Physicochemical and Catalytic Properties of Vanadia/Titania Catalysts. III. Acidic Properties

FT-IR spectral studies of titania calcined at 400°C or 600°C indicated the presence of four important absorption bands with maxima located at 1630, 2920, 3670 and 3740 cm⁻¹, respectively. These absorption bands have been ascribed to undissociated, coordinated water, hydrogen-bonded OH, acidic OH and basic OH groups, respectively. For vanadia/titania samples containing 6 wt% and 12 wt% V₂O₅ calcined at 400°C or 600°C, the band at 1630 cm⁻¹ still existed in the corresponding spectra whereas that at 2920 cm⁻¹ disappeared, thereby indicating an interaction between the vanadia and the hydrogen-bonded OH groups.

The FT-IR spectra of pyridine adsorbed on to titania or titania/vanadia samples exhibited bands at 1420 cm⁻¹ and 1540 cm⁻¹, respectively, the former being ascribed to Lewis acidity and exhibiting a shift to lower wavenumbers with increasing vanadia content, while the latter was ascribed to the presence of Brönsted acid sites.

Temperature programmed desorption (TPD) of pyridine from titania exhibited three desorption peaks extending over the temperature ranges 267–288°C, 335–370°C and 480–510°C, respectively. Impregnation of the initial titania samples with vanadia led to an increase in both the amount of acid sites present on the surfaces of the samples and in the acid strength. Besides reacting with Brönsted and Lewis acid sites, pyridine also underwent hydrogen bonding, with the hydrogen atom serving as a bridge between two electronegative nitrogen atoms.