The precipitation and characterization of cobalt (II) oxalate tetrahydrate

Mat. Res. B u l l . , Vol. 18, p p . 993-1000, 1983. P r i n t e d in t h e USA. 0025-5408/83 $3.00 + . 00 C o p y r i g h t (e) 1983 Pergamon P r e s s L t d .

THE PRECIPITATION

AND CHARACTERIZATION

OF C O B A L T

(If) O X A L A T E T~I'MAHYDRATE

E. Wisgerhof and J.W. Geus

Department of Inorganic Chemistry, University of Utrecht, Croesestraat 77A, 3522 AD Utrecht (The Netherlands)

( R e c e i v e d May 2, 1983; Communicated b y G. H. J o n k e r )

ABSTRACT

Mixing of solutions of Co 2+ and C2042- ions can lead to crystallization of cobalt (II) oxalate both as a tetrahydrate and a dihydrate. Up to about 60°C acicular crystallites of a greyish pink tetrahydrate initially result. In a period depending mainly on the temperature and to a lesser extent on the degree of supersaturation and the recrystallizes [~042-]/[co2+]ratio, the precipitated tetrahydrate to B-cobalt (II) oxalate, a bright pink dihydrate. The two cobalt (II) oxalates have been distinguished by means of X-ray diffraction (XRD), (electron)microscopy and thermal analysis. The d-values and relative peak heights of the most pronounced maxima of the XRD-pattern of the tetrahydrate are presented, as these are not available in standard reference works.

Introduction Small metal particles are utilized in catalytic reactions. Especially liquid phase hydrogenations can be done easily by utilization of suspended small metal particles. Instances are the hydrogenation of fatty oils by suspended Ni particles and the Fischer-Tropsch synthesis by Co particles. To deal with the considerable heat of Fischer-Tropsch reactions a catalyst suspended in a liquid is favourable. Small ferromagnetic particles also have important applications. As examples we mention ferrofluids (1) and magnetic tapes (2). Small metal particles can be smoothly produced by decomposition of metal oxalates. Some carbon remaining in the metal impedes surface migration of metal atoms and consequently restricts sintering. The size-distribution of the metal particles obtained by thermal decomposition of metal oxalates strongly depends on the size of the original metal oxalate particles. A control of the size of metal oxalates is therefore important to produce metal particles of a desired particle size. We therefore studied the precipitation of cobalt (II) oxalate. Up till now the tetrahydrate of cobalt (II) oxalate has hardly been investigated. Deakin et al. (3) were the first to report on a difference in 993

994

E. W I S G E R H O F ,

et 81.

Vol. 18, No.

8

appearance of cobalt (II) oxalate according to the drying procedure. They attributed the formula COC204.4H20 to the compound obtained on drying in atmospheric air at room temperature. At about the same time Ephraim (4) demonstrated the existence of cobalt (IT) oxalate tetrahydrate as one of the series CoC20~(NH~) (H~O) 4 , where x = 0, i, 2, 3, 4. More recently Avond (5) estabJ x lishe~ that cobalt ~II) oxalate dihydrate obtained by dehydration of the tetrahydrate had a crystal structure differing from that of a precipitated dihydrate. They reported on some factors that influence the stability of the tetrahydrate with respect to dehydration to the dihydrate, but they did not provide detailed information on the preparation of the tetrabydrate. They only mentioned that the reaction should be carried out at or below room temperature. If a suspension of the tetrahydrate in water was heated a sudden change in colour was observed at 75 °C. They ascribed the change in colour to transformation of the tetrahydrate to the dihydrate. This paper deals with the experimental conditions leading to precipitation of cobalt (If) oxalate tetrahydrate from solutions of cobalt (II) and oxalate ions.

