Surface properties of Ni-Al mixed oxide catalysts

Surface Technology,

26 (1985) 235 - 244

235

SURFACE PROPERTIES OF Ni-Al MIXED OXIDE CATALYSTS G. A. EL-SHOBAKY and A. N. AL-NOAIMI Laboratory (Egypt)

of Surface Chemistry and Catalysis, National Research Centre, Dokki, Cairo

(Received January 2, 1985)

Summary A series of Ni-Al mixed oxides having different compositions was prepared by coprecipitation and by impregnation. The solids obtained were heated in air at 450 - 1000 “C. The surface properties, namely, SBET,S,, VP and F, of each adsorbent were determined from nitrogen adsorption isotherms. The change in SBET as a function of calcination temperature permitted the calculation of the apparent energy of activation of sintering. The results obtained revealed that the catalysts prepared by coprecipitation had higher surface areas than those obtained by impregnation. The mixed oxide solids seemed to be composed of wide pores. However, the pores of the adsorbents prepared by impregnation were narrower than those measured for the adsorbents prepared by coprecipitation. Also, the specific surface area depended on the NiO/Al,Os ratio. The presence of A1203 increased the degree of dispersion of NiO grains leading to a significant increase in its surface area, with effective retardation of sintering of the NiO phase. The apparent energy Es of activation of sintering indicated that the increase in alumina content was followed by a corresponding increase in the magnitude of E,.

1. Introduction The influence of the method of preparation on the porous structure of Ni-Al mixed oxides is of great importance since these properties determine the catalytic and adsorptive activities of these solids [ 11. The effects of chemical composition and thermal treatment on the surface and catalytic activity of binary NiO-Al,O, solids have been investigated by several authors [2 - 61. The addition of A1203, catalytically inactive for oxidation-reduction reactions, to NiO might improve its surface properties by increasing its degree of division thus retarding its sintering [ 2, 61. A study of the effects of the method of preparation and of the calcination conditions on the surface properties of NiO-A1203 mixed oxides is 0376-4583/85/$3.30

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236

reported in the present paper. The surface characteristics, namely, the specific surface area, total pore volume and mean pore radius, of various adsorbents were determined from nitrogen adsorption isotherms measured at --196 “C. The activation energy of sintering for different solids was calculated from the measured specific surface areas at different temperatures. 2. Experimental

details

2.1. Apparatus The specific surface areas of the various mixed oxides were determined from nitrogen adsorption isotherms at -196 “C using a conventional volumetric apparatus under a reduced pressure of lo-’ Torr. Before carrying out the measurements the samples were outgassed at 200 “C for 3 h. An X-ray investigation of various Ni-Al mixed oxide samples was made using a Philips PW 1051 diffractometer. The patterns were obtained using nickel-filtered copper radiation (h = 1.5405 A) at 36 kV and 16 mA with a scanning speed in 20 of 2’ min-‘. The differential thermal analysis (DTA) of Ni-Al mixed oxides was carried out using a Du Pont 990 thermal analyser with a differential scanning calorimeter cell. The rate of heating was 20 “C! min-’ and the sensitivity was 1 mV in-‘. 2.2. Ma teds Five Ni-Al mixed oxides of various compositions were prepared by the thermal decomposition of Ni-Al mixed hydroxides in air at various temperatures (450 - 1000 “C) for 4 h. The mixed hydroxide specimens were prepared by coprecipitation from the mixed nitrate solution using 0.1 M NH,OH at 70 “C. Additional mixed oxide samples were obtained by impregnating Al(OH)s with different amounts of Ni(NO& and heating in air at temperatures varying between 450 and 1000 “C. The amount of NiO in the various adsorbents was measured using atomic absorption techniques, the compositions of the solids prepared by these methods are given in Table 1. TABLE 1 Composition of different Ni-Al mixed oxide specimens Sample

Mode of preparation

NiO content (mol.%)

AIz03 content (mol.%)

Adsorbent composition

I II III IV V VI VII VIII

Coprecipitation Coprecipitation Coprecipitation Coprecipitation Coprecipitation Impregnation Impregnation Impregnation

