Formation of Flame Ions, Cluster, Nanotubes and Soot in
Hydrocarbon Flames
H. Jander and H.Gg. Wagner; Institut fuer Physikalische Chemie, Tammannstr. 6,
37077 Göttingen, Germany
Abstract
The actual work deals on the present state of research about flame ions, polyaromatic
hydrocarbons (PAH), nanotubes, fullerenes and soot particles in premixed flames.
Experimental arrangements for the detection and quantitative investigation of flame ions are
presented. In addition, the influence of ions on flame chemistry and carbon particle formation
at non-sooting and sooting conditions is discussed. The study also focuses on the formation
pathway from the flat PAH to fullerenes, nanotubes and soot particles. In this connection, it is
reported on the features of arched “aromeres”, high reactive metastabile species and leading
candidates for soot precursors and fullerene formation. These aromeres seem to be a kind of
“switch” capable to produce either fullerenes or soot particles depending on the reaction
conditions: at lower flame temperature and a high number density of small unsatured
hydrocarbons, bimolecular reactions are favored and thus the formation of soot particle
exceeds that of fullerenes. Also, it is shown how the further growth of soot particles can be
described, namely by soot mass growth and by coagulation processes, respectively by
aggregation in strong sooting flames.
Finally, the work concentrates on fullerene, nanotube and soot particle measurements. Typical
values for soot volume fractions and particle diameters at various reaction conditions are
given.
Introduction
In general, a flame is a very complex system due to its chemical and physical processes.
During the combustion process, many different substances are formed. There are very shortlived species such as radicals and flame ions. Besides, very stable compounds such as the
watergas compounds CO, CO2, H2 and H2O are produced. Additionally, in a fuel rich flame,
the PAH and soot can be found. Among these hundred of different species within a flame,
many compounds are well-known since a long time. On the other hand, highly crystallized
carbon particles, the fullerenes and nanotubes, exist which were discovered not until 1985 [1].
Although flame ions are known since Haber’s work [2], quantitative experiments about the
flame ions were performed for the first time by the Homan group [3,4,5,6,7]. In a sooting
premixed flame, for all neutral hydrocarbon species and also soot particles ionic counterparts
were found. But the concentrations of the HC-ions are too low to play an important role in
the flame chemistry [8].
The nanotubes and fullerenes were at first developed not in flames but by the method of
vacuum vaporization of graphite [1]. Already two years later fullerenes could be detected in
premixed flames [3]. The formation of the high crystallized carbon particles takes place only
in special flames at special flame conditions and the “formation windows” where the highordered carbon particles can grow are small. They can be formed in the low pressure
-2C2H2/O2- and C6H6/O2-flame [3,5,7], in the pyrolysis flame [9] and in the counterflow
diffusion flame [10]. The easiest way to produce nanotubes is that by multi wall nanotubes
(MWNT) obtained with and without catalysts. But the scientists aim at the production of
single wall nanotubes (SWNT) which succeeds by seeding the flames with metal catalysts
[5,9,11].
Open questions on the formation mechanism of the fullerenes and nanotubes in flames
remain, especially about the influence of the catalysts on the their growth. Based on own
experimental measurements, solely the groups of Homann and Howard proposed pathways to
fullerene formation [6,11]
In contrast to the formation of carbon nanoparticles that of soot in premixed flames is better
understood. Soot volume fraction, particle number density and particle diameter at different
flame parameters such as mixture composition, flame temperature and pressure are available
[12]. The growth of soot particles in the post flame gases can be described by soot mass
growth and coagulation process [13]. In the recent years, the formation of soot aggregates was
studied in different flame types [14,15,16].
The present state of the research about the flame ions, PAH nanotubes, fullerenes and soot
particles shall be reviewed in what follows.
Experimental Conditions and Techniques
The experimental set up consists of a differentially pumped molecular beam sampling system
which is coupled with a linear time of flight mass spectrometer (TOF-MS). Flat premixed
laminar acetylene-oxygene of benzene-oxygene-flames burnt at low pressure, in general at 2.7
kPa. In the case of flame ion detection the molecular beam which contains the ions were
collimated through a skimmer into a modified Bendix ion source without electron gun [3]. In
the case of neutral compound detection (high molecular PAH) the flame gases were ionized
on-line through REMPI at a wavelength λ = 208 nm [4]. The ions were than separated with a
reflectron and detected with microchannel plates in a chevron arrangement [3,4].
