The influence of the Ni/Al ratio on the properties of coprecipitated Nickel/Alumina catalysts

THE INFLUENCE OF

THE Ni/Al RATIO

ON THE PROPERTIES

OF COPRECIPITATED

NICKEL/ALUMINA

CATALYSTS

TiTdiss

Paul de Korte

( « v V

THE INFLUENCE OF THE Ni/Al RATIO

ON THE PROPERTIES OF

COPRECIPITATED

NICKEL/ALUMINA CATALYSTS

PROEFSCHRIFT

TER VERKRIJGING VAN DE GRAAD VAN DOCTOR AAN DE TECHNISCHE UNIVERSITEIT DELFT,

OP GEZAG VAN DE RECTOR MAGNIFICUS, PROF. DR. J.M. DIRKEN, IN HET OPENBAAR TE VERDEDIGEN

TEN OVERSTAAN VAN EEN COMMISSIE AANGEWEZEN DOOR HET COLLEGE VAN DEKANEN,

OP DONDERDAG 9 JUNI 1988 TE 16.00 UUR DOOR

PAULUS HENRICUS MARIA DE KORTE

SCHEIKUNDIG DOCTORANDUS GEBOREN TE GOUDA

Dit proefschrift is goedgekeurd door de promotor

Prof. Dr. Ir. L.L. van Reijen

STELLINGEN

I

Burch en Warburton bepleiten het gebruik van pillared clays voor het demetalliseren van reslduale oliën op basis van activiteit, stabiliteit en regenereerbaarheid. Zij gaan hierbij ten onrechte voorbij aan het geringe porievolume en de dientengevolge geringe metaalopname-capaciteit van deze alternatieve verbindingen.

R. Burch & Cl. Warburton, Appl. Cat. 33. (1987) 395

II

Bij de verklaring van de door Nita et al. gevonden verbanden tussen temperatuur en activiteit voor residu conversie van zeolitische katalysa­ toren houden deze auteurs geen rekening met de mogelijkheid, dat adsorptie (desorptie) evenwichten van stikstofverbindingen hierbij een rol spelen.

K. Nita, S. Nakal, S. Hidaka, T. Hibuchi, H. Shimakawa, K. Ii & K. Inamura, Cat. Deact. 1987, B. Delmon and G.F. Froment, Eds., Elsevier, Amsterdam

(1987) 501

lil

Het invoeren van 56 correlatiefuncties door Tanaka et al., teneinde de waargenomen ionengeleiding in AgCrS„ beter te beschrijven, is niet zinvol.

T. Tanaka, M.A. Sawtarie, J.H. Barry, N.L. Sharma & C.H. Hunera, Phys. Rev. B 34 (1986) 3773

IV

De kinetische methode ontwikkeld door H.L. Friedman zou meer toegepast moeten worden voor de interpretatie van complexe thermo-analytische data.

V _V Arnoldy en Moulijn interpreteren een deel van hun TPR data van CoO/Al-0.

katalysatoren onjuist, doordat zij geen rekening houden met het feit dat de reductie van (cobalt deficient) cobalt aluminaat alleen bepaald wordt door cationdiffusie in het spinel rooster.

P. Arnoldy & J.A. Moulijn, J. CaCal. 93. (1985) 38

VI

De door Radovanovic voorgestelde procedure om de samenstelling van de onzuiverheidsfase in gesinterd MgO te berekenen, is, ondanks de aanbeveling van de onderzoeker, ongeschikt voor uit zeewater bereid MgO.

S. Radovanovic, Proc. 2nd. Int. Conf. Refractories, Tokyo, (1987) 813

VII

De door Nemudry et al. voorgestelde structuur van de gevormde lithium aluminium hydroxyde verbindingen.is niet in overeenstemming met de gepresenteerde röntgendiffractie gegevens.

A.P. Nemudry, V.P. Isupov, N.P. Kotsupalo & V.V. Boldyrev, React. Solids, I (1986) 221

VIII

In tegenstelling tot wat Kitamura et al. beweren, wijzen de hydroxyl bevattende vlak-defecten in het door hen onderzochte olivijn niet op een nieuw waterreservoir in de bovenste delen van de aardmantel.

M. Kitamura, S. Kondoh, N. Horimoto, G.H. Miller, G.R. Rossman & A. Putnis, Nature, 328 (1987) 143

IX

I t ' s a l l b e h i n d me now,

t h e work i s d o n e . . .

VOORWOORD

Wijn dank gaat uit naar allen die een bijdrage hebben geleverd aan de

totstandkoming van dit proefschrift. Enkelen van hen wil ik graag apart noemen:

Hennie Schaper, die ook na ons beider Delftse leven bleef fungeren als mijn belangrijkste "wetenschappelijke sparring-partner" .

Giel Doesburg, voor zijn steun, vooral in de beginfase van het onderzoek, en voor de vele nuttige discussies.

Cees de Winter en Antoon Roovers. Hun vele reductie- en methaniserings-metingen zijn een belangrijk onderdeel geworden van dit proefschrift.

Dick Amesz, voor de bereiding en (thermische) analyse van de sproeidroogpreparaten.

Ben Sonneville en IJsbrand Timmermans, voor hun assistentie bij de katalysatorbereiding en de thermische analyse.

Gerrit Hakvoort, voor de verleende faciliteiten op het gebied van de thermische analyse.

Hans Grondel, voor zijn technische adviezen en ondersteuning. De medewerkers van de instrumentmaker ij en de glasblazerij van de afdeling, voor, met name, de fraai uitgevoerde thermobalans.

Jaap Teunisse en Nico van Westen; zij stonden immer klaar voor het bepalen van de specifieke oppervlakken.

N.M. van der Pers en J.F. van Lent voor de röntgen analyses en J.P. Koot voor de chemische analyses.

Louis Bakker en dhr.J.C. Ruis voor het verzorgen van de meeste figuren in dit proefschrift en Fred Hammers voor zijn snelle en mooie fotowerk.

Verder dank ik mijn ex-collega's in Delft, met name hen uit de

katalysatorbereidingsgroep en de werkgroep dunne lagen, voor de plezierige werksfeer gedurende mijn vierjarig verblijf in de vakgroep, en mijn

huidige'collega's in Arnhem voor de interesse waarmee zij de totstand­ koming van dit proefschrift hebben gevolgd.

Mijn werkgever Billiton Research B.V. ben ik zeer erkentelijk voor de faciliteiten, mij geboden om dit proefschrift te voltooien.

Tenslotte ben ik natuurlijk de meeste dank verschuldigd aan jou Cora. Je geduld en begrip gedurende de lange (soms moeilijke) periode die nu achter

CONTENTS

1. INTRODUCTION 1 REFERENCES 4 2. COMPOSITION AND STRUCTURE OF DRIED AND CALCINED COPRECIPITATES

A literature reviev 5 2.1. INTRODUCTION 5 2.2. STRUCTURE OF THE DRIED PHASES 6

2.2.1. Nickel hydroxide 6 2.2.2. Aluminium hydroxide 6 2.2.3. Nickel aluminium hydroxy carbonate 6

2.3. NICKEL ALUMINIUM COPRECIPITATE, DRIED FORM 11

2.4. STRUCTURE OF CALCINED PHASES 13

2.4.1. Nickel oxide *~ 13

2.4.2. Nickel aluminate 14 2.4.3. Aluminium oxide 16

2.5. NICKEL ALUMINIUM COPRECIPITATE, CALCINED FORM 17

2.6. CONCLUDING REMARKS 18

2.7. REFERENCES 20 3. COMPOSITION AND STRUCTURE OF DRIED AND CALCINED COPRECIPITATES

Experiments with variation of the Ni/Al ratio 23

3.1. INTRODUCTION 23 3.2. EXPERIMENTAL 24

3.2.1. Sample preparation 24 3.2.2. Sample characterisation 26

3.3. RESULTS AND DISCUSSION 26

3.3.1. The dried coprecipitates 26 3.3.2. Coprecipitates, calcined at 600°C 37

3.3.3. Coprecipitates, calcined at 900°C 42

3.4. CONCLUSIONS 43

3.4.1. The dried coprecipitates 43 3.4.2. Coprecipitates, calcined at 600°C 44

4. REDUCTION BEHAVIOUR OF PURE AND SUPPORTED NICKEL OXIDE

A literature review 47 4.1. INTRODUCTION 47 4.2. PURE NICKEL OXIDE 48 4.3. SUPPORTED NICKEL OXIDE 51 4.4. NICKEL COMPOUNDS 55 4.5. CONCLUDING REMARKS 60 4.6. REFERENCES " 61 5. INFLUENCE OF THE Ni/Al RATIO ON THE REDUCTION BEHAVIOUR OF

