Luận Án tiến sĩ sythesize and investigate the catalytic activity of three way catalysts based on mixed metal oxides for the treatment of exhaust gases from internal combustion engine

Synthesize and investigate the catalytic activily of three-way catalysis based on mixed metal oxides for the treatment of exhaust gases from internal combustion engine 2.2.2 Scanning El

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HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

NGUYEN THE TIEN

SYNTHESIZE AND INVESTIGATE THE CATALYTIC ACTIVITY OF THREE-WAY CATALYSTS BASED ON MIXED METAL OXIDES

FOR THE TREATMENT OF EXHAUST GASES FROM

INTERNAL COMBUSTION ENGINE

CHEMICAL ENGINEERING DISSERTATION

HANOLI-2014

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MINISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

NGUYEN THE TIEN

SYNTHESIZE AND INVESTIGATE THE CATALYTIC ACTIVITY OF

THREE WAY CATALYSTS BASED ON MIXED METAL OXIDES FOR THE TREATMENT OF EXHAUST GASES FROM INTERNAL COMBUSTION ENGINE

Speciality: Chemical Fngineering

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ACKNOWLEDGEMENTS

‘This PhD thesis has been carried out at the Laboratory of Environmental Friendly Material and Technologies, Advance Institute of Science and Technology, Department of Organic and Petrochemical Technology, Laboratory of the Petrochemical Retinering and Catalytic Materials, School of Chemical Engineering, Hanoi University of Science and

‘Technology (Vietnam) and Department of inorganic and Physical Chemistry, Ghent University (Belgium) The work has bean completed under supervision af Associate Prof,

I want to thank Prof, Isabel and all staff in Department of Inorganic and Physical Chemistry, Ghent University for their kind help and friendly attitude when | lived and studied in Ghoul

gratefully acknowledge the receipt of grants fiom VLIR (Project ZEIN2009PR367) which enabled the research team to camry out this work

T acknowledge to all members in my rescarch group for their ffiondly altitude and (heir

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Synthesize and investigate the catalytic activily of three-way catalysis based on mixed

metal oxides for the treatment of exhaust gases from internal combustion engine

COMMITMENT

T assure that this is my own rescarch, All the data and results in the thesis arc completely true, was agreed to use in this paper by co-author ‘This research hasn’t been published by other authors than me

Nguyen The ‘Tien

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1.1.4 Air pollution from exhaust gases of internal combustion engine

1.3.1 Gatalytic systems based on noble metals (NMs} 20

1.3.3.1 Metallic oxides based on CeO; 23

1.3.3.3 Catalytic systems based on cobalt oxides 25

14.1 Mechanism of hydrocarbon oxidation over transition metal oxides

28

14.2 Mechanism ofthe oxidation reaction of carbon monoxide 23

2 EXPERIMENTAL 37

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2.2.2 Scanning Electron Microscopy {SEM) and Transmission

2.2.3 BET method for the determination of surface area 40

3.1 Selection of components for the three-way catalysts 48

37.27 Catalysis based on single and bi-metailic oxide 53

31.23 Imiuence of MnO›, Co¿O„ CeO; conten† on catalylic actvfy of

3.2 MnO -Co;0,-CeQ, based catalysts for the simultaneous

3.24 MnO,-Co30,-CeO, catalysts with MnOfCos0.=1/3 66

3.2.2 MnOz-Co:O,-GeO; with the other Mn0,/Co;0, ratio 68

3.23 Influence of different reaction conditions on the activity of

3.24 Activity for the treatment of scot and the influence of scot on

3.2.5 Influence of aging condition on activity of MnCoCe catalysts 74

325.7 The influence of steam at high temperature 74

325.2 The characterization and catalytic activity of MnCoCe 1-30.75

3.2.6 Activity of MnCoCe 1-3-0.75 at room temperature 80

3.3 Study on the improvement of NO, treatment of MnQ2-

3.4 Study on the improvement of the activity of MnO2-Co30,-

3.5 Comparison between MnO,-Co;0,-CeO, catalyst and noble

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ABBREVIATION

‘TWCs: ‘Three-Way Catalysts

NO,: Nitrous Oxides

‘VOCS: Volatile Organic Compounds

PM10: Particulate Matter less than 10 nm in diameter

NMVOCs: Non-Methane Volatile Organic Compounds

HC: hydrocarbon

AVF tlio: Air/Fuel ratio

+: the theoretical stoichiometric value, defined as ratio of actual A/F to stoichiomettic; > can

be calulateđA (20;+NOY (10CjH_-CO); 2 Lat stoichiometry (A/F 14.7)

SOF: Soluble Organie Fraction

DPM: Diesel Particulate Matter

CRT Continupusly Regenerating Trap

NM: Noble Metal

Cpsi: Cell Per Inch Square

in: inch

C7, (Ce-72): mixtures of CeO and 720+

C7.ALa: mixtures of CcOz, 7102, Al:O;, La2O;

NGVs: natural gas vehicles

OSC: oxygen storage capacity

WGS: water gas shaft

INTs: Lean NO, traps

Tivo: the temperature that correspond to the pollutant was completely treatment

‘Tree: ‘he maxium peak temperature was presented as reference temperature of the maximum reaction rate in TG-DI'A (DSC) diagram

ppm: part per million

ppb: part per billion

Nguyen The ‘Tien

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LIST OF TABLES Table 1.1 Example of exhaust conditions for tvo- and four-stroke, diesel and lean-four-stroke

Table 1.2 Adsorplion/derorption reactions on Pt catalyst {101}

Table 1.3 Surface reactions af prapylene oxidation [101]

Table 1.4 Surface reactions of CO oxidation [101]

Table 1.5 Surface reactions of hydroxyl spices, NO and NO; [101 i

Table 2.1 Aging conditions of MnCoUe catalyst

Table 2.2 Strong line of some metallic axides

Table 2.3 Binding energy of some atoms [702]

Table 2.4 Specific wave manber of some function group ar compatends

Table 2.5 Composition of mixture gases at different! reaction conditions for CyHe oxidation

Table 2.6 Composition of mixture gases al different reaction conditions for CO oxtdthion 0 Af

Table 2.7 Composition of mixture gases at different reaction conditions for treatment of CO, CH

Table 2.9 Retention time of some chemicals as

Table 21 Quantity of hydrogen cansianed valune (ml/g) at different reduction peaks in TPR-H,

Table 3.3 Adsorbed oxygen vohune (ml/g) of some pure single oxides (MnO, Co

chemical mixed oxides MnCoCe 1-3-0

‘Table 3.4 Surface atomic composition of the sol-gel and mechanical sample

5 Tacx of mixtiae of single oxides and soot in TG-FITA (DSC} diagrams

Table 2.6 Catalytic activity af single oxides for soat treatment a

Table 87 Taoy of mixture af mnaltiple oxides and soot determined from TG-DTA diagrams 6

Table 3.8 Catalytic activity of waltiple oxides for soot treatment al 500°C

Table 3.9 Soot conversion of some mixture of MnCeCe 1-3-0.75 anid sovt in the flow condaining CO: 4.359%, Ox: 7.06%, Calle: 1.1394, NÓ: 1.77% ab SOPC for 425 nửn ¬—- Table 3.10 Specific surface area of MnCoCe catalysis before and after aging in the flow: containing

Tnble 3.11 Connnned hydrogen vohone tml/g) oƒ the MnCoCE ]-3-0.73 /heah and agìng at BUŒC

Table 212 Specific sunface area of MnCot'e 13-075 fresh and after aging in different conditions

79 Table 3.13 Specific surface area of catalysts containing MnO, Cor, CeO, BaO and WO} 81 Table 3.14 Specific surface avea of some catalyst containing MnO:, Co;0s, CeO, ZrO; before and

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LIST OF FIGURES

Figure 1.1 Micrograph of diesel saat, showing particles consisting of clumps of spherules [110] 13 Figure 1.2 A typical arrangement for abatement of NO from a heavy-duty diesel engine wring urea

Figure 1.3 Principle of filter operation (1; and filter re-generation (2) fora soot removal sysiew,

Kigure 1.8 Wash-coais on automotive catalyst cam have different surface structures as shown with

io , Rj to Re tahile

Figure 1.9 Inprovensent trend of calatvtic converter [43]

Figure 1.10 Scheme of catalytic hydracarbon cxidation: H-hydracarbon, C-catalys

Figure 1.11 Reaction cycle and potential energy diagram for the catalytic oxidation of CO by Os

Figure 1.72 Reaction patinvays of CO axidalion over the metallic oxides [34]

Figure 1.73 Chemical reaction pathways of selective catalylic reduction of NO, by propane [99] 32 Higure 1.14 Principle of operation of em NSR catalyst: NO, are stoved under axidising conditions (1) and then reduced ona TWC when the A/V is tempavarily switched to rich conditions (2) [67}.33 Figure 1.75 Schematic representation of the seven main steps involved in the conversion af the

