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Title:Proton-coupled electron transfer reactions in bio-inspired catalysis
Author(s):Huynh, Mioy T
Director of Research:Hammes-Schiffer, Sharon
Doctoral Committee Chair(s):Hammes-Schiffer, Sharon
Doctoral Committee Member(s):Rauchfuss, Thomas B; Trinkle, Dallas R; Vura-Weis, Josh
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Proton-coupled electron transfer
solar energy conversion
hydrogen evolution
density functional theory
rate theory
kinetic isotope effects
proton transfer
electron transfer
benzimidazole phenol
photosystem II
artificial photosynthesis
catalyst design
design principles
Abstract:Nature often serves as a seminal source of inspiration for the design of catalysts, which is likely due to the efficiency of many biological systems. Bio-inspired design is especially applicable in many energy conversion processes, such as artificial photosynthesis, and in the development of alternative renewable energy technologies. Often proton-coupled electron transfer (PCET) is central to the interconversion of energy. Consequently, understanding the role of PCET in the mechanisms of bio-inspired catalysts can aid in the design of next generation catalysts and the elucidation of guiding design principles. In nature, hydrogenases are the most active catalysts for hydrogen production, with rates and overpotentials comparable to leading, synthetic Pt catalysts while utilizing earth abundant metal centers in their active sites. Density functional theory calculations, in conjunction with complementary experiments, were conducted to understand the underlying physical principles of hydrogenase-based models. Our collective studies on hydrogenase-based models have revealed two emerging themes: (1) one metal center is acid-base active while the adjacent metal center is redox active, and (2) the importance of considering thermodynamically less stable but active intermediates. Nature also often relies on mediators to couple electrons and protons between various cycles in catalysis. A prime example of the importance of electron-proton transfer mediators is photosynthesis, where the slow water splitting reaction and the fast photoinitation of a reaction center are mediated by a Tyr-His redox pair in Photosystem II. Understanding the fundamental PCET processes in Tyr-His models is helpful for the design of photoelectrochemical water splitting cells, where water oxidation provides protons and electrons for hydrogen production via a hydrogen-evolving catalyst, such as those based on hydrogenases. To understand the kinetics of this reaction, a nonadiabatic PCET rate theory is applied to predict and interpret kinetic isotope effects for Tyr-His model systems. Importantly, theory predicted new mediators capable of two proton transfers coupled to an electron transfer, and these predictions were later validated experimentally. In addition to the Tyr-His redox mediator, Photosystem II also employs a quinone derivative to mediate the transfer of electrons and protons across the membrane. This plastoquinone cycles between the oxidized quinone state and the doubly-reduced, doubly-protonated hydroquinone state (i.e., a 2 e–/2 H+ PCET process). Our analysis of fundamental redox behaviors of over one hundred quinones using linear scaling relationships, such as Hammett correlations, revealed linear correlations between 1 e– reduction potentials, pKa values, and 2 e–/2 H+ reduction potentials with an effective Hammett constant. More importantly, key deviations resulting from hydrogen-bonding, halogenated, charged, and sterically-bulky substituents were identified and analyzed. In principle, these types of deviations can be leveraged to further develop and tune quinone-based catalysts, mediators, or devices beyond the redox properties predicted by standard linear scaling relationships.
Issue Date:2017-07-13
Rights Information:Copyright 2017 Mioy T. Huynh
Date Available in IDEALS:2017-09-29
Date Deposited:2017-08

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