Scandinavian Journal of Metallurgy 2005; 34: 108–115 Copyright C© Blackwell Publishing 2005Printed in Denmark. All rights reserved SCANDINAVIAN

JOURNAL OF METALLURGY

Hydrogen reduction of complex oxides—a novel routetoward the production of nanograined alloys

and intermetallicsRicardo Morales1, Francisco J. Tavera1, Ragnhild E. Aune2 and Seshadri Seetharaman2

1Instituto de Investigaciones Metalurgicas, Universidad Michoacana, Santiago Tapia, Morelia, Mich., Mexico; 2Department of Materials Science andEngineering, Royal Institute of Technology, Stockholm, Sweden

One of the major obstacles in the incorporation of nanoma-terials in high technology is the lack of new processes for thebulk production of the materials with compositions tailoredto suit the application. Oxides can potentially be reduced tometals, intermetallics or alloys by hydrogen or natural gas.The formation of homogeneous alloys and intermetallics bythis method has been confirmed by a number of experimen-tal studies. The kinetics of hydrogen reduction of pure oxidesof transition metals as well as complex tungstates, molyb-dates, titanates, aluminates and chromate were investigatedby thermogravimetry. The formation of homogeneous alloysand intermetallics was confirmed by these studies. Arrhenius

activation energies of the reduction reactions could be linkedto the stabilities of the complex oxides. The products werefound to have nanograin structure. Bulk processing throughhydrogen reduction route was examined in the case of ironmolybdate using a fluidized bed reactor.

Key words: hydrogen reduction, nanograined alloys, reduc-tion kinetics.

C© Blackwell Publishing, 2005

Accepted for publication 27 October 2004

Nanomaterials play a very important role in the cur-rent trends of materials science. The unique proper-ties of nanomaterials and structures on the nanometerscale have sparked the attention of materials develop-ers. Hence, there has already been much progress in thesynthesis, assembly and fabrication of nanomaterials,and, equally important, in the potential applications ofthese materials in a wide variety of technologies. Theexplosive growth of nanoscience and technology in thelast decade has been due primarily because of the avail-ability of new methods of synthesizing nanomaterials,as well as tools for characterization and manipulation.Because our capability to synthesize, organize and tailormake materials at the nanoscale is of very recent origin,the next decade is likely to witness major progress inthe discovery and commercialization of nanotechnolo-gies and devices. Even though commercialization of avariety of nanomaterials is already available, the costscontinue to be high due to the lack of bulk productioncapabilities of the current manufacturing technologies.Toward the production of nanoscale materials, the poten-tialities of gas–solid reactions have not been seriouslylooked into so far. In fact, in comparison to the tech-

niques for nanoalloy production being investigated to-day, the gas–solid reaction route offers an interestingalternative toward the production of these materials insubstantial quantities with great scope for quality assur-ance by the control of kinetic parameters.

Gas–solid reaction route is a versatile technique toproduce metallic alloys in the form of powder. This tech-nique allows tailor-made materials with large-scale pro-duction and rather low energy consumption per ton ofproduct. The gas–solid route toward the production ofmetallic and composite powder materials has been apart of a long-term research in the Division of MaterialsProcess Science, Royal Institute of Technology, Sweden.The establishment of the hydrogen reduction kinetics ofthis route toward the production of ultra-fine particlesis the main scope of the present work.

Experimental

Thermogravimetric studiesAll the precursor materials used were grade reagentswith the exception of NiAl2O4, FeCr2O4 and Fe2MoO4,which were prepared by solid-state synthesis. The

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Fig. 1. The thermogravimetric assembly.

synthesized powders were subjected to X-ray diffrac-tion analysis (XRD) to verify their purity before beingused.

