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.