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Real-time observation of metastable polymericspecies formed from precursor soot

Article in Chemical Physics Letters October 2004

Impact Factor: 1.9 DOI: 10.1016/j.cplett.2004.08.126

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Peter T A Reilly

Washington State University

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Real-time observation of metastable polymeric speciesformed from precursor soot

Ryan P. Rodgers, Peter T.A. Reilly *, William B. Whitten, J. Michael Ramsey

Laser Spectroscopy and Microinstrumentation Group, Oak Ridge National Laboratory, P.O. Box 2008, MS 6142,

Oak Ridge, TN 37831-6142, USA

Received 13 August 2004; in final form 27 August 2004

Available online 16 September 2004

Abstract

Micrometer-sized precursor soot particles were formed by pyrolysis of pure atmospheric-pressure acetylene in a flow tube reac-

tor. Real-time analysis of both the gas and particle phases of the reactor effluent was performed with an ion trap-based aerosol mass

spectrometer. Off-line analyses were also performed on the filter-collected reactor effluent. The real-time particle mass spectra

revealed the presence of a polymeric mass distribution starting near 1000 Da and continuing to greater than 1750 Da. The spacing

between the polymer peaks is approximately 70 Da. Comparison of real-time and off-line analyses reveals the metastable nature of

the polymer-like high mass species.

2004 Elsevier B.V. All rights reserved.

1. Introduction

Precursor soot as its name implies is an integral step

in the formation of mature soot. It forms during pyroly-

sis of hydrocarbons and has been observed in flames

[1,2] and during the commercial production of carbon

black used in the production of tires[3]. Though precur-

sor soot was observed more than 50 years ago [4], its

very existence is still controversial with most critics sug-

gesting it results from sampling artifacts.

Though the origins of the controversy have never (to

our knowledge) been put in print, it probably is due to

the analysis of the composition of precursor soot. Pre-

cursor soot is an oil containing polycyclic aromatichydrocarbons (PAHs). These same PAHs have been ob-

served to reside in the gas phase of pyrolytic processes.

The controversy arises because the boiling points of

the vast majority of the components of the condensed

species are well below the formation temperature of

the precursor soot. It is therefore logically concluded

that these components cannot physically condense atatmospheric pressure at those temperatures. Hence, it

can and has been argued that precursor soot does not

exist it must result from sampling-induced condensa-

tion of the gas phase species.

However, the above arguments against the existence

of precursor soot are not supported by the definitive

work performed by Sweitzer and Parker (members of

the carbon black industry) in the 1950s [3]. In these

experiments, a white mist was observed to form inside

a huge furnace (used to industrially produce carbon

black) when the hydrocarbon fuel was passed through

the furnace. The white mist was sampled and found tobe an oil of low boiling PAHs upon analysis. The ob-

served mist evolved into mature soot (carbon black) as

the temperature of the furnace was raised. Since there

was no sampling involved in this experiment, the mist

was not formed by sudden cooling of the vapor phase

PAHs. Consequently, a high temperature stable form

of hydrocarbon must exist that is a precursor to soot.

Hence, precursor soot must exist in high temperature

environments.

0009-2614/$ - see front matter 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.cplett.2004.08.126

* Corresponding author. Fax: +1 865 574 8363.

E-mail address: [email protected] (P.T.A. Reilly).

www.elsevier.com/locate/cplett

Chemical Physics Letters 397 (2004) 324328
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The above arguments suggest an apparent contradic-

tion or paradox. However, in reality, they are an indica-

tion that some very interesting chemistry is occurring

during the pyrolysis process. In this work, we present

compelling evidence for the existence of precursor soot

by examining in real time the gas and particle phases

of the same pyrolytic process. We show, for the firsttime, the existence of transient high mass polymer-like

species that could not be produced by gas phase conden-

sation. This evidence will be used to provide a rational

explanation of the chemistry of precursor soot.

2. Experimental

Pure atmospheric pressure acetylene was passed

through a 3.85-mm ID, 45-cm long quartz tube reactor

at a constant temperature (700 C). The flow through

the reactor was laminar at 40 ml/min. Upon exiting

the reactor, the effluent was rapidly diluted and cooledby mixing with a 4 l/min flow of pure room temperature

nitrogen. A portion of the diluted effluent was then sam-

pled into our aerosol mass spectrometer. The details of

the configuration and operation of the aerosol instru-

ment have been published elsewhere[5].