In the case of thermogravimetric studies, all exper-iments were conducted under comparable conditions.Such studies were carried out using a SETARAM TGA92 U (France) having a detection limit of 1 µg. Theschematic diagram of the assembly is shown by Fig. 1.The balance is controlled by a PC through a CS 92 con-troller. Shallow alumina crucibles were used to holdpowdery compounds. The crucible containing the sam-ple was suspended from one arm of the balance by Ptsuspension wire, long enough to reach the hot zone ofthe graphite furnace. The crucible was carefully centeredto keep it away from the walls of the reactor tube. Thetemperature of the furnace was controlled by a Pt-10%Rh/Pt (S type) thermocouple placed below the crucible.At the onset of the experiments, the reaction chamberwas evacuated after the sample was introduced. There-after, the reaction chamber was filled with argon gas anda constant argon flow was maintained. The sample washeated up to the experimental temperature at a heatingrate of 25 K/min under argon atmosphere. When thesample temperature was stabilized, the flow of argonwas stopped and hydrogen gas was introduced into thereaction chamber to start the reduction. The experimentwas terminated by cooling down the furnace at a highcooling rate. Hydrogen flow was maintained until thefurnace reached room temperature to avoid the oxida-tion of the reduced sample during the cooling of thefurnace.

To focus on the chemical reaction as the rate-controlling step, preliminary experiments were carriedout on each system to establish the optimum param-eters. For example, a hydrogen flow rate far above thestarvation rate was determined by varying the flow rate.Additionally, the aspect of mass transfer through the

powder bed on the reduction rate was examined bychanging the height of the bed using 2 different sam-ple weights for a given isothermal temperature. Also,to observe the buoyancy effect of the hydrogen gas ve-locity on the results of the thermogravimetric experi-ments, some experiments were performed using alu-mina powder under similar conditions. Repetition ofsome selected experiments showed a high degree ofreproducibility. Because of the small particle sizes em-ployed, both the pore diffusion and heat transfer effectsare expected to be negligible, especially at earlier stagesof reduction.

Fluidized bed studiesIn the case of fluidized bed studies, only Fe2MoO4 pow-der was studied. The main reactor of the fluidized bedapparatus was made of quartz of 15-mm i.d. and 1-m long. The gas distributor was a 2-mm thick porousquartz plate (average pore size = 200 µm). The flowrates of hydrogen and argon before entering the reac-tor were controlled by rotameters. The extent of the re-action was estimated quantitatively by measuring thewater phase in the off-gases produced by the reactionof iron molybdate with hydrogen using a gas chromato-graph (Shimadzu GC-9AM with a ThermoconductivityDetector, Kyoto, Japan), abbreviated hence forth as GC.

Before starting the actual studies at high temperature,preliminary studies were carried out on a cold modelto arrive at some suitable design criteria. These studieshelped to establish conditions of different variables forgood fluidization. Thus, the minimum fluidization ve-locity (Umf ) was determined experimentally by pressuredrop measurements at room temperature. Fluidizationvelocity at a higher temperature was then determinedfrom these calculated values by taking into account thedecrease in density and increase in viscosity of H2 dueto temperature increase. The fluidized bed reductionswere conducted isothermally. Typically, a charge weightof 2 g was used, which gave a static bed of H/D ratioof about 1. This charge was placed in the even temper-ature zone of the furnace. A type-S thermocouple wasplaced at the exterior of the fluidized bed reactor. Thereactor was heated electrically using a vertical tube fur-nace whose temperature was controlled by a tempera-ture controller (Eurotherm-94, Arlov, Sweden). Duringthe heating ramp, argon was passed through the re-actor. When the predetermined reduction temperaturewas stabilized, argon gas was replaced by hydrogen.Then, gas chromatography measurements were carriedout on-line. The average particle size of Fe2MoO4 usedin this study was about 100 µm. The minimum fluidiza-tion velocity depends on the diameter and density ofthe solid particles as well as the physical properties of

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the gas. Thus, the estimated values of gas velocities ofhydrogen at different temperatures ranged from 2.47 to2.75 cm/s.

