CHAPTER I
INTRODUCTION
1.1
Air Pollution
Air pollution is the presenFe oI material in air in Tuantities large enough to
produFe harmIul eIIeFts. The undesirable materials may damage, vegetation, human
property, or the global environment as well as Freate aesthetiF insults in the Iorm oI
brown or ha]y air or unpleasant smells >1@.
Air pollution Faused by mobile sourFe had been reFeived muFh attention by
most oI the environmentalist. In the last 6 years the world vehiFles Ileet has
inFreased Irom about 4 million vehiFles to over 7 million, this Iigure is projeFted
to inFrease to 92 million by the year 21 >2@. This will lead to inFrease oI more
and more air pollutant. The engine e[haust Fontains prinFipally three main
pollutants, unburned or partially burned hydroFarbons (HCs), Farbon mono[ide (C2)
and nitrogen o[ides (N2[), mostly N2, in addition to other Fompounds suFh as
water, hydrogen, nitrogen, o[ygen, etF. Sulphur o[ides, though polluting, are
normally not removed by the post Fombustion treatments, sinFe the only eIIeFtive
way is to reduFe them to elemental sulphur, whiFh would aFFumulate in the system
>2@.
2
1.2
Definition and Types of Pollutants
Any solid, liTuid, or gas that is present in the air in a FonFentration that
Fauses some deleterious eIIeFt is Fonsidered an air pollutant. However, there are
several substanFe that, by virtue by their massive rates oI emission and harmIul
eIIeFts, are Fonsidered the most signiIiFant pollutants >3@.
SubstanFes emitted direFtly Irom sourFes are Falled primary pollutants. The
pollutants manuIaFtured in the lower atmosphere by FhemiFal reaFtions among
primary pollutants are Falled seFondary pollutants, they are responsible Ior most oI
the smog, ha]e, and eye irritation and Ior many oI the Iorms oI plant and material
damage attributed to air pollution >4@.
1.3
Effects of Air Pollution
Carbon mono[ide (C2) is a gaseous pollutant whiFh enters the atmosphere
Irom Iour sourFes: Iossil Iuel Fombustion and industrial emissions, biomass burning,
o[idation oI CH4, and o[idation oI non-methane hydroFarbon >5@.
C2 is a Folorless and odorless gas. It is very stable and has a liIetime oI 2 to 4
months in the atmosphere. High FonFentration oI C2 Fan Fause physiologiFal and
pathologiFal Fhanges and ultimately death. C2 is a poisonous inhalent that deprives
the body tissues oI neFessary o[ygen >6@.
N2[ in the atmosphere Fauses environment problems, suFh as photoFhemiFal
o[idant and aFid rain >7@. Nitrogen dio[ide (N22) Fan reaFt with moisture present in
the atmosphere to Iorm nitriF aFid, whiFh Fan Fause Fonsidered Forrosion oI metal
surIaFe. Nitrogen o[ide (N2) is a Folorless gas and its ambient FonFentration is
usually Ior less than .5 ppm. N2 is a preFursor to the Iormation oI nitrogen dio[ide
and is an aFtive Fompound in photoFhemiFal smog Iormation as well >6@.
3
Unburned hydroFarbons in Fombination with the o[ides oI nitrogen in the
presenFe oI sunlight Iorm photoFhemiFal o[idants, Fomponents oI photoFhemiFal
smog, that do have adverse eIIeFts on human health and on plants >6@.
1.4
Catalyst
A Fatalyst is a substanFe that inFreased the rate at whiFh a FhemiFal reaFtion
approaFhes eTuilibrium without itselI beFoming permanently involved in the reaFtion
>@.
BasiFally, Fatalysts are Fonsidered as FhemiFal Fompounds Fapable oI
direFting and aFFelerating thermodynamiFally Ieasible reaFtions while remaining
unaltered at the end oI the reaFtion, whose thermodynamiF eTuilibrium they
FonseTuently Fannot Fhange >9@.
Catalysis is homogeneous when the Fatalyst is soluble in the reaFtion
medium, and heterogeneous when Fatalyst is e[isting in a phase distinFtly diIIerent
Irom the phase oI the reaFtion medium. In most instanFes oI heterogeneous Fatalysis,
the Fatalyst is a solid that is brought into FontaFt with gas or liTuid reaFtants to bring
about a transIormation >9@.
1.5
Catalytic Converter
Nowadays, more than 95 oI vehiFles produFed in the world are eTuipped
with a FatalytiF Fonverter, whiFh Ior the gasoline-Iuelled engines, is almost
e[Flusively based on the so-Falled three-way Fatalyst (T:C). T:Cs are Fapable oI
simultaneously and eIIiFiently Fonverting C2, hydroFarbon (HC) and N2[ into
harmless C22. H22 and N2, provided that the so-Falled and air-to-Iuel ratio (A/)) is
Fonstantly kept at the stoiFhiometry >1@. A typiFal design oI a modern three-way
FatalytiF Fonverter is reported in )ig. 1.1. BasiFally, it is a stainless steel Fontainer
4
whiFh inForporates a honeyFomb monolith made oI Fordierite
(2Mg2.2Al223).5Si22) or metal >2@.
