Development and characterization of a thermostable nckel-alumina methanation catalyst

DEVELOPMENT AND CHARACTERIZATION

OF A THERMOSTABLE

NICKEL-ALUMINA

METHANATION CATALYST

HENNIE SCHAPER

DEVELOPMENT AND CHARACTERIZATION

OF A THERMOSTABLE

NICKEL-ALUMINA

METHANATHON CATALYST

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DEVELOPMENT AND CHARACTERIZATION

OF A THERMOSTABLE

NICKEL-ALUMINA

METHANATION CATALYST

ter verkrijging van de graad van

doctor in de technische wetenschappen

aan de Technische Hogeschool Delft

op gezag van de Rector Magnificus,

Prof.ir. B.P.Th. Veltman,

in het openbaar te verdedigen

ten overstaan van het College van Dekanen

op dinsdag 18 september 1984

PROEFSCHRIFT

te 16.00 uur

door

HENNIE SCHAPER

geboren te Enschede

scheikundig ingenieur

Dit proefschrift is goedgekeurd door de promotor

Prof.dr.ir. L.L van Reijen

This investigation was supported by the Netherlands Foundation for Chemical

Research (SON) with the financial aid from the Netherlands Organization for

Don't you miss it.' Don't you miss itJ

David Byrne

CONTENTS

Chapter I INTRODUCTION 1 Chapter II SYNTHESIS OF METHANATION CATALYSTS BY

DEPOSITION-PRECIPITATION 5

2.1. Introduction 5

2 . 2 . An examination of Ma Arthurs preparation method 8

2 . 2 . 1 . Introduction 8 2 . 2 . 2 . Experimental 9 2 . 2 . 3 . Results and discussion 9

2 . 3 . Deposition-precipitation on gamma alumina 10

2 . 3 . 1 . Experimental 10 2.3.2. Results and discussion 11

2 . 3 . 3 . Conclusion 12

2.4. The mechanism of deposition-precipitation 12

2 . 4 . 1 . Introduction 12 2 . 4 . 2 . Experimental 12 2 . 4 . 3 . Results and discussion 13

2.4.4. Conclusion 13

2 . 5 . Activity and stability 14

2 . 5 . 1 . Introduction 14 2.5.2. Experimental 16 2 . 5 . 3 . Results and discussion 18

2.5.4. Conclusions 20

2.6. Successive deposition-precipitation 20

2 . 6 . 1 . Introduction 20 2 . 6 . 2 . Experimental 20 2 . 6 . 3 . Results and discussion 21

2. 7. The influence of the support 22

2 . 7 . 1 . Introduction 22 2 . 7 . 2 . Experimental 24 2.7.3. Results and discussion 25

2.7.4. Conclusions 26

2.8. Scaling-up of deposition-precipitation 21

2 . 8 . 1 . Introduction 27 2 . 8 . 2 . Experimental 28 2.8.3. Results and discussion 29

2 . 8 . 4 . Conclusion 29

2.9. Conclusions 29 2.10. References 30

Chapter III SURFACE AREA LOSS OF GAMMA ALUMINA 33

3.1. Introduction 33 3.2. Development of a kinetic equation for the sintering

of high surface area oxides 34 3. 3. The kinetics of the sintering of gamma alumina 37

3 . 3 . 1 . Experimental 37 3 . 3 . 2 . Results and discussion 37

3 . 3 . 3 . Conclusions 39

3.4. The influence of additives 39

3 . 4 . 1 . Introduction 39 3 . 4 . 2 . Experimental 40 3 . 4 . 3 . Results and discussion 40

3.5. The sintering of lanthanum oxide promoted alumina in

air 41

3 . 5 . 1 . Experimental 41 3 . 5 . 2 . Results and discussion 41

3 . 5 . 3 . Conclusions 45

3.6. The sintering of lanthanum oxide promoted gamma

alumina in steam 45

3 . 6 . 1 . Introduction 45 3 . 6 . 2 . Experimental 46 3 . 6 . 3 . Results and discussion 46

3.6.4. Conclusion 47

3. 7. Identification of the lanthanum compound 48

3 . 7 . 1 . Introduction 48 3 . 7 . 2 . Experimental 49

3 . 7 . 3 . Results and discussion 49

3 . 7 . 4 . Conclusion 50

3.8. Conclusions 50 3.9. References 51

Chapter IV THE PHASE TRANSFORMATION OF GAMMA TO ALPHA ALUMINA 53

4.1. Introduction 53 4.2. Determination of kinetic parameters from

differential thermal analysis curves 54 4. 3. The kinetics of the phase transformation to alpha

alumina 58

4 . 3 . 1 . Experimental 58 4 . 3 . 2 . Results and discussion 58

4 . 3 . 3 . Conclusions 63

4.4. The influence of additives on the phase transformation 64

4 . 4 . 1 . Introduction 64 4 . 4 . 2 . Experimental 65 4 . 4 . 3 . Results and discussion 65

4.5. The influence of lanthanum oxide on the phase

transformation 65

4 . 5 . 1 . Experimental 65 4 . 5 . 2 . Results and discussion 65

4 . 5 . 3 . Conclusions 69

4.6. Conclusions 69 4. 7. References 69

Chapter V THE INFLUENCE OF LANTHANUM OXIDE ON NICKEL/ALUMINA

METHANATION CATALYSTS 73

5.1. Introduction 73

5 . 2 . Activity and selectivity 74

5 . 2 . 1 . Experimental 74 5 . 2 . 2 . Results and discussion 75

5.2.3. Conclusions 76

5 . 5 . Kinetics and mechanism 77

5 . 3 . 1 . Introduction 77 5 . 3 . 2 . Experimental 79 5.3.3. Results and discussion 79

5.4. Thermostability 84

5 . 4 . 1 . Introduction 84 5.4.2. Experimental 84 5.4.3. Results and discussion 85

5.4.4. Conclusion 85 5.5. Conclusions 86 5.6. References 86 Chapter VI CONCLUSIONS 89 SUMMARY 91 SAMENVATTING 93 NASCHRIFT 95 LIST OF PUBLICATIONS 97

Chapter I INTRODUCTION

In the f i e l d of heterogeneous catalysis much attention has been given to the conversion of synthesis gas (carbon monoxide/hydrogen mixtures). Depending on the catalyst used, the products range from methane and methanol to high molecular weight p a r a f f i n s , olefins and oxygenates. These reactions have in common that they can be used to produce synthetic fuels from the products of coal g a s i f i c a t i o n .

One of the reactions that are studied is methanation, the synthesis of methane from carbon monoxide and hydrogen:

CO + 3H2 •+ CH4 + H20, A H2 g 8 ^ = - 205 kJ/mol (1.1)

This process is mainly used for the production of substitute natural gas (SNG), but i t can also be applied in an energy transport system that i s being investigated in West Germany [ 1 - 9 ] .

In a power plant energy is released in the form of heat. Currently the energy is distributed amongst the consumers in the form of e l e c t r i c i t y .

However, the energy loss due to transportation via the high-tension network i s s i g n i f i c a n t . Therefore the overall efficiency from plant to consumer, i s rather low: less than 30% [10]. This has led to investigations into other means of energy transport. One p o s s i b i l i t y i s to use a chemical c i r c u i t : the heat released in the power plant is used for an endothermic chemical reaction; the products are transported to centres of energy consumption and the chemi-c a l l y stored heat is released by the reverse exothermichemi-c reachemi-ction. The advantage of this alternative energy transport system is a very high e f f i -ciency (up to 75%), especially when the heat released by the exothermic reaction is used for heating purposes.

In West Germany (KFA J ü l i c h ) such a system, the NFE process, also known as the 'Adam and Eve process', has been studied on p i l o t plant scale. The

reactions used in this process are steam reforming of methane

CH4 + H20 •* CO + 3 H2, A H2 9 8 K = + 205 kJ/mol (1.2)

and the reverse reaction, the exothermic methanation of carbon monoxide. The steam reforming is carried out in 'Eve' (E.V.A. = Einzelrohr Versuchs Anlage = single tube p i l o t plant) at 800°C over conventional nickel catalysts. The heat is supplied by a high temperature nuclear reactor; in the United States a similar system that uses solar energy is investigated [11]. The methanation section, nicknamed 'Adam' (A.D.A.M. = Anlage mit Drei Adiabaten Methanisie-rungsreaktoren = plant with three adiabatic methanation reactors), constitutes the main problem of the process: due to economic considerations the exit temperature of the methanation reactor must be as high as possible (600 -780°C). Under these circumstances, especially in the presence of steam (up to 15 bar partial pressure), conventional methanation catalysts tend to deacti-vate very rapidly, due to sintering. However, the catalyst used in this reactor must be s u f f i c i e n t l y active to ignite the reaction at much lower temperatures (250 - 350°C), even after prolonged exposure to these severe conditions.

