576
Chem. Mater. 1993,5, 576-582
Cobalt-Iron Hydroxide Carbonate as a Precursor for the Synthesis of High-Dispersity Spinel Mixed Oxides Elly Uzunova,* Dimitar Klissurski,' Ivan Mitov,+ and Plamen Stefanov Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113,Bulgaria Received October 8, 1992. Revised Manuscript Received January 13, 1993 Iron-cobalt hydroxide carbonates of pyroaurite-like structure have been synthesized by a coprecipitation technique. The structure is preserved within the limits 0.33 I Fe/Co 5 2 with no additional phases being found. Up to 10% of the iron cations are stabilized as Fe(II), replacing Co(I1) in the coprecipitate, but the main cation substitution in the lattice is Co(I1)-Fe(III), evidenced by X-ray diffraction study, differential thermal analysis, IR and Mossbauer spectroscopy. The spinel mixed oxides, syflthesized at 570 K, are of high dispersity, with specific surface areas 90-185 m2/g and particle sizes 6-12 nm, cationic vacancies being detected for samples with prevailing iron content. A good correlation exists between the bulk and surface iron/cobalt ratios of the mixed oxides, determined by X-ray fluorescence analysis and ESCA. The oxygen-to-metal ratios on the surface are for all oxide samples higher than expected on the basis of the formulas of the spinel or sesquioxides.
Introduction Precursors play an important role in the synthesis of transition-metal mixed oxides.' In many cases the use of a particular precursor may affect the materials structure on a molecular level and also ensure high dispersity and homogeneity. Hydroxide carbonates, obtained by coprecipitation, have been employed successfully in the synthesis of various types of oxide catalysts.2-8 There is a large number of investigations on hydrotalcite-likephases as precursors of catalysts for CO hydrogenation, containing Cu, Zn, Co, Ni, and A1 (or Cr).%14The lattices of pyroaurite and hydrotalcite type are rather tolerant toward variations in composition. Substitution of the elements is allowed, with alteration of the intercalated anions type and also the kind and ratio of metal cations over a wide range. Their composition can be generalized by the formula + Institute of Kinetics and Catalysis, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria (1) Courty, P.;Marcilly, C. Studies in Surface Science and Catalysis, Preparation of Catalysts III; Poncelet, G., Grange, P., Jacobs, P., Eds.; Elsevier: Amsterdam, 1983; Vol. 16, p 485. (2) Porta, P.; Fierro, G.; Lo Jacono, M.; Moretti, G. Catal. Today 1988, 2, 675. (3) Porta, P.; De Rossi, S.; Ferraris, G.; Lo Jacono, M.; Minelli, G.; Moretti, G. J . Catal. 1988, 109, 367. (4) Porta, P.; Dragone, R.; Del Fierro, G.; Inversi, M.; Lo Jacono, M.; Moretti, G. J . Mater. Chem. 1991, 1, 531. (5) Porta, P.; Moretti, G.; Musicanti, M.; Nardella, A. Catal. Today 1991, 9, 211. (6) Klissurski, D.; Uzunova, E. J . Mater. Sci. Lett. 1990, 9, 576. (7) Klissurski, D.; Uzunova, E. J . Mater. Sci. Lett. 1990, 9, 1255. ( 8 ) Klissurski, D.; Uzunova, E. Chem. Mater. 1991, 3, 1060. (9) Gherardi, P.; Ruggeri, 0.;Trifiro, F.; Vaccari, A.; Del Piero, G.; Manara, G.; Notari, B. Studies in Surface Science and Catalysis, Preparation of Catalysts III; Poncelet, G., Grange, P., Jacobs, P., Eds.; Elsevier: Amsterdam, 1983; Vol. 16, p 723. (10) Courty, P.; Durand, D.; Freund, Ed.; Sugier, A. J . Mol. Catal. 1982, 17, 241. (11) Gusi, S.;Pizzoli, F.; Trifiro, F.; Vaccari, A.; Del Piero, G. Studies i n Surface Science and Catalysis, Preparation of Catalysts IV; Delmon, B., Grange, P., Jacobs, P., Poncelet, G., Eds.; Elsevier: Amsterdam, 1987; Vol. 31, p 753. (12) Fornasari, G.;Gusi, S.; Trifiro, F.; Vaccari, A. Ind. Eng. Chem. Res. 1987, 29, 1500. (13) Clause, 0.;Gazzano, M.; Trifiro, F.;Vaccari, A.; Zatorski, L. Appl. Catal. 1991, 73, 217. (14) Clause, 0.:Rebours, B.: Merlen, E.: Trifiro, F.: Vaccari, A. J . Catal. 1991, 133, 231.
