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| Titel |
The structural changes of silica gel by compressions up to 10 GPa |
| VerfasserIn |
A. Arasuna, M. Okuno, X. Xue, T. Mizukami, M. Akaogi, M. Nozawa, H. Okudera, S. Arai |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250061794
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| Zusammenfassung |
| Structural change of TEOS-derived silica gel by compression up to 10 GPa has been
investigated by XRD, Raman, IR, and 1H and 29Si MAS NMR spectroscopy, in
order to gain insight into the compression behavior of silica gel. Silica gels were
compressed under 5 and 10 GPa for 1 hr at room temperature by 6–8 Kawai-type
multi anvil apparatus. All the measurements have been performed on the recovered
samples.
Raman spectrum of silica gel has a broad band at around 460cm-1 assigned to a
symmetrical Si-O-Si stretching mode. For samples compressed above 5GPa, this broad band
loses its intensity in region of about 480cm-1 and becomes much sharper than that before
compression. This observation indicates depolymerization of the network structure, decrease
of the average Si-O-Si angle and narrowing of the angle distribution after compression. Such
distinct spectral changes of silica gel could be related to the presence of Si-OH groups,
which causes depolymerization of the structure. Silica gel contains a large quantity
of molecular water and silanols. Both the Raman and 1H MAS NMR spectra of
silica gel showed decreasing water content with increasing pressure. 1H MAS NMR
spectra of silica gel contain a prominent narrow peak near 4.9 ppm and a very weak
tail to higher frequency. The former may be largely attributed to relatively mobile
molecular H2O, and the latter to more strongly hydrogen-bonded SiOH/H2O groups.
For the compressed samples, the narrow peak near 4.9 ppm becomes much less
intense, whereas the high-frequency shoulder (extending to about 10 ppm) becomes
prominent, suggesting increased proportion of hydrogen-bonded SiOH/H2O groups after
compression.
Furthermore, 29Si MAS NMR spectra for both the uncompressed and compressed silica
gels revealed three peaks near -110, -101 and -92 ppm, which may be, respectively, attributed
to Si of Q4, Q3 and Q2 speciation (Si bonded to 0, 1 and 2 OH groups). The relative
proportion of Q3 vs. Q4 is higher in the sample compressed at 10 GPa than the uncompressed
sample, despite significantly lower total water content. This suggests that some of the
molecular H2O might have been converted to OH groups during compression, resulting in
further depolymerization of the network structure.
In conclusion, this study has revealed significant irreversible structural changes for silica
gels compressed at high pressure. These include significantly decreasing bulk water
content and increasing proportion of strongly hydrogen-bonded water species, overall
depolymerization and changes in the Qn speciation distribution, and narrowing of the Si-O-Si
angles and their distribution. These structural changes are expected contribute to the
compression behavior of silica gels. |
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