Zirconia

ZIRCONIA

P R O E F S C H R I F T

TER V E R K R I J G I N G VAN DE G R A A D VAN D O C T O R IN DE T E C H N I S C H E W E T E N S C H A P P E N AAN DE T E C H N I S C H E H O G E S C H O O L DELFT, OP G E Z A G VAN DE R E C T O R M A G N I F I C U S IR. H. R. VAN N A U T A LEMKE, HOOG-LERAAR IN DE A F D E L I N G DER E L E K T R O T E C H N I E K , VOOR EEN COMMISSIE U I T DE SENAAT TE V E R D E D I G E N

OP D O N D E R D A G 4 MAART I 9 7 I TE I 6 U U R

DOOR

HENDRIK THEODORUS RIJNTEN scheikundig ingenieur

geboren te Arnhem

1971

DIT PROEFSCHRIFT IS GOEDGEKEURD DOOR DE PROMOTOR

Aan mijn ouders Aan Greetje

C O N T E N T S

Page

1 INTRODUCTION 9

2 LITERATURE SURVEY OF. ZIRCONIUM CHEMISTRY

2.1 General chemistry of zirconium i l 2.2 Zirconyl chloride solutions 12

2.2.1 Structure of ZrOCU.SHaO 14 2.2.2 Hydrolysis and polymerization in ZrOCh solutions 14

2.2.3 The anions in ZrOCU solutions 16 2.2.4 The charge of the zirconyl species in solutions 16

3 STUDIES ON HYDROLYSIS AND POLYMERIZATION

3.1 Formation of zirconyl solutions from zirconium salts 18

3.2 The hydrolysis of ZrOCU solutions 18 3.3 Polymerization in ZrOCU solutions 20 4 CONDUCTIVITY MEASUREMENTS IN Z r O C b SOLUTIONS

4.1 Experimental 22 4.2 Influence of concentration on hydrolysis of ZrOClj solutions 22

4.3 Influence of neutralization on hydrolysis and polymerization of Z r O C h solutions . . 25

4.4 Neutralization of heated Z r O C h solutions 29 4.5 Influence of neutralization and heating on percentage of bridged zirconium ions . . 29

4.6 A three-dimensional picture of the polymerization 32 4.7 The polymerization of group iv elements 36

4.8 Summary 37 5 PREPARATION OF HYDROUS ZIRCONIA

5.1 Method I 38 5.2 Methods 11 and iia 38 5.3 Method III 40 5.4 Method IV 40 6 CHARACTERIZATION OF HYDROUS ZIRCONIA

6.1 Differential thermal analysis 42 6.2 Density measurements 44 6.3 Dehydration of Zr(OH)4 46 6.4 Nitrogen adsorption 48 6.5 Crystallization of zirconia gels 49

7 PREPARATION OF ZrO(OH)2 FROM DECOMPOSITION PRODUCT OF ZrOCh.SHiO

7.1 Dehydration of ZrOCU.SHzO 55 7.2 Decomposition of ZrOCh.4H20 ' . . . . 57

7.3 Stability of Zr(OH)4 and related compounds 59 8 THE CRYSTAL STRUCTURES OF ZIRCONIA

8.1 Crystal Structure of Baddeleyite (monoclinic ZrOi) 61

8.2 Crystal structure of tetragonal zirconia 63 8.3 Crystal growth and sintering of M-type ZrOj 64 8.4 Occurrence of low-temperature tetragonal monoclinic transformation . . . . 65

Page 8.5 Explanation of low-temperature tetragonal monoclinic transformation . . . . 67

8.6 The mechanism of the tetragonal monoclinic transformation 70 8.7 Infrared spectra of monoclinic and tetragonal Zr02 71

8.8 Summary and conclusions 73 9 THE SINTERING OF ZIRCONIA

9.1 Introduction 74 9.2 The decomposition of solids 75

9.3 Kinetic expressions for the rate of sintering 76

9.4 Sintering 79 10 THE TEXTURE OF ZIRCONIA PREPARATIONS

10.1 Introduction 86 10.2 Analysis of adsorption isotherms by the t-method 86

10.3 Origin of hysteresis phenomena 89 10.4 Considerations on hysteresis loops in zirconia preparations 92

10.5 Texture of sintered A-preparations 94 10.6 Pore size distribution calculations 96 10.7 Pore shape of sintered A-preparations 99 10.8 Microporous texture of M-preparations 102 10.9 Microporous texture of T-preparations 105 11 THE CATALYTIC DECOMPOSITION OF 2-PROPANOL ON ZIRCONIA CATALYSTS

I I.I Introduction 109 11.2 Experimental 109 11.3 Kinetics of the catalytic decomposition reaction 112

11.4 Diffusion phenomena in the catalyst mass during the catalytic decomposition 116

11.5 Pore difl"usion phenomena with A-type zirconia catalysts 120

11.6 Catalytic properties of M-preparations 123 11.7 Catalytic properties of T-preparations T25 11.8 Comparison of the kinetic data for zirconia catalysts 126

n .9 Comparison of the literature results with the activity and selectivity of zirconia catalysts 128

SUMMARY 133 SAMENVATTING 137 LIST OF SYMBOLS I4O REFERENCES I42 ACKNOWLEDGEMENT 145

I I N T R O D U C T I O N

In the Middle Ages hyacinth and jargoon were well-known gem stones. These minerals have a very attractive colour as a result of small impurities in the mineral zircon. The chemical composition of zircon was later found to be ZrSi04. One of the twelve stones in the breastplates of the priests of Israel is thought to have been hya-cinth or jahya-cinth. In Revelation the hyahya-cinth and other minerals are mentioned as the precious stones which garnish the walls of new Jerusalem. In the eighteenth century, colourless zircons were regarded as inferior or imperfect diamonds; they were known as Matara diamonds because many were derived from the Matara district of Ceylon.

The name zircon was first applied by Werner, and was probably derived from the Arabic 'zerk', a precious stone, or Persian 'zargün', gold-coloured.

By fusing the zircon with sodium hydroxide and extracting the melt with hydrochlo-ric acid, the solution contains some element which is precipitated with a base. Surpri-sing the precipitate does not dissolve when excess base is added. Klaproth proposed the German name 'Zirkonerde' and the Latin 'terra circonia' for this new oxide. The English equivalent is zirconia. Unless more information is available, the term hydrous zirconia may be given to the precipitate which contains an undefined amount of bound water. The same term may also be assigned to the dried precipitate still containing a certain amount of bound water.

The appearance of hydrous zirconia on the addition of a base to a solution of a zirconium salt is often designated by the term polymerization. A large amount of water may be included during the polymerization process when ZrOa is formed. On the other hand, zirconium hydroxide is supposed to precipitate from the weak acidic solution on the addition of the base. These questions of polymerization, precipitation, and the question whether the oxide or hydroxide is precipitated from the zirconium solution, will be studied thoroughly in this thesis.

Once the precipitate has been obtained from the aqueous solution, it can be filtered and dried. The white solid thus obtained can be studied in numerous ways, depending on the interests and the available techniques. In this thesis we shall try to give a con-tribution to the questions mentioned above. The studies on the dried precipitate mean an open field since little is known on the physico-chemical properties of zirconia. We have tried to select the parts of this thesis in such a way that they form a logical combination. The solution of the interesting question on the phase transformation, which is very important in the beginning of this thesis, is solved in the last chapters.

The extensive knowledge of silica has meant the starting point of this thesis. The change in size, coordination number and type of bonding in the Group iv ions lead to interesting differences in the behaviour of these oxides. Sometimes a striking resemblance is found, however, for certain properties of these oxides. Throughout the whole thesis, for instance, the resemblance between hafnia and zirconia will come emerge.

2 _{LITERATURE SURVEY OF ZIRCONIUM CHEMISTRY}

2.1 GENERAL CHEMISTRY OF Z I R C O N I U M

The literature concerned with zirconium chemistry is largely a conglomeration of empirically determined facts. While assuming the basic laws of chemistry and physics to apply to zirconium, relatively httle has been done to make the behaviour of zirco-nium intelligible within this framework, although Blumenthal' encouraged efforts in this direction. For orientating purposes, some general fundamentals of the properties of zirconium are outlined below.

