Nanoscale surface chemistry

Abstract

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 Pd͞W 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 C 2H2 over planar and faceted Ir(210) surfaces also exhibits structure sensitivity; temperature programmed desorption of H 2 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 (Ϸ30 nm in width). Fractional monolayers of vapordeposited 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.A n important issue in surface chemistry and catalysis is how surface structures and features with nanometer dimensions affect reactivity in heterogeneous systems (1-3). The focus of our work has been on several aspects of nanoscale phenomena that influence surface chemistry, including faceting of metallic and model bimetallic catalyst surfaces, and nucleation of subnanometer metallic clusters on sulfided surfaces. We study atomically rough substrates [bcc (111) surface of W, and fcc (210) surface of Ir] that are morphologically unstable, that is, the initially planar substrate becomes covered with nanoscale facets when covered with monolayer films of gases or other metals, and heated to elevated temperature. Major objectives of this work have been (i) to determine how the surface transition from planar to faceted affects the surface reactivity of metallic and bimetallic systems, and (ii) to characterize the nucleation and growth of metals on sulfided W surfaces. The three main components of this effort are surface structure, surface chemistry, and surface electronic properties.The importance of bimetallic catalysts based on Pt-group metals has been increasing in recent decades (4, 5). These catalysts display important advantages over classical reforming catalysts, including better stability, as well as improved activity and selectivity. In particular, refractory metals (W, Mo, Re, . . .) in combination with ...

Effect of reducible oxide–metal cluster charge transfer on the structure and reactivity of adsorbed Au and Pt atoms and clusters on anatase TiO2

“…The presence of co-adsorbed O 2 appears to have opposite effects on the computed CO adsorption energies on supported Pt 3 and Au 3 : it reduces that of CO on Pt 3 /TiO 2 by ∼0.3 eV and increases E CO ads on Au 3 /TiO 2 by approximately the same amount (Table IV) in the absence (Table III) and presence (Table IV) …”

“…On stoichiometric TiO 2 , two Pt atoms of the trimer bind with two neighboring O 2c at distance 2.056 Å and two Ti 5c at distance 2.664 Å. Consequently, the Pt-Pt bond parallel to the surface is slightly longer (2.516 Å) than the other two Pt-Pt bonds (2.508 Å) pointing away from Similarly Figure 4. For Pt 3 , we can see a large number of metal states in the TiO 2 band gap, which makes the overall band gap nearly vanishing. On the stoichiometric surface (Figure 4(a)), no Ti occupied state is present near the Fermi level (E F ), as expected.…”

“…This implies that the TiO 2 excess electron is transferred to the cluster, which then transforms to the closed shell Au 3 anion. Finally, the PDOS in Figure 4(f) for Au 3 on r 2 TiO 2 indicate that one of the two excess electrons is transferred to the cluster, which becomes Au 3 , while the other remains on the surface. with increasing surface reduction.…”

“…Noble metal nanoparticles (NPs) supported on oxide surfaces are widely used in heterogeneous catalysis [1][2][3][4][5][6][7][8][9] and photocatalysis, where they can effectively inhibit the recombination of photogenerated electron-hole pairs in photocatalytic processes, 10,11 and at the same time facilitate/enhance the catalytic activity. 11,12 Among oxide supports for metal NPs, titanium dioxide (TiO 2 ) is known for showing significant promoting effects of the NPs' catalytic activity.…”

Hydrogen mediated transport of Sn to Ru film surface

“…It has been observed that atomically rough, high energy surfaces may be morphologically unstable during reaction conditions. 9 In contrast to the closed packed faces, the high-index surfaces may undergo massive structural rearrangements ͑e.g., faceting͒ when covered by a metallic film. It is also shown that film growth mode on nanofaceted surface may be very different from a growth mode on the flat substrate.…”