Site segregation in small rhodium bimetallic aggregates: A combined catalytic and quantum chemical study

Article abstract of DOI:10.1021/ja00005a002

The spatial distribution of the two components at the surface of small bimetallic aggregates has been investigated by using the hydrogenolysis of model alkanes and quantum chemical calculations. The catalysts were supported on γ-alumina and consisted of rhodium particles (size ? 1-2 nm), modified by Sn, Pb, Sb, or Ge. They were prepared by using the surface reaction of rhodium hydride with organometallic compounds. On pure Rh catalysts, the conversions of n-hexane and methylcyclopentane show a low sensitivity to changes of the surface topology, corresponding either to a modification of the particle size or to the addition of a small amount of Sn or Ge modifier. At variance, the hydrogenolysis of 2,2,3,3-tetramethylbutane is highly sensitive to the topology of the surface and yields selectively 2,2,3-trimethylbutane and methane on small particles, which have mainly sites of low coordination (corners and edges), whereas isobutane is selectively formed on large particles, which show more sites of high coordination (facets, planes). The addition of a small amount of Sn or Pb favors the formation of isobutane, but the addition of an equivalent amount of Ge promotes demethylation. Therefore, Sn and Pb inhibit selectively the reactions catalyzed by the sites of low coordination, and this demonstrates that there is a topological segregation to these sites, while Ge atoms appear to be randomly distributed at the surface. Quantum chemical calculations of the relative stabilities of Rh₁₀M₄, Rh₁₀M₃, and Rh₁₂M model clusters (M = Sn, Ge), with different geometries and different localizations of M, support the conclusion that the most stable aggregates are those where the M atoms are localized at the periphery. However, Sn atoms are shown to segregate essentially to the corner sites, whereas Ge atoms do not have as definite a preference. Moreover, Ge differs from Sn in the sense that Ge atoms tend to disperse in the Rh lattice, while Sn atoms tend to form Sn-Sn bonds.