Hydrogen Reduction of Cobalt-Chromium Spinel Oxides
2395
Hydrogen Reduction of Cobalt-Chromium Spinel Oxides. 1. Stoichiometric Cobalt Chromite Pierre Bracconi’ and Louis-Claude Dufour Laboratoire de Recherches sur la Reactivite des Solides (associ6 au Centre National de la Recherche Scientifique), apartement B, Faculte des Sciences Mirande, 21000-Dijon, France (Received June 10, 1974; Revised Manuscript Received June 5, 1975)
.
Publication costs assisted by the University of Dyon
The reduction by hydrogen of pure stoichiometric cobalt chromite CoCr204 is investigated by thermogravimetry, X-ray diffractometry, magnetic analysis, and scanning electron microscopy. The following reaction model is proposed the reaction products formation processes (fcc-Co and a-Cr2O3) are assumed to take place repeatedly in a few atomic planes thickness and discontinuous “interfacial layer” of the initial spinel structure; the unreducible Cr3+ ions rearrange by a short-range inward migration on the sites determined by the fcc oxygen sublattice, the number of which decreases proportionally with the oxygen removal; the Co2+ ions counterdiffuse toward the solid-gas interface where they are reduced to the metallic state simultaneously with the formation of water molecules. This diffusion would be the rate-determining step characterized by the experimental activation energy for the reaction E = 40 kcal/mol. The metal nucleates and grows on the surface, whereas a-Cr2O3 precipitates directly from the spinel structure. The mechanism of this precipitation can be described clearly assuming that a favorable and well-defined cation rearrangement previously occurs in the disturbed spinel structure of the “interfacial layer”. Such a favorable rearrangement might be equivalent to the formation of y-Cr2O3 zones from which the equilibrium a-CrzOs phase would crystallize by the well-known shear mechanism: y-MezO3 (spinel structure) a-MezO3 (corundum structure). Mechanical strains due to the cation concentration gradients and the associated variation of lattice parameter between the bulk and the “interfacial layer” would greatly assist this precipitation, i.e., prevent the intermediate cation order (of y-Cr2O3 type) from extending to the whole bulk of the grain. The momentarily rearranged zones in the spinel could be regarded as a-Cr203 pre-precipitates. Above 750°C, a partial reduction of a-Cr203 is observed; it is due to the presence of Co metal: fcc Co-Cr solid solutions are formed and the maximum fraction of Cr2O3 reduced at a given temperature is determined by the limit of chromium solubility in the fcc-Co lattice at that temperature. This reduction is made possible because, contrary to the reduction of pure a-Cr2O3, it does not require the formation of Cr nuclei (or Cr/CrzO3 interface). Thus it ceases when the fcc-Co lattice is saturated with Cr, that is to say, when a new intermetallic phase nucleation would be required so that it might continue.
-
I. Introduction This work concerns the reduction of three cobalt-chromium oxide specimens of various cation compositions or stoichiometries, and is divided in two parts: this first part deals with the reduction of pure stoichiometric chromite CoCrzO4 (specimen 1); the second with that of two specimens in which Cr3+ ions are partly substituted by Co3+ ions. All three specimens have in common the spinel structure, a paramagnetic behavior, and a p-type semiconductivity above room temperature. Until now, little work has been done on the hydrogen reduction of ternary oxides, particularly of chromites. The work of Hussein et a1.l is concerned with the reduction of ferrochromite ores with high aluminum and magnesium contents, while Manchinskii et a1.2 and Gel’d and Esin3 reduced iron chromite. These authors observed that a-Cr2O3 formed simultaneously with iron was easily reducible in the temperature range where pure Cr2O3 is not, and that this also occurs in the reduction of mixtures FeO-Cr203 or Fe( 3 2 0 3 . Hussein explained this (as did previously Baukloh and Henke4) by a catalytic effect of the metal, while Manchinskii noted that, in the temperature range 1100-140O0C, Fe-Cr alloys form and that the quantity of iron in the initial mixture as well as the quality of the contact between the metal and cr203 influence the reduction rate.
