Chapter I

Introduction

1.1. General aspects

Metal oxides play a very important role in diverse areas of chemistry,

physics and materials science. They have long attracted attention because of

their several unique materials properties, which make them versatile and

useful materials in various industries. They find extensive applications as

structural materials, refractories, adsorbents, insulators and corrosion

resistant materials. Metals are able to form a variety of oxide derivatives,

which can adopt a vast number of structural geometries with electronic

structures that can exhibit either metallic, semiconductor or insulator

characteristics. In technological applications metal oxides are used in the

fabrication of microelectronic circuits, sensors, piezoelectric devices, fuel

cells, coating for the passivation of surfaces against corrosion and as

catalysts. Majority of the catalysts used in chemical and petrochemical

industries involve an oxide as an active phase, promoter or support. Oxide

catalysts fall in to two general categories. They are either electrical insulators

or semiconductors. Insulator oxides are generally those in which the cationic

material has a single valence and have stoichiometric M:O ratios. The simple

oxides MgO, Al2O3 and SiO2 fall in to this category. These materials are not

effective as oxidation catalysts and find more use as solid acids or bases.

Semiconductor oxides are more commonly used in oxidation. They are

materials in which the metallic species are relatively easily cycled between

two valence states as in Fe2O3, V2O5, TiO2 and CuO or the inter-conversion

between the positive ion and the neutral metal as with the more easily

reduced oxides such as ZnO and CdO. In general, the simple semiconductor

2 Chapter 1

oxides are not very good catalysts for synthetic reactions1. The insulator

oxides however, can be used as solid acids and bases for a number of

reactions. Furthermore, the most active areas of the semiconductor industry

involve the use of oxides. Thus, most of the chips used in computers contain

an oxide component. The continued growth in industry and technology leads

to an ever-increasing demand for materials with specific properties. Hence in

the current scenario, further diversification of applications of metal oxides

through modifying their properties has added relevance.

Research interest has shifted from metal oxides to mixed metal

oxides since the properties of the metal oxides can be tuned and modulated

by judicious mixing with suitable other metal oxides. The two metal centers

in a mixed metal oxide can work in a cooperative way catalyzing different

steps of a chemical process, or they can have an enhanced chemical activity

due to effects of metal-metal or metal-oxygen-metal interactions. In

principle, the combination of two metals in an oxide matrix can produce

materials with novel structural or electronic properties that can lead to

superior materials with enhanced properties2. Although the importance of

this class of materials have been known for many years, it is only in the

relatively recent past, with the advancement of analytical techniques, studies

related to their structure and structure property relationship have attained

momentum. The preparation of specific tailor made mixed metal oxide,

which can perform complex functions is one of the main topics of research

today. The intense interest in these materials is driven to a large extent by

their unique properties and potential in various applications relating to high

temperature superconductors, 3

ceramic pigments4

and catalysts5-9

. Mixed

metal oxide thin films offer a strong and versatile material base for the

Introduction 3

development of novel technologies such as ferro-electric memories, 10

infrared detectors, 11

magnetic sensors12

and gas sensors13

.

The large application spectrum of mixed metal oxides requires

tailoring of the properties, such as particle size, morphology and phase

homogeneity. Low temperature route to mixed metal oxides have attracted a

great deal of interest since in the traditional high temperature ceramic

method there is little scope for control over homogeneity of components,

particle size and morphology. In the present work we propose to exploit sol-

gel technology as a route to process mixed metal oxide catalysts. Sol-gel

synthesis of mixed metal oxides provides an approach to control structures at

the nano-scale, thus enabling the formation of highly homogeneous new

materials, generally having improved or entirely new properties14

. Since our

focus is on metal ion doped sol-gel derived alumina, it is worth to mention

some of the relevant details in this area.

1.2. Alumina

One of the well-known metal oxide alumina, Al2O3, which exhibits

diverse phase characteristics, is known for its versatile material properties

and wide applications. It is an important and key material in ceramic industry

because of its unique mechanical, electrical, thermal and chemical properties.

Some of the important applications of alumina are listed below in Table1.1.

4 Chapter 1

Table 1.1. Important applications of alumina

Property Applications

Mechanical Abrasives, wear resistant applications, thread guides,

cermets etc

Thermal Refractory material, substrate for electronic materials,

good thermal conductor

Electrical High temperature insulator, high voltage insulator

Chemical Corrosion resistant applications, coatings on metals

Surface Adsorbents, catalysts

Seven crystalline phases have been reported for alumina which are ,

, , , , and 15

.The transformation between alumina phases strongly

depends on the precursors and the thermal treatment used in their

stabilization. -Alumina is known to have a d- spacing of 2.12Aº and is

probably cubic16

. -Alumina has a cubic spinel structure while -alumina

consists of oxygen anions in fcc lattice with Al ions occupying both

octahedral and tetrahedral sites. The cubic structure resembles a defective

spinel in which both octahedral and tetrahedral cation positions are

occupied17

. -Alumina has been identified as either tetragonal or as pure

amorphous phase18

. It was also suggested that -alumina has a tetragonal

superstructure of the spinel lattice with one unit cell parameter tripled17

. The

-form is an orthorhombic phase and -alumina is monoclinic, 19

its structure

is based on a face centered cubic packing of anions similar to the -phase and

contain both octahedral and tetrahedral Al ions17

. The thermodynamically

stable form of alumina is rhombohedral α-alumina with cell parameters

a = 4.7758Aº and c =12.991Aº. The oxygen positions approximate to

Introduction 5

hexagonal close packing (HCP) with the trivalent cations occupying two-

third of the octahedral interstices. The details of the crystal structure of

various alumina phases and hydroxides of aluminium are given in Table 1.2.

Table 1.2. Crystal structure of aluminium hydroxides and alumina phases

Phase Crystal system

Molecules

per unit

cell

Unit cell parameters Aº

a b c

Bayerite Monoclinic 2 4.72 8.68 5.06

Gibbsite Monoclinic 4 8.64 5.07 9.72

Diaspore Orthorhombic 2 4.40 9.43 2.84

Boehmite Orthorhombic 2 2.87 12.2 3.70

Chi Cubic 10 7.95 - -

Eta Cubic spinel 10 7.90 - -

Gamma Tetragonal - 7.95 7.95 7.79

Delta Tetragonal 32 7.97 7.97 23.5

Theta Monoclinic 4 2.95 2.95 11.9

Kappa Orthorhombic 32 8.49 12.7 13.4

Alpha Rhombohedral 2 4.76 - 12.9

Generally alumina is prepared by calcination of hydroxides of Al,

gibbsite, boehmite, bayerite and diaspore. Among various hydroxides,

boehmite and diaspore are both monohydroxy oxides of aluminium with

different crystal structures. Boehmite is of particular importance because it is

easily dispersible in water around pH 3-3.5. The various hydroxides of Al

6 Chapter 1

can be transformed in to -Al2O3 through different routes by temperature

control as shown below.

300ºC 970ºC 1100ºC

Gibbsite χ κ α

450ºC 750ºC 1000ºC 1200ºC

Boehmite γ δ θ α

230ºC 850ºC 1200ºC

Bayerite η θ α

500ºC

Diaspore α

900ºC 1000ºC

Tohdite κ α

In general, boehmite acts as an important precursor or intermediate for

the synthesis of transition aluminas. Boehmite transforms on heating through

a series of meta-stable transition alumina phases including -, -, -Al2O3,

finally forming thermodynamically stable -Al2O3 as shown above.

Introduction 7

Fig. 1.1. Boehmite polyhedra.

The crystal structure of boehmite (Fig.1.1) consists of Al-O octahedral

double layers, which are connected by hydrogen bonds. The octahedra are

nearly regular and have four of their corners occupied by oxygen atoms and

the other two by hydroxyls20

. The dehydration path inside the boehmite

structure is therefore believed to be along the ac plane. In large crystallite of

boehmite, this path becomes longer and the interlayer spacing is narrow.

Dehydration occurs slowly and suppresses the conversion of boehmite to -

Al2O3, elevating the formation temperature of -Al2O3. The chemical water

of this monohydroxide is made by combination of one terminal OH group on

top of a crystallographic boehmite layer, with one H atom from a

neighboring OH group. Thus the OH groups on the surface of each atomic

boehmite layer combine to produce water molecules, which are expelled.

The oxygen anions, which are not involved in this dehydration basically,

remain unaffected; hence the type of anion crystallographic packing and the

crystal boundaries remains fundamentally unchanged. Such transformations

are known as topotactic transformations. Since the transformation from -

through - to -Al2O3 proceeds topotactically21

it involves only a small

8 Chapter 1

energy difference. The transition of - to -Al2O3 does not require a re-

constructive re-crystallization process because of the similar cubic close

packed oxygen sub lattice. The transformation from - to -Al2O3, however,

requires a reconstruction of the structure involving a change in oxygen sub-

lattice from cubic to hexagonal and hence can occur only with a large energy

difference.

