MÖSSBAUER STUDIES OF THE REDUCTION OF α-F2O3 CONTAINING ALKALI METAL HYDROXIDES

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MÖSSBAUER STUDIES OF THE REDUCTION OF α-F2O3 CONTAINING ALKALI METAL

HYDROXIDES

D. Klissurski, I. Mitov

To cite this version:

D. Klissurski, I. Mitov. MÖSSBAUER STUDIES OF THE REDUCTION OF α-F2O3 CONTAINING ALKALI METAL HYDROXIDES. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-353-C2-354.

�10.1051/jphyscol:19792125�. �jpa-00218496�

JOURNAL DE PHYSIQUE Collogue C2, suppl6ment au n o 3, Tome 40, mars 1979, page C2-353

MOSSBAUER STUDIES OF THE REDUCTION OF

CONTAINING ALKALI METAL HYDROXIDES

D.G. Klissurski and I.G. Mitov

I n s t i t u t e of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia III3, Bulgaria

Rdsum6.- Le mCcanisme de r6duction par lrhydrogPne de a-Fez03 contenant les hydroxydes de mdtaux al- calins (Li, Na, K, Rb, Cs) est BtudiC par spectroscopie Mzssbauer. On montre que l'unique produit in- termsdiaire de la r6duction est la magn6tite (Fe301,) et que le produit final est a-Fe.

Les hydroxydes alcalins accClPrent la rdduction sans influencer son m6canisme.

Abstract.- The mechanism of reduction with hydrogen of a-Fez03 and Fez03 containing hydroxides of va- rious alkali metals (Li, Na, K, Rb and Cs) has been studied by Miissbauer spectroscopy. It was shown that during reduction a-Fez03 successively passes into magnetite (Fe304) and a-iron.

The alkali hydroxides accelerate the reduction without changing its mechanism.

1. Introduction.- The kinetics and mechanism of re- ductionof a-Fe203 with hydrogen have been the sub- ject of a number of studies 11-61. There are, howe- ver, contradictions in the conclusions on the basic stages of the reduction / 1,3,4/.

The influence of different additives on the re- duction process is also of interest. The present communication contains results concerning the influ- ence of alkali metal hydroxides. Systems with these components are usually the starting compositions in the preparation of a new type impregnated catalysts for ammonia synthesis.

2. Experimental.- High purity a-Fez03 was additio- nally calcined for 5 h . a t 800°C and the alkali metal hydroxides were introduced by impregnation.

A gradientless static-circulation method was used for elucidating the kinetic regularities and

minima at a reduction degree of 1 1 and 33 %, which correspond to the iron/oxygen ratio in magnetite and wiistite /1,5/.

As is evident from figure I, the characteristic minimum for a reduction degree of 1 1 % is clearly expressed under our experimental conditions.

bf6ssbauer spectroscopy technique was applied to the

Fig. 1 : Dependence of the reduction rate on the re- study of the process mechanism 151. The water va- duction degree of : I-a-FezOs; 2-a-Fe203+ LiOH;

3-a-Fe203 + NaOH; 4-a-Fe203 + KOH; 5-a-Fe203 + RBOlt;

pours formed during the reduction was frozen by a

6-a-Fez03 + CsOH.Conditions : P = 80 torr t = XX)Oc,

trap cooled in liquid nitrogen. The amount of the Hz

hydrogen used was measured volumetrically. The re- duction process was performed at a constant pressure.

The Miissbauer spectra were registered both at room and liquid nitrogen temperature.

3. Results and Discussion.- The activation energy of the process showed a value of 17.9 2 0.7 kcal.

mol-'. The order of the reaction calculated from the experimental data was 0.86

+

A number of previous studies showed that when a-Fez03 is reduced to iron it successively passes into magnetite (FesOc) and then into wiistite (FeO).

As a rule these phase transitions are accompanied by changes in the rate of reduction. The reduction rate versus reduction degree curves show characteristic

Figure 2 shows the Gssbauer spectrum of a a-Fez03 sample reduced to 1 1 %. As is evident this

spectrum completely coincides with the Gssbauer spectrum of pure magnetite.

It is worth noting that with a reduction of 33 % there is no clearly expressed minimum in the reduction rate versus reduction degree curves. AS was already pointed out, this minimum is associated with the appearance of the wiistite phase.

The possibility of formation of wiistite when the reduction proceeds under our experimental condi- tions has still not been elucidated by previous in

-

vestigations. According to the phase diagram of the iron-oxygen system, wiistite may exist as a stable

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792125

C2-354 JOURNAL DE PHYSIQUE phase only at temperatures higher than the point of

the eutectoid transition (570'~).

cesium hydroxide the reduction has its highest rate.

A more general tendency was observed. The rate of reduction increases with the increase in the atomic number of the alkali metal.

It is worth noting that the reduction process has a mechanism which is analogous to the reduction of a-FezOs. In the case of these specimens again at a reduction degree of 1 1 X (reclaculated with res- pect to the a-Fez03 component) the Mzssbauer spectra correspond to that of pure magnetite Pig. 3-5). At a reduction degree of 33 9. no formation of fistite pha- se was observed. A strong promoting effect of the alkali hydroxides on the reduction of a-Fez03 was observed up to 500°C.

The studies performed by Miissbauer spectrosco- Fig. 2 : Gssbauer spectra of : l*-FezOa; 2-a-FezO3

py showed that under the described experimental con- reduced to a degree q = 1 1 %; 3-a-Fez03 reduced to

rl = 33 %. ditions, the reduction of a-Fez03 proceeds via con-

On the other hand Rostovtsev et /I/ have obser- secutive formation of magnetite phase and a-Fe. No ved the appearance of w;istite at temperatures around accumulation of wiistite phase was observed. The ex- 400~C. According to the thermodynamic calculations, perimental results also show that the alkali metal however, the probability of formation of wiistite at hydroxides influence strongly the kinetics but not lower temperatures strongly decreases. Our investi- the basic mechanism of the reduction

gations showed that the Mzssbauer spectra of hemati- te samples reduced to 33

X

References

Fig. 3 : ~ijssbauer spectra of : 4-a-Fe203 + KOH (10 at.% K) reduced to a reduction degree q = I 1 X;

5-a-Fe203 + KOH (10 at.% K), reduced to ll = 33 X;

6-a-Fez03 + KOH (10 at.% K) reduced to r~ = 92

X.

This result shows that at the described experimental conditions no accumulation of the wiistite phase is observed, i.e. the reduction of magnetite proceeds without a phase transition in wustite.

The kinetic curves of the reduction of hemati- te(a-FezOs), containing different alkali metal hy- droxides are also shown in figure 1. The obtained experimental data showed, that reduction of the spe- cimens containing lithium hydroxide proceeds with

/I/ Rostovtsev. S.T., Em, A.P., Doklady Akad. Nauk SSR (Russ.)

93

/2/ Colombo, U., Gazarini, F., Lanczavecchia, G., Mater. Sci. Eng.

2

/3/ Romanov, V.P., Tatsienko, P.A., Checherskaya, L.F., Phys. Status Solidi (a)

15

/4/ Graham, M.J., Channing, D.A., Swallow, G.A., J. Mater. Sci.

10

/5/ Mitov, I.G., Ph. D. Thesis, BAS, Sofia.

/6/ Boreskov, G.K., Kinetika i kataliz (Russ.)

2

(1970) 374.

the lowest rate whereas with specimens containing