Kinetics and mechanism for the oxidation of CS2 and COS at high temperature

Article abstract of DOI:10.1246/bcsj.74.1233

The rate constant for the thermal decomposition of CS₂ and COS were investigated by measuring the time profiles of S atom using atomic resonance absorption spectroscopy behind the incident shock of CS₂/Ar or COS/Ar mixtures, and the rate constants were determined to be k = (4.0 ± 0.1) × 10¹⁴ exp (-295 ± 20 kJ/RT) cm³ mol^-1 s^-1, T: 2200-2900 K and k = (1.4 ± 0.3) × 10¹⁵ exp (-290 ± 25 kJ/RT) cm³ mol^-1 s^-1, T: 2000-3200 K, respectively. The oxidation mechanism of COS was also investigated behind the incident shock of COS/O₂/Ar mixtures and the rate constant for the reaction S + O₂ → SO + O was obtained to be k = (9.5 ± 0.7) × 10¹³ exp (-41 ± 28 kJ/RT) cm³ mol^-1 s^-1, T: 2200-2900 K. From a numerical analysis for time profiles of S atom behind the incident shock of CS₂/O₂/Ar mixtures, the rate constant for the reaction O + CS → CO + S was expressed as k = (3.2 ± 1.0) × 10¹³ cm³ mol^-1 s^-1, T: 2100-3000 K. Similarly, the rate constant for the reaction CS + O₂ → COS + O was also determined to be k = (6.1 ± 0.6) × 10¹² exp (-51.0 ± 68 kJ/RT) cm³ mol^-1 s^-1, T: 2000-2900 K by measuring the time profiles of O atom behind the reflected shock of CS₂/O₂/Ar mixtures. Ab initio calculations were carried out and the reaction pathways of CS + O₂ → COS + O and CS + O₂ → CO + SO were also investigated.

Full text of DOI:10.1246/bcsj.74.1233

c. Jpn.
© 2001 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 74, 1233–1240 (2001) 1233
e Chemical
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Kinetics and Mechanism for the Oxidation of CS2 and COS at High
Temperature
Oxidation of CS2 and COS
Y. Murakami et al.
Yoshinori Murakami,* Masami Kosugi, Kenji Susa, Takaomi Kobayashi, and Nobuyuki Fujii
01
33
40
SJA8
09-2673
412
Department of Chemistry, Nagaoka University of Technology, Kamitomioka, Nagaoka 940-2188
(Received December 15, 2000)
The rate constants for the thermal decompositions of CS2 and COS were investigated by measuring the time pro-
files of S atom using atomic resonance absorption spectroscopy behind the incident shock of CS2/Ar or COS/Ar mix-
tures, and the rate constants were determined to be k = (4.0 0.1) × 1014 exp (−295 20 kJ/RT) cm3 mol−1 s−1, T:
2200–2900 K and k = (1.4 0.3) × 1015 exp (−290 25 kJ/RT) cm3 mol−1 s−1, T: 2000–3200 K, respectively. The ox-
idation mechanism of COS was also investigated behind the incident shock of COS/O2/Ar mixtures and the rate constant
for the reaction S + O2 → SO + O was obtained to be k = (9.5 0.7) × 1013 exp (−41 28 kJ/RT) cm3 mol−1 s−1, T:
2200–2900 K. From a numerical analysis for time profiles of S atom behind the incident shock of CS2/O2/Ar mixtures,
the rate constant for the reaction O + CS → CO + S was expressed as k = (3.2 1.0) × 1013 cm3 mol−1 s−1, T: 2100–
3000 K. Similarly, the rate constant for the reaction CS + O2 → COS + O was also determined to be k = (6.1 0.6) ×
1012 exp (−51.0 68 kJ/RT) cm3 mol−1 s−1, T: 2000–2900 K by measuring the time profiles of O atom behind the re-
flected shock of CS2/O2/Ar mixtures. Ab initio calculations were carried out and the reaction pathways of CS + O2 →
COS + O and CS + O2 → CO + SO were also investigated.
