Toward Effective CO2 Reduction in an Acid Medium: Electrocatalysis at Cu2O-Derived Polycrystalline Cu Sites Immobilized within the Network of WO3 Nanowires

A hybrid catalytic system composed of copper (I)-oxide-derived copper nanocenters immobilized within the network of tungsten oxide nanowires has exhibited electrocatalytic activity toward CO2 reduction in an acid medium (0.5 mol dm-3 H2SO4). The catalytic system facilitates conversion of CO2 to methanol and is fairly selective with respect to the competing hydrogen evolution. The preparative procedure has involved voltammetric electroreduction of Cu2O toward the formation and immobilization of catalytic Cu sites within the hexagonal structures of WO3 nanowires which are simultaneously partially reduced to mixed-valence hydrogen tungsten (VI, V) oxide bronzes, HxWO3, coexisting with sub-stoichiometric tungsten (VI, IV) oxides, WO3-y. After the initial loss of Cu through its dissolution to Cu2+ during positive potential scanning up to 1 V (vs RHE), the remaining copper is not electroactive and seems to be trapped within in the network of hexagonal WO3. Using the ultramicroelectrode-based probe, evidence has also been provided that partially reduced nonstoichiometric tungsten oxides induce reduction of CO2 to the CO-type reaction intermediates. The chronocoulometric data are consistent with the view that existence of copper sites dispersed in WO3 improves electron transfers and charge propagation within the hybrid catalytic layer. The enhanced tolerance of the catalyst to the competitive hydrogen evolution during CO2R should be explained in terms of the ability of HxWO3 to consume protons and absorb hydrogen as well as to shift the proton discharge at Cu toward more negative potentials. However, the capacity of WO3 to interact with catalytic copper and to adsorb CO-type reaction intermediates is expected to facilitate removal of the poisoning CO-type adsorbates from Cu sites.