Selectivity for HCO₂^– over H₂ in the Electrochemical Catalytic Reduction of CO₂ by (POCOP)IrH₂

Abstract

It has been demonstrated experimentally that electrochemical CO2 reduction catalyzed by (POCOP)­IrH2 ([C6H3-2,6-[OP­(tBu)2]2]­IrH2) produces formate without significant H2. We use first-principles density functional theory (M06) including Poisson–Boltzmann solvation to determine the detailed atomistic mechanism and illuminate strategies for designing formate-selective catalysts. A mechanism involving hydride transfer from IrIII dihydride explains the selectivity for formate over H2 and is corroborated by reduction potential (irreversible reduction of (POCOP)­Ir­(H)­(NCMe)2 + at ca. −1.3 V vs NHE, in comparison to −1.31 V vs NHE calculated for one-electron reduction of IrIII(H)­(NCMe)2 +) and turnover frequency. We find that several thermodynamically favorable pathways exist for the hydrogen evolution reaction (HER) from both IrIII(H)2 and IrI–H– but are kinetically hindered, posing computed activation barriers above 25 kcal/mol at pH 7. However, with formate or bicarbonate acting as cocatalyst, the barriers are lowered to 18.8 kcal/mol. The preference of (POCOP)Ir to form a dihydride instead of a dihydrogen adduct also disfavors the HER and facilitates catalyst regeneration. In contrast, substituting cobalt for iridium raises the barrier for hydride transfer to CO2 by 12.0 kcal/mol and lowers the required reduction potential to −1.65 V vs NHE. Calculated driving forces for hydride transfer from IrI and IrIII intermediates illustrate different strategies for positioning the hydricity relative to the thermodynamic hydricities of H2/H+ and HCOO–/CO2. The data support an approach of selecting a hydricity that is just thermodynamically able to reduce CO2. The effect of solvation on calculated driving forces for hydride transfer is also discussed.

Directing the reactivity of metal hydrides for selective CO 2 reduction

“…4 and 7, respectively) the only two molecular catalysts with >90% selectivity for formate generation function optimally at modest to high pK a /pH conditions, with decreasing selectivity at lower proton activities (16,20). Yet, both maintain fairly high selectivity under more acidic conditions, indicating that the product distribution is also under kinetic control (64).…”

Single Iridium Pincer Complex for Roundtrip Electrochemical Conversion between Carbon Dioxide and Formate

“…Theoretical calculations by Cao et al. suggested that a moderate hydricity is critical in achieving high selectivity in this system ,. Nozaki et al.…”

“…[7] Theoretical calculations by Cao et al suggested that a moderate hydricity is critical in achieving high selectivity in this system. [8,9] Nozaki et al reported iridium complexes of PNP pincer ligands to hydrogenate CO 2 to formate, showing that Ir PNP trihydride complex can insert/de-insert CO 2 reversibly under suitable conditions, [14] thus suggesting that PNP type Ir complex has great potential for reversible electrochemical conversion between CO 2 and formate.…”

Mapping free energy regimes in electrocatalytic reductions to screen transition metal-based catalysts

“…The free energy of the transition metal hydride intermediate relative to reactants, products and other intermediates, an important thermodynamic quantity that determines the driving force towards hydride transfer to CO 2 and protons to produce HCO 2 À and H 2 respectively, has been well studied as an important descriptor of catalytic activity. [28][29][30][31] For instance, we previously showed, using experimentally calibrated DFT, how specic ligand environments change the free energy of CO 2 insertion into a ruthenium hydride complex. 24 The hydride complex we were studying despite being an active transfer hydrogenation catalyst for ketones 32 was not found to be a catalyst for CO 2 reduction due to product inhibition.…”