The electronic configurations of zirconium and its 'huge isotope' hafnium are 4d^5s^ and 5d-^6s^ in the outer shells. The remarkable chemical similarity between Zr and Hf is due in part to the analogous arrangements of electrons in the outermost quantum levels and to the near identity of the atomic radii: 1.452 A and 1.442 A respectively. It is also pertinent that zirconium and hafnium occur in their compounds almost exclusively with the oxidation number of 4+ and with the maximum number of bonds that is sterically possible. This results in a maximum coordination number of 8. The zirconium ion can use various hybrid orbitals to give strongly directional bonds. For eightfold coordination the orbitals used are d^sp^ leading to a dodecahe-dral arrangement of bonds, and d'p^, giving an antiprismatic configuration ^. These two configurations, which have been found experimentally for various ions, are illu-strated in Fig. I.

The classical example of dodecahedral coordination is given by the Mo(CN)^ and W(CN)8" complexes^. The square antiprism has been found in TaFg" (Ref 4) and ReFg" (Ref. 5). A similar arrangement of ligands is found in the acetylacetone com-plexes of Ce(iv), Th(iv), U(iv) and Zr(iv) (Ref. 6). The square-antiprismatic configu-ration in zirconium compounds is found in ZrOCh.SHaO and ZrOBr2.8H20 (Ref. 7). In this coordination the 4d and 5p vacant orbitals of the zirconium ion form coordi-nate links with the lone pair of electrons in the hgands present. These coordicoordi-nate linkages are essentially covalent, since they involve the sharing of two electrons between two atoms or ions. The term coordinate merely signifies that both electrons are provided by one atom or ion. In ZrOCU.SHaO, in which the zirconium ion is coordinated by 4OH and 4H2O groups, the lonepair electrons are provided by the oxygen.

H H

Formula (i) shows the formation of a Zr-OH2 bond. When the bond is strictly a covalent one this results in a charge of 4-coordination number of 8. The electrostatic approach, which in the case of water would correspond to a dipole bond, leaves the charge of the zirconium ion unchanged. When the final charge on the zirconium ion is adjusted to zero, the actual bonding results in a configuration in which the electrons are for the greater part situated at the oxygen ion. When in aqueous solutions a proton is transferred from a H2O ligand to a water molecule, the negative charge of the resulting OH-ligand shifts the electrons in the direction of the zirconium ion.

The general principles that can be given for zirconium include the absence of partly filled d-shell in the zirconium (iv) ion. This limits the possibilities of spectroscopic and magnetic investigations during ligand exchange of the zirconium complexes.

The behaviour of zirconium complexes in aqueous solutions is characterized by hydrolysis and polymerization. In recent years interest in these phenomena has in-creased. However, the literature on the aqueous chemistry of zirconium frequently contains inconsistencies.

The behaviour of hydrous zirconia which has been discussed very poorly in the literature, will be discussed extensively in close relation to the picture we gave for the formation of hydrous zirconia.

2.2 ZIRCONYL CHLORIDE SOLUTIONS

Aqueous solutions of zirconium and zirconyl salts show some characteristic features. In the literature polymerization and hydrolysis are used to describe the behaviour of

zirconyl chloride solutions. We shall define these terms here as they will be used throughout this chapter^.

Hydration. Hydration occurs when a zirconium salt is dissolved in water. The Zr''+

ion is 8-coordinated with water molecules, forming a square antiprism: Zr''+ + 8H2O ^ [Zr(H20)8]''+.

Hydrolysis. Hydrolysis involves a proton transfer of a Zr-OH2 linkage in aqueous

solution. This results in the liberation of H+ into the solution: Zr(H20)^+ + H2O ^ [Zr(H20)7 (OH)]3+ + H3O+.

A second hydrolysis step can occur which reduces the charge of the complex [Zr(H20)7 (OH)]^+ while a second proton is liberated into the solution:

[Zr(H20)7(OH)]3+ + H2O ^ [Zr(H20)6(OH)2]^+ + H3O+.

Polymerization. When two complexes have the right charge and composition, they

can polymerize. The polymerization is achieved by the formation of two OH-ligands between two zirconium ions:

2[Zr(H20)6(OH)2]^+-^ [(H20)5(OH)Zr(OH)2Zr(OH)(H20)5]''+ + 2H2O. Zirconyl solutions are frequently chosen as the starting material for the precipi-tation of hydrous zirconia. In this survey we shall confine ourselves to the formation of hydrous zirconia from zirconyl chloride solutions.

When a zirconyl salt is dissolved in water and enough base is added, a gelatinous precipitate indicated as hydrous zirconia is formed. Zirconyl chloride solutions can be obtained when zirconium dioxide is dissolved in a hot, strongly acidic HCl solution:

ZrOz + 2HCl*!f^ZrOCl2.

Another method of preparing zirconyl solutions is the dissolution of a zirconium salt in water. This dissolution is accompanied by a vigorous reaction:

ZrCU + H2O -> ZrOCU + 2HCI.

It indicates that a zirconium salt is transferred into a zirconyl salt when the zirco-nium ion is in aqueous medium.

When a zirconyl chloride solution is obtained, the solid zirconyl salt can be crystal-lized out by concentrating the solution; the amount of solid zirconyl salt can be increased by making a suitable choice of the HCl concentration. The solubility of

zirconyl chloride in water is shown in Table i (Ref 9), from which it can be concluded that a solubihty minimum appears at an HCl concentration of about 8.5 mole/i.

TABLE I Solubility of ZrOCb.SHiO in HCl solutions at 20° C (Ref. 9) HCl concn. ZrOCU.SHjO concn.

(mole/1) (mole/1) 0.2 2.91 1.47 2.14 4.97 0.329 8.72 0.0547 10.14 0.0988 10.94 0.205 11.61 0.334 2.2.1 Structure of ZrOCh.%HiO

The compound crystallizing from the concentrated HCl solution has the compo-sition ZrOCl2.8H20 (Ref 9). This salt is thought to be built up from the zirconyl ion ZrO^+ and two chloride ions held together by eight water molecules. The structure of zirconyl chloride octahydrate was clarified by Clearfield and Vaughan''. They showed the presence of a tetrameric unit, four zirconium ions lying at the corners of a square, each bound to its nearest neighbours through two OH groups, one of which located above the plane of the zirconium ions and one below. The other four linkages of the zirconium ion to complete the square antiprismatic configuration are formed with water molecules. Fig. 2 shows a projection on (001) of the unit cell of ZrOCl2.8H20, containing eight groups. From this projection it can be seen that the tetrameric complexes are surrounded by the remaining water molecules and chloride ions. The composition of the tetrameric complex in the unit cell was found to be [Zr4(OH)8 (H20),6]8+.

Another interesting aspect of the structure of ZrOCl2.8H20 is the absence of an actual Zr-Cl bond''. When the zirconyl chloride octahydrate is dissolved in water, the same tetrameric complex is present as that in the solid zirconyl chloride octahydrate'°. There are also indications of the presence of larger species, owing to hydrolysis and polymerization. The presence of the tetrameric cations in water solutions indicates that this configuration is apparently stable both in water solutions and in strongly acidic solutions.

2.2.2 Hydrolysis and polymerization in ZrOCh solutions

Water solutions of ZrOCl2.8H20 show acid properties. Ermakov et a l . " studied the acid strength of zirconyl chloride solutions. Table 2 shows the activities of H+ and HCl in solutions of zirconyl chloride kept for 24 h at 25° C.

o

OH-Oer

FiG. 2 Projection on (ooi) of the unit cell of ZrOCl2.8H20.

The activity of HCl in an m-molar solution of ZrOCl2.8H20 is almost equal to the activity of an m-molar solution of HCl. The formation of the acid solutions is due to hydrolysis.

The presence of tetrameric complexes in water solutions of zirconyl chloride'" is at present widely accepted.

TABLE 2 Activities of H+ and HCl in ZrOCh solutions " ZrOCb concn. (mole/1) 0.00617 0.01239 0.02474 0.2161 0.3868 Activity of H+ 0.00595 0.01127 0.02171 0.1737 0.2806 HCl 0.00642 0.01191 0.02284 0.164 0.273

Many efforts have been made to determine the exact size and composition of the complexes in solution. Investigations on this subject were carried out by diffusion, cryoscopy, distribution and spectrophotometric methods. Table 3 taken, from the article by Ermakov et a l . " , gives a survey of these investigations with their main results. From this Table it can be seen that not only tetrameric complexes of a molecular weight of 800 are present. An increase in molecular weight of the species in solution is observed when the acidity of the solution decreases. This increase in molecular weight of the species is indicated as polymerization.

