OPTIMIZATION OF MODIFIED SOL-GEL METHOD FOR THE FORMATION OF TETRAGONAL PHASE ZIRCONIA IN Zr(IV)/Ce(IV)

CATALYST

LEIW SOOK FUN

UNIVERSITI TEKNOLOGI MALAYSIA

OPTIMIZATION OF MODIFIED SOL-GEL METHOD FOR THE

FORMATION OF TETRAGONAL PHASE ZIRCONIA IN Zr(IV)/Ce(IV)

CATALYST

LEIW SOOK FUN

A dissertation submitted in partial fulfillment of the requirements for the award

of the degree of Master of Science (Chemistry)

Faculty of Science

Universiti Teknologi Malaysia

MARCH, 2005

iii

This dissertation is dedi ated to my beloved amily and Choong Meng

Thanks or being there when I needed you.

iv

ACKNOWLEDGEMENTS

I would like to e press my deepest and sin ere thanks to my proje t

supervisor, Asso iate Pro essor Dr. Nor A iah Buang or valuable ideas and help ul

guidan e given sin e the beginning o this resear h.

Sin ere gratitude also goes to all those who have helped me dire tly or

indire tly in this resear h, Pro essor Dr. an A elee an Abu Bakar, Mrs. Mariam

Hassan, Mr. Hanan, Mr. A mi, and Mr. Junaidi Bin Mohamad Nasir rom the

Department o Chemistry, Mr. Jaa ar Raji rom the Department o Physi s,

Mr. ainal Abidin Abbas and Mr. Je ri rom the a ulty o Me hani al Engineering.

Thanks are also due to Mr. Lim Kheng ei rom the Institute Ibnu Sina or an easy

a ess to RD analyti al instrument.

inally, I am grate ul or the motivation and support o my amily and riends

in ompleting my thesis proje t. Thank you or your support and aith in me and or

making it all worthwhile.

v

ABSTRACT

ir onium- erium mi ed metal o ides is widely used as promoters in

automobile emissions ontrol atalyst systems (Three- ay Catalyst, T C). In order

to improve the per orman e o ir onia- eria atalysts, a resear h has been done to

develop a ir onia-based atalyst doped with eria by modi ied sol-gel method with

ir onium o y hloride and erium (III) nitrate he ahydrate as the pre ursors. This

resear h investigated the optimum onditions on the ormation o single phase

tetragonal r 2. A series o r1- Ce 2 atalyst by varying the atomi per entage

ratios were prepared in di erent amount o water and al inations temperature. The

properties o atalysts were hara teri ed with -Ray Di ra tion ( RD), sur a e

area analysis, and ourier-Trans orm In rared Spe tros opy ( TIR). The atalyst

showed the best result is sele ted to test or C onversion. The r .7 Ce .3 2

atalyst prepared in 5 mL o water dried or two days at 7 C and al ined at 4 C

or 17 hours gives the optimum ondition on the highest ormation o single phase

tetragonal r 2. The RD analysis showed that this atalyst has rystallinity

property, onsists o tetragonal r 2 as the main phase and average rystallite si e o

appro imately 12.9 nm serve as the a tive phases or the atalyst. Sur a e area

analysis showed that this atalyst has 35.36 m2/g o sur a e area. The TIR analysis

showed the e isten e o sur a e hydro yl group on the sur a e o atalyst. The

bands that were assigned to the water de ormation mode and hydro yl de ormation

mode o sur a e hydro yl groups have been eliminated as the al ination temperature

in reased. The r .7 Ce .3 2 atalyst prepared in 5 mL o water was reported to

e hibit the best atalyti a tivity towards C onversion, it showed the lowest TL at

room temperature while 1 C onversion was a hieved at T1 32 C.

vi

ABSTRAK

Campuran logam oksida irkonium- erium diguna se ara meluas sebagai

penyokong dalam sistem pemangkinan bagi pengawalan bahan pen emar daripada

kenderaan bermotor (Pemangkinan Tiga Arah, TWC). Demi meningkatkan

keupayaan mangkin irkonia- eria, satu kajian telah dilakukan untuk menyediakan

mangkin berasaskan irkonia yang didopkan dengan eria melalui kaedah

pengubahsuaian sol-gel di mana irkonium oksiklorida dan erium (III) nitrat

heksahidrat sebagai bahan pemula. Kajian ini dijalankan bertujuan menyelidik

keadaan yang optimum bagi pembentukan asa tunggal r 2 berstruktur tetragonal.

Satu siri mangkin r1- Ce 2 telah disediakan dengan mempelbagaikan nisbah

peratus atom dan menggunakan kuantiti air serta suhu pengkalsinan yang berbe a.

Analisis pen irian dilakukan terhadap mangkin dengan menggunakan teknik

pembelauan sinar- ( RD), analisis luas permukaan dan TIR. Mangkin yang

menunjukkan keputusan yang terbaik dipilih untuk kajian penukaran gas karbon

monoksida (C ) kepada C 2. Mangkin r .7 Ce .3 2 yang disediakan

menggunakan 5 mL air dan dikering pada suhu 7 C selama dua hari serta dikalsin

pada suhu 4 C selam 17 jam menunjukkan keadaan yang paling optimum dalam

pembentukan asa tunggal r 2 berstruktur tetragonal. Analisis RD menunjukkan

mangkin ini terdiri daripada asa tetragonal r 2 sebagai asa utama dengan si at

hablur serta purata sai hablur kira-kira 12.9 nm yang bertindak sebagai asa akti

mangkin. Analisis luas permukaan menunjukkan mangkin ini mempunyai luas

permukaan sebanyak 35.36 m2/g. Keputusan TIR menunjukkan kehadiran hidroksil

pada permukaan mangkin. Pun ak-pun ak bagi air, hidroksil telah disingkirkan

apabila suhu pengkalsinan meningkat. Penyediaan mangkin r .7 Ce .3 2 dalam

5 mL air memberikan keputusan aktiviti pemangkinan terhadap penukaran gas C

yang paling baik di mana menunjukkan TL yang paling rendah pada suhu bilik

sementara 1 penukaran gas C di apai pada T1 32 C.