We show, here, that the corresponding phenomenon in the system Co-Cr203 takes place as low as 750°C, and that it is not due to a catalytic effect of the metal supported on the oxide, but to the existence of a metal-oxide interface. A detailed model of cationic rearrangement is also proposed to explain the formation of the corundum type CrzO3 structure from the CoCr204 spinel structure in accordance with reaction kinetics.
11. Experimental Section Apparatus and Procedure. Cobalt and chromium concentrations in the initial specimens were determined by the Service de Microanalyse du CNRS. These oxides lattice parameters were measured with a Seeman Bohlin type camera (double monochromator, Cu Ka1 radiation), while the X-ray analysis of partially reduced samples quenched to room temperature was done either with a Siemens diffractometer or a Debye-Scherrer type camera (in both cases Cu K a radiation filtered with Ni). In a few high-temperature diffractometry experiments, an apparatus of the Compagnie GBn6rale de Radiologie5 was used. Magnetic susceptibilities and magnetizations were measured by the Faraday method with a Cahn RG electrobalance (temperature and pressure controlled) calibrated The Journal of Physical Chemistry, Vo/. 79, No. 22, 1975
Pierre Bracconi and Louis-Claude Dufour
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with C O H ~ ( S C N )The ~ . ~magnetic fields up to H = 10,000 Oe were measured by graphical integration of the experimental curves H dH/dz vs z . Pycnometry was done with ethyl phthalate; the samples were degassed and the pycnometer was filled under vacuum. A silica spring thermobalance (sensitivity 0.01 mg) was used for continuous recording of the samples weight losses in reducing conditions (water condensed in liquid nitrogen trap), and for measurement of the BET surface areas by argon adsorption at 77 K. A Cambridge Mark I1 scanning electron microscope (SEM) was used for observation of the solids. Materials. The preparation method is due to Whipple and Wold.' The complex CoCr207-4CsH5N is prepared from 99% pure CoClz, 6Hz0, and 99% pure (NH4)2Cr207. Its decomposition in air at 12OOOC during 24 hr yields a spinel type oxide of composition C01.06Cr1.8204.This product is made stoichiometric by addition of the required amount of (NH4)zCr~07,followed by a second treatment at 1200°C for 24 hr, in air. The chromite obtained proves to be stoichiometric with the composition Col.ooCrz,ooOs. Its lattice parameter 8.330 f 0.007 8, is in good agreement with the literature data (8.332 8.3359 As), and its Curie constant is 0.0281 f 0.0004 K cm3/g. As Cr3+ ions always have an invariable (spin only) magnetic moment, pcr3+= 3.87 p ~ , 9the magnetic moment per Co atom in SI can be calculated. The obtained value p c o = 4.58 f 0.11 p~ is only consistent with Co2+ ions. All these observations confirm that, as expected, SI is a normal spinel with the ionic structure CoZ+[CrZ3+]B04'- (the index B means that the ions between brackets are located on the octahedral sites of the structure; tetrahedral sites will be referred to as A). The specimen, a blue color, is crystallized in the form of small octahedra, the edges of which have an average length of 1.5 & 0.5 pm. The measured density is 5.26 g/cm3, in very good accordance with that calculated from the experimental composition and lattice parameter (5.24 f 0.04 g/cm3). The corresponding geometrical surface area is S = 0.7 f. 0.2 m2/g. 111. Results
Thermograuimetric Analysis carried out at P(H2) = 50 Torr and constant rate temperature increase (120°C/hr) between room temperature and 1000°C shows that the reduction is not limited to CoCrzO4
+ H2
-
Co
+ Crz03 + HzO
(i)
From the isothermal reaction curves of a vs. time (Figure 1, where a is the weight loss in oxygen gram atoms per mole of CoCrzO4), it is observed that 75OOC is approximately the maximum temperature at which the reaction rate becomes zero when 01 1 g-atom. At higher temperatures the extra reduction proceeds with continuously decreasing rate and stops for an a value which depends on the reactipn temperature but always remains much smaller than a = 4 g-atom corresponding to the overall reduction CoCrzOr + 4Hz Co Cr 4&0. These observations show that the chromium oxide formed by reaction i, along with Co metal, is partially reducible in a temperature range where pure aCrzO3 is n0t.l-4 X - R a y Diffraction Analysis. High temperature diffraction confirms that the reaction products at TR < 75OOC are fcc-Co and a-CrzO3. After quenching the samples, the cubic -+
+
+
The Journal of Physical Chemisfty, Vol. 79, No. 22, 1975
-+
5
Isothermal therrnogravirnetric curves for the reduction, at constant pressure P H ~= 50 Torr, of specimen 1, a represents the weight loss in oxygen gram atoms per CoCr204 mole, a = 1 corresponding to the reaction: CoCr204 H2 Co + Cr203 H20. Figure 1.
