Effect of doping on the sintering of finely divided NiO catalyst

Surface Technology, 13 (1981) 197 - 203

197

E F F E C T OF DOPING ON THE SINTERING OF F I N E L Y D I V I D E D NiO C A T A L Y S T

G. A. EL-SHOBAKY and N. SH. PETRO Laboratory of Surface Chemistry and Catalysis, National Research Centre, Dokki, Cairo (Egypt) (Received September 26, 1980)

Summary The effect of lithium, sodium and iron dopants on the s i n t e r i n g of finely divided NiO was studied. P u r e and doped oxides were p r e p a r e d by the t h e r m a l d e c o m p o s i t i o n in air at t e m p e r a t u r e s of 300 - 1000 °C of pure Ni(OH) 2 and of Ni(OH) 2 mixed with the doping a g e n t in the form of the h y d r o x i d e respectively. The a p p a r e n t a c t i v a t i o n energies E s of s i n t e r i n g for the different solids were c a l c u l a t e d from the s i n t e r i n g coefficient d a t a at different t e m p e r a t u r e s and also from m e a s u r e m e n t s of the specific surface a r e a of p u r e and doped oxides which had u n d e r g o n e t h e r m a l t r e a t m e n t s at various t e m p e r a t u r e s . A l t h o u g h the two methods gave different values for E~, the results showed the saree trends. The observed differences in the m a g n i t u d e s of Es are explained in terms of modifications in the pore sizes of the different solids. The i n c o r p o r a t i o n of sodium ions in the NiO lattice was found to d e c r e a s e the a p p a r e n t a c t i v a t i o n e n e r g y of sintering, thus facilitating the diffusion of lattice ions in the o u t e r m o s t surface layers of the NiO. The addition of lithium and iron ions to the NiO lattice had no n o t i c e a b l e effect on the s i n t e r i n g process.

1. I n t r o d u c t i o n The i n c o r p o r a t i o n of small a m o u n t s of m o n o v a l e n t and t r i v a l e n t ions in a NiO lattice g e n e r a l l y affects its e l e c t r o n i c s t r u c t u r e [1] and h e n c e modifies its catalytic, a d s o r p t i v e and electrical p r o p e r t i e s [2 - 5]. Doping m a y also p r o d u c e s t r u c t u r a l defects such as c a t i o n i c or anionic v a c a n c i e s [6, 7], the p r e s e n c e of which m a r k e d l y affects the diffusion of the NiO lattice ions and h e n c e modifies its s i n t e r i n g b e h a v i o u r . The effect of doping on the electrical, c a t a l y t i c and a d s o r p t i v e p r o p e r t i e s of NiO has been studied by a n u m b e r of w o r k e r s [1 - 7]. H o w e v e r , m u c h less effort has been devoted to i n v e s t i g a t i o n s of the effects of doping on the s i n t e r i n g of NiO [8, 9]. The most f r e q u e n t l y 0376-4883/81/0000-0000/$02.50

© Elsevier Sequoia/Printed in The Netherlands

198

used m e t h o d s for i n v e s t i g a t i n g the s i n t e r i n g of metal oxide systems are to study the kinetics of the d e c r e a s e in the specific surface area [10, 11], to d e t e r m i n e the p a r t i c l e size d i s t r i b u t i o n from X-ray diffraction profiles [12] and to m e a s u r e the density and s h r i n k a g e [13, 14]. In this paper we r e p o r t a study of the effects of lithium, sodium and iron d o p a n t s on the s i n t e r i n g of NiO. The d o p a n t s were i n c o r p o r a t e d in the NiO lattice by h e a t i n g mixed h y d r o x i d e s in air at t e m p e r a t u r e s in the r a n g e 300 - 1000 °C. The sintering of the NiO samples was studied using measurements of the specific surface area o b t a i n e d after h e a t i n g the samples to various temperatures.

