Influence of morphology and composition of fumed oxides on changes in their structural and adsorptive characteristics on hydrothermal treatment in steam phase at different temperatures

A series of fumed oxides such as silica, titania, alumina, silica/alumina (SA), silica/titania (ST), and alumina/silica/titania (AST), initial and hydrothermally treated (HTT) in the steam phase at T(HTT)=150, 250, and 350 degrees C was studied by adsorption, AFM, XRD, FTIR, and theoretical methods. Diminution of the size of primary particles (corresponding to increasing S(BET)) of initial silica and mixed oxides results in enhancement of their structural changes on HTT with elevating T(HTT) and increasing density of packing of primary particles in the secondary structures. Relative changes in the texture of treated fumed silicas are smaller than those of mesoporous silica gels occurring under similar HTT conditions. On HTT, aggregates of primary particles and their agglomerates become denser but their surface layers become looser because of transfer of silica fragments from one particle to another, and the smaller the initial primary particles, the greater the relative diminution of the specific surface area S(BET) for the same type of primary particle packing in aggregates. Relative changes in the pore volume V(p) (or V(BJHd)) on HTT are more complex than that of S(BET), as for many samples the V(p) value increases especially at T(HTT)=150 degrees C. Alumina and titania partially inhibit structural changes on HTT, which decrease in the series silica > SA > AST approximately ST.