Pressure Dependent Kinetics of the Reaction between CH3O2 and

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A: Kinetics, Dynamics, Photochemistry, and Excited States

Pressure Dependent Kinetics of the Reaction between CHO and OH: Triox Formation 3

2

Chao Yan, and Lev Krasnoperov J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b03861 • Publication Date (Web): 28 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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The Journal of Physical Chemistry

Pressure Dependent Kinetics of the Reaction between CH3O2 and OH: TRIOX Formation Chao Yan1 and Lev N. Krasnoperov2* 1 Mechanical

Aerospace Engineering Department, Princeton University, New Jersey, USA, 08540

2

Department of Chemistry and Environmental Science New Jersey Institute of Technology University Heights, Newark, NJ 07102

*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 1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract

Reaction of methyl peroxy radicals with hydroxyl radicals (1) was studied using pulsed laser photolysis coupled to transient UV-vis absorption spectroscopy at 296 K over the 1 – 100 bar pressure range (bath gas He): CH3O2 + OH  CH3O + HO2 (1a), CH3O2 + OH  CH2OO + H2O (1b) and CH3O2 + OH  CH3OOOH (TRIOX) (1d). Channel 1a is the dominant channel under ambient conditions, while the contribution of channel 1b is less than 5 % at 1 bar and 296 K. Channel 1c is strongly pressure dependent and becomes dominant at high pressures. The measured branching ratio of channel 1c is α1c = 0.87 ± 0.20 at 100 bar and 296 K. The chain termination channel 1c forming important product TRIOX is experimentally evaluated over an extended pressure range (1 - 100 bar) for the first time. The stabilization channel 1c might play a role at ambient pressures and low temperatures as well as high pressures at ambient and elevated temperatures.

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Introduction Alkylperoxy radicals, RO2, play important roles as reaction intermediates in the low-temperature combustion as well as pre-ignition period of combustion.1 Reactions of hydroxyl radicals, OH, with RO2 are considered to be important in combustion systems due to the relatively large reaction rate constant.2 These reactions might compete with unimolecular isomerization and decomposition reactions

of

RO2,

which

form

hydroperoxyalkyl

(QOOH)

and

hydroperoxyalkylperoxy (O2QOOH) radicals, which in turn, play important roles in the low temperature oxidation of hydrocarbons. According to the recent studies, the reaction of CH3O2 radicals with hydroxyl radical, OH (reaction 1) might be important in the atmospheric chemistry.3-6 Until recently, limited data on the rate constants and the branching ratios have been reported, especially over elevated temperature and extended pressure ranges. In the previous experimental study, Yan et al.2 measured the rate constant and an upper limit on the branching ratio of the Crigee intermediate formation channel 1b of the target reaction using excimer laser pulsed photolysis coupled with transient UV absorption spectroscopy.2 The experiments were performed at pressure 1 bar over the 292 - 526 K temperature range. A relatively large overall rate constant was reported (k1 = (8.4 ± 1.7) × 10-11(T / 298 K)-0.81 cm3 molecule-1 s-1). The reported major channel is channel 1a, while the minor channel is 1b.2 The branching ratio of channel 1b is less than 5%.2 3 ACS Paragon Plus Environment


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CH3O2

+

OH



CH3O

+

HO2

(1a)

CH3O2

+

OH



CH2OO

+

H2O

(1b)

Bossolasco et al.5 studied the target reaction using laser induced fluorescence and cw-cavity ring down spectroscopy coupled to pulsed laser photolysis. The experiments were performed at 294 K and pressures 50 and 100 Torr in helium. Very large rate constant was reported k1 = (2.8 ± 1.4) × 10-10 cm3 molecule-1 s-1. Hydroxyl radicals, OH, were generated either by direct photodissociation of H2O2 at 248 nm or in the photolysis of ozone in the presence of H2O. CH3I was used as the precursor to provide methyl radical, CH3, at 248 nm, which react with excess O2 molecules to produce methyl peroxy radicals, CH3O2. The rate constant of reaction 1 was obtained based on the OH temporal profiles. Based on this very large rate constant, the conclusion of the importance of reaction 1 in atmospheric chemistry was derived.7 Two years later, the same group (Assaf et al.8) reported a different rate constant (two times lower) of the reaction CH3O2 with OH. The same experimental set-up was used in this study. Possible source of the error was indicated as due to the copious production of excited iodine atoms, I*(2P1/2), with subsequent kinetic interferences, as suggested in Ref.2 Instead of using CH3I photolysis as a 4 ACS Paragon Plus Environment

