- Email: [email protected]
Acetylene oxidation: The reaction C2H2+O at high temperatures
Twenty-first Symposium (International) on Combustion/The Combustion Institute, i986/pp. 8 8 5 - 8 9 3
A C E T Y L E N E O X I D A T I O N : T H E R E A C T I O N CzH2 + O A T H I G H TEMPERATURES P. FRANK, K.A. BHASKARAN- axn TH. JUST
DFVLR, Inst. f. phys. Chemie d. Ferbrennu~g, Stuttgart, W. German3
Shock heating, together with atomic and molecular resonance absorption spectrometry was used for simultaneous monitoring of H and O or CO in highly diluted acetylene-nitrous oxideargon mixtures. The investigations covered a temperature and pressure range of 1500 to 2500 K and 1.5 to 2.0 bar respectively. New measurements are presented for the primary step: CH~ + CO
(R2a)
HCCO + H
(R2b)
C2H2 + O =
The channel ratio, k2a/(k2a + k2b) was found to be about 0.64 in the temperature range investigated. Best fits for the measured H and O profiles were achieved with k20 = 1.6 • l014 exp (-4,975/T), k2b = 4 X 1014 e x p ( - 5 , 3 6 5 / T )
and
(All rate coefficients in cm 3 mol 1 s-l). The fact that channel 2b is faster than 2a is in qualitative agreement with theoretical predictions. To reach the above conclusions, high temperatures ketyl and methylene oxidation kinetics were investigated. The following rate coefficients were determined tbr some significant reactions. CO + 2 H
(R9a)
k = 8 ~ l • 10 l:~
CO + H2
(R9b)
k = 4 -+ 1 x 10 ~:~
(R3) (R4) (R5) (R6)
k k k k
3CH2 + O (3p)
HCCO + O (3p) HCCO + H HCCO + Ar CH + O
= 2 CO + H = ICHz + CO = CH + CO + Ar = CO + H
= = = =
1x 1.5 6 x 1-
l014
x 1014 10 ~3 exp(-29600/T) 1.5 x 10 H CH2 + C O
Introduction
C2H 2 + O = HCCO + H
R e a c t i o n s o f a c e t y l e n e a r e an i m p o r t a n t s u b s y s t e m in t h e o x i d a t i o n o f m a n y h y d r o c a r b o n s , u n d e r b o t h rich a n d l e a n flame conditions, T h e r e has b e e n a c o n s i d e r a b l e c o n t r o versy in the l i t e r a t u r e r e g a r d i n g the r e a c t i o n p r o d u c t s o f the p r i m a r y step:
+Present address: Indian Institute of Technology, Madras, India
(R2a) (R2b).
J o n e s a n d Bayes I f o u n d a c o n t r i b u t i o n o f 85 to 50% for c h a n n e l R2b. B a s e d o n theoretical c o n s i d e r a t i o n s , H a r d i n g 2 s h o w e d c h a n n e l R2b to be d o m i n a n t , w h e r e a s H o y e r m a n n et aDs gave e x p e r i m e n t a l e v i d e n c e f o r a 95c/c contrib u t i o n via c h a n n e l R2a. In a r e c e n t theoretical s t u d y o n C2Hz + O, H a r d i n g a n d W a g n e r 4 p r e d i c t e d the overall rate o f r e a c t i o n at temp e r a t u r e s below 1000 K satisfactorily, but calculated a substantially l o w e r r a t e coefficient t h a n
885
886
REACTION KINETICS
that observed in high temperature experiments. The b r a n c h i n g ratio k2b/(kza + k2b) was definitely greater than 0.25 and most probably exceeded 0.5. The high temperature experiments of Loehr and Roth 5 on acetylene oxidation were explained by assuming that R2a was the d o m i n a n t channel and using the available rate coefficients for the CH2 subsystem. I n view of some recent investigations on methylene and ketyl radical kinetics,6,z's a reinvestigation of the reaction C2H2 + O seems worthwhile and may well lead to an improved u n d e r s t a n d i n g of this complex reaction system. According to H o y e r m a n n et al. 3, the low temperature C2H2/O/H measurements of H o m a n n and Wellmanng indicated that R2a was the d o m i n a n t channel below 500K. In the present work, the R2b channel was found to be most important with a branching ratio of 0.65. Consequently the subsequent ketyl radical reactions assume greater significance. In view of recent work 7 on CH2 kinetics, an attempt is made in this paper to arrive at some quantitative data for high temperature ketyl radical reactions with O- and H-atoms.
Acetylene gas was supplied by Deutsche L'Air Liquide and certified to contain less than 0.4% impurities (mainly air). Experiments performed earlier 13with additional purification by activated carbon showed that only in the case of higher acetylene mole fractions (X > 200 ppm) was some influence of acetone impurity on H-atom profiles detectable. Experiments with C2H2 c o n c e n t r a t i o n s < 100 p p m were performed without further purification. T h e argon gas used as diluent was certified 99.9999% pure. T h e model calculations showed that in mixtures with low C2H 2 concentration (-<5 ppm) at high temperatures, the H-atom concentration in the later stage of the observed reaction time frame is practically equal to the n u m b e r of H-atoms stemming from the initial C2H2. From this and the measured H-concentration it was deduced that the calculated initial acetylene concentration was reduced by about 10-20% due to wall adsorption. T h e magnitude of this effect is in accordance with previous experiments with other molecules in the shock tube. Concentrations given in Table I for mixtures A, B, and C have been corrected for adsorption effects.
Results Experimental Details of the shock tube as well as the optical setup have been described elsewherea~ only a brief summary is presented below: The brass shock tube consists of a test section of about 7m long and a driver section of 4m with an internal tube diameter of 7.5 cm. In preparation for each experiment, the test section was evacuated by a turbomolecular p u m p to less than 10 -~ mbar. Gas mixtures were prepared and stored in a stirred 56 litre stainless steel vessel which may be baked at temperatures up to 523 K. All measurements were performed behind the reflected shock, near the end flange. Atomic and molecular resonance absorption spectrometry was used to monitor time-dependent H-, O- and CO-concentrations. Microwave-excited discharge lamps were used as light sources for the absorption measurements. The transmitted intensity signals were recorded by a 2-channel transient recorder a n d fed to a computer for averaging and computation of particle density values. Further details of the measurements of H, O and CO are given in refs. 11 and 12. N20 was the source of O-atoms in all C2H2/Oexperiments. Dissociation kinetics of N20 are important below 1800 K. The rate coefficient for this reaction was recently reconfirmed 11 for the pressure range of the present experiments.
The primary goal of this work is to determine the channel ratio for C2H2 + O = CH2 + CO HCCO + H
(R2a) (R2b)
This required careful selection of C 2 H 2 / N 2 0 mixture ratios which exhibited high sensitivity to R2a and R2b. I n a first attempt to model the measured H a n d O profiles, a 48 step reaction mechanism involving 25 species was used. Sensitivity studies demonstrated that the profiles for the different mixtures could be simulated successfully by a much smaller set of reactions. This was possible because the reactant mixtures were highly diluted, which reduced the influence of subsequent reactions. The final reaction scheme is given in Table II and the computed best fits for the measured O,
TABLE I C~H2 - N20-mixtures, diluted in argon (in ppm).
Mixture
A
B
C
D
X(C~H~) 2 2.5 4.5 10 (2.5)+ (3,0)+ (5.0)* X(N20) 20 20 5 100 +: uncorrected concentrations
E
F
G
15
20
100
100 20
100
H I G H T E M P E R A T U R E CzH~ + O~ R E A C T I O N
887
T A B L E II Reaction m e c h a n i s m a n d rate coefficient expressions Reaction R1 N20 + Ar R2a C2Hz + O R2b C~H2 + O R3 HCCO + O R4 HCCO + H R5 HCCO+Ar R6 CH + O R7 CH2 + CH2 R8 CHz + H R9a CH2 + O R9b CH~ + O RI0 ICH2 + A r Rll N 2 0 4- CH2 R12 N~O + H R13 CH9 + C2H2 R14 1CH2+C2H~
= = = = = = = = = = = = = = = =
N2 + O + A r CH2 4- CO HCCO + H 2 CO + H ICH2 + C O C H + CO + A r CO + H C2H2 + 2H C H + H2 CO + 2H C O + H2 3CH2 + A r N~ + CO + 2 H N~ + O H CsHs + H C3Hs + H
A n 9.3 x 1014 1.6 x 1014 -4.0 x 1014 i -+ 0.2 x 1014 - 1.5 X 1014 6 x 1015 1 - 1.5 x 1014 - 1 x 1014 -8 x 1012 8-+ 1 x 10 Is 4-+ 1 x 1013 4 - 3 5 x 101l - 4 x 1011 1.9 x 106 2.42 1.5 x 109 2 x 10 TM -
-
-
-
-
-
-
-
Ea 30061 4975 5365 --
-
29600 ---
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
6790
-
-
-
-
-
-
-
-
T - r a n g e (K) 1450-2200 1500-2500 1500-2500 300-2500 300-2500 1500-2500 1500-2500 2100-2700 2100-2700 2100-2600 2150-2600 300-2100 1500-1700 300-1700 300-1900 300-1900
Ref, 10 44-
+,14 +,14 a
+,27 13 13 44-
13,20,21 +b +,19,26 8,+ c
+c
All rate coefficients in c m 3 mol -l s -1 (k = A 9 T " . exp (-Ea/T)). +: T h i s work a: estimated f r o m u n i m o l e c u l a r - t h e o r y , low p r e s s u r e limit. b: H C O is an i n t e r m e d i a t e , which d e c o m p o s e s very fast above 1500K. c: negligible in this work. H a n d C O c o n c e n t r a t i o n s a r e s h o w n as s o l i d l i n e s i n Figs. 1 - 6 . T h e r e a c t i o n s c h e m e s h o w n in T a b l e II is t h e r e s u l t o f s y s t e m a t i c s e n s i t i v i t y a n a l y s i s a n d t h e e x a m p l e s s h o w n in Figs. 1 - 6 , w e r e c h o s e n to d e m o n s t r a t e t y p i c a l r e s p o n s e s o f t h e m o d e l to c h a n g e s in t h e i m p o r t a n t r a t e coefficients. S i n c e t h e m e t h y l e n e r a d i c a l ( t r i p l e t 3CH2) is a m a j o r p r o d u c t d u r i n g a c e t y l e n e o x i d a t i o n , 5'9 a s e r i e s o f e x p e r i m e n t s w e r e f i r s t c a r r i e d o u t to 4
10 ~3 e m -3
20
3
...-;:L2
15
.........
