Vanadium oxide nanostructures on Rh(111): Promotion effect of CO adsorption and oxidation

The adsorption of CO and the reaction of CO with pre-adsorbed oxygen at room temperature has been studied on the (2 x 1)O-Rh(1 1 1) surface and on vanadium oxide-Rh(1 1 1) inverse model catalyst surfaces using scanning tunnelling microscopy (STM) and core-level photoemission with synchrotron radiation. Two types of structurally well-defined model catalyst V3O9-Rh(1 1 1) surfaces have been prepared, which consist of large (mean size of similar to 50 nm, type I model catalyst) and small (mean size < 15 nm, type II model catalyst) two-dimensional oxide islands and bare Rh areas in between; the latter are covered by chemisorbed oxygen. Adsorption of CO on the oxygen pre-covered (2 x 1)O-Rh(1 1 1) surface leads to fast CO uptake in on-top sites and to the removal of half (0.25 ML) of the initial oxygen coverage by an oxidation clean-off reaction and as a result to the formation of a coadsorbed (2 x 2)-O + CO phase. Further removal of the adsorbed O with CO is kinetically hindered at room temperature. A similar kinetic behaviour has been found also for the CO adsorption and oxidation reaction on the type I inverse model catalyst surface. In contrast, on the type II inverse catalyst surface, containing small V-oxide islands, the rate of removal of the chemisorbed oxygen is significantly enhanced. In addition, a reduction of the V-oxide islands at their perimeter by CO has been observed, which is suggested to be the reason for the promotion of the CO oxidation reaction near the metal-oxide phase boundary. (c) 2005 Elsevier B.V. All rights reserved.