Figure 2.1 Aging process of the catalyst (I> air pump; 2.6: tube furnace, 3: water tank, 4 heater,

igure 2.3 The relationship benveen concentration of CsHs and peak arec eee

Figure 2.4 The velationship hetween concentration of CO; and peak area 16 Figure 2.5 The relationship hetween concentration af CO and peak area th Figure 2.) Catalytic activity of aome môed oxide Ma, CoCt and single metallic oxide in

Figure 3.2 Catalytic activity of Mntv 1-3 and CeCo 1-t catalysts in excess oxygen contlition 49 Figure 3.3 CsHs conversion of CeCol-4 in different reaction conililions (condition a: excess oxygen condition with the presence of CO: 0.9% Cslls, 0.3% CO, 586 Q2, Nz balance, condition b: excess oxygen condition with the presence of CO and HO: 0.995 Cille, 0.396 CO, 2% ILO, 38% Os,

Figure 3.4 XRD patterns of CeCo=I-4, MnCo=1-3 chemical mixtures and same pure single oxides

eo 50 Figure 3.5 Conversion of CHa, CoH and CH an MnCoCe 1-3-0.75 catalyst under sufficient

Figure 3 SEM images of MuCo 1-3 fresh ‘), MncoCe 1-3-0.75 before (a) and after (b; reaction under ngficiant oxygen condition (OxCs1Tg=5!1) - ¬ 32 Figure 3.7 XRD pattern of MnŒoCk 1-3-0 73 and origimal ovides 3 Figure 3.6 CO conversion of same catalysts in sufficient oxygen condition 3 Figure 3.9 SEM images af MnCo 1-3 before {a} and after (b) reaction under sufficient oxygen

Figure 3.10 CO conversion af original oxides (MnOs, CuiQs, CeO;) and mixtures of these oxides in

Figure 3.11 LPR Hy profiles of the mixtiwe MnCoCe 1-3-0.75, MaCo 1-3 and pioe Ma0;, Co:Os

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Figure 3.13 XRD pattern of MuCoCe 1-3-0.75 synthesized hy sol-gel and mechanical mixing

Pigure 3.14 XPS measurement of Co 2p region (al, Ce 3d region (b), Mn 2p region (c) and O Ls region (d) of the mechanical mixture (J; and chemical MnCaCe 1-3-0.75 sample (2) oe SB Figure 3.15 XRD patterns of Mnt?:-Co:OrCeO; samples with MnQy-Co:O.=1-3(MnCoCe 1-3 0.17 (a), MnCoCe 1-3-0.38 (B), MnCoCe 1-30.75 (c), MnCoCe 1-3-1.26 (di; MaCoCe 1-31.88 (e)

60 MnCoCe 7-3

Figure 3.16 XRD patterns of MnOrCo;OrCeO; samples with MnQ:-Co,0,—

Higure 3.17 Specific surface avea of MnCoCe catalysts with different MnOyC0:Oe Yabi0S 00.61

Higure 3.48 Teiperature to reach 100% (0 canversion (Tyee of mixed MnO: 20,0,-Cey

samples with the malar vatio of MnO,-Co.O, of 1-3 (a) and MnO,Co,O.=7-3 (6) with different

Figure 3.19 TG-DSC and TG-DTA of soot (a), mixture of soot-Co.Oy 'B), s00-MAO: (c), voof- FQ: (a) with the weight vatio of soot-eatalyst of IAL - „62 Figure 3.20 XRD patterns s4 MnCoCe 1-3-0.75 (1), MhCoCeV 1<#0.75-0.53 (

Figure 3.23 C:Hy and CO conversion of MaCoCe catalyst with MuOy/CasO, 1-3 low containing

Higure 3.24 Catalytic activity of MnCoCe catalyst th hẳn Qà-CoyQ¿ =1-5 (le containing 4.3506

Figure 3.25 SEM images of MuCoCs 1-3 0.75 (a), May 1-4-1 36 (BỊ, MnCaCe

Figure 3.26 Catalytic activity of MnCoCe catalysts with ratio MnO-Co;0.=7-A(flow containing

Figure 3.27 Catalytic activity of MnCoCe 1-3-0.75 with different lambda values

Figure 3.28 CO and C:Hy conversion of MnCoCe 1-3-0.75 in different condition (non-CƠ; and

Higure 2.29 € vatalytie activity of MinC‘ot‘e 1340.75 at a high temperatures in 4.35% CO, 765% QO,

Figure 3.30 Catalytic activity qƒ MnCoCt 1-3-0.75 with the different mass ratio of catalylic!saot

(a: CsHy conversion, b: NO conversion, «: CO, concentration in outlet flow; d: CO concentration

Figure 3.32 Catalytic activity of MnCoCe (MnO-Co:0, 1-3) catalysis before and after aging at

Ploure 3.32 XRD patterns of MnCoCe catalysis before and after aging in a flow containing 521% volli:O at S002C for 24h (MI: MnCoUe 1-34.75 fresh, M2: MnCoCe 1-30.75 aging, M3: MnCioCe 1-3-1.66 fresh, Md: MuCoCe 1-3-1.86 agingi, Ce: Ces, Co:Ca 75 Figure 3.33 SEM images of MnCoCe catalysts before and after aging at 800°C in flow containing

3726 steam for 24h (ak MnCoCe 1-3-0.75 fresh and aging, bye: MnCoCe 1-3 26 fresh and aging, off MnCoCe 1-3-1.88 fresh and aging, respectively) ¬- Figure 3.34 TPR-E patiern of MuCoCe 1-3-0.75 fresh and aging al 800°C in flow conlaininy 3786

Figure 3.35 Catalytic activity of Mnt‘ot'e 1-3-0.75 fresh and afier aging in different conditions 78

Higure 3.38 XR1) pattarn of MnCote 1-30.75 in different aging canditions 79

Figure 3.37 SEM images of MuCoCe 1-3-0.75 fresh and after aging in different conditions 0 Figure 2.38 Activity of MaCoCe 1-3-0.75 after activation 80 Figure 3.39 CO and CE, conversion of MrCoCe 1-3-0.75 at roi iemperature after activation 2h

im gas flow 4.35% CO, 7.6595 O., 1.15% CaHhy, 0.59% NO with and without COg Sĩ

Higure 3.40 XRD pattern of catalysts based on MnO, CozO,, CeO;, BaO and IWO: 82

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Figure 3.41 Catalytic activity catalysts based on MnU;, Ca;Q,, CeO;, BaO and WO, in the flow containing 4.25% CO, 7.066 Oz, 1.1596 CsH and 4.77 ¥6 NO " 83 Higure 3.42 SEM images of catalysts containing MnQs, Coss, CeO #aO and WO: senna A Figure 4.43 Catalytic activity of MuCoCe 1-3-0.75 added 26, 586, 71⁄4 Z2; fresh ‘a, ¢, ¢) and aged (b, df) in flow containing 4.358% CO, 7.65% 02, 1.1595 CsH and 0.59% NO 4 Figure 3.44 XRD patiern of MnCoCe 1-3-0.75 added 296 and 5% Zr; before and after aging al

Figure 3.45 SEM images of MuCoCe 1-3-0.75 aided 595 Zr before (a) andl fier (0) aging al

Higure 3:48 SEM image [0.154 P4-4lO) (A), 0.595 Pd9-Al,O; (B) and 10% MnC'oCe/ ALO: (c)

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INTRODUCTION

Enviromental pollution fiom cngine in Victnan was morc and more scrious since the

number of motoreycles used in Vietnam is incteasing significantly The development of the automotive industry attracts more attention on the atmosphere pollution from exhaust gases, and three-way catalysts (I'WC) are the best way to remove these pollutants They

can conver! completely pollutants to Teach Ihe Furo standards,

In the world, precious metallic catalysts such as Pt, Rh and Pd were focused for three

way catalyst application and sepresented the key component, as the catalytic activity

occurs at the noble metal (NM) centre Furthermore, this catalytic category was applied broadly in commercial calalysl and investigated in delail [15-21, 23, 29, 33, 85] Tigh prices and casy losl aclivily when contacting with sulfir compound are the most

disadvantages of this catalyst category [18, 19, 72] In Victnam, NM catalysts (from

Emitee company-Germany) hav2 been tested for the treatment of exhaust gases of some

43% and 60%, respectively by

using these catalysts [13, 11] However, the price of these catalysts is expensive for smotorbike’s users Therefore, the recent research trends is the partial or complelz

substitution of precious metals in the catalytic converter by a less expensive components

Perovskites were reported as the most efficient structures in oxidation reactions and they were even proposed as an alternative to NM supported catalysts since they present similar

aciivities in oxidation and a lower synthesis cost However, the low specific surface arsa

28, 60, 78, 79]

Meanwhile, metal oxides are an alternative to NMs as catalysts for pollutant treatment

