An examination of the relative stabilities of MgxNi1−xO and NiO on spherical carbon particles

Mat. R e s . B u l l . , Vol. 22, p p . 609-615, 1987. P r i n t e d in t h e USA. 0025-5408/87 $3.00 + .00 C o p y r i g h t (c) 1987 Pergamon J o u r n a l s L t d .

AN EXAMINATION OF THE RELATIVE STABILITIES OF MgxNil_xO AND NiO ON SPHERICAL CARBON PARTICLES by Michael Schwartz, Robert gershaw, Kirby Dwight and Aaron Wold Department of Chemistry, Brown University Providence, RI 02912

( R e c e i v e d December 12, 1986; R e f e r e e d ) ABSTRACT The H 2 reductions of MgxNil_x 0 and NiO dispersed on spherical carbon molecular sieve particles were studied using a combined magnetic-thermogravimetric technique. It was found that the NiO was greatly stabilized by solid solution formation with MgO, but the carbon support did not stabilize the Ni0 towards H 2 reduction. MATERIALS INDEX:

MgxNil: x, NiO/carbon particles, oxides, nickel, magneslum Introduction

It has been shown (1) that stabilization of hexagonal iron oxide on rutile TiO 2 only occurs if ternary phases such as Fe2TiO 4 or FeTiO 3 are formed under a reducing atmosphere. The stabilization of a transition metal oxide can also be achieved by the formation of a solid solution without a change of crystal structure. A simple example of such solid solution formation is the system MgxNil_xO where all of the members crystallize with the rock salt structure. The system MgxNil_x O was chosen for this study because of the ease of reduction of Ni(II) to metallic nickel in a hydrogen atmosphere. Furthermore, the Curie point of nickel, 358°C, makes it convenient to study the magnetic properties as the reduction proceeds as a function of temperature. In addition, the solid solution MgxNil_x O is an ideal system to study for several reasons. First, MgO is not reduced by hydrogen up to 2500°C (25, and therefore any weight changes can be attributed solely to the reduction of NiO. Second, the magnetic properties of NiO-MgO solid solutions ave readily understood. They have been reported to be paramagnetic at low nickel oxide concentrations (35 and antiferromagnetic at higher nickel oxide concentrations (45. Finally, there have been no reports of other nickel-magnesium oxides which would interfere with the interpretation of the experimental results. A recent paper by Gallagher et al. concerns a study of the H 2 reduction of NiO using thermogravimetric and evolved gas analyses (5). For NiO with low surface areas, 1.0 m2/g, and using pure H 2 as the reductant, they found that initial reduction began at approximately 250°C, depending upon the rate of heating. Their results were consistent with other reports and presented a clear picture of the temperature dependence of the reduction of NiO. A second relevant paper described the effect that doping small amounts of MgO into NiO 6O9

610

M. SCHWARTZ, et al.

Vol. 22, No. 5

h a d upon t h e r a t e o f H2 r e d u c t i o n o f NiO ( 6 ) . S m a l l a m o u n t s o f MgO, 1.5% a n d 7.1%, g r e a t l y d e c r e a s e d t h e r a t e o f r e d u c t i o n a t c o n s t a n t h i g h t e m p e r a t u r e s . However, the study did not correlate the magnetic properties of the phases formed on reduction with the increased stabilization of the NiO. I t was t h e p u r p o s e o f t h i s s t u d y t o i n v e s t i g a t e this correlation and to compare the stabilization o f n i c k e l o x i d e i n a s o l i d s o l u t i o n c o n t a i n i n g MgO w i t h a s a m p l e o f NiO d i s p e r s e d on s p h e r i c a l carbon molecular sieve particles w h e r e t h e r e a p p e a r t o be no i n t e r a c t i o n s present (7).

