Luận Án tiến sĩ study on the loading procedures of the support on the substrates to prepare catalytic complexes for the treatment of motorbike's exhausted gases

nh nhieu 2.3.5 Double đeposilions 2.4 Deposition of support on metal substrates 2.5 Deposilion of active eatalylic phase on supportisubstale 2.6 Preparation of the real catalytic con

MISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

PHẠM THỊ MAI PHƯƠNG

STUDY ON THE PROCEDURES OF THE SUPPORT ON THE SUBSTRATES TO PREPARE CATALYTIC COMPLEXES FOR THE TREATMENT OF MOTORBIKE’S EXHAUSTED GASES

DOCTOR OF PHILOSOPHY THESIS: CHEMICAL ENGINEERING

HANOI- 2014

MINISTRY OF EDUCATION AND TRAINING

HANOIE UNIVERSITY OF SCIENCE AND TECHNOLOGY

PHẠM THỊ MAI PHƯƠNG

STUDY ON THE PROCEDURES OF THE SUPPORT ON THE SUBSTRATES TO PREPARE CATALYTIC COMPLEXES FOR TIE

TREATMENT OF MOTORBIKE’S EXIIAUSTED GASES

Chuyên ngành: Kỹ thuật hóa học

Commitment

1 assư that tha is my own rescarch, All the data and results in the thesis arc completely frue, was agreed to we in this paper by co-authors, his research hasn't been published by other authors thane

Phạm Thị Mai Phương

Acknowledgement

This PhD thesis has been carried out at the Department of Organic Synthesis and Petrochemistry, School of Chemical Engineering, Hanoi University of Science and

‘Technology during the period July 2010 to September 2013 The work has been completed

under the supervision of Assoc Prof Dr Le Minh Thang,

Firstly, I would like to express my deepest and most sincere gratitude to my promotors: Assoc Prot Dr, Le Minh Thang, She has been helping me a lot not only in the scienfific work but also in my private life Without her guidance, her encouragement, her enthusiastic and kind help, it would have been difficult to overcome the difficulties | met during the proscnt work

I want to thank my colleagues in the lab Environment friendly Materials and

‘Technologies for their fiendly attitude towards me and their help in my work,

T would Tike to thank all members of the Depsrimenl of Inorganic and Physical Chomistry, ospocially the group of Solid Stale Chemistry far their support and guidance during the porind T was in Belgium

Tam gratefid to the onfirc member in he Advanced Institute of

Technology for their help, and nice cnvironment they ercatod for me

T especially want to i

bebween Flemish Tnteruniv

cree aud

gratitude for (he cooperation program

sity Council (VIR) ard Hanoi Universily of Technology (HUT) for the Grancial support for this study Tacknowlodge lo Prof Isabel Van Dricssche (Coordinator of the ecopcration program) fix the administrative help

Firally, I lovingly thank my fumily for their love and encowagements during the whole long study period

CONTENT OF FIGURE§ 22222220111 rereuorTÚ

1.1 Ait pollution caused by vehicles emission „14 1.1.1 Over the world and in Vietnam dxg82sðnnyeosstsdbf

1.1.3 Solutions for air pollution

1.4.1 Principles of some synthesis methods

1.4.2 Synthesis methods of substrates and supports

1.5 Preparation the catalytic converters

1.5.1 Coating a monolith witha catalysis support material

1.5.2 Deposition of active phase on monolithic support

Literature review's COnCIUSION -.- :s:0sscsssieeccssenneeessnnennnie si¿23AGi026/09 40

2.1 Preparation the substrates

2.1.1 Preparation of the cordierite substrate

2.1.2 Preparation of Cordierite using additives

2.1.3 Preparation of cordierite with the addition of dolomite

2.1.4 Surface treatment of prepared cordierite

2.1.5 Surface treatment of FeCr alloy substrate

2.2.3 AlCey Zte «sO, mixed oxide

2.3 Deposition methods of support on cordierite substrate

2.3.1 Direel combustion

2.3.2 Hyttrid deposition

2.3.3 SUSPORSION weiss sieememmneneteveneiinstnanetnninenen nineteen

2.3.4 Secondary B[OWTL nh nhieu

2.3.5 Double đeposilions

2.4 Deposition of support on metal substrates

2.5 Deposilion of active eatalylic phase on supportisubstale

2.6 Preparation of the real catalytic converter cecceeeeeeerrre

2.7.1 Xtay diffraction (XRD)

2.7.2 Characterization of surtace properties by physical adsorption

2.7.3 Scanning electron microscopy (SEM)

2.7.4 Thermal Analysis

2.7.5 X-tay photoelectron Spectroscopy (XPS)

2.8 Calalylic activily measurement

2.8.1 Measurement of catalytic activity in the micro-reactor connected with GC

online

2.8.2 Mcasurentcrf oỀ cxÌat8fd 88808 oioon this

CHAPTER 3 RESULTS AND DISCUSSION

3.1 Synthesis of cordierik

3.1.1 Influence of synthesis methods on the preparation of cortticrite

3.1.2 The influence of burnable additives on the synthesis of cordierite

3.1.3 The influence of dolorrite on synthesẽ of cordierite

3.1.4 Influgnce of acid treatment on surface area of cordicrite

3.2 Preparation of FeCr metal substrate neseominnenennenenenn erst

3.3 Synthesis of supports .sesseremninenenenmennieninanenminnnennes

3.3.1 Synthesis of boehmite and Y-ALOs „ co nennonosrerrerroie

3.3.2 Synthesis of Ceo 2Ztp Or mixed Ode oe sessneteteninenentmnneannn

3.3.3 Ale Are nO» mixed oxides

3.4 Deposition of support on substrates .ecaiensneteteeneesietseeses

3.4.1 Preparation of Cep 2Ztp gs on COMETH eeesieeestseenneninis

3.4.2 Preparation of y-AlOs support on cordierite substrate

$7

3.4.3 Preparation of AlCeg »Z1, «,0 support on cordierite substrate -81

3.6 Catalytie activities ofthe complets calalysfs à co sec mùi

LIST OF ABREVIATES

VOCs Volatile organic compounds

Tic Tonburned hydrocarbons

TWCs Three-way catalysts

AF “Aur to fel

osc Oxygen slorage capacity

AOZ — ARO; —CeOn — ZrO, mined oxides

Œ CEO —ZTÕ- mixed oxides

BET Brunaucr, Emmett and Teller

SEM Scanning clacton microscopy

TỔA Thermogravimetric analysis

DIA Differential thermal amalysis

XE§ Xaay photoelectron Specttoscopy

CTAB Coty timethyl ammonium bromide

SDS Sodium dodecyl sulfate

PEG polyethylene glycol

CONTENT OF TABLES

Table: 12 Panission Siantards for in-used vehicles in Vietnam 15 Table 1.3: Characteristic properties of Coudierite

Table 1.4 TWC microkinetic scheme used in the model |66, ế7|

Table 2} The content (weight %) of main metal oxides inkaolin after activation .43 Table 2.2 Synthesis condition of substrates sarmples - eee AS Table 2.3 Synthesis conditions of supports samples

Table 24 Synthasis conditions of supports deposited on substrates samples

Tabk 2.5 Synthesis conditions of catalyst samples

Table 2.6 Standard XRD reflections of the synthesized matenials Tabk 3.1 Properties of cordierite samples synthesized trom different methods 61 'Tabk 3.2 Propertias of synthesized Cordierite using additive 64 Table 3.3 The BET surtice areas of the cordierite prepared by conventional sintering ftom kaolin with different addition of cellulose betore sintering -65

‘Table 3.4 Compositions of precursors to prepare cordierite -66

‘Table 3.5 Content of cordierite phase in the product and impurities in the precursor .66

‘Table 38, BET specific surfice areas, pore sizes, pate volumes of the CZ samples 76 Table 3.9 BRT surface area of AC7 samples synthesized using different precipitants 79 Table 3.10 Th: BRT surface area of samples synthesived with and wihouL aging 2 Table 3.11 The BUT results of mixed oxides with differcut surfactants 83 Tabk: 3.12 Surfics of Ci jcorducrite samples proparcd by differcel

Table 3.13 Characterization of 7-AbOs supporl on cordierite substrate .80 Table 3.14 Alomic conpasitions (%) of components in Ca.2 and Ca3 oalalysls !3 Tabk: 3.15 Alomnic conpasitions (%) af components in Ca.2 and Ca.3 catalysts by XPS 95 Table 3.16 Resulls of BET surfiice ara of MnO2-Co,04-Ce0, catalysts 97 Table 3.17, Atomic composition (%4) of the commereial catalyst CAT-920 based on metal

‘fable 3.18 ‘The content of emission gases with and without catalytic complex (Ca.11 -

Table 3.19 Emission of motorbike Vespa installed the commercial catalysts fom Vespa based on metal substraff§ neeetenetensieesneinineniesititenentniesienetee see kLO

CONTENT OF FIGURES

Fig.1.3: The formation of various alumina at different calcination temperature 22

bÕy,

Tip.1.4: Structure ofy-/

Fig 1.5: Phase diagram ofthe CeO; 20; system

Tig2.1 Tsollerm adsorption

Fig.2.2 TUPAC clissification of hysteresis loops (revised in 1985)

Fig.2.3 Schema of micro-reactar set up

Fig 2.4, Schema ofexhavst tube witha fixed catalytic converter

Fig 2.5 Schema of measuring motorbike’s exhaust gases

Fig 3.1: XRD palterns of Cudierile samples prepared by vations methods 256 Fig3.2 SEM image of Cordierite produced by sol-gel processing’ SG-0 (@) and conventional sintering of kaolin: CV-0 (b) ceneseereeeo.ỔT Fig3.3 TGA-DSC pfeordierils sarnples prebarod Bom soi-gel methộ 62 Fig 3.4 XRD pattern of cordierite sample prepared by conventional sintering caleined at

Hgl5, XRD pademe of cordierie prepared by conventional sintering with different

ig3.6 XRD pattems of cordierite prepared by sol-gel wilh different additionof 64 activated carbon

Fig3.7 SEM image of cordierite produced ffom kaclin without -

Vig3.8 SEM image of cordiesite produced by sol-gel processing without - SG-0 (a) and with- SG-SAC (b) the addition of activated carbon to the preforms 65 Fig3.9 XRD patterns of cordicrite samples propared with different dolomile contont

