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

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.