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Direct determination of the rate constant for the reaction CO + N2O → CO2 + N2
DIRECT DETERMINATION CO t N,O + CO, + N, Nobuyuki
FUJII,
20 December 1985
CHEMICAL PHYSICS LETTERS
Volume 122,:number 5
Tomohisa
Deporrmenr of Chemisry,
OF THE RATE CONSTANT
KAKUDA.
Tire Technological
Takeo
Urricenicv
Received 4 July 1985; in final form 24 Sepkmber
SUGIYAMA
of Ivagooka.
FOR THE REACTION
and Hajime
hlmiromioka,
MIYAMA
h’agooka. h’iigora 919-M
f apun
1985
The rake cons~~m for lhe bimolecular rexlion CO + N20 -+ CO1 +N2 vzas determined by comparison of calculaled infrared in the lempersture range 1350-2100 K for emission profiles of CO? \rirh Ihose observed in shock-Iubc experiments CO-N?O-He-Ar mixrures. The rate consran~ was found IO be X-, = 3.2 X lO”sxp( - 85 kJ/RT) cm3 mol-’ ‘.
s-
1. Introduction Recently,
the oxidation of CO with N,O at high has received much attention as a possible system for CO, gas-dynamic lasers [ 11. Shock-heated CO-+ N20 diluted with inert gas was forced to expand through a nozzle, where the small signal gain obtained was higher than a conventiynal CO2 + N2 system [24]. In order to confirm whether this high gain is due to the reaction between CO and N20, it is necessary to clarify the reaction in the shock wave and the expansion flow. As a first step, the reaction kinetics of the CO + N20 system at high temperature is examined in this study. It has generally been accepted that the reaction of CO with N,O proceeds via two channels; (i) highly exothermic bimolecular reaction CO + N20 + CO2 + N2, and (ii) the set of reactions initiated by the dissociation of N20 [5]. Especi$ly, the former reaction plays an important role below about 2000 K, and precise measurement of the rate con!stant for the reaction is essential. High-temp.erature measurements! of the rate constant for this reaction have been perfoded by several researchers [S-S] : Lin et al. [6] and Milks et al. [7] measured the rate constant by the &ngle-pulse shocktq,be _rnethod.
0 OO9-2614/85/s 0330 0 Elsevier Science Publishers B-V. (North-Holland Physics Publishing Division)
ment of the critical ignition pressure_ However, there are large discrepancies among those reported rate constants. In the present study, the rate constant was determined directly by measuring infrared emission of CO, in the high-temperature reaction of CO with N20 in the reflected shock waves.
2. Experimental The experiments were conducted in a 78 mm stainless-steel shock tube, consisting of a test section 3.5 m long and a high-pressure se&ion I .5 m long. The shock tube had a leak rate less than 2 X 10e3 Torr min-1 and was evacuated to less than 1 X 10m3 Torr before each experiment_ The incident shock-wave velocity was measured by piezo-electric gauges and a time counter, and the shock parameters behind the reflected shock wave were calculated from the incident shock velocity. The observation window was set at 35 mm from the end plate of the shock tube. The time history of CO or CO2 was monitored by measuring infrared emission isolated with an interference filter (4.87 m with fwhm = 0.24 m for CO arid 4.25 pm with fwhm = 0.16 m for CO,). The emission signal obtained from a liquid-nitrogen-cooled InSb detector and the pressure change from the piezoelectric gauge were displayed on a.digital storage oscilloscope. 489
Volume 122. number 5
CHEhiICAL
Experiments were carried out on mixtures of highpurity CO and N,O diluted with high-purity He and Ar (CO/N20/Ar/He = 2-6/l-6/46-38/50), which were heated behind the reflected shock waves in the temperature range 1350-2100 K and about 2.5 atm of total pressure. Here, He was added to reduce the vibrational relaxation time of CO.
3 _Results and discussion The vibrational relaxation of CO in CO-Ar and systems is known to be very slow [9]_ Then, as a preliminary experiment, the relaxation time was observed by measuring the CO infrared emission at 4.87 pm on mixtures of CO/Ar = l/99 and CO/Ar/He = l/49/50. It was found that the relaxation time of CO in the former mixture was 300 ps at To = 184 ‘T, PO = 2.4 atm, and in the latter mixture it was shortened to 20 ps at To = 1885 K, PO = 2-4 atm. Accordingly, He was added to Ar as a diluent (He: 50%) in the following kinetic experiments in order to reduce the effect of the vibrational relaxation time of CO on the kinetics. A typical profile of 4.25 pm emission on a mixture of CO-N20-Ar-He behind the reflected shock wave CO-CO
FiE.1 Typical oscihgam of 4.25 pm infrared emission (up’@cr trace) and the pressure (lower trace). CO/N20/Ar/He=
4/4/42/S&
490
TO = 1600 K. PO = 2.60 atm.
