SURFACE SEGREGATION IN SUPPORTED PD-PT NANOCLUSTERS AND ALLOYS

Surface segregation processes in Pd-Pt alloys and bimetallic Pd-Pt nan oclusters on alumina and carbon supports (technical catalysts) have be en investigated by determining the metal surface composition of these systems by low-energy ion scattering (LEIS). Both Pd-rich (Pd80Pt20) a nd Pt-rich (Pd20Pt80) systems have been studied. The surface of the Pd -Pt alloys is enriched in Pd after heating in ultrahigh vacuum and the rmodynamic equilibrium is reached at about 700 degrees C. Pd surface s egregation is enhanced by heating the alloys in hydrogen or oxygen, an d thermodynamic equilibrium is reached already at about 400-500 degree s C. For Pd-Pt catalysts with low metal dispersions of al,out 0.3 and 0.8, Pd surface segregation does take place during heating in hydrogen to approximately the same extent as in the Pd-Pt bulk alloys. For Pd- Pt catalysts with a high metal dispersion close to 1, however, surface segregation is completely suppressed during heating in hydrogen and o xygen. We attribute this to the limited supply of Pd atoms from the bu lk to the surface of the nanoclusters.