[M11~-,M11*,(OH)21 [Xn-lxlnmHzO,where M(I1) = Mn, Zn, Ni, Co, etc., M(II1) = Fe, Al, X = cos, NOS,SO4, C1,etc.15 The structure of pyroaurite is hexagonal, space group (R3m)l6and is constituted of positively charged brucitelike cationic layers [Mglll-,Felll,(OH)~lx+, where x = 0.33, alternating with disordered anionic interlayers [C0y4H2012-. The metal cations are statistically distributed in the octahedral sites of the cationic layers. A wide variety of di- and trivalent cation combinations,that form similar layered-type structures after precipitation in alkali media, have been studied. The mechanism of thermal decomposition of such precursors makes possible preservation of structural elements from the initial phase in the final product, that is in most cases a mixed oxide. Hydroxide carbonates containing Fe(II1)-Fe(I1) can be scarcely obtained by coprecipitation due to the easy oxidation of Fe(I1) from the atmospheric oxygen. An iron hydroxide carbonate with pyroaurite-type structure of is known as an composition Fe114Fe111~(0H)1~C03.3H20 unstable compound, isolated from corrosion producta.17 The hydroxide carbonates of cobalt(I1) and iron(III), obtained by coprecipitation, are amorphous compounds, the latter having layered type morphology.'* The chemical composition of both compounds depends strongly on the preparation conditions.lg Their thermal decomposition at 570-1170 K leads to the formation of a-Fez03and Co304 spinel, respectively. The synthesis of pyroaurite-like cobalt-iron hydroxide carbonates with different iron/cobalt ratios by the coprecipitation method, the stabilization of divalent iron in the pyroaurite structure, and their applicability as precursors of high-dispersity iron-cobalt mixed oxides with comparison between bulk and surface characteristics are investigated in the present paper. (15) Allmann, R. Chimia 1970, 24, 99. (16) Allmann, R. Acta Crystallogr. B 1968,24, 972. (17) Stampel, P. Corrosion Sci. 1969, 9, 185. (18) Markov, L.;Blaskov, Vl.; Klissurski, D.; Nikolov, S. J. Mater. Sci. 1990, 25, 3096. (19) Gmelins Handbuch der Anorganischen Chemie; Verlag Chemie GMBH: Weinheim, 1959; Vol. 59 B2, p 512.
1993 American Chemical Society
Chem. Mater., Vol. 5, No. 4,1993 677
Cobalt-Iron Hydroxide Carbonate
Table I. Chemical Composition and Unit Cell Parameters of IronCobalt Hydroxide Carbonates sample HFC33 HFC5O HFCl HFC2 HFCIa
a (nm) 0.3122 0.3114 0.3109 0.3086 0.3091
c (nm) 2.2772 2.2666 2.2464 2.2347 2.2280
Fe(I1) (%)
chemical formula
C~O.~~F~*~~O.Z~(OH)Z(C~~)O.~Z~~O.~ 3.4 h 0.6 8.6 f 0.6 8.0 f 0.5 ndb
Coo.~9Fe111~.33Fe11~.~l~(OH)~(C03)~.l~~.0.55H~ Coo.~~Fe1110.~6Fe110.~~~OH~~~C0~~~,~~~0.