13.79 25.93 37.89

86.21 74.07 62.11 39.17 26.32 86.21 74.07 51.02

0.16 0.35 0.61 1.46 2.8 0.16 0.35 0.96

60.83 73.68 13.79 25.93 48.98

NiO:1A1203 Ni0:1A1203 Ni0:1A1203 Ni0:1A1203 NiO:lAI203 Ni0:1A1203 Ni0:1A1203 NiO:1A1203

237

3. Results and discussion The DTA of pure and various mixed hydroxides revealed that the presence of Ni(OH)* together with Al(OH)s exerted a mutual effect on the decomposition of each hydroxide. Pure Ni(OH), decomposed at 330 “C while the samples mixed with A1(OH)3 decomposed at 360 “C. The main endothermic peak of the thermal decomposition of Al(OH)s had its maximum at 280 “C while that for the mixed hydroxide was shifted to 300 “C showing that hydrated aluminium oxide retarded the decomposition of nickel hydroxide. The X-ray investigation showed that the presence of NiO with A1,03 enhanced the degree of crystallinity of the r-AlzO, phase. However, the presence of A120s appreciably retarded the crystallization of the NiO phase. The thermal treatment of the mixed oxides in air at 900 “C led to the formation of NiA1204 spinel; its crystallinity was increased by increasing the calcination temperature from 900 “C!to 1000 “C. For these reasons, the calcination temperatures were chosen to be above the decomposition temperatures of the .mixed oxides (450 - 1000 “C). 3.1. Nitrogen adsorption on the thermal products of Ni-Al mixed hydroxides prepared by coprecipitation The adsorption isotherms of nitrogen were measured on Ni-Al mixed hydroxides calcined in air at temperatures varying between 450 “C and 1100 “C. These isotherms belong to type II of the Brunauer classification [ 71. Figures 1 and 2 show representative adsorption isotherms measured for

0

01

0

01

0

01

0.2

0.3

04

P/ d

Fig. 1. Nitrogen adsorption isotherms of sample I heated at 450, 550 and 650 “C.

238

0

0

0

m

0

01

0.2

0.3

OL P/P

Fig. 2. Nitrogen adsorption isotherms of sample I heated at 750, 850,lOOO and 1100 “C.

sample I. Adsorption of nitrogen is irreversible and exhibits hysteresis loops of different areas depending on the chemical composition and calcination temperatures. The specific surface areas of various adsorbents were calculated from the linear plots of the BET equation which was satisfactorily obeyed in a p/p0 range between 0.01 and 0.40. The data for SBETobtained are given in Table 2. Also included in Table 2 are the total pore volumes VPwhich were calculated using [ 81 VP= 15.47 x 10-4& where V,, is the volume of nitrogen adsorbed at saturation; and the mean pore radius T;of various adsorbents was also calculated using [ 9, lo] V r= 2.5 2

s

X lo4

The St values of the adsorbents were calculated from the slope of the VI-t plots. The C constant of the BET equation was calculated for different adsorbents and found to be in the range 26 - 125. So the t curves of de Boer [ll] were used. The VI-t plots, however, give an upward deviation in all cases which indicates wide pores. The VI--t plot of sample I is shown in Fig. 3. The comparison of S, and SBETshows that these values are very close to each other which justifies the choice of the t curve for the analysis, and also reveals the absence of ultramicropores. The thermal treatment of different adsorbents calcined in air at different temperatures (450 - 650 “C) caused a decrease in their adsorption capacities. Increasing the calcination temperature to 750 “C led to a slight

239

increase in the surface area of sample V and a slight decrease in the surface area of the other adsorbents. It can be seen from Table 2 that VP is increased by increasing the alumina content of different adsorbents heated at 450 “C. V, suffers a progressive decrease with increasing calcination temperature in the range 450 750 “C. Table 2 shows also that the mean pore radius of different adsorbents calcined at various temperatures is almost independent of their chemical composition. It can also be seen that F decreases, in most cases, on increasing the calcination temperatures from 450 to 550 “C, then increases with further increase in the calcination temperatures of the various adsorbents. The observed increase in F (widening of the pores) on increasing the calcination temperature to 750 “C contributed to the consequent decrease in their surface areas. The alumina in the Ni-Al mixed oxide catalysts plays the role of catalyst support through increasing the degree of dispersion of the solids, TABLE