Experimental The s t a r t i n g m a t e r i a l s were Mg(NOj)2-6H20 , Ni(NO3)2-6H20 ( F i s h e r C e r t i f i e d Reagents) and carbon particles (Spherocarb carbon molecular sieve, Analab, GCA-012). The c a r b o n p a r t i c l e s h a d a p o r e s i z e o f 1SA a n d a s u r f a c e a r e a o f 1200 m 2 / g . MgxNil_xO s o l i d s o l u t i o n s were p r e p a r e d by t h e c o d e c o m p o s i t i o n o f the nitrates. Typically, the appropriate amount of the metal nitrates were dissolved in distilled H20, 2 ml o f H20 p e r gram o f s t a r t i n g material. This s o l u t i o n was t h e n d r i e d 12 h r a t 150°C. The r e s u l t i n g s o l i d was g r o u n d a n d t h e n heated in a porcelain crucible in air for 24 hr at 600°C. A temperature of 600°C was chosen as the preparation temperature in order to ensure complete decomposition of the dried magnesium nitrate precursor. X-ray powder diffraction patterns showed the resulting products to be single phased with the rock salt structure, NiO was also prepared by the same procedure. The samples of NiO dispersed on Spherocarb were prepared according to the method of Kim et al. (7) using Ni(NO3)2-6H20 (Fisher Certified Reagent) as the source of Ni. The dried Ni nitrate/C precursor was heated at 450°C in a wet N 2 atmosphere in order to prevent reduction. The resulting product consisted of NiO as determined by x-ray powder diffraction. No Ni metal was detected in the samples by either x-ray powder diffraction or magnetic measurements. A loading of I0 atomic percent of Ni was used. The t h e r m o g r a v i m e t r i c balance used in this study combines magnetic measurements with thermogravimetric analysis. As t h e r e a c t i o n p r o c e e d s , t h e w e i g h t o f t h e s a m p l e c a n be d e t e r m i n e d a l t e r n a t e l y in a magnetic field gradient and without the magnetic field gradient. This allows for the constant monitoring of the appearance and growth of a magnetic phase in conjunction with weight changes associated with the reaction. The new p h a s e s c a n be i d e n t i f i e d on t h e b a s i s o f t h e i r C u r i e t e m p e r a t u r e s . Although x-ray powder diffraction can be u s e d t o i d e n t i f y new p h a s e s , t h i s t e c h n i q u e i s more u s e f u l b e c a u s e i t monitors the reaction as it occurs. M a g n e t i c m e a s u r e m e n t s a r e a l s o much more sensitive than x-ray powder diffraction. Therefore, new information on reactions can be gained using this combined technique. A schematic diagram of the apparatus used for the combined magnetic and thermogravimetric a n a l y s e s i s shown i n F i g . 1. The m a g n e t i s m o u n t e d on a s h a f t w h i c h i s c o n n e c t e d t o a m o t o r t h r o u g h a c r a n k . The m a g n e t moves from the vertical position ( s a m p l e o u t o f t h e f i e l d ) up t o t h e h o r i z o n t a l position (sample in the field) once a minute. The s a m p l e i s p o s i t i o n e d s o t h a t i t s i t s i n t h e maximum f i e l d g r a d i e n t . The w e i g h t o f t h e s a m p l e was d e t e r m i n e d

i

m

SPHERICAL CARBON PARTICLES

Vol. 22, No. 5

611

using a Cahn electrobalance (model RG). The temperature was measured by a type S thermocouple which was positioned just below the sample.

ITO BALANCE

QUARTZ TUBE

QUARTZ FIBER

_

SAMPLE MAGNET

3-"

~i--

MAGNET

_

'

I

,'

I

I

I

% t

,L

I

I

'

:1

'

Fig. 1 Schematic Diagram of Magnetic-Thermogravimetric

Balance

Typical sample weights were 35-40 mg. 85%Ar/15%H 2 was used for the reductions. The gas was dried by passing through a P205 column. The flow rate was 30 cm3/min and the samples were heated at 50°C per hour. X-ray powder diffraction patterns were taken with a Philips diffractometer using copper radiation (~ = 1.5405 A) and a single crystal graphite monochromator. The scan rate was i ° 20 per minute with a chart speed of 30 in/hr. For cell constant determination, the scan rate was I/4 ° 28 per minute.

Results and Discussion Samples having the composition MgxNil_×O (x = 0.l-0.3) were prepared by double decomposition of the nitrates. The minimum temperature for complete decomposition of the nitrates was established from TGA data. X-ray analysis indicated that all members of the system crystallized with the rock salt structure. The variation in the cell parameters vs. nickel content is shown in Fig. 2. A comparison of the stability of bulk nickel oxide was made with the compositions in the system HgxNil_xO. The TGA results obtained under an 85%Ar/15%H 2 atmosphere are shown in Fig. 3. In order to avoid discrepancies due

612

M. SCHWARTZ, et al.

Vo]. 22, No. 5

H ~ x N i l-x O 4.24

~ v~

4.

i

,

~

I

i

i

,

,

i

.

.

.

~

•

•

i

~

.

.

.

I

r

'

'

'

1

.

.

,

i

22

L.

o

4. 20

cJ

/

j~

0 O_

4.1B

4.

18

. . . .

O.O

I

'

~

0.2

I

'

'

0.4

0.8

I

0.8

1.0

X

Fig. 2 Plot of Cubic Cell Parameter vs. Composition Parameter x

H~xNil_x0 i

ii0

i00

.

.

.

.

Reduction i

.

.

.

.

I

i

i

i

i

""****'*'°°'*',,i~::o..~;~;~;~

r-

90 0 >

A~ A

x=O. 0

0

80

x=O.

7 0

i

200

l

300

b

~

I

i

,

'

*

x=O. 2

o

x=O. 3

'

400

Temperature

1

I

i

,

,

500

(°C)

Fig. 3 Thermogravimetric Results for H2 Reduction of MgxNil_x0

600

Vol. 22, No. 5

SPHERICAL CARBON PARTICLES

613

MgxNii_x O R e d u c t i o n . . . .

