¹H NMR spectroscopy of titania: Chemical shift assignments for hydroxy groups in crystalline and amorphous forms of TiO₂

¹H magnetic-angle spinning (MAS) NMR measurements have been performed on a number of crystalline titanias, and on amorphous silica-supported titania and titania-silica, with the aim of measuring the characteristic proton chemical shifts of hydroxy groups bound to titanias of different crystalline form. In the case of anatase, signals observed at δ = 2.3 and 6.7 ppm correspond to terminal and bridging hydroxy groups, the results of deuterium exchange experiments (using D₂O) and IR data supporting these assignments. For rutile, signals observed at δ = 2.2 and 5.3 ppm are similarly assigned. Hydroxy groups bound to amorphous titania supported on silica (containing tetrahedrally coordinated Ti^IV) are found to possess a characteristic chemical shift of δ = 3.3 ppm. Deconvolution of ¹H NMR spectra of titania-silica (containing 8 wt.% Ti) indicate the presence of a signal at δ ≈ 3.3 ppm, which is similarly assigned to hydroxy groups bound to tetrahedrally coordinated Ti^IV, together with signals assigned to anatase and silanol groups. These observations are consistent with literature reports indicating the presence of two main titania phases in titania-silicas: an amorphous phase containing isolated Ti sites tetrahedrally coordinated by Si-O and OH groups, and segregated nanodomains of TiO₂ (anatase or rutile).