 14% of the total ion count, and they have not been reported in previous studies. Ion mass 263 is the most prominent of these and is likely to be a fragment of ion mass 264. A number of tests from samples taken at Z = 20 mm in the ethene diffusion flame showed a wide range of carbon and c a r b o n hydrogen clusters that were similar to the test results of carbonaceous soot. Low in the diffusion flame there is an annular region of high concentration of carbonaceous soot through which the sampling grid passes briefly before coming to rest at an interior region at the center line. This deposition of unwanted soot, termed contamination, is unavoidable, and tests displaying a wide range of carbon and carbon-hydrogen clusters were not used in the analysis of the precursor particles. The opaque particles present in Fig. 1A are likely to be carbonaceous contaminants. Carbonaceous Particles
Soot aggregates sampled at the Z = 50 mm level were found to consist of CxHy with x ranging from 3 to as large as 24 and usually y = 0, 1, 2, or 3. The largest values of x were normally accompanied by y = 0 while values of y as high as 4 or 5 were found for the smaller values of x. The smaller carbon dusters are likely to have been created by the fragmentation and are thus unrelated to the material present in the flame. The approximate correspondence of the largest value of x to its
ANALYSIS OF S O O T PARTICLES value for the PAl-Is found lower in the flame suggests that the largest carbon clusters are formed by the dehydrogenation, without substantial further acetylenic or polymeric growth, of the P A l l species that are found lower in the flame. The count fraction of the carbon-hydrogen clusters CxHy in the carbonaceous sample is shown in Table 3 based on the total count of 18 _< x _< 24. This range brackets the value of x for the dominant PAl-Is found in the precursor samples at z = 20 mm. Table 3 shows that the smaller values of x are more strongly populated. This trend is displayed in Fig. 5 where the count fraction is plotted against the carbon number x for both precursor and carbonaceous particles. The even-numbered values of x are more populated for the precursor particles sampled from the lower flame as is expected from thermodynamic stability considerations. The absence of this trend in Fig. 5 for the carbonaceous particles may be the result of fragmentation of the larger carbon-hydrogen clusters by the laser microprobe. A small fraction of the tests of carbonaceous soot display PAl-I-like species in the 418-444 number range suggesting that limited PAH growth does occur. It is further noted that masses 600, 720, 840, and 1105 corresponding to the fullerenes C50, C60 , C70 , and C92 w e r e found in two of the 27 tests of carbonaceous aggregates. The low frequency of occurrence of these species supports the view that they do not play a role in the formation of carbonaceous soot [38].
TABLE 3 Count Fraction of C a r b o n - H y d r o g e n Clusters in Carbonaceous Soot a
x
Count Fraction
18 19 20 22 23 21 24
0.325 0.293 0.142 0.0925 0.0794 0.0559 0.0118
a Based on aggregate count for CxHy of indicated values of x, s u m m e d over all y.
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Fig. 5. Count fraction versus carbon number x in CxH~ for 18 < x < 2 5 .
DETERMINATION OF HYDROGEN MOLE FRACTION The ratio of the moles of hydrogen to the moles of carbon plus hydrogen X H is a useful parameter that relates to the history of soot particle formation in flames. The value of X a can be calculated from the LMMS tests if the count peak area c~ is representative of the actual number of ions at each mass number and if the fraction of species ionized is independent of mass number. The value of X H is calculated by. idmitifying x~ and y~ for each mass number having count c~ and using the expression ~ i ciYi X H = Ei cixi q- E i c i y i .