00
01
.2
Carbon disulfide is known to be an important sulfur-contain-
ing species, and understanding the mechanism for the oxida-
tion of carbon disulfide is also important for understanding
such environmental problems as acid rain. Since the 1930’s,
several researchers have tried to determine the explosion limits
of CS2 oxidation as a function of temperature, and have dis-
cussed the explosion limits based on the kinetic model of CS2
oxidation.1,2 To explain the explosion limits of CS2/O2 mix-
tures, Wood and Heicklen3 proposed the importance of the fol-
lowing branching chain reaction:
reaction. This reaction is known to produce highly vibrational-
ly excited CO molecules via
O + CS → CO + S ∆H = −351 kJ mol−1.
(4)
The vibrational distribution of CO produced by this reaction
was investigated by several researchers,8,9 the results of which
showed that CO was populated even up to high vibrational lev-
els (v = 15). Therefore, such a population inversion of CO
was applied for chemical-pumped CO lasers.10 On the other
hand, the rate constant for the reaction O + CS → CO + S
was first investigated by Slage et al.11 They determined the rate
constant for this reaction to be k = 1.24 × 1013 cm3 mol−1 s−1
using a flow-tube technique with photoionization mass spec-
trometry. Bida et al.12 also measured the rate constant by mon-
itoring the decay of CS by a multipath absorption technique at
257.6 nm. They concluded that the reaction O + CS → CO +
S is very rapid and that the obtained rate constant is in good
agreement with that obtained by Slage et al. The temperature
dependence of the reaction rate was measured by Lienfeld and
Richardson13 using a fast-flow reactor with an electron-spin
resonance mass spectrometer, but the temperature range was
below 300 K, which was far below the flame temperature.
Similar to the case of CS2 oxidation, the oxidation of COS is
known to play the important roles in the combustion of sulfur-
containing compounds. Above 2000 K, the initial process of
COS oxidation is considered in the following reaction scheme:
O + CS2 → CS + SO
∆H = −88 kJ mol−1,
(1)
and subsequent reactions of CS with O2 molecule:
CS + O2 → O + COS ∆H = −161 kJ mol−1,
CS + O2 → CO + SO ∆H = −377 kJ mol−1.
(2)
(3)
On the other hand, some researchers4,5 concluded that CS radi-
cal didn't play important roles in the oxidation mechanism of
CS2, and that CS radical was lost at the wall of the reactor dur-
ing the oxidation of CS2 because of the low reactivity of CS
radical with O2 molecule. A recent kinetic study of CS + O2
reaction suggested that the overall rate constant of this reaction
is very slow (i.e. k = 1.7 × 105 cm3 mol−1 s−1) at room tem-
perature.6 Richardson7 measured the rate constant for this re-
action under the temperature range between 290–500 K using
a discharge flow tube with a mass spectrometer. However, the
rate constant for this reaction has not been measured at temper-
atures higher than 500 K.
COS + Ar → CO + S + Ar ∆H = + 305 kJ mol−1, (5)
S + O2 → SO + O
∆H = −23 kJ mol−1.
(6)
The other possible pathway of CS removal is the O + CS

1234 Bull. Chem. Soc. Jpn., 74, No. 7 (2001)
Oxidation of CS2 and COS
Pioneering work on determining the rate constant for the ther-
mal decomposition of COS was carried out by Schecker and
Wagner.14 Recently, Woiki and Roth15 measured this rate con-
stant; their result was found to be in good agreement with each
other. Oya et al.16 also measured the rate constant, and it was
also confirmed that the rate constant for reaction (5) was simi-
lar to the results obtained by previous researchers. On the oth-
er hand, measurements of the rate constant for reaction (6) at
high temperature have been limited until recently. Saito et al.17
measured the rate constant by monitoring the O-atom produc-
tion in COS/O2/Ar mixtures for the first time at a temperature
range higher than 1000 K. Later, Miyoshi et al.18 and Woiki
and Roth19 re-investigated the rate constant for reaction (6) by
monitoring S atoms behind the reflected shock waves of COS/
O2/Ar mixtures or after 248-nm photolysis of COS/O2/Ar mix-
tures behind the reflected shock waves.
Fig. 1. Schematic figure of experimental setup.
