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CH3O+CO removal rate constant measurements over the 473–973 K temperature range
Volume 138, number 6
CHEMICAL PHYSICS LETTERS
7 August 1987
CH,O+CO REMOVAL RATE CONSTANT MEASUREMENTS OVER THE 473-973 K TEMPERATURE RANGE Paul J. WANTUCK, Richard C. OLDENBORG, Steven L. BAUGHCUM and Kenneth R. WINN Chemical and Laser Sciences Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
Received 28 April 1987
Removal rate constants for CH30 by CO have been measured over the temperature range 473-973 K using a laser photolysisl laser-induced fluorescence technique. For temperatures exceeding 773 K, the removal rate constant begins to exhibit a non-linear Arrhenius behavior suggesting that other removal processes, in addition to oxidation of CO by CHsO, are important at elevated temperatures.
1. Introduction Methoxy radicals are thought to be important.contributors to chemical processes occurring in both atmospheric and combustion environments. The reaction of methoxy with CO by the reaction CHsO+CO+C02+CH3
(1)
has been identified as one of importance to methane combustion under some conditions [ 11. Lissi, Massiff, and Villa [ 21 conducted a study of reaction (1 ), over the temperature range 396-426 K, by monitoring the rate of CO2 production during thermal decomposition of dimethyl peroxide in the presence of carbon monoxide. They derived the Arrhenius expression k1 =2.6x 10-l
exp( -5940/T)
cm3 molecule-’
s-’ ,
where T denotes temperature in K. Wiebe and Heicklen [ 31 also investigated the reaction of CH30 with CO at 298, 353, and 423 K by photolyzing methyl nitrite in mixtures of CO and NO, concluding that the CH30 + CO reaction rate was both independent of temperature and approximately a factor of 5 x 1Om4slower than that of the CH30 + NO reaction. Sanders, Butler, Pasternack, and McDonald [ 41 used a combined laser photolysis/laser-induced fluorescence (LIF) technique to estimate an upper limit 548
of 1 x lo-l4 cm3 molecule-’ s-’ for removal of CH30 by CO at room temperature. We have reinvestigated the removal of CH30 by CO to determine accurate removal rate constants at elevated temperatures. This work provides, what we believe are, the first direct measurements of removal rate constants for CH30 by CO over the temperature range 473-973 K. A laser photolysis/LIF technique was used to perform these measurements.
2. Experimental Methoxy radicals are produced by phoiolyzing methanol (CH,OH) with the 193 nm output of an excimer laser (Lambda Physik EMG- 102) operating on ArF. Using methanol as a precursor allowed the investigation of CH,O+CO to be extended to high temperatures. Methyl nitrite (CH,ONO) has been used as a photolytic source of CH30 by several investigators; however, it decomposes at high temperatures. In our apparatus the maximum temperature for effective use of this precursor is x 573 K. Methanol was found to be stable for temperatures up to and exceeding 1000 K. Methoxy radicals are produced in a temperatureregulated gas mixture containing a known concentration of CO and 25 Torr of argon diluent. The argon serves to translationally relax the methoxy photofragments. The methanol pressure is held at x30
0 009-2614/87/S 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume
138, number
6
CHEMICAL
PHYSICS
mTorr. The CHJO radical is detected by LIF using a Nd:YAG laser-pumped dye laser (Quanta-Ray). The A *A,, v;= 4+-X *E, v’;=O transition is excited at 292.8 nm [ 51. A temporal history of the radical is produced by recording the fluorescence emission as a function of the delay time between the excimer laser pulse and the probe laser pulse. These temporal profiles are well represented by single exponential decays. Reaction rates are determined by the variation of these exponential decay rates with systematic changes in CO concentration and temperature. A high-temperature cell (HTC), based upon the design of Felder and co-workers [ 6 1, and described previously by Baughcum and Oldenborg [ 71, is utilized for these experiments. The HTC consists of an alumina ceramic tube