‘The aim of the thesis is to study on a catalytic system that exhibit high activity, high thermel resistance, ow cost and easy to apply in trealment of exhaust gases Therefore,

metallic oxides were choosen for investigation in this study The most active single metal oxides are the oxides of Cu, Co, Mn, and Ni Among all metal oxides studied, manganese

and cobalt containing catalysts are low cost, environmentally friendly and relatively highly

active ‘I'he catalytic properties of Mn(,-based catalysts are attributed to the ability of

manganese to form oxides of different oxidation statcs and to their high oxygen storage

capacity Appropriate combinations of metal oxides may exhibit higher activity and

the maximum treatment of toxic components in exhaust gas to enhance the application

catalyst in order to obtain the best catalyst The influcnee of activation, aging proccss to

catalytic activity of the samples were also studied Then, the optimized catalysts will be

supported on y-Al.O; in order to compare with the noble catalysts

The thesis conlains four chaplers The firs! chapter, the literature review, summarizes

mechanism of exhaust treatment The aims of this thesis will be then proposed

‘The second chapter introduces basic principles of the physico-chemical methods used in

the thesis, catalyst synthesis, aging processes and catalytic measurement

The most important chapter (chapter 3) is focused on calalytic activily of metallic oxide for ctimination of single pollutants (hydrocarbon, CO, sool) and the simultaneous

treatments of these pollutants (CO, HC, NO,, soot) Furthermore, the influence of aging

and activation processes to the activity of the catalysts was investigated in details in this

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1 LITERATURE REVIEW

1.1 Air pollution and air pollutants

Now a day, air pollution from exhaust gases of inlermal combustion engine 48 one of srious problems in the world and immediate consaquenccs are hazards such as: aciel rain, the greenhouse effect, ozone hole, ete |2|, An air pollutant is known as a substance in the air that can canse harm to humans and the environment Pollutants can be in the form of solid particles, liquid droplets, or gases [126]

1.1.1 Air pollution from exhaust gases of internal combustion engine in

Vietnam

Vietnam is a developing country reaching the next stage of cconomical level

Motorbikes are the main way of transportation for the moment The number of motorbikes

is about 90% of all vehicles in Viemam In 2006, there were eighteen million operating

more and more serious The air in Hanoi and Ho Chi Minh City (HCMC) also contains

dangerous levels of benzene and sulfur dioxide and PM | 127]

4.4.2 Air pollutants

Pollutants for which health criteria define specific acceptable levels of ambient concentrations are known as "criteria pollutants." The major criteria pollutants are carbon monoxide (CO), nitrogen dioxide (NO;), volatile organic conpounds (VOCS), ozone,

PM10, sulfur dioxide (SO,), and lead (Pb) Ambient concentrations of NO: are usually controlled by limiting emissions of both nitrogen oxide (NO) and NO:, which combined

are referred to as oxides of nitrogen (NO,) NO, and SQ are important in the formation of acid precipitation, and NO, and VOCs can teal react in the lower atmosphere to form ozone, which can cause damage to lungs as well as to property [42]

HC (hydrocarbon), CO and NO, are the major exhaust pollutants, HC and CO occur

because the combustion efficiency is <100% due to incomplete mixing of the gases and the wall quenching effects of the colder cylinder walls ‘fhe NO, is formed during the very high temperatures (71500 °C) of the combustion process resulting in thermal fixation of

the nitrogen in the air which forms NO, [43]

4.1.2.1 Carbon monoxide (CO)

Carbon monoxide (CO): is a colores, odorless, non-irtitaling but very poisonous gus Carbon monoxide emissions are typically the resuit of poor combustion, although there are several processes in which CO is formed as a natural byproduct of the process (such as the refining of oil) In combustion processes, the most effective method of dealing with CO is

to cnsure that adcquate combustion air is available in the combustion zone and that the air and fick arc well mixed at high temperatures [41]

1.1.2.2 Volatile organic compounds (VOCs)

Volatile organic compounds (VOCs) are an important outdoor air pollutant, VOCs are emitted from a broad variety of stationary sources, primarily manufacturing processes, and are of concern for two primary reasons In this field they are often divided into the separate Nguyen ‘The ‘Tien

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categories of methane (Cll) and non-methane (NMVOCs) Methane is an extremely

efficient greenhouse gas which contribules lo entumnce global warming Other hydrocarbon

VOCs are also significant greenhouse gases via their role in creating ozone and in

prolonging the life of methane in the atmosphere, although the effect varies depending on local air quality VOCs react in the atmosphere in the presence of sunlight to form photochemical oxidants (including ozone) that are harmful to human health [41]

1.1.2.3 Nitrous oxides (NO,)

Nitrous oxides: (NO,) - especially nitrogen dioxide arc emmitled ftom high temperature combustion “itrogen dioxide is the chemical compound with the formula NO; It is one of the several nitrogen oxides ‘This reddish-brown toxic gas has a characteristic sharp, biting odor NO; is one of the most prominent air polluants Nitrous oxides can be formed by some reactions:

1.1.2.4 Some other pollutants

Sulfur oxides; (SO,) especially sulfur dioxide, a chemical compound with the formula SOs, Further oxidation of S:, usually in the presence of a catalyst such as NO», forms

1804, and thus acid rain, ‘This is one of the causes for concern over the environmental imnpacl of the usc of these fuels as power sources [1, 41]

Particle matter (PM10): Particulates altematively referred to as particulate matter (PM)

or fine particles, are tiny particles of solid or liquid suspended in a gas In contrast, aerosol refers to particles and the gas together, Increased levels of fine particles in the air are inked to health hazards such as heart discases, allored hang function and lung cancer [1 At] Sool as sampled, e.g froma dilution tunel, is fannd to be im the form of agglonne

which are around 100 un in size, These agglomerates are composed of smaller, very open

‘particles’, which are in tum a collection of smaller carbonaceous spherules The terms agglomerate (100 mm typical size), particle (0.1—1 mm) and spherule (10-50 nm) will be used for these Uhrec scales of particulate The fundamental unit of the soo! agglomerates are the sphcrules with diameters of 10-30 mm, Most of these parlicles are almwsl spherical, but a number of less regular shapes may be found, The surface of the spherules has adhering hydrocarbon material or soluble organic fraction (SOI?) and inorganic material (mostly sulphates) The SOF and other adsorbed species such as sulphates and water are cuplured by the sool in the gas cooling phass c.g in the exhaust pipe of a diescl engine The spherules are joined together by shared carbon deposition to form loose particles of

0.1—1 mm size The nitrogen BET area of a soot was found to be only 40% of the external

surfice area calculated for spherules whose diameter was measured by electron microscopy as seen in Figure 1.1 [110]

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Figure 1.1 Micrograph of diesel soot, showing particles consisting of clumps of spherules [110]

1.1.3, Composition of exhaust gas

As shown in Table 1.1, the exhaust contains principally three primary pollutants,

unburned or partially burned HCs, CO and nitrogen oxides (NO, ), mostly NO, in addition

to other compounds such as water, hydrogen, nitrogen, oxygen, SO: etc In exhaust gas of engine, the flow rate was very high with GHSV of 30000-100000 h' [67] The concentrations of NO,, in exhaust gas of diesel engine and four-stroke engines were very high meanwhile two-stroke spark ignited engine emit large amount of HC The second and

fourth engine types emit massive concentration of CO It can be seen that the amount of

H,O was high (7-12%) but the oxygen concentration in exhaust gas was significantly lower

than that in air However, the 4 value of all of engine was equal or higher than 1

Table 1.1 Example of exhaust conditions for two- and four-stroke, diesel and lean-four-stroke engines [67]

Exhaust Diesel engine Four-stroke Four-stroke Two-stroke

components spark ignited- lean-burn spark ignited-

and condition * engine spark ignited- engine

(test cycle) | temperature- | temperature- | temperature- | temperature-

b For comparison: diesel fuels with 500 ppm of sulphur produce about 20 ppm of SO»,

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¢ Close-coupled caialyst

di, the theoretical stoichiomewie value, defined as mass ratio of actual A/F io stoichiometric A/F; A

can be calculated = (20;+NO) (LOCsHy+CU), a = 1 at stoichiometry (A/F = 14.7)

¢ Pari of the fuel is employed for scavenging of the exhaust, which does not allow to define a precise definition af the A/F

1.2 Treatments of air pollution

With the development of science and technology, there are many methods for exhaust gas Lreatinenl, They were devided ina lwo calegories: Ireatmenls of single pollntant and

simutlancous ircatrent of pollutants

Method 2: water gas stifl process could converl CO wilh parlicipalion of sleante

‘This reaction was catalyzed by catalysts based on precious metal [53]

by using addilional catalyst or higher temperatures (and thus more supplemental fucl) Because catalysts may be poisoned by contactmg improper compounds, catalytic oxidizers are neither as flexible nor as widely applied as thermal oxidation systems Periodic replacement of the catalyst is necessary, even with proper usage [41] Catalytic systems based on NM, perovskile or, motal and metallic oxide [26, 27, 35-40, 55-57]

1.2.1.3 NO, treatments

the rate of NO, formation is so highty dependent upor temperature as well as sity within the combustion cnvirommiont, NO; és ideally snited to contrat by means of modifying the combustion conditions, There are several methods of applying these combustion modification NO, controls, ranging from reducing the overall excess air ievels in the combustor fo burners specifically designed for low NO; emissions [41] NOx can be trealed by sore reductions occurred in exhaust gas such as CO, VOCs or soot with using NM, perovskite catalysts and metallic oxide systems |23, 28, 54, 58-66]

