Structure and catalytic activity of palladium–platinum aggregates obtained by laser vaporisation of bulk alloys

The European emission legislation concerning the pollution abatement of the exhaust gas from light cars imposes a reduction of the concentration of aromatics in gasoline, down to 2%. The hydrogenation of these hydrocarbons can be performed on Pd or Pt catalysts. However, these catalysts are poisoned by traces of sulphur — 100–300 ppm — remaining in the fuels. It is claimed in the literature that bimetallic Pd–Pt catalysts are less affected by this poisoning than the pure metals. To verify this point, both Pd–Pt and pure metal aggregates were synthesised by laser vaporisation of bulk alloys and deposited on alumina. Analytical microscopy and EXAFS have shown that these clusters have a very narrow composition distribution and form alloys. The study of their activity in the hydrogenation of Tetralin, a model molecule, in the presence of variable amounts of H2S, has shown that Pt is more active than Pd at low sulphur concentration, whereas Pd becomes more active for the highest H2S content. Contrary to what is claimed in the literature, no synergetic effect has been found found by alloying these two metals; actually their activity is the simple additivity of that of the two metals.