Figure 1.1: Diagram oI a typiFal FatalytiF Fonverter
1.6
Three-way Catalysts
Automobiles under severe driving Fonditions suFh as a long-distanFe
Fontinuous running usually generate high temperature in the FatalytiF Fonverter, that
may Fause thermal sintering oI both metal aFtive phases and Fatalyst support, and
Iinally leads to Fatalyst deaFtivation. 2n the other hand, at Fool start, the Fatalyst
usually shows low FatalytiF aFtivity. ThereIore, a satisIaFtory FatalytiF eIIiFiently at
high temperature and a promoted light-oII reaFtion behavior at Fool start are
demanded Ior the Furrent three-way Fatalysts (T:Cs) >11@.
Three-way Fatalysts (T:Cs) have been widely used to reduFes pollutant
emissions Irom gasoline engine powered vehiFles >12@. 9arious three-way Fatalysts
(T:Cs) are widely used in Far e[hausts Ior the Fonversion oI poisonous gases (C2,
N2[ and hydroFarbons) to harmless H22, C22 and N2. These Fonverters, beFause oI
their eIIeFtiveness, beFome Fompulsory part oI the modern automobile >13@.
5
The FonFept oI using Fatalyst to Fonvert C2, N2[, and HC to less
environment aFtive Fompounds suFh as nitrogen, water and Farbon dio[ide was well
established praFtiFe prior to the need arising on motor vehiFles. The prinFipal
reaFtions are shown in Table 1.1 >14@.
Table 1.1: ReaFtions oFFurring on automobile e[haust Fatalyst
Oxidation Reactions
2C2 22 ĺ 2C22
HC 22 ĺ C22 H22
Reduction Reactions
2C2 2N22 ĺ 2C22 N2
HC N2 ĺ C22 H22 N2
SinFe C2 and HC emission are removed by o[idation whereas N2[ may only
be removed by reaFtion with reduFing agents, the elimination oI all three pollutants
Fan only be aFhieved by separating the o[idation and reduFtion IunFtions in
appropriate reaFtors or by design oI seleFtive Fatalyst on whiFh the two reaFtions
may proFeed simultaneously >15@.
The development oI the three-way Fatalyst (T:C) was diFtated by the need
to simultaneously Fonvert the HC, C2, and N2[, present in the automobile e[haust
to H22, C22 and N2 >16@.
1.7
Zirconium Oxide or Zirconia
=irFonium o[ide or ]irFonia is widespread in nature as a traFe mineral,
baddeleyite. In its pure Iorm, properties suFh as high hardness, low wear resistantFe,
low FoeIIiFient oI IriFtion, high elastiF modulus, FhemiFal inertness, good ioniF
FonduFtivity, low thermal FonduFtivity, and high melting temperature have made
]irFonia a teFhnologiFally important material. These uniTue properties enable a wide
range oI appliFation oI ]irFonia suFh as struFtural materials, thermal barrier Foatings,
6
solid-state eleFtrolytes, solid Iuel Fells, gate dieleFtriFs, o[ygen sensors, and
heterogeneous Fatalysts >17@.
SinFe ]irFonia has a lattiFe with many vaFanFies, it is a good ioniF FonduFtor
at high temperature. These vaFanFies allowed it to be used as an o[ygen sensor to
measure the air-to-Iuel ratio in internal Fombustion engines in order to Fontrol the
emission oI N2[, C2 and hydroFarbon Fompounds >1@.
=r22- based materials have attraFted Fonsiderable interest in reFent deFades.
In a FatalytiF sense, they appear to have some advantages in area oI praFtiFal
appliFation over traditional o[ides, suFh as Si22 and Al223. More signiIiFant is the
IaFt that additives Fan bring about a strong modiIiFation oI the surIaFe struFture oI
]irFonia, in whiFh Fase substitution oI =r4 with dopant Fations results in a rise in
anion vaFanFy FonFentration and FonduFtivity >19@.
The tailored physiFo-FhemiFal properties oI ]irFonia inFluding struFture,
redo[, and aFid-base FharaFteristiFs make it as an attraFtive Fatalyst and Fatalyst
support Ior a number oI reaFtions >2@. In IaFt, due to its aFid-base biIunFtional
properties, it Fatalyses the hydrogenation oI diIIerent substrates suFh as Ior e[ample
ben]oiF aFid, Farbo[yliF aFids, and Farbon mono[ide. However, the main use oI
]irFonia is as an eIIiFient support Ior a variety oI FatalytiF systems >21@.
