Structure and composition of Cu(hkl) surfaces exposed to O_2 and emersed from alkaline solutions: Prelude to UHV-EC studies of CO_2 reduction at well-defined copper catalysts

Abstract

The ability of copper electrodes to catalyze the reduction of carbon dioxide better than any single-metal material is now well known. However, it is also an established fact that copper is an efficient scavenger of dry oxygen. Hence, the initiation of the CO_2 reduction reaction at copper must contend with the presence of surface oxides spontaneously formed when the metal is exposed to ambient air. In this regard, the interfacial structures and compositions of Cu(1 0 0), Cu(1 1 0) and Cu(1 1 1), before and after exposure to gaseous oxygen and emersion from in mildly alkaline media (pH 8 and 10), were characterized by a combination of electrochemistry and electron spectroscopy (low-energy electron diffraction and Auger electron spectroscopy). The affinity of the low-index copper planes to oxygen gas was found to decrease in the order Cu(1 1 0) > Cu(1 0 0) > Cu(1 1 1). The same reactivity trend was exhibited by the electrodes emersed from alkaline K_2SO_4 solution. The initial stages of the anodic oxidation of copper, prior to formation of bulk oxides, span a wide potential window that is pH-sensitive; within this precursory region, submonolayer coverages of oxygen tended to form surface domains with long-range order. At potentials far below the anodic-oxidation region (E < −0.90 V), the surface compositions and structures of Cu(hkl) are expected to mimic those of zerovalent copper. These results may bear significant implications in the generation as well as identification of surface-bound intermediates that define the electrocatalytic selectivity of copper towards the reduction of molecular species such as CO_2 and CO in alkaline media.