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Multiphoton ionization of methyl radicals in CH4 pyrolysis
Volume
114, number
CHEMICAL
3
MULTIPHOTON IONIZATION
PHYSICS
1 March
LETTERS
OF METHYL RADICALS
1985
IN CH, PYROLYSIS
Mau-Song CHOU Corporate Research-Science
Laboratories, Exxon Research and Engineering
Annandale, NJ 08801,
USA
Received
1984; in final form 7 December
9 November
Company’,
1984
has been applied to in situ detection of methyl radicals The technique of resonance enhanced multiphoton ionization in high-temperature atmospheric pressure CH, pyrolysis. The detection is via three-photon ionization using two-photon resonances to the 3p *A;’ state ‘of CH; at 333.5 and 329.5 nm for the 0: and 2: bands, respectively. Deuterium isotope shifts in both bands are also observed in CD4 pyrolysis. The detection sensitivity is estimated to be at the ppm level. The methyl radical concentration profiles are measured as a function of reaction time.
1. Introduction
2. Experimental
The methyl radical is one of the most important intermediates in several chemical systems, especially hydrocarbon pyrolysis. Detection of methyl radicals in bulk gas samples has previously relied upon absorption of 3s 2A; + 2p 2Ay at 216 nm [l-4] . The interferences from C2H2, C2H4, and aromatics often limits the use of the absorption technique in many practical systems. Very recently, resonance enhanced multiphoton ionization (REMI) spectroscopy was employed for the detection of methyl radicals [5-l 0] . However, to our knowledge, the investigations so far were carried out only in “collisionless” molecular beam conditions. We wish to report successful in situ detection of methyl radicals in high-temperature atmospheric pressure methane pyrolysis. The use of longer wavelengths here reduces the problems of the background interferences at shorter wavelengths. The detection is via three-photon ionization using two-photon resonance to the 3p 2A;’ state of methyl radicals [8,10] . The detection sensitivity is estimated to be at the ppm level. Measurements are also reported here for CHj radical concentration profiles as a function of reaction time in CH,/Ar/He pyrolysis.
The flow reactor is made of a cold-rolled steel tube with a quartz lining (12 mm internal diameter), and is heated by a three-zone controlled Thermcraft furnace. The temperature is maintained at 1100°C with deviation less than 15°C for the entire 76 cm reaction zone. The gas mixtures are prepared in a stainless steel manifold using MKS flow controllers. Spectra Gases ultrahigh purity CH, (99.995%) and Merck CD, (99%) are used. The Quanta Ray YAG-pumped dye laser system (model DCR-lA/PDL-1) which is equipped with a WEX system provides tunable laser output between 328 and 335 nm by doubling the DCM dye laser. The laser system has a nominal linewidth of ~0.5 cm-l at these wavelengths, and the laser energy is ~0.8 mJ for all the data reported here. The laser beam is focused by a 50 cm lens through the exhaust end into the high-temperature zone of the flow reactor. A quartz tube, purged with N, gas, is used to guide the laser beam in order to minimize attenuation of laser energy along the beam path. The laser beam is focused at ~2 cm in front of the quartz tube to minimize any perturbation in pyrolysis chemistry caused by insertion of the quartz tube. The electrons and ions produced by the ionization are detected with a pair of high-temperature alloy wires inserted into the high-temperature zone. The signal is measured by a Tektronix (model AM501)
0 009-2614/&j/$03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
279
Volume
114, number
3
CHEMICAL
preamplifier with a load resistor of 200 kS1. No significant changes in signal are observed for a bias voltage between 15 and 300 V. Attempts to detect methyl radicals in neat atmospheric pressure CH, pyrolysis met problems of noise which is probably caused by carbon deposit on the surface of the two electrodes, presumably via thermionic emission and/or field emission. The noise persists even when the CH, flow is turned off, and is eliminated by introducing air or 0, to burn off the carbon deposit. Fortunately, we can delay the onset of the noise by using dilute CH, feeds. For example, the delay lasts about 2-5 min for a flow ratio of CH,/He/Ar = 2.6/32.4/65, and about 60 min for CH,/Ar = 2/1000.