Fig.3.12, XRD patterns ofsamples treated cordiarite by hydrochloric acid

Fig 3.13 Effect of HCI acid treatment on cordierite’s content

Fig 3.14 XRD palterns of samples with 8.69 wi% af dolomite bef

Fig 3.15, XRD patterns of cordierite samples with 16.27 wt% of dolomite before (TD2)

Fig 3.16, Influence of acid treatment on cordierite content (a) and BET surface area (b) of the cordierite samples with addition of dolomite (8.69 wt % - TDI, 16.27 wh.% - TD2) 71 1ig3.17 ‘The determination of contact angle of untreated (a) and treated (b) metal

substrates by B3 procedure (calcined at 800°C, ten immersed in NaOH 10 wt%⁄4) 72 Fig.3.18 XRD pattern of boehimtte sesesesseeseeesenetete -

Eig23.20 Adsorptien-desorption isotherm plots of boehmile and 7-AhO; A

10

Hig 321 XHD pafern of CZ2&CTAH and CZ28-non templadie (T: teragonal

Tig322 Ne adsorplion-desorption isolhenm of samples wilt and wihou CTAR, and urcalcined and calcined (C728-CTAB, C728-CTAB as-prepared, C7.28-non template and

Fig, 3.23 XRD spectra of samplzs prepared using these different preeipitants caleined at

Fig.3.24, Isothzrm plots of samples prepared using, these different prccipitants (2) ACZ08, () ACZ09, (e) ACZ10 cakeined at 550°0 -79 Fig.3.25 SEM bmagss ofsamplas tsìng wrth diBbront prseipiters caleied ai 5505C 80 Tig.3.26 XRD patlarns of ACZ samples with diferent aging conditions calcined at 550°C

„81 Ghonaged - ACZ08, aged aL90°C - ACZ11, aged at 160°C - ACZ12) „81 Fig3.27, XRD patterns of ACZ samples prepared using different surietus calcined at 500°C (non surfactant - ACZ08, SDS surfietant-ACZ13, CTAB

Fig 3.29 SEM inages of mixed oxides wilhout (AC708) and with surficlant SDS

Fig 332 XRD patio of the Coy 121) 4Oz/oordicrits sample (DD) 90

Fig 3.35 XRD pattern of the complete catalyst with Mu, — NiO - Cod, /

Hig 3.36 SEM images of final catalysts: Ca 2 (Mi 4Ð —Co;O¿ /cordkritz) and Ca

Fig 337 XPS Swucy of the nseprupared sample Ca 2 (MnO, —NiO — Caj0, ‘eordierite) and Ca 3 (MnO2 — NiO — CosOq/ Coy 2Zry sO-/cordicrite)

Fig 3.39 XRD pattern of Mn0)2-Co,Q4-CeO: /cordierite (Ca 4) _ 96

Vig 3.41: SEM images of MnX).-Co.04-Ce0; AlC eg «Zto as O-/ oordisrite (Ca.7) 9B ig.3.42, XRD pattern oFMnOz-Co;O¡-CeOs /AlCea sZre osO-/FeCr alloy (Ca.10) 99

Hig3.44, Microscopy images of MnO>-Co;04-CoO, deposited on FeCr substrates withand

Vig 3.46 Catalytio activilies Er te treatmeni o£CO (ã), C;1s (B), NÓ (6) of MnO — NiO

— CazOsleordierite (Ca 2), Mn©+ — NỈO ~ CosQ./ Can vZrọ ;Os/cordierite (Ca, 3) L03 Fig 3.47 Catalytic activily of C29 »7re sOveurdierile (DD-C7) 103 Fig 3.48 Catalytic activities for the bealment of (a) Calls, (6) CO of MuO — Core CeO2/ y-ALO; /cordierite (CaS), MnO2 Co;O¿CeO¿/ Cea;ZrysO:/ cordierite (Ca.6), MnO› Co;Or.CeO-/ AlCea 2ZroœO2/ oordisrite (Ca.7) "4

Fig.3.49 Catalytic activities for the meatment of C:Hs (a),CO (b) ofFMnO; Cos CcOz/AbO¿/ EcCt foil (Ca, 9), MtOy — CosOy-CeOy/Al-Ce-Zr-O/ FeCr foil 106 Fig 3.30 XRD patternof ground CAT-920, CatCo, U§A 106 Fig 3.51 SEM images ofthe hole inside area ofa CAT-920, CatCo, USA „107 Fig 3.52 Catalytic activity of commercial noble catalyst on cordierite (CATCO) 108

TNTRODUCTIOX

Air pollution, especially fiom automobile exhaust gases has become more and more serious problems over the world In a developing country tke Vietam wilh the tremendous increase of vehicles every year, it is urgent to control the emission which consisted of ai pollutants as carbon monoxide (CO), mirogen oxides (NO;), unburnt hydrecarbon (HC), suliu oxides (§Oz), volatile organic compounds (VOC) tor Protection of air environment

One of the most effective ways to control the vehicular pollution is catalytic converter which could treat simultaneously NOx, CO and HC Most of catalytic converters contain three main components as substrate, support material and active phase, It is well- known that the dispersion of rare metals as Pt, Pd, Rh on y-AbOs support exhibited high activity for the treatment of exhausted gases Therefore, the commercul catalytic converters have been produced with rare metals as active phase, y-AlO: as support and cordicrite as substrate, Moreover, the addition of CeO; which has been proved to be an excellent promoter in the catalytic converters improved the catalytic activity for the

treatment of NO,, CO and HC

However, the sensitive poisoning property and the cost of Pt-group are the reasons for the replacement of Pl-group by transition metals as active phase in calalylic converters

Many investigations both im the world and Vietnam praved the high abilily of Co, Ni, Mn

or Cu for the conversion of CO, NOx and HC Thus, it may be possible to prepare the inexpensive, sffective catalytic converters for a developing coumiry like Vietnam with the use of these bansilion metals

‘The catalytic aclivily is influcnced by not only the compositions of «

lyst bul also the deposition method fr loading active phase and support material aw substrates Te is obvious tbat the catalytic activity would be decreased sharply if the layer of active phase and support is detached forn substrate’s surface

Neverlheless, compared with the mmber

of studiss of calatysl’s composition, the investigation on deposi

altracted much altetiion Thus, in this thes

lon method fasn’t

s, the methad of impregnation process would be sludicd systematically lo prepare the catalytic complexes

‘The goal of this thesis is “Study on the loading pracedures of the support on the substrates to prepare catalytic complexes for the treatment of motorbike's exhausted gases” The thesis includes three parts The first part summarizes the aspects about the catalyst converter, and the preparation of catalyst in the literature The sccond part

describes the synthesis of scparated components as substrate, supports, and the method to

Final are the general conclusions of the performed work

13

CHAPTER 1 LITERATURE REVIEW

1.1 Air pollution caused by vehicles emission

1.1.1 Over the world and in Vietnam

With the rapid growth of the number of vehicks in operation, the air pollutants emitted fiom these vehicles lave contributed to urban air pollution in recent years, especially in large cities such as Sao Paulo, Detroit, and Tokyo In New York, the fine

particulate matter (PM) concentrations in the morning with traffic were 58% higher than those in the morning without traffic in 2011 A model simnation indicated that the

contripution of NO, fom vehicular sources accounted for a range of 9% ta 39% of thai concentration in atmosphere, In China, vehicle emissions in Beijing contributed to

approximately 71%-85% of the total CO concentration, 67% —71% of the total NO, concentration, and 26%—45% of total VOCs emission amount NO, emissions from

vehicles accounted for 35.4% to 75.7% of the total emissions, The bansporlation sector has

become a tmjor source of urban air pollution Ths

pollutants ernitied from vehickss [1]

Recently, the munber of vehicles in Victnam has inercascd tremendously In 2013,

fore, it is necessary lo control air

It is estimated that traffic activities contribute nearly 85% of CO emission and 95% of

VOCs, 30% of NO», In consideration of diffe

from motorcycles is quite low, being on average as little as @ quarter of the cmission volume of car transport However, due to the higher number of motorcycles and their often poor quality, motorcycles are the main contributor of contaminants, especially of CO and VOC Meanwhile, trucks and buses release larger volumes of $02 and NO: [3]

Therefore, it is urgent to apply the European emission standard to control the emission of vehicles Ewopean emission standards define the acceptable limits for exhaust emissions of new vehicles sold in Furopean member states The emissions of nitrogen oxide (NO,), total hydrocarbon (TTC), nowmethane hydrocarbon (NMIIC), carbon anonoxide (CO) and particulate mutter (PM) are regulated tor most vehicles, including cars, motorcycles, trucks For each vehicle type, different standards are also applied At the present, the Euro 5 standard has been applied with the limits of toxic emission fiom motorcycles listed int lable 1.1 [4]

‘Vietnanfs ewrent emissions limits for two- and three-wheelers, refered to as Type

2 standards, are equivalent to Ewo 2 standards These regulations were implemented via Government Decision No: 249/2005/QD-TTg, 10% October in 2005 Two- and three- wheclers must moct Ewo 2 standards ftom beginning 1" July in 2007 [5]

Table 1.2 Ruropean Enussion Standard

‘Vietnam planned to apply future Policies as following:

+ Type 3 - Standards (~Evro 3) are to be in place by I* January in 2017

+ Type 5- Vietnam will skip Type 4 (~Euro 4) standards and move ahead to Type 5

(~Evro 5) Standards starting 1 January in 2022

At the present, emission slankrd for Vietnam vehicles im volume percenlage ave required as in table 1.2

Table 1.2 Kmission Standards far in-used vehicles in Vietnam

Vehicle types

Level 1] Level 2 [Level 3|Level 1 [Level 2| Level 3|

CO@voD: | 45 | 35 | 30 | 45 | 35 | 25

HC (ppm vol):

Fourstrokes [1200 [800 [600 |1500 [1200 [800 Two strokes [7800 [7800 [7800 [10000 [7800 |7800

- For the moloreycles has non-cortrolfed extaust crission treatment system

«Level 1 for motorcycles with first registration date before 1 July in 2008,

= Level 2 for motorcycles with first registration date from 1* July in 2008;

- For the molore:

+ Tevel 3 is appl 8 tes conibolted oxtaust omission treatment sysism

ed

1.1.2 Air pollutants from emission

‘The mnjor criteria pollutants are carbon monoxide (CO), nitrogen dioxide (NO-), ozone (03), particulate matter Jess than 10 nm in diameter (PM10), sulfur dioxide (S02), and lead (Pb), Ambient concentrations of NO2 are usually controlled by limiting emissions

of both nitrogen oxide (NO) and NO2, which combined ae reférred to as oxides of nittogen (NO,) NO, and SO) are important in the formation of acid precipitation, NO, and volatile organic compounds (VOCs) can real react in the lower atmosphere to form ozone, which can cause damage to lungs as well as to property [6] In addition, PM also aifeet ibe jung when inhaling Carbon monoxide is mostly emitted from mobile sources (up to 90%)

‘The high levels of carbon monoxide found in traffic congested areas (20 - 30 mg/m*) can lead to levels of 3% carboxyhemoglobin, These levels can produce adverse cardiovascular and neurobelaviowzal efiécts and seriously aggravate the condition of individuals with ischemic heart disease The toxic benzene, polycyclic aromatic hydrocarbons in the

VOCS cause cancer [7]

Dre to incomplete combustion in the engine, there ate a number of incomplete combustion products Typical exhaust gas composition at the normal engine operating conditions is [8]

+ Carbon monoxide (CO, 0.5 vol %);

© Sulfur dioxides (SO 0.01% vol):

«Particulate matter (PM10 0.05% vol),

HC, CO and NOx are the major exhaust pollutants HC and CO occur because the combustion efficiency is <100% dus to incomplete mixing of the gases and the wall qrenching effects of the colder cylinder walls ‘fhe NO,, is formed during the very high temperatures (21500 C) of the combustion process resulting in thermal fixation of the nittogen in the air which forms NO; [8]

1.1.3 Sobutions far air poTution

Because of the large vehicle population, significant amounts of HC, CO and NO, are emitted to the atmosphere, it is extremely urgent to treat the exhaust gases before emission to the environment, There have been many ways to convert these toxic compounds to harmless ones, such as treating separated pollutants by catalyst or simultaneously by three-ways catalyst

1.1.3.1 Separated treatments for pollutants

1 CO treatments

Method I: Carbon monoxide can be converted by oxidation:

CO 1 O, —— COr

The catalysts base on noble metals [9, 10] Moreover, some transition metal oxides (Co,

Ce, Cu Fe,W, Mn) canbe used for treating CO [11, 12]

Method 2: water gas shift process can converted CO with participation of stsam:

This reaction was oalalyzcẻ by oaltnlysls base on provious metal [13]

Method 3: NO clinmuation:

NO + CO———C0: + AN:

‘The mos! active calalysl is Rh [1] Be

ii, VOCs treatments

Some control technologi

is bio-filters relying on microorganisms to ffcd on and thus destroy the VOCs And catalytic oxidizers we a catalyst to promote the reaction of the organic compounds with

16

oxygen, thereby requiring lower operating tempertures and reducing the med for supplemental fuel Destruction efficiencies are typically near 95%, but can be increased by using additional catalyst or higher lemperalimes (and (hus more supplemental fuel) {16]

ii NO, treatments

NG, formed by the combustion of fiel in air is typically composed of greater than 90% NO, with NO, malting up the remainder Unfortunately, NO is not amenable to flue gas scrubbing processes, as SO» is An understanding of the chemistry of NO; formation and destruction is helpful in understanding emission-control technologies for NO,,

Because the rate of NO; formation is so highly dependent upon temperature as well

as local chemistry within the conibustion environment, NO; is ideally suited to control by means of modifying the combustion conditions, There are several methods of applying these combustion modification NO controls, ranging ftom reducing the overall excess air levels in the combustor to burners specifically designed for low NOx emissions [16] NOx can be treated by some reductions occured in exhaust gas such as CO, VOCs or soot with using noble metal, perovskite catalysts and metallic oxide systems [17, 18,19

2.13.2 Simuitaneous treatments of three pollutants

i Two successive converters

It ean be treated simultancously three pollutants (NO;, CO, HC) by designing successive oxidation and reduction converters The main reactions in weatment process are: Reduction reaction: NO + 4 No + 40>

Tn this method, reduction converter only operiles well in cxcess fucl condition Furthermore, NH; can be formed in reduction condition This pollutant will be converted inlo NO-another pollulant in axidation modia [20]

fi Three-way catalytic (IWC) systems

The basic reactions for CO and LIC in the exhaust are oxidation with the desired product being CƠa, while the NO, reaction is a reduction with thị product being No

and 11:0 A calalyst promotes these reactions al lower Lemperatures than a thermal process

giving the following desired reactions for HC, CO and NOx

Oxidation:

CyHa— (14 1/4 30: + yCO2 + n’2 HO COBO, 1COr

CÓ + HạO —› CÓ: + Hạ

Reduction:

NÓ @GrNO;)+ CÓ —: 2N: + CÓ

NO (orNOs) + Hy + 4 No+H:O

(2+ W2) NO (or NÓ) + CyH, — (110/4) N: + yCO: + 82 HàO

There axe some common components, which represent the state-ofat of the composition of a catalytic converter:

+ Cordierite ceramic or metal fil as popular substrate

+ Alumina, which is employed as a high surface area support, CeO.—Zr0: mixed oxides, principally added as oxygen storage promolers, Tarium and/or lanlhanum oxides as stabilizers of the alumina surfice arca

+ Noble metals (Rh, Pt and Pd) as active phases [8]

1.2 The catalytic converter

‘The thuee-way catalytic monolith converter for abatement of automobile cmissions operated inherently ina transient regime is the most common multifinctional reactor Llere oxidation of CO and hydrocarbons and reduction of mitogen oxides (NOs) take place simultaneously im the complex porous struclwe of calalylic washcoat layer, which is formed by 7 -ALOs support (alumina) with dispersed crystalllites of nob: metals (typically Ptand Rh) as catalytic sites, particles of oxygen storage materials (CeO or mixed Ce-Zr oxides) and stabilizers of surface structue (¢.g oxides of Ba and La) Storage (deposition) and release of different exhaust gas components, renetion intermediates and produets take place concurenity with reactions on specific sites on the washeoat surface, Not only chemisorption of gas components on noble metal sites (Pt, Rh), but also oxygen storage on ceria and zirconium compounds, CO, and HC adsorption on y-AbOs support and other adsorption processes participate in TWC operation They become important in the transient regime, when inlet flow rate, temperature and concentrations of components vary with time

(eg city driving) [21, 32]

‘The three-ways catalysts have three imin components as substrates, support materials and active phase as following figure

Honcssamb:

canlierie

Fig.1.2, Structure of three-ways catalyst [23]

The top of the catalyst is the catalytic phase where the reactions happen The rare

metallic elements such as Pt, Pd and Rh has been wsed for a long time for the application of

catalyst, but now, peroskite, and transition metals (Cu Nii Mn, Co ) has attracted the

attention for its high efficiency and low cost As mentioned above, the y-AbO; plays an

important role of dispersion noble metals’ crystallite as catalytic sites Thus, 7-ALO; has

been used as the most popular support material for years However, the excellent properties

of CeO: or Ce,Zr 0: make this substance plays not only as the support material but also a part of active phase The essential component of three-ways catalyst is a monolith substrate This monolith has been prepared in the form of honeycomb for the low pressure

drop Cordierite and metal foil were chosen to produce monolith substrate because they

have high mechanical strength, a good ability to stand high temperatures and temperature

shocks, and a low thermal expansion coefficient

1.2.1 Substrates

The first success of the monolithic catalyst was in the automobile exhaust

treatment, After that, other applications became available, the environmental ones being by

far those most demanded The following environmental applications have been listed as

three-way catalysts; diesel catalysts for the abatement of liquid particulate (soluble organic

fraction) and CO, HC; O; abatement in aircraft, ,, [23] The monolithic reactors have clear advantages over the conventional shury and fixed-bed reactors, especially in application of

automobile exhaust treatment, because of low presswe drop, high thermal stability, easy

preparation [24]

1.2.1.1 Ceramic monoliths

First, the most commonly uses as a catalyst substrate of porous ceramic material are

easier to use than the metal of the conventional structured packings (the bonding of the catalyst to the ceramic substrate is mote facile), When coating metal substrates with a catalyst or catalyst supported layer, an intermediate layer of a ceramic material is often wed for a better binding, Second, the cost of monolithic substrates is relatively low, mainly due to the large-scale production for the automotive industry The cost for a basic

19

smonolithic structure can be as low as US$ 3 per dm’, mainly due to the relatively simple

production method (ie via an extrusion process) [24]

In the application of a monolithic

requirernenls for the substrate are The most common material for monolithic substrates is cordierite (a ceramic material consisting of magnesia, silica, and alumina in the ratio of 2Mg0.2Ab05.58i0.), because this material is very well suited for the requirements of the automotive industry (high mechanical strength, ability to high temperatures and

temperature shocks, and a low thermal expansion coc fficicnt) [24]

Other materials whose ceramic monolith substrates are conunercially available are amullite (mixed oxide of silica and almnina, ratio 2:3) and silicon carbide Disadvantages of all these materials are that, similar to cordierite, they have a low specific surfice area (e.g,

for cordizrite, typically 0.7 m/g), they are rarely used as support materials for

conventional catalysts, and the metal — support interaction is usually very low Monolithic elements out of carbon, silica, and y-alunina are available as research samples and can be produced once a significant derrand exists For these materials, surface areas of 200 mn”/g are easily available; the mechanical strength, however, is significantly lower than that of cordierite The most important characteristics of ceramic monoliths are listed in table 1.3 (24)

l@lyst, one should first determine whal the

Table 1.3: Characteristic properties of Cordierite

Surface atea (Xx BET, mL7g) A

As the cordierie mineral is not abundant, for indistral production usually it has to

be synthesized ‘Thus, there are many raw materials that may be used for the preparation of cordicrite monoliths where the employment of aluminum silicates, such as kaolin or clays, and the use of tale together with alumina is ffequent The simplest composition is a mixture ofkaolin and tale that can be kneaded with the aid of a dispersant (sodium lignosulfate), an agglomerant (polyvinyl alcohol) and a lubricant (water), ‘The paste is extruded, dried and subsequently calcined at 1300eC for 2h Nevertheless in the majority of the procedures described in patents over the preparation of monoliths ffom mixtures of preewsors, three or more components are utilized in proportions that are adequate to obtain a SiO~AhOxMg0

ratio equal to 51.4:34.9-13.7 (ratio of weight), that is close fo that corresponding to

eordierite, the most fiequently used being mixtures of talc | kaolin or clay | aluminum hydroxide [23]