vii

CONTENTS

PAGE

TITLE i

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENTS iv

ABSTRACT v

ABSTRAK vi

CONTENTS vii

LIST OF TABLES

LIST OF FIGURES i

LIST OF SYMBOLS / ABBREVIATIONS /

NOTATIONS / TERMS iii

LIST OF APPENDICES iv

CHAPTER I INTRODUCTION

1.1 Air Pollution 1

1.2 De inition and Types o Pollutants 2

1.3 E e ts o Air Pollution 2

1.4 Catalyst 3

1.5 Catalyti Converter 3

1.6 Three- ay Catalysts 4

1.7 ir onium ide or ir onia 5

viii

1. Cerium ide or Ceria 7

1. .1 Role o Ce 2 in the T Cs 9

1.9 Catalysis by ir onia-Ceria 9

1.1 Sol-Gel Chemistry 11

1.1 .1 The Advantages o Sol-Gel

Approa h ver the Traditional

Methods o Preparation 14

1.1 .2 E e ts o ariables on the

Chemistry o the Sol-Gel Synthesis 15

1.1 .3 Preparation o Catalysts by Sol-Gel

Method 16

1.11 Statement o the Problem and the Needs o

the Study 1

1.12 Resear h bje tives 19

1.13 S ope o Resear h 19

CHAPTER II EXPERIMENTAL

2.1 Chemi als 2

2.2 E periment 2

2.3 Preparation o ir onia Based Catalysts

Doped with Ceria by Modi ied Sol-Gel

Method 21

2.4 Drying and Cal ination 22

2.5 Chara teri ation Te hini ues 23

2.5.1 -Ray Di ra tion ( RD) 23

2.5.2 Sur a e Area Analysis 25

2.5.3 ourier-Tran orm In rared

Spe tros opy ( TIR) 26

2.6 Catalyti A tivity Testing Measurement 2

i

CHAPTER III RESULTS AND DISCUSSIONS

3.1 In luen e o ariables on the ormation o

Tetragonal Phase r 2 3

3.1.1 E e t o Amount o ater on the

ormation o Tetragonal Phase

r 2-Based Catalyst Doped with

Ce 2 ( r .95:Ce . 5) 31

3.1.2 E e t o Loading o Ce 2 on the

ormation o Tetragonal Phase r 2 34

3.1.3 E e t o Cal ination Temperature

on the ormation o Tetragonal Phase

r 2 in r .7 Ce .3 2 Catalyst 4

3.2 ourier Trans orm In rared ( TIR) Analysis 46

3.3 Catalyti A tivity o r .7 Ce .3 2 Catalyst

Towards Carbon Mono ide idation

Rea tion 49

CHAPTER IV CONCLUSIONS

4.1 Con lusions 52

4.2 uture orks 53

REFERENCES 55

APPENDICES 64

LIST OF TABLES

TABLE NUMBER TITLE PAGE

1.1 Rea tions o urring on automobile e haust atalyst 5

2.1 Classi i ation o In rared Radiation 27

3.1 RD peaks assignment or r .95Ce . 5 2 atalyst

sample in di erent amount o water 33

3.2 RD peaks assignment or r Ce1- 2 atalyst

in mL o water 36

3.3 RD peaks assignment or r Ce -1 2 atalysts

in 5 mL o water 39

3.4 Parti le si es o the sample with di erent amount

o eria per entage in 5 mL and mL o water

al ined at 4 C 4

3.5 RD peaks assignment or r .7 Ce .3 2 atalyst

prepared in 5 mL and mL o water al ined at

4 C and 6 C 44

3.6 Parti le si es and sur a e areas o r .7 Ce .3 2

atalysts prepared in 5 mL and mL o water

al ined at 4 C and 6 C 46

3.7 Catalyti a tivity o r .7 Ce .3 2 atalyst

towards C onversion 5

i

LIST OF FIGURES

FIGURE NUMBER TITLE PAGE

1.1 Diagram o a typi al atalyti onverter 4

2.1 S hemati diagram o the atalyst preparation

set-up using modi ied sol-gel method 21

2.2 Basi eatures o a typi al RD e periment 24

2.3 S hemati diagram o the mi rorea tor sample tube 29

3.1 Di ra tograms o r .95Ce . 5 2 using di erent

amount o water: (a) mL and (b) 5 mL 32

3.2 Di ra tograms o r Ce1- 2 atalyst with di erent

amount o eria in mL o water (a) r .95Ce . 5 2,

(b) r .9 Ce .1 2, ( ) r . Ce .2 2, and

(d) r .7 Ce .3 2 35

3.3 Di ra tograms o r Ce1- 2 atalyst with di erent

amount o eria in 5 mL o water (a) r .95Ce . 5 2,

(b) r .9 Ce .1 2, ( ) r . Ce .2 2, and

(d) r .7 Ce .3 2 3

3.4 Di ra tograms o r .7 Ce .3 2 prepared using

5 mL o water dried at (a) 7 C, and al ined at

(b) 4 C and ( ) 6 C 42

3.5 Di ra tograms o r .7 Ce .3 2 prepared using

mL o water and al ined at (a) 4 C and

(b) 6 C 43

3.6 TIR spe trum o r .7 Ce .3 2 atalyst prepared

in 5 mL o water dried at 7 C 47

ii

3.7 TIR spe trum o r .7 Ce .3 2 atalyst prepared

in 5 mL o water al ined at 4 C 4

3. TIR spe trum o r .7 Ce .3 2 atalyst prepared

in 5 mL o water al ined at 6 C 49

3.9 Per entage onversion o C versus temperature

or the r .7 Ce .3 2 atalyst prepared in

mL and 5 mL o water 51

iii

LIST OF SYMBOLS/ ABBREVIATIONS/ NOTATIONS/ TERMS

A/ - Air-to- uel ratio

- Hal angle o di ra ted beam

- avelength o the -rays

- Peak width

L - Parti le si e

- Cubi

Ce 2 - Cerium o ide/ eria

C - Carbon mono ide

d - Interplaner spa ing in a rystal

TIR - ourier-trans orm in rared

HC - Hydro arbon

m - Mono lini

N - Nitrogen o ides

SC - ygen storage apa ity

PD - Power Di ra tion ile

RT - Room temperature

t - Tetragonal

T1 - 1 onversion temperature

TL - Light-o temperature

T C - Three-way atalyst

RD - -ray di ra tion

r 2 - ir onium o ide/ ir onia

iv

LIST OF APPENDICES

APPENDIX TITLE PAGE NUMBER

A Cal ulation o Parti le Si es 64

