Limits in the use of pyridine adsorption, as an analytical tool to test the surface acidity of oxidic systems. The case of sulfated zirconia catalysts

When high-area oxidic systems carry surface anionic species (in the present case: stable sulfate groups at the surface of sulfated zirconia catalysts) , pyridine adsorption leads to an overestimated and thus partly artefact ev aluation of the surface Lewis acidity. LR spectroscopic evidence is present ed for the existence of a li:and competition at surface cationic centres (L ewis acid sites) between strong Lewis bases (pyridine) and surface sulfates . In the case of weak Lewis bases, like CO, there is no competition. The us e of a "model" sulfated zirconia system, obtained by sulfation of an yttria -stabilized tetragonal zirconia preparation, turned out to be vital for obt aining the mentioned evidence, as this preparation allowed to compare the s urface Lewis acidity in an homogeneous series of non-sulfated, sulfated, an d sulfated-and-calcined systems of virtually unchanged crystal phase, surfa ce area and crystal morphology. Also the conventional sulfated zirconia cat alysts, prepared from an amorphous hydrate precursor, present the same type of surface ligand competition and the partly artefact evaluation of surfac e Lewis acidity, but in that case a piece of information is missing in that physical and chemical properties of the non-sulfated amorphous precursor c annot be compared with those of the crystalline sulfated systems.