Tale is present in the composition described in most patents The contribution of magnesium in some procedures is made by the addition of magnesium hydroxide, The second component (kaolin or clay) contributes with the silica and some of the alumina, The same effect may be obtained with the addition of halloysife or saponite The third component (aluminum hydroxide) is used to provide the aluninum necessary to complete the condicrite composition, although the use of mixtures of this hydroxide with alumina is ako tiequent [231

Generally, the multi-component mixtwes are prepared for extrusion with the aid of anagelomerant and water Once extruded, the monolith is dried and then calcined at 1200—

1450sÖ for 2-3 h

Sometimes, the overall composition is designed lo obtain cordierite plus other amaterials such as spinel, mmlite or similar, in order to improve the thenml shock yesistance of the monolith, It is also very important to control the particle size of the raw materials to achieve a good contact between the solids that take part in the reactions during, this process [23]

1.2.1.2 Metallic monoliths

Beside the initial pellet beds and cordieritz monoliths, metallic monoliths were soon proposed duc to their higher mochanical resistance and thermal conductivity, the possibility of thinner walls allowing higher cell density and lower presswe drop But additional advantages of the metallic substrate were soon discovered, in particular, the easy wayto prodwe diffzrent and complicated forms adapted to a wide variety of problems and wes [23]

Many different metals and alloys have been proposed for the manufacture of anonoliths in search for mechanical, chemical and thermal stability, availability in thin foils and good suface adherence of the catalytic coating, Ln addition to some Ni and Cr alloys, steel is the most widely wed alloy, in particular fémritic alloys containing AL(S 7%) that can produce alumina protecting coatings with excellent properties for anchoring the calalylic coating TL is important to note thal during the high temperature use of the alloy, the alumina protective layer continues growing until the alumirum is consumed, Preakdown of this thermally grown alumina would Isad to broskaway oxidation conditions and rapid comporent failure This is especially important fic the new ultra-thin foils (20 nin) available for the high cell density monoliths (1600 epsi) Reducing the thicknisss from

70 to 20 paw means that the componcul Tile will be reduced However, it is quite difficult and ustelly unccanomical to increas

because such an alloy is Lritlls, hence inducing difficelly during production or lowering productivity It is gencrally casicr to produce the thin foil or cven the monolith fiom an alloy having low Al conteut ant hones good mechanical and mamuficluring properties, and

subsequently ta treal il 1a inercase Ihe Al content Tn addition to Ue main components of

these ferrilic

qrantitics bocause they arc fundamental to improve the oxidation resistance of the alloy and to aid oxide adhesion 23}

“The new stricter emission limits for car exhausts all around the world demand more

effective catalylic solutions Metal catalyst

combustion engine applications:

* Significant reductions of all emissions (HC, CO, NO, and PM) canbe achiz

both spark ignition and dics] engines

* New, high cell density, ultra-thin fbil substrates further increase catalyst efficiency The formation of a self-healing protective “skin” of alumina allows the ultra-thin stee! to withstand the high temperatures and corrosive conditions in auto exhaust and other environmental uses These materials also have high thermal shock resistance and high

he Al concentration to a valuc more then 5 mass%,

21

melting and softening points and facilitate the development of high cell densities with very low-pressure losses [23]

1.2.2 Supports

‘Phe first and important role of support materials in the three-ways catalyst is a host

of active phase, mostly noble metals Without support material, it is extremely difficult for the dispersion of crystallite of noble metals, which act as catalytic sites in the reactions It

is welkknown thal 7-AbOs has been used as lhe support for PL Rh in the application of catalysis becanse of its high surface area, and its stability Since the beginning of 1980s, the researchers have focused on the CeQn- based materials or it has been called the oxygen storage material, which can improve the catalytic activity This material has been rsed not only as the support but also as a part of active phase, Recently, a now generation of materials as ALOs-CeO.-ZrO; was investigated, With the aim of combination the advantages of alumina and CeZ1.20:, this material is expected to become the optimal support for the catalytic application

12.2.1 Amina

in 1950, Stumpf et al reported that apart ffom a-AbQs (corundum), another six crystal structures of alumina occw: y, 8, «1, 0, y-AbO2 The sequence of particular type formation under the thermal processing of gibbsite, bayerite, bochmite and diaspore is as follows [251:

Fig.d.3: The formation of various alumina ai different caleination temperature

The temperature of aluminum hydroxide formation 1s the basis of this system of classification ‘I'he two groups of alumina are: (i) low-temperature alumina: ALO; 1lL0 (0<n<6) obtained by dehydrating at temperatwes not exceeding 600°C (y-g1owp) ¥ 1 AbOs belong to ths group; (ii) high-temperatwe alumina: nearly anhydrous Al:O3 obtained at temperatures between 900 and 1000°C (&-group) x, @ and 8-AhOs belong to this group

All thess structures are based on a more or tess close-packed oxygen lattice with aluminum iors in the octahedral and tetrahedral inlerstices Low-temperature alumina is characterized by cubic close-packed oxygen lattices; however, high-temperature alumina is characterized by hexagonal close-packed lattices In terms of catalytic activity, high-

22

temperature alumina is less active than low-temperature alumina ‘his results not only

lower surface area (higher order and larger particle size) but also the different population

of surfice active siles of high-temperature ahunina when compared to low-lemperatare ones [25]

The most common form of alumina used for catalyst support is 7 form, which possesses a surface area more than 300 an'/g, a pore size ranged fiom 30 to 120 A anda pore volume from 0.5 + 1 cm*/g, The structure of y-ALOs is built from single layers of packing spheres, the laycrs have the ionic O° at position 1 The spheres of the second layer sit in half of the hollows of the first layer There are two cases for the distribution of third layer, but in case of y-ALOs, the third layer was distributed on the hollows of the first one, following the number: 1,2,3,1,2,3 Therefore, cation AF* was placed in the space between these layers of oxide anion packing The structure of'7-AbO: was illustrated in the

figure 1.4:

Fig.14: Structure of y-s

1 first layer; 2— second layer 2; 3— third laver

Because of'y-AhO;'s polahedral cubic erystalliic, the structure inchules oviahadral and tetragonal AL The structure of y-AkO; is pscdo-spinen differing only in oxide anion packing density ‘The surfice of y-AbOs contains both Bronsted and Lewis active sites, which pliys imporlint tole in calalylic rection [I4]

In vonchsion, 7-ALO, tes becn wed extensively in the application of aulomobile cxhaust catalyst beoausc of its normal incxpensiveness, workability, long life oxiteria, arz those allowing the preatest activity of the active catalytic agent, namely high specific surface and adequate porosity on one hand, and on the othar band that of the highest structural stability [26

provides oxygen for oxidizing CO and L1C under rich A/F conditions and removes it ftom the exhaust gas phase for reducing NO under lean A/D Ilowevar, the surface of CeO is collapsed umder elevated lemperature: the addition of Zz can nol only prevent this phenomenon bul also itmprove the oxygen toobility in the CeO2 lailice For years, the phase

of CesZi1.0r is still a huge argunent between many researchers in the world Besides the oxygen storage capacity (OSC), ceria exhibits metal support interaction with precious metals such as Pt, Pd or Rh enhancing their catalytic activity |27{ These effets are noticeable as long as a high surfacc arca, and consequently low temperature reduction featues are present in the CeO.-based catalysts Accordingly, the research activity in the 1990s las been focused mainly on the improvement of the surface area stability in the CeO> promoter Among different systems tested, ZO appeared to be the most effective thermal stabilizer of CeO, particularly when it forms a mixed oxide with ceria, This material has been investigated since the early 1990 and is now generally known that the incorporation of zirconium into the ceria lattice creates a higher concentration of deibots improving, thus, the O2 mobility, such mobility would explain the outstanding ability to store and release oxygen [28]

Many researchers have reported the phase diagrams of promoters for IWC, phase diagram of Ce,7r 0> is present in fig.1.5 In the Ce-rich region of the diagrams, a cubic solid sohution af Ca,7m; sO appears, whilz

inthe tight regions of the Zr-richregion

CeO 210; system exhibited following properties:

- Inhibiting sintering:

CeOs powder readily sinters at elevated temperatures, although it is a good reftactory

oxide with a high melting point The addition of zirconium, especially the formation of CeOrZr0 mixed oxides, is effective in the inhibition of the sintering of ceria Simple

experiments indicate that Zr modification of CeO powder, followed by solid state reactions, has the effect of improving the thermal stability of CeO- promoter [30] For examples, Eduardo L Crepaldi et al prepared the crystalline phase of Cexs:.0: which

was stable al termperalure above 800°C and wo phase segregation [3) |

2

- Redox properties:

Oxygen evolution anor uptake originate fiom the norstoichiometry and oxygen diffusion in the surface add lattice of Ce,7r ,Oo The OSC promoter should satisfy two Gciors: a wide operation range for redox belween Ce™* and Ce’ in reducing and oxidizing atmospheres, and a high reaction zate over the modified CeO, particles The redox behavior and catalytic activity of five different CeO+ZrO2 mixed oxides and CeO, were investigated using a series of temperature programmed experiments

Samples containing at cast 50 mol% ceria were reduccd at similar temperatures

as 581 598°C, while samples with lower catia content were reduced at significantly higher temperatues as 666 690°C [32]

- High performance of precious metal given by high oxygen storage capacity: The activities of precious metallic (Pt, Rh) catalysts were enhanced by the presence

of CexZm.sOs, Temperature-programme reduction in a H./Ar mixture of Rh-loaded CeO+- Z4O- solid solution witha ZrO contert varying between 10 and 90% mol was carried out

It is shown that incorporation of Zr0- into solid solution with CeO stongly promotes bulk reduction of the Rh-loaded solid solution in comparison to a Rh/CeO, sampk In the seaction of redustion NO by CO, bulk oxygen vacancies play an important role of in promoling NO conversion over melal-loaded CeO,-7702 An oxygen vacancy gradisut ix indicated as the driving foree for NO dissociation, suggesting that i may be

the enhanced NO and CO conversions |33, 34]

