Oxidation mechanism of manganese-substituted magnetite

Reactivity of Soli&, 1 (1986) 139-152 EIsevier Science Pubhshers B.V., Amsterdam - Printed in The Neth~rIands

139

OXIDATION MECHANISM OF MANGANESE-SUBSTITUTED MAGNETITE

B. GILLOT * and M. EL GUENDOUZI Laboratoire de Recherches sur la RkactivitL; des So/ides, Fact&& des Sciences Mirande, B, P. 138, F-21004 Dijon Cedex (Frances and P. TAILHADES

and A. ROUSSET

Laborutoire de Chimie des Matkriaux Inorganiques, Universitb Paul-Sabatier, Toulouse III, 118, route de Narbonne, F-31062 Toulouse Cedex (France) (Received February 2&t, 1985; accepted August 16th, 1985)

ABSTRACT During oxidation in air of finely grained manganese-substituted magnetites in defect phase y of the same spineI , OsxMn~‘&)aOj(Mn~+8xFe:_+,.,,)A(Fe:=0.sxFe2_+ structure, the availability for oxidation of Mn2+ tons in tetrahedral sites (A sites) is much less than that of Fe’+ and Mn3+ tons in octahedral sites (B sites). The oxidation kinetic of Mn2+ ions is well interpreted by diffusion under variable working conditions with an activation energy of about 140 kJ mol-‘. The defect phases y obtained at 400°C undergo stoichiometry changes due to manganese ions in the temperature range 450-55O*C as a function of oxygen pressure. Above 6OO”C, Mn2+ ions . that are not oxidized at lower temperatures are transformed into Mn3’ with a phase change from a spinet to a corundum structure. The kinetic curves for this tr~sformation are sigmoidal with an activation energy depending on the amount of Mn3’ ions,

INTRODUCTION

Several mixed oxides with a spine1 structure containing iron(I1) of type (M~+Fe:TXFe~+)O~-, with 0
0 1986 Elsevier Science Publishers B.V.

140

leads to the formation of defect spinels y [l-4]. These phases are obtained only by the oxidation of Fe”+ ions, whereas the divalent or trivalent substituted cations maintain their initial oxidation state. Deficient cation iron-spine1 oxides are formed: X (8 2,&J~,-,),,)O,2cM2+Fe3t_

and

!Fe~~,,,,3M~,‘,3o1,3)0,2-

where 0 K x < 1, 0
EXPERIMENTAL

Samples

The conditions for the preparation of finely grained manganese-substituted magnetites do not differ from those in the previous method used to prepare M3+- or M2+ -substituted magnetite [S]. Radioc~stallographic analysis at room temperature shows that the samples contain only the spine1 phase with the totality of Mn2’ ions on tetrahedral sites (A sites) and a small amount of Mn3’ ions on octahedral sites (B sites), resulting in the structural formula

(Mn2df,,Fe:+,.,,),(Fe:=,.~,Fe:_+,.*.~Mn3d’,, >@4’The lattice parameter increases approximately linearly with the extent of substitution x (Fig. 1). This distribution agrees reasonably well with the neutron diffraction work of Hastings and Corliss 191, which indicates that MnFe,O, is a normal spinel.

Fig, 1.. Evolution of experimental and theoretical parameter (ref. (Mn~~~~,,Fe:_+,,,)A(Fe:=,6xFe:‘(l.,xM-.

lattice parameter 22) (- - - - - -)

(a} of Fe3_,Mn,04 (--) for the cation distribution

The nmpholagical study of the smples carried out by mans uf electron microscopy shows that the shape d the particks was spherical ur at least close ta spherid, as iEustrated in Fig 2, The awxage diameters calculated

142 TABLE

1

Sample characteristics Sample Fex_,Mn,O,

Specific area

Particle size

(m*/s)

(nm)

x x x x x x x

22 23 25 21 20 24 17

54 51 45 59 63 48 71

= = = = = = =

0.14 0.21 0.37 0.50 0.67 0.93 0.97

from the BET specific surface area are listed in Table ening was consistent with this trend.

1. X-ray line broad-

Measurement

The samples were oxidized isothermally in a Setaram MTB 10-8 microbalante using 6 mg of powder, or the temperature was increased at a linear rate. Before every experiment, great care had to be taken while degassing to ensure that the powder was not even partially oxidized. This necessitated a vacuum of 10e4 Pa and a very slow temperature rise, during which time no weight change occurred. The Fe3+, Mn4’ and Mn3+ contents of the samples at various levels of oxidation (or reduction) were calculated from the gravimetric data.

