THE JOURNAL OF
PHYSICAL CHEMISTRY (Registered in ;.v--
U. S. Patent Office)
VOLUME62
(0Cogyright, 1958,by the American Chemical Society)
NUMBER 10
OCTOBER 24, 1958
THE MAGNETIC PROPERTIES OF THE COBALT OXIDES AND THE SYSTEM COBALT OXIDE-ALUMINA BY J. T. RICHARDSON AND L. W. VERNON Humble Oil and Refining Company, Baytown, Texas Received March 3,1968
Magnetic susceptibility and X-ray diffraction measurements have been made on the oxides CozOa*HzO,CoaOc and COO, prepared by the thermal decom osition of Co(OH)2 at various temperatures. The magnetic moment and Weiss constant are reported for each sample ancfthe magnetic properties shown to be consistent with the structures of the oxides. Similar measurements on a series of cobalt oxide-alumina samples prepared at 700" indicate that each sample may be re resented by the spinel form COa+zmA1'+,n( OtCoa+ (1--)mAl'+ ( ~ - y ) ~ ) Owhere 4 the cationic distribution parameters may g e determined from magnetic measurements and confirmed by X-ray diffraction intensities. The variation of these parameters with cobalt concentration is discussed.
Introduction The structures of cobaltous hydroxide and its products of thermal decomposition have been reported intermittently in the literature. Cobaltous hydroxide, CO(OH)~,is precipitated in two forms when excess alkali is added to cobalt salt solutions.1 The blue form, a-Co(OH)z, is precipitated a t 0" and is less stable than the pink form, P-CO(OH)~,which is produced a t ordinary temperatures. Both have the same hexagonal structure (D3ad-CFm, G = 3.19, co = 4.66, C = 1.46). 2 Upon heating in air a t loo", P-CO(OH)~is oxidized to Co203.Hz0. Kondrashev and Fedarova reported that CozOa.HzO has a hexagonal structure (Did-RG, 30 = 2.849, co = 13.13, C = 4.61) with each Coa+ ion surrounded by an octahedron and OH- ions. This is a different hexagof 02onal lattice to that reported earlier by Natta and Strada4 (ao = 4.64, ca = 5.75, C = 1.24) for anhydrous Co203,which, according to these authors, also had the same structure as the monohydrate Coz03.Hz0. However, Huttig and Kassler6 reported that, regardless of the method of (1) H. B. Weiser and W. 0. Milligan, THIEJOURNAL, 36, 729, 732 (1932). (2) W. Lotmar and W. Feitknecht, Z.Krist., 93, 374 (1936). (3) Yu. D.Kondrashev and N. W. Fedarova, Daklady Akad. Nauk S.S.S.R., 94, 229 (1954). (4) G. Natta and M. Strada, G'azz. chim. ital., 58, 419 (1928). (5) G. F. HLtttig and R. Kassler, 2. onorg. allgem. Chem., 184, 279 (1929).
preparation, they were unable to prepare anhydrous Coz03 and that all attempts to dehydrate CoZO3. zHzO resulted in the removal of water to give Coz03.Hz0, followed by decomposition with loss of both water and oxygen. These results were confirmed by Pagel, et aZ.6 Above 240-300", Coz03.H20 decomposes to form Co3O4.' This oxide has a cubic spinel structure and may be either Co4+(Co2+2)04or Co2+(CO~+~)O ~ . ~ decomposes above 750" to Co304 give COO,^.^,' which has a face-centered cubic NaCl structure. A further endothermic transformation occurs in the range 960-1000" without any weight loss or structure change.' It also has been reporteds that COO prepared below 1000" adsorbs 02 a t 18" up to a composition of Co304,but retains the COO structure until heated above 100" to give the Co304structure. The cobalt hydroxides and oxides that have been investigated by magnetic methods were all found to be paramagnetic; ie., the susceptibility, x, follows the Curie-Weiss law x=-
C T--8
(6) H. A. Pagel, W. K. Noyce and M. T. Kelley, J . A m . Chem. Sac., 87, 2552 (1934).
(7) T. M. Ovchinnikova, E. Sh. Ioffe and A. L. Rotinyan, Daklady Akad. Nouk S.S.S.R., 100, 469 (1955). ( 8 ) E. J. W. Verwey and J. H. de Boer, Rec. trau. chim., 66, 531 (1936).
