57Fe MÖSSBAUER SPECTROSCOPIC STUDIES OF STRUCTURAL CHANGES OF MONTMORILLONITE ON HEATING IN REDUCING ATMOSPHERE

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57Fe MÖSSBAUER SPECTROSCOPIC STUDIES OF STRUCTURAL CHANGES OF MONTMORILLONITE

ON HEATING IN REDUCING ATMOSPHERE

M. Takeda, O. Kawakami, T. Tominaga

To cite this version:

M. Takeda, O. Kawakami, T. Tominaga. 57Fe MÖSSBAUER SPECTROSCOPIC STUD- IES OF STRUCTURAL CHANGES OF MONTMORILLONITE ON HEATING IN REDUC- ING ATMOSPHERE. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-472-C2-474.

�10.1051/jphyscol:19792163�. �jpa-00218538�

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JOURNAL DE PHYSIQUE Colloque C2, supplt!ment au n o _3,Tome 40, mars 1979, page C2-472

Fe MOSSBAUER SPECTROSCOPI C STUD I € S OF STRUCTURAL CHANGES OF MONTMOR I LLON I T E ON H E A T I N G I N REDUCING ATMOSPHERE

M. Takeda, 0. Kawakami and T. Tominaga

Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Tokyo, Japan

R5sum5.- Les changements de structure de la montmorillonite, induits par chauffa e en atmosphere rd- ductrice, ont dtd analys6s par spectroscopie Mgssbauer sur les formes Fe2+ et Fe8+ du fer.

Abstract.-The structural changes of montmorillonite on heating in reducing atmosphere was investigated by examining NGssbauer parameters of both ~ e and Fe3+ components in the fired products. ~ +

I. Introduction.- Dehydration of clay minerals cau- for the data obtained both at 77 K and 293 K was im- ses the loss of adsorbed, interlayer, or lattice OH proved by assuming that two iron sites exist in the water and alters their structures significantly. sample fired at 500'~ for ~ e ~ + , and in those fired

5 7 ~ e Gssbauer spectroscopy has been used effective- at 800-1000°c for ~ e ~ + . ly for the study of structural changes during heat

treatments in air for clay minerals such as kaolini- te /1,2/, biotite /3/, chlorite /4/, and montmorillonite /5,6/. However, little attention has been directed to the dehydration reactions in reducing atmospheres. Very recently, Mackenzie et al. /7/

studied the thermal transformations of nontronite in a reducing atmosphere (hydrogen/r.itrogen) by means of ~gssbauer technique. In this study, the structural changes in montmorillonite during heat treatment in reducing atmosphere was investigated.

2. Experimental.- The montmorillonite sample was supplied by Prof. H. Takahashi, and described as 48 W 1250 montmorillonite No.25 (Upton, Wyoming) from Ward's Natural Science Establishment, Rochester, New York. The pulverized sample (300 mg) mounted on a carbon boat was heated in an electric furnace for 5 h at 400, 500, 600, 700, 800, 900 and IOOO'C in pure nitrogen stream. After cooling, ~gssbauer spectra and X-ray powder patterns of the fired products were taken. The data obtained at 293 K and 77 K by a Shimadzu MEG-2 Mgssbauer spectrometer (5-20 X 10' counts per each channel) were computer-fitted with, Lorentzians

.

3. Results and Discussion.- Figure 1 illustrates MEssbauer spectra obtained at 293 K of unfired montmorillonite and the fired products at various temperatures up to 1000~~. No magnetic components (e.g.

magnetite and iron metal) were observed even in the spectra taken at 77 K. The quadrupole splitting values for paramagnetic Fe2+ and Fe3+ both at 77 K and 293 K in these samples are plotted against the firing temperature in figure 2. The curve-fitting

I

~ l l l l ~ ~ ~ t ~

l

~ ~ ~ l l r ~ . ~ I ~

-10

-

⁵ ⁰ ⁵ 10

Velocity (mmls

Fig. I : Mgssbauer spectra (at 293 K) of montmorillonite fired at various temperatures in reducing atmosphere.