RESULTS

Oxidation

AND DISCUSSION

in air

The effect of temperature on the oxidation characteristics was demonstrated directly by observing the differential thermal analysis (DTA) curves and the weight change when the samples were heated in air at a constant rate of 2.5”C min-’ from 20 to 750°C (Fig. 3). The first exothermic peak between 200 and 300°C which is accompanied by a weight change corresponds to the oxidation of Fe2+ and Mn3+ ions in octahedral sites. Such an exothermic DTA peak has also been found in other substituted magnetites undergoing oxidation of Fe2+ ions in octahedral sites [lo]. Following this oxidation, there is, for x > 0.50, a second exothermic DTA peak at about 350°C, which is again accompanied by a weight change; this peak could be due to the oxidation of Mn 2t ions in tetrahedral sites. The X-ray diffractograph has shown that for all compositions one phase with a

143

0

100

300

500

700

TEMPERATURE

‘C

) and DTA curves (- - - - - -) for manganese-substituted Fig. 3. TG curves (heated in air at 2.5”C min-‘. 1, x = 0.14; 2, x = 0.50; 3, x = 0.67; 4, x = 0.97.

magnetites

spine1 structure is present with only a decrease in the lattice parameter. Above 400°C the compound starts to lose weight and there is also, for x > 0.50, an onset of a small endothermic peak. Based on this loss in weight, the reaction corresponds to the reduction of Mn4+ to Mn3+ ions on octahedral sites. Indeed, it is known that in the Mn-0 system, MnO, is continuously converted into the lower oxide Mn,O, at about 450°C [ll]. A study of copper-substituted lanthanum manganates has also indicated the reduction of Mn4+ ions in the range 400-500°C from gravimetric data [12]. This behaviour suggests that the spine1 at point B contained only Fe3+, Mn3+ and Mn2+ ions. For the temperature range 400-550°C X-ray analysis should show a single phase compound of spine1 structure with a lattice constant that steadily increases with increasing degree of reduction, i.e., with increasing x. At this stage, for complete oxidation of Fe2+ and Mn” ions in 6 mg of Fe3_,Mn,04 with x = 0.50, the total weight gain should be 0.2 mg, whereas the actual weight gain is only 0.15 mg. The difference of 0.050 mg seems due to incomplete oxidation of Mn2+ ions. Above 500°C we again have a weight gain and an exothermic peak due to the oxidation of Mn2+ ions that were not completely oxidized at lower temperature.. On examination of the powder diffraction pattern of the compound heated at 600°C a larger number of lines was observed and identified with those of cu-Fe,O, with only a slight shift. Hence at this temperature, instead of decomposing into the respective oxide, Fe,O, and Mn,O,, the defect phase y is converted into the rhombohedral phase a-(Fe, - xMnx) 2/3 0,. At higher temperature (about 750°C), no weight gain is observed but the solubility of Mn,O, in Fe,O, decreases markedly and orthorhombic Mn,O, is precipitated.

1 100

200

Fig. 4. DTG curves, dAm/dr

so0

400

TEMPERATURE

“c

= f(T).

Mechanism of oxidation of manganese-substituted magnetites to cation-deficient spinels Differential thermograuimetric (DTG) curues The effect of the distribution of Fe’+, Mn3* and Mn2” between B and A sites on the oxidation can be studied by plotting dAm/dt against temperature (132= mass, t = time), which for convenience is usually normalized to the starting weight. Fig. 4 shows the results of this experiments as a function of the extent of substitution X. The pure magnetite (X = 0) containing all Fe”’ ions on the octahedral sites exhibits only one peak centred at about ‘160°C. As x increases, the peak at 160°C decreases and the curves show the progressive development of two other peaks centred at about 270 and 340°C indicating that further oxidation takes place. The size of the third peak at 340°C increases with increase in x, which is consistent with an increasing number of Mr?’ ions on tetrahedral sites, having only limited availability for oxidation in the lower temperature range. Then. comparison may be made with the results for chromium- or aluminium-substituted magnetites [5] where all Fe*+ ions are on tetrahedral sites (most oxidation temperatures are in the range 350-450°C) and whose covalent bonding in tetrahedral sites of the structure renders tetrahedrally sited Fe’+ ions more stable towards oxidation than Fe2+ ionically bound in octahedral sites. The second peak at about 270°C is almost non-existent for x = 0.14 and only a “knee” is present at x = 0.67. This peak is considered to be due to the oxidation of Mn3+ into Mn4’ ions on octahedral sites. Hence the temperature of each peak connected with the oxidation process allows the increase in stability towards oxidation to be envisaged as follows: IFeZ+-- 02- Is < \Mn3+- 02- 1B < (Mn*+-- CP 1A

