Pyridine adsorption is used to study the acidic sites of silica-supported CuO-MoO 3 catalysts, with
reference to CuO/SiO2 and MoO3/SiO2 catalysts. At low wt% of copper, both Lewis and Bronsted acid sites
are present on the CuO-MoO3/SiO2 surface. A substantial decrease and then vanishing of the Bronsted acidity
is obtained as a result of increasing the copper loading. An approach confining the prevalence of Lewis acidity
over the Bronsted ones is described and discussed. A new band of Lewis acidity is observed at 1574 cm -1 and
attributed to the presence of CuMoO4 species. This band is not observed in the individual oxide form of both
CuO/SiO2 and MoO3/SiO2 catalysts. Carbon monoxide adsorption on CuO-MoO3/SiO 2 catalysts reflects a
major dispersion if it is compared with CuO/SiO: and MoO3/SiO~ catalysts and also gives a picture about the
expected structures of these supported oxide catalysts. Copper is found also to exert a major control on the
mode of interaction of molybdenum species with the silica surface. Moreover, important correlations and shifts
in the IR bands are evaluated and discussed.