CALCINED COPRECIPITATES 65 5.1. INTRODUCTION 65 5.2. INTERPRETATION OF KINETIC DATA 65

5.3. EXPERIMENTAL 70

5.3.1. Sample preparation 70 5.3.2. Thermogravimetrlc measurements 70

5.4. RESULTS AND DISCUSSION 71

5.4.2. Pure nickel oxide 71

5.4.2. Coprecipitates, calcined at 600°C 73

5.4.2.1. General 73 5.4.2.2. 90 mol% nickel 79

5.4.2.3. 82 and 64 mol% nickel 81 5.4.2.4. 50, 33 and 16 mol% nickel 82 5.4.2.5. 90, 64 and 16 mol% nickel, effect of

boiling treatment 83 5.4.2.6. Conclusions for the reduction mechanism 84

5.4.3. Coprecipitates, calcined at 900°C 90

5.4.3.1. General 90 5.4.3.2. Coprecipitates with Ni mol% > 33 93

5.4.3.3. Coprecipitates with Ni Mol% < 33 96

5.5. CONCLUSIONS 102

5.5.2. Pure nickel oxide 102

5.5.2. Coprecipitates, calcined at 600°C 102 5.5.3. Coprecipitates, calcined at 900°C 103

6. INFLUENCE OF THE Nl/Al RATIO ON THE SINTER STABILITY OF

REDUCED COPRECIPITATES 107 6.1. INTRODUCTION 107 6.2. EXPERIMENTAL 108

6.2.1. Pretreatments of the samples 108 6.2.2. Measurements of methanation activities 109

6.3. CHARACTERISATION BY X-RAY DIFFRACTION 111

6.3.1. Reduced samples 111 6.3.2. Sintered samples 113 6.4. METHANATION ACTIVITIES 116 6.5. CONCLUSIONS 122 6.6. REFERENCES 124 7. CONCLUDING REMARKS 125 REFERENCES 129 SUMMARY 131 SAMENVATTING 133

C H A P T E R 1 I N T R O D U C T I O N

About twelve years ago a cooperative research was started between a group at the Kern Forschungs Anlage (KFA) at Jülich (West Germany), a group at the University of Bradford (England) and our group at the University of Technology at Delft with the aim to develop a thermostable nickel/alumina methanation catalyst. The need for the thermostability emanated from the way the methanation was carried out in the "ADAM"-reactor of the_"Nukleare Fern Energie" (NFE) process in Jülich [1-3]. Due to economic considerations the reaction had to be performed at tempera­ tures equal to or higher than 600°C. Most conventional methanation cata­ lysts would readily sinter and hence deactivate at this temperature, especially in the presence of high partial steam pressures. A second requirement was that the same catalyst had to be sufficiently active to ignite the reaction at low temperatures (~300°C). Hence thermostable nickel catalysts that were used for steam reforming [4,5], were not applicable either.

The best alternative seemed to be the catalyst used for the steam reforming of naphta, a process developed by British Gas [6,7]. This type of nickel/alumina catalyst, made by co-precipitation, could still catalyse the methanation reaction in the process, although it had been exposed to relatively high temperatures (=500°C). Therefore it was decided to investigate the applicability of this type of catalyst for the high

temperature methanation reaction and it was agreed that our group in Delft would be concerned with the chemical aspects related to the preparation and characterisation of these catalysts.

Kruissink et al., who made a start with the project, investigated the influence of the various precipitation parameters on the composition of the coprecipitated catalysts in all subsequent stages [8-12] . According to these authors the main phase in the dried coprecipitate is a nickel aluminium basic compound with variable Ni/Al ratio, which crystallised in the same double layer structure as the mineral hydrotalcite (magnesium

aluminium basic carbonate). It was assumed that the alumina carrier is formed during calcination, as the mixed basic compound transforms into finely divided alumina and nickel oxide. The reduced coprecipitated catalyst was described "as consisting of porous alumina particles,

throughout which nickel crystallites are present, attached to the alumina surface".

The same authors succeeded in determining the dependence of the

methanation activity on the preparation parameters of the coprecipitated catalysts [8-12]. They found that the nickel oxide crystallite size is directly related to both the nickel surface area and the methanation activity. Furthermore, the nickel (oxide) crystallite size was shown to be affected by parameters like the nature of the anion present in the

coprecipitate, the calcination temperature and the degree of reduction. They also found that the presence of a small amount of sodium (0.5%(m/m))

is detrimental to the methanation activity of the catalyst. The influence of the nickel content on the nickel dispersion was found to be such that an increase of the former from 50% up to 75% results in a slight decrease in nickel dispersion. The nickel dispersion remains more or less constant for nickel contents lower than 50%. This latter effect was explained by assuming that "the nickel dispersion of the catalyst is determined by the nickel concentration in the nickel aluminium basic compound, rather than by the overall nickel concentration" .

A disadvantage of coprecipitated catalysts is the fact that the powder has to be converted to mechanically strong pellets and extrudates. This requires an extra, difficult synthesis step. However, Schaper et al.

[13-15] showed that this problem can be circumvented successfully, by incorporating the nickel aluminium basic carbonate within the pores of a preformed support by means of deposition precipitation.

In spite of the results obtained by Kruissink et al. some obscurities remained:

- a detailed description of the structure of the oxide phases formed after calcination at low temperature (<;600°C) was still lacking. - the reducibility of the coprecipitates was found to be very low,

behaviour with that of nickel oxide/a-alumina mixtures were not success­ ful. Although several possible' causes were suggested, a definite con­ clusion concerning the low reducibility could not be drawn on basis of the reduction measurements.

- another important aspect that was left for further research was the sinter stability of the coprecipitated catalysts under reaction conditions at temperatures between 600°C and 800°C.

In this thesis an attempt has been made to solve the above given problems. As can be concluded from the title of this thesis for each subject emphasis will be laid on the influence of the Ni/Al ratio on the investigated properties of the coprecipitated catalysts.

In chapter 3 the composition and structure of the dried and calcined

(600°C and 900°C) coprecipitates will be central. Mainly based on X-ray diffraction studies some new results will be given, obtained by examining the changes that take place in the composition, as the Ni/Al ratio is (gradually) changed. This chapter is preceded by chapter 2, which will

review the literature data reported on the structure and composition of the dried and calcined phases that are present in the coprecipitates. To facilitate the interpretation of the various (disordered) structures, possibly present in the coprecipitates, descriptions of the structures of related well-crystallised stoichiometric phases are also given.

Chapter 5 will give a detailed quantitative description of the reduction

behaviour of the coprecipitates calcined at 600°C and 900°C, studied by thermogravimetric analysis. It will be shown that by use of a special kinetic method certain sub-phases can be identified, which cannot be distinguished by means of X-ray diffraction. Since the reduction processes of the various nickel phases, present in the calcined coprecipitates, may be similar to those already reported for nickel oxide (unsupported and/or

supported), the literature data on this subject are reviewed in the preceding chapter 4.

In chapter 6 the sinter stability of the calcined and reduced coprecipi­

tates will be discussed. X-ray analysis as well as methanation activity measurements will be used to determine the change in composition and structure that has taken place during sintering under severe conditions.