=irFonia is also widely used in optiFal Iields beFause oI its high reIraFtive
inde[, large optiFal band gap, and low optiFal loss and sFatter in the inIrared region.
It is also useIul Ior making high-reIleFtivity mirrors, laser gyros and broadband
interIerenFe Iilters >22@. Depending on IaFtors suFh as preparation method, pH,
temperature, and kinetiF meFhanism, synthetiF ]irFonia shows three Frystalline
phases: monoFliniF (m), tetragonal (t), and FubiF (F) it has also been shown that a
high-pressure orthorhombiF Iorm e[ists >23@. At room temperature, t- and F-phases
metastable, while m-phase is stable. t- and F-phases are stable at high temperature.
They Fan be partially or Iully stabili]ed at room temperature by addition a small
amount oI o[ides (e.g. Ca2, Mg2 and types oI rare earth o[ides) >24@.
7
The monoFliniF phase is stable up to 14 K where it transIorms to the
tetragonal phase, whiFh is stable up to 157 K while the FubiF phase e[ists up to the
melting point oI 29 K. However, the FubiF phase Fan be stabili]ed by the addition
oI divalent Fations suFh as Ca2, Mg2 or Y3 >2@. Several methods have been
developed to stabili]e the tetragonal phase at low temperature. These inFlude the
addition oI dopants (Y223, Ca2, Mg2, Ce22), or the use oI soIt-Fhemistry methods,
Irom whiFh nanoFrystalline materials Fan be obtained >23@.
It is known that tetragonal ]irFonia as a metastable phase e[ists at well below
its normal transIormation temperature as a result oI the Frystalline si]e eIIeFt, the
presenFe oI impurities, or the presenFe oI strain at the domain boundaries in
polydomain tetragonal partiFles >25@.
Tetragonal- =r22 is used as a Fatalyst and Fatalyst support Ior various gas
phase reaFtions >26@. Several studies have shown that the perFentage oI tetragonal
phase oI =r22 (t-=r22) is FruFial Ior the aFtivity and seleFtivity in Fatalysis >27@.
However, a phase transIormation Irom the tetragonal phase to the less reaFtive
monoFliniF phase oI Frystalline =r22 (m-=r22) during the reaFtion prevents the
Iurther appliFation oI =r22. ThereIore, stabili]ing the tetragonal ]irFonia during
reaFtion beFomes an important topiF in Fatalysis Iields >2@. =r22 stabili]ed by Ce22
and other rare earth is known to inFrease durability and removal eIIiFienFy Ior
automotive emission Fontrol >29@.
1.8
Cerium Oxide or Ceria
Cerium o[ide or Feria (Ce22) has been Fonsidered and largely used in the last
years as one oI the most important promoters oI heterogeneous FatalytiF reaFtions
>3@. Cerium o[ide is one oI the most important Fomponents in high-perIormanFe
three-way Fatalyst (T:C) Ior its ability in enhanFing the removal oI Farbon
mono[ide (C2), nitrogen o[ides (N2[) and hydroFarbons (HC) >31@.
Cerium o[ide has been Fonsidered oI potential interest Ior solid o[ide Iuel
Fells and three-way automobile Fatalysts beFause oI its good o[ygen ion
FonduFtivity. Its ability to aFt as an o[ygen buIIer and thereIore to operate
eIIeFtively under Fonditions oI osFillating o[ygen FonFentration has promoted its use
in automobile FatalytiF appliFations >32@.
However, pure Ce22 alone is known to be poorly thermostable. The loss in
surIaFe area is usually related to the Fhanges in the pore struFture and to Frystalline
growth. It is, thereIore, very important to improve its te[tural stability. Doping with
Fations suFh as Al3, =r4, or Si4 may signiIiFantly improve the stability oI the
surIaFe area oI Feria at high temperature >33@.
Among the lanthanide elements, Ferium is the only one that Iorms stable
Fompounds in a tetravalent o[idation (i.e., Ce4) and the Foordination number oI
surIaFe Ce4 Fan vary between Iour and eight, si[ and eight being the Foordination
number oI bulk Ce4. )urthermore, Cerium ions Fan behaves a large 22 trap: due to
its low redo[ potential and Ferium ion Fan easily pass Irom Ce4 to Ce3 >34@.
Ce22 has a FubiF Iluorite-like struFture in whiFh Ferium Foordinates eight
o[ygen anions at the Forners oI a Fube, while o[ygen is tetrahedrally surrounded by
Iour Ferium Fations. It has been demonstrated that the 2[ygen Storage CapaFity
(2SC) oI Ferium o[ide is improved by suitable doping with diIIerent FationiF speFies
and in partiFular with lanthanum >35@.
