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

Abstract

Electrocatalytic CO 2 reduction (e-CO 2 RR) to CO is replete with challenges including the need to carry out e-CO 2 RR at low overpotentials. Previously, a tricopper-substituted polyoxometalate was shown to reduce CO 2 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 CO 2 in the first coordination sphere of the polyoxometalate, forming heterometallic polyoxometalates, e.g., [SiCu II Fe III Ga III (H 2 O) 3 W 9 O 37 ] 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 13 CO 2 revealed that CO 2 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 CO 2 binds to [SiCu II Fe III Ga III (H 2 O) 3 W 9 O 37 ] 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 CO 2 species at both −2.5 and −1.5 V through series of electron transfer, proton-coupled electron transfer, protonation and CO 2 binding steps, the active site for reduction, and the role of protons in facilitating the reactions.