Soot particles were measured in a premixed C2H4/air-flame at 1 bar [12]. The flames were
stabilized on a cooled porous sinter plate or on a steel capillary burner. They were surrounded
by a second non sooting flame. Absorption measurements were performed at λ = 488 up to
850 nm. For light scattering measurements an argon-ion line of λ = 488 nm was used. When
the particle size exceeds the Rayleigh regime ( π ⋅ dp / λ < 0.1) Mie corrections were applied.
Soot volume fraction, fv, particle number density, N, and particle diameter, d, were evaluated
from scattering and absorption signals. The complex refractive index from Sarofim and
Dalzell were taken (1.57-0.56i) [24].Flame temperature was measured by the Kurlbaum
method [25].
For the gas analysis a quartz sampling probe was inserted along the flame axis. The flame
gases were analyzed gaschromatographically. The PAH were condensed (N2 liq. cooling) and
analyzed with GC-MS using two standards.
Hydrocarbon Flame Ions under different Flame Conditions
Since a long time, the presence of flame ions is well-known and extensive results about ions
in flames were published [3,7,8]. In the pemixed flames, positive and negative ions as well as
electrons are formed at the beginning of the main oxidation zone. The flame itself is electric
neutral. The positive ions arise through chemiionisation reactions. The negative charged ions
are formed through dissociative or associative electron attachments [8].
-3The absolute concentrations of individual kinds of ions can be obtained through the overall
detection sensitivity μi, where μi is the ratio of signal intensity Ii of an ion I to its absolute
number density ni in the flame. It depends on various geometric factors of the sampling beam
and the pulsed beam in the TOF-MS [3,17].
In lean and slightly fuel rich flames, only very few ions dominate, whereas the H3O+ with μH20
= 0.54ּ10-10 V· cm3 has a relatively high value. The polyynic ion and the PAH+ 179 – 289
amu have a detection sensitivity of μPAH = 0.54ּ10-10 V· cm-3 while μi of C60+ = 0.38ּ10-10 V·
cm-3 [3]. The hydrocarbon ions in the fuel rich and sooting flames prevail due to the detection
sensitivity μi and also due to the number of ions which can be seen from the compressed
mass-spectrum in Fig. 1, where nearly every peak appears as an ion. In the mass range of
2·103 amu there is a minimum in the distribution. This observation can be interpreted as being
due to coagulation of PAH+ of this mass range with neutral PAH of similar mass to form the
first charged soot particles. Otherwise if only growth by addition of small hydrocarbon
molecules took place one a smoothly increase of the PAH+ peaks would be expected.
Fig. 1. Compressed mass spectrum of charged species in a sooting C2H2/O2-flame, C/O = 1.0, vu = 24 cm/s,
P = 27 mbar [4].
Fig. 2 shows profiles of two PAH. C20H12 and C59H19 in their neutral and charged state. The
profiles are shifted to larger distances from the burner with increasing C-numbers. The degree
of ionization increases from about 0.2·10-4 for C20H12 to 0.5·10-3 for C59H19, which is
qualitatively in line with presumably decreasing ionization potential.
-4-
Fig.2. The concentrations profiles of neutral and positively charged PAH in flames are similar. Shown are the
PAH of C20H12+ /C20H12 and C59H19+ /C59H19 in a C6H6/O2-flame [7].
Further in the limit of error there is no difference in the relative shape and position of the
profiles of neutral and ionic PAH. This has also been tested for larger PAH. It shows clearly
that PAH ions apart from their much lower level of concentration behave like their neutral
counterparts.
In the sooting flames, the mole fraction of the ions is in general about 10-9 – 10-10. The mole
fraction of the neutral counterparts is about 10-3 – 10-6. It means concentration of the flame
ions are lower by a factor of 104 – 106 in comparison to that of the neutral PAH. Based on
these results, an unimportant role of the higher PAH ions in the flame chemistry [8] has to be
assumed.