2.2.3 The anions in ZrOCh solutions

An interesting aspect of the chemistry of zirconyl solutions is the role of the anions. From the investigations by Muha and Vaughan'" it followed that in aqueous solutions of ZrOCl2.8H20 no actual Zr-Cl bond is present. The experiments by Ermakov et a l . " indicated that one chloride ion per zirconium ion is free in solution. The other chloride ion is bound in a certain way to the complex. From the thermometric titration curves of TumbulP^ it was concluded that no Zr-X bond is present in water solutions of zirconyl chloride, zirconyl bromide and zirconyl iodide. These zirconyl solutions were obtained by dissolving the accessory zirconium salts. Moreover, the solutions obtained by dissolving the zirconium salts are very similar to those obtained by dis-solving zirconyl salts. These inconsistencies will be discussed in greater detail in Chapter 3.

2.2.4 T^he charge of the zirconyl species in solutions

A point which is frequently discussed in zirconium literature is the charge of the species in solution. Lister and McDonald'^ noted electromigration of zirconium spe-cies in solution towards the cathode.

An elegant method of determining the charge of the species in solution is ion exchange on a cation-exchanger. However, the ionexchanger is often blocked by the large zirconium species'^"''. A positive charge of the zirconium species could be determined.

Matyevic et al.' * studied the coagulation of negatively charged AgBr sols with zirconyl chloride solutions. From the influence of charge on the coagulation floccu-lation, the charge of the zirconium species was calculated to be 4+.

TABLE 3 Summary of published information on the formation of polynuclear species in solutions of zirconium salts " .

Method Compound Medium Diffusion Zirconium nitrate HCIO4,

HNO3

Diffusion Zirconium nitrate HNO3

Diffusion Zirconium nitrate HNO3, and oxide-chloride HCl solutions

Dialysis Zirconium oxide- HCl, chloride solutions HCIO4, of zirconium HNO3 nitrate and

perchlorate

Potentiometry, Zirconium Water cryoscopy oxide-chloride

Ultra- Zirconium tetra- HCl centrifuge chloride and ( + M C I )

oxide-chloride

X-ray MOX2.8H2O Water structural (M = Zr, Hf;

analysis X = CI, Br)

Distribution Zirconium HCIO4 perchlorate

Spectro- Zirconium HCIO4 photometry perchlorate

Main results

Continuous polymerization occurs in nitrate solns. on decreasing the HNO3 concn., Dmax = 2.54 X 1 0 - ' cm^ s"' at 12 moles HNO3/ mole ZrO(N03)2 and Dmin = 9.3 x io~''cm^ S-' at 0.5 mole NaOH/mole ZrO(N03)2. Perchlorate solns. contain products of low mol. wt; D = 2.08 x lO"* cm^ s~' and is almost independent of the HCIO4 concn. When the acidity is increased from 0.002 to 6 M, the self-diffusion coefficient of Zr in-creases from 3.5 X io~° to 7-8 x lO"* cm^ s"S which corresponds to an approximately sixfold decrease in the weight of the ionic groups.

Nitrate solns. contain species of mol. wts. from 800 to 1500 ([Zr] = 0.08-0.16 M, [HNO3] = 10-1 M, [NH4NO3] = 0-8 M). In HCl solns. the mol. wt. is ~ 2600. Degree of polymerization (p) depends on the zirconium and acid concn. but is independent of the anion (C\-, C I O ; , N O ; ) : p = 1-7for [Zr] = 1 0 - ' to i o - \ [H+] = 2 M ; p = 2.5-7 for [Zr] = 10-^ to 10-^ [H+] = o.i M. Complexes with different compositions exist, for example [Zr(OH)4ZrOCl2 ZrO : CI2] and [Zr(OH)4Cl2 : ZrO].

In 0.08 M HCl the degree of polymerization Ne ^ 7-11 ([Zr] = 0.05 M, L = 2). In 0.2 M HCl, Ne = 4.2 and 5.5. From 0.2 to 2 M HCl, Ne does not vary and apparently trimers and tetramers predominate.

Tetramers predominate in solns. of hafnium oxide-chloride between 0.5 and 2.04 M. In zirconium oxide-chloride solns. the degree of polymerization > 4.

Continuous polymerization occurs in perchlo-rate solns.: with increasing zirconium concn. the mol. wt. of the polymeric species increases. Trimers and tetramers predominate in perchlo-rate solns., the threshold of polymerization depending on the acidity of the soln.

3 STUDIES ON HYDROLYSIS AND POLYMERIZATION

From the results put forth in the literature (Section 2.2) and those obtained from some preliminary experiments, the following picture of hydrolysis and polymerization in zirconyl chloride solutions can be drawn. This picture is confirmed by the experi-mental results described in Chapter 4.

3.1 F O R M A T I O N OF Z I R C O N Y L S O L U T I O N S FROM Z I R C O N I U M SALTS

When ZrCU is dissolved in water, a zirconyl chloride solution is obtained. This solution behaves identically to zirconyl solutions obtained from ZrOCl2.8H20. The reactions which occur can be represented by hydration, hydrolysis and further com-plex formation:

ZrCU + 8H2O -* [Zr(H20)8]''+ + 4CI-. [Zr(H20)8]''+ + H2O ^ [Zr(H20)7(OH)3+ + H3O+. [Zr(H20)7(OH)]3+ + H2O - . [Zr(H20)6(OH)2]^++ H3O+.

The cation [Zr(H20)6(OH)2]'+ forms the tetrameric complex [Zr4(OH)8(H20)i6]'+: 4[Zr(H20)6(OH)2]^+ -> [Zr4(OH)8(H20)i6]«+ + 8H2O.

It is known that Zr(iv) in ZrOCU solutions occurs in tetrameric complexes (Section 2.2). From the similar behaviour of ZrCU and ZrOCh solutions it follows that this scheme also holds for ZrCU solutions as given in the above scheme.

3.2 THE H Y D R O L Y S I S OF ZrOCU S O L U T I O N S

When ZrOCl2.8H20 is dissolved in water, the zirconylchloride solution is acidic because of hydrolysis of the tetrameric complex:

[Zr4(OH)8(H20),6]8+ + 4H2O ^ [Zr4(OH)8(OH)4(H20),2]''+ + 4H3O+. The hydrolysis reaction is given for one group of the tetrameric complex in Formula (2). The charge on the complex is reduced to 4+ in the hydrolysis reaction. We now

\ ~ 0 H OH2 \ " 0 H OH2

0 H ^ _ ^ . 0 H 2 ^^^ 0 H ^ ^ > H 2 ^ ^^ (^)

assume that, dependent on the acid concentration, the charge of the complex 4+ can be reduced further. The tetrameric complex contains four zirconium ions which can hydrolyse independently. When one group is involved in a second hydrolysis step, the total charge on the tetrameric complex decreases to 3+:

[Zr4(OH)8(OH)4(H20),2]^+ + H2O ^ [Zr4(OH)8(OH)5(H20)ii]3+ + H3O+. This reaction of one of the groups on the tetrameric complex is shown in greater detail in Formula (3). Some experiments were carried out to determine the state of

°"^,^°"^ H,0 " " 3 ^ ° " . H® (3) O H - ^ ^>0H2 " OH-y y-OHj

^ O H OH 1^0» OH

the chloride ion in zirconyl chloride solutions to give a complete description of hydro-lysis in zirconyl chloride solutions. When ZrOCl2.8H20 (Merck) solutions were ti-trated potentiometrically with AgN03 solutions, the chloride ions were quantitatively precipitated as AgCl. From the shape of the titration curves no indication of a Zr-Cl bond could be observed. When an m-molar zirconyl chloride solution is titrated with a NaOH solution, 2 mmoles NaOH are required to obtain the equivalence point at pH 9. During this neutralization the free chloride ion concentration, as measured potentiometrically, increases. Ermakov et a l . " proved that one chloride ion per zirconium ion is free in solution. The second chloride ion, which enters the solution during neutralization, must be bound very weakly to the tetrameric complex. As the positive charge of the complex is firmly established, we assume that the chloride ion acts as counterion for the positive charge of the tetrameric complex. When the positive charge of the complex decreases by further hydrolysis, caused by the addition of NaOH, the counterions enter the solution. Assuming that the second hydrolysis step can occur, more than one HCl is liberated per mole zirconium in the solution. The hydrolysis can be described now in greater detail as the Cl-ion is included as coun-terion of the tetrameric complex:

[Zr4(OH)8(H20)?+ . . . . 8 C l - ] " i [Zr4(OH)i6-n(H20):;t8.... nCh] + (8-n)Cl- + (8-n)H+.

