Nanoscale surface chemistry

We report evidence in several experiments for nanometer-size effects in surface chemistry. The evidence concerns bimetallic systems, monolayer films of Pt or Pd on W(111) surfaces. Pyramidal facets with {211} faces are formed on annealing on physical monolayer of Pt, Pd on a W(111) substrate, and facet sizes increase with annealing temperature. We used synchrotron radiation-based soft x-ray photoemission to show that monolayer films of Pt, Pd, on W "float" on the outer surface, whereas multilayer films form alloys on annealing. Acetylene reactions over bimetallic planar and faceted Pd/W surfaces exhibit size effects on the nanometer scale, that is, thermal desorption spectra of reactively formed benzene and ethylene (after acetylene adsorption) change systematically with facet size. In the second case, the decomposition of C₂H₂ over planar and faceted Ir(210) surfaces also exhibits structure sensitivity; temperature programmed desorption of H₂ from C₂H₂ dissociation depends on the nanoscale surface structure. Finally, we have characterized interactions of Cu with the highly ordered S(4 × 4)/W(111) surface. The substrate is a sulfur-induced nanoscale reconstruction of W(111) with (4 × 4) periodicity, having broad planar terraces (≈30 nm in width). Fractional monolayers of vapor-deposited Cu grow as threedimensional clusters on the S(4 × 4) surface over a wide coverage range. At low Cu coverage (≤0.1 ML), Cu nanoclusters nucleate preferentially at characteristic 3-fold hollow sites; we find a clear energetic preference for one type of site over others, and evidence for self-limiting growth of nanoclusters.