The dynamic catalytic surface: probing bimetallic active sites with mediumenergy ion scattering

The shadowing and blocking effects exhibited by medium energy ion scatterin g (MEIS) make it a uniquely effective technique for the analysis of the lay er composition of bimetallic surfaces. We use MEIS to study adsorbate-induc ed segregation effects at the Cu50Pd50(110) surface. We show that the clean surface composition is strongly dependent on sample preparation. In additi on, the adsorption of chlorohydrocarbons causes substantial surface layer C u enrichment (up to 100% Cu), whereas hydrocarbon adsorption causes Pd enri chment. We propose an explanation for the magnitude of these segregation ef fects in terms of the surface chemistry of these molecules. (C) 1999 Publis hed by Elsevier Science B.V. All rights reserved.