2. E x p e r i m e n t a l details

2.1. Materials P u r e NiO samples were o b t a i n e d by the t h e r m a l d e c o m p o s i t i o n of v e r y pure Ni(OH) 2 [15] in air at t e m p e r a t u r e s r a n g i n g from 300 to 1000 'C. Samples doped with Li + and Na ÷ were p r e p a r e d by adding c o n c e n t r a t e d lithia and soda solutions r e s p e c t i v e l y to solid Ni(OH) 2 (10 a t . % Li and 20 a t . % Na) and mixing to a paste with the m i n i m u m a m o u n t of distilled water. The paste was.dried to c o n s t a n t w e i g h t at 60 °C u n d e r a r e d u c e d pressure. The doped NiO specimens were o b t a i n e d by h e a t i n g for a f u r t h e r 5 h at different t e m p e r a t u r e s . The oxides doped with Fe 3 ÷ were p r e p a r e d by adding Fe(OH)z (which was o b t a i n e d by adding a m m o n i a to ferric a m m o n i u m s u l p h a t e [16]) to solid Ni(OH) 2 (10 a t . % Fe); the m i x t u r e was t h e n given the same t r e a t m e n t as the oxides doped with lithium and sodium. 2.2. Apparatus and techniques The specific surface areas of the v a r i o u s oxides were d e t e r m i n e d from n i t r o g e n a d s o r p t i o n isotherms at 77 K using a c o n v e n t i o n a l v o l u m e t r i c a p p a r a t u s . The effectiveness of the i n c o r p o r a t i o n of the d o p a n t ions in the NiO lattice was tested by the i o d o m e t r i c d e t e r m i n a t i o n of the active o x y g e n in the different solids [17].

3. R e s u l t s a n d d i s c u s s i o n

3.1. The effect of the sintering temperature on the specific surface areas of pure and doped NiO In Fig. 1 the specific surface areas of the various solids are plotted as f u n c t i o n s of TIT m w h e r e T and T m are the s i n t e r i n g and melting temperat u r e s respectively. The melting t e m p e r a t u r e of NiO was t a k e n as 2270 K [14] and was assumed to be u n a f f e c t e d by the d o p a n t ions used in this investigation. It can be seen from Fig. 1 t h a t t h e r e is a steady decrease in the surface area of sodium-doped NiO o v e r the whole r a n g e of t e m p e r a t u r e s u n d e r investigation. The o t h e r oxides showed a r e l a t i v e l y large decrease in

199

150

1.8

100

~

50

I 4

'-0.25 0

1.2

. -' 4

5 0.35 'T

/ ' '

'

T~'

600

700

800

900

1()00

K

F i g . 1. P l o t s o f t h e s p e c i f i c s u r f a c e a r e a o f p u r e a n d d o p e d N i O vs. T / T m : c u r v e 1, N i O ; c u r v e 2, N i O : F e ; c u r v e 3, N i O : Li ; c u r v e 4, N i O : N a . F i g . 2. P l o t s o f l o g ( S T - - Sf) vs. t h e s i n t e r i n g c u r v e 3, N i O : Li ; c u r v e 4, N i O : N a .

temperature:

c u r v e 1, N i O ; c u r v e 2, N i O : F e ;

the surface area when the sintering t e m p e r a t u r e was increased from 0.250Tm to 0.275Tm, followed by a progressive decrease as the sintering temperature was increased further. It is well known t hat the sintering of solids takes place by three different processes [18, 19, 20]: (1) adhesion of the solid particles, causing grain growth; (2) migration of the lattice ions along the outermost surface layers of the solid, causing the surface irregularities and cracks between the neighbouring crystallites to be filled up; (3) collapse or shrinkage of the pores. Adhesion dominates at lower temperatures and may occur at room t e mp er atu r e [14] provided t hat the grain surface is free from adsorbed ionic or molecular species. Surface diffusion operates in the temperature range 0.20Tm - 0.35Tin and bulk diffusion (migration of lattice ions t h r o u g h o u t the whole volume of the solid) becomes effective at temperatures above the Tammann temp er a t ur e (0.5Tin). Consequently each of these three sintering mechanisms will have a different effect on the rate and extent of change in the surface area. In the range of temperatures under investigation (0.25T~ 0.50T~,) the role played by bull~ diffusion can be neglected and sintering can be assumed to occur mainly by adhesion and surface diffusion. We can conclude from the results given in Fig. 1 t hat the doping of NiO with alkali lowers its surface area considerably. This effect is most pronounced for sodium doping in the t em pe r at ure range 0.250Tm - 0.375T~. In contrast, doping with iron increases the surface area of NiO at temperatures above 0.3T~. These results are in good agreement with those published by other workers [21, 22]. The effectiveness of the incorporation of monovalent and trivalent ions in NiO was tested by the iodometric determination of the excess oxygen in different oxide samples, M onova l e nt doping was accompanied by an increase in the c o n c e n t r a t i o n of the active oxygen while trivalent doping had the opposite effect. Details of the results obtained have been reported elsewhere [4, 23].