+
-
+
structure of the cobalt is retained, and, even after very long ageing time at room temperature, no traces of hexagonal cobalt are observed. More detailed X-ray analysis of samples reduced in a thermobalance for kinetic purpose, and quenched to room temperature, gives the following results. (1) On one hand, the (11I) interplanar spacing of fcc-Co is independent of the temperature of reduction, as well as of a; i.e., it remains unchanged even for an a value,that would correspond to the reduction of 0.25 g-atom of chromium to the metallic state along with 1 g-atom of cobalt. On the other hand, no reflection lines attributable to Cr metal are observed, even in this last case. This can be understood in three ways. First, it is possible that the strongest Cr reflection line (dl10 = 2.0390 8,) is overlapped by the (111) fcc-Co line (dl11 = 2.0476 A). This possibility however may be discarded by observing that the relative intensity of the (200) Co reflection ( Z ~ O o l Z ~is~ also ~ ) unmodified when a > 1 g-atom whereas it should be lowered. Second, one may imagine that the Cr particles form under the Co particles and are thus unaccessible to X-rays. Last, if no Cr metal is precipitated, Co-Cr fcc solid solutions, with a lattice parameter practically similar to that of pure fcc-Co, may be considered to form. It is clear that X-ray analysis cannot distinguish between these two last possibilities, and that the use of other investigational means is required. (2) The interplanar spacings dhkl of CrzO3 are also constant with TR,corresponding to the following parameters at room temperature: a = 4.946 8, and c = 13.571 A. According to the work of Greenwald,lo one may conclude from such values that this phase is antiferromagnetic at room temperature, as could be normally expected, but could not be demonstrated directly, here, because of the presence of ferromagnetic cobalt in the samples. Magnetic Analysis. The first magnetization curves u VS. H of small fractions of the samples used for the X-ray analysis were plotted: the following results were obtained, shown in Figure 2. (1) For a < 1 g-atom, the maximum magnetization per gram of cobalt in the samples, increases with a (i.e., with the amsunt of cobalt reduced to the metallic state) and reaches value u = 173 emu cgs/g for a = 1 g-atom; this
Hydrogen Reduction of Cobalt-Chromium Spinel Oxides I
1
loor
2397
rived curves daldt vs. a have a maximum between a = 0.3 and a = 0.4 g-atom, but the initial rates a t zero time have a small positive value. Although the reduction of CrzO3 may be expected to interfer at T R > 75OoC, for 0.1 < a < 0.7 gatom however, plotting log daldt vs. 1 l T ~ gives straight lines over the whole temperature range for all a values (Figure 3), and the corresponding activation energy E = 40 kcal/mol may be regarded as characteristic of the reaction rate of CoCrzO4
+ H2
-
fcc-Co
+ a-Cr203+ H2O
(i)
between 700 and 845°C. Moreover the reaction rate is hardly affected by pressure. The second part of the reaction for a > 1 g-atom, corresponding to the reduction of Cr203 according to Co + Cr203 + xH2
-
(Co,Crzx/3)
Figure 2. First magnetization curves (recorded at room temperature) of partially reduced samples of specimen 1: curves 1 and 2, the
magnetization increases with the amount of metallic Co in the samples at a < 1; curves 3 and 4, it decreases at a > 1 by dissolution of Cr atoms in Co. value is higher than that generally given for pure cobalt (161 emu cgslg) but the excess remains in the limit of experimental error. (2) For a > 1 g-atom a strong decrease of u is observed (curves 3 and 4, Figure 2) with increasing a. This is due to the dissolution of Cr atoms in the fcc-Co lattice: our values of saturation magnetization obtained by extrapolation to infinite magnetic field are compared to the data of Farcas” and Crangle12 concerning magnetization variation at room temperature of Co-Cr alloys. The discrepancy between our value and that of Farcas and Crangle for the saturation magnetization of pure Co has been taken into account. This allows the compositions (in atom percent Cr) of the solid solutions formed to be calculated, which are in good agreement with those deduced from a if the reduced Cr atoms are supposed to be homogeneously “distributed” in the Co lattice:
