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.