J . Phys. Chem. 1990, 94, 5381-5391
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Ions and Charged Soot Partlcles in Hydrocarbon Flames. 2. Positive Allphatic and Aromatic Ions in Ethyne/Oxygen Flames Ph. Gerhardtt and K. H. Homann* Institut f u r Physikalische Chemie der Technischen Hochschule Darmstadt, Petersenstrasse 20, 0 - 6 1 0 0 Darmstadt, FRG (Received: October 18, 1989)
Positive and negative ions up to masses >lo3 u in premixed low-pressure ethyne/oxygen flames were analyzed by means of a time-of-flight mass spectrometer. This work deals with positive ions. Absolute concentrations of different groups of ions were calculated from comparison with absolute total ion concentrations under different conditions. After a very fast formation of aliphatic C,H5+ and substituted oxomethylium ions, mono- and polyaromatic hydrocarbon ions (PAH') were formed in the oxidation zone even at C/O below the threshold of soot formation. In nonsooting flames they were totally consumed by thermal decomposition, generating polyynic ions C3+ZnH3+and C4+ZnH3+,n = e - 6 . At lower temperatures c-C3H3+was present while at maximum temperature the linear form probably prevailed. The polyynic ions were in equilibrium with ethyne and polyynes. Enthalpies of formation were obtained for ions up to CI4H3+from the equilibrium constants. Mass distributionsof odd- and even-carbon-numberedC,H3+ could clearly be distinguished and their resemblance with Poisson distributions is discussed. In sooting flames, PAH+ up to masses of about 325 u were decomposed before the flame reached its maximum temperature while larger species continued to grow forming charged soot. After thermal decomposition of the lower mass PAH+ at temperatures 21400 K they were formed again at a much slower rate when the burned gas cooled. However, they did not exceed a mass of about 400 u. The different mechanisms of PAH' formation in the oxidation zone and in the postflame gas is discussed.
1. Introduction The ionic structure of fuel-rich hydrocarbon flames is of interest for a number of reasons. The large variety of ions which are formed in a relatively dense hydrocarbon atmosphere at different temperatures gives information on the types of ion-molecule reactions which occur under these conditions, and provides answers to the question which of these ions are stable at low or at high temperature. Of particular interest are condensation reactions leading to large aromatic ions and the thermal stability of these products. The transition of large ions to charged soot particles can be studied. Other questions concern possible partial equilibria between ions and the corresponding neutral components of the flame.' Mass spectrometry of flame ions has revealed the existence of polyhedral carbon molecules (fullerenes) in sooting flames and allows a study of their positive and negative ionization and their mass distributions under conditions of high temperature.2 A large fraction of the negative charge carriers are negative ions.3 Their different structure and behavior allows one to distinguish between certain types of reactions which are characteristic for ions of the respective sign. A number of papers have been published which report on the relative concentration profiles of ions in fuel-rich and sooting flame^.')^-^ However, a systematic investigation up to masses of several ten thousand mass units is missing. Using time-of-flight mass spectrometry and analysis of kinetic energy and velocity of ions in a sample beam, we have studied positive and negative flame ions in this mass range. In the first paper, we reported on the properties of the ion beam from a probe, on mass distributions of positively charged soot particles, and profiles of total positive and negative ionization in low-pressure C 2 H 2 / 0 2flames.8 The present paper focuses on the formation of positive hydrocarbon ions up to masses of about IO3 u. Special attention is given to polyynic ions and their equilibria with neutral polyynes and to polycyclic aromatic hydrocarbon ions (PAH+) and their growth and decomposition in the oxidation zone and in the burned gas.
2. Experimental Section 2.1. Flames and Sampling Method. Low-pressure flat premixed ethyne-oxygen flames burning on a cooled sintered disk burner of 75 mm diameter were investigated. The standard *To whom correspondence should be addressed. 'Present address: Forschungszentrum Jiilich, GmbH, ICH 3, Postfach 1913,D-5170 Jiilich. FRG.
