J. C. Biordi, C. P. Lazzara, and J. F. Papp
1042
Mass Spectrometric Observation of Difluorocarbene and Its Reactions in Inhibited Methane Flames Joan C. Blordl,* Charles P. Lazzara, and John F. Papp Pittsburgh Mining and Safety Research Center, Bureau of Mines, U.S.Department of the Interior, Pittsburgh, Pennsylvania 152 13 (Received November 14, 1975) Publication costs assisted by the U.S.Bureau of Mines
Difluorocarbene has been identified in methane-oxygen-argon flames initially containing 11% CF3Br and burning at 0.04 atm. Profiles of the concentration of this species (maximum 1014molecules ~ m - through ~) the reaction zone of the flame have been determined, and from these, profiles of the net chemical reaction rates have been calculated. These data, combined with knowledge of the complete microstructure of the flames studied, suggest that CF2 is formed in this flame principally by the reaction CF3 H HF CF2 (2), and k z = 2 X 1014cm3 mol-l s-l at 1540 K. The decay of CF2 is via reactions with the major flame radicals, CF2 (0,H, OH) products, with rate constants of the order of (1-5) X 1013cm3 mol-l s-l. The implication of these reactions for the mechanism of flame inhibition by fluorocarbons is discussed.
+
+
-
+
--+
Introduction
Near stoichiometric methane-oxygen-argon flames and their analogues containing 11% CF3Br are burned on a cooled, The reactivity of difluorocarbene continues to be of interest porous plug copper burner with a diameter of 10 cm. The in the field of carbene chemistry in genera1,l and it is of direct pressure is maintained at 32 (f0.02) Torr. Under these conpractical interest from the viewpoint of the mechanism of the ditions, the flame appears as a steady luminous disk about 3 high-temperature pyrolysis and oxidative degradation of mm thick and sitting about 2-3 mm above the surface of the fluorocarbons and fluorocarbon polymer^.^,^ The CF2 radical burner. The burner can be moved vertically, and its distance has been produced by p y r ~ l y s e s , land - ~ ~by ~ hot from a fixed sampling cone can be measured precisely. This atomg and shock tube2J0-15 techniques. It has been reported sampling cone, constructed from quartz, has an outside angle as a product of hydrocarbon-fluorine flamed6 and has been of about 38', and the orifice at its tip has a diameter of 87 M . investigated spectroscopically using a matrix isolation techIt is fixed to a cooled flange, which forms the interface between nique.17The radical has been generated in or introduced into the low-pressure flame housing and a differentially pumped the ion source of mass spectrometers by several techniques, two stage quadrupole mass spectrometer (Extranuclear and its appearance potential is well established.1G20 SpecLaboratories EMBA 11). trophotometric monitoring of CF2 in flash photolyses and As the flame gases expand into the low-pressure (-5 x shock tube studies has provided the most direct determinaTorr) region downstream of the orifice, their temperature tions of rate constants for recombination of CF24>5and, redrops rapidly, and chemical reactions are effectively cently, for reaction with 0 2 at high temperature.15 Rate data quenched. The expansion, initially in the slip-transition flow are also available for reaction between CF2 and HX? C Z F ~ , ~ ,regime, ~ makes the transition to molecular flow upstream of and CF3H.14 There have been little data reported on reactions a second cone, through which the central portion of the moof CF2 with atomic and radical species;21it appears that such lecular beam is admitted to the mass spectrometer proper. reactions are important in high-temperature C/F/O systems.15 The beam is modulated by a toothed chopper wheel upstream We report here observations of difluorocarbene in lowof the electron impact ionizer. An electron multiplier (14-stage pressure methane flames to which small amounts of CF3Br Cu-Be or Channeltron) is used to detect the mass analyzed have been added. Considerable information is available for ions. Detection of signals in phase with a reference signal from radical and stable species composition profiles for these flames the chopper wheel permits distinction between ions origias well as aerodynamic and temperature profiles.22This innating from molecules within the beam and those randomly formation, together with the fact that the data permit a scattered off surfaces in the mass spectrometer. quantitatively meaningful kinetic analysis, leads to mechaTemperature profiles were determined using fine wire nisms for the formation and decay of CF2 in the flame and to (0.0025 cm) Pt/Pt-10% Rh thermocouples coated with silica estimates of rate coefficients for reactions between difluoroto reduce'catalytic reactions on the metal surface. Radiation carbene and the major flame radicals. The mechanism implies corrections were calculated using the formulation by Kasthat H atom scavenging reactions by the fluorocarbon part of kan;27the emissivity of the coated wire was measured to be the CF3Br molecule contribute to its observed effectiveness 0.22 f 0.02. Area expansion ratios, required for velocity profile as a flame inhibitor. calculations, were measured on unignited gas using hot wire anemometers calibrated in situ.28 The sampling system mass spectrometer is calibrated diExperimental Section rectly for the major stable species from mixtures of known composition passed through the burner without ignition, and The apparatus used was constructed to study the chemical otherwise sampled in exactly the same manner as with the microstructure of low-pressure flat flames. The details of construction and its performance have been described,22-26 flame. The procedures used to identify, calibrate for, or estimate the concentration of minor stable species are specified so only a limited description will be given here. The Journal of Physical Chemistry, Vol. 80,No. IO, 1976
1043