B Cal ulation o Gas Conversion in Per entage 65

C Di ra tograms o r .9 Ce .1 2 prepared by using

5 mL o water al ined at: a) 4 C and b) 6 C 66

D Di ra tograms o r . Ce .2 2 prepared by using

5 mL o water al ined at: a) 4 C and b) 6 C 67

CHAPTER I

INTRODUCTION

1.1 Air Pollution

Air pollution is the presen e o material in air in uantities large enough to

produ e harm ul e e ts. The undesirable materials may damage, vegetation, human

property, or the global environment as well as reate aestheti insults in the orm o

brown or ha y air or unpleasant smells 1 .

Air pollution aused by mobile sour e had been re eived mu h attention by

most o the environmentalist. In the last 6 years the world vehi les leet has

in reased rom about 4 million vehi les to over 7 million, this igure is proje ted

to in rease to 92 million by the year 21 2 . This will lead to in rease o more

and more air pollutant. The engine e haust ontains prin ipally three main

pollutants, unburned or partially burned hydro arbons (HCs), arbon mono ide (C )

and nitrogen o ides (N ), mostly N , in addition to other ompounds su h as

water, hydrogen, nitrogen, o ygen, et . Sulphur o ides, though polluting, are

normally not removed by the post ombustion treatments, sin e the only e e tive

way is to redu e them to elemental sulphur, whi h would a umulate in the system

2 .

2

1.2 Definition and Types of Pollutants

Any solid, li uid, or gas that is present in the air in a on entration that

auses some deleterious e e t is onsidered an air pollutant. However, there are

several substan e that, by virtue by their massive rates o emission and harm ul

e e ts, are onsidered the most signi i ant pollutants 3 .

Substan es emitted dire tly rom sour es are alled primary pollutants. The

pollutants manu a tured in the lower atmosphere by hemi al rea tions among

primary pollutants are alled se ondary pollutants, they are responsible or most o

the smog, ha e, and eye irritation and or many o the orms o plant and material

damage attributed to air pollution 4 .

1.3 Effects of Air Pollution

Carbon mono ide (C ) is a gaseous pollutant whi h enters the atmosphere

rom our sour es: ossil uel ombustion and industrial emissions, biomass burning,

o idation o CH4, and o idation o non-methane hydro arbon 5 .

C is a olorless and odorless gas. It is very stable and has a li etime o 2 to 4

months in the atmosphere. High on entration o C an ause physiologi al and

pathologi al hanges and ultimately death. C is a poisonous inhalent that deprives

the body tissues o ne essary o ygen 6 .

N in the atmosphere auses environment problems, su h as photo hemi al

o idant and a id rain 7 . Nitrogen dio ide (N 2) an rea t with moisture present in

the atmosphere to orm nitri a id, whi h an ause onsidered orrosion o metal

sur a e. Nitrogen o ide (N ) is a olorless gas and its ambient on entration is

usually or less than .5 ppm. N is a pre ursor to the ormation o nitrogen dio ide

and is an a tive ompound in photo hemi al smog ormation as well 6 .

3

Unburned hydro arbons in ombination with the o ides o nitrogen in the

presen e o sunlight orm photo hemi al o idants, omponents o photo hemi al

smog, that do have adverse e e ts on human health and on plants 6 .

1.4 Catalyst

A atalyst is a substan e that in reased the rate at whi h a hemi al rea tion

approa hes e uilibrium without itsel be oming permanently involved in the rea tion

.

Basi ally, atalysts are onsidered as hemi al ompounds apable o

dire ting and a elerating thermodynami ally easible rea tions while remaining

unaltered at the end o the rea tion, whose thermodynami e uilibrium they

onse uently annot hange 9 .

Catalysis is homogeneous when the atalyst is soluble in the rea tion

medium, and heterogeneous when atalyst is e isting in a phase distin tly di erent

rom the phase o the rea tion medium. In most instan es o heterogeneous atalysis,

the atalyst is a solid that is brought into onta t with gas or li uid rea tants to bring

about a trans ormation 9 .

1.5 Catalytic Converter

Nowadays, more than 95 o vehi les produ ed in the world are e uipped

with a atalyti onverter, whi h or the gasoline- uelled engines, is almost

e lusively based on the so- alled three-way atalyst (T C). T Cs are apable o

simultaneously and e i iently onverting C , hydro arbon (HC) and N into

harmless C 2. H2 and N2, provided that the so- alled and air-to- uel ratio (A/ ) is

onstantly kept at the stoi hiometry 1 . A typi al design o a modern three-way

atalyti onverter is reported in ig. 1.1. Basi ally, it is a stainless steel ontainer

4

whi h in orporates a honey omb monolith made o ordierite

(2Mg .2Al2 3).5Si 2) or metal 2 .

