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

Abstract

¹ H 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).