[~] I 2
s t u d y t h e r e a c t i o n o f 3 C H 2 w i t h O - a t o m s 6. M e t h y l e n e r a d i c a l s w e r e f o r m e d by t h e t h e r m a l decomposition of ketene and the H, O and CO concentrations were monitored. Figure 1 compares the measured and computed H and CO profiles produced by shockheated ketene/ nitrous oxide mixtures. Despite the high rate of the reaction 3CH2 + 3CH2 = C2H2 + 2 H 2.0
1013 em-a
1o I [co]
(R7)
I0
8
10 is e m -3 1.5
1012 em -3
6 I [o]
In] I 1.0
4 0.5
2 0
0.0 200
400
time (psee)
600
800
-
Fro. 1. C o m p a r i s o n o f H - a t o m (triangles) a n d C O - m o l e c u l e (rhombs) m e a s u r e m e n t s with c o m p u t a tional results, s h o w i n g t h e i n f l u e n c e o f CH2 4- O
(R9a, b). 4 p p m H2CCO + 12.5 p p m N~O in A r T = 2280 K P = 1.57 b a r Full line:calc, u s i n g t h e k-values o f table II . . . . . . : calc. without R9a,b .... : calc. with kg~ x 0.8, kgb = 0
0
200 400 time (~ see)
600
800
-
Fro. 2. C o m p a r i s o n o f H- (triangles) a n d O - a t o m (circles) m e a s u r e m e n t s with c o m p u t a t i o n a l results, s h o w i n g the influence o f both c h a n n e l s o f C2Hz + O (R2a, b) a n d o f H C C O + H, O (R3,R4). Mixture E T = 1550 K P = 1.85 bar Full line:calc, u s i n g t h e k-values o f table II .... : calc. with k~a = 0, k2b x 1.1 . . . . . . : calc. with k2b = 0, k2a • 3 . . . . . : calc. without R3-R4
888
REACTION KINETICS
the CH2 + O reaction c o n t r i b u t e d substantially to the formations of CO a n d H, d u e to the chosen excess of O-atoms. T h e dashed-dottedcurves show the CO a n d H concentrations d u e solely to ketene pyrolysis. T h e model calculations indicate that a n alternative CH2 + O c h a n n e l which does n o t p r o d u c e H-atoms directly must also be taken into account. If this second reaction c h a n n e l is neglected a n d the rate coefficient for the H - p r o d u c i n g c h a n n e l is chosen to fit the e x p e r i m e n t a l H-profile early in the reaction time (t < 200 ~sec), then the calculated H-profiles are definitely too large later in the reaction (dashed curve). T h e evalu a t i o n of a series of e x p e r i m e n t s on CH2 with O-atoms resulted in two c h a n n e l s CO + 2 H ~CH2 + O = CO + H2
(R9a) (R9b)
with a b r a n c h i n g ratio kg,,/(kga + kgb) of about 0.65. In view of these findings a n d the results of previous studies, 6'7 the high t e m p e r a t u r e reaction m e c h a n i s m of m e t h y l e n e may now be considered to be k n o w n . T h e r e f o r e the m a i n emphasis of the p r e s e n t investigation is o n the initiation reaction, a n d o n the reactions involving HCCO. Figure 2 shows the c o m p a r i s o n between m e a s u r e d a n d calculated H- a n d O-profiles at a relatively low t e m p e r a t u r e . Due to the slow f o r m a t i o n rate of O-atoms we have conditions of a "rich mixture" that is, [ C 2 H 2 ] / [ 0 ] ~ 4.5 at 600 I*sec, if acetylene would n o t be c o n s u m e d by C2H 2 + 0 . O a n d H profiles are sensitive to variations of kl to kl2, but, w h e n t < 300 p~secthe profiles are within the e x p e r i m e n t a l scatter (ca. + / - 15%) exclusively d e t e r m i n e d by R2a, R2b, a n d the well k n o w n reaction R1.11 Both acetylene channels are n e e d e d to describe the measured profiles. I f the CH2-channel R2a is switched off a n d the k-value for the HCCOc h a n n e l R2b is chosen to match the e x p e r i m e n tal H-profile, t h e n the p r e d i c t e d O-atom concentrations (dashed line) are too high. If R2b is switched off a n d k~a is chosen to fit the measured O-profile, t h e n the H - a t o m concentrations (dashed dotted line) are too low. T h e reactions of H a n d O with ketyl radical have some influence on the profiles w h e n the reaction time exceeds 300 p,sec. I f the HCCO-reactions (R3 a n d R4) are neglected then the calculated profile is r e p r e s e n t e d by the dotted curve in fig. 2. Ketyl d e c o m p o s i t i o n (R5) has no influence at this t e m p e r a t u r e . U n d e r the chosen e x p e r i m e n t a l conditions, only the Oprofile is sensitive to R3 a n d R4. I n Figure 3, the m i x i n g ratio [C2H2]/[O] is a b o u t 1. Both profiles are sensitive to changes
!
2.0 1013 cm-3 1.5 [HI I
1.0
[0]
0.5
0.0
200 400 time (p see)
0
600
BOO
--
FIG. 3. Comparison of H- (triangles) and O-atom (circles) measurements with computational results, showing the influence of both channels of C2H~ + 0
(R2a, b). Mixture C T = 1970 K P = 1.65 bar Full line:calc, using the k-values of table II . . . . . . calc. with k2~ = 0, k2b x 1.1 . . . . : calc. with k~b = O, k~ x 2.6
2.5
_ 25
2.0
[HI
:
1.5
/
1.0
x"
0.5
/./
0.0
~ 200 time
........... 7 .~. . . . . . . .
~ ......
' 400
' 600
~see)
"
T- 20
i 15
10 lz e r a - 3
[o]
' - 10
' 800
FIG. 4. Comparison of H- (triangles) and O-atom (circles) measurements with computational results, showing the influence of C~H2 + O = HCCO + H (R2b) and the HCCO-reactions (R3-R5). Mixture A T = 2460K P = 1.51 bar Full line:calc, using the k-values of table II ---: calc. with k2a = 0, k2b x 1.5 . . . . . . : calc. with k2b x 0.5, k~a x 2 . . . . . : calc. without R3-R5 in the rate constants k 2 to k0. I f the CH2-channel
R2a is again switched off a n d the value ofk2b is adjusted to fit the e x p e r i m e n t a l H-profile, the calculated O-profile is r e p r e s e n t e d by the dotted curve. If k2b equals zero a n d k2a fitS the e x p e r i m e n t a l O-profile, t h a n the model predicts the dashed curves for H. U n d e r the conditions of F i g u r e 4 (extremely lean mixture, high t e m p e r a t u r e ) , the O-profile is insensitive to variations in k2 to kg. I n this case, it is possible to fit b o t h e x p e r i m e n t a l profiles without reaction R2a by increasing k2b by 10%.
HIGH TEMPERATURE C2H2 + 02 REACTION 7
1013
cm -3
[o]
5
14 3
9
0
0
100
200 300 time (~ sec)
400
500
=
Fro. 5. Comparison of O-atom measurements with computational results, showing the influence of C2H2 + 0 (R2a, b) Mixture F T = 2475 K P = 1.64 bar Full line:calc, using the k-values of table II ---: calc,with (k9~ + k~b) x 0.8 . . . . . . calc. with (k2~ + k2b) x 2.2 ...... : calc. with k2b = 0 , k2a x 8 6
I
i
I
j
5
1014 cm-3 4
[CO] 2 1 0 0
200 400 time (~tsec)
600
800
=
Fro. 6. Comparison of CO-molecule measurements with computational results, showing the influence of the HCCO-reactions (1/3 - R5). Mixture G T = 1956K P = 1.69 bar Full line:calc, using the k-values of table II : cal. without R3-R5 For k2a->0, changing kzb by 1.5 or 0.5, with k2a 4-
k2b remaining constant, shows the sensitivity of the H-profile with respect to the ketyl channel. If channel R2b is switched off, and kza is set nearly equal to the gas kinetic collision number, good agreement is achieved initially with the measured H-profile, but not in the later stage. For an experiment with comparable conditions, but with a mixing ratio of 1:1, where the O-profile is sensitive to variations of k2~ + kzb, O-atom concentrations (Fig. 5, dashed-dotted curve), which are by a factor of 2 - 3 smaller than the measured ones, are calculated for the upper limit of k2a (k2a ~ 2 x 1014).
889
For such high temperature experiments (fast N20-decay) with rich to near stoichiometric mixtures, we can deduce the uncertainty in the evaluation of k9o + k2b. Figure 5 shows the changes in the O-profile when k2o + kh is varied by about + 20%. If additionally the reactions of ketyl (R3 and R5) are switched off, larger O-concentrations are calculated, which can be compensated for by increasing k2a + k2b by about 20%. The strong influence of the ketyl reactions (R3-R5) on the CO-production is shown in Figure 6. If R3-R5 are neglected then we calculate CO-concentrations that are as much as a factor of 2.5 smaller than the measured values. Discussion
Results at high temperatures Of the 3 primary steps CH2 + CO C2H 2 4- O = HCCO 4- H CzH + O H
(R2a) (R2b) (R2c)
the OH-channel proposed b~, Bradley and Kistiakowsky, 14 and Glass et all 15 is not ~mportam u n d e r the present conditions. This channel is about 23 kcal/mol endothermic while the CHz- and HCCO-channels are exothermic. Hence, R2c probably has very little influence. From the initial shape of the measured H-profiles, it was apparent that there was an early H-producing channel. All attempts to model the H-profile neglecting the ketyl channel were unsuccessful. This is considered excellent proof of the existence of this channel. To achieve a good fit between the measured and computed H-profiles in the investigated temperature range, it was necessary to increase the rate coefficient of the HCCO channel by a factor of about 1.5, as compared to that of Loehr and Roth. 5 The activation energy remained practically unchanged. T h e rate coefficient obtained for C2Hz + O = HCCO + H was: k~b = 4 x 1014 exp (-5365/T) cm 3 mo1-1 s -1. It is estimated that the error in this rate coefficient due to contributions by the CHz-channel (R2a), and caused by subsequent HCCO- and CHz-reactions, and experimental scatter in the initial temperatures, should not exceed 20% for temperatures above 1900 K, and 30% for temperatures in the 1500-1700 K range, Based on flame studies, Fenimore and Jones 16 concluded that the oxygen atom should directly attack an acetylene carbon atom which would make the methylene channel dominant.
