Surface segregation processes in Pd-Pt alloys and bimetallic Pd-Pt nanoclusters on alumina and carbon supports (technical catalysts) have been investigated by determining the metal surface composition of these systems by low-energy ion scattering (LEIS). Both Pd-rich (Pd80Pt20) and Pt-rich (Pd20Pt80) systems have been studied. The surface of the Pd-Pt alloys is enriched in Pd after heating in ultrahigh vacuum and thermodynamic equilibrium is reached at about 700 C. Pd surface segregation is enhanced by heating the alloys in hydrogen or oxygen, and thermodynamic equilibrium is reached already at about 400-500 C. For Pd-Pt catalysts with low metal dispersions of about 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 oxygen. We attribute this to the limited supply of Pd atoms from the bulk to the surface of the nanoclusters.
|Journal||The Journal of physical chemistry B|
|Publication status||Published - 1998|