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Title: Development of an apparatus for time resolved measurement of OH and HO₂, with application to atmospherically relevant reactions
Author: Speak, Thomas Henry
ISNI:       0000 0004 8506 0554
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2019
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Abstract:
The primary focus of this work has been to develop and characterize an instrument for the time resolved (millisecond timescale) measurement of OH and HO2, to monitor reactions occurring under high pressures (500 - 4000 mbar), and over a wide range of temperatures (290 - 700 K). This instrument was then used in conjunction with PTR-TOF-MS and conventional slow-flow reactors to study the OH initiated oxidation of a range of nitrogen containing compounds. Chapter 3 describes the development of this system and its characterization. Of particular importance to later work, it was noted that measuring the temperature of a flowing gas with a thermocouple could be highly inaccurate. Following this observation, chemical thermometers (e.g. the reaction of OH and methane) were instead used to assign the temperature at which reactions were occurring in the high-pressure system (and this was later extended to conventional slow-flow systems). HO2 detection was carried out indirectly via titration with NO to form OH, which was then detected by LIF. Efficient and selective titration of HO2 could only be achieved beyond the breakdown of the jet in the low-pressure detection cell, and as such, time resolution of the HO2 signal was reduced compared with the OH measurements. However, HO2 yield could be achieved with high accuracy and precision. This was validated for the reaction of OH with methanol under high (6 ×1018 cm-3) and low oxygen (< 1×1016 cm-3) conditions; where the measured yields of 87 ± 5 % at low oxygen, and 99 ± 2 % at high oxygen were in excellent agreement with the expected yields of 85 ± 8 % and 100 % from McCaulley et al..(1) In chapter 4, this apparatus was applied to study the OH initiated oxidation of n-butanol (a potential 'drop in' biofuel); where, the measurement of OH recycling, and HO2 yields allowed for the assignment of α and β branching fractions. The measured branching ratios were β = 0.24 ± 0.04 (616 - 640 K), and α= (0.57 ± 0.06) at 293 K, and (0.54 ± 0.04) at 616 K). Understanding the ratio of the α and β branching fractions for OH abstraction from n-butanol is important in modelling its ignition delay time, with α leading to the formation of HO2 a chain inhibiting reaction. Chapter 5 and 6 describe the study of the OH initiated oxidation of methyl formamide (MF), dimethyl formamide (DMF), tertiary butyl amine (tBA), and methyl propane diamine (diamine). In chapter 5, the temperature dependence of the OH oxidation of dimethyl formamide was studied, and particular emphasis was placed on the source of OH recycling that becomes prevalent above 450 K. The overall temperature dependence of OH and DMF was assigned as "k" _"OH+DMF" "=(1.317± 0.117)×" 〖"10" 〗^"-11" 〖"(" "T" /"298" ")" 〗^"-0.5" "e" ^"37.16/RT" , with an ambient temperature value of k298 K= (1.30 ± 0.12) × 10-11 cm3 molecule-1 s-1. High level computational calculations (CCSD(T)/CBS//mp2/6-311++g(3df,3pd)) were carried out to generate potential energy surfaces for the abstraction reactions and the subsequent R + O2 surfaces. The results of both the experimental and computational work carried out, indicated that the majority abstraction over all temperatures was from the aldehydic position. This is of atmospheric significance as it is a route to the formation of a nitrogen centred radical, which can lead to the formation of dimethyl nitramine and dimethyl nitrosamine. Nitramines and nitrosamines are potent carcinogenic compounds. Chapter 7 used the reaction of OH and isoprene to illustrate potential differences in the chemistry observed when the reaction was carried under different experimental conditions, where the reactions were monitored using PTR-TOF-MS. The well-defined ambient temperature rate coefficient of OH and isoprene allowed it to be used as a reference compound for a relative rate study of OH and 1,3,5- tri methyl benzene (TMB) which was carried out in EUPHORE and HIRAC giving kOH+TMB = (5.77 ± 0.05) × 10-11 cm3 molecule-1 s-1. From reactions carried out in EUPHORE, an ambient pressure quartz reactor, and the high pressure system (at 1000 mbar) the combined yield of methacrolein and methyl vinyl ketone was shown to be > 70 % in good agreement with that observed by Sprengnether et al. (72 ± 7 %).(2) In addition, from the reactions carried out in HIRAC, an ambient pressure quartz reactor, and the high pressure system the yield of m/z 83 was assigned as 3-7 % at room temperature and was identified as the product of the abstraction, not of 3-MF (3-methyl furan). The homogenous pathway to 3-MF is not well known, and at room temperature when the reaction were carried out in the high-pressure reactor, and the quartz reactor secondary OH chemistry and heterogeneous routes were minimized.
Supervisor: Seakins, P. W. ; Blitz, M. A. Sponsor: NERC
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.797998  DOI: Not available
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