890
REACTION KINETICS
Vandooren and Van Tiggelen 17 from their data on low pressure flames (700 to 1430 K) also found the methylene channel to be faster than the ketvl channel. However, in the present work a satisfactory fit of the measured O and H profiles was possible only when a channel ratio (k2jk2,) of approximately 65/35 was assumed The fact that the methylene channel was slower than the ketvl channel was observed in all experiments with different mixing ratios over the entire temperature range of 1500 to 2500 K. Harding's theoretical predictions 2'4 very strongly support this. The rate coefficient obtained for the CH2-channel is k2,, = 1.6 x 10 I4 exp (-4975/T) cm :~ mol -t s-1. For temperatures below 2100 K, the Oprofiles are sensitive to k2,,, and an error of about -+ 30% for k2,, is estimated. For higher temperatures, the O-profiles become increasingly insensitive to variations of k>,. The rate of O-consumption was measured for temperatures above 2200 K with C2H2/O = 1 (mixture F of Table I). The O-consumption is mainly determined by reactions R2a and R2b in the first 200 Ixsec. From k ~,~,t(,/,and k2b, the values for k2~ could be evaluated with an uncertainty of less than 50% at 2400 K. The experiments indicate that the overall rate coefficient for CeH2 + O is higher for temperatures above 2000 K (about a factor 3 at 2500 K) than the theoretical predictions of Ref. 4. From the above, it can be concluded that the primary reaction C2H2 + O exhibits a channel ratio of 0.7 to 0.6 for R2b and 0.3 to 0.4 for R2a. This channel ratio is not expected to change significantly at lower temperatures (see below). The shape of the measured profiles later in the reaction time frame (t > 300 Ixsec) is determined to some extent by secondar~ reactions.
Secondam: reactions of HCCO Very little is known quantitatively about reactions of HCCO with atomic oxygen at high temperatures. U n d e r certain experimental conditions (mixtures C, D, E), measured H and O profiles often tend to decrease at the end of the reaction time, suggesting that there are some important O-atom removal processes. Channel R3 (HCCO + O = 2 CO + H) was found to be necessary in r e p r o d u c i n g the shapes of both O and H profiles. Due to the fast decay of HCO (HCO + Ar = CO + H + A r ) i n the experiments, it was not possible to discriminate between possible product channels leading to 2 CO + H or HCO + CO. In rich C2H2/O mixtures, at relatively low temperatures, the effect of R3 on the O-profile is p r o n o u n c e d in
the last stages of the reaction (Fig. 2). T h e concurrent decomposition reaction of ketyl HCCO + Ar = CH + CO + Ar
(R5)
and the fast consecutive step CH + O = CO + H
(R6)
cannot influence the O-profile because of the slow decomposition rate of HCCO below 1700 K. This allows one to set the value for k3 in the temperature range 1500 - 1700 K at about 1014 cm s tool -1 s, -a, which is in excellent agreement with the room temperature results of Vinckier et al. 8 Our experimental conditions do not allow a very accurate study of the reaction of H-atoms with HCCO (R4). U n d e r lean conditions, the influence of R4 is negligible even though it is very fast. 8 For near stoichiometric to rich mixing ratios, contributions of R4 become apparent. T h r e e product channels are discussed in the literature: HCCO + H
= 1CH2 + CO (R4) AHo~ = - 19 kcal/mol C20 -{- H2 (R4a) AHo~ = - 4 kcal/mol HCCO + H + M = HCCOH* + M (R4b) A H ~ = - 5 5 kcal/mol followed by 1CH2 + Ar = 3CH2 + Ar 18'm and HCCOH = C2H + O H 2~ In the present experiments the total rate (k4 + k4a + k4b) of HCCO consumption by H-atoms is evaluated. From the re-evaluation of the flow reactor experiments, 9 it was deduced (see below) that the d o m i n a n t product channel should be the carbene radical, singlet ICH2, and CO. Faubel and W a g n e r 2~ found that k4b should have a value less than 10% of k4. The effect of channel R4a, which was also proposed by Faubel and W a g n e r 2~ and Becker et al, 21 was also considered negligible (k4a<-O.Ixk2, ref. 20) in the high temperature range. For all experiments there was good agreement between experimental results and model calculations using the high rate coefficient for R4, performed recently by Vinckier et al. 8 No direct m e a s u r e m e n t of the decomposition of ketyl is available. T h e rate coefficient for the unimolecular decay was calculated using Troe's theory 22 and structural and frequency data. 4 For 13c, a value of 0.04 was chosen, close to that of the ketene decay. T h e average energy transferred by collisions (zXE) total, was assumed to be a r o u n d - 0 , 2 kcal/mol, d e p e n d i n g on temperature as - T ~ This is in accordance with high temperature results for the decay of
HIGH TEMPERATURE C2H2 + 02 REACTION other small molecules such as C2H2, CH4, and CH~, which were also investigated in this laboratory. It is estimated that the uncertainty in k5 should not exceed factors/>3 to < 0.3. U n d e r the experimental conditions above 2000 K the contribution of the ketyl decay in the ketyl reaction mechanism becomes increasingly stronger. It is impossible to differentiate between the H-production a n d the O-consumption caused by reaction R3 and the reaction sequence R5 and R6. T h e value of k6 used was based on the results of Messing et al. 23, adjusted to the high temperature range.
Subsequent reactions coupled to the CH2-channel Reactions of methylene with O-atoms consume O-atoms and contribute significantly to H-atom formation via e l l 2 + O = CO + 2 H CH2 + O = CO + H2
(R9a) (R9b).
T h e combined rate coefficient at room temperature suggested by Boehland et al. 24 is about 8 • 101~ cmS~mo1-1 s -1. I n the present work it was necessary to increase the total rate to a value of about 1.2 x 1014 cm 3 mo1-1 s-! with a branching ratio of kga/kgb = 2, based on the best fit to the measured profiles. For example, the decrease in the concentration of H in the C 2 H 2 / 0 experiments a r o u n d 1600 K is up to 25% at 8001xsec, if only channel R9b is considered. The reactions: 3CH9 + C2H2 = C3H3 + H
k12 = 1.88 x 106T2'42 exp (-6790/T) cm 3 mo1-1 s-1. T h e reaction N20 + CH2 = N2 + 2H + CO
(Rll)
is an additional H source at low temperatures, and it is estimated t h a t the rate coefficient for this reaction is kll = 4 x 1011 cm 3 mo1-1 s-a.
Relating High and Low Temperature Results Some preliminary calculations were done in an attempt to predict H o m a n n s experimental results o n C2H 2 + O (low pressure, 300 < T < 500 K) assuming approximately the branching ratio determined in the present high temperature experiments. T h e results are encouraging. H o m a n n had to use a relatively high rate coefficient for 3CHz + C2H2 = C3H3 + H, (k = 1.8 • 1012 cm 3 mo1-1 s-1) in order to model his measured C4H2 a n d C3H4 profiles. This was disputed recently by Walsh et al. 28 who gave an u p p e r limit of 1.5 • 109 cm ~ mo1-1 s-1 for this reaction at 300 K. T h e high rate coefficient used by H o m a n n and W e l l m a n n9 may, in fact, be the combined rate for two channels namely: ~CH2 + C2H2 = C3H3 + H k = 1.5 x 109 cm ~ mo1-1 s -1
(R13)
ICH2 + C2H2 = C3H3 + H k = 2 x 1014 cm 3 mol -a s -1
(R 14)
(R14)
have little importance at the high temperatures even though the rate coefficient for R139 is unrealistically high. Similarly the contribution to H from R7 (CHz + CH2 -~ C2H2 + 2 H) is small though it has a very high rate coefficient of 1 x 101~cm ~ mo1-1 s -1 derived from ketene pyrolysis experiments. 7 This was due to the low absolute concentration of CH~ in the high temperature experiments, which is evident from the low branching ratio (0.35) for the formation yield of CH2 in the C2H2 + O reaction. At temperatures a r o u n d 1500 K, reactions involving N20 also become important due to its slow decomposition rate. For the reaction N20 + H = N2 + OH
present work indicates that the rate coefficient is twice that calculated from Baulch et al. expression at 1500 K. Fittin6g the low temperature values of Albers et al. 2 with recent values from CH20 + O experiments 27 and the present work, gives the following expression:
(R13)
and 1CH2 + C2H2 = C3H3 + H
891
(R12),
Baulch et al. 25 have evaluated a rate coefficient of k12 = 7.6 x 1013 exp (-7600/T). But, the
T h e singlet ICH2 in the second channel results very probably from the reaction of the ketyl radical with H-atoms: HCCO + H = ICH2 + CO
(R4).
Calculations based on 1CH2 production via R4, reaction R10 and the fast reaction R14 gave reasonably good predictions of the reported profiles of Ref. 9. At present, it is unclear how well the assumption may hold to keep the branching ratio of C2H~ + O constant over a large temperature interval. According to Harding and Wagner 4 the two potential barriers which separate the addition complex 3A" (OHCCH) from the two product channels are well below, by about 18 kcal/mol, the entrance
892
REACTION KINETICS
b a r r i e r on the C2H2 + O side. In view o f the b r a n c h i n g ratio t e m p e r a t u r e d e p e n d e n c e , relatively small rotational effects are e x p e c t e d since all three barriers are o f the n o n - M o r s e type. At present, it may be acceptable to assume that the b r a n c h i n g ratio is i n d e p e n d e n t of t e m p e r a t u r e . See also Ref. 4. H a r d i n g and W a g n e r have tried to determ i n e the structure a n d the frequencies o f the T S T c o m p l e x for C2H2 + O. With their calculated f r e q u e n c i e s the t e m p e r a t u r e d e p e n d e n c e o f the total r e a c t i o n coefficient for C2H 2 + O = p r o d u c t s is s o m e w h a t smaller t h a n e x p e r i m e n t a l l y o b s e r v e d at high t e m p e r a t u r e s . H o w e v e r , a r e d u c t i o n by about 15% in the non-stretch vibration f r e q u e n c i e s gives m u c h better a g r e e m e n t with all m e a s u r e d data between 300 and 2500 K. It is felt that such a f r e q u e n c y r e d u c t i o n falls well within the accuracy of this type o f ab-initio calculation.