The results presented in chapters 2-6 will be reviewed in chapter 7

REFERENCES

1. K. Kugeler, H.F. Niesen, M. Roth-Kamat, D. Boeker, B. Rüter and K.A. Theis, Nucl. Eng. Design 34 (1975) 65

2. B. Höhlein, Berichte der Kernforschungs Anlage, Jülich, Nr.1512 (1978)

3. H. Harms, B. Höhlein and A. Skov, Chem. Ing. Tech 52 (1980) 504 4. J.R. Rostrup-Nielsen,"Steam reforming catalysts", Teknisk Forlag

A/S, Copenhagen (1975)

5. J.R.H. Ross in "Surface and Defect Properties of Solids", Vol.IV, M.W. Roberts and J.M. Thomas Eds., Specialist Periodical Reports, Chem S o c , London (1975), 34

6. R.G. Cockerhaa, G. Percival and T.A. Yarwood, Inst. Gas. Eng. J. 5

(1965) 109

7. J.A. Lacey, "C.R.G.-based SNG: Principles and Process Routes", British Gas booklet (1971)

8. G. van Veen, E.C. Kruissink, E.B.M. Doesburg, J.R.H. Ross and L.L. van Reijen, React. Kin. Catal. Lett. £ (1978) 143 9. E.C. Kruissink, L.E. Alzamora, S. Orr, E.B.M. Doesburg,

L.L. van Reijen, J.R.H. Ross and G. van Veen, Prep. Cat. II, B. Delmon et al. Eds., Elsevier, Amsterdam (1979) 143

10. E.C. Kruissink, L.L. van Reijen and J.R.H. Ross, J. Chem. S o c , Faraday Trans. I, 21 (1981) 649, 665

11. E.C. Kruissink, H.L. Pelt, J.R.H. Ross and L.L. van Reijen, Appl. Cat. 1 (1981) 23

12. E.C. Kruissink, Thesis (Delft), 1981 13. H. Schaper, Thesis (Delft), 1984

14. H. Schaper, E.B.M. Doesburg, J.M.C. Quartel and L.L. van Reijen, Prep. Cat. Ill, G. Poncelet et al. Eds., Elsevier, Amsterdam (1983), 301 15. H. Schaper, D.J. Amesz, E.B.M. Doesburg, P.H.M, de Korte,

CHAPTER 2

COMPOSITION AND STRUCTURE OF DRIED AND CALCINED COPRECIPITATES

A literature review

2.1. INTRODUCTION

Catalysts prepared via coprecipitation are known to be very thermo­ stable [1,2]. This can be ascribed to the fact that the chemical inter­ action between support and catalytically active phase is more intense as compared to catalysts prepared otherwise. This special property is intro­ duced in the first preparation stage: during coprecipitation the precur­ sors of the alumina support and the nickel oxide are formed simultaneous­ ly, which gives rise to a very intense mixing of the aluminium and nickel ions. For certain Nl/Al ratios a combined single precursor compound is formed and in those cases the degree of mixing of both metal ions, being at atomic scale, is maximum.

Studies carried out in the past [2] indicate that the composition and structure of this particular precursor compound, nickel aluminium basic carbonate, is of crucial importance to the performance of the final catalyst. Hence a more detailed description of the structure of the mixed basic carbonate and of its decomposition product would be very helpful to get more insight into the characteristic properties of the final catalyst. Such a study is carried out for nickel aluminium coprecipitates as a function of the Ni/Al ratio and the results will be presented in the next chapter. To facilitate the interpretation of these results, in this chapter, a review is made of the literature data on the structure and composition of nickel aluminium coprecipitates. This is done for the dried and the calcined form separately. Each of these sections will be preceded by descriptions of the structures of the well-crystallised forms of the phases, that can be present in the samples studied.

2.2. STRUCTURE OF THE DRIED PHASES

2.2.1. Nickel hydroxide

Nickel hydroxide crystallises in the brucite (Mg(OH)„) structure. In this structure the hydroxyl ions form a hexagonal close-packing (stacking sequence ABABAB....). The nickel ions occupy the interstitial octahedral lattice sites in such a way that each two close-packed hydroxyl layers are alternately filled and empty.

2.2.2. Aluminium hydroxide

The structure of aluminium hydroxide resembles that of brucite. Because of the higher valency of aluminium one third of the cations is absent. Three different modifications are known of aluminium hydroxide, viz gibbsite (or hydrargillite), bayerite and nordstrandite. The difference between these three structures is related to the stacking sequences of the hydroxyl layers. Gibbsite has a slightly distorted monoclinic structure with a packing sequence according to ABBAABBA.... Bayerite has also a monoclinic structure, but instead of the head to tail arrangement observed for gibbsite, the packing sequence is hexagonal, according to ABABABA.... The structure of nordstrandite is related to that of gibbsite, but with a shift of the layers with respect to each other. Hence the stacking sequence is best presented by ABBCCAABBCCA For more detailed information concerning the structural characteristics of the aluminium hydroxides we refer to the work of Lippens [3] and van Dijk [4].

2.2.3. Nickel aluminium hydroxy carbonate

The structure of nickel aluminium hydroxy carbonate is similar to that of a group of mixed basic compounds of which magnesium aluminium hydroxy carbonate, the mineral hydrotalcite, is the best known member. Since a large number of minerals crystallise in this particular structure, its characteristics have been outlined in detail in the'literature [5-10]. Reviews on this subject are given by Allman [6] and Taylor [7]. A

[110]-A

(^3> &$)

V

brucite

layer

inter

layer

brucite

layer

inter

layer

brucite

layer

inter

layer

brucite

layer

O = metal Ions [Nr^.AI3*]

( ) = hydroxyl Ions

( J = Interlayer compounds [CO/~ ,H20]

like layers, accommodating both divalent and trivalent cations, are alter­ nated by interlayers comprising water molecules and anions. The latter are

intercalated to compensate for the positive net charge of the brucite layers, caused by the partial substitution of the divalent by trivalent cations. Two modifications are known of this structure differing only in hydroxyl layer stacking: a rhombohedral modification (AABBCCAABBCCA ) ,

of which the unit cell consists of three double layers, and a hexagonal one (ABBAABBA...), with two double layers.

The ratio of di- to trivalent cations in the brucite-like layers is variable and therefore usually the general formula of these mixed basic compounds is expressed as:

M2+(l-x)Me3+x<OH)2(An">x/n-yH2° ™

X.* u «2+ „ 2+ „.2+ „ 2+ „ 2+ „ 2 + „ 2 + „ 2 + in which M = Mg , Ni , Fe , Mn , Co , Zn , Cu

„ 3+ A13+ . 3+ „ 3 + „ 3 + „ 3 + . 3+

Me - Al , Cr , Fe , Ga , Mn , Co

and An" = CC>32", SO^2", N03", CrC^2", C104", Cl", Br", OH", . . .

Hence, a large number of di- and trivalent metal ions can be incor­ porated in the brucite-like layers, as long as they have approximately the

2+

same size as Mg . The anions in the interlayers are weakly bound to the hydroxyl ions via hydrogen bridges, and as a result they can be exchanged readily by virtually every negatively charged species. Reichle [9], for instance, recently reported on the synthesis of a magnesium/aluminium mixed compound in which the interlayers contain huge hydrocarbon ions

(1,12 dodecane-di-carboxylic acid di-anion). However, in most cases the exchange of inorganic ions is studied [10-12]. According to these studies there exists a pronounced preference for carbonate ions. Presumably this can be ascribed to the nearly flat structure of this ion, which enables a favourable interaction with the hydroxyl ions of the brucite layers.

The cell parameters of the structure of the mixed basic compounds depend on the size and ratio of the cations and on the size of the anions. For the mineral hydrotalcite a =.305 run and c =2.281 nm [61.

J o o L '

first value being the most observed one [6,7].

For synthetic samples the range of x often is broader, but the exact limits largely depend on the way the coprecipitate is treated. To obtain samples with a high degree of crystallinity, most mineralogists subject their samples to a hydrothermal treatment. For such well-crystallised compounds a range of 0.20<x<0.33 is most reported,

According to Brindley and Kikkawa [8] a highest value of x near 0.33 is quite understandable from a crystallographic point of view. For higher values the Me ions have to occupy adjacent sites, which seems unlikely to occur, unless it is accompanied by vacant cation sites. In that case units with a gibbsite/bayerite-like structure are introduced within a brucite layer. A lower limit of x corresponding to 0.17 can be explained on similar grounds. Lower values would imply the development of

unsubstituted parts in the brucite layers and hence would also give rise to a disordered structure.

For freshly prepared samples usually wider ranges of x are reported, but the limits depend on the nature of the cations and the applied preparation method. Anyway, it can be expected that the crystal structure of compounds for which x<0.20 or x>0.33 will be partially disordered.

