Direct reduction of CO2 catalyzed by a formate dehydrogenase immobilized on carbon nanotubes without NADH cofactor

This paper describes the electroenzymatic reduction of CO2 to formate catalyzed by formate dehydrogenase from Candida boidinii (CbFDH) immobilized on carbon nanotube-modified gold electrodes. Cyclic voltammetry indicates that CbFDH could catalyze CO2 reduction to formate without protonated nicotinamide adenine dinucleotide (NADH) as a cofactor, exhibiting diffusion-controlled, quasi-reversible kinetics on both multi-walled carbon nanotube (MWCNT) and single-walled carbon nanotube (SWCNT) substrates. Surface-enhanced infrared absorption (SEIRA) spectra indicate that CbFDH adopts a near-parallel orientation on the CNT-modified gold surface, positioning its active site for the direct electron transfer between CO2 and tht 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 CO2 reduction. 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 CO2 could 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.