Oxidation of BrCN in shock waves and formation of NO

COMBUSTION

AND FLAME

61: 167-175

(1985)

167

Oxidation of BrCN in Shock Waves and Formation of NO TETSUO Matsue

Technical

College,

HIGASHIHARA Nishiikuma-cho,

Matsue 690, Japan

and HIROYUKI KURODA, KO SAITO, and ICHIRO

MURAKAMI

Department of Chemistry,Faculty of Science, Hiroshima University, Higashisenda-machi,

Hiroshima

730, Japan

The oxidation of BrCN has been studied in shock waves over the temperature range 1680-2550K. The kinetics of the oxidation was monitored by following ir emission from NO at the wavelength of 5.34 Fm. A computer simulation study was performed to determine the important elementary reactions. It was found from this computer study that the NO formation from BrCN oxidation is mainly governed by 9 elementary reactions, and the main reaction in producing NO is NCO+O=NO+CO. The rate constant

of this reaction

was determined

as

k5=1()“.‘*‘J.4

The rate constants

cm3

mol-’

s-~.

of Br + BrCN = Br:! + CN, NCO+Ar=N+CO+Ar

were also determined

as k~=10’5~0*“.2 exp[-(157.4k8.7) k~=10’3~8’o~4 exp(-159

kJ/RT)

INTRODUCTION It is well known that HCN (or the CN radical) is an important intermediate in the formation of prompt or fuel NO. For the conversion of HCN or CN to NO, it is necessary that the HCN or CN is first converted to NC0 by the reactions CN+O,-tNCO+O, HCN + O(OH)-+NCO(HNCO)

+ H.

Copyright 0 1985 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc. 52 Vanderbilt Avenue, New York, NY 10017

~J/RT]

cm3 mol-1

cm) mol-1

s-r,

s-r,

However, the fate of the NC0 radical certain. Two routes have been postulated disappearance of NCO: NC0 NC0

TNO+CO 02 NH ol

is less for the

(A) NO.

Among recent studies, Puechberty and Cottereau [ 11, for example, have taken into account route (B) but not route (A). On the other hand,

OOlO-2180/85/$03.30

TETSUO HIGASHIHARA ET AL.

168 Crowhurst and Simmons [2] and the present authors [3] have taken into account both routes (A) and (B). However, because of the lack of rate data for the reactions of the NC0 radical, it has been impossible to determine the relative importance of routes (A) and (B). For example, in our previous study [3] on the HCN-Op and HCN-02-NO2 reactions, even if route (A) was eliminated, experimental findings were explained, although they were more completely explained by taking into account route (A). In this paper, we studied the reaction of BrCN + 02 in order to verify the importance of the reaction NC0 + 0 = NO + CO. In this system, because of the absence of an H-atom, route (B) cannot occur and the reaction mechanism is expected to be rather simple. Moreover, as far as we know, there has not been any study of the BrCN + O2 reaction. Therefore it is also interesting to study the mechanism of this reaction for its own sake.

The compositions of the reaction mixtures were (A) 2% BrCN-2 % O,-96% Ar; (B) 1% BrCN-1% O,-98% Ar; (C) 2% BrCN-4% 0294% Ar; and (D) 1% BrCN-2 % 02-97 % Ar. The shock tube was described previously [4]. All experiments were performed behind reflected shock waves. The initial sample pressure was kept at 30 or 60 Torr and the pressure of driver gas (Hz) was varied. The temperature range studied was 1650-225OK. The kinetics of the BrCN + O2 reaction was monitored by following the emission intensity at 5.34 pm detected by an InSb detector through an interference filter (half-width = 0.4 pm). The ir emission at this wavelength is mainly due to NO, but some contribution from BrCN is observed (see Fig. 1). So that the experimental profile of the ir emission can be compared with the calculated one, it was necessary to obtain the emission intensities per unit concentration of NO and BrCN. These were determined by shock heating 2% NO-98% Ar and 2% BrCN-98% Ar mixtures, respectively.

EXPERIMENTAL Cyanogen bromide from Yoneyama Kogyo Corp., having a purity of 98 % , was degassed at dry ice-acetone temperature and purified by trap to trap distillations, with the middle fraction being retained for mixture preparation. High-purity oxygen from Takachiho Chemistry Corp. and argon from Nihon Sanso Corp. were used without further purification.