At this point it has to be emphasised, that the above given arguments actually relate to short-range order. Values of x much higher than 0.33 may for instance be possible if only "long-range order" is considered, viz. if a systematic substitution of a part of the hydrotalcite double layers by un-charged bayerite/gibbsite layers is assumed to be possible. Likewise values of x lower than 0.17 would be conceivable, if groups of unsubstituted brucite layers would be present, alternated by hydrotalcite layers. In this respect it is quite interesting to refer to a publication of Mumpton et al. [13], in which the occurrence of two minerals,

coalingite and coalingite-K, is reported. Further examination of these minerals by Taylor et al. [14] revealed that their structures largely resemble that of sjögrenite (=magnesium iron hydroxy carbonate) but than with two, respectively three brucite-like layers on each site of every interlayer instead of one (see figure 2.2.).

As was mentioned earlier the nature of the cations has a large' effect on the range of x. Using divalent cations that normally are not coordinated

2+ 2+

octahedrally, like Cu and Zn , the range of x can be very small [15]. However, as was recently reported by Doesburg et al. [16], addition of

2+

small amounts of Mg ions stabilises such a double layer compound. Various claims have been made concerning the substitution of the magne­ sium or aluminium ions by much larger metal ions, but it is difficult to trace whether these attempts actually have been successful. In a recently granted patent, Miyata [17] claims to have synthesised a hydrotalcite compound, in which a part of the divalent magnesium ions are substituted by combinations of on the one hand other alkaline earth ions and on the other lead, cadmium or tin ions. Additional proof, however, that these large metal ions actually have been incorporated in the brucite-like layers was not given. Ross et al. [18] reported on the manufacture of nickel aluminium basic carbonates in which lanthanum ions substituted a part of the aluminium ions. Again convincing evidence of an actual substitution within the brucite-like layers was not given.

CO/"+H20 OH" MgJ ++Fe3 + OH" C032"+H20 OH" Mg2++Fe3+ OH" OH" Mg2 ++Fe3 + OH" . ■C032"+H20 Sjoegrenite & pyroaurite

B

fig.2.2. Schematic representation of layer sequences in (A) sjögrenite and pyroaurite, (B) coalingite, and (C) coalingite-K (suggested)

For the sake of completeness it has to be mentioned that there exists a particular modification of the hydrotalcite structure in which monovalent and trivalent metal ions occupy the octahedral sites in the brucite-like layers. However, up to now this deviating composition has only been observed for combinations of which the monovalent cations were Li ions

[19,20]. Apparently other monovalent metal ions are too large to be incorporated in the brucite layers.

The temperature at which the mixed basic compounds decompose depends on 2+ 3+

the nature of the cations and anions as well as on the Me /M ratio [2,5,21]. Usually between 150 °C and 250 °C the molecular water is lost. The total breakdown of the layer structure generally takes place at temperatures between 300 °C and 400 °C.

2.3. NICKEL ALUMINIUM COPRECIPITATE, DRIED FORM

In earlier studies carried out in our laboratory Kruissink et al. [2] found that the composition of the dried coprecipitate mainly depends on the nickel (or aluminium) content "of the starting metal salt (nitrate) solution. Using sodium carbonate as the precipitating agent it appeared that within the range of 1.0<Ni/Al<5.5 nickel aluminium basic carbonate

[Ni Al (OH) (CO.) ...nH.0] is formed as the only phase. For Ni/Al ratios higher than 5.5 (x<0.15) in addition nickel hydroxide forms, whereas for Ni/Al ratios lower than 1.0 (x>0.5) aluminium hydroxide is precipitated as the second phase. Upon hydrothermal treatment at 150 'C the above given range is narrowed to that for which the natural minerals exist, viz. 0.25< x<0.33, and the (oxy-)hydroxide of the excess cation is separated out.

.It appeared furthermore that the best results are obtained if precipita­ tion is carried out at a constant pH value of 7 and a temperature of 80"C. At lower pH values nitrate ions, introduced via the metal salt solution, are included in the interlayers instead of the carbonate ions, while at higher pH values the morphology of the coprecipitate becomes coarse giving rise to an increased contamination of sodium nitrate. In the latter case the sodium nitrate can only be washed out adequately during a second washing stage, applied to the dried coprecipitate.

The cell parameters of the hexagonal double layer structure are strongly influenced by the composition of the nickel aluminium basic carbonate. Actually this is the strongest indication for the occurrence of composi­ tions with variable Ni/Al ratios in the mixed basic carbonates. Kruissink found that the a -parameter increases with decreasing amount of aluminium ions, from .300 nm for x=0.5 up to .306 nm for x-0.15. This was ascribed

3+ 2+ to the smaller ionic radius of Al (53 pm) with respect to Ni (72 p m ) .

Further extrapolation to x=0 gives an a -parameter of 0.309 nm, which is considerably lower than the a -parameter of nickel hydroxide. This difference was explained in terms of a higher contraction of the a -para­ meter of the double-layer compounds with respect to the a -parameter of the brucite-type compounds.

A similar influence of the Al substitution was observed for the

c -parameter (from 2.265 nm for x=0.5 up to 2.391 nm for x=0.15). This was explained by assuming a stronger attraction between the two layers accord­ ing as their charge increases. If other interlayer ions are incorporated in the structure the c-axial length may become considerably larger. An exchange of carbonate ions by nitrate ions for instance results in an increase of the c -parameter of 17% for Ni/Al-3 (x=0.25). This large effect predominantly can be ascribed to the fact that besides the water molecules twice as many (monovalent) nitrate ions have to be accommodated in the interlayers to compensate for the same positive net charge of the brucite-like layers.

A comparison of the results reported by Kruissink et al. with the outcome of other studies on the coprecipitation of nickel aluminium basic carbonate is difficult. As was mentioned in the foregoing section in most investigations the samples are subjected to some sort of ageing treatment. This narrows the composition range considerably and therefore in the present case comparison with those results is irrelevant. Furthermore, the

influence of the applied preparation method is substantial. The structure of mixed basic carbonates with compositions outside the natural cationic range are inherently disordered and as a consequence very sensitive to variations of the precipitation parameters. Finally, these phases often have a low degree of crystallinity and as a result the usage of X-ray diffraction to analyse the structure may lead to erroneous conclusions.

Puxley et al. [22], for instance, claimed to have coprecipitated a mixed basic compound with a Ni/Al ratio of 0.5, of which the chemical composi­

tion closely agrees with the formula "Ni^NO.) (OH) 4A1(0H) 4H20". Their

X-ray powder data however, perfectly correspond with those reported by Kruissink et al. [2] for nickel aluminium basic nitrate, with the formula Ni_Al(0H),(N0_).4H„0. Since their coprecipitates are prepared with in-creasing pH, by addition of the basic solution to the metal salt solution, X-ray amorphous aluminium hydroxide may be formed [23] , that is not detectable with X-ray analyses. Moreover, since ammonium bicarbonate was used as precipitating agent this assumption is even more valid, as van Dijk [4] showed that formation of gelatinous (X-ray amorphous) aluminium hydroxide is considerably enhanced in the presence of ammonium ions.

2.4. STRUCTURE OF CALCINED PHASES

2.4.2. Nickel oxide

Nickel oxide crystallises in the cubic NaCl structure. The relative size of cations and anions is such, that it can be considered as a cubic

close-packed array of oxygen anions, with the nickel cations filling the appropriate octahedral interstices. Looking along the [111]-direction the layers of anions show the same configuration as in Ni(0H)„. However, as the cation/anion ratio is 1:1 instead of 1:2 Ni-ions are found between every two successive anion layers and not alternating as in the hydroxide. Due to the fact that the nickel ion radius is somewhat larger than would fit into an octahedral site, the lattice can gain more stability by a small distortion along the 3-fold axis. As a result the angle becomes 90°4* instead of 90°. Fired at high temperatures in an oxygen rich atmosphere NiO departs from the ideal stoichiometry, obtaining a light excess of oxygen. As a consequence cation vacancies are introduced, which are compensated by trivalent nickel ions. Such a deviation from stoichi­ ometry makes the oxide black instead of the light-green colour often observed in calcined coprecipitates.

2.4.2. Nickel aluminate

Nickel aluminate crystallises in the spinel structure. This structure, that is named after the mineral MgAl.O, (magnesium aluminate) has been investigated thoroughly [24-29]. In the ideal spinel structure the anions constitute a cubic close-packed lattice, similar to the oxygen ions in nickel oxide. In fig.2.3. a part of the unit cell is given. The whole cell contains 32 anions which form 64 tetrahedral and 32 octahedral inter­ stitial sites. The cations occupy one eighth of the tetrahedral and one half of the octahedral sites. The general formula of spinel is AB„0,, which means that the unit cell contains eight formula units.