For kinetic measurements of combustion phenomena, using
a shock-tube is convenient method to measure the rate con-
stants for elementary reactions relevant to the combustion pro-
cesses because of the uniform temperature profile and no dis-
turbance from heterogeneous reactions at the wall of the reac-
tor. Especially, atomic resonance absorption spectrometry
(ARAS), which monitors the time profiles of the absorption of
atomic species, like H and O during high-temperature reac-
tions, is successfully used to determine the rate constants of el-
ementary reactions behind the shock waves at temperatures
higher than 1000 K.20 Although the shock-tube apparatus was
already applied to study the oxidation mechanism of carbon
disulfide by Sheen et al,21 no kinetic investigations on the time
profiles of atomic species were carried out using the shock-
tube apparatus. Since S and O-atoms are important carriers of
chain reactions, the temporal behaviors of these species are of
particular importance for understanding the combustion of
CS2/O2 mixtures. In the present work, the time profiles of S
and O-atoms were monitored using ARAS techniques behind
the incident or reflected shock of CS2/O2/Ar mixtures. By ki-
netic simulations of these temporal profiles of atomic species,
the rate constants for the reactions O + CS → CO + S and CS
+ O2 → COS + O were mainly determined above 2000 K.
The oxidation mechanism of COS was also investigated by
monitoring S atoms behind the incident shock of COS/Ar and
COS/O2/Ar mixtures and the rate constants for the reactions
(5) and (6) were determined. All of these higher-temperature
results were compared with previous works; further, the fate of
CS2 and COS at the flame temperature was discussed.
Fig. 2. Relationship between O- and S-ARAS signal intensi-
ties and their concentrations.
lamp, a vacuum-UV monochromator (Ritsu MCV-20) and a solar-
blind photo-multiplier (Hamamatsu R-1080). The gases for the
light sources for the absorption measurements of O and S atoms
were 1% O2 and 0.1% H2S diluted in helium, respectively. Each
time profile of the atomic absorption signal obtained in the solar-
blind photo-multiplier was stored in an oscilloscope (Tektronix
2201) and sent to a personal computer for further analysis.
Each absorption signal was calibrated to obtain the relationship
between the signal intensities and the concentrations. In the case
of the calibration of O atoms at 130.5 nm, the thermal decomposi-
tion of N2O was used. By varying the concentration of N2O/Ar
mixtures, the relationship between the O-atom concentration and
1 − I/I0 was determined. The relationship between the O-atom
concentration and 1 − I/I0 is shown in Fig. 2. Here, I and I0 are
the transmitted and incident lights of a microwave discharge lamp,
respectively. Similarly, a calibration of S atom at λ = 182.6 nm
was performed using CS2/Ar and COS/Ar mixtures. Since all of
CS2 and COS thermally decompose to produce S atoms, the rela-
tionship between the signal intensities and S-atom concentrations
can be determined by calculating the initial concentrations of CS2
or COS behind the shock waves. The results are also shown in
Experimental
A schematic of the experimental apparatus is shown in Fig. 1.
The shock-tube apparatus is a conventional diaphragm shock-tube
of 46 mm internal diameter and 3.6 m long at the low-pressure
section. Shock was driven with high-pressure hydrogen or helium
gas, and before each run the driven section was pumped down to
pressures below 5 × 10−5 Torr by a diffusion pump. The tempera-
ture and pressure behind the shock waves were calculated from
the incident shock velocities. The incident shock velocities were
measured by four piezoelectric transducers, and the mean shock
velocity was used for a calculation.
The shock tube was equipped with atomic resonance absorption
spectroscopy diagnostics consisting of a microwave discharge

Y. Murakami et al.
Bull. Chem. Soc. Jpn., 74, No. 7 (2001) 1235
Table 1. Reaction Scheme and Rate Constantsa) in the CS2/
O2/Ar Mixtures
the rate constant for the reaction was determined to be k7 =
(4.0 0.1) × 1014 exp (−290 20 kJ/RT) cm3 mol−1 s−1 in
the temperature range of 2200 to 2900 K, and was in good
agreement with those obtained by Gaydon et al.24 and by
Olschwski et al.,25 but about three-times larger than the value
obtained by Saito et al.26
The rate constant for the thermal decomposition of COS
was also investigated by measuring the time profiles of S at-
oms behind the incident shock of COS/Ar mixtures. Figure 5
shows an example of the S-atom time profile. Based on a ki-
netic calculation it was confirmed that S atoms were solely
A
Ea
Reaction
Reference
cm3 mol−1 s−1
3.6 × 1013
6.1 × 1012
3.2 × 1013
1.4 × 1015
9.5 × 1013
4.0 × 1014
1.7 × 1012
7.1 × 1012
1.0 × 1014
4.5 × 1011
1.6 × 1013
4.0 × 1012
1.2 × 1014
1.0 × 1012
kJ
O + CS2 = CS + SO
OS + O2 = O + COS
O + CS = CO + S
COS + Ar = CO + S + Ar
S + O2 = SO + O
CS2 + Ar = CS + S + Ar
O + CS2 = CO + S2
O + CS2 = COS + S
S + CS2 = CS + S2
SO + O2 = SO2 + O
COS + O = CO + SO
S2 + O = SO + S
7.1
51
0.0
290
41
290
5.0
8.8
17
27
19
0
46
130
31
this work
this work
this work
this work
this work
11
11
30
31
31
32
32
33
COS + O = CO2 + S
CS2 + O2 = CS + SO2
a) Rate constants in the form, k = A exp (−Ea/RT) in cm,
mol, kJ and K units.