C~O.~~F~~~'O.~ZF~~~O.O~~(OH)Z(C~~)
The sample with Fe/Co = 3 contains amorphous iron hydroxide carbonate. b Not determined. M 0
0
28 70 6 0 50 40 30 20 10 Figure 1. X-ray diffraction pattern of sample HFC33. n
4 Q
194. 0 0 -
E
1 0
\L
* 190.003
a V
* .+ 186. 0 0 d
5
323 L23
182.00
~
0. 00
1.00
2. 00
I
3. 0 0
523
623
T,K
Fe/Co m o l a r r a t i o Figure 2. Unit cell volume dependence on the composition of iron-cobalt hydroxide carbonates. Table 11. Interplane Distances and Miller Indexes of Sample HFC33 exptl 0.756 0.381 0.268 0.263 0.255 0.245 0.232 0.209 0.1962 0.1901 0.1743 0.1643 0.1561 0.1529 0.1468
calcd 0.759 0.380 0.268 0.263 0.253 0.244 0.232 0.207 0.1960 0.1898 0.1847 0.1742 0.1644 0.1561 0.1529 0.1518 0.1470
index (hexagonal) 003 006 101 012 009 104 015 107 018 0012 109 1010 0111 110
113 0015 1013
I/Io 100 45 10 65 3 7 35 5 25 3 10 7 18 20
5
Experimental Section Synthesis. The hydroxide carbonates were synthesized as follows: a solution of Fe(I1) and Co(I1)chlorides,taken in a desired molar ratio and total Me(I1) concentration 0.6 mol/dm3,was added to a sodium carbonate solution with a 2-fold excess volume a t pH 9. The initially formed blue precipitates were kept under continuous stirring for 1 h at 300-310 K. After filtration and
373
473
573
673
T, K Figure 3. TG, DTG, and DTA profiles of iron-cobalt hydroxide carbonates: (a) HFC33; (b) HFCBO; (c) HFC1; (d) HFC2; (e)
HFC3. careful washing, during drying at room temperature, Fe(I1) is oxidized to Fe(II1) and yellow-brown substances with pyroauritelike structure are formed. Characterization. Atomic absorption analysis of iron and cobalt was performed; the OH/C03 ratio was determined by
578 Chem. Mater., Vol. 5, No. 4, 1993
Uzunova et al.
W
W @L
::I 80' -6
1600
1200
60.0
800 c m-'
F i g u r e 4. IR spectra of iron-cobalt hydroxide carbonates. a-e as in Figure 3. Table 111. Assignment of the IR Bands, Registered in the Spectra of IronCobalt Hydroxide Carbonates registered bands for samples" (cm-') HFCl HFC2 HFC33 HFC5O HFC3 ==1640vw 1630vw 1508w 1508vs 1560w 1520w 1576vw 1352s 1352s 1356s 1372s 1376s 1070vw 1075vw 1056vw 1068w 840sh 838sh 840w 835w 835sh 776m, b 788m, b 764m, b 512w 472m 520w 510vw 497m 3405, sp 3485, sp 3589, sp =350m, b 332s, sp
" w = weak, v = very, m = medium, s = strong, sp = sharp, b = broad, sh = shoulder. absorption of the CO2 and H20 evolved in natron asbestos. X-ray diffraction analysis was performed on a diffractometer with Co K a monochromatized radiation. Differential thermal analysis was carried out with a heating rate of 10 K/min and a-Al201 as a reference. The IR spectra were taken on a Perkin-Elmer 983 G spectrometer in KBr pellets. Mossbauer spectra were recorded on a spectrometer operating in constant-acceleration mode at room temperature. The source used was a 57Co/Crmatrix, and the isomer shifts were referred to a-Fe. XPS spectra were recorded on a ESCALAB MkII spectrometer with Mg K a radiation (1253.67 eV). Transmission electron microscopy was used for determining mean particle size. Specific surface areas were measured according to the BET method, by Nzphysisorption a t 77 K.
Results and Discussion
Bulk Characterization of Precipitates. The diffraction patterns of the samples with ironlcobalt ratio in the range 0.33-3 are similar to those of pyroaurite (Mg6Fe2(OH)16C0~4H20)and hydrotalcite (Mg&12(0H)16-
I
-4
'
-2
'
'
0
I
2
'
I
4
'
'6
V E LOC ITY [ m m / s ] F i g u r e 5. Mossbauer spectra of iron-cobalt hydroxide carbonates. a-e as in Figure 3.
S,
1'75
m2/g
-
150 -
125 -
75 0. 00
1. 00
2. 00
Fe/Co Figure 6. Specific surface areas of iron-cobalt mixed oxides, synthesized a t 570 K.
