PNNL

Abstract

The effective design of an artificial photosynthetic system entails the optimization of several important interactions. Herein we report stopped-flow UV-Vis spectroscopy, X-ray crystallography, DFT, and electrochemical kinetic studies of the Re(bipy-tBu)(CO)3(L) catalyst system. A remarkable selectivity for CO2 over H+ was observed by stopped-flow UV-Vis spectroscopy of [Re(bipy-tBu)(CO)3]-. The pseudo-first order rate constant for the reaction with 10 mM CO2 in THF is 35 s-1. This is ca. 15-20 times faster than the reactions with water or methanol at the same concentration in THF. X-ray crystallography and DFT studies of the doubly-reduced anionic species suggest that the HOMO has mixed metal-ligand character rather than being purely dz 2, which is thought to aid catalytic selectivity by favoring binding of CO2 over H+. 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 catalytic rates up to seven-fold, indicating that the properties of the electrode surface should not be overlooked when developing a homogeneous artificial photosynthetic system. This research was supported at the University of Washington, Seattle by the Camille and Henry Dreyfus Postdoctoral Program in Environmental Chemistry (for a fellowship to A. J. M. M.), and, for funds to purchase the stopped-flow instrument, the U.S. National Institutes of Health 13 (Grant GM-50422 to JMM), and the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.