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(eon pre Urea

Due to the limited success of HCs as efficient reducing agent under lean conditions, the

use of urea as an alternative reducing agent for NO, from heavy-duty diesel vehicles has

received attention Selective catalytic reduction of NO, with NH; in the presence of excess

O: is a well-implemented technology for NO, abatement from stationary sources

Typically, vanadia supported on TiO:, with different promoters (WO; and MoQ;) are

employed in monolith type of catalysts A sketch of an arrangement for the urea based NO,

abatement technology was shown in Figure 1.2 Typically, the urea solution is vaporized

and injected into a pre-heated zone where hydrolysis occurs according to the reaction:

H:N-CO-NH: + H:O — CO:+ 2NH;

Ammonia then reacts with NO and NO, on the reduction catalyst via the following reactions:

4NO + 4NH; + O: — 4N; + 6H:O

6 NO: + 8 NH; + 7N2 + 12 H:0 [67]

1.2.1.4 Soot treatment

Diesel particulate matter (DPM) is the most complex of diesel emissions Diesel

particulates, as defined by most emission standards, are sampled from diluted and cooled

exhaust gases Removal of soot may be achieved by means of filtration Even though

different types of filters can be employed the filtration efficiency is generally high

However, the continuous use under the driving conditions leads to filter plugging

Regeneration of the filter is therefore a crucial step of the soot removal systems This can

be achieved thermally, by burning the soot deposits on the filter, using, for example a dual

filter systems such as depicted in Figure 1.3 However, such systems may be adopted only

in the trucks where space requirements are less stringent compared to passenger cars In

addition, there are problems arising from the high temperatures achieved during the

regeneration step when the deposited soot is bummed off In fact, local overheating can easily occur leading to sintering with consequent permanent plugging of the filter To

overcome these problems, development of catalytic filters has attracted the interested of

many researchers [67]

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Synthesize and investigate the catalytic activity of three-way catalysts based on mixed

drawback of these systems is related to the capability of Pt catalysts to promote SO:

oxidation as well The sulphate thus formed is then deposited on the particulate filter interfering with its regeneration Moreover, the NO; reacts with the soot to reform NO whilst reduction of NO; to Nz would be the desirable process, Accordingly, it is expected

that as the NO, emission limits will be pushed down by the legislation, less NO will be

available in the exhaust for soot removal, unless the engine is tuned for high NO, emission that are used in the CRT and then an additional DeNO, trap is located after the CRT device

1.2.2 Simultaneous treatments of three pollutants

There are two solutions for simultaneous treatment of pollutants In particular, two successive converter possessed drawback that incomplete NO, treatment Meanwhile,

three-way catalyst is the best solution when converting toxic gas (CO, HC, and NO,) into

N:, COs, and H:0

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4.2.2.1 Two successive converters

NO,, CO, HC could be treated by designing successive oxidation and reduction converters (Figure 1.5) The main reactions in treatment process are:

Reduction reaction: NO could he reduced into N> and NIT;

Oxidalion reaclions CO+% O; › CÓ;

C.Hy+ 0ey/4)O: >xCO;+y/2 HO

Steam formed in process reacts with CO to form CO› and H: Thus, some reactions

Addition air

Figure 1.5 Scheme of successive heo-converter model [1]

1.2.2.2 Three-way catalytic (TWC) systems

The basic reactions for CO and HC in the exhaust are oxidation with the desired product being COz, while the NO, reaction is a reduction with the desired produet being Nz and

HO A catalyst promotes these reactions at lower temperatures than a thermal procass giving the following desired reactions for HC, CO and NOx

Oxidation

CyHa+ (y+ 4) 0: > yCO2+ 0/2 HO

CO + 1⁄4 0; —> CO;

CO + HạO — CO: + Hạ Reduction:

NO (orNO)+CO šN‡+ CO;

NO (œrNO)+H; +1⁄2N:+H, (2-1/2) NO (er NO.) + CH, (n4) N: + yCO: + m2 HạO All the above reactions required some heat or temperature on the catalyst surface for the reaction lo occur When the aulomobile fist starts, both (he engine and catalyst are cold, Afler startup, the heat of combustion is tansferred from the engine and the exhanst piping begins to heat up, Finally, a temperature is reached within the catalyst that initiates the catalytic reactions ‘This light-off temperature and the concurrent reaction rate is kinetically controlled, is depends on the chemistry of the catalyst since the transport reactions are fasl, Typically, the CO reaction begins first followed by the TIC and NO,

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reaction When all three reactions are occurring, the term three-way catalyst or TWC is

used Upon further heating, the chemical reaction rates become fast and pore diffusion

and/or bulk mass transfer control the overall conversions

70.98 Air/tucl weight ratio (wet Avgt) AEl01 A=l01

Figure 1.6 Three- way catalyst performance determined by engine air to fuel ratio [43]

Figure 1.6 shows a typical response of a TWC catalyst as a fiction of the engine air to fuel ratio [43] Today the required conversion of pollutants is greater than 95%, which is

attained only when a precise control of the A/F (air to fuel ratio) is maintained, ie within a

narrow operating window Accordingly, a complex integrated system is employed for the control of the exhaust emissions, which is aimed at maintaining the A/F ratio as close as possible to stoichiometry (Figure 1.6) To obtain an efficient control of the A/F ratio the

amount of air is measured and the fuel injection is controlled by a computerized system

which uses an oxygen sensor located at the inlet of the catalytic converter The signal from this sensor is used as a feedback for the fuel and air injection control loop A second sensor

is mounted at the outlet of the catalytic converter (Figure 1.7) [43]

Electronic contre,

Figure 1.7 Diagram of a modern TWC/engine/oxygen sensor control loop for engine

exhaust control [67]

Catalyst system included some common components

* Noble metals e.g Rh, Pt and Pd as active phases

+ Alumina, which is employed as a high surface area support

+ CeO;-Z10 mixed oxides, principally added as oxygen storage promoters

+ Barium and/or lanthanum oxides as stabilizers of the alumina surface area

*Metallic foil or cordierite as the substrate which possess high mechanical and thermal strength The dominant catalyst support for the auto exhaust catalyst is a monolith or honeycomb structure The use of bead catalyst has been studied in the beginning of history

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Synthesize and investigate the catalytic activitv of three-way catalysts based on mixed

of three-way catalyst The monolith can be thought of as a series of parallel tubes with a

cell density ranging from 300 to 1200 opsi Figure 1.8 shows the surface coating on a

+ Stabilized alumina

TY

Tee see Low emission Vehicles All Palladium three -

with no Ce Tieeetbe Volbdie je “Noce - Underfloor catalyst underfloor catalyst, high

precious metal loading

Figure 1.9 Improvement trend of catalytic converter [43]

Along with the advances in catalyst technology, the automotive engineers were

developing new engine platforms and new sensor and control technology (as seen in Figure

1.9) This has resulted in the full integration of the catalyst into the emission control

system The catalyst has become integral in the design strategy for vehicle operation [43]

1.3 Catalyts for the exhaust gas treatment

TWC is one of the best solutions for treatment of exhaust gas It can transform polluted agents approximately 100% in large temperature range to reach Euro III and IV standards

[15] Catalysts are classified to some groups beyond metallic characteristic

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1.3.1 Catalytic systems based on noble metals (NMs)

‘NM catalysts have received considerable attention tor more than 20 years for used in automotive emission control systems, essentially base on Pt group, such as Pt, Pd and Ri

on supports [69] Supports can be CeQr7rO, AlOs, mixtures of some oxides: Ce0:-7.10, (C7), CcOs-7TO-ALO; (CZA), CeOs-7rOs-fiO- (C78), CcO;-7TOs-AlsOr-lazO; (CZALa) CeO: in the three-way catalysis sinec multiple effects have been attributed to this promoter Ceria was suggested to: promote the NM dispersion, increase the thermal stability of the ALLO; support, promote the water gas shift (WGS) and steam reforming reactions, promote CO removal through oxidation employing lattice oxygen, store and release oxygon under, respectively, tan and rich conditions Among differonl, systcrs tested, ZrO; appeared to be the most effective thermal stabilizer of C2O., particularly when

it forms a mixed oxide with cena [31, 32, 81] For the øtabihzation of the cubic structure even for lmgh Zr content at elevated temperatures many researchers [8S_ 86] have suggesled the addition of trivalent cations M** (La, ¥*, Ga®*) in the oxide mixture CeO; ZrO2 Catalyst based on NM exhibited high catalytic activity in pollutant treatment and these catalysts were used extensively |15, 18-22, 29, 33, 44-47, 69, 70, 73-76]

