XANES Study of Ti Coordination in Heat-Treated (TiO₂)_x(SiO₂)_1-x Xerogels

Abstract

Accurate measurements of the height and position of the preedge peak in Ti K-edge XANES (X-ray absorption near-edge structure) can distinguish 4-, 5-, and 6-fold coordination of Ti by O ([4]Ti, [5]Ti, and [6]Ti, respectively), and mixtures thereof. This approach has been applied to titania−silica xerogels, (TiO2) x (SiO2)1 - x , prepared from Si and Ti alkoxides using a prehydrolysis step and acid catalyst, with 0.18 < x < 0.75. As expected, samples with x = 0.75 contain phase-segregated, amorphous, pre-anatase [6]Ti, which is converted to anatase after heating at 500 °C. Samples with x = 0.18 contain a majority of isolated, distorted [6]Ti before heating, which is converted to [4]Ti substituted for Si after heating at 750 °C. This is the first time that the changes in the coordination of isolated Ti have been unambiguously described. Xerogels with 0.25 < x < 0.35 and heat treatment at 750 °C showed concentration limits of [4]Ti substitution for Si of 10−15mol %. Slightly higher values were obtained for xerogels prepared using acetylacetone, but the main difference was that the latter had a significantly larger content of distorted [6]Ti before heat treatment. There is no evidence of significant amounts of [5]Ti.

Environment of titanium and aluminum in a magnesium alumino-silicate glass

“…27 Al nuclear magnetic resonance (NMR) results revealed that the proportion of highly coordinated aluminum atoms in this titanium-bearing glass was higher than in the titanium-free sample. RMC modeling was used to interpret the structural role of these [5] Al species and we show a trend for preferential bonding between [5] Al and Ti atoms. This favored linkage is important to understand the role of titanium dioxide as a nucleating agent in inorganic glass-ceramics fabrication.…”

“…In the COR glass, enough Mg 2+ ions are present to charge-compensate the negative charge of the aluminate tetrahedra and, looking only to the glass composition, all aluminum atoms are expected to be [4] Al. However, it appears that significant amounts of [5] Al Figure 5. 27 Al 1D NMR experimental spectra and the fitting models obtained with parameters given in table 2 (top) and contour plots of 27 Al MQ-MAS NMR spectra (bottom) for the (a) 2MgO-2Al 2 O 3 -5SiO 2 (COR) glass and for the (b) 2MgO-2Al 2 O 3 -5SiO 2 -TiO 2 (Ti-COR) glass.…”

“…The environment of titanium has been mainly investigated in the TiO 2 -SiO 2 binary system [3][4][5] and Ti-bearing alkali or alkaline-earth silicate glasses [6][7][8][9][10][11][12]. All these studies show that titanium atoms are present in the glass with multiple coordination numbers from four to six.…”

“…At low TiO 2 content (up to 6 mol% TiO 2 ), TiO 2 -SiO 2 glasses contain predominantly fourfold-coordinated titanium atoms, [4] Ti, and Ti is assumed to substitute for silicon in tetrahedral units. As the TiO 2 content increases and exceeds 6 mol%, the amount of fivefoldcoordinated, [5] Ti, or sixfold-coordinated, [6] Ti, titanium atoms increases.…”

“…Computer modeling of the diffraction data was performed using the reverse Monte Carlo method to provide a structural model of the 2MgO-2Al 2 O 3 -5SiO 2 -TiO 2 glass. The detailed Ti coordination is obtained showing the existence of a mixture of [4] Ti, [5] Ti and [6] Ti. The presence of TiO 2 modifies the alumino-silicate network, favoring the formation of fivefoldcoordinated aluminum atoms, [5] Al.…”