Difluorocarbene Reactions in Inhibited Methane Flames elsewhere.29 The procedure fon estimating radical concentrations will be discussed presently. Cylinder gases (Ar, 0 2 , CH4, and CF3Br) were of the highest purity available commercially;all had stated purities of 99.99% except CF3Br (99%).The gases passed through Drierite and molecular sieve traps before entering the critical flow system, and their purities were checked periodically by mass spectrometric analyses. The use of molecular beam sampling techniques allows the possibility of sampling atom and radical species, as well as highly reactive stable species, in addition to the normal flame gases. We routinely sample quantitatively species such as H, OH, CH3, and 0 from flames, and others have successfully applied similar techniques to flames as well as a variety of dynamic sampling problem^.^^-^^ In flame-microstructure studies, species concentration, temperature, and gas velocity are determined along an axis, here called z , perpendicular to the flame front as a function of distance from a fixed reference point in the here taken to be the burner surface. Meaningful analyses of these profiles for rate data require either that the sampling probe presents acceptably small perturbation to the flame, or that quantitative corrections be made for the perturbation. In either case, it is necessary to demonstrate, by calculation or experiment, the perturbing effect of the probe. We have empirically characterized the molecular-beam probe used in this studyz5and have assessed the accuracy with which one might expect to determine rate constants for elementary reactions at flame temperatures using this sampling technique?6 A properly constructed probe offers no visual perturbation to the flame and shows concentrations of species and temperature at a given point in the flame to be those characteristic of a position slightly upstream of the actual physical location of the probe tip.34For certain well-known flame reactions for which the radical concentrations can be determined to f50% or better (e.g., H 0 2 OH 0)rate constants can be evaluated in good agreement with literature values measured in the same temperature range.29 For other, less well-defined reactions, the limitations of accuracy for the rate constants are dominated by the lack of definition of the mechanism and the uncertainties in knowledge of the absolute radical concentrations involved.26The limitations specific to the reactions examined here will be noted as those reactions are discussed.
+
+
-
Identification of the CF2 Radical An unignited mixture of CH4 (10.1%)-02 (21.2%)-Ar (67.6%)-CF3Br (l.l%), passed through the burner and sampled in the usual fashion, gives no signal a t 50 amu with an electron energy of 14.5 eV and other adjustable instrument parameters optimized for high sensitivity. When the flame is ignited without CF3Br, no signal is observed at 50 amu, 14.5 eV, anywhere across the flame front. When CF3Br is added to the flame, a signal is easily detected a t mass 50. The variation of the intensity of the signal at mass 50 was measured as a function of distance from the burner surface using several electron energies. These profiles are shown in Figure 1. The 70-eV profile is typical of the CF3Br decay profilez2usually monitored a t the base peak, 69 amu, and the intensity of the signal at mass 50 at 70 eV is everywhere directly proportional to that at mass 69. The CF2+ ion is a fragment ion in the normal electron impact spectrum of CF3Br and at high electron energy dominates the profile of mass 50. At lower electron energy, the 50-amu profile has the character of an intermediate flame species, but with some contribution from CF3Br close to the burner. Finally, a t low enough electron energy, a
3,0001
t
i
B
2,000
-1
1 30 -
0-
-
01
03 04 05 06 OISTANCE FROM BURNER SURFACE,
02
07
08
09
cm
Flgure 1. Profiles of the mass spectral intensity at 50 amu at various energies of the ionizing electrons. The initial composition of the flame gases was 10.1% CH4-21.2% 02-67.6% Ar-1.1% CF3Br; pressure, 32 Torr; initial linear flow velocity, 48 cm s-'.
profile is observed of a species initially absent from the reacting mixture, maximizing in the primary reaction zone, and decaying to zero. Thus, the signal at 50 amu at low electron energy is associated with the presence of CF3Br in the flame, but does not arise from dissociative ionization of the inhibitor molecule. Similar results were obtained from a flame containing 0.3%CF3Br. T o further characterize the source of the signal at 50 amu, a number of appearance potential measurements were made. The electron energy distribution in the high efficiency ion source of the mass spectrometer is broad relative to that of conventional ionizers, and it has been previously demon&ratedz4 that the accuracy of appearance potential measurements using a simple linear extrapolation is about f 0 . 5 eV. This will also be apparent from the succeeding discussion. The argon constituent of the flames provides a convenient reference gas for calibration of the electron energy scale, and its ionization efficiency curve was determined simultaneously with that of the species in question. An ionization-efficiency curve determined at z 0.5 cm, where the maximum occurs in the profile, is shown in Figure 2. At this point in the flame, the appearance potential for the species observed at 50 amu was found to be 11.6 f 0.2 eV, where the limits refer to the precision of the measurement. This is in good agreement with values expected for ionization of CF2. The ionization potential for the CF2 radical has been recently reported to be 11.5 eV,20 and appearance potential measurements with conventional ion sources have yielded values of 11.7 eV.lSJ9The ionization efficiency curve of Figure 2 shows distinct structure, suggesting contributions to mass 50 from other sources above approximately 20 eV, presumably CF2+ from CF3Br. To verify this, an ionization efficiency curve was determined for 50 amu at a point in the flame close to the
-
The Journal of Physical Chemistry, Vol. 80, No. IO, 1976
1044
500 6oo!