The Growth of the PAH
To day it is well-known that the precursors of the high crystallized carbon cluster are the high
molecular PAH and the aromeres [6]. The PAH are formed in the main oxidation zone of the
flame where they grow by successive C2H2 addition and later on by combinative reactions
(direct reaction of two PAH) to the high molecular species [6]. In this process, the H-rich
PAH are the most reactive ones, that is the PAH with 4 carbon- and 5 carbon bays and/or with
pentagons carrying CH2-groups.
The discovery of the direct precursors of the fullerenes was possible as this class of
compounds form easily negative ions, in contrast to the PAH in this mass range [6], see Fig.
3. It shows the existence of a continous background together with the negative charged
fullerenes peaks. This continous mass spectrum appears always intermediary when fullerenes
or soot are formed.
-5-
Fig. 3. Overview of the mass spectrum of negative fullerene ions with a continous background of negative
charged aromeres [6].
Homann infered of a new class of C-compounds from these results and named them
aromeres (aromate and oligomere) [6,18].
The aromeres are highly reactive metastable species which can be read off from their
continuous mass spectrum. Their structure differ from that of the flat PAH, otherwise discrete
peaks in the mass spectrum should appear.
The aromeres with their arched structures are the direct precursors of the fullerenes and soot
particles. That is, aromeres are the switch in the mentioned formation pathways and it
depends on the flame conditions whether fullerenes or soot particles are formed.
The Field of Cluster, Nanotubes and Fullerenes
The fullerenes and nanotubes were detected at first time not in flames but through laser
ablation applied by Kroto et.al. [1]. Already two years later Homann could identify fullerenes
in premixed low pressure flames in form of their negative ions [3]. In 1991, the fullerenes
could be analyzed in premixed low pressure flames in concentrations high enough for being
extracted from soot particles and analyzed by the HPLC-method [11].
Research on clusters has been neglected by the scientists for a long time although this type of
compounds is the bridge between molecules and the high molecular weighted species such as
the soot particles or crystals. However, from 1985 onwards, the research on clusters received
a vigorous development due to the formation of the high crystallized carbon particles of the
nanotubes and fullerenes.
Fullerenes and nanotubes are only formed in some special types of flames with special flame
conditions. Two types of flames are most promising for producing nanotubes and fullerenes
without catalyzed metal compounds, that is the low pressure C2H2- or C6H6/O2/Ar- flames
[6,11,19] and the counterflow diffusion flames [10]. In the pyrolysis flame, the formation of
nanotubes succeeds only with catalysts [9].
In the premixed 1 bar hydrocarbon/air/O2-flames, with and without catalyzed metal
compounds, neither nanotubes nor fullerenes could be detected. Only the hyperfullerenes [20]
were analyzed. Obviously, the residence time at the end of the particle formation, respectively
at the beginning of the sooting zone is too short for building up high crystallized C-particles.
Fig. 4 shows a low magnification overview of MWNT formed in the pyrolysis C2H2-flame
using cobalt nitrate as catalyst precursors [9]. Many tubes extended for length of several
microns possess a generally uniform cross-sectional area and exhibit a small amount of
-6curvature. The inner core of some tubes are partially filled with cobalt otherwise they are
hollow.
Fig 4. Low magnification imaging of MWNT formed in a pyrolysis flame with catalyst [9].
In the literature, many experimental and modeling results about the formation mechanism of
nanotubes are given. Most of the contributions refer to results which were obtained by the
methods of vacuum vaporization of graphite [1,20,21]. A good compilation about nanotubes
is presented in [22].
On the experimental results about the formation of fullerenes in flames, a detailed formation
mechanism of the fullerenes is given by Homann et al. [6]. In flames, the high crystallized Cparticles are formed by the high molecular PAH, by the aromeres together with small HCspecies in a hydrogen rich environnement. Homann suggested a closing of the cage by a rapid
decomposition of H-atoms at the periphery of the aromeres molecules, namely by
unimolecular reactions, the so called “zipper-mechanism”. If the flame temperature is not too
high and a high number density of small unsaturated HC are available than the way to soot
particles through bimolecular reactions is favoured.
Possibly, the nanotubes are formed in a similar way as the fullerenes.
Up to now there is no answer how the metal catalysts influence the formation and growth
mechanism of the nanotubes or fullerenes in flames.
