The quest for carbon neutrality requires the development of technologies for the transformation of CO₂ and its derivatives into commodity products from low-carbon energy. In this context, the design of new efficient and environmentally friendly catalytic processes for CO₂ reduction is at the heart of societal concerns. In the field of catalysis for sustainable chemistry, biocatalysts have the advantage of being highly selective and efficient under mild conditions (ambient temperature and pressure, aqueous solvents). However, despite their benefits, the cost of large-scale production of enzymes and their lack of stability still limit the development of such approaches..

This is especially true for complex redox metalloenzymes, such as Carbon Monoxide Dehydrogenase (CODH), which remains to date the most efficient catalyst for the activation of small molecules like CO and CO₂. CODH has an active site that is unique in biology. It is composed of a multi-metallic NiFe₄S₄ center whose biosynthesis involves several molecular steps, requiring a specific multi-protein machinery which is still poorly understood. Over the last few years, IRIG researchers have studied in detail the chaperone proteins involved in this mechanism. These studies have allowed them to develop a heterologous CODH production system in which the gene of the carboxydotrophic bacterium Rhodospirillum rubrum (capable of using CO as a source of carbon and energy) is expressed in Escherichia coli. This process produces, in a single purification step, an enzyme that is as stable and active as a natural CODH. The researchers went further by immobilizing this recombinant CODH on functionalized carbon nanotubes [in collaboration with the DCM-BEA of UGA Grenoble], which allowed them to develop a bioelectro-catalytic system that is stable for several hours to achieve the inter-conversion of CO₂ to CO (Figure), reaching current densities of 4.2 mA.cm² for CO₂ reduction and 1.5 mA.cm² for CO oxidation.


Compared to existing electrochemical CO2 reduction processes based on molecular catalysts, this enzymatic process achieves similar performances while bringing the advantages of operating in a reversible manner, under mild conditions and with very low overpotentials. These results allow to consider their use for CO₂ reduction as well as for CO oxidation, in the context of synthesis gas purification processes for the production of various chemicals or fuels.