MICROSTRUCTURES OF PT-CE AND RH-CE PARTICLES ON ALUMINA AND SILICA

The effects of adding Ce to 20-200 Å diameter particles of Pt and Rh on planar Al2O3 and SiO2 substrates are examined using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and electron energy-loss spectroscopy (EELS), following treatments in H2 and O2 atmospheres. Samples were transferred repeatedly between furnace and TEM so that the time evolution of the microstructure of individual particles could be followed. TEM shows that upon heating Pt alone on Al2O3 or SiO2 in H2, Pt forms cubic particles, while when Pt and Ce are codeposited, the Pt particles are about the same size but no longer have square outlines. XPS shows that the Pt peak shifted to Pt+2 when heated in O2 on both Al2O3 and SiO2 when Ce was present and returned to Pt0 after heating in H2. The Pt particles tend to shrink or disappear after heating in O2 at 650C, and this is much faster without Ce, showing that Ce retards the volatilization of Pt oxides. When Rh-Ce on Al2O3 of SiO2 is heated in H2, Rh forms particles of no preferred shape. After heating in O2 to 600°C, Rh oxidizes to Rh2O3 and remains particulate on Al2O3, but spreads to a film on SiO2 in the presence of Ce. Upon reduction, Rh2O3 reduces to metallic Rh and forms particles of approximately the same size as the original particles on A1203 and larger than the original size on SiO2. Ce2Si2O7 has been characterized on SiO2 adjacent to Rh particles after reduction of an oxidized sample, but CeAlO3 has not been observed on Al2O3. © 1992.