1.3. Alumina based mixed oxides

Incorporation of other metal oxides into alumina is an area of great

interest since it improves the properties of alumina with regards to many

aspects. A scan of literature reveals that many workers have attempted to

modify the properties of alumina by incorporating other metal oxides in its

matrix. Some of the important property modifications achieved by doping

other metal ions in alumina are listed in Table 1.3.

Table 1.3. Property modulation of alumina by incorporating other metal

oxides

System Property

ZrO2/Al2O3 Wear resistance22

Nb2O5/Al2O3 Thermal conductivity23

CeO2/Al2O3 Oxygen storage capacity24

La2O3/Al2O3 Surface area25

B2O3/Al2O3 Surface acidity26

MgO/Al2O3 Creep resistance27

Introduction 9

1.4. MAl2O4 spinels

The spinels are a group of oxides with the general formula

AIIB2

IIIO4. The structure of normal spinel consists of cubic close packing of

anions with the A atoms in tetrahedral sites and the B atoms in octahedral

sites; i.e., A(T)B2(O)O4 (Fig. 1.2). Each AO4 tetrahedron shares corners with

four BO6 octahedra, which are linked to each other through the edges. It is

possible to have various degree of randomization where both types of sites

are occupied partly by A cations and partly by B cations. These are known as

random spinels; i.e., [AxB(1-x)]T[A(1-x)B(1+x)]OO4.

Fig.1.2. Spinel polyhedra

Spinel oxides constitute an important class of advanced ceramic

materials, which exhibit interesting electronic, magnetic and catalytic

properties28-31

. Aluminium based spinels MAl2O4 (M = Cu2+

, Mn2+

, Co2+

and

Ni2+

etc) are also important as ceramic pigments32-

35

since the constituent

transition metal ions can form MO4 and MO6 polyhedra which can exhibit

the d-d transitions within the M2+

ion that get modulated by the ligand field

environment around the metal ion which result in colour modification within

the system.

10 Chapter 1

1.5. General methods of preparation of mixed metal oxides

Multi-component materials exist in various forms and their

preparation involves different protocols with a multitude of possible

preparation schemes. It has long been recognized that the properties of mixed

metal oxides often depend on their method of preparation. The dispersion

and size distribution of metal dopant oxide, their spatial distribution on the

support, homogeneity of components in a multi-component system, porosity,

surface area and pore size distribution of a support are all sensitive functions

of precursors used. The treatment temperature and atmosphere, as well as

other preparation variables such as pH of the preparation solution or the use

of aqueous or organic medium also contribute substantially to their

properties36-38

. In recent years significant progress has been made towards

understanding the relationship between the preparation method and the final

properties of the mixed metal oxides. Some of the most common methods of

preparation of mixed oxide catalysts are described below.

1.5.1. Solid- solid blending

Various methods based on solid-state reaction between powder oxides

are frequently used for the preparation of mixed oxide39

. In high temperature

ceramic method, the mixed oxide phase results from heating intimately

mixed powders at temperatures high enough to allow inter-diffusion and

solid state reactions. A major shortcoming of the high temperature ceramic

method is the lack of homogeneity of the material prepared since the solid-

state reactions between the precursor oxides occur at slow rates. The high

temperature required for completing solid-state reactions between oxides

lead to a drastic decrease in surface area of the resulting catalytic material by

sintering. To overcome this problem, precursor compounds such as

Introduction 11

carbonates and oxalates, which decompose at lower temperatures, have been

used instead of the corresponding oxides.

1.5.2. Co-precipitation method

In the co-precipitation method, the formation of the precipitate from a

homogeneous liquid phase may occur as a result of physical transformations

like change of temperature, solvent or solvent evaporation but most often is

determined by chemical processes like addition of acids or bases and use of

complex forming agents40

. The precipitate is treated in subsequent

preparation stages to form the active catalyst. In a multi-component system,

the composition of the precipitate depends on the difference in solubility

between the components and the chemistry occurring during precipitation.

One possibility is the sequential precipitation of separate chemical

compounds. This occurs whenever there is a large difference in the solubility

products of the compounds involved. The co-precipitates of hydroxides,

hydroxocarbonates, oxalates and formates containing two or more different

metals are generally non-homogeneous in composition and only very seldom

generate a homogeneous mixed oxide41

.

1.5.3. Multi-component complexes as precursors

If preformed complexes are used as precursors in which the desired

ratio of components is already present, it should be possible to prepare a

homogeneous multi-component material with lower temperature treatment.

Only a limited number of examples are available in the literature using this

approach to prepare multi-component oxides. In one example, anhydrous

metal acetate is dissolved in an organic medium in the presence of

isopropoxide of another metal to form a compound. Calcination of this

compound produces a well-mixed multi-component oxide37

. The use of

12 Chapter 1

compound precursors guarantees that the different metal components are

automatically mixed in the precursor. Thus it is expected that homogeneous

multi-component oxides can be obtained subsequently with rather mild

treatments. However, because stochiometric compounds are formed, the

range of composition is limited. Furthermore, the number of compounds that

can be formed readily limits the mixed oxides that can be prepared.

1.5.4. Hydrotalcite oxides as precursors

MgxAl1-x(OH)2(CO3)1-x is the most commonly used hydrotalcite oxide

precursor for other oxides42

. It is made up of layers of MgxAl1-x(OH)2 that carry

a net positive charge when x is greater than 0. This positive charge is balanced

by negatively charged CO32-

ions located between the layers. The Mg and Al

ions can be readily substituted by other cations, which makes hydrotalcite a

convenient precursor for the preparation of homogeneous mixed metal oxides or

samples of metal highly dispersed on an oxide support.

1.5.5. Impregnation method

In the impregnation route, a solid phase preformed in a separate

process is used as a support, and the catalytically active material is mounted

and stabilized on it.43, 44

In this method, a certain volume of solution

containing the precursor of the active element of the catalyst is contacted

with the solid support. If the volume of the solution is either equal or less

than the pore volume of the support, the technique is referred to as incipient

wetness. When the interaction of the active precursor in solution with the

support is weak, the method of incipient wetness impregnation followed by

drying is used to apply high loading of precursors. The maximum loading is

limited by the solubility of the precursors in pore filling liquid. However, in

the absence of sufficiently strong interactions, the drying step usually result

Introduction 13

in severe redistribution of the impregnated species and the support become

non-homogeneously covered by the active material. In the wet impregnation

technique, an excess of solution is used. After a certain time the solid is

separated from the solution and the excess solvent is removed by drying. The

amount of active component introduced onto the support depends on the

equilibrium concentration of the impregnating solution, the pore volume of

the carrier and the binding of precursor on to the support surface45

.

1.5.6. Equilibrium adsorption technique

Equilibrium adsorption is a method used to deposit metal containing

species onto the surface of a porous oxide support. Deposition of species

onto the surface can occur by interaction of protonated or deprotonated

surface hydroxyl with oppositely charged solution species or by

condensation of neutral hydroxyl with hydrolyzed solution cation. Thus, the

pH of the solution is an important parameter that determines the

concentration and the nature of the deposited species, the extent of

hydrolysis of the solution cation, and the extent of protonation and

deprotonation of surface hydroxyls46

.

1.5.7. Chemical vapor deposition method

The chemical vapor deposition method designates any process of

deposition using reaction between surface sites, such as –OH groups on

inorganic supports and vapor of an active material. For preparation of

dispersed metal catalysts, volatile metal chlorides such as SnCl4 or NiCl2

were used to obtain Sn/Al2O3 or Ni/Al2O3 catalysts47

. The method

circumvents the process of impregnation, washing, drying, calcination and

activation, which are involved in solvent-assisted catalyst preparations. Other

complications caused by solvent during liquid phase preparation such as

14 Chapter 1

dissolution, surface poisoning and redistribution of the active material during

drying are also eliminated.

The methods described above however, have limitations with respect

to the chemical compositions, homogeneity, and ease of control of

stoichiometry, porosity and surface area of the final oxide.

1.5.8. Sol-gel processing

Sol-gel process is a very versatile technique, which is used in the

synthesis of nanosized composites, monoliths, thin films, optical ceramics,

fibers and catalysts. 48, 49

One of the most important requirements in the

preparation of multimetallic oxide catalysts is obtaining a good inter-

dispersion of different phases and components, which constitute the catalyst.

Preparation methods based on sol-gel techniques have been well known to

result in highly homogeneous multi-component material of molecular or

atomic levels. Such mixed metal oxides have been widely reported to have

many fold increase in catalytic activity. Table 1.4 provides a summary of

comparison of various methods for the preparation of multi-component

catalysts highlighting both their advantages and disadvantages.