which is heated resistively by Pt/40°h Rh resistance wire. The alumina core is surrounded by zirconia fiber insulation and the entire assembly is enclosed in a water-cooled vacuum chamber. Cell temperature is monitored with stainless-steel sheathed chromel-alumel thermocouple probes. The output of these probes is sent to a Micricon model 823 microprocessor which automatically regulates the heater current. Temperatures in the observation zone can be different from the set point value, particularly at higher temperatures, but are always within 1% of the set point value and held constant to within 1“C. The methanol precursor is premixed with argon (2% CH,OH) and enters the HTC through a watercooled inlet at the bottom of the cell. The argon diluent (high purity, 99.995Oh) and the CO (UHP, 99.99%) are mixed in a gas handling system, enter the HTC through an inlet located at the cell base, and flow slowly upward ( x 5 cm/s) through the cell. Such a flow rate ensures that a fresh sample of precursor is present for each laser shot and is yet essentially static on the timescale of the reaction ( < 1 ms) . Cell pressure is measured with a capacitance manometer and gas flows are regulated-with calibrated Tylan mass flowmeters. The excimer and probe laser beams are introduced collinearly into the HTC. To minimize diffusional mixing effects, the probe beam is focused to approximately 0.5 mm while the excimer beam is approximately 3 x 3 mm at the same location. The excimer and dye laser fluences are typically 0.15 and 0.25 J/cm*, respectively. The broadband methoxy fluo-
7 August
LETTERS
1987
rescence emission, monitored at right angles to the photolysis and probe beams, is imaged through a 305 nm longpass filter onto the photocathode of a RCA 7265 gated photomultiplier. The signal from the PMT is amplified and processed with a PAR model 162 boxcar signal averager and then recorded and analyzed with an interfaced Data General Nova/Eclipse computer system.
3. Results Experiments were performed to measure CH30 + CO removal rate constants as a function of temperature. Fig. 1 shows a typical decay curve for ground-state CH30 in the presence of 40 Torr of CO for a temperature of 673 K along with the logarithm of the decay curve and the best-fit single exponential (which establishes a decay time, 7). A representative Stern-Volmer plot of the inverse decay time for CH30 as a function of CO pressure, recorded for a cell temperature of 673 K, is shown in fig. 2. The slope of the plot is used to calculate the bimolecular removal rate constant, k,, for CH,O by CO at 673 K. The quality of the fit is a good indication of the precision of the measurements and the standard deviation of the slope represents the uncertainty for these measurements. Similar plots were constructed for the other temperatures investigated. The removal rate ---T
-- -I-
I
’ T
=
-
= BEST FIT SINGLE EXPONENTIAL i
I
0
I
80
I
I 160
I
I 240
=
I
673 K
1769
I 320
,,s
I
1 400
TIME (ps) Fig. 1. CH,O decay curve together with its logarithm and best-fit single exponential, PO=40 Torr, PAr= 25 Torr, T= 673 K.
549
Volume 138, number 6
CHEMICAL PHYSICS LETTERS
7 August I987
0.006
0
5
10
15
20
CO PRESSURE Fig.2.Stem-Volmer
plot of inverse first-order decay constant, t -I,
constants determined from such plots are presented in table 1. In general the use of the LIF technique to study methoxy radical reactions is limited by quenching of the fluorescence emission. For the present study, quenching of CH30( A 2A, ) fluorescence emission by CO combined with the relatively slow CH,O+CO reaction rate, prevented the measurement of removal rate constants below a temperature of 473 K. The room-temperature CO quenching rate constant for such fluorescence emission, k,, has been measured [8] tobe(9.5+0.2)xlO-“cm3moleculee’s_’.The Table 1 Measured removal reaction rate constants, k,, for CH,O by CO
550
T(K)
k, (cm3 molecule-’ s-l)
473 573 673 773 873 973
(8.4_+3.0)~10-I6 (4.3_+0.1)xlO-” (9.2*0.4)x IO-l5 (2.1~0.2)x10-‘4 (7.2+0.4)x IO-l4 (1.6kO.I)x10-”
25
30
35
(torr)
versusCOpressure,PAT= 25 Torr, T= 673 K.