200

3.2. Calculation of the activation energy E s of sintering 3.2.1. Determination of Es from m e a s u r e m e n t s of the sintering coefficient V a r i o u s i s o t h e r m a l rate expressions w h i c h give r e l a t i o n s b e t w e e n the time o f s i n t e r i n g at c o n s t a n t t e m p e r a t u r e and the g r a i n size, pore volume, pore r a d i u s and surface area h a v e been proposed [24 - 26]. Of the different r e l a t i o n s b e t w e e n the s i n t e r i n g r a t e and the surface area, the following two are the simplest [27, 28]: dA dt -

Ks(A-At)

(1)

dS dT

K(ST-Sf)

(2)

w h e r e Ks is the rate c o n s t a n t , A is the initial surface area, At is the final s u r f a c e area, t is the time of sintering, ST is the surface a r e a at t e m p e r a t u r e T, S t is the final surface a r e a a t t a i n e d at the h i g h e s t s i n t e r i n g t e m p e r a t u r e and K is the s i n t e r i n g coefficient. If eqn. (2) holds, a plot of l o g ( S T - Sf) a g a i n s t T should give a s t r a i g h t line. In fact, as c a n be seen in Fig. 2, a s t r a i g h t line is o b t a i n e d for all the solids investigated.

l -I

3

1.7

1

1.5

1.3

1.1

o.g 1000/T

;.7

1.5

1.3

1.1

0.9 1(700/1

Fig. 3. Plots of logK vs. 1/T: curve 1, NiO; curve 2, NiO:Fe; curve 3, NiO:Li; curve 4, NiO : Na. Fig. 4. Plots of logSBEw vs. 1/T: curve 1, NiO; curve 2, NiO:Fe; curve 3, NiO:Li; curve 4, NiO : Na. Since the s i n t e r i n g coefficient is a f u n c t i o n of both time and temperature, the A r r h e n i u s e q u a t i o n c a n be used to c a l c u l a t e the a p p a r e n t activ a t i o n e n e r g y of s i n t e r i n g if the v a l u e of K c a n be d e t e r m i n e d at different t e m p e r a t u r e s . N u m e r i c a l values of K a v e r a g e d over the entire t e m p e r a t u r e r a n g e were c a l c u l a t e d from the slopes of the s t r a i g h t lines in Fig. 2. W h e n these values are s u b s t i t u t e d in eqn. (2), d S / d T can be d e t e r m i n e d for each oxide over the whole r a n g e of the observed surface a r e a s S T - - S f . Values of K at different t e m p e r a t u r e s are t h e n o b t a i n e d from the c a l c u l a t e d values of d S / d T for each solid. F i g u r e 3 shows plots of l o g K versus 1/T. The a p p a r e n t a c t i v a t i o n

201

TABLE 1 The apparent activation energy of sintering for various NiO samples determined from measurements of the sintering coefficient

Solid

Es (kcal tool 1)

Temperature range (K)