(1)
The summation indicated in Eq. 1 is taken over all mass numbers in any one spectrum. A mixture of hydrocarbons may have lighter species of higher vapor pressure which would evaporate more readily at the high vacuum conditions of the LAMMA-500 and thus be lost prior to the delivery of the laser pulse. In this event only the heavier species would be detected and the indicated value of X n would be less than the true value, hence a lower bound value. This method of determining X H was tested using perylene and the standard PAH mixture whose true X n are 0.375 and 0.423, respectively. The corresponding lower bound values from the L A M M A test are 0.270 and 0.354, respectively. Thus, the lower bound
308 X n were less than the true values by 28% and 16% respectively. The lower bound value of X H were found by using Eq. 1 for the data from the two samples extracted from the ethene flame. The data for seven LMMS spectra on precursor particles yield X H = 0.36, a value representative of the intermediate PAHs. For 16 spectra of carbonaceous soot a value of 0.15 is obtained which is in general agreement with the values reported in the literature for mature soot released from flames. In calculating the value of X n for the carbonaceous material collected from Z = 50 mm, it is necessary to account for the isotopic composition of carbon. On the assumption that the total carbon ion population in any one spectrum has the natural isotopic abundance, the value of X H for carbonaceous soot is found by subtracting from Y,ciy i in both the numerator and the denominator of Eq. 1, the magnitude 0.011 E c i x i which represents the total number of 13C ions therein. This correction is significant for the larger carbon-hydrogen clusters, but only perceptibly reduces the calculated hydrogen mole fraction. SUMMARY AND CONCLUSIONS OF SOOT EVOLUTION The LMMS analysis of soot samples from the axis of the ethene diffusion flame yields an abundance of information notwithstanding the limitations of this method. Precursor particles from low in this flame, Z = 20 mm, display mass spectra characteristic of the intermediate PAHs (200-300 u) and have a value of X H > 0.36 that is representative of these species. The presence of the mass increment of 24 suggests that the growth of the P A l l species within the precursor species is by an acetylenic growth process. The detection of the 252, 276, and 300 tz mass sequence as components of the soot precursor particles suggests that soot formation proceeds via the reaction path formed by the stabilomer PAils whose existence was predicted by Stein and Fahr [31] based on chemical thermodynamic considerations. Carbonaceous soot from the upper region, Z = 50 mm, produces mass spectra of large carbon-hydrogen clusters, C 18H y to C 24H y with
R . A . DOBBINS E T AL. y = 0, 1, or 2 along with low levels of PAl-I-like species in the 418 to 444 u range. These carbon atom numbers predominantly correspond to those of the P A H species that are found to be present in the lower flame. The lower value of X n > 0.15 at Z = 50 mm indicates that the carbonization of the soot precursor particles is a dehydrogenation process that is favored in the higher temperature regions of the flame. Trace levels of mass numbers corresponding to C50, C60, C70, and C92 are also found in samples of the carbonaceous soot. These species are found in the ion mass spectra from the upper flame region and are considered to be minor products of the soot formation process. This work has been supported at Brown University by the A r m y Research Office under Grant No. DAALO3-92-G-O023. The temperature measurements were conducted by Mr. George J. Govatzidakis. Helpful discussions with Drs. D. E. Newbury, D. S. Simons, Profs. J. E. Baird, E. F. Greene, and E. M. Suuberg are acknowledged. The authors express their thanks to Mr. A. Schwartzman for assistance with the electron microscopy and to Mr. W. D. Lilly for technical assistance with the instrumentation. The L A M M A - 5 0 0 is located at N I S T in the Microanalysis Research Group.
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Received 1 December 1993; revised 19 April 1994
Comments A. F. Sarofin, MIT, USA. The results obtained with the laser microprobe provide very useful insights on the nature of soot precursor particles and aged soot particles, which help clarify the soot formation mechanisms. It appears unlikely, however, that the isomers of C2OH12, C22H12, and C24H12 would condense at the flame temperature of 1450K. Is it possible that these isomers form part of higher molecular weight polyaryl chains that are broken up by laser microprobe?
Lewis and Singer [1] describe thermal carbonization as consisting of oligomer formation involving single and multiple aryl bonds that contribute to a progressive polymerization process. If this description is here applicable, then the rupture of the aryl bonds existing among the PAils detected in precursor particles by the laser microprobe is a distinct possibility. There exists preliminary experimental evidence that aryl sites are present in the PAHs detected in the precursor particles at the z = 20
Authors' Reply. The gaseous PAl-/species have been proposed as the direct precursors of young soot particles through the aryl radical growth mechanism. Their detection as components of the precursor particles reinforces with this hypothesis.
m m level.
REFERENCE 1. Lewis, I. C., and Singer, L. S., in PolynuclearAromatic Compounds, L B. Ebert, Editor, Advances in Chemistry 217, American Chemical Society, Washington, D.C., 1986, pp. 269-285.
