CERIA ON SILICA AND ALUMINA CATALYSTS - DISPERSION AND SURFACE ACID-BASE PROPERTIES AS PROBED BY X-RAY-DIFFRACTOMETRY, UV-VIS DIFFUSE-REFLECTANCE AND IN-SITU IR ABSORPTION STUDIES

X-ray diffractometry and UV-Vis diffuse reflectance spectroscopy revea led that fluorite-structured CeO2 crystallites (mean size 22.3 ma) are dispersed on silica surfaces (CeSi) into microcrystallites (11.2-8.1 nm) and dispersed further on alumina surfaces (CeAl) into nanocrystall ites (<8.5 nm) and CeOx monolayers. Consequently, IR spectroscopy of a dsorbed pyridine found Lewis acid sites to be far more strengthened on CeAl. Bronsted acid sites (proton-donors) were probed exclusively on CeAl. On the other hand, IR spectroscopy of adsorbed deuterated chloro form (CDC,) showed the originally moderate Lewis base sites (low-coord inated OH- and O2-) to be weakened on CeSi, but markedly strengthened on CeAl. Lewis base sites exposed on ceria surfaces assume a strong nu cleophilic reactivity. (C) 1997 Elsevier Science B.V.