Pure NiO NiO : Fe NiO : Na NiO : Li NiO : Li

9.45 8.78 3.60 6.25 9.85

673 - 1073 673 - 973 573 - 973 573 - 773 773 - 973

e n e r g i e s of s i n t e r i n g c a n be d e t e r m i n e d f r o m t h e slopes of the lines in Fig. 3. A single v a l u e of Es was o b t a i n e d for p u r e N i O a n d for NiO doped w i t h s o d i u m a n d w i t h iron, a n d t w o different v a l u e s of Es w e r e f o u n d for lithiumdoped NiO. The v a l u e s o b t a i n e d a r e g i v e n in T a b l e 1. T h e i n c o r p o r a t i o n of s o d i u m in the NiO l a t t i c e c a u s e d a d r a s t i c d e c r e a s e in the a c t i v a t i o n energy, w h i l e d o p i n g w i t h iron h a d little effect on the m a g n i t u d e of E s. In c o n t r a s t , the a d d i t i o n of lithium" to the N i O l a t t i c e led to a d e c r e a s e in Es w h e n t h e doped oxide w a s s i n t e r e d a t t e m p e r a t u r e s of 773 K a n d below, a n d to a m a r k e d i n c r e a s e in E~ to a v a l u e s l i g h t l y g r e a t e r t h a n t h a t for p u r e NiO at h i g h e r s i n t e r i n g t e m p e r a t u r e s . It m a y be a r g u e d t h a t the d o p i n g of N i O w i t h l i t h i u m c a n be d e s c r i b e d by t w o d i f f e r e n t m e c h a n i s m s : the first r e s u l t s in a r e d u c t i o n in the a c t i v a t i o n energy, t h u s e n h a n c i n g t h e diffusion of l a t t i c e ions, a n d the s e c o n d c a u s e s t h e diffusibility of the l a t t i c e ions to r e t u r n to t h e i n i t i a l v a l u e found for p u r e NiO. T h e effect of m o n o v a l e n t a n d t r i v a l e n t d o p i n g on t h e electrical, m a g n e t i c , c a t a l y t i c a n d s u r f a c e p r o p e r t i e s of N i O h a s b e e n e x t e n s i v e l y studied by a n u m b e r of w o r k e r s [1 - 7]. H o w e v e r , no d a t a h a v e b e e n r e p o r t e d for the a c t i v a t i o n e n e r g y of s i n t e r i n g of doped NiO. T h e r e p o r t e d v a l u e s of E S for n o n - s u p p o r t e d p u r e NiO v a r y b e t w e e n 6.5 a n d 13 k c a l m o l - 1 [28 - 31] d e p e n d i n g m a i n l y on t h e h i s t o r y of the solid a n d the s i n t e r i n g conditions. T h e v a l u e of E~ o b t a i n e d f o r t h e finely divided p u r e m e t a l oxide e m p l o y e d in this i n v e s t i g a t i o n is t h u s in a g r e e m e n t w i t h p r e v i o u s results. It s h o u l d be n o t e d t h a t D o l l i m o r e a n d J o n e s [14] o b t a i n e d t w o e x t r e m e l y h i g h v a l u e s of E~ for s u p p o r t e d NiO (9.7 m o l . % o v e r ~/-A1203). T h e s e values, w h i c h are 23.44 k c a l m o l - 1 a n d 110.05 k c a l m o l - 1 in t h e t e m p e r a t u r e r a n g e s 1100 - 1164 K a n d 1182 - 1200 K r e s p e c t i v e l y , c o r r e s p o n d to t h e s u r f a c e a n d b u l k diffusion of the l a t t i c e ions of NiO. A c o m p a r i s o n of t h e s e d a t a w i t h the' p u b l i s h e d d a t a (6.5 - 13 k c a l m o l - 1 for n o n - s u p p o r t e d NiO) d e m o n s t r a t e s the role of the c a r r i e r (7-A12Oz) in m a k i n g the s i n t e r i n g of s u p p o r t e d N i O e n e r g e t i c a l l y difficult.

3.2.2. Determination of Es from measurements of the surface area As h a s b e e n n o t e d earlier, the diffusion of ions of the solid a l o n g the o u t e r m o s t s u r f a c e l a y e r s of its l a t t i c e m a y be t h e m a i n s i n t e r i n g m e c h a n i s m

202

a t t e m p e r a t u r e s b e l o w 0.5Tin. I f w e n e g l e c t t h e e f f e c t o f c o l l a p s e o r s h r i n k age of pores of the sintered solid which results in pore widening and a consequent decrease in the surface area, the relation between surface area a n d t e m p e r a t u r e is g i v e n b y SBET = A e x p - R T

(3)

w h e r e A is a c o n s t a n t a n d Es is t h e a p p r o x i m a t e a c t i v a t i o n e n e r g y o f s i n t e r i n g . W h e n l o g SBET is p l o t t e d a g a i n s t 1 / T a s t r a i g h t l i n e i s o b t a i n e d t h e s l o p e o f w h i c h d e t e r m i n e s Es. F i g u r e 4 s h o w s p l o t s o f 1OgSBET a g a i n s t 1 / T f o r p u r e a n d d o p e d o x i d e s . T a b l e 2 i n c l u d e s t h e Es v a l u e s o b t a i n e d f o r t h e different oxide samples. The doping of NiO with sodium reduced the activ a t i o n e n e r g y o f s i n t e r i n g m a r k e d l y w h i l e t h e a d d i t i o n o f i r o n h a d n o effect. In contrast, the incorporation of Li ÷ in the NiO lattice at temperatures b e l o w 773 K e x e r t e d n o d e t e c t a b l e e f f e c t o n E s w h i c h i n c r e a s e d s u b s t a n t i a l l y at higher sintering temperatures. The results given in Tables 1 and 2 show s i m i l a r t r e n d s ; i n d e e d E s f o r s o d i u m - d o p e d N i O is a l m o s t t h e s a m e i n b o t h t a b l e s . T h e d i f f e r e n c e s i n t h e m a g n i t u d e s o f Es f o r t h e o t h e r o x i d e s a m p l e s may arise from shrinkage of the pores which results in pore widening and c a u s e s a d e c r e a s e i n t h e s p e c i f i c s u r f a c e a r e a o f t h e s i n t e r e d o x i d e [32]. T h i s e f f e c t w a s n e g l e c t e d i n t h e d e d u c t i o n o f eqn. (3) w h i c h w a s u s e d to c a l c u l a t e E s. TABLE 2 Apparent activation energy of sintering for various NiO samples calculated from specific surface area measurements Solid