The aim of the investigations presented in this thesis is to design a thermostable methanation catalyst. At f i r s t the major components of the c a t a l y s t , the active metal and the support, have to be selected. For reasons of a c t i v i t y , s e l e c t i v i t y and p r i c e , nickel is the obvious choice for the active component. Because the catalyst w i l l be exposed to high partial steam pressures at high temperatures, s i l i c a containing supports can not be used. This leaves alumina as the most logical choice of a support.

The next step is to select a preparation procedure for the nickel/ alumina c a t a l y s t . The common method of catalyst preparation is impregnation. Impregnated nickel/alumina c a t a l y s t s , which are for instance used in the removal of traces of carbon oxides present in the ammonia synthesis gas, are s u f f i c i e n t l y active at low temperatures, but they sinter very rapidly above 600°C. Therefore other preparation methods have to be considered. Kruissink et a l . [12-15] investigated the p o s s i b i l i t y to use coprecipitated nickel/alumina c a t a l y s t s , as used in the steam reforming of naphthas [16], for the methana-tion of carbon monoxide. They found that calcinamethana-tion and reducmethana-tion of nickel aluminium hydroxycarbonate results in very active and thermostable methanation c a t a l y s t s , due to the large interaction between nickel and aluminium ions during a l l preparation stages.

hydroxycarbonate as the precursor for the c a t a l y s t . However, the coprecipita-tion method has two disadvantages: scaling-up is problematic and an extra synthesis step is necessary to convert the powder obtained by coprecipitation to mechanically strong pellets or extrudates. This led to the conclusion that i t is necessary to incorporate the nickel aluminium hydroxycarbonate catalyst precursor within the pores of a strong preformed (commercial) support. The development of a method to accomplish this goal (deposition-precipitation) is the subject of chapter II of this thesis.

A common method to improve the thermostability of catalysts is to add a suitable promoter, preferably based on the knowledge of the mechanism of s i n t e r i n g . In our laboratory preliminary investigations on the sintering behaviour of coprecipitated nickel alumina catalysts were carried out [17,18], The results indicate, in accordance with work in other laboratories [19,20], that the sintering of nickel c r y s t a l l i t e s is probably caused by the sintering of the alumina support. Therefore i t was decided to investigate the mechanism of the sintering of gamma alumina and the accompanying phase transformation to alpha alumina. The knowledge of these mechanisms should allow a selection of a suitable promoter. These subjects are dealt with in chapters III and IV.

Chapter V presents the investigations on the influence of the promoter selected in chapters III and IV on the a c t i v i t y and thermostability of nickel/ alumina c a t a l y s t s , synthesized by the method developed in chapter II. The results of these investigations w i l l show whether the design of a thermostable methanation catalyst has been succesful. F i n a l l y , chapter VI gives a review of the most important conclusions and a survey of further applications.

References

1. H.W. Nürnberg & G. Wolff, Naturwiss. 63 [4] (1976) 190.

2. K. Kugeler, H.F. Niessen, M. Röth-Kamat, D. Bocker, B. Rüter & K.A. Theis, Nucl. Eng. Design 34 (1975) 65.

3. U. Boltendahl, H.F. Niessen & K.A. Theis, GWF - Gas/Erdgas 117 (1976) 517. 4. C.B. von der Decken & B. Höhlein, Erdöl und Kohle-Erdgas-Petrochemie 33_

(1980) 305.

5. H. Teggers, Ber. Bunsenges. Phys. Chem. 84 (1980) 1013.

6. H. Harms, B. Höhlein, E. Jörn & A. Skov, Oil & Gas J . (1980) 120. 7. H. Harms, B. Höhlein & A. Skov, Chem.-Ing.-Tech. 52 (1980) 504. 8. B. Höhlein, R. Menzer & J . Range, Appl. Catal. 1_ (1981) 125. 9. J . Range & H.G. Harms, Chemie-Technik 11 (1982) 894.

10. Ullmanns Encyklopädie der Technischen Chemie, 4e A u f l . , 14, p. 357-474, Verlag Chemie (Weinheim/Bergstrasse) 1978.

11. J.H. McCrary, 6.E. McCrary, T.A. Chubb, J . J . Nemeck & D.E. Simmons, Solar Energy 29 (1982) 141.

12. E.C. Kruissink, Thesis (Delft) 1981.

13. E.C. Kruissink, L.E. Alzamora, S. Orr, E.B.M. Doesburg, L.L. van Reijen, J.R.H. Ross & G. van Veen, Preparation of Catalysts II, B. Delmon et a l . Eds., E l s e v i e r , Amsterdam (1979) 143.

14. G. van Veen, E.C. Kruissink, E.B.M. Doesburg, J.R.H. Ross & L.L. van Reijen, React. Kinet. Catal. Lett. 9 (1978) 143.

15. E.C. Kruissink, L.L. van Reijen & J.R.H. Ross, J . Chem. S o c , Faraday Trans. I, 77 (1981) 649, 665.

16. R.G. Cockerham, G. Percival & T.A. Yarwood, Inst. Gas. Eng. J . 5 (1965) 109.

17. H. Windmeijer, Graduation Report (1980). 18. E. van der Heeft, Graduation Report (1981).

19. A. Williams, G.A. Butler & J . Hammonds, J . Catal. 24 (1972) 352. 20. B. Höhlein (KFA J ü l i c h ) , Private communication.

Chapter II SYNTHESIS OF METHANATION CATALYSTS BY DEPOSITION-PRECIPITATION

2.1. Introduction

Nickel/alumina catalysts can be synthesized in several ways. The most common procedure is impregnation. An aqueous solution of a nickel s a l t i s added to a preformed porous alumina support. The product is d r i e d , calcined and reduced. This is a quite simple preparation method, without scaling-up problems. Strong and rather active methanation catalysts with up to 15 wt % nickel may be obtained in this way. However, due to the small interaction between the highly dispersed nickel c r y s t a l l i t e s and the alumina support, the thermostability of these catalysts i s i n s u f f i c i e n t for use in the methanation reactor of the Adam and Eve process (see chapter I).

A t o t a l l y different procedure is coprecipitation. Under controlled pre-paration conditions a nickel aluminium hydroxycarbonate i s precipitated. This

'Feitknecht compound' [1,2] has the general formula: Ni, vA l ( 0 H ) , ( C 0 , )l v. n H90 with 1/4 < x < 1/3

Its structure, as shown in figure I I . 1 , consists of brucite (MgfOHJg) type layers separated by interlayers. The brucite layers are formed by a close packing of hydroxide anions, with nickel and aluminium cations distributed homogeneously over octahedral s i t e s . The interlayers consist of anions (carbonate, under some circumstances nitrate) and water molecules. Upon calcination at moderate temperatures (up to 700°C, to prevent NiAl formation) and reduction the f i n a l nickel/alumina catalyst is formed.

Previous investigations in our laboratory [3,4,5,6] have shown that nickel/alumina catalysts made by coprecipitation are very active and in spite of their very high nickel content (about 70 wt %) remarkably thermostable. This may be explained by the large interaction between nickel and aluminium

brucite

layer

inter

layer

brucite

layer

inter

layer

brucite

layer

inter

layer

brucite

layer

O

metal ions ( N i

, A l

)

hydroxyl i o n s

-interlayercompoundsCHjO^COg )

ions during a l l preparation stages. The nature of the reduced catalyst is s t i l l uncertain. Based on neutron d i f f r a c t i o n experiments of Wright et a l . [7], Puxley et a l . [8] suggest a paracrystal1ine nickel skeleton containing atomically dispersed alumina, comparable to the iron/alumina ammonia synthesis catalyst. Our opinion [9], supported by surface area measurements and nickel extraction experiments, is that small nickel c r y s t a l l i t e s (5 - 10 nm), which are probably paracrystalline, are dispersed on an amorphous alumina support. The results obtained thus far for coprecipitated nickel/alumina catalysts apply to powders prepared by laboratory methods. For industrial applications pellets or extrudates in the size range of several mm are required. Thus an extra preparation step is necessary in which a s u f f i c i e n t mechanical s t a b i l i t y of the product, not only under normal handling conditions, but also under the conditions of the reaction, should be obtained. Preliminary experiments [10] have shown that this is a d i f f i c u l t step. Therefore i t is desirable to i n -corporate the nickel aluminium hydroxycarbonate precursor within the pores of a preformed mechanically stable alumina support.