20 December 1985’
PHYSICS LEITERS
I
I
EXD.
L:
L/c:: 100 1 015)
--
200
Fig. 2. An example of comparison of 4.25 pm emission profde with calculated ones. Exp.: experimental emission intensity. Other lines are calculated values, Marks (1) and (6) correspond to the emission from CO2 produced by reactions (1) and (6) respectively, NsO: emission due to N20, and total: sum of the emission intensities from CO2 and N20. CO/M20/Ar/He = 6/6/X3150, TO = 1430 K, PO = 2.60 atm.
is shown in fig. 1. The initial rise of the emission is due to shock-heated N20, and the subsequent linear rise is due to CO, production. The emission profiles were compared with those obtained by a simulation, in which the concentrations of N20 and CO, calculated from the reaction scheme were converted to the emission intensity by using the calibration curves obtained from the shock-heated N20- Ar and COz- Ar experiments. An example of the comparison is shown in fig. 2. The scheme of the high-temperature reaction of CO with N,O used in the simulation is shown in table L Here; two channels are responsible for the formation of CO,: (i) the highly exothermic bimolecular reaction of CO with N,O (reaction (1)); and (ii) the set of elementary reactions initiated by the dissociation of N20 (reactions (2)-(7)) [S]. In this calculation, differential equations for reactions (l)-(7) and their reverse reactions were solved numerically, coupled with a heat balance equation and gas-flow equations for the reflected shock waie [ 121. Since the hightemperature decomposition of N20 has been studied by many researchers and its reaction scheme is well established [I 1,131, the rate of C02.formation can be calculated precisely.
Vblume 122, number 5
20 December 1985
CHEh%tCALPHYSICS LETTERS
Table 1 Reaction scheme and rate parameters a) Reaction
1ogA
B
(1) N20 + CO = N2 +‘COz (2)N20+M=Nz+O+M
11.50 15.21
0 0
(3)NzO+O=Na+02
13.84 14.00 13.30
0 0 0
(4)NzO+O=NO+N0 (5)N02+O=NO+02 (6)CO+O+M=C02+hl
15.64 14.95
(7)NO+O+M=NOz+M a) k
= AT8
exp (-&/RT)
(in units of cm3 mol-’
kJ/RT)
cm3 mol-l
s-1;
cm6 moP2
s-l
and(2)CO+O+M+C02+M(M=Ar+He):
k6 = 4.4 X 1015 exp(-18 (1700-2100
kJ/RT)
this work [lOI
85.0 257.7 111.3 117.2 4.6
1111
1111 171 this work [71
18.0 -7.5
s-l).
Below about 1700 K, CO2 formation by reaction (6) was confirmed to be negligible under the present conditions because of the slow decomposition of N20, as shown in fig. 2. Accordingly, the rate constant Icl of reaction (1) can be determined directly by a comparison of the profties of CO, formation from the experiments with those obtained by the calculations, as shown in fig. 2. Above 1700 K, CO2 formation via the second cannel becomes important, Accordingly, the rate constant kg was determined by comparison of the 4.25 m emission profiles. Here, the extrapolated value of kl determined below 1700 K was used in the calculation_ The values of k, and k6 were further corrected by the comparison over the wide temperature range. Thus, the following rate constants were determined: (1) CO + N20 + CO2 + N,:
klc3.2X 10” exp(-85
0 0
Ref.
EO;I)
K).
An Arrhenius plot of the rate constants k, thus obtained is shown in_fig. 3. The figure also shows the values reported by previous researchers [S-S]. The values obtained by the single-pulse shock-wave technique [6,7] generally have some uncertainty. For other data [5,8] obtained mainly on CO-rich mixtures (CO > 90%) the effect of the vibrational relaxation time on the kinetics must be taken into account. Therefore, the values of kl obtained by the present experiment, in which the effect of the relaxation time of
4!
.!
:
1
1
:
Fig. 3. Arrhenius plots of the rate constants for CO + NaO + COa + Na. (1) This work, (2) Milks et al. [7], (3) Lin et al. [6]. (4) Zaslonko et al. [a]. (5) Loirat et al. [S].
CO was reduced by the addition of He, seem to be more reliable. Also, the values of k6 obtained by the present experiment agree well with those recommended by Baulch et al. [14]; a detailed discussion on the k6 will be reported elsewhere. References
[ 11 N.N. Kudryavtsev, S.I. Kryuchkov, SS.