Figure 1.1: Diagram o a typi al atalyti onverter

1.6 Three-way Catalysts

Automobiles under severe driving onditions su h as a long-distan e

ontinuous running usually generate high temperature in the atalyti onverter, that

may ause thermal sintering o both metal a tive phases and atalyst support, and

inally leads to atalyst dea tivation. n the other hand, at ool start, the atalyst

usually shows low atalyti a tivity. There ore, a satis a tory atalyti e i iently at

high temperature and a promoted light-o rea tion behavior at ool start are

demanded or the urrent three-way atalysts (T Cs) 11 .

Three-way atalysts (T Cs) have been widely used to redu es pollutant

emissions rom gasoline engine powered vehi les 12 . arious three-way atalysts

(T Cs) are widely used in ar e hausts or the onversion o poisonous gases (C ,

N and hydro arbons) to harmless H2 , C 2 and N2. These onverters, be ause o

their e e tiveness, be ome ompulsory part o the modern automobile 13 .

5

The on ept o using atalyst to onvert C , N , and HC to less

environment a tive ompounds su h as nitrogen, water and arbon dio ide was well

established pra ti e prior to the need arising on motor vehi les. The prin ipal

rea tions are shown in Table 1.1 14 .

Table 1.1: Rea tions o urring on automobile e haust atalyst Oxidation Reactions

2C 2 2C 2

HC 2 C 2 H2

Reduction Reactions

2C 2N 2 2C 2 N2

HC N C 2 H2 N2

Sin e C and HC emission are removed by o idation whereas N may only

be removed by rea tion with redu ing agents, the elimination o all three pollutants

an only be a hieved by separating the o idation and redu tion un tions in

appropriate rea tors or by design o sele tive atalyst on whi h the two rea tions

may pro eed simultaneously 15 .

The development o the three-way atalyst (T C) was di tated by the need

to simultaneously onvert the HC, C , and N , present in the automobile e haust

to H2 , C 2 and N2 16 .

1.7 Zirconium Oxide or Zirconia

ir onium o ide or ir onia is widespread in nature as a tra e mineral,

baddeleyite. In its pure orm, properties su h as high hardness, low wear resistant e,

low oe i ient o ri tion, high elasti modulus, hemi al inertness, good ioni

ondu tivity, low thermal ondu tivity, and high melting temperature have made

ir onia a te hnologi ally important material. These uni ue properties enable a wide

range o appli ation o ir onia su h as stru tural materials, thermal barrier oatings,

6

solid-state ele trolytes, solid uel ells, gate diele tri s, o ygen sensors, and

heterogeneous atalysts 17 .

Sin e ir onia has a latti e with many va an ies, it is a good ioni ondu tor

at high temperature. These va an ies allowed it to be used as an o ygen sensor to

measure the air-to- uel ratio in internal ombustion engines in order to ontrol the

emission o N , C and hydro arbon ompounds 1 .

r 2- based materials have attra ted onsiderable interest in re ent de ades.

In a atalyti sense, they appear to have some advantages in area o pra ti al

appli ation over traditional o ides, su h as Si 2 and Al2 3. More signi i ant is the

a t that additives an bring about a strong modi i ation o the sur a e stru ture o

ir onia, in whi h ase substitution o r4 with dopant ations results in a rise in

anion va an y on entration and ondu tivity 19 .

The tailored physi o- hemi al properties o ir onia in luding stru ture,

redo , and a id-base hara teristi s make it as an attra tive atalyst and atalyst

support or a number o rea tions 2 . In a t, due to its a id-base bi un tional

properties, it atalyses the hydrogenation o di erent substrates su h as or e ample

ben oi a id, arbo yli a ids, and arbon mono ide. However, the main use o

ir onia is as an e i ient support or a variety o atalyti systems 21 .

ir onia is also widely used in opti al ields be ause o its high re ra tive

inde , large opti al band gap, and low opti al loss and s atter in the in rared region.

It is also use ul or making high-re le tivity mirrors, laser gyros and broadband

inter eren e ilters 22 . Depending on a tors su h as preparation method, pH,

temperature, and kineti me hanism, syntheti ir onia shows three rystalline

phases: mono lini (m), tetragonal (t), and ubi ( ) it has also been shown that a

high-pressure orthorhombi orm e ists 23 . At room temperature, t- and -phases

metastable, while m-phase is stable. t- and -phases are stable at high temperature.

They an be partially or ully stabili ed at room temperature by addition a small

amount o o ides (e.g. Ca , Mg and types o rare earth o ides) 24 .

7

The mono lini phase is stable up to 14 K where it trans orms to the

tetragonal phase, whi h is stable up to 157 K while the ubi phase e ists up to the

melting point o 29 K. However, the ubi phase an be stabili ed by the addition

o divalent ations su h as Ca2 , Mg2 or Y3 2 . Several methods have been

developed to stabili e the tetragonal phase at low temperature. These in lude the

addition o dopants (Y2 3, Ca , Mg , Ce 2), or the use o so t- hemistry methods,

rom whi h nano rystalline materials an be obtained 23 .

It is known that tetragonal ir onia as a metastable phase e ists at well below

its normal trans ormation temperature as a result o the rystalline si e e e t, the

presen e o impurities, or the presen e o strain at the domain boundaries in

polydomain tetragonal parti les 25 .

Tetragonal- r 2 is used as a atalyst and atalyst support or various gas

phase rea tions 26 . Several studies have shown that the per entage o tetragonal

phase o r 2 (t- r 2) is ru ial or the a tivity and sele tivity in atalysis 27 .

However, a phase trans ormation rom the tetragonal phase to the less rea tive

mono lini phase o rystalline r 2 (m- r 2) during the rea tion prevents the

urther appli ation o r 2. There ore, stabili ing the tetragonal ir onia during

rea tion be omes an important topi in atalysis ields 2 . r 2 stabili ed by Ce 2

and other rare earth is known to in rease durability and removal e i ien y or

automotive emission ontrol 29 .