The reduction curve obtained from both themogravi-metric and fluidized bed studies was describe in terms ofthe degree of reaction defined as the ratio of the weightchange (�W) to the theoretical final weight change(�W∞):

X = �W�W∞ . (1)

Argon and hydrogen gases (plus grade, maximum 10ppm impurities) were used for both the thermogravi-metric and the fluidized bed experiments. These gaseswere supplied by AGA Gas, Stockholm, Sweden. Addi-tionally, argon gas was purified by passing it consecu-tively through columns of silica gel, ascarite and mag-nesium perchlorate.

Results and discussion

TungstatesHydrogen reductions of tungstates were carried out inthe same temperature range, e.g. 873–1183 K. The mech-anism of reduction of CoWO4 [1] toward the formationof tungsten-reinforced cobalt alloys was found to pro-ceed in 1 step. The thermogravimetric studies along withthe chemical analysis of samples partially reduced indi-cated that cobalt tungstate reduces directly to Co7W6

and W according to the following reaction,

14 CoWO4 + H2 = 1

28 Co7W6 + 128 W + H2O. (2)

The activation energy for the chemically controlledreaction was obtained from the rates of reaction obtainedat different temperatures using an Arrhenius plot. Suchactivation energy was found to be 90 kJ/mol. Similarly,FeWO4 [2] was found to be reduced in a single stepdirectly to Fe3W2 and W, e.g.

FeWO4 + 4H2 = 13 Fe3W2 + 1

3 W + 4H2O. (3)

Scanning electron studies revealed that the tungstenphase was well dispersed in the matrix. The activationenergy of iron tungsten by hydrogen was evaluated tobe 85 kJ/mol. In contrast to the above tungstates, themechanism of reduction of NiWO4 [3] by hydrogen wasfound to proceed in 2 steps as follows:

NiWO4 + 2H2 = Ni + WO2 + 2H2O (4)

followed by

WO2 + 2H2 = W + 2H2O. (5)

The activation energies of reactions, eqs. (4) and (5),were calculated to be 95 and 80 kJ/mol, respectively.

As a continuation of the reduction series of tungstatesand aiming at the production of heavy alloys with finemicrostructures, 2 NiWO4–FeWO4 solid solutions [4]were prepared by mixing and sintering appropriate pro-portions of nickel tungstate and iron tungstate. Thus, 2target compositions were obtained, namely, 40 mass%NiWO4–60 mass% FeWO4 and 80 mass% NiWO4–20mass% FeWO4 (henceforth referred as S40 and S80, re-spectively). The kinetics studies indicated that both re-duction in S40 and S80 proceed in 1 step according tothe following equation:

14 (Fe, Ni)WO4 + H2 = Alloy + H2O, (6)

where ‘Alloy’ in the eq. (6) stands for different phasemixtures for a given (Fe, Ni)WO4 solid solution. In thecase of the reduction of S40, the alloy obtained is a 3-phase mixture, whereas in the reduction of S80 the alloyproduced consisted of 2-phase mixture. The hydrogenreduction of the solid solutions resulted in uniform andvery fine-grained structures.

Despite the different reduction products of S40 andS80, if was found that the reduction curves are very sim-ilar as shown in Fig. 2. It should be noted that this simi-larity was obtained irrespective of the reduction temper-ature. The similarities of the reduction curves of S40 andS80 suggest that the breakdown of the chemical bond byhydrogen might take place in similar fashion. From thereduction curves obtained at different temperatures, the

Fig. 2. Reduction curves for 2 solid solutions reduced at the same tempera-ture (933 K). S40: 40 mass% NiWO4–60 mass% FeWO4; S80: 80 mass%NiWO4–20 mass% FeWO4.

