Adsorption at Interfaces

16 Surface Properties o f N i c k e l H y d r o x i d e Before and

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After Dehydration to N i c k e l O x i d e M.TOPIC,*F. J. MICALE, C. L. CRONAN, H. LEIDHEISER, JR., and A. C. ZETTLEMOYER Center for Surface and Coatings Research, Lehigh University, Bethlehem, Penn. 18015 Introduction It is a pleasure for us to take part in this symposium honoring Professors Marjorie and Robert Vold. They have done much for colloid science in teaching and in research through almost forty years of service; they have developed leaders in their field on several continents. As they come to well deserved retirement, we note their many contributions which will stand as a monument to their efforts. The technique of thermal decomposition of precipitated Ni(OH)2 for the preparation of NiO is often employed in studies (1-4) of this useful oxide. During surface characterization studies of Ni(OH) and NiO a relationship has been found between the specific surface areas of the product and precursor. This relationship has allowed the proposal of a mechanism describing the process of thermal decomposition of Ni(OH)2 at moderate temperatures. Nickel hydroxide has the CdI crystal structure (5) with the basic repeating layer structure. Each hexagonally packed layer of Ni(OH)2 is made up of a hexagonal layer of N i sandwiched between two hexagonal layers of OH-. Nickel oxide has the NaCl structure. 2

2

++

Experimental Nickel hydroxide samples were prepared using various techniques and are designated 1 through 4. Samples 1 and 2 were precipitated with NH3 gas from solutions of Ni(NO3) at 80°C for sample 1 and at 25°C for sample 2. These samples were repeatedly washed with deionized distilled water and centrifuged until the wash water attained a constant conductivity of approximately 3 x 10 mho/cm. Samples 3 and 4 were obtained from Dr. Velimir Pravdic (Rudjer Boskovic Institute, Zagreb, Yugoslavia) and were 2

-6

* Current address - Rudjer Boskovic Institute, Zagreb, Yugoslavia. 225 Mittal; Adsorption at Interfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

ADSORPTION A T INTERFACES

Time of Heating at 200°C, Hours

Figure 1. Percent weight loss of Ήΐ(ΟΗ) vs. time at 200°C in vacuum ζ

Heating Time @200°C, Hours

Figure 2. Specific surface area of sample vs. time of activation at 200°C in vacuum

Mittal; Adsorption at Interfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

16.

TOPIC

ET AL.