Fig. 2. As shown in Fig. 2, no pressure and temperature depen-
dences of these calibration curves were observed. In the present
work, all experiments were performed at 1 − I/I0 < 0.5 because a
higher 1 − I/I0 didn’t give a linear relationship between 1 − I/I0
and the atomic concentrations.
For simulating the temporal profiles of atomic species behind
the incident or reflected shock by numerical calculations, we used
the Chemkin22 and Chemkin thermodynamic database.23 The ki-
netic schemes used in the present work are shown in Table 1. The
program can also calculate the rate constant for the backward re-
action for each reaction given in Table 1 through the equilibrium
constant using the thermodynamic functions of the species in-
volved.
CS2 (99%; Nacalai tesque) and COS (97.5%; Matheson) used
in this work were purified by trap-to-trap distillation using dry ice
and liquid N2. Ar (99.9995%; Taiyo Sanso) and O2 (99.995%;
Taiyo Sanso) were used without further purification.
Fig. 3. Typical absorption profile at 182.6 nm for CS2/Ar
system.
Conditions: T = 2653 K, P = 0.7 atm, CS2 = 6 ppm.
Results and Discussion
1 Thermal Decomposition of CS2 and COS. Several re-
searchers have already investigated the rate constant for the
thermal decomposition of CS2. We also tried to determine the
rate constant for the reaction CS2 + Ar → CS + S + Ar by
measuring the time profiles of S atoms behind the incident
shock of CS2/Ar mixtures. Figure 3 is a typical example of S-
atom time profiles. Here, the time axis is converted to the real
gas time. As shown in the figure, S atoms produced rapidly be-
hind the incident shock, reaching an equilibrium value after
700 µs, where CS2 decomposed completely at temperatures
higher than about 2500 K. Since the initial concentration of
CS2 diluted in Ar was as low as few ppm, the subsequent reac-
tion, S + CS2 → CS + S2, had a negligible contribution on the
time profile of S atoms. A comparison of the kinetic calcula-
tion with the experimental profile is shown in Fig. 3, and the
calculated profiles using the k1 values doubled and halved are
also shown in the figure. From these calculated lines, the accu-
racy of the values is shown to be high. The resultant Arrhenius
form of the rate constants obtained for the reaction CS2 + Ar
→ CS + S + Ar is shown in Fig. 4. As shown in this figure,
Fig. 4. Arrhenius plot of measured second-order rate con-
stant for CS2 + Ar → CS + S + Ar.

1236 Bull. Chem. Soc. Jpn., 74, No. 7 (2001)
Oxidation of CS2 and COS
Fig. 5. Typical absorption profile at 182.6 nm for COS/Ar
system.
Conditions: T = 2337 K, P = 0.8 atm, COS = 4 ppm.
Fig. 7. Typical absorption profile at 182.6 nm for COS/O2/Ar
system.
Conditions: T = 2315 K, P = 0.9 atm, COS = 20 ppm, O2
= 200 ppm.
