RATIONAL DESIGN OF BIOINSPIRED WATER OXIDATION CATALYSTS TO UNDERSTAND IMPORTANT STRUCTURAL FEATURES IN THE MECHANISM OF HOMOGENEOUS AND HETEROGENEOUS WATER OXIDATION

Abstract

A limiting technology needed for creation of solar fuels derived from renewable feedstock is the development of efficient water oxidation catalysts made from earth abundant materials for the oxygen evolving half-reaction (OER). My research efforts have focused on developing both inorganic and organometallic water oxidation catalysts using abundant 3d-transition metals and testing them in both homogeneous solution and heterogeneous interfaces. The most active system for water oxidation is found in nature, the photosystem II enzyme water-oxidizing complex (PSII-WOC). This universally conserved catalytic core of the enzyme is comprised of a cubical CaMn3O4 cluster with a fourth oxo-bridged Mn atom. The chemical principles that may govern catalysis by this inorganic core provide a blueprint for rational design of new robust, abundant catalysts.

In Chapter 1, a new organometallic water oxidizing catalyst, Co4O4(py)4(OAc)4 is described in detail. This Co4O4-cubane catalyst provides considerably higher turnover rates in homogenous aqueous solution compared to previous Mn4O4-cubane type catalysts used in heterogeneous photoelectrolysis cells, although considerably slower than the PSII-WOC. The structural importance of the M4O4 cubical subunit in the mechanism of water oxidation has been extended to cobalt systems. In Chapter 2, the catalytically inert spinel metal oxide, LiMn2O4, is converted to the isostructural &#955;-MnO2 by delithiation and found to produce an active water oxidation catalyst. The retention of Mn4O4 cubical units in this spinel structure and the more flexible lattice arising from conversion of tetrahedral O atoms to pyramidal 3-coordination are postulated as the structural features responsible for promotion to active catalyst. In Chapter 3, the Mn oxide heterogeneous catalysts are further developed in a systematic study of eight polymorphs of manganese oxides, synthesized as pure polycrystalline materials. Analysis of all eight materials, only three of which are catalytically active, reveals a correlation between activity and structure. More active polymorphs possess a greater number of longer inter-manganese separations and lower coordination number O atoms, suggesting that the presence of weak Mn-O bonds and deformable (flexible) lattice are attributes needed for catalysis. These correlations point to more reactive catalysis from sp<super>3</super>-hybridized O atoms at the corners of non-planar µ3-oxo (M3O) or M4O4 cubes, vs unreactive sp<super>2</super> hybridized planar µ2-oxos. In Chapter 4, the ligand exchange kinetics of the previously developed Mn4O4L6 cubanes are explored, revealing the ability of the L=diphenylphosphinate ligands to rapidly exchange with free ligands in solution, both other diphenylphosphinates and phenylphosphonates.

Throughout the thesis, the importance of designing and understanding catalysts for water oxidation in relation to their structural elements is emphasized. The work extends support to the mechanistic theory of water oxidation requiring a cubical subunit of M4O4 to provide both four oxidizing equivalents and a structural guide to O-O bond formation across an open face of the cube, and open coordination sites for O2 formation/release.