PNNL

Abstract

The conversion of solar energy to fuels in both natural and artificial photosynthesis requires components for both light harvesting and catalysis. The light-harvesting component generates the electrochemical potentials required to drive fuel-generating reactions that would otherwise be thermodynamically uphill. This review focuses on work from our laboratories on developing molecular electrocatalysts for CO2 reduction and for hydrogen production. A true analog of natural photosynthesis will require the ability to capture CO2 from the atmosphere and reduce it to a useful fuel. Work in our laboratories has focused on both aspects of this problem. Organic compounds such as quinones and inorganic metal complexes can serve as redox active CO2 carriers for concentrating CO2. Catalysts for CO2 reduction to form CO have also been developed based on a [Pd(triphosphine)(solvent)]2+ platform. A required feature for catalytic activity is the presence of a weakly coordinating solvent molecule that can dissociate during the catalytic cycle and provide a vacant coordination site for binding water and assisting C-O bond cleavage. Participation of a second metal in CO2 binding also appears to be required for achieving very active catalysts as suggested by structures of [NiFe] CO dehydrogenase enzymes and the results of studies on complexes containing two [Pd(triphosphine)(solvent)]2+ units. Molecular electrocatalysts for H2 production and oxidation based on [Ni(diphosphine)2]2+ complexes are also described. These catalysts require the optimization of both first and second coordination spheres similar to that of the palladium CO2 reduction catalysts. In this case, structural features of the first coordination sphere can be used to optimize the hydride acceptor ability of nickel needed to achieve heterolytic cleavage of H2. The second coordination sphere can be used to incorporate pendant bases that assist in a number of important functions including H2 binding, H2 cleavage, and the transfer of protons between nickel and solution. These pendant bases or proton relays are likely to be important in the design of catalysts for a wide range of fuel production and fuel utilization reactions involving multiple electron and proton transfer steps. The work described in this review has been supported by the Chemical Sciences program of the Office of Basic Energy Sciences of the Department of Energy. The Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.