Ly
At R C r (magnetic)
At % C r (gravimetric)
1.oo
0 0 6.4 6.2 1.42 23a 21.5 a Obtained by extrapolation of the data of ref 11and 12.
1.10
An attempt to measure the corresponding Curie temperature T,, which would also be indicative of the Cr concentrations, was unsuccessful. For low Cr contents, T, would be too high to be measured: T,(Co) = 1121OC; for higher Cr contents the fcc solid solutions quenched to room temperature are metastable. Consequently they decompose, when reheated, into an hexagonal e solution (with lower Cr content and greater magnetization and T,), together with the p a r a m a g n e t i ~ lu~ phase of the Co-Cr phase diagram,14 making T, determination meaningless. Perfectly similar decompositions are known to occur with supersatured fcc solid solution of tungsten in cobalt.15 Kinetics. Isobaric (P(H2) = 50 Torr) and isothermal a vs. t curves (TR = 702, 719, 749, 778, 808, and 845OC) are recorded (Figure 1).They are slightly sigmddal: the de-
+
.
“ I
is characterized by a constantly decreasing rate. The concentration of chromium C, in the solid solution formed (assumed to be homogeneous), can be calculated in terms of a
c,
2/3(a
= 1
- 1) =- a - 1
+ 2/3(a- 1)
a
+ 0.5
expressed in atomic fraction. Figure 4 shows that at 845OC the reaction rate is proportional to the difference between the saturation concentration C, and concentration C, daldt
K(Cs - C,) = KC, - K
a-1 a + 0.5
Indeed, as shown in the figure, daldt becomes zero for C, = 0.244, being in accordance with the C, value at 845OC (0.215) given by the Co-Cr phase diagram.14 Morphology. As shown by the SEM micrographs (Figure 5), the aggregates of reaction products crystallites keep the octahedral form of the initial crystals, as generally observed in metallic oxides reductions. Indeed, their morphology (particles size and shape) is fundamentally determined by two types of adverse phenomena both depending directly on temperature: on one hand the reactive processes and on the other sintering (affecting more effectively the metal). The fragmentation due to the chemical reaction increases with the latter rate and thus with temperature, but it is opposed by sintering.
IV. Discussion (A) We will first consider the reaction according to eq i. In the a-Cr203 structure the oxygen ions are arranged in an hexagonal close-packed sublattice; the chromium ions occupy two-thirds of the distorted octahedral sites, and thus have a similar anionic nearest neighbor arrangement in both the initial and final phases. As they are unreducible (in the present conditions) they must remain bonded to oxygen ions throughout the reaction; we shall then assume that they consequently diffuse back into the solid matrix so as to maintain this bonding state. Since the number of normally occupied octahedral sites decreases proportionally to the number of oxygen atoms removed, they will have to locate on other available sites, where a new distribution will occur, at least temporarily. More precisely, this means that the chromium ions have to rearrange inward by short-range diffusion either on normally unoccupied B sites or on both A and B sites. In the first case the diffusion path for a C ~ B to ~ a+ normally unoccupied B site is merely through a The Journal of Physical Chemistry, Vol. 79, No. 22, 1975
Pierre Bracconi and Louis-Claude Dulour
2398
figure 3. S1 reductban rate (g-atomslhr) variation in an Arrhenius plot for 0.2 < a < 0.5. The corresponding results concerning S2 (cf. part II) are also given. here, allowing direct comparison. In both cases activation energy is E = 4 0 kcallmol.