AgraIiotis et al >32@ studied deposition oI nanophase doped-Feria systems on
FeramiF honeyFomb Ior automotive FatalytiF appliFations. A remarkable
enhanFement on the FatalytiF aFtivity oI Ferium o[ide has been observed when the
power is oI nanophase dimensions beFause oI the large number oI lattiFe deIeFts suFh
as o[ygen vaFanFies present, whiFh provide a large number oI aFtive sites Ior gassolid Fatalysis. The number oI deIeFts Fan be inFreased by partial substitution
(doping) oI the Ferium atoms in the Ferium o[ide lattiFe with other metal atoms oI
lower o[idation state suFh as Ca or Nd. BeFause oI these properties, doped Ferium
nano-o[ides Fan aFt as Fatalyst themselves, reduFing signiIiFantly the need Ior noble
metals.
9
1.8.1
Role of CeO2 in the TWCs
It is an ambitions task to deIine the role oI the Ce22 in the three-way Fatalysis
sinFe multiple eIIeFts have been attributed to this promoter. Ceria was suggested to
>16@:
1.9
(i)
promote the noble metal dispersion
(ii)
inFrease the thermal stability oI the Al223 support
(iii)
promote the water gas shiIt (:GS) and steam reIorming reaFtions
(iv)
Iavour FatalytiF aFtivity at the interIaFial metal-support sites, etF
(v)
promote C2 removal through o[idation employing a lattiFe o[ygen
(vi)
store and release o[ygen under, respeFtively, lean and riFh Fonditions.
Catalysis by Zirconia-Ceria
Ceria-]irFonia solid solutions Fomprise an interesting system with great
potential Ior appliFations in various teFhnologiFal Iields. Depending on Fomposition,
Feria-]irFonia may be used Ior the manuIaFture oI solid o[ide Iuel Fells, gas sensors,
o[ygen semi-permeable membranes Ior steam eleFtrolysis and hot eleFtrodes Ior
magnetohydrodynamiF systems >36@.
Not long ago, in the middle oI 199s, Ce22-=r22 mi[ed o[ides has been
proFlaimed as one oI the most perspeFtive materials Ior appliFation as a Fomponent
oI so-Falled ³three-way Fatalysts´ (T:C) in automotive pollution Fontrol. These
solids are able to neutrali]e simultaneously C2, hydroFarbons and N2[. High
FatalytiF aFtivity oI suFh system Fombines with enhanFed thermal stability and high
2[ygen Storage CapaFity (2SC). The latter is due to the presenFe oI Ferium ions
able to Fhange easily their o[idation state depending on the environment >37@.
Cerium o[ide (Ce22) is used in automotive emissions Fontrol Fatalysts to
regulate the partial pressure oI o[ygen near the Fatalyst surIaFe. Despite its
widespread use and appliFation, pure Ferium o[ide has poor thermal stability and is
1
known to sinter at 1123 K. In order to inFrease its thermal stability, and ability to
store and release o[ygen during operation, ]irFonium is substituted into the FubiF
struFture oI Feria. The addition oI ]irFonium to the FubiF struFture oI Feria is
reported to inFrease the o[ygen storage FapaFity oI the system while enhanFing the
thermal stability under high temperatures as Fompared to pure Feria >3@.
In the last Iew years, it has been eIIeFtively shown that Ce[=r1-[22 solid
solutions in the Feria riFh regions ([
.5) are the most eIIeFtive redo[ promoters at
low temperature (i.e engine Fold start), and so Fan greatly reduFe emissions oI C2,
N2[, and hydroFarbon when aFtive metal is dispersed on the surIaFe >39@.
=irFonia-riFh Fompositions, Ce1-[=r222-į ( [ .3) Iind appliFations as
FeramiF materials and ioniF FonduFtors whereas Feria-riFh o[ides ( [ ! .3) are
e[ploited above all as FatalytiF materials >36@.
Mello et al >4@ studied the perIormanFe oI Pd / Ce.75=r.2522 Fatalyst Ior the
reduFtion oI N2 with ethanol and Fompare with the Pd-Mo Fatalysts. The Pd /
Ce22-=r22 sample showed higher aFtivity Ior the Fonversion oI N2 and higher
seleFtivity Ior N2 Iormation when Fompared to the Pd-Mo23 / Al223 sample. The
C2 Fhemisorption results showed that the Ce=r mi[ed o[ide Fhemisorbed a higher
amount oI C2 , and this is related to a higher reduFibility FapaFity oI this Fompound
when Fompared to Mo23.