HU Chunming et al [15] showed the Pt/Pd/Rh three-ways catalyst was prepared using a highperformanee CeassZrosYazsLaser©¬ soHd solution and high surface area La- stabilized alumina (La/ALO;) as a wash-coat layer The activity and durability of the catalysts under simulated conditions and actual vehicle test conditions were studied The sesults revealed that CeassZtv ssYoosLacos2 solid solution maintains superior textual and oxygen storage properties, and La/AbOs has superior textual properties ‘The catalyst had high low-lcmperalure activity, wide sir-lo-lel ratio windows, and good {hermal slabilily The results fom the cmission test of a motorcycle showed that the catalyst could mect Euro IIT emission requirements

F Dong and colleagues research the OSC performance of PYCeOrZ10:-Y.0s catalysts

by CO oxidation and 0/0 isotopic exchange reaction and obtained good results They

indicated thal the development of a more efficient axygen storage matcrisl is a very important approach for the optimization of automotive catalysts [17]

Daniela Meyer Fernandes and co-worker used the commercial Pd/Rh-based automotive catalyst The catalysts were evaluated for CO and propane oxidation with a stoichiometric gas mixture similar to engine exhaust gas The catalytic activity results, reported as Ts

(comverL 50% gas) valics, were consistent with aging Lemperature and Girne In spite of

severe therinal impacts caused by aging, evidenced by the characterization results, the commercial catalyst could still convert 100% of CO at 450 °C [18]

Ana Iglesias et al [54] showed the behaviors of a series of Pd-M (M=Cu, Cx) bi- matallic catalysts for CO oxidation and NO reduction processes has heen tested and compared wilh thal of monomelallic Pd references The catalytic proportics display a strong dependence on the degree of interaction, which exists between the metals in the calcinations state For CO oxidation with oxygen, the second metal plays no significant role except in the case of PA-CwCZ

Li-Ping Ma et al.[69] proved thal the catalylic activily of PAR (1.6% NM, Pd

Rh 5:1) supported by alumina system is very good for treating exhaust gas

Containing Pd catalyst was researchad by Jiangiang Wang et al.|70] For fresh catalyst

it can be observed that both PA/CZ and Pd/(2ZS show the almost same oxidation activity for CO, the conversion of which can reach almost 100% under % > 1 conditions, but descend as decreasing 4 -valuc under 2 < 1 conditions

Pd supported on CZALa was used for transformmg CO, C:Hy, NO With these fresh catalytic systems, the conversions are 100% at about 240, 300, 340 °C for CO, NO, C;H;

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respectively Operating temperatures for aging catalysts (the catalyst was undergone in some

condition such as: high temperature, contact with gases: seam, SO, CO, ele) are higher

than that for fiesh ones |76| Furthermore, palladium catalysts were prepared by

impregnation on CZA and CZALa for CH., CO and NO, treatment in the mixture gas simulated the exhaust ftom natural gas vehicles operated under stoichiometric condition

was investigated by Xiaoyu Zhang [71]

U Lassi indicated that catalytic activity of catalyst base on Rh depends on the nature of

aging atmosphere and temperature These catalysts reach their maximum conversions by

the temperature of 400°C [72]

Sudhanshu Sharma showed catalytic activily of cordisrite honeycomb by a completely new coating method for the oxidation of major hydrocarbons in exhaust gas, Weight of active catalyst can be varied from 0.02 wi% to 2 wt% whichis sufficient but can be loaded even up to 12 wi% by repeating dip dry combustion Adhesion of catalyst to cordierite surface is via oxide growth, which is vary strong [73]

Binary metallic activity is higher than single one Some metals are added to promote activity or reduce price but properties preserving or increase activity Guo Jiaxiu and co-

worker investigated influence of Cer 3310 «Ye 19 Solid solution on the performance of Pt-

Rh three-way catalyst The results revealed that Cen 23ZtessYe1o had cubic structure similat to Ceo sZr9 50, and its specific surface area can maintain higher than Cec sZ19 501

after 1000°C calcinations for 5h Being hydrothermal aged at 1000°C for Sh, the catalyst

containing Ces 3s4tessYo1o still exhibited higher conversion of CsHs, CO and NO and lower light-off temperature in comparison with Ceo 44100.’ WC [74]

Hyuk Jac Kwon reported thal the lighl-off lemperature of the oxidations of CO and

C3Hg over a commercial three-way catalyst (TWC) was shifted to a lower temperature by

the addition of water to the feed stream The formation of carboxylate and carbonate by a reaction between adsorbed CO and -OH on the catalyst surface was observed during the

[75]

In Vietnam, Tran Que Chi et al [6] show the catalytic activity of Au/Co;0, for CO and propylene oxidation under excess of oxygen It can be seen that, CO and C3H; was treated completely from room temperature and 200 °C, respectively owing to the presence of Au

nanomeler particles

Le Thi Hoi Nam studied on Au-Z8MS5 catalysis for carbon monoxide oxidation to

carbon dioxide The result showed that catalytic activity can be affected at low

temperature Catalytic activity increases when temperature increases and it is more preeminent than some other systems (Awo-FeO; AmFe=1:19}, Pd/y-AlO;) [3]

Turlhermore, Au-7.SMS was applied for complete oxidation of loluenz The conversion of

this catalyst 1s abont 11% at low demperature (150°C) [7]

Pham Minh Tuan and Duong Viet Dung applied the catalyst Pt-Rh=5-1 supported on metallic foil for treatment of exhaust gases of some Vietnamese motorbikes ‘The catalysts

converted 30%-50% amount of pollutants [13, 14]

41.3.2 Catalytic systems based on perovskite

Peroyskile-Lype tixed oxides have been widely studisd far the las! four decades The

iualcrials prescul an ABOs formula, wilh Lhe lolcrance factor defined by Goldsclunidt as:

t= Ga + to) V2 Ue + re), where ra, rp and ro are the ionic radii for the ions A, B and O Perovskite structures are obtained at 0.8 <t < 1 Their high catalytic activity was reported for a wide sel of reactions and particularly for oxidation reactions of hydrocarbons and volatile organic compounds Coball- and manganese-based perovskites were usually

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reported as the two most efficient structures in oxidation reactions and they were even

proposed as an allernalive to NM supported catalysis since they present sirnilar activities in

oxidation and a lower synthesis cost, However, the low specific surface area generally

displayed by these solids is still the major impediment to their use [27]

1D Fino and colleague realized that the LaMnosee ;Os catalyst was found to provide the best performance of combustion of methane Further catalyst development allowed to

maximize the catalytic activity of this compound by promoting it with CeO (1:1 molar

satio) and with 1 wt% Pd This promoted catalyst was lined on cordierite monoliths ina y-

Al:Os-supported form [26 |

Following L Forni’s invesligalion, series of La: Ce,CoOzs peravskile-type calalysts, wilh x ranging (rom 0 to 0.20, showed lo bs quile active far reduction of NO by CO and

for oxidation of CO by air oxygen at temperatures sanging from 373 to 723 K [24]

Hirchisa Tanaka et al.[25| showed that one of the most important issues of automotive

catalysts is the endurance of fluctuations between reductive and oxidative (redox) almospheres al high lemperaiures exceeding 1173 K The calalytic activity and structural stability of LagsCec aC = 0, 0.2, 0.4, 0.6, 08 and 1.0), both

in powder and monolithic forms, were investigated after aging ticatments in real and

simulated “model” automotive exhaust gases

Some anthor improved the specific surface area of perovskite system by impregnating on SBACI5 m order to enhance the complete oxidalion of elhylene and ethyl acelale Mesoporous

direct hydrothermal treatment, and excellent performance was observed over the

40LaCoO;/SBA-15 catalyst in the combustion of toluene and ethyl acetate it is believed

that the excellent performance is due to good dispersion of highly reducible LaCoO, embedded in SRA-L5 [77]

The nanosized La; K-NiMnO¢ perovskite-like complex oxides have good catalytic

performances on diesel soot particulates combustion under loose contact conditions The

catalyst was investigated by W.Shan In the Lax,K,NiMnO catalysts, the partial

substitution of La with K at A-site enhances their catalytic activity, which can be attributed

to the production of high valence metal ions at B-site and nonstoichiometry of oxygen

vacancies The oxygen vacancy concentration has an important effect on the catalytic

activity because the oxygen vacancy is beneficial to enhance the adsorption and activation

of molecular oxygen ‘The optimal substitution amount of K is equal to x=0.4 among these

samples [78]

Lei Li investigated perovskite La-Mn-O bascd catalysts coated on houcycomb ceramic

in practical diesel exhaust, Nanosized perovskite LaMnOs, LaysKa.MnQ; and LagyKo2

CocsMing sO; have been prepared by the citrate gel process and their coatings on the

these three catalysts, Tap ¢Ka2MnO; shows the bes! comprehensive catalytic performance,

with the best soot trapping cffcet, the lowest Tsc value (414 °C) and a very small smoke

opacity, and the Lao aKaxMn©; coated honeycomb ceramic is a promising device for diesel

exhaust gas emissions [79]

Tn Vietnam, Tran Thi Minh Nguyel studied deNO, properties of Tay St,CoO;

perovskilevcormplex oxides The resmlls showsd that catalyst with molar ralios

La:$r.Co-0.4:0.6:1; a single phase perovskite exhibited only an oxidation function, while

the product with three phases realized three fictions of DeNO, reaction The conversion

was 10% [4]