In an international cooperation between the universities of Bradford (UK) and Delft two possible synthesis routes for the incorporation of the F e i t -knecht compound within the pores of an alumina support were investigated. At Bradford, Ross et a l . [11] applied an impregnation/precipitation procedure. Porous alpha alumina extrudates were impregnated with a solution of nickel n i t r a t e , aluminium nitrate and urea. Upon heating at 100°C the urea decompo-ses, resulting in a homogeneous precipitation of a nickel aluminium hydroxy-s a l t within the porehydroxy-s of the alpha alumina hydroxy-support. Due to i t hydroxy-s low nickel content and the corresponding low a c t i v i t y per gram of c a t a l y s t , this catalyst has found a commercial application in steam reforming of naphthas rather than high temperature methanation. At Delft research was started on deposition-p r e c i deposition-p i t a t i o n . This deposition-predeposition-paration method was develodeposition-ped by Geus et a l . [12,13]. When a homogeneous precipitation is carried out in the presence of a high surface area support, the surface of the support w i l l provide nuclei" and the precipitate w i l l almost exclusively be formed within the pores of the support. A homogeneous precipitation can for instance be realized by a gradual change in pH (e.g. urea decomposition) or a gradual removal of a complexing agent (e.g. edta).

A very interesting result was found by Geus et a l . [12] for n i c k e l / s i l i c a catalysts made by deposition-precipitation. When the decomposition of urea is used, the s i l i c a support reacts with the precipitating nickel hydroxide and nickel hydrosi1icate is formed. This is caused by the low i n i t i a l pH of the s o l u t i o n , which activates the surface of the s i l i c a support. When alumina is

used instead of s i l i c a , the corresponding nickel aluminium hydroxysalt is not formed.

In view of these results for n i c k e l / s i l i c a i t seemed interesting to investigate deposition-precipitation on alumina using a solution with a high i n i t i a l pH. Under these circumstances i t is more l i k e l y that the alumina surface can be activated. A homogeneous precipitation starting at a high pH may be realized by the thermal decomposition of nickel ammino complexes. Such a method is claimed in a patent to McArthur [14] for the production of thermo-stable methanation catalysts using aluminium(oxy)hydroxide as a support. The aim of the investigations presented in this chapter i s to modify the prepara-tion procedure claimed in this patent so that preformed aluminium oxide may be used as a support.

2.2. An examination of McArthurs preparation method

2 . 2 . 1 . Introduction

In an United States patent McArthur [14] claims a novel method for the production of thermostable nickel/alumina methanation catalysts. This method involves digesting a slurry of aluminium(oxy)hydroxide in an aqueous solution of an ammino complex of nickel nitrate at a temperature s u f f i c i e n t l y high to decompose the complex and release the nickel ions. This results in the gradual precipitation of nickel hydroxide in the pores of the aluminium(oxy)hydroxide support. The product is f i l t e r e d , washed, d r i e d , calcined, extruded and reduced. The resulting catalysts are claimed to have a much higher degree of thermostability than conventional coprecipitated nickel/alumina c a t a l y s t s .

An obvious disadvantage of this method is that no preformed aluminium oxide supports can be used. According to McArthur aluminium oxide is not a suitable support, because i t is ' r e l a t i v e l y non-reactive with nickel hydroxi-de'. This statement implies that when aluminium(oxy)hydroxide is used, a chemical reaction occurs between the support and the precipitating nickel hydroxide. The most probable product is a nickel aluminium hydroxysalt (Feit-knecht compound). In this section we w i l l analyse the product of McArthurs preparation method, using both aluminium(oxy)hydroxides and aluminium oxide as a support.

2.2.2. Experimental

To an aqueous nickel nitrate s o l u t i o n , prepared by dissolving 20 grams of nickel nitrate hexahydrate in 20 ml of d i s t i l l e d water and 1 ml of concen-trated n i t r i c a c i d , 25 ml of concenconcen-trated ammonia were added slowly under vigorous s t i r r i n g . A l l chemicals used were of pro analysis q u a l i t y . The n i t r i c acid is added to prevent precipitation of either nickel hydroxide (when ammonia is added to nickel nitrate) or hexammine n i c k e l ( I l ) n i t r a t e (when nickel nitrate is added to ammonia).

Ten grams of support material were put into this s o l u t i o n . After waiting for 30 minutes, the solution was heated to 90°C, while an a i r flow was led through i t to obtain some s t i r r i n g . Mechanical s t i r r i n g could not be applied since this would destroy the preformed alumina support. The out-flowing gases were refluxed in a water-cooled condensor to avoid excessive water l o s s . The evaporation of ammonia results in a rapid decrease of the pH to a constant level of 7.5 + 0 . 5 . After a few hours a further decrease of the pH was observed and at that point the synthesis was stopped. The product was f i l t e -red, washed with 2 1 of hot d i s t i l l e d water and dried for one night at 80°C.

Three types of supports were used in this investigation:

A) bayerite (A^OH)^), prepared by precipitation of an aluminium nitrate solution with sodium carbonate at pH = 7, followed by f i l t r a t i o n , washing with 2 1 of hot d i s t i l l e d water and drying at 80°C for one night.

B) boehmite (A100H), a commercial f l u i d i z e d bed catalyst (Ketjen Alumina grade D).

C) gamma alumina, a commercial preformed catalyst support (Ketjen 000 - 1.5 E extrudates).

The products were characterized by X-ray d i f f r a c t i o n , using an Enraf Nonius Guinier-De Wolff camera mark II.

2 . 2 . 3 . Results and discussion

The results of the X-ray d i f f r a c t i o n analyses are presented in table I I . 1 . The Feitknecht compound (Nij Al (0H)2(C03), .nH20) was i d e n t i f i e d by

broad X-ray d i f f r a c t i o n lines at 0.80, 0.40 and 0.15 nm [3]. The f i r s t l i n e indicates that the carbonate ions are partly substituted by nitrate ions: for a pure carbonate compound d = 0.76 nm and for a pure n i t r a t e compound d = 0.89 nm [3]. These results show that when bayerite or boehmite is used, the p r e c i -pitating nickel hydroxide reacts with a part of the support to form nickel aluminium hydroxysalts. When alumina is used this reaction does not occur.

Table II. J Results of X-ray diffraction analyses

sample support phase composition of the product A B C bayerite boehmi te y-alumi na

bayerite, Feitknecht compound

boehmite, Ni(OH)2> Feitknecht compound y-alumina, Ni(OH),

As stated in section 2 . 1 . the aim of this investigation is to incorporate the nickel aluminium hydroxycarbonate catalyst precursor within the pores of a preformed mechanically stable support. Therefore the use of bayerite or boeh-mite is excluded. We w i l l try to modify this preparation method, so that aluminium oxide may be used as a support. In the next section we w i l l describe the effect of increasing the i n i t i a l pH of the s o l u t i o n , by adding sodium hydroxide. In our opinion this is the most promising method to activate the alumina surface. Furthermore i t was decided to s t i r with carbon dioxide i n -stead of a i r , to promote the incorporation of carbonate ions rather than nitrate ions. Previous investigations have shown that the decomposition of nitrate ions during calcination of nickel aluminium hydroxysalts may lead to sintering of nickel oxide c r y s t a l l i t e s [6].

2 . 3 . Deposition-precipitation on gamma alumina

2 . 3 . 1 . Experimental

An ammoniacal nickel nitrate solution was prepared as described in section 2 . 2 . 1 . The pH of this solution is approximately 9. A 2 M aqueous solution of sodium hydroxide was added very slowly to this solution until the pH was 10.5. Ten grams of preformed gamma alumina extrudates (Ketjen 000 - 1.5

2

E, S = 200 m / g , V = 0.5 ml/g) were put into this solution. After waiting for 30 minutes, during which time the pH decreased to 10.0, the solution was heated to 90°C, while carbon dioxide was led through i t . The remainder of the synthesis procedure was the same as described in section 2 . 2 . 1 .

The product was characterized by X-ray d i f f r a c t i o n , using an Enraf Nonius GuinierDe Wolff camera mark II. The carbonate content was analysed by d i s -solving the sample in phosphoric acid and t i t r a t i n g the C02 evolved. The

nitrate content was determined by reduction to NH3 > which was then t i t r a t e d

nickel and the sodium content. 2 . 3 . 2 . Results and discussion

The X-ray d i f f r a c t i o n pattern of the product shows lines of gamma alumina and a Feitknecht compound (d-values 0.766, 0.384 and 0.151 nm). This shows that the increase of the i n i t i a l pH of the solution makes i t possible to use gamma alumina as a support for McArthurs deposition-precipitation procedure. The f i r s t d-value of the d i f f r a c t i o n pattern indicates that mainly carbonate ions are incorporated in the nickel aluminium hydroxysalt structure. This i s confirmed by chemical analyses: carbonate content 3 wt % and nitrate content 0.05 wt Ï.

