Abstract
Multinuclear 1 H, 13C, 17O, 29Si MAS and 93Nb static NMR is reported from a series of sol–gel prepared (Nb2O5)x(SiO2)1x materials with x ¼ 0:03; 0.075 or 0.30. 13C NMR shows that by 500 1C the organic precursor fragments have been removed although some residual carbon remains as a separate phase. The 29Si NMR typically shows three Q-species (Q2,3,4) in the initial gels, and that with increasing heat treatment the average n of the Qn -species increases as the organic fragments and hydroxyl groups are removed. 17O shows unequivocally that the x ¼ 0:03 and 0.075 samples are not phase separated, while at the much higher niobia-content of x ¼ 0:30 Nb–O–Nb signals are readily detected, a definite indication of the atomic scale phase separation of Nb2O5. Th e x ¼ 0:03 and 0.075 samples heated to 750 1C are thus representative of amorphous niobium silicates. Comparison is made to other sol–gel prepared metal silicates especially with another Group Va metal tantalum. The effects of tantalum and niobium on the silica network are very different and it is suggested here that most of the niobium is present as NbO4, forming part of the silicate network.