NOV~!XOV,
V-N. Shcheglov and RI. Soloukhin, 5th Gas Flow and Chemical Lasers Symposium (Hi&r, Bristol, 1984) p. 425. [2] N.N. Kudryavtsev, S.S. Novikov and LB. Svetlichnyi, Soviet Phys. DokL 19 (1975) 831. [3] N-N. Kudryavtsev, S.S. Novikov and LB. SveUichnyi, Soviet Phys. DokL 21 (1976) 748. 141 V-V. Kovtun. N-N. Kudryavtesev. S.S. Novikov, LB. Svetlichnyi and P.N. Shagov, Soviet Phys. DokI. 23 (1978) 503.
491
Volume 122, number
5
CHEMICAL
(51 H. Loirat. F. Caralp and M. Destrian, J. Phys. Chem. 87 (1983) 2455. [6] M.C. Lin and S.H. Baucr, J. Chem. Phys. 50 (1969) 3377. [7] D. Milks and R.A. Matula, 14th Symposium (International) On Combustion (Combustion Institute, Pittsburgh. 1973) p. 83. [8] I..% Zaslonko. A.S. Losev, E.V. Mozzhukhin and Yu.K. Mukoseev, Kinet. Katal. 20 (1979) 1385 (EngJ. Transl. 80 (1979) 1144. 191 R.C. Miuikan and D-R. White, J. Chem. Phys. 39 (1963) 3209.
492
PHYSICS [lo]
LETTERS
--i0 December
1985
N. Fujii. H. Sate, S. Fujimoto and H. Miyama. Bull; Chem. Sot. Japan 57 (1984),277. .. [ll] RX. Hanson and S. Sahian, in: Combustion chemistry, ed. WC Gardiner Jr. (Springer, B~lin, 1984) p. 361. [12] W.C. Gardener Jr.. in: Shqck wave in chemiftry. ed. A. Lifshitz (Dekker, New York, 1981) p_ 319. [ 131 T. Just, in: Shock waves in chemistry, ed. A. Lifshitz (Dekker, New York, 1981) pi279. [14] D.L. Baulch, D.D. Drysdale, J. Duxbury and S. Grant, in: Evaluated kinetic data for high temperature reactions. Vol. 3 (Buttenvorths. London, 1976) p. 275.
FUJII,
20 December 1985
CHEMICAL PHYSICS LETTERS
Volume 122,:number 5
Tomohisa
Deporrmenr of Chemisry,
OF THE RATE CONSTANT
KAKUDA.
Tire Technological
Takeo
Urricenicv
Received 4 July 1985; in final form 24 Sepkmber
SUGIYAMA
of Ivagooka.
FOR THE REACTION
and Hajime
hlmiromioka,
MIYAMA
h’agooka. h’iigora 919-M
f apun
1985
The rake cons~~m for lhe bimolecular rexlion CO + N20 -+ CO1 +N2 vzas determined by comparison of calculaled infrared in the lempersture range 1350-2100 K for emission profiles of CO? \rirh Ihose observed in shock-Iubc experiments CO-N?O-He-Ar mixrures. The rate consran~ was found IO be X-, = 3.2 X lO”sxp( - 85 kJ/RT) cm3 mol-’ ‘.
s-
1. Introduction Recently,
the oxidation of CO with N,O at high has received much attention as a possible system for CO, gas-dynamic lasers [ 11. Shock-heated CO-+ N20 diluted with inert gas was forced to expand through a nozzle, where the small signal gain obtained was higher than a conventiynal CO2 + N2 system [24]. In order to confirm whether this high gain is due to the reaction between CO and N20, it is necessary to clarify the reaction in the shock wave and the expansion flow. As a first step, the reaction kinetics of the CO + N20 system at high temperature is examined in this study. It has generally been accepted that the reaction of CO with N,O proceeds via two channels; (i) highly exothermic bimolecular reaction CO + N20 + CO2 + N2, and (ii) the set of reactions initiated by the dissociation of N20 [5]. Especi$ly, the former reaction plays an important role below about 2000 K, and precise measurement of the rate con!stant for the reaction is essential. High-temp.erature measurements! of the rate constant for this reaction have been perfoded by several researchers [S-S] : Lin et al. [6] and Milks et al. [7] measured the rate constant by the &ngle-pulse shocktq,be _rnethod.
