Text

theses

The “holy grail” of renewable, sustainable energy is artificial replication of the only natural process capable of storing sun energy into chemical bonds: Photosynthesis. The oxidation of water to molecular O2 is the thermodynamic bottleneck to this process. As such, viable catalysts for water oxidation are warranted. These materials ideally 1) are constructed of abundant elements (e.g., first row transition metals) and thus affordable, 2) operate efficiently and effectively with little applied bias (overpotential), and 3) maintain high activity for useful lifetimes. In Chapter 1, Nature’s CaMn4O5 “heterocubane” catalyst is introduced. Principles of this structure which must translate into artificial catalysts are discussed: O-O bond formation, sacrificing oxidizing strength for long lifetimes, and effective storage of oxidizing equivalents via proton-coupled electron transfer (expanded in Chapter 5). In Chapters 2-3, this thesis addresses the reactivity of cobalt based catalysts. Crystalline and amorphous cobalt oxides are well-known oxygen evolving catalysts, but up to three different mechanisms are proposed to occur on their surfaces. While the “cubane” topology is stressed as biomimetic, these mechanisms commonly only feature a single metal active site- seemingly negating the cubane topology as necessary for catalysis. The results in these chapters- via studies on discrete Co2O2, Co3O3 and Co4O4 clusters- demonstrate that the cubane topology optimally stabilizes the Co4+ oxidation state via delocalization across all metal centers. This stored oxidizing equivalent reacts with terminally bound OH- sites and facilitates oxidation fully to O2. In Chapter 4, this thesis addresses the reactivity of manganese-based catalysts. Paradoxical observations are known: Nature’s effectiveness at utilizing Mn have predominantly translated into poor artificial Mn catalysts. While partially explained by the ~30 possible structures of Mn-oxides (many of which are minerals), promising results have correlated activity with stabilization of Mn3+, as opposed to Mn4+. The studies shown here rationalize these paradoxes by comparing structural polytypes of Mn3+, clearly demonstrating that corner-sharing, labile Mn3+ centers capable of facile water binding correlate with catalytic activity as found in both layered and tunnel Mn oxides. Conversely, Mn of any oxidation state in strongly coupled structures are effective at storing charge but not transferring it to water.

ETD_6847

Ph.D.

Includes bibliographical references

by Paul F. Smith

doi:10.7282/T39P33NP

ETD doctoral

The author owns the copyright to this work.