Heterometallic Transition Metal Oxides Containing Lewis Acids as Molecular Catalysts for the Reduction of Carbon Dioxide to Carbon Monoxide with Bimodal Activity.

Electrocatalytic CO reduction (e-CORR) to CO is replete with challenges including the need to carry out e-CORR at low overpotentials. Previously, a tricopper-substituted polyoxometalate was shown to reduce CO to CO with a very high faradaic efficiency albeit at -2.5 V versus Fc/Fc. It is now demonstrated that introducing a nonredox metal Lewis acid, preferably Ga, as a binding site for CO in the first coordination sphere of the polyoxometalate, forming heterometallic polyoxometalates, e.g., [SiCuFeGa(HO)WO], leads to bimodal activity optimal both at -2.5 and -1.5 V versus Fc/Fc; reactivity at -1.5 V being at an overpotential of ∼150 mV. These results were observed by cyclic voltammetry and quantitative controlled potential electrolysis where high faradaic efficiency and chemoselectivity were obtained at -2.5 and -1.5 V. A reaction with CO revealed that CO disproportionation did not occur at -1.5 V. EPR spectroscopy showed reduction, first of Cu to Cu and Fe to Fe and then reduction of a tungsten atom (W to W) in the polyoxometalate framework. IR spectroscopy showed that CO binds to [SiCuFeGa(HO)WO] before reduction. In situ electrochemical attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) with pulsed potential modulated excitation revealed different observable intermediate species at -2.5 and -1.5 V. DFT calculations explained the CV, the formation of possible activated CO species at both -2.5 and -1.5 V through series of electron transfer, proton-coupled electron transfer, protonation and CO binding steps, the active site for reduction, and the role of protons in facilitating the reactions.