The value of the third d i f f r a c t i o n l i n e , corresponding to the (110) d i f -fraction of nickel aluminium hydroxycarbonate, can be used to determine the molar nickel/aluminium r a t i o . A comparison with the results of Kruissink [3] gives a ratio of 1.5 + 0 . 3 . The atomic adsorption spectroscopy results show that the product contains 14 wt X n i c k e l . From these data we can calculate that 10% of the alumina support has reacted to form nickel aluminium hydroxy-s a l t hydroxy-s . A BET hydroxy-surface area of 200 m /g impliehydroxy-s that the top molecular layer of the alumina corresponds to 20% of the bulk. Furthermore, since the nickel aluminium hydroxysalt product can be detected by X-ray d i f f r a c t i o n , i t has to be several layers t h i c k . Combining these facts we may conclude that only a small part of the surface reacts. This point w i l l be investigated in more detail in the next paragraph.

Calculations show that in order to get a product with 14 wt % n i c k e l , the pore volume has to be replaced four times. Assuming that the diffusion of ammonia molecules i s the rate determining step, we can calculate the reaction time:

*NH3 = DNH3/H20 ^

where (j)^ i s the molar ammonia flow, D ^Hy ^ Q i s the diffusion c o e f f i c i e n t of

ammonia in water (2 x 10 ^ m^/s, according to Arnold [15]), AC i s the concen-3 concen-3

tration gradient (5 x 10 mol/m ) and 1 i s the mean diffusion length. For 1 we may take the product of 25% of the extrudate diameter (2 mm) and the tortuo-s i t y factor (2), which givetortuo-s a value of 1 mm. With thetortuo-se data we can calculate

o

<t>;',u = 0.01 mol/m s , from which i t follows that the time needed to replace the

3 3 3 total pore volume i s 0.5 x 10 s . The total reaction time would be 2 x 10 s.

5 x 103 s.

F i n a l l y , i t should be mentioned that a possible disadvantage of this modified preparation method is the introduction of sodium ions into the solution. Kruissink et a l . [16] have shown that nickel/alumina catalysts made by coprecipitation, using sodium hydroxide or sodium carbonate, may be conta-minated by sodium oxide. This results in a large decrease of the methanation a c t i v i t y . Therefore i t is necessary to wash the products thoroughly. Atomic absorption spectroscopy measurements show that the sodium ion content of thoroughly washed deposition-precipitates is very low.

2.3.3. Conclusion

McArthurs [14] preparation method can be applied using alumina as a support, i f the i n i t i a l pH of the solution is increased to 10.5. The product consists of nickel aluminium hydroxycarbonate incorporated within the pores of the preformed gamma alumina catalyst support.

2.4. The mechanism of deposition-precipitation

2 . 4 . 1 . Introduction

The results presented in section 2.3. show that increasing the i n i t i a l pH of the solution 'activates' the alumina support. It was also shown that only a small part of the alumina surface is activated. A logical explanation is that pits are etched in the alumina surface by 0H~ ions. When the pH decreases the aluminium ions dissolved during the etching process w i l l precipitate as bayerite. It was shown in section 2.2. that bayerite may react with p r e c i p i -tating nickel hydroxide to form nickel aluminium hydroxysalts. Similar reac-tions were also observed when precipitated bayerite and magnesium hydroxide were mixed in their mother liquors at 80°C for one hour [17]. Obviously, these hydroxides are very reactive, due to dissolution/precipitation cycles, whereas the aluminium oxide is inreactive.

The model of pits being etched in the alumina surface was investigated by several experiments. These w i l l be discussed in this section.

2 . 4 . 2 . Experimental

A sample made by deposition-precipitation on gamma alumina was heated slowly in concentrated n i t r i c acid until a l l nickel compounds were dissolved

(75 min). A sample of the alumira support was treated in the same way. Both samples were washed with hot d i s t i l l e d water and dried for one night at 80°C. The BET surface areas were determined by nitrogen adsorption at - 196°C.

Scanning Electron Micrographs (magnification 10.000 x) were made of the gamma alumina support and of a sample made by deposition-precipitation, using a Jeol 200 C microscope. A 'model support' was made by anodizing aluminium f o i l . The product, a non porous alumina f i l m (30 nm t h i c k ) , was put into a sodium hydroxide/sodium carbonate solution of pH = 10.5 for 30 minutes, washed with hot d i s t i l l e d water and dried for one night at 80°C. This sample was analysed by transmission electron microscopy, using a P h i l i p s 201 TEM (magni-f i c a t i o n 33.000 x), scanning electron microscopy (magni(magni-fication 6.000 x) and X-ray fluorescence spectroscopy, using a Jeol 200 C microscope.

2 . 4 . 3 . Results and discussion

?

The BET surface area determinations give values of 200 m /g for the gamma 2

alumina support, 218 m /g for the alumina treated with n i t r i c acid and 236 2

m /g for the sample made by deposition-precipitation, treated with n i t r i c acid. This is in accordance with the model of pits etched in the alumina surface, since the model predicts an increase in surface area when the nickel aluminium hydroxycarbonate is dissolved.

The SEM photographs of the support and the sample made by deposition-precipitation show no differences. This is probably due to the r e l a t i v e l y small magnification of 10.000 x and the natural roughness of the support material. Therefore i t is much more interesting to investigate the effect of a treatment at pH = 10.5 on the non porous model support. The TEM photographs of the untreated alumina f i l m show only a plain grey layer, whereas the photo-graphs of the treated f i l m show that a roughening of the surface occurs (figure I I . 2 ) . The SEM photographs of this sample show that hemispherical particles have been formed. Microanalysis points out that these particles have the same composition as the f i l m , and do not consist of sodium compounds. An obvious explanation of these results is that part of the f i l m dissolves during the treatment at pH = 1 0 . 5 , followed by precipitation of aluminium hydroxide during the washing step.

2 . 4 . 4 . Conclusion

The results presented in this section favour the following model for deposition-precipitation: the treatment of the alumina support at pH = 10.5

Fig. II. 2 Transmission Electron Micrograph (magnification 33.000 x) of a non-porous alumina film, treated, at pH = 10.5

results in pits being etched in the alumira surface. During the subsequent decrease in pH the dissolved aluminium ions precipitate as bayerite. The bayerite reacts with the precipitating nickel hydroxide and nickel aluminium hydroxycarbonate is formed.

2.5. Activity and stability

2 . 5 . 1 . Introduction

In the previous sections we have developed a deposition-precipitation procedure that leads to the incorporation of the nickel aluminium hydroxy-carbonate catalyst precursor within the pores of a preformed gamma alumina support. In this section we w i l l compare the methanation a c t i v i t y and the thermostability of catalysts prepared in this way with conventional coprecipi-tated and impregnated samples.

consists of two steps. The f i r s t step is c a l c i n a t i o n , i . e . heating the precur-sor at a temperature s u f f i c i e n t l y high to decompose the anions (hydroxides, carbonates, n i t r a t e s ) . The second step is reduction, heating the calcined precursor in hydrogen until (nearly) a l l nickel oxide is reduced. A third step, passivation, i s necessary i f the reduced samples are to be investigated ex s i t u . As Kruissink [3] pointed out, this has the advantage that a l l analy-ses can be done on the same reduced and passivated samples. Passivation is carried out by slowly reoxidizing the surface layer(s) of the nickel c r y s t a l -1ites.

It is also possible to reduce the uncalcined precursor. Several authors [18-22] have shown for impregnated n i c k e l / s i l i c a catalysts that larger nickel surface areas may be obtained in that way. It was therefore decided to inves-tigate the effect of direct reduction on our c a t a l y s t s .