2

Some surface coprecipitation Samples

characteristios and calcined Calcination temperature

of various at different

specimens of Ni-Al temperatures

SBET b2 g-‘1

St

mixed

oxides

b

(m2g-‘1

(ml

prepared

by

F

g-‘1

(‘Q

(“C) I

II

450 550 650 750 850 1000 1100

316 309 200 187 131 71 48

312.5 297.0 200.0 173.0 126.7 66.7 44.4

0.557 0.486 0.433 0.433 0.201 0.130 0.135

44.06 39.30 54.15 57.91 38.38 45.74 70.10

450 550 650 750

346 322 211 205

346.1 341.1 210.7 201.1

0.588 0.495 0.433 0.424

42.48 38.43 51.33 51.70

355 332 251 208 60.86

355.4 327.04 250.8 206.2 62.16

0.526 0.511 0.501 0.356 0.071

37.04 38.44 49.92 42.76 29.23

III

450 550 650 750 1000

IV

450 550 650 750

288 282 179 154

284.3 281.2 173.1 154.7

0.483 0.415 0.347 0.334

41.90 36.76 48.39 54.25

V

450 650 750

297 178 183

298.4 179 185.6

0.464 0.396 0.387

39.07 55.62 52.84

240

0

0

0

0

0

0

0

2

4

6 ttA1

Fig. 3. Volume-thickness 1100 “C.

curves of sample I heated at 450, 550,650,750,850,1000

and

which leads to an increase in the surface area. Furthermore, it resists grain growth of the adsorbents, as has been confirmed from the X-ray investigation, thus opposing the sintering of the supported catalyst. It may be expected that the dispersion effect of alumina in the mixed oxide adsorbents is proportional to the concentration of the supp’ort. On this basis, the greater the concentration of alumina the higher will be the surface area. It may be argued that at temperatures below 800 “C, the increase in NiO content would enhance a solid-solid interaction, especially in the outermost surface layers of the mixed oxides leading to the formation of a surface spinel. However, the X-ray diffraction technique revealed the presence of nickel aluminate spine1 in the case of Ni-Al mixed oxides heated at temperatures above 800 “c. 3.2. Nitrogen adsorption on Ni-Al mixed oxides prepared by impregnation Representative adsorption isotherms of nitrogen on sample VIII, heated at different temperatures (450 - 1000 “C), are shown in Fig. 4. The adsorption isotherms obtained lie between types I and II of the Brunauer classification [ 71. The nitrogen adsorption isotherms for sample VIII calcined at 450 “C and 650 “C are of type II. Adsorption is irreversible and exhibits a hysteresis loop closing at a relative vapour pressure of about 0.50. The initial regions (0.05 - 0.40) were found to obey the BET equation. The specific surface areas were calculated and the data obtained are given in Table 3. It can be seen that the SBET and VP values of all adsorbents heated at 550 “C are increased by increasing the alumina content of the various adsorbents. This signifies the role of alumina in increasing the degree of dis-

241

0

0

02

04

Fig. 4. Nitrogen adsorption isotherms of sample VIII heated at 550, 650 and 1000 “C. TABLE 3 Some surface characteristics of different specimens of NCAl mixed oxides prepared by impregnation and calcined at different temperatures Samples

Calcination temperature (“C)

SBET

St

F

(m* g-l)

(m* g-i)

21 g-*)

(A)