If n = 4, the results of Ermakov et a l . " are obtained, i.e. the resulting tetrameric complex [Zr4(OH)i2(H20)f2 • • • • 4^1"] and one HCl free in solution per zirconium ion. The second step in the hydrolysis reaction can be represented with values of 3, 2, I and 0 for n. If n = o, the tetrameric complex obtained is Zr4(OH)i6(H20)8. This represents the fully-hydrolysed zirconium hydroxide. If only one group on the tetra-meric complex is involved in the second hydrolysis step, then n = 3, giving the complex: [Zr4(OH)i3(H20)n . . . . 3CI-]. The amount of HCl liberated is 5 mole

from 4 mole zirconium. This second hydrolysis reaction on one of the zirconyl groups is represented in Formula (3).

3.3 P O L Y M E R I Z A T I O N IN ZrOCU S O L U T I O N S

The reduction of the charge of the tetrameric complex, achieved by the second step of hydrolysis, n = 3, in the foregoing equation, results in one neutral group on the tetrameric complex. We assume that this electrical neutrality is localized on one of the zirconium sites and is not spread over the complex, where the three other zirco-nium sites retain their single positive charge. This neutral site is assumed to be the starting point for the polymerization reaction, and reacts with a singly-charged site of another tetrameric complex in such a way that the zirconium ions are connected by two OH groups. A similar mechanism is observed for the polymerization of sihca^''. The OH groups which connect the zirconium ions have a configuration similar to that present in the tetrameric complex itself. Clearfield and Vaughan'' showed that the OH groups are present when ZrOCl2.8H20 is crystallized from strongly acidic solutions. Zaitsev ^^ suggested that these OH groups exist in neutralized ZrOCh solu-tions. We assume here that the OH groups which connect the zirconium ions of two tetrameric complexes are resistant to strongly acidic solutions and are fairly stable. The justification for this conclusion is that the amount of neutral sites on tetrameric complexes in zirconyl chloride solutions will be very small since they are one of the reactants in the polymerization reaction (Formula 4). The OH groups connecting

^ O H ^ Z-OH2

V~OH pH2 ' \ P" / OH^^^OH OH2

OH^4-°"2 , ° " ^ V / ° " ^ / 0 H / \ H ^ / / 0 H *^H20 (4)

lj,i \ OH2 O H ^ ^ ^ / \ ^ ^

two tetrameric complexes are called bridging ligands. When zirconium ions are con-nected by two OH bridging ligands, they are said to be 'bridged'. When they are not connected by bridging ligands, they are said to be 'free' or 'non-bridged'. As the tetrameric complexes are the smallest units in zirconyl chloride solutions, we call a solution with only tetrameric complexes unpolymerized. A polymerized solution consists of species in which tetrameric complexes are connected by bridging ligands resulting in bridged zirconium ions which can no longer play a role in the hydrolysis reaction.

From Table 3 it was concluded that a decreasing acid strength increases the degree of polymerization. When ZrOCl2.8H20 is dissolved in water, at least one mole HCl is formed per mole salt. Neutralization of this acid by the addition of a NaOH

solu-tion causes an increase in the degree of polymerizasolu-tion. When the neutralizasolu-tion is continued to pH 3, a precipitate begins to form. At the equivalence point (pH = 9) the precipitation is complete. The gelatinous precipitate is hydrous zirconia (Zr(OH)4). We assume that during the neutralization process of zirconyl chloride solutions, continuous polymerization occurs, finally resulting in the formation of hydrous zirconia.

Another method for preparing hydrous zirconia also strongly suggests continuous polymerization in zirconyl chloride solutions. ClearfieW obtained hydrous zirconia by refluxing a partly neutralized zirconyl chloride solution at 100° C. We prepared hydrous zirconia in a similar way by refluxing a zirconyl chloride solution at 100° C for three weeks without any added base. The acid strength of the solution increased very rapidly, which points to progressive hydrolysis. After one week the solution was coloured light blue. On further refluxing, the colour of the solution became progres-sively whiter and after three weeks the formation of the positively charged sol was complete. The acid strength obtained in the m-molar zirconyl chloride solution reached a value of almost 2 m-molar. According to the hydrolysis equation given in Section 3.2, the n value is then almost zero. Nearly complete hydrolysis was thus achieved at 100° C even without any added base.

4 C O N D U C T I V I T Y M E A S U R E M E N T S I N ZrOCh S O L U T I O N S

The pictures we sketched above for hydrolysis and polymerization in zirconyl chloride solutions, based on the literature survey given and on some preliminary experiments, are still rather rough, and need more experimental evidence. We shall now describe our conductivity studies on zirconyl chloride solutions and conductivity measurements during the titration of zirconyl chloride solutions with NaOH solutions. This method offers a very good opportunity to determine the changes in ion concen-trations during titration. When an assumption is made for the mobility of the tetra-meric complexes in zirconyl solutions, the HCl concentration of zirconyl chloride solutions can be calculated. We assumed the conductivity of the big complexes, with a charge which is shielded off by the counterions, to be zero. The ions which are measured in conductivity experiments are H+ and CI" in equal concentrations. This method of investigation is more reliable than potentiometric measurements, which can give rise to inconsistent measurements (the suspension effect) due to the presence of very small sol particles.

4.1 E X P E R I M E N T A L

Conductivity measurements were carried out with a conventional cell at a fre-quency of 1000 Hz and at 25° C ± 0.05° C. The ZrOCh solutions were all prepared from de-ionized water and Merck ZrOCh.8H2O. The conductivity of de-onized water was subtracted from the conductivities of the ZrOCh solutions.

The conductivity of an m-molar ZrOCh solution is often compared with the con-ductivity of a 2 m-molar HCl solution. The data of the concon-ductivity of HCl and NaCl solutions at 25° C were taken from Conway ^". From experiments with heated ZrOCh solutions it appeared that polymers in ZrOCh solutions influence the conduc-tivity measurements. Although these polymers are non-conducting, they influence, the conductivity such that a lower value is measured than that calculated from the amount of free HCl in solution. The correction is dependent on the concentration but does not exceed 4 %. Conway's data, which are given in different tables, are corrected for this effect to make comparison of the right values for the conductivity possible.

When 50 ml portions of ZrOCh solutions were titrated conductometrically with NaOH, some dilution of the solution occurred due to the addition of titrant. The conductivity was corrected for this dilution.

4.2 I N F L U E N C E OF C O N C E N T R A T I O N ON H Y D R O L Y S I S OF ZrOCh S O L U T I O N S

Solutions of ZrOCh were prepared by dissolving various amounts of ZrOCl2.8H20 in 500 ml de-ionized water. The zirconium concentrations of these solutions varied

from 0.183 >< io~' to 13.59 >< 'o~^ mole/i. The solutions were allowed to stand for one month at 25° C, after which their conductivity was measured. Table 4 shows the conductivity and the concentration of these solutions. On complete hydrolysis the conductivity of an m-molar ZrOCh solution should be equal to the conductivity of a 2 mM HCl solution. Table 4 compares the ZrOCh solutions to the HCl solutions, indicated by Kmax. It can be seen that Kmax > K > ^ Kmax- The ratio K/Kmax increa-ses as the concentration of the ZrOCh solution decreaincrea-ses. Obviously the amount of HCl liberated on hydrolysis of an mM ZrOCh solution increases with decreasing concentration of the ZrOCh solution. This leads to an acid strength of between m and 2 m mole/1. The conclusions of Ermakov et a l . " can be now extended to lower con-centrations. In the concentration region from 6.17 X 10^^ to 386.8 x io~^ mole/i Ermakov et al. found that the acid strength of an mM ZrOCh solution and the acid strength of an mM HCl solution are equal.