The determination of c a t a l y t i c a c t i v i t i e s is usually carried out by leading the reactants over the c a t a l y s t , followed by a quantitative analysis of the products. In the case of carbon monoxide methanation this can be accomplished by the use of a fixed bed flow reactor and gas chromatographic analysis [3]. As an a l t e r n a t i v e , d i f f e r e n t i a l scanning calorimetry (DSC) may be used [23,24]. In DSC different heat fluxes are supplied to a sample and an inert reference, so that both maintain the same temperature. The difference between these heat fluxes is recorded as a rate of heat flow (dQ/dt). If this difference is caused by a chemical reaction occurring in the sample, a simple relationship exists between reaction rate r (mol/s), reaction enthalpy AH (J/mol) and the rate of heat flow dQ/dt (J/s):

r = (dQ/dt)/AH (2.2) It is therefore possible to determine the reaction rate and the corresponding

c a t a l y t i c a c t i v i t y from a DSC s i g n a l . There are two important r e s t r i c t i o n s : the reaction enthalpy must be s u f f i c i e n t l y large and the s e l e c t i v i t y of the reaction must be nearly 100%. For the highly exothermic nickel catalyzed methanation of carbon monoxide these conditions are f u l f i l l e d . Furthermore Kruissink [3] has shown that a linear relationship exists between a c t i v i t i e s determined in a conventional flow reactor and by DSC. The main advantage of DSC is the short duration of the measurements, which allows a c t i v i t y determi-nations of up to five samples a day. Therefore i t was decided to determine a l l a c t i v i t i e s in this investigation by DSC.

To investigate the thermostability of the catalysts a standard sinter test has to be selected. The conditions of this test have to be severe enough

to cause a considerable deactivation, without completely k i l l i n g the cata-l y s t s . Precata-liminary investigations showed that this can be accompcata-lished by sintering for 20 hours at 800°C in a 30% steam/70% hydrogen mixture of atmospheric pressure.

2 . 5 . 2 . Experimental

The a c t i v i t y and thermostability of the following samples were deter-mined:

sample_A-l,

made by deposition-precipitation on a gamma alumina support (Ketjen 000 - 1.5 E extrudates), as described in section 2 . 3 . 1 . ; the sample was calcined at 450°C in a i r for 16 hours. Reduction was carried out at 600°C (heating rate 2°C/min) in a hydrogen flow (18 Nl/h) for 16 hours. The reduced sample was passivated at room temperature in a 5% oxygen/95% nitrogen mixture. sarriDle_A:2,

prepared in the same way as sample A - l , but the sample was not calcined before the reduction step.

sample_B-l,

made by coprecipitation of an aqueous solution of nickel nitrate (0.6 M) and aluminium nitrate (0.3 M) with sodium carbonate at pH = 7 and 80°C. The product (nickel aluminium hydroxycarbonate) was f i l t e r e d , washed with 2 1 of hot d i s t i l l e d water and dried at 80°C. Calcination, reduction and passivation were carried out as described for sample A - l .

sample B;2,

prepared in the same way as sample B - l , but the sample was not calcined before the reduction step.

sample_C,

made by impregnation to incipient wetness of a batch of 10 grams of gamma alumina extrudates (Ketjen 000 - 1.5 E) with an aqueous nickel nitrate solu-tion (1.5 M). The sample was dried at 80°C for one night. Calcinasolu-tion, reduc-tion and passivareduc-tion were carried out as described for sample A - l .

prepared by the special impregnation/precipitation procedure developed by Ross 2

et a l . [11]. A batch of 10 grams of alpha alumina (Dyson, = 1 m /g, pore volume = 0.3 ml/g) was evacuated and impregnated with 10 ml of an aqueous solution of nickel nitrate (0.10 M), aluminium nitrate (0.033 M) and urea (0.33 M) at 80°C. After 30 minutes the excess solution was removed and the urea was decomposed by increasing the temperature to 95°C. This results in the

precipitation of nickel aluminium hydroxynitrate (as determined by X-ray d i f f r a c t i o n ) in the pores of the alpha alumina support. After 16 hours at 95°C the sample was washed with 1 1 of hot d i s t i l l e d water and dried at 80°C. To increase the nickel content the whole procedure was repeated. C a l c i n a t i o n , reduction and passivation were carried out as described for sample A - l . sam2]e_D-2,

prepared in the same way as sample D - l , but the sample was not calcined before the reduction step.

The methanation a c t i v i t i e s of the reduced and passivated samples were determined by DSC (Du Pont Instruments 910). The samples (approximately 2 mg) were rereduced at 400°C in hydrogen. A baseline was recorded by heating sample and reference (2 mg a - A l203) from 180°C to 400°C at a rate of 10°C/min in pure

hydrogen. The same procedure was repeated in a 2% carbon monoxide/98% hydrogen flow (2 Nl/h). The resulting curve is shown in figure I I . 3 . The conversion at 250°C was determined as the r a t i o of the heat flow at 250°C and the heat flow at 400°C, where the conversion is 100%. If the heat flow at 250°C was outside of the k i n e t i c region defined in figure 11.3, the value was determined by extrapolation from a In (dQ/dt) versus T * plot within the kinetic region.

The c a t a l y t i c a c t i v i t y at 250 C was calculated by multiplying the conversion at 250°C and the carbon monoxide flow rate, devided by the weight of the catalyst. The a c t i v i t y per gram of nickel was also calculated, using nickel contents determined by atomic absorption spectroscopy.

To determine the thermostability of the c a t a l y s t s , reduced and passivated samples were sintered at 800°C for 20 hours in a 30% steam/70% hydrogen flow (18 Nl/h). This gas mixture was obtained by leading hydrogen through boiling water, followed by saturation at 70°C. Sintered samples were passivated in the

usual way and the methanation a c t i v i t y was determined. 2 . 5 . 3 . Results and discussion

The results of the DSC measurements are l i s t e d in table II.2.

Table II.2 Methanation activities before and after sintering

sample Ni content a c t i v i t y (250°C) a c t i v i t y (250°C) (wt %) before sintering after sintering

(mol C 0 / gc a t n) (mol C0/gN i h) (mol C 0 / gc a t h) (mol CO/g

A - l 16 0.15 0.94 0.075 0.47 A-2 16 0.23 1.44 0.075 0.47 B - l 70 0.54 0.77 0.11 0.16 B-2 70 0.47 0.67 0.10 0.15 C 4 0.025 0.63 0.000 0.00 D-l 7 0.040 0.57 0.010 0.14 D-2 7 0.055 0.79 0.011 0.15

When we compare the i n i t i a l a c t i v i t i e s , expressed per gram of n i c k e l , of the 'calcined samples' ( A - l , B - l , C and D - l ) , we observe that the preparation method does not have a very large influence. The sample made by deposition-precipitation (A-l) is somewhat more active than the others. However, very large differences are found for the a c t i v i t i e s after the sinter t e s t . The impregnated catalyst (C) has no a c t i v i t y l e f t at a l l , confirming the opinion that impregnation is not a suitable method to produce thermostable methanation c a t a l y s t s . The coprecipitated catalyst (B-l) and the sample made by the Ross/ Dyson procedure (D-l) show a large decrease in a c t i v i t y (75 - 80%). The sample made by deposition-precipitation loses only 50% of the i n i t i a l a c t i v i t y and is

therefore by far the most thermostable. This result is rather s u r p r i s i n g , since deposition-precipitation and coprecipitation lead to the same catalyst precursor: nickel aluminium hydroxycarbonate. The difference is not caused by diffusion l i m i t a t i o n of steam during the sinter t e s t , since extrudates and powdered extrudates show the same sintering behaviour. However, there is a very important difference between the two preparation methods: during deposi-t i o n - p r e c i p i deposi-t a deposi-t i o n deposi-the aluminium ions are provided by deposi-the alumina suppordeposi-t. Ideposi-t is therefore probable that a large interaction exists between the nickel aluminium hydroxycarbonate (and subsequently the nickeloxide/alumina and nickel/alumina mixtures) on one hand and the remaining alumina support on the other hand. This interaction could result in the observed increase in thermo-s t a b i l i t y .

The results in table II.2 show also that the i n i t i a l a c t i v i t y of cata-lysts made by deposition-precipitation ( A - l and A-2) or by the Ross/Dyson procedure (D-l and D-2) can be increased by omitting the calcination step. This i s not observed for the coprecipitated samples ( B l and B2). The f o l -lowing explanation may be offered: the nickel aluminium hydroxysalt that is formed during deposition-precipitation contains mainly carbonate ions, but also a small amount of nitrate ions. Upon calcination the nitrate ions de-compose, resulting in the presence of nitrogen oxide vapours in the pores of the extrudates. This may lead to sintering of nickel oxide c r y s t a l l i t e s [ 6 ] , probably via v o l a t i l e nickel-nitrogen-oxygen compounds. However, i f the de-composition of the nitrate ions occurs in a hydrogen flow, as is the case during reduction of uncalcined samples, the nitrogen oxide vapours may be re-duced to nitrogen or ammonia and water, thus preventing the sintering of nickel oxide. This would result in smaller nickel c r y s t a l l i t e s and higher a c t i v i t i e s . The Ross/Dyson sample contains mainly nitrate ions and is there-fore also more active when the calcination step is omitted. Since the copre-cipitated sample contains no nitrate ions we would expect l i t t l e or no influence of direct reduction, as confirmed by the experimental r e s u l t s . However, for a coprecipitate prepared at pH = 5 (which contains mainly n i t r a t e ions) we found a 502 increase of a c t i v i t y upon direct reduction.