1.8 Cerium Oxide or Ceria

Cerium o ide or eria (Ce 2) has been onsidered and largely used in the last

years as one o the most important promoters o heterogeneous atalyti rea tions

3 . Cerium o ide is one o the most important omponents in high-per orman e

three-way atalyst (T C) or its ability in enhan ing the removal o arbon

mono ide (C ), nitrogen o ides (N ) and hydro arbons (HC) 31 .

Cerium o ide has been onsidered o potential interest or solid o ide uel

ells and three-way automobile atalysts be ause o its good o ygen ion

ondu tivity. Its ability to a t as an o ygen bu er and there ore to operate

e e tively under onditions o os illating o ygen on entration has promoted its use

in automobile atalyti appli ations 32 .

However, pure Ce 2 alone is known to be poorly thermostable. The loss in

sur a e area is usually related to the hanges in the pore stru ture and to rystalline

growth. It is, there ore, very important to improve its te tural stability. Doping with

ations su h as Al3 , r4 , or Si4 may signi i antly improve the stability o the

sur a e area o eria at high temperature 33 .

Among the lanthanide elements, erium is the only one that orms stable

ompounds in a tetravalent o idation (i.e., Ce4 ) and the oordination number o

sur a e Ce4 an vary between our and eight, si and eight being the oordination

number o bulk Ce4 . urthermore, Cerium ions an behaves a large 2 trap: due to

its low redo potential and erium ion an easily pass rom Ce4 to Ce3 34 .

Ce 2 has a ubi luorite-like stru ture in whi h erium oordinates eight

o ygen anions at the orners o a ube, while o ygen is tetrahedrally surrounded by

our erium ations. It has been demonstrated that the ygen Storage Capa ity

( SC) o erium o ide is improved by suitable doping with di erent ationi spe ies

and in parti ular with lanthanum 35 .

Agra iotis et al 32 studied deposition o nanophase doped- eria systems on

erami honey omb or automotive atalyti appli ations. A remarkable

enhan ement on the atalyti a tivity o erium o ide has been observed when the

power is o nanophase dimensions be ause o the large number o latti e de e ts su h

as o ygen va an ies present, whi h provide a large number o a tive sites or gas-

solid atalysis. The number o de e ts an be in reased by partial substitution

(doping) o the erium atoms in the erium o ide latti e with other metal atoms o

lower o idation state su h as Ca or Nd. Be ause o these properties, doped erium

nano-o ides an a t as atalyst themselves, redu ing signi i antly the need or noble

metals.

9

1.8.1 Role of CeO2 in the TWCs

It is an ambitions task to de ine the role o the Ce 2 in the three-way atalysis

sin e multiple e e ts have been attributed to this promoter. Ceria was suggested to

16 :

(i) promote the noble metal dispersion

(ii) in rease the thermal stability o the Al2 3 support

(iii) promote the water gas shi t ( GS) and steam re orming rea tions

(iv) avour atalyti a tivity at the inter a ial metal-support sites, et

(v) promote C removal through o idation employing a latti e o ygen

(vi) store and release o ygen under, respe tively, lean and ri h onditions.

1.9 Catalysis by Zirconia-Ceria

Ceria- ir onia solid solutions omprise an interesting system with great

potential or appli ations in various te hnologi al ields. Depending on omposition,

eria- ir onia may be used or the manu a ture o solid o ide uel ells, gas sensors,

o ygen semi-permeable membranes or steam ele trolysis and hot ele trodes or

magnetohydrodynami systems 36 .

Not long ago, in the middle o 199 s, Ce 2- r 2 mi ed o ides has been

pro laimed as one o the most perspe tive materials or appli ation as a omponent

o so- alled three-way atalysts (T C) in automotive pollution ontrol. These

solids are able to neutrali e simultaneously C , hydro arbons and N . High

atalyti a tivity o su h system ombines with enhan ed thermal stability and high

ygen Storage Capa ity ( SC). The latter is due to the presen e o erium ions

able to hange easily their o idation state depending on the environment 37 .

Cerium o ide (Ce 2) is used in automotive emissions ontrol atalysts to

regulate the partial pressure o o ygen near the atalyst sur a e. Despite its

widespread use and appli ation, pure erium o ide has poor thermal stability and is

1

known to sinter at 1123 K. In order to in rease its thermal stability, and ability to

store and release o ygen during operation, ir onium is substituted into the ubi

stru ture o eria. The addition o ir onium to the ubi stru ture o eria is

reported to in rease the o ygen storage apa ity o the system while enhan ing the

thermal stability under high temperatures as ompared to pure eria 3 .

In the last ew years, it has been e e tively shown that Ce r1- 2 solid

solutions in the eria ri h regions ( .5) are the most e e tive redo promoters at

low temperature (i.e engine old start), and so an greatly redu e emissions o C ,

N , and hydro arbon when a tive metal is dispersed on the sur a e 39 .

ir onia-ri h ompositions, Ce1- r2 2- ( .3) ind appli ations as

erami materials and ioni ondu tors whereas eria-ri h o ides ( .3) are

e ploited above all as atalyti materials 36 .

Mello et al 4 studied the per orman e o Pd / Ce .75 r .25 2 atalyst or the

redu tion o N with ethanol and ompare with the Pd-Mo atalysts. The Pd /

Ce 2- r 2 sample showed higher a tivity or the onversion o N and higher

sele tivity or N2 ormation when ompared to the Pd-Mo 3 / Al2 3 sample. The

C hemisorption results showed that the Ce r mi ed o ide hemisorbed a higher

amount o C , and this is related to a higher redu ibility apa ity o this ompound

when ompared to Mo 3.