TABLE 4 Influence of concentration on the conductivity of ZrOCh solutions

No. 5 I 3 4 6 10 7 [Zr] X 10^ (mole/1) 0.183 0.416 1.182 2.768 4-673 6.355 1 3 5 9 K a t 25° C (fï-' m-') 0.0139 0.0309 0.0807 o . i 6 i 0.249 0.325 0.650 Kmax a t 25° C (11-' m-') 0.0154 0.0350 0.0986 0.224 0.375 0.503 1.034 K/Kmax 0.903 0.883 0.818 0.719 0.664 0.646 0.629 % bridged Zr ions 80.5 76.6 63.7 43-8 33.2 295 25.7

The hydrolysis reaction following from these experiments are given in the equation: H2O

[Zr4(OH)8(H20)?J . . . . 8C1-] ^ [Zr4(OH)i2(H20)tJ . . . . 4CI-] + 4HCI The second hydrolysis step, which follows from our measurements, results in a va-rying amount of HCl being liberated into the solution:

[Zr4(OH)i2(H20)^+ . . . . 4CI-] ^ [Zr4(OH)i6(H20)8] + 4HCI

Considering one tetrameric complex, this reaction can be assumed to occur on the four individual sites of the complex. The foregoing overall equation thus consists of four equations which describe the hydrolysis on the four individual sites of the com-plex:

H2O

[Zr4(OH),3(H20)?t • •

[Zr4(OH)i4(H20)f+ . .

[Zr4(OH)i5(H20); . . .

The forward reactions in the above equations lead to the formation of neutral sites on the tetrameric complexes. The amount of such sites is determined by the rate constants in both directions and the HCl concentration in the solution. When an equihbrium solution is obtained, the reaction should proceed in a direction reverse to that observed when the equilibrium is disturbed by the addition of HCl. This was done by conductometric titration of the solutions given in Table 4 with 0.0303 n HCl. The increase in conductivity of these solutions, plotted against ml HCl added, showed a strictly linear curve with a slope corresponding, within the experimental error, to the slope of the curve, obtained when the conductivity of similar HCl solutions was measured as a function of the added 0.0303 n HCl solution. The values of K of the titrated ZrOCh solutions remained constant over a period of 24 h. These experiments show that no acid consumption occurs when the solutions are titrated with 0.0303 n HCl solutions. This result contradicts the assumption of equilibrium reactions given above. In Section 4.3 the hydrolysis reaction will be shown to be reversible. The obvious conclusion is that neutral sites on the tetrameric complexes are short-lived intermediates. A reaction must be assumed then in which the neutral sites are in-volved. From Table 4 it can be seen that a certain amount of singly-charged zirco-nium sites is always present in solution. A reaction between these singly-charged sites and neutral sites can explain the irreversibility of the hydrolysis reaction in ZrOCh solutions. The reaction which occurs is the polymerization reaction, shown in For-mula (4).

If this reaction is irreversible, the observed facts can be explained completely. From the experimental data it appears that no acid consumption was observed during the addition of the HCl solution to the ZrOCh solution. Obviously no depolymerization occurs and the polymerization reaction is irreversible, under the conditions of our experiments.

The conclusions given indicate that there is a direct relation between the degree of hydrolysis and polymerization. When one mole HCl is liberated on hydrolysis of one mole ZrOCh, the charge of the individual zirconium sites on the tetrameric complex is i-|-; only tetrameric complexes with a total charge of 4-(- are present in the ZrOCh solution. When hydrolysis proceeds further, neutral sites are formed and the amount of HCl liberated into the solution increases. The amount of HCl formed in the second step of the hydrolysis reaction is equivalent to the number of neutral sites on the tetrameric complexes. As these neutral sites are consumed in the irreversible

polyme-. 3CI-] =1: Zr4(OH)i4(H20)^+ polyme-. polyme-. polyme-. polyme-. 2CI-] + HCl H2O

. 2CI-] ^ [Zr4(OH)i5(H20)+ . . . . C1-] + HCl H2O

rization reaction, the amount of HCl formed in the second step of the hydrolysis reaction is equivalent to the number of bridged zirconium ions. In Table 4 the con-ductivity of m-molar ZrOCh solution (K) was compared with that of a 2 m-molar HCl solution (Kmax). When only the first hydrolysis reaction occurs, K = ^Kmax and the amount of bridged zirconium ions is zero, since only tetrameric complexes are present in solution. When K increases, the amount of bridged zirconium ions also increases. Since K—^Kmax stands for the number of zirconium ions, the percentage bridged zirconium ions is given by the expression:

, . , , . . . K—^Kmax _ . percentage bridged zirconium ions = 100 %

iKmax

Table 4 summarizes the percentage bridged zirconium ions for varying ZrOCh solu-tions. Obviously the percentage bridged zirconium ions is related to the degree of polymerization.

Summarizing we can conclude that zirconyl chloride solutions can be hydrolysed to a larger extent than was found by Ermakov et a l . ' ' and that ZrOCh solutions are polymerized to a certain extent depending upon concentration. These results will be discussed in greater detail in Section 4.5.

4 . 3 I N F L U E N C E OF N E U T R A L I Z A T I O N ON H Y D R O L Y S I S AND P O L Y M E R I Z A T I O N OF ZrOCh S O L U T I O N S

In Section 2.2 we suggested that increased polymerization occurs when ZrOCh so-lutions are neutralized to give hydrous zirconia. The possibilities of calculating the percentage of bridged zirconium ions we suggested above will be used here. Conducto-metric titration curves of ZrOCh solutions with NaOH and HCl were recorded. For the experiments described below, 1.0750 g Merck ZrOCh.8H20 was dissolved in 500 ml deionized water. The 6.677 ^ io~^ molar solution was allowed to stand for one week; 50 ml portions of this stock solution were used for the conductometric titration with 0.0607 n NaOH. 11.00 ml of the NaOH solution were required for complete neutralization of the ZrOCh solution. Back-titrations were carried out with 0.0303 n HCl solution, 5, 8 and 11 ml portions of the NaOH solution being added with 10, 16 and 22 ml of the HCl solution. This results in the formation of NaCl, the amount following from the added amount of NaOH and HCl. The contribution of the amount of NaCl to the conductivity can be calculated from the data given by Conway^". Fig. 3 shows the conductivity of the ZrOCh solutions, corrected for dilution, as a function of the added volume of 0.0607 n NaOH. The curves with the arrows directed to the right are the forward-titration curves, those with the arrows directed to the left the back-titration ones.

0 6 r 0.5 0.4 0.3 •E 0.2 'G it .0.1 O o 2 4 6 8 10 12 ^ — ml 0.0607N NaOH

FiG. 3 Titration curves of 50 ml 6.677 x lO"^ M ZrOCU solutions against 0.0607 n NaOH.

indicated by this point is titrated direct with HCl, a straight line is obtained having a slope as that discussed in Section 4.2. The titration curve with NaOH (ABCDE) has part a negative slope and part a positive one. The equivalence point is obtained in point E. When HCl solutions are titrated with NaOH, the conductivity of the solution decreases linearly with the amount of NaOH added, until the equivalence point is reached. This is caused by the replacement of H+ ions by an equivalent amount of Na+ ions which have a lower mobility than H+ ions. The first part of the titration curve shows such a decrease in conductivity, although this decrease is less than that observed in a HCl-NaOH titration. Moreover, the negative slope of the curve (ABC) varies during titration, indicating that no distinct change in ion composition can be conclu-ded from this curve. From the positive slope of the curve (CDE) it can be calculated that the increase in conductivity is due to formation of additional Cl-ions. The solu-tion, however, was titrated with 0.0607 n NaOH solution. Obviously, in the second part of the titration curve NaCl is formed on the addition of NaOH. When the titra-tion was stopped after the addititra-tion of 5 ml 0.0607 n NaOH (point B) back-titratitra-tion was carried out with 10 ml 0.0303 n HCl solution, curve BF in Fig. 3. This curve shows a linear increase in conductivity while its slope is equal to that of the curve obtained by titrating 50 ml HCl solution of a conductivity of about 0.15 Q - ' m~^ with 0.0303 n HCl solution. From this result it can be concluded that the non-bridged zirconium ions are all singly charged until point B of the titration curve at the least (see Section

4.2). This was true for the starting point of the titration curves of solutions of a con-ductivity of 0.279 Q - ' m - i . The concon-ductivity of the titrated solution indicated by point F, corrected for the conductivity of 0.303 mmole NaCl in 50 ml water, is calcu-lated to be 0.328 Q-*m-'.