iGR2 5.10'
2.110-
0 0
0.1
a2
1.0
a-~ a-as
Figure 4. Linear relation between the reaction rate daldt. at 845OC and a > 1 gatom. and the atomic fraction of drromium C, = (a l ) l ( m 0.5) in the fcc Co-Cr soli solution fwmed.
+
normally unoccupied A site; this is clear from a description of the spinel structure in terms of space filling anion coordination polyhedra (Stone and Tilley'% If the new distribution could be such that a mere local rearrangement of the oxygen planes from the ABCABC to the ACAC close packing was sufficient to give rise to a more or less important volume of the a-Cr203 structure, a satisfactory explanation of the formation mechanism of the embryos of this phase would be given, showing certain similarities with the a-AI203 precipitation mechanism from Al203-superseturated Mg-AI spinel oxide proposed by Jag~dzinski.'~However, a redistribution of the C$+ ions on the two types of sites of the spinel matrix and especially on those freed by the reduced cobaltous ions will be more probably energetically easier. Indeed, Cr203 is known to crystallize under certain conditions in the spinel structure with a lattice parameter (a = 8.36 A) slightly higher (0.4%) than that of C0Cr201.'~ Furthermore. in the iron oxides homologous system, the transformation from the spinel to the corundum structure of Fez03 is known to occur by associated movements of the oxygen planes [in the (112) direction, from the ABCABC to the ACAC close pa~king'~] and of the Fe3+ ions. Thus, the y-Cr203 unstable structure can be proposed here not as an actual intermediate state, hut as a model for the ions temporary rearrangement, in e few atomic planes of the spinel matrix near the solid-gas interface, which will be called the "interfacial layer". This arrangement would give rise to the equilibrium phase n*Jamaiorm+irChaniPhy. vol. 79, NO. 22, 7875
Flpun 5. SEM micrographs of (top)CoCr204 inRmi grains. X 19,000; (center)CoCr20. reduction products (at 749OC for 19 hr. a = 1.00 g-atom). X 10.000; and (bottom) CoCrlOI reduction products (at 845OC for 17 hr. a = 1.42 g-atom). X 20,000.
through a mechanism similar to that of the y-Fe203 transformation into a-Fe203 according to C$+[Cr23+]B042-
+ H2
-
+ Coo + H 2 0
Hydrogen Reduction of Cobalt-Chromium Spinel Oxides
2399
Besides a y-CrzO3 type arrangement specific instability (under our experimental conditions), mechanical strains, compensating for the misfitting of the oxygen sublattices in the bulk and in the interfacial layer, must appear. They probably initiate or a t least assist the shear mechanism, and prevent the intermediate arrangement from extending to the entire bulk of the grain, i.e., beyond this interfacial layer. In other words, this intermediate arrangement can be considered here as a superficial pre-precipitate and the above model, then, nears that proposed by Bansal and Heuer for the Mg0-3.5A1203 spinel decomposition.20 The direct experimental demonstration of such mechanisms is obviously linked, on one hand, to the chemical nature of the system and on the other, to specific experimental conditions, among which the crystallite size is certainly important relative to the mechanical strains produced. The diffusion coefficients of chromium and cobalt in CoCr204 were measured in air between 1400 and 160OOC by Sunz1:Dc0= 1X 10-3exp(-51,000/RT),D~,=2exp(-70,000/
