Structural investigation of cobalt oxide seeded silica xerogels under harsh hydrothermal condition

This work reports the hydrothermal treatment effect on cobalt silica-seeded xerogels under harsh conditions at 550 °C for 100 h in a humid gas stream containing 25 mol% H₂O_(v). A series of two seeded xerogels were prepared by two distinct methods, namely, xerogel seeding (XG) and sol–gel seeding (SG). Post-hydrothermal structural analysis showed type I sorption isotherms for both seeded xerogel, associated with microporous domains after harsh hydrothermal treatment. However, the SG series loaded with ≥25 mol% Co resulted in the formation of hysteresis, a characteristic of mesoporous silica. As a consequence, the SG series resulted in higher pore volume and surface area for higher Co loading than the XG series. Interestingly, surface area and pore volume retention (ratio of prior over post-hydrothermal treatment) increased with Co loading. For instance, seeded xerogels (0 mol% Co) resulted in ~45% retention whilst those containing 40 mol% Co reached structural retention values up to 70% (surface area) and 80% (pore volume). Further analysis of the pore size distribution showed densification for the pore size ranges of <14 and 17–28 Å, though pore size enlargement in the pore ranges of 14–17 and 28–387 Å. A mechanistic model is proposed indicating that cobalt oxide particles confer a region of protection to the adjacent silica structure that opposes densification. [Figure not available: see fulltext.]