Scandinavian Journal of Metallurgy 2003; 32: 263–272 Copyright C© Blackwell Munksgaard 2003Printed in Denmark. All rights reserved SCANDINAVIAN

JOURNAL OF METALLURGYISSN 0371-0459

Kinetics of reduction of FeO–Fe2MoO4–MoO2 mixturestoward the production of Fe–Mo alloys

Ricardo MoralesDivision of Metallurgy, Department of Materials Science and Engineering, Royal Institute of Technology, Sweden

The kinetics of reduction of thin powder beds of oxide mix-tures by hydrogen was investigated using a thermogravimet-ric technique. Four oxide mixtures with different Fe/Mo ratioswere investigated. The experiments were carried out isother-mally in the temperature range 873–1023 K. The reductantgas flow rate was kept at 0.5 l/min, which was above thestarvation rate for the reaction. Thin beds of the oxide pow-ders with particle sizes in the micron range were used. Un-der these conditions, the chemical reaction rate is expectedto control the reduction processes. It was found that the re-duction reaction consisted of two steps. The activation en-ergies were calculated for every reduction step involved andthe values are compared. XRD analyses of the obtained alloys

showed two-phase compositions, namely FexMoy and Fe orMo. SEM analyses show the formation of bigger Fe particlessurrounded by extremely fine Fe–Mo intermetallic phase, indi-cating thereby that FeO gets reduced in the first step to metal-lic iron and undergoes grain growth while the iron molybdatecomplex oxide gets reduced in the second step.

Key words: hydrogen reduction, iron–molybdenum alloy,microcrystalline, thermogravimetry.

C© Blackwell Munksgaard, 2003

Accepted for publication 15 January 2003

Reduction of metal oxides toward the production ofnovel alloys requires an optimization of initial oxidecompositions, temperature of reduction, particle sizeand more importantly the feasibility of bulk produc-tion. Gas–solid reactions route offers the advantage ofemploying fairly low processing temperatures and theversatility of using different gaseous reductants suchas methane, carbon monoxide and hydrogen, the lat-ter having a positive impact on the environment. Fur-ther, the method leads to the production of powderswith particle sizes in the micro- and nanoregion. Recentwork in this laboratory [1] has shown the possibilityof bulk production of Fe2Mo alloy with nano- and mi-crocrystalline structure by reducing Fe2MoO4 with hy-drogen gas using a fluidized bed reactor. The presentwork focuses on the possibilities of extending the hy-drogen reduction route toward the synthesis of Fe–Moalloys with varying Fe/Mo ratios. The literature reviewshows that reduction of MoO3–30 wt% Fe was stud-ied by Gasik & Orstrik [2] using a stepwise differen-tial isothermal analysis. One of the objectives of thepresent work is to investigate the kinetics of hydro-gen reduction of oxide mixtures with different Fe/Moratios by thermogravimetry to arrive at the reactionmechanisms.

Experimental

Preparation of materialsThe starting materials used in the present work are Fe,Fe2O3, MoO2 and MoO3. The specifications of these ma-terials along with their sources are presented in Table 1.The compositions of the oxides prepared are summa-rized in Table 2. FeO was prepared at the stoichiometriccomposition by mixing Fe and Fe2O3 powders in re-quired proportions and sintering the mixture. The pro-cedure adopted is described in an earlier work in thislaboratory [3]. Fe2MoO4 was prepared by mixing andsintering suitable amounts of Fe, Fe2O3 and MoO3 pow-ders as described in an earlier work [4], Fe2MoO4 wasmixed with either FeO or MoO2 in various proportionsand sintered to generate the oxide phases with differentFe/Mo ratios. The procedure adopted in preparing theoxide mixtures was as follows. The mixture of the ox-ides was placed into an iron crucible and an iron lid waswelded on to the top. The sealed crucible was heated inargon atmosphere for at least 24 h at 1273 K and thenquenched in water. In case of sample A, which was pre-pared by mixing Fe, Fe2O3 and MoO3, the sample washeated first to 973 K till all MoO3 had reacted before thetemperature was raised to 1273 K. This procedure was

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Table 1. Specifications of the starting materials used in this work