RT). In our temperature range of experiments (70O-85O0C) the Dcr extrapolated value is smaller than that of Dco, but the short-range displacements of the chromium ions required for the formation of cy-CrzO3 can hardly be assumed to control the overall reaction rate; instead, the cobalt ions have to be transferred to the external surface through the interfacial layer. The energetically most favorable diffusion path for Co2+ in CoCrzO4 according to Stone and Tilley16 is through an octahedral unoccupied site to one of the four nearest tetrahedral sites (two of which are normally empty). CoCrzO4 being a p-type semiconductor,22 vacancies are likely to exist (associated with Co3+ ions), on the tetrahedral sites sublattice, favoring that diffusion path even more. Then, the overall reaction can be considered, a t this point of the discussion, as a repetition of the following elementary steps: Co2+ diffusion and Cr3+ reordering mechanical strains cy-CrzO3 formation. The sigmoidal shape of the kinetic curves can be regarded as due to the variation, with time, of the interfacial layer stretch within which the above processes take place, and the experimental activation energy, 40 kcallmol, as related to Co2+ diffusion through that interfacial layer. Consecutively to such a mechanism, one deduces that the precipitated a-CrzO3 embryos cannot grow in terms of the common viewpoint of nucleation and growth models, i.e., by matter transfer through an a-Cr203-CoCr204 interface. They can, however, sinter independently of the reactive mechanism itself, which also applies to cobalt nuclei. Therefore the eventual solid morphology (cf. Figure 5 ) is mainly determined by sintering (homogeneous and heterogeneous) and all the more as the reduction temperature is higher. Now, the hydrogen adsorption and water formation and desorption may be discussed, although these processes are at equilibrium, as shown by the reaction rate invariability with hydrogen pressure. Gillot and Barret have shown in a recent work on iron chromite reductionz3 that adsorbed hydrogen on that compound was dissociated: on the other hand there are some reasons to think that hydrogen adsorption on a p-type semiconductor such as CoCr204 involves a conductivity decrease corresponding to the destruction of the defect centers. This can be written using R e e ~notation: ’~~
-
+
H z ( ~ ) 20,+
-+
-
2HC/o,+ + 202-/0,-
2H+/o,+ -+
+ 2e-
20H-/o,-
+ 20,+
(a) (b)
2Co3+1o,+ 4. O + + 2e-
+
+
2C02+l~,+ O+
(c)
Tikkanen25suggested that the destruction, by reduction, of such point defects (at their limit concentration) in cobalt monoxide must be opposed by a process regenerating them in order to maintain this concentration: 3Co2+/0,+
-+
2c03+10,+ x o+
+ coo
(dl
In our case, however, the fact that an equilibrium concentration of Co3+ point defects is maintained in the interfacial layer is hardly conceivable. It may rather be said that the two following processes cannot be dissociated and require Co2+ ions to be available on superficial sites:
+ o,+ HzO(,) + 02-/Os- + 0,-
Co2+/0,+ 20H-/Os-
-+
+ 2e-
-+
Coo
(e) (f)
(B) Any attempt to explain the partial reducibility of aCrzO3 has to take into account the following points. (1) Under our experimental conditions and in the absence of metallic cobalt, cy-CrzOa is unreducible. (2) When cobalt is present, the rate of the reduction is continuously decreasing with time up to an cy value that corresponds to the saturation of the fcc cobalt lattice with Cr. Then, an explanation of the phenomenon on the basis of a catalytic effect due to the metal (considered here as “supported” on CrzO3) may be discarded, although Co metal is likely to favor the adsorption processes, especially