For most spinel compounds the anion positions slightly deviate from those of the ideal spinel structure. This deviation is characterised by the oxygen parameter "u", which designates the oxygen displacement along the [111]-direction. This value is usually somewhat larger than .375, which is the value for the ideal spinel lattice, and as a result the tetrahedral sites are somewhat larger and the octahedral sites are a little smaller.

In most spinels the anion is oxygen, but spinel compounds with other anions are also known [28]. Concerning the cations, a large number of combinations are possible and the valency can range from +1 up to +6. In the following only oxygen spinels with di- and trivalent cations (the so-called 2-3 spinels) will be considered.

The sizes of the A and B cations do not differ widely and both can occupy tetrahedral and octahedral lattice sites. If square brackets are used to designate the cations situated at octahedral sites two extreme distributions can be defined, viz. A[B„]0, an B[AB]0,. The first one represents a "normal' spinel, for which all the trivalent metal ions occupy octahedral sites. The second formula refers to an ^inverse" spinel, in which all divalent cations and only half of the trivalent metal ions are situated at octahedral sites. In most cases the exact cation distribu­ tion is defined by an additional parameter "x", according to:

A B,, vfA-T ,B.- , . ]0. . This cation distribution appears to be a

x (1-x) (1-x) (l+x)J 4 vv

function of the temperature and the pressure and is governed by factors that are related to the crystal lattice, such as the lattice energy, the

Which factor predominates for a particular spinel depends largely on the nature of the cations [25-26,28].

The cation distribution of nickel aluminate was investigated by several authors [30-32]. Unlike most aluminium spinels, this compound is for about 75% inverted, corresponding to the formula: N iQ 2 5 A lo . 7 5 'N i0 . 7 5A ll . 2 5 ' ° 4 '

This inversion is ascribed to the excess CFSE value of octahedrally coordinated nickel (85 kJ/mol). At temperatures higher than 1400°C the entropy factor becomes more important and the distribution is forced towards a "random" spinel, corresponding to approximately 67% of the nickel ions at octahedral lattice sites.

O

2.4.3. Aluminium oxide

Besides the thermodynamically stable e»-Al„0,, a large number of

meta-stable crystal structures of aluminium oxide are known. Lippens [3] as well as van Dijk [4] classified them in low temperature (p, X, fj and 7) and high temperature (5, 8 and X ) aluminas. The first group is formed by

decomposition of precipitated hydroxides at temperatures lower than 800°C, whereas the high temperature aluminas are formed at temperatures between 800°C and 1200°C. At higher temperatures a-A1.0, is formed. Based on the studies of the aforementioned authors the structural characteristics of the various meta-stable aluminas can be summarised as follows:

- p- and X-alumina have highly disordered structures of which the position of the cations is unknown. They are formed at relatively low temperatures (<400°C).

- 7- and 7/-alumina crystallise in a defective spinel structure. One ninth of the cation sites which are normally occupied in spinels are vacant in this structure. Whether these vacancies are predominantly located at octahedral or at tetrahedral positions is not clear. Some authors [33,34] assume all vacancies to be situated in the tetrahedral sub-lattice. The difference between 7- and «j-alumina is related to the order in the anion sub-lattice, which appears to be lower for

ij-alumina.

- S-alumina has also a cation deficient spinel structure, but here the

vacancies are better ordered. 5-alumina is formed exclusively from 7-alumina.

- In 9-alumina the anions constitute a deformed cubic close-packed

sub-lattice, which is also found in 0-Ga„O-. The cation arrangement is distinctly different from that in spinels. The aluminium ions occupy octahedral and tetrahedral lattice sites in equal amounts.

- The structure of K'-alumina is not known in detail. The cation sub-lattice has a layer structure parallel to the close-packed anion layers. The cations in adjacent layers are alternately octahedrally or tetrahedrally coordinated.

- a-alumina, finally, is the most stable modification and crystallises in the corundum structure. The anions in this structure constitute a hexagonal close-packing: ABABAB... In this structure the aluminium

2.5. NICKEL ALUMINIUM COPRECIPITATE, CALCINED FORM

The structure of the calcined or oxidic form of the nickel aluminium co-precipitates eludes easy determination, as can be judged from the differ­ ent results reported in the literature [22,35-38]. Partly this can be ascribed to the fact that the oxide phases formed at low calcination temperatures often exhibit a poor crystallinity, making a reliable X-ray analysis rather difficult. A second reason for the observed dissimilari­ ties can be ascribed to differences in preparation conditions. As argued in section 2.3. variations in precipitation parameters affect the struc­ ture and composition of the final product markedly.

For the latter reason the results reported by Kruissink et al. [2] will also form the main basis of the short literature review in this section. These authors found that if coprecipitatlon is carried out as described in section 2.3., calcination between 300 'C and 600 °C results in the forma­ tion of two oxide phases, namely a nickel oxide phase containing dissolved aluminium ions and an alumina phase in which nickel ions are present. The latter phase, sometimes described as a solid solution of nickel aluminate and Tj-alumina (both have the spinel structure, see section 2.4.), is only detected for samples with nickel contents (=Ni/(Ni+Al)) lower than 0.50, coprecipitated at pH-7. Samples with similar nickel contents but

coprecipitated at pH=10, contain ^-alumina instead of a nickel deficient spinel phase. This latter feature was ascribed to the fact that the sam­ ples precipitated at pH=7 are much more reactive, being in a more finely divided state than those precipitated at pH=10. At nickel contents higher than 50 mol% no alumina phase is detected by means of X-ray analysis. Higher calcination temperatures (>800 °C) give rise to formation of both pure nickel oxide and nickel aluminate (spinel structure).

The dissimilarities observed between the results of Kruissink et al.[2] and those of other investigators mainly relate to the structure of samples calcined at low temperatures. Especially the presence of the nickel

deficient spinel phase appeared to be difficult to establish [35-38]. A short review on the structure of the oxide form of nickel aluminium

coprecipitates was recently given by Puxley et al [22] . Based on their own experiments these authors propose a model for the oxide form calcined at low temperatures. It is supposed to have a continuously variable

composition from almost pure nickel oxide at extremely high nickel content down to r)- or 7-alumina in the case that all nickel ions are substituted

by aluminium ions. With decreasing nickel content (-"increasing aluminium content) of the coprecipitate they name these intermediate stages: substi­ tuted nickel oxide (S.N.O.), disordered oxide-spinel intermediate

(D.O.S.I.)., spinel-like material (S.L.M.), nickel aluminate,7-alumina. According to the same authors such a continuous oxide phase would be possible since both nickel oxide and nickel aluminate contain a cubic close-packed lattice of oxide ions. However, such an assumption implies that the dried form of the coprecipitates also consists of one single phase, independent of the nickel (or aluminium) content. A theory, for which they have not given convincing evidence (see section 2.3.).

2.6. CONCLUDING REMARKS

Based on the literature data discussed in the foregoing sections the following conclusions can be drawn:

1. The hydrotalcite structure, that is adopted by nickel aluminium basic carbonate, consists of brucite-like layers, in which a part of the divalent metal ions is substituted by trivalent cations, alternated by interlayers, accommodating water molecules and anions. The latter are incorporated to compensate for the positive net charge of the brucite-like layers.

2. The cationic range for which synthetic compounds with the

hydrotalcite structure are formed as the only phase is usually much broader than the natural composition range, according to which the

2+ 3+

minerals crystallise (=2< M /Me < 3 ) . However, the limits of this "synthetic range" largely depend on the nature of the cations and the applied preparation method.

If the samples are treated hydrothermally the composition range is narrowed to the natural composition range.

3. Concerning the composition range of synthetic nickel aluminium basic carbonate little information can be obtained from the literature, since systematic studies on this subject are hardly available and moreover, most of the results are incomparable, owing to the differences in applied preparation method.

