Kinetic and structural studies, origins of selectivity, and interfacial charge transfer in the artificial photosynthesis of CO

Abstract

The effective design of an artificial photosynthetic system entails the optimization of several important interactions. Herein we report stopped-flow UV-visible (UV-vis) spectroscopy, X-ray crystallographic, density functional theory (DFT), and electrochemical kinetic studies of the Re(bipy-tBu)(CO) 3(L) catalyst for the reduction of CO2 to CO. A remarkable selectivity for CO2 over H+ was observed by stopped-flow UV-vis spectroscopy of [Re(bipy-tBu)(CO)3]-1. The reaction with CO2 is about 25 times faster than the reaction with water or methanol at the same concentrations. X-ray crystallography and DFT studies of the doubly reduced anionic species suggest that the highest occupied molecular orbital (HOMO) has mixed metal-ligand character rather than being purely doubly occupied dz2, which is believed to determine selectivity by favoring CO2 (σ + π) over H+ (σ only) binding. Electrocatalytic studies performed with the addition of Brönsted acids reveal a primary H/D kinetic isotope effect, indicating that transfer of protons to Re-CO2 is involved in the rate limiting step. Lastly, the effects of electrode surface modification on interfacial electron transfer between a semiconductor and catalyst were investigated and found to affect the observed current densities for catalysis more than threefold, indicating that the properties of the electrode surface need to be addressed when developing a homogeneous artificial photosynthetic system.