3. Results and discussion The ionization spectra from CH, and CD, pyrolysis at atmospheric pressure and 1100°C are shown in figs. l-3. The ionization peaks are observed at 333.5 and 329.5 nm in CH, pyrolysis and 333.9 and 330.7 nm in CD, pyrolysis. These peaks agree well with those reported by Hudgens et al. [lo] , indicating that the signals are indeed due to CH; and CD; radicals, respectively. The 333.5 nm in CHj and 333.9 nm in CD; are assigned as two-photon resonance of the 3p 2As + 2P 2Ai (0:) transition
PHYSICS
1 March 1985
LETTERS
Laser Wavelength Fig. 2. The resonance
CH,
ionization at 1100°C.
pyrolysis
(nm)
spectrum
near 329.5
nm in
whereas the 329.5 nm in CH; and 330.7 nm in CD; are the corresponding hot band (2;) transitions [lo] . Note that the spectral bandwidth for the 0: transition is substantially narrower than that of 2; transition, and is narrower for CD; as compared to CH; for the same transtion. The utility of the present technique for in situ measurements of CH; radical concentration profiles in hydrocarbon pyrolysis is illustrated by the data of
I
/
I
CDs (0:)
I
i
From CD4
From CH4
1
333.6 Laser Wavelength
Fig. 1. The resonance ionization CH, pyrolysis at 1100°C.
280
(nm)
spectrum
_/I
8’ 333.8
I
Laser Wavelength
near 333.5
nm in
Fig. 3. The resonance ionization CD, pyrolysis at 1100°C.
L
1
z
334.2
334.0 (nm)
spectrum
near 333.9 nm in
Volume
114, number
CHEMICAL
3
PHYSICS
LETTERS
1 March
1985
ground at 333.30 nm has a relatively long induction period followed by a rapid increase beyond ~600 ms. These background signals may be attributed to large molecular species (such as tar) and soot particles which are produced at longer reaction times. The contribution from soot particles may be differentiated from the large molecular species using the difference in arrival times of the ion signals as has been demonstrated by Smith and Mallard [ 121.
Acknowledgement The mental Bomse, Woodin
ll
author wishes to thank K. Putman for experiassistance with these measurements, and D.S. D.M. Cox, A.M. Dean, R.L. Whetten and R.L. for helpful discussions.
References
0
Reaction
Time (ms)
Fig. 4. Ionization signals as a function of reaction time in CH, pyrolysis for a CH4/Ar/He ratio of 2.6j65132.4 at atmospheric pressure and 1100°C: (0) methyl radical signal at resonance maximum of 333.53, (m) off-resonance background (tar and soot) at 333.30 nm.
fig. 4 for a flow ratio of CH,/He/Ar = 2.6132.4165 at various reaction times. The reaction time is changed by varying the flows while keeping the same flow ratio. The signal is taken as the difference between those on resonance at 333.53 nm and off-resonance at 333.30 nm. The trend of rapid rise followed by a slower increase in the CH; concentration profile is consistent with kinetic modeling calculations [ 1l] . The CHj radical concentration is estimated to be several ppm based on modeling calculations, suggesting that the detection sensitivity is at the ppm level. We have also observed a broadcontinuum background. As shown in fig. 4, the off-resonance back-
[II G. Herzberg, Proc. Roy. Sot. A262 (1961) 291. (21 K. Glinzer, M. Quack and J. Troe, 16th International Symposium on Combustion (Combustion Institute, Pittsburgh, 1977) p. 949. [31 T. Tsuboi, Japan J. Appl. Phys. 17 (1978) 709. 141 0. Kondo, K. Saito and I. Murakani, Bull. Chem. Sot. Japan 53 (1980) 2133. [51 T.G. DiCiuseppe, J.W. Hudgens and M.C. Lin, Chem. Phys. Letters 82 (1981) 267. [61 J. Danon, H. Zacharias, H. Rottke and K.H. Welge, J. Chem. Phys. 76 (1982) 2399. [71 T.G. DiGiuseppe, J.W. Hudgens and M.C. Lin, J. Phys. Chem. 86 (1982) 36. [81 T.G. DiGiuseppe, J.W. Hudgens and M.C. Lin, J. Chem. Phys. 76 (1982) 3337. [91 B.H. Rockney and E.R. Grant, J. Chem. Phys. 77 (1982) 4257. IlO1 J.W. Hudgens, T.G. DiGiuseppe and M.C. Lin, J. Chem. Phys. 79 (1983) 571. (1984). [Ill A.M. Dean, private communication iI21 K.C. Smith and W.G. Mallard, Comb. Sci. Tech. 26 (1981) 35.