The CeO2-Z10, mixed oxide was also a part of active phase a8 Cea ;sPđ, ,sO› ¿ ft the oxidation of uajor hydrocarbons in extanst gases [ydrocarhou oxidation over the mountilh catalysl is carried out with a mixture having the composition, 470 ppin of both propenc andl propane and 870 ppm of both ethylene and acctylene with (he varying amount

of Qs, Three-way catalytic test is done by putting hythocarbon mixture along with CO (10000 ppm), NO (2000 ppiỳ and O: (15000 ppm Below 350°C full conversion is achieved [27]

Tn order to improve the Ce

prccipitadion Thịs nwterial caloined at 600°C has surftwec arca oŸ131.5 Im”⁄g, por: voliune

of 0.23 ml/g, mean pore diameter of 8.5 nm, and OSC of 478 mol/g; alter L000°C aging

for 5 h, still bas surface area of 44.4 nr/g, pore volume of 0.11 ml’g, mean pore diameter

of 16.8 nm, and OSC of[ 368 pnolg [36] Another cxample is Cty 3570 33 You which was

prepared by Guo Jiaxiu cf al Cec 3:2re 55 ¥o1¢ had cubic structure similar to Ccy ;Ziy 502

and its specific surface area can maintain higher than Ce 5Zr);0 after 1000°C

calcination for 5h [37] ‘hus, these materials are suitable to prepare a motorcycle catalyst that can work at high space velocities and larger fluctuations of the air-to-firel

Tao

1.22 3.lạ; — Cea— Zr, materials

710; mixed oxide, the clemenl such as Ta, Y was

Recently the support of three-ways catalyst is directed to the combination between the high surface area ALO; and the effective oxygen storage capacity (OSC) material CeQ~Zr0., This material has attracted the attention of many researchers

‘The ALO; — CeO, — 420, (ACZ) samples with the alumina contents of 10, 25, 50, 75

wt%, were prepared by co-precipitation method (the afom ratio of Ce to Zr is 1:1) For all the samples wilh 10-75 wL% Als, only the single CeO; fluorite slructure phase was

observed and no Ah(; phase was detecled in XRD palierns As alumina conlenis incr

the peak intensities of the ACZ samples become weaker and the 2@ values of these peaks show no shift The xeasons maybe ACZ compounds form homogengous solid solutions and the highly dispersed alumina in ACZ solid solutions can-not be detected by KRD Based

on the calculation of lattice parameters by the Scherrer formula, the lattice parameters of

ACZ samples are larger than that ofthe CeO; 2103 (CZ) sample and inctease with ALO» content increasing, which indicates that alumina enters into the crystal lattice of CZ and makes lattice parameter become larger However, after calcination at 1273 K, the increase

degree of lattice parameter of the ACZ samples is less than that of the CZ sample It is

plausible that the CeO-, ZrO» and AbOs are well dispersed at the nanometer level and can serve as a barrier lo each other and thus sintering is impeded | 38}

AO; CeO ZrO support exhibits the following properties

- Texture properties

Specific surface area and pore volume both increase with ALO content increasing

and are larger Han thal of CZ [means Al;O3 doped into CZ mixed oxides can stabilive

and improve their textural properties The textural properties such as

face arca, pore volume and average pore diameter, play an important roe in the performance of catalytic supports, especially for the catalytic supports used in high space velocity, which requires the supports to have larger surface area, pore volume and good pore diameter distribution Compurad with CoO: oxides, C7 solid solutions are widcly omployed in TWCs, dus lo

th cellent redox behaviour and higher Ihermal stability, However, C7 mixed oxides slill tuve drawbacks in stbilily of structure, cspocially in thermal slabitily of texture, which obviously need be improved as catalytic supports cmployed in high space velocity Sie ACZ samples oxhibil cxoelicnt thermal sinbility in structure and texture, thoy shoukd

‘be more suitable as calalytic curiors and can be employed in high space velocity [39]

+ Reduction behaviour and oxygen storage capability

‘The H.-TPR of the ACZ samples indicated their reduction peaks also shift to lower

temperature to some extent, which is attributed to an increase of oxygen mobility in the

bulk induced by solid solttion sintering ‘The results indicate that these materials have more sible reduction performance at high temperature

Affcr oxygen uptakes measurements, with alumina contents increasing, itcan be scon

that the OSC of the fresh ACZ sample increases fiom 439.5 to 728 pmolg ', and the OSC

of the aged sample, which were calcined in air at LOO0°C for 5h, increases from 351.5 to

700 jumolg ', ‘This indicates that the surface area of ACZ sample becomes larger and the ratio of utilizable cerium also increases correspondingly with AlO, content increasing

t0]

Akia Morikawa et al proved the beneficial effect of Al to the mixed oxide

Ce; #IgQ: It was the improvement of the desoxption rate of oxygen (OSC-1) in the fiesh

catalyst and inhibition of the decrease in the OSC-r after durability testing were achieved

‘by suppression of particle growth of (Ce,Z1)0+ in ACZ by introducing AO, asa diffusion barrier with resuitant inhibition of sintering of Pt particles [AI] This material was used as

26

the suppori for hosting the active phase l'or example, Pt was dispersed on Al:Ox — CeO; —

ZrO to make a catalyst in the reaction of partial oxidation of methane [42] It is also to be applied in the three way calalyst as well [43, 44]

Trivalent La has been: used lo romote the thermal slabilily and oxygen mobilily (ionic conductivity) of Ce0>-Z10; solid solution and to restrict the phase transformation of y-AbO; to o-AbO; at high temparatwe Therefore, the introduction of lanthanum into the sysiems of ACZ composite oxide was expected to improve the thermal stability and oxygen storage capacity, and then its usc as the support of Pd catalyst Pd supported on Ce-Zi-La- ALO was ted for transtorming CO, CsHs, NO With these tiesh catalytic systems, the conversions are 100% at above 240, 300, 340°C tor CO, NO, CsHy respectively, Operating temperatures for aged catalysts, which were calcined at 1000°C for 3h, are higher [SI Furthermore, palladium catalysts were prepared by impregnation on ACZ and CeO2-ZrO

AbLOs-La:O3 for CHy, CO and NO, treatment in the mixture gas simulated the exhaust

fiom natural gas vehicles operated under stoichiometric condition was investigated by Xiaoyu Zhang |45 |

1.2.3 Active phase

The primary precious metals for a TWC were Pt and Rh with the latter being xostly responsible for reduction of NOx and the former for oxidation of CO and HC, Palladium is less resistant to deactivation by poisons such as Pb and § versus Prand Rh With 0.5 1 wt% of precious loading on substrates, the catalysts still exhibit completely conversion of CO, HC and NOx However, the noble metals were limited by cost and sensitivity to poisoning, especially by chiorine/chloride products Thus composite mixed oxides, mostly with perovskite-type structure (ABO; or A:BO,), can be seen as interesting

alternatives as they are low-cost materials and stable even at above 1000°C Another

alternative way is wansition metals as Mn, Co, Cu, V, which are inexpensive, high

aclivity and resistance Lo poisoining,

1.2.3.1 Noble metals:

Noble metal catalysts have received considerable attention for more than 20 years for wseid in aufomplive emission control systems, essenlially base on Pi, Pd, Rh over supporls Supports can be CeO ZrO2, AbO:, mixtures of some oxides Catalyst based on noble

ametal exhibited high catalytic activity in pollutant treatment and these catalysts were used

extensively [16, 17]

Containing Pd calalyst was researched hy Jianqiamg Wang et al[48] For fresh calatyst it can be observed that hath Pdi Cog 6/7t0.s5Or ard PCy 5 :7ty 35Sty940r gs show the almost same oxidation activity for CO, the conversion af which can rach almost 100% under %.> 1 conditions, but descend as decreasing? -valne undsr 4 < | contitions (@: the Usoretical sloichiometric yalue anđ 2 can be calculaled 4= (20+ NO) (10G:Tf; L CO)

U Lassi indivalod (hat catalyhe activity of catalyst base on Rh depends on the ralure

of ageing almosphere ani temperature Those calalysts reach their maximum conversions (100%) by the termperature of 400°C [49]

Sudlanshu Stara showed Ihal catalytic activity of cordierite honeycomb by a completely new coating method for the oxidation of major hydrocarbons in exhaust gas

Weight of active catalyst can be varied ftom 0.02 wt.% to 2 wt.% which is sufficient but

2

can he loaded even up to 12 wt % by repeating dip dry combustion Adhesion of catalyst to cordicrite surface is via oxide growth which is very strong [27]

Binary metallic activily is higher than single one Furthermore, some metals are

added to promote activily or reduce price bul properties preservirys or increase activily,

‘Ana Iplesias et al, studied the behaviors of a series of Fd M with M as Cx, Cụ and

‘Ni bimetallic catalysts for CO oxidation and NO reduction processes has been tested and compared with that of monometallic Pd references The catalytic properties display a strong dependence on the degrce of interaction whieh cxists between the metals in the calcinations state, For CO oxidation with oxygen, the second metal plays no significant role except in the case of Pd-Cuy Cop sZi0 50 [50]

Hyuk Jae Kwon investigated the light-otf temperature of the oxidations of CO and CsHe over a commerezal TWC was shifted to a lower temperature by the addition of water

to the fed stream ‘he formation of carboxylate and carbonate by a reaction between adsorbed CO and OH on the catalyst surface was observed during the couse of the reactions, The catalysts are containing Pd only and Pt-RhCe catalysts |51 |

In Vietnam, the catalyst with the dispersion of Pd on y- AkOs far the treatment of

exhaust gases was investigated by Nguyen Duc Khien, ‘Tran Van Nhan since 2000 [20]

Other noble metals were also studied in Vietnam, Le Thi Hoai Nam studied on Aw

7SM-S catalysis for carbon monoxide axidation to carbon dioxide The result showed that catalytic activity can be afizcted at very low temperature, Catalytic activity increases when temperature increases and it is more preeminent than some other systems (Aw/a-Fe.0; AuFe=1:19) [52] Nanoparticles gotd was atso loaded on support Co,O4 in the investigation of Tran Que Chi amd her co-workers ‘This materia was prepared by co- precipitation The products had sphere shape and the size about | — 3 nm, the BET surface arca of 69 1'/g, The catalytic activity was studied on the oxidation reaction of CO and CoH, the results showed that the complete conversion of CO was at ambicnt temperature Q7°C), andl total conversion of CsHy was at 200°C [53]