It should be noted that the i n i t i a l a c t i v i t y may be increased by direct reduction, but that the a c t i v i t y after the sinter test is not influenced. This confirms the results of Doesburg et a l . [25], who showed that for the same precursor the nickel c r y s t a l l i t e size after sintering is independent of the i n i t i a l c r y s t a l l i t e s i z e .

2 . 5 . 4 . Conclusions

The results presented in this section show that nickel/alumina methana-tion catalysts made by deposimethana-tion-precipitamethana-tion are active and very thermo-stable. Omitting the calcination step results in an increase of the i n i t i a l a c t i v i t y , but does not change the a c t i v i t y after sintering.

2.6. Successive deposition-precipitation

2 . 6 . 1 . Introduction

As was shown in section 2 . 5 . deposition-precipitation on gamma alumina in the way developed in this investigation results in a nickel/alumina catalyst containing approximately 16 wt % n i c k e l . For some applications i t may be desi-rable to increase the nickel loading. For impregnated catalysts this is usually realized by successive impregnation steps, alternated by calcination steps. In this section we w i l l investigate whether a similar procedure can be applied to catalysts made by deposition-precipitation.

2.6.2. Experimental

In the f i r s t step deposition-precipitation was carried out as described in section 2 . 3 . 1 . , using 40 grams of gamma alumina (Ketjen 000 - 1.5 E extru-dates) as a support and the corresponding amounts of chemicals. The product was calcined at 450°C in a i r for 16 hours. For the second

deposition-precipi-tation step 75% of the calcined product of the f i r s t step was used as a support. For the third deposition-precipitation step 67% of the calcined product of the second step was used and for the f i n a l fourth step 50% of the calcined product of the third step. In each case the amounts of the chemicals were adjusted to the amount of the support.

A l l samples were calcined at 450°C in a i r , reduced at 600°C in hydrogen and passivated; the thermostability was tested by sintering at 800°C in 30% steam/70% hydrogen. The procedures described in section 2 . 5 . 2 . were used. The nickel content of the reduced samples was determined by atomic absorption spectroscopy. The methanation a c t i v i t y before and after sintering was measured by DSC, as described in section 2 . 5 . 2 . The reduction behaviour of calcined samples was determined in a thermobalance. The samples were dried at the calcination temperature until constant weight was reached, cooled down to 200°C and subsequently reduced in hydrogen in the temperature range 200

-1000°C. A linear heating rate of 6°C/min was applied. The weight versus temperature curve was differentiated to obtain a DTG ( d i f f e r e n t i a l thermo-gravimetry) curve. Four samples were investigated: two made by coprecipita-t i o n , as described in paragraph 2 . 5 . , concoprecipita-taining 16 wcoprecipita-t % and 33 wcoprecipita-t % n i c k e l ; one made by deposition-precipitation, containing 16 wt % nickel and one made by successive deposition-precipitation, containing 33 wt % n i c k e l . A l l samples were calcined at 600°C in a i r for 18 hours.

2.6.3. Results and discussion

Figure II.4 presents the methanation a c t i v i t i e s before and after the sinter test as a function of nickel content. The results show that i t is possible to increase the nickel content to 37 wt % by successive deposition-precipitations. However, the increase in methanation a c t i v i t y is not propor-tional to the increase in nickel content. After the sinter t e s t , the methana-tion a c t i v i t i e s of a l l samples, expressed per gram of c a t a l y s t , are even approximately the same.

s i n t e r t e s t

L | | — i 1 1 1 1 15 2 0 25 30 35

A M O U N T O F N i ( W T °/0)

Fig. II.4 Methanation activities (250°C) of samples made by successive deposition-precipitation

A possible explanation of these results is that only during the f i r s t step nickel aluminium hydroxycarbonate is formed and that the succeeding steps lead to precipitation of nickel hydroxide, because no 'active' alumina is available. That would explain the s l i g h t increase of methanation a c t i v i t y with nickel content: nickel obtained from nickel hydroxide w i l l be less active than nickel from nickel aluminium hydroxycarbonate. It also explains the results of the sinter t e s t : i t is probable that after sintering only the nickel from nickel aluminium hydroxycarbonate is s t i l l c a t a l y t i c a l l y active.

The X-ray d i f f r a c t i o n patterns of the samples after the second, third and fourth deposition-precipitation step show neither nickel aluminium hydroxy-carbonate nor nickel hydroxide l i n e s . This favours our explanation, since nickel hydroxide precipitates are often X-ray amorphous. The DTG curves

(figure II.5) show a similar reduction behaviour for the samples containing 16 wt % n i c k e l , as was to be expected. However, there is a large difference in

the reduction behaviour of the samples containing 33 wt % n i c k e l . The sample made by successive deposition-precipitation has two extra peaks, at 440°C and 750°C. The f i r s t one may be attributed to nickel oxide obtained by the decom-position of nickel hydroxide rather than nickel aluminium hydroxycarbonate. The extra peak at 750°C is caused by the reduction of nickel aluminate

(NiAlgO^). This is consistent with the observation that nickel aluminate formation is slower in samples obtained by the decomposition of nickel aluminium hydroxycarbonate than i n , for instance, impregnated samples [26]. Therefore the DTG curves confirm our opinion that successive deposition-pre-c i p i t a t i o n does not result in the formation of nideposition-pre-ckel aluminium hydroxy-carbonate.

2 . 6 . 4 . Conclusions

Successive deposition-precipitation leads to the precipitation of nickel hydroxide rather than nickel aluminium hydroxycarbonate, and does not increase the methanation a c t i v i t y after the sinter t e s t .

2 . 7 . The influence of the support

2 . 7 . 1 . Introduction

Thus f a r , only results have been reported for one type of support: gamma alumina extrudates. In this section we w i l l investigate other supports. At

200 360 520 680 840 1000

> T ( ° C )

Fig. II. 5 DTG curves of samples made by coprecipitation and (successive) deposi tion-precipitation

f i r s t the influence of the shape of the support ( f l u i d i z e d bed powder/extru-dates/pellets) is considered. Then we w i l l determine whether alpha alumina can be used as a support for this particular deposition-precipitation procedure. The mechanical strength of alpha alumina supports is much larger than that of gamma alumina. Alpha alumina is therefore a more attractive support material. The drawbacks of alpha alumina are a low surface area and usually a low pore volume. In this investigation a special high pore volume alpha alumina was used.

F i n a l l y , we w i l l investigate whether i t is possible to use p a r t i a l l y sintered gamma alumina supports for this deposition-precipitation method. If the gamma alumina support is precalcined at high temperatures (900 - 1100°C), the mechanical strength is increased by s i n t e r i n g . Furthermore, a more thermo-stable support i s obtained, which may result in more thermothermo-stable nickel/ alumina c a t a l y s t s . The obvious drawback of precalcination is that i t causes a substantial surface area loss of the support. Therefore deposition-precipita-t i o n , a surface process, may become more d i f f i c u l deposition-precipita-t or even impossible. 2 . 7 . 2 . Experimental

The standard deposition-precipitation procedure described in section 2 . 3 . was applied. The properties of the supports used in this investigation are

l i s t e d in table I I . 3 . Samples E - H were obtained by calcining sample D at the

Table II. 3 Properties of the supports

code origin shape pore volume surface area alumina (ml/g) (m2/g)

A Ketjen Alu-es powder 0.48 150 Y B Ketjen 000-3P pellets 0.52 166 Y C Dyson rashig rings 0.30 1 a D Ketjen 000-1.5 E extrudates 0.50 200 Y E D - 900°C i b i d . n.d. 110 e F D - 1000°C i b i d . n.d. 80 6 , G D - 1050°C i b i d . n.d. 45 e, H D - 1100°C i b i d . n.d. 8 a n.d. = not determined

indicated temperature in a i r for 17 hours. The surface areas of these samples were determined by measuring the nitrogen adsorption isotherm at - 196°C. The alumina phases were determined by X-ray d i f f r a c t i o n , using an Enraf-Nonius GuinierDe Wolff camera mark II. The other data in table II.3 are s p e c i f i c a -tions of the manufacturers.

