Raman spectroscopic technique has been used to characterize a Ru/TiO 2 catalyst and to follow in-situ their structural changes during the CO selective methanation reaction (S-MET). In order to a better comprehension of the catalytic mechanism, the in situ Raman study of the catalysts activation (reduction) process, the isolated CO and

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Structural differences which occur on heating La-montmorillonite have been studied and compared with those for montmorillonite saturated with Na(I) and Li(I). Rehydration experiments show that La-montmorillonite swells after preheating up to 500~ However, FT-IR spectra suggest partial migration of the La(III) cations from the interlamellar region to the structural sheets and(or) deprotonation of hydrated La(III) cations, followed by migration of the resulting protons to vacant octahedral sites. Additional measurements of X-ray absorption spectra (EXAFS, XANES) and heating to 700~ suggest that interlamellar hydrated La(III) partially deprotonates producing polyoxocations.Smectites and their pillared derivatives made with trivalent lanthanide cations have recently attracted much attention as catalysts (Shabtai et al., 1984). In addition, La simulates Am in studies on the interaction of the latter with clay minerals. Determination of the interaction mechanism upon heating is important for the design of safe repositories for high-level nuclear wastes. Their storage in continental or marine repositories usually involves claybearing packing materials that act as a barrier.The aim of this paper is to study significant structural differences upon heating Na-, Liand La-montmorillonites. It has already been observed that lanthanide-montmorillonites show a decrease in cation exchange capacity (CEC) upon heating at temperatures above 160~ (Mozas et al., 1980). Miller et al. (1982 examined the effects of temperature on the sorption of Eu(III), Ho(III) and Yb(III) on an Upton, Wyoming montmorillonite and claimed that hydrolysis and migration into octahedral sites are likely mechanisms for 4lion fixation. Results already obtained in our laboratory contradict the migration mechanism on heating at 300~ (Trillo et al., 1990).The nature of the materials used demands a variety of experimental approaches and X-ray diffraction (XRD), Fourier Transform infrared spectroscopy (FT-IR), nitrogen adsorption isotherms and X-ray absorption spectroscopy techniques such as extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) have been used. EXPERIMENTAL MaterialsThe <2/~m fraction of a Trancos smectite mineral from Gador, Almeria, was saturated with Na using a 1 M solution of NaCI; chemical analysis, as described elsewhere (Trillo 9 1992 The Mineralogical Society

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