Comparison of microstructures in oxidation and reduction: Rh and Ir particles on SiO 2 and Al 2O 3

Abstract

X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) are used to compare the microstructures and fractions of metal and oxide formed in oxidation and subsequent reduction of ~ 100-Å particles of Rh and Ir on SiO 2 and Al 2O 3 by treating identically prepared samples under identical conditions in O 2 and H 2. XPS shows that both metals are oxidized at ~200 °C higher temperatures on SiO 2 than on Al 2O 3, and reduction in H 2 occurs at a slightly lower temperature on Al 2O 3. For TEM, metal particles are deposited on curved regions of SiO 2 and Al 2O 3 on microscope grids so that particle shapes perpendicular to the substrates can be examined. Rh oxidation occurs largely by film growth and is complete by 600 °C, while Ir does not oxidize significantly until 500 °C, and complete oxidation does not occur even at 700 °C. At higher temperatures the mode of oxide formation changes from film growth to nonuniform oxide penetration into the particles. Oxidation of Ir produces protrusions which grow out from the original metal particles. Subsequent reduction of both oxides produces clusters of 10- to 20-Å particles which only sinter into their original shapes after heating to above 500 °C.