Surface features of P-doped silica explored with CD3CN adsorption: can Si atoms act as Lewis centres?
Articolo
Data di Pubblicazione:
2005
Abstract:
Surface features of a P-doped nonporous pyrogenic silica (A200P) were analyzed by means of in-situ
FTIR spectroscopy. P-doped silica finds application in different fields and can be considered a reference
system for silica-based P-containing bioactive glasses. The presence of hydroxyl groups and Lewis acid
centers was tested with perdeuterated acetonitrile (CD3CN), a medium strength Lewis base often used as
probe molecule in acid/base IR analyses. Experimental results were compared with computational ones,
obtained with ab initio calculations on small Si/P-containing clusters. An IR band at 2290 cm-1 was
observed upon CD3CN adsorption on A200P outgassed at increasing temperatures up to 500 °C. On
A200P outgassed at T g 500 °C, another band, located at 2340 cm-1, was also observed. These results
were compared with î(CN) and î(OH) harmonic frequency shifts computed on molecular clusters. The
comparison suggested that the occurrence of the band located at 2290 cm-1 is due to the formation of
both linear and “tilted” H-bonds between POH and acetonitrile molecules. The assignment of the intriguing
spectral î(CN) feature found at 2340 cm-1 has been attributed to CD3CN interacting with Si atoms
directly bound to at least three PO4 groups. The strong nucleophilic character of the O atoms of the PO4
groups allows for the expansion of the Si atom coordination upon CD3CN adsorption (from fourfold to
sixfold), which increases its CN frequency by a sizable amount. At temperatures of sample treatment
higher than 700 °C, a fraction of Si atoms already bound to PO4 groups may become sixfold coordinated
by extra bonds to the available surface PO4 groups, inhibiting further interaction with CD3CN. It is
hypothesized that a similar phenomenon may occur at the surface of silica-based bioactive glasses, when
a layer of hydroxycarbonated apatite precipitates on their surface.
Tipologia CRIS:
03A-Articolo su Rivista
Elenco autori:
M. CERRUTI; C. MORTERRA; P. UGLIENGO
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