Abstract
The effect of adding Sn to Pt and Rh particles on SiO{sub 2} is examined by transmission electron microscopy following heat treatments in H{sub 2} and O{sub 2} atmospheres. With both metals, after heating in H{sub 2}, no fcc metal is observed but rather a series of intermetallic compounds with multiple phases coexisting within most 20 to 200-{angstrom} particles. The surfaces of these particles are the Sn-rich MSn{sub 4} phases. When Pt-Sn is oxidized, the only phases observed are Pt metal and SnO{sub 2}. Dark field imaging shows that the SnO{sub 2} forms 10 to 30-{angstrom} crystals which surround the single-crystal Pt particles and are in contact with the SiO{sub 2}. When Rh-Sn is oxidized Rh, Rh{sub 2}O{sub 3}, and SnO{sub 2} are detected, but SnO{sub 2} forms before Rh{sub 2}O{sub 3}, and the presence of Sn retards the oxidation of Rh.
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