The same procedure was followed when 8 ml and 11 ml of the NaOH solution were added. These back-titration curves are indicated in Fig. 3 by CG and EH respectively. Curve CG slightly deviates from linearity at the beginning of the back-titration. Curve EH shows initially a horizontal part while further on a straight line is obtained with a slope equal to BF. The horizontal part of curve EH indicates that the ion con-centration of the suspension remains constant despite the addition of HCl.

Table 5 summarizes the conductivities of the solutions before and after titration. It is clear that the percentage of bridged zirconium ions increases with the degree of neutralization. As the degree of polymerization is related to the percentage of bridged zirconium ions we can conclude that polymerization occurs when these ZrOCh solu-tions are neutralized with NaOH to give Zr(OH)4.

TABLE 5 Conductivity of ZrOCh solutions after back-titration with HCl, corrected for the amount of NaCl formed during the titrations

ZrOCh % Bridged K(fl-' m-') Zr ions

Starting solution, i week at 25° C (point A in Fig. 3) . 0.279 5.7 Partly neutralized with 5 ml NaOH 0.328 24.2 Partly neutralized with 8 ml NaOH 0.384 45.5 Completely neutralized with 11 ml NaOH . . . . 0.414 56.8 Heated for 4 h at 80° C 0.399 5I-I Heated for 4 h at 80° C completely neutralized

with 11 ml NaOH 0.402 52.3

Complete hydrolysis (calculated) 0.528 loo.o

The last part of the titration curve (CDE) shows only a slight increase in the percen-tage of bridged zirconium ions. We saw that the slope of this curve indicated the for-mation of NaCl which can now be explained by the neutralization of the non-bridged zirconium ions in this part of the curve. That the neutralization is accompanied by the liberation of Cl-ions, indicates that the Cl-ion is a counterion for the positive charge of the complexes. During the neutralization a sol is formed, which coagulates at the end of the titration. The point of coagulation lies on the line CE in the titration curve.

We stated in Section 4.2 that the second hydrolysis reaction is reversible. This can be proved as follows. At point E of the titration curve, all zirconium ions are neutra-lized and the formation of Zr(OH)4 is complete. On back-titration, indicated by curve EH, these non-bridged zirconium ions became singly-charged again, over the first part of the titration curve, due to the adsorption of HCl forming H2O from one singly bonded OH group; the reverse of Eq. (3).

100 9 0 80 70 .2 6 0 E I 50 _u (-« T3 ï 30 20 10 0 1 3 5 7 9 11 ^ _ ml0.0607NNoOH

FIG. 4 Percentage of bridged zirconium ions during neutralization with 0.0607 n NaOH.

The increase in the percentage of bridged zirconium ions against the addition of NaOH is shown in Fig. 4. The increase from 5.7 up to 56.8 % shows a sigmoid charac-ter. The formation of bridged zirconium ions decreases when the neutralization point at 11 ml NaOH is reached in this part of the curve the number of singly-charged zirconium sites decreases. This conclusion shows that singly-charged zirconium sites are one of the reactants in the polymerization reaction. The upper curve in Fig. 4 shows the percentage of neutralized zirconium ions in the complexes. This curve varies from 5.7 to 100 %, while in the first part of the titration curve the percentage of neutralized zirconium ions and the percentage of bridged zirconium ions are equal. Obviously, bridged zirconium ions are neutralized zirconium ions.

The formation of hydrous zirconia by neutralizing ZrOCh solutions involves a polymerization reaction, as hydrous zirconia settles by coagulation.

f I I I I I I I I I I I I I I I I ^ -/ -/^ / /

4-4 N E U T R A L I Z A T I O N OF HEATED ZrOCh S O L U T I O N S

The preparation of hydrous zirconia on refluxing ZrOCh solutions was accompa-nied by an increasing acid strength of the ZrOCh solution. The increased hydrolysis was promoted by higher temperatures, resulting in a polymerization reaction, as discussed before. The effect of heating ZrOCh solutions on the titration curve was studied by heating 50 ml samples of the 6.677 x io~^ m ZrOCh solution described above. The solution was heated for 4 h at 80° C, and subsequently allowed to cool down. It was titrated with 0.0607 n NaOH solution, and back-titrated from the equivalence point with 0.0303 n HCl solution. The titration curves IDE and EJ are shown in Fig. 3. From the conductivity of the starting solution, 0.399 fi^^rn~\ the percentage of bridged zirconium ions was calculated to be 51.1. Curve IDE shows a distinct slope which is almost equal to that of an HCl-NaOH titration. Part DE of curve IDE coincides with that of the non-heated solutions. Between points D and E of the titration curve of the heated solution, coagulation suddenly occurs. The back-titration curve for this heated solution, EJ, shows the adsorption of HCl at the begin-ning of the back-titration accompanied by peptization of the suspension, as observed in Section 4.3. The conductivity of point J, from which the value for the conductivity of 0.6677 m-mole NaCl in 50 ml water is subtracted, is found to be K = 0.402 Q-'m~'. Table 5 summarizes the data of the heated ZrOCh solution. The conductivi-ties of the heated solution before and after neutralization are almost equal (0.3999 and 0.402 Q-'m~'). Neutralization of these heated ZrOCh solutions leads only to the neutralization of non-bridged zirconium ions. This neutralization occurs in the second part of curve DE.

The polymerization of the species in solution occurs during heating of the solution (Table 5). The precipitation of hydrous zirconia from heated solutions involves only a coagulation of the zirconium complexes.

4.5 I N F L U E N C E OF N E U T R A L I Z A T I O N AND H E A T I N G ON P E R C E N T A G E OF B R I D G E D Z I R C O N I U M IONS

From Table 5 it can be seen that the percentage of bridged zirconium ions for the completely neutralized non-heated solution is 56.8 and that of the heated solution only 52.3. Two experiments were carried out to show that the percentage of bridged ions can reach higher values, both on neutrahzing and on heating ZrOCh solutions. The solutions mentioned in Section 4.2 were titrated conductometrically. In order to compare the titration curves of ZrOCh solutions of more different concentra-tions, they were based on a io~^ m ZrOCh solution. This was done by multiplying both the actual conductivity and the number of ml NaOH added, by the ratio of the concentration of the standard solution, io~^ m ZrOCh, and the actual concentration

0.7 0.6 0.5 O.i 0.3 0.2 ; \ • ^ ,

j ^ \ \

\ /A

W

Titration curves of ZrOCh so-lution based on 50 ml o.oi m ZrOCh solution against 0.0607 n NaOH: (1)0.416,(3)1.182,(6)4.673,(10) 6.355,(7) 13-59 x lO"^ m ZrOCh. o 6 0 -9 0 BO 7 0 6 0 5 0 4 0 3 0 2 0 1 0 0 -- ^

--y

--y

Xx^

//yv

yy/

FIG. 6 Percentage of bridged zirconium ions against percentage of neu-tralization for ZrOCh solutions given in Tables 5 and 6.

of the solution. The equivalence point is thus found at 16.47 ml 0.0607 N NaOH when 50 ml samples of the solution would have been titrated. From the titration curves the percentage of bridged zirconium formed can be calculated during titration (see Section 4.3). A straight line with a slope corresponding to that of the HCl-NaOH titration curve indicates that only neutralization of free HCl in the solution occurs.

When during titration polymerization occurs, a bended line results, since Cl-ions are liberated by the polymerization reaction. The increase in the percentage of bridged zirconium ions during titration is plotted against the percentage of neutralization in Fig. 6. The differences in the percentage of bridged zirconium ions of these aged solutions (one month at 25° C) remain on neutralization. The lower curve refers to a solution with a concentration comparable to that of solution 7, but with a time of standing of only i week at 25° C. The difference in the percentage of bridged zirco-nium ions due to the time of standing decreases during titration of these solutions.

The percentage of bridged zirconium ions is increased by neutrahzation, a long period of standing and heating. The titration curves of the heated ZrOCh solutions showed that the percentage of bridged zirconium ions did not vary during titration. (see dashed line in Fig. 6). Heating offers a possibility to prepare hydrous zirconia with a fixed amount of bridged zirconium ions. If the percentage of bridged zirconium

ions in the heated solution can be varied by varying the time of heating, this difference will remain when the solutions are neutralized.

The influence of the time of heating was studied as follows: 1.2085 g Merck ZrOCh. 8H2O was dissolved in 500 ml de-ionized water and allowed to stand for one month at 25° C. During periods varying from i to 6 days, 50 ml samples of this solution were then heated at 80° C.