The C2 o[idation on Ce22-=r22 mi[ed o[ides prepared via sol-gel teFhniTue
using urea as a hydrolysis Fatalyst was studied >41@. Highly uniIorm nano-si]e solid
solution partiFles oI Feria-]irFonia were attained under the Fonditions oI this study
(Fa 1 žC). The C2 o[idation aFtivity oI the mi[ed o[ides was Iound to be
dependent on Ce/=r ratio, whiFh relates to the degree oI reduFibility. The FatalytiF
aFtivity Ior C2 o[idation deFreases with a deFrease in Ce/=r ratio. This might be
due to the diIIerent in phase Fompositions oI the mi[ed o[ides. It Fan be postulated
that the FubiF phase, Iluorite struFture, whiFh is mainly Iound in Ce1-[=r[22 (where [
.5) Fould be reduFed easily than the tetragonal phase Iound in Ce1-[=r[22 (where
[ ! .5) >41@.
11
Martorana et al >42@ reported on the struFtural modiIiFations oI two Pt/Feria]irFonia Fatalysts in diIIerent Fonditions oI Ilowing gas mi[tures and oI isothermal
treatment temperature this behavior is Fompared with that oI a metal-Iree Feria]irFonia sample. In a Iirst phase, o[ygen Foming Irom the surIaFe layers oI the Feria]irFonia mi[ed o[ide is Fonsumed and no struFtural variation oI the support is
observed. AIter this induFtion time, bulk reduFtion oI Pt/Feria-]irFonia takes plaFe as
a step-like proFess, while the C22 produFtion Fontinues at a nearly Fonstant rate.
This behavior is totally diIIerent Irom that oI the metal-Iree support in similar
reaFtion Fonditions, that show a gradual bulk reduFtion.
Ko]lov et al >43@ investigated the eIIeFt oI diIIerent synthesis methods on the
redo[ property oI Ce22-=r22-Al223. Related to the important oI the preparation
method is the observation that enhanFed reduFibility aIter redo[ FyFling is only
present in bulk o[ides using Fertain preparation methods. Al223 was able to stabili]e
the partiFle si]e oI the mi[ed o[ide during severe redo[ and thermal treatmant.
1.10
Sol-gel Chemistry
Heterogeneous Fatalysts are oIten prepared by wet Fhemistry method suFh as
preFipitation, FopreFipitation, hydrothermal synthesis or sol-gel proFess. The main
advantages oI these low temperature proFesses are to give solids with large speFiIiF
surIaFe area and high porosity in the meso and maFropore ranges. The solid network
is Iormed, Irom the solution, via the hydrolysis and Fondensation oI moleFular
preFursors in solution >44@.
The sol-gel proFess is a useIul synthesis approaFh Ior the preparation oI
amorphous, as well as struFturally ordered materials. Through sol-gel proFess, it is
possible to Fontrol the properties oI the synthesi]ed samples, suFh as porosity and
surIaFe area to obtain homogeneous matriFes >45@.
12
The sol-gel is also Fonsidered to be the most praFtiFal method Ior the
IabriFation oI porous FeramiF membranes. During the sol-gel proFess, the properties
oI the sol, the surIaFe and pore struFture oI the support, the method oI preparing gel
membrane and its drying and Iiring Fonditions are the key IaFtors inIluenFing the
pore struFture and perIormanFe oI membranes >46@.
The sol-gel proFess Ior preparing o[ide glasses is attraFtive as the preparation
is aFFomplished at room temperature without high-temperature melting or hot
pressing. SinFe the preparation are FonduFted in solutions, the homogeneity attained
in the initial reaFtion mi[ture Fan also be retained. The synthetiF proFedure Ior solgel preparation based on aFid or base Fataly]ed hydrolysis oI alko[y-silane
preFursors in aTueous solution, Iollowed by Fondensation, gelation, aging, drying,
and densiIiFation. The teFhniTue is Fapable oI providing speFialty glasses with
optiFally aFtive organiF or inorganiF materials embedded in the matri[, whiFh oIten
have high optiFal purity and e[Fellent dieleFtriF eIIeFts while possessing
transparenFy over a wide visible range. Thus, glasses prepared by the sol-gel proFess
are attraFtive Fandidates Ior optiFal and eleFtro-optiFal appliFations >47@.
The sol-gel teFhniTue works well Ior the synthesis oI Fomple[ metal o[ides
with high phase purity beFause the polymeri]ing gel traps the various metal ion
Fomponents spatially, permitting preFipitation Irom solution where all the metal ions
oFFupy near-neighbor positions in the gel matri[, upon Iurther proFessing and high
temperature FalFinations, the resultant amorphous mi[ture oI metal o[ides,
hydro[ides, and metal salts deFomposes with M-2-M bond Iormation, while having
to diIIuse only a Iew angstroms to their lattiFe positions in a homogeneous solid
solution. )urthermore, the gel matri[ isolates the individual metal o[ide partiFles,
giving rise to nanostruFtured grains aIter the high temperature FalFinations to Iorm
the Iinished metal o[ide >4@.