Experimental To prepare samples of cobalt ( I I ) o x a l a t e we used the following three procedures at various temperatures. The procedures 2 and 3 were used in order to reduce local supersaturation (6) as much as possible. Procedure I~ classical ~reci~itation. A solution of 12.0 g of oxalic acid in 90 ml water (1.06 M) was added relatively rapidly and under continuous stirring to a solution of 17.0 g cobalt (II) chloride hexahydrate in I00 ml water (0.72 M). Hence , the ratio [C2042-]/[Co 2+] was 4/3. After a period varying from half a minute up to about a week or more, the sample (or a part of the sample) was filtered off and washed thoroughly with demineralized water. Owing to the instability of the tetrahydrate with respect to dehydration, the samples were not dried. Precipitates, expected to contain some tetrahydrate, were carefully kept in a refrigerator at 4 oc and transported in a Dewar vessel above melting ice, until the XRD-pattern could be recorded (usually within a few hours). Procedure 2, precipitation by means of dimethyl oxalate. A solution of cobalt (If) chloride hexabydrate (0.28 M) and dimethyl oxalate (0.28 M) was vigorously stirred. Hydrolysis of the dimethyl oxalate made available oxalate ions in a homogeneous solution. After a period from 0.5 to about I00 h the cobalt (If) oxalate (or a part of it) was filtered off and washed similarly as in procedure I. Procedure 3 F slow dro~wise addition. 500 ml o f 0.56 M oxalic acid was added dropwise to 500 m l 0.56 M cobalt (II) chloride hexahydrate by means of a peristaltic pump through a polythene tube ending above the level of the liquid. The solution was continuously agitated. The remainder of this procedure is the same as procedure 2. Some samples have been prepared using slightly different procedures as are indicated in the tables. A thermostated water bath kept the temperature in the precipitation vessel constant within a few degrees. Frequently the course of the precipitation was studied by filtering off a small amount of the precipitate. The successive samples thus obtained were denoted by a, b, c, etc. During the precipitations according to the procedures 2 and 3 the pH of the solution was recorded continuously. In the beginning of the preparation the bright solution was circulated through a spectrophotometer which enabled ns to distinguish the onset of the precipitation (nucleation and growth of the nuclei). When the transmission of the solution had dropped from 100% to 0%, the first sample of the precipitate, denoted by a, was taken. The filtrate (about

Vol. 18, No. 8

COBALT (II) OXALATE

995

20% of the solution) was r e t u r n e d into the p r e c i p i t a t i o n vessel. X-ray d i f f r a c t i o n p a t t e r n s were r e c o r d e d by means of a Philips diffractometer (~ = 0.15405 nm). The crystal h a b i t was i n v e s t i g a t e d in a Leitz light microscope, at m a g n i f i c a t i o n s up to 1,300. A Cambridge scanning electron microscope (150 S) was also used. The solids were m o u n t e d onto a l u m i n u m stubs and s u b s e q u e n t l y coated by a s p u t t e r e d gold film. T h e r m o g r a v i m e t r i c studies w e r e carried out in ambient air using samples of about 8 mg using the Perkin Elmer TGS-2 (heating rate 5 °C min-l). Differential Scanning C a l o r i m e t r y was p e r f o r m e d w i t h a Dupont 900 at a heating rate of 10 °C min -I u s i n g samples of about 10 mg kept in an aluminum pan also in ambient air.

Results i. C h a r a c t e r i z a t i o n

and d i s c u s s i o n

of the p r e c i p i t a t e s

To avoid d e h y d r a t i o n of the p r e c i p i t a t e s the diffraction p a t t e r n s were recorded as soon as possible. Four d i f f e r e n t d i f f r a c t i o n patterns can be distinguished. The patterns are r e p r e s e n t e d in figure i.

2

FIG.

1

Intensities of d i f f r a c t i o n m a x i m a versus 2@ (@ glancing angle) for h y d r a t e d cobalt (II) oxalates

L

III

30

25

15

20

I pure tetrahydrate II dihydrate o r i g i n a t e d from solid state transformation of tetrahydrate III ~-dihydrate (7), c r y s t a l l i z e d in aqueous suspension IV ~-dihydrate (7), crystallized in aqueous suspension

28 (e glancing angle)

We used the p e a k at 2@ = 18.7 ° to e s t a b l i s h the presence of ~-dihydrate in the precipitate. When this p e a k was absent we a s s u m e d the tetrahydrate to be present exclusively. When a p r e c i p i t a t e consisted of a m i x t u r e we calculated a "main p e a k ratio" R d e f i n e d as R=

I(14.9) I I(18.7)ii I'

where I(14.9) I r e p r e s e n t s the peak h e i g h t in p a t t e r n I of figure 1 at 2@ = 14.9 ° and I(18.7)ii I the p e a k h e i g h t in p a t t e r n III of figure 1 at 2@ = 18.7 ° . This ratio was u s e d to characterize the composition of these mixtures. To e s t a b l i s h the p r e s e n c e of t e t r a h y d r a t e in d i h y d r a t e XRD appeared to be rather sensitive. W h e n a m i x t u r e of dihydrate and 2.4 w e i g h t % tetrahydrate was investigated, d i f f r a c t i o n m a x i m a at 28 = 14.9 ° and 28 = 16.1 ° were clearly observed. The m i x t u r e e x h i b i t e d a R-value of 0.09. Since the X R D - p a t t e r n of cobalt (II) oxalate tetrahydrate has not been p u b l i s h e d so far, we will here report the m o s t intensive diffraction m a x i m a

