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

Abstract

High Resolution Image Download MS PowerPoint Slide 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.

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