the hydrogen molecules dissociation, by a spill-over mechanism. (3) The activity of Cr in Co-Cr solid solutions is not known, but, by analogy with the Ni-Cr ~ y s t e m , it~ ~ is , ~ ~ probable that in the thermodynamic conditions of our experiments, the affinity of the reaction ii will be higher than that of pure CrzO3 reduction. Hence, the most likely explanation for the observed reduction of chromium oxide is that no Cr nuclei (or CrCr203 interface) need be formed as would be required for the pure Cr2O3 reduction. A “host” lattice (or a metaloxide interface) already exists and plays a similar part. It is thus easy to understand that the reaction stops when the maximum concentration of the solid solution is reached: further reduction would require a phase change in the metallic matrix, Le., the precipitation of another alloy. The observations of Manchinskii, indicating that increasing the quantity of iron in a mixture Fe-CrzOs increases the fractional reduction attainable in standard conditions, would be due to the higher absolute amount of chromium that can be dissolved in more iron, and those on the part played by the quality of the contact between Fe and Cr203 could be explained in terms of size of the interfacial area. References and Notes (1) M. K. Hussein, G. A. Kolta, I. F. Hewaidy, and A. M. El Roudi, Rev. Chirn. Miner., 8, 463 (1971); UAR J. Chem., 14, 289 (1971). (2) V. G. Manchinskii, A. P. Lyuban, and A. M. Semenov. Nauchno Tekh. hf.Byull Leningf. Politekh. Insf., 10, 92 (1961). (3) P. V. Gel’d and 0. A. Esin, J. Appl. Chem. USSR (Engl. Trans.), 23, 1351 (1950); 23, 1271 (1950). (4) W. Baukioh and G. Henke, 2. Anorg. Allg. Chem., 234, 307 (1937). (5) N. Gerard, J. Phys. E, 5 (1972). (6) S. A. Klzhaev, P. V. Usachev, and V. M. Yudin, Sov. Phys. Sol. State, 13, 2380 (1972); H. St Rade, J. Phys. Chem., 77, 424 (1973). (7) E. Whipple and A. Wold, J. lnorg. Nucl. Chem., 24, 23 (1962). (8) R . J. Makkonen, Suomen Chem. B,3 5 , 2 3 0 (1962). (9) M. M. Schieber. “Experimental Magnetochemistry”, North Holland Publishing Co., Amsterdam, 1967, p 25. (10) S . Greenwald, Nature(London), 177, 286 (1956). (11) T. Farcas, Ann. Phys. Paris, 8, 146 (1937). (12) J. Crangle, Phil. Mag., 2, 659 (1957).
The Journal of Physical Chemistry, Vol. 79. No. 22, 1975
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(16) (17) (18)
(19)
Pierre Bracconi and Louis-Claude Dufour
N. Mori and T. Mitsui, J. Phys. SOC.Jpn., 26, 1087 (1969). M. Hansen and P. Anderko, "Constitution of Binary Alloys", 2nd ed, McGraw-Hill, New York, N.Y., 1958, p 466. L. N. Larikov and 0. A. Shmatko, Fiz. Met. Metalloved., 30 (6), 1173 11970). F. S. Stone and R. J. D. Tilley, React. Solid, Proc. lnt. Symp., 7th. 1972, 262 (1972). H. Jagodzinski, 2.Kristallogr., 109, 368 (1957). K. P. Sinha and A. P. B. Sinha, J. Phys. Chem., 61, 758 (1957); A. W. Laubengayer and H. W. Mc Cune, J. Am. Chem. Soc., 74,2362 (1952). A. Deschanvres and B. Raveau, Rev. Chim. Miner., 5 , 201 (1958).
(20) G. K. Bansal and A. H. Heuer. Phil. Mag., 30, 709 (1974). (21) R. Sun, J. Chem. Phys., 28, 290 (1958). (22) T. E. Bradburn and G. R. Rigby, Trans. Brit. Ceram. SOC., 52, 417 11953). (23) B. Gillot and P. Barret, C.R. Acad. Sci. Paris, Ser. C,278, 57 (1974). (24) A. L. G. Rees, "Chemistry of the Defect Solid State", Methuen, London, 1954. (25) M. H. Tikkanen, Genie Chim., 92 (3), 57 (1964). (26) G. Grube and M. Flad, 2.flektrochem., 42 (7), 377 (1942). (27) H. Davies and W. W. Meltzer, J. flectrochem. SOC., 121 (4), 543 (1974).
