Coadsorption and interactions of O and H on Pd(111)

The interactions between oxygen and hydrogen coadsorbed on Pd(111) have been studied by scanning tunneling microscopy (STM) in the temperature range from 25 to 230 K. Atomic oxygen without coadsorbed hydrogen forms a (2 x 2) structure in a continuous layer or in the form of islands. Individual oxygen atoms are also observed between the islands at low temperature. Coadsorbed hydrogen modifies the STM image contrast and enhances the diffusion of the isolated oxygen atoms. Above 120 K hydrogen modifies the structure of the oxygen islands, transforming them into rows of oxygen atom pairs at nearest neighbor distances. Above 150 K, hydrogen causes all (2 x 2)-O islands to convert into a (root3 x root3) structure, which is stable up to 210 K. Above 210 K the (root3 x root3) structure transforms back to (2 x 2) due to dissolution of the surface hydrogen into the bulk. During these transformations the number of oxygen atoms on the surface remains unchanged. Above 220 K the oxygen population decreases by reaction with dissolved hydrogen to form H2O which desorbs from surface. (C) 2002 Published by Elsevier Science B.V.