The samples made by deposition-precipitation were characterized by X-ray d i f f r a c t i o n . The samples were d i r e c t l y reduced and passivated as described in section 2 . 5 , 2 . The nickel content was determined by atomic absorption spec-troscopy. The methanation a c t i v i t y of powdered samples was measured by DSC (section 2 . 5 . 2 . ) . The samples supported on D - H were sintered at 800°C in 30% steam/70% hydrogen (section 2 . 5 . 2 . ) , and the remaining methanation a c t i v i t y was determined.

2 . 7 . 3 . Results and discussion Ib§_§b§E§_2f-th§_support

Deposition-precipitation on f l u i d i z e d bed material (A) leads to the for-mation of a Feitknecht compound (nickel aluminium hydroxycarbonate), as shown by X-ray d i f f r a c t i o n . The nickel content of the reduced material is somewhat higher than for extrudates: 21 wt %. This is probably due to a lower amount of d i f f u s i o n - l i m i t a t i o n in the powdered material during deposition-precipitation. The methanation a c t i v i t y is proportional to the nickel content: 0.28 mol

C 0 /9Ca th

-When pellets (B) are used as a support, the X-ray d i f f r a c t i o n pattern again shows that a Feitknecht compound has been formed. However, the nickel content of the reduced material is only 5 wt %, with a corresponding low a c t i v i t y of 0.08 mol C 0 / gc a th . A cross-section through a reduced p e l l e t shows

that the nickel is concentrated in the outer layers of the p e l l e t . For extru-dates an almost homogeneous d i s t r i b u t i o n is observed. Obviously, there is a much larger d i f f u s i o n - l i m i t a t i o n for the deposition-precipitation process in pellets than in extrudates. The reason for this can be found in the different methods of preparation of these supports. P e l l e t i z i n g i s accomplished by forces perpendicular to the p e l l e t surface, resulting in a dense outer layer with very small pores [27]. During extrusion, the forces are parallel to the surface of the extrudates, so no densification of the outer layers occurs. These results show that pellets can only be used as a support for this de-p o s i t i o n - de-p r e c i de-p i t a t i o n de-process i f 'eggshell tyde-pe' catalysts are required (for processes with large gas diffusion l i m i t a t i o n ) .

Q§B2§I5ion=9recipitation_on_alpha_§1umina

The X-ray d i f f r a c t i o n pattern of a sample made by deposition-precipita-tion on alpha alumina (C) shows no nickel compound at a l l . The a c t i v i t y of the reduced sample, containing 1.6 wt % n i c k e l , was zero. Apparently alpha alumina is not a suitable support for this process, because of i t s low surface area. Attempts to 'activate' the alpha alumina by a five hour treatment at 100°C in a 1 M sodium hydroxide solution or a 1 M hydrochloric acid solution were un-succesful.

Br§£§l9lD§d_gamma_alumina_supports

X-ray d i f f r a c t i o n shows that in a l l samples (D - H) nickel aluminium hydroxycarbonate is formed. Figures II.6 and II.7 show the methanation a c t i v i -ties of these samples, before and after the sinter t e s t , as a function of the calcination temperature of the support and the surface area of the support, respectively. The nickel content decreases gradually with increasing c a l c i n a -tion temperature, from 17 wt % to 15 wt % at 1050°C (G). Precalcining the support at 1100°C (H) results in a much lower nickel loading of 7 wt %.

No data could be obtained for the effect of using precalcined alumina supports on the mechanical strength of the f i n a l catalysts.

These results show that i t is possible to precalcine the gamma alumina support at temperatures up to 1000°C without substantial (> 25%) loss of a c t i v i t y or s t a b i l i t y of the nickel/alumina c a t a l y s t s . When higher precalcina-tion temperatures are used, the properties of the catalysts become clearly i n f e r i o r , due to the large surface area loss of the support and the formation of inreactive alpha alumina.

At f i r s t sight i t is rather surprising that the use of precalcined sup-ports does not increase the thermostability of the nickel/alumina c a t a l y s t s . A logical explanation is that the thermostability of the nickel c r y s t a l l i t e s is mainly determined by the alumina obtained by the decomposition of the Feitknecht compound.

2.7.4. Conclusions

The investigations on the influence of the support lead to the following conclusions for the deposition-precipitation procedure:

1. Gamma alumina supports of a l l shapes can be used; however, the use of pellets leads to egg-shell type c a t a l y s t s .

2. Alpha alumina is not a suitable support.

0.3

-Fig. II.6 The influence of the calcination temperature of the support on ac-tivity (250 C) and stability of samples made by deposition-precipi-tation

this does not increase the thermostability of the c a t a l y s t .

2.8. Scaling-up of deposition-precipitation

2 . 8 . 1 . Introduction

Catalyst preparation is a science that is not very popular in academic c i r c l e s . That is understandable since i t is often considered an a r t , rather than a science [28]. The scaling-up of catalyst preparation i s even more neglected as a research subject. Unjustly so, since the development of a new catalyst preparation method is only valuable i f scaling-up i s possible.

It is therefore necessary to investigate whether the scaling-up of the deposition-precipitation procedure, developed in this chapter, is possible. Of course, due to limitations in equipment and financial resources, even the scaling-up experiments must s t i l l be on laboratory scale. However, i t i s our

> 0.3 -<

Fig. II. 7 The influence of the surface area of the support on activity

(250°C) and stability of samples made by deposition-precipitation

opinion that a scaling-up of a factor 100 (1 kg of support in stead of 10 g) should provide a suitable t e s t .

2 . 8 . 2 . Experimental

A c y l i n d r i c a l double-walled glass vessel of 10 1 was used. The bottom consists of a P 4 glass f i l t e r , the top is equipped with a water-cooled con-densor. The space between the double-walls i s connected with a thermostatic heater. A batch of 1 kg of gamma alumina extrudates (Ketjen 000 - 1.5 E) was placed on the f i l t e r . An ammoniacal nickel nitrate solution with pH = 10.5 was prepared as described in section 2 . 3 . 1 . , using 100 times the given amounts of chemicals and solutes. The solution was put into the vessel. After 30 minutes the temperature was increased to 90°C and carbon dioxide was led through the glass f i l t e r at a rate high enough to obtain some movement in the bed of extrudates.

After 100 hours the synthesis was complete as indicated by the pH. The product was f i l t e r e d , washed with 10 1 of hot d i s t i l l e d water and dried for

one night at 80°C. The dried product was analysed by X-ray d i f f r a c t i o n ; the nitrate content was determined as described in section 2 . 3 . 1 .

A 10 gram portion of the dried product was d i r e c t l y reduced at 600°C and passivated. The nickel content was determined. A sinter test at 800 C in 30% steam/70% hydrogen was applied. The methanation a c t i v i t y before and after sintering was determined by DSC. The procedures given in section 2 . 5 . 2 . were applied.

2.8.3. Results and discussion

The scaling-up of the deposition-precipitation procedure posed one problem: the deposition of a white s o l i d in the cooler. The s o l i d was i d e n t i -fied as ammonium carbonate, which is obviously formed by condensation of the amrnonia/carbon dioxide gas mixture. If the cooler is not properly designed, with wide passages, this may lead to clogging and eventually explosion.

The product of the scaling-up experiment is remarkably homogeneous in colour. The X-ray d i f f r a c t i o n pattern shows that the expected nickel aluminium hydroxycarbonate has been formed. The nitrate content is very low (0.07 wt %).

The reduced and passivated product contains 18 wt % n i c k e l . The methana-tion a c t i v i t y is 0.26 mol c° / 9c a t n before the sinter test and 0.09 mol CO/

gc a th after the sinter t e s t . These data are comparable to samples made by

deposition-precipitation on 10 grams of gamma alumina.

An outstanding difference between the scaling-up experiment and the usual procedure is the length of time of the reaction: 100 hours in stead of 4 hours. An appropriate cooler design would reduce this period. Preliminary experiments have shown, however, that a very rapid deposition-precipitation leads to a rather inhomogeneous product.

2.8.4. Conclusion

The deposition-precipitation procedure developed in this chapter can be scaled-up by a factor of 100 without substantial changes in the important properties of the c a t a l y s t s .

2.9. Conclusions

The most important conclusions that can be drawn from the results presen-ted in this chapter are:

1. A new deposition-precipitation procedure has been developed to incorporate nickel aluminium hydroxycarbonate within the pores of a preformed alumina support; therefore the main goal of this part of the investigation has been reached.