145

Kinetic study of the oxidation of tetrahedral Mn’ + ions It has been shown earlier [13,14] that during the oxidation of slightly aluminium- or chromium-substituted magnetites (inverse spinels) where all Fe’+ ions are on octahedral sites, the reaction kinetics can be well interpreted by the diffusion of vacancies generated at the solid-gas interface under variable working conditions. The results indicate that an activation energy of 88.5 kJ mol-’ is associated with the low-temperature peak. In contrast, the diffusion of ferrous iron located at A sites of FeCr,O, or FeAl,O, (normal spinel) proceeds with an activation energy of 144.2 and 163.4 kJ mol-‘. These results provide further support that B site Fe*+ ions will be more rapidly oxidized than A site Fe2+ ions. The purpose of the kinetic study was to measure the activation energy of Mn2+ ions located on A sites of the spine1 structure and to compare it with the activation energy of Fe*+ ions in A sites. The oxidation of octahedral Fe*+ and Mn3+ ions occurs quickly at low temperature (l-2 h at 170°C). At this stage, when all the octahedral Fe*+ and Mn3+ ions have been oxidized, the cation distribution in the partial oxidation product will be governed by the initial cation distribution, and the vacancies will occur largely on octahedral sites according to the formula

At higher temperatures, the reaction can only proceed by the oxidation of tetrahedral Mn2+ ions (280-38O”C), and this further oxidation, retaining the spine1 structure, may be possible because the oxidation products depend on particle size: a single-phase, cation-deficient spine1 for the finely divided substituted magnetite and multi-phase oxides for the coarsely substituted magnetite [15]. The reaction kinetics were studied by heating about 6 mg of pre-oxidized sample at 200°C in the thermobalance at constant temperatures between 260 and 360°C and observing the change in mass with time. The curves (Fig. 5) show that the reaction starts immediately with a maximum rate and are best explained as a diffusion-controlled process, involving a composition gradient through particles of a non-stoichiometric spinel. Under these conditions for spherical particles, the experimental curves can be described by the expression [16] (y = 1 _ 6/=*

5 e-n’kt (I) fl=l where k = r2b/r2, b is the chemical diffusion coefficient, r the mean grain radius and cy = M,/M, the degree of conversion, with M, as the amount of Mn2+ oxidized to Mn3+ in time t and M, as the corresponding amount after infinite time. For (Y< 0.70 and for sufficiently long times ((Y > 0.20) we have shown [16] that eqn. 1 may be written as log (1 - LY)= log 6/m2 - kt = f (t)

(2)

I

I

1

3

5 TIME

Fig. 5. Kinetic

curves,

Am = f(t), of tetrahedral

IN HOURS

Mn*+ ions.

A plot of log (1 - a) versus kt gives a straight line in the middle region, i.e., the region between approxima_tely (Y= 0.15 and 0.65. The activation energy calculated from a plot of log D vs. l/T was 130 kJ mol-’ for x = 0.5 and 147 kJ mol-’ for x = 0.93, a value which appears to be of a reasonable magnitude for a diffusion process within the solid phase. The present results are in good agreement with activation energies which are thought to represent the oxidation of tetrahedral-site manganese. The implication is therefore that the oxidation of an A-site cation will be less rapid than of a B-site cation, which agrees with previous studies [5]. The chemical diffusion coefficients 2, determined at 300°C from the slope of log (1 - CX)= f(t) were averaged and are given in Table 2. These values can be compared with those observed in several other spinels (Table 2). Thus, the diffusion of iron from Fe,O, (Fe *+ in B sites) appears to be faster than that from FeCr,O, and FeAl,O, (all Fe*+ in A sites) and that from the diffusion of manganese from Fe 3_,Mn,O, (all Mn*+ in A sites).

