12, pp. 155-158, 1999 Else&r Science. Ltd 8 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved 09659113l99l.$-see front matter
IN X AND Y ZEOLITES
K. LBdrl, H.K. Beyerl, G. Onyestyhk2, B J. Jiinsson3, L.K. Varga4, S. Pronier5 * Institute of Isotope and Surface Chemistry, Budapest, P.O.B. 77, H-1525, Hungary 2 Institute of Chemistry, Budapest, P.O.B. 17, H-1525, Hungary 3 Dept. Condensed Matter Physics, Royal Inst. Technology, S-10044 Stockholm, Sweden 4 Institute of Solid State Physics, Budapest, P.O.B. 49, H-1525, Hungary 5 LACCO, URA CNRS 350, Universite de Poitiers, F-86022 Cedex, Poitiers, France Abstract Strong reduction of extra-framework iron ions was attempted by sodium azide to form metallic iron particles with dimensions of a few nanometers. Na-X and Na-Y zeolites were partially ion exchanged to 5 wt % Fe content. The iron-zeolites were mixed with sodium azide, evacuated at 550 K and heated to 800 K in nitrogen to form sodium vapour by decomposing NaN3. The sodium exerts strong reduction, a part of iron can be reduced to zerovalent state to AC form particles in the nanometer size range, as in situ Miissbauer spectroscopy, susceptibility, X-ray diffraction and transmission electron microscopy studies revealed. Simultaneous partial recrystallization of the zeolite is also observed. 01999 Acta Metallurgica Inc.
INTRODUCTION Zeolites may provide an appropriate hosting media for stabilising high dispersion metallic phase in nanometer size region. For preparation, as a possible way, strong reduction of extraframework ions can be suggested. The method was attempted for iron, eg. by using sodium vapour: at 1066 K (I), and at lower temperature (673 K) with prolonged periods (5 - 48 h) (2); a part of iron ions was reduced to metallic state in both cases. To attain the same reducing effect the “local generation” of sodium vapour can also be suggested by decomposing sodium azide in a mixture: 2 Fe3+-Z + 6 NaN3 + 2 Fee-Z + 6 Na+ + 9 N2
where -Z refers to the X or Y zeolite matrix. It is supposed that the reduction may proceed in the cages, at a temperature exceeding slightly that necessary to decompose NaN3, and the metallic particles formed are retained in the cages. In fact, upon the decomposition of sodium azide Na43+, Nag5+ and Naxo clusters are formed (3); they have the capability for reduction. In the present work we report on the results of the attempted reduction of iron in Fe-X and Fe-Y by Eq. 111. Methods of X-ray diffraction (XRD), AC susceptibility, in situ Mossbauer spectroscopy and transmission electron microscopy (TEM) are used for characterisation of the products. 155
Sodium ions in X and Y zeolites (with SiO2/Al203 moduli 2.75 and 5.11, respectively) were ion exchanged partially for iron (5 wt % Fe content). After drying the adsorbed water was removed by an overnight heating at 620 K. Then the iron-zeolites were mixed mechanically with NaN3 (10 % excess to stoichiomeuy) in a dry box under argon atmosphere. Finally the mixtures were placed to an in situ Mossbauer cell, then they were evacuated (10-l Pa) at 550 K, and the azide was decomposed by heating at 800 K in nitrogen. For characterisation of products methods of in situ Mijssbauer spectroscopy and ex situ XRD, in phase AC susceptibility, and TEM combined with electron diffraction were used.
RESULTS X-ray difraction (XRD)
X-ray diffractograms reveal partial recrystallization for X zeolite, with appearance of nonporous aluminium silicate phase(s), with typical reflections at 28 = 21.1 and 34.8 degree. In the Y zeolite the original crystallinity was essentially preserved. Reflections of metallic iron crystals were not observed (the size limit to detect reflections is ca. 10 nm). AC susceptibility
The dependence of the in-phase AC susceptibility on temperature for the Fe-X + NaN3 sample is shown in Fig. 1. A characteristic break appears at 120 K at different frequencies: the exponential dependence changes to a linear one. The exponential increase is characteristic of metallic iron, the break indicates the presence of nanoparticles (4). In case of oxidized iron nanoparticles a similar behaviour was found (with a break at 100 K) in Ref. (5).