0022-3654/90/2094-5381$02.50/0
burning pressure, p, was 2.7 kPa (20 Torr). Commercial gases, ethyne (98%) and oxygen (>99.5%, Messer-Griesheim), were used without further purification. Only for measurements within the early oxidation zone was it advisable to purge the ethyne from acetone (2%) by passing it through a cold trap of -50 OC since certain ions originated from this impurity. The C / O ratio was varied from 0.40 (stoichiometric) to 1.12 (sooting), and the unburned gas velocity, u,, ranged from 35 to 63 cm/s. The burning conditions of the flame are given in the captions to the respective figures. The probing of the flame has been described earlier;* in short (Figure l), a conical uncooled quartz probe covered at the tip and plated inside with a thin layer of platinum was used to form a supersonic free jet at a background pressure of less than 0.1 Pa. The burner and the conducting parts of the probe were kept electrically at ground potential. A beam with a rectangular cross section was formed by means of a skimmer-like diaphragm with an opening of 0.85 X 7.35 mm2 placed at a distance of 45 mm from the sampling nozzle. The intensity of ions with masses SI00 u in the collimated beam was improved when the skimmer was biased by a few volts oppositely to the sign of the ions. 2.2. The Time-of-Flight Mass Spectrometer (TOF-MS). The setup of the TOF-MS is shown schematically in Figure 1. The mass spectrometer was evacuated by a 3000 L/s oil diffusion pump with a machine-cooled baffle to a pressure of normally 2 X IO4 Pa. The skimmed beam was shaped by an entrance slit (1.0 X 12 mm2) to a first accelerating field. In this field the positive ions were drawn toward a first grid (1 2 mm diameter) of fine nickel mesh to which a rectangular negative pulse (-1 50 to -320 V, 1-5 ~s duration, 5 or 10 kHz) was applied. Opposite to this drawing grid was a back plate to which the pulse was applied for negative ( I ) Michaud, P.; Delfau, J. L.; Barassin, A. 18th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1981;p
443. (2) Gerhardt, Ph.; Loffler, S.; Homann, K. H. Chem. Phys. Lett. 1987, 137, 316. (3) Green, J. A. AGARD ConJ Proc. 1965, I , No.8, 191. Hayhurst, A. N.;Jones, H. R. N. 20th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1985;p 1121. (4) Olson, D. B.; Calcote, H. F. 18th Symposium (Internarional) on Combustion; The Combustion Institute: Pittsburgh, PA, 1981;p 453. (5) Goodings, J. M.; Bohme, D. K.; Ng, C.-W. Combust. Flame 1979,36, -2 1 , 45_ . (6)Tanner, S. D.; Goodings, J. M.; Bohme, D. K. Can. J . Chem. 1981, 59, 1760;1982, 60,2766. (7) Hayhurst, A. N.;Jones, H. R. N. J . Chem. Soc., Faraday Trans. 2 1987, 83, -1.
(8)Gerhardt, Ph.; Homann, K. H. Combust. Flame To be published.
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 13, 1990
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Gerhardt and Homann TABLE I: Detection Sensitivities of Various Groutis of Ions
*,--a
i\
detection sensitivity, positive ions
1
'
icr
CI
Figure 1. Sampling system and time-of-flightmass spectrometer. (The pumping lines for the nozzle probe are arranged out of plane.)
ion analysis. The drawing grid was followed by two similar grids (IO mm diameter) through which the ions were accelerated to an energy of 2.6 keV, which was slightly mass dependent. After acceleration, the ions were deflected vertically and (if necessary) horizontally by two plate capacitors. Vertical deflection compensated their kinetic energy from the sampling process. The total flight length was 530 mm. The detector consisted of an aluminum plate placed at a right angle to the flight tube axis, and a channeltron (single channel multiplier CEM 4830, Galileo). The whole detector system was movable in the vertical direction to optimize the impact of ions on the aluminum plate. Secondary electrons from the AI plate were accelerated by a potential difference of 250 V to the cathode of the channeltron. When positive ions were measured, the anode of the multiplier was at ground potential. The signals were amplified by a broad-band amplifier and fed into a boxcar averager (Model 162, PAR) which was connected to an x-y recorder. Simultaneously the amplified signals could be displayed on a 100-MHz oscilloscope (Tektronix). For measurements of negative ions the respective accelerating and deflecting potentials were reversed and the potential of the multiplier anode was +5.0 kV. In this case the output of the multiplier was connected to the amplifier by a ceramic capacitor (1 nF, 6 kV). Because of the width of the incoming ion beam a method of longitudinal (or time) focusing was needed. The beam width caused a distribution of arrival times at the first draw grid, particularly for heavy ions. But with longer arrival times the kinetic energy attained by the draw pulse also increased. For all ions in the beam the later arrival time at the draw grid could largely be compensated by accelerating to a higher velocity. This was achieved by using a suitable pulse shape and biasing of the draw grid and the back plate so that the spread of arrival times at the detector was minimized.