C 0 ~ 4 H 2 0Figure ) ~ ~ 1. All reflections can be indexed in a hexagonal unit cell (Tables I and 11). Beyond the limita 0.33 < FeICo < 3, there is a trend to formation of an amorphous substance. The unit cell volume dependence on the ironlcobalt ratio obeys Vegard's law and indicates a continuous Co(I1)-Fe(II1) cation substitution within the (20) ASTM file 25-521 (pyroaurite), ASTM files 14-191 and 22-700
(hydrotalcite).
Chem. Mater., Vol. 5, No. 4, 1993 579
Cobalt-Zron Hydroxide Carbonate
Table IV. Mtissbauer Parameters (mm/s) of IronCobalt Hydroxide Carbonates fwhma
a
sample
6Fe(II)
AFe(I1)
HFC033 HFCO5O HFCl00 HFC200
1.15 f 0.04 1.14 f 0.02 1.14 f 0.02
2.06 f 0.09 2.10 f 0.04 2.12 f 0.03
8Fe(III) 0.349 f 0.004 0.344 f 0.004 0.361 f 0.003 0.362 f 0.003
AFe(II1) 0.502 f 0.006 0.549 f 0.008 0.758 f 0.006 0.818 f 0.005
Fe(II1) 0.452 f 0.005 0.484 f 0.006 0.486 f 0.004 0.518 f 0.004
Fe(I1) 0.25 f 0.05 0.35 f 0.03 0.32 f 0.02
Full widths at half-maximum.
T
*
501
/
2 00
/
/
/ * /
, 0
1.
50
e
/
u
/
/
\
/
/
a, 1 00CL
0
/
/e
v
/ / @/
0 50-
e /
0
/
/ o
/ o /
0 00-f
1200
800
40 0
c m-I Figure 7. IR spectra of iron-cobalt spinel mixed oxides, synthesized at 570 K: (a) Fe/Co = 0.33; (b) Fe/Co = 0.495; (c) Fe/Co = 0.99; (d) Fe/Co = 1.985; (e) Fe/Co = 3.
limits 0.33 IFe/Co I2.00, this volume change being associated with the simultaneous decrease of parameters a and c (Figure 2 and Table I). For hydrotalcite-like compounds a narrower interval of M(I1)-M(II1) substitution was determined in most cases, 0.33 < Me(II)/Al(III)< 0.5, while at higher Al(II1) content an oxyhydroxide (boehmite) phase was found.14~21The synthesis of cobaltaluminum and nickel-aluminum hydroxide carbonates with Me(II)/Al(III)ratios as low as Co/A1 = 0.5 and Ni/Co = 1 was recently rep0rted.~Z~23The comparison of the high-spin octahedral Co(I1) (0.745 A) and Fe(II1) (0.645 A) ionic radiiz4is in agreement with the strong decrease in unit cell volume of the iron-cobalt hydroxide carbonates in the range 0.33 IFe/Co I2. The substitution of a divalent by a trivalent cation increases the positive charge of the cationic layer and is compensated by the anionic interlayers negative charge. The distance between two nearest cationic layers equals c/3, and a difference of 0.016 (21) Sato, T.;Fujita, H.; Endo, T.; Shimada, M.; Tsunashima, A. Reactiu. Solids 1988,5, 219. (22) Busca, G.;Lorenzelli,V.; Bolis, V. Mater. Chem. Phys. 1992,31, 221. (23) Busca, G.; Lorenzelli, V.; Escribano, V. Chem. Mater. 1992, 4, 595. (24) Shannon, R. Acta Crystallogr. A. 1976,32, 751.