1153
(9)
M.Leblanc and E. Mobins, Z . physik. Chem.. 142, 151 (1929).
J. T. RICHARDSON AND L. W. VERNON
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Vol. 62
ments on dispersed systems such as chromiaalumina16 and manganese oxide-aluminale have been used to study "surface clustering." I n more dispersed systems, such as Ni0-A1z0317 and Fe304-A1~03,~*X-ray diffraction measurements have indicated that each sample in the series may be considered as a mixed spinel, in which case the measurement of magnetic susceptibility is a useful tool in the determination of spinel cationic distributions. l7 It is the purpose of this paper to report the results of a magnetic study of the various cobalt oxides discussed above. Magnetic and X-ray diffraction measurements have also been used to determine the composition and structure of the cobalt oxide-alumina system. Experimental
" < F , q , 'I , sd / Go 0
e
g
I
-100
-300
io0
0
700
500
300
TEMPERATURE ('C.),
Fig. 1.-The
magnetic properties of Co(OH)2 ( p = 4.88, e = -io); C O ~ O ~ H( p~ =O 4.47, e = -370); coro4(,. = 2.79, e = -1050); COO (,., = 4.92, e = -2800).
"
Y X
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I
Preparation of Samples.-Samples of Co203.Hz.0,Coa04 and COO were prepared by heating Co(0H)p in air a t 100, 700 and lOOO', respectively. The original Co(OH)2 was precipitated from C O ( N O ~ solution )~ using NaOH. The gel was washed in a centrifuge until no nitrate ions remained, and then air dried before heat treatment. The samples of the cobalt oxide-alumina series were prepared from gels co-precipitated from controlled mixtures of cobalt and aluminum nitrate solutions. The gels were washed thoroughly, dried in air, and heat treated six hours in nitrogen a t 700". Magnetic Measurements.-Magnetic susceptibility measurements were made by the Faraday method using an apparatus similar to one previously described.18 Measurements were made at five different magnetic field values up to 5000 oersted to check for any field dependence. The temperature range was varied from - 196 to 700' by means of suitable dewar flasks and furnaces. Quartz springs with sensitivities of 1.00 and 2.00 mg./mm. were used in the force balance. This assembly also served as a thermogravimetric balance, so that simultaneous weight loss and magnetic data could be obtained. Approximately 50 mg. of sample was contained in a small spherical silica bucket. Appropriate corrections for the diamagnetism of the bucket, alumina, oxygen and cobalt ions were made in calculating the susceptibility per gram of cobalt. Cobalt Analyses.-As each sample was loaded into the magnetic apparatus, an aliquot portion was prepared for cobalt analysis, using both standard electrodeposition and colorimetric techniques. The amount of adsorbed water in the sample was determined from the weight loss following heat&g to constant weight in the temperature range 500700 The percentages of cobalt in the dry samples were easily calculated from these data. X-Ray Diffraction Measurements.-X-Ray diffraction measurements were made using a North American Philips Co. X-ray spectrometer with Mo Koc radiation.
.