Es (kcal mol 1)

Temperature range (K)

Pure NiO NiO : Fe NiO : Na NiO : Li NiO :Li

6.40 6.53 3.48 6.20 8.10

573 673 573 573 773

- 873 - 973 - 973 - 773 - 1073

References 1 E . J . W . Verwey, P. W. Haaijam, F. C. Romeign and O. Van, Philips Res. Rep., 5 (1950) 173. 2 G.A. E1-Shobaky, P. C. Gravelle and S. J. Teichner, J. Catal., 14 (1969) 4 - 22. 3 A. Bielanski and A. Nabjar, J. Catal., 25 (1972) 398. 4 N. Sh. Petro, G. A. E1-Shobaky, N. Z. Misac and R. Sh. Mikhail, Surf. Technol., 5 (1977) 315. 5 G.A. E1-Shobaky, Surf. Technol., 7 (1978) 375 - 384. 6 A. Bielanski, K. Dyrek and Z. Kluz, Bull. Acad. Pol. Sci., Ser. Sci. Chim. 13 (1965) 285; 14 (1966) 775. 7 P. C: Gravelle, G. A. E1-Shobaky and S. J. Teichner, in K. Hauffe and Th. Volkenstein

203

8 9 10 11 12 13 14 15

(eds.), Proc. Symp. on the Electronic Phenomena in Chemisorption and Catalysis on Semiconductors, Moscow, 1968, de Gruter, Berlin, 1969, pp. 124 - 137. S.B. Boskovic and B. M. Zivanoc, J. Mater. Sci., 9 (1) (1974) 117. S.B. Boskovic and M. Stevanvic, J. Mater. Sci., 10 (1) (1975) 25. R.M. German, J. Am. Ceram. Soc., 61 (5, 6) (1978) 272. R . M . German, J. Sci. Sintering, 10 (1) (1978) 11 - 25. P. Ganesan, H. K. Kuo, A. Saaverda and R. J. De Angelis, J. Catal., 52 (2) (1978) 310. A. Anderson and U. Harlan, Phys. Sintering, 5 (3) (1973) 149 - 162. D. Dollimore and Th. E. Jones, J. Appl. Chem. Biotechnol., 23 (1973) 29. G.A. E1-Shobaky, P. C. Gravelle and S. J. Teichner, Bull. Soc. Chim. Fr., 9 (1967) 3244 3251. J . W . Mellor, Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol. 13, Longman's Green, London, 1957, p. 860. P.C. Gravelle, G. A. E1-Shobaky and H. Urbain, C.R. Acad. Sci., 26 (1966) 549. D. Roberts, Metallurgia, (August 1950) 1. G . F . Huttig, Kolloid-Z., 98 (1942) 263; 99 (1942) 262. S . J . Gregg, J. Chem. Soc. (1953) 3940. M . E . Dry and F. Stone, Discuss. Faraday Soc., 28 (1959) 192. A. Bielanski, J. Deren, J. Haber and J. Sloczinski, Trans. Faraday Soc., 58 (1962) 166. G.A. E1-Shobaky and N. Sh. Petro, Surf. Technol., 9 (1979) 415 - 426. S . J . Gregg, R. K. Packer and K. H. Loheat, J. Chem. Soc. (1955) 46. M . H . Tikkanen and S. A. Makipritti, Int. J. Powder Metall., 1 (1965) 15. H . J . Oel, Proc. 2nd Int. Powder Metallurgy Conf., New York, 1965, Vol. 1, p. 345. D. Nicholson, Trans. Faraday Soc., 61 (1965) 990. B.R. Arora, R. K. Banerjee, N. K. Mandal, N. C. Ganguli and S. P. Sen, Fert. Technol. (India), 8 (3, 4) (1971) 207 - 210. G.A. E1-Shobaky, I. F. Hewaidy and Th. E1-Nabarawy, Surf. Technol., 12 (1981) 309. J. Moriyama, A. Yamaguchi and Y. Yamashita, Yoshihisa Shiyokama Shi, 15 (1965) 385. T. Iomoto, Y. Harano and Y. Nishi, Nippon Kagaku Zasshi, 86 (1965) 644. N. Sh. Petro, Ph.D. Thesis, Ain-Shams University, Cairo, 1974. -

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