Titrating surface acidity of sulfated zirconia catalysts: is the adsorption of pyridine a suitable probe?

A model system of sulfated zirconia (SZ) has been studied, obtained by sulfation of ZrO stabilized in the 2 tetragonal form by forming a solid solution with Y2O3. It has been thus possible to compare the surface properties of non-sulfated tetragonal zirconia (t-ZrO2), of sulfated t-ZrO2, and of sulfated and calcined t-ZrO2 systems possessing constant crystal phase, crystal morphology, and surface area. When the surface acidity of SZ is probed by the IR study of pyridine (py) adsorption at ambient temperature, as currently done in surface chemistry, it is observed that : (i) the estimated BrÔnsted acidity is probably correct (provided the adopted extinction coefficients are correct) ; (ii) the estimated Lewis acidity is usually overestimated, and to an extent that depends on the surface concentration of sulfates. As the surface concentration of sulfates depends on the morphology of the ZrO crystallites, and in a discontinuous way on the thermal treatment (calcination) undergone by the SZ system, it is deduced that the actual concentration of Lewis acid sites at the surface of SZ systems is difficult to attain by the adsorption of a strong (hard) Lewis base such as py.