Introduction 15

Table 1.4. A comparison of important methods of preparation of multi-

component oxides

Synthesis methods Advantage Disadvantage

Solid-solid blending Powder of about one

micron or more in size

Aggregation of grains,

no perfect

homogeneity, high

temperature is needed

Co-precipitation

Complex process,

insufficient

homogeneity

From preformed

complexes

Homogeneous multi-

component catalyst

Limited range of

composition

Hydrotalcite method Good stichiometry Medium homogeneity

Impregnation method Poor homogeneity

Equilibrium adsorption

technique

Limited range of

composition

Chemical vapour

deposition High product purity

Limited range of

composition

Sol-gel processing

High homogeneity,

product purity and

flexibility

Simple and requiring no

sophisticated facilities

1.6. Sol-gel processing: An overview

Interest in sol-gel processing of ceramic materials began as early as in

the mid 1800s with Ebelman and Graham’s studies on silica gels. In these

early investigations, the hydrolysis of tetraethyl orthosilicate, under acidic

conditions yielded SiO2 in the form of a glass like material. During late

1800s to the 1920s gels became of considerable interest to chemists

16 Chapter 1

stimulated by the phenomenon of Liesegaing50

rings formed from gels. Many

noted chemists including Ostwald and Lord Raleigh investigated the problem

of the periodic precipitation phenomena that led to the formation of

Liesegaing rings and the growth of crystals from gels. Roy et al51

recognized

the potential for achieving very high levels of chemical homogeneity in

colloidal gels and used the sol-gel method in the 1950s and 1960s to

synthesize a large number of novel ceramic oxide compositions that could

not be made using traditional ceramic powder methods. During the same

period Iler’s52

pioneering work on silica chemistry led to the commercial

development of colloidal silica powder. Stober et al53

extended Iler’s

findings to show that using ammonia as a catalyst for the TEOS hydrolysis

reaction could control both the morphology and size of the powders

Overbeek54

and Sugimoto55

showed that nucleation of particles in a

very short time followed by growth without super saturation will yield

monodispersed colloidal oxide particles. Matijevic et al56-59

employed these

concepts to produce enormous range of colloidal powders with controlled

size and morphologies, including oxides (TiO2, Fe3O4, BaTiO3, CeO2),

hydroxides (AlOOH, FeOOH) and various mixed phases or composites (Ni,

Co, Sr ferrites) and coated particles (Fe3O4 with Al(OH)3).

Sol-gel powder processes have also been applied to fissile elements60

where spray formed sols of UO2 and UO2-PuO2 were formed as rigid gel

spheres during passage through a column of heated liquid. Both glass and

polycrystalline ceramic fibers have been prepared by using the sol-gel

method. Compositions include TiO2-SiO2 and ZrO2-SiO2 glass fibers60, 61

. A

variety of coatings and films have also been developed using sol-gel method.

Of particular importance are the antireflection coatings of indium tin oxide

applied to glass windowpanes to improve insulation characteristics.62, 63

Introduction 17

1.6.1. Sol-gel process: Steps

Sols are dispersions of solid particles in a liquid with particle size of

1-100 nm64

A gel is an interconnected, rigid network with pores of sub

micrometer dimensions and polymeric chains whose average length is

greater than a micrometer. The term gel embraces a diversity of

combinations of substances that can be classified in four categories: (i) well-

ordered lamellar structure, (ii) covalent polymeric net-works completely

disordered, (iii) polymer networks formed through physical aggregation,

predominantly disordered and (iv) particular disordered structure

Generally three approaches are used to make sol-gel monoliths: (I)

gelation of a solution of colloidal powders, (II) hydrolysis and poly-

condensation of alkoxide or nitrate precursors followed by hypercritical

drying of gels and (III) hydrolysis and poly-condensation of alkoxide

precursors followed by aging and drying under ambient atmosphere.

1.6.1.1. Mixing

In method I, a sol is formed by mechanical mixing of substances in

water at a suitable pH. In method II and III, a liquid metal alkoxide precursor

is hydrolyzed by mixing with water65

. The possible reactions are,

2M(OH)4M

OH

O

OH

HO M

OH

OH

OH

+ H2O --------(2)

M(OR)4 + 4H2O M(OH)4 + 4ROH -------(1)

18 Chapter 1

The hydrolysis and poly-condensation reactions eventually result in

M-O-M network. The water/alcohol expelled from the reaction remains in

the pores of the network. Yoldas66

observed that the hydrolysis reaction and

the condensation reaction are not separated in time but takes place

simultaneously. It has been established that the presence of H3O+ in the

solution increases the rate of hydrolysis, whereas OH- increases the

condensation reaction. Orcel et al67

showed that the shape and size of

polymeric structural units are determined by the relative values of the rate

constants for hydrolysis and poly-condensation reactions. Fast hydrolysis

and slow poly-condensation favor formation of linear polymers. On the other

hand, slow hydrolysis and fast condensation result in large, bulkier, and more

ramified polymers68

.

1.6.1.2. Gelation

With time the sol particles link together to become a three

dimensional network. As the sol particles grow and collide, condensation

occurs and macro particles form. The sol becomes a gel when it can support

a stress elastically69

. This is typically defined as the gelation point or gelation

time. One cannot precisely define the point when the sol changes from a

viscous fluid to an elastic gel. The change is gradual as more and more

particles become interconnected. The physical characteristics of the gel

network depend greatly upon the size of the particles and extent of cross-

linking prior to gelation. During gelation, the viscosity increases sharply.

The structure of a gel is established at the time of gelation. The size and the

degree of branching of the inorganic polymer and the extent of cross-linking

have strong influence on porosity of gel, and later on the surface area, pore

volume, pore size distribution, and thermal stability of the final oxide after

calcinations70

.

Introduction 19

In general, if the gel contains polymeric chains with significant

branching and cross-linking (Fig.1.3), it will have large void regions and the

resulting oxide after calcinations will mostly be macro-porous and

mesoporous71

. Conversely, if the gel contains polymeric chains with little

branching and cross-linking, it will have smaller void regions and the

resulting oxide will mostly be micro-porous with low surface area. The

relative rates of hydrolysis and condensation determine the extent of branching

of the inorganic polymer and colloidal aggregation in the gelation mixture.

In general, in the presence of high concentration of metal cation monomer,

Fig. 1.3. Schematic diagram of gels (a) with significant branching and cross-

linking and (b) with little branching and cross-linking

20 Chapter 1

if hydrolysis is slow relative to condensation, long, highly branched and

cross-linked polymeric chains are formed and the resulting oxide can be

macro porous. If condensation and hydrolysis occur at comparable rates,

short polymeric chains with less branching and less cross-linking are formed,

and the final oxide can be micro porous in nature. Since hydrolysis and

condensation are chemical reactions, their relative rates are functions of

many parameters such as pH, temperature, nature and concentration of the

metal ion precursors and concentration of water.

1.6.1.3. Aging

When a gel is maintained in its pore liquid its structure and properties

continue to change long after the gel point. The process is called aging. Four

processes can occur singly or simultaneously during aging poly-

condensation, synersis, 72

coarsening52

and phase transformation. Poly-

condensation reactions continue to occur within the gel network and this

increase the connectivity of the network. Usually in alkoxide-based gels,

chemical hydrolysis reactions are very rapid and are completed in the early

stages of sol preparation, especially when the sol is acid catalyzed. Since the

chemical reactions are faster at high temperature, aging can be accelerated by

hydrothermal treatment, which increases the rate of condensation reaction.

The shrinkage of the gel and the resulting expulsion of liquid from

the pores are called syneresis. Syneresis in alcoholic gel systems is greatly

attributed to formation of new bonds through condensation reactions, which

increases the bridging bonds and causes the interaction of the gel network. In

aqueous gel systems, the structure is controlled by the balance between

electrostatic repulsions and attractive van der waal’s forces. Coarsening or

“Ostwald ripening52

is the irreversible decrease in surface area through

Introduction 21

dissolution and re-precipitation processes. Since the convex surfaces are

more soluble than concave surfaces, if a gel is immersed in a liquid in which

it is soluble, dissolved materials will tend to precipitate on to regions of

negative curvature. Necks between particles will grow and small pores will

get filled in, resulting in an increase in the average pore size of the gel and

decrease in the specific surface area. Washing the pore liquid out of a gel is

also an aging step73

. The pH of the wash water is critical in the case of gels

made from acid catalyzed precursors. The final properties of such gels

depend on both the pH at which the gel was formed and the pH in which it

was washed before drying.

During aging there are changes in most physical properties of the gel.

Since the condensation reaction creates additional bridging bonds, the

stiffness of the gel network increases, as does the elastic modulas74

, the

viscosity and the modulus of rupture. The strength of the gel also increases

with aging. West et al75

. showed that gel strength increased logarithmically

with time ranging between 1 and 32 days. Therefore large monolithic gels

are subjected to aging process before drying to reduce the chance of

cracking. The greater stiffness of the aged gel reduces the shrinkage during

drying, especially if the aging treatment is performed under hydrothermal

conditions.

