Spectroscopic study of the coordination and valence of Fe and Mn ions in and on the surface of aluminas and silicas

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Abstract
A comparative study of the first-row transition metal ions Fe3+ and Mn2+ on the surfaces of aluminas and silicas has been made using the conventional techniques e.s.r., X-ray diffraction, magnetic susceptibility and in addition a relatively new technique phosphorescence spectroscopy. The results indicate that Fe3+ ions dissolve readily in γ-alumina up to 1 w% in samples calcined at 200°C and up to at least 10 % in samples calcined at 800°C. In silica not more than 0.1 w% Fe3+ could be dissolved, the remainder crystallizing as α-Fe2O3 on the surface. E.s.r. and phosphorescence indicate that Fe3+ ions on γ-Al2O3 calcined below 400°C are mostly octahedrally coordinated on the surface, but above this temperature the majority of the Fe3+ ions take up tetrahedral coordination. Mn2+ ions almost exclusively occupy tetrahedral holes at the surface. At the phase transition θ→α-Al2O3 above 1000°C, Fe3+ ions move into trigonally distorted octahedral sites whereas Mn2+ changes valence to Mn4+ in octahedral sites. Also in silica, Fe3+ ions preferentially take up tetrahedral coordination above 400°C heat treatments and remain in these sites up to 1200°C calcination temperatures. Most of these results can be explained on the basis of the differences between the structures of silicas and aluminas.

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