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Kinetics of the Reaction of CHO Radicals With OH Studied Over the 292 – 526 K Temperature Range Chao Yan, Stefani Kocevska, and Lev N. Krasnoperov J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b04213 • Publication Date (Web): 11 Jul 2016 Downloaded from http://pubs.acs.org on July 14, 2016
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Kinetics of the Reaction of CH3O2 Radicals with OH Studied over the 292 – 526 K Temperature Range
Chao Yan, Stefani Kocevska and Lev N. Krasnoperov* Department of Chemistry and Environmental Science New Jersey Institute of Technology University Heights, Newark, NJ 07102
Running title:
Reaction CH3O2 + OH
Journal sub-section: Laser Chemistry and Chemical Kinetics
*
Author to whom correspondence should be addressed:
Lev N. Krasnoperov Department of Chemistry and Environmental Science New Jersey Institute of Technology, University Heights, Newark, NJ 07102 U.S.A. Tel:
(973)-596-3592
E-mail:
lev.n.krasnoperov@ njit.edu
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Abstract Reaction of methyl peroxy radicals with hydroxyl radicals, CH3O2 + OH CH3O + HO2 (1a) and CH3O2 + OH CH2OO + H2O (1b) was studied using pulsed laser photolysis coupled to transient UV-vis absorption spectroscopy over the 292 – 526 K temperature range and pressure 1 bar (bath gas He). Hydroxyl radicals were generated in the reaction of electronically excited oxygen atoms O(1D), produced in the photolysis of N2O at 193.3 nm, with H2O. Methyl peroxy radicals were generated in the reaction of methyl radicals, CH3, produced in the photolysis of acetone at 193.3 nm, and subsequent reaction of CH3 with O2. Temporal profiles of OH were monitored via transient absorption of light from a DC discharge H2O/Ar low-pressure resonance lamp at ca. 308 nm. The absolute intensity of the photolysis light was determined by accurate in situ actinometry based on the ozone formation in the presence of molecular oxygen. The overall rate constant of the reaction k1a+1b = (8.4 ± 1.7)x10-11(T/298 K)-0.81 cm3molecule-1s-1 (292 – 526 K). The branching ratio of channel 1b at 298 K is less than 5%.
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Introduction Methyl peroxy radical, CH3O2, plays an important role as a reaction intermediate in the low-temperature combustion as well as atmospheric oxidation of hydrocarbons.1 Peroxy radicals are generated in the reactions of free radicals (mainly hydroxyl radicals) with hydrocarbons in the presence of molecular oxygen.1 Hydroxyl radicals then might be regenerated in the further reactions of peroxy radicals in a chain reaction.1 In the atmosphere, radical species initially produced photochemically from carbonyl compounds, hydroperoxides, etc., are converted to peroxy radicals in subsequent reactions with molecular oxygen.1-2 In the atmosphere, the reaction of peroxy radicals with nitric oxide (NO) plays an important role, since it leads to the production of ozone by further reactions. The photostationary state between NO, NO2 and O3 is sustained in the sunlit atmosphere by a reaction cycle which involves NO2, O, O2, NO and O3.1-3 Oxidation of NO to NO2 mainly occurs in the reaction of NO with ozone. Peroxy radicals (such as CH3O2) provide additional channels for oxidation of NO to NO2.1 When the abundance of NOx is low (such as in the remote atmosphere and lowtemperature combustion), the self reactions of peroxy radicals as well as the cross reactions with other peroxy radicals are the predominate pathways for removing peroxy radicals.1-2, 4 One of the potential additional pathways for the consumption of CH3O2 radicals could be the reaction with hydroxyl radical, OH (reaction 1).5-8 Only very limited studies have been performed on this reaction. Recently reaction 1 was studied theoretically.6 Among the exothermic channels only two were found to be of importance at near ambient conditions, channels 1a and 1b:
CH3O2
+
OH
CH3O
+
HO2
(1a)
3
+
H2O
(1b)
CH2OO
Channel 1a is barrierless; according to the calculations6 it is exothermic by 16.8 kJ mol-1. Channel 1b has a barrier of 30 kJ mol-1.6 The three other channels suggested in the literature5-7