,
I
I
I
1
nm is observed between samples HFC33 and HFC2, the shrinking exceeding more than 5 times that of the a unit cell parameter. As the total formula composition [CoI11-*.Fe111,(OH)21[C03Ix/~mHz0 is governed by the value x/2, the number of carbonate groups from the interlayers would increase when the iron(II1) content is higher (Table I). Neither hydroxide nor oxyhydroxide phases of iron and cobalt were identified in the diffraction patterns. If, however, the samples were dried at a temperature above 370 K after coprecipitation, a cobalt-substituted Fe30r (magnetite) phase appeared along with the pyroauritelike phase for cobalt-rich samples. In the samples with Fe/Co > 1,the pyroaurite patterns were completely missing and an a-FezO3 (haematite) admixture was also found. In the TG-DTG and DTA profiles of all samples a single endothermal transition is registered between 370-450 K (Figure 3). Weakly bound water molecules are lost at temperatures below and around 370 K, but the main decomposition process, causing destruction of both the cationic and anionic layers with evolution of water and carbon dioxide, is found to proceed in a single step. With increasing iron/cobalt ratio, the DTA peak maxima shift toward lower temperatures. The thermal decomposition of cobalt(I1) hydroxide carbonate takes place at 513 K, while that of iron(II1) hydroxide carbonate is registered at 408 K, the latter peak being rather broad. Both are amorphous compounds; the shoulder of the broad endothermal peak in the DTA profile of sample HFC3 (Figure 3e) may be attributed to the presence of iron(II1)hydroxide carbonate. This can be deduced indirectly from the change in unit cell volume with composition. Since no phases other than pyroaurite-like were detected, excess of iron
Uzunova et al.
580 Chem. Mater., Vol. 5,No. 4, 1993 781.1
A
A
A
w 711.8
Fe2p3'2
J
BE ,eV
700
7 0 4 7 0 8 712
716
720 724
770 774
770 7 8 2 7 0 6
790
794
BE, eV 71 0.7
780.6
B
B
Fe 2 p 3 / 2
710.2
710.8
770 771, 778 782 786 790 7 0 0 7 0 4 708 712 716 720 BE e" B E . ~. V. Figure 9, XPS spectra of iron/cobalt hydroxidecarbonates and the corresponding mixed oxides. A: precursor hydroxidecarbonates, a-d as Figure 3. B: mixed oxides, a-d as in Figure 7. I
over the ratio Fe/Co = 2 should be in the form of hydroxidecarbonate, or other amorphous compound. The bands, characteristic of carbonate group vibrations25126 appear in the IR spectra (Figure 4,Table 111). The four vibrational modes of a free carbonate group are u(OCOz),asymmetric CO stretching vibration; 6(co3),outof-plane deformation vibration; and 6(0C02), in-plane deformation vibration. The existence of a low-intensity band at 1515 cm-l (@OB)) indicates the presence of -.
(25)Busca, G.;Lorenzelli, V. Mater. Chem. 1982, 7, 89. (26) Nakamoto, K. Infrared and,Ruman Spectroscopy of Inorganic and Coordinution Compounds; Wiley: New York, 1978 pp 283,380.
carbonate groups with lower than Dsh symmetry, which may be a result of interaction with adjacent water molecules or hydroxyl groups from the cationic layer. In that case the symmetric v(C=O) vibration is activated in the IR spectrum and a very weak band appears a t 1087 cm-'. Carbonate groups of C, and CzUsymmetry show splitting of the two doubly degenerated vibrations (asymmetric stretching and in-plane deformation). Since the difference Av = &Os) - u(OCO2), which is used for estimating the type of carbonate group bonding (monodentate, bidentate, bridging, or p0lydentate),~5,~6 is too small (40-100cm-l), direct coordination of the carbonate
Chem. Mater., Vol. 5, No. 4,1993 581
Cobalt-Iron Hydroxide Carbonate