Material Purity% Manufacturer

Fe 99.95 Merck, Darmstadt, GermanyFe2O3 99.8 Alfa Aesar, Karlsruhe, GermanyMoO2 99 Aldrich, Milwaukee, USAMoO3 99.95 Alfa Aesar, Karlsruhe, Germany

essential in view of the high vapor pressure of MoO3.The phases of the products obtained after sintering andquenching were identified by x-ray diffraction analy-sis (XRD) using a Philips X-pert instrument, PW 1830.Cu Kα radiation was used for all the diffraction stud-ies. The results of the XRD analyses of the productsobtained after sintering are also presented in Table 2.X-ray analyses of samples B, C and D showed a smallBragg peak of iron. The presence of small amounts ofFe is due to the fact that the starting material was cor-

Table 2. Phases identified by XRD in the compositions used in this work

Stoichiometric ratio Temp. of heat treatment, K Phases identified after heat treatment

Oxides synthesizedA Fe, Fe2O3, MoO3 4:1:3 973, 1273 Fe2MoO4B Fe, Fe2O3 1:1 1273 FeO–Fe

Mixtures heat treatedC Fe2MoO4, FeO 1:1 1273 Fe2MoO4–FeO–FeD Fe2MoO4, FeO 1:7 1273 Fe2MoO4–FeO–FeE Fe2MoO4, MoO2 1:1 1273 Fe2MoO4–Fe2Mo3O8F Fe2MoO4, MoO2 1:2.25 1273 Fe2Mo3O8–MoO2

Fig. 1. Isothermal section of the Fe–Mo–O system at 1273 K.

responding to stoichiometric FeO which does not existbut the material consisted of nonstoichiometric wustite,Fe0.98O, and metallic iron. The targeted oxide mixturessynthesized for the reduction experiments were all lo-cated along the FeO–Fe2MoO4–Fe2Mo3O8–MoO2 mix-ture and have been marked in the Fe–Mo–O isothermal(1273 K) ternary phase diagram in Fig. 1 [5].

Reduction studies by thermogravimetryIsothermal reduction was carried out using a thermo-gravimetric unit (SETARAM TGA 92, France) which hasan accuracy of 1 µg. The balance is controlled by a PCcomputer through a CS 92 controller. The schematic di-agram of the assembly is shown in Fig. 2. An Al2O3

crucible with 8-mm inner diameter and 1-mm heightwas used to hold the sample. About 10 mg were used ineach reduction experiment so that it formed a thin ox-ide layer. The reactor tube was made of dense alumina

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Kinetic reduction of mixture

Fig. 2. The thermogravimetric assembly.

with 16-mm inside diameter. The crucible containingthe sample was suspended from one arm of the balanceby Pt suspension wire, long enough to reach the hotzone of the graphite furnace. The crucible was carefullycentered to keep it away from the walls of the reactiontube. The temperature of the furnace was controlled bya Pt–10% Rh/Pt (S type) thermocouple placed below thecrucible. Before heating the furnace, the reaction cham-ber was evacuated to less than 10 Pa. Then, the chamberwas filled with argon gas at a flow rate of 0.05 l/minthrough the auxiliary gas inlet and heated up to the de-sired temperature of reduction. When the temperaturewas stable, the inert gas was stopped and hydrogen wasintroduced through the carrier gas inlet at a flow rate of0.5 l/min. The hydrogen flow rate was set above thestarvation rate in order to avoid gas phase mass trans-fer affecting the reduction rate. The starvation rate wasdetermined in preliminary studies and is in conformitywith the reduction studies carried out by the present au-thor earlier [4, 6]. After the completion of reduction, hy-drogen flow was maintained until the furnace reachedthe room temperature. This was in order to avoid anyoxidation of the reduced sample during the cooling pe-riod. Both argon and hydrogen gases were Grade Plus(10 ppm impurities, supplied by AGA Gas, Stockholm).XRD analyses were carried out on samples before and af-ter reduction as well as partially reduced samples. Somereduction experiments were also conducted in the non-isothermal mode in the case of samples C, D, E and F,where the sample was heated from room temperatureup to 1050 K in a stream of hydrogen flowing at 0.5l/min. The heating rate used in these experiments was10 K/min.