CH3O2
+
OH
CH3OH
+
O2 (1∆g, 3Σg+) (1c)
1
+
H2O
CH2OO
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have high activation barriers (193 and 87 kJ mol-1 for 1c, singlet and triplet oxygen molecule respectively, and 188 kJ mol-1 for channel 1d).6 There are two more exothermic channels which also have very large activation barriers.6 Both potentially important channels 1a and 1b are the chain propagation steps forming free radical species. Channel 1a produces HO2 and CH3O radicals which participate in the consumption of CH3O2 as well as other reactions.9 The Criegee intermediate CH2OO, formed in channel 1b, is relatively active in the troposphere10 where these species undergo further reactions to produce secondary organic aerosols and phytotoxic compounds.11 There is only very limited information on the kinetics and the branching ratios in reaction 1. Tsang and Hampson7 estimated the rate constant of reaction 1 based on the analogy with the reaction of HO2 with OH radicals. Channel 1c was suggested as the major channel of reaction 1. They recommended a rate constant of k1 = 1.0×10-10 cm3 molecule-1 s-1 with an estimated uncertainty of a factor of 5.7 In the only recent experimental direct study of reaction 1, Bossolasco et al.8 employed laser induced fluorescence and cw-cavity ring down spectroscopy coupled to laser photolysis to study reaction 1 at 294 K at pressures 50 and 100 Torr in helium as buffer gas. Methyl peroxy radicals, CH3O2, were generated in the photolysis of CH3I at 248 nm in the presence of O2; OH radicals were generated either in the reaction of O(1D) atoms formed in the photolysis of ozone at 248 nm with H2O or by photodissociation of H2O2 at 248 nm. Methyl peroxy radicals were monitored by time-resolved continuous-wave cavity ring-down spectroscopy at the v12-transition of the A ← X band at 7489.16 cm-1. OH radicals were detected using high repetition rate (10 kHz) laser induced fluorescence (LIF). The absorption cross section of CH3O2 radicals in the near-infrared was obtained indirectly in separate experiments based on the literature rate constant of the self-reaction of CH3O2 radicals.12 Subsequently, these cross-sections obtained based on the
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decay rates of CH3O212 were used to assess the concentrations of CH3O2 in the kinetic measurements of reaction 1.8 The rate constant of reaction 1 was obtained based on the OH temporal profiles. The reported rate constant is k1 = (2.8 ± 1.4)x10-10 cm3 molecule-1 s-1 at 294 K and 50 and 100 Torr pressures at He as a bath gas. Based on this relatively large rate constant of reaction 1, the conclusion of the importance of reaction 1 in atmospheric chemistry was derived.13 In this work, the rate constant of reaction 1 was measured over the temperature range 292 – 526 K at pressure 1 bar. In addition, the branching ratio of channel 1b at ambient temperature was evaluated.
Experimental Section Experimental set-up. The experimental set-up is described in detail elsewhere.14-18 therefore only a brief description critical for the current experiments is given here. The technique of the excimer laser photolysis coupled to UV-Vis transient absorption spectroscopy is used. The measurements were performed over the 19 – 253 °C (292 – 526 K) temperature range and ambient pressure (1.02 ± 0.01) bar in He as a bath gas. The heatable high-pressure flow reactor is described in Ref.14, 19 Pre-heated reactants entered the reactor in the middle and exited via two outlets. The reactants zone was defined by two unsealed internal windows. The photolysis beam formed by two lenses has uniformity of ±7%. More details of the experimental arrangements and the signal accumulation are described elsewhere.14-18 Gas mixtures were slowly pumped through the reactor. The gas flow rates were controlled by high pressure mass flow controllers (Brooks, model 5850) which were periodically calibrated using the soap film method. The total flow rates of the reactant mixtures with helium were in the range 340-4100 sccm (standard cubic centimeters per minute). Additional flush flows to the cold reactor windows were in the range 270 - 600 sccm.