J. C. Biordi, C. P. Lazzara, and J. F. Papp
CF2 from CF, Br +
o
’ 18 ELECTRON ENERGY, eV
20
o
a
22
Flgure 2. Ionization efficiencycurve for 50 arnu determined at L cm for the flame described in Figure 1.
1 24
-
0.5
burner surface, where the low electron volt profile data (Figure 1) indicate that [CF2] = 0. This curve is shown in Figure 3. The low-energy contributor is clearly absent from the curve and the appearance potential measured is 20.5 eV, within the range of literature values quoted for formation of CF2+ from CF3Br (18.3,21.4 eV).35Shown as an insert in Figure 3 is an appearance potential measurement for 69 amu, CF3+, determined where the CF2 is at a maximum. The value measured is 12.1 eV, compared with 12.1-12.5 eV reported for the formation of CF3+ from CF3Br.35 In methane flames containing CFsBr, several intermediate stable species associated with the inhibitor have been observed;22these species might be expected to make some contribution to the mass 50 peak at normal electron energies. It is necessary to examine the possibility that they might be responsible for or contribute to the low electron energy signal a t 50 amu. The species in question are CH2CF2, F2C0, and, if sufficient inhibitor is present, CF3H. For the flames containing 1%CF3Br initially, the maximum mole fractions observed (and the distance from the burner surface at which the maximum occurs) for these intermediates are 1.0 X (z = 0.52 cm), 1.8 X low3( z = 0.68 cm), and 6.5 X ( z = 0.57 cm), for CH2CF2, FzCO, and CFsH, respectively. Appearance potential measurements for CF2+ from these species have been reported only for CF3H (14.7 eV, 20.2 eV),35and these are well outside the experimental error of the appearance potential measurements made here. Furthermore, the intensity at 51 amu, which is about seven times that of 50 in the “pattern” of CF3H:G is zero everywhere across the flame front under the measurement conditions of the CF2 profile. Therefore, no contribution is made by CFBHto the signal at 50 amu a t 14.5 eV. Appearance potentials for CF2+ from CH2CF2 and F2CO can be estimated from enthalpy of formation data. For CH2CF2, AP(CF2+, CH2CFp) AH0f(CF2+)+ AH’f(CH2) AH’f(CH2CF2). Using the values recommended by Lifshitz and Long37 for the enthalpies of formation, AP(CF2+, CH2CF2) is about 17.5 eV compared with 11.6 eV observed. The CF2+ ion is a very minor species in the fragmentation of excited CH2CF2+ and, in fact, is not reported at all in standard compilations of mass spectral data.36 Using the The Journal of Physlcal Chemistry, Vol. 80, No. IO, 1976
ELECTRON ENERGY, eV
-
Figure 3. Ionization efficiencycurve for 50 amu determined at L 0.06 cm for the flame described in Figure 1. The insert shows an ionization efficiency curve for 69 arnu determined near the [CF,]maximum.
patterns reported by Lifshitz and Long, and taking into account the discrimination of our mass filter over the range 50-64 m u , we estimate that the contribution to 50 amu from CH2CF2 at 70 eV is less than one-third the signal actually observed at 14.5 eV, and is bound to be negligible at the lower electron energy. Finally, one can calculate the appearance potential for CF2+ from FzCO to be 19.3 eV when AHoffor 0 and FzCO are taken from B e n ~ o n The . ~ ~ CF2+ ion is also reported to be a very minor species in the F2CO mass spectrum.36However, in this case, the profiles for mass 50 and for mass 47 or 66 (Le., F2CO) are sufficiently different that it is clear by inspection alone that they do derive from the same species in the flame. The F2CO profile reaches a maximum around z = 0.7, well downstream of that for 50 amu; the maximum is broad and the decay is slow, so that the profile has a long tail on the downstream side, reaching zero a t z 2 cm. All of these characteristics are in contrast to the profile shown in Figure 4. In summary, the appearance-potential measurements, together with a consideration of the profiles and nature of other possible interfering species, lead to the conclusion that the species observed a t low electron energy a t 50 amu is the CF2 radical. As will be shown later, the absolute concentration of this radical in this system is low, about two orders of magnitude lower than in the shock tubelo and photolytic4 studies, where the radical was monitored via its optical absorption. Optical detection of CF2 in flames might be possible under certain optimum conditions, but for our system it would require instrumentation sufficiently stable and sensitive to detect a 0.5%attentuation.
