Comproportionation of CO2 and Cellulose to Formate Using a Floating Semiconductor-Enzyme Photoreforming Catalyst.

Formate production via both CO2 reduction and cellulose oxidation in a solar-driven process is achieved by a semi-artificial biohybrid photocatalyst consisting of immobilized formate dehydrogenase on titanium dioxide (TiO2 |FDH) producing up to 1.16±0.04 mmolformate g TiO 2 ${{_{\ {\rm TiO}{_{2}}}}}$ -1 in 24 hours at 30 °C and 101 kPa under anaerobic conditions. Isotopic labeling experiments with 13 C-labeled substrates support the mechanism of stoichiometric formate formation through both redox half-reactions. TiO2 |FDH was further immobilized on hollow glass microspheres to perform more practical floating photoreforming allowing vertical solar light illumination with optimal light exposure of the photocatalyst to real sunlight. Enzymatic cellulose depolymerization coupled to the floating photoreforming catalyst generates 0.36±0.04 mmolformate per m2 irradiation area after 24 hours. This work demonstrates the synergistic solar-driven valorization of solid and gaseous waste streams using a biohybrid photoreforming catalyst in aqueous solution and will thus provide inspiration for the development of future semi-artificial waste-to-chemical conversion strategies.

Electrochemical Kinetics Support a Second Coordination Sphere Mechanism in Metal-Based Formate Dehydrogenase.

Metal-based formate dehydrogenases are molybdenum or tungsten-dependent enzymes that catalyze the interconversion between formate and CO2 . According to the current consensus, the metal ion of the catalytic center in its active form is coordinated by 6 S (or 5 S and 1 Se) atoms, leaving no free coordination sites to which formate could bind to the metal. Some authors have proposed that one of the active site ligands decoordinates during turnover to allow formate binding. Another proposal is that the oxidation of formate takes place in the second coordination sphere of the metal. Here, we have used electrochemical steady-state kinetics to elucidate the order of the steps in the catalytic cycle of two formate dehydrogenases. Our results strongly support the "second coordination sphere" hypothesis.

Engineering Electro- and Photocatalytic Carbon Materials for CO2 Reduction by Formate Dehydrogenase.

Semiartificial approaches to renewable fuel synthesis exploit the integration of enzymes with synthetic materials for kinetically efficient fuel production. Here, a CO2 reductase, formate dehydrogenase (FDH) from Desulfovibrio vulgaris Hildenborough, is interfaced with carbon nanotubes (CNTs) and amorphous carbon dots (a-CDs). Each carbon substrate, tailored for electro- and photocatalysis, is functionalized with positive (-NHMe2+) and negative (-COO-) chemical surface groups to understand and optimize the electrostatic effect of protein association and orientation on CO2 reduction. Immobilization of FDH on positively charged CNT electrodes results in efficient and reversible electrochemical CO2 reduction via direct electron transfer with >90% Faradaic efficiency and -250 µA cm-2 at -0.6 V vs SHE (pH 6.7 and 25 °C) for formate production. In contrast, negatively charged CNTs only result in marginal currents with immobilized FDH. Quartz crystal microbalance analysis and attenuated total reflection infrared spectroscopy confirm the high binding affinity of active FDH to CNTs. FDH has subsequently been coupled to a-CDs, where the benefits of the positive charge (-NHMe2+-terminated a-CDs) were translated to a functional CD-FDH hybrid photocatalyst. High rates of photocatalytic CO2 reduction (turnover frequency: 3.5 × 103 h-1; AM 1.5G) with dl-dithiothreitol as the sacrificial electron donor were obtained after 6 h, providing benchmark rates for homogeneous photocatalytic CO2 reduction with metal-free light absorbers. This work provides a rational basis to understand interfacial surface/enzyme interactions at electrodes and photosensitizers to guide improvements with catalytic biohybrid materials.