PART 1: NON-TECHNICAL SUMMARY The goal of this project is to develop hybrid biotic/abiotic functional materials that achieve light driven, multi-electron reduction of carbon dioxide (CO2) to formate - an anion derived from formic acid. CO2 is the inevitable oxidation product of the primary fossil fuel energy vectors of modern society and reducing it back to useful hydrocarbon products such as formate is desirable for both environmental and economic reasons. It is difficult to activate CO2 towards reduction, however, because it is a very stable molecule, both thermodynamically and kinetically. New catalysts and a method for coupling them to a renewable energy source are required to achieve this goal. Man-made materials such as metal oxide surfaces are inefficient catalysts and suffer from lack of specificity for CO2 substrate over H+ and the interfering hydrogen evolution reaction. In contrast, nature has evolved highly efficient catalysts for selective CO2 reduction without competitive H2 evolution. Formate dehydrogenases (FDH), are a class of enzymes from prokaryotes that reversibly catalyze the reduction of CO2 to formate, a precursor to methanol or methane production and a potential energy source itself. In nature, however, FDH enzymes are not naturally activated by light, but require some other input of energy. The Dyer group will develop hybrid biotic/abiotic materials that integrate a nanocrystalline semiconductor (quantum dot) with the FDH enzyme to achieve light driven CO2 reduction. Biotic/abiotic interfaces have evolved in nature to achieve functional designs that range from biofilms to photonic crystals and structural materials. Artificial hybrid abiotic/biotic materials may be rationally designed to achieve novel and emergent functions. Integrating biomolecular and abiotic structures is a powerful approach to create functional materials. The major goal of this work is to couple the biological enzyme FDH to an abiotic photosensitizer component (quantum dot) that converts solar energy into reactive electrons, to produce functional materials that can be optimized for highly efficient light driven conversion of CO2 to fuel.
PART 2: TECHNICAL SUMMARY The central goal of this grant is to develop hybrid biotic/abiotic functional materials based on a formate dehydrogenase (FDH) enzyme coupled to a nanocrystalline semiconductor (NCS) photosensitizer for light driven, multi-electron reduction of CO2 to formate. The first objective is to design, synthesize and characterize hybrid NCS:FDH materials that convert light to reactive electrons and then efficiently transfer them to the catalyst. The interfacial electron transfer (ET) will be optimized by controlling the NCS:FDH interaction using three different approaches: 1) direct attachment to the NCS surface via an N-terminal His-tag; 2) optimization of ET efficiency using a viologen derivative redox mediator and 3) a covalent "molecular wire" to the distal FeS cluster. Interfacial ET will also be optimized by wave-function engineering of the NCS structure (dot, core shell, rod, dot-in-rod structures) and the nature of the capping ligands and interfacial charge. Finally, the efficiency of light-driven CO2 reduction will be correlated to the interfacial ET efficiency and to the underlying structure of the hybrid interface. The second objective of the grant is to elucidate the mechanism of light driven CO2 reduction by hybrid NCS:FDH materials. The intrinsic photosensitivity of these hybrid materials will be exploited to optically trigger the ET and enzyme turnover and thereby study the dynamics of all of the relevant processes, including multi-exciton generation and extraction, interfacial electron transfer and enzyme turnover. These experiments will test the important hypothesis that a quantum confined NCS may act as a multi-electron photosensitizer by multi-exciton generation and extraction. This approach will also be used to elucidate the CO2 reduction mechanism by the FDH enzymes. The Dyer lab has developed a unique capability for this purpose, based on the laser induced potential jump coupled with structure-specific, time-resolved methods including ultrafast infrared and fluorescence spectroscopy. Finally, these materials will be integrated into photo-electrodes to demonstrate sustained light-driven CO2 reduction without the limitations imposed by the use of a sacrificial electron donor.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