996

E. WISGERHOF, e t 8].

VoI. 18, No. 8

of a sample freshly p r e p a r e d according to p r o c e d u r e 1. In v i e w of scattering of peak heights of d i f f e r e n t p r e p a r a t i o n s only the heights of the strongest peaks are given. The resulting d-values and relative p e a k heights are p r e s e n t e d in table I. Though the p o s i t i o n of the reflections reproduces very well, the height of the peaks displays some scatter. From the m a n y X R D - p a t t e r n s that have been recorded to identify the samples, we calculated the m e a n relative p e a k h e i g h t for the four strongest peaks b e l o w glancing angle 8 = 15 ° (see tabel II). These m a x i m a are denoted by I, 2, 3, 4 in p a t t e r n I of figure 1. The standard deviation is given as a m e a s u r e of the scatter of the peak heights. TABLE X-ray data for C o b a l t heights I/I0) d

(nm)

0.596 0.549 0.449 0.369 0.347 0.301

(II) Oxalate

I/I 0

d

1.00 0.81 0.46 0.29 -0.05

0.285 0.274 0.270 0.262 0.254 0.250

(nm)

I

Tetrahydrate

I/I 0

d

(nm)

0.30 0.04 0.06 0.03 ---

0.242 0.227 0.224 0.218 0.213 0.212

TABLE

(d-values

(D/a)

I/I 0

d

0.03 -0.05 ----

0.207 -0.202 -0.1857 . . . . 0.1849 . . . . 0.1778 0.03 0.1743 . . . .

number of the peak number in fig. I, I ipatterns u s e d

procedure

I

3

I/I 0

d

(rim)

0.1735 0.1714 0.1627 0.1431 0.1428

peak

I/I 0

0.07 0.05

II

Scatter of the Relative Peak Height of four p r o n o u n c e d C o b a l t (II) Oxalate T e t r a h y d r a t e

procedure

and relative

XRD-maxima

m e a n relative peak h e i g h t

of

standard deviation

27 27 27 25

1.00 0.79 0.49 0.32

0.09 0.09 0.06

23 23 23 23

i .00 0.73 0.31 0.22

0.15 0.18 0.08

There is a correlation b e t w e e n the habit of the crystallites, as observed (electron)microscopically, and the XRD-pattern. Samples d i s p l a y i n g XRD-pattern I or II always show an acicular habit (figure 2a). This agrees w i t h the findings of A v o n d et al. (5) that the m o r p h o l o g y of the crystallites is retained during the solid state dehydration of cobalt (If) oxalate tetrahydrate. They carried out the d e h y d r a t i o n of the tetrahydrate to the dihydrate in an evacuated desiccator above p h o s p h o r p e n t o x i d e and by m e a n s of w a s h i n g w i t h dioxane. The original m o r p h o l o g y was m a i n t a i n e d better d u r i n g w a s h i n g w i t h dioxane. For samples e x h i b i t i n g X R D - p a t t e r n s III or IV the appearance of the crystallites generally is as shown in figure 2b. Owing to the better resolution and depth of focus we p r e s e n t here scanning electron m i c r o g r a p h s only. The crystal habits can, however, also readily be d i s t i n g u i s h e d with the light microscope. Especially interference contrast is very suitable to display the m o r p h o l o g y of the crystallites. Figure 3 gives the T G - p l o t of a sample of cobalt (II) oxalate ide.~tified

Vol. 18, No. 8

COBALT ( I ! ) OXALATE

FIG.

997

2

Scanning electron m i c r o g r a p h s of h y d r a t e d cobalt (II) oxalate. a. T e t r a h y d r a t e or dihydrate o b t a i n e d from solid state t r a n s f o r m a t i o n of tetrahydrate (left). b. Dihydrate o b t a i n e d by r e c r y s t a l l i z a t i o n in aqueous suspension (right).

I 10(\ 8( WEI (%)GHT

CoC2O&2H~O.~

L_

IEXO I ENDO

COC20&

~ Co30~.

z,.o

i

2~o

1;o

~o

I

i

100

3

T G - p l o t (weight % versus temperature) of cobalt (II) oxalate tetrahydrate. (8 mg was h e a t e d in ambient air at a h e a t i n g rate of 5 °C min-1).

.

.

