Morphology and composition of PtPd alloy crystallites on SiO 2 in reactive atmospheres

Abstract

Scanning transmission electron microscopy and energy dispersive X-ray microanalysis are used to examine the structure, sintering rates and chemical composition of 20 to 200-Å-diameter PtPd alloy crystallites on planar amorphous SiO 2. Oxygen causes PtPd alloy crystallites to separate into distinct crystallites of metal and tetragonal PdO, both of which are in contact with the SiO 2 support. Oxide is first observed at 300 to 400 °C depending on composition, and the oxide begins to decompose at 650 °C and completely disappears between 740 and 850 °C. For high Pt alloys the oxide nucleates and grows at one side of the metal particles but for high Pd alloys small (20 to 40 Å) Pt particles form around the edges of the PdO crystal. The oxide can be reduced by H 2 even at room temperature and this produces distinct and separated Pt and Pd-rich particles as shown by single particle microanalysis. These morphologies are presumably caused by stronger interfacial interactions between PdO and SiO 2 compared to that between metal and the oxides. A stability diagram for formation of oxide shows that oxide is formed only for alloys containing at least 15% Pd. Evaporation of Pt as PtO 2 is inhibited by the presence of Pd and growth of crystallites is slower for alloys than for pure Pt metal. Implications of these results on the structures of supported alloy catalysts will be discussed.