Acknowledgements The authors wish to thank Mrs. M. Behringer and Mrs. B. Rudo for technical assistance. We are very grateful to Dr. F. Bachmaier and Mr. M. Kapernaum for preparing and analyzing the ketene samples, and to Dr. L.B. Harding and Dr. A.F. Wagner for sending us their results prior to publication. We thank the Deutsche Forschungsgemeinschaft for its financial support. REFERENCES 1. JONES, I.T.N. AND BAYES, K.D.: Proc.R.Soc.A 335, 547 (1983) 2. HARDING,L.B.: J.Phys.Chem. 85, 10 (1981) 3. BLUMENBERG,B. HOYERMANNK. AND SIEVERT R: 16th Syrup. (Int.) on Combustion, 841 (1976) 4. HARDING,L.B. AND WAGNER, A.F.: J.Phys.Chem. 90, 2974 (1986) 5. LOEHR, R. AND ROTS, P.: Ber. Bunsenges. Phys.Chem. 85, 153 (1981) 6. FRANK,P. BHASKARAN,K.A. ANDJUST, TH.: 20th Symp. (Int.) on Combustion, Ann Arbor (1984) poster session 7. FRANK, P. BHASKARAN, K.A. AND JUST, TH.: J.Phys.Chem. 90, 2226 (1986)
8. VINCKIER, C. SCHAEKERS, M. AND PELTERS, J.:
J.Phys.Chem. 89, 508 (1985) 9. HOMANN, K.H. AND WELLMANN, CH.: Ber. Bunsenges.Phys.Chem. 87, 609 (1983) 10. JusT, TH.: ARAS in Shock Tubes, in A. Lifshitz: Shock Waves in Chemistry, M. Decker, New York 1981 11. FRANK, P. AND JUST, TH.: Ber. Bunsenges. Phys.Chem. 89, 181 (1985) 12. FRANK, P. AND JUST, TH.: Proc. 14th Symp. on Shock Tubes and Waves, Sydney, 705 (1983) 13. FRANK,P. ANDJUST, TH.: Comb. and Flame, 38, 231 (1980) 14. BRADLEY,J.N. AND KISTIAKOWSKY,G.B.: J.Chem. Phys. 35, 264 (1961) 15. GLASS, G.P. KISTIAROWSKY,G.B. MICHAEL,J.V. AND NIKI, H.: 10th Symp. (Int.) on Combustion, 513 (1965) 16. FENIMORE, G.P. AND JONES, G.W.: J.Chem.Phys. 39, 1514 (1963) 17. VANDOOREN, J. AND VAN TIGGELEN, P.J.: 16th Symp. (Int.) on Combustion, 1133 (1976) 18. BRAUN, W. BASS, A.M. AND PILLING, M.: J.Chem.Phys. 52, 5131 (1970) 19. LANGFORD, A. PETEK, H. AND MOORE, C.B.: J.Chem.Phys. 78, 6650 (1983) 20. FAUBEL, C. AND WAGNER, H.GG.: Ber.Bunsenges.Phys.Chem. 81,684 (1977) 21. BECKER, K.H. KLEY, D. AND NORSTROM,J.R.: 12th Symp.(Int.) on Combustion, 405 (1968) 22. TROE, J.: J.Chem. Phys. 66, 4758 (1977) 23. MESSING, I. FILSETH, S.V. SADOWSKI,C.M. AND CARRINGTON, T.: J.Chem.Phys. 74, 3874 (1981) 24. BOEHLAND, T. TEMPS, F. AND WAGNER, H.GG.: Ber. Bunsenges. Phys. Chem. 88,455 (1984) 25. BAULCH,D.L. DRYSDALE, D.D. HORSE, D.G. AND LLOYD, A.C.: Evaluated kinetic data for high temperature reactions, Vol. I, Butterworths, London 1972 26. ALBERS, E.A. HOYERMANN, K. SCHACKE, H. SCHMATJKO,K.J. WAGNER, H.GG. WOLFRUM,J.: 15th Syrup. (Int.) on Combustion, 765 (1974) 27. RIMPEL,G.: DFVLR Stuttgart, W. Germany, to be published 28. CANOSA-MAs, C.E. ELLIS, M. FREY, H.M. AND WALSH, R.: Int.Journal of Chem. Kinet. 16, 1103 (1984)
COMMENTS David Gutman, Dept. of Chem., Illinois Inst. of Techn., Chicago, Ill. 60616 USA. In your high temperature studies of H-atom production by the 0 + C2H2 reaction, could the decomposition process HC~O-~H + C20 provide additional hydrogen atoms?
Author's Reply. We considered the contribution of the ketyl channel leading to C20 + H as negligible because this reaction pathway is about 25 kcal/mol more endothermic than the channel leading to CH + CO.
H I G H T E M P E R A T U R E C9H2 + 02 R E A C T I O N
W.C. Gardiner, Prof. Univ. of Texas at Austin. Do the rate c o n s t a n t expressions derived in y o u r m o d e l i n g also r e p r o d u c e the profiles in t h e cited p a p e r of L 6 h r a n d Roth? Author's Reply. We find g o o d a g r e e m e n t between the m e a s u r e d profiles o f ref. 5 (LOhr a n d Roth) a n d o u r calculated ones at h i g h e r t e m p e r a t u r e s . At the lower e n d ( 1 5 0 0 - 1 6 2 5 K) o f the investigated temp e r a t u r e r a n g e o u r reaction m e c h a n i s m r e p r o d u c e s perfectly o u r m e a s u r e d profiles, w h e r e a s the profiles o f Ref. 5 show u n d e r similar e x p e r i m e n t a l conditions smaller H - a t o m concentrations.
F. Temps, MPI f. StrOmungsforschung, Bunsenstr. 10, 3400 GOttingen BRD. 1) Did t h e a u t h o r s include the reaction o f CH2 with C2H2 in their analysis a n d did it play a role? 2) I w o u l d like to c o m m e n t o n t h e p r o d u c t yields in the reaction O + C2H2. U s i n g N M R detection o f O (se/ a n d sCH2 we have carried o u t p r e l i m i n a r y studies o f the p r o d u c t distribution. W e have f o u n d the yield o f sCHz to be - 50% in a g r e e m e n t with the a u t h o r ' s findings.
Author's Reply. O f course we have incorporated in o u r reaction s c h e m e both the reactions o f C2H2 with triplet a n d singlet C H > U n d e r the e x p e r i m e n t a l conditions t h e effect o f both these reactions was f o u n d negligible (see discussion a n d table II).
Szabo Zoltan Gabor, Prof. EOTVOS University, Budapest, H-1443 POB 123. You have p r e s e n t e d several pathways for acetylene oxidation. I have missed the discussion a b o u t t h e m e.g. h o w the principle of Rice & Teller, the principle o f least motion, has been r e g a r d e d as valid
Author's Reply. Discussion o f d i f f e r e n t p r o d u c t pathways o f t h e reaction C2H2 + O u n d e r theoretical aspects was not the subject o f o u r work.' More inf o r m a t i o n o n detailed theoretical t r e a t m e n t o f the reaction C2H2 + O ---+ CH2 + C O / H C C O + H can be f o u n d in the work o f H a r d i n g a n d W a g n e r (L.B. H a r d i n g a n d A.F. W a g n e r , J. Phys. C h e m . 90, 2974 (1986)
AI Wagner, Argonne Nat. Laboratory, Argonne, IL 60439, USA. My first c o m m e n t is a reply to C. Melius'
893
question about the abstraction O (3p) + H C C H ~ O H + C C H c o n t r i b u t i n g to t h e overall rate of loss of O (3p). We e x a m i n e d that c h a n n e l by applying T S T to a m o d e l o f the O . . . H . . . C C H transition state that is derived f r o m o u r ab initio calculated H...H...CCtt transition state (which is consistent with rate constant m e a s u r e m e n t s o f H2 + CCH). We used fbr the e n e r g y o f the transition state a value largely based on the Lee a n d W o d t k e m e a s u r e m e n t o f the heat of f o r m a t i o n o f CCH. T h e result is that at 2500 K, abstraction is not significant (only a 5% effect). T h e activation e n e r g y for this process is quite high ( - 3 0 kcallmole at T = 0 K). Even i n c l u d i n g the loose rocking m o d e s at the transition state ( - 1 5 0 200 cm 1) the high activation e n e r g y m a k e s abstraction a m i n o r channel. Since the d i f f e r e n c e between the m e a s u r e m e n t s p r e s e n t e d in this p a p e r and o u r calculations based on addition are large, abstraction a p p e a r s to be too m i n o r to explain t h e difference. My second c o m m e n t c o n c e r n s y o u r calculations based on a 15% reduction o f o u r ab initio frequencies for addition. As we u n d e r s t a n d your calculations, you r e d u c e d all transition state f r e q u e n c i e s by 15c7c while leaving the reactant frequencies u n c h a n g e d . T h e correlation o f transition state frequencies between reactant a n d transition state would make this r e d u c t i o n o f only the transition state frequencies not likely except for the single b e n d i n g frequency that evolves o u t o f free rotations o f the reactants. O u r calculations s u g g e s t that varying that o n e frequency (and a d j u s t i n g the reaction barriers to preserve the a g r e e m e n t at r o o m t e m p e r a t u r e ) requires a 50% r e d u c t i o n to place the calculated rate into your e r r o r bars on the overall rate. T h i s r e d u c t i o n seems too e x t r e m e to us. We presently conclude that neither variation o f o u r frequencies for addition n o r inclusion o f the abstraction c h a n n e l can b r i n g your results a n d o u r calculations into a g r e e m e n t for the overall rate. However, this does n o t influence the good a g r e e m e n t b e t w e e n y o u r m e a s u r e m e n t s and o u r calculations for the b r a n c h i n g rates.
K. Mahmud and A. Fontijn, Rensselaer Polytechnic Inst., Troy, NY 12180-3590, USA. We have used o u r H T P ( h i g h - t e m p e r a t u r e p h o t o c h e m i s t r y technique) to m e a s u r e k for overall O - a t o m d i s a p p e a r a n c e in the O -t- C 2 H 2 reaction f r o m 3 0 0 - 1 5 0 0 K. We worked with this isolated e l e m e n t a r y reaction m e t h o d u n d e r conditions o f [C2H2] >~" [O]. O u r m e a s u r e m e n t s a p p e a r to be a n e a r perfect e x t e n s i o n o f yours a n d are also in a g r e e m e n t with work by o t h e r s from 3 0 0 1000 K. At the h i g h e r t e m p e r a t u r e s T ~> 1200 K, the a g r e e m e n t with the theoretical work j u s t m e n t i o n e d is thus n o t good.