0 OO9-2614/85/s 0330 0 Elsevier Science Publishers B-V. (North-Holland Physics Publishing Division)
ment of the critical ignition pressure_ However, there are large discrepancies among those reported rate constants. In the present study, the rate constant was determined directly by measuring infrared emission of CO, in the high-temperature reaction of CO with N20 in the reflected shock waves.
2. Experimental The experiments were conducted in a 78 mm stainless-steel shock tube, consisting of a test section 3.5 m long and a high-pressure se&ion I .5 m long. The shock tube had a leak rate less than 2 X 10e3 Torr min-1 and was evacuated to less than 1 X 10m3 Torr before each experiment_ The incident shock-wave velocity was measured by piezo-electric gauges and a time counter, and the shock parameters behind the reflected shock wave were calculated from the incident shock velocity. The observation window was set at 35 mm from the end plate of the shock tube. The time history of CO or CO2 was monitored by measuring infrared emission isolated with an interference filter (4.87 m with fwhm = 0.24 m for CO arid 4.25 pm with fwhm = 0.16 m for CO,). The emission signal obtained from a liquid-nitrogen-cooled InSb detector and the pressure change from the piezoelectric gauge were displayed on a.digital storage oscilloscope. 489
Volume 122. number 5
CHEhiICAL
Experiments were carried out on mixtures of highpurity CO and N,O diluted with high-purity He and Ar (CO/N20/Ar/He = 2-6/l-6/46-38/50), which were heated behind the reflected shock waves in the temperature range 1350-2100 K and about 2.5 atm of total pressure. Here, He was added to reduce the vibrational relaxation time of CO.
3 _Results and discussion The vibrational relaxation of CO in CO-Ar and systems is known to be very slow [9]_ Then, as a preliminary experiment, the relaxation time was observed by measuring the CO infrared emission at 4.87 pm on mixtures of CO/Ar = l/99 and CO/Ar/He = l/49/50. It was found that the relaxation time of CO in the former mixture was 300 ps at To = 184 ‘T, PO = 2.4 atm, and in the latter mixture it was shortened to 20 ps at To = 1885 K, PO = 2-4 atm. Accordingly, He was added to Ar as a diluent (He: 50%) in the following kinetic experiments in order to reduce the effect of the vibrational relaxation time of CO on the kinetics. A typical profile of 4.25 pm emission on a mixture of CO-N20-Ar-He behind the reflected shock wave CO-CO
FiE.1 Typical oscihgam of 4.25 pm infrared emission (up’@cr trace) and the pressure (lower trace). CO/N20/Ar/He=
4/4/42/S&
490
TO = 1600 K. PO = 2.60 atm.
20 December 1985’
PHYSICS LEITERS
I
I
EXD.
L:
L/c:: 100 1 015)
--
200
Fig. 2. An example of comparison of 4.25 pm emission profde with calculated ones. Exp.: experimental emission intensity. Other lines are calculated values, Marks (1) and (6) correspond to the emission from CO2 produced by reactions (1) and (6) respectively, NsO: emission due to N20, and total: sum of the emission intensities from CO2 and N20. CO/M20/Ar/He = 6/6/X3150, TO = 1430 K, PO = 2.60 atm.
is shown in fig. 1. The initial rise of the emission is due to shock-heated N20, and the subsequent linear rise is due to CO, production. The emission profiles were compared with those obtained by a simulation, in which the concentrations of N20 and CO, calculated from the reaction scheme were converted to the emission intensity by using the calibration curves obtained from the shock-heated N20- Ar and COz- Ar experiments. An example of the comparison is shown in fig. 2. The scheme of the high-temperature reaction of CO with N,O used in the simulation is shown in table L Here; two channels are responsible for the formation of CO,: (i) the highly exothermic bimolecular reaction of CO with N,O (reaction (1)); and (ii) the set of elementary reactions initiated by the dissociation of N20 (reactions (2)-(7)) [S]. In this calculation, differential equations for reactions (l)-(7) and their reverse reactions were solved numerically, coupled with a heat balance equation and gas-flow equations for the reflected shock waie [ 121. Since the hightemperature decomposition of N20 has been studied by many researchers and its reaction scheme is well established [I 1,131, the rate of C02.formation can be calculated precisely.
Vblume 122, number 5
20 December 1985
CHEh%tCALPHYSICS LETTERS
Table 1 Reaction scheme and rate parameters a) Reaction
1ogA
B
(1) N20 + CO = N2 +‘COz (2)N20+M=Nz+O+M
11.50 15.21
0 0
(3)NzO+O=Na+02
13.84 14.00 13.30
0 0 0
(4)NzO+O=NO+N0 (5)N02+O=NO+02 (6)CO+O+M=C02+hl
15.64 14.95
(7)NO+O+M=NOz+M a) k
= AT8
exp (-&/RT)
(in units of cm3 mol-’
kJ/RT)
cm3 mol-l
s-1;
cm6 moP2
s-l
and(2)CO+O+M+C02+M(M=Ar+He):
k6 = 4.4 X 1015 exp(-18 (1700-2100
kJ/RT)
this work [lOI
85.0 257.7 111.3 117.2 4.6
1111
1111 171 this work [71
18.0 -7.5
s-l).
