Surface and bulk characterisation of titanium-oxo clusters and nanosized titania particles through O-17 solid state NMR

For the first time, titanium-oxo based nano-objects have been probed using O-17 MAS and 3Q-MAS NMR. Three titanium oxo-organo clusters {[Ti12O16(OPri) (16)], [Ti16O16(OEt)(32)] and [Ti18O22(OBun)(26)(acac)(2)]}, and monodisper se nanoparticles of titania anatase having 20 and 30 Angstrom oxide core di ameters, have been characterised by O-17 NMR. This study shows that in tita nium oxo-based compounds, the mu(2)-O sites are dominated by a significant chemical shift anisotropy, an interaction that is much weaker for the other mu(n)-O sites (n=3, 4, 5). For all mu(n)-O sites (n=2, 3, 4 or 5), the 1(7 O) NMR linewidths are dominated by chemical shift distribution with a minor contribution from second order quadrupolar broadening. However, depending on the degree of distortion from tetrahedral geometry, the mu(3)-O sites ca n also be sensitive to second order quadrupolar effects. Bulk mu(3)-O and s urface oxo species (mu(3)-O, mu(2)-O and acac-Ti) present in titania anatas e nanoparticles are identified and clearly assigned. The ratio between bulk and surface species decreases as the particle size is increased. The surfa ce reconstruction of the nanoparticles in the presence of (OH2)-O-17 enrich ed moist air, at room temperature, is demonstrated by O-17 MAS NMR experime nts. From these experiments the mu(3)-O, mu(2)-O, Ti-OH, acac-Ti and H2O--> Ti surface species were identified.