Quach Thi Hoang Yen et al [11] showed the catalytic aclivity of La; Na,CoOy series

for CO and diesel sool treatment, Amongst these catalysts, Taig7Niz3CoO, exhibited the

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best performance ‘he sample can convert CO and soot from 216°C and 400°C

respectively Il is suilable for treatment of exhausl gas of diesel engine

Tran Thi Thu Huyen studied Lag2Sry;MnO; supported on 7-AlLO; for complete

oxidation of m-xylene The best catalyst was 30% Lao7Sr3MnO: on support This catalyst

can convert m-xylene completely from 300 °C [12]

1.3.3 Catalytic systems based on metallic oxides

active single metal oxides for combustion of VOCs arc the oxides of Cu, Co, Mn, and Ni

‘Some typical oxades will be mentioned in more detail,

1.3.3.1 Metallic oxides based on CeO»

As seen in section 1.3.1, C202 was reported the most popular metallic oxides for the support and promoter of noble catalyst This oxide possessed high OSC due to the redox of Ceé“/ce* Moreover, when combining with other metallic oxides, CeO, exhibited high activity for CO, hydrocarbon, soot oxidation and NO, reduction

H Zou investigated the catalytic system CuO-CeO, add some elements (Zn, Mu, Fe) for CO in reduction condition (65% H:, 25% CO», 1% CO, 9% H:O, 0/CO-1.5)

Cu(CezO¿ and Cu⁄n(CezO; catalysts exhibited the highest activity at 160 °C and CO selectivity of 100% at 100-140 °C The doping of Zn remarkably improved the catalytic

activily, while Fe:O3 or MnO, deteriorated the ewlalytie properties Addition of ZnO to CuO CeO: catalyst stabilized the reduced Cur species and inereased the amounts of CO adsorption and lattice oxygen [51

A series of Cu,Ce,0+ nanocomposite catalysts with various copper contents were

synthesived by a simple hydrothermal method at low temperature without any surfactanis using mixed solutions of Cu(II) and Ce(iII) nitrates as metal sources The optimized performance was achieved for the Cue Cec 20: nanocomposite catalyst, which exhibited

superior reaction rate of 11.210 * mmolg 's ' and high turnover frequency of 7.53*10 7

=1 (1% CO balanced with air at arate of 40 ml min at 90 °C) [52]

F.Lin ob al [65] show the catalytic activity of CuO: tern added BaQ for soot treatment in the gas Dow 1000 ppmiNO/10%O-/N: (1 Lan) im loose contael When the amount of BaO was fiom 6% to 10%, the catalyst exhibited the highest activity with the onset temperatures Tie (the maximum peak temperature was presented as reference temperature of the maximum reaction rate) were 400 °C and 483 °C: for fresh and aging

catalyst, respectively

Mu; :CcozO, tú Mni ¡Cca 6Zto30 samples synthesized by sol-gel method were tested for redox properties through the dynamic oxygen storage measurement, The results showed that redox performances of ceria-based materials could be enhanced by synergetic effects between Mn-© and Ce-O Tresh and aged samples were characterized with the fluarite- {ype cubic structure similar to CoO,, and furthermore, the thermal stabilily of Ming ,Co9 sO; materials was improved by the introduction of some Zr atoms |92|

M Casapu used the system based on Niobia-Ceria to reduce NO, ‘The catalyst was

able to convert 72% NO already at 250 +C and showed almost full NO reduction between

300 and 450 °C The new nichia-ceria exhibited a similar urea hydrolysis activity as compared to a conventional TiO, catalyst A significant decrease of the soot oxidation temperature was also noticed with this catalyst [94|

‘A superior Ce-W-Ti mixed oxide catalyst prepared by a facile homogeneous

precipitation method showed excellent NIl:-SCR (selective catalytic reduction) activity

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and 100% N selectivity vith broad operation temperature window and extremely high resislance lo space velocity This is a very promising calalyst for NO, abalernenl from

diesel engine exhaust The excellent catalytic performance is associated with the highly

dispersed active Ce and promotive W species on TiOs The introduction of W species

could increase the amount of active sifes, oxygen vacancies, and Bronsted and Lewis acid

sites over the catalyst, which is also beneficial to improve the activity aat low temperature

[95]

1.3.3.2 Catalytic systems based on MnQ>

MnO: was one of the most popular metallic oxides that exhibited very high catalytic activity for CO and hydrocarbon oxidation due to high OSC ‘The catalyst based on MnO has higher oxygen storage capacity and demonstrates faster oxygen absorption and oxide reduetion tates than current commercial coria-stabilizcd materials [80] Among all metal oxides studied, manganese and cobalt conlaming catalysis are low cost, onvironmentally friendly and relatively highly active for VOC combustion The catalytic properties of

‘Mn0,-based catalysts are attributed to the ability of manganese to form oxides of different oxidation states and to their high oxygen storage capacity (OSC) Chang and McCarty claim thal MnO, has higher oxygen slorage capacity and dowonstrales faster oxygen absorption and oxide reduction raves than current commercial ceria-stabilized materials [30]

Catalytic activity of the Co-Mn-Al mixed oxide catalyst (Co:Mn:Al molar ratio of A:1:1) modified with various amounts of potassium (0-3 wtt) was examined in total oxidation of toluene and ethanol with the concentration of 1g/m’, The catalyst added 1% K,O can convert 90% these organic compounds at 160 °C [35]

MnO+-Co;O, supported on SiO: for complete oxidation in air of n-haxane (2.5 g/m’) was investigated by S Todotova The catalytic activity of both single component cobalt

ad nhangan mnpls is simibw, howevcr, a cornbinalion bclwesn the two clement changes significantly the activily and this depends on the method of preparation The

catalyst with $% MnO»-L5% CoO exhibited the highest activity with the conversion of n

CoH, of 100% al 265 °C: [36]

Copper-containing mesoporous manganese oxides were prepared by the sol gel method Catalyst synthesized by maleic acid sol-gel method possessed high specific surface area (170-230 m’/g, pore diameter of 6 nm) Using these samples as catalysts, CO oxidation was caried out as a saodel reaction (1% CO, 20% O-, N¬ balance) Copper- containing mesoparous manganese oxide prepared by the sol gel inethod showed a very high activity The catalyst CwMn 1/2 exhibited the highest activity when converting completely CO at 160 °C On the other hand, copper-supported manganese oxide prepared

by the impregnation method using copper sulfate showed a low activity Differences in aclivilics were correlated with the mobilily of lattice oxygen [49]

The MnO, CeO, AlO; mixed oxides catalyst exhibited the maxinmum soot oxidation rate at 455 °C, which shifts upwards by 53 °C after exposure to flow air at 800 °C tor 20 h with the mass ratio of catalyst/scot of 10/1 Compared with MnO,—CeO:, the superior (hormal stability of (he AlsOs-modificd catalyst should be mainly ascribed lo retarding the sintering uf MnO, and CeO, crystalliles as well as preventing Ihe phase separation of MnO, CeO; solid solutions to some extent These maintain a rather strong synergistic effect between Mn and Ce species on the nanometer scale for the aged alumina-modified catalyst, and increase the amount of available active oxygen for XO and soot oxidations at relatively low temperatures A good accordance is found between the low-temperature

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redox property (<600 -() and soot oxidation activity in G- (10% ON.) A similar

consistency appears between the redox property af lower Lermperatures (<400 eC) and soot

oxidation activity in XO+0O, (1000 ppm NO, 10% O;inN;) |63|

Said Azalim studied on the catalyst based on MnO», CeO» and ZrO tor complete VOC

(a-butanel with the concentration of 800 ppm in air condition) ‘the best catalyst

#ZaaCeasaMnazzO¬ can convert completely 100% this organic compound from 200 °C [122]

MnO, supported on Al:O; was applied for VOCs treatment, e.g, ethyl acetate, ethanol

and toluene in air with the concentration of 0.5-1% The order of VOCs conversion was ethanol>ethyl acelate> Loluene will the (emperalure of complete oxidation of 250, 300 and

380 °C, respectively [56]

Furthermore, MnO,/Al:O3 was deposited on FeCrAl metallic foil The reactant flow

contained ethanol, ethyl acetate, toluene with the gas flow of 300 ml/min and the concentration of 1000 ppmC diluted in air The powdered catalyst has demonstrated an

excellenl calalylic performance in VOCs combustion, hawever, supporting if on a metallic

activity of monoliths in VOCs combustion incrcascd with the amount of catalyst retained

‘The lowest temperature of the best catalysts for 80% conversion of ethanol, ethyl acetate

and toluene was 201 °C, 240 °C, 340 °C, respectively [123]