R. A. FLETCHER Chemical Science and Technology Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899
W. LU Division of Engineering, Brown University, Providence, RI 02906 Samples of soot precursor particles and carbonaceous soot from a nonsmoking ethene diffusion flame have been analyzed by means of laser microprobe mass spectrometry (LMMS). The mass spectra of soot precursor particles from the lower flame display many peaks in the 200-300-amu range that are characteristic of polycyclic aromatic hydrocarbons (PAHs). The most prominent of these are the masses 252, 276, and 300, which correspond to the isomers of C20H12, C22H12, and C24H12 , respectively. These masses are among those predicted by Stein and Fahr to be the most thermodynamically stable ("stabilomers") under typical hydrocarbon flame conditions, and they have been previously reported as components of soot collected from a variety of fuels and combustion configurations. Carbonaceous soot from the upper region of the flame yields mass spectra composed of carbon-hydrogen clusters, CxHy with x = 3 to 24 and y usually 0, 1 or 2. Smaller amounts of PAH-like species with masses in the 418 to 444 ainu range are also found. These results suggest that the carbonization process observed in this flame is the dehydrogenation of the PAH species formed in the lower flame unaccompanied by polymeric growth. The LMMS technique provides a lower bound value of the hydrogen mole fraction X H of 0.36 ( C / H = 1.8) for precursor particles from the lower diffusion flame and 0.15 ( C / H = 5.6) for carbonaceous soot aggregates.
INTRODUCTION--SOOT PRECURSOR PARTICLES AND CARBONACEOUS SOOT
The discovery, using thermophoretic sampling and transmission electron microscopy (TEM), of soot precursor particles in hydrocarbon diffusion flames has recently been reported [1, 2]. Similar findings result from experiments using laser sources to study the scattering and fluorescence in the lower regions of premixed flames [3, 4]. TEM micrographs (Fig. 1A), show that precursor particles are polydisperse, singlet particles that are more transparent to the electron beam than are the mature soot aggregates. The precursor particles coalesce upon collision to form new singlet particles. In contrast, carbonaceous soot (Fig. 1B) is highly * Corresponding author. Presented at the Twenty-Fifth Symposium (International) on Combustion, Irvine, California, 31 July-5 August 1994.
opaque, consists of clusters made up of monomers that are approximately both spherical and monodisperse. The discovery of precursor particles in both diffusion and premixed flames is consistent with earlier observations of soot formation formed by pyrolysis in flow reactors [5, 6], shock tubes [7], and with earlier studies of premixed acetylene flames at reduced pressure [8]. The discovery of soot shells surrounding burning droplets in microgravity [9] can also be explained as the accumulation of precursor particles at a radial position where the drag force due to Stefan flow is equal to the opposing thermophoretic force. Precursor particles are observed in diffusion flames on the fuel side of the flame front and undergo conversion to carbonaceous soot in the high temperature regions of the flame by the process of carbonization. The purpose of this work has been to seek information on the chemical nature of soot precursor particles and
COMBUSTION AND FLAME 100:301-309 (1995) Copyright © 1995 by The Combustion Institute Published by Elsevier Science Inc.
0010-2180/95/$9.50 SSDI 0010-2180(94)00047-V
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R. A. D O B B I N S E T AL.
(a)
Co) Fig. 1. TEM micrographs of particles sampled from two heights on the axis of the ethene diffusion flame, a. Soot precursor particles from Z = 20 mm. b. Carbonaceous soot from Z = 50 mm.
carbonaceous soot, and ultimately on the nature of the carbonization process. The particles examined in this work were taken by the thermophoretic sampling technique [10, 11] from the nonsmoking laminar
diffusion flame. Although this technique is of relatively recent origin, it has been used by several groups [12-18] to obtain detailed information on the carbonaceous and refractory particles formed in flames. In the present work,
ANALYSIS OF SOOT PARTICLES
303
thermophoretic sampling is employed as part of a procedure for securing information about the chemical composition of soot particles. The coannular burner previously described [19] has been used in this work. The burner tube of 11.1 mm i.d. is surrounded by a ceramic grid of 102 mm o.d. through which the coannular air stream flows. The fuel and air flow rates were 3.85 m L / s and 715 mL/s, respectively, which results in a nonsmoking diffusion flame of 88 mm height that profusely emits the continuum radiation characteristic of soot. The measured axial velocity points vs. height for this flame [20] are well fitted by a curve of constant acceleration equal to 32.1 m / s 2 from which the height-time conversion is made. Temperatures were measured by the rapid insertion technique [20, 21], and they agree with previously published values [20] for this flame. Particles were extracted from the axis of the coannular flame at two heights, Z = 20 and Z = 50 mm above burner mouth (see sketch, Fig. 2). At Z = 20 mm the micrographs, Fig. 1A, reveal the particles to be singlet precursors at an early stage of development. At this point the flame temperature was measured to be 1428 K and the time elapsed from Z = 0 is 35 ms. In contrast, at Z = 50 mm the particles, Fig. 1B, are clusters of various chain lengths characteristic of mature soot particles, whose monomer units have a volume mean diameter of 29.7 nm. At this height the measured temVisible Height 88 mm
P°"
II
50
mm
20 turn
Fig. 2. Sketch of coannular burner showing TEM grid on the flame axis.