There is Fonsidered evidenFe that sols, or Folloidal o[ides, Fan play a FritiFal
role as preFursors to the primary partiFles oI Frystalline or amorphous o[ides >49@.
Sols have played a very FritiFal role in the ability to prepare FatalytiF o[ide
struFtures, suFh as washFoats on monolith struFtures, as the sol struFtures play a
13
FritiFal role as the binder phase between aggregates oI o[ides within suFh struFtures
>49@.
Sol struFtures Fan, however, just as likely be mere speFtators in the
Frystalli]ation proFesses whiFh Iorm the Iinal struFtures within gelled sols. A Fase
Fan be made that the term sol-gel is very misleading, in that although sols may be
Iormed as intermediate struFtures in some Fases, they are Flearly not neFessarily
transIormed into the primary partiFles, or elementary struFtures, within the gelled
o[ide >49@.
Gel synthesis involves polymeri]ing moleFular preFursors into a three
dimensional network and subseTuently Fonverting the wet gel into a [erogel by
removing the solvent >5@.
In the sol-gel route synthesis, a stepwise reaFtion sFheme has been
undertaken to Fontrol the ratio oI hydrolysis to Fondensation rates. In general, the
rate oI hydrolysis is Iast Fompared to that oI Fondensation in strong aFidiF Fondition.
ThereIore, a well-ordered he[agonal arrangement oI mesopores is Iormed at low-pH
in aFidiF Fonditions. Meanwhile, in neutral or basiF Fonditions ranging Irom pH 7 to
pH 9, the rate oI Fondensation is Iaster than that oI hydrolysis, and eventually the
materials prepared by a single-step reaFtion at high pH display gel-like struFture
without mesopores. It is henFe an interesting attempt to synthesi]e ordered
mesoporous materials by two-step sol-gel route at lower aFidiF pH Iollowed by a
higher pH >51@.
As in previous work >52, 53@, modiIied sol-gel method is used to prepare a
]irFonia based Fatalyst in this study. ModiIied sol-gel is a method whiFh varies the
teFhniTue to prepare a sol-gel as well as reaFtion Fondition.
14
1.10.1 The Advantages of Sol-gel Approach Over the Traditional Methods of
Preparation
Sol-gel proFessing provides a new approaFh to the preparation oI supported
metal Fatalysts. A well-deIined pore si]e distribution Fan be obtained using this
approaFh. The potential advantages oI sol-gel proFessing inFlude: purity,
homogeneity, and Fontrolled porosity Fombined with the ability to Iorm large surIaFe
area materials at low temperature >54@.
The advantage oI the sol-gel method oI synthesis is that virtually any metal
o[ide system Fan be e[amined, and no speFial apparatus or eTuipment is reTuired.
Besides, its advantages are the Iormation oI FeramiFs oI high purity and good Fontrol
over miFrostruFture and partiFle morphology in the synthesis, typiFally at room
temperature >4@.
1.10.2 Effects of Variables on the Chemistry of the Sol-gel Synthesis
The important variables whiFh must be Fonsidered in the synthesis oI
supported metals by the sol-gel method inFlude pH, water/metal alFo[ide ratio (R),
gelation temperature, metal loading, addition oI dopants, solvent removal, and
pretreatment Fonditions >55@.
A large number oI studies have been perIormed in an eIIort to understand the
eIIeFt oI water on the properties oI the gel. :hen non-stoiFhiometriF amounts oI
water were added, an analysis oI the reaFtion produFts showed that the e[tent oI
polymeri]ation was largely depends on the number oI moles oI water, whiFh was
added >54@. :hen the H22 / alFo[ide ratio is inFreased, the time reTuired Ior the
Iormation oI the gel is also observed to inFrease. This same result is also obtained
when pH is deFreased >54@. An inFrease in the H22 / alFo[ide ratio Flearly leads to
an inFrease in the pore diameter. The surIaFe area appears to go through a ma[imum
when the H22 / alFo[ide ratio is Flose to that Forresponding to stoiFhiometry >54@.
15
The synthesis oI solid Fatalysts using sol-gel proFessing may be perIormed
under aFid Fondition (pH 1.5-6), basiF Fonditions (pH -11) or neutral Fonditions
(pH 7). DiIIerent Fatalysts Fan used to perIorm the hydrolysis reaFtions >54@. Under
strongly aFidiF Fonditions, hydrolysis oFFurred very rapidly and that gel Iormation
times were inFreased substantially. Hydrolysis is slower and the polymeri]ation is
Fataly]ed by the base. Under these Fonditions, gelation times are Fonsiderably slower
>54@.
The reaFtion pH is very important in determining the Iinal properties oI the
material whiFh is synthesi]ed. Under basiF Fonditions, the partiFles whiFh are
initially Iormed have a diameter oI appro[imately 1 nm. These partiFles inFrease in
si]e during the synthesis. The resultant gel tends to be mesoporous or maFroporous.