The C o idation on Ce 2- r 2 mi ed o ides prepared via sol-gel te hni ue

using urea as a hydrolysis atalyst was studied 41 . Highly uni orm nano-si e solid

solution parti les o eria- ir onia were attained under the onditions o this study

( a 1 C). The C o idation a tivity o the mi ed o ides was ound to be

dependent on Ce/ r ratio, whi h relates to the degree o redu ibility. The atalyti

a tivity or C o idation de reases with a de rease in Ce/ r ratio. This might be

due to the di erent in phase ompositions o the mi ed o ides. It an be postulated

that the ubi phase, luorite stru ture, whi h is mainly ound in Ce1- r 2 (where

.5) ould be redu ed easily than the tetragonal phase ound in Ce1- r 2 (where

.5) 41 .

11

Martorana et al 42 reported on the stru tural modi i ations o two Pt/ eria-

ir onia atalysts in di erent onditions o lowing gas mi tures and o isothermal

treatment temperature this behavior is ompared with that o a metal- ree eria-

ir onia sample. In a irst phase, o ygen oming rom the sur a e layers o the eria-

ir onia mi ed o ide is onsumed and no stru tural variation o the support is

observed. A ter this indu tion time, bulk redu tion o Pt/ eria- ir onia takes pla e as

a step-like pro ess, while the C 2 produ tion ontinues at a nearly onstant rate.

This behavior is totally di erent rom that o the metal- ree support in similar

rea tion onditions, that show a gradual bulk redu tion.

Ko lov et al 43 investigated the e e t o di erent synthesis methods on the

redo property o Ce 2- r 2-Al2 3. Related to the important o the preparation

method is the observation that enhan ed redu ibility a ter redo y ling is only

present in bulk o ides using ertain preparation methods. Al2 3 was able to stabili e

the parti le si e o the mi ed o ide during severe redo and thermal treatmant.

1.10 Sol-gel Chemistry

Heterogeneous atalysts are o ten prepared by wet hemistry method su h as

pre ipitation, opre ipitation, hydrothermal synthesis or sol-gel pro ess. The main

advantages o these low temperature pro esses are to give solids with large spe i i

sur a e area and high porosity in the meso and ma ropore ranges. The solid network

is ormed, rom the solution, via the hydrolysis and ondensation o mole ular

pre ursors in solution 44 .

The sol-gel pro ess is a use ul synthesis approa h or the preparation o

amorphous, as well as stru turally ordered materials. Through sol-gel pro ess, it is

possible to ontrol the properties o the synthesi ed samples, su h as porosity and

sur a e area to obtain homogeneous matri es 45 .

12

The sol-gel is also onsidered to be the most pra ti al method or the

abri ation o porous erami membranes. During the sol-gel pro ess, the properties

o the sol, the sur a e and pore stru ture o the support, the method o preparing gel

membrane and its drying and iring onditions are the key a tors in luen ing the

pore stru ture and per orman e o membranes 46 .

The sol-gel pro ess or preparing o ide glasses is attra tive as the preparation

is a omplished at room temperature without high-temperature melting or hot

pressing. Sin e the preparation are ondu ted in solutions, the homogeneity attained

in the initial rea tion mi ture an also be retained. The syntheti pro edure or sol-

gel preparation based on a id or base ataly ed hydrolysis o alko y-silane

pre ursors in a ueous solution, ollowed by ondensation, gelation, aging, drying,

and densi i ation. The te hni ue is apable o providing spe ialty glasses with

opti ally a tive organi or inorgani materials embedded in the matri , whi h o ten

have high opti al purity and e ellent diele tri e e ts while possessing

transparen y over a wide visible range. Thus, glasses prepared by the sol-gel pro ess

are attra tive andidates or opti al and ele tro-opti al appli ations 47 .

The sol-gel te hni ue works well or the synthesis o omple metal o ides

with high phase purity be ause the polymeri ing gel traps the various metal ion

omponents spatially, permitting pre ipitation rom solution where all the metal ions

o upy near-neighbor positions in the gel matri , upon urther pro essing and high

temperature al inations, the resultant amorphous mi ture o metal o ides,

hydro ides, and metal salts de omposes with M- -M bond ormation, while having

to di use only a ew angstroms to their latti e positions in a homogeneous solid

solution. urthermore, the gel matri isolates the individual metal o ide parti les,

giving rise to nanostru tured grains a ter the high temperature al inations to orm

the inished metal o ide 4 .

There is onsidered eviden e that sols, or olloidal o ides, an play a riti al

role as pre ursors to the primary parti les o rystalline or amorphous o ides 49 .

Sols have played a very riti al role in the ability to prepare atalyti o ide

stru tures, su h as wash oats on monolith stru tures, as the sol stru tures play a

13

riti al role as the binder phase between aggregates o o ides within su h stru tures

49 .

Sol stru tures an, however, just as likely be mere spe tators in the

rystalli ation pro esses whi h orm the inal stru tures within gelled sols. A ase

an be made that the term sol-gel is very misleading, in that although sols may be

ormed as intermediate stru tures in some ases, they are learly not ne essarily

trans ormed into the primary parti les, or elementary stru tures, within the gelled

o ide 49 .

Gel synthesis involves polymeri ing mole ular pre ursors into a three

dimensional network and subse uently onverting the wet gel into a erogel by

removing the solvent 5 .

In the sol-gel route synthesis, a stepwise rea tion s heme has been

undertaken to ontrol the ratio o hydrolysis to ondensation rates. In general, the

rate o hydrolysis is ast ompared to that o ondensation in strong a idi ondition.

There ore, a well-ordered he agonal arrangement o mesopores is ormed at low-pH

in a idi onditions. Meanwhile, in neutral or basi onditions ranging rom pH 7 to

pH 9, the rate o ondensation is aster than that o hydrolysis, and eventually the

materials prepared by a single-step rea tion at high pH display gel-like stru ture

without mesopores. It is hen e an interesting attempt to synthesi e ordered

mesoporous materials by two-step sol-gel route at lower a idi pH ollowed by a

higher pH 51 .