Fig. 1. Temperature dependence of the inphase AC susceptibility of the Fe-X + NaN3 sample at various frequencies
Fig. 2. 77 K in situ Mdssbauer spectra of Fe-X + NaN3 (top) and Fe-Y + NaN3 (bottom) samples
77 K in situ Mdssbauer spectra are presented in Fig. 2. Two types of zerovalent iron are identified (with and without magnetic splitting), and Fe2+ ions were found in various coordination states (Table 1). Fe3+ component (indicative for oxides) was not detected. Transmission
TEM measurements were performed on the X zeolite sample. Iron containing particles were observed dominantly in a narrow, 4 - 7 nm size range. Electron diffractograms obtained on larger particles revealed the presence of Fe0 (12 nm) and Fe304 (36 nm).
DISCUSSION The analysis of data clearly attests for formation of metallic iron nanoparticles. Their existence is directly manifested in the Mossbauer spectra (recorded in situ). The metallic component formed exists primarily in two characteristic sizes; particles in 4-6 nm range, and clusters smaller than 1 nm. (Prior to the other, ex situ measurements, probably secondary oxidation of particles had taken place due to exposure to air.) AC susceptiblity curves and TEM and electron diffraction images indicate presence of iron-containing particles. The first size range can be directly obtained from TEM images, and can be estimated from the lack of the reflections of metallic iron (or oxides) in the X-ray diffractograms (< 10 nm), combined with the appearance of the magnetic sextet in Mossbauer spectra ( > 4 nm (6)). As for
TABLE 1 Spectral Components in the 77 K In Situ Mossbauer Spectra (IS: isomer shift, related to o-iron, mm/s; QS: quadrupole splitting, mm/s; MHF: magnetic hyperfine field, Tesla; RI: relative intensity, %) Fe-X + NaN3 IS
Fe0 Fe0 Fe2+ Fe2+ Fe2+ Fe2+ Fe2+
0.10 0.02 1.01 1.13 1.20
2.44 1.75 2.54
Fe-Y + NaN3
0.10 0.03 0.96 1.08 1.48
24 28 28
33.7 0.52 2.33
12 18 12 35
the existence of iron clusters we may rely primarily on the Mossbaucr spectra (singlet component at IS 0.0 mm/s). These clusters do not appear in TEM (the estimated lower limil for detection of particles is ca. 2 nm), the most appropriate assumption is that they are stabilised inside the zeolite cages (pore diameter 1 nm). It should be noted that considerable recrystallization of the zeolite lattice has also taken place simultaneously in the X zeolite (as X-ray diffractograms attested for formation of nonporous aluminosilicate phase(s)). In contrast, the structure of Y zeolitc was practically preserved. In correspondence, different Fe2+ species arc identified for the two samples in the Mijssbauer spectra; some are characteristic for the non-porous aluminosilicates in the transformed Fe-X sample, the others are occuping various extra-framework positions in the FeY sample. Thus, the greater proportion of reduction in Fe-Y can also be interpreted by the different extent of lattice transformation. The recrystallization of X lattice is more pronounced by two reasons: first, the amount of sodium is ca. 1.5 half times as large as that in the Y zeolite, and second, the X lattice originally is less stable due to the smaller Si02/A1203 modulus. Thus, the Fe2+ ions are less accessible in the X structure since they are incorporated partially to the aluminosilicate before they could be reduced. In Y zeolite the porous structure is preserved in a larger extent, the extra framework iron ions are longer accessible for reduction.
In summary, it may be concluded, that iron nanoparticles can successfully be prepared in zeolite cages by sodium generated upon heating the Fe-Z + NaN3 mixtures. Two size ranges are dominating: particles in 4 - 6 nanometer range and clusters smaller than 1 nm. The reduction process is accompanied with partial recrystallization of the zeolite lattice (dominantly in the X zeolite). This process results in a limitation to the accessibility of iron ions and thus, to the extent of reduction.
The financial supports provided by the Commission of the European Communities in the COST D5 project (CIPE CT 92 6107) and obtained from the Hungarian National Research Fund (OTKA TO21 13 1) are thankfully acknowledged.
1. 2. 3. 4. 5.
Lee, J.B., J. Catal.,&, 27, (1981). Schmidt, F., Gunsser, W., Adolph, J., A.C.S. Symp. Series, a, 293, (1977). Brock, M., Edwards, C., Forster, H., Schrtider, M., Stud. Surf. Sci. Catal., 84, 1515, (1994) Dormann, J.L., Bessais, L., Fiorani, D., J. Phys. C: Solid State Phys., U,2015, (1988). Turkki, T., Jiinsson, B.J., Striim, V., Medelius, H., El-Shall, M.S., Rao, K.V., J. Korean Magnetics Society, 5,745, (1995). 6. Clausen, B.S., Tops& H., Morup, S., Appl. Catal., 48,327, (1989).