:hen the reaFtion is perIormed at a pH oI 7, the partiFles in the sol vary between 2.5
and 2 nm. )or this reason, the resultant gel has a non-uniIorm pore si]e
distribution. Under aFid Fonditions the partiFles in both the sol and the gel are very
uniIorm. 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 Forresponding to meso
(2-5 nm) and maFro (!5 nm) pores inFrease at the e[pense oI miFropores (2.
nm) with inFreasing pH >54@.
In the sol-gel proFess, drying is regarded as an important proFedure in the
determination oI the perIormanFe oI resultant [erogels >56@. Among the many
drying teFhniTue, thermal drying is employed most IreTuently Ior FonvenienFe, and a
preFeding ambient drying is usually perIormed. It is assumed that shortening the
ambient drying period, along with shortening the total drying proFedures, Fan tailor
the FharaFteristiFs oI the resultant [erogels >56@.
In some Fases, the gel is aged beIore drying. Aging Fan take the Iorm oI a
³redispersion treatment´ whiFh allows dissolution and preFipitation oI material Ior a
degree oI ³Iine tuning´ oI the network¶s struFture. The solvent is subseTuently
removed by either (i) evaporative drying or (ii) a superFritiFal drying step.
SuperFritiFal drying has the advantage oI eliminating the liTuid/vapor interIaFe
during drying that Fould otherwise lead to partial Follapse oI the network struFture-
16
most notably loss oI surIaFe area and pore volume-due to high diIIerential Fapillary
pore pressures >57@.
1.10.3 Preparation of Catalysts by Sol-gel Method
The preparation oI supported metal Fatalysts by sol-gel method is uniTue
beFause the metal preFursor is mi[ed homogeneously together with the moleFular
preFursor oI the support, resulting in a greater degree oI Fontrol over the Iinal
properties oI the Fatalyst >55@.
There are two types oI sol-gel method depending on the FhemiFal nature oI
the preFursors suFh as inorganiF based sol-gel method and organiF based sol-gel
method. InorganiF based sol-gel method involved inorganiF salt whiFh easily
dissolved in water whereas organiF based sol-gel method involved organometalliF
salts/ Fomple[es suFh as alko[ides whiFh Fan be dissolved in organiF solvent.
Sol-gel proFessing in whiFh metalorganiF preFursors are mi[ed with metal
preFursors to Iorm a homogeneous solution. The metalorganiF is hydroly]ed through
the addition oI water FareIully Fontrolling the pH and the reaFtion temperature. As
hydrolysis and polymeri]ation oFFur, Folloidal partiFles or miFelles with an
appro[imate diameter oI 1 nm are Iormed. These partiFles Fontinue to inFrease in
si]e until a metal o[ide gel (alFogel) is Iormed. The solvent Fan be eliminated by
heat treatment in air to Iorm a [erogel. The metal o[ide gel (hydrogel) is Iormed by
using inorganiF salt as a preFursor. The solvent Fan be dried to Iorm a [erogel >54@.
Most ]irFonia Fompounds Fan be used as preFursors oI ]irFonium o[ide in the
sol-gel proFess. These preFursors oI ]irFonia Fan be FlassiIied as ]irFonium
alko[ides (]irFonium propo[ide, ]irFonium tetra-n-buto[ide etF.) and inorganiF
]irFonium salts (]irFonium o[yFhloride, ]irFonium aFetate etF.) >46@.
17
:hen ]irFonia is prepared using the sol-gel method, the tetragonal phase Fan
be stabili]ed at low temperature, depending on pH and the type oI Fatalyst used in
the synthesis. The 2H groups retained in the bulk in this method Iavour stabili]ation
oI the tetragonal phase, whereas when positive ions are present (e.g., H, Li, Na)
the monoFliniF phase is produFed at low temperature >23@.
The Frystalline properties oI ]irFonia synthesi]ed by the preFipitation and solgel methods were Fomparatively studied >1@. Both synthesis methods gave rise to
nanoFrystalline ]irFonia. The samples FalFined at žC and prepared by the
preFipitation had average Frystalline si]es less than 1 nm, whiFh were two times
smaller than the Forresponding value obtained in the sol-gel samples. Both
tetragonal and monoFliniF nanoFrystalline phases had atomiF deIeFts in
FonFentrations that depended on the synthesis method and annealing temperature.
Rossignol et al >5@ studied the preparation oI ]irFonia-Feria solid solutions
(=r1-[Ce[22) with ” [ ”1) by two diIIerent methods: Fonventional ³FopreFipitation´
and ³modiIied sol-gel´ preparation. Both the struFture and the te[ture oI those solids
depend on the synthesis and the ]irFonium preFursor. )or solids prepared by the
modiIied sol-gel method, with a surIaFe area oI 6 m2g-1, a new FubiF phase
(=r.25Ce.7522) is obtained, while an orthorhombiF ]irFonia phase was identiIied Ior
solids prepared either by FopreFipitation or sol-gel methods. The sol-gel method was
partiFularly eIIiFient Ior preparation oI Ce-=r-2 mi[ed o[ides with high Ferium
Fontents.
