| Popis: |
Interaction of single γ-Al2O3 and γ-Ga2O3, and mixed γ-Al2O3/γ-Ga2O3 xerogels with CHF3 at intermediate temperatures results in partial fluorination. Fluorinated oxides remain amorphous and retain a considerable part of the initial surface area; for the fluorinated Al-based materials surface areas in all cases exceed 100 m2 g-1. Lewis acidity of mixed oxides, either before or after fluorination, is strongly influenced by the presence of surface Ga3+ ions, mainly due to their strong preference to replace highly acidic Al3+ ions in tetrahedral positions. Ion replacement leads to the formation of acidic sites with lower strengths what is confirmed by the model catalytic reaction, isomerisation of CCl2FCClF2. XPS investigations indicate that fluorination of mixed oxides is accompanied by substantial surface reconstructions and preferential formation of Al-F based phases with Ga remaining mainly in O environments. Further segregation processes, such as slow crystallisation of Al(F,OH)3·nH2O phases, are probably promoted by water adsorption. |
