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XANES Study of Ti Coordination in Heat-Treated (TiO2) x (SiO2)1- x Xerogels

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 xe...

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Bibliographic Details
Published in:Chemistry of materials 1999-05, Vol.11 (5), p.1253-1258
Main Authors: Mountjoy, G, Pickup, D. M, Wallidge, G. W, Anderson, R, Cole, J. M, Newport, R. J, Smith, M. E
Format: Article
Language:English
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Summary: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.
ISSN:0897-4756
1520-5002
DOI:10.1021/cm980644u