RESULTS AND DISCUSSION Figure 1 shows typical emission profiles from NO at 5.34 pm (A is for a relatively lower temperature and B is a higher temperature). Slight emission observed just behind the shock front is due to BrCN. In order to elucidate the reaction mechanism of the BrCN + O2 reaction,

tlme profiles of the Fig. 1. Typical experimental 02-94% Ar mixture, ps = 1.046 x 10 -5 mol 97% Ar mixture, ps = 2.129 x lo-’ mol maximum rate of increase of the IR emission

ir emission at 5.34 hrn: (A) 2% BrCN-4% cm - 3, r5 = 1796K; (B) 1% BrCN-2% 02~rn-~, T5 = 1907K. (dl/dt),,, indicates intensity.

BrCN OXIDATION AND NO FORMATION

169 TABLE

Elementary

Reactions

I

and Their Rate Constants

A&s ’ Reaction

(1) (2) (3) (4) (5) (6) (7) (8) (9)

(kJ/mol)

BrCN + Ar = Br + CN + Ar Br + BrCN = Brz + CN BrCN + 0 = NC0 + Br CN + 02 = NC0 + 0 NC0 + 0 = CO + NO NC0 + Ar = N + CO + Ar NO + N = Nz + 0 O2 + N = NO + 0 BrZ + Ar = 2Br + Ar

r?Units:

cm3 mol -I s-r

for rate; kJ/mol

Rate Constant”

366.8 167.6 123.5 14.0 469.4 162.0 313.9 133.3 193.2

-

Ref.

I61

lOr5 z exp( - 320.5/RT) 10r5.” exp( - 157.4/RT) lO13.16 exp( -40.58/RT) lO’3.5’ exp( - 4.2/RT) 10,x.5

This work

[101 [Ill This work This work

103.s exp( - 159/RT) 10,X,9

[I21 I121

109.99T’.0 exp( -27.7IRT) 10”33T0.5 exp(-13l.O/RT)

I131

for energy.

B

k2 = 5.0~101~ cm3nW-1~-1

75.

I 50-

25

k6

25-

0

250

0

750

500

250

timeps

0

9x10L3

2.63~10'

b

3~10~~

8.77~10~

c

3~10~~

2.63~10'

d

3~101~

7.89x10'

e

1~10~~

2.63~10~

500

750 time/p

C

z z 50

25

0

9x1013

4.26~10~

b

3x1013

1.42x109

c

1x1013

4.73x108

25

0

time&s

Fig. 2. Profiles calculated by using (B) k5 or k6 was varied; (C) and (D) other reactions, rate constants listed (A)-(C) are the same as those of Fig. those of Fig. IA.

time&

various values of kr, kS, and kg: (A) k2 was varied; k, and k6 were varied with constant ks/k5 ratio. For in Table 1 were used. Experimental conditions of 1B. Experimental conditions of(D) are the same as

TETSUO HIGASHIHARA ET AL.

170 we compared the experimental profiles with the calculated ones. In preliminary simulations, 35 elementary reactions containing the 17 independent chemical species of BrCN, 02, C2N2, NO, CO, NZ, Br2, N20, NOz, COz, NCO, CN, OBr, 0, N, Br, and Ar were used. In these simulations we found that the reaction is mainly governed by the 9 elementary reactions listed in Table 1. Among these reactions, there were three reactions [(2), (5), and (6)] for which rate constants had not yet been measured. [For reactions (5) and (6), while we were revising this paper, Louge and Hanson [5] reported their rate constants, which will be compared with the rate constants determined in this study later.] In order to investigate how these three reactions influence the calculated profiles of the ir emission, we calculated profiles by using various values of kz, kS, and kg, and show them in Fig. 2 (A: kz was varied; B: k5 or k6 was varied; C and D: k5 and k6 were varied with constant k6/kS ratio). Figures 2A and 2B show that the maximum rate of increase of the ir emission intensity, (cZZ/dt),,, (see also Fig. l), is mainly governed by reaction (2) and is slightly influenced bj reactions (5) and (6). On the other hand, reactions (5) and (6) influence mainly the yield of NO. Moreover, it is seen from Fig. 2C that the yield of NO is governed by the k6/kS ratio and does not depend on the individual rates of reactions (5) and (6). Figures 2C and 2D show that when kS and k6 are varied but the kg/ kS ratio is unchanged, the calculated profile shifts in parallel, maintaining constant (cZZ/ dt),,, and yield of NO. This phenomenon is more obvious at lower temperatures. These facts imply that we can determine kz by comparing the calculated and the experimental (dZ/dt),,, . And when kz is determined, the k6/ks ratio can be determined by comparing the calculated and the experimental yields of NO. Finally, by changing k5 and kc (but keeping the k6/k5 ratio constant) and fitting the calculated profile to the experimental one, we can determine kS and k6 at temperatures where the influence of k5 and k6 on the profile is large. In order to estimate the uncertainties in kZ, kG/ks, kS, and k6 determined by the method