As in previous work 52, 53 , modi ied sol-gel method is used to prepare a

ir onia based atalyst in this study. Modi ied sol-gel is a method whi h varies the

te hni ue to prepare a sol-gel as well as rea tion ondition.

14

1.10.1 The Advantages of Sol-gel Approach Over the Traditional Methods of

Preparation

Sol-gel pro essing provides a new approa h to the preparation o supported

metal atalysts. A well-de ined pore si e distribution an be obtained using this

approa h. The potential advantages o sol-gel pro essing in lude: purity,

homogeneity, and ontrolled porosity ombined with the ability to orm large sur a e

area materials at low temperature 54 .

The advantage o the sol-gel method o synthesis is that virtually any metal

o ide system an be e amined, and no spe ial apparatus or e uipment is re uired.

Besides, its advantages are the ormation o erami s o high purity and good ontrol

over mi rostru ture and parti le morphology in the synthesis, typi ally at room

temperature 4 .

1.10.2 Effects of Variables on the Chemistry of the Sol-gel Synthesis

The important variables whi h must be onsidered in the synthesis o

supported metals by the sol-gel method in lude pH, water/metal al o ide ratio (R),

gelation temperature, metal loading, addition o dopants, solvent removal, and

pretreatment onditions 55 .

A large number o studies have been per ormed in an e ort to understand the

e e t o water on the properties o the gel. hen non-stoi hiometri amounts o

water were added, an analysis o the rea tion produ ts showed that the e tent o

polymeri ation was largely depends on the number o moles o water, whi h was

added 54 . hen the H2 / al o ide ratio is in reased, the time re uired or the

ormation o the gel is also observed to in rease. This same result is also obtained

when pH is de reased 54 . An in rease in the H2 / al o ide ratio learly leads to

an in rease in the pore diameter. The sur a e area appears to go through a ma imum

when the H2 / al o ide ratio is lose to that orresponding to stoi hiometry 54 .

15

The synthesis o solid atalysts using sol-gel pro essing may be per ormed

under a id ondition (pH 1.5-6), basi onditions (pH -11) or neutral onditions

(pH 7). Di erent atalysts an used to per orm the hydrolysis rea tions 54 . Under

strongly a idi onditions, hydrolysis o urred very rapidly and that gel ormation

times were in reased substantially. Hydrolysis is slower and the polymeri ation is

ataly ed by the base. Under these onditions, gelation times are onsiderably slower

54 .

The rea tion pH is very important in determining the inal properties o the

material whi h is synthesi ed. Under basi onditions, the parti les whi h are

initially ormed have a diameter o appro imately 1 nm. These parti les in rease in

si e during the synthesis. The resultant gel tends to be mesoporous or ma roporous.

hen the rea tion is per ormed at a pH o 7, the parti les in the sol vary between 2.5

and 2 nm. or this reason, the resultant gel has a non-uni orm pore si e

distribution. Under a id onditions the parti les in both the sol and the gel are very

uni orm. They vary in si e between .5 and 3. nm and in general have very high

porosities. It is also important to note that the pore volume orresponding to meso

(2-5 nm) and ma ro ( 5 nm) pores in rease at the e pense o mi ropores ( 2.

nm) with in reasing pH 54 .

In the sol-gel pro ess, drying is regarded as an important pro edure in the

determination o the per orman e o resultant erogels 56 . Among the many

drying te hni ue, thermal drying is employed most re uently or onvenien e, and a

pre eding ambient drying is usually per ormed. It is assumed that shortening the

ambient drying period, along with shortening the total drying pro edures, an tailor

the hara teristi s o the resultant erogels 56 .

In some ases, the gel is aged be ore drying. Aging an take the orm o a

redispersion treatment whi h allows dissolution and pre ipitation o material or a

degree o ine tuning o the network s stru ture. The solvent is subse uently

removed by either (i) evaporative drying or (ii) a super riti al drying step.

Super riti al drying has the advantage o eliminating the li uid/vapor inter a e

during drying that ould otherwise lead to partial ollapse o the network stru ture-

16

most notably loss o sur a e area and pore volume-due to high di erential apillary

pore pressures 57 .

1.10.3 Preparation of Catalysts by Sol-gel Method

The preparation o supported metal atalysts by sol-gel method is uni ue

be ause the metal pre ursor is mi ed homogeneously together with the mole ular

pre ursor o the support, resulting in a greater degree o ontrol over the inal

properties o the atalyst 55 .

There are two types o sol-gel method depending on the hemi al nature o

the pre ursors su h as inorgani based sol-gel method and organi based sol-gel

method. Inorgani based sol-gel method involved inorgani salt whi h easily

dissolved in water whereas organi based sol-gel method involved organometalli

salts/ omple es su h as alko ides whi h an be dissolved in organi solvent.

Sol-gel pro essing in whi h metalorgani pre ursors are mi ed with metal

pre ursors to orm a homogeneous solution. The metalorgani is hydroly ed through

the addition o water are ully ontrolling the pH and the rea tion temperature. As

hydrolysis and polymeri ation o ur, olloidal parti les or mi elles with an

appro imate diameter o 1 nm are ormed. These parti les ontinue to in rease in

si e until a metal o ide gel (al ogel) is ormed. The solvent an be eliminated by

heat treatment in air to orm a erogel. The metal o ide gel (hydrogel) is ormed by

using inorgani salt as a pre ursor. The solvent an be dried to orm a erogel 54 .

Most ir onia ompounds an be used as pre ursors o ir onium o ide in the

sol-gel pro ess. These pre ursors o ir onia an be lassi ied as ir onium

alko ides ( ir onium propo ide, ir onium tetra-n-buto ide et .) and inorgani

ir onium salts ( ir onium o y hloride, ir onium a etate et .) 46 .

17

hen ir onia is prepared using the sol-gel method, the tetragonal phase an

be stabili ed at low temperature, depending on pH and the type o atalyst used in

the synthesis. The H groups retained in the bulk in this method avour stabili ation

o the tetragonal phase, whereas when positive ions are present (e.g., H , Li , Na )

the mono lini phase is produ ed at low temperature 23 .

