F.E. Huggens, M.L. Occelli, J.M. Dominguez, J.M. Sencel and S.A.C. Gould. 1995. Characterization of Iron Impurities in Pillared Rectorite Catalysts. Microporous Materials 4: 291-300.
|X-Ray mapping, energy dispersive spectrometry (EDS), X-ray photoelectron spectroscopy (XPS) and Mössbauer effect spectroscopy (MES) together with atomic force microscopy (AFM) have been used to examine iron impurities in natural rectorites pillared with alumina clusters. EDS results have shown that Fe is preferentially located near the uppermost 0.5 m of the clay crystal and that high-temperature thermal or hydrothermal treatments reduced by almost 50% the Fe concentration in this region. However, the presence of surface Fe impurities could not be inferred from AFM images nor by XPS measurements indicating that Fe, on the top 2.5 nm of the clay crystal is present below the 0.5 wt.-% level. Mössbauer spectroscopy has shown that, before pillaring, iron in rectorite is predominantly ferric and in octahedral coordination. After pillaring and upon heating, a fraction of the ferric iron exhibits a lower coordination number, and eventually, at the highest temperatures, hematite is formed from the clay structure. Thus, Fe appears to migrate from the octahedral to the tetrahedral layer and onto the clay silicate layer where it can form hematite. Hydrothermal treatment at 760°C converts the octahedral ferric iron in the pillared clay to octahedral ferrous, tetrahedral ferric, and hematite, in approximately equal amounts.|
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