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

Electrocatalytic CO₂ reduction (e-CO₂RR) to CO is replete with challenges including the need to carry out e-CO₂RR at low overpotentials. Previously, a tricopper-substituted polyoxometalate was shown to reduce CO₂ 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^III, as a binding site for CO₂ in the first coordination sphere of the polyoxometalate, forming heterometallic polyoxometalates, e.g., [SiCu^IIFe^IIIGa^III(H₂O)₃W₉O₃₇]^8-, 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 ¹³CO₂ revealed that CO₂ disproportionation did not occur at −1.5 V. EPR spectroscopy showed reduction, first of Cu^II to Cu^I and Fe^III to Fe^II and then reduction of a tungsten atom (W^VI to W^V) in the polyoxometalate framework. IR spectroscopy showed that CO₂ binds to [SiCu^IIFe^IIIGa^III(H₂O)₃W₉O₃₇]^8- 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 CO₂ species at both −2.5 and −1.5 V through series of electron transfer, proton-coupled electron transfer, protonation and CO₂ binding steps, the active site for reduction, and the role of protons in facilitating the reactions.