In order to identify any intermediate product duringthe reduction, some of the reduction experiments werearrested at intermediate stages. In these cases, the re-ductant gas flow was replaced by argon at the desired

stage and the furnace was cooled at the maximum al-lowable rate. The morphology of the powder was de-termined by scanning electron microscopy (SEM) usinga JEOL JSM-840 (Japan Electronic Optics Ltd., Tokyo,Japan) with an energy-dispersive spectroscopy (EDS)Link AN-10000 (Oxford Instruments, UK) used for de-termining compositions.

Results

All the powder mixtures used in the present investiga-tion were essentially of few microns in size. Hence, thethickness of the product layer is small and it is not likelyto offer any appreciable resistance to the reactant andproduct gas species. In order to observe the effect of thediffusion of reactant and product gas species throughthe bed, the height of the bed was changed using dif-ferent amounts of powders, such as 10, 15 and 20 mg. Itwas found that the bed height had no appreciable effecton the reaction rate, within the detection limits of thethermoanalyzer. However, in all the experiments, only10 mg of powder was used for the kinetic studies. Fur-ther, as the amount of the oxide powder used is verysmall, about 10 mg, the impact of the heat of the reac-tion on the temperature of the particles is likely to benegligible.

Figure 3 shows the reduction curves of all the mixturesat four different temperatures, that is 873, 923, 973 and1023 K. The dimensionless mass change X represents theratio of the instant mass loss �Wt to the theoretical finalmass loss �W∞, the latter corresponding to the loss of alloxygen atoms from the oxide mixtures. In all the cases,the reaction rate was found to increase with tempera-ture, which is typical of a thermally activated process.In the case of mixture C, the reduction curves show abreak at about X = 0.2 at lower temperatures which getsmoothened at higher temperatures. Below X = 0.2, thereduction rate is not significantly affected with increas-ing temperature, while beyond X = 0.2, the reductionrate is affected dramatically with temperature. In thecase of mixture D, the reduction occurs in similar fash-ion. However, at about X = 0.7, reduction curves cor-responding to 923, 973 and 1023 K converge at longertime intervals. In contrast to compositions C and D, com-positions E and F do not show discontinuities in theirslopes.

The results of the nonisothermal experiments are pre-sented in Figs. 4–7. In the case of compositions E andF (Figs. 6 and 7, respectively), the differential reductioncurves dX/dt against time have also been presented. Thenonisothermal reduction curves in Figs. 4 and 5 clearlyshow that both the reductions go through an intermedi-ate step in conformity with the results of the isothermal

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0 100 200 300 400 500

0.0

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400 500

0.0

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400 500

0.0

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400 500

0.0

0.2

0.4

0.6

0.8

1.0

4

32

1

Co

nver

sio

n, X

43

2

1

iii) System E

43

2

1

iv) System F

Time (sec)

43

21

Time (sec)

ii) System Di) System C

Co

nver

sio

n, X

Fig. 3. Isothermal reduction curves for the systems investigated. Experimental temperatures: (1) 873, (2) 923, (3) 973 and (4) 1023 K.

0 200 400 600 800 1000 1200 1400 1600 1800

0.0

0.2

0.4

0.6

0.8

1.0

Composition C

Con

vers

ion,

X

Time (sec)

700 800 900 1000Temperature (K)

Fig. 4. Nonisothermal reduction curve for composition C at 10 K/min.

experiments shown in Figs. 3(a) and (b). The differen-tial reduction curves dX/dt against time in Figs. 6 and7 indicate peak values around X = 0.4 and X = 0.05, re-spectively. At longer time, the reaction rate shows anincrease implying that the reduction may be occurringin two steps.

In the present work, an x-ray diffraction method wasemployed in order to identify the phases present inthe fully and partially reduced specimens. The phases

0 500 1000 1500 2000 2500

0.0

0.2

0.4

0.6

0.8

1.0

Time (sec)

Temperature (K)

Composition DCon

vers

ion,

X

600 700 800 900 1000

Fig. 5. Nonisothermal reduction curve for composition D at 10 K/min.

present in various reduced samples are presented inTable 3.