are shown in Fig. 7. As shown in Fig. 7, S-atom formation fol-
lowed by fast decay was observed. Because the decay of S at-
oms is only responsible for the reaction S + O2 → SO + O,
the rate constant for this reaction can be determined by simu-
lating the decay of the time profile of S atoms behind the inci-
dent shock of COS/O2/Ar mixtures using all of the reactions
tabulated in Table 1. A sensitivity analysis using the reaction
schemes shown in Table 1 was performed, and it was con-
firmed that only reactions (5) and (6) were responsible to the
time profile of S atom. Therefore, the other reactions had little
influence on the determination of the rate constant for reaction
(6). The rate constant obtained in this work was k6 = (9.5
0.7) × 1013 exp (−41 28 kJ/RT) cm3 mol−1 s−1 between T =
2200 and 2900 K, and the present rate expression compared to
previous ones is plotted in Fig. 8. As can be seen in the figure,
the rate constant obtained in this work is much higher than that
obtained by Saito et al.17 In addition, a slight curvature of the
Arrhenius forms of the rate constant was observed, combined
with previous works by Miyoshi et al.18 and Woiki and Roth.19
3 High-Temperature Oxidation of CS2. A typical ex-
ample of the time profiles of S atoms behind the incident shock
of CS2/O2/Ar mixtures is shown in Fig. 9. The time profile of
S atom in the figure showed a fast increase at an early reaction
time, followed by a slow decay compared with the case of
COS oxidation. To simulate the time profiles of S atom ob-
tained in this work, numerical simulations were performed us-
ing the reaction scheme in Table 1. The results are also shown
by line (a) in Fig. 9. It can be seen that the agreement between
the experiment and computation is not good. To find the rea-
son for such a disagreement, a sensitivity analysis of the time
profiles of S atom on the rate constants for reactions were car-
ried out under the same experimental conditions as in Fig. 9.
Here, the sensitivity is expressed as (d[S]/dk) (k/[S]), where (k/
Fig. 6. Arrhenius plot of measured second-order rate con-
stant for COS + Ar → CO + S + Ar.
produced by the thermal decomposition, COS + Ar → CO +
S + Ar, and that a subsequent reaction, such as S + COS →
CO + S2, had negligible contributions on the time profiles of S
atoms under our experimental conditions. The Arrhenius form
of the rate constant is shown in Fig. 6, and the rate constant
was determined to be k5 = (1.4 0.3) × 1015 exp (−290 25
kJ/RT) cm3 mol−1 s−1 in the temperature range of 2000 to 3200
K. The Arrhenius forms of the rate constant for this reaction
obtained by previous workers are also shown in Fig. 6. As
shown in Fig. 6, the present results are in good agreement with
previous works using the S-ARAS technique by Woiki et al.15
and Oya et al.16
2 High-Temperature Oxidation of COS. Time profiles
of S atoms behind the incident shock of COS/O2/Ar mixtures

Y. Murakami et al.
Bull. Chem. Soc. Jpn., 74, No. 7 (2001) 1237
Fig. 8. Arrhenius plot of measured rate constant for S + O2
→ SO + O.
Fig. 10. Sensitivity analysis of time profile of S atom in the
CS2/O2 reaction system.
Conditions: T = 2614 K, P = 0.8 atm, CS2 = 20 ppm, O2
= 200 ppm.
Fig. 9. Typical absorption profile at 182.6 nm for CS2/O2/Ar
system.
Conditions: T = 2614 K, P = 0.8 atm, CS2 = 20 ppm, O2
= 200 ppm.
Fig. 11. Arrhenius plot of measured rate constant for O +
CS → CO + S.
[S]) is used to normalize the (d[S]/dk) values. The results are
shown in Fig. 10. From a sensitivity analysis the following re-
actions:
ready obtained in this work, we can determine the rate constant
for reaction (4). Although the rate constant for reaction (4)
was determined previously by Slage et al.11 and by Bida et al.12
below 300 K, it is doubtful that the rate constant for reaction
(4) is correct, even under our high-temperature experimental
conditions. Therefore, the rate constant for reaction (4) was
re-investigated by simulating the time profiles of S atom ob-
tained behind the incident shock of CS2/O2/Ar mixtures. Line
(b) in Fig. 9 shows the simulated profile using the rate constant
for reaction (4) obtained by fitting work. The best fit to our ex-
perimental profiles gave the rate constant k4 = (3.2 1.0) ×
1013 cm3 mol−1 s−1 for reaction (4) in the temperature range of
T = 2100 to 3000 K. Figure 11 shows Arrhenius plots of the
CS2 + Ar → CS + S + Ar,
O + CS → CO + S,
S + O2 → SO + O,
(7)
(4)
(6)
were found to be sensitive to the slow-decay part of the S atom
profiles, and the other reactions were less sensitive to that of S
atom compared to the above three reactions.