described above, we checked the sensitivity of the calculated profile to the rate constants other than kz, kS, and k6. The calculated profile was insensitive to the changes of kl (X 1.3, x 0.6), kq (X 4, x 0.25), and kg (X 2, x 0.5), but it was affected by the changes of k3 (x 2, x 0.5), k, (x1.5, x0.67), and kg (x1.5, x0.67). The effect of uncertainties in k3, k7, and kg on the determined rate constants are presented later. Figure 3 shows the Arrhenius plot of kz. The error bars on each point indicate the uncertainty in k2 (X 1.3, x 0.77), which results from the uncertainties in k7 and kg and from the uncertainty in fitting. The solid line in this figure is the result of least-squares analysis and is represented as k2=

1015.0'0.2

X

exp[-(157.4k8.7)

kJ/Z?T]

cm3 mol-’ s-l. In Fig. 3 k2 values estimated by Kayes and Levitt [6] and by Tabayashi et al. [7] are also shown by broken lines, both of which were estimated in their BrCN decomposition studies so as to explain their experimental results. The activation energy, 157.4 kJ/mol, is about 10 kJ smaller than the heat of reaction (AHz9so = 167.6 kJ/mol). A survey of the kinetic data for bimolecular exchange reactions has shown that the activation energy for atom-molecule reactions such as reaction (2) lie generally around the endothermicity. Therefore, the activation energy found for k2 seems to be reasonable. Figure 4 shows the yield of NO obtained experimentally. The yield of NO is dependent on the compositions of the reaction mixtures, as is seen in Fig. 4. The solid lines in this figure are the results of least-squares analysis. For each reaction mixture, we determined the most suitable ks/ks ratio so as to reproduce the result of this least-squares analysis at the temperatures of 1850, 1900, 1950, 2000, 2100, and 2200K. Figure 5 shows the Arrhenius plot of k6/kS thus determined. The error bars on each point indicate the uncertainty in k6/ks (x2.3, x 0.43), which results mainly from the uncer-

BrCN OXIDATION AND NO FORMATION

2200

2000

1800

T/K

Fig. 4. Yield of NO. Solid lines represent the results of least-squares analysis

4.8

BrCN-2% O2 mixtures. Accordingly, pose the value of k6/k5 as

6 .O

5.6

5.2 lO'K/T

Fig. 3. Arrhenius plot of kf: KL, estimated value by Kayes and Levitt; TKF, estimated value by Tabayashi, Kajimoto, and Fueno. Solid line represents the result of least-squares analysis.

&/k, =

[email protected]*".3

exp( - 159 kJ/R T),

which is the mean value for these mixtures. Louge and Hanson [5] reported recently the kd k5 ratios at three different temperatures. These values were also plotted in Fig. 5. The agreement between their value and ours is good. Finally, we determined k5 (and kc) by changing k5 and k6 with the constant ka/ks ratio and

tainties in kZ, k3, k,, and ks. For the mixtures 1% BrCN-1 % O2 and 2% BrCN-2% 02, we obtained the same ka/k5 ratio. But the ratio for the 1% BrCN-2% O2 mixture is somewhat larger than that for the 1% BrCN- 1% 02 and 2 %

2% BrCN-2% 02-962 Ar +'l%

t

+: Lowe

-3

and Hanson

BrCN-1% 02.98% Ar 1% BrCN-2% 02-97% Ar Lowe

and Honson

s -k Y B

-4

-5

L

4.0

4.5

5.0 104K/T

Fig. 5.

we pro-

Arrhenius plot of ks/ks.

