Characterization of Pd-β-Cyclodextrin Colloids as Catalysts in the Photosensitized Reduction of Bicarbonate to Formate

Photosensitized reduction of bicarbonate. HCO₃^-, to formate. HCO₂^-, proceeds in an aqueous system composed of deazariboflavin, dRFl (1), as photosensitizer, N,N'-dimethyl-4,4'-bipyridinium, MV²⁺, as primary electron acceptor, sodium oxalate as sacrificial electron donor, and in the presence of a Pd colloid stabilized by β-cyclodextrin, Pd-β-CD. The process proceeds with a quantum efficiency, ϕ = 1.1. Kinetic characterization of the Pd-β-CD catalyst activity reveals the presence of active sites for bicarbonate activation and reduction as well as catalytic sites for H₂ evolution. The HCO₃^- activation sites are specifically inhibited by thiols. The catalytic reduction of HCO₃^- to HCO₂^- and the respective inhibition processes exhibit enzyme-like kinetic properties. The Pd-β-CD colloid shows reversible activities and effects the reduction of MV²⁺ by formate. Kinetic characterization of the catalyzed reduction of HCO₃^- to HCO₂^- and the reverse oxidation of HCO₂^- provides a sequential mechanism for the reactions.