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precursor, photolysis of XeF2 at 248 nm was used to produce F atoms in Ref. 8. Photolysis of XeF2 in the presence of CH4, O2 and H2O was used to simultaneously generate CH3O2 and OH radicals. A rate constant of k1 = (1.60 ± 0.4) × 10−10 cm3 s −1 has been determined at 295 K, which is still ca. factor of 1.8 higher than the results of Yan et al.2 In the most recent publication from the same group, the experimental data on reaction 1 were re-analyzed to result in the rate constant of reaction 1 of 1×10−10 cm3 molecule-1s −1, in agreement within the combined errors with the results of Yan et al.9 Bian et al.4 investigated the reaction pathways at the QCISD(T)/aug-cc-pVTZ// B3LYP/6–31111G(d, p) level of theory. Among all channels, channel 1a is the major channel. This channel is barrierless and exothermic by 16.8 kJ mol-1. Channel 1b might potentially play a minor role at ambient conditions. Channel 1b has a relatively small barrier of 30 kJ mol-1. The other channels of reaction 1 suggested in the literature have very large activation barriers.4-6 Most recent theoretical study was performed by Muller et al.10 Detailed potential energy surfaces were calculated using suitable high-level density functional theory (DFT) and ab initio methodologies for the singlet and triplet intermediates. The Criegee pathway (1b) is negligible at atmospheric conditions. Channel 1a is expected to dominate, whereas both methanol formation (channel 1c) and



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stabilization of the methylhydrotrioxide CH3OOOH (TRIOX) (channel 1d) are minor channels:

CH3O2

+

OH



CH3OH

CH3O2

+

OH



CH3OOOH

+

O2

(1c) (1d)

The formation of methanol occurs through the decomposition of the activated TRIOX. Channel 1c is more important at lower pressures, while less important at higher pressures due to the collisional stabilization of TRIOX.10 The theoretically estimated branching ratios reported at 298 K and 1 atm pressure are α1a = 0.82, α 1c = 0.069 and α 1d = 0.107.10 These estimates have been confirmed in the subsequent studies.11-12 As indicated in all recent theoretical studies, channel 1a is the major channel at ambient conditions. Channels 1a and 1b are the chain propagation steps forming free radical species, while channels 1c and 1d are the chain termination steps. Channel 1a produces HO2 and CH3O radicals which participate in the consumption of CH3O2 as well as other reactions.13 The Criegee intermediate CH2OO, formed in channel 1b, is relatively active in the troposphere.14 The most recent modeling study indicates that at 298 K with He as the bath gas, the yield of TRIOX increases from 0.04 to 0.86 when the pressure increases from 1 to 100 atm.15 Even through some experimental information on the kinetics of reaction 1 has been obtained at low pressures, very limited information is available at elevated 6 ACS Paragon Plus Environment

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pressures. Specifically, the yield of TRIOX in channel 1d (which has relatively deep well), is expected to be strongly pressure dependent.12 In this work, the branching ratios of reaction 1 were measured over the 1 – 100 bar pressure range at ambient temperature (296 K).

Experimental Section

Experimental set-up. The experimental set-up is described in detail elsewhere.2, 16-19

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 high-pressure flow reactor is described in previous studies.2, 16-19 The experimental measurements were performed over the 1 – 100 bar pressure range and ambient temperature in helium as a bath gas. Excimer laser (EX100) was used to initialize the radical-radical reactions (193.3 nm, ArF laser mixture). A 150 W xenon short arc lamp was used as a light source to monitor time resolved radical profiles A low-pressure radio frequency discharge mercury lamp was used to detect ozone at 253.6 nm. The concentrations of the precursors used were: (0.95 – 7.04) × 1015 molecule cm-3 for (CH3)2CO, (0.74 – 4.29) × 1017 molecule cm-3 for N2O, (5.9 – 11.8) × 1016 molecule cm-3 for O2 and (3.11 – 4.61) × 1017 molecule cm-3 for H2O. The initial radical concentrations were (0.54 – 2.41)×1014 molecule cm-3 for O(1D), (1.80 – 7 ACS Paragon Plus Environment


17.2)×1013 molecule cm-3 for CH3, (2.17 – 17.77)×1013 molecule cm-3 for CH3O2 and (0.64 – 2.14)×1014 molecule cm-3 for OH. The photon fluence of photolysis laser inside the reactor was varied in the range (2.3 – 12.7) × 1015 photon cm-2 pulse-1. In order to completely replace the gas mixture in the reactor between the laser pulses, the repetition rate of the photolysis laser was set to 0.1 Hz. The gas flow rates were controlled by high pressure mass flow controllers (Brooks, model 5850) which were periodically calibrated. The total flow rates of the reactant mixtures with helium were in the range 400-4300 standard cubic centimeters per minute (sccm). A digital precision syringe pump (Harvard Apparatus, Model PHD 4400) was used to inject the precursor solutions (acetone-water) into the evaporator through a 1/16” stainless steel capillary tube. In this way, steady and stable flows of acetone vapor are achieved. The details can be found in the previous studies.2, 16-19 All other precursors (N2O, O2) and bath gas (He) were supplied to the reactor as gases using high pressure mass flow controllers. The photon fluence inside the reactor was measured based on the in-situ actinometry. Details of this technique as well as the accounting for the laser energy drift in the course of measurements are described in the previous work.16