1.6.1.4. Drying

During drying, the liquid is removed from the interconnected pore

networks. When the pore liquid is removed, shrinkage occurs and the

monolith is termed as xerogel76

(Fig.1.4). If the pore liquid is primarily

alcohol based, the monolith is often termed as an alcogel77

. A gel is defined

as dried when the physically adsorbed water is completely evacuated. This

22 Chapter 1

occurs between 100 and 1800C. Large capillary stresses can develop during

drying and this can cause the gels to crack unless the drying process is

controlled. It has been generally accepted that there are three stages of

drying. During the first stage of drying, the decrease in volume of the gel is

equal to the volume of liquid lost by evaporation. The complete gel network

is deformed by the large capillary forces, which causes the shrinkage of the

material. For large or small pore gels the greatest change in volume, weight,

density, and shrinkage occur during stage 1 drying. Stage 1 drying ends

when shrinkage ceases. Stage 2 begins when the strength of the material has

increased due to the greater packing density of the solid phase, sufficient to

resist further shrinkage. As the network resistance increases the radius of the

meniscus is reduced. Eventually the contact angle approaches zero and the

radius of the meniscus equals to the radius of the pore. This condition creates

the highest capillary pressure, and unable to compress the gel any further, the

pores begin to empty which is the start of stage 2. In stage 2 liquid transport

by flow through the surface films that cover partially empty pores. The liquid

flows to the surface where evaporation takes place. The flow is driven by the

gradient in capillary stress78

. The third stage of drying is reached when the

pores are substantially emptied and surface films along the pores cannot be

sustained. The remaining liquid can escape only by evaporation from within

the pores and diffusion of vapor to the surface. During this stage there are no

further dimensional changes but just a slow progressive loss of weight until

equilibrium is reached, determined by ambient temperature and partial

pressure of water79

. A dried gel still contains a very large concentration of

chemisorbed hydroxyls on the surface of the pores. Thermal treatment in the

range 500-8000C desorbs the hydroxyls and thereby decreases the contact

Introduction 23

angle and the sensitivity of the gels to re-hydration stresses resulting in a

stabilized gel79

.

Fig. 1. 4. Various stages of gel (a) stabilized gel, (b) wet-aged gel and (c)

xerogel

1.6.1.5. Densification

Densification is the increase in bulk density that occurs in a material

as a result of a decrease in volume fraction of pores. Heating the porous gel

at high temperature causes densification. Densification of a gel network

occurs between 10000C and 1700

0C depending up on the radii of the pores,

connectivity of the pores and surface area. Controlling the gel-glass

transition is a difficult problem if it is necessary to retain the initial shape of

the starting material. It is essential to eliminate volatile species prior to pore

closure and to eliminate density gradients due to non-uniform thermal or

atmosphere gradients. The amount of water in the gel has a major role in the

sintering behavior. The viscosity is strongly affected by the concentration of

water, 80

which in turn determines the temperature of the beginning of

densification. For example, a gel prepared in acidic conditions has a high

24 Chapter 1

surface area and water content than a gel prepared in basic conditions and

starts to densify about 2000C sooner than the base catalyzed gel

81. During

sintering, the driving force is a reduction in surface area82

. Most authors

report a diminution of the specific surface area when the densification

temperature increases. However, it was shown that certain samples display

first an increase of surface area until a temperature between 3000C and

4000C and then a decrease with further increase of temperature

83. Several

studies show that despite the complex manner in which the gel evolves

towards a glass its structure and properties become indistinguishable from

those of a melt-derived glass84-87

once the gel has been densified and heated

above the glass transition temperature. Generally the mechanisms

responsible for the shrinkage and densification of gels are capillary

contraction, condensation, structural relaxation and viscous sintering. It is

possible that several mechanisms operate at the same time.

Using different models one can describe the sintering behavior of a

gel. Frenkel’s theory, 88

which is derived for spheres, is valid for the early

stages of sintering, because of the geometrical assumptions. It is based on the

fact that the energy dissipated during viscous flow is provided by the

reduction in surface area. Scherer89

developed a model for describing the

early stages as well as the intermediate stage of sintering. It is assumed that

the microstructure consists of cylinders interacting in a cubic array. To

reduce their surface area, the cylinders become shorter and thicker. The

above models predict reasonably well the behavior of gels upon heating.

1.6.2. Advantages of sol-gel process

The motivation for sol-gel processing is primarily due to higher purity,

homogeneity of the products and the lower processing temperature compared

Introduction 25

with traditional glass melting or ceramic powder methods90

. During the past

decade, there have been enormous growths of interest in sol-gel processing.

This growth has been stimulated by several factors. On the basis of

Kistler’s91

early work, several teams produced very low-density silica

monoliths called aerogels and large monolithic pieces of alumina by sol-gel

methods. These demonstrations of potentially practical route for production

of new materials with unique properties coincided with the growing

recognition that powder processing of materials has inherent limitations in

homogeneity due to difficulty in controlling agglomeration92

.

The goal of sol-gel processing is to control the surface and interfaces

of materials during the early stages of production. Long-term reliability of

materials is usually limited by localized variations in the physical chemistry

of the surface and interfaces within the material. Each step of the process can

be controlled and modified in order to obtain a specific material, with better

characteristics than those obtained by traditional methods of preparation. The

important advantages of sol-gel processing are compositional homogeneity

and the ability to prepare shaped materials such as spherical particles, fibers

and thin films. The sol-gel process presents inherent advantage for the

preparation of porous ceramic oxides, because the nano structure of the

derived materials can be controlled together with their porous structure. At

present sol-gel processing is employed for the preparation of new generation

of advanced materials for structural, electrical, optical, electronic and

catalytic applications.

The sol-gel process is of particular interest for designing catalytically

active materials with specific properties and/or with improved performances

compared to those of classical catalysts. It allows preparation of catalytically

active materials, which can be directly cast in/on supports at the sol stage

26 Chapter 1

itself. This is a great advantage for catalytic membrane development93

. The

classical synthesis methods for conventional catalysts often start from salts

or oxide precursors and involve precipitation, impregnation, or even

solid/solid reactions. These methods are not adapted to a homogeneous

casting of the catalyst on a support and lead to limited specific surface areas

and to a heterogeneous distribution of active species. The sol-gel process

starting from a homogeneous distribution of the precursors at the molecular

level can in many cases improve these specific criteria. Indeed a large range

of methods and precursors can be investigated to obtain the required powders

or films either as pure catalyst or homogeneously doped or dispersed in a

matrix. Further more, the specificity of the process can lead to original

materials which can help in a better understanding of the catalytically active

sites in a specific catalyst.

Since the work embodied in the thesis involves developing various

metal ion doped aluminas through sol-gel route and also the structural

evolution of aluminas and mixed metal oxides including spinels of the type

MAl2O4. We thought it worthwhile to discuss some of the known aspects of

Cu2+

, Mn2+

, Co2+

, Ni2+

, Ce4+

and Ti4+

ions doped aluminas. These are

presented below in separate sections as CuO/Al2O3, MnOx/Al2O3,

CoOx/Al2O3, NiO/Al2O3, CeO2/Al2O3 and TiO2/Al2O3 mixed oxides.

1.7. CuO/Al2O3 mixed oxides

CuO/Al2O3 mixed oxides are widely used in industry as catalysts and

in electronic fields. CuAlO2 is a p-type transparent conducting oxide that has

been proposed for a number of technological applications94

. Another mixed

oxide CuAl2O4, which crystallizes in the spinel form, is a potential catalyst

for the oxidation of phenol in aqueous solutions95

and for the selective

Introduction 27

catalytic reduction of NO with propane96

. The study of CuO/Al2O3 system is

considered to be very important especially in the field of catalysis where the

CuO phase is the most active species for certain catalytic process.

Selim et al97

reported the formation of CuAl2O4 spinel in the

temperature range of 700-8000C, and the decomposition of the spinel to

CuAlO2 at higher temperatures, for a copper-aluminium mixed oxide

prepared by impregnation of -Al2O3 with copper nitrate solution in an

equimolecular ratio. It was also observed that the incorporation of sodium

ions in the mixed oxides stabilized most of the CuAl2O4 spinel in the

crystalline form, and prevented the decomposition to CuAlO2. The rate of

metal aluminate formation in MOx/Al2O3 systems (M = Ni, Co, Cu and Fe)

at 500-10000C in oxygen or nitrogen atmosphere was compared by Boit et

al.98

It was observed that spinel formation rate is higher for CuO/Al2O3

system compared to other systems.

Among the most promising new technologies of power generation

from coal are gasification integrated with combined cycle power generation,

i.e., gas turbine in series with a steam turbine and coal gasification followed

by oxidation in a molten carbonate fuel cell. Both technologies require

removal of H2S from the fuel gas prior to combustion. Patrick et al99

reported

the preparation of copper- aluminium mixed oxide sorbents by the citrate

process, and the resulting product consisted of varying amounts of

compound oxide CuAl2O4 and mixed oxides CuO and Al2O3. The sulfidation

rate was much slower for those sorbents that consisted mainly of the

compound oxide CuAl2O4 compared to those that consisted mostly mixed

oxides CuO and Al2O3. The CuAl2O4 has a partly inverse spinel structure

with Cu2+

occupying both tetrahedral and octahedral sites, the Cu2+

that

28 Chapter 1

occupies tetrahedral sites were responsible for the slow sulfidation kinetics

observed.

Youssef et al100

reported the formation of CuAl2O4 in CuO/Al2O3

system when the sample containing 20 w % of CuO was calcined at 900ºC.