perature is 1570 K and the elapsed time is 56 ms.
LASER MICROPROBE MASS SPECTROMETRY
While various microanalytical techniques can provide detailed information on chemical composition, many require microgram size sampies. Laser microprobe mass spectrometry (LMMS) has the capability of providing qualitative and semiquantitative information [22, 23] on samples as small a s 10 -12 to 10 -18 g. This technique in combination with the thermophoretic sampling procedure provides the basis for the study of the evolution of soot particle composition in flames. A great advantage of the LMMS method lies in its ability to detect the presence of a wide range of cluster or fragment masses in microscopic samples. The application of the LMMS technique to the analysis of PAHs has been described in the literature [24-29]. The method has been used to explain the ultrafine particle emissions from a nonsmoking acetylene flame [30]. In the LMMS technique the output of the Nd:YAG laser is frequency quadrupled to achieve the wavelength of 265 nni in a pulsed mode. Microscope optics can be used to focus the beam to a circular pattern as small as 1 /xm diameter. Energy levels of 1-100 /xJ can thus produce an irradiance of 10 +l° W / c m 2 for pulse durations of 10 ns. These levels of radiation cause the sample to be ablated and singly ionized. The LAMMA-5001 (LeyboldHeraeus, Cologne, Germany) [22] employs a collinear He-Ne search laser with an optical microscope to permit the definition of the analytical region of the sample which is mounted on a movable x - y stage. The sample is mounted in a vacuum (10 4 Pa) on an uncoated copper electron microscope grid or on a quartz cover glass that has been cleaned to remove foreign materials. The ionized ablation products are
1 Commercial equipment, instruments, materials, or software are identified in this report to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement of these items by the NIST, nor does it imply that they are the best available for the purpose.
304 mass analyzed by a time-of-flight (TOF) mass spectrometer whose output is stored in a computer memory for subsequent analysis. The mass spectrometer achieves a mass resolution m/Am of ~ 500 at m/z values of 200 and, using 30,720 channels, covers the mass range of 0 to 5500 u. Data are recorded and stored in a 100-mHz transient recorder and are analyzed using software developed at the University of Antwerp. In the present work, a defocused beam was used to produce a nominal spot diameter of 50 to 100 micrometers resulting in irradiance levels of 10 +8 W / c m 2. The sample material was ablated from the grid surface that faces the laser source, and the resulting ions passed through the grid openings on their way to the spectrometer. Reproducibility of the LMMS technique is much poorer than other analytical methods [22] as a result of (a) variations of the sample composition on the microscopic scale, (b) imperfect control of the laser pulse resulting in variable irradiance levels, and (c) variations of the sample thickness. In view of these factors, it is necessary to compile the results of many LMMS spectra to obtain reliable data. LMMS ANALYSIS OF HYDROCARBON SAMPLES
In this work the positive ion mass peaks are + identified as consisting of CxHy with certain exceptions. The copper of the mounting grid will produce counts at 63 and 65 u. Lower levels of sodium and potassium contamination mass peaks at 23, 39, and 41 u. Counts at these masses are disregarded in the data reduction. If a quartz substrate is used, then mass numbers corresponding to Si +, SiO2+, and SiO3 + (28, 60, and 76) are discounted. All remaining + masses peaks are identified as CxHy . Since a mass with x carbon atoms plus 12 hydrogen atoms cannot be distinguished from the carbon cluster Cx+ a with no combined hydrogen, the information conveyed by the surrounding mass peaks was used to guide the interpretation. Thus, the 252 mass in the sequence 248, 250, 252 (in the absence of major peaks at 253, 254) is interpreted as Cz0H~2 while 252 in the sequence 252, 253, 254 (with no 248 or 250