4. For a description of the composition of both the dried and calcined nickel aluminium basic carbonate we have to rely on the findings made earlier in our laboratory by Kruissink et al. [2]. These authors came to the following conclusions:

- Coprecipitation of mixed nickel/aluminium salt solutions with sodium carbonate and/or sodium hydroxide, at constant pH (7) and constant temperature (80°C) results in the formation of nickel aluminium basic carbonate, with the general formula

Ni Al (OH) (CO,) ,,.nH 0 with 0.15<x<0.50. For lower values of x additionally nickel hydroxide is formed and for higher values of x aluminium hydroxide is formed as a second phase.

- Calcination of the coprecipitates at temperatures between 300°C and 600°C results in the formation of a nickel oxide phase containing dissolved aluminium ions and an alumina phase possibly containing dissolved nickel ions.

- Calcination at temperatures higher than 800°C yields pure nickel oxide and nickel aluminate.

2.7. REFERENCES

1. J.R.H. Ross in "Surface and Defect Properties of solids", Vol IV., M.W. Roberts and J.M. Thomas Eds., Specialist Periodical Reports, Chemical Society, London (1975), 34

2. E.C. Kruissink, thesis (Delft) 1981;

E.C. Kruissink, L.E. Alzamora, S. Orr, E.B.M. Doesburg,

L.L. van Reijen, J.R.H. Ross and G. van Veen, Prep. Cat. II, B. Delmon et al. Eds., Elsevier, Amsterdam (1979) 143;

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

3. B.C. Lippens, thesis (1961) Delft 4. J. van Dijk, thesis (1976) Delft

5. M.C. Gastuche, G. Brown and M.M. Mortland, Clay Miner. 7 (1967) 177; G. Brown and M.C. Gastuche, ibid. 7 (1967) 193

6. R.Allman, Chimia 24 (1970) 99

7. H.F.W. Taylor, Mineral. Mag. 39 (1973) 377

8. G.W. Brindley & S. Kikkawa, Am. Mineral. 64 (1979) 836 9. W. Reichle, J. Cat. 94 (1985) 547

10. D.L. Bish, Bull. Mineral. 103 (1980) 170

11. G.W. Brindley and S. Kikkawa, Clays and Clay Miner. .28 (1980) 87 12. S. Miyata, ibid. 31 (1983) 305

13. F.A. Mumpton, H.W. Jaffe and C.S. Thompson, Am. Mineral. 50 (1965) 1893

14. J. Pastor-Rodriguez and H.F.W. Taylor, Mineral. Mag. 38 (1971) 286 15. P. Gherardi, O. Ruggeri, F. Trifiro, A. Vaccari, G. del Piero,

G. Manara and B. Notari, Prep. Cat. Ill, G. Poncelet et al. Eds., Elsevier, Amsterdam (1983) 723;

16. E.B.M. Doesburg, R.H. Höppener, B. de Koning, Xu Xiaoding and J.J.F. Scholten, Prep. Cat. IV, B. Delmon et al. Eds., Elsevier, Amsterdam (1987) 767

17. S. Miyata, EP Patent 0189899 (1986)

18. M.R. Gelsthorpe, B.C. Lippens, J.R.H. Ross and R.M. Sambrook, Proc. 9th Iberoamerican Symp. Cat., M.F. Portela Ed., Lisbon (1984), 1082; B.C. Lippens Jr., P. Fransen, J.G. van Ommen, R. Wijngaarden,

19. C.J. Serna, J.L. White and S.L. Hem, Clays and Clay Miner. 25 (1977) 384;

C.J. Serna, J.L. Rendon and J.E. Iglesias, ibid. 30 (1982) 180 20. M.A. Ulibarri, M.J. Hernandez, J. Cornejo and C.J. Serna, Mat. Chem.

Phys. 14 (1986) 569

21. S. Miyata & A. Okada, Clays and Clay Miner. 25 (1977) 14

22. D.C. Puxley, I.J. Kitchener, C. Komodromos and N.D. Parkyns, Prep. Cat. Ill, G. Poncelet et al. Eds., Elsevier, Amsterdam (1979) 89 23. R.J. Stol, thesis (1978) Utrecht

24. W.H. Bragg, Nature 95 (1915) 561

25. E.J.W. Verwey and E. Heilmann, J. Chem. Phys. 15 (1947) 174

26. E.W. Gorter, Philips Res. Reports 9 (1954) 295; Thesis (1954) Leiden 27. S. Hafner, Schweiz. Min. Petrogr. Mitt. 40 (1960) 207

28. G. Blasse, Philips Res. Rep. Suppl. No.3 (1964); Thesis (1964) Leiden 29. T.J. Gray, ch.4 in "High temperature oxides", part IV, Ed. A.M. Alper,

Academic Press, London (1971)

30. D. McClure, J. Phys. Chem. Solids 1 (1957), 311 31. F.C. Romeijn, Philips Res. Rep. 8 (1953) 304; 321

32. R.K. Datta and R. Roy, J. Amer. Ceram. Soc. 50 (1967) 578

33. H. Saalfeld and B. Mehrotra, Ber. Deut. Keram. Ges. 42 (1965) 161 34. L. Bragg, G.F. Claringbull and W.H. Taylor, Cryst. Struct, of Miner.,

Cornell Univ. Press (1965)

35. L.G. Simonova, V.A. Dzis'ko, M.S. Borisova, L.G. Karakchiev & L.P. Olen'kova, Kin. Cat. (Engl. Transl.) 14 (1973) 1380;

M.S. Borisova, V.A. Dzis'ko 6c L.G. Simonova, ibid. 15 (1974) 425, 667. 36. A.M. Rubinshtein, V.M. Akimov 6. L.D. Kretalova, Bull. Acad. Sci. USSR,

Div. Chem. Sci. (1958) 903

37. K. Rikhter, S.V. Ketchlk, L.G. Simonova & M.S. Borisova, Kin. Cat. (Engl. Transl.) 16 (1975), 1121

38. V.V. Levlna, V.Y. Danyushevskii, E.A. Boevskaya & V.I. Yakerson, Izv. Akad. Nauk. SSSR, Ser. Khim. (1975) 1003; Bull Acad. Sci. USSR, Chem. J. (1975) 918, 2533

CHAPTER 3

COMPOSITION AND STRUCTURE OF DRIED AND CALCINED COPRECIPITATES Experiments with variation of the Ni/Al ratio

3.1. INTRODUCTION

From previous studies carried out in our laboratory by Kruissink et al.[l] (see section 2.5.) it turned out that the composition and structure of the coprecipitates are very difficult to determine. Since both largely depend on the cation ratio, this determination may be improved by a study based on a wide range of Ni/Al ratios. From the summary of the data given in the foregoing chapter it can be concluded that up to now a detailed study on the influence of the cation ratio on the composition of the dried and calcined coprecipitate has not been reported in the literature. The only more detailed study was carried out by Kruissink et al. [1], but these authors did not scan the entire composition range step by step. It was therefore decided to carry out such a study and in this chapter the results will be presented.

For this purpose a number of coprecipitates with different Ni/Al ratios were prepared and after drying, calcined at two different temperatures, viz. 600°C and 900°C. The first temperature was chosen as a sort of compromise. According to the work of Kruissink et al. [1] it is the highest temperature for which phase separation has not yet taken place, whereas it is possibly the lowest temperature to get meaningful informa­ tion from the X-ray data. The temperature of 900°C was chosen to inves­ tigate to what extent spinel formation has occurred at such a relatively low temperature.

The X-ray diffraction work on the dried coprecipitates is supported by thermo-gravimetric analysis of the decomposition behaviour of these coprecipitates. In addition to the X-ray diffraction results of the products calcined at 600°C, the accessible surface areas are measured by means of the B.E.T.-method.

3.2. EXPERIMENTAL

3.2.1. Sample preparation

Samples with nickel contents (- Ni/(Ni+Al) * 100%) between 10 and 90% were prepared via coprecipitation according to the recipe given by Kruissink et al [1]: two aqueous solutions, one containing nickel and aluminium nitrate and the other containing sodium carbonate, both with

3

concentrations of 1.0 mol/dm , were added simultaneously to a beaker with a small amount of distilled water. The rate of addition of the salt

3

solution was 0.5 dm /hr, the rate of addition of the basic solution was varied in order to keep the pH of the suspension at 7. During the co-precipitation the suspension was well stirred and the temperature was maintained at 80°C. After the co-precipitation was completed, the sus­ pension was kept at 80°C for one hour, filtered over a glass filter (P4) and washed thoroughly with hot distilled water to remove any sodium ni­ trate contamination; afterwards the samples were dried overnight at 80°C.

