Photoelectrocatalytic Reduction of CO 2 to Formate Using Immobilized Molecular Manganese Catalysts on Oxidized Porous...

The selective reduction of ${\text{CO}}_2$ to formate using molecular catalysts immobilized on high surface area porous silicon is described. Manganese complexes of the form (^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 Mn@SiO_x-porSi photoelectrodes are photoelectrocatalysts for ${\text{CO}}_2$ 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 ${\text{CO}}_2$ reduction to formate. This work presents rare examples of immobilized molecular catalysts for ${\text{CO}}_2$ reduction to formate, and the first on semiconducting silicon.