2% Oy itr Ar The calalysl synthesized Rom oxalate salt possessed very high specific surface area (525 nig) The catalyst exhibited superior activity when converting

completely CO in room temperature (300 K) [124]

a ÓC

1.3.3.3 Catalytic systems based on cobait oxides

Catalysts based on cobalt oxides are of great importance for catalytic processes like Tischcr-Tropach synthesis, low Lemperalure CO oxidation, No decomposition,

reforming of ethanol and other industrially important hydrogenation and oxidation reactions It is also established that such materials are effective combustion catalysts for

VOC removal, diesel soot oxidation, and particularly total oxidation of light hydrocarbons,

generation and crrissions control As a result, cobalt oxides and their preparation lave

been extensively studied The high catalytic activity in reactions oxygen involving of the

Co;:Os-based catalysts is most likely related to the high bulk oxygen mobility and facile

formation of highly active electrophilic oxygen (O° or O°) species for hydrocarbon oxidation [34-38, 87, 90, 91]

AY Sulker investigalcd Coy ,Fo,WO¢ catalysis fin comptcle oxidation of CO (5%

CO, 5% On, Nx balance) Before reaction, the catalyst was activated in O, with the gas low

250 ml/h for 30 minutes at 150 °C ConWOs catalyst exhibited the highest activity with the

CO conversion of 100% at the temperature lower than 200°C [34]

Ascrics of mamosized Ca;0-/y-AlO; catalysts have been prepared using a combination

‘the observed influence of the initial precursors cobalt acetate, mixtures of cobalt acetate/cobalt nitrate, and mixhwes of cobalt nitrate with fuels such as urea, citric acid, glycine, and glycerine on the catalytic performance correlates well with their combustion behaviour Calalysls obtained wilh the combustion method al the lighest velocities and the

lowest temperatures during the synthesis were found to have the highest activity (complete conversion of methane at 400-425 °C) [37]

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A % K Sinha studied CoO/SiQ, for n-hexane in air This catalyst can convert

completely hydrocarbon from 553K The activity reduced slightly aller 30h and maintains the conversion of 80% for 90h | 39]

Quian Liu demonstrates that nanocrystalline cobalt oxides prepared by citrate- precursor-based soft reactive grinding procedure are exceptionally active for total

oxidation of light hydrocarbons ‘The gas flow contain 1% C:Il,, 10% O> in Na Prior, the catalyst was activated in air flow 30 ml/min at 300 °C The best catalyst can convert 100% CsHy from 240 °C, Kinetic results show that these grinding-derived cobalt spinel catalysts

ars among the most active catalysts yet reported for propane combustion, being considerably mote active than the previously best reporled calalytic activity of eoball- based calulysts for complete hydrocarbon removal, The superior activity of the present

grinding-derived cobalt oxide catalyst has becn attributed to the bencficial formation of

highly strained cobalt spinel nanocrystals as a consequence of prolonged mechanochemical

activation during the dry citrate-precursor synthesis process [10]

F Wyrwalskr investigated a new and simple synthesis method for oblaining a highly dispersed Co/710y catalyst is described Introduction of yltrium (3 mol) into the support

and addition of an aqueous solution of cthylencdiamine to a cobalt nitrate solution during

the catalyst preparation leads to a strong increase of the catalytic performance of these new solids in the toluene total oxidation ‘The catalytic results have been explained in terms of cobalt oxides (Cos) dispersion which is strongly muproved whsn the support and/or the

associated with a low interaction of these species with the zirconia support [88]

Cobalt-aluminate spinel catalyst (Co; ¢sAl; 3404) exhibited the perfect activity for CO

treatment it can convert CO at room temperature and at low temperature with the present

of some gases (COh, Th, SO», Calle and XO.) When all compounds were added to the feed

gas simultaneously, their combined effect resulted in an almost total loss of the catalytic

activity for CO oxidation at temperatures below 500 °C [89]

In Vietnam, the catalyst Co-Al/Bentonite was studied by ‘ran Dai An for complete

oxidation of toluene 50% and 100% toluene were treated at 362 °C and 110 °C

respectively [8]

Tran Thi Minh Nguyet investigated the activity of Co;O4/ZrO/Cordierite The lowest

temperature of complete oxidation of CO was 170 °C and equal to that of active phase nano- Co30,, ‘The catalyst can be applied in exhaust gas treatment [9]

1.3.3.4 Other mefailic oxides

Some other metallic oxides such as CuO, V-Os and WO; were also investigated for CO

oxithation by NO or NO reduction by NU

CuO supported on 7103 and y-AlO3 for CO oxidation was sludicd by RewXian Zhou CuO/Zr0; sample can convert completely fiom 125 °C in the gas flow 2.8% CO, 8% Os

‘The addition of ZrO, would also increase the reduction ability and desorptibility of surface

oxygen spices of CuO/-Al,O; [50]

CHOCO, and CuO/eOy MgO wore applied for oxidation of CO by NO (3000 ppm

NO, 6000 pựm CO) Cu/MgO-CeO; was treated in redox proves Cu/MgO-CeO, was firstly reduced by 1% CO/He (20 ml/min) at 350 °C for Ih, subsequently, the sample was

cooled to 300 °C in He stream and then oxidized with 20 % ©./He (10 m/min) for half an

hour This catalyst can convert 80% CO and 95% NO with Ne selectivity approximate 100% from 250 °C [58]

Lean-bum engines provide more efficient fuel combustion and Jower CO, emissions

compared with traditional stoichiometric engines However, the effective removal of NOx

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from lean exhaust represents a challenge to the automotive industry Lean NO, traps

technology, particularly tor light duty diesel and gasoline lean-bumn applications

Moreover, recent studies have shown that the performance of LNT’ can be significantly improved by adding a selective catalytic reduction (SCR) catalyst in series downstream In

industry, SCR catalysts promote the selective reduetion of NO, with ammonia (N1Js) in the presence of excess oxygen: 4NO + 4NHs + O + 4Ny+ GH2O Many catalytic systems

‘based on metallic oxides or metals were investigated to treat XO with the presence of NH; [93-95] Typical industrial catalysts contain V2Os and WO; supported on TiO: (anatase) with the amount of V2Os is lawer than 2% [93]

SCR techmology is believed to be one of the most promising options for deNO,

However, SCR usually requires rather high reaction temperature (over 300 -C) when

hydrocarbons (HCs) or CO are used as reducing agents Low-temperature removal of NO;

by SCR can be achieved with the application of the toxic reducing agent NU; if SCR of

XO, will TIC occurs over catalyst al law temperature (< 200 oC) with high deNO, activity,

NO, at stationary or mobile sources Low-temperature SCR of XO, with HCs has been

extensively studied and a large number of catalysts have been evaluated [96]

1n Vietnam, some authors also studied some metallic oxides for treatment of pollutants

Tran Thi Nhu Mai and co-worker used of V-0s-TiO-/Me-O, (Me= Mo, Cu, Ce) catalysl

advanced oxidation reaction The reaction temperature range was trom 350 to 400 °C to

reach 100% conversion [5]

Jloang ‘Tien Cuong performed CuO-Cr.0:/Al.0,/cordierite catalyst for CO elimination

CO was converted al 230 °C by the bes sample The conversion of thts catalyst was higher

than 90% when using ina pilot set-up [10]

1.3.4 Other catalytic systems

Some researchers are interested in some other kind catalysts such as: Cu-ZSM-5, complexes catalyst MAX (M: transition metals such as: Cu, Fe, Co, Ni; A: 80," 80

anion, X: 3-amino-1,2,4 triazol) was condensed with formaldehyde on porous supporter (

silicagel, Al:Os, bentonite, zealile), Ag catalyst or Ag compound (X: halogen, oxide, sulfile, phospleite) an Sn oxide; and may he added ALOs, TiOs, cle [82, 83]

Kaiski showed that metal substrate ZSM-5 zeolites ion-exchanged with copper are

effective catalysts in the elimination of nitiogen oxides ffom ean automotive exhaust gases

when propene works as a reductant Some co-cations improve the catalytic activity of Cu-

Z§M5 [R4]

Te Minh Thang synthesized some trasiion melal calalyals such as: Nify-

Al,O,/Cordierite (5% Ni, 10% y-Al;O;), Coly-Al.O,/Cordiarite (5% Co, 10% y-Al:O,), Ni-Cofy-AlOyCordierite (2.5% Ni, 2.5% Co, 10% y-AlO:) for complete oxidation of

hydrocarbon They have suitable operation temperature is from 350 to 400°C Containing

Co catalysts are beticr than Ni-calalysts in n-hexane oxidation The maximum conversion

was 80 % [2]

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1.4 Mechanism of the reactions