R. A. DOBBINS ET AL. present) is interpreted as C21. These judgments were invariably reinforced by the presence of homologous sequences at many mass numbers in the same spectra. Mass peaks that are integer multiples of 12, i.e., peaks of CxHy with y = 0, represents ~2C, the abundant isotope of carbon. The 1.10% natural abundance of the ~3C isotope would produce 12x + 1 peaks even if hydrogen were not present. Since the ion counts corresponding to y = 1 and 2 that were substantially higher than is accounted for by the presence of ~3C, it is concluded that these peaks are predominantly carbon-hydrogen clusters. While the mass 300 does correspond solely to one species, namely, coronene C24H~2, many other PAHs have one or more isomers and an identification of the particular species is not possible. For example, no fewer than 16 isomers are given in one list of PAH species for the atomic mass of 252. Chromatographic methods have lead to the identification of five specific 252 isomers that have been found in hydrocarbon flames. The reliability of the above described interpretation procedure was tested using both a single known PAH and a mixture of PAH species analyzed with the LAMMA-500. Tests on perylene, C20H12 , showed that the parent 252 produced a strong signal and that the fragments C20H1~, and C20H10 were also produced in lesser quantities by the laser microprobe action. Many small carbon fragments, typically with y = 0, 1, and 2 were also produced from PAHs as previously noted [22-25]. The NIST Standard Reference Material 1491 (SRM) consisting of 24 PAHs of 14 different molecular masses ranging from 128 to 278 u was also tested. This mixture is provided in a hexane/toluene solvent with a complete gravimetric analysis. The mass spectrum of the SRM is shown in Fig. 3. The 11 lightest PAHs, 128 to 170 u, were not detected by the LAMMA-500 presumably because they evaporated in the test chamber vacuum. The 13 heaviest PAII were detected long with significant amounts of ion masses 250 and 274 that were not present in the PAH mixture. This result bears significantly on the purposes of this study as noted below. No large species of mass greater than 278 u were detected when the standard PAH
ANALYSIS OF SOOT PARTICLES 300
I
I
305
i
¢q
eq
i
250
/
200
(b)
e,i 250
200
150
eq 150
100 1(},9
50 5O
0
I
I
r~
__=J.. . . . . . . .
I
m]z
Fig. 3. Mass spectra of standard reference material No. 1491 which contains 14 PAH masses.
i
0
I
I
~ ...... I
(a) 250
150
mixture was tested. The ability of the LAMMA-500 to give semiquantitative results was tested on the standard PAHs mixture. An approximate correlation between the known mole fraction and the LAMMA-500 count fraction was obtained provided the runs with heavy conversion to small CxHy fragments were not included. For five prominent PAHs the rms deviation of count fraction from the known mole fraction was 52% over a mole fraction range of 6.5. The mass spectra shown in Figs. 4a and 4b show the contrasting results yielded by precursor particles and carbonaceous soot. In the former case there is a large population of masses throughout the 200-300 u range with mass numbers that correspond to PAH species. The mass spectra for carbonaceous soot consists mainly of carbon clusters with 0, 1, or 2 hydrogen atoms.
Precursor Particles Seven LMMS spectra of precursor particles were analyzed in detail and the results are presented in Table 1. The total mass counts (actually count peak area which is proportional to ion count) for each species in all seven runs were calculated from which the count fraction for each mass could be found. The count fractions of the various masses given in Table 1 are considered to be approximately correlated with the respective mole fractions. Three prominent masses, 252, 276, and 300, listed therein consti-
I O0
5O
1)
m/z
Fig. 4. Mass spectra of particles sampled from the ethene diffusion flame, a. Soot precursor particles from Z = 20 mm. b. Carbonaceous soot aggregates from Z = 50 mm.