5.5

172

TETSUO HIGASHIHARA ET AL.

‘:

1XlOlQ r

explain their findings in their study of C2Nz oxidation as

5x1013

Ug= [NCO][Ar]0.4101’

m ‘:

;; E

“E ? s

-

xexp(-167.4 2x1013

lxl!P

:

6.1

5.7

5.3

104K/T

Fig. 6.

Arrhenius plot of k~.

fitting the calculated profile to the experimental one (see Fig. 2D). Because the calculated profile is not so sensitive to kS and kg, the k5 and k6 values thus determined have some error ( x 1.3, x 0.75). Other uncertainties in k5 and k6 result from the uncertain knowledge of k3 and kc/k5 (x 2.5, x 0.4). Figure 6 shows the Arrhenius plot of k5 determined. Although this figure shows some scatter, it seems that k5 has no temperature dependence. Moreover, reaction (5) is highly exothermic (m2s8” = - 469.4 kJ/ mol) and should exhibit only a small activation energy, if any. We propose then the k5 value as kg=

1013.5t0.4

cm3

mol-’

s-I_

By combining this k5 value with the k6/k5 ratio, we obtain the k6 value as k6=

1013.8kO.4

xexp(-

159 kJ/RT) cm3 mol-’ s-l.

The activation energy reaction is close to the = 162 kJ/mol) [8]. reported the k5 and k6 k5=

1013.75

k6=

1016.8

(+O.Z,-0.26)

(159 kJ/mol) for this heat of reaction (All,,,” Louge and Hanson [5] values as

cm3

mol-l

s-1,

(+0.36,-0.4)T-0.5

xexp(199 WIRT) cm3 mol-’ s-l. These values agree with our values within the error limits. Moreover, Lifshitz and Frenklach [9] estimated the rate of reaction (6) so as to

WIRT) mol cmm3 s-l.

This value is also close to ours within our experimental range of conditions, although the rate expressions are different from each other. It is thus expected that the k5 and k6 values determined by following the procedure described above are not so different from the true values. Now, we have all the rate constants for the reactions that control the formation of NO from BrCN oxidation. They are listed in Table 1 with the heats of reaction at 298K. As the next step, in order to visualize the mechanism of NO formation from BrCN, we calculated the formation rate of each chemical species. Figure 7A shows the formation rate of NO, indicating that the main reactions in producing NO are reactions (5) and (8) and that about 65% of NO is produced by reaction (5). The NO thus produced is partly converted to N2 by reaction (7). Figure 7B shows the formation rate of NCO. It is seen from this figure that NC0 is mainly produced by reactions (3) and (4) and is converted to NO by reaction (5) and to N by reaction (6). The Natom thus produced is converted to NO in an early stage of the reaction by reaction (8) and then to N2 by reaction (7), as Fig. 7D shows. The CN radical, which is converted to NC0 by reaction (4), is mainly produced by reaction (2) (see Fig. 7C). Figure 7E shows the formation rate of the O-atom. Thus, the O-atom, which is produced by reactions (4), (7), and (8), reacts with BrCN by reaction (3) and with NC0 by reaction (5). The reactants BrCN and O2 are consumed by reactions (2) and (3) and by reactions (4) and (8), respectively, as Figs. 7G and 7H show. The rate of NO formation is largely affected by the value of k2, as Fig. 2A shows. In order to clarify why the rate of NO formation is very sensitive to k2, we calculated the formation rate of each chemical species by reducing k2 to half and showed this in Figs. 7A-7C by broken lines.

BrCN OXIDATION AND NO FORMATION

r

173

D

6

C

‘\\ .. \

‘0 /

, ,’

0

2

--4

--6

-6

-_I

0

5

250

0

250

5(JO

250

5013

0

t ime/ps

t ime/rs

t ime/ps

250

500 time/p

---7

H

1

1

/

Ii

0

I 250

50 time/p5

0

250

250

50(

time/p

Fig. 7. Calculated formation rates Experimental conditions are the same using the rate constants listed in Table half. The numbers in the parentheses

ttme/ps

of NO, NCO, as those of Fig. 1; broken lines, are the reaction

500

250

time/w

CN, N, 0, Br, BrCN, and 02. IB. Solid lines, rates calculated by rates calculated by reducing k2 to numbers indicated in Table 1.