A C E T Y L E N E O X I D A T I O N : T H E R E A C T I O N CzH2 + O A T H I G H TEMPERATURES P. FRANK, K.A. BHASKARAN- axn TH. JUST
DFVLR, Inst. f. phys. Chemie d. Ferbrennu~g, Stuttgart, W. German3
Shock heating, together with atomic and molecular resonance absorption spectrometry was used for simultaneous monitoring of H and O or CO in highly diluted acetylene-nitrous oxideargon mixtures. The investigations covered a temperature and pressure range of 1500 to 2500 K and 1.5 to 2.0 bar respectively. New measurements are presented for the primary step: CH~ + CO
(R2a)
HCCO + H
(R2b)
C2H2 + O =
The channel ratio, k2a/(k2a + k2b) was found to be about 0.64 in the temperature range investigated. Best fits for the measured H and O profiles were achieved with k20 = 1.6 • l014 exp (-4,975/T), k2b = 4 X 1014 e x p ( - 5 , 3 6 5 / T )
and
(All rate coefficients in cm 3 mol 1 s-l). The fact that channel 2b is faster than 2a is in qualitative agreement with theoretical predictions. To reach the above conclusions, high temperatures ketyl and methylene oxidation kinetics were investigated. The following rate coefficients were determined tbr some significant reactions. CO + 2 H
(R9a)
k = 8 ~ l • 10 l:~
CO + H2
(R9b)
k = 4 -+ 1 x 10 ~:~
(R3) (R4) (R5) (R6)
k k k k
3CH2 + O (3p)
HCCO + O (3p) HCCO + H HCCO + Ar CH + O
= 2 CO + H = ICHz + CO = CH + CO + Ar = CO + H
= = = =
1x 1.5 6 x 1-
l014
x 1014 10 ~3 exp(-29600/T) 1.5 x 10 H CH2 + C O
Introduction
C2H 2 + O = HCCO + H
R e a c t i o n s o f a c e t y l e n e a r e an i m p o r t a n t s u b s y s t e m in t h e o x i d a t i o n o f m a n y h y d r o c a r b o n s , u n d e r b o t h rich a n d l e a n flame conditions, T h e r e has b e e n a c o n s i d e r a b l e c o n t r o versy in the l i t e r a t u r e r e g a r d i n g the r e a c t i o n p r o d u c t s o f the p r i m a r y step:
+Present address: Indian Institute of Technology, Madras, India
(R2a) (R2b).
J o n e s a n d Bayes I f o u n d a c o n t r i b u t i o n o f 85 to 50% for c h a n n e l R2b. B a s e d o n theoretical c o n s i d e r a t i o n s , H a r d i n g 2 s h o w e d c h a n n e l R2b to be d o m i n a n t , w h e r e a s H o y e r m a n n et aDs gave e x p e r i m e n t a l e v i d e n c e f o r a 95c/c contrib u t i o n via c h a n n e l R2a. In a r e c e n t theoretical s t u d y o n C2Hz + O, H a r d i n g a n d W a g n e r 4 p r e d i c t e d the overall rate o f r e a c t i o n at temp e r a t u r e s below 1000 K satisfactorily, but calculated a substantially l o w e r r a t e coefficient t h a n
885
886
REACTION KINETICS
that observed in high temperature experiments. The b r a n c h i n g ratio k2b/(kza + k2b) was definitely greater than 0.25 and most probably exceeded 0.5. The high temperature experiments of Loehr and Roth 5 on acetylene oxidation were explained by assuming that R2a was the d o m i n a n t channel and using the available rate coefficients for the CH2 subsystem. I n view of some recent investigations on methylene and ketyl radical kinetics,6,z's a reinvestigation of the reaction C2H2 + O seems worthwhile and may well lead to an improved u n d e r s t a n d i n g of this complex reaction system. According to H o y e r m a n n et al. 3, the low temperature C2H2/O/H measurements of H o m a n n and Wellmanng indicated that R2a was the d o m i n a n t channel below 500K. In the present work, the R2b channel was found to be most important with a branching ratio of 0.65. Consequently the subsequent ketyl radical reactions assume greater significance. In view of recent work 7 on CH2 kinetics, an attempt is made in this paper to arrive at some quantitative data for high temperature ketyl radical reactions with O- and H-atoms.
Acetylene gas was supplied by Deutsche L'Air Liquide and certified to contain less than 0.4% impurities (mainly air). Experiments performed earlier 13with additional purification by activated carbon showed that only in the case of higher acetylene mole fractions (X > 200 ppm) was some influence of acetone impurity on H-atom profiles detectable. Experiments with C2H2 c o n c e n t r a t i o n s < 100 p p m were performed without further purification. T h e argon gas used as diluent was certified 99.9999% pure. T h e model calculations showed that in mixtures with low C2H 2 concentration (-<5 ppm) at high temperatures, the H-atom concentration in the later stage of the observed reaction time frame is practically equal to the n u m b e r of H-atoms stemming from the initial C2H2. From this and the measured H-concentration it was deduced that the calculated initial acetylene concentration was reduced by about 10-20% due to wall adsorption. T h e magnitude of this effect is in accordance with previous experiments with other molecules in the shock tube. Concentrations given in Table I for mixtures A, B, and C have been corrected for adsorption effects.
Results Experimental Details of the shock tube as well as the optical setup have been described elsewherea~ only a brief summary is presented below: The brass shock tube consists of a test section of about 7m long and a driver section of 4m with an internal tube diameter of 7.5 cm. In preparation for each experiment, the test section was evacuated by a turbomolecular p u m p to less than 10 -~ mbar. Gas mixtures were prepared and stored in a stirred 56 litre stainless steel vessel which may be baked at temperatures up to 523 K. All measurements were performed behind the reflected shock, near the end flange. Atomic and molecular resonance absorption spectrometry was used to monitor time-dependent H-, O- and CO-concentrations. Microwave-excited discharge lamps were used as light sources for the absorption measurements. The transmitted intensity signals were recorded by a 2-channel transient recorder a n d fed to a computer for averaging and computation of particle density values. Further details of the measurements of H, O and CO are given in refs. 11 and 12. N20 was the source of O-atoms in all C2H2/Oexperiments. Dissociation kinetics of N20 are important below 1800 K. The rate coefficient for this reaction was recently reconfirmed 11 for the pressure range of the present experiments.
The primary goal of this work is to determine the channel ratio for C2H2 + O = CH2 + CO HCCO + H
(R2a) (R2b)
This required careful selection of C 2 H 2 / N 2 0 mixture ratios which exhibited high sensitivity to R2a and R2b. I n a first attempt to model the measured H a n d O profiles, a 48 step reaction mechanism involving 25 species was used. Sensitivity studies demonstrated that the profiles for the different mixtures could be simulated successfully by a much smaller set of reactions. This was possible because the reactant mixtures were highly diluted, which reduced the influence of subsequent reactions. The final reaction scheme is given in Table II and the computed best fits for the measured O,
TABLE I C~H2 - N20-mixtures, diluted in argon (in ppm).
Mixture
A
B
C
D
X(C~H~) 2 2.5 4.5 10 (2.5)+ (3,0)+ (5.0)* X(N20) 20 20 5 100 +: uncorrected concentrations
E
F
G
15
20
100
100 20
100
H I G H T E M P E R A T U R E CzH~ + O~ R E A C T I O N
887
T A B L E II Reaction m e c h a n i s m a n d rate coefficient expressions Reaction R1 N20 + Ar R2a C2Hz + O R2b C~H2 + O R3 HCCO + O R4 HCCO + H R5 HCCO+Ar R6 CH + O R7 CH2 + CH2 R8 CHz + H R9a CH2 + O R9b CH~ + O RI0 ICH2 + A r Rll N 2 0 4- CH2 R12 N~O + H R13 CH9 + C2H2 R14 1CH2+C2H~
= = = = = = = = = = = = = = = =
N2 + O + A r CH2 4- CO HCCO + H 2 CO + H ICH2 + C O C H + CO + A r CO + H C2H2 + 2H C H + H2 CO + 2H C O + H2 3CH2 + A r N~ + CO + 2 H N~ + O H CsHs + H C3Hs + H
A n 9.3 x 1014 1.6 x 1014 -4.0 x 1014 i -+ 0.2 x 1014 - 1.5 X 1014 6 x 1015 1 - 1.5 x 1014 - 1 x 1014 -8 x 1012 8-+ 1 x 10 Is 4-+ 1 x 1013 4 - 3 5 x 101l - 4 x 1011 1.9 x 106 2.42 1.5 x 109 2 x 10 TM -
-
-
-
-
-
-
-
Ea 30061 4975 5365 --
-
29600 ---
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
6790
-
-
-
-
-
-
-
-
T - r a n g e (K) 1450-2200 1500-2500 1500-2500 300-2500 300-2500 1500-2500 1500-2500 2100-2700 2100-2700 2100-2600 2150-2600 300-2100 1500-1700 300-1700 300-1900 300-1900
Ref, 10 44-
+,14 +,14 a
+,27 13 13 44-
13,20,21 +b +,19,26 8,+ c
+c
All rate coefficients in c m 3 mol -l s -1 (k = A 9 T " . exp (-Ea/T)). +: T h i s work a: estimated f r o m u n i m o l e c u l a r - t h e o r y , low p r e s s u r e limit. b: H C O is an i n t e r m e d i a t e , which d e c o m p o s e s very fast above 1500K. c: negligible in this work. H a n d C O c o n c e n t r a t i o n s a r e s h o w n as s o l i d l i n e s i n Figs. 1 - 6 . T h e r e a c t i o n s c h e m e s h o w n in T a b l e II is t h e r e s u l t o f s y s t e m a t i c s e n s i t i v i t y a n a l y s i s a n d t h e e x a m p l e s s h o w n in Figs. 1 - 6 , w e r e c h o s e n to d e m o n s t r a t e t y p i c a l r e s p o n s e s o f t h e m o d e l to c h a n g e s in t h e i m p o r t a n t r a t e coefficients. S i n c e t h e m e t h y l e n e r a d i c a l ( t r i p l e t 3CH2) is a m a j o r p r o d u c t d u r i n g a c e t y l e n e o x i d a t i o n , 5'9 a s e r i e s o f e x p e r i m e n t s w e r e f i r s t c a r r i e d o u t to 4
10 ~3 e m -3
20
3
...-;:L2
15
.........