Hydrogen Reduction of Cobalt-Chromium Spinel Oxides. II. CoCr204-Co304 Solid Solutions Pierre Bracconi* and Louis-Claude Dufour Laboratoire de Recherches sur la Reactivite des Solides (associe au Centre National de la Recherche Scientifique),Departement B, Faculte des Sciences Mirande, 21000-Dijon, France (Received June 10, 1974; Revised Manuscript Received June 5, 1975) Publication costs assisted by the Universlty of Dijon
The hydrogen reduction of stoichiometric cobalt chromite (Sl)was investigated in the first part of this series. Here, we are concerned with the reduction of two cobalt chromite-cobalt cobaltite solid solutions, Co2+ [ c r ~ - ~ 3 + c o ~ 3 + ] B owith 4 2 - x = 0.06 in specimen S2 and x = 0.45 in specimen S3; S2 contains, in addition, a large amount of vacancies on the octahedral (B) sites and of associated Cr6+ ions. From the results concerning the reduction of S3 (lattice parameter a = 8.281 A) it is obvious that, below 55OoC,the x COB^+ can be reduced to CO$+, but that, simultaneously, a fraction (x/2) of the tetrahedral C O A ~is+ reduced to the metallic state, x water molecules are formed and the lattice parameter increases continuously; a spinel lattice ( a = 8.320 A) with an abnormal cationic distribution is obtained, which can be formally regarded as an intermediate between CoCr204 and COO.At higher temperatures these COB^+ are reduced to the metallic state and the spinel rearranges into C O ~ + [ C ~ ~ ~ +( a] B ~ ~ A) - which finally reduces to fcc-Co and =O 8.330 a-CrzO3. These two steps have not been separated thermodynamically, but, as they proceed consecutively, it is clear that the reactivity of the cobaltous ion is here related to the type of crystallographic site it occupies. The presence of octahedral vacancies in S2 does not modify the absolute rate of reduction into Co and Cr203 as compared with that measured for S1. Such an observation is in agreement with the conclusion that the rate-determining step of these reactions is a diffusion phenomenon of cobaltous ions through the spinel lattice tetrahedral sites.
I. Introduction The authors studying the reduction of ternary metallic oxides (ref 1-3, see also ref 1-3 and 23 in part I, preceding article in this issue) did not consider the possible relationships between the kinetics and mechanism of such reactions and the particular structure of the initial oxides studied. Dobrovinskii and Balakirev4 observed that some of the successive equilibrium states occurring in the reduction of the spinel CoFe1.75Cr0.2504a t 1000°C and decreasing oxygen partial pressure corresponded to different successive compositions of the spinel phase. On the contrary, the reduction of Co304 by HZ5and by CHd6 is known to yield COOas an intermediate phase. The purpose of this paper is to investigate, in comparison with CoCi-204, the reduction kinetics and mechanism of other cobalt-chromium spinel oxides with different compositions, S2 and S3, in which cobaltic ions replace part of the Cr3+ on the octahedral (B) sites. In addition, one of them, S2, contains octahedral vacancies and associated Cr6+ ions. The Journal of Physical Chemistry, Vol. 79, No. 22, 1975
Speculation about the reduction behavior of such compounds gives rise to interesting questions. For instance, if the CoB3+can be reduced to Co2+,these ions might either migrate on tetrahedral (A) sites (their normal location in the spinel structure) or be expelled in a separate phase such as COO. Concerning S2, the reduction kinetics and mechanism can be, a priori, expected to depend upon the vacancies existence. 11. Experimental Section Techniques. This work was carried out with the same experimental techniques as those already described in part I. Materials. Preparation and Characterization. Specimen 2 (S2) was obtained by decomposition of CoCrz07-4C5H5N in air at 120OOC for 24 hre7Similar to specimen S1 (see part I, section 11) it is crystallized in octahedral grains (see Figure 1 (top)) of average edge size a = 1.5 f 0.5 wm. Its composition is Co~.0~Cr1.~104.00 and its structure of spinel type with a parameter equal to 8.305 f 0.003 A. It is obvious,