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The mole fraction of acetone in water was varied from 0.001 to 0.03. Acetone - water mixtures were injected into the evaporator of the flow system using a precision syringe pump (Harvard Apparatus, Model PHD 4400) through a stainless steel capillary tube. The temperature of the evaporator was kept at 90 °C; it was observed that this approach produces steady and stable flows of acetone-water gas mixtures. Gas mixtures were supplied to the reactor through a heated transfer line (heated up to ca. 140 °C). This was necessary to provide high concentration of water at elevated temperatures. The flow rate of acetone-water mixtures was varied in the range of 0.5 – 30 µL/min depending on the reactor temperature. The concentrations of the precursors used: (0.47 – 6.90)x1016 molecule cm-3 for N2O, (1.00 – 9.97)x1017 for O2, (1.37 – 3.80)x1015 for (CH3)2CO and (0.85 – 12.8)x1017 for H2O. The laser photon fluence inside the reactor was varied in the range (5.2 – 10.5)x1015 photon cm-2 pulse-1. The initial concentrations of radicals were in the range is (0.43–1.94)x1014 molecule cm-3 for CH3 radicals, (4.85 – 109.1)×1012 for OH radicals and (4.01 – 55.14)×1012 for O(1D) atoms. The measurements were performed at ambient pressure (1 bar He) over the 292 – 526 K temperature range. No noticeable depletion of the radical precursors (acetone and nitrous oxide) was observed at and below 526 K. At higher temperatures, depletion of the precursors was observed. Separately, these compounds are stable up to the highest temperature of the reactor of 834 K.15 The repetition rate of the laser was set to 1 Hz to ensure complete replacement of the gas mixture in the reactor between the pulses. In situ actinometry. Reliable determination of the absolute concentrations of free radicals is one of the most important requirements in the studying of radical-radical reactions. This involves in situ measurements of the photon fluence inside the reactor. The photon fluence was determined using the in situ actinometry based on the measurements of the ozone formed in the
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photolysis of N2O/O2/N2 mixtures. Details of this technique as well as the accounting for the laser energy drift in the course of measurements are described in the previous works.17-18 Reagents. In the experiments BIP®Helium from Airgas with 99.9999% purity with reduced oxygen content ( HO2 (+Ar) in the Range 1-900 bar and 300-700 K. Phys. Chem. Chem. Phys. 2004, 6, 1997-1999. 32. Johnson, D.; Krasnoperov, L. N.; Raoult, S.; Lesclaux, R. UV Absorption Spectra of Methyl-Substituted Hydroxy-Cyclohexadienyl Radicals in the Gas Phase. J. Photochem. Photobiol., A 2005, 176, 98-106. 33. SCIENTIST, Micromath Scientific Software. Saint Louis. MO. 1995. 34. Sheps, L. Absolute Ultraviolet Absorption Spectrum of a Criegee Intermediate CH2OO. J. Phys. Chem. Lett. 2013, 4, 4201-4205. 35. Buras, Z. J.; Elsamra, R. M. I.; Green, W. H. Direct Determination of the Simplest Criegee Intermediate (CH2OO) Self Reaction Rate. J. Phys. Chem. Lett. 2014, 5, 2224-2228. 36. Fernandes, R. X.; Luther, K.; Troe, J. Falloff Curves for the Reaction CH3 + O2 (+ M) → CH3O2 (+ M) in the Pressure Range 2−1000 Bar and the Temperature Range 300−700 K. J. Phys. Chem. A 2006, 110, 4442-4449. 37. Vranckx, S.; Peeters, J.; Carl, S. Kinetics of O(1D) + H2O and O(1D) + H2: Absolute Rate Coefficients and O(3P) Yields between 227 and 453 K. Phys. Chem. Chem. Phys. 2010, 12, 92139221.