The quadrupole splittings corresponding to the major components are plotted along the broken lines for Fe2+ and Fe3+ ions, respectively, in figure 2.

Isomer shifts at 293 K for ~e~+(82 % of total

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792163

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iron) and Fe2+ (18%) in the original montmorillonite are 0.36 mm/s and 1.14 mm/s (relative to iron metal at 293 K), respectively, indicative of octahedral coordination around the iron ions.

Fjg. 2 : Dependence of quadrupole splitting on fi-

ring temperature. Mksbauer measurements made at 293 K (0) and 77 K (A).

This suggests that Fe3+ and ~e'+ ions have substitu- ted for A:'+ and Elg2+ ions (in octahedral sheets) coordinated with two OH groups (lattice OH water) and four oxygen atoms.

Actually, the isomer shift and quadrupole splitting were not changed significantly on heating the sample up to 40o0c, where adsorbed water and interlayer water between silicate sheets were elimina- ted.

On heating at ~OO'C, the relative amount of Fe3+ was decreased from 82 % to 78 X due to reduction of Fe3+ to F'e2+. The relative amounts of Fe3+

remaining in the products fired at 600, 700 and 800°C were 61, 44 and 13 X, respectively, and the reduction was complete at 900°C.

In the sample heated at 500ec, the quadrupole splitting for octahedral Fe2+ (22 % of total iron) was decreased whereas the quadrupole splitting for octahedral Fe3+ (65 %) was increased. The isomer shift of a minor Fe3+ component ( 13 X) was 0.23 and 0.37 mm/s at 293 and 77 K, respectively, smaller than those commonly observed for octahedral Fe3+

ions. We may tentatively ascribe this component to tetrahedral Fe3+.

In the samples dehydrated completely at 600- 700°~, both octahedral Fe2+ (AE = 1.99 mm/s at

Q

293 K, 1.98-2.11 mm/s at 77 K) and octahedral Fe3+

(BE = 1.27-1.33 m / s at 293 K, 1.34-1.39 mm/s at Q

77 K) may presumably be located at strongly distor- ted sites in the octahedral sheets of layer structure, the existence of which was confirmed by X-ray powder patterns of the samples heated up to 700°c.

The quadrupole splitting values of ~e~~ ions in the disbrdered phase were in agreement with those repor- ted by Plachinda et al. /6/ for montmorillonite fired at 700-800'~ in air.

X-ray powder patterns indicated destruction of the montmorillonite lattice at 800-900~~, and appea- rance of spinel and cristobalite instead, at 1000~~.

The curve-fitting of MEssbauer spectra of the sample heated at IOOOOC suggests one of the following possi- bilities : either 27 % and 73 X of Fe2+ are at tetrahedral and octahedral sites, respectively, with the quadrupole splittings shownin figure 2, or Fe2+ is located in two octahedral sites with different quadrupole splitting values, 1.43 and 2.15 mm/s at 293 K, and 2.04 and 2.80 mm/s at 77 K. Further study will be needed to explain unequivocally the structures of the iron-containing phase appearing at 1000~C,

In conclusion, the structural changes of montmorillonite on firing in reducing atmosphere may be

summarized as follows :

1 ) The quadrupole splitting is increased for ~ e ~ +

and decreased for Fe2+ on heating at 600 and 700°c, indicating strong distortion of octahedra in this disordered phase caused by the loss of lattice OH.

2) Since the quadrupole splittings of Fe2+ on heating at 800 and 900°c are different from those for the disordered phase (600-70O0C), it is likely that further structural change may have taken place before spinel formation.

3) Smaller isomer shift of a minor Fe3+ component on heating at 500'~ implies structural change from the original octahedron to tetrahedron, even before formation of the octahedron in the disordered phase

(600-700'~)

.

The present authors wish to thank for Pr.Hirosh Takahashi, The Institute of Industrial Science, The University of Tokyo, for generous supply of the mont morillonite sample. They are also indebted to Dr. T.

Yamanaka for his assistance in X-ray measurements.

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