Examples of transition metal complexes capable of the dual roles of light harvesting and catalysis of CO2 reduction are rare. This self-sensitized approach simplifies systems and efficiencies; therefore, complete understanding of mechanistic principles is essential for improving catalysts. Here, we present a comprehensive study of dark reactions using electrochemical techniques to understand the multiple pathways for the selective reduction of CO2 to CO by an example self-sensitized photocatalyst: [Ir(bip)(ppy)(CH3CN)]2+ (bip = 2,6-bis(benzimidazole)pyridine, ppy = 2-phenylpyridine). Cyclic voltammetry (CV) in acetonitrile under anhydrous conditions reveals electrocatalysis by a two electron cycle at -1.7 V vs Fc+/0 (denoted the cat-1 region) in which the metallocarboxylate formed by binding of Ir(I) to CO2 is cleaved by CO2 as the oxide acceptor. At -1.9 V (denoted the cat-2 region), the Ir(CO2) intermediate is reduced and catalysis is accelerated. In the presence of water, Ir(CO2) is protonated to Ir(CO2H), which is reduced at a potential less negative than -1.7 V, and then, the oxide acceptor is either CO2 to form HCO3- or protons to release H2O and the conjugate base of the acid source. Further reduction of Ir(CO2H) at cat-2 again accelerates catalysis. Rates vary widely in these various regimes with the minimum kobs of 0.3 s-1 for anhydrous cat-1 to a maximum cat-2 rate of 2100 s-1 with 1% water. Competitive deactivation pathways were discovered as Ir-Ir dimerization without reacting with CO2 or the formation of a hydride-bridged dinuclear complex during extended electrolysis at a high water concentration. The Ir-Ir dimer was characterized by high resolution mass spectrometry and X-ray absorption spectroscopy (XAS).
- cyclic voltammetry
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