I00

i

'

'

'

'

J

J

~ x=O.

0

A x=O.

1

v

80 C O .,--4 4J O N

~

* x=O. 2 6O

0 C

°%

0 ~E

413

.

o

o

> o j.

0

o

20

~o

~ 0

A

,~ "

;

"

÷"

~, "

200

XXXXXXXXX~XXXXXXXXXXX

T ~,~

,

,

,

300

,

,-

,

,

Fig. Results

for

. . . .

J

,

600

(°C)

4 H 2 Reduction

NiO/C i00

,

500

TemperaturQ

Thermomagnetic

,

400

of

MgxNil_x

0

Reduction

. . . .

x

i

~

~

•

,

'

. . . .

x x x

80

x x

x

x

v

•

60

E o u

x

x 40

>

o

20

j

@

x

Mognetizotion

~

Weight

o£

NiO

×

x

0

x

...... *--*-~~--~-----~--~

-20

.

200

.

.

.

........................................................................................................

,

.

.

.

.

250

,

.

Temperoture Fig. Thermogravimetric

.

.

.

300

and Thermomagnetic

5 Results

,

.

.

.

350

.

.

400

(°C) for H 2 R e d u c t i o n

of Ni0/C

i

614

M. SCHWARTZ, et al.

Vol. 22, No. 5

to kinetic effects, samples of at least 35-40 mg were used for the stability determinations. Nhereas bulk NiO begins to reduce at 255°C, samples with the composition MgxNil_xO (x = 0.I-0.3) begin to reduce at temperatures between 312 365°C. However, the initial weight loss is so small that it is not apparent on the scale of Fig. 3. The rate of temperature increase was 50°C per hour. Thermomagnetic studies (Fig. 4) were carried out simultaneously with the TGA experiments using the thermomagnetic balance previously described. Reduction of bulk NiO with 8S%Ar/IS%H 2 resulted in the formation of metallic nickel which was detected at 2SS°C. This is the same temperature at which TGA results indicate reduction of NiO. For samples containing approximately 30 mg of metallic nickel, the Curie point was found to be 3S8°C. This agrees with the reported Curie point of metallic nickel. For the compositions x = 0.I and 0.2, magnetic studies clearly indicate the formation of nickel at temperatures which correspond to those reported above where the barely detectable weight losses began. It is evident that the thermomagnetic studies are far more sensitive in determining the onset of reduction. For x = 0.3, the temperature required for the formation of nickel occurs above the Curie temperature. It can be seen that appreciable stabilization of nickel occurs when only a small fraction of magnesium is substituted into the rock salt structure. The degree of stabilization of nickel towards reduction in the system MgxNil_x O (x = 0.1-0.3) was compared to nickel dispersed on Spherocarb carbon where no solid solution occurs. The magnetic TGA results for the reduction in 8S%Ar/IS%H 2 of NiO dispersed on carbon are shown in Fig. S. Both the formation of the magnetic phase and weight loss were first detected at 225°C. A comparison of these results with those obtained for reduction of bulk nickel oxide and nickel-magnesium oxides indicate that there is a lowering of the reduction temperature of nickel oxide. Furthermore, it is clear that there is no stabilization of NiO when it is supported on Spherocarb.

Conclusion MgO, which interacts with NiO through solid solution formation, greatly stabilize~ the NiO. ~/hen NiO is dispersed on C, no stabilization is observed. These results also show the utility of the combined magnetic-TGA experiment. Moreover, the barely detectable weight loss is concident with the pronounced appearance of a magnetic phase.

Acknowledgments This work was supported in part by the Eastman Kodak Company and the Office of Naval Research. In addition, acknowledgment is made to the National Science Foundation for the partial support of K. Dwight. The authors also express their appreciation for the use of Brown University's Materials Research Laboratory which is supported by the National Science Foundation.

i

ii

Vol. 22, No. 5

SPHERICAL

CARBON

615

PARTICLES

References I.

J. Yu, R. Kershaw, K. Dwight and A. Wold. J. Sol. State Chem.

2.

E. Newberry and J. N. Pring,

3.

A. Cimino, M. LoJacono, 14 (1967).

Submitted for publication

in

Proc. Royal Soc. London A92, 276 (1916).

P. Porta and M. Valigi,

Z. Phys. Chem.

(Weisbaden}

55, 4.

O. Evrard, J. Francois and J. M. Lecuine, Rev. Chim. Miner.

9, 463

(19723. 5.

P. K. Gallagher,

E. M. Gyorgy and W. R. Jones, J. Thermal Anal.

23,

185 (1982). 6.

M. H. Tikkanen,

B. O. Rosell and W. Wiberg, Aeta Chem. Scand.

17, 513

(1963). 7.

K. Kim, R. Kershaw, K. Dwight, A. Wold and K. C o l l e , Mat. Res. B u l l . 591 (1982).

17,