I-

5013

TETSUO HIGASHIHARA ET AL.

174

-6 250

0

500

250

750

500 timeps

time/p

Fig. 8. Influence of k6 on the formation rates of NO (A) and NC0 (B). Experimental conditions are the same as those of Fig. 1B. Solid lines, rates calculated by using the rate constants listed in Table 1; broken lines, rates calculated by reducing k6 to l/3. Numbers in the parentheses are the reaction numbers indicated in Table 1.

Thus, the rates of all the reactions except for reaction (1) were retarded, when k2 was reduced to half. This fact implies that reaction (2) is one of the rate determining steps. The yield of NO is sensitive to the values of k5 and kg, as Fig. 2B shows. This phenomenon is easily recognized by looking at Fig. 8. That is, reactions (5) and (6) are competition reactions, and if k6 is reduced, the relative rate of reaction (6) to reaction (5) is retarded, as Fig. 8B shows. As a result, the rate of reaction (7) is retarded and the yield of NO increases (Fig. 8A). And as long as the k6/k5 ratio is unchanged, the yield of NO remains unchanged, as Fig. 2C shows. As Fig. 2D shows, below 18OOK, when both of k5 and k6 were varied but the k6/kS ratio was unchanged, the calculated profile shifts in parallel by a larger amount. Figure 9 shows the formation rates of NO and NC0 for the conditions b (solid lines) and c (broken lines) of Fig. 2D. Figure 9B shows that when we reduced both of k5 and k6 to l/3, both the disappearance rates of NC0 through reactions (5) and (6) were delayed about 100 ps. Consequently, as Fig. 9A

3.

B

0250 ttme/ps

Fig. 9. Influence of the variation of k5 and k6 with constant k6/kI, ratio on the formation rates of NO (A) and NC0 (B). Experimental conditions are the same as those of Fig. 1A. Solid lines, rates calculated by using the same rate constants as those of b in Fig. 2D; broken lines, rates calculated by using the same rate constants as those of c in Fig. 2D. Numbers in the parentheses are the reaction numbers indicated in Table 1.

shows, both of the rates of NO formation through reactions (5) and (8) were delayed about 100 ps, but the maximum rate of NO formation and the yield of NO were unchanged. We conclude from the above discussion that reaction (5)) NCO+O=NO+CO,

(9

BrCN OXIDATION AND NO FORMATION is a major reaction in production of prompt and fuel NO and should not be excluded from the formation mechanism for NO. The rate constant determined for this reaction, however, has some error and further studies using other diagnostics will be necessary to determine k5 more pre-

175 6. 7. 8. 9.

J. Chem.

Puechberty,

11. D.,

and Cottereau,

M. J.,

Combust.

Tabayashi,

K.,

Kajimoto,

O.,

and

Fueno,

Crowhurst,

D.,

and

Simmons,

R.

Combust.

F.,

Higasbihara, T., Saito, K., and Murakami, I., J. Phys. Chem. 87:3707 (1983). Saito, K., Yokubo, Y., Fuse, T., Tahara, H., Kondo, O., Higashihara, T., and Murakami, I., Bull, Chem.

Sullivan,

B. J.,

Smith,

G. P., and Crosley,

Lifshitz,

A.,

and

Frenklach,

M.,

Int.

J.

D. R..

J. Chem.

Davies,

P. B., and Thrush,

B. A.,

Trans. Faraday

Albers, E. A., Hoyermann, K., Schacke, H., Schmatjiko, K. J., Wagner, H. Gg., and Wolfrum, J.,

12. 13.

765. Kuratani, K.. Bull. Inst. Space Aeronaut. Sci. 1 I:755 (1975). Warshay, M., J. Chem. Phys. 54:4060 (1971).

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T.,

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and Levitt,

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10.

2.

P. J.,

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cisely.

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Flame Received 7 February 1984; revised 6 March 1985