Results and Discussion A. Cobalt Oxides.-The X-ray diffraction pattern of the original material confirms that it is the hexagonal CO(OH)~. The magnetic results are shown in Fig. 1. The Curie-Weiss constants give p = 4.88 POand e = - 1OK. These values correspond to a normal Co2+ion with a large amount of magnetic dilution. When this sample was heated in situ for two hours in air a t loo", the weight loss data indicated the composition of the oxide to be Coz03~Hz0. (15) R. P. Eischens and P. W. Selwood, J . Am. Chem. Soc., 7 0 , 2271 (1048). (16) P. W. Selwood, T. E. Ilioore, M. Ellis and K. Wethington, ibid., 71, 693 (1940). (17) J. T. Richardson and W. 0. Milligan, THWJOURNAL,60, 1223 (195G). (18) A. €Iof%man,Z. phglsik. Chem., 7 , 80 (1956). (19) W. 0. Milligan and H. B. Whitehuret. Rev. Sci. Instr., 2 3 , 618 (1952)
'P
MAGNETIC PROPERTIES OF COBALT OXIDES
Oct., 1958
The X-ray diffraction pattern, shown in Table I, gives the same structure as that reported by Kondrashev and Fedorova. The magnetic properties of COz03.HzO are shown in Fig. 1. The magnetic moment, p = 4.47 PO, is lower than that expected for free Co3+ ions. This is probably due to the splitting of the free ion energy levels by the crystalline field of the 02-and OHions surrounding each C03+ ion, adthough the effect is not as great as that found in Co3+spinels.l2 Figure 2 illustrates the decrease in weight and corresponding changes in room temperature susceptibility after heating for one hour at the temperatures indicated. The original sample was I C 0 ~ 0 ~ . 1 . 3 4 H ~As 0 . the temperature was raised to 2 01 io0 150" the valence of the Co3+ ion remained the same but the adsorbed 0.34 HzO was removed. As Fig. 3.-Weight the temperature increased above 150", further loss of weight below that corresponding t o Coz03.HzO was accompanied by an increase in susceptibility, ie., some of the Co3+ ions were reduced to Coz+ ions with a loss of oxygen. These facts are consistent with previous observations that heating COZO3.H2O results in a loss of both HzO and 0 2 . 6 Above 200" the loss of weight and changes in magnetic susceptibility were more rapid, until at 250" the weight and susceptibility of the sample corresponded to Co304. TABLE I X-RAYDIFFRACTION PATTERN OF Co2Oa.H& OR Dfi3d-RTm,Uo = 2.85, d/n I/Io hkl
4.42 2.43 2.31 1.97 1.80 1.50 1.43
100 32 80 15 47 18 32
Co
= 13.25,
003 101 102 104 105 107 110
c
coo (OH)
= 4.65
d/n
I/Io
hkl
1.36 1.21 1.16 1.12 1.03 0.925 0.880
28 13 11 8 4 6 7
113 202 204 205 207 122 125
The magnetic properties of Co304 are shown in Fig. 1. Below 300" the results p = 2.79, 0 = -105°K. agree with those of Cossee.12 A similar curvature of the 1/x line toward the T axis also occurs. As pointed out by Cossee, these results indicate the structure to be Co2+(Co3+z)O4. Moreover, Cossee determined the magnetic moment of Zn(Co3+z)04to be p = 1.61 Po, due t o the degeneracy not removed by the crystalline field a t the octahedral spinel sites. If this Co3+ contribution is considered in the case of Co304,then the relationship pzexp =
+ (2/3)p2~((ha+)(2)
(1/3)p'~(GO*+)
gives a value of p.4(Co2+) = 4.28 PO. This agrees with the range of p*(Co2+) values, 4.20-4.35 Po, determined from various cobalt spinels.zo When heated above 750", Co304 decomposes to give COO. However, when COO prepared in this manner was heated in air, the weight and magnetic data shown in Fig. 3 indicated that the COO remained constant in weight and followed the Curie-Weiss curve until about 300". At this temperature, the Coz+ ions began to oxidize to Co3+ ions with the subsequent increase in weight and (20) J. T. Richardson, t o be published.
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I I I 1 1 300 500 700 TEMPERATURE ("C.).
1
5
1
900
and susceptibility changes in COO after heating in air.
J. T. RICHARDSON AND L. W. VERNON
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Vol. 62
sumptions give structures whose calculated X-ray diffraction intensities agree very well with those measured experimentally. This is considered a justification for the model. Each sample, therefore, may be represented by C~~+zrnAl'+un( OtCo*+(,-z)mAla+(l-
(3)
y)n)04
The parameter 2 may be calculated from the measured magnetic moments using the expression p* =
0
20 30 40 50 60 WEIGHT PER CENT COBALT,
10
80
70
Fig. 5.-Variation of magnetic moments and Weiss constants of the cobalt oxide-alumina samples. I
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E? L=
v)
ea 0
81.0
2 CL
- ~)(1.61)'
(4)
b.