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36 38. Atkinson, R.; Baulch, D. L.; Cox, R. A.; Crowley, J. N.; Hampson, R. F.; Hynes, R. G.; Jenkin, M. E.; Rossi, M. J.; Troe, J. Evaluated Kinetic and Photochemical Data for Atmospheric Chemistry: Volume I - Gas Phase Reactions of Ox, HOx, NOx and SOx Species. Atmos. Chem. Phys. 2004, 4, 1461-1738. 39. Carl, S. A. A Highly Sensitive Method for Time-Resolved Detection of O(1D) Applied to Precise Determination of Absolute O(1D) Reaction Rate Constants and O(3P) Yields. Phys. Chem. Chem. Phys. 2005, 7, 4051-4053. 40. Nishida, S.; Takahashi, K.; Matsumi, Y.; Taniguchi, N.; Hayashida, S. Formation of O(3P) Atoms in the Photolysis of N2O at 193 nm and O(3P) + N2O Product Channel in the Reaction of O(1D) + N2O. J. Phys. Chem. A 2004, 108, 2451-2456. 41. Dunlea, E. J.; Ravishankara, A. R. Kinetic Studies of the Reactions of O(1D) with Several Atmospheric Molecules. Phys. Chem. Chem. Phys. 2004, 6, 2152-2161. 42. Bahng, M.-K.; Macdonald, R. G. Determination of the Rate Constant for the OH(X2Π) + 2 OH(X Π) → O(3P) + H2O Reaction over the Temperature Range 293-373 K. J. Phys. Chem. A 2007, 111, 3850-3861. 43. DeMore, W. B.; Sander, S. P.; Golden, D. M.; Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R.; Kolb, C. E.; Molina, M. J. Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling. JPL Publication 97-4 1997, Evaluation Number 12. 44. Baulch, D. L.; Cobos, C. J.; Cox, R. A.; Frank, P.; Hayman, G.; Just, T.; Kerr, J. A.; Murrells, T.; Pilling, M. J.; Troe, J. et al. Evaluated Kinetic Data for Combusion Modelling. Supplement I. J. Phys. Chem. Ref. Data 1994, 23, 847-1033. 45. Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, J. R. F.; Kerr, J. A.; Rossi, M. J.; Troe, J. Evaluated Kinetic and Photochemical Data for Atmospheric Chemistry: Supplement VI. IUPAC Subcommittee on Gas Kinetic Data Evaluation for Atmospheric Chemistry. J. Phys. Chem. Ref. Data 1997, 26, 1329-1499. 46. Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, R. F., Jr.; Kerr, J. A.; Troe, J. Evaluated Kinetic and Photochemical Data for Atmospheric Chemistry. Supplement IV. IUPAC Subcommittee on Gas Kinetic Data Evaluation for Atmospheric Chemistry. J. Phys. Chem. Ref. Data 1992, 21, 1125-1568. 47. Hippler, H.; Rahn, R.; Troe, J. Temperature and Pressure Dependence of Ozone Formation Rates in the Range 1--1000 bar and 90--370 K. J. Chem. Phys. 1990, 93, 6560-6569. 48. Manion, J. A.; Huie, R. E.; Levin, R. D.; Jr., D. R. B.; Orkin, V. L.; Tsang, W.; McGivern, W. S.; Hudgens, J. W.; Knyazev, V. D.; Atkinson, D. B. et al. NIST Standard Reference Database 17, Version 7.0 (Web Version), Release 1.6.8; National Institute of Standards and Technology, Gaithersburg, Maryland, 20899-8320: 2015. 49. Tables, A. T. Active Thermochemical Tables Version 1.118; U.S. Department of Energy: 2015. 50. Linstrom, P. J.; Mallard, W. G.; Eds. NIST Chemistry WebBook; National Institute of Standards and Technology, Gaithersburg MD, 20899: 2016. 51. Yamada, T.; Taylor, P. H.; Goumri, A.; Marshall, P. The Reaction of OH with Acetone and Acetone-d[sub 6] from 298 to 832 K: Rate Coefficients and Mechanism. J. Chem. Phys. 2003, 119, 10600-10606. 52. Keyser, L. F. Kinetics of the Reaction Hydroxyl + Hydroperoxo → Water + Oxygen from 254 to 382 K. J. Phys. Chem. 1988, 92, 1193-1200. 53. Davies, J. W.; Green, N. J. B.; Pilling, M. J. Association Reaction of CH3 and NO: Evidence for the Involvement of the Triplet Surface. J. Chem. Soc., Faraday Trans. 1991, 87, 2317-2324. 54. Fulle, D.; Hamann, H. F.; Hippler, H.; Troe, J. Temperature and Pressure Dependence of the Addition Reactions of HO to NO and to NO2. IV. Saturated Laser-Induced Fluorescence Measurements up to 1400 bar. J. Chem. Phys. 1998, 108, 5391-5397.
