In this project funded by the Chemical Catalysis Program of the Chemistry Division, Stefan Bernhard of Carnegie Mellon University will design, synthesize and study novel light-absorbing metal complexes combined with reduction catalysts able to generate hydrogen from water and sunlight. Coordination complexes of biologically innocuous and inexpensive first-row transition metal ions will be investigated for use as catalysts and chromophores. Strongly chelating hemi-cage and wrap-around ligand topologies that stabilize these normally labile structures will be explored. Advanced spectroscopic and electrochemical techniques will be used to understand the structure and function of these complexes and their application to water-splitting systems, and parallel photoreactors will be employed to study large numbers of candidate complexes and reaction conditions. Through a combination of these spectroscopic and synthetic methods, quantitative structure-activity relationships of photocatalytic systems will be determined. These insights will be used to further improve the robustness and reaction rate of hydrogen evolution. The solution-based chemistry of these water reduction catalysts will then be modified into scalable photoelectrochemical systems immobilized on transparent electrodes suitable for use in full water-splitting systems. The broader impacts involve training undergraduate and graduate students, hosting visiting high school teachers for summer research opportunities, teaching a course on the chemistry of chocolate, and the potential societal impact of renewable energy technology.
This work will provide fundamental and applied knowledge about catalysts that generate hydrogen fuel from water and sunlight, and potentially could show how earth abundant metals can be used for such technologies. Results from this research would be important to the energy industry and to industries that manufacture commercial consumer products that require fuels sources, such as the automotive and aerospace sectors.
Photosynthesis is a carefully orchestrated sequence of photon absorption and electron transfer processes that power the catalyzed oxidation of water to oxygen and the reduction of CO2 to carbohydrates. The work in the Bernhard laboratory funded by this NSF grant progressed towards artificial systems that mimic this intricate machinery. The development of novel systems for transforming sun light into fuels such as hydrogen and metals was the principal objective of this work. The judicious design and synthesis of novel light absorbing molecules and catalysts initiated all the projects. Novel iridium metal complexes were synthesized as light absorbers for the photocatalytic hydrogen and zinc metal generation. Combinatorial techniques were utilized to uncover and understand structure activity relationships in these photocatalytic reactions. Another major portion of the work involved the development and study of catalysts facilitating the generation of oxygen. These water oxidation reactions are the source of oxygen in plants and are critical to the development of the majority solar fuel systems. These catalysts undergo very demanding redox transformations during these reactions and the development of resilient structures is critical for progress in this field. The Bernhard laboratory published a wide range of structurally diverse catalysts over the last years. The champion material exhibits over 38,000 catalyst turnovers, which indicates that such a large number of catalytic cycles could be sustained without significant material degradation. All the work was disseminated in a variety of periodicals and book chapters. Future work will focus on the development of full solar fuel systems with practical applications. Many undergraduate and graduate students performed this research and they all transitioned to successful careers in industry, military laboratories, academia, or medical school.