TABLEI1 CALCULATED AND EXPERIMENTAL I(220)/I (311) FOR Co'+ZmCo'+(~-z)mAl*+n to4
0
-
z
2.O,vlb
+
~(4.28)' (1
The parameter m may be calculated from the cobalt content, n from valence requirements and y and t follow from the consideration of the spinel structure. The results of these calculations are shown in Fig. 6. As Ala+ions are substituted into the C O ~ + ( C O ~ +lattice, ~ ) O ~ they first replace Coa+ ions on the B sites, the A sites remaining occupied ' only by Go2+ ions. Below about 45 weight % cobalt, Ala+ ions begin to replace Go2+ ions on the A sites with the,appearance of vacancies on the B lattice. Finally, below about 12 weight %, the cobalt exists o d y as Go2+ ions on A sites. The B sites are occupied by AI3+ions and vacancies. I n order to test the validity of the above model, the ratios of the intensities of the (220) and (311) lines were calculated using standard procedures.22
do
J o b
'$0
WEIGHT PER CENT COBALT.
, i
Fig. &-Variation of cationic parameters of the series co2+ztnA1~+,n(otco* +(L .)mAla+(I- u,n)O4.
The variation of the magnetic moments and Weiss constants with cobalt concentration is shown in Fig. 5. The magnetic moment increases from 2.79 ,BOas the cobalt content is lowered, leveling off at about 4.20 ,&. The Weiss constants initially increase, pass through a maximum and decrease steadily to zero. The approach to zero with decreasing cobalt content indicates the dispersion or dilution of the cobalt ion in the lattice. In considering the expression Co,AI, 0 as representative of this cobalt oxide-alumina series, it is desirable to determine not only the parameters, m, n and t as functions of cobalt concentration, but also the relative amounts of Co2+ and C03+ ions and their distribution over the spinel sites. This problem may be simplified by making two assumptions, which may be shown to be not only reasonable but also justified in view of the resulting conclusions. It is assumed that the Co2+ and the C03+ ions occupy A and B spinel sites, respectively. This assumption follows from the cationic distributions found in cobalt spinels such as C O ~ + ( C O ~ +and ~)O Zn(Coa+z)04. ~ It is also assumed that the vacancies, 0 , occupy B sites only. This follows from the reasoning of Sinha, et uE.,~' who found AI( I,.IA1S/g)04as the best structure for r-Al2o3. It will be shown that these two as-
.
(21) K. P. Sinha and A. P. B. Sinha, THIS JOUICNAL,61, 758 (1957).
Wt. Yo co
Vao. on A
69.4 64.2 GO. 6 53.1 43.5 32.6 26.8 22.9 18.9 14.5 13.5 11.9 7.1 4.1
0.28 .32 .35 .39 * 45 .45 .39 .36 .32 .30 .28 .27 .23 .19
1(220)/1(311) Vao. on A Vac. on and B B
0.28 .32 .35 .39 .45 .54 .48 .45 .41 .41 .40 .38 .33 .30
0.28 .32 .35 .39 .45 .54 .50 .47 .44 .44 .43 .42 .37 .34
Exp.
0.25 .35 .35 .39 .33 .39 .43 .44 .43 .39 .45 .44 .50 .53
These were compared with similar calculations for the cases of vacancies on A sites only and vacancies randomly distributed among A and B. It may be seen from Table I1 that the case for vacancies on B sites only gives the best overall agreement with experimental data, although random distribution cannot be ruled out completely. Conclusions The thermal decomposition scheme Co(0H)z -+CozOa.H20 -* CoaO, -+COO 100"
250 O
750'
has been confirmed by thermogravimetric, magnetic and X-ray diffraction measurements. The magnetic properties of these oxides have been measured and the structure C O ~ + ( C O ~ +confirmed ~)O~ for Co304. (22) "Internationale Tabellen zur Bestimmung von Kristallstruokturen," Vol. 2.
1
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Oct., 1958
NUCLEAR MAGNETIC RESONANCE OF ADSORBED WATERON SILICAGEL
1157
Magnetic and X-ray diffraction measurements have been used to calculate spinel cationic distribution in the cobalt oxide-alumina series
It has been shown that, as the series approaches low cobalt concentrations, the cobalt exists only as Co2+ions on A or tetrahedral sites. Furthermore, X-ray diffraction intensity measurements favor the case with lattice vancancies on B sites only. Acknowledgments.-The authors wish t o acknowledge the assistance of Mr. R. C. Damron in making the magnetic measurements, Mr. A. E. Walters for the cobalt analyses and Miss Virginia Harleston for X-ray diffraction measurements. DISCUSSION L. W. VERrioN.-In
Richardson and Vernon's pa er no explanation is given for the Weiss constant curve in &g. 5. Arguments are presented to explain the variation of the Weiss constant in the system cobalt oxide-alumina with the cobalt concentration. The Weiss constant 0 is a measure of the average exchange energy (expressed in degrees temperature) of the unpaired electrons of a given cobalt ion with those of neighboring cobalt ions. This energy depends on the magnetic environment around each cobalt ion and will, therefore, vary in some systematic way with the cationic distribution. To obtain the average magnetic exchmge energy, we sum over all possible interactions in the crystal latt,ice and divide by
ATOMIC FRACTION OF Go, mJ7.
