Preparation of colloidal platinum/palladium alloy particles from non-ionic microemulsions: characterization and catalytic behaviour

Bimetallic particles of platinum and palladium have been prepared by reduction, with hydrazine at arabient temperature, of H2PtCl6 and PdCl2 dissolved in microemulsions consisting of water, hexadecane and pentacthylene glycol dodecyl ether. The particles were deposited onto Al2O3. Examination by scanning transmission electron microscopy and extended X-ray adsorption fine structure measurements of particles with atomic ratio Pt:Pd = 35:65 showed that the particles are true Pt/Pd alloys. Indirect evidence was also obtained that particles containing only 4% Pd are alloys. The diameter of the particles ranges from 10 to 100 nm; there is evidence that they are formed by aggregation of particles in the diameter range 2–5 nm.

The catalytic properties of the Pt/Pd/Al2O3 catalysts prepared from microemulsions were similar to those of catalysts prepared by conventional impregnation. Catalysts with low Pd content had the same selectivity in the isomerization of 2-methylpentane and hydrogenolysis of methylcyclopentane as a highly dispersed Pt catalyst (or a pure Pd catalyst). The catalytic cracking of 2-methylpentane indicates that the particle surfaces behave as Pt surfaces up to a Pd concentration of 60% in the particles. Isomerization of hexanes is dominated by the cyclic mechanism even at Pd concentrations as low as 4%; for these very low Pd concentrations hydrogenolysis of methylcyclopentane is the same as observed for very small Pt particles. It is concluded that Pd is enriched in the particle surfaces in such a way that they consist of Pd and some highly dispersed Pt. even at bulk Pd concentrations as low is 4%.