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Hydrogen reduction route toward novel alloys

Table 1. Activation energies for the hydrogen reduction of tungstates studied in the present work along with the relevant thermodynamicdata

Reaction Activation Heat of Gibbs Energy,(Equation Energy Formation, Ho

298, Go298,

Number) Oxide (kJ/mol) kJ/ g. atom O kJ/g. atom O

(2) CoWO4 90 −286 −355 at 1000 K(3) FeWO4 85 −296 −346 at 1000 K(4) NiWO4 95 −282 −329 at 1000 K(5) WO2 85 −284 −337 at 1000 K(6) (Fe, Ni)WO4 S40: 90; S80: 97(7) ZnWO4 90 −284 −362 at 1000 K(8) MnWO4 92 −302 −378 at 1000 K

Arrhenius plot for S40 yields an activation energy of 90kJ/mol, whereas the Arrhenius plot for S80 leads to anactivation energy value of 97 kJ/mol. When comparingthese 2 values with the activation energies for reactions,eqs. (3) and (4), it seems that there is a trend that sug-gests that higher nickel content in the solution wouldresult in higher activation energy value.

The reduction of zinc tungstate, ZnWO4, is of spe-cial interest as the thermodynamic information indicatesthat there is no intermediate phase in the Zn–W system.The immiscibility of these 2 elements up to 1623 K hasbeen documented elsewhere [5]. However, in this work,the reduction of zinc tungstate by hydrogen in the tem-perature range 823–1073 K leads to an important discov-ery [6]. At 1073 K, the only phase in the reduced samplewas tungsten, but at lower temperatures a W–Zn binaryphase was detected. The reduction curves showed thatthere is significant loss of zinc due to evaporation. Thisfinding was confirmed by EDS analysis performed onsamples reduced at different temperatures. Thus, the fol-lowing equation for the hydrogen reduction of ZnWO4

is proposed:

ZnWO4 + H2 = W + Zn(solidorgas) + H2O. (7)

It can be assumed that, as soon as Zn is formed dur-ing reduction, a part of it is likely to react with W dueto the highly reactive state of both elements. The ap-parent activation energy of reaction, eq. (7), was evalu-ated using the initial reaction rates leading to a value of90 kJ/mol.

The reduction of MnWO4 [7] by hydrogen is found tobe a promising choice for the in situ production of metalmatrix composites. The reduction curves showed no dis-continuities in their slopes indicating that the reductionreaction occurs in 1 step. The EDS analysis carried outof on samples partially reduced confirmed that the hy-drogen reduction of MnWO4 takes place according tothe following reaction,

13 MnWO4 + H2 = 1

3 MnO + 13 W + H2O (8)

MnO cannot be reduced by hydrogen to any signif-icant extent. Thus, the end product constitutes a metalmatrix composite. If MnO was reduced by hydrogen, itwould be interesting if Mn and W could form an alloyas in the case of reaction, eq. (7). According to the exist-ing literature [6], no mutual solubility of Mn and W hasbeen reported. The experimental conditions enable thefocusing on the chemical reaction rate at initial stages.The activation energy of reaction, eq. (8), was evaluatedfrom the Arrhenius plot. The activation energy valuewas found to be 92 kJ/mol.

To avoid confusion, Table 1 presents the activation en-ergies of reactions involving tungstates and other sys-tems along with thermodynamic data. It can be seen inTable 1 that the activation energies for the tungstates re-actions studied in this work are all in the same level. Inthe case of single-step reductions, the reaction is likely toproceed by the breakdown of the WO4 complex. To ex-pose the possibility of correlating the reduction kineticsof complex oxides with that of the corresponding sim-ple oxides under identical experimental conditions, theactivation energies of CoO [1], FeO [2], WO3 [3], WO2

[8] and NiO [9] were evaluated in the present laboratory.The values of the activation energies for simple oxidesare summarized in Table 2. It should be pointed out thatthe sum of the activation energies of WO3 to WO2 [3]and NiO [9] is 93 kJ/mol. This value is very close to theactivation energy of the reaction, eq. (4), viz. 95 kJ/mol.In the 2-step reaction for NiWO4, the breakdown of the

Table 2. The activation energy for the reduction of Fe, Co and Ni oxidesby hydrogen and the thermodynamic data for the pure oxides

Activation Heat of Gibbs Energy,Energy Formation, Ho

298, Go298,

Oxide ( kJ/mol) kJ/g. atom O kJ/g. atom O

Fe0.96O 44 −265 −364 at1100 KCoO 54 −237 −248 at 700 KNiO 18 −239 −277 at 700 KWO3 75 −281 −327 at 1000 KWO2 83 −284 −337 at 1000 K