2. Gamma alumina and theta alumina are suitable supports; alpha alumina is not.

3. With this new method catalysts can be produced that are s l i g h t l y more active and much more thermostable than conventional nickel/alumina cata-lysts made by impregnation or coprecipitation.

4. Scaling-up of this new preparation method is not problematic.

2.10. References

1. W. Feitknecht, Helv. Chim. Acta 25^ (1942) 555. 2. R. Allmann, Chimica 24 (1970) 99.

3. E.C. Kruissink, Thesis (Delft) 1981.

4. G. van Veen, E.C. Kruissink, E.B.M. Doesburg, J.R.H. Ross & L.L. van Reijen, React. Kinet. Catal. Lett. 9 (1978) 143.

5. E.C. Kruissink, L.L. van Reijen & J . R . H . Ross, J . Chem. S o c , Faraday Trans. I, 77 (1981) 649, 665.

6. E.C. Kruissink, L.E. Alzamora, S. Orr, E.B.M. Doesburg, L.L. van Reijen, J . R . H . Ross and G. van Veen, Preparation of Catalysts II, B. Delmon et a l . Eds., E l s e v i e r , Amsterdam (1979) 143.

7. C.J. Wright, C G . Windsor & D.C. Puxley, J . Catal. 78 (1982) 257.

8. D.C. Puxley, I.J. Kitchener, C. Komodromos & N.D. Parkyns, Preparation of Catalysts III, G. Poncelet et a l . Eds., Elsevier, Amsterdam (1983) 237. 9. E.B.M. Doesburg, G. Hakvoort, H. Schaper & L.L. van Reijen, Appl. Catal.

7 (1983) 85.

10. E.B.M. Doesburg, unpublished r e s u l t s .

11. K.B. Mok, J . R . H . Ross & R.M. Sambrook, Preparation of Catalysts III, G. Poncelet et a l . Eds., Elsevier, Amsterdam (1983) 291.

12. J.A. van D i l l e n , J.W. Geus, L.A.M. Hermans & J . van der Meijden, Proc. Vlth Int. Congress on C a t a l y s i s , G.C. Bonds et a l . Eds., The Chemical Society, London (1977) 677.

13. J.W. Geus, Preparation of Catalysts III, G. Poncelet et a l . Eds., Else-v i e r , Amsterdam (1983) 1.

14. D.P. McArthur, U.S. Patent 4,042,532 (1977). 15. J . H . Arnold, J . Amer. Chem. Soc. 52 (1930) 3937.

16. E.C. Kruissink, H.L. P e l t , J.R.H. Ross & L.L. van Reijen, Appl. Catal. I (1981) 23.

17. E.B.M. Doesburg & B. Sonneville, Unpublished r e s u l t s . 18. G.C.A. Schuijt & L.L. van Reijen, Adv. Catal. 10 (1958) 242. 19. R. van Hardeveld & F. Hertog, Adv. Catal. 22 (1972) 75. 20. C.H. Bartholomew & R.J. Farrauto, J . Catal. 45 (1976) 41. 21. R. Burch & A.R. Flambard, J . Catal. 78 (1982) 389.

22. R. Burch & A.R. Flambard, Preparation of Catalysts III, G. Poncelet et a l . Eds., Elsevier, Amsterdam (1983) 311.

23. T. Beecroft, A.W. M i l l e r & J . R . H . Ross, J . Catal. 40 (1975) 281. 24. G. Hakvoort & L.L. van Reijen, 7th Int. Conf. Therm. A n a l . , Kingston

(1982) 1175.

25. E.B.M. Doesburg, P.H.M. de Korte, H. Schaper & L.L. van Reijen, Accepted for publication in Applied Catalysis (1984).

26. P.H.M. de Korte & L.L. van Reijen, Unpublished results.

27. R. Toei, M. Okazaki, K. Nakanishi, Y. Kondo, M. Hayashi & Y. Shiokazi, J . Chem. Eng. Japan 6 (1973) 50.

Chapter III SURFACE AREA LOSS OF GAMMA ALUMINA

3.1. Introduction

Coprecipitated nickel/alumina methanation catalysts deactivate during prolonged exposure to high temperatures (600 - 800 C), especially in the presence of steam. This is caused by a decrease of the nickel surface area due to sintering of the nickel c r y s t a l l i t e s . Under these circumstances also s i n -tering of the gamma alumina support occurs. The surface area loss of the sup-port causes a decrease in the s t a b i l i z i n g effect of the supsup-port on the highly dispersed nickel c r y s t a l l i t e s and may therefore contribute to the sintering of n i c k e l . There are indications in the l i t e r a t u r e [1-2] and from experiments in our laboratory [3-4] that indeed the surface area loss of the alumina support is one of the main reasons for the deactivation of coprecipitated nickel/ alumina catalysts.

The aim of the investigations presented in this thesis i s the development of a thermostable methanation c a t a l y s t . The usual procedure in c a t a l y t i c research to improve the thermostability of a catalyst is to add a suitable promoter, selected on a t r i a l and error basis. In this investigation the f o l -lowing approach is followed. We assume that a promoter that s t a b i l i z e s the gamma alumina support w i l l also s t a b i l i z e the nickel/alumina c a t a l y s t . If the • mechanism of the sintering of gamma alumina is known, a suitable promoter may be selected. It is therefore important to determine this mechanism.

Gamma alumina may loose surface area by two processes: the phase trans-formation to alpha alumina and s i n t e r i n g . The phase transtrans-formation w i l l be discussed separately in chapter IV. S i n t e r i n g , the rearrangement of material in order to reduce the surface energy, requires material transport in the s o l i d state. This may proceed via surface diffusion or volume diffusion of both aluminium and oxygen ions. According to Bevan et a l . [5] the ions in the surface of a s o l i d become mobile at temperatures exceeding 0.3 times the

melting-point. For alumina this corresponds to 420°C. Volume diffusion can only occur when bulk vacancies become mobile, at temperatures above 0.5 times the melting-point (the 'Tammann temperature'). For alumina this is 890°C.

The sintering of gamma alumina has been the subject of many publications [6-18]. In a l l cases the surface area was determined as a function of the temperature. L i t t l e research has been done on the kinetics or mechanism of this process. Without substantial evidence several authors [10,14,15,17,18] conclude that surface diffusion is the predominant mechanism, either because the pore volume is constant [14,15,18] or because a low value for the activation energy is calculated [10,17]. Levy and Bauer [10] derived a very s i m p l i -fied equation for the activation energy of s i n t e r i n g :

Ea = - RT In [(SQ - S ) / kQ] , or SQ - S = kQ exp - ^ (3.1)

in which S is the surface area after s i n t e r i n g , SQ is the i n i t i a l surface area

and kQ is the pre-exponential factor. Using this equation, values of 13 kJ/mol

[10] and 25 kJ/mol [17] were calculated. In the next section we w i l l try to derive a general kinetic equation for the sintering of high surface area oxides.

3.2. Development of a kinetic equation for the sintering of high surface area oxides

In order to derive a kinetic equation for the sintering of high surface area oxides the following assumptions are made:

(1) The oxide consists of uniform spherical c r y s t a l l i t e s (for gamma alumina this is a reasonable assumption).

(2) The loss of surface area is caused by the formation of necks between these c r y s t a l l i t e s .

(3) Sintering proceeds either via surface diffusion or volume d i f f u s i o n . In ceramic research much attention has been paid to the formation of necks between spherical grains. Kuczynski [19] derived a relationship between the radius of the neck (x), the time (t) and the temperature (T) for material transport by surface d i f f u s i o n . He used the model given in figure III. 1 * In the region of the neck the radius of the curvature p is small. This corres-ponds to a high surface energy. In order to decrease the surface energy the volume of the neck has to be increased. This leads to a large vacancy concen-tration in the neck, resulting in material transport to the neck. This can be

Fig. III..2 Model used in the development of a kinetic equation for the sin-tering of high surface area oxides

described by F i c k ' s law:

dV/dt = D1 A C * ( 3 - 2 )

in which V i s the volume and A i s the area of the neck, D' i s the vacancy diffusion coefficient and C i s the vacancy concentration gradient. Further-more, i f 5 i s the interatomic distance, we can w r i t e :

V = irx4/2a and A = 2nx6 (3.3)

For C * Kuczynski chooses the difference in vacancy concentration (AC) devided by the transport distance p . According to Kelvins equation we can write for

A C :

AC = ( 2 o 63/ k T ) ( p "1 - x"1) exp (- E /RT) (3.4) a

where a i s the surface tension and E= i s the activation energy of d i f f u s i o n .

a Now, for x >> p , integrating (3.4) leads to