-
Estimation of Concentration of CFz The concentration of CF2 can be estimated using a technique that has been applied previously to the major flame radicals.24It is assumed that the ionization cross sections for the species in question and another chemically similar species have the same ratio a t equivalent energies above their appearance potentials as they have at 70 eV. With a knowledge of those cross sections and by measuring the relative intensi-
1045
DifluorocarbeneReactions in Inhibited Methane Flames
where -/~XIO-~
ir 3X
10'4
?
x/
DISTANCE FROM BURNER SURFACE, cm
Figure 4. Concentration prohle for CFp and CF3Br. The flame Is that described for Figure 1.
ties a t the appropriate electron energies, the relative concentrations can be calculated. In this case, we chose CO2 for comparison with CF2. This may not be an optimum choice from the viewpoint of similar chemistries (although it is not as bad as it might, at first, appear because of the remarkably low reactivity of the radical4), but an equally critical requirement in this quadrupole system is that ions with nearly the same mass are compared. There is the additional advantage to using CO2 that the required total ionization cross section for CO2, QcoZ,has been measured directly. The cross section for CF2 must be estimated. A simple additivity calculation using Mann's41 atomic cross sections gives a value of cm'2(based on 70 eV &d Q A =~ 3.62 X cm2). 4.82 X However, Beran and Kevan have found that perfluorocarbons generally have measured ionization cross sections less than additivity calculations predict. TJsing their empirical correlation, eq 111,a value of 3.39 X cm2is calculated for QcF~, and is taken to be the better approximation. Then
where X denotes mole fraction; E O , the appearance potential; and b , a constant. The mole fraction of COz is known from calibration, and (159/144) was measured to be 0.017 f 0.001 a t the maximum of the CF2 profile. This gives XcF2(max) 4.5 X in the 1%CF3Br containing flame. This value is used, together with argon as an internal standard, to generate a concentration profile from the intensity profiles. I t is difficult to accurately estimate the uncertainty in this value but, considering the various assumptions made, a factor of 2 would be appropriate. Figure 4 shows the concentration profile for CF2 in the 1%flame together with that of CF3Br for orientation purposes. The maximum X C Fin~ the flame containing 0.3% CF3Br was estimated by comparing intensities a t 50 amu directly with that of the 1%flame, both measured under the same conditions. The maximum CF2 mole fraction in this flame was 1.6 x 10-4.
-
Kinetic Analyses In order to obtain rate data from a profile such as that shown in Figure 4, it is necessary to take into account the fact that in a system with concentration and temperature gradients as large as in the flame, mass and thermal diffusion contribute importantly to the observed concentration at a point. The net chemical reaction rate, Ki, of a flame species can be determined by evaluation of the following equation:
In these equations, p is the mean density; u , the bulk flow velocity; A, the area expansion ratio; M, the molecular weight; X,the mole fraction; T , the temperature; D, a binary diffusion coefficient; and kT,the thermal diffusion ratio. The subscript i associates the quantity with the species i, and subscript 0 refers to unburnt gas conditions. The development of these equations from physical modeling of the flame may be found in standard reference the specific computational techniques used to analyze profiles from low-pressure flames have been documentedt4 and some results for reaction rate coefficients are also available.22p26We apply the techniques here to the CF2 radical and use the derived information to construct a mechanism for the formation and decay of CF2 in the flame and to estimate rate coefficients for some of the reactions involved. D c F ~ - Aand ~ ~ T ( c Fwere ~ ) evaluated using the Lennard-Jones potential with the parameters suggested by S ~ e h l a . 4The ~ net reaction rate for CF2 calculated in the flame containing 1%CF$r is shown as a function of distance from the burner surface in Figure 5. The same quantity for CF3Br is also shown. It has been found that in stoichiometric flames containing 0.3%CF3Br,22the reaction responsible for 90% of the disappearance of the inhibitor is CF3Br
+H
-
HBr
+ CF3
(1)
We have been unable to detect unambiguously the CF3 radical in these or the 1%CF3Br containing flames. The CF2 radical is readily detectable in both cases, however; this, together with the fact that in the 0.3% flame, no species is observed containing the CF3 group intact, leads to the conclusion that the CF3 radical is rapidly destroyed. It seems reasonable to suppose that CF2 is a product of this destruction and, further, that it is the principal source of CF2 in the early part of the flame. Of all of the reactions that can be written forming CF2 from CF3 in this system (including those with CH4, OH, 0,Br, and H o d , only CF3
+H
-
HF
+ CF2 + 40.5 kcal molv1
(2)
is exothermic and therefore may be fast. Since H atoms diffuse rapidly into the low-temperature region of the flame and since H is the dominant radical in these flames, reaction 2 is probably the most important source of CF2. Two other reactions could conceivably contribute to the formation of CF2: CF3H
-+
HF
+ CF2
(3)
and CH2CF2
+0
+
HzCO
+ CF2
(4)