The surface area decreases and means pore radius increases with increase of

Cu loading and rise of calcination temperature. Hydrogenation of

cyclopentadiene to cyclopentane on the reduced catalyst depends on the

efficiency of the dispersion of the metal at the surface of the support. Wang

et al101

prepared sol-gel derived -Al2O3 supported CuO granular sorbents

which exhibited flue gas desulfurization capacity greater than those of the

similar commercial sorbents. Catalytic combustion of methane is one of the

most promising processes for energy production. The high temperature

methane combustion requires thermally stable catalysts. According to

Artizzu et al102

CuO deposited on to high surface area MgAl2O4 spinel

prepared from alumina and magnesium nitrate showed very good activity for

the methane combustion with a selectivity of 100% to carbon dioxide.

Preparation and activity of copper-aluminium mixed oxides from

hydrotalcite like precursors, for the oxidation of phenol aqueous solutions

was reported by Alejandre et al95

. Phenol conversion decreased continuously

over time for the samples calcined at lower temperatures. Samples calcined

at higher temperatures and after HCl treatment (to avoid the CuO phase)

have pure CuAl2O4 spinel phase and showed higher activity for phenol

conversion. In another study, Alejandre et al103

prepared copper-aluminium

hydrotalcite (HT) like compounds from Cu(NO3)26H2O and Al(NO3)39H2O

using trimethylamine and CO2 as precipitants. Pure CuAl2O4 with high

surface area and catalytic property was obtained from the calcination of the

‘HT 0.5’ samples. Contrary to the reports of Shimizu et al104

and Maritisius

Introduction 29

et al105

, they observed that the rate of formation of CuAl2O4 considerably

decreased when the amount of copper in the sample increased. Woelk et al4

reported the formation of brown and blue pigments from Cu-Al-Si-O

systems in the temperature range 800-14000C. The pigment formation was

dependent on the temperature as well as composition of the sample. It was

suggested that the colour is due to two possible reactions: (i) the reaction of

copper ions in a glass matrix that formed during the calcination process and

(ii) the reaction between Al2O3 and Cu2O/CuO or SiO2 and Cu2O/CuO.

Transparent conducting oxides simultaneously exhibit both high

transparency through the visible spectrum and high electrical conductivity.

Transparent conducting oxides are utilized in a variety of commercial

applications, such as flat panel displays, photovoltaic devices and

electrochromic windows, in which they serve as transparent electrodes.

Delafossite type CuAlO2 exhibits both p-type conductivity and transparency.

The interfacial reaction between copper and alumina, which occurs during

the eutectic bonding process was observed by Yi et al106

During eutectic

bonding the compound CuAlO2 is formed at the interface between the copper

and alumina in specimens containing solid copper at temperatures between

1068 and 1072ºC. However, in specimens containing heavily oxidized

copper in which the solid copper phase was not present at the bonding

temperature, CuAlO2 formation was observed at temperatures higher than

10750C. Buijan et al

107 compared the electronic structures of CuAlO2 and

Cu2O, two Cu+

oxides that exhibit weak homoatomic d10

-d10

interactions by

the semiemprical extended Huckel’s method. It revealed the important role

of weak d10

-d10

interactions in the origin of colour and conductivity in this

family of compounds.

30 Chapter 1

A low temperature hydrothermal technique was used to synthesize

CuAlO2 by Shahriari et al108

. In this method, appropriate amounts of

aluminium metal, CuO and Al2O3 were placed along with ground sodium

hydroxide pellets in a Teflon pouch, sealed and placed in an autoclave,

heated to 210ºC and subsequently cooled to room temperature. The

formation of Cu2O, CuAl2O4 and CuO at 700ºC and CuAlO2 at temperature

greater than 800ºC in nitrogen (neutral) atmosphere was reported by Ohashi

et al109

from copper acetate hydrate and aluminium tri-sec-butoxide. Ingram

et al110

reported direct preparation of CuAlO2 by conventional high

temperature solid state reaction. The pellets prepared using high purity

constituent oxides in stoichiometric ratios were heated in air to 1100ºC for

24-36h and quench-cooled in air. This process of firing was repeated until an

X-ray phase-pure sample of CuAlO2 was achieved.

Copper compounds in the environment usually exist in two common

oxidation states Cu+

and Cu2+

. If not properly treated before discharge,

copper-containing wastewater has a great potential to contaminate the soil

and water systems. Thermal treatment has recently been emerging as a

promising environmental technology to stabilize heavy metal-containing

industrial sludge. Aluminosilicate and silica sand are the main inorganic

constituents of the earth’s upper crust, and they are acceptors of heavy metal

contaminants. Wei et al111

reported that copper could be thermally

immobilized with silica or kaolinite, and reported the formation of a less

soluble copper solid phase containing CuO in the copper contaminated

kaolinite heated at 9000C for 1 h. Wei et al

112 reported the formation of

CuAl2O4, while investigating the species distribution of copper in copper

contaminated alumina heated at 900ºC for 1h.

Introduction 31

Nitric oxides (NOx) are commonly produced by various combustion

processes, and contribute largely to a variety of air pollution such as

formation of acid rain, smog, green house effect etc. Strict environmental

regulations necessitates effective after treatments. Abatement is generally

achieved through the selective catalytic reduction. Several authors have

reported the catalytic ability of copper- aluminium mixed oxides for the

selective reduction of NOx. Fernandez-Garcia et al96

reported the reduction

of NO with CO over alumina supported copper oxide catalysts, which is less

expensive than the noble metals (Rh, Pt, Pd etc). From the correlation of

copper surface species present on the catalyst with its catalytic performance

they deduced that a redox type mechanism Cu2+

/Cu+ is working at the

reaction conditions. Shimizu et al104

prepared copper-aluminium mixed

oxide catalysts from hydroxides obtained by co- precipitation of copper

acetate and aluminium nitrate, for the selective reduction of NO with

propane. The UV- vis reflectance spectra showed bands at 1450nm and 750

nm, due to the electronic transitions in Cu2+

in tetrahedral and octahedral

sites of spinel lattice respectively. Low copper samples exhibited a band at

750 nm which indicated that Cu2+

was predominantly in octahedral sites. As

the copper content increased the intensity of band at 1450nm due to the

tetrahedral Cu2+

increased indicating the formation of aluminates. Surface

Cu2+

cations in octahedral sites were responsible for the catalytic activity.

Maritisius et al105

also observed that the amount of copper aluminate

increased with increasing CuO concentration.

Copper aluminate catalysts for abatement of NO with propylene or

ammonia in presence of oxygen was reported by Kim et al113

. The catalysts

were prepared by the incipient wetness impregnation of copper nitrate

solution on commercial alumina. The catalyst calcined at high temperature

32 Chapter 1

showed enhanced activity in the presence of propylene but had no effect on

NO conversion when ammonia was used as the reducing agent. According to

Metelkina et al 114

the catalyst prepared by the above method with 2 %

copper loading showed about 90% NO conversion at 350-400ºC. The

catalytic performance of the system appears to be determined by the degree

of clustering of copper cations. Zhu et al115

prepared copper oxide

supported on alumina by impregnation of copper nitrate on alumina at

different calcination temperatures. The N2O reduction activity increased with

increasing calcination temperature of supports from 450 to 900ºC. However,

further increase in calcination temperature of supports resulted in a reduction

in activity. Several copper species, surface spinel, bulk CuAl2O4 and CuO

were present in the CuO-Al2O3 system. Bennici et al116

reported the surface

properties of deposited CuO phase on silica modified with alumina, titania

and zirconia. All the catalysts possessed high surface area and homogeneous

coverage of the relevant support. The CuO phase deposited on the most

acidic support showed the best activity and selectivity in NOx reduction.

1.8. MnOx/Al2O3 mixed oxides

The increasing stringent environmental regulations limiting emissions

of halogenated hydrocarbons have increased enormously the demand for

technology to remove effectively halogenated compounds from waste

streams. Tseng et al117

reported the decomposition of trichloroethane (TCE)

over an MnOx/γ-Al2O3 catalyst, prepared by the incipient wetness

impregnation method with aqueous solution of manganese nitrate, in a fixed

bed reactor. Einaga et al118

reported the catalytic oxidation of cyclohexane

with ozone at room temperature over alumina supported manganese oxide

catalyst prepared by impregnation of manganese acetate on γ-Al2O3.

Trawezynski et al119

reported the oxidation of ethanol over alumina

Introduction 33

supported MnOx catalyst, prepared by a conventional impregnation of Al2O3

support with aqueous solution of manganese acetate. In another study,

Einaga et al120

reported the oxidation of benzene with ozone at room

temperature over alumina supported manganese oxide catalyst prepared by

the impregnation of γ-Al2O3 with aqueous solution of manganese acetate and

observed a linear correlation between the amount of ozone decomposed and

that of the COx formed. Alumina supported Mn catalysts with Mn loading

ranging from 3.9 to 18.2% were prepared by impregnation of γ-Al2O3

substrate with aqueous solution of manganese nitrate and tested its catalytic

activity in the combustion of formaldehyde/methanol mixture in an air

stream by Arias et al121

and reported that the activity increases with

increasing manganese loadings. Barrio et al122

reported a method for the

preparation of homogeneous coatings of manganese oxide on alumina by a

redox deposition-precipitation method using acetone as solvent and observed

that the catalyst prepared by this method is very active in the complete

oxidation of oxygenated volatile organic compounds.

