Mechanistic insights into biomimetic carbonic anhydrase action catalyzed by doped carbon nanotubes and graphene

Electronic structural analyses of hydrogen terminated metal doped carbon nanotube/graphene (M-CNT/Gr, MN₃-CNT/Gr, M = Ru/Rh) and ruthenium cluster decorated carbon nanotube/graphene (Ru₄-CNT/Gr) were carried out for examining the biomimetic catalytic activity towards CO₂ hydration reaction. The carbonic anhydrase action was followed for the reaction of CO₂ with H₂O resulting in a bicarbonate ion and a proton. All the catalysts were found to be active for CO₂ hydration and the mechanism proved them to be biomimetic. Interconversion of CO₂ to a HCO₃^- ion took place with five elementary steps viz. OH^- formation by H₂O dissociation, linear CO₂ complexation, CO₂ bending by nucleophilic attack of an OH^- ion over CO₂, HCO₃^- ion formation by intramolecular proton migration and HCO₃^- ion displacement by H₂O addition. Free energy landscapes over the catalysts were developed for CO₂ hydration reaction. The activation energies of H₂O dissociation and CO₂ bending were observed to be substantially smaller over Ru₄-CNT when compared to those over the other catalysts. Ru₄-CNT was found to be the best catalyst for CO₂ hydration with the rate limiting step being HCO₃^- ion formation.