Small amounts of CF3H were observed in the 1%CF3Br containing flames. However, reaction 3 is unlikely to be anywhere near its high-pressure limit under the conditions of the flame;14 therefore, the use of the reported rate coefficient14 for (3) will overestimate its importance in this system. In fact, we can estimate the rate constant for (3) in our system from the minimum value of KCF~H, where reactions forming CF3H can be considered negligible. The rate constant so calculated is six times less than that expected from the literature value. The Journal of Physlcal Chemlstry, Vol. 80, No. 10, 1976
J. C. Biordi, C. P. Lazzara, and J. F. Papp
1046
Using the calculated X C F we ~ , evaluate the rate coefficient, k z , of reaction 2 a t the point of maximum net formation of CF2 to be 2 X 1014cm3 mol-l s-l at 1540 K. For this calculation, all of the uncertainty in the determination of X C F K ~ ,c F ~and , Q X C F propagate ~ into k2, so that this number is good only to x within an order of magnitude. Since the calculated value corresponds to the reaction occurring about every tenth gas kinetic collision, it probably errs on the high side. i The decay of CF2 in the flame can proceed by several routes. We have considered the possibility of CF2 reacting with all the major and minor observed constituents of the flame, and the results of these considerations are summarized in Table I. The table is divided into three parts, and each will be discussed in turn. Part A lists those reactions that are primarily responsible for the consumption of CF2 in the flame. These reactions are all thermodynamically favorable and although there is no quantitative information available regarding their rates, there is qualitative evidence that they can be fast. Bauer et a1.2 in their shock tube studies of the oxidation of perfluoroethylene -803 04 05 06 07 08 09 suggest that reaction 7 has a rate constant very much larger than that for the reaction of CF2 and molecular oxygen, reDISTANCE FROM BURNER SURFACE, cm action 11. Other shock-tube studies of the CzF4-02 system Figure 5. The net chemical reaction rate profiles for CF2 and CF3Br have led to similar conclusions, but favor reaction 61° or redetermined from analyses of the profiles shown in Figure 4. action 815 as the dominant reaction path for CF2 0. Reactions of CF2 with H and OH have not been characterized, but must be considered here since H and OH concentrations are Reaction 4 is assumed to occur with a rate coefficient equal appreciable (actually greater than [O] everywhere in the to that of the parallel reaction giving F2CO CH2 as product; flame) and, sterically, reaction with H should be more favorthis coefficient has been estimated at flame temperature from able than with 0. It is not possible to distinguish among reprevious studies.22 Using these rate coefficients and the measured reactant concentrations and temperatures, it is actions 5-10 because they do not lead to unique products in found that reaction 3 could account for 4%and reaction 4 for this system. At the point in the flame where CF2 is disappearing rapidly, there are parallel reactions forming HF, 19% of the observed maximum net formation rate of CF2. F2C0, and C0.22 Thus, reaction 2 accounts for >75% of the CF2 formation. Part B of Table I lists reactions that could occur but can be If the concentration of CF3 can be determined, then an esshown quantitatively to be unimportant. For each of the retimate for the rate coefficient of reaction 1 can be made. The actions shown, some rate constant data are available, and are observation of CF3H in the 1%CF3Br containing flame profound in the references cited. This data, together with the vides a means of determining [CF3] if the reactions of CF3H measured concentrations of the reactants, permit calculation can be properly evaluated. CF3H is formed by H atom abof the rate of disappearance of CF2 in the flame by the reaction straction reactions between the CF8 radical and species such in question. The calculated rate may be compared with the as H2, CH4, H20, and HBr in the flame. Rate constants for all rate of disappearance of CF2. None of the reactions these abstraction reactions except H2O are a ~ a i l a b l eWe . ~ ~ ~ observed ~~ 11-17 can account for as much as 1%of the observed decay take the rate constant for the analogous reaction with H2S47 rate and usually not even 0.1%. For example, the most imas a reasonable approximation to that with H2O. A direct reportant of these, reaction 12, has a rate constant of 1.1 X 1O1O combination of CF3 with H has also been postulated as a forcm3 mol-l s-l 1800 K,15 where K C Fis~ a minimum (see mation route in these systems.48The consumption of CF3H Figure 5). The mole fraction of CF2 at this point in the flame is most likely via reaction with H atom since the decay occurs is 8 X and of 02,6.9 X The rate of disappearance relatively early in the flame, where the concentration of that of CF2 via reaction 11 is 5.3 X mol cm-3 s-l. The observed atom is large relative to 0, OH, and HO2. A rate coefficient for mol cm-3 s-l and thus rethis reaction was calculated by Skinner and R i n g r ~ s e . ~ ~ rate , ~ ~of decay of CF2 of 6.2 X action 11 is of negligible importance in this flame. Analogous Reaction 3 also contributes to the net rate of reaction of CF3H. calculations for the other reactions listed in part B lead to Assuming these seven are the only important reactions insimilar conclusions. volving CF3H in the flame, and with