1.2.3.2 Perovskite

Perovskite-type mixed oxides have been widely studicd for the last four decades

‘These waterials present an ABO; formula, with the tolerance factor defined by Goldschumidt as: t= (tq, +10)! V2 (a+ To), Whore Ta, Ty and Ty are the ionie radii for the

jons A B and O Perovskil

activily was reported for a wide set of teactions and particularly for oxidation reactions of hydrocarbons and volatile organic compounds Cobeli- and manyanesc-bascd peravskites wore usually roperled as the two most cflicion, structures in oxidation reactions and they wore even propased as an alicrnative to noble metat supported calalysts sim

similar activitics in oxidation and a lower synthesis cost, However the low specific surface area generally displayed by these solids 1s still the major impediment to their use [54]

D Fino and colloaguc tealived thal the LaMmy yFcy 10s ealalys was found lo provide the best performaree of combustion of methane, Further calityst development alfowed ta maximize the catalytic activity of this compound by promoting it with CeO (1:1 molar ratio) and with 1 wt,% Pd This promoted catalyst was lined on cordierite monoliths ina y- AbOs-supported form [55]

uctures are obiained al 0.8 < <1 Their high catalytic

hey presead

Kollowing L Vorni’s investigation, series of La; „Ce,oO:„; perovekite-type

catalysts, with x ranging ftom 0 to 0.20, showed to be quite active for reduction of NO by

CO and for oxidation of CO by air oxygen al lemperatures ranging from 373 lo 723 K [56]

Hirohisa Taraka of al showed that one af the most important issues of aulomotive catalysts is the endurance of fluctuations between reductive and oxidative (redox) atmospheres at high temperatures exceeding 1173 K The catalytic activity and stractural stability of Lac sCeyiCo xFecOs perovskite catalysts (x — 0, 0.2, 0.4, 0.6, 0.8 and 1.0), both

in powder and monolithic forms, were investigated after aging treatments in ral and simulated “model” automotive exhaust gases [57]

In Vietam, Tran Thi Minh Neuyet studied deNOx properties of LaizSiCoO; perovskite‘compkx oxides The results showed that catalyst with molar ratios La:Sr:Co-0.40.6:1; a single phase perovskite exhibited only an oxidation function, while the product with three phases realized three functions of DeNOx reaction ‘I'he conversion was 40% [58] In 2011, she and coworkers prepared nanoparticles of perovskite Lay <NacCoOs by sol-gel using citric acid The obtained materials had sphere shape, the particks size of about 30

—40 nm, surface area in the range of 12 — 14 nig, La, Na,CoOs catalyst exhibited maximum

activity when x is 0.2 — 0.3 The temperatures of the total conversion of CO and diesel soot were 215°C axl 400°C, respectively [59]

1.2.3.3 Transition metallic oxidas:

‘The high price of noble metals and their sensitivity to higher temperatures have long inolivaled investigators to search (or subslitule catalysis Metal axides are am allermative to noble metals as calalysis for the camnplete oxidation They are reported less active at low

femperalures, bul al higher temperatures their activity is sirnilar fo that of the noble metals

‘The most active single metal oxides for combustion of VOCs are the oxides of Cu, Co,

Mn, and Ni, Among all metal oxides studied, manganese and coball containing catalysts are Tow cost, cnvirommentally friendly and ralatively highly active Gr VOC combustion The catalytic proporlics of MnO,-based catalysis are allribuled to the ability of manganese

ta Srm oxides of diffe and to their high oxygen storage capacity (OSC) [39]

Scloctivo catalylie ređuelion oŸ NO by propone in the prescnee of exeoss of oxygen

ơn Cu, Có, NỈ anE Mn ion-exchangod ZSM-5 and trordsrrile catalysts was it

A De Lucas clal The resulis showed thal catalylic activitics of all catalysts

seached to a maximum NO, conversion when metal content increased to a certain value However, the activity decreased at higher metal contents Similarly, NO, conversion peaked at a specific reaction temperature (in the examined range from375 to 425°C) due to the combustion af the hydrecarbon [60]

In Victnam, the sekctive reduction of NO; by CsHs on CwZSM-5 catalyst was investigated by Tran Van Nhan and co-workers, It was found that the presence of oxygen

in the reactant gas flow influenced badly on the reduction of NOx [61]

CuO and CrQ; based catalysts for CO oxidation ina micro reactor were investigated

by Lloang ‘Tien Cuong et al At 180°C, 100% of CO was treated completely on the catalyst containing LO%CuO | 10% Cr.O; ony-AhO; support [62]

C0304, Zz and CosO,- Zi: nano oxide catalysts were prepared by citrate sol-gel method and dispersed on the cordierite in the study of Tran Thi Minh Neuyet et al The catalysts were studied for the oxidation of CO Co;0, exhibited high catalytic activity but less durability, while ZrO oxide was thermally stable but exhibited limited catalytic activity, However, CoxO, dispersed on the surface of 4202 on cordierite exhibited the highest activity durability The complete conversion of CO was obtained at 170°C |64]

Harms

performed, far cxample, by nob at (66, 67], who also cvalualed the resulls in the form of delailed micro-kinctie schon, including umiber of reaetion steps and intermediates for important componants of exhaust gascs (Table 1.4) All kinctic sub-models (CO and C:H; oxidation, oxygenstorage

on CeO; and NO; transtormation) have been proposed and evaluated by the same research group for a typical TWC washcoa therefore, the TWC reaction scheme and th valucs of Kinclic parame

li OF +20* 3 2C0* + HO + * Geactions 16 23) on noble netals (*

13 COT+ OS CO + * 48

This model of CO treatment vonsists of three different pathways: (1) adsorption of

carbon monoxide and oxygen on noble metal, followed by a Langmuir - Hinselwood

surface reaction, (2) adsorption of carbon monoxide on an oxygen-covered site, with subsequent carbon dioxide desorption from so-called OCO species; and (3) reaction between carbon monoxide adsorbed on the noble metal and oxygen adsorbed on ceria

For the treatment of two acetylene surface species, the work was followed: (1)

p-acetylene, adsorbed on one catalytic site (atop), is the most unstable and therefore most reactive species, (2) di8 + ji acetylene, triangular adsorbed, is stable and therefore refractory towards oxidation

‘Thus, the reversible adsorption of acetylene (step 6) leads to j1 acetylene on the surface and 1s first order in the vacant sites Step 7 describes the reversible conversion of the two types of adsorbed aectylene info cach other The conversion ofa acetylene fo di-8 + L acetylene requires two vacant sites, making the latter species less abundant when acetylene is in the gas phase he rate of this step is second order in the fraction of vacant sites The direct partial combustion of both acetylene species to adsorbed carbon monoxide

is deseribed by steps 8 and 9, i.c reaction paths A and B

Although not clementary, the first and rate-determining step is assumed to be the oxygen-assisted abstraction of the weakest bonded hydrogen atom, whereupon oxidation to

carbon monoxide occurs instanfanconsly Decomposition of acetylene on the surface is unlikely because of the small number of vacanl sites, and the relatively large molar flow

rates in the reactor Steps 10 and 11 show an Eley-Ridcal type of reaction for the combustion of acetylene Acetylene is able to adsorb on an oxygen-covered surface to

‘yield oxidation products immediately Therefore, the species formed in step 10 is believed

to be highly unstable and will react rapidly through step 11 to adsorbed carbon monoxide Step 14, finally, describes the influence of ceria in the catalyst on the oxidation of

acetylene Only the reaction between the a acetylene and the ceria oxygen lus been used in

the model

For the teatment of NO, initially NO remains fully adsorbed At 548 K, CO slowly desorbs, leaving vacant sites onfo which NO can adsoth (step 16) The production of NO sequires the dissociation of NO, which involves extra vacant sites (step 17) then N20 is

then formed by step 18, and subsequence desoption in step 19 Nitrogen can be produced

in two diffzient ways, where the first (step 20) is known to be more important at low temperature than step 21 And finally, NO was form by the reaction between NO and oxygen adatom ( step 22 and 23)

Besides the above reaction kinetic of three-ways catalyst researched by Larmsenet

al, the kinetic model based on Langmmuiz-Llinderwood was investigated by Ilytk Jas Kwon

el al [68] The reaction kinetics

las been developed on the basis of the following assumptions: (i) all reaction steps ave assumed to be a first-order reaction with respect lo each of the reactants involved, (ii) all reactants except C02 adsorb on the eatalyst surface, (iti) oxygen and hydrogen adsorb dissociative on the catalyst surface, (ix) the surface seaction is a rate-determining step and described by a dual-site Langmuir Hinshelwood amechanism Table 1.5 shows the surface zaction mechanism considered in the proscnt study First, it has been assumed that all reactants are adsorbed reversibly on the active reaction site, 8, as reactions (1) (8) And then, the adsorbed species can react through reactions (9) (24), mainly occurring in the commercial automotive catalytic converter For the Hh oxidation, it has been reported that the reaction proceeds via H: $ + OH: $ as

illustrated in reaction (14) after forming OH - $ fiom the H- $+ Q - $ reaction On the

ofher hand, water and hydrogen enhanced the activity of the reactions involving CO by moderating the selfpoisoning of CO adsorbed on the catalyst surthce The enhancement effects of water and hydrogen have beenaccounted for in reaction (15), which involves the xeaction intermediate, OH - S, derived ftom water dissociation and the H S+0-S$ reaction Note thal the water-gas shill and steam relormning reactions have nol been explicitly considered in Ihe model, in view of Ibcir relatively small contributions la TWC Kinetics af low reaction temperatures, The reduction of NO to N20 and N:: canbe described

‘by reactions (16}-(18) and (23), Moreover, the reaction (19) was included since hydrogen- assisted NO dissociation plays an important role during NO reduction by Th NTs formed

