Toward Effective CO 2 Reduction in an Acid Medium: Electrocatalysis at Cu 2 O-Derived Polycrystalline Cu Sites Immobilized within the Network of WO 3 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 CO 2 reduction in an acid medium (0.5 mol dm -3 H 2 SO 4 ). The catalytic system facilitates conversion of CO 2 to methanol and is fairly selective with respect to the competing hydrogen evolution. The preparative procedure has involved voltammetric electroreduction of Cu 2 O toward the formation and immobilization of catalytic Cu sites within the hexagonal structures of WO 3 nanowires which are simultaneously partially reduced to mixed-valence hydrogen tungsten (VI, V) oxide bronzes, H x WO 3 , coexisting with sub-stoichiometric tungsten (VI, IV) oxides, WO 3- y . After the initial loss of Cu through its dissolution to Cu 2+ 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 WO 3 . Using the ultramicroelectrode-based probe, evidence has also been provided that partially reduced nonstoichiometric tungsten oxides induce reduction of CO 2 to the CO-type reaction intermediates. The chronocoulometric data are consistent with the view that existence of copper sites dispersed in WO 3 improves electron transfers and charge propagation within the hybrid catalytic layer. The enhanced tolerance of the catalyst to the competitive hydrogen evolution during CO 2 R should be explained in terms of the ability of H x WO 3 to consume protons and absorb hydrogen as well as to shift the proton discharge at Cu toward more negative potentials. However, the capacity of WO 3 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.