The influence of preadsorbed oxygen on the adsorption of CO on two-dimensional Pd islands on a Rh(111)surface

The influence of preadsorbed oxygen on the CO adsorption properties of a la terally heterogeneous bimetallic surface consisting of Pd islands on Rh(111 ) at a Pd coverage of 0.5 monolayers has been studied by high resolution co re level photoemission. A surface consisting of clean Pd islands surrounded by oxygen-covered Rh(111) patches was prepared by predosing oxygen at room temperature. By applying core level photoemission to the Pd 3d(5/2) and th e C 1s levels, the adsorption of CO on this surface was studied with partic ular attention being paid to possible CO diffusion between the two surface parts. The CO molecules are found to diffuse from the oxygen-covered Rh(lll ) patches onto the Pd islands for low to medium CO exposures. This diffusio n direction is opposite of that found previously for the 0.5 ML Pd on Rh(11 1) system with no oxygen predosing. This reversal of the diffusion directio n is argued to be due to a large reduction of the CO adsorption energy on t he Rh patches of the surface caused by the preadsorbed oxygen. (C) 1998 Els evier Science B.V. All rights reserved.