TABLE1 Principal Mass Numbers Found Among Precursor Particles from Low in the Ethene Diffusion Flame, Z = 20 mmg' b Mass (/x)
Formula
Count Fraction
276 252 274 250 300 263 248 240 296 288 283 261 226 264 290 298
C22 Hlz C2oHlz C 22 H 1o C2oHlo C 24H I2 C21 H 11 C20 H 8 C 19H 12 C24 H 8 C 23H 12 C23 H 7 C21H 9 C 1~H 1~ C 21H 12 C 2 3 H 14 C24Hio
0.201 0.163 0.0958 0.0911 0.0896 0.0886 0.0673 0.0289 0.0286 0.0274 0.0260 0.0231 0.0213 0.0200 0.0148 0.0128
Count fraction is based on the total counts of all mass numbers given for seven LMMS spectra. b Small counts for ion masses 202, 237, 239, 275, and 285 are also observed.
306
R . A . DOBBINS E T AL.
tute 45% of the total count population and are noteworthy in that they correspond to the molecular masses of the particular species found to be the most stable by Stein and Fahr [31], who studied the thermodynamic stability of PAHs under flame conditions. Most of the even numbered PAHs enumerated in Table 1 are found i~n the chart of "stabilomers" [31] which is reproduced in part in Table 2. The numbers shown in parentheses represent the count fraction of the various masses based on the total ion count for the 16 most prominent masses. Many of the m / z values with even mass numbers correspond to PAHs that have been reported in various studies of PAl-Is or soot formed by hydrocarbon fuels. While the PAHs identified in many previous studies were reported as gaseous species, the ion masses given in Table 1 specifically apply to soot precursor particles. The occurrence of the mass increment of 24 that is displayed by the data suggests that growth of the PAH species can be accounted for by the addition of an acetylene molecule with the release of hydrogen. The presence of the 252-276-300 u PAIl triad as the dominant P A H components of the precursor particle material is consistent with other reports of the chemical composition of flame generated soot from v.arious gaseous and liquid fuels in diverse flame configurations [32-37]. In all of these studies, the 252276-300 u species were found to coexist along with other P A H and a more carbonaceous component in relatively large soot samples captured on a cold surface or by a filter medium TABLE 2
Masses of PAH Stabilomers and Count Fraction Found in Soot Precursor Particlesa 14 Number 8 of Hydrogen 10 Atoms 12 14
176 (nd) 178 (nd)
Number of Carbon Atoms 16 18 20 22
24
200 (nd) 202 226 250 274 298 (t) (0.021) (0.091) (0.096) (0.013) 204 228 252 276 300 (rid) (nd) (0.16) (0.20) (0.090) 302 (t)
and, not detected, t, trace. In parenthesis are count traction data.
for subsequent analysis by chromatography. The frequent occurrence of these species is evidence that the route to the formation of carbonaceous soot is through the intermediate 200-300 u P A H stabilimers irrespective of the fuel composition or the nature of the combustion process. Masses 250, 274, and 298 that are found in the LMMS spectra also correspond to stabilimers, and they are expected on thermodynamic grounds. However, the tendency of the LAMMA-500 to form 250 and 274 from 252 and 276, respectively, has been noted in the above test of the SRM and also noted by others [27, 28]. This action may bias the counts shown in Tables 1 and 2. Odd-numbered masses listed in Table 1, viz., 261,263, and 283, represent 14% of the total ion count, and they have not been reported in previous studies. Ion mass 263 is the most prominent of these and is likely to be a fragment of ion mass 264. A number of tests from samples taken at Z = 20 mm in the ethene diffusion flame showed a wide range of carbon and c a r b o n hydrogen clusters that were similar to the test results of carbonaceous soot. Low in the diffusion flame there is an annular region of high concentration of carbonaceous soot through which the sampling grid passes briefly before coming to rest at an interior region at the center line. This deposition of unwanted soot, termed contamination, is unavoidable, and tests displaying a wide range of carbon and carbon-hydrogen clusters were not used in the analysis of the precursor particles. The opaque particles present in Fig. 1A are likely to be carbonaceous contaminants. Carbonaceous Particles
Soot aggregates sampled at the Z = 50 mm level were found to consist of CxHy with x ranging from 3 to as large as 24 and usually y = 0, 1, 2, or 3. The largest values of x were normally accompanied by y = 0 while values of y as high as 4 or 5 were found for the smaller values of x. The smaller carbon dusters are likely to have been created by the fragmentation and are thus unrelated to the material present in the flame. The approximate correspondence of the largest value of x to its
ANALYSIS OF S O O T PARTICLES value for the PAl-Is found lower in the flame suggests that the largest carbon clusters are formed by the dehydrogenation, without substantial further acetylenic or polymeric growth, of the P A l l species that are found lower in the flame. The count fraction of the carbon-hydrogen clusters CxHy in the carbonaceous sample is shown in Table 3 based on the total count of 18 _< x _< 24. This range brackets the value of x for the dominant PAl-Is found in the precursor samples at z = 20 mm. Table 3 shows that the smaller values of x are more strongly populated. This trend is displayed in Fig. 5 where the count fraction is plotted against the carbon number x for both precursor and carbonaceous particles. The even-numbered values of x are more populated for the precursor particles sampled from the lower flame as is expected from thermodynamic stability considerations. The absence of this trend in Fig. 5 for the carbonaceous particles may be the result of fragmentation of the larger carbon-hydrogen clusters by the laser microprobe. A small fraction of the tests of carbonaceous soot display PAl-I-like species in the 418-444 number range suggesting that limited PAH growth does occur. It is further noted that masses 600, 720, 840, and 1105 corresponding to the fullerenes C50, C60 , C70 , and C92 w e r e found in two of the 27 tests of carbonaceous aggregates. The low frequency of occurrence of these species supports the view that they do not play a role in the formation of carbonaceous soot [38].