argon diluent has no deleterious effect on the methoxy A-state fluorescence emission since it is a very ineffrcient quencher (kQc3.5x10-‘4 cm3 molecule-’ s-‘) [ 81. The measured CH30 + CO removal rate constants are plotted versus inverse temperature in fig. 3. The removal rate constant is not described by a simple Arrhenius expression over the entire temperature range. The solid line represents a best fit to our removal rate constant data using the empirical expression kJ T) =AT-”
exp( -E/RT)
,
whereA=4.5X10-‘4cm3molecule-‘s-‘K”,n=9.2, and E/R = - 1434 K. These parameters appear to be physically unrealistic; however, the expression does provide a means of interpolating removal rate constants for CH,O by CO over the 473-973 K range. Also shown in fig. 3 are the reported rate constants of Lissi and co-workers [ 21 and the line corresponding to an extrapolation of the Arrhenius expression they formulated to describe the temperature dependence of their rate constant measurements over the
7 August 1987
CHEMICAL PHYSICS LETTERS
Volume 138, number 6 10-1:
,
k,(T)
= 4.5x10-14
T-g.2exp(1434/T)
a
E
\
c)
5
10-16
\
Y
L
0
CURRENT
0
LISSI.
n
WIEBE & HEICKLEN
et
al
\
2 lo-‘1
1 o-1” 1.0
1.5
2.5
2.0
1000/T
3.0
3.5
(K)
Fig. 3. Arrhenius plot of CH,O+CO removal rate constants. Error bars where not shown are smaller than the symbol.
396-426 K range. Their results are at least a factor of ten lower than might be expected on the basis of our results. The reasons for this discrepancy are not clear; however, the experiments do differ in two important ways. First, Lissi and co-workers [2] measure the production rate of CO* and therefore obtain a rate constant for reaction (1). Our experiment measures the total rate constant for the sum of all removal reactions including reaction (1). To the extent that other remove1 pathways are important, our measured rate constant would be expected to be larger. However, at their low temperatures and at high CO* concentrations, they see little or no CHIO and CH,OH, which are expected to be products for other reactive pathways. Therefore, reaction (1) must dominate the removal process at the temperatures of their experiment. The second difference between the experiments is that we are directly measuring the removal rate constant, while their rate constants are determined from indirect measurements and depend on the values assumed for the reactions CH300CHJ+2CH30 and
Consequently, the accuracy of the rate constants they deduce depend directly on the accuracy of the rate constants for these aforementioned reactions. Wiebe and Heicklen [ 31 also investigated the total removal of CH30 by CO. They concluded that the removal rate constant is essentially independent of temperature (at 298,353, and 423 K) and is approximately 5 x 10e4 slower than the rate constant for the bimolecular reaction of CH30 and NO. Their results are consistent with a CH30 removal rate constant by CO of lo-l4 cm3 molecule-’ s- ’ as displayed in fig. 3. Calculation of these temperature-independent rate constants is done using a value for the CH30 + NO reaction rate constant measured by Sanders and coworkers [4], namely, (2.08~0.12)~10-~~ cm3 molecule - ’ s- ’ (a value in good agreement with that obtained by Batt, Milne and McCulloch [9]). We believe this NO removal rate constant value is more reliable than the value recommended by Wiebe and Heicklen (based on the results of Arden, Phillips and Shaw [lo], i.e. 8.3x lo-r4 cm3 molecule-’ s-r). If the CH30 + CO removal rate constant had a value of IO-l4 cm3 molecule-’ s-’ in the 298-423 K range as their data predict, then it should be possible to
CH30+CH30+CH30H+CH10. 551
Volume I38, number 6
CHEMICAL PHYSICS LETTERS
directly measure such a rate constant with the LIF technique. However, our results, as well as those of Sanders et al. [ 41, indicate a slower rate for methoxy removal by CO in this temperature range. The curvature in the Arrhenius plot (fig. 3) may indicate the onset, at increasing temperatures, of another mechanism for methoxy removal by CO. One such mechanism may be the reaction of methoxy with CO to produce the formyl radical and formaldehyde, i.e. CH~O+CO~HCO+CH~O.
