n-Hexane conversion on γ-alumina supported palladium-platinum catalysts

Two series of Pd-Pt/Al2O3 catalysts, characterized by high metal dispersions were prepared and investigated in the reaction of n-hexane conversion at the temperature < 300 degrees C. Incipient wetness co-impregnation of gamma-alumina with the solutions of Pd(acac)(2) and Pt(acac)(2) led to relatively good Pd-Pt alloying. However, a similar catalyst preparation using solutions of PdCl2 and H2PtCl6 resulted in unsatisfactory homogenization; furthermore, significant amounts of chlorine were retained in the catalysts. The Pd-Pt alloy homogeneity has a significant effect on the relations of catalytic activities and product selectivities with Pd-Pt alloy composition. The relationship between the catalytic activity of chlorine-free Pd-Pt/Al2O3 catalysts and bulk Pd-Pt composition matches the accepted relation between the surface composition and bulk composition of Pd-Pt, reflecting a high surface enrichment in palladium. In contrast, for the chloride series of Pd-Pt/Al2O3, variations of the turnover frequency are directly correlated with the bulk composition, indicating that unalloyed, more active, Pt particles decide about the catalytic behavior. Pd/Al2O3 shows better than Pt/Al2O3 selectivity for isomerization, especially after reduction at higher temperatures, >= 500 degrees C. On alloying with Pt, for the Cl-free series the isomerization selectivity was very high for nearly all bimetallic catalysts and showed a synergistic effect for 20 at.% Pt. In contrast, for the Cl-containing series, the isomerization selectivity was proportional to the palladium content. The effect of reduction temperature on the isomerization propensity of Cl-containing Pd-Pt/Al2O3 catalysts was rather small, suggesting that possible variations in support acidity associated with chloride presence do not have large catalytic consequences, confirming the metal-only catalytic action in alkane isomerization carried out at the temperatures below 300 degrees C.