TABLE 3 Count Fraction of C a r b o n - H y d r o g e n Clusters in Carbonaceous Soot a
x
Count Fraction
18 19 20 22 23 21 24
0.325 0.293 0.142 0.0925 0.0794 0.0559 0.0118
a Based on aggregate count for CxHy of indicated values of x, s u m m e d over all y.
307 0 5 0
. . . . .
. . . .
I
. . . .
I
. . . .
I
. . . .
[
. . . .
~
r
l
....
Pr~rsor Part,de Carbonac~us $oQt
23
24
0.40
020
0.10
0.00 18
19
20
21
22
25
Carbon Number x
Fig. 5. Count fraction versus carbon number x in CxH~ for 18 < x < 2 5 .
DETERMINATION OF HYDROGEN MOLE FRACTION The ratio of the moles of hydrogen to the moles of carbon plus hydrogen X H is a useful parameter that relates to the history of soot particle formation in flames. The value of X a can be calculated from the LMMS tests if the count peak area c~ is representative of the actual number of ions at each mass number and if the fraction of species ionized is independent of mass number. The value of X H is calculated by. idmitifying x~ and y~ for each mass number having count c~ and using the expression ~ i ciYi X H = Ei cixi q- E i c i y i .
(1)
The summation indicated in Eq. 1 is taken over all mass numbers in any one spectrum. A mixture of hydrocarbons may have lighter species of higher vapor pressure which would evaporate more readily at the high vacuum conditions of the LAMMA-500 and thus be lost prior to the delivery of the laser pulse. In this event only the heavier species would be detected and the indicated value of X n would be less than the true value, hence a lower bound value. This method of determining X H was tested using perylene and the standard PAH mixture whose true X n are 0.375 and 0.423, respectively. The corresponding lower bound values from the L A M M A test are 0.270 and 0.354, respectively. Thus, the lower bound
308 X n were less than the true values by 28% and 16% respectively. The lower bound value of X H were found by using Eq. 1 for the data from the two samples extracted from the ethene flame. The data for seven LMMS spectra on precursor particles yield X H = 0.36, a value representative of the intermediate PAHs. For 16 spectra of carbonaceous soot a value of 0.15 is obtained which is in general agreement with the values reported in the literature for mature soot released from flames. In calculating the value of X n for the carbonaceous material collected from Z = 50 mm, it is necessary to account for the isotopic composition of carbon. On the assumption that the total carbon ion population in any one spectrum has the natural isotopic abundance, the value of X H for carbonaceous soot is found by subtracting from Y,ciy i in both the numerator and the denominator of Eq. 1, the magnitude 0.011 E c i x i which represents the total number of 13C ions therein. This correction is significant for the larger carbon-hydrogen clusters, but only perceptibly reduces the calculated hydrogen mole fraction. SUMMARY AND CONCLUSIONS OF SOOT EVOLUTION The LMMS analysis of soot samples from the axis of the ethene diffusion flame yields an abundance of information notwithstanding the limitations of this method. Precursor particles from low in this flame, Z = 20 mm, display mass spectra characteristic of the intermediate PAHs (200-300 u) and have a value of X H > 0.36 that is representative of these species. The presence of the mass increment of 24 suggests that the growth of the P A l l species within the precursor species is by an acetylenic growth process. The detection of the 252, 276, and 300 tz mass sequence as components of the soot precursor particles suggests that soot formation proceeds via the reaction path formed by the stabilomer PAils whose existence was predicted by Stein and Fahr [31] based on chemical thermodynamic considerations. Carbonaceous soot from the upper region, Z = 50 mm, produces mass spectra of large carbon-hydrogen clusters, C 18H y to C 24H y with