[~] I 2
s t u d y t h e r e a c t i o n o f 3 C H 2 w i t h O - a t o m s 6. M e t h y l e n e r a d i c a l s w e r e f o r m e d by t h e t h e r m a l decomposition of ketene and the H, O and CO concentrations were monitored. Figure 1 compares the measured and computed H and CO profiles produced by shockheated ketene/ nitrous oxide mixtures. Despite the high rate of the reaction 3CH2 + 3CH2 = C2H2 + 2 H 2.0
1013 em-a
1o I [co]
(R7)
I0
8
10 is e m -3 1.5
1012 em -3
6 I [o]
In] I 1.0
4 0.5
2 0
0.0 200
400
time (psee)
600
800
-
Fro. 1. C o m p a r i s o n o f H - a t o m (triangles) a n d C O - m o l e c u l e (rhombs) m e a s u r e m e n t s with c o m p u t a tional results, s h o w i n g t h e i n f l u e n c e o f CH2 4- O
(R9a, b). 4 p p m H2CCO + 12.5 p p m N~O in A r T = 2280 K P = 1.57 b a r Full line:calc, u s i n g t h e k-values o f table II . . . . . . : calc. without R9a,b .... : calc. with kg~ x 0.8, kgb = 0
0
200 400 time (~ see)
600
800
-
Fro. 2. C o m p a r i s o n o f H- (triangles) a n d O - a t o m (circles) m e a s u r e m e n t s with c o m p u t a t i o n a l results, s h o w i n g the influence o f both c h a n n e l s o f C2Hz + O (R2a, b) a n d o f H C C O + H, O (R3,R4). Mixture E T = 1550 K P = 1.85 bar Full line:calc, u s i n g t h e k-values o f table II .... : calc. with k~a = 0, k2b x 1.1 . . . . . . : calc. with k2b = 0, k2a • 3 . . . . . : calc. without R3-R4
888
REACTION KINETICS
the CH2 + O reaction c o n t r i b u t e d substantially to the formations of CO a n d H, d u e to the chosen excess of O-atoms. T h e dashed-dottedcurves show the CO a n d H concentrations d u e solely to ketene pyrolysis. T h e model calculations indicate that a n alternative CH2 + O c h a n n e l which does n o t p r o d u c e H-atoms directly must also be taken into account. If this second reaction c h a n n e l is neglected a n d the rate coefficient for the H - p r o d u c i n g c h a n n e l is chosen to fit the e x p e r i m e n t a l H-profile early in the reaction time (t < 200 ~sec), then the calculated H-profiles are definitely too large later in the reaction (dashed curve). T h e evalu a t i o n of a series of e x p e r i m e n t s on CH2 with O-atoms resulted in two c h a n n e l s CO + 2 H ~CH2 + O = CO + H2
(R9a) (R9b)
with a b r a n c h i n g ratio kg,,/(kga + kgb) of about 0.65. In view of these findings a n d the results of previous studies, 6'7 the high t e m p e r a t u r e reaction m e c h a n i s m of m e t h y l e n e may now be considered to be k n o w n . T h e r e f o r e the m a i n emphasis of the p r e s e n t investigation is o n the initiation reaction, a n d o n the reactions involving HCCO. Figure 2 shows the c o m p a r i s o n between m e a s u r e d a n d calculated H- a n d O-profiles at a relatively low t e m p e r a t u r e . Due to the slow f o r m a t i o n rate of O-atoms we have conditions of a "rich mixture" that is, [ C 2 H 2 ] / [ 0 ] ~ 4.5 at 600 I*sec, if acetylene would n o t be c o n s u m e d by C2H 2 + 0 . O a n d H profiles are sensitive to variations of kl to kl2, but, w h e n t < 300 p~secthe profiles are within the e x p e r i m e n t a l scatter (ca. + / - 15%) exclusively d e t e r m i n e d by R2a, R2b, a n d the well k n o w n reaction R1.11 Both acetylene channels are n e e d e d to describe the measured profiles. I f the CH2-channel R2a is switched off a n d the k-value for the HCCOc h a n n e l R2b is chosen to match the e x p e r i m e n tal H-profile, t h e n the p r e d i c t e d O-atom concentrations (dashed line) are too high. If R2b is switched off a n d k~a is chosen to fit the measured O-profile, t h e n the H - a t o m concentrations (dashed dotted line) are too low. T h e reactions of H a n d O with ketyl radical have some influence on the profiles w h e n the reaction time exceeds 300 p,sec. I f the HCCO-reactions (R3 a n d R4) are neglected then the calculated profile is r e p r e s e n t e d by the dotted curve in fig. 2. Ketyl d e c o m p o s i t i o n (R5) has no influence at this t e m p e r a t u r e . U n d e r the chosen e x p e r i m e n t a l conditions, only the Oprofile is sensitive to R3 a n d R4. I n Figure 3, the m i x i n g ratio [C2H2]/[O] is a b o u t 1. Both profiles are sensitive to changes
!
2.0 1013 cm-3 1.5 [HI I
1.0
[0]
0.5
0.0
200 400 time (p see)
0
600
BOO
--
FIG. 3. Comparison of H- (triangles) and O-atom (circles) measurements with computational results, showing the influence of both channels of C2H~ + 0
(R2a, b). Mixture C T = 1970 K P = 1.65 bar Full line:calc, using the k-values of table II . . . . . . calc. with k2~ = 0, k2b x 1.1 . . . . : calc. with k~b = O, k~ x 2.6
2.5
_ 25
2.0
[HI
:
1.5
/
1.0
x"
0.5
/./
0.0
~ 200 time
........... 7 .~. . . . . . . .
~ ......
' 400
' 600
~see)
"
T- 20
i 15
10 lz e r a - 3
[o]
' - 10
' 800
FIG. 4. Comparison of H- (triangles) and O-atom (circles) measurements with computational results, showing the influence of C~H2 + O = HCCO + H (R2b) and the HCCO-reactions (R3-R5). Mixture A T = 2460K P = 1.51 bar Full line:calc, using the k-values of table II ---: calc. with k2a = 0, k2b x 1.5 . . . . . . : calc. with k2b x 0.5, k~a x 2 . . . . . : calc. without R3-R5 in the rate constants k 2 to k0. I f the CH2-channel
R2a is again switched off a n d the value ofk2b is adjusted to fit the e x p e r i m e n t a l H-profile, the calculated O-profile is r e p r e s e n t e d by the dotted curve. If k2b equals zero a n d k2a fitS the e x p e r i m e n t a l O-profile, t h a n the model predicts the dashed curves for H. U n d e r the conditions of F i g u r e 4 (extremely lean mixture, high t e m p e r a t u r e ) , the O-profile is insensitive to variations in k2 to kg. I n this case, it is possible to fit b o t h e x p e r i m e n t a l profiles without reaction R2a by increasing k2b by 10%.
HIGH TEMPERATURE C2H2 + 02 REACTION 7
1013
cm -3
[o]
5
14 3
9
0
0
100
200 300 time (~ sec)
400
500
=
Fro. 5. Comparison of O-atom measurements with computational results, showing the influence of C2H2 + 0 (R2a, b) Mixture F T = 2475 K P = 1.64 bar Full line:calc, using the k-values of table II ---: calc,with (k9~ + k~b) x 0.8 . . . . . . calc. with (k2~ + k2b) x 2.2 ...... : calc. with k2b = 0 , k2a x 8 6
I
i
I
j
5
1014 cm-3 4
[CO] 2 1 0 0
200 400 time (~tsec)
600
800
=
Fro. 6. Comparison of CO-molecule measurements with computational results, showing the influence of the HCCO-reactions (1/3 - R5). Mixture G T = 1956K P = 1.69 bar Full line:calc, using the k-values of table II : cal. without R3-R5 For k2a->0, changing kzb by 1.5 or 0.5, with k2a 4-
k2b remaining constant, shows the sensitivity of the H-profile with respect to the ketyl channel. If channel R2b is switched off, and kza is set nearly equal to the gas kinetic collision number, good agreement is achieved initially with the measured H-profile, but not in the later stage. For an experiment with comparable conditions, but with a mixing ratio of 1:1, where the O-profile is sensitive to variations of k2~ + kzb, O-atom concentrations (Fig. 5, dashed-dotted curve), which are by a factor of 2 - 3 smaller than the measured ones, are calculated for the upper limit of k2a (k2a ~ 2 x 1014).
889
For such high temperature experiments (fast N20-decay) with rich to near stoichiometric mixtures, we can deduce the uncertainty in the evaluation of k9o + k2b. Figure 5 shows the changes in the O-profile when k2o + kh is varied by about + 20%. If additionally the reactions of ketyl (R3 and R5) are switched off, larger O-concentrations are calculated, which can be compensated for by increasing k2a + k2b by about 20%. The strong influence of the ketyl reactions (R3-R5) on the CO-production is shown in Figure 6. If R3-R5 are neglected then we calculate CO-concentrations that are as much as a factor of 2.5 smaller than the measured values. Discussion
Results at high temperatures Of the 3 primary steps CH2 + CO C2H 2 4- O = HCCO 4- H CzH + O H
(R2a) (R2b) (R2c)
the OH-channel proposed b~, Bradley and Kistiakowsky, 14 and Glass et all 15 is not ~mportam u n d e r the present conditions. This channel is about 23 kcal/mol endothermic while the CHz- and HCCO-channels are exothermic. Hence, R2c probably has very little influence. From the initial shape of the measured H-profiles, it was apparent that there was an early H-producing channel. All attempts to model the H-profile neglecting the ketyl channel were unsuccessful. This is considered excellent proof of the existence of this channel. To achieve a good fit between the measured and computed H-profiles in the investigated temperature range, it was necessary to increase the rate coefficient of the HCCO channel by a factor of about 1.5, as compared to that of Loehr and Roth. 5 The activation energy remained practically unchanged. T h e rate coefficient obtained for C2Hz + O = HCCO + H was: k~b = 4 x 1014 exp (-5365/T) cm 3 mo1-1 s -1. It is estimated that the error in this rate coefficient due to contributions by the CHz-channel (R2a), and caused by subsequent HCCO- and CHz-reactions, and experimental scatter in the initial temperatures, should not exceed 20% for temperatures above 1900 K, and 30% for temperatures in the 1500-1700 K range, Based on flame studies, Fenimore and Jones 16 concluded that the oxygen atom should directly attack an acetylene carbon atom which would make the methylene channel dominant.