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The details of the radicals (OH, CH3O2) generation were discussed in the previous study:2 CH3O2 radicals were generated by excimer laser pulse photolysis of acetone in the presence of oxygen:

(CH3)2CO

CH3

+

+

h(193 nm)

O2



2CH3



+

CO

(P1a)

CH2COCH3 +

H

(P1b)



CH4

CH2CO

(P1c)



CH3O2

+

(2)

OH radicals were generated by excimer laser pulse photolysis of N2O in the presence of H2O at 193 nm:

N 2O

+

O(1D) +

h (193 nm) 

O(1D)

+

N2

(P2a)



N(4S)

+

NO(2Π)

(P2b)



2OH(v = 0, 1)

H 2O

(3)



The absorption cross sections of acetone and N2O at 193.3 nm and the quantum yields were taken from the previous studies.16-17 The absorption cross sections of H2O and O2 at 193.3 nm are taken from the previous work2 as 1.51 × 10-21 cm2molecule-1 and ca. 3 × 10-22 cm2molecule-1, respectively. The temperature and pressure dependences of the absorption cross-sections of N2O and (CH3)2CO have been characterized previously.16, 18 For these molecules the wavelength 193 nm lies on the wings of the absorption bands, and the effect of temperature and pressure on the cross-section due to the line broadening is significant. In the case of CH3O2 and HO2, where the wavelengths of interest (210 nm and 224 nm) lie close to the maxima, only minor effects of pressure are expected. The absorption cross-sections of CH3O2 and HO2 were measured at ambient temperature and four different pressures (1 bar, 10 bar, 30 bar and 100 bar. A 150 W xenon short arc lamp was used as a light source to monitor time resolved radical profiles. Mixtures of (CH3)2CO/O2/He were photolyzed at 193.3 nm using an uniform beam. The transient intensity profiles were recorded and processed according to the reaction mechanism at two different wavelengths (210 nm and 224 nm). The initial concentrations of CH3O2 were determined via the photon fluence, the absorption cross sections of (CH3)2CO16 and the fraction of CH3 radicals converted to CH3O2 radicals by reaction with O2. The excess oxygen was provided in the


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experiments in order to have almost unit conversion ratio. The photon fluence was measured in separate in situ actinometry experiments. The absorption cross sections of CH3O2 are slightly pressure dependent. The absorption cross section of CH3O2 at 224 nm at 100 bar is about 4% lower than that at 1 bar. At 210 nm the absorption cross sections of CH3O2 slightly increases

Figure 1. The pressure dependent absorption cross-sections of CH3O2 at 210 nm and 224 nm.

with pressure. Figure 1 shows the results of the measurements.

To

determine

the

absorption

cross-sections

of

HO2

gas

mixtures

(COCl)2/CH3OH/ O2/He were used. The transient intensity profiles were recorded 11 ACS Paragon Plus Environment


and processed according to the reaction mechanism. The initial concentration of HO2 were determined via the photon fluence, the absorption cross sections of (COCl)220 and the fraction of Cl atoms converted to HO2 radicals by reaction with CH3OH and O2. The excess methanol and oxygen were used to make sure that almost all Cl atoms are converted to HO2 radicals.


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The absorption cross sections of HO2 at both 210 nm and 224 nm wavelength slightly decrease with pressure. The absorption cross sections at 210 nm and 224

Figure 2. The pressure dependent absorption cross-sections of HO2 at 210 nm and 224 nm.

nm of HO2 at 100 bar are about 4% and 6 % lower than at 1 bar, respectively. These experimentally determined cross-sections were used in the data processing and the branching ratio determination. The kinetics of hydrogen peroxy radicals, HO2, were monitored by absorption in the UV at different wavelengths (210 nm and 224 nm) using 150 W (Oriel) short



arc xenon lamp combined with an imaging spectrometer (Acton 300i). The imaging spectrometer setting is 1200 groove/mm grating, 300 mm focal length, both slits 0.25 mm, triangle slit function, FWHM = 0.64 nm.

Reagents. In the experiments BIP®Helium from Airgas with 99.9999% purity with reduced oxygen content (

Pressure Dependent Kinetics of the Reaction between CH3O2 and

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