Nickel Hydroxide

227

prepared by a m o d i f i e d s o l - g e l process (6). S p e c i f i c s u r f a c e area values were obtained u s i n g argon gas and a c l a s s i c a l v o l u m e t r i c BET apparatus. Gas pressures were measured w i t h a c a p a c i t i v e e l e c t r o n i c manometer equipped w i t h a 1000 t o r r d i f f e r e n t i a l p r e s sure head (Datametrics, I n c . ) . A l l isotherms were determined w i t h the samples immersed i n a l i q u i d n i t r o g e n bath. Weight l o s s values were obtained through the use o f a quartz s p r i n g m i c r o balance (Worden) w i t h a d e f l e c t i o n s e n s i t i v i t y o f approximately 1 mg/cm and sample c a p a c i t y o f about 30 mg. R e s u l t s and D i s c u s s i o n Studies o f the t r a n s i t i o n between N i ( 0 H ) and NiO i n v o l v e d the d i r e c t observations o f weight l o s s and s u r f a c e areas. F i g u r e 1 i l l u s t r a t e s a t y p i c a l weight l o s s v s . time curve f o r a sample o f Ni(OH)2 a t 200°C under vacuum c o n d i t i o n s . The process i n c l u d e s a p e r i o d o f i n d u c t i o n , passing through a maximum r a t e a t about 1-1/2 hours and l e v e l i n g o f f a t &5% completion, corresponding t o a s t o i c h i o m e t r y Ni0-0.15H 0. This l i m i t compares w e l l w i t h p r e v i o u s l y r e p o r t e d values o f NiO-0.l6H 0 (j) and Ni0-0.l8H 0 (13). The s u r f a c e areas o f a sample o f N i ( 0 H ) d u r i n g the 200°C thermal treatment were measured a t v a r i o u s a c t i v a t i o n times and are i l l u s t r a t e d i n F i g u r e 2. As demonstrated i n F i g u r e 1, t h e decomposition i s v i r t u a l l y complete a f t e r 3 hours o f 200°C a c t i v a t i o n . On the b a s i s o f t h i s work the thermal decomposition c o n d i t i o n s f o r a l l samples were 200°C f o r 8 hours i n vacuum. The BET argon s u r f a c e a r e a s , assuming 1 3 . 8 2 f o r the c r o s s s e c t i o n a l area o f argon, o f the f o u r N i ( 0 H ) samples and the r e s u l t a n t NiO samples prepared under the above c o n d i t i o n s a r e g i v e n i n Table I . F i g u r e 3 i s a p l o t o f the r a t i o o f the s p e c i f i c s u r f a c e areas o f NiO o t those o f the corresponding p r e c u r s o r Ni(OH)2 vs. the r e c i p r o c a l o f the s p e c i f i c s u r f a c e area o f the Ni(OH)2 sample. The s t r a i g h t l i n e drawn through the f o u r sample p o i n t s has a l e a s t squares slope and i n t e r c e p t o f 73-9 m /g and 0.72, r e s p e c t i v e l y . The s u r f a c e area r e l a t i o n s h i p presented i n F i g u r e 3 and e l e c t r o n micrograph r e s u l t s , which show s p l i t t i n g Ni(0H)2 hexa gonal c r y s t a l s i n t o hexagonal l a y e r s , suggests a model o f the thermal decomposition process. F i g u r e h i l l u s t r a t e s the proposed p h y s i c a l process f o r the decomposition. I n the f o l l o w i n g d e r i v a t i o n the s u b s c r i p t 1 r e f e r s t o N i ( 0 H ) and 2 r e f e r s t o NiO. E l e c t r o n micrographs showed t h a t the p r e c i p i t a t e d N i ( 0 H ) occurs as hexagonal p l a t e l e t s which can be represented as having a r a d i u s R^, and a t h i c k n e s s H j ( g r e a t l y exaggerated i n F i g u r e h). The process o f 200°C a c t i v a t i o n r e s u l t s i n the N i ( 0 H ) c r y s t a l e x f o l i a t i n g i n t o η i d e n t i c a l l a y e r s o f NiO, each o f r a d i u s R and t h i c k n e s s H . The s p e c i f i c s u r f a c e a r e a , Σ, o f a m a t e r i a l con s i s t i n g e n t i r e l y o f i d e n t i c a l hexagonal p l a t e s and d e n s i t y ρ can be shown t o be equal t o : 2

2

2

2

2

2

2

2

2

2

2

2

2


ADSORPTION AT INTERFACES

Table I . S p e c i f i c Surface Areas o f Ni(OH)2 and NiO Prepared a t 200°C Sample

Ni(0H)

NiO

2

1

1*

8*

2

28

96

3

k9

102

h

128

177

Table I I . P h y s i c a l and X-ray P r o p e r t i e s Ni(0H) M.W. (g/mole) pX-ray (g/cc) V (cc/mole) c a

Q

Q

NiO

2

92.71

7*.69 6.809

3.950

10.97

23.*7 ο U.605A 3.126A

ο = 4.1769 A = 2.1*10 A

a ά

1 1 λ

220

d

=

1-^76 A


16.

TOPIC E T A L .

Figure

Nickel Hydroxide

4. Schematic

229

diagram Ni(OH)

of the exfoliation

of

2


230

ADSORPTION

L

AT

INTERFACES

U

RHp

;

4/3 where C = ^ = 2.309, and R and H a r e the c r y s t a l r a d i u s and t h i c k n e s s . Table I I con t a i n s v a r i o u s p h y s i c a l and X-ray p r o p e r t i e s (£) N i ( 0 H ) and NiO. Most n o t a b l e , the molar volume decreases by 53% d u r i n g the decomposition process. A n a l y s i s o f X-ray data and e l e c t r o n micro graphs i n d i c a t e t h a t t h e broad hexagonal face o f Ni(0H)2 i s the (001) plane. A f t e r thermal decomposition, t h e broad hexagonal face becomes the ( i l l ) plane o f NiO. I t can be shown t h a t the f o l lowing r e l a t i o n s h i p s e x i s t between t h e c r y s t a l dimensions o f N i ( 0 H ) and NiO: o f

