Direct reduction of CO2catalyzed by a formate dehydrogenase immobilized on carbon nanotubes without NADH cofactor.

This paper describes the electroenzymatic reduction of CO2to formate catalyzed by formate dehydrogenase fromCandida boidinii(CbFDH) immobilized on carbon nanotube (CNT)-modified gold electrodes. Cyclic voltammetry indicates thatCbFDH could catalyze CO2reduction to formate without protonated nicotinamide adenine dinucleotide (NADH) as a cofactor, exhibiting diffusion-controlled, quasi-reversible kinetics on both multi-walled CNT and single-walled CNT substrates. Surface-enhanced infrared absorption spectra indicate thatCbFDH adopts a near-parallel orientation on the CNT-modified gold surface, positioning its active site for the direct electron transfer between CO2and the conductive carbon support. The IR spectra reveal an increase in the formate band's intensity in the potential region from -0.3 V to -0.6 V vs Ag/AgCl, confirming efficient CO2reduction. Below -0.6 V vs Ag/AgCl, the hydrogen evolution reaction competitively suppresses formate yield. This study demonstrates that CNTs serve as an effective support for enzyme immobilization and confirms that CO2could be directly reduced to formate at the CNT-modified electrode without a cofactor at potentials close to the equilibrium potential (minimum of overpotential). This represents a novel and unexpected finding.