(2)
Thermochemical calculations for reactions (1) and (2)) using the enthalpies of formation tabulated by Okabe [ 111, show that reaction (1) is thermodynamically favored (exothermic, AH= - 35 kcal mol-‘) over an approximately thermoneutral reaction (2). To the extent that this reaction is important, one should observe either CHzO as a product as well as CHLIOH. As noted before, Lissi et al. [2] observed that neither CH20 nor the further reaction product CHsOH were detectable in their system. Consequently, reaction (2) is probably not an important methoxy removal channel in the 396-426 K range. However, this result does not preclude the increased importance of reaction (2) at our higher temperatures. Sabe, Radom and Schaefer [ 121 predict that decomposition of CH,O to CHIO and H requires an activation energy of 34.4 kcal mol- ’ while isomerization to CHzOH has an activation energy of 36 kcal mol-‘. Thus, at elevated temperatures, collisioninduced decomposition of CH30 by the reaction CH~O+CO~CH~O+H+CO,
(3)
or isomerization of CHjO via the reaction CHJO + CO-CH,OH
+ CO
(4)
may also represent important methoxy removal channels. The results presented in this paper would seem to indicate that at least two removal channels for CH30+C0 are important. Currently, it is not possible to distinguish between the different channels. Studies of reactions of CH30 with non-reactive species such as argon, nitrogen, and CF, are currently
5.52
7 August 1987
in progress in our laboratory. Such studies will help to ascertain the importance of reactions (3) and (4). Preliminary measurements for methoxy removal by such unreactive collision partners indicate that processes like (3) and (4) can be quite important at elevated temperatures. However, further experiments and analysis are required to fully deconvolute the fundamental rate constant parameters for the reaction of CH30 with CO.
Acknowledgement This work was supported by the Morgantown Energy Technology Center (DOE) and was performed under the auspices of the Department of Energy. PJW gratefully acknowledges the support provided by the LANL Photochemistry and Photophysics Group (CLS-4) during his postdoctoral appointment.
References [ 1] W. Tsang and R.F. Hampson, J. Phys. Chem. Ref. Data 15 (1986) 1087. [2] E.A. Lissi, G. Massiff and A.E. Villa, J. Chem. Sot. Faraday Trans. I69 (1973) 346. [ 31 H.A. Wiebe and J. Heicklen, J. Am. Chem. Sot. 95 (1973) [4] k. Sanders, J.E. Butler, L.R. Pastemackand J.R. McDonald, Chem. Phys. 48 (1980) 203. [5] G. Inoue, H. Akimoto and M. Okuda, J. Chem. Phys. 72 (1980) 1769. [ 61 W. Felder, A. Fontijn, H.N. Volltrauer and D.R. Voorhees, Rev. Sci. Instr. 51 (1980) 195. [ 71 S.L. Baughcum and R.C. Oldenborg, in: The chemistry of combustion processes, ed. Th. M. Sloane, Am. Chem. Sot. Symp. Series, No. 259 (Am. Chem. Sot., Washington, 1984) pp. 257-266. [8] P.J. Wantuck, R.C. Oldenborg, S.L. Baughcum and K.R. Wmn, J. Phys. Chem, to be published. [9] L. Batt, R.T. Milne and R.D. McCulloch, Intern. J. Chem. Kinetics 9 (1977) 567. [ IO] E.A. Arden, L. Phillips and R. Shaw, J. Chem. Sot. ( 1964) 5126. [ II] H. Okabe, Photochemistry of small molecules (Wiley, New York, 1978) pp. 375-380. [ 121 S. Ssebe, L. Radom and H.F. Schaefer III, J. Chem. Phys. 78 (1983) 845.
CHEMICAL PHYSICS LETTERS
7 August 1987
CH,O+CO REMOVAL RATE CONSTANT MEASUREMENTS OVER THE 473-973 K TEMPERATURE RANGE Paul J. WANTUCK, Richard C. OLDENBORG, Steven L. BAUGHCUM and Kenneth R. WINN Chemical and Laser Sciences Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
Received 28 April 1987
Removal rate constants for CH30 by CO have been measured over the temperature range 473-973 K using a laser photolysisl laser-induced fluorescence technique. For temperatures exceeding 773 K, the removal rate constant begins to exhibit a non-linear Arrhenius behavior suggesting that other removal processes, in addition to oxidation of CO by CHsO, are important at elevated temperatures.
