Hyperfine Interactions 131: 43–50, 2000. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. 43 Mössbauer Spectroscopic Study of Iron Containing Hydrotalcite Catalysts for the Reduction of Aromatic Nitro Compounds J. SANCHEZ-VALENTE 1 , J. M. M. MILLET 1 , F. FIGUERAS 1 and L. FOURNES 2 1 Institut de Recherches sur la Catalyse, CNRS, associé à l’Université Claude-Bernard, Lyon I, 2 avenue A. Einstein, F-69626 Villeurbanne cedex, France 2 ICMCB,Château Brivazac, av. du Dr Schweitzer, F-33608 Pessac cedex, France Abstract. Carbonated layered double magnesium hydroxides containing Fe with hydrotalcite struc- ture which are active and selective catalysts for the reduction of aromatic nitro compounds by hydrazine hydrate have been prepared and characterized by X-ray diffraction and Mössbauer spec- troscopy. Using these techniques we have shown that when the Fe/(Fe + Mg) ratio was greater than 0.2, which corresponds to the natural phase composition, ferrihydrite Fe 5 HO 8 ·4H 2 O was formed. This last phase, characterized at room temperature by a superparamagnetic doublet, leads after ac- tivation under nitrogen at 723 K to small particles of ferric oxide. An in-situ study of the activated catalysts allowed us to show that only iron cations present in this phase could undergo an oxido- reduction under the conditions of catalysis and that the catalytic properties must be related to this last phase. 1. Introduction The present work reports the results of the characterization by Mössbauer spec- troscopy of iron containing Mg-hydrotalcites used as catalysts for the reduction of aromatic nitro-compounds to the corresponding amines by hydrazine hydrate. These catalysts have been shown to be as active as iron oxide hydroxides catalysts previously described [1]. However, they present the great advantage of having a very low selectivity into undesired intermediate products such as hydroxylamines, leading to a safer and cleaner process [2]. The catalysts have been characterized after synthesis and after activation. An in-situ oxido-reduction study of a catalyst has also been performed in order to ascertain the redox mechanism of the catalytic reaction. 2. Experimental The catalysts were synthesized as described elsewhere [3]. They were obtained from Mg(NO 3 ) 2 , Fe(NO 3 ) 3 , or FeCl 3 and NaOH + Na 2 CO 3 by coprecipitation at low supersaturation, pH 9 and 293 K. The precipitates dried at 333 K showed all