Physicochemical characterisation and hydrothermal stability investigation of cobalt-incorporated silica xerogels

The hydrothermal stability of the cobalt oxide silica xerogels was comprehensively investigated, including the effect of Co/Si molar ratio (0.00-0.50), vapour content (0-75 mol%), exposure time (0-100 h) and temperature (250-550 degrees C). Physicochemical properties of the xerogels were characterised by nitrogen sorption, FTIR, solid-state Si-29 NMR (CP/MAS), micro-Raman, XRD and HR-TEM techniques. The structural characterisation indicated that increasing cobalt incorporation inhibited the degree of condensation in the silica network, and that the formation of tricobalt tetroxide (Co3O4) nanocrystals in the silica matrix was only observed in high cobalt loading samples (Co/Si >= 0.25). The hydrothermal stability of the xerogels assessed by N-2 sorption was found to be strongly dependent on the cobalt loading; particularly when the presence of Co3O4 in the silica matrices was implicated. For the unstable xerogels (Co/Si < 0.25), the material's stability was significantly decreased by both vapour content and exposure time, resulting in an almost 90% surface area reduction. On the other hand, the high cobalt loading xerogels (Co/Si >= 0.25) were found to contain Co3O4 and were much more stable, losing less than 25% of surface area and maintaining microporous structure after exposing to a harsh condition of 75 mol% vapour at 550 degrees C for 40 h. A structural model is proposed whereby the cobalt oxide particles `shield' the silica matrix and inhibit the hydrolysis and condensation of the silica in the pores walls. This effectively limits the structural rearrangement that hydrothermal treatment typically invokes and therefore confers improved hydrothermal stability.