Lope] et al >59@ investigated the improvement oI struFtural and
morphologiFal properties oI Mn-=r mi[ed o[ides by sol-gel method with varing
Mn/=r ratio in the entire Fompositional range. This proFedure allows an e[Fellent
Fontrol over relevant properties oI the synthesi]ed materials, namely Fompositional
homogeneity, purity, and porous struFture.
Aguilar et al >23@ reported a systematiF analysis oI the Frystalli]ation proFess
oI amorphous sol-gel samples and the thermal behavior oI ]irFonia Fogelled with
siliFa, in the Fomplete =r22-Si22 system range, annealing the samples with long
thermal treatments at temperature between 1 žC-14 žC. =r22-Si22 mi[ed o[ides
1
prepared using the sol-gel teFhniTue are promising Fatalysts, or Fatalyst supports, in
petroFhemiFal proFesses. )or e[ample, it has been shown to be useIul in the nhe[ane isomeri]ation reaFtion to high oFtane, employing biIunFtional ]irFonia-siliFa
Fatalysts in produFts suFh as 2,2-dimethylbutane and 2,3-dimethylbutane.
)arias et al >45@ investigated on the sol-gel synthesis oI alumina and 1:1
mi[ed o[ides oI Al223-=r22, Si22-Ti22, =r22-Ti22 and Al223-Ti22. Due to their
speFies with aFidiF sites oI diIIerent types (mainly Br|nsted sites Ior Si22 and Lewis
sites Ior the other o[ides) and strengths, the mi[ed o[ides are interesting matriFes
Irom a FatalytiF point oI view.
1.11
Statement of the Problem and the Needs of the Study
The automobile e[haust is identiIied as one oI the major sourFes oI air
pollution >29@. Due to inFomplete Fombustion oI Iuel, automobiles emits to[iF gases
suFh as Farbon mono[ide (C2), hydroFarbon (HC), and nitrogen o[ides (N2[) to the
environment. The way to reduFe these to[iF gases released to the air is by Fonverting
them simultaneously to non-to[iF gases and to do this, a three-way Fatalyst is
developed. Tetragonal =r22 is used as Fatalyst and Fatalyst support Ior various gas
phase reaFtion >26@. Several studies have shown that the perFentage oI tetragonal
phase oI =r22 is FruFial Ior the aFtivity and seleFtivity >27@.
SinFe the tetragonal phase oI =r22 Fan serve a good FatalytiF site, thereIore
this researFh will be Farried out to study the optimum Fondition on the highest
Iormation oI single phase tetragonal =r22. This researFh will be FonFentrated on the
preparation oI the three-way Fatalyst based on ]irFonium o[ide using modiIied solgel method.
19
1.12
Research Objectives
The objeFtives oI this researFh are:
1. To prepare a single-phase tetragonal ]irFonium o[ide doped with
Ferium o[ide Fatalyst by modiIied sol-gel method.
2. To eluFidate the physiFal FharaFteristiFs oI Fatalyst using various
analytiFal methods Ior Iurther understanding oI the properties oI the
Fatalyst Fonsisting oI single-phase tetragonal ]irFonia.
3. To evaluate the perIormanFe oI the Fatalyst obtained towards C2
Fonversion/ o[idation.
1.13
Scope of Research
This researFh is Farried out to study the optimum Fonditions on the highest
Iormation oI single phase tetragonal =r22. Three parameters will be studied in this
study to prepare a single phase tetragonal ]irFonia doped with Feria Fatalyst, vi]
amount oI water, amount oI dopants and FalFination temperatures. In this study, the
preFursors will be dissolved in a minimum amount oI water to prepare a Fatalyst. A
series oI =r[Ce1-[22 Fatalysts will be prepared by varying the atomiF perFentage
ratios, over the Fomposition range Irom 5-5 oI Ferium loading. The FalFination
temperature start at 4 ÛC will be Fhosen based on previous studies on Fatalyst
based on metal o[ides, whiFh reported that the Fatalysts were mostly aFtive at this
temperature >6@.
The physiFal properties oI Fatalysts will be FharaFteri]ed with ;-Ray
DiIIraFtion (;RD), surIaFe area analysis, and )ourier-TransIorm InIrared
SpeFtrosFopy ()TIR). The best atomiF perFentage ratio oI Fatalyst Ior the highest
Iormation oI single phase tetragonal =r22 will be seleFted Ior Iurther study on the
FatalytiF aFtivity towards C2 Fonversion.