9 NỔ .SIII.S->N.SIOII 5

20 N.S13II S>NH: 5145 2L 2NH;.S+5O.5—>2NO S+3H:O.S+28

22 2NH;:.5+3O §->N:†+3H:OQ.S+28

23 2NIEL.53I2NO.SIO.S5-+2N: 13IhO S128

24 NIO.S>N.LIO.S

tị conchsion Ihe hler kmebe reacHon model reveals fou typical reactions as

oxidation of CO, HC, reduction of NO and water-gas shift which not indicated in the

former model Ilowever, both models was investigated on catalysts based on noble metals (Pt Rh, Pad), therefore, there is consistent between the modeling of oxidation CO and reduction of NO In the exhaust gascs of internal conibustion engine, C2Hs is the main pollutant, but the first model didn’t discuss, while the second one didn’t explain more detail

Solid state reaction

‘The solid-state reaction is a conventional method to prepare the mixed oxides In order to prepare the product, the precursors are ground to fine powder and mixed thoroughly Afterward, the powder was calcined at high temperature In the solid state reaction method, the particles react through the grain’s houndary, leading to the less homogenzows products

33

mostly micropores and mesopores oxide affer calcinations If the gel contains polymeric chains with little branching and cross-linking, the gel fas smaller void regions, is sbuetirally weak, and thus collapse readily upon calcinalions The resulting oxide has inosily macropores arel low surfice area, However, due io the mixing the reactanls al the atomic scale, it is possible to prepare nano- material by using, sol-gel method

Hydrothermal method

Hydrothermal is a non-conventional method to obtain nanocrystalline inorganic materials A direct precursor-product comelation exists allowing the tailoring of almost any material synthesis without the presence of further structure direeting agerts

At a given hydrothermal temperature and presse which are convertient for the

synthesis (eg Ikbar water presse and temperatures of about 300°C), the precursor

material is continuously dissolved in the hydrothermal fluid ‘The formation of gels is not observed at any time diming the process - even if alumosilicate materials are used because bigger molecular mits are hydrolysed at elevated temperature and pressure

Inan aqueous solution under autogensous pressure conditions fir below the critical point, diffrent states of dissolution might be existent and, most important, not only the basic structural building units can be present, but even colloidal states Iligh pressure hydrothermal synthesis implements, therclirc, a first step of crackdown of possibly present “macromokcula:” units by chemical reaction, cxisting ¢.g, as colloidal solution,

as precipitated colloidal solution (crystalline, partially crystalline (c.g gel), glassy and amorphows) or solid state precursor materials of the same kind, because bigger units excocding the size present in te solutions arc not stable under figh presse hydrothermal conditions The formation of a truc solution is therefore assumed in which the smallest possible structual building units as well as cations with their respective hydration spheres are transported

1.4.2 Synthesis methods of substrates and supports

1.4.2.1 Synthesis methods of cordierite

Cordierite is well-known to be used as the substrates of three-ways catalyst ‘There are many ways to prepare this substance, for example: solid state reaction, conventional

sintering, sol-gel, with various precursors All these methods were proved to prepare

ss Fully at 1300 — 1400°C

B.P Saha et al prepared cordierite honeycombs with oxide compositions of 49.5—

51 wt% SiOn, 35.5-36 wil Als Og and 14-14.5 wh% MpO fiom clay, fale, and alumina

using conventional extrusion process The extruded honeycombs were sintered with

various healing and cooling rates varying from 80 to 180°C/h up Lo a peak lermperalure of

1420°C for various soaking times ranging from 4 to 8h XRD studies reveal that all the investigated sintered honeyocombs contain >90% cordierite phase with smaller quantities of

mullite, spinel and o- ALO; [96]

Cristina Ghitulica ct al utilized solid state reactions to prepare Cordicrite ecramic

powders fiom SiO:, y-AlOs, MeCOs The powders were thermally treated at 750 and 950¢C, respectively, for the decomposition of the volatile compounds, and then at

temperatures between 1050 and 1400+C for the synthesis of cordierite, X-ray diffraction cordierile su

34

methods proved the apparition of cordierite at temperatures as low as 1200°C in the

presence of cristobalite silica At higher temperatures, cordierite was identified as the main phase [97]

AM Menchi and co-worker investigaled the mechanism of conticrile formation obtained by sol gel method was studied using DTA and XRD techniques to #llow the yeactions ocotring during gel calcination up to full cordierite conversion (1300°C) Spinel and magnesium aluminum silicate are formed at the beginning of calvination, followed

by the formation of quartz, sapphitinc and cristobalite At 1203°C, crystallization of ctistobalite is observed as an exothermic reaction; then, at 1246°C, other exothermic reactions between spinel and the remaining amorphous silica, and lastly between ctistobalite, sapphirine and magnesium aluminum silicate produce cordierite When the calcination was performed in a strongly reducing atmosphere followed by a second

calcination at 1000° C inair, a porous cordierite material witha sharp pore size distribution

The acid method wilh the inexpensive precursors as waste aluminium, aluminiam trineral was used fo produce boclunite efficicatly In this methods, the impurities was eliminated by filtration afterward the obtained solution was mixed with NaOII to

precipilale the Cu, Pe Then, 1804 was alded ty prepare boehrmite al pl 8.5 The

bouhinite was dricd at 80°C, then calcined at 590°C te yield y-ALOs which tad surface arca as 200 m'/g [1]

While Osama Suber prepared ALO, renoparlicles (hrough a sol-gel method

by reacting aqueous solutions of both aluminum prcewsor and ammonium Bicarborale im thơ prescnce of cetyl trimethyl ammonium bromide (CTAR) surfictan, as lemplale [99], ternptato-ficc sol-gel mnolhnd under rorravidie conditions to Propare 1rso-ttweroporots ramteerysialline -y-ALO, was studied by Abbas Khu

Starting with aluminum isopropoxide in alcoholic solvents, where colloidal solutions wore initially obtained, resulted in meso-macroporous y-ALO, withzelatively high surfice areas,

350-500 mi/g, and large total pore volumes, I.4—2.0 ce/g [102]

In the shudy of Qian Liu alat,, bochimile sats

cl ct al

sre uscd as alurtinam procursors for

preparing y-ALO; having crystalline framework walls in the prescice of non-ionic

surfactants as structure directing agents The sample with a comugated platelet-like

smorphology exhibited a large surface area of 463 nv/g, which was reduced to $1 nr/g after

calcination at 1200°C, indicating a strong resisiance to sintering This material, with its improved textural properties, crystalline framework walls and high thermal stability, not only could inetease the dispersion of the aetive catalytic species, but also could enhance

the diffusion efficiency and mass transfer of reactant molecules when employed as catalyst

supports [100]

Qian Liu el at also investigaled the mosphologically conholled synlhesis of

.O5 using hydrothernal route Afler calcination al 800°C, the amorphous

mesoporous y-Al

fiamework walls of the samples transformed fo crystalline y -AbOs, as revealed by their wide XRD patterns, With the aid of surfactants, mesoporous y-AhO; with spheres, rods, fibers and three-dimensional dumbbell, tlower-like hierarchical superstructures on the microscopic scake has been obtained [101]

b CepZt1 202

‘The observation of the ability of CeO, to promote metal dispersion in comparison with conventional supports such as ALO; traces back to the Inte 1970s Since then multiple roles of CeO have been identified and it is believed that CeO is able to have extremely positive effect on catalytic activity of catalyst, Therefore, the CeO.-based material, especially Ce,Z1.,02 has been attracted much attention fiom many researchers They put amch effort to prepared CayZrxO: wih advanced properties as nano-particles, mesoporous framework by many methods, for instance, co-precipitation, hydrothermal

roule, sol-gel, Trñ©Trperrtulsiom

In case of wilboul using terrplale, Sonia Letichevsky wilived co-preeipilation was

as the standard method The use of Ce(NO3); leads to the formation of cubic -CeO> and tetragonaLZxO; mixed oxide whereas a solid solution is achieved by using (NEA pCe(NOs)s [28] Rui Si ef al also prepared mesoporous Ceo-7i sO ranosived powilers withoul any iemplaics via # hydrothermal method in the presence of urca The

propacd powders tad high surface areas (232-281 m/g) and narrow pore s

distribulions (3.54.0 1m), according to Ih: measurements of nitrogen adsorption Afler

calcining at 773 K, this mesostructure was retained to some degree with lower surface

arcas (66-75 w/z) and larger pore sizes (5.1—7.2 mnt) [89]

Yucheng Du studied nanosad-like Ccy-;7ay 30, solid sotulions synthesized by a sodium dodecyl sulfite-assisted precipitation method Typical Ceo 71y sO2 nanoreds were

AO nm in average diameter and 450 nm in length, with specific surface arca and oxygen

storage capacity of 194 m'/g and 374 umol/g, respectively [93]

LI Hongmei et al prepared a series of Ce,Zte so-zAlle so O-as (0.05<x«0.45) mixed oxides with different Ce/Zr ratio by co-precipitation method ‘The XRD results showed that all samples kept the single CeO» cubic fluonte structure after calcination at 600 and 1000°C for 5 h ‘The results of BET revealed that CerZio so xl so 1 75 with Ce/Zr molar ratio 1/1 exhibited higher specific surfice area (212 m'/g) and larger pore volume (0.40 mig) For all aged samples, Ce,7to s0-«Ale se O15 with Ce/7x molar ratio 3/7 presented the

36

highest specific swface arca (104 m’/g) and pore volume (0.34 mrl/g) The cormpounds could still keep prominent structural and textwal stability with excellent redox properties

even calcined al 1000 °C [40]

The stadiss of Zhenting Wei et al [38] and Akira Morikawa el al [41] proved the advantages of aluminum oxide’s presence in the mixed oxides The mixed oxides CesZio sozAbsoOi7s was prepared by co-precipitation and characterizad by X-ray diffraction, BET The results showed that at 1273K, specific surtace area of CexZtoso- Alo s¢ O75 after duability testing in air at 1273K was 20 m’/g, which is higher than that

ot conventional CZ (2 '/2) composed of (Ce, Z1)02 without ALO

L.SPreparation the catalytic converters

‘The cordierite and metal substrates have many excellent properties such as high mechanical strength, low thermal expansion constant, resistant to pressure shock, but they have disadvantages on surfice as low surface area, Therefore, it is essential to coat the support material on substrates ‘Then, the active phase should be deposit later The advantage of using the coating technique is that the catalyst is used more efficiently, because the diffusion distance toward the active species will be small