TABLE 2 Chemical

diffusion

coefficients

at 300°C in several spinels

Sample

b (cm2/s)

Fe@, FeCr,O, FeAl 204 Fe,_,Mnx04 Fe,_,Mn,O,

3.5.10-14 2.8.10P” 4.1 .10-19 8.2.10-16 5.6.10-”

(x = 0.5) (x = 0.93)

147

P.3

.,g_____c%,=1.3

i 700 TEMPERATURE

Fig. 6. TG curves for manganese-substituted magnetites heated and x = 0.93 (- - - - - -). oxygen pressures, x = 0.50 ( -)

OC

at 2.5”C min-’

at different

Influence of oxygen pressure on oxidation-reduction phenomena ~educt~~~ process in the temperature range 45~-55~‘C The effect of oxygen pressure on the o~dation-reduction phenomena was demonstrated directly by observing the weight change when the samples were heated at a constant rate of 2S°C min-’ from 300 to 720°C (Fig. 6). When the oxygen pressure is decreasing, especially when this oxygen pressure in the environment during the oxidation is much lower than that in air,

1

2

3

TIME IN HOURS

Fig. 7. Reduction kinetics at 5 Pa () for specimens reduced between and at 48O’C f- - - - - -) for specimens reduced between 80 and 5 Pa.

420 and 490°C

148

the maximum at point A is diminished as a result of the decrease in the amount of unoxidized phase due to the reduction of Mn3’ into Mn2’ ions. From point A, a further loss in weight occurred at higher temperature according to the reduction of Mn4’ into Mn3+ ions as shown in Fig. 2. At point B, the decrease is then caused by a combination of these two effects. From X-ray measurements we determined that no phase changes took place up to point B, but the value of the lattice constant was found to have increased slightly for the AB portion. The kinetics of reduction as a function of temperature and pressure corresponding to the AB portion are shown in Fig. 7. The reaction was characterized by a rapid initial stage, declining regularly according to a parabolic law, suggestive of a typical diffusion-controlled process [16] with production of a cation-deficient spinel. Reoxidation process above 600” C From the TG curves (Fig. 6) it is apparent that above about 550°C (point B) the compound starts to gain weight very rapidly and there is also an onset of a large DTA peak (Fig. 1). This point B moves towards higher temperatures as the oxygen pressure decreases, contrary to the reduction, the amount of BC increasing with increasing oxygen pressure as shown in Fig. 6. The reaction corresponds to the oxidation of Mn2+ ions which were not completely oxidized below 400°C and is accompanied by a phase change from a cation-deficient spine1 structure to a corundum structure, but the nature of the inversion products based on the X-ray data depends of oxygen pressure and temperature. Depending on these parameters, one or two of the following phases were found: (1) at a temperature of 700°C and for 2 < PO2< 2.1 +lo4 Pa, a phase with a corundum structure which is a solid solution of a-(Mn, Fe),O,; (2) at higher temperatures (T > 75O”C), the solubility of Mn 203 in Fe,O, decreases markedly and orthorhombic Mn,O, is observed as a precipitate with a rhombohedral (Yphase; (3) at low oxygen pressure ( PO2< 10-l Pa), Mn2’ is not oxidized and some of the X-ray diffraction lines could be identified with those of Fe,O, and MnFe,O,. The transformation that occurs above 700°C could be represented by (MnxFeo_2x),30~1_x),3)0qZ-

-j 4/3(1 - x)Fe,O,

+ xMnFe,Q

The kinetics of transformation were studied for PO, = 2.1 . lo4 Pa by heating at a constant temperature between 550 and 650°C and observing the change in mass with time. Fig. 8 shows the kinetic curves during oxidation of two selected compositions, x = 0.37 and x = 0.97. These curves display a two-stage oxidation process. The initial pattern is a smooth parabolic curve, the amount oxidized (shown by an arrow in Fig. 8) being due to the experimental procedure and

149

TIME

IN HOURS

Fig. 8. Kinetic curves showing the two steps of oxidation at PO, = 2.1 .104 Pa after treatment at low oxygen pressure ( Psz= 0.3 Pa). The arrow shows the change of kinetic law due to the transformation from a spmel to a corundum structure. x = 0.50 () and x =0.93 (_ _ _ - _ _).