Formation of Soot Particles at different Flame Conditions
At the beginning of the sooting zone at about 4 –5 mm above the burner in a 1 bar HC-flame,
soot particles are formed indicated by the yellow luminosity of the post flame gases. In this
region of the flame, soot particles are of small diameter (d ~ 2 nm) and of high number
density (N ~ 1012 cm-3) resulting in a soot volume fraction of fv < 10-8. On their way through
the flame, soot particles grow by coagulation and surface growth processes. At the end of the
combustion process particle diameter in a meaning sooting flame is about d ∞ = 30 –50 nm
while particle number density has decreased to N ∞ ~ 109 cm-3, and soot volume fraction fv ∞
amounts to > 10-7.
The carbon content of soot related to the carbon content of the unburned gas in an
atmospheric C2H4/air-flame amounts to 1.7 % , while the sum of all hydrocarbons in this
flame is about 15.8 %. The main part of the fuel carbon is transferred into the compounds of
CO and CO2 , see also Table I.
-7-
Products
CO + CO2
CO2
Soot
ΣHC
CH4
C2H2
PAH
% Carbon related to the
unburnt Carbon content
82.5
25.0
1.7
15.8
4.0
11.0
~ 0.02
Table III. Carbon containing post flame products in a C2H4/air flame.
The sooting region of a hydrocarbon flame in a temperature-mixture composition field forms
a kind of “dome”, see Fig. 5. A three-dimensional plot of the soot volume fraction at the end
of the combustion process shows fv vs flame temperature and mixture composition (C/Oratio). The soot region is framed by threshold of soot formation, the thick line. From this plot,
the regions with and without soot can be read off.
If the flame pressure is increased to about 10 bar or higher, the sooting field becomes larger,
that is, the “soot dome” becomes higher and broader.
Fig. 5. Sooting region in the temperature-mixture composition field. C2H4/air-flame C/O = 0.68, p = 1 bar
[12,13].
Between 1 – 10 bar, an increasing soot volume fraction with pressure, fv ∝ p2, is found.
Against that, for p > 10 bar fv is proportional to p.
Different results are obtained for C2H2. While at 1 bar C2H2 is the main hydrocarbon
compound of all analyzed hydrocarbons in the post flame gases, it decreases rapidly with
flame pressure. At 70 bar, the concentration of C2H2 is below the detection limit (detection
limit xi ~ 1 ppm). Also, the sum of all detected PAH carbon concentrations is about two
-8orders of magnitudes smaller than that of soot.
The post flame gases of high pressure C2H4/air-flames consist of only the water gas
components, CH4 and soot which is seen in Fig.6.
Fig. 6. Carbon balance of the post flame gases for different pressures. ρci is the carbon density of the species i in
g/ cm3.
Conclusions
Flame ions are always formed in the flames, in the non-sooting as well as in the sooting
flame. Nearly every neutral HC-compound does exist also in its ionic form. The mole
fractions of the ions are in general about 10-9 – 10-10 which is approximately hundred times
lower than that of their neutral counterparts. Besides their low level of concentrations, the
ions behave like their neutral counterparts. In particular, they do not supply faster growth of
carbon particles.
The field of cluster has received a rapid development through the detection of the nanotubes
and fullerenes, substances which are highly crystallized. In flames, this class of compounds
could be analyzed in low pressure C2H2/O2- and C6H6/O2-flames, in the pyrolysis and in the
counterflow diffusion flame with and without catalytic substances, such as Fe(CO)5,
ferrocene. SWNT and MWNT could be detected. The type of nanotube which are produced
in the flame depends on the flame conditions and the catalytic compound. Many tubes
extended for length of several microns, while the diameter varies between 10 - 100 nm.
At the end of the particle formation zone soot particles are formed. The young soot particles
are very mobile with a small diameter of about 2 nm. In the first part of the post flame gases,
soot particles grow rapidly, in the later phase of the combustion process the growth of the
particles ceases. In this part of the flame, they have reached diameters of d = 30 – 50 nm.
Soot volume fraction amounts to fv > 5ּ10-7. Soot volume fraction depends on mixture
composition, flame temperature and pressure. For a constant flame pressure, the sooting
region in the temperature-mixture composition plot forms a kind of “dome”.
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