Discussion

The phase diagram for the binary system Fe–Mo pub-lished recently [7] is reproduced in Fig. 8. It is to bepointed out that this diagram differs significantly fromthat presented by Hansen [8]. The phases present in

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Kinetic reduction of mixture

0 200 400 600 800 1000 1200 1400

0.0

0.2

0.4

0.6

0.8

1.0

X=0.4Composition E

Temperature (K)

Con

vers

ion,

X

Time (s)

0.0000

0.0004

0.0008

0.0012

0.0016

dX/d

t

850 900 950 1000

Fig. 6. Nonisothermal reduction curve and its differential for composition E at 10 K/min.

0 200 400 600 800 1000 1200 1400

0.0

0.2

0.4

0.6

0.8

1.0

X=0.05

Time (s)

Con

vers

ion,

X

-0.0005

0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

dX/dt

800 850 900 950 1000 1050

Composition F

Temperature (K)

Fig. 7. Nonisothermal reduction curve and its differential for composition F at 10 K/min.

various reduced products as well as their composi-tions agree well with Fig. 8. The XRD analyses of thepartially reduced samples E and F, presented in Ta-ble 3 show that the reduction path is through threephase regions involving Fe2MoO4–Fe2Mo3O8–Fe3Mo2

as well as Fe2Mo3O8–Mo–MoO2, respectively. Thesethree-phase intermediate regions are in accordance withthe Fe–Mo–O ternary phase diagram in Fig. 1 eventhough this diagram corresponds to 1273 K. It is tobe noted that the fully reduced products are furnace-cooled and would have the phase compositions corre-sponding to Fig. 8 and will not be in agreement with

Fig. 1. The reduction paths through the intermediatethree-phase regions would also be in agreement withthe results of the nonisothermal reduction experimentsshown in Figs. 4–7, namely each system goes throughan intermediate step. From the discussion above andparticularly by visualizing the reduction paths inFig. 1, the following intermediate reactions are proposedfor the various compositions investigated in the presentwork:Composition C:

FeO + Fe2MoO4 + H2 = Fe + Fe2MoO4 + H2O. (1)

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Fig. 8. The Fe–Mo phase diagram. λ = F e2 Mo and µ = F e3 Mo2.

Table 3. Phases identified by XRD on specimens processed at 873 K

CompletePartially reduced reduction

Composition C Fe2Mo, FeComposition D Fe2Mo, FeComposition E Fe2MoO4, Fe2Mo3O8, Fe3Mo2 Mo, Fe3Mo2

(quenched at X = 0.42)Composition F Fe2Mo3O8, Mo, MoO2 Mo, Fe3Mo2

(quenched at X = 0.06)

Theoretical value for conversion; Xtheo = 0.2. (Ob-served experimentally; X exp = 0.17.)

14 Fe2MoO4 + H2 = 1

4 Fe2Mo + H2O. (2)

Composition D:

FeO + 17 Fe2MoO4 + H2 = Fe + 1

7 Fe2MoO4 + H2O. (3)

Theoretical value for conversion; Xtheo = 0.63. (Ob-served experimentally; Xexp = 0.58.)

14 Fe2MoO4 + H2 = 1

4 Fe2Mo + H2O. (4)

Composition E:

528 Fe2MoO4 + 5

28 Fe2Mo3O8 + H2

= 428 Fe2Mo3O8 + 4

28 Fe3Mo2 + H2O. (5)

Theoretical value for conversion; Xtheo = 0.46. (Ob-served experimentally; Xexp = 0.40.)

18 Fe2Mo3O8 + H2 = 1

12 Fe3Mo2 + 524 Mo + H2O. (6)

Composition F:

12 MoO2 + 2Fe2Mo3O8 + H2

= 12 Mo + 2Fe2Mo3O8 + H2O. (7)

Theoretical value for conversion; Xtheo = 0.058. (Ob-served experimentally; Xexp = 0.05.)