1. Introduction Methoxy radicals are thought to be important.contributors to chemical processes occurring in both atmospheric and combustion environments. The reaction of methoxy with CO by the reaction CHsO+CO+C02+CH3
(1)
has been identified as one of importance to methane combustion under some conditions [ 11. Lissi, Massiff, and Villa [ 21 conducted a study of reaction (1 ), over the temperature range 396-426 K, by monitoring the rate of CO2 production during thermal decomposition of dimethyl peroxide in the presence of carbon monoxide. They derived the Arrhenius expression k1 =2.6x 10-l
exp( -5940/T)
cm3 molecule-’
s-’ ,
where T denotes temperature in K. Wiebe and Heicklen [ 31 also investigated the reaction of CH30 with CO at 298, 353, and 423 K by photolyzing methyl nitrite in mixtures of CO and NO, concluding that the CH30 + CO reaction rate was both independent of temperature and approximately a factor of 5 x 1Om4slower than that of the CH30 + NO reaction. Sanders, Butler, Pasternack, and McDonald [ 41 used a combined laser photolysis/laser-induced fluorescence (LIF) technique to estimate an upper limit 548
of 1 x lo-l4 cm3 molecule-’ s-’ for removal of CH30 by CO at room temperature. We have reinvestigated the removal of CH30 by CO to determine accurate removal rate constants at elevated temperatures. This work provides, what we believe are, the first direct measurements of removal rate constants for CH30 by CO over the temperature range 473-973 K. A laser photolysis/LIF technique was used to perform these measurements.
2. Experimental Methoxy radicals are produced by phoiolyzing methanol (CH,OH) with the 193 nm output of an excimer laser (Lambda Physik EMG- 102) operating on ArF. Using methanol as a precursor allowed the investigation of CH,O+CO to be extended to high temperatures. Methyl nitrite (CH,ONO) has been used as a photolytic source of CH30 by several investigators; however, it decomposes at high temperatures. In our apparatus the maximum temperature for effective use of this precursor is x 573 K. Methanol was found to be stable for temperatures up to and exceeding 1000 K. Methoxy radicals are produced in a temperatureregulated gas mixture containing a known concentration of CO and 25 Torr of argon diluent. The argon serves to translationally relax the methoxy photofragments. The methanol pressure is held at x30
0 009-2614/87/S 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume
138, number
6
CHEMICAL
PHYSICS
mTorr. The CHJO radical is detected by LIF using a Nd:YAG laser-pumped dye laser (Quanta-Ray). The A *A,, v;= 4+-X *E, v’;=O transition is excited at 292.8 nm [ 51. A temporal history of the radical is produced by recording the fluorescence emission as a function of the delay time between the excimer laser pulse and the probe laser pulse. These temporal profiles are well represented by single exponential decays. Reaction rates are determined by the variation of these exponential decay rates with systematic changes in CO concentration and temperature. A high-temperature cell (HTC), based upon the design of Felder and co-workers [ 6 1, and described previously by Baughcum and Oldenborg [ 71, is utilized for these experiments. The HTC consists of an alumina ceramic tube which is heated resistively by Pt/40°h Rh resistance wire. The alumina core is surrounded by zirconia fiber insulation and the entire assembly is enclosed in a water-cooled vacuum chamber. Cell temperature is monitored with stainless-steel sheathed chromel-alumel thermocouple probes. The output of these probes is sent to a Micricon model 823 microprocessor which automatically regulates the heater current. Temperatures in the observation zone can be different from the set point value, particularly at higher temperatures, but are always within 1% of the set point value and held constant to within 1“C. The methanol precursor is premixed with argon (2% CH,OH) and enters the HTC through a watercooled inlet at the bottom of the cell. The argon diluent (high purity, 99.995Oh) and the CO (UHP, 99.99%) are mixed in a gas handling system, enter the HTC through an inlet located at the cell base, and flow slowly upward ( x 5 cm/s) through the cell. Such a flow rate ensures that a fresh sample of precursor is present for each laser shot and is yet essentially static on the timescale of the reaction ( < 1 ms) . Cell pressure is measured with a capacitance manometer and gas flows are regulated-with calibrated Tylan mass flowmeters. The excimer and probe laser beams are introduced collinearly into the HTC. To minimize diffusional mixing effects, the probe beam is focused to approximately 0.5 mm while the excimer beam is approximately 3 x 3 mm at the same location. The excimer and dye laser fluences are typically 0.15 and 0.25 J/cm*, respectively. The broadband methoxy fluo-
7 August
LETTERS
1987
rescence emission, monitored at right angles to the photolysis and probe beams, is imaged through a 305 nm longpass filter onto the photocathode of a RCA 7265 gated photomultiplier. The signal from the PMT is amplified and processed with a PAR model 162 boxcar signal averager and then recorded and analyzed with an interfaced Data General Nova/Eclipse computer system.
