MECHANISTIC EVALUATION OF HYDROGEN EVOLVING CATALYSTS THROUGH ELECTROCHEMICAL AND SPECTROSCOPIC KINETIC ANALYSIS Public Deposited

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  • March 19, 2019
Creator
  • Rountree, Eric
    • Affiliation: College of Arts and Sciences, Department of Chemistry
Abstract
  • The desire to generate carbon neutral fuels from renewable energy sources prompted researchers to turn their attention to biological systems several decades ago in hopes of being able to emulate the processes that allow nature to so effectively perform these fuel generating chemical reactions. Because of its energy density and chemical simplicity, hydrogen, and thus, the hydrogenase enzymes have been the focus of many of these studies. The process began by attempting to synthesize mimics of the enzyme active sites, however, after countless structural mimics were prepared, it became clear that the surrounding enzyme was crucial to the activity. The failure of the structural mimics to perform has more recently led to the search for functional mimics; molecules that, while not similar in appearance, are designed to function as the hydrogenase active site would in the midst of the surrounding enzyme. With the focus on functional mimicry, two families of hydrogen evolution catalysts have risen to prominence; the cobaloximes, and the Ni(P_2^R N_2^(R^' ) )_2 catalysts (where P_2^R N_2^(R^' ) represents 1,5-R’-3,7-R-1,5-diaza-3,7-diphosphacyclooctane). Within this dissertation, examples of each of these catalysts are thoroughly analyzed for mechanistic understanding. In addition, study of the Ni(P_2^Ph N_2^Ph )_2 catalyst revealed that the hydride intermediate could be isolated and reacted with proton sources independently. This allowed for a unique study of what role the proton source plays in catalytic turnover and thoroughly demonstrated that the carboxylic acid based proton sources can react with the catalyst while in their dimerized state. This was shown to artificially suggest second order reactivity and significantly increase the acidity of the proton source. The intense scrutiny of these catalytic mechanisms had the natural consequence of a focus on proton-coupled electron transfers, which have the potential to eliminate the need for high energy intermediates if the proton and electron can be transferred in a single step, and can open the door to catalysts that operate at a fixed overpotential, regardless of solution pH. This work carried the mechanistic study forward to a catalyst that has electron and proton transfers sufficiently coupled to generate a catalytic Pourbaix diagram. The diagram prepared in this study has shown that under catalytic conditions, the experimental Pourbaix diagram does not necessarily depict the most thermodynamically stable species, but rather only those capable of reaching equilibrium on the electrochemical timescale. These mechanistic insights will be useful to researchers designing functional mimics of the hydrogenase enzymes.
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  • In Copyright
Advisor
  • Lockett, Matthew
  • Templeton, Joseph
  • Dempsey, Jillian
  • Wightman, R. Mark
  • Meyer, Gerald
Degree
  • Doctor of Philosophy
Degree granting institution
  • University of North Carolina at Chapel Hill Graduate School
Graduation year
  • 2017
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