Individual particle mass spectra were obtained by

ablating each detected particle as it passed through the

center of the ion trap with a focused pulse from a

308-nm laser (1 J/cm2). Gas phase mass spectra of the

diluted effluent were obtained by deliberately mistiming

the laser pulse to miss the detected particle. Gas phase

mass spectra were also obtained by electron impact ion-

ization for the sake of comparison and validation.

The diluted effluent was collected on hydrophobic

polyvinylidene fluoride membrane filters (Millipore,

0.22 lm pore size) for further off-line analysis. The efflu-

ent was completely extracted from the filter with methyl-

ene chloride and analyzed by several techniques.

GCMS was used to identify the low mass components

(200 Da). The extractedeffluent was also deposited on C18-coated, 5-lm silica

beads by solvent evaporation. These beads are standard

packing for liquid chromatography columns. The beads

were analyzed individually with our aerosol mass spec-

trometer by sprinkling them into the inlet to yield a

direct comparison between real-time and off-line particle

analysis.

3. Results and discussion

The gas phase species from the diluted effluent of the700 C reactor were examined in real time by laser and

electron impact ionization mass spectrometry. The re-

sults of this study are presented in Fig. 1a, b, respec-

tively. Both ionization techniques yield similar spectra

with the primary differences in the degree of hydrogena-

tion and the intensities of the PAH ions in the mass

spectral distributions. These differences can be explained

by differences in the ionization energies and cross-

sections of the two techniques. Ionization by electron

impact leaves the ions with greater internal energy than

the laser ionization process and therefore results in more

fragmentation. The consistent difference in the degree of

hydrogenation of the ions by the different ionization

techniques suggests the presence of labile hydrogen even

in the gas phase species. There is no ion signal exceeding

Laser Ionization

of Gas Phase Species61

74

87

115

141

154

166

180

216

202

50 100 150 200 250 300 350 400 450 500

(a)

Electron Impact Ionization

of Gas Phase Species

6374

89 1

15

128

141152

202

192

178

165

50 100 150 200 250 300 350 400 450 500

m/z

(b)

Fig. 1. Comparison of the gas phase ion distributions created by laser ionization (a) and electron impact ionization (b) of the diluted gas phase

species from 700 C pyrolysis of atmospheric pressure acetylene.

R.P. Rodgers et al. / Chemical Physics Letters 397 (2004) 324328 325
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the noise level above 216 Da in any of the gas phase

spectra.

Fig. 2a shows the averaged particle mass spectrum in

the ever-present gas phase background. No attempt was

made at background subtraction. Comparison of the

particle and gas phase laser ionization spectra in Figs.

1a and 2a show similar ion distributions with three fun-damental differences. First, the local distributions

around each PAH mass line are broadened. This type

of broadening has been observed before [5] and results

from rapid hydrogen transfer and exchange. Because

hydrocarbon radicals can readily rearrange their carbon

skeletal structure, any given hydrocarbon formation is

just as likely to lose hydrogen as gain it as indicated

by the symmetric distribution around each peak in the

mass distribution (see Fig. 2b). The rate of transfer or

the density of hydrocarbon radicals is indicated by the

width of the distribution around each of the mass peaks

in the spectrum (see [5]). Second, the ion distribution

proceeds beyond 216 Da. Third, there is an overall in-

crease in the degree of hydrogenation as seen by the

mass shift of most intense peaks in the particle phase

spectrum. Given the similarity in the ion distributions

of the gas and particle phases, it could be suggested that

the particles that we observe in our experiment might

actually result from the dilution process creating a

supersaturation (due to the rapid change in tempera-

ture) and causing rapid condensation of the gas phase

species. Sampling-induced condensation has often been

suggested to explain the presence of particles containing

low-boiling PAHs from high temperature environments

such as flames. However, the observed mass shift be-tween the gas phase PAHs and the condensed phase

PAHs (compare Figs. 1a and 2a) negates this supposi-

tion because it would require that the gas phase PAHs

condense along with hydrogen to yield the observed

mass shift. Since hydrogen does not condense at room

temperature, the particles must have been produced by

some other process. Moreover, we have also performed

experiments at higher temperatures where the differences

in the mass spectral distributions of the gas and particle

phases become much more pronounced. Consequently,

the suggestion of rapid condensation of the gas phase

species producing our results is not credible.Given the differences between our real-time results

with the off-line measurements from other techniques,

we thought a direct comparison of real-time and off-line

techniques would be prudent. After the effluent was col-

lected on a filter and extracted with methylene chloride,

5 lm, C18-coated silica beads were deposited in the

extractant. These beads are routinely used to pack liquid

chromatography columns. The solvent was then evapo-

rated driving the effluent species into the C18 matrix on

the silica beads. The beads were then dropped into our

aerosol mass spectrometer where they were analyzed.