3. Results 3.1. Criteria for Ion Assignments. While there are only a few kinds of ions in stoichiometric and lean ethyne/oxygen flames, their number increases drastically when the flame is made more fuel-rich. In sooting flames the mass range extends to thousands of atomic mass units (u). This makes an assignment of a molecular formula to an ion increasingly difficult. There are, however, a number of criteria that enable one to interpret flame mass spectra correctly or at least to make some reasonable assumptions as to the nature of the ions. For some ions the hydrogen content is known from experiments with perdeuterated f ~ e l . ' * ~ In * ' ~many cases ions differ from related molecules only by an additional proton or missing H-, or by a missing proton in the case of negative ions which will be dealt with in another paper. There are no fragmentation patterns in flame ion mass spectra. However, many ionic (and also uncharged) hydrocarbons are almost always members of a series. Thus the repeatedly occurring differences by the mass of a C, CH, CH2,or C2 increment is a clue to the presence of a series. Within a series. ions usually have similar concentration profiles in the flame. Temperature dependence gives additional information. It may be assumed that larger hydrogen-rich aliphatic hydrocarbon ions are unstable at high temperature, as are the related neutral (9) Calcote, H. F.; Keil, I).G . Combusr. Flame 1988, 7 4 , 131. (10) Loffler, S . Dissertation Technische Hochschule Darmstadt, 1990,D 17.
polyynic ions oxygenated ions aromatic ions up to 165 u
V cm3 0.54 f 0.08 0.50 f 0.08 0.57 f 0.1 0.45 f 0.07
PAH', 179-289 u larger PAH* C,,+ (polyhedral carbon ions)
0.54 f 0.08 0.50 f 0.1 0.38 f 0.15
H30+
radicals" and molecules,24 and may only be expected in lowtemperature zones. Oxygenated aliphatic hydrocarbon ions do probably only occur in flame zones where molecular oxygen is still present and where the temperature has not reached its maximum. One may expect that large hydrocarbon ions are aromatic, as are the respective neutral flame components. A strong increase of positive ionization in the soot forming zone is due to species which become thermally ionized, Le., soot particles. 3.2. Determination of Absolute Ion Concentrations. The ratio of signal intensity, Zi, of an ion i to its absolute number density, n,, in the flame is the overall detection sensitivity, F ~ for , a given status of operation of the sampling system and the mass spectrometer. The F~ depended on various geometric factors of the sampling beam and the pulsed beam in the TOF-MS as well as on the characteristics of the detector and the preamplifier. They were determined by a comparison of the mass spectrum with the total ion concentration measured via the total ion current in the nozzle beam.* The intensities of all kinds of ions i are described by a system of linear equations
Summation is made over all kinds of ions, combined in groups of similar species (if necessary), at various heights, hj, above the burner and in different flames labeled by the parameter, Fk. n is the total concentration of ions of the respective sign. The overdetermined equation system was solved for the pi by Gauss-Seidel iteration. The relative errors of the k i were comparable to the reproducibilities of the single measurements only if the number of different kinds of ions in the equations was not greater than five. In slightly fuel-rich flames only a few kinds dominated, and consequently their sensitivities could be determined fairly accurately. If related ions were combined in a group, mean sensitivities were obtained for the group. Absolute concentrations of polyhedral carbon ions resulted from energy distributions in the unskimmed nozzle beam since these ions appeared as a distinct part of the distribution.s Table I gives the detection sensitivities obtained in this way for certain groups of ions. 3.3. Polyynium and Polyenylidenium Ions (Groups A1 and A2). These ions have the general formulas C4+ZnH3+(AI) and C3+2nH,+(A2), n = k 6 . Together they are occasionally termed polyynic ions or C,H3+ in this work. (The letters AI, A2, B, ... refer to Table 11 in which most ions are listed.) While Delfau et al. detected ions up to C9H3+at low burning pressure' Hayhurst et al. reported ions up to C,,H,+ in nonsooting atmospheric pressure flame^.^ These are ions with a largely linear carbon skeleton as have the uncharged polyynes, CkH2. The unprotonated analogues of the A2 group would be polyenic diradicals. Figures 4-6 show profiles of these ions for different C/O. The profiles of C3H3+had a double maximum. This is interpreted as being due to the formation of the two isomers, cyclic and linear.12 Since the enthalpy of formation is less for the cyclic isomer it is supposed that it belongs to the first maximum where the temperature is lower. In fuel-rich flames, the most noticeable ions at the end of the oxidation zone were of the AI and A2 groups, where they reached their maxima, and in the burned gas where they decreased much less than other ions; see Figures 3, 5 , and 6. However, they were ( 1 1 j Warnatz, J. 20th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1984; p 845. (12) Ausloos, P.; Lias, S. G . J . Am. Chem. Sac. 1981, 103. 6505.