281
114, number
CHEMICAL
3
MULTIPHOTON IONIZATION
PHYSICS
1 March
LETTERS
OF METHYL RADICALS
1985
IN CH, PYROLYSIS
Mau-Song CHOU Corporate Research-Science
Laboratories, Exxon Research and Engineering
Annandale, NJ 08801,
USA
Received
1984; in final form 7 December
9 November
Company’,
1984
has been applied to in situ detection of methyl radicals The technique of resonance enhanced multiphoton ionization in high-temperature atmospheric pressure CH, pyrolysis. The detection is via three-photon ionization using two-photon resonances to the 3p *A;’ state ‘of CH; at 333.5 and 329.5 nm for the 0: and 2: bands, respectively. Deuterium isotope shifts in both bands are also observed in CD4 pyrolysis. The detection sensitivity is estimated to be at the ppm level. The methyl radical concentration profiles are measured as a function of reaction time.
1. Introduction
2. Experimental
The methyl radical is one of the most important intermediates in several chemical systems, especially hydrocarbon pyrolysis. Detection of methyl radicals in bulk gas samples has previously relied upon absorption of 3s 2A; + 2p 2Ay at 216 nm [l-4] . The interferences from C2H2, C2H4, and aromatics often limits the use of the absorption technique in many practical systems. Very recently, resonance enhanced multiphoton ionization (REMI) spectroscopy was employed for the detection of methyl radicals [5-l 0] . However, to our knowledge, the investigations so far were carried out only in “collisionless” molecular beam conditions. We wish to report successful in situ detection of methyl radicals in high-temperature atmospheric pressure methane pyrolysis. The use of longer wavelengths here reduces the problems of the background interferences at shorter wavelengths. The detection is via three-photon ionization using two-photon resonance to the 3p 2A;’ state of methyl radicals [8,10] . The detection sensitivity is estimated to be at the ppm level. Measurements are also reported here for CHj radical concentration profiles as a function of reaction time in CH,/Ar/He pyrolysis.
The flow reactor is made of a cold-rolled steel tube with a quartz lining (12 mm internal diameter), and is heated by a three-zone controlled Thermcraft furnace. The temperature is maintained at 1100°C with deviation less than 15°C for the entire 76 cm reaction zone. The gas mixtures are prepared in a stainless steel manifold using MKS flow controllers. Spectra Gases ultrahigh purity CH, (99.995%) and Merck CD, (99%) are used. The Quanta Ray YAG-pumped dye laser system (model DCR-lA/PDL-1) which is equipped with a WEX system provides tunable laser output between 328 and 335 nm by doubling the DCM dye laser. The laser system has a nominal linewidth of ~0.5 cm-l at these wavelengths, and the laser energy is ~0.8 mJ for all the data reported here. The laser beam is focused by a 50 cm lens through the exhaust end into the high-temperature zone of the flow reactor. A quartz tube, purged with N, gas, is used to guide the laser beam in order to minimize attenuation of laser energy along the beam path. The laser beam is focused at ~2 cm in front of the quartz tube to minimize any perturbation in pyrolysis chemistry caused by insertion of the quartz tube. The electrons and ions produced by the ionization are detected with a pair of high-temperature alloy wires inserted into the high-temperature zone. The signal is measured by a Tektronix (model AM501)
0 009-2614/&j/$03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
279
Volume
114, number
3
CHEMICAL
preamplifier with a load resistor of 200 kS1. No significant changes in signal are observed for a bias voltage between 15 and 300 V. Attempts to detect methyl radicals in neat atmospheric pressure CH, pyrolysis met problems of noise which is probably caused by carbon deposit on the surface of the two electrodes, presumably via thermionic emission and/or field emission. The noise persists even when the CH, flow is turned off, and is eliminated by introducing air or 0, to burn off the carbon deposit. Fortunately, we can delay the onset of the noise by using dilute CH, feeds. For example, the delay lasts about 2-5 min for a flow ratio of CH,/He/Ar = 2.6/32.4/65, and about 60 min for CH,/Ar = 2/1000.