The averaged results of our off-line measurement are

shown inFig. 2b. Once again, the primary difference is

the greater degree of hydrogenation seen in the real-time

measurement. The remarkable similarity between the

two techniques suggests that we have a credible off-line

technique.

Unfortunately, ion traps have a limited dynamic

range in terms of the masses that can be stored. Conse-

quently, the high mass species are not as readily ob-

served under the same conditions as the lower mass

species. However, they can be observed in a separate

experiment without the low mass species. The high mass

range was scanned for both the real-time and off line

measurements. The averages from these measurementsare shown inFig. 3a, b, respectively. It is here that the

differences between the two measurements become obvi-

ous. On the left-hand side of the real-time mass spec-

trum, the tail end of the PAH distribution can be

observed. Out of that distribution, a fullerene distribu-

Off-Line Analysis

of the Extracted Effluent

from the C18 Matrix

110

98

86

74

61

154

215

202

180

166

50 100 150 200 250 300 350 400 450 500m/z

(b)

Real-Time Analysis

of the Particle Plus Gas Phase

216

203

1931

81

154

907

6

167

64

62

50 100 150 200 250 300 350 400 450 500

(a)

Fig. 2. Comparison of the averaged mass spectra from real-time measurement of single particles plus gas phase species in the effluent (a) with the

filter-collected effluent deposited on liquid chromatography column packing beads (b). It is a direct comparison of real-time and off-line techniques.

326 R.P. Rodgers et al. / Chemical Physics Letters 397 (2004) 324328
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tion emerges showing the typical 24 Da mass separation

and C60 being the most intense peak. Above 1000 Da, a

polymer-like distribution of ions appears with a very

regular 70 Da peak separation. The width of the local

distribution around each polymer mass appears to be

very broad and becoming sharper toward higher mass.

These ions have never been observed in any pyrolyticprocess, to our knowledge.

Turning our attention to the off-line measurement, we

observe very different results. First, the fullerene distri-

bution is not present. This is not surprising since fullere-

nes are not particularly soluble in the C18 matrix. What

is surprising is the loss of the structure in the polymer-

like distribution. The ion intensity is still there, but the

distribution has been completely scrambled. Since the

C18-coated particles are a packing material standard

used in liquid chromatography columns, a catalytic

process converting the high mass hydrocarbon species

into something else at room temperature in this medium

is unlikely. This suggests that the polymeric structures of

the high mass species are metastable! Further examina-

tion of the off-line spectrum reveals significant ion inten-

sity in the vicinity of 500 Da that is not observed in the

real-time spectrum. We speculate that these ions are

fragments of the metastable polymeric species.

Observation of the metastable nature of the high

mass polymer-like species suggested that other off-line

techniques do not observe these species because they

rearrange before the measurement is made. To check

this hypothesis and have a valid comparison with other

off-line techniques, we later decided to perform laser

desorption/ionization time-of-flight (LD-TOF) mass

spectrometry on the same extractant sample that was

used in the C18 off-line measurements. A few drops of

the extractant were deposited on a stainless steel probe.

The solvent was allowed to evaporate and the probe was

inserted into the spectrometer and pumped down. A fo-

cused nitrogen laser was used to desorb and ionize thesample. The averaged mass spectrum is shown in Fig.

4. The observe LD-TOF distribution does not look at

all similar to our C18 off-line measurements in Figs.

2b and 3b. However, it does look similar to the distribu-

tion observe by Dobbins and coworkers [2]. In the

Real-Time

Particle Mass Spectrum

1045

1109

1178 1

248

1318

1388

1458 1

528

1597

1670 1

740

300 500 700 900 1100 1300 1500 1700

PA

C60

C70

C50C

40

(a)

Off-Line Extractant

Mass Spectrum

From C18 Particles

300 500 700 900 1100 1300 1500 1700

m/z

PA

Polymer

Fragments?

(b)

Fig. 3. Comparison of the averaged high mass spectra from real-time measurement of single particles plus gas phase species in the effluent (a) with

the filter-collected effluent deposited on liquid chromatography column packing beads (b). It is a direct comparison of real-time and off-line

techniques.

150 250 350 450 550 650 750

m/z

202

217

230

242

252/4

266

276

290

302

316

330

340

350

364

378

181

LD-TOF

of Extractant

Fig. 4. The averaged laser desorption time-of-flight mass spectrum of

the filter-collected effluent extract that has been deposited on a

stainless steel probe.

