However, in most industrial catalysts preparations, a number of components are deposited
on the surface of the support besides the active phase(s), such as stabilizers, activators,
etc. As discussed years ago for the V2O5-WO3-K2O/TiO2 system, in some way
supported species compete for anchoring on the more reactive surface sites at the surface
of support. According to recent studies, it seems likely that on the alumina surface the
most active surface sites are on edges, corners and defects of the oxide structure, the
sites located on flat faces being likely less reactive. Over the defect sites the deposited
species would first interact, while at higher coverages they also occupy less reactive sites.
Silica is a common additive of aluminas mainly to stabilize Al2O3 against loss of surface
area. The presence of small amounts of silica on the alumina support may modify the
dispersion of the active phases with a resulting modification of the final catalyst behaviour.
This has been recently shown e.g. for Co/Al2O3 Fischer Tropsch catalysts and for Ni-Mo
sulphide HDS catalysts.