| Scanning transmission electron microscopy (STEM) and X-ray photoelectron spectroscopy (XPS) are used to examine the effects of air, H(,2), H(,2)S and SO(,2) on the structure and surface composition of 20-200 A particles of Pt, Pd, Rh and Ir and their alloys on SiO(,2) and (gamma)-Al(,2)O(,3).; It is found using STEM that iridium oxide can first be observed to form in air at pressure of 1 atm at 300C, and by 700C substantial amount of oxide form around metal particles and spread as a thin layer over the SiO(,2) support. At 750C the residual metal particles are observed to break up into clusters of several smaller 10-30 A particles. Oxidation is complete only at 900C. When small particles formed by heating to 750C in air are subsequently heated in H(,2), all oxide is reduced by 400C to form clusters of small Ir particles. These remain stable to 600C, above which sintering to larger particles occurs.; Morphologies produced by oxidation-reduction of 20-200 A diameter Rh and Pt-Rh alloy particles on planar SiO(,2) and Al(,2)O(,3) are examined by STEM. It is found that redispersion of small crystallites of Rh can be achieved by first oxidizing Rh to Rh(,2) O(,3) in air at 600C, and then reducing the oxide particles at lower temperatures. Particles after reduction are converted to small clusters of metal 10-20 A in diameter or form crack. For Pt-Rh alloys, Rh(,2)O(,3) forms at the edge of Pt metal cores. Reduction at 25C then produces small Rh particles at the edges of the original particle. Upon heating to 500C, particles coalesce into roughly their original morphologies, but the cores are primarily Pt and the surfaces of particles are enriched in Rh. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of school.) UMI... |