group with metal cations is not likely to occur. A characteristic feature of the layered-type hydroxide carbonates is that the sharp band of the carbonate group out-of-planedeformation vibrations at 820-900 cm-l, found in most basic carbonates- and transition-metal carbonate complexes,26appears as a shoulder to the broad band of M-O-H bending vibrations at -760 cm-l. As the last band is rather broad, it eventually obscures also the band at 670 cm-l, that corresponds to the p(c03) vibrations. In the region of hydroxyl group vibrations a broad band at 3450 cm-l exists, due to the water molecules, but no presence of free hydroxyls was detected. The latter result is in agreement with previous studies on zinc-cobalt, nickelcobalt, and copper-cobalt hydroxide carbonates,- also on hydrohlcite-like compounds,13and allows the suggestion that all OH groups from the cationic layers are hydrogen bonded. No hydroxide (or oxyhydroxide) admixtures were found in the IR spectra of all samples. The strong sharp band at 340 cm-l is due to the v(Me-0) stretching mode of octahedrallycoordinatedcentral cation, The shift @ward higher where Me = Fe(II1) or C0(11).2~-28 frequencies of the band v(Me-0) with incressing the ratio iron/cobalt is in agreement with the statement of Co(I1)Fe(II1) substitution in the cationic layers. As the masses of iron and cobalt do not differ substantially, a stronger force constant of the Fe(II1)-0 bond, compared with the Co(I1)-0 bond, should be responsible for this effect. The Mdssbauer spectrum of the sample with Fe/Co = 1/3 consists of a doublet with isomer shift 6 typical for high-spin Fe(II1) ions in octahedral c o ~ r d i n a t i o nTable ~~ IV. Asymmetry of the doublet may be related torelaxation p h e n ~ m e n a .In ~ ~all samples with Fe/Co > 1/3,divalent iron was present (Figure 5 and Table I). Only part of the iron, not exceeding lo%, replaces Co(I1) in the pyroaurite structure (Table I). From all Mdssbauer parameters, the iron(II1) quadrupole splitting is most clearly influenced by the alterations in composition;for Fe(I1)the differences are in most cases within the error limits. The growth of Fe(1II) quadrupole splitting with increasing iron content can be ascribed to the building of a more asymmetric environment of Fe(III), as the number of neighboring iron cations increases. In the cationic layers of the pyroaurite structure the first neighbors remain the same, since all cations are octahedrallycoordinatedwith hydroxyl groups. The second-order neighboring cations produce the effect of asymmetry, because the average number of nearest iron cations increases from samples a to d. As all cations are statistically distributed among octahedral positions, this is also related to the larger line width of the Mdssbauer components of Fe(II1) and Fe(I1). Evolution of Hydroxide Carbonate Precursors to Mixed Oxides: Bulk and Surface Characterization. The thermal decomposition of sampleswith different iron/ cobalt ratios yield high-dispersity spinels with specific surface areas above 90 m2/g (Figure 6). The spinel solid solutions obtained at 570 K are monophasic, no traces of cr-FezO3 being detected in the range 0.33 < Fe/Co < 3. However, as the ratio iron/cobalt increases, the line width of the diffraction patterns becomes larger, due to the samples' high dispersity. In the IR spectra ( F i w e 7a,b) of samples with Fe/Co I0.5, two bands characteristic of (27) Tarte, P.Spectrochim. Acta 1962, 18, 467. (28) Ishii, M.;Nakahira, M.; Yamanaka, T. Solid State Commun. 1972,II,209. (29) Greenwood,N.; Gibb, T.M6ssbauer Spectroscopy; Chapman and Hall: London, 1971;p 148.
5 . 001
-2.
8
50-
0
u+
0,
h2 00-
Y
I
0
1 50-
l
, 0 00
O 0 . 50
O
l
+eye0
1 . 50
I
, 2.00
Figure 10. Oxygen-to-metal ratio for the iron-cobalt mixed oxides, synthesized a t 570 K, compared with the nominal ratio for spinels and sesquioxides.
spinel phases are present. Both bands are due to vibrations, governed mainly by octahedral symmetry modes.22~23~2~~30 The higher frequency band is due to the AOB3, and that of lower frequency to the BOB2vibrations in the spinel lattice (A denotes tetrahedrally coordinated cations, while B stands for octahedral positions). The IR spectra of the mixed oxides with Fe/Co 1 1 (Figure 7c-e) display broad bands, typical for spinel mixed oxides with cationic va~ancies.