ALUMINA-SUPPORTED PT-RH CATALYSTS .1. MICROSTRUCTURAL CHARACTERIZATION

Analytical electron microscopy was performed on several Pt-Ph bimetall ic catalysts. All catalysts prepared for this investigation contained alloy particles, but their composition distributions varied with prepa ration procedure. A catalyst with approximately 1 wt% total metal cont ent and a relative metal composition of 60 wt% Pt and 40 wt% Rh, reduc ed at 300 degrees C for 12 h, displayed a bimodal particle composition distribution with particle compositions centered at approximately 10 wt% Pt and 90 wt% Pt. Few particles possessed the expected average com position of 60 wt% Pt. The same catalyst when reduced at 700 degrees C for 72 h displayed a broad particle composition distribution centered at about 60 wt% Pt. Together, these composition distributions and tho se observed for several other catalysts are consistent with the presen ce of a low-temperature miscibility gap in the Pt-Rh equilibrium phase diagram. Catalysts were also prepared that were observed to contain o nly Pt-rich or only Rh-rich particles. High-resolution electron micros copy identified two discrete particle morphologies, particles >7 mn in diameter were faceted and often internally faulted or twinned, while particles <7 nm displayed no internal faults and appeared to be homoge neous, and roughly circular in cross-section. Because of the small met al particle size, X-ray photoelectron spectroscopy was unable to deter mine if the metal particles were uniformly alloyed or if the particle surfaces were enriched with one alloy component. (C) 1995 Academic Pre ss, Inc.