890
REACTION KINETICS
Vandooren and Van Tiggelen 17 from their data on low pressure flames (700 to 1430 K) also found the methylene channel to be faster than the ketvl channel. However, in the present work a satisfactory fit of the measured O and H profiles was possible only when a channel ratio (k2jk2,) of approximately 65/35 was assumed The fact that the methylene channel was slower than the ketvl channel was observed in all experiments with different mixing ratios over the entire temperature range of 1500 to 2500 K. Harding's theoretical predictions 2'4 very strongly support this. The rate coefficient obtained for the CH2-channel is k2,, = 1.6 x 10 I4 exp (-4975/T) cm :~ mol -t s-1. For temperatures below 2100 K, the Oprofiles are sensitive to k2,,, and an error of about -+ 30% for k2,, is estimated. For higher temperatures, the O-profiles become increasingly insensitive to variations of k>,. The rate of O-consumption was measured for temperatures above 2200 K with C2H2/O = 1 (mixture F of Table I). The O-consumption is mainly determined by reactions R2a and R2b in the first 200 Ixsec. From k ~,~,t(,/,and k2b, the values for k2~ could be evaluated with an uncertainty of less than 50% at 2400 K. The experiments indicate that the overall rate coefficient for CeH2 + O is higher for temperatures above 2000 K (about a factor 3 at 2500 K) than the theoretical predictions of Ref. 4. From the above, it can be concluded that the primary reaction C2H2 + O exhibits a channel ratio of 0.7 to 0.6 for R2b and 0.3 to 0.4 for R2a. This channel ratio is not expected to change significantly at lower temperatures (see below). The shape of the measured profiles later in the reaction time frame (t > 300 Ixsec) is determined to some extent by secondar~ reactions.
Secondam: reactions of HCCO Very little is known quantitatively about reactions of HCCO with atomic oxygen at high temperatures. U n d e r certain experimental conditions (mixtures C, D, E), measured H and O profiles often tend to decrease at the end of the reaction time, suggesting that there are some important O-atom removal processes. Channel R3 (HCCO + O = 2 CO + H) was found to be necessary in r e p r o d u c i n g the shapes of both O and H profiles. Due to the fast decay of HCO (HCO + Ar = CO + H + A r ) i n the experiments, it was not possible to discriminate between possible product channels leading to 2 CO + H or HCO + CO. In rich C2H2/O mixtures, at relatively low temperatures, the effect of R3 on the O-profile is p r o n o u n c e d in
the last stages of the reaction (Fig. 2). T h e concurrent decomposition reaction of ketyl HCCO + Ar = CH + CO + Ar
(R5)
and the fast consecutive step CH + O = CO + H
(R6)
cannot influence the O-profile because of the slow decomposition rate of HCCO below 1700 K. This allows one to set the value for k3 in the temperature range 1500 - 1700 K at about 1014 cm s tool -1 s, -a, which is in excellent agreement with the room temperature results of Vinckier et al. 8 Our experimental conditions do not allow a very accurate study of the reaction of H-atoms with HCCO (R4). U n d e r lean conditions, the influence of R4 is negligible even though it is very fast. 8 For near stoichiometric to rich mixing ratios, contributions of R4 become apparent. T h r e e product channels are discussed in the literature: HCCO + H
= 1CH2 + CO (R4) AHo~ = - 19 kcal/mol C20 -{- H2 (R4a) AHo~ = - 4 kcal/mol HCCO + H + M = HCCOH* + M (R4b) A H ~ = - 5 5 kcal/mol followed by 1CH2 + Ar = 3CH2 + Ar 18'm and HCCOH = C2H + O H 2~ In the present experiments the total rate (k4 + k4a + k4b) of HCCO consumption by H-atoms is evaluated. From the re-evaluation of the flow reactor experiments, 9 it was deduced (see below) that the d o m i n a n t product channel should be the carbene radical, singlet ICH2, and CO. Faubel and W a g n e r 2~ found that k4b should have a value less than 10% of k4. The effect of channel R4a, which was also proposed by Faubel and W a g n e r 2~ and Becker et al, 21 was also considered negligible (k4a<-O.Ixk2, ref. 20) in the high temperature range. For all experiments there was good agreement between experimental results and model calculations using the high rate coefficient for R4, performed recently by Vinckier et al. 8 No direct m e a s u r e m e n t of the decomposition of ketyl is available. T h e rate coefficient for the unimolecular decay was calculated using Troe's theory 22 and structural and frequency data. 4 For 13c, a value of 0.04 was chosen, close to that of the ketene decay. T h e average energy transferred by collisions (zXE) total, was assumed to be a r o u n d - 0 , 2 kcal/mol, d e p e n d i n g on temperature as - T ~ This is in accordance with high temperature results for the decay of
HIGH TEMPERATURE C2H2 + 02 REACTION other small molecules such as C2H2, CH4, and CH~, which were also investigated in this laboratory. It is estimated that the uncertainty in k5 should not exceed factors/>3 to < 0.3. U n d e r the experimental conditions above 2000 K the contribution of the ketyl decay in the ketyl reaction mechanism becomes increasingly stronger. It is impossible to differentiate between the H-production a n d the O-consumption caused by reaction R3 and the reaction sequence R5 and R6. T h e value of k6 used was based on the results of Messing et al. 23, adjusted to the high temperature range.
Subsequent reactions coupled to the CH2-channel Reactions of methylene with O-atoms consume O-atoms and contribute significantly to H-atom formation via e l l 2 + O = CO + 2 H CH2 + O = CO + H2
(R9a) (R9b).
T h e combined rate coefficient at room temperature suggested by Boehland et al. 24 is about 8 • 101~ cmS~mo1-1 s -1. I n the present work it was necessary to increase the total rate to a value of about 1.2 x 1014 cm 3 mo1-1 s-! with a branching ratio of kga/kgb = 2, based on the best fit to the measured profiles. For example, the decrease in the concentration of H in the C 2 H 2 / 0 experiments a r o u n d 1600 K is up to 25% at 8001xsec, if only channel R9b is considered. The reactions: 3CH9 + C2H2 = C3H3 + H
k12 = 1.88 x 106T2'42 exp (-6790/T) cm 3 mo1-1 s-1. T h e reaction N20 + CH2 = N2 + 2H + CO
(Rll)
is an additional H source at low temperatures, and it is estimated t h a t the rate coefficient for this reaction is kll = 4 x 1011 cm 3 mo1-1 s-a.
Relating High and Low Temperature Results Some preliminary calculations were done in an attempt to predict H o m a n n s experimental results o n C2H 2 + O (low pressure, 300 < T < 500 K) assuming approximately the branching ratio determined in the present high temperature experiments. T h e results are encouraging. H o m a n n had to use a relatively high rate coefficient for 3CHz + C2H2 = C3H3 + H, (k = 1.8 • 1012 cm 3 mo1-1 s-1) in order to model his measured C4H2 a n d C3H4 profiles. This was disputed recently by Walsh et al. 28 who gave an u p p e r limit of 1.5 • 109 cm ~ mo1-1 s-1 for this reaction at 300 K. T h e high rate coefficient used by H o m a n n and W e l l m a n n9 may, in fact, be the combined rate for two channels namely: ~CH2 + C2H2 = C3H3 + H k = 1.5 x 109 cm ~ mo1-1 s -1
(R13)
ICH2 + C2H2 = C3H3 + H k = 2 x 1014 cm 3 mol -a s -1
(R 14)
(R14)
have little importance at the high temperatures even though the rate coefficient for R139 is unrealistically high. Similarly the contribution to H from R7 (CHz + CH2 -~ C2H2 + 2 H) is small though it has a very high rate coefficient of 1 x 101~cm ~ mo1-1 s -1 derived from ketene pyrolysis experiments. 7 This was due to the low absolute concentration of CH~ in the high temperature experiments, which is evident from the low branching ratio (0.35) for the formation yield of CH2 in the C2H2 + O reaction. At temperatures a r o u n d 1500 K, reactions involving N20 also become important due to its slow decomposition rate. For the reaction N20 + H = N2 + OH
present work indicates that the rate coefficient is twice that calculated from Baulch et al. expression at 1500 K. Fittin6g the low temperature values of Albers et al. 2 with recent values from CH20 + O experiments 27 and the present work, gives the following expression:
(R13)
and 1CH2 + C2H2 = C3H3 + H
891
(R12),
Baulch et al. 25 have evaluated a rate coefficient of k12 = 7.6 x 1013 exp (-7600/T). But, the
T h e singlet ICH2 in the second channel results very probably from the reaction of the ketyl radical with H-atoms: HCCO + H = ICH2 + CO
(R4).
Calculations based on 1CH2 production via R4, reaction R10 and the fast reaction R14 gave reasonably good predictions of the reported profiles of Ref. 9. At present, it is unclear how well the assumption may hold to keep the branching ratio of C2H~ + O constant over a large temperature interval. According to Harding and Wagner 4 the two potential barriers which separate the addition complex 3A" (OHCCH) from the two product channels are well below, by about 18 kcal/mol, the entrance
892
REACTION KINETICS
b a r r i e r on the C2H2 + O side. In view o f the b r a n c h i n g ratio t e m p e r a t u r e d e p e n d e n c e , relatively small rotational effects are e x p e c t e d since all three barriers are o f the n o n - M o r s e type. At present, it may be acceptable to assume that the b r a n c h i n g ratio is i n d e p e n d e n t of t e m p e r a t u r e . See also Ref. 4. H a r d i n g and W a g n e r have tried to determ i n e the structure a n d the frequencies o f the T S T c o m p l e x for C2H2 + O. With their calculated f r e q u e n c i e s the t e m p e r a t u r e d e p e n d e n c e o f the total r e a c t i o n coefficient for C2H 2 + O = p r o d u c t s is s o m e w h a t smaller t h a n e x p e r i m e n t a l l y o b s e r v e d at high t e m p e r a t u r e s . H o w e v e r , a r e d u c t i o n by about 15% in the non-stretch vibration f r e q u e n c i e s gives m u c h better a g r e e m e n t with all m e a s u r e d data between 300 and 2500 K. It is felt that such a f r e q u e n c y r e d u c t i o n falls well within the accuracy of this type o f ab-initio calculation.