The adsorption capacity for NO of alumina-supported oxides and

hydroxides of Mn have been studied by Spasova et al123

and reported that the

samples with Mn3+

ions on the surface of the support show enhanced

adsorption capacity. The structural characteristics of manganese oxides

supported on alumina during ozone decomposition reaction in the presence

of water vapour was studied by Einaga et al124

and reported that Mn was

oxidized to higher oxidation state, along with the coordination of water to

Mn. The phase transformation features of MnOx/Al2O3 system, prepared by

impregnation of manganese nitrate on γ-Al2O3 on calcination at 950ºC and

the effect of these phases on the catalytic oxidation of n-pentane was

reported by Tsyrulnikov et al125

. Arena et al126

studied the effect of the oxide

34 Chapter 1

carriers (γ-Al2O3, SiO2, ZrO2 and TiO2) and Mn-precursors (KMnO4,

manganese nitrate and manganese acetate) loading on the structure and

reduction pattern of MnOx based catalysts prepared by incipient wetness

impregnation method by PXRD and temperature programmed reduction

measurements.

Hernandez et al127

reported the synthesis of nano-particles of Mn-

doped alumina from hydrolysis of manganese and aluminium salts at 60ºC in

presence of urea. Sintered density, grain size and microstructure formation of

alumina doped with manganese oxide have been investigated by

Sathiyakumar et al128

and reported that the addition of manganese oxide

produces a relatively uniform dense microstructure at high temperature.

1.9. CoOx/Al2O3 mixed oxides

Liotta et al129

prepared CoOx catalysts supported on alumina and

BeO/Al2O3 by incipient wetness impregnation method and the catalytic

activity was tested in the selective catalytic reduction of NO with C3H6 in the

presence of excess of oxygen, and found that the presence of BeO in the

alumina network is effective in the stabilization of the dispersed Co2+

ions,

which are active and selective for NO reduction and clusters of cobalt oxide

present as Co3O4 in the alumina are active mainly for C3H6 combustion.

Konova et al130

investigated an alumina supported cobalt oxide system with

respect to catalytic decomposition of ozone, complete oxidation of volatile

organic compounds and oxidation of CO in the temperature range –40 to

250ºC in an isothermal reactor. A very high activity of cobalt towards ozone

decomposition at temperatures below -40ºC and significant increase in the

catalytic activity and decrease in the reaction temperature for oxidation of

VOCs and oxidation of CO were observed.

Introduction 35

Wang et al131

prepared a series of Co/Al2O3 catalysts with various

metal loadings by incipient wetness impregnation method and investigated

the reducibility of cobalt oxides in these samples by the temperature

programmed reduction technique and reported that for catalysts with low

cobalt loadings the cobalt phase present is CoAl2O4 and for high Co contents

it is bulk Co3O4. Ji et al132

prepared Co/Al2O3 catalyst with conventional and

sol-gel methods to vary the chemical reactivity of the alumina support and

found that the reactivity of the surface changes the surface structure and

chemical composition of the catalyst. When the metal-support interaction is

weak, Co3O4 was the predominant surface phase and with an increase in

surface reactivity the presence of CoO and CoAl2O4 on the surface were

observed. The performances of some cobalt based catalysts such as mixed

oxides derived from Co-Al hydrotalcite precursor, cobalt supported on

alumina and Co-Mn perovskite in the selective oxidation of toluene to

benzaldehyde were studied by Centi et al133

and reported that the sample

CoOx/Al2O3 mixed oxide derived from hydrotalcite precursor having Co: Al

ratio of 2 was active and selective.

Vakros et al134

modified the equilibrium deposition filtration

technique to allow the deposition via adsorption of a relatively large amount

of Co2+

ions on the γ-Al2O3 surface and prepared CoO/ γ-Al2O3 catalysts

with a very high active surface compared to that achieved by conventional

pore volume impregnation technique. Ataloglou135

prepared CoO/γ-Al2O3

catalysts using three methods. Pore volume impregnation, equilibrium

deposition filtration and pore volume impregnation by adding nitrilotriacetic

acid in the impregnation solution to study the influence of methodology used

for mounting Co(II) species on the γ-Al2O3 surface on the catalytic activity

of the catalyst for complete oxidation of benzene and observed that ‘pvi’

36 Chapter 1

catalysts exhibited the highest activity among the samples with low or

medium Co content, whereas the ‘edf’ catalysts exhibited the highest activity

among the catalysts with the maximum Co content. In another study,

Ataloglou et al136

investigated the influence of the initial concentration of the

impregnating solution used for mounting Co(II) species on γ-Al2O3 surface

by equilibrium deposition filtration method on the catalytic activity of

catalysts for the complete oxidation of benzene and observed higher catalytic

activity for catalysts prepared using high initial concentration of Co(II)

concentration.

1.10. NiO/Al2O3 mixed oxides

Alumina doped with Ni2+

ions is an important system since it

contains a d8 metal ion having interesting electronic, magnetic and catalytic

properties. This system is used in industry as heterogeneous catalysts and

ceramic pigments. Salagre et al137

reported the chemical preparation, surface

areas, pore distributions of several γ-alumina supported nickel samples active

for the catalytic hydrogenation of hexanedinitrile. XPS results showed the

presence of surface Ni2+

in the form of stoichiometric and non-stoichiometric

NiO, and stoichiometric and non-stoichiometric nickel aluminates and

surface reduced nickel. Nickel aluminate is detected at calcination

temperatures greater than 3500C Catalytically active reduced nickel (for

nitrilehydrogenations) either as naked crystallites or as encapsulated nickel

inside voided non-stoichiometric aluminate shells lie on top of the

underlying catalytically inactive NiAl2O4 when precursor calcination

temperatures are higher than 3500C and reduction temperatures of 400

0C.

Dimitrijewits et al138

prepared two series of NiO/γ-alumina catalysts

with Ni content between 3.5% and 14% and their crystalline phases, porous

Introduction 37

microstructure, and methane dry-reforming reaction behavior were studied.

In the case of the impregnated series, the results could be related with the

formation of two kinds of Ni sites: one derived from free-nickel oxide on the

support surface and another produced from NiAl2O4 due to the oxide-support

interaction. The ratio between both types of sites might change from the

beginning of the catalytic reaction to the stationary state (12h of reaction). In

the other series, which had the Ni included in the alumina matrix, an increase

of the specific activity of the sample with the highest Ni content at stationary

state, could indicate changes in the Ni sites during the reaction process. The

conversion values indicated, at both initial and stationary states, the

effectiveness of increasing the Ni-content in the two series.

Goncalves et al139

prepared and characterized various nickel based

catalysts derived through sol–gel method and reported that their surface area

decrease in the sequence Ni-SiO2 > NiAl2O3 > Ni-TiO2. Li et al140

prepared

and characterized three Ni-Al2O3 catalysts, with nickel loadings of 10-13

wt.%, by co-precipitation, impregnation on a sol–gel derived alumina, and

impregnation on a commercial γ-Al2O3 and reported that sol–gel derived

Al2O3 structure was the most stable support up to temperatures of 1000ºC.

Han et al141

investigated the influence of various factors on the existence of

the excess NiO in the sintered sample on preparing spinel from 1:1 mixture

of NiO and Al2O3 and reported that the milling time and starting materials

had an important effect on the excess of NiO while the sintering temperature

and sintering time had little effect on it.

Amini et al142

demonstrated that the coprecipitated mixture of nickel

and aluminum hydroxide precursor absorbs microwave effectively, and that

leads to the formation of nickel aluminate spinel with dominant species are

nickel ions in the tetrahedral coordinated sites. Peelamedu et al143

reported

38 Chapter 1

the simultaneous synthesis and sintering of NiAl2O4 from NiO +Al2O3

powder mixture in just 15 min in a 2.45-GHz microwave field and suggested

that the anisothermal reaction condition that was achieved in a microwave

field appears to be responsible for the observed enhancement in the reaction

kinetics of NiAl2O4 formation. Jeevanandam et al144

reported the synthesis of

nano-sized nickel aluminate spinel particles with the aid of ultrasound

radiation by a precursor approach. Sonicating an aqueous solution of nickel

nitrate, aluminium nitrate and urea yields a precursor, which on heating at

950°C for 14h yields nano-sized NiAl2O4 particles with a size of ca. 13nm

and with a surface area of about 108m2 g

-1. Arean et al

145 prepared NiAl2O4

and Al2O3-NiAl2O4 spinel solid solutions at 500ºC by means of a sol–gel

route, starting from mixed metalalkoxides, with materials having a surface

area greater than 200m2 g

_1 and reported that mixing at the molecular level

between the precursor materials seems to be the key factor allowing spinel

formation at a relatively low temperature

Thermal treatment has recently been emerging as a promising

environmental technology to stabilize heavy metal-containing industrial

sludge. Shih et al146

incorporated Ni-laden waste sludge into kaolinite-based

construction ceramic materials and evaluated incorporation efficiency and

nickel leachability of the products. Results from prolonged leachability tests

of NiO and NiAl2O4 indicate the NiAl2O4 containing samples showed

stronger intrinsic resistance to acidic attack and revealed the superiority of

NiAl2O4 over NiO in stabilizing nickel over long time periods.