3. Results and discussion The ionization spectra from CH, and CD, pyrolysis at atmospheric pressure and 1100°C are shown in figs. l-3. The ionization peaks are observed at 333.5 and 329.5 nm in CH, pyrolysis and 333.9 and 330.7 nm in CD, pyrolysis. These peaks agree well with those reported by Hudgens et al. [lo] , indicating that the signals are indeed due to CH; and CD; radicals, respectively. The 333.5 nm in CHj and 333.9 nm in CD; are assigned as two-photon resonance of the 3p 2As + 2P 2Ai (0:) transition
PHYSICS
1 March 1985
LETTERS
Laser Wavelength Fig. 2. The resonance
CH,
ionization at 1100°C.
pyrolysis
(nm)
spectrum
near 329.5
nm in
whereas the 329.5 nm in CH; and 330.7 nm in CD; are the corresponding hot band (2;) transitions [lo] . Note that the spectral bandwidth for the 0: transition is substantially narrower than that of 2; transition, and is narrower for CD; as compared to CH; for the same transtion. The utility of the present technique for in situ measurements of CH; radical concentration profiles in hydrocarbon pyrolysis is illustrated by the data of
I
/
I
CDs (0:)
I
i
From CD4
From CH4
1
333.6 Laser Wavelength
Fig. 1. The resonance ionization CH, pyrolysis at 1100°C.
280
(nm)
spectrum
_/I
8’ 333.8
I
Laser Wavelength
near 333.5
nm in
Fig. 3. The resonance ionization CD, pyrolysis at 1100°C.
L
1
z
334.2
334.0 (nm)
spectrum
near 333.9 nm in
Volume
114, number
CHEMICAL
3
PHYSICS
LETTERS
1 March
1985
ground at 333.30 nm has a relatively long induction period followed by a rapid increase beyond ~600 ms. These background signals may be attributed to large molecular species (such as tar) and soot particles which are produced at longer reaction times. The contribution from soot particles may be differentiated from the large molecular species using the difference in arrival times of the ion signals as has been demonstrated by Smith and Mallard [ 121.
Acknowledgement The mental Bomse, Woodin
ll
author wishes to thank K. Putman for experiassistance with these measurements, and D.S. D.M. Cox, A.M. Dean, R.L. Whetten and R.L. for helpful discussions.
References
0
Reaction
Time (ms)
Fig. 4. Ionization signals as a function of reaction time in CH, pyrolysis for a CH4/Ar/He ratio of 2.6j65132.4 at atmospheric pressure and 1100°C: (0) methyl radical signal at resonance maximum of 333.53, (m) off-resonance background (tar and soot) at 333.30 nm.
fig. 4 for a flow ratio of CH,/He/Ar = 2.6132.4165 at various reaction times. The reaction time is changed by varying the flows while keeping the same flow ratio. The signal is taken as the difference between those on resonance at 333.53 nm and off-resonance at 333.30 nm. The trend of rapid rise followed by a slower increase in the CH; concentration profile is consistent with kinetic modeling calculations [ 1l] . The CHj radical concentration is estimated to be several ppm based on modeling calculations, suggesting that the detection sensitivity is at the ppm level. We have also observed a broadcontinuum background. As shown in fig. 4, the off-resonance back-
[II G. Herzberg, Proc. Roy. Sot. A262 (1961) 291. (21 K. Glinzer, M. Quack and J. Troe, 16th International Symposium on Combustion (Combustion Institute, Pittsburgh, 1977) p. 949. [31 T. Tsuboi, Japan J. Appl. Phys. 17 (1978) 709. 141 0. Kondo, K. Saito and I. Murakani, Bull. Chem. Sot. Japan 53 (1980) 2133. [51 T.G. DiCiuseppe, J.W. Hudgens and M.C. Lin, Chem. Phys. Letters 82 (1981) 267. [61 J. Danon, H. Zacharias, H. Rottke and K.H. Welge, J. Chem. Phys. 76 (1982) 2399. [71 T.G. DiGiuseppe, J.W. Hudgens and M.C. Lin, J. Phys. Chem. 86 (1982) 36. [81 T.G. DiGiuseppe, J.W. Hudgens and M.C. Lin, J. Chem. Phys. 76 (1982) 3337. [91 B.H. Rockney and E.R. Grant, J. Chem. Phys. 77 (1982) 4257. IlO1 J.W. Hudgens, T.G. DiGiuseppe and M.C. Lin, J. Chem. Phys. 79 (1983) 571. (1984). [Ill A.M. Dean, private communication iI21 K.C. Smith and W.G. Mallard, Comb. Sci. Tech. 26 (1981) 35.
281