Surface features of P-doped silica explored with CD3CN adsorption: can Si atoms act as Lewis centres?

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.