CO oxidation on inverse CeO(x)/Cu(111) catalysts: high catalytic activity and ceria-promoted dissociation of O2.

A Cu(111) surface displays a low activity for the oxidation of carbon monoxide (2CO + O(2) → 2CO(2)). Depending on the temperature, background pressure of O(2), and the exposure time, one can get chemisorbed O on Cu(111) or a layer of Cu(2)O that may be deficient in oxygen. The addition of ceria nanoparticles (NPs) to Cu(111) substantially enhances interactions with the O(2) molecule and facilitates the oxidation of the copper substrate. In images of scanning tunneling microscopy, ceria NPs exhibit two overlapping honeycomb-type moiré structures, with the larger ones (H(1)) having a periodicity of 4.2 nm and the smaller ones (H(2)) having a periodicity of 1.20 nm. After annealing CeO(2)/Cu(111) in O(2) at elevated temperatures (600-700 K), a new phase of a Cu(2)O(1+x) surface oxide appears and propagates from the ceria NPs. The ceria is not only active for O(2) dissociation, but provides a much faster channel for oxidation than the step edges of Cu(111). Exposure to CO at 550-750 K led to a partial reduction of the ceria NPs and the removal of the copper oxide layer. The CeO(x)/Cu(111) systems have activities for the 2CO + O(2) → 2CO(2) reaction that are comparable or larger than those reported for surfaces of expensive noble metals such as Rh(111), Pd(110), and Pt(100). Density-functional calculations show that the supported ceria NPs are able to catalyze the oxidation of CO due to their special electronic and chemical properties. The configuration of the inverse oxide/metal catalyst opens new interesting routes for applications in catalysis.