,

,

300

TEMPERATURE (°C}

TEMPERATURE (°El

FIG.

t 200

FIG.

4

D S C - p l o t (AT versus temperature) of cobalt (II) oxalate tetrahydrate (I0 m g was h e a t e d in ambient air at a h e a t i n g rate of i0 °C min -i).

as t e t r a h y d r a t e by means of X R D - a n a l y s i s and microscopy. A n o t h e r p a r t of this sample was used in the D u p o n t t h e r m o b a l a n c e for a D S C - r e c o r d i n g (figure 4). Both figure 3 and 4 indicate the p r e s e n c e of the tetrahydrate. The two separate w e i g h t losses (figure 3) caused by two d e h y d r a t i o n steps c o r r e s p o n d to the two e n d o t h e r m i c heat effects (figure 4). Both the XRD and the thermal analysis m u s t be carried out immediately after the samples have b e e n r e m o v e d from the Dewar vessel. At 20 °C d e h y d r a t i o n of the t e t r a h y d r a t e can be d i s c e r n e d a l r e a d y after 15 m i n u t e s d e p e n d i n g on the surface area of the sample e x p o s e d to atmospheric air. At 4 °C in a closed vessel d e h y d r a t i o n p r o c e e d s m u c h m o r e slowly.

19 25 25 25 25 29 29 29 29 29 29 29 4O 47 47 59 59 68

s 15 h 25 47 h 169 h h 220 7 min 4.6h 125 h h 92 h 216 92 h 92 h 7 min 2.0 min min 6 0.5 min 2.0 min 0.5 min

1.33 1.33 1.33 1.33 1.33 13.3 13.3 13.3 0.88 0.88 1.00 1.14 1.33 1.33 1.33 1.33 1.33 1.33

I I I +III I +III III I I III I +III Ill III III I+III I +III III I +III III III

precipita~ecipita- ~ 2 0 4 2 1 / [ C ~ i XRDzempera- precJ :ure (Oc) tion time ratio >attern

(II) Oxalate according to

~03

51 3.9

9.2

143 1.8

R

i) A[C2042-] /[Co 2+] ratio different from 1.33 was obtained by mixing the required volumes of 1.06 M oxalic acid and 0.72 M cobalt (II) chloride. )) For this precipitation the Co 2+ solution was added to the solution of oxalic acid. z) While the transmission o f t h e solution was measured the equilibrium temperature of the suspended precipitate in the reaction vessel was higher. This was caused by the fact t h a t t h e circulating solution was heated during passage through the spectrophotometer.

54a g h k 1 52a b c 41a b 42a 43a 47a 48a b 50a b 49a

3ample %umber

?reparations of Cobalt ?rocedure Ia)

TABLE III

1.0 64

60 60

28a b

III IV

I III

xRnpattern

55a b 38a b 39a b 57ab) b 40a b 53a b 56a b 46a b

0.21 0.21 0.21 0.21 0.21 0.21 0.21 0.21 0.21 0.21 0.34 0.34 0.21 0.2] 0.21 0.21

6 ic ) 37 37 45 45 48 48 53 53 58 58 67 67 90 90

3.7 72 3.5 69 3.5 93 2.8 121.0 4.0 75 2.0 72 6.0 121 3.0 126

h h h h h h h h h h h h h h h h

I I I III I III III III I + I I I 2.0 IIl I + I I I 0.3] III III IV III IV

~reparations of Cobalt (II) Oxalate according to pro:edu.re 3 (final [C ~42J / tco2+ ratio 1.01 precipitationlXRDI ha~ture samplelrate of time pattern number addition nl min -1

h h

0.92 h 96 h

30 30

1

precipitation time

TABLE V

Zemperature (°C)

27a b

~ample lumber

Preparations of Cobalt (II) Oxalate according to procedure 2 (final [C2042-]/[Co 2+] ratio 1.0)