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Ni–O bonding could have been the main reaction stepfollowed by the reduction of WO3 to WO2. This couldprobably explain the additive behavior of the activationenergies. On the other hand, it is well known that thesolubility of hydrogen in Ni is significant. As gases dis-solve in metals as a monoatomic species, the reductionof NiO within NiWO4 may be due to the atomic hy-drogen dissolved in the metallic layer after the initialstages. On the other hand, the value of the activationenergy obtained earlier by Bustnes et al. [8] for the pureWO2 to W is very close to that obtained for the reaction,eq. (5), viz. 83 and 80 kJ/mol. This would imply that, inthe case of the reduction of the complex oxide, NiWO4,the nickel metal in the second step does not affect thereduction mechanism of WO2 to W. By comparing thevalue of the activation energies of reductions of CoO toCoWO4 and FeO to FeWO4, it may be said that the en-ergy barrier hindering the start of reactions, eqs. (2) and(3) is mainly due to the strength of the tungsten–oxygenbond.

TitanatesIlmenite (FeTiO3) was chosen as a first part of a seriesof studies on the reduction of transition metal titanatesystems. Ilmenite is the most abundant ore in natureamong the titanium bearing ores. The extraction of ironfrom ilmenite requires the initial separation of iron fromthe ores by pyrometallurgical routes. The reduction ofFeTiO3 by hydrogen gas would provide an insight intothe reaction mechanism. The reduction of FeTiO3 [10]by hydrogen was carried out in the temperature range1034–1487K. The kinetic experiments showed that be-low 1236 K, all the reductions proceeded in 1 step ac-cording to the following equation:

FeTiO3 + H2 = Fe + TiO2 + H2O. (9)

The results from EDS and XRD analyses were in con-formity with the mechanism proposed. For the reduc-tion experiments carried out at 1236, 1286 and 1387 K,the characterization analyses along with the kinetic datasuggest the following reaction in addition to reaction, eq.(9),

3TiO2 + H2 = Ti3O5 + H2O. (10)

It was found that the behavior of the reduction at1487 K differs from the lower temperatures. This as-pect could be attributed to the formation of ferrouspseudo-brookite FeTi2O5 before the sample is reducedto TiO2–Fe and finally to Ti3O5–Fe. The activation en-ergy for reaction, eq. (9), was estimated from the initialstages of reduction for reduction temperatures for up to1186 K. The experimental data at and above 1236 K were

left out of the calculations due to the multi-step chem-ical reactions involved. Thus, the activation energy forthe reduction of FeTiO3 to iron and titanium dioxidewas estimated to be 108 kJ/mol. This reduction processleads to a separation of iron from the titanium dioxideat temperatures below 1186 K.

The reduction kinetics of titanates containing Co2+, orNi2+ ions were investigated [11] to compare the effect ofcobalt and nickel on the reduction. It was found that thereduction of CoTiO3 and NiTiO3 are similar as in the caseof FeTiO3, i.e. the reduction mechanism depends on thereduction temperature range. For example, the chemicalreaction for the reduction of CoTiO3 in the temperaturerange 934–1135 K is as follows:

CoTiO3 + H2 = Co + TiO2 (11)

while in the temperature range 1186–1387 K, the reduc-tion proceed as follows,

3CoTiO3 + 4H2 = 3Co + Ti3O5 + 4H2O. (12)

In the case of CoTiO3, it is likely that above 1278 Kthe reduction process undergoes an additional chemicalreaction such as:

2Ti3O5 + H2O = 3Ti2O3 + H2O (13)

From the initial stages of reduction, the activationenergy for reaction, eq. (11), was evaluated to be 151kJ/mol.