Acknowledgements The authors wish to thank Mrs. M. Behringer and Mrs. B. Rudo for technical assistance. We are very grateful to Dr. F. Bachmaier and Mr. M. Kapernaum for preparing and analyzing the ketene samples, and to Dr. L.B. Harding and Dr. A.F. Wagner for sending us their results prior to publication. We thank the Deutsche Forschungsgemeinschaft for its financial support. REFERENCES 1. JONES, I.T.N. AND BAYES, K.D.: Proc.R.Soc.A 335, 547 (1983) 2. HARDING,L.B.: J.Phys.Chem. 85, 10 (1981) 3. BLUMENBERG,B. HOYERMANNK. AND SIEVERT R: 16th Syrup. (Int.) on Combustion, 841 (1976) 4. HARDING,L.B. AND WAGNER, A.F.: J.Phys.Chem. 90, 2974 (1986) 5. LOEHR, R. AND ROTS, P.: Ber. Bunsenges. Phys.Chem. 85, 153 (1981) 6. FRANK,P. BHASKARAN,K.A. ANDJUST, TH.: 20th Symp. (Int.) on Combustion, Ann Arbor (1984) poster session 7. FRANK, P. BHASKARAN, K.A. AND JUST, TH.: J.Phys.Chem. 90, 2226 (1986)
8. VINCKIER, C. SCHAEKERS, M. AND PELTERS, J.:
J.Phys.Chem. 89, 508 (1985) 9. HOMANN, K.H. AND WELLMANN, CH.: Ber. Bunsenges.Phys.Chem. 87, 609 (1983) 10. JusT, TH.: ARAS in Shock Tubes, in A. Lifshitz: Shock Waves in Chemistry, M. Decker, New York 1981 11. FRANK, P. AND JUST, TH.: Ber. Bunsenges. Phys.Chem. 89, 181 (1985) 12. FRANK, P. AND JUST, TH.: Proc. 14th Symp. on Shock Tubes and Waves, Sydney, 705 (1983) 13. FRANK,P. ANDJUST, TH.: Comb. and Flame, 38, 231 (1980) 14. BRADLEY,J.N. AND KISTIAKOWSKY,G.B.: J.Chem. Phys. 35, 264 (1961) 15. GLASS, G.P. KISTIAROWSKY,G.B. MICHAEL,J.V. AND NIKI, H.: 10th Symp. (Int.) on Combustion, 513 (1965) 16. FENIMORE, G.P. AND JONES, G.W.: J.Chem.Phys. 39, 1514 (1963) 17. VANDOOREN, J. AND VAN TIGGELEN, P.J.: 16th Symp. (Int.) on Combustion, 1133 (1976) 18. BRAUN, W. BASS, A.M. AND PILLING, M.: J.Chem.Phys. 52, 5131 (1970) 19. LANGFORD, A. PETEK, H. AND MOORE, C.B.: J.Chem.Phys. 78, 6650 (1983) 20. FAUBEL, C. AND WAGNER, H.GG.: Ber.Bunsenges.Phys.Chem. 81,684 (1977) 21. BECKER, K.H. KLEY, D. AND NORSTROM,J.R.: 12th Symp.(Int.) on Combustion, 405 (1968) 22. TROE, J.: J.Chem. Phys. 66, 4758 (1977) 23. MESSING, I. FILSETH, S.V. SADOWSKI,C.M. AND CARRINGTON, T.: J.Chem.Phys. 74, 3874 (1981) 24. BOEHLAND, T. TEMPS, F. AND WAGNER, H.GG.: Ber. Bunsenges. Phys. Chem. 88,455 (1984) 25. BAULCH,D.L. DRYSDALE, D.D. HORSE, D.G. AND LLOYD, A.C.: Evaluated kinetic data for high temperature reactions, Vol. I, Butterworths, London 1972 26. ALBERS, E.A. HOYERMANN, K. SCHACKE, H. SCHMATJKO,K.J. WAGNER, H.GG. WOLFRUM,J.: 15th Syrup. (Int.) on Combustion, 765 (1974) 27. RIMPEL,G.: DFVLR Stuttgart, W. Germany, to be published 28. CANOSA-MAs, C.E. ELLIS, M. FREY, H.M. AND WALSH, R.: Int.Journal of Chem. Kinet. 16, 1103 (1984)
COMMENTS David Gutman, Dept. of Chem., Illinois Inst. of Techn., Chicago, Ill. 60616 USA. In your high temperature studies of H-atom production by the 0 + C2H2 reaction, could the decomposition process HC~O-~H + C20 provide additional hydrogen atoms?
Author's Reply. We considered the contribution of the ketyl channel leading to C20 + H as negligible because this reaction pathway is about 25 kcal/mol more endothermic than the channel leading to CH + CO.
H I G H T E M P E R A T U R E C9H2 + 02 R E A C T I O N
W.C. Gardiner, Prof. Univ. of Texas at Austin. Do the rate c o n s t a n t expressions derived in y o u r m o d e l i n g also r e p r o d u c e the profiles in t h e cited p a p e r of L 6 h r a n d Roth? Author's Reply. We find g o o d a g r e e m e n t between the m e a s u r e d profiles o f ref. 5 (LOhr a n d Roth) a n d o u r calculated ones at h i g h e r t e m p e r a t u r e s . At the lower e n d ( 1 5 0 0 - 1 6 2 5 K) o f the investigated temp e r a t u r e r a n g e o u r reaction m e c h a n i s m r e p r o d u c e s perfectly o u r m e a s u r e d profiles, w h e r e a s the profiles o f Ref. 5 show u n d e r similar e x p e r i m e n t a l conditions smaller H - a t o m concentrations.
F. Temps, MPI f. StrOmungsforschung, Bunsenstr. 10, 3400 GOttingen BRD. 1) Did t h e a u t h o r s include the reaction o f CH2 with C2H2 in their analysis a n d did it play a role? 2) I w o u l d like to c o m m e n t o n t h e p r o d u c t yields in the reaction O + C2H2. U s i n g N M R detection o f O (se/ a n d sCH2 we have carried o u t p r e l i m i n a r y studies o f the p r o d u c t distribution. W e have f o u n d the yield o f sCHz to be - 50% in a g r e e m e n t with the a u t h o r ' s findings.
Author's Reply. O f course we have incorporated in o u r reaction s c h e m e both the reactions o f C2H2 with triplet a n d singlet C H > U n d e r the e x p e r i m e n t a l conditions t h e effect o f both these reactions was f o u n d negligible (see discussion a n d table II).
Szabo Zoltan Gabor, Prof. EOTVOS University, Budapest, H-1443 POB 123. You have p r e s e n t e d several pathways for acetylene oxidation. I have missed the discussion a b o u t t h e m e.g. h o w the principle of Rice & Teller, the principle o f least motion, has been r e g a r d e d as valid
Author's Reply. Discussion o f d i f f e r e n t p r o d u c t pathways o f t h e reaction C2H2 + O u n d e r theoretical aspects was not the subject o f o u r work.' More inf o r m a t i o n o n detailed theoretical t r e a t m e n t o f the reaction C2H2 + O ---+ CH2 + C O / H C C O + H can be f o u n d in the work o f H a r d i n g a n d W a g n e r (L.B. H a r d i n g a n d A.F. W a g n e r , J. Phys. C h e m . 90, 2974 (1986)
AI Wagner, Argonne Nat. Laboratory, Argonne, IL 60439, USA. My first c o m m e n t is a reply to C. Melius'
893
question about the abstraction O (3p) + H C C H ~ O H + C C H c o n t r i b u t i n g to t h e overall rate of loss of O (3p). We e x a m i n e d that c h a n n e l by applying T S T to a m o d e l o f the O . . . H . . . C C H transition state that is derived f r o m o u r ab initio calculated H...H...CCtt transition state (which is consistent with rate constant m e a s u r e m e n t s o f H2 + CCH). We used fbr the e n e r g y o f the transition state a value largely based on the Lee a n d W o d t k e m e a s u r e m e n t o f the heat of f o r m a t i o n o f CCH. T h e result is that at 2500 K, abstraction is not significant (only a 5% effect). T h e activation e n e r g y for this process is quite high ( - 3 0 kcallmole at T = 0 K). Even i n c l u d i n g the loose rocking m o d e s at the transition state ( - 1 5 0 200 cm 1) the high activation e n e r g y m a k e s abstraction a m i n o r channel. Since the d i f f e r e n c e between the m e a s u r e m e n t s p r e s e n t e d in this p a p e r and o u r calculations based on addition are large, abstraction a p p e a r s to be too m i n o r to explain t h e difference. My second c o m m e n t c o n c e r n s y o u r calculations based on a 15% reduction o f o u r ab initio frequencies for addition. As we u n d e r s t a n d your calculations, you r e d u c e d all transition state f r e q u e n c i e s by 15c7c while leaving the reactant frequencies u n c h a n g e d . T h e correlation o f transition state frequencies between reactant a n d transition state would make this r e d u c t i o n o f only the transition state frequencies not likely except for the single b e n d i n g frequency that evolves o u t o f free rotations o f the reactants. O u r calculations s u g g e s t that varying that o n e frequency (and a d j u s t i n g the reaction barriers to preserve the a g r e e m e n t at r o o m t e m p e r a t u r e ) requires a 50% r e d u c t i o n to place the calculated rate into your e r r o r bars on the overall rate. T h i s r e d u c t i o n seems too e x t r e m e to us. We presently conclude that neither variation o f o u r frequencies for addition n o r inclusion o f the abstraction c h a n n e l can b r i n g your results a n d o u r calculations into a g r e e m e n t for the overall rate. However, this does n o t influence the good a g r e e m e n t b e t w e e n y o u r m e a s u r e m e n t s and o u r calculations for the b r a n c h i n g rates.
K. Mahmud and A. Fontijn, Rensselaer Polytechnic Inst., Troy, NY 12180-3590, USA. We have used o u r H T P ( h i g h - t e m p e r a t u r e p h o t o c h e m i s t r y technique) to m e a s u r e k for overall O - a t o m d i s a p p e a r a n c e in the O -t- C 2 H 2 reaction f r o m 3 0 0 - 1 5 0 0 K. We worked with this isolated e l e m e n t a r y reaction m e t h o d u n d e r conditions o f [C2H2] >~" [O]. O u r m e a s u r e m e n t s a p p e a r to be a n e a r perfect e x t e n s i o n o f yours a n d are also in a g r e e m e n t with work by o t h e r s from 3 0 0 1000 K. At the h i g h e r t e m p e r a t u r e s T ~> 1200 K, the a g r e e m e n t with the theoretical work j u s t m e n t i o n e d is thus n o t good.