Thesis presented October 17, 2024

Abstract:
Syngas is a gaseous mix mainly composed of hydrogen gas (H₂), carbon monoxide (CO), carbon dioxide (CO₂) and other components in various concentrations (nitrogen gas N₂, sulfur impurities…). Used as a fuel source in the 19th century, synthetic application of syngas truly started in the first half of the 20^th century with the Haber-Bosch process (ammonia synthesis from H₂ and N₂), methanol production and the Fischer-Tropsch process. A prerequisite to these transformations is the control of the H₂ / CO ratio to one appropriate for the reaction. The balance of this ratio is achieved through the water gas shift reaction (WGSR). The syngas reacts with water vapor to produce the following reaction: CO + H₂O = CO₂ + H₂. This reaction is carried out at industrial scale using high temperature and pressures. The catalysts used (iron and chromium oxides mainly) lack selectivity and are only active in these harsh conditions. Remarkably, the WGSR is also present in vivo and allows carboxydothrophic micro-organisms to use CO as an energy source. This biologically mediated WGSR is carried out at ambient temperature and pressure enabled by the use of two enzymes: a nickel/iron carbon monoxide dehydrogenase ([NiFe] CODH) and a nickel/iron hydrogenase ([NiFe] H₂ase). Each enzyme catalyzes half of the total reaction: CO + H2O = CO₂ + 2H⁺ + 2e^- and 2H⁺ + 2e^- = H₂. A physiological partner allows for the electron transfer between the two. Inspired by this selective version of the WGSR carried out in mild conditions, this PhD thesis presents a bio-hybrid system for the WGSR. This system uses a [NiFe] CODH from Rhodospirillum rubrum and a hydrogenase-inspired nickel complex with diphosphine ligand containing pendant amines ([Ni(P₂^CyN₂^Arg)₂]⁸⁺). The electron transfer between the two species is achieved though their adsorption on a carbon nanotube matrix. An electrochemical study of the catalysts adsorbed at a carbon nanotube matrix surface is presented. The bio-hybrid system was designed and studied in various conditions. The impact of CO quantity, catalyst quantity and pH on the system’s activity was analyzed. The impact of carbon nanotube surface modification is also presented. This bio-hybrid system is compared with an enzymatic system using a CODH and a H₂ase adsorbed on a conductive surface. The use of a bio-inspired catalysts gives the system a better CO tolerance. Future studies could modulate the CODH or H₂ production catalyst to increase stability and overall performance of the system.​

Keywords:
carbon nanotubes, electrocatalysis, metalloenzyme, bio-inspired
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