In the case of NiTiO3, the kinetic studies along withthe characterization analyses of samples partially andfully reduced revealed that nickel titanate is reducedaccording to the following reactions:

NiTiO3 + H2 = Ni + TiO2 + H2O 884 K ≤ T ≤ 1135 K

(14)

3NiTiO3 + 4H2

= 3Ni + Ti3O5 + H2O 1186 K ≤ T ≤ 1387 K. (15)

Similar to CoTiO3, above 1278 K, Ti3O5 is further re-duced by reaction, eq. (13). The activation energy for thereduction of NiTiO3 to Ni and TiO was estimated to be153 kJ/mol.

Table 3 summarizes the activation energy of titanatesby hydrogen along with relevant thermodynamic infor-mation. It is interesting to mention that there is a simi-larity between titanates and tungstates containing Fe2+,Co2+ or Ni2+ ions, e.g. the activation energy for the com-plex iron oxide is lower in both titanates and tungstates.

AluminatesThe reduction of complex oxides is an important processroute for the production of ceramic matrix composites.

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Hydrogen reduction route toward novel alloys

Table 3. Activation energies for the hydrogen reduction of titanates studiedin the present work along with the relevant thermodynamic data

Activation Heat of Gibbs Energy,Energy Formation, Ho

298, Go298,

Reaction Oxide (kJ/mol) kJ/ g. atom O kJ/g. atom O

(9) FeTiO3 108 −413 −495 at 1100 K(11) CoTiO3 151 −402 −463 at 1100 K(14) NiTiO3 153 −400 −454 at 1100 K

For example, alumina-impregnated Ni-based alloys canbe produced by the reduction of NiAl2O4 by hydrogen.The reduction kinetics of nickel aluminate [9] with hy-drogen was carried out in the temperature range 1287–1537 K. The reduction occurs according to the followingreaction:

NiAl2O4 + H2 = Ni + Al2O3 + H2O (16)

SEM analysis revealed that the end product consistsof fine particle size Al2O3 uniformly distributed in thenickel phase. These results provide the fundamentals inthe production of alumina-based nickel catalysts. Theactivation energy calculated from the initial stages ofreduction lead to a value of 134 kJ/mol.

The reduction kinetics of CoAl2O4 is also of relevancein the production of metal matrix composites. It is of the-oretical interest to investigate the reduction characteris-tics of this complex oxide. The experiments were carriedout in the temperature range 1173–1472 K [12]. Fig. 3shows the reduction curves, the term ‘f ’ denoting theratio of instant mass loss to the theoretical mass change.A value of f = 1 corresponds to the loss of 1 oxygen atomfrom CoAl2O4. As it can be seen, the reduction curvesat temperatures higher than 1273 K cross the expected

Fig. 3. Weight loss curves for the reduction of CoAl2O4.

maximum value of 1 indicating that some amount of alu-mina was reduced to Al. SEM and EDS analyses in thereduced sample at 1473 K confirmed that part of the alu-mina was reduced and the produced aluminum reactedwith Co forming an Al–Co alloy. According with thethermodynamics, the reduction of Al2O3 by hydrogenis not possible at the experimental temperatures usedin this work. The high affinity of cobalt for aluminumwould explain the reduction of Al2O3 at so low temper-atures. The reduction of CoAl2O4 by hydrogen can beexpressed as:

CoAl2O4 + (1 + 3

2 (1 − α))H2

= XCoαAl1−α + (1−Xα)Co + (1− 1

2 (1 − α)x)Al2O3

+ (1 + 3

2 (1 − α)x)H2O (17)

where α stands for the composition of the observed Al–Co alloy and X represents a constant. The apparent ac-tivation energy estimated for the reduction of CoAl2O4

by hydrogen was found to be 102 kJ/mol.It is felt that more aluminate systems are to be in-

vestigated in order to draw the differences in activationenergies. Moreover, it would be interesting to investi-gate the possible formation of aluminium alloys otheraluminate systems.