3. Results Experiments were performed to measure CH30 + CO removal rate constants as a function of temperature. Fig. 1 shows a typical decay curve for ground-state CH30 in the presence of 40 Torr of CO for a temperature of 673 K along with the logarithm of the decay curve and the best-fit single exponential (which establishes a decay time, 7). A representative Stern-Volmer plot of the inverse decay time for CH30 as a function of CO pressure, recorded for a cell temperature of 673 K, is shown in fig. 2. The slope of the plot is used to calculate the bimolecular removal rate constant, k,, for CH,O by CO at 673 K. The quality of the fit is a good indication of the precision of the measurements and the standard deviation of the slope represents the uncertainty for these measurements. Similar plots were constructed for the other temperatures investigated. The removal rate ---T
-- -I-
I
’ T
=
-
= BEST FIT SINGLE EXPONENTIAL i
I
0
I
80
I
I 160
I
I 240
=
I
673 K
1769
I 320
,,s
I
1 400
TIME (ps) Fig. 1. CH,O decay curve together with its logarithm and best-fit single exponential, PO=40 Torr, PAr= 25 Torr, T= 673 K.
549
Volume 138, number 6
CHEMICAL PHYSICS LETTERS
7 August I987
0.006
0
5
10
15
20
CO PRESSURE Fig.2.Stem-Volmer
plot of inverse first-order decay constant, t -I,
constants determined from such plots are presented in table 1. In general the use of the LIF technique to study methoxy radical reactions is limited by quenching of the fluorescence emission. For the present study, quenching of CH30( A 2A, ) fluorescence emission by CO combined with the relatively slow CH,O+CO reaction rate, prevented the measurement of removal rate constants below a temperature of 473 K. The room-temperature CO quenching rate constant for such fluorescence emission, k,, has been measured [8] tobe(9.5+0.2)xlO-“cm3moleculee’s_’.The Table 1 Measured removal reaction rate constants, k,, for CH,O by CO
550
T(K)
k, (cm3 molecule-’ s-l)
473 573 673 773 873 973
(8.4_+3.0)~10-I6 (4.3_+0.1)xlO-” (9.2*0.4)x IO-l5 (2.1~0.2)x10-‘4 (7.2+0.4)x IO-l4 (1.6kO.I)x10-”
25
30
35
(torr)
versusCOpressure,PAT= 25 Torr, T= 673 K.
argon diluent has no deleterious effect on the methoxy A-state fluorescence emission since it is a very ineffrcient quencher (kQc3.5x10-‘4 cm3 molecule-’ s-‘) [ 81. The measured CH30 + CO removal rate constants are plotted versus inverse temperature in fig. 3. The removal rate constant is not described by a simple Arrhenius expression over the entire temperature range. The solid line represents a best fit to our removal rate constant data using the empirical expression kJ T) =AT-”
exp( -E/RT)
,
whereA=4.5X10-‘4cm3molecule-‘s-‘K”,n=9.2, and E/R = - 1434 K. These parameters appear to be physically unrealistic; however, the expression does provide a means of interpolating removal rate constants for CH,O by CO over the 473-973 K range. Also shown in fig. 3 are the reported rate constants of Lissi and co-workers [ 21 and the line corresponding to an extrapolation of the Arrhenius expression they formulated to describe the temperature dependence of their rate constant measurements over the
7 August 1987
CHEMICAL PHYSICS LETTERS
Volume 138, number 6 10-1:
,
k,(T)
= 4.5x10-14
T-g.2exp(1434/T)
a
E
\
c)
5
10-16
\
Y
L
0
CURRENT
0
LISSI.
n
WIEBE & HEICKLEN
et
al
\
2 lo-‘1
1 o-1” 1.0
1.5
2.5
2.0
1000/T
3.0
3.5
(K)
Fig. 3. Arrhenius plot of CH,O+CO removal rate constants. Error bars where not shown are smaller than the symbol.
