ALKANE CONVERSION AND TOPOLOGICAL SEGREGATION IN PTM AL2O3 CATALYSTS/

The site distribution of the two components at the surface of small bi metallic aggregates has been investigated by using the hydrogenolysis of model alkanes. The catalysts were supported on gamma-alumina and co nsisted of platinum particles (size 1-2 nm), modified by Sn, Pb, Ge, A l or Zn. They were prepared by using the surface reaction of platinum hydride with organometallic compounds. On platinum catalysts, the conv ersion of n-hexane shows a low sensitivity to changes of the surface t opology, corresponding either to a modification of the particle size o r 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. T eMB conversion yields large amounts of 2,2,3-trimethylbutane and metha ne on small particles, which have essentially sites of low coordinatio n (corners and edges), whereas isobutane is selectively formed on larg e particles, which show more sites of high coordination (facets, plane s). 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 thes e elements to these sites. Further tin addition shifts the catalytic p roperties of platinum towards those of smaller platinum particles, sho wing the dilution of the platinum surface by tin.