The rystalline properties o ir onia synthesi ed by the pre ipitation and sol-

gel methods were omparatively studied 1 . Both synthesis methods gave rise to

nano rystalline ir onia. The samples al ined at C and prepared by the

pre ipitation had average rystalline si es less than 1 nm, whi h were two times

smaller than the orresponding value obtained in the sol-gel samples. Both

tetragonal and mono lini nano rystalline phases had atomi de e ts in

on entrations that depended on the synthesis method and annealing temperature.

Rossignol et al 5 studied the preparation o ir onia- eria solid solutions

( r1- Ce 2) with 1) by two di erent methods: onventional opre ipitation

and modi ied sol-gel preparation. Both the stru ture and the te ture o those solids

depend on the synthesis and the ir onium pre ursor. or solids prepared by the

modi ied sol-gel method, with a sur a e area o 6 m2g-1, a new ubi phase

( r .25Ce .75 2) is obtained, while an orthorhombi ir onia phase was identi ied or

solids prepared either by opre ipitation or sol-gel methods. The sol-gel method was

parti ularly e i ient or preparation o Ce- r- mi ed o ides with high erium

ontents.

Lope et al 59 investigated the improvement o stru tural and

morphologi al properties o Mn- r mi ed o ides by sol-gel method with varing

Mn/ r ratio in the entire ompositional range. This pro edure allows an e ellent

ontrol over relevant properties o the synthesi ed materials, namely ompositional

homogeneity, purity, and porous stru ture.

Aguilar et al 23 reported a systemati analysis o the rystalli ation pro ess

o amorphous sol-gel samples and the thermal behavior o ir onia ogelled with

sili a, in the omplete r 2-Si 2 system range, annealing the samples with long

thermal treatments at temperature between 1 C-14 C. r 2-Si 2 mi ed o ides

1

prepared using the sol-gel te hni ue are promising atalysts, or atalyst supports, in

petro hemi al pro esses. or e ample, it has been shown to be use ul in the n-

he ane isomeri ation rea tion to high o tane, employing bi un tional ir onia-sili a

atalysts in produ ts su h as 2,2-dimethylbutane and 2,3-dimethylbutane.

arias et al 45 investigated on the sol-gel synthesis o alumina and 1:1

mi ed o ides o Al2 3- r 2, Si 2-Ti 2, r 2-Ti 2 and Al2 3-Ti 2. Due to their

spe ies with a idi sites o di erent types (mainly Br nsted sites or Si 2 and Lewis

sites or the other o ides) and strengths, the mi ed o ides are interesting matri es

rom a atalyti point o view.

1.11 Statement of the Problem and the Needs of the Study

The automobile e haust is identi ied as one o the major sour es o air

pollution 29 . Due to in omplete ombustion o uel, automobiles emits to i gases

su h as arbon mono ide (C ), hydro arbon (HC), and nitrogen o ides (N ) to the

environment. The way to redu e these to i gases released to the air is by onverting

them simultaneously to non-to i gases and to do this, a three-way atalyst is

developed. Tetragonal r 2 is used as atalyst and atalyst support or various gas

phase rea tion 26 . Several studies have shown that the per entage o tetragonal

phase o r 2 is ru ial or the a tivity and sele tivity 27 .

Sin e the tetragonal phase o r 2 an serve a good atalyti site, there ore

this resear h will be arried out to study the optimum ondition on the highest

ormation o single phase tetragonal r 2. This resear h will be on entrated on the

preparation o the three-way atalyst based on ir onium o ide using modi ied sol-

gel method.

19

1.12 Research Objectives

The obje tives o this resear h are:

1. To prepare a single-phase tetragonal ir onium o ide doped with

erium o ide atalyst by modi ied sol-gel method.

2. To elu idate the physi al hara teristi s o atalyst using various

analyti al methods or urther understanding o the properties o the

atalyst onsisting o single-phase tetragonal ir onia.

3. To evaluate the per orman e o the atalyst obtained towards C

onversion/ o idation.

1.13 Scope of Research

This resear h is arried out to study the optimum onditions on the highest

ormation o single phase tetragonal r 2. Three parameters will be studied in this

study to prepare a single phase tetragonal ir onia doped with eria atalyst, vi

amount o water, amount o dopants and al ination temperatures. In this study, the

pre ursors will be dissolved in a minimum amount o water to prepare a atalyst. A

series o r Ce1- 2 atalysts will be prepared by varying the atomi per entage

ratios, over the omposition range rom 5 -5 o erium loading. The al ination

temperature start at 4 C will be hosen based on previous studies on atalyst

based on metal o ides, whi h reported that the atalysts were mostly a tive at this

temperature 6 .

The physi al properties o atalysts will be hara teri ed with -Ray

Di ra tion ( RD), sur a e area analysis, and ourier-Trans orm In rared

Spe tros opy ( TIR). The best atomi per entage ratio o atalyst or the highest

ormation o single phase tetragonal r 2 will be sele ted or urther study on the

atalyti a tivity towards C onversion.

54

Other characterization techniques can be used for further understanding of the

properties of the catalyst. Nitrogen adsorption analysis at 77 K should be carried out

to determine the types and shapes of pores for catalysts. The chemical composition

of the prepared model catalysts can be analyzed with Auger electron spectroscopy

(AES) to characterize the surface species segregation compared with the bulk

composition. Besides, the catalyst systems can also be further analyzed with X-ray

photoelectron spectroscopy (XPS) to verify surface composition and elemental

oxidation states [35].

Finally, the catalytic activity of Zr0.70Ce0.30O2 catalyst towards the oxidation

of CO and hydrocarbon with the reduction of NOx is suggested to carry out in order

to get better performance of the three-way catalyst.

55

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