Since the rate constants for reactions (7) and (6) have al-

1238 Bull. Chem. Soc. Jpn., 74, No. 7 (2001)
Oxidation of CS2 and COS
rate constant for the reaction O + CS → CO + S. Compared
to the results obtained by Lienfeld et al. below 300 K,13 our re-
sult was about one third of their rate constant. Also, the activa-
tion energy for the reaction was found to be small, which is the
same conclusion as that by Lienfeld et al.
The time profiles of the O atom in the CS2/O2/Ar reaction
system were also measured using the atomic resonance absorp-
tion spectrometry technique. Because of a modification of the
experimental apparatus, the measurement was carried out be-
hind the reflected shock of CS2/O2/Ar mixtures, and diaphrag-
mless shock-tube was used instead of the conventional dia-
phragm shock-tube. A typical example of the time profile of O
atom is shown in Fig. 12. Because of some absorption of CS2
at 130.5 nm, a certain amount of absorption was observed just
behind the reflected shock wave at the t = 0 position. Hence,
such molecular absorption of CS2 at 130.5 nm was included in
the kinetic simulations by using the absorption coefficient
measured by Hatarajan and Roth.27 Other molecular absorp-
tion was found to be of little influence on the time profiles of O
atom. Similar to the case of S-ARAS in the CS2/O2/Ar system,
a sensitivity analysis of the O-atom time profiles in the CS2/O2/
Ar system were performed using the reaction scheme shown in
Table 1. As shown in Fig. 13, the reactions O + CS2 → CS +
SO and CS + O2 → COS + O as well as the reactions CS2 +
Ar → CS + S + Ar and S + O2 → SO + O were sensitive to
the time profile of O atom compared to the other reactions.
Since the rate constant for the reaction O + CS2 → CS + SO
has already been investigated by several researchers11,28–31 us-
ing discharge-flow techniques, and their results are almost in
good agreement with each other, we used the recommended
value of this rate constant derived from their experimental re-
sults, and tried to determine the rate constant for the reaction
CS + O2 → COS + O by simulating the O-atom time profiles
in the CS2/O2/Ar system. In these kinetic simulations of O at-
om, the present results were used for the rate constants for the
reactions CS2 + Ar → CS + S + Ar and S + O2 → SO + O.
By a slight modification of the rate constant for the reaction CS
+ O2 → COS + O tabulated in Table 1, agreements between
the experimental profiles and kinetic simulations were ob-
tained. The results of kinetic simulations using Table 1 are
also shown in Fig. 12. Figure 14 shows the Arrhenius plots of
the rate constant of the reaction CS + O2 → O + COS. A
least-squares fit of the rate constants lead to the following Ar-
rhenius expression: k2 = (6.1 0.6) × 1012 exp (− 51.0 68
kJ/RT) cm3 mol−1 s−1 in the temperature range of 2000 to 2900
K. As shown in Fig. 14, the rate constant obtained in this work
is almost similar to a previous estimate by Howgate et al.32
Fig. 12. Typical absorption profile at 130.5 nm for CS2/O2/
Ar system.
Conditions: T = 2474 K, P = 0.9 atm, CS2 = 100 ppm, O2
= 500 ppm.
Fig. 13. Sensitivity analysis of time profile of O atom in the
CS2/O2 reaction system.
Conditions: T = 2474 K, P = 0.9 atm, CS2 = 100 ppm, O2
= 500 ppm.
Fig. 14. Arrhenius plot of measured rate constant for CS +
O2 → COS + O.

Y. Murakami et al.
Bull. Chem. Soc. Jpn., 74, No. 7 (2001) 1239
Table 2. Vibrational Frequencies of Transition States Calcu-
lated at the Level of UHF/6-31G(d)
Transition states
Vitrational frequenciesa)/cm−1
TS1
TS2
1294i,
308i,
263,
169,
341,
197,
544,
390,
639,
736,
1922
2022
a) Vibrational frequencies scaled with 0.893 are given.
Fig. 15. Energy diagram for reaction of CS + O2 at the level
of MP2/6-31G(d) theory.
The reaction of CS with O2 molecule is considered to have
the following two reaction channels,
CS + O2 = O + COS,
CS + O2 = CO + SO.
