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

The selective reduction of to formate using molecular catalysts immobilized on high surface area porous silicon is described. Manganese complexes of the form (bpy)Mn(CO)Br (bpy = 2,2'-bipyridine) were prepared with silatrane groups on the bpy ligand for attachment to oxide-coated porous silicon (SiO-porSi). SiO-porSi wafers were formed by heating hydrogen-terminated p-type porous silicon wafers under air and the manganese complexes were immobilized on SiO-porSi by heating at 80 °C. The resulting Mn@SiO-porSi photoelectrodes are photoelectrocatalysts for 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/cm), 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 reduction to formate. This work presents rare examples of immobilized molecular catalysts for reduction to formate, and the first on semiconducting silicon.