Cooperative Stabilization of the [Pyridinium-CO₂-Co] Adduct on a Metal–Organic Layer Enhances Electrocatalytic CO₂ Reduction

Abstract

Pyridinium has been shown to be a cocatalyst for the electrochemical reduction of CO₂ on metal and semiconductor electrodes, but its exact role has been difficult to elucidate. In this work, we create cooperative cobalt-protoporphyrin (CoPP) and pyridine/pyridinium (py/pyH⁺) catalytic sites on metal-organic layers (MOLs) for an electrocatalytic CO₂ reduction reaction (CO₂RR). Constructed from [Hf₆(μ₃-O)₄(μ₃-OH)₄(HCO₂)₆] secondary building units (SBUs) and terpyridine-based tricarboxylate ligands, the MOL was postsynthetically functionalized with CoPP via carboxylate exchange with formate capping groups. The CoPP group and the pyridinium (pyH⁺) moiety on the MOL coactivate CO₂ by forming the [pyH⁺-^-O₂C-CoPP] adduct, which enhances the CO₂RR and suppresses hydrogen evolution to afford a high CO/H₂ selectivity of 11.8. Cooperative stabilization of the [pyH⁺-^-O₂C-CoPP] intermediate led to a catalytic current density of 1314 mA/mgCo for CO production at -0.86 V_RHE, which corresponds to a turnover frequency of 0.4 s^-1.