Iron ions supported on oxides: Fe/Al2O3 vs. Fe/SiO2

The effect of the surface structure of the support on the nature and dispersion of iron ions was studied by comparing low-loaded Fe/SiO2 and Fe/Al2O3 catalysts, prepared by the "adsorption-precipitation" method. This method, based on the electrostatic interaction between iron ions and the negatively charged support surface, is effective in the preparation of highly dispersed iron-supported catalysts, active in alkanes selective oxidation reactions. The samples were characterized by DR. UV-Vis and IR, spectroscopies, coupled with NO adsorption and to temperature programmed reduction (TPR) measurements. DR UV-Vis and TPR show that after calcination in both catalysts mainly isolated Fe3+ ions, a minor fraction of 2-d FeOx dimers/oligomers and a, very small fraction of 3-d oxidic particles are present. The results indicate a higher ability of alumina, in dispersing iron-supported species. Despite the multiple sites possibly available for hosting iron cations offered by the silica support, only two types of iron species were identified by In, of adsorbed NO, differing for the ability to form mono or trimitrosyl complexes (that evolve into dinitrosyl ones, by decreasing NO pressure). The same technique shows that at least five different types of iron species are formed on the alumina surface, four of them able to form only mononitrosyl complexes and one giving di-or trinotrisylic adducts.