1.11. CeO2/Al2O3 mixed oxides

Cerium oxide is an important component of fine grade abrasives for

polishing glass, thin film optical divices, 147

gas sensors148

superconductors, 149

Introduction 39

and heterogeneous catalysts150-152

. Alumina supported ceria forms part of the

classical three-way catalysts used for the elimination of pollutants in

automobile exhausts. Ceria acts as a catalytic activity promoter and this

promoting effect can be classified in two general types. First, it works as a

structure promoting component in the stabilization of the alumina support

toward thermal sintering153

and second as a chemical promoter enhancing the

oxygen storage capacity of the catalysts. The oxygen storage capacity of

CeO2/Al2O3 catalysts is attributed to reversible change in the cerium oxidation

state24.

The change in oxidation state is associated with reversible removal and

addition of oxygen and hence the designation oxygen storage.

Martinez-Arias et al154

characterized and studied the catalytic activity

of Pt/ CeO2/Al2O3 in CO-O2 reaction. The support was prepared by incipient

wetness impregnation of -Al2O3 with an aqueous solution of

Ce(NO3)36H2O. The platinum was incorporated by impregnation of support

with solution of [Pt(NH3)4](OH)2 previously neutralized with nitric acid.

Characterization of the surface sites carried out by means of EPR using

oxygen as the probe molecule indicates that platinum is located on

bidimensional ceria patches present at the alumina surface. The enhancement

of low temperature reduction of platinum by ceria at these sites under

CO+O2 reacting mixture led to the conclusion that the above sites are active

for the CO oxidation reaction. Damyanova et al155

reported that CeO2

loading results in the improvement of catalytic performance of CeO2/Al2O3

supported platinum catalysts for the reforming of methane with CO2.

X-ray absorption spectroscopic studies on CeO2/Al2O3 and

Pd/CeO2/Al2O3 catalysts in the reduction of NO by CO indicated that for

stoichiometric mixtures of NO + CO, the average oxidation state of cerium

was between +3 and

+4and that of Pd was between 0 and

+2

156. Cracium et

40 Chapter 1

al157

reported that the rate of steam reforming of methane was two order of

magnitude higher on Pd/CeO2/Al2O3 than on Pd/Al2O3, which was attributed

to a catalytic synergism between Pd and CeO2. Propane oxidation was

studied on a Pd supported alumina catalyst promoted with ceria, by

Guimaraes et al158

. X-ray photoelectron spectroscopy and temperature

programmed surface reaction were used to characterize the surface sites. It

was observed that the Pd/CeO2/Al2O3 catalyst was less active for oxidation of

propane at low temperatures than the Pd/Al2O3 catalyst, but became very

active at higher temperatures. However, at high temperatures large quantities

of H2 were released, which suggest a drastic change in selectivity. XPS

results showed that the catalyst containing CeO2 form the highest oxidation

state palladium species, PdO after the oxidation of propane. Xiao et al159

reported that the CeO2-MO2 (M = Zr4+

, La3+

, Ca2+

or Mg2+

) solid solution

modified Pd/-Al2O3 catalysts are efficient for the catalytic combustion of

methane and are comparable in activity with other conventional catalysts.

The active sites in the catalyst system are contacts between Pd and the CeO2-

MO2 mixed oxide component.

Kurungot et al160

reported that catalytic reactor system consisting of

integrated micro porous silica with a sand-witched type Rh/-Al2O3 catalyst

layer can give better efficiency for reforming of methane. They found that

promoting the Rh/-Al2O3 catalyst with ceria resulted in significant

improvement in the catalytic stability, possibly due to the kinetic and

oxidative stabilization of the catalyst matrix with ceria.

During a comparative study of Cu/Al2O3 and Cu/CeO2/Al2O3

catalysts with redox processes induced by thermal treatments in CO on

preoxidized samples, Martinez-Arias et al161

observed that the reduction of

copper species is favored when it is in contact with more dispersed two-

Introduction 41

dimensional ceria entities. This easier reduction of copper interacting with

ceria is presumed to favor the catalytic activity of Cu/CeO2/Al2O3 for CO

oxidation. Andreeva et al162

reported that gold-vanadia catalysts supported

on ceria-alumina (1:1) exhibited higher benzene oxidation activity in

comparison with the corresponding ones on ceria-alumina (1:4). It was also

observed that the presence of alumina prevents the agglomeration of gold

and ceria.

Djuricic et al163

reported the preparation and properties of alumina-

ceria nano-nano composites by the calcination of gels obtained by

homogeneous precipitation from dilute chloride and nitrate solutions,

followed by either microwave treatment, autoclave treatment or air drying.

They observed a variety of morphologies depending on the processing route.

The calcined microwave treated gels consisted of nano-size particles of ceria

in nanostructured transition alumina, suitable for catalysis applications.

Decarne et al164

studied the reducibility of Ce-Al-O solids using the H2-TPR

and EPR and found that dispersion of ceria on alumina stabilizes surface

ceria and improve the reducibility of bulk ceria. When ceria content

increases, reduction temperature of bulk ceria increases, whereas reduction

temperature of surface ceria decreases.

Fetter et al165

reported the selective formation of diisopropyl ether

from 2-propanol on alumina- ceria pillared clay catalysts and found that the

reaction depends on the structure and acidity of active sites. The addition of

ceria promotes selectivity and conversion rate of the reaction. Rajiah et al166

reported the synthesis of 3-nitrothphalic acid from 1-nitronaphthalene using

-alumina supported ceria catalyst at 900C with 80 mol% yield and 98%

selectivity.

42 Chapter 1

Saab et al 167

investigated the catalytic oxidation of carbon black in the

presence of CeO2 and Al2O3 catalysts and reported that the rate of carbon

black combustion increases considerably in tight contact conditions. For tight

contact CB-catalyst mixtures, a new EPR signal, designated by S2, is

observed. The appearance of the new signal indicates the formation of new

paramagnetic species consistent with localized paramagnetic spins on the

carbon particles and catalyst interface and can be considered as a fingerprint

of the contact between the two solids. These new species increase the

reactivity of the tight contact CB + CeO2 mixtures in the catalytic reaction of

CB combustion. Sun et al 168

prepared a series of alumina doped ceria

catalysts by citric acid complex combustion method and used for soot

combustion and reported that a strong interaction between CeO2 and Al2O3 in

the mixed oxide developed.

Piras et al169

reported that under oxidizing conditions at 12000C, ceria

is almost ineffective as a stabilizing agent for alumina whereas under

reducing conditions its effects are remarkably enhanced and surface area in

the order of 60 m2/g can be retained after heat treatment at 1200ºC for

several hours. This is directly related to the formation of Ce3+

present mainly

as CeAlO3, which inhibits crystal growth and prevent formation of the -

alumina responsible for reduction in surface area. The ceria entities present

in alumina supported ceria system can be classified according to their

different dispersion degrees: dispersed ceria entities, which include isolated

cerium ions, clusters thereof and or more or less thick CeOx two dimensional

patches (or 2D-Ce entities) and non-dispersed ceria species i.e. ceria

aggregates or 3D-Ce entities. The formation and properties of dispersed ceria

entities depend strongly on the interaction established with underlying

alumina support. The ceria aggregates shows properties close to that of

Introduction 43

unsupported ceria entities. Martinez-Arias et al170

reported that the relative

amounts of each of these species did not show a linear relationship with the

ceria loading. As the ceria loading increased a trend towards formation of

3D-Ce was observed.

1.12. TiO2/Al2O3 mixed oxides

Alumina-titania mixed oxide system is of interest for many

applications such as catalyst, catalyst supports and as an advanced ceramic

material171-174

. It was reported that TiO2/Al2O3 system show high catalytic

activity in the synthesis of aniline from phenol and ammonia and to possess

increased acidity compared to the single oxides175

. Aluminium titanate has

been identified as an important high temperature ceramic in view of its low

thermal expansion coefficient, low thermal conduction, excellent thermal

shock resistance and high melting point.176, 177

A number of investigations have identified alumina-titania

composites as potential catalyst supports for application in coal liquefaction.

Parker et al178

evaluated alumina-titania supported molybdenum

hydrogenation catalysts for coal liquefaction. The catalyst support was

prepared by co-precipitation from aqueous AlCl3 and TiCl4. The optimum

titania loading was found to be 30 mol.%.

Mostafa et al179

reported that TiO2/Al2O3 catalysts, prepared by

calcination of hydro-gels precipitated from the respective chloride solutions

were more active in dehydrating alcohol than the corresponding single oxide

catalysts alumina or titania. Dehydration of alcohols is related to the amount

of surface acidity rather than to the strength of surface acid sites. The

chemical composition and calcination temperature of mixed oxides also

affect the performance of the catalysts in the dehydration of alcohol.

