Strategic Applications of Electrochemistry in Ammonia Oxidation and Alkyl Halide Reduction

Citation

Zott, Michael David (2023) Strategic Applications of Electrochemistry in Ammonia Oxidation and Alkyl Halide Reduction. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/4fr8-7r78. https://resolver.caltech.edu/CaltechTHESIS:04282023-205948012

Abstract

This thesis describes the strategic application of electrochemistry in the development of catalytic systems for two challenging processes: alkyl halide reduction and ammonia oxidation. In the case of alkyl halide reduction, the ability to precisely tune electrochemical potential favored the use of electrochemistry as compared to chemical reagents. By contrast, for ammonia oxidation, electrochemistry was specifically targeted due to motivations in the eventual development of ammonia fuel cell technology. The first chapter introduces these and other advantages of electrochemistry, as well as details regarding the thermodynamic potentials and kinetic barriers associated with alkyl halide reduction or ammonia oxidation. The second chapter details our development of photoelectrochemical methodology to employ a strongly luminescent dicopper system for outer-sphere, single-electron transfer reduction of benzyl chlorides. The third chapter marks the beginning of our work in molecular iron-mediated ammonia oxidation catalysis, in which we develop our hypothesis that catalyst structures featuring cis-labile coordination sites should mediate ammonia oxidation. We disclose the first iron electrocatalyst ([(TPA)Fe(MeCN)₂]²⁺) as well as a framework for the analysis of metrics such as overpotential, catalytic rate, and catalyst stability. The fourth chapter introduces a hypothesis for catalyst improvement—favoring low-spin electronic structures—and a model system for testing: ([(BPM)Fe(MeCN)₂]²⁺). Using this second-generation catalyst, improved stability, enhanced activity, and lowered overpotential were observed. The fifth chapter explores the validity of the cis-labile and low-spin hypotheses via Hammett-type substituent studies on both the [(TPA)Fe(MeCN)₂]²⁺ and the [(BPM)Fe(MeCN)₂]²⁺ platforms. This study resulted in the development of a further enhanced molecular electrocatalyst for ammonia oxidation and revealed mechanistic information pertinent to the development of future catalytic systems.

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