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 evaluation of the surface Lewis acidity. IR spectroscopic evidence is presented for the existence of a ligand competition at surface cationic centres (Lewis acid sites) between strong Lewis bases (pyridine) and surface sulfates. In the case of weak Lewis bases, like CO, there is no competition. The use of a “model” sulfated zirconia system, obtained by sulfation of an yttria-stabilized tetragonal zirconia preparation, turned out to be vital for obtaining the mentioned evidence, as this preparation allowed to compare the surface Lewis acidity in an homogeneous series of non-sulfated, sulfated, and sulfated-and-calcined systems of virtually unchanged crystal phase, surface area and crystal morphology. Also the conventional sulfated zirconia catalysts, prepared from an amorphous hydrate precursor, present the same type of surface ligand competition and the partly artefact evaluation of surface Lewis acidity, but in that case a piece of information is missing in that physical and chemical properties of the non-sulfated amorphous precursor cannot be compared with those of the crystalline sulfated systems.
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
MORTERRA, Claudio;CERRATO, Giuseppina;
2001-01-01
Abstract
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 evaluation of the surface Lewis acidity. IR spectroscopic evidence is presented for the existence of a ligand competition at surface cationic centres (Lewis acid sites) between strong Lewis bases (pyridine) and surface sulfates. In the case of weak Lewis bases, like CO, there is no competition. The use of a “model” sulfated zirconia system, obtained by sulfation of an yttria-stabilized tetragonal zirconia preparation, turned out to be vital for obtaining the mentioned evidence, as this preparation allowed to compare the surface Lewis acidity in an homogeneous series of non-sulfated, sulfated, and sulfated-and-calcined systems of virtually unchanged crystal phase, surface area and crystal morphology. Also the conventional sulfated zirconia catalysts, prepared from an amorphous hydrate precursor, present the same type of surface ligand competition and the partly artefact evaluation of surface Lewis acidity, but in that case a piece of information is missing in that physical and chemical properties of the non-sulfated amorphous precursor cannot be compared with those of the crystalline sulfated systems.
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