TABLE 6 Measured conductivities K of ZrOCh solutions [Zr] = 0.0075 mole/i, K max. exptl. = o.593fi^' m-' Temp.

rc)

Table 6 lists the conductivities of these solutions. The percentage of bridged zirco-nium ions, calculated from the conductivities increases on heating. The increase is highest in the early periods of the heat treatment. On prolounged heating however only a small further increase is observed. At this point the percentage in the neutrali-zed zirconium solutions with moderate concentrations varies from 55-75 % (Fig. 6). This difference is not very great and it must be expected that differences in hydrous zirconia obtained on neutralization will be difficult to detect.

The ZrOCh solutions discussed in the different parts of this section are characte-rized by the percentage of bridged zirconium ions. These percentages have been plotted against the concentration in Fig. 7.

When solutions are heated, an irreversible increase is observed in the percentage of bridged zirconium ions. It is expected that prolonged heating for six days show a further increase, as refluxing results in the formation of a hydrous zirconia sol. This is obviously not the most stable form, since a sol can be electrically neutralized irrever-sibly on further raising the temperature. Hydrous zirconia in an acid solution, obtai-ned from complete hydrolysis of ZrOCh, will be the most stable condition.

Fig. 7 shows one peculiarity. On decreasing the concentration all curves show an increasing percentage of bridged zirconium ions as a result of increasing hydrolysis. The increased hydrolysis at low concentrations, which can be described by an equili-brium reaction, results in a higher percentage of neutral sites than is present in more concentrated solutions. The percentage of bridged zirconium ions finally obtained will therefore be higher in more dilute solutions. The rate of formation of polymeric species, which can be calculated from the solutions given in Fig. 7, increases, however,

100 90 8 0 7 0 60 o 50 E c o 4 0 rsj ? 3 0 at •D n Jï 20 i l , » o 0 5 10 15 ^— tZr] nnole/in^j x1o3

FIG. 7 Percentage of bridged zirconium ions against zirconium concentration.

with the concentration. This is to be expected since, despite the low rate of formation, all curves must reach a percentage of bridged zirconium ions approaching lOO.

It is necessary to indicate time and temperature when hydrolysis data of ZrOCh solutions are given, since ZrOCh solutions are not stable. Kinetic factors merely determine the actual percentage of bridged zirconium ions in a ZrOCh solution.

4 . 6 A T H R E E - D I M E N S I O N A L P I C T U R E OF THE P O L Y M E R I Z A T I O N

The formulation we gave for the polymerization reaction in Formula (4) can now be extended with models. The tetrameric complex Zr4(OH)8(H20)f5 was illustrated in Fig. 2 in a projection on (001) of the unit cell of ZrOCh.8H20. Taking into account the size of the Zr''+ ion and the OH-ion, it can be seen that the eight OH-groups in the tetrameric complex are packed in a way similar to that of the F-ions in CaF2. These eight OH-groups can be enveloped by a cube. The Zr''+ ion shown in Fig. 8b lies in the cavity of four OH-groups. Four planes of this cube contain such a Zr''+ ion. In

6 days 80"C

4 h 8 0 ' C

• 1 month! 2 5 ' C

(a) (b)

(c)

FIG. 8 a The tatrameric complex [Zr4(OH)8(H20)i6]'^ (i) cube consisting of eight OH-groups (2) slice consisting of four H20-groups

b The zirconium ion in the cavity of four OH-groups

c The representation of the tetrameric complex used in visualizing the polymerization products.

FIG. 9 The polymeric species formed from two tetrameric complexes.

Fig. 2 it can be seen that four H2O groups are linked with the zirconium ion, leading to a square antiprismatic coordination of the Zr''+ ion. The four H2O groups lying in one plane can be represented by a slice, while two slices form a cube similar to that already discussed. Every zirconium ion is linked with these four H2O groups. The way in which the slices are placed on four planes of the cube is indicated by the coordination of the Zr''+ ion. Fig. 8a shows the complete tetrameric complex Zr4(OH)8 (H20)f6 • The cube is indicated by i and the H2O slices by 2. Fig 8c shows the tetra-meric complexes where the H2O groups are found at a certain distance from the cube. This representation will be used for visualizing the polymerization reaction, as it shows the important parts of the complex more clearly.

The hydrolysis reaction has no influence on the representation of the tetrameric complexes, since the proton is very smafl compared with the H2O group. According to Formula (4), polymerization results in the formation of two bridging ligands. The product which is formed in Formula 4 can be represented as shown in Fig. 9. The two tetrameric complexes are connected by two OH-bridging ligands. The configu-ration which is created now shows that the bridging ligands and the remaining OH-and H2O groups form a good point for further polymerization. Fig. 10 shows a poly-meric species which is formed from four tetrapoly-meric complexes. Comparising Figs. 9 and 10 it is clear that the OH-bridging ligands formed between the zirconium ions can be represented in Fig. 10 by a cube. The (0) in the figure indicates where polymeri-zation on the polymer represented in Fig. 9 took place. The bridging ligands between four tetrameric complexes form a similar arrangement as that in the stable tetrameric complex itself. The representation of polymerization in ZrOCh solutions obviously results in a three-dimensional polymer.

The size of the polymers can be related to the percentage of bridged zirconium ions. When a polymer is built up from the tetrameric complexes represented in Fig. 8c, the number of bridged zirconium ions can be counted. The polymer given in Fig. 9 shows two tetrameric complexes while one zirconium ion is a bridged zirco-nium ion; 8 zircozirco-nium ions are present in this polymer and the percentage of bridged zirconium ions is 12.5. The polymer shown in Fig. 10 consists of four tetrameric complexes. Here 16 zirconium ions are present, while 4 zirconium ions are bridged. The percentage of bridged zirconium ions is 25. Further increase in the size of the polymers results in an increasing percentage of bridged zirconium ions.

4.7 THE POLYMERIZATION OF GROUP IV ELEMENTS

The precipitation of oxides and hydroxides can be described by processes such as polymerization and crystallization. Group iv elements exhibit the polymerization processes typical of those which can occur in solutions.

of polymerization processes in silica sols depend on the pH of the solution. Formula (5) iflustrates a simplified polymerization reaction, finally leading to Si02. The

OH \ S OH OH OH OH 0^ OH \ / \ / \ / , , SI — SI Si •2HjO (5) / \ / \ / \ / \ OH2 OH OH OH OH2 0 OH

kinetics of the polymerization of silica showed that complex information also plays an important role in the mechanism of the polymerization.

The polymerization of titanyl complexes in solution is very difficult to study expe-rimentally. The anion S04~ present in titanyl sulfate solutions is strongly bonded to the titanium ion^'. If this anion is not taken into consideration, some generalizations can be formulated. The charge of the titanium ion in normal solutions is 4 + , its coordination number is 6. When the polymerization occurs on neutralization of a titanyl solution, formation of TiO(OH)2 must be assumed. The charge of the titanium ion is just compensated for, while the bridging ligands 0^~ and OH" account for the coordination number 6. The reaction between a neutral site and a singly-charged site is suggested by the results obtained for silica and zirconia. Formula 6 illustrates one

OH OH OH2 OH OH , 0 H > OH

\ ® \ / \ ® \ /

,,-OH2 ^Ti OH • OH Ti ,,-OH2 — O H 2 — T i — 0 — ^ T i OHo • 3 H 2 O (6)

/ \ / \ / \ / \

OH2 OH2 OH OH OH2 O H ' OH

of the possibihties for the polymerization reaction in titanyl solutions. A comparison of the polymerization reaction of group iv elements shows that the increase in the coordination number results in the formation of oxide, oxide-hydroxide or hydroxide. Polymerization of other elements leads to similar considerations. Some differences can arise by the metal-anion combination. An anion with a high affinity for the metal ion can act as a bridging ligand in the polymerization reaction as does O^ orOH~. This was clearly demonstrated by Wyatt' and Hermans ^^ for palladium and uranyl solutions.

In the ZrOCh solutions studied we showed that the chloride ion acts only as coun-terion. The picture we gave for the ZrOCh solutions can be extended to other zirconyl solutions where no actual metal-anion bond is present. From the similarity between Zr and Hf (Section 2.1) it is expected that the chemistry of zirconyl solutions also applies to hafnyl solutions.

