Morphologies of Sn and PtSn phases on thin films of alumina and graphite

Abstract

Transmission electron microscopy studies of Sn and Pt-Sn morphologies were conducted on model catalysts, prepared by the vacuum evaporation of metal onto thin films of γ-alumina and sheets of graphite. All supported Sn samples showed nonwetting metallic particles following treatment in hydrogen. Small (<20 nm) particles assumed a core/shell morphology on alumina, due to the existence of a thin oxidic tin-support phase surrounding or beneath a metallic core, whereas the metallic particles on graphite showed uniform contrast. Upon oxidation, metallic Sn particles transformed into toroidal-shaped particles, indicative of an interaction between tin oxide and the substrates. Upon addition of Pt to the supported Sn samples, Pt-Sn alloy formation was observed under reducing conditions. On alumina, the particles were round and surrounded by a light apron. Graphite-supported particles were round and featureless. Upon oxidation of all specimens, the alloy phase was destroyed, as the tin was preferentially oxidized to SnO 2 while the platinum remained metallic, forming a characteristic core/shell appearance. The presence of tin retarded the catalytic oxidation of the graphite substrate by platinum at 770 K. A fraction of the surface tin oxide layer formed during the oxidation treatment of Pt-Sn/Al 2O 3 did not re-alloy with the platinum upon reduction, due to interaction with the alumina support.