Calorimetric and FTIR study of the acid properties of sulfated titanias

Titanium oxides of different surface areas were sulfated then calcined to convert the solid to a strong acid. The amount of sulfur retained by the solid and the thermal stability of the resulting sulfate are controlled by the dispersion of the initial oxide. The acid properties were determined by gravimetry at 383 K, calorimetry using ammonia adsorption at 353 K, and by quantitative analysis of the infrared spectra of pyridine retained after evacuation at 423 K. A good agreement was observed between the different determinations. At low coverage of ammonia, sulfated titanias show a much lower heat of adsorption, and the IR study of NH3 adsorption shows that the first doses of NH3 dissociate at the surface with the formation of OH species. The lower heat of adsorption is then attributed to the contribution of NH3 dissociation to the differential heat of adsorption. IR spectroscopy indicates that NH3 reacts with sulfates and may lead to the transformation of disulfate species into monosulfate species on sulfated titania dioxide. A band at ca. 3574 cm-1 has been assigned to nu(OH) of monosulfate species. This particular behavior makes it difficult to appreciate the initial acidity of these sulfated oxides.