increasing with temperature. Briefly, the procedure was as follows: after oxidation of sample at about 400°C the gas was pumped out at the end of a run, the temperature was raised to the transformation temperature and oxygen was admitted at a known pressure but lower than its partial pressure in air. Thus, samples heated in this cycle and brought to the final temperature at about 600°C were partially reduced under vacuum or at a low oxygen pressure as mentioned above (point A in Fig. 6). As result of this thermal treatment, a certain proportion of the manganese ions must be reduced to lower valency, i.e., to Mn2’ ions. We have also before oxidation two different Mn*+ ions: Mn2+ ions created by reduction, the amount of which depends on the thermal treatment, and Mn 2f ions not completely oxidized at 400°C. The parabolic curves correspond to the oxidation of Mn2+ ions generated by reduction and the phase may be considered to have a spine1 structure, as confirmed by X-ray diffraction. This reduction can be eliminated (and thus the parabolic stage) if the temperature is raised to the transformation temperature while the oxygen pressure is maintained at lo* Pa or higher. For the second stage, corresponding to Mn2+ ions that are not totally oxidized, the Am vs.t plots are sigmoidal and the isotherms were best fitted with the equation 1 - (1 - a) 1’3 = kt where (Y, k and t are the fractional precipitation, temperature-dependent ‘proportionality constant and time, respectively. In our case, the degree of precipitation, QI,is defined by Am(t)

-Am(p)

100

200

TIME(min)

Fig. 9. Relationship between time and 1 - (1 - a) ‘I3 for the reoxidation ing the transformation from a spine1 to a corundum structure.

process

accompany-

where Am(t), Am(p) and Am( cc) are represented in Fig. 8. Such curves are usually explained on the basis of rapid nucleation but of no uniform probability with isotropic growth [17]. The plots of 1 - (1 - (Y)“~ vs. t are shown in Fig. 9. It can be seen that they are linear in the (Yrange 0.2-0.8. An Arrhenius plot of the constant k vs. reciprocal temperature gives an activation energy of 388 kJ mall’ for x = 0.50 and 238 kJ mol-’ for x = 0.93. The reported values for the transformation y-Fe,O, -+ a-Fe,O, show a wide spread, from a minimum of 153 kJ mol-’ to a maximum of 360 kJ

0.1

0.3

0.5

0.7

0.9

X

Fig. 10. Variation spinels.

of transformation

temperature

with composition

for different

defective

151

mol-’ 118,191. This failure to obtain consistent results has been attributed, among other factors, to the presence of impurities [19] and to the influence of particle size and shape [20]. The lower activation energy for high manganese concentration can be attributed to the larger amount of Mn3+ ions created by reoxidation, which involve instability of the spine1 structure compared with the corundum structure. In the corundum structure, Mn3” ions with the outer electronic configuration t&e: are located in a favourable octahedral environment [21]. Moreover, the presence of Mn3+ ions appears to provide an explanation for the fact that the stabilization of defect spine1 is less pronounced for manganese than for zinc or cobalt (Fig. 10). In manganese-substituted magnetites, an Mn3’ ion created from the oxidation in situ of an Mn2+ ion may produce a chemical potential gradient for the migration in these highly defective phases.

CONCLUSIONS

The reactivity in oxygen of finely grained manganese-substituted magnetites leads to complex phenomena associated with the charge and position of manganese ions in the spine1 structure. Below 400°C the dAm/dt = f(T) curves clearly show a three-stage oxidation process. The oxidation temperatures are related to the distribution of Fe2+ and Mn3’ ions on octahedral sites and Mn2’ ions on tetrahedral sites and are in agreement with the fact that covalent bonding in tetrahedral sites of the spine1 structure renders tetrahedrally sited Mn2+ ions less available for oxidation than Fe2+ and Mn3” ions ionically bound in octahedral sites. After three oxidation steps, the material retains the original spine1 structure with a decrease in the lattice parameter. No boundary of solid phase is present and the process may be considered as an interdiffusion inside the oxide itself, the only result being a change in stoi~hiometry with conservation of the oxygen lattice, resulting in a defect spinel. This is due to the preparation method whichs governs the crystallite size, whose mean diameter lies between 40 and 70 nm. These defect phases are partially reduced in the temperature range 450-55OOC according to the diffusion law, while the spine1 structure is preserved, and are then oxidized above 550°C with a phase change from the spine1 to the corundum structure. However, because of the oxidation of Mn2+ to Mn3+ ions and to the affinity of Mn3’ ions for octahedral sites, the stabilization of these defect phases is less pronounced than for defect phases containing cobalt or zinc. The kinetic curves related to the phase change show that the transformation can be described by the equation 1-(1--6X) Ii3 = kr , indicating a nucleation growth mechanism with a process characterized by an activation energy depending on the spine1 composition.

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