ChromateA knowledge of the reaction mechanism of the reduc-tion of FeCr2O4 by hydrogen is, on the one hand, oftheoretical interest, and on the other hand, essential inexploring alternative process routes for the reductionof iron chromate. Thus, the reduction experiments ofsynthetic FeCr2O4 were carried out in the temperaturerange 1173–1423 K [13]. From the thermodynamic pointof view, the available phase diagram of the Cr–Fe–O sys-tem [14] at 1273 K shows that the reduction of FeCr2O4

should go through the 3-phase equilibrium before beingreduced to Cr–Fe solid solution. This is in accordancewith the observations made in the kinetic experimentsand the results obtained from EDS and XRD analysesperformed on samples partially reduced. Thus, the re-duction of FeCr2O4 occurs in 2 steps, viz.

FeCr2O4 + H2 = Fe + Cr2O3 + H2O (18)

Cr2O3 + 3H2 = 2Cr + 3H2O. (19)

At and above 1373 K, the reduction of Cr2O3 appearedto occur before the iron chromate was fully reducedto Fe and Cr2O3. The activation energy for the reduc-tion of FeCr2O4 to Fe and Cr2O3 was evaluated to be131 kJ/mol.

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Morales et al.

MolybdateThe reduction kinetics of iron molybdate (Fe2MoO4) byhydrogen was investigated using both a thermobalance[15] and a fluidized bed reactor [16]. The thermogravi-metric studies were carried out in the temperature range823–1073 K [15]. X-ray diffraction analysis performed on3 partially reduced samples showed Bragg peaks corre-sponding to Fe2MoO4 and Fe2Mo. This would indicatethat Fe2MoO4 was directly reduced to Fe2Mo, viz.

14 Fe2MoO4+H2 = 1

4 Fe2Mo + H2O (20)

The XRD results of the completely reduced sam-ple showed broad diffraction peaks corresponding toFe2Mo. The TEM analysis indicated the end productconsisted of strongly ordered crystal structure havingnano- and microcrystalline regions. The existence ofnano and micro grains would account the broadeningfound in the XRD peaks. It should be point out that theexistence of Fe2Mo phase was point of controversy inthe evaluation of the Fe–Mo phase diagram. The con-troversy aroused from the experimental difficulties en-countered by several researchers in obtaining the Fe2Mophase. In the light of this, it may be said that the gas–solid reaction route could be the only viable route inproducing monolithic Fe2Mo. From the kinetic studies,the activation energy for the reduction of Fe2MoO4 toFe2Mo intermetallic was found to be 173 kJ/mol.

In view of the potentialities of the gas–solid reactionroute in producing completely pure Fe2Mo with grainsize in the nanoscale level, it would be interesting touse a fluidized-bed process as an attractive alternativefor the bulk production of fine-grained alloy materials.The 2 most important aspects identified in this processare the initial particle size and the processing tempera-ture. With this in mind, fluidized bed experiments werecarried out in order to evaluate the feasibility of bulkproducing fine-grained Fe2Mo alloy by the reduction ofFe2MoO4 by hydrogen [16]. Another objective of thiswork was to establish the optimum kinetic parametersfor the reduction of Fe2MoO4 in a fluidized bed reactor.The temperature range used in the reactor was from 923to 1173 K. The activation energy for the process of re-action, eq. (20), was found to be 158 kJ/mol. This highactivation energy value suggests that the rate of chemi-cal reaction is likely to control the process. This value isclose to the activation energy obtained from the ther-mogravimentric studies, viz. 173 kJ/mol. The use offine particles with an average particle size of 100 µmand a fluidizing-gas velocity, just 1.5 times higher thanthe minimum fluidization velocity, leads to a processcontrolled by the rate of the chemical reaction. Figure 4shows a TEM micrograph of a sample produced at 1173

Fig. 4. TEM micrograph of a Fe2Mo pellet pressed at 1 GPa showing domainsof different orientations with perfect coherency at the particle interface.

