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

Identifier:  imarina:9386581

Handle: https://hdl.handle.net/20.500.11797/imarina9386581

Authors:  Azaiza-Dabbah, D; Wang, F; Haddad, E; Solé-Daura, A; Carmieli, R; Poblet, JM; Vogt, C; Neumann, R

Abstract:
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., [(SiCuFeGaIII)-Fe-II-Ga-III(H2O)(3)W9O37](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 similar to 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 (CO2)-C-13 revealed that CO2 disproportionation did not occur at -1.5 V. EPR spectroscopy showed reduction, first of Cu-II to CuI and Fe-III to Fe-II and then reduction of a tungsten atom (WVI to WV) in the polyoxometalate framework. IR spectroscopy showed that CO2 binds to [(SiCuFeGaIII)-Fe-II-Ga-III(H2O)(3)W9O37](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.