MICROSTRUCTURES OF PT-SN AND RH-SN PARTICLES ON SIO2

The effect of adding Sn to Pt and Rh particles on SiO, is examined by transmission electron microscopy following heat treatments in H2 and O2 atmospheres. With both metals, after heating in H2, no fcc metal is observed but rather a series of intermetallic compounds with multiple phases coexisting within most 20 to 200-A particles. The surfaces of these particles are the Sn-rich MSn4 phases. When Pt-Sn is oxidized, the only phases observed are Pt metal and SnO2. Dark field imaging shows that the SnO2 forms 10 to 30-Å crystals which surround the single-crystal Pt particles and are in contact with the SiO2. When Rh-Sn is oxidized Rh, Rh2O3, and SnO2 are detected, but SnO2 forms before Rh2O3, and the presence of Sn retards the oxidation of Rh. © 1991.