18 Fe2Mo3O8 + H2 = 1

12 Fe3Mo2 + 524 Mo + H2O. (8)

The conversion steps observed in Figs. 4–7 agree wellwith the theoretical ones. While in the case of compo-sitions C and D (Figs. 4 and 5, respectively), the stepcorresponding to the complete reduction of wustite tometallic iron are clearly seen, the conversion steps incompositions E and F (Figs. 6 and 7, respectively) are bestidentified by plotting the derivative of X with respect totime. In contrast, the stepwise reduction is not obviousin the results of the isothermal experiments for compo-sitions E and F. A likely explanation is the merging of

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0.00096 0.00104 0.00112-8.0

-7.5

-7.0

-6.5

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

0.00096 0.00102 0.00108 0.00114

-4.9

-4.8

-4.7

-4.6

-4.5

-4.4

-4.3

1/T (K-1)

Second step (Reac. 2)Q= 153.3 kJ/mol

ln k

1/T (K-1)

First step (Reac. 1)Q=26.7 kJ/mol

Fig. 9. Arrhenius plots for composition C. The transition region at about X = 0.17 indicates the completion of the reduction of FeO to Fe (reaction (1)).

the second step with the last stage of step one. Similarobservations in the case of the reduction of MoO3 werenoted in an earlier work by the present author [6].

From the rates of reaction obtained by isothermal re-duction experiments at different temperatures, the ap-parent activation energies for the two reaction stepscould be calculated using Arrhenius equation, eq. (9).

k = k0 exp(

− QRT

), (9)

where k0 is a constant, Q stands for the activation en-ergy for the reaction, T is the temperature in kelvin, Ris the gas constant and k is the rate constant. Plots of

0.00096 0.00102 0.00108 0.00114

-4.9

-4.8

-4.7

-4.6

-4.5

-4.4

-4.3

-4.2

0.00096 0.00104 0.00112

-7.0

-6.5

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5First step (Reac. 3)Q= 20.7 kJ/mol

ln k

1/T (K-1)

Second step (Reac. 4)Q= 131.8 kJ/mol

1/T (K-1)

Fig. 10. Arrhenius plots for composition D. The transition region at about X = 0.58 indicates the completion of the reduction of FeO to Fe (reaction (3)).

ln k against 1/T for the first and second steps for thecompositions C, D and E are given in Figs. 9, 10 and 11,respectively. In order to avoid experimental errors ow-ing to the transient region when the reaction steps aremerging, the rate for each reduction was taken at ear-lier stages of the first step and at the point where therate was maximum in the second step. For compositionF (Fig. 12), the apparent activation energy was calcu-lated over the experimental range that encompasses thetwo reaction steps due to the low transition step (X =0.05). Table 4 shows the experimental range in the plotsX against the time used for calculating the apparent acti-vation energies. It is to be noted that in the calculation of

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0.00096 0.00102 0.00108 0.00114

-8.0

-7.5

-7.0

-6.5

-6.0

-5.5

-5.0

-4.5

0.00096 0.00102 0.00108 0.00114

-7.0

-6.5

-6.0

-5.5

-5.0

-4.5Second Step (Reac. 6)Q= 160.4 kJ/mol

1/T (K-1)

First step(Reac. 5)Q= 118 kJ/mol

ln k

1/T (K-1)

Fig. 11. Arrhenius plot for composition E. The maximum in the reaction rate (dX/dt) at about X = 0.4 implies the completion of reaction (5).

0.00096 0.00100 0.00104 0.00108 0.00112 0.00116

-6.5

-6.0

-5.5

-5.0

-4.5

-4.0

Reactions 7 and 8Q= 114.8 kJ/mol

ln k

1/T (K-1)

Fig. 12. Arrhenius plot for composition F. The maximum in the reaction rate(dX/dt) at about X = 0.05 implies the completion of reaction (7).

Table 4. Summary of the activation energies for reactions (1)–(8)

ActivationRange used energyfor calculation (kJ/mol)

Composition C Reaction (1) 0 < X < 0.17 26 ± 4Reaction (2) 0.22 < X < 0.6 153 ± 3

Composition D Reaction (3) 0 < X < 0.3 20 ± 5Reaction (4) 0.35 < X < 0.6 131 ± 7

Composition E Reaction (5) 0 < X < 0.4 118 ± 6Reaction (6) 0.45 < X < 0.7 160 ± 9

Composition F Reactions (7) and (8) 0 < X < 0.6 114 ± 4

the apparent activation energy of the first step in Fig. 10,the experimental point corresponding to 923 K was leftout as it deviated strongly from the regression line. Thereason for this scatter is not quite clear.