Because some samples still contained a rather large amount of sodium nitrate, a part of each sample was boiled in distilled water for three hours after which they were filtered and dried as described above. This boiling treatment was applied for two reasons. In the first place It was assumed to be more effective for the removal of the sodium contamination than a second washing stage. Secondly we wanted to investigate if such a severe treatment would affect the composition' of the dried coprecipitate in a similar way as was observed for hydrothermal treatments (see section 2.3).

In the following sections the coprecipitates will be specified according to their atomic nickel percentage, viz. Ni/(Ni+Al) * 100%, determined by chemical analysis. Furthermore, the words "boiled" and "untreated" will be used to indicate whether the samples were boiled in distilled water after their drying stage or not.

The coprecipitates 'were calcined in air at two different temperatures, viz. 600 °C and 900 °C, in both cases during 16 hours. The heating rate being 2°C/min. Calcination was performed in a quartz tube placed in a vertically positioned tube furnace.

3.2.2. Sample characterisation

The dried as well as the calcined samples were characterised by X-ray diffraction, using an Enraf-Nonius Guinier-de Wolff camera, mark II. Diffractograms were recorded by means of a Siemens D 500-B goniometer. In both cases Cu-Ka radiation was used.

B.E.T.-surface area measurements were carried out by nitrogen adsorption at -196 °C.

Nickel, aluminium and sodium contents were determined by atomic absorption spectroscopy.

The decomposition of the coprecipitates was examined thermogravi-metrically, by heating the samples in air at a rate of 10°C/min. The thermobalance was a Stenton-Redcroft device, type TG-770.

3.3. RESULTS AND DISCUSSION

3.3.1. The dried coprecipitates

Table 3.1. gives the mol-percentages nickel and the weight percentages sodium of the untreated and boiled samples, calculated from the chemical analysis data. It can be seen that after this latter treatment the sodium content of all samples is below 0.10% (m/m).

X-jray diffraction

The composition of 14 coprecipitates with different nickel contents has been determined by means of X-ray diffraction. The main results are:

- Nickel aluminium basic carbonate is detected as the only compound for nickel contents between 55 and 85 mol%. In formula this means that

N i( l - x )A 1x( O H )2( C 03)x / 2n H2 °) e x l s t s f o r 0.15<x<0.45.

- For nickel contents lower than 55 mol% (x>0.45) in addition aluminium hydroxide is formed.

- For nickel contents higher than 85 mol% the composition is difficult to determine, due to the diffuse character of the X-ray diffraction patterns. Presumably nickel hydroxide is formed as a second phase.

Initial Ni mol% 10 14 16 21 25 28 33 40 50 55 60 66 75 80 90 90 Untreated samples Ni mol% 11 15 17 22 24 29 34 40 50 55 60 64 75 82 92 90 Na %m/m 0.04 0.03 0.01 0.04 0.05 1.00 0.85 0.04 0.15 0.03 0.02 0.04 0.22 0.04 1.85 0.03 Boiled Ni mol% -22 25 28 33 -50 -75 82 92 90 samples Na %m/m -<0.01 0.02 0.10 0.08 -0.02 -0.02 0.01 0.02 <0.01

table 3.1. Nickel mol contents and sodium mass contents of the dried coprecipitates, as determined by atomic absorption spectroscopy.

- Except for the samples with nickel contents higher than 85 mol% these results are not influenced by the boiling treatment.

In fig.3.1. the above given results are idealised in the form of a composition diagram. For comparison the narrowed composition range of x obtained for the coprecipltates after hydrothermal treatment is also shown (dotted lines).

Ni

*(mol7.)<

100 80 60 40 20

v Ni

_

Al

(OH)

(C0

)1

nH

0

//

Ni (0H)

\

20

80

100

» AI (mol%)

Fig.3.1. Composition diagram of the coprecipitate as a function of molair nickel content (aluminium content).

The dotted lines define the region of the basic carbonate for hydrothermally treated samples.

C.P.S. 1400 1 100 800 500 1.57 1.55 1.53 1.51 1.49 1.47 1.45 1.43 — d(A) 55 % Ni 1.57 1.55 1.53 1.51 1.49 1.47 1.45 1.43 C.P.S. 1700 1.57 1.55 1.53 1.51 1.49 1.47 1.45 1.43 :\ — d(A) 66 % Ni 1.57 1.55 1.53 1.51 1.49 1.47 1.45 1.43 d(A)

fig.3.2. X-ray diffraction patterns of samples with nickel contents between 50 mol% and 64 mol%, representing the [110] and [113] reflections of nickel aluminium hydroxy carbonate.

untreated samples boiled samples

The upper limit of x (0.45) is determined from the shift of the [110] reflection line in the diffraction pattern of the mixed basic carbonate (see section 2.4.). This is shown in fig.3.2., which gives the [110] and the [113] reflection lines of the X-ray pattern of nickel aluminium basic carbonate for four boiled and untreated coprecipitates with nickel

contents between 50 and 55 mol%. It can be clearly seen that the mentioned shift towards higher d-values starts at 55 mol% nickel.

This is somewhat higher than reported by Kruissink et al [1], who estimated this limit at 50 mol% nickel. However, this latter value was derived on the basis of interpolation over a wider gap in Ni/Al ratios. Fig.3.3. shows the dependence of a parameter on nickel content, reported by Kruissink. The results of the present study are included as well and show that they form a good fit with those found by Kruissink. However, the slope of the line representing the linear relationship between the

a -value (=2*[110]) and the nickel content has to be adjusted. In the new situation extrapolation to x .=1.0 gives a value of 0.311 nm (instead of 0.309). This agrees better with the literature value of nickel hydroxide, being 0.3125. The value of 0.311 for x —1.0 is also reported by Brindley and Kikkawa [2].

The lower limit of x is more difficult to establish, owing to the highly diffuse X-ray patterns of the samples with nickel contents higher than 85 mol%. Since the few vague lines in the X-ray pattern of the untreated sample with 90 mol% nickel only suggest the presence of a (disordered) mixed basic carbonate phase, it has been decided to estimate the lower limit of x at 0.15. This is in close agreement with the findings of Kruissink et al. [l](see also section 2.3.).

With respect to the foregoing discussion it should be emphasised that, as already was argued in section 2.2., the nickel aluminium basic carbonates with nickel contents lower than 66 mol% (x > 0.33) may have disordered structures, as a consequence of the fact that the number of aluminium ions becomes too large to get them sufficiently separated from each other. With decreasing nickel content this feature will become more apparent and at a certain nickel content parts of the brucite-like layers will be substituted by gibbsite/bayerite-like units. Likewise in mixed basic carbonates with nickel contents higher than 75 mol% (x<0.25) parts of the brucite-like layers may solely contain nickel ions.

t

(pm)

310

305

300

ih

40

60

80 100

Ni mol%

Fig.3.3. Dependence of a parameter on nickel content O hydroxy carbonates [ref.1]

A hydroxy nitrates [ref.1]

D solid solution of hydroxy carbonate

and hydroxy nitrate [ref.1] V hydroxy chloride [ref.1]

• hydroxy carbonates [this study] : this study

The aluminium hydroxide, formed at nickel contents lower than 50% is present as bayerite and/or nordstrandite. The intensity of the bayerite diffraction pattern is very strong for samples with nickel contents between 30 and 40 mol%; for lower nickel contents the intensity of this pattern rapidly decreases. The X-ray pattern of the nordstrandite phase is very vague and is only well perceptible for nickel contents lower than 30%. This trend is observed for the boiled as well as the untreated samples.