2

2

H

2

=

J

and

R

where

r = ^

and

h=

2

%

(2)

=

(3)

d-i

C

= |^|| =

0.9UU3

(U)

= 2.1*10=

0.5233

(5)

τη o

k.605

Expressions f o r N i ( 0 H ) and NiO, u s i n g the form o f Equation ( l ) , can be combined i f both and Σ a r e w r i t t e n as f u n c t i o n s o f Rj_ and H". E l i m i n a t i o n o f R]_ r e s u l t s i n t h e equation: 2

2

2

where

r = 0.9*1+3 h = 0.5233

Equation (6) represents a s t r a i g h t l i n e w i t h an i n t e r c e p t o f P l / r p and slope o f fe/H2P2) ( l - [h/rn]) i f the s u r f a c e areas are p l o t t e d as i n F i g u r e 3. E v i d e n t l y t h e slope i s a constant over the range o f s u r f a c e areas s t u d i e d , even through i t i s a f u n c t i o n of two p o s s i b l e v a r i a b l e s , H2 and n. Given t h a t the experimental v a l u e o f the slope i n F i g u r e 3 i s 73-9 m /g and as shown i n Equation (6): 2

2

S = slope = * H P2 2

(1 - — ) rn


(7)

16. TOPIC ET AL.

Nickel Hydroxide

231

Replacing η by 2

and rearranging

L

2

=

r

H

l Spgi^ + 2

(8)

_ 1.889 % " O.Oi+752 E + 2 ±

where and H2 are expressed i n 2 u n i t s . Figure 5 i s a plot of the thickness of an NiO c r y s t a l , H , v s . the thickness of i t s parent Ni(0H) hexagonal plate, Ητ_, as calculated from Equation (8). The distance H i s the ( i l l ) directional thickness of the NiO hexagonal plate. The experimental intercept of 0.72 agrees with the theoreti c a l value of 0.6li* calculated from Equation (6). The difference i n the intercept value may he due to a r e a l value of p lower than the X-ray value presented i n Table I I , especially since the activation was only 85$ complete. Other author's (2,10) have measured the NiO density as 6.0 g/cc. I f t h i s density applies to these samples the theoretical intercept should be 0.70, which i s i n excellent agreement with the experimental intercept. Analysis of Equation (6) indicates that polydisperse Ni(0H) samples yields a scatter of data i f plotted as i n Figure 3. Since the scatter i s not severe i n Figure 3 i t can be concluded that either there i s l i t t l e polydispersity or the four samples are approximately equally polydispersed. 2

2

2

2

2

Literature Cited 1. 2.

Richardson, J.T., Milligan, W.O., Phys. Rev. (1956), 102, 1289. Larkins, F . P . , Fensham, P.J., Sanders, J . V . , Trans. Faraday Soc., (1970), 66, 1748.

3.

Nicolaon, G.A., Teichner, S . J . , J . Colloid Interface S c i . , (1972), 38, 172.

4.

Fahim, R.B., Abu-Shady, A.I., J. Catal., (1970), 17, 10.

5.

Huckel,W.,"Structural Chemistry of Inorganic Compounds", Vol. I I , p. 547ff, Elsevier Publishing Co., Amsterdam, 1951.

6.

Bonacci, Ν., Novak, D.M., Croat. Chem. Acta, (1973), 45, 531. Teichner, J., Morrison, J . A . , Trans. Faraday Soc., (1955), 51, 961. K l i e r , Κ., Kinet. Katal. (Eng. trans.), (1962), 3, 51.

7· 8.


232

ADSORPTION A T INTERFACES


16. TOPIC ET AL. Nickel Hydroxide 9. 10.

Smith, J . V . , Ed., "X-ray Powder Data F i l e , " ASTM Phila delphia, 1967. Helms, W.R., Mullen, J . G . , Phys. Rev. Β, (1971), 4, 750.


233

Adsorption at Interfaces

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