Table 4. Activation energies for the hydrogen reduction of aluminates,chromate, molybdate and MoO3 studied in the present work along with therelevant thermodynamic data

Reaction Activation Heat of Gibbs Energy,(Equation Energy Formation, Ho

298, Go298,

Number) Oxide ( kJ/mol) kJ/ g. atom O kJ/g. atom O

(16) NiAl2O4 134 −480 −537 at 1400 K(17) CoAl2O4 102 −487(18) FeCr2O4 131 −365 −495 at 1100 K(20) Fe2MoO4 173 −293 −345 at 1000 K

MoO3 175 −248 −283 at 900 K

K. This figure shows micro- and nanocrystalline grainsof Fe2Mo with different texture orientations and perfectcoherence among themselves. In order to achieve thismicrostructure, both crystal size of the product as well asthe reaction time are to be optimized. The optimum tem-perature for this process was found to be 973 K. Theseresults make the gas–solid reaction route a promis-ing alternative in the production of nanostructurematerials.

Table 4 summarizes the activation energies obtainedin the present work for aluminates, chromate, molyb-date and the single molybdenum oxide MoO3 [17] in-vestigated earlier. It is noteworthy to mention that theactivation energy for the reduction of iron molybdateis very close to the activation energy for the reduc-tion of MoO3 to MoO2 by hydrogen, 172 kJ/mol, es-timated earlier in the present laboratory under verysimilar conditions [17]. It is important to point out thatthis comparison seems to follow the same observationmarked earlier on the reduction kinetics of some tran-sition metal tungstates by hydrogen under comparableconditions. These results suggest that the breakdown ofthe molybdenum–oxygen bond plays a significant role

114

Hydrogen reduction route toward novel alloys

240 300 360 420 480 5400

30

60

90

120

150

180 Fe2MoO

4MoO

3

ZnWO4

MnWO4

FeWO4

WO2

CoWO4

NiWO4

MoO2

WO3

FeO

NiO

CoO

NiTiO3

FeTiO3

NiAl2O

4

FeCr2O

4

CoTiO3

Act

ivat

ion

Ene

rgy,

kJ/

mol

- Gibbs Energy, kJ/g. atom O

Fig. 5. The activation energies for the hydrogen reduction of some complexand pure oxides a function of the standard Gibbs energies of the compoundsat the averages reduction temperatures.

in all the reactions. Further investigations are requiredin order to confirm these conclusions.

In order to find a relationship between the thermo-dynamic stability of the transition metal oxides and thechemical activation energy, it is necessary to compare theactivation energies obtained with the thermodynamicinformation of their corresponding reactions. Figure 5shows the kinetic against the thermodynamic data forthe hydrogen reduction of transition metal oxides pre-sented in this work. For the reduction of metals, assum-ing the chemical reaction as the rate controlling, it is rea-sonable to expect that the activation energy to be propor-tional to the strength of metal-oxygen bond (M–O) andconsequently to the Gibbs energy of the compound. InFig. 5, it is seen that most of the experimental point gen-erally follow a straight line relationship implying thatthe chemical activation energies depend on the thermo-dynamic stabilities. The values of the activation energyfor Mo+6 oxides fall farther above the regression line,indicating a deviation in the bond strength compared toother oxides investigated.

Summary

In view of the growing demand from high technology,the need for the development of newer process routestoward the production of bulk nanomaterials is acutelyfelt. Reduction of complex oxides by means of gas–solid

reactions offers an elegant process for the synthesis ofnanoalloys, intermetallics and composite materials inbulk. Reduction of oxides by hydrogen or natural gasoffers a green process route with fewer unit processestoward materials with targeted properties. The kinet-ics of hydrogen reduction of a number of tungstates,molybdates, titanates, chromates as well as pure transi-tion metal oxides were investigated in the present lab-oratory. Bulk production of Fe2Mo intermetallics wassuccessfully carried out by using a fluidized bed reac-tor. Correlations were established between the thermo-dynamic stabilities of the oxides and the Arrhenius ac-tivation energies of hydrogen reduction.

References

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Address:Ricardo Morales, Instituto de Investigaciones MetalurgicasUniversidad Michoacana, Santiago Tapia403, C.P. 58000, Morelia, Mich., Mexicoe-mail: rmorales@umich.mx

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