Autocatalytic Reduction of a Cu₂O/Cu(111) Surface by CO: STM, XPS, and DFT Studies

The reduction of a Cu 2 O surface layer on Cu(111) by CO was studied using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and calculations based on density functional theory. Real-time XPS measurements show the existence of an induction time and an autocatalytic reaction for the reduction of the Cu 2 O/Cu(111) surfaces. The reduction of the Cu 2 O surface layer goes through two stages, the “slow reaction” regime and the “fast reaction” regime. During the “slow reaction” regime, an “O-deficient Cu 2 O” phase forms and propagates on the surface, which lowers the reaction barrier for the removal of lattice oxygen in Cu 2 O. The propagation of the “O-deficient Cu 2 O” phase across the surface leads to the “fast reaction” regime, through which the reduction rate is approximately constant due to a “step-only” reaction mechanism. STM studies provide an atomic level picture for the intermediate structures and the reaction pathway during the reduction of the Cu 2 O surface oxide. Upon the loss of chemisorbed oxygen from the typical Cu 2 O honeycomb structure, a reconstruction occurs, and Cu 2 O heptagons are formed. The “O-deficient Cu 2 O” phase is a disordered phase linked to the formation of Cu 2 O heptagons. The Cu 2 O heptagons provide adsorption sites for CO, which reacts with chemisorbed oxygen hopped from neighboring Cu 2 O honeycombs.