a knowledge of the net Part C of Table I lists those possible reactions that are reaction rate of CF3H calculated from the profile data, X C F ~ considered to be slow, but for which it is not possible to can be estimated. Its maximum value was found to be 3 X quantitatively demonstrate that fact. Some of these reactions a t z 0.5 cm. The value changes by about a factor of 2 for an are assumed to be relatively slow because of their endotherorder of magnitude change in the rate coefficient for either the micity. recombination reaction, CF3 H CF3H, or the abstraction That CF2 is remarkably unreactive toward alkanes, olefins, reaction with water, CF3 H20 CF3H OH, the two reand gases such as 02, H2, and CO has been noted by various actions that are the least well defined. Based on our maximum investigators.4,5.50 The rate constant for the reaction of CF2 sensitivity for radicals species in general24and on the fact that, with either C2F45 or C Z His~found ~ to be smaller than that for to avoid interference from CFsBr, it is necessary to work at reaction with 0 2 at room temperature. electron energies only a few tenths of an electron volt above Even at 500 K, little reaction was observed between CF2 the appearance potential for CF3+ from CF3 in the search for produced in a discharge and reagents such as C3H8, CO, or H2, this radical, it is not surprising that we were unable to detect although small amounts of the recombination product, C2F4, CF3, even in the flame containing 1%CF3Br. TEMPERATURE,
OK
x 10-2
18.65
I
,
+
+
N
-
+
+
-+
+
The Journal of Physical Chemistry, Vol. 80, No. 10, 1976
1047
Difluorocarbene Reactions in Inhibited Methane Flames
TABLE 11: Rate Coefficients Determined for Reactions of CF, in Inhibited Methane Flames Reaction T,K k , cm3 mol s-’
TABLE I: Possible Reactions of CF, in Inhibited Methane Flames A. Major Reactions Consuming CF, ( 5 ) CF, + H + HF + CF (6) CF, + O + C O + F + F + F,CO (7) +FCO+F (8) (9) CF, + OH + CO + HF + F (10) + F,CO + H
1540
2x
1800
1 x 101.3, if k H = k o
1800
2 x 1013,
CF, + 0 + products
1800
CF, + OH
1800
5 x 1013, if k o % k H , k o H 3 x 1013,
-+
-+
Ref + 0, + F,CO + 0 15 + CO + 2F + 0 + CF, + C,F, 4, 5, 6 11 + M + CF + F + M 9 + HF CF,H 9 + HBr + CF,HBr 14 + CF,H + C,F, + HF C. Reactions that Are Qualitatively Slow Endothermic (18) CF, + CH, + HF + CFCH, Endo thermic 119) CF. + Br + CF + BrF Endo thermic (20; CF; + CF,Br + C,F, + BrF (21.) CH, 50 CF, + + products
-+
products
--f
[ :e\ co,
’
(22) CF, + CH,CF,
+
H,C-CF,
5, 6
‘c’
F,
+ F,CO + CF, + FCO
Endothermic Endo thermic Endo thermic
+ 0 + CF + FO + OH CF + FOH -f
+ H,CO + products + CH,Br + products
were observed30 There appear to be no data reported on the reactions of CF2 with H2CO or CH3Br, for which exothermic reactions can be written. However, even if these reactions were to have rate coefficients equal to that of reactions with the .major radical species, it is unlikely that they could be significant in this system since, a t the point of the maximum rate of CF2 decay, the concentrations of CH3Br and H2CO are 100 and 10 times smaller, respectively, than any one of the major radical species. Although it is not possible to distinguish among reactions 5-10, we can estimate a rate constant for reaction of CF2 with H, 0,and/or OH as follows, We consider that at 1800 K, where is a minimum, the observed net reaction rate is the rate of decay of CF2, i.e., CF:! is no longer being formed. This is reasonable since [CF3] 0 at this point in the flame. If we assume that the rate coefficient for reaction with each of the radicals are equal, then
-
K C F=~ -d[CFz]/dt = k,[CF2]
[i]
i = H, 0, OH
1
and k, can be calculated. It is found to be 1.0 X 1013cm3 mol-l s-l. Alternatively, we may assume that reaction with a single radical dominates the decay of CF2. Then KCF2
= Izi[CFz][i]
i = H, 0, or OH
We find that ki is 2.4 X 1013,4.9 x 1013,or 2.5 X 1013 when the dominant radical reactant is assigned to H, 0, or OH, respectively. The results of these two types of calculations permit a bracketing of the rate coefficient for the reaction between CF2 and these species. It should be noted that errors in the absolute concentration of CF2 do not propagate into the rate constant here because of the KCF$[CF~]term. However,
=koH
> ko, koH
if k o H
I
(11) CF, (12) (13) CF, (14) CF, (15) CF, (16) CF, (17) CF,
1014
if k H
B. Reactions that Are Quantitatively Slow
(23) CF, (24) CF, (25) CF, (26) CF, (27) CF,
CF, + H-+ HF + CF, CF, + (H,O,OH) products CF, + H products
%-
ko, k H
the net reaction rate for CF2 does depend upon the value of the diffusion coefficient for CF2 in argon, and, by necessity, this is calculated from a model for molecular interactions that is not very realistic for radical species. We estimate that the rate constants calculated for CF2 H, 0, or OH, an average between the two types of calculations, are good to a factor of 5 or better. A summary of the rate coefficients determined for reactions relating to CF2 in the flame is given in Table 11. There are no literature data available for comparison of the rate coefficients for CF2 reactions estimated here, formation or decay. Keating and Matula15 conclude that the reaction CF2 0 FCO F can be important in shock tube studies of C2F4-02 oxidation as the oxygen concentration is increased. The (gas kinetic) value for the reaction rate constant of this reaction used by them in modeling their experiments is ten times higher than that found here.