~'These broad bands are constituted by a number of band@ that may not be clearly distinguished in most cases. The band at 635 cm-' is considered as characteristic for substituted cobalt ferrites with vacancies of ordered type.31 The increasing iron-cobalt substitution causes a shift toward lower frequencies of the IR bands, as was observed by other authors in the case of Co- and Cr-substituted magnetites.33 The surface composition of the precursors display a deviation in favor of cobalt when the Fe/Co ratio increases (Figure 8). This may be explained, having in mind the higher solubility of Fe(I1) present at higher concentration in these samples. In the mixed oxides, synthesized at 570 K, the surface iron/cobalt ratio grows in comparison with the corresponding precursors and is still higher for cobalt ferrite, synthesized at 870 K. These results indicate the tendency of iron to segregate on the surface, reported for some ferrites, e.g., N i , F e ~ , 0 4 . ~ ~ The binding energies of cobalt and oxygen exhibit no substantial changesassociated with the Fe/Co ratio (Figure 9). The Co 2p3/2 peak of samples with high cobalt content which is is unresolved, due to multiplet ~plitting,3~1~~ characteristic of the paramagnetic compounds of divalent cobalt. This effect is more pronounced for the hydroxide carbonates, where the total cobalt quantity is in a divalent, (30)Farmer, V. C. Infrared Spectra of Minerals; Mineralogic Society: London, 1974;p 191. (31)Gillot, B.; Jemmali, F.; Rousset, A. J. Solid State Chem. 1983, 50,138. (32)White, W.; De Angelis, B. Spectrochim. Acta A 1967,23, 986. (33)Gillot, B.; Bouton, F.; Ferriot, J.; Chassagneux, F.; Rousset, A. J. Solid State Chem. 1977,21,375. (34) Allen, G.; Harris,S.; Jutaon, J.; Dyke, J. Appl. Surf. Sci. 1989, 37,111. (35) Chuang, T.;Brundle, C.; Rice, D. Surf. Sci. 1976,59,413. (36) Frost, D.; McDowell, C.; Woolaey, I. Chem. Phys. Lett. 1972,17, 320.
Uzunova et al.
582 Chem. Mater., Vol. 5, No. 4,1993 high-spin state. The O(1s) binding energy for the hydroxide carbonates (531.2 f 0.2 eV) is higher by 1eV than that of the spinel oxides (530.1f 0.2 eV). A similar effect was established for the oxides and hydroxides of nickel, copper, and cobalt.37 The values of Fe(3p3p) binding energy (BE) are close to the data reported by McIntyre and Z e t a r ~ and k ~ ~do not indicate presence of a- (or y-) Fez03 on the surface, although this cannot be absolutely excluded, having in mind the close values of their binding energie~.3~ The oxygen/metal ratio on the surfaces of all mixed oxides exceed both the values for the spinel oxides and the sesquioxides (Figure 10). This is associated either with the existence of cationic vacancies, already indicated by the IR spectra or with the existence of oxygen surface species that are not destroyed under the high vacuum of the spectrometer. The first assumption seems more reasonable, as the O(1s) peak is rather symmetric and does not indicate presence of different oxygen forms. The oxygen/metal ratio is highest for cobalt ferrite. Iron-cobalt mixed oxideswith compositions Co,Fesx04, 1< x I 2.25, are metastable and in the temperature interval
670-900 K segregate to two spinel phases-one
rich in
(37)McIntyre, N.;Cook, M. Anal. Chem. 1975,47, 2208. (38)McIntyre, N.;Zetaruk, D. Anal. Chem. 1977,49, 1521. (39) Brundle, C.; Chuang, T.; Wandelt, K. Surf. Sci. 1977,68,459.
cobalt, and the other rich in iron. Above 670 K in samples with compositionFe/Co > 2 a haematite admixture appears that may result from the presence of additional amorphous iron(II1) hydroxide carbonate in the precursor for these samples.
Conclusion Cobalt(I1) and iron(II1) hydroxide carbonates, which are amorphous as individual compounds, form a solid solution, having a pyroaurite structure in the ratio range 1/3 I Fe/Co I 2. In all samples with Fe/Co > 1/3partial Fe(I1)-Co(I1) substitution takes place, but the divalent iron does not exceed 10% of the total iron quantity. The Co(I1)-Fe(II1) substitution in the cationic layers is evidenced by the lattice parameters dependence on the iron content, the change in frequency of the IR v(M-0) vibrations, the shift in DTA peaks, and Mossbauer parameters. Thermal decomposition of the hydroxide carbonate precursors yields high-dispersity iron-cobalt spinel mixed oxides CoZFe3-,Or, within the limits 1 Ix I 2.25. Above 870 K, haematite admixtures are found in samples with x < 1,which may be due to the amorphous iron(II1) hydroxide carbonate present in the coprecipitate. Ferrite samples synthesized at 570 K contain cationic vacancies, identified in their IR spectra by the broad bands in the region 300-800 cm-l and the characteristic band at =635 cm-l. Excess of oxygen was detected on the surface by XPS.
