The conversion of biomass into synthesis gas would make it possible to exploit the energy contained in wood in the absence of any direct combustion. This process involves gasifying the organic matter by pyrolysis. The resulting mixture consists of recoverable gases (CH₄ and CO), as well as numerous impurities (CO₂, H₂S, etc.) which must be eliminated, in particular by reduction of CO₂ (to CO) or oxidation of CO (to CO₂).

In this context, biocatalysts of CO₂ reduction have the advantage of being highly selective and efficient under mild conditions (ambient temperature and pressure, aqueous solvents). However, they suffer from a lack of stability and the high cost of large-scale enzyme production. This is especially the case for "CO dehydrogenase" (CODH), the main biocatalyst that activates small molecules such as CO and CO₂.

The distinctive feature of CODH is its active site consisting of a multi-metallic NiFe₄S₄ center, whose biosynthesis requires a specific multi-protein machinery that is still poorly understood.

In their previous work, researchers at Irig developed a CODH production system that uses the gene from a CO metabolizing bacterium (Rhodospirillum rubrum) expressed in the bacterium Escherichia coli. This process can produce an enzyme that is as stable and active as natural CODH, in a single purification step.

Now, the scientists are going even further by immobilizing the recombinant CODH on functionalized carbon nanotubes. They can thus create a stable bioelectro-catalytic system for several hours, which allows them to inter-convert CO₂ into CO and vice versa. Current densities of 4.2 mA.cm^-2 for CO2 reduction and 1.5 mA.cm^-2 for CO oxidation have thereby been achieved.

The performance of this system compares favorably with electrochemical CO₂ reduction processes using molecular catalysts. Furthermore, it has the added advantage of operating reversibly, under mild conditions and with very low overpotentials.

This work was supported by the Labex Arcane and was conducted in collaboration with researchers from Aix-Marseille University and Grenoble Alpes University.