Below about 1700 K, CO2 formation by reaction (6) was confirmed to be negligible under the present conditions because of the slow decomposition of N20, as shown in fig. 2. Accordingly, the rate constant Icl of reaction (1) can be determined directly by a comparison of the profties of CO, formation from the experiments with those obtained by the calculations, as shown in fig. 2. Above 1700 K, CO2 formation via the second cannel becomes important, Accordingly, the rate constant kg was determined by comparison of the 4.25 m emission profiles. Here, the extrapolated value of kl determined below 1700 K was used in the calculation_ The values of k, and k6 were further corrected by the comparison over the wide temperature range. Thus, the following rate constants were determined: (1) CO + N20 + CO2 + N,:
klc3.2X 10” exp(-85
0 0
Ref.
EO;I)
K).
An Arrhenius plot of the rate constants k, thus obtained is shown in_fig. 3. The figure also shows the values reported by previous researchers [S-S]. The values obtained by the single-pulse shock-wave technique [6,7] generally have some uncertainty. For other data [5,8] obtained mainly on CO-rich mixtures (CO > 90%) the effect of the vibrational relaxation time on the kinetics must be taken into account. Therefore, the values of kl obtained by the present experiment, in which the effect of the relaxation time of
4!
.!
:
1
1
:
Fig. 3. Arrhenius plots of the rate constants for CO + NaO + COa + Na. (1) This work, (2) Milks et al. [7], (3) Lin et al. [6]. (4) Zaslonko et al. [a]. (5) Loirat et al. [S].
CO was reduced by the addition of He, seem to be more reliable. Also, the values of k6 obtained by the present experiment agree well with those recommended by Baulch et al. [14]; a detailed discussion on the k6 will be reported elsewhere. References
[ 11 N.N. Kudryavtsev, S.I. Kryuchkov, SS.
NOV~!XOV,
V-N. Shcheglov and RI. Soloukhin, 5th Gas Flow and Chemical Lasers Symposium (Hi&r, Bristol, 1984) p. 425. [2] N.N. Kudryavtsev, S.S. Novikov and LB. Svetlichnyi, Soviet Phys. DokL 19 (1975) 831. [3] N-N. Kudryavtsev, S.S. Novikov and LB. SveUichnyi, Soviet Phys. DokL 21 (1976) 748. 141 V-V. Kovtun. N-N. Kudryavtesev. S.S. Novikov, LB. Svetlichnyi and P.N. Shagov, Soviet Phys. DokI. 23 (1978) 503.
491
Volume 122, number
5
CHEMICAL
(51 H. Loirat. F. Caralp and M. Destrian, J. Phys. Chem. 87 (1983) 2455. [6] M.C. Lin and S.H. Baucr, J. Chem. Phys. 50 (1969) 3377. [7] D. Milks and R.A. Matula, 14th Symposium (International) On Combustion (Combustion Institute, Pittsburgh. 1973) p. 83. [8] I..% Zaslonko. A.S. Losev, E.V. Mozzhukhin and Yu.K. Mukoseev, Kinet. Katal. 20 (1979) 1385 (EngJ. Transl. 80 (1979) 1144. 191 R.C. Miuikan and D-R. White, J. Chem. Phys. 39 (1963) 3209.
492
PHYSICS [lo]
LETTERS
--i0 December
1985
N. Fujii. H. Sate, S. Fujimoto and H. Miyama. Bull; Chem. Sot. Japan 57 (1984),277. .. [ll] RX. Hanson and S. Sahian, in: Combustion chemistry, ed. WC Gardiner Jr. (Springer, B~lin, 1984) p. 361. [12] W.C. Gardener Jr.. in: Shqck wave in chemiftry. ed. A. Lifshitz (Dekker, New York, 1981) p_ 319. [ 131 T. Just, in: Shock waves in chemistry, ed. A. Lifshitz (Dekker, New York, 1981) pi279. [14] D.L. Baulch, D.D. Drysdale, J. Duxbury and S. Grant, in: Evaluated kinetic data for high temperature reactions. Vol. 3 (Buttenvorths. London, 1976) p. 275.