4.8 SUMMARY

The conductivity measurements of zirconyl solutions reconfirmed the picture of hydrolysis and polymerization sketched in Chapter 3. The hydrolysis of ZrOClj

solutions increases as the concentration decreases. From the experimental results it followed, however, that the hydrolysis reaction cannot be described by the reversible dissociation of a weak acid. The degree of hydrolysis is directly related to the percen-tage of bridged zirconium ions. During neutrahzation of ZrOCl2 solutions the per-centage of bridged zirconium ions increases as the hydrous zirconia is settled down by coagulation. The precipitation of hydrous zirconia from heated solutions only involves a coagulation of the zirconium complexes in solution. The differences in percentage bridged zirconium ions from a varying concentration remain on neutra-lization of the solutions.

5 PREPARATION OF HYDROUS ZIRCONIA

5.1 METHOD I

250 g ZrCU was dissolved in 25 1 de-ionized water, and a diluted solution of am-monium hydroxide was added dropwise to the zirconyl solution with vigorous stirring. The precipitation was followed potentiometrically, so that it could be stopped at any required pH.

Three samples were prepared with final pHs of the solutions at 4, 6, and 8. The suspensions were left for one week at their final pH, and the gelatinous precipitates filtered and washed with de-ionized water until Cl~-free. The filtered products were dried for 48 h at 120° C. On drying, they shrank and formed hard lumps, which were difficult to handle, but, when placed in water, separated into small particles of about I mm, which are far more easily manipulated. Filtration and drying again at 120° C yielded more or less translucent products similar to splinters of glass. The preparations obtained in this way will be indicated by A-4, A-6, and A-8.

5.2 METHODS II A N D IIA

Precipitation of zirconium hydroxide with a dilute solution of ammonium hydro-xide, as by method i, is not an ideal procedure. When a drop of the ammonia solution falls into the zirconium solution, flakes of zirconium hydroxide are formed due to the high hydroxyl concentration around the drop of ammonia. As these flakes are dispersed in the solution by vigorous stirring, homogeneous precipitation should be preferred.

For this reason a perspex precipitation vessel was constructed, as shown in Fig. 11, in which the zirconium hydroxide can be precipitated by means of gaseous ammonia. The cyhndric vessel is closed by means of a lid that is sealed with an O-ring and pro-vided with openings for stirrer, ammonia inlet, and pH-measuring cell.

Five htres of de-ionized water, containing 100 g Merck ZrOCh.8H20, were used for every preparation. The ammonia was purified and its flow regulated with a needle valve at 91/h. Precipitation took i^ h. The small air buffer above the solution led to dilution of the gaseous ammonia. Mixing in this vessel was very good, as was proved by neutralizing HCl solutions in this way with methyl red as indicator. After neutralization of the ZrOCh solution, easily filtering precipitates were obtained. The gelatinous products were washed, filtered, and dried at 120° C. The hard lumps were again immersed in water, where they crumbled into small, easy-to-handle particles, and dried at 120° C.

pH mete^z^^ q_,>

E ^

i i w

• N H ,

\z=i

^

FIG. 11 Reaction vessel for the precipitation of zirconia i

The hard lumps of zirconium oxide-hydroxide obtained after drying at 120° C can present some difficulties in catalytic investigations of Zr02 with respect to diffusion problems, even when the particles are about i mm. According to Van der Giessen ^^, dehydration of iron oxide-hydrate gels occurs on freezing at liquid nitrogen tempera-tures. When the mass is thawed out at room temperature, the gel separates into a water phase and a brown precipitate. After washing and drying, a finely divided powder is obtained. This way of dehydrating gels was therefore also adopted for zirconia gels obtained by method 11. The zirconium oxide-hydroxide gel was frozen at liquid nitrogen temperature, and after 24 h it had separated into a water phase and a layer of precipitate. Filtration, washing, and drying of this precipitate at 120° C affor-ded a finely-diviaffor-ded powder. This method will be indicated by iia.

Drying at room temperature over P2O5 pellets gives a higher water content than drying at 120° C. Table 7 lists the data for some products obtained by methods 11 and iia. The differences between frozen and normal preparations are obviously very smaU, and differential thermal analysis (DTA) yields identical thermograms. The table shows a distinct correlation between the water content and the surface area. When water is liberated, the surface area increases, indicating that at these high water contents, water is present in micropores. The composition of the dried gels wifl be discussed in greater detail in Chapter 6. Preparations obtained by methods i, 11, and iia will subsequently be designated as A-preparations.

TABLE 7 Characteristics of dried zirconia preparations Method u na Temp. of drying 120 20 120 20 (°C) (P205) (P205) Surface area (mVg) 308 243 344 177 Loss on ignition (7o) 10.9 16.9 10.5 20.8 Density (g/cm^) 4.51 3.63 4.56 3.38 5.3 METHOD III

Direct formation of hydrous zirconia from zirconyl chloride solutions was described by Clearfield " : a i m zirconyl chloride solution was adjusted to pH 2.5 by the addi-tion of ammonia. The soluaddi-tions were refluxed and the resulting colloidal sol was coagulated with ammonia or collected by centrifuging. We dissolved 100 g Merck ZrOCl2.8H20 in 2 litre de-ionized water and refluxed the solution for three weeks. After one week the solution was blue, and finally a white colloidal sol was obtained. This sol was coagulated with ammonia, filtered, washed, and dried at 120° C. A finely-divided powder was obtained. DTA showed a crystalline thermogram, and the X-ray diffraction pattern had monoclinic spacings. These preparations will be designa-ted as M-preparations.

5.4 METHOD IV

As was pointed out by Clearfield'', crystalline hydrous zirconia can be obtained by refluxing the gelatinous precipitate obtained by method i. When the precipitates were washed thoroughly, no crystallinity was observed during refluxing and the pH of the suspension did not decrease. When, however, they were washed until a chlorine content of 0.1-0.4 %, a decrease in pH was observed during refluxing, and monoclinic zirconia was obtained. Refluxing gelatinous precipitates in 20 % NaOH solution gave tetragonal hydrous zirconia.

We prepared gels according to method 11. The gelatinous products were washed thoroughly until Cl~-free. Each preparation of 100 g ZrOCh.8H20 was mixed with 21 de-ionized water, and the resulting slurries were refluxed until no amorphous zirconia could be detected by DTA. The development of crystallinity was followed this way.

Preparation method IVa. The slurry was refluxed in a basic medium. When

Preparation method IVb. The slurry was refluxed in a neutral medium. When

crys-taUinity was complete, the slurry was filtered, washed, and dried at 120° C.

Preparation method IVc. The slurry was refluxed in an acid medium. When the

crystallinity was complete, the resulting colloidal sol was coagulated with ammonia solution, washed, and dried at 120° C.

6 C H A R A C T E R I Z A T I O N O F H Y D R O U S Z I R C O N I A

6.1 DIFFERENTIAL THERMAL ANALYSIS

As was pointed out in Chapter 5, hydrous zirconia can be made in many ways, resulting in various modifications of the product, such as amorphous, tetragonal, and monoclinic zirconia.

The DTA curve of amorphous zirconia shows two characteristic parts (Fig. 12). The endothermic effect caused by dehydration of the amorphous product is followed, when the dehydration is almost complete, by a large exothermic effect. At different points of the analysis, samples were taken for investigation by X-ray diffraction. The diffraction patterns of preparations removed from the furnace before the exothermic effect show an amorphous character. The preparation taken just after the exothermic effect shows a fine tetragonal pattern (Fig. 13), and the preparation taken at the end of the analysis (800° C) evidently contains a mixture of tetragonal and monoclinic Zr02 (Fig. 14). From these experiments we concluded that the exothermic effect is due to crystaflization of the amorphous Zr02 into the tetragonal form. The conversion of tetragonal into monoclinic Zr02 on prolonged heating is accompanied by small heat effects, not detectable by DTA. Kommissarova et al. ^'' obtained a similar thermogram obtained from amorphous Zr02. The exothermic peak at 405° C was due to

crystalli-+

t

AT

100 300 500 700 ' » - T °C

FIG. 12 Differential thermal analysis of a m o r p h o u s zirconia.

1

4 0 38 36 34 32 3 0 2 8 26

2 8 - ^

FIG. 13 X-Ray diffraction pattern of tetragonal zirconia.

35 3 4 33 32 31 3 0 2 9 2 8 27 26 2 e •