VI

450 550 650

269.28 248.55 178.48

264.0 237 173.3

0.2321 0.2119 0.2011

21.55 21.31 28.17

VII

450 550

256.78 219.06

240.0 200.0

0.2166 0.2243

21.09 25.60

550 650 1000

189.9 138.92 33.48

183.3 140.0 33.33

0.1609 0.1547 0.0325

21.18 27.84 24.26

VIII

persion of the supported NiO. Table 3 shows also that the mean pore radius P of various adsorbents is almost independent of their chemical composition and calcination temperature. The VI--t plot for nitrogen adsorption on the Ni-Al mixed oxides calcined at temperatures between 450 “C and 1000 “C gives an upward deviation in all cases which indicates wide pores. The representative VI-t plots of sample VIII are given in Fig. 5. It can be seen from Tables 2 and 3 that Ni-

242

1 0.2 }

Fig. 5. Volume-thickness

curves of sample VIII heated

at 550,

650 and 1000

“C.

Al mixed oxides prepared by coprecipitation have significantly higher surface area and total pore volume than those measured for the samples prepared by impregnation. 3.3. Sintering of mixed oxide adsorbents The apparent energies of activation of sintering were calculated using the method proposed by one of the authors [12, 131 from plots of the logarithm of surface area uersus l/T. Plots of log Saxr uersus l/T for different adsorbents prepared by coprecipitation and calcined at various temperatures are shown in Fig. 6. The slopes of these plots enable the calculation of the apparent energy of activation of the sintering process (E,). The data for E, are given in Table 4. These results indicate clearly that the increase in the Al,Os content of Ni-Al mixed oxide adsorbents leads to an increase in the activation energy of the sintering process to an extent proportional to the concentration of the support TABLE

4

The activation

energy

of sintering

Samples

Concentration AI203 (mol.%)

I II IV V

for various

of

adsorbent6

prepared

by coprecipitation

Activation energy E, (kcal mol-‘)

Range of temperature WI

86.21

10.05

750 - 1100

74.07 40.65 26.3

7.36 5.33 2.64

450 550 450

- 750 - 750 - 750

243

2.5 I 2.3i

2.1.

1.9

1.7.

I

1.4

1.3

1.2

1.1

1

0.9

0.8

0 * 7 163/T

Fig. 6. Variation of log SBET as a function of l/T for different adsorbents.

employed. The observed increase in the activation energy of sintering may reasonably be ascribed to the effect of A1203 in rendering the process of grain growth of NiO energetically more difficult. References 1 N. F. Krmolenko and N. Z. Efras, Zh. Fiz. Khim., S.S.S.R., 38 (1964) 1353. 2 A. M. Robinstein, Yu. Eltekova and K. J. Slovetzkaya, J. Chem. Phys., 33 (1959) 310. 3 S. Yoshitomi, K. Kodaka, Y. Morita and K. Yemoto, J. Chem. Sot. Jpn., 66 (1963) 1303. 4 M. Samaane and S. J. Teichner, Bull. Sot. Chim. France, 10 (1968) 1927, 1934, 1944. 6 D. Dollimore and T. E. Jones, J. Appl. Biotechnol., 23 (1973) 29. 6 G. Wendt, E. Fritsch and R. Scholiner, React. Kinet. Katal. Lett., 16 (1981) 137. 7 S. Brunauer, R. Mikhail and E. E. Bodor, J. Colloid Interface Sci., 24 (1967) 351. 8 J. C. P. Brokhoft and B. G. Linsen, in B. G. Linsen (ed.), Physical and Chemical Aspects of Adsorbents and Catalysts, Academic Press, London, 1970, p. 18.

244 9 A. V. Kiselev, Capillary Condensation

Heat Maxima. In D. H. Everett and F. S. Stone (eds.), The Structure and Properties of Porous Materials, Academic Press, London, 1958, p. 189. 10 A. V. Kiselev, Usp. Khim., SSSR, 14 (1945) 361. 11 B. C. Lippens, B. G. Linsen and J. H. de Boer, J. Catal., 3 (1964) 32. 12 G. A. El-Shobaky, I. F. Hewaidy and Th. El-Nabarawy, Surf. Technol., 12 (1981) 309. 13 G. A. El-Shobaky and N. Sh. Petro, Surf. Technol., 13 (1981) 197.