1.5.1 Coating a monolith with a catalysis support material

For deposition the support material on substrate, there are many methods can be used depending on the properties of support and substrate In cases of cordierite and metal foil, the deposition methods were investigated as direct synthesis techniques [69, 70), suspension, hydrid deposition, and sol-gel

15.4.4 Direct synthesis techniques

Direct combustion synthesis has alrendy proved to be anefficient, quick, cheap and straightforward preparation process, suitable for producing a layer of catalyst which excellent adherence on ceramic substrates I'he catalytic loading content (up to 10%) and thickness of the catalyst layer are dependent on the concentration of the mother solution However, surface areas of the catalysts remain small although an improvement compared

to the original ceramic is observed,

When the coating layer is zeolite (ZSM-5), the in-situ crystallization method was

chosen, in which zeolite can be deposited by direct orystallization fiom a gel on substrates Placed together with the gel in an aulochivs The thickness of the layer varied from 70-100

pm For the samples with a dense layer of zeolite, the adhesion was good but in some case, less dense films were formed, which showed crystal detachments |69| Some modifications

of the in-situ crystallization method have been reported [70], where a seeding step was performed prior to the in-situ crystallization of ZSM-5 by hydrothermal synthesis

According to this report, after the first synthesis, the cordierite subswate gained 28 wt%,

and L1 wt.% more after the second, The porosity of the final eoated cordizrite was 16%

15.1.2 Suspension

All methods based on the dispersion of a finished material (catalyst support or catalyst itself) have been gathered under the term “‘sespension method” It is the most largely used method, namely for ceramic monoliths Powder (catalyst support or catalyst

3

itself), binder, acid and water (or another solvent) are the standard ingredients ‘rhe concentration of all ingredients depends on the nature of the surface to coat and on the desivedd layer thickness [72]

‘The shury method has also been applied to coat different powders such as y-AbOs,

Z20., TiO2, CeO, on cordierite ceramics It was proved that a homogeneous and good

adhesive coating layer on the ceramic could be obtained with small particle sizes (doo < puss) [73] y-ALO, was ulso deposited an cordicrite fiom sats of y-ALO, iiself [75], by a

sol-gel method The viscosity of the solution, which was adjusled by binders and sol

concentzation, influenced significantly the loading content and thickness of the layer (Sm)

[74] Compared with bochmnite precursors, the wash coating of cordicrite monoliths with y-

AkOs suspensions of suitable patlick sive allows a higher alumina loading, and the deposition of a homogeneous wash coal layer with good adhesion properties and surface

areas ofabout 50 1g [75]

Zapf ct al [76] prepared the suspension with 20g ALO; (um particles), 75 g watcr,

5g polyvinyl alcohol and 1g acetic acid and obtained a very adherent Al,O3 layer on

stainless steel microchannels

In the case of Phifer ct al, dhe suspension contained a cellulose derivative (1 wt.%

of hydroxy ethyl (or propyl cellulose) and a solvent (water or isopropyl alcohol) The

ranoparticles (20 wt% in the suspension) of CuO, ZnO and TiO or P/ZnO catalyst were

axed together with this solution The celhiose derivative was found to efficiently avoid the particles agglomeration ‘rhe resulting suspension was filled into microchammels, dried and calcined at 450°C A complete burn off of the polymer was obtained [77, 78]

Liguras et al prepared a dense suspension of catalyst (Ni/La205) powder im de-

ionized water A simple immersion of ceramic substrates in the suspension followed by

drying at 120°C and calcinations (550°C and 1000°C:) allowed obtaining the catalytic

material [79]

LF Liotta et al deposited the support over cordierite by suspension Commercial cordierite witha cell densily of 400 cpsi was cul lo oblai samples with 25 channels on the cross-section and differest lengths up to 40 nan In ortler to obtain a washcoal kyer of veria—irconia onto the cordierite surface, highly dispersed pseudobohemite alumina powder was used as a binder The monoliths were dipped in a slurry composed of diluted nitric acid solution (1.4 wi.%) ard appropriate amounts of finely grounded ceria—rconia and pseutoboherile powders in order to oblain a final washcoal cormposilion of ceria—

zuconia (80 wt%)- y-ALO; (20 wt%) Several dips were needed to obtain the desived amount of washcoat loading (25% ot the total weight) In each cycle the excess shury was

rempved by blowing air through the channels ard Lhen the sarnples were dried al 120°C for

1 hand calcined at 550°C for 3 h for binder decomposition and y-ALO; stabilization The

deposition over the cordierite doesn’t produce any structural modification [80|

15.1.3 Hybrid method between suspension and sol-gel deposition

This method is a combination of suspension and sol-gel method Inthis method, a sol

of support’s precursors act as the binders while a prior prepared support powders are suspended in the deposition solution and participate in the chemical and textural properties

of the final deposited layer For example, fo obtain a silica layer, metallic monoliths have

3

been dipped in a suspension of silica powder (0.7—7 jum) with a silica sol ‘Ihe layer obtained after drying and calcination steps is 20-30 mm thick {72]

In order lo oblain thicker films, A Ronge eft al added y-alumina powder lo an aqueous suspension of boskmite, the boehmile acting as a binder for the particles Tw the washcoating procedure, an aqueous suspersion containing 15% boehmite, 15% 7-alunina, 1% acetic acid and 4% polyvinylalcohol was wed, In this way, considerable thicker films consisting of an agglomerate of particls instead of a homogengous layer were obtained [81]

L, Villegas abo wed boehmite as the binder in the suspension of alumina tor washcoating, The washcoating procedures were developed with 230 and 400 cpsi cordierite monoliths with square channels The boehmite and y-AbO; (3 mm average particle size powder) were used tor the washcoat preparation HCl has been used as the dispersant for bochmite powder ‘The 7-AbO; powder was dispersed by HNOs After vigorous stirting tor 15 h at room temperatime, the suspensions were used for washcoating

‘The monoliths were dipped vertically into the suspension tor 2 min, then removed and the excess suspension was evacuated fiom the channels by a tlow of compressed air The monoliths were dried at 100°C for 1.5 hand weighted The procedure was repeated until a 13-15 wL.% increase was obtained, The washcoaled monoliths were calcined in air al 800°C fir 4h The uss suitable particles siva of y-ALOs suspensions allows a fastor alumina loading and the deposition of a washcoat layer with better adhesion properties than those obtained with bochmite precursors [74]

In Vietram, the deposition wethod af support material on the substrale hast heen fully paid altcntion The numiber of studiss on the deposition method is vory rare The most

nol cffeutive although it is usually ignored

in lke pubbeations, Some autlirs have choscn mixing supporl powder with substraie powder materials befor extruding monolith substrate to increase the adfxsion of support onthe substrate [64]

1.5.2 Deposition of active phase om monolithic support

of the monoliths contains cxcass liquid, causing an uncverrdistibulion In this procedie, the monolith is allowed to suck up the Hyuid by capillary forecs,

excess liquid is allowed to dip out of the channels [24]

In the case of wet impregnation, first the amount of liquid that will adsorb on a ronptith must be determined The concentration of the dissalved metal precursor should

be suilable lo produce the desired metal loading A dry monolith is imumersed in this solution, removed, anit exvess liquid is blown out To prevent an uncven distribution of metal, especially if the metal precursor shows a significant interaction with the support, the dipping procedure should be carried out in as short a time as is practically Êasible to

3g

prevent an excess metal adsorbing on the support Since drying can also result in maldistribution, it is important to continue this step immediately after impregnation because trany solvenls will slarl evaporating inmnedialely afer the monobith is removed from the liquid Tpossible, the wet manoliths shoukl be kept ina horizontal position while

continuously being rotated te prevent gravity from causing the liquid to flow to one side of the monolith [24]

L Villegas deposited Ni on alumina support by wet impregnation The alumina-

washcoated monoliths were dipped into a stirred aqueous solution of 0.5M Ni{NO3);.6H:0

for 2 h After removal, the excess solution in the channels was evacuated with an air flow

‘The wet impregnated monoliths were dried by three different methods: (i) in a ventilated oven at 100°C for 2 h; (ii) at room temperature and atmosphere pressure for 2 weeks; (iii)

in a microwave oven operating at 200W for 50 min Th: monoliths were then calcined at 550°C for 4 hinair ‘The Ni distribution is strongly mfluenced by drying method applied to

wet-impregrated monoliths At a macroscopic scale, the Ni distribution is more Homogerzous after microwave treated than those obtained after room temperature drying

At a microscopic scale, there is a surface enrichment of Ni in the washcoat, whatever the

applied drying methods In all catalysts, Ni is present in two forms: (j) it is predominantly incorporated in a Ni-ahanina spinel phase, bul sorne large metal particles (10-20 mt) are also present, (ii) in a largor extent in the monoliths drisd by microwaves and oven procedures [74]

LF Lioita et al prepared ceria—zirconia supported alumina monolithic samples impregrated with platinum (1 wL% with respect fo the ceria—zirconia weight conterd) by

using a solution of Pl(acac) in tolusne al 70°C, then calcined al 400°C for S fh The

vesulling catalyst bad howogencous distribution of the acti

[80]

¢ compancnls over Cey.671y40%

Literature review’s conclusion

When vehicles are in rmming condition, toxic components such as CO, HC, and NO,

ate expelled fiom gasoline engine in large amounts The catalytic unit capable of simultaneously and efficiently converting carbon monoxide (CO) , urburned hydrocarbon

(HC), nitrous oxides (NO) into harmless CO:, H:O, and Ni

For automotive applications, cordierite (2MgO.2A1.05.5SiO2) ceramic monoliths and inetallic substrate are offen the matztial of choice due to their excellent properties The cordierite is often prepared by conventional sintering, solid-state reaction, sol-gel Amongst these methods, the conventional sintering has been wed extensively because the

precumsors are popular sources as tale, kaolin

It is wellHnown that yAbO; Is been used as the support for Pt, Rh in the application of catalysis because of its high surface area, and its stability The most simple and effective method to prepare 7-ALO; is bozhmite Since the beginning of 1980s, the

researchers have focused on the Ce().- based mmterials or it has been called the oxygen

storage material, which can improve the catalytic activity Recently, a new generation of

materials as AhO3-C2O.-7101 which is expected to becore the optinal support for the catalytic application, was investigated ‘[he mixed oxides are prepared by co-precipitation