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

Analytical electron microscopy was performed on several Pt-Ph bimetallic catalysts. All catalysts prepared for this investigation contained alloy particles, but their composition distributions varied with preparation procedure. A catalyst with approximately 1 wt% total metal content and a relative metal composition of 60 wt% Pt and 40 wt% Rh, reduced 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 composition 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 those observed for several other catalysts are consistent with the presence of a low-temperature miscibility gap in the Pt-Rh equilibrium phase diagram. Catalysts were also prepared that were observed to contain only Pt-rich or only Rh-rich particles. High-resolution electron microscopy 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 homogeneous, and roughly circular in cross-section. Because of the small metal particle size, X-ray photoelectron spectroscopy was unable to determine if the metal particles were uniformly alloyed or if the particle surfaces were enriched with one alloy component. (C) 1995 Academic Press, Inc.