CHARACTERIZATION OF IRON IMPURITIES IN PILLARED RECTORITE CATALYSTS

X-Ray mapping, energy dispersive spectrometry (EDS), X-ray photoelectr on spectroscopy (XPS) and Mossbauer effect spectroscopy (MES) together with atomic force microscopy (AFM) have been used to examine iron imp urities in natural rectorites pillared with alumina clusters. EDS resu lts have shown that Fe is preferentially located near the uppermost 0. 5 mu m of the clay crystal and that high-temperature thermal or hydrot hermal treatments reduced by almost 50% the Fe concentration in this r egion. However, the presence of surface Fe impurities could not be inf erred from AFM images nor by XPS measurements indicating that Fe, on t he top 2.5 nm of the clay crystal is present below the 0.5 wt.-% level . Mossbauer spectroscopy has shown that, before pillaring, iron in rec torite is predominantly ferric and in octahedral coordination. After p illaring and upon heating, a fraction of the ferric iron exhibits a lo wer coordination number, and eventually, at the highest temperatures, hematite is formed from the clay structure. Thus, Fe appears to migrat e from the octahedral to the tetrahedral layer and onto the clay silic ate layer where it can form hematite. Hydrothermal treatment at 760 de grees C converts the octahedral ferric iron in the pillared clay to oc tahedral ferrous, tetrahedral ferric, and hematite, in approximately e qual amounts.