(2)
(3)
In this work we assumed that the rate constant for the latter re-
action (3) had negligible contributions compared with the
former reaction (2), as proposed by Sheen et al.21 To make
sure that this assumption was correct, ab-initio calculations at
the level of UMP2(full)/6-31G(d)//UMP2(full)/6-31G(d) were
performed to find the pathways of the CS + O2 reaction. All
calculations were performed by Gaussian98.33 The energy dia-
gram for the CS + O2 reaction is shown in Fig. 15. Although
the level of calculations is not sufficiently high, a quantitative
picture of the reaction pathways is usefully described in this
energy diagram. As shown in the energy diagram, the first step
of the CS + O2 reaction is the formation of an adduct, CSO2.
The planar adduct CSO2 has two pathways. One is cleavage of
the CO bond to form COS and O (see TS1); the other is to go
through the triangular transition state TS2 to form CO and SO.
The structures and vibrational frequencies of the transition
states (TS1 and TS2) are shown in Fig. 16 and Table 2, respec-
tively.
Fig. 17. Calculated temperature dependence of branching ra-
tios for reactions.
CS + O2 → O + COS (2) and CS + O2 → CO + SO (3).
is the sum of states at the activated complex with energies of
between 0 to E*. Similarly, G(E+) is the density of states at
the transition state with excess energy, E+. Since both reac-
tions (2) and (3) go through the same activated complex,
CSO2, the sum of states, N(E*), is the same for both reactions,
and therefore the branching ratios between reactions (2) and
(3) are determined by the ratio of the sum of states, G(E+), at
the transition states. Using the Whitten–Rabinovitch approx-
imation37 to calculate the sum of states, G(E+), the branching
ratios for reactions (2) and (3) were obtained. The temperature
dependence of the branching ratio is shown in Fig. 17. From
these calculations, it was concluded that reaction (2) was the
dominant channel for the reaction CS + O2 below 3000 K, and
reaction (3) was found to make negligible contributions to the
CS + O2 reaction, as was proposed by Sheen et al.21 In this
work, the activation energy for reaction (2) was determined to
be about 50 kJ mol−1, but the activation energy of TS1 is much
below the total energy of the reactants. The high activation en-
ergy for reaction (2) was also inferred by Black et al.6 and Ri-
chardson et al,7 because the rate constant for reaction (2) was
very slow at room temperature. The most probable explana-
tion for such conflicts is that the reaction CS + O2 → CSO2
has an activation energy of about 50 kJ mol−1, and this reaction
is a rate-controlling process for the reaction of CS + O2. On
the other hand, exit barriers of the transition states TS1 and
TS2 only determine the branching rations for reactions (2) and
(3). In this calculations, the transition state for the entrance
barrier of CS + O2 → CSO2 could not be determined. Further
work is necessary for more accurate predictions of these rate
According to RRKM theory, the microcanonical rate con-
stant is expressed as k(E*) = G(E+)/(h × N(E*)). Here N(E*)
Fig. 16.
Calculated structures of transition states at
UMP2(full)/6-31G(d) level of theory.

1240 Bull. Chem. Soc. Jpn., 74, No. 7 (2001)
Oxidation of CS2 and COS
Int. J. Chem. Kinet., 18, 871 (1986).
constants and a comparison with the experimental results.
18 A. Miyoshi, H. Shiina, K. Tuchiya, and H. Matsui, 26th
Symposium (International) on Combustion, The Combustion In-
sititute, 1996, p. 535.
Conclusion
In the present work we measured the kinetic behavior of S
and O atoms, which were the important products of the CS2
combustion process at high temperature. To explain the tem-
poral behavior of these atomic species, the rate constants for
both reactions, CS + O → CO + S and CS + O2 → COS +
O, were determined. The oxidation mechanism of COS has
also been carried out, and the rate constant for the reactions
COS + Ar → CO + S + Ar and S + O2 → SO + O were de-
termined.
19 D. Woiki and P. Roth, Int. J. Chem. Kinet., 27, 59 (1995).
20 T. Just, in “Shock Waves in Chemsitry,” ed by A. Lifshitz,
Marcel Dekker Inc, New York and Basel (1981), Chap. 6.
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22 R. J. Kee, F. M. Rupley, and J. A. Miller, CHEMKIN II, A
Fortran Chemical Kinetics Package for Analysis of Gas-Phase
Chemical Kinetics, Sandia Report SAND89-8009B,1989.
23 R. J. Kee, F. M. Rupley, and J. A. Miller, The Chemkin
Thermodynamics Data Base, Sandia Reprot SAND87-8215B,
1987.
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