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37 55. Zellner, R.; Erler, K.; Field, D. Kinetics of the Recombination Reaction OH + H + M → H2O + M at Low Temperatures. Symp. (Int.) Combust., [Proc.] 1977, 16, 939-948. 56. Jasper, A. W.; Klippenstein, S. J.; Harding, L. B. Theoretical Rate Coefficients for the Reaction of Methyl Radical with Hydroperoxyl Radical and for Methylhydroperoxide Decomposition. P. Combust. Inst. 2009, 32, 279-286. 57. Hassinen, E.; Koskikallio, J. Flash Photolysis of Methyl Acetate in Gas Phase. Products and Rate Constants of Reactions between Methyl, Methoxy and Acetyl Radicals. Acta Chem. Scand. Ser. A 1979, 33, 625-630. 58. Orlando, J. J.; Tyndall, G. S.; Wallington, T. J. The Atmospheric Chemistry of Alkoxy Radicals. Chem. Rev. 2003, 103, 4657-4690. 59. Pilling, M. J.; Smith, M. J. C. A Laser Flash Photolysis Study of the Reaction Methyl + Molecular Oxygen .Fwdarw. Methylperoxy (CH3O2) at 298 K. J. Phys. Chem. 1985, 89, 47134720. 60. Zellner, R.; Hartmann, D.; Karthauser, J.; Rhasa, D.; Weibring, G. A Laser Photolysis/LIF Study of the Reactions of O(3P) Atoms with CH3 and CH3O2 Radicals. J. Chem. Soc. Farad. 2 1988, 84, 549-568. 61. Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, R. F.; Kerr, J. A.; Troe, J. Evaluated Kinetic and Photochemical Data for Atmospheric Chemistry: Supplement III. J. Phys. Chem. Ref. Data 1989, 18, 881-1097. 62. Jiménez, E.; Gierczak, T.; Stark, H.; Burkholder, J. B.; Ravishankara, A. R. Reaction of OH with HO2NO2 (Peroxynitric Acid): Rate Coefficients between 218 and 335 K and Product Yields at 298 K. J. Phys. Chem. A 2004, 108, 1139-1149. 63. Bardwell, M. W.; Bacak, A.; Teresa Raventos, M.; Percival, C. J.; Sanchez-Reyna, G.; Shallcross, D. E. Kinetics of the HO2 + NO Reaction: A Temperature and Pressure Dependence Study Using Chemical Ionisation Mass Spectrometry. Phys. Chem. Chem. Phys. 2003, 5, 23812385. 64. Baulch, D. L.; Cobos, C. J.; Cox, R. A.; Esser, C.; Frank, P.; Just, T.; Kerr, J. A.; Pilling, M. J.; Troe, J.; Walker, R. W. et al. Evaluated Kinetic Data for Combustion Modelling. J. Phys. Chem. Ref. Data 1992, 21, 411-429. 65. Zellner, R. Recent Advances in Free Radical Kinetics of Oxygenated Hydrocarbon Radicals. J. Chim. Phys. 1987,84, 403-407. 66. Dobe, S.; Berces, T.; Szilagyi, I. Kinetics of the Reaction between Methoxyl Radicals and Hydrogen Atoms. J. Chem. Soc., Faraday Trans. 1991, 87, 2331-2336. 67. Ting, W. L.; Chen, Y. H.; Chao, W.; Smith, M. C.; Lin, J. J. The UV Absorption Spectrum of the Simplest Criegee Intermediate CH2OO. Phys Chem Chem Phys 2014, 16, 1043810443.
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TOC graphics / cm3molecule-1s-1
5E-10
kOH+CH3O2
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4E-10
OH + CH3O2 −−> HO2 + CH3O
3E-10
(1a)
−−> H2O + CH2OO
2E-10
(1b)
Bossolasco et al., 2014
1E-10 9E-11 8E-11 7E-11 6E-11 5E-11
This work
4E-11 3E-11
-11
kOH+CH3O2 = (8.4 ± 1.7)x10 (T/298 K)
-0.81
2E-11
250
300
350
400
T
/
450
500
550
K
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