Date of Award

5-31-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Chemistry - (Ph.D.)

Department

Chemistry and Environmental Science

First Advisor

Lev N. Krasnoperov

Second Advisor

Joseph W. Bozzelli

Third Advisor

Alexei Khalizov

Fourth Advisor

Yong Yan

Fifth Advisor

Lei Zhu

Abstract

Combustion mechanisms consist of hundreds elementary reactions of free radicals and stable molecules. Radical-radical elementary reactions play important roles due to the high concentration in which free radicals are accumulated in combustion systems. Radical-radical reactions are typically multi-channel. Some of the channels might be of chain propagation or even chain branching nature, while other channels might be of chain termination nature. The relative importance of different channels is pressure dependent. Compared to radical-molecule reactions, radical-radical reactions are much less studied. This is due to the difficulties of well characterized quantitative production of radical species as well as due to the multi-channel nature of the majority of such reactions. This study is focused on the kinetics and mechanism of several elementary radical-radical reactions of combustion importance.

The kinetics of several free radical-radical reactions was studied using pulsed laser photolysis coupled to transient UV-vis absorption spectroscopy over the 292 – 714 K temperature range and the 1- 100 bar pressure range. Free radicals such as CH3, OH and Cl are generated by photodissociation of parent molecules; some other radicals such as CH3O2 and HO2 are generated in secondary reactions following the initial photodissociation. Different free radicals are monitored at different wavelengths depending on their UV absorption spectra. Quantitative measurements of the concentrations of free radicals relay upon the absolute intensity of photolysis laser light determined by well characterized in situ actinometry. Experimental temporal absorption profiles are fitted by numerical solutions of a system of differential equations (ODE) which correspond to the reaction mechanism.

The reactions studied are CH3 + CH3 ? Products (kCH3+CH3), CH3O2 + OH ? Products (kCH3O2+OH), CH3 + Cl ? CH3Cl (kCH3+Cl) and CH3 + HO2 ? Products (kCH3+HO2). The reaction of recombination of CH3 radicals is studied over the 1 – 100 bar pressure range and the 292 – 714 K temperature range. The rate constant is determined as kCH3+CH3, 8 = (5.66 ± 0.43) × 10-11 (T / 298 K)-0.37 cm3 molecule-1 s-1. The overall rate constant of CH3O2 with OH is kCH3O2+OH = (8.4 ± 0.43) × 10-11 (T / 298 K)-0.81 cm3 molecule-1 s-1 at pressure 1 bar and 292 – 526 K temperature range. The branching ratios for three major channels were measured at 298 K over the 1 – 100 bar pressure range. High-pressure limit rate constants for reaction of CH3 radicals with Cl atoms are measured over extended temperature (296 – 558 K) and pressure (1 – 100 bar) ranges. The pressure falloff was characterized by combining the high pressure data with limited literature low pressure data for reaction CH3 + Cl. The reaction CH3 + HO2 is studied over the 292 – 558 K temperature range and a single pressure (1 bar). The rate constant is kCH3+HO2 = (5.10 ± 0.95) × 10-11 (T / 298 K)-1.01 cm3 molecule-1 s-1.

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