44 Chapter 1

Stringent environmental regulations are exerting pressure to reduce the

maximum allowable sulfur content in diesel. Almost complete removal of 4,6-

dimethyl dibenzothiophene will perhaps be inevitable for reducing the sulfur

content of diesel to a level of 50 wppm and lower. Lecrenay et al180

reported

that the activity of alumina -itania supported Co Mo catalysts were higher than

that of the alumina supported catalysts for hydrodesulfurization of 4,6-

dimethyl dibenzothiophene. It was concluded that the increased acidity caused

the enhancement of activity. Cecilio et al181

observed that sol-gel alumina-

titania supports with TiO2 mol.% between 30 and 50 show good mesoporous

texture and it was particularly valuable for the preparation of well dispersed

MoS2 active phase which leads to a hydrodesulfurization catalyst with higher

activity than that prepared using commercial alumina support.

Effects of titania on the catalytic property of Pd/Al2O3 towards methane

combustion were examined by Wang et al182

. It was observed that the catalytic

activity of Pd/Al2O3 catalysts was considerably improved by pre-coating the

alumina support with titania. Hydrogen chemisorption and BET measurements

revealed that titanium modified alumina could modify the support characteristics

to achieve a high dispersion of Pd. Temperature programmed reduction and

temperature programmed desorption studies demonstrated that the coating of

Pd/Al2O3 with titania can weaken the bond strength of Pd-O and enhance the

catalytic activity towards methane combustion.

Reduction of nitrogen oxides emission has become one of the most

pressing issues to be solved for environmental protection. Catalytic reduction

of nitrogen oxides by hydrocarbon is a promising method for the removal of

dilute nitrogen oxides from engine exhausts. Kawabata et al183

prepared

alumina-titania catalysts from various alumina and titania sources, metal

alkoxides and metal alkoxide modified with organic groups by sol-gel

Introduction 45

method. It was observed that specific surface area, pore size distribution,

solid acidity and catalytic activity of NO reduction for the alumina-titania

catalysts depended on the alumina and titania sources. The catalyst prepared

from metal alkoxide for alumina source and metal alkoxide modified with

organic groups for titania source exhibited higher activity of NO reduction

than other alumina-titania catalysts.

Farias et al184

prepared 1:1 mixed oxides of Al2O3-ZrO2, Al2O3-TiO2,

SiO2-TiO2 and ZrO2-TiO2 at 6000C from sol-gel derived precursors and all

samples had amorphous structures. The oxides are active catalysts in the

epoxidation of cyclooctene with tert-butylhydroperoxide as oxidant. The

titania containing catalysts are the most active and selective ones. Gutierrez-

Alejandre et al185

reported a comparative study of alumina-titania mixed

oxides prepared by sol-gel and impregnation procedures. It was observed that

the samples prepared by sol-gel process showed an increase of surface area

with titania loading, in contrast to their impregnation counterparts, where a

decrease was observed. Better titania dispersion was obtained by the sol-gel

technique than by impregnation technique.

Montoya et al186

reported the effect of temperature on the structural

and textural evolution of sol-gel alumina-titania mixed oxides. Mixed oxides

calcined at 500ºC were amorphous and formed by nanometric particles and

pore sizes of about 2 nm. For these samples the surface area increased as the

titania concentration increased. The sintering process started at 7000C,

resulting in particle and pore size growing from 2nm to about 10 nm and the

powders remained amorphous. At 900ºC both sintering and crystallization

process of TiO2 and Al2O3 were accelerated by the Ti4+

segregation, rutile

and -Al2O3 were present as separate phases.

46 Chapter 1

Linacero et al187

studied a series of xTiO-(1-x)Al2O3 mixed oxides

prepared by sol-gel method with variable amounts of titania, from pure

alumina to pure titania. The textural results showed that at low titania

contents, higher surface area than those of pure alumina were obtained, and

in titania rich samples, higher surface areas than for pure titania were

stabilized. The titanium content also affected the crystallization process. It

was suggested that changing the composition of the mixed oxide might vary

the strength and distribution of the acid sites.

A series of xTiO-(1-x)Al2O3 mixed oxides with different molar ratios

were prepared by a non-alkoxide sol-gel route. It was found that alumina

introduces structure and textural modification to titania and the resultant

properties of the mixed oxides vary with the composition. The anatase to

rutile phase transformation temperature increased with increasing mole

percent of alumina. The surface areas of mixed oxides were about 80%

higher than that of pure alumina 188

. Alumina-titania nano mixtures with

molar ratio 1:0.5 and 1:5 were synthesized by a sol-gel technique. The

precursor powders were characterized by TGA, BET and TEM. The results

showed that as the titania content increases the specific surface area

increased. TEM monographs showed a highly homogeneous distribution of

the constituents with fine needle like particles of ~10nm size189

. In another

report190

they observed a specific surface area as high as 291m2g

-1 for 1:5

Al2O3/TiO2 composite calcined at 400ºC.

I.13. Scope and objectives

The vast majority of catalysts used in modern chemical industry are

based on mixed metal oxides .The preparation of specific tailor made mixed

oxides which can perform complex functions is one of the main topics of

Introduction 47

research in the field of heterogeneous catalysis. Achieving complex catalytic

reactions require a poly-functional catalyst with appropriate morphological

properties. The mixed metal oxide catalyst preparation has focused on a

tuning of the chemical reactivity of the oxide surface to modify the

interaction with adsorbed reacting species. Catalytic activities of oxides have

been associated with surface defects such as surface vacancies. Surface

vacancies are basic or acidic, depending on the type of missing ions. Cation

vacancies can be highly basic while anion vacancies are acidic. Defects in

solid oxides are generally made by doping it with a compound in which the

metal ion has a valence other than the metal in the host oxide. Over the past

decade, research on nano-crystalline materials has been greatly accelerated

by the advances in the ability to manipulate structures on the molecular or

atomic level. Direct synthesis and successful stabilization of nano-crystalline

ceramic materials has only recently been investigated in detail for some

catalytic applications. When a ceramic is fabricated from “nano” powders,

the resulting advanced nano-phase material has dramatically improved

properties; it may conduct electrons, ions and heat more readily than

conventional materials, the features that will have impact on catalyst

efficiency.2 The needs for better catalysts will only increase as environmental

and economic concerns motivate the development of them.

From the discussion made earlier it is clear that in most of the reports

the multi-component oxides were prepared by impregnation of the metal salt

on γ-Al2O3 or solid-solid blending of metal oxide and γ-Al2O3 at high

temperatures. These methods have inherent weakness regarding homogeneity

and surface properties of the products. In these methods the matrix

component used was commercially available γ-Al2O3 and it is likely that this

may considerably affect the reproducibility of the samples prepared. Use of

48 Chapter 1

hydroxides of aluminium as precursor for Al2O3 is fundamental to obtain

pure and reproducible matrix component. Boehmite remains the most

important and versatile precursor for the synthesis of transition aluminas.

Further no study was reported by systematically varying the level of

incorporation of metal ions into the matrix component and calcination

temperature and the influence of these factors on the structural evolution of

various phases and surface characteristics of mixed metal oxides generated.

The present investigation aims to modulate the surface properties of alumina

by doping with Cu2+

, Mn2+

, Co2+

, Ni2+

, Ce4+

and Ti4+

ions using boehmite as

the source of alumina through sol-gel route. For a better understanding of the

properties of the mixed metal oxides developed, it is necessary to know the

structural transformations that occur when the metal ion doped boehmite

xerogels are calcined over a range of temperatures. Therefore we have

focused more on the effect of dopants on the structural evolution of alumina

from boehmite. Thus the main objectives of our study are:

Generation of Cu2+

, Mn2+

, Co2+

, Ni2+

, Ce4+

and Ti4+

ions doped

boehmite xerogels containing varying concentrations of the dopants

and the study of the thermal characteristics of the mixed xerogels to

monitor the chemical processes and phase evolution in them.

Preparation of Cu2+

, Mn2+

, Co2+

, Ni2+

, Ce4+

and Ti4+

ions doped

alumina mixed oxides by the calcination of the above xerogels at

various temperatures.

Investigation of the effect of dopants on the structural evolution of

alumina from boehmite by spectroscopic technique and PXRD

method.

Introduction 49

Monitor the formation characteristics of alumina based spinels

MAl2O4 (M = Cu2+

, Mn2+

, Co2+

and Ni2+

) on calcination of the mixed

xerogels and study the influence of these compounds on the structural

evolution of alumina from boehmite.

Investigation of the influence of the insitu formed MAl2O4 spinels on

the surface characteristics of the alumina mixed oxides.

Investigation of the surface acidity features of the doped and undoped

oxides developed through pyridine adsorption by employing TGA,

FTIR and CHN analysis.

Study of the surface characteristics such as specific surface area, pore

volume and pore size distribution of the selected mixed metal oxides

developed which are relevant for catalytic functions.

50 Chapter 1

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