Received: September 6, 2007

In Final Form: November 21, 2007

Abstract:

In this work, we report on the characterization by Mössbauer spectroscopy, X-ray diffraction, and transmission electron microscopy of nanoparticulate iron phases and their precursors synthesized by a wet chemical route within the framework of a silicate matrix (sepiolite). These results show that the nature and location of the different iron species within the sepiolite structure are dependent on the conditions of the wet chemical preparation and, specifically, on the pH value of the acidic treatment to which the sepiolite powder was subjected prior to the impregnation with the iron(III)-containing solution. A subsequent thermal treatment at 500 C (2 h) of the iron-containing sepiolites in a reducing H₂/Ar atmosphere leads to the formation of different iron-reduced species. The nature of these reduced species is also dependent on the initial acidic treatment of the sepiolite. So, in the case of the iron-containing materials produced after treatment in strong acidic conditions of the original sepiolite, the reducing conditions bring about the formation of a significant amount of metallic iron nanoparticles. These metallic iron nanoparticles have been shown to be strongly resistant to oxidation at high temperatures. In the case of milder acidic initial conditions, the reduced samples do not contain metallic iron particles but mainly nonstoichiometric Fe₃O₄ nanoparticles.

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