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.