Structural properties of an unsupported model Pt–Sn catalyst and its catalytic properties in cyclohexene transformation

Abstract

Unsupported PtSn powder was prepared by direct reduction of a solution containing both H 2PtCl 6 and SnCl 4 using hydrazine as the reducing agent. The dark gray powder was characterized with scanning electron microscopy (SEM), EDX analysis, Mössbauer spectroscopy, X-ray diffraction, XPS depth profiling after different treatments: presintering, O 2 and H 2 treatments. SEM showed a conglomerate of small spherical particles (0.2–1.5 μm). They contained Pt, various PtSn alloy phases and tin-oxide(s). EDX showed 70–75% Pt and 25–30% Sn on various grains. The mixture of Pt 3Sn and SnO 2 represented the final stabilized state obtained upon repeated heating in air and, finally, H 2. This mixed Pt–Sn was catalytically inactive in “structure-sensitive” reactions, such as methylcyclopentane ring opening or cyclohexane dehydrogenation, but was active in “structure-insensitive” hydrogenation and also in the dehydrogenation of cyclohexene. The relative importance of the latter two reactions depended strongly on the previous treatments of the catalyst—i.e., on its composition, the final stage (Pt 3Sn and SnO 2) being most active, with cyclohexane as the main product.