The reduction of FeO by hydrogen was studied earlierby Bustnes [3] in the temperature range 1023–1073 K.This author found that the activation energy for thechemical reaction was 42.7 kJ/mol This value is higherthan the apparent activation energies for the reductionof FeO to Fe in Figs. 9 and 10; however, the tempera-ture range at which the present investigation is carriedout is lower. Further, the composition of FeO used byBustnes was Fe0.96O. In the present experiments, the ap-parent composition of FeO is stoichiometric FeO, indi-cating excess iron in the system. A similar decrease in theactivation energy is found in the case of the reductionof Fe2MoO4 calculated from the present results as com-pared to the earlier work by the present author [4]. It isnot clear if the presence of metallic iron has an impact onthe reaction mechanism. Gasik & Orstrik [2] reported,in their reduction studies of MoO3–30 wt% Fe mixturethat the presence of iron increases the reaction rate dueto the fact that iron is reducing MoO3. The lowering ofthe activation energy observed in the present studiesseems to be in agreement with their results. However,the amount of free iron metal present is much lower inthe present case.

The rate of reduction for the composition D in Fig. 3decreases dramatically at about X = 0.7. This behaviorcould be due to the sintering of the iron particles on theFe2MoO4 as FeO gets reduced.

SEM micrographs of compositions D and F after re-duction are shown in Figs. 13(a) and (b), respectively.The compositions of the phases present were deter-mined by EDS. In Fig. 13(a), the light phase corre-sponds to Fe2Mo while the dark phase corresponds toFe. As it can be seen, the iron particles are larger than

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Kinetic reduction of mixture

Fig. 13. SEM micrographs of Fe–Mo alloys after reduction. (a) product obtained from composition D and (b) product obtained from composition F.

the Fe2Mo particles. Additionally, the sintering effectis quite evident in the iron particles. In Fig. 13(b), thedark phase marked by the arrow corresponds to molyb-denum while the light phase has a composition corre-sponding to Fe3Mo2. Here the difference in particle sizeis less significant.

Conclusion

Oxide mixtures Fe2MoO4–FeO–MoO2 of desired Fe/Moratios were prepared and reduced by hydrogen in thetemperature range 873–1023 K. The rate of the chemi-cal reaction was followed using thermogravimetry. Allreductions were found to proceed in two steps. In thecase of FeO–Fe2MoO4 mixtures, the apparent activation

energies for the reduction of both the component ox-ides were found to be lower than values reported ear-lier. XRD along with SEM analyses indicated that themetallic phase such as iron or molybdenum in the prod-ucts was of crystalline structure while the intermetallicphase had microcrystalline structure.

Acknowledgments

The author acknowledges the encouragement and ad-vice of Professor S. Seetharaman in preparing thismanuscript. Valuable comments by Professor Du Sichenare gratefully acknowledged. The author wishes to ex-press his sincere gratitude to the National Council for

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Science and Technology (Conacyt, Mexico) for its finan-cial support.

References

1. Morales R, Sichen Du, Seetharaman S. Metall Mater TransB (in press).

2. Gasik MM, Orstrik PN. J Thermal Anal 1993: 40: 3313–3319.3. Bustnes JA. Metall Mater Trans B 1997: 28B: 613–618.4. Morales R, Sichen Du, Seetharaman S. Metall Mater Trans

B 2002: 33B: 589–594.5. Kleykamp H, Schauer V. J Less-Common Met 1981: 81: 229–

238.

6. Morales-Estrella R, Arvanitidis I, Seetharaman S. Z Metal-lkd 2000: 91: 589–593.

7. Massalski TB. Binary Alloys Phase Diagrams, American So-ciety for Metals, OH 1986: 1080.

8. Hansen M. Constitution of Binary Alloys. McGraw-Hill,London 1958: 668.

Address:Division of MetallurgyDepartment of Materials Science and EngineeringRoyal Institute of TechnologySE-100 44 StockholmSwedene-mail: ricardo@mse.kth.se

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