Heterometallic\nTransition Metal Oxides Containing\nLewis Acids as Molecular Catalysts for the Reduction of Carbon Dioxide\nto Carbon Monoxide with Bimodal Activity

Electrocatalytic\nCO<sub>2</sub> reduction (e-CO<sub>2</sub>RR)\nto CO is replete with challenges including the need to carry out e-CO<sub>2</sub>RR at low overpotentials. Previously, a tricopper-substituted\npolyoxometalate was shown to reduce CO<sub>2</sub> to CO with a very\nhigh faradaic efficiency albeit at −2.5 V versus Fc/Fc<sup>+</sup>. It is now demonstrated that introducing a nonredox metal\nLewis acid, preferably Ga<sup>III</sup>, as a binding site for CO<sub>2</sub> in the first coordination sphere of the polyoxometalate,\nforming heterometallic polyoxometalates, e.g., [SiCu<sup>II</sup>Fe<sup>III</sup>Ga<sup>III</sup>(H<sub>2</sub>O)<sub>3</sub>W<sub>9</sub>O<sub>37</sub>]<sup>8–</sup>, leads to bimodal activity optimal\nboth at −2.5 and −1.5 V versus Fc/Fc<sup>+</sup>; reactivity\nat −1.5 V being at an overpotential of ∼150 mV. These\nresults were observed by cyclic voltammetry and quantitative controlled\npotential electrolysis where high faradaic efficiency and chemoselectivity\nwere obtained at −2.5 and −1.5 V. A reaction with <sup>13</sup>CO<sub>2</sub> revealed that CO<sub>2</sub> disproportionation\ndid not occur at −1.5 V. EPR spectroscopy showed reduction,\nfirst of Cu<sup>II</sup> to Cu<sup>I</sup> and Fe<sup>III</sup> to\nFe<sup>II</sup> and then reduction of a tungsten atom (W<sup>VI</sup> to W<sup>V</sup>) in the polyoxometalate framework. IR spectroscopy\nshowed that CO<sub>2</sub> binds to [SiCu<sup>II</sup>Fe<sup>III</sup>Ga<sup>III</sup>(H<sub>2</sub>O)<sub>3</sub>W<sub>9</sub>O<sub>37</sub>]<sup>8–</sup> before reduction. In situ electrochemical attenuated\ntotal reflection surface-enhanced infrared absorption spectroscopy\n(ATR-SEIRAS) with pulsed potential modulated excitation revealed different\nobservable intermediate species at −2.5 and −1.5 V.\nDFT calculations explained the CV, the formation of possible activated\nCO<sub>2</sub> species at both −2.5 and −1.5 V through\nseries of electron transfer, proton-coupled electron transfer, protonation\nand CO<sub>2</sub> binding steps, the active site for reduction, and\nthe role of protons in facilitating the reactions.