the total number of cobalt atoms. (The details of this treatment will be published later.) After performing this summation and simplifying, this treatment gives the result p2e =
+ - z)%RB + ~
x Z ~ R A (1
(1 ~ ) ~ K A B
The constants K A , K B and K A B are evaluated from the Weiss c0nstant.s of: (1) CoAl2O4,(2) Co804, and (3) the 60.6ojO cobalt sample in the cobalt oxide-aluminn series. The figure shows the theoretical curve for the variation of the Weiss constant with the atomic fraction of cobalt for the cobalt oxide-alumina system. The circles repi-esent the experimentally determined values. The good agreement between the experimental data and the theoretical curve supports the principles on which the derivation is based, and, furthermore, there is also an indication that the cationic distribution parameters, deduced from other considerations, are correct.
NUCLEAR MAGNETIC RESONANCE RELAXATION STUDIES OF ADSORBED WATER ON SILICA GEL. I11 BYJ. R. ZIMMERMAN AND J. A. LASATER Magnolia Petroleum Company, Field Research Laboratory, Dallas, Texas Received March PI, 1868
A refined experimental investigation of the nuclear magnetic resonance relaxation phenomena of water vapor adsorbed on silica gel is described. Two phase behavior for both longitudinal and transverse relaxation measurements is observed to exist simultaneously. The two adsorbed phases in longitudinal relaxation data are shown to be identical with the corresponding two phases in transverse relaxation data. The adsorption at low coverage is identified only with the short time component; the coverage at which the second adsorbed phase begins can be determined. An accurate relaxation (T2) value for the average lifetime of a hydrogen nucleus in an adsorbed phase for a particular coverage is determined. A minimum value of TIis established; reasonable values of nuclear correlation times are obtained. A new and possibly accurate method for determining the monolayer is discussed.
Previous of adsorbed water vapor on silica gel by means of nuclear magnetic resonanceap4 spin-echo6 techniques have demonstrated in a small way the future role that this physical tool may have in probing the phenomena of molecular sorption on solids. These investigations have pointed out the simultaneous existence of two phase behavior from transverse (T2) relaxation measurements and single phase behavior from longitudinal ( T I ) relaxation measurements. The existence of two phases in Tz measurements was interpreted as proof of two distinct adsorbed phases for the water vapor on the silica gel, whereas the existence of a single phase in T I measurements was explained on the basis that average lifetimes in the adsorbed phases (1) J. Zimmerman, B. Holmes and J. Lasater, THIE JOURNAL, 60,1157 (1956). (2) J. Zimmerman and W. Brittin, ibid., 61, 1328 (1957). (3) F. Bloch, Phye. Rev., 70, 460 (1946). (4) N. Bloembergen, E.Purcell and R. Pound, ibid., 78, 679 (1948). (51 E. Hahn, ibid., 80, 580 (1050).
were so short that only a Tlcav, could be experimentally determined. From a combination of these experimental interpretations and the theory of stochastic processes in relaxation phenomena, the average lifetime of a water molecule in one of the adsorbed phases on silica gel was estimated. Another experimental investigation of adsorbed water on a silica gel sample by spin-echo techniques has been completed. The relaxation measurements and the adsorption techniques are considerably more refined in comparison with previous investigations. The general features observed in the initial studies of water vapor adsorbed on silica gel have been confirmed and further clarified. This paper places special emphases on (a) evidences of multiphase systems and phase transition phenomena, (b) quantitative analyses of relative populations of adsorbed phases, (e) lifetimes of hydrogen nuclei in an adsorbed phase, (d) evaluation of nuclear correlation times, and (e) adsorbed phases and dielectric loss phenomena.
