The site distribution of the two components at the surface of small bimetallic aggregates has been investigated by using the hydrogenolysis of model alkanes. The catalysts were supported on gamma-alumina and consisted of platinum particles (size 1-2 nm), modified by Sn, Pb, Ge, Al or Zn. They were prepared by using the surface reaction of platinum hydride with organometallic compounds. On platinum catalysts, the conversion of n-hexane shows 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 the modifier. At variance, the conversions of 2,2,3,3-tetramethylbutane (TeMB) and methylcyclopentane (MCP) are highly sensitive to the topology of the metallic surface. TeMB conversion yields large amounts of 2,2,3-trimethylbutane and methane on small particles, which have essentially 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, Zn or Al (M/Pt < 0.2, at./at.) favors the formation of isobutane. Therefore, Sn, Zn and Al inhibit selectively the reactions catalyzed by the sites of low coordination, and this demonstrates that there is a topological segregation of these elements to these sites. Further tin addition shifts the catalytic properties of platinum towards those of smaller platinum particles, showing the dilution of the platinum surface by tin.
