Steric Effects on CO<sub>2</sub> Reduction with Substituted Mn(bpy)(CO)<sub>3</sub>X-Type Catalysts on Multiwalled Carbon Nanotubes Reveal Critical Mechanistic Details
Emile E. DeLuca, Thomas Chan, James M. Taylor, Byunghoon Lee, Rajiv Ramanujam Prabhakar, Clifford P. Kubiak
Abstract
A series of Mn(bpy-R)(CO) 3 Br (bpy-R = 4,4′-R-2,2′-bipyridine) complexes with systematic substituent variations (R = H, –Me, –Et, t Bu, and –Ph) are immobilized on multiwalled carbon nanotubes (MWCNTs) and investigated as electrocatalysts for CO 2 reduction to study substituent effects on heterogenized molecular electrocatalysis. The electrochemical response and catalytic activity of each heterogenized complex are characterized, unveiling clear trends across the series investigated. Mn(bpy-Ph)(CO) 3 Br/MWCNT exhibited the best catalytic performance, producing CO with a Faradaic efficiency of 72% and a current density ( J CO ) of 7.0 mA/cm 2 at low overpotential (η = 0.65 V). Adding steric bulk to the bpy ligands is shown to restrict Mn 0 –Mn 0 dimerization and cause a shift to two-electron reduction occurring at less negative potentials. The apparent quantity of electroactive catalyst scales inversely with steric bulk, where Mn(bpy-Ph)(CO) 3 Br exhibits no distinguishable Faradaic features in CV under normal conditions. These results indicate that catalytic performance is optimized by the confinement of electroactive species to the MWCNT interface.