The Journal of Physical Chemistry, Vol. 94, No. 13, 1990 5383
Ions and Charged Soot Particles in Flames h=Lmm
4
I
i
C
2
4
6
0
2 4 6 8
2 4 5 8
2 4 6 8 2 4 6 Height above \he burnerimm
Figure 4. Concentration profiles of various positive ions in the oxidation zone of a stoichiometric flame. The numbers denote ion masses in atomic mass units. The lettering refers to the different groups of ions listed in Table 11, C / O = 0.40, u, = 63 cm/s, p = 1.5 kPa. I
107
"
"
"
I
"
8.E
AI
40 100 200 300 400 M a s s / u Figure 2. Mass spectrum of positive ions from the beginning of the oxidation zone. C/O = 1.12, u, = 42 cm/s, p = 2.7 kPa; ACE = protonated acetone (impurity).
m
CrnG
=S
h = 12"
1
9 I
8
I
I
t
1
8
8
,
I
I
I
I
3 Figure 5. Concentration profiles of various positive ions in the oxidation zone of a fuel-rich flame. For numbers and letters cf. Figure 4. C/O = 0.80, u, = 42 cm/s, p = 2.7 kPa.
PAH +
40 100 200 300 400 500 M a s s h Figure 3. Mass spectrum of positive ions from the end of the oxidation
zone; same flame as in Figure 2.
absent from the lower temperature part of the oxidation zone, cf. Figure 2. Their profiles (with the exception of C3H3+)showed a uniform behavior. Their relative concentrations resembled those of a Poisson distribution and can be explained by partial equilibria with ethyne and polyynes; see section 4.1. The mass distributions were dependent on the C/O ratio and of C4+2nH3+and C3+2nH3+ on the maximum flame temperature. With increasing C/O the
center of mass of the distributions was shifted to greater n, while an increase of temperature had the opposite effect. Figure 7 (left, middle) shows the maximum concentrations of the C3+znH3+as a function of C/O. Ions with the general formula C,,,H,+ ( m L 3) were also formed, and occurred in greater abundance than the neighbouring C,H3+ for m = 4 and m > 6 in nonsooting flames at low pressure and high temperatue (not included in Table 11). Positive ions with only one H atom were not detected in general. Only in a very hot flame with u, = 220 cm/s, C/O = 0.6 was an ion with mass 61 u, presumably CSH+,detected. 3.4. C,HS Ions (Group B ) . These ions with m = 4-1 are grouped together because of their hydrogen content, but their profiles in the flames did not show the similarity that was found within the groups AI and A2. Two of them, C4H5+(53 u) and C6HS+(77 u), had double maxima in their profiles (Figures 4 and 5). They were among those ions found earliest in the flame. Their first maxima lay between 1.8 and 3.2 mm above the burner depending on the C/O ratio and the unburned gas velocity; see Figures 4 and 5. A possible interpretation is the existence of cyclic and linear isomers of C4H5+and C6H5+. Plausible cyclic ions would be methylcyclopropenylium and phenylium while the linear C,Hs+ probably are protonated vinylethynes. The profiles of CsHS+(65 u) and C7HS+(89 u) displayed only one maximum lying between about 3.5 and 6 mm, which coincides with the second maximum of C4HS+and C6HS+.These relatively hydrogen-rich
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TABLE 11: Groups of Hydrocarbon and Oxygenated Hydrocarbon Ions _______-_________._________________ macs/u mol formula presumed struct remarksu