+
+
-
+
Comments on the Involvement of t h e Fluorocarbon Adduct in Flame Inhibition The use of chemical extinguishants for fire protection is now common. These agents are considered effective by virtue of their interference with the normal chain reactions responsible for flame propagation. The exact nature of this interference has been the object of a number of investigations (see, e.g., ref 51 and the references therein). Until very recently the fluorine containing part of the halohydrocarbon inhibitors was viewed as an essentially nonreactive adduct whose function was to transport bromine to the flame reaction zone; bromine is considered to be the principal reactive agent in retarding flame propagation. For some time, CF3Br has been known to be rather more effective than one might expect on the basis of its bromine contentb2and, in that sense, has been described as a synergistic system.53 In this discussion, it is important to bear in mind that there are significant differences between flames maintained at low pressure and at atmospheric pressure, but they are differences that are understood and can be modeled. Once the basic chemistry of the system is identified, the relative importance of any reaction under a variety of imposed conditions can be assessed. The reactions proposed for the formation and decay of CF2 are relevant to the mechanism of inhibition of CF3Br to the extent that inhibition does involve the fluorocarbon adduct. Reactions such as (2) and (5) are true atom scavenging reactions since, once formed, HF does not react further in the flame. Reaction 7 could also have a similar effect since the decay of F2CO is relatively slow in the flame and could itself provide for a relatively rapidly bimolecular route to remove H atoms from the secondary reaction zone.22The early appearance of both H F and F2CO in the flame is consistent with the notion that these three reactions are important.22The fact The Journal of Physical Chemistry, Vol. 80, No. 10, 1976
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that the CF2Br2 is only as effective as CH2Br2 and Br2 in reducing the burning velocity of atmospheric methane flames52 suggests that, for inhibition, reaction 2 is the most significant of the three. However, other reactions proposed for the decay of CF2, those producing F atoms, e.g., could result in promotion, rather than inhibition, Rapid F-atom abstraction reactions, such as F CH4 HF CHs, could hasten the decay of fuel. Promotion of fuel reaction at low temperature has been observed in HBr-propane flames burning at 1 atm54 and was partly attributed to reaction of Br atoms with C3Hs. We are unable to observe F atoms, supposing they are present in detectable concentrations, because of interference at 19 amu by H20, which has an appearance potential well below that for F+ from F atoms and whose concentration is relatively large throughout the flame. Since we do observe inhibition at low pressures55 where, according to current ideas regarding the Br mechanisrnFl the effectiveness of the Br part of the molecule should be greatly reduced, such promotion reactions are unlikely to be important in this system. This further implies that reactions 6,8, and 9 are not important here, or that the F atoms produced react so rapidly that their concentration in the region where methane is being attacked remains very low.
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References and Notes (1) R. Mitsch and A. Rodgers, lnt. J. Chem. Kinet., 1, 439 (1969). (2) S.H. Bauer, K. C. Hou, andE. L. Resler, Jr., fhys. Fluids. Suppl. I, 12,l-125 11969). (3)k. E. Kupel, M. Nolan, R. G. Keenan, M. Hite, and L. D. Scheel, Anal. Chem., 36,386 (1964). (4) F. W. Dalby, J. Chem. fhys., 41,2297 (1964). (5) W. J. R. Tyerman, Trans. Faraday Soc., 65, 1188 (1969). (6) N. Cohen and J. Heicklen, J. Chem. fhys., 43,871 (1965). (7)G. R.Barnes, R. A.Cox,andR. F. Simmons, J. Chem. SOC.8 , 1176(1971). (8)R. A. Cox and R. F. Simmons, J. Chem. SOC.s,1625 (1971). (9)T. Smail and F. S.Rowland, J. fhys. Chem., 74, 1866 (1970). (IO) A. P. Modica and J. E. LaGraff, J. Chem. fhys., 43,3383 (1965). (11) A. P. Modica, J. Chem. fhys., 44, 1565 (1966). (12)A. P. Modica and S. J. Sillers, J. Chem. fhys., 48,3283 (1968). (13)G. A. Carlson, J. fhys. Chem., 75, 1625 (1971). (14)E. Tschuikow-Roux and J. E. Marte, J. Chem. fhys., 42,2049 (1965). (15)E. L. Keating and R. A. Matula, paper presented at the 1973 Fall Meeting
of the Eastern States Section of the Combustion Institute, McGill University, Montreal, Quebec, Canada, Oct 11-12, 1973. (16)E. S.Fleming, D. I.MacLean. and J. M. Smist, paper presentedat the 169th National Meeting of the American Chemical Society, Philadelphia, Pa., Apr
4-11, 1975.
