Investigations of ruthenium transfer hydrogenation catalysts as electrooxidation catalysts for alcohols [electronic resource]

One of the premier challenges in energy research is the development of efficient methods to convert chemical fuels into useful forms of energy with reduced greenhouse gas emissions. The reversible electrodehydrogenation of liquid fuels, such as alcohols, would provide a means for the efficient release of energy and energy storage, if electrocatalysts that operate at the appropriate potentials can be developed. Currently, even the most highly optimized alcohol electrooxidation catalysts suffer from kinetic limitations that require high overpotentials to achieve reasonable rates despite significant effort in the design of heterogeneous methanol and isopropanol electrooxidation catalysts. With the perspective that the fundamental gaps in understanding of the energetic requirements of critical chemical steps for reversible electrodehydrogenation are the major impediments to further progress, mechanistic examination of the relevant chemical and electrochemical events of molecular complexes may illuminate the properties of a catalyst required to effect energy efficient conversion of chemical fuels. A new conceptual approach to the discovery of catalysts is proposed: The approximate thermoneutrality of chemical transfer hydrogenation catalysis implicates their catalysts as viable candidates for reversible electrodehdyrogenation catalysts. In transfer hydrogenation reactions, alcohols are oxidized with zero driving force, the thermodynamic equivalent of an electrochemically reversible alcohol oxidation at the thermodynamic potential of the alcohol. Ruthenium arene transfer hydrogenation catalysts are investigated for the electrooxidation of alcohols. They are shown to serve as active electrocatalytic precursors and exhibit rapid rates for alcohol oxidation when supported on electrodes in basic aqueous solution. The first examination of organometallic reactions and of species on electrode surfaces via desorption ionization electrospray mass spectrometry are performed. These studies provide support for the proposed electrocatalytic mechanism for alcohol oxidation by the Ru-arene complexes. Additionally, Ru transfer hydrogenation catalysts are shown to reversibly and catalytically oxidize methanol to methyl formate. Mechanistic studies of octahedral ruthenium transfer hydrogenation catalysts are carried out to gain insight into the types of homogeneous reaction mechanisms are amenable to translate into catalytic electrochemical mechanisms. The octahedral complexes are studied extensively by cyclic voltammetry; however, preparative electrolysis results in the presence of alcohols did not provide clear evidence for the electrooxidation of methanol or isopropanol. Possibilities for the absence of oxidation products are discussed as well as the challenges in developing molecular electrocatalysts, particularly in translating catalytic homogeneous two electron chemistry to heterogeneous one electron electrochemistry.

Keywords: Chemistry
Advisor: Waymouth Robert M. primary advisor. Thesis advisor
Huestis Wray advisor. Thesis advisor
Stack T. (T. Daniel P.)