R . A . DOBBINS E T AL. y = 0, 1, or 2 along with low levels of PAl-I-like species in the 418 to 444 u range. These carbon atom numbers predominantly correspond to those of the P A H species that are found to be present in the lower flame. The lower value of X n > 0.15 at Z = 50 mm indicates that the carbonization of the soot precursor particles is a dehydrogenation process that is favored in the higher temperature regions of the flame. Trace levels of mass numbers corresponding to C50, C60, C70, and C92 are also found in samples of the carbonaceous soot. These species are found in the ion mass spectra from the upper flame region and are considered to be minor products of the soot formation process. This work has been supported at Brown University by the A r m y Research Office under Grant No. DAALO3-92-G-O023. The temperature measurements were conducted by Mr. George J. Govatzidakis. Helpful discussions with Drs. D. E. Newbury, D. S. Simons, Profs. J. E. Baird, E. F. Greene, and E. M. Suuberg are acknowledged. The authors express their thanks to Mr. A. Schwartzman for assistance with the electron microscopy and to Mr. W. D. Lilly for technical assistance with the instrumentation. The L A M M A - 5 0 0 is located at N I S T in the Microanalysis Research Group.
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1. Dobbins, R. A., and Subramaniasivam, H., in Mechanisms and Models of Soot Formation, Heidelberg, October 1991, to appear in Lecture Notes in Physics, Springer-Verlag. 2. Dobbins, R. A., Glassman Symposium,October 1993, to appear. 3. D'Alesio, A., Barbella, R., Ciajolo, A., D'Anna, A., D'Orsi, A., Minutolo, P., in Mechanisms and Models in Soot Formation, Heidelberg, October 1991, to appear in LectureNotes in Physics, Springer-Verlag. 4. D'Alesio, A., D'Anna, A., D'Orsi, A., Minutolo, P., Barbella, R., Ciajolo, A., Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1992, pp. 973-980. 5. Sweitzer, C. W., and Heller, G. L., Rubber World 34:855-865 (1954). 6. Prado,G., and Lahaye, J., Carbon 12:27-35 (1974). 7. Graham, S. C., Sixteenth (International) Symposium on Combustion, The Combustion Institute, Pittsburgh, 1977, pp. 663-669. 8. Wersborg, B. L., Howard, J. B., and Williams, G. C., Fourteenth (International) Symposium on Combustion,
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Received 1 December 1993; revised 19 April 1994
Comments A. F. Sarofin, MIT, USA. The results obtained with the laser microprobe provide very useful insights on the nature of soot precursor particles and aged soot particles, which help clarify the soot formation mechanisms. It appears unlikely, however, that the isomers of C2OH12, C22H12, and C24H12 would condense at the flame temperature of 1450K. Is it possible that these isomers form part of higher molecular weight polyaryl chains that are broken up by laser microprobe?
Lewis and Singer [1] describe thermal carbonization as consisting of oligomer formation involving single and multiple aryl bonds that contribute to a progressive polymerization process. If this description is here applicable, then the rupture of the aryl bonds existing among the PAils detected in precursor particles by the laser microprobe is a distinct possibility. There exists preliminary experimental evidence that aryl sites are present in the PAHs detected in the precursor particles at the z = 20
Authors' Reply. The gaseous PAl-/species have been proposed as the direct precursors of young soot particles through the aryl radical growth mechanism. Their detection as components of the precursor particles reinforces with this hypothesis.
m m level.
REFERENCE 1. Lewis, I. C., and Singer, L. S., in PolynuclearAromatic Compounds, L B. Ebert, Editor, Advances in Chemistry 217, American Chemical Society, Washington, D.C., 1986, pp. 269-285.