Photoelectrocatalytic reduction of CO2 to formate using immobilized molecular manganese catalysts on oxidized porous silicon

Abstract

The reduction of carbon dioxide (CO₂) to formate using molecular catalysts immobilized on high surface area porous silicon is described. Manganese complexes of the type (^Rbpy)Mn(CO)₃Br (bpy = 2,2′-bipyridine) were prepared with silatrane groups on the bpy ligand for attachment to oxide-coated porous silicon (SiO_x-porSi). SiO_x-porSi wafers were formed by heating hydrogen-terminated p-type porous silicon wafers under air, and the manganese complexes were immobilized on SiO_x-porSi by heating at 80°C. The resulting hybrid photoelectrodes are photoelectrocatalysts for CO₂ reduction in acetonitrile containing 2.0 M triethylamine and 2.0 M isopropanol, yielding formate with high selectivity (>96%) and current density (∼0.6 mA/cm²), excellent reproducibility, and a photovoltage of 280 mV at −1.75 V (versus ferrocenium/ferrocene) under 1 sun illumination. The applied potential is close to the equilibrium potential for CO₂ reduction to formate. This work presents rare examples of immobilized molecular catalysts for CO₂ reduction to formate and the first on semiconducting silicon.