Selective electrocatalytic reduction of carbon dioxide to formate by a water-soluble iridium pincer catalyst

Abstract

A water-soluble Ir PCP-type pincer catalyst was developed to reduce CO₂ to formate electrocatalytically in water with high efficiency and selectivity. Formate is the only reduced carbon product, formed in 93% Faradaic yield with no formation of CO. A small fraction of “background” H₂ (ca. 7%) is directly produced at the electrode by solvent reduction. Detailed kinetic information relevant to the catalysis was obtained. The high selectivity for formate production over H₂ originates from the aqueous stability of Ir dihydride species, the active species for hydride reduction of CO₂. Under neutral pH, the Ir pincer complex does not catalyze the reduction of protons to H₂ making water a viable solvent for use with this catalyst system. Addition of small amounts (ca. 1%) of acetonitrile reduces the over-potential and renders the catalysis sustainable. Mechanistic studies suggest that acetonitrile is a key ancillary ligand that ionizes formate effectively preventing catalyst deactivation.