The high degree of crystallinity of the bayerite phase at relatively high nickel contents is quite remarkable and suggests that the formation of this phase is initiated by the presence of the mixed basic carbonate. It is however not entirely clear what mechanism underlies this initiation. It might be possible that both the nickel aluminium basic carbonate and the bayerite phase originate from one and the same precursor. Support for this assumption is provided by the work of Pakhomov et al. [3], who studied the formation of Zn/Al coprecipitates and observed such a bayerite-like precursor compound. Furthermore, it is known that for the

formation of hydrotalcite as well as bayerite, the aluminate ions (Al(OH), ) are the essential solution species [4,5]. Additional support for this "special-precursor" theory is provided by van Straten et al. [6], who studied the crystallisation of aluminium hydroxides from aluminate solutions. They observed that the formation of bayerite is strongly enhanced if lithium ions are present in the solution. Based on a series of precipitation experiments they convincingly showed that the mechanism of this enhancement involves the fast nucleation of a lithium-bearing precursor phase onto which bayerite rapidly grows. At high Li/Al ratios the precipitate appeared to contain in addition lithium aluminium hydroxide (LiAl„(0H)7.2H.0). This compound crystallises in the same

hydrotalcite structure [7] as nickel aluminium basic carbonate. Since the basic carbonates of sodium/aluminium and potassium/aluminium mixtures do not crystallise in this structure, it is interesting to note that van Straten et al. observed no acceleration of the growth of the bayerite phase in the presence of 8a or K .

With decreasing nickel content nucleation of the bayerite phase becomes more difficult, due to the reduced formation of a combined precursor compound and nordstrandite forms. It is not clear why for lower nickel

contents this latter modification of aluminium hydroxide is formed in preference to bayerite or gibbsite.

The additional formation of nickel hydroxide for nickel contents higher than 85 mol% can only be assumed. As argued before the X-ray patterns of the samples with such high nickel contents are too diffuse to obtain reliable information concerning the composition.

For the samples with nickel contents lower than 85 mol% the composition is not affected by the boiling treatment. Only a slightly higher degree of crystallinity is observed for the boiled samples as compared to the untreated samples. This means that either the size of the crystallites is larger or the internal disorder in the crystallites is reduced, for instance by a more uniform (re)distribution of the cations. Since the surface areas of the boiled and untreated samples do not differ signifi­ cantly, the latter possibility is the most plausible one.

For the samples with nickel contents higher than 85 mol% the boiling treatment does have influence on the composition. In fig.3.4. the X-ray photographs of the boiled and the untreated 82% and 90% samples are shown. It can be clearly seen that, apart from the slightly higher intensity, the pattern of the 82% sample is not affected. For the sample with 90 mol% nickel, however, the diffraction pattern indicates a change in

composition, after the boiling treatment. Two of the new lines in the X-ray pattern (at 0.46 and 0.27 run) suggest the formation of nickel hydroxide, but this assumption is questionable since two other strong

diffraction lines of this phase are absent. Another possibility may be that these lines belong to some kind of superstructure similar to coalingite or coalingite-K (see section 2.2), viz. an ordered mixture between nickel aluminium basic carbonate an nickel hydroxide. However, the

remaining part of the diffraction pattern is too diffuse to check the validity of this assumption.

Except for one very sharp line at 0.303 nm, present in the samples with sodium contents higher than 1.0 %(m/m) and indicating the presence of sodium nitrate, no other diffraction lines than those belonging to the phases described were observed.

Fig.3.4. Guinier photographs of four dried coprecipitates. a: 90 mol% - untreated b: 82 mol% - untreated c: 82 mol% - boiled d: 90 mol% - boiled Thermal decomposition

The composition of the coprecipitates as a function of the nickel content is also studied by following their decomposition in a thermo-balance. In figs.3.5-3.9. the decomposition curves of eight coprecip­ itates are presented. The decomposition curves of bayerite and nickel hydroxide are included for comparison. It should be emphasised that due to

the variable amount of physically bound water (<150°C) the curves cannot be interpreted quantitatively and therefore have to be regarded as "fingerprints" of the coprecipitates. For the sake of clearness only the differentiated weight-temperature (DTG) curve of each analysis is

presented. Since there are no significant differences with respect to the curves of the untreated samples only the decomposition curves of the boiled samples are presented.

The.characteristic decomposition peak of bayerite at 275°C, is present in the coprecipitates with nickel contents lower than 55 mol% nickel. The shoulder at the left side of this peak most probably originates from the decomposition of nordstrandite, since its size increases with decreasing nickel content. In the sample containing 55 mol% nickel the latter two peaks are virtually gone and the decomposition peaks of the mixed basic

T

r

dT

F i g . 3 . 5 . DTG-decomposition curves of b a y e r i t e and t h e c o p r e c i p i t a t e with 14 mol% n i c k e l . - I 1 1 1 16% Ni 24% Ni da dT

u

Fig.3.6. DTG-decomposition curves of the coprecipitates with 16 and 24 mol% nickel.

carbonate become more pronounced. Around 200°C the water molecules are expelled from the interlayers and at 350°C the remaining part of the interlayers (carbonate ions) decompose together with the brucite-like layers. The poor crystallinity of the mixed basic carbonate for nickel contents lower than 66 mol% is emphasised by the broad nature of the peaks. The thermograms of the samples with 66 and 75 mol% nickel represent the decomposition of well ordered basic carbonates. The peak around 350"C, which represents the decomposition of the carbonate and hydroxyl groups, consists of two separate sub-peaks for the sample with 66 mol% nickel. This phenomenon is also observed for Mg/Al samples by Miyata [8]. This author showed by means of chemical analysis of the outgassed samples, that after the first peak only part of the hydroxyl ions have disappeared, but a further clarification concerning this indirect decomposition was not given. In our opinion this two-stage decomposition is related to the fact that the total breakdown of the double layer structure results in the formation of a highly disordered oxide. Hence the onset of this transfor­ mation will be postponed to temperatures higher than in unsubstituted

da

cF

t

100 200 300 400

> T(°C)

Fig.3.7. DTG-decomposition curves of the coprecipitates with 40 and 55 mol% nickel.

n i i i i i i i

4 0 % Ni 6 5 % Ni

da

dT

100 200 300 400

> T(°C>

Fig.3.8. DTG-decomposition curves of the coprecipitates with 64 and 75 mol% nickel.

da dT

!

'

Fig.3.9. DTG-decomposition curves of the coprecipitate with 90 mol% nickel and of nickel hydroxide.

nickel hydroxide and only those hydroxyl ions will be expelled at the usual temperature, that have very little interaction with the aluminium and/or carbonate ions. According as the amount of aluminium ions decreases this postponement becomes less important and finally the mixed hydroxy carbonates with very low aluminium contents decompose at the same tempera­ ture as nickel hydroxide, as is shown in fig.3.9.

The results discussed above largely correspond to those obtained by means of X-ray diffraction. This is especially noticeable for the trend of increasing amount of bayerite and decreasing amount of nordstrandite with increasing nickel content. The fact that the aluminium hydroxide

decomposition peaks no longer are perceptible for the sample with 55 mol% nickel supports the conclusion that this nickel content corresponds with an upper limit of x (= Al mol%) is 0.45 for the mixed basic carbonate.

3.3.2. Coprecipitates, calcined at 600''C

Just as was observed for the dried samples with nickel contents lower than 85 mol%, the composition of the calcined samples is also unaffected by the boiling treatment. Since furthermore no differences were observed for the samples with 90 mol% nickel, this paragraph only deals with the boiled samples.

X-ray. diffraction

According to the X-ray results the coprecipitates calcined at 600°C consist of one or two oxide phases of which the concentration depends on the Ni/Al ratio of the sample. The diffraction lines of both phases are broad, indicating a low degree of crystallinity. They are situated in between the ASTM values of nickel oxide and nickel aluminate on the one hand and nickel aluminate and 7-alumina on the other. This is shown in fig.3.10., which gives a representative part of the diffractograms of eight boiled samples.

The diffraction line, that lies between the [220] reflection of nickel oxide and the [440] reflection of nickel aluminate is only detected for samples with a nickel content higher than 20%. With increasing nickel content this diffraction line shifts towards higher d-values and becomes

more and more pronounced. For samples with nickel contents equal to or higher than 85 mol* this reflection exactly coincides with the ASTM value of the [220] reflection of nickel oxide.

The diffraction line of the second oxide phase, that lies between the [440] reflections of nickel aluminate and 7-alumina is only perceptible for samples with nickel contents of 50% and lower and its intensity increases with decreasing nickel content.

By comparison of these results with the X-ray results of the dried coprecipitates (see also fig.3.4.) the following remarks can be made.

The oxide phase with reflections between those of nickel oxide and nickel aluminate must be the decomposition product of the mixed basic

440 440 220 Y A I203 NiAI204 NiO

mol

% Ni