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(17) D. E. Milligan, D. E. Mann, and M. E. Jacox, J. Chem. phys., 41,1 1 99 (1964). (18)I. P. Fisher, J. B. Homer, and F. P. Lossing, J. Am. Chem. Soc., 87, 957 (1965). (19)T. C. Ehlert, J. fhys. Chem., 73,949 (1969). (20)D. L. Hildenbrand, Chem. fhys. Left., 30,32 (1975). (21)M. C. Lln, paper presented at the 169th National Meeting of the American Chemical Society, Philadelphia, Pa., Apr 4-1 1, 1975. (22)J. C. Biordi, C. P. Lazzara, and J. F. Papp, Symp. (ht.)Combust.,[ f r o c . ] , 15th, 917-931 (1975). (23)J. C. Blordi, C. P. Lazzara, and J. F. Papp, US. Bureau of Mines RI 7723, 1973. (24)J. C. Biordi, C. P. Lazzara, and J. F. Papp, Combust. Flame, 21,371 (1973). (25)J. C. Biordi, C. P. Lazzara, and J. F. Papp, Combust. Flame, 23, 73 (1974). (26)J. C. Biordi, C. P. Lazzara, and J. F. Papp, Combust. Flame, 26,57 (1976). (27)W. E. Kaskan, Symp. (lnt.) Combust., [ f r o c . ] , 6th, 134-143 (1957). (28)J. Peeters and G. Mahnen, Symp. (lnt.) Combust., [ f r o c . ] , 14th, 133-141 (1973). (29)J. C. Biordi, C. P. Lazzara, and J. F. Papp, US. Bureau of Mines RI 8029, 1975. (30)E. L. Knuth in “Engine Emissions: Pollutant Formation and Measurement”, G. Springer and D. J. Patterson, Ed., Plenum Press, New York, N.Y., 1973, pp 319-363. (31)G. J. Williams and R. G. Wilkins, Combust. Flame, 21,325 (1973). (32)K. H. Homan and D. I. MacLean, J. fhys. Chem., 75,3645(1971). (33)H. Gg.Wagner, C. Zetzsch, and J. Warnatz, 8er. Bunsenges. fhys. Chem., 76,526 (1972). (34)R. M. Fristrom and A. A. Westenberg, “Flame Structure”, McGraw-Hill, New York, N.Y., 1965. (35)F. H. Field and J. F. Franklin, “Electron Impact and IonizationPhenomena”, Revised edition, Adademic Press, New York, N.Y., 1970,pp 239-493. (36)A. Cornu and R. Massut, “Compilation of Mass Spectral Data”, Heyden and Son, Ltd., London, 1966. (37)C. Lifshitz and F. A. Long, J. fhys. Chem., 69,3731 (1965). (38)C. Lifshitz and F. A. Long, J. fhys. Chem., 67,2463 (1963). (39)S.W. Benson, “Thermochemical Kinetics”, Wiley, New York, N.Y., 1968. (40)D. Rapp and P. Englander-Golden, J. Chem. Phys., 43, 1464 (1965). (41)J. B. Mann, J. Chem. fhys., 46, 1646 (1967). (42)J. A. Beran and L. Kevan, J. fhys. Chem., 73,3866 (1969). (43)J. 0. Hirschfelder, C. F. Curtiss, and R. B. Bird, “Molecular Theory of Gases and Liquids”, Wiley, New York, N.Y., 1965. (44)J. F. Papp, C. P. Lazzara, and J. C. Biordi, U.S. Bureau of Mines RI 8019, 1975. (45)R. A. Svehla, NASA Technical Report R-132,1962. (46)V. N. Kondratiev, National Bureau of Standards, No. COM-72-10014(1972). (47)A. F. Trotman-Dickenson and G. S.Milne, Natl. Stand. Ref. Data. Ser., Natl. Bur. Stand., No. 9 (1967). (48)G. B. Skinner, ACS Symp. Ser., No. 16,295-312 (1975). (49)G. B. Skinner and G. H. Ringrose, J. Chem. Phys., 43,4129(1965). (50)W. Fielding and H. 0. Pritchard, J. Phys. Chem., 64,278 (160). (51)R. G. Gann, Ed., ACSSymp. Ser., No. IS(1975). (52)W. A. Rosser, H. Wise, and J. Miller, Symp. (lnt.) Combust., [ f r o c . ] , 7th, 175-182 (1959). (53)J. W. Hastie and C. L. McBee, ACS Symp. Ser., No. 16, 118-146 (1975). (54)C. Pownall and R. F. Simmons, Symp. (lnt.) Combust., [ f r o c . ] , 13th 585-592 (1971). (55)J. C. Biordi, C. P. Lazzara, and J. F. Papp, Combust. Flame, 24,401 (1975). (56)Skinner48 has recently reanalyzed these data and recommends a sub-
stantially higher activation energy than in the earlier work. The change has negligible effect on the calculated maximum [CF3] here, since at this point in the flame primarily CF3H is being formed: decay is relatively very slow.