R.P. Rodgers et al. / Chemical Physics Letters 397 (2004) 324328 327
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LD-TOF spectra, we speculate that the most intense low

mass species are absent or greatly diminished by evapo-

ration that occurs in the vacuum during pump down.

No ion intensity was observed in the high mass region

above 1000 Da.

The polymer-like high mass species must have de-

graded into PAHs and perhaps other evaporable lowmass species. This is the reason that these high mass spe-

cies are not observed by typical off-line measurements.

Why do we see large mass species in the C18 off line

measurements and not in the LD-TOF measurements?

Time and perhaps the nature of the desorption media.

The C18 measurements were done the same day the

effluent was collected whereas the LD-TOF experiments

were done a couple months later on the same sample.

Additionally, it is conceivable that the stainless steel

probe surface may have facilitated rearrangement

whereas the C18 matrix did not.

The above comparisons may explain the previously

described apparent paradox. It is clear that the chemical

nature of precursor soot in high temperature environ-

ments is very different from that measured under room

temperature conditions. In pyrolytic environments, pre-

cursor soot is not an oil of low boiling PAHs. It is prob-

ably not a polymer either. The reason for this suggestion

comes from acknowledging the rapid hydrogen transfer

and exchange observed in real-time PAH distributions.

Rapid hydrogen transfer and exchange results in a large

concentration of radicals in the precursor soot. Radical/

neutral reactions are very rapid and preserve the radical.

Consequently, a system of radicals continuously rear-

ranges its carbon skeletal structure by making andbreaking carbon bonds. Such a system is best thought

of as one big molecule whose substructure is continually

changing. It therefore cannot be thought of as a polymer

because a polymers substructure does not change.

So why do we see a polymer-like distribution in the

mass spectrum? Our guess is that the metastable struc-

ture results from the rapid cession of hydrogen transfer

and exchange upon cooling of the particle. The radicals

are then rapidly eliminated by radicalradical recombi-

nation. The resulting structure is not thermodynamically

stable at room temperature and probably neglects rules

of aromaticity for the expedience of radical elimination.

This is a good reason for the existence of the short-lived

metastable species. The nature of the polymer-like struc-

ture still eludes us. We have tried many combinations,

but still cannot account for the 70 Da (C5H10) peak sep-

arations. We are reasonably sure that the metastable

structure evolves into the more stable PAHs observed

in the low mass region as the polymeric structure rear-

ranges itself in order to obey the rules of aromaticity.

Consequently, the creation of precursor soot is another

path to the production of PAHs in pyrolytic processes.

4. Conclusion

Comparison of real-time and off-line measurements

has permitted the nature of the chemistry of precursor

soot to be revealed. Observation of the metastable poly-

mer-like mass distribution yields conclusive proof that

precursor soot in the high temperature environment is

chemically different from that observed at room temper-

ature. The polymer-like high mass species indicate that

precursor soot in the high temperature environment is

polymeric with its substructure in a continuous state

of flux due to the presence of hydrocarbon radicals. This

is what gives precursor soot its remarkable high temper-

ature stability and phenomenal growth rate. The metast-

able high mass species apparently evolve into the small

(low boiling point) PAHs typically observed from pyro-

lytically condensed hydrocarbon species when the tem-

perature is reduced. These observations support the

existence of precursor soot and help elucidate its

chemistry.

Acknowledgements

Research Sponsored by the Division of Chemical Sci-

ences, Geosciences, and Biosiences, Office of Basic En-ergy Sciences, US Department of Energy, under

Contract DE-ACo5-00OR22725 with Oak Ridge

National Laboratory, managed and operated by UT-

Battelle. We also thank Dr. Christopher L. Hendrickson

and other members of the Marshall group who helped

obtain our LD-TOF data.

References

[1] R.A. Dobbins, H. Subramaniasivam, in: H. Bockhorn (Ed.),

Formation in Combustion, vol. 59, Springer, Berlin, 1994, pp.290301.

[2] R.A. Dobbins, R.A. Fletcher, W. Lu, Combust. Flame 100 (1995)

301.

[3] C.W. Sweitzer, G.L. Heller, Rubber World 134 (1956) 855.

[4] W.G. Parker, H.G. Wolfhard, J. Chem. Soc. (London) (1950) 2038.

[5] P.T.A. Reilly, R.A. Gieray, W.B. Whitten, J.M. Ramsey, Combust.

Flame 122 (2000) 90.

328 R.P. Rodgers et al. / Chemical Physics Letters 397 (2004) 324328

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