Characterization of the Brønsted acidity of PtSn/Al₂O₃ surfaces by adsorption of 2,6-di-tert-butylpyridine

The characterization of acid sites on solid catalysts is a key to understanding reaction mechanisms at the molecular level. This is generally done indirectly using a model probe molecule able to coordinate or be protonated on surface acid sites. For instance, Temperature Programmed Desorption (TPD) of the pyridine probe molecule followed by IR spectroscopy has become a standard method for the quantification and identification of Brønsted and Lewis acid sites found in zeolites. On the other hand, there is a lack of molecules able to be protonated on very weak Brønsted acid sites. In this work 2,6-di-tert-butylpyridine is shown to be a suitable probe for characterizing weak Brønsted acidity. The proof of concept is performed on a series of Pt catalysts supported on a chlorinated alumina support (Pt-γ-Al₂O₃-Cl) generally used in the reforming process. In particular, we show that the area of the 1617 cm⁻¹ vibration band is correlated to the concentration of Cl introduced into the catalyst. We also underline the relative insensitivity of pyridine as an IR probe molecule and the inadequacy of NH₃ TPD for the characterization of acid sites of reforming catalysts, making 2,6-di-tert-butylpyridine a particularly important and specific probe molecule.