M. Benaissa et al., INTERACTION OF SULFATE GROUPS WITH THE SURFACE OF ZIRCONIA - AN HRTEMCHARACTERIZATION STUDY, Journal of catalysis, 161(2), 1996, pp. 694-703
High-resolution transmission electron microscopy (HRTEM) has been used
to characterize the morphology and the surface structure at an atomic
level of sulfated zirconia and sulfate-free zirconia. Our study shows
that HRTEM can be used to directly observe sulfate layers adsorbed on
the surface of zirconia crystallites. The results indicate that the p
resence of sulfate groups not only stabilizes the tetragonal zirconia
phase, but also induces the formation of well-faceted small zirconia c
rystallites. In particular, it is observed that the presence of sulfat
e groups induces the preferential formation of relatively long-flat (1
10) plane of tetragonal zirconia. HRTEM images of this plane revealed
the presence of an adsorbed sulfate layer. It is proposed that the geo
metry of the (110) plane is such that it can accommodate sulfate group
s in a two- or threefold coordination. Although, zirconia crystallites
with relatively long-flat {110} planes containing adsorbed sulfate-gr
oups were clearly predominant in the sulfated zirconia catalyst; its r
ole in the formation of the highly acidic sites is not clear. Our stud
y also reveals the presence of few zirconia crystallites containing ro
ugh surfaces, crystallographically speaking high-Miller-index surfaces
, which if they were to contain sulfate groups, they could be the locu
s of the highly acidic sites. Thus, it is suggested that caution must
be taken when performing spectroscopic studies using techniques such a
s IR, NMR, XPS, and Raman to distinguish between spectator sulfate gro
ups, which could be a majority if they were the ones observed on the (
110) plane, and those participating in the formation of the highly aci
dic site, which could be associated to the few high-Miller-index-conta
ining zirconia crystallites. (C) 1995 Academic Press, Inc.