TABLE IV

O

00

o

z

D

m

Vol. 18, No. 8

COBALT (II) OXALATE

999

2. Transformation of the tetrahydrate to the dihydrate in aqueous suspension. In table III, IV and V a selection of the numerous precipitations is shown. In each table the samples are presented in order of increasing temperature during the precipitation. The samples 54, 47, 48, 50, 49 (table III), 27, 28 (table IV), 55, 38, 39, 40, 56 and 46 (table V) show the effect of the temperature. Below about i0 °C the precipitated tetrahydrate was stable. During a period of several days after the onset of the precipitation we did not find any dihydrate. Between room temperature and about 60 °C the initially precipitated tetrahydrate recrystallizes to dihydrate. This transformation of suspended tetrahydrate proceeded more rapidly at higher temperatures. At room temperature the tetrahydrate could still be detected after several days or a week after the start of the precipitation. At about 60 °C this time had decreased to about a minute. It is hence obvious that Avond (5) states that the preparation of cobalt (II} oxalate tetrahydrate should be carried out at room temperature or below. Above the limit of about 60 °C and within our experimental conditions we could not detect any tetrahydrate even if a sample was taken immediately after the solutions had been mixed (15 ~ 30 s). The effect of the temperature appeared to predominate over the other factors to be dealt with. The morphology and the crystal structure of the dihydrate were found to depend on the mode of dehydration. With solid state dehydration, the morphology of the crystals remained the same as that of the original tetrahydrate, which is shown in figure 2a. The resulting dihydrate exhibited the XRD-pattern II of figure I. Transformation of the tetrahydrate suspended in the original liquid generally led to the morphology of figure 2b and the XRD-pattern III of figure i. Apparently the conversion of suspended tetrahydrate proceeds by intermediate dissolution of the cobalt and oxalate ions leading to a completely different shape of the particles. We now will deal with the other experimental variables affecting the rate of recrystallization of the tetrahydrate to the dihydrate, viz. the [C2042-]/[Co 2+] ratio and the degree of supersaturation. The samples 41, 42 and 43 (table III) demonstrate that an excess of oxalate ions also accelerates the recrystallization of the tetrahydrate to dihydrate. This conclusion is supported by the fast recrystallization observed during the precipitation of sample 57 (table V). Here, exceptionally, the Co 2+ ions were added to oxalic acid and thus a large excess of oxalate ions was present at the start of the precipitation (compare samples 39 and 40 with 57, table V). With procedure 3 the nucleation takes place at a much lower supersaturation. Consequently larger particles have precipitated. As a result the tetrahydrate initially precipitated dissolves more slowly. Moreover the [C2042-]/[Co 2+] ratio is relatively low, when the oxalate ions are added over a period of a few hours according to procedure 3. These facts explain that if procedure 3 is followed, the recrystallization is slower compared with procedure 1 (sample 47, table III and sample 39, table V). To produce small metal particles thin acicular crystallites are better suited than the more symmetrical crystallites obtained from suspended transformation to the dihydrate. The needles decompose into metal particles smaller than about 0,i ~m. Decomposition at an elevated temperature results into particles oriented in chains. Finally our results show that the crystallites of 8-cobalt

(II) oxalate

E. WISGERHOF, et al.

1000

Vol. 18, No. 8

dihydrate eventually will recrystallize to the most stable phase ~-cobalt (II) oxalate dihydrate (table IV, sample 28 and table V, samples 56 and 46, with XRD-pattern IV, figure i). We intend to report on this and on the thermal decemposition of the oxalates exhibiting different morphologies in subsequent papers.

Conclusions At temperatures u p t o about 60 °C cobalt (II) oxalate initially precipitates as a tetrahydrate with a greyish pink colour. Both solid state dehydration and dehydration in aqueous suspension lead to dihydrates with a bright pink colour, but the crystal habit and the XRD-pattern of the two dihydrates are quite different. In aqueous suspension dehydration proceeds by means of recrystallization which can be accelerated by i. an increase of the temperature, the most important factor. 2. an excess of oxalate ions over cobalt (II) ions. 3. a high degree of supersaturation. Production of small cobalt particles from cobalt (II) oxalate calls for prevention of liquid phase recrystallization to the dihydrate.

Acknowledgement We thank Mr. A. Broersma who carried out the TG and DSC measurements.

References 1.

S.W. Charles and J. Popplewell,

Endeavour 6_, 153

(1982).

2.

Ampex Corp. Neth. AppI. 6,400,382, 1963, 4 pp. Chem. Abstr. 62 6000c.

3.

S. Deakin, M. Scott and B.D. Steele, Z. phys. Chem. 69, 123

4.

F. Ephraim, Ber. Deut. Chem. Ges. 42, 3850

5.

G. Avond, H. Pezerat, J.P. Lagier and J. Dubernat, Rev. Chim. Miner. 6, 1095 (1969).

6.

P.F.S. Cartwright, 92, 663 (1967).

7.

R. Deyrieux,

July 22, 1964; U.S. Appl. Jan. 21,

(1909).

(1910).

E.J. Newman and D.W. Wilson, The Analyst,

C. Berro and A. Peneloux, Bull. Soc. Chim. Fr. 25 (1973).