396-426 K range. Their results are at least a factor of ten lower than might be expected on the basis of our results. The reasons for this discrepancy are not clear; however, the experiments do differ in two important ways. First, Lissi and co-workers [2] measure the production rate of CO* and therefore obtain a rate constant for reaction (1). Our experiment measures the total rate constant for the sum of all removal reactions including reaction (1). To the extent that other remove1 pathways are important, our measured rate constant would be expected to be larger. However, at their low temperatures and at high CO* concentrations, they see little or no CHIO and CH,OH, which are expected to be products for other reactive pathways. Therefore, reaction (1) must dominate the removal process at the temperatures of their experiment. The second difference between the experiments is that we are directly measuring the removal rate constant, while their rate constants are determined from indirect measurements and depend on the values assumed for the reactions CH300CHJ+2CH30 and
Consequently, the accuracy of the rate constants they deduce depend directly on the accuracy of the rate constants for these aforementioned reactions. Wiebe and Heicklen [ 31 also investigated the total removal of CH30 by CO. They concluded that the removal rate constant is essentially independent of temperature (at 298,353, and 423 K) and is approximately 5 x 10e4 slower than the rate constant for the bimolecular reaction of CH30 and NO. Their results are consistent with a CH30 removal rate constant by CO of lo-l4 cm3 molecule-’ s- ’ as displayed in fig. 3. Calculation of these temperature-independent rate constants is done using a value for the CH30 + NO reaction rate constant measured by Sanders and coworkers [4], namely, (2.08~0.12)~10-~~ cm3 molecule - ’ s- ’ (a value in good agreement with that obtained by Batt, Milne and McCulloch [9]). We believe this NO removal rate constant value is more reliable than the value recommended by Wiebe and Heicklen (based on the results of Arden, Phillips and Shaw [lo], i.e. 8.3x lo-r4 cm3 molecule-’ s-r). If the CH30 + CO removal rate constant had a value of IO-l4 cm3 molecule-’ s-’ in the 298-423 K range as their data predict, then it should be possible to
CH30+CH30+CH30H+CH10. 551
Volume I38, number 6
CHEMICAL PHYSICS LETTERS
directly measure such a rate constant with the LIF technique. However, our results, as well as those of Sanders et al. [ 41, indicate a slower rate for methoxy removal by CO in this temperature range. The curvature in the Arrhenius plot (fig. 3) may indicate the onset, at increasing temperatures, of another mechanism for methoxy removal by CO. One such mechanism may be the reaction of methoxy with CO to produce the formyl radical and formaldehyde, i.e. CH~O+CO~HCO+CH~O.
(2)
Thermochemical calculations for reactions (1) and (2)) using the enthalpies of formation tabulated by Okabe [ 111, show that reaction (1) is thermodynamically favored (exothermic, AH= - 35 kcal mol-‘) over an approximately thermoneutral reaction (2). To the extent that this reaction is important, one should observe either CHzO as a product as well as CHLIOH. As noted before, Lissi et al. [2] observed that neither CH20 nor the further reaction product CHsOH were detectable in their system. Consequently, reaction (2) is probably not an important methoxy removal channel in the 396-426 K range. However, this result does not preclude the increased importance of reaction (2) at our higher temperatures. Sabe, Radom and Schaefer [ 121 predict that decomposition of CH,O to CHIO and H requires an activation energy of 34.4 kcal mol- ’ while isomerization to CHzOH has an activation energy of 36 kcal mol-‘. Thus, at elevated temperatures, collisioninduced decomposition of CH30 by the reaction CH~O+CO~CH~O+H+CO,
(3)
or isomerization of CHjO via the reaction CHJO + CO-CH,OH
+ CO
(4)
may also represent important methoxy removal channels. The results presented in this paper would seem to indicate that at least two removal channels for CH30+C0 are important. Currently, it is not possible to distinguish between the different channels. Studies of reactions of CH30 with non-reactive species such as argon, nitrogen, and CF, are currently
5.52
7 August 1987
in progress in our laboratory. Such studies will help to ascertain the importance of reactions (3) and (4). Preliminary measurements for methoxy removal by such unreactive collision partners indicate that processes like (3) and (4) can be quite important at elevated temperatures. However, further experiments and analysis are required to fully deconvolute the fundamental rate constant parameters for the reaction of CH30 with CO.
Acknowledgement This work was supported by the Morgantown Energy Technology Center (DOE) and was performed under the auspices of the Department of Energy. PJW gratefully acknowledges the support provided by the LANL Photochemistry and Photophysics Group (CLS-4) during his postdoctoral appointment.
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