1.4.1 Mechanism of hydrocarbon oxidation over transition metal oxides:

Any catalytic mechanism implies that adsorption represents the primary step of catalysis and controls the transition of a reactant molecule to the active state Molecules of oxygen

or of a hydrocarhon are adsorbed on the calalyst surface during hydrocarbon oxidation The stale of these adsorbed molecules, their interaction, and their reactions wilh the gas

phase molecules would account for different routes of the process

mmctals may occur cven at low temperature The chemisorption of oxygen on various metals

over a range of different temperatures is so fast as to make kinetic measurement impossible; this is indication that the activation energy for chemisorption is very low Vast chemisorption is followed by slow uptake of oxygen by the metal Wilh NMs, such as platinum and silver, oxygen wilt be dissolved in faycrs adjacent lo the surface, bringing about changes in the electronic properties of the latter

the whole surface of the metal, and chemisorption of hydrocarbons occurs either on a thin

layer of the given metal oxide formed as an individual phase, or on oxygen that was sorbed

on the surface and has filled the adjacent-to-surface layers

The essential points of the hydrocarbon oxidation scheme, as derived publish dala and from the clectronie theary of catalysis

(2) A molecule with a double bond adsorbed on a semi-conducting catalyst surface converts into a radical bound with the lathice and having a free valence A molecule with a single bond emerging from the gas phase may react with the free valence of sucha radical and dissociale

@) An adsorbed saturated molecule with a single bond may dissociate into radicals,

one saturated with the surface valence, and the other having a fee valence, free radicals will be generated by desorption of the latter radical into gas phase

(3) It may be considered fiom isotopic data and electron work fiction measurement

that negatively charged ions of molccular and atomic oxygen are present on scmi-

condusting surfaces The ratio of these is a function of temperatue and chemical properties

of the solid

(4) Hydrocarbons are sorbed on semi-conducting catalysts either weakly-reversibly,

or strongly-irreversibly The ratio of weak lo strong adsorption is 4 fimelion of lermperalure

and the chemical composition of the catalyst

(5) Various types of ion-radicals are formed in adsorption of reactant molecules on the

semi-conducting surface; the formation of these is a function of the electronic propertizs of the solid, and the structure and kind of bots

(6) The ealslyst surface is markedly heterogeneous both with respect to oxygen and to

hydrocarbon adsorption

(7) The heterogeneous-homogeneous step oceurs only for certain catalysts, such as

platinum and spinels, and is not observed with oxide catalysts over the temperature rangs

up Lo 400°C

(8) Reaction products, such as aldchydes, olefine oxides, cle., are strongly sorbed on

the catalyst surface, contributing to formation of the organic residne and representing

additional sources of carbon dioxide generation

(9) It may be considered on the basis of data obtained by means of the radioactive tracer lechnique thal the various stable oxyger-containmg producls on sermi-conductinys oxides are gencraled by different routes, though active intermediates

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(10) The oxygen in metal oxide lattices, as well as that sorbed on lattice surfaces is ofa low mobilily Under certain conditions, the hydrocarbon will react wilh oxygen of the

catalyst lattice At low temperature, this side reaction is of small importance for oxidation

Figure 1.10 Scheme of catatytic hydrocarhan axidation; F-hydrocarhan, C-catalyst, Ry ta Rs-labile intermediate,

prohably of the peroxide type [97]

AI types of reactions between oxygen and hydrocarbon yield oxygen-containing compounds, such as akdchydes, avid, zlc.„ Ti logelher with Ihe product of complete

oxidation, ic, with carbon monoxide and watcr The reaction selectivity seems to bc

determined by the strength of bonding between the surface and the ion-radicals formed,

and may be increased solely by changing the chemical composition of the catalyst [97]

ILis very difficult lo estabtish kinetic laws for hydrocarbon oxidation first of all due to the high endothermicily of this reaction resulling in sinstering of the calatysl, in surface

changes, and in the intensification of side proccss, This is probably the reason why the

Kinetics of a number of hydrocarbon oxidation reaction is insufficiently know, and data

reported in literature are scarce

A number of physical side processes, such as the diffusion of initial compounds and reaction products, the Hiberalion and distribution of hzal, the dynamic of gascs and liquids

exert an influence on hydrocarbon oxidation under working condition, All these factors arc

of prime importance for the design of catalytic apparatus, and moreover, may bring @

change in the main oxidation characteristic, ie., in the selectivity

‘The formal kinetics of high conversion of hydrocarbon is primarily a fiuction of molecular struclure and is but slightly affected by the nalurs of catalysts The greater the

number of carbon atoms in a molecule the higher the pre-exponential factor and the

activation energy for high conversion ‘The regularity holds both for saturated and

unsaturated, as well as for simple cyclic hydrocarbons Change in order of the kinetic

equation as 2 finction of the molecular structure of a hydrocarbon provides evidence for a

rate-determining step that seems to be related to the nature of hydiocarbon radicals formed

in adsorption, In certain cases the rate-determining step is the chemisorptions of oxygen

97]

3.4.2 Mechanism of the oxidation reactien of carbon monoxide

“The catalytic oxidation of CO on the surface of NMs such as platinum, palladium and rhodium, In order lo describe the process, the metal surface consists of acliva sites were

denoted as “*” The calalytre reaction oyelc begins with the adsorption of CO and Os on the

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surface of platinum, whereby the O: molecule dissociates into two O atoms (X* indicates

that the atom or molecule is adsorbed on the surface, i.e bound to the site *)

Q2:+2* =—=20°

CcOr* = CO’

The adsorbed O atom and the adsorbed CO molecule then react on the surface to form

CO;, which, being very stable and relatively unreactive, interacts only weakly with the platinum surface and desorbs almost instantaneously:

co"+0" = Co, +2°

Note that in the latter step the adsorption sites on the catalyst are liberated, so that these

become available for further reaction cycles Figure 1.11 shows the reaction cycle along with a potential energy diagram Once these radicals are available, the reaction with CO to CO; follows instantaneously

‘The activation energy of the gas phase reaction will be roughly equal to the energy

required to split the strong O-O bond in O:, ie about 500 kJ mol” In the catalytic reaction, however, the O, molecule dissociates easily — in fact without an activation energy

— on the surfaee of the catalyst The activation energy is associated with the reaction

between adsorbed CO and O atoms, which is of the order of 50-100 kJ mol” Desorption

of the product molecule CO; costs only about 15-30 kJ mol”' (depending on the metal and its surface structure) It can be seen that the most difficult step of the homogeneous gas phase reaction, namely the breaking of the O-O bond is easily performed by the catalyst Consequently, the ease with which the CO, molecule forms determines the rate at which the overall reaction from CO and O, to CO: proceeds This is a very general situation for

catalyzed reactions, hence the expression: A catalyst breaks bonds, and lets other bonds form The beneficial action of the catalyst is in the dissociation of a strong bond, the

subsequent steps might actually proceed faster without the catalyst [98]

adsorption reaction ‘desorption

reaction coordinate Figure 1.11 Reaction cycle and potential energy diagram for the catalytic oxidation of CO by O; [98]

Figure 1.12 shows the reaction path ways based on the investigations This is analogous

in parts to those proposed by others and is one of the possible reaction pathways Here M(a)-O and M(b)-O are considered as two types of active sites on metal oxides namely acidic and basic sites respectively Where M(a) as surface active acidic site and O(b) as

basic active site on metal oxides M(a) is considered as an acidic site which is electron

deficient CO having lone pair of electrons directing the C-end of CO gets chemisorbed

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with acidic site of metal oxide to form a bond as shown in Eq (1).The adsorbed CO interacls with the lallice oxygen of the melal oxide The partially bonded CO gels

desorbed leaving the reduced acidic metal oxide on the swthce as shown in Eq (2)

Subsequently reduced site takes oxygen ftom the gas phase to fill the oxygen vacancy as seen in kq, (3) The oxygen molecule takes electrons from the basic site forming Q” species

in lig (4) The adsorbed species may interact to give intermediate as shown in £q.(5),

subscquently giving CO, and regeneration of the catalyst in Eqs (6) and (7) If acidic and

basic sites are present on the same metal oxide, then the reaction paths ways follow as

below The Eq (8) indicates the presence of both acidic and basic sites on the same metal oxide The carbon monoxide adsorbed on fhe acidic sile and oxygen on basic site to form

igure 1.12 Reaction patincays of CO oxidation aver the metallic oxides [3-4]

1.4.3 Mechanism of the reduction of NO,

Two main chemical reaction pathways of HC-SCR (hydrocarbon-selective catalytic

reduction) are complete oxidation of hydrocarbons and selective reduction of NOx by oxygenated species that are produced from such hydrocarbons (as seen Figure 1.13) On the basis of complete oxidation pathway, methoxy radical which is variously derived from

i-propoxy radical, acctate and acctyl radical is the csscntial intermediate species for this

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via formyl radical In terms of selective reduction of NO, route, nitromethane is the first intermediate which contain both carbon and nitrogen species Mitromethane is created by either the reaction between surface acetate and nitrogen diode or the reaction of surface acetyl radical and nitrate Both surface acetate and acetyl radical are derivative products that generate ftom the same source, acetaldehyde Acetaldehyde appears as the surface intermediate species which is produced from propane by oxidation processes via i+

propanol and i-propoxy radical specics Onec nitromethane is formed, it is further

chemically converted to nitrogen through nitromethylene, formaldiminowy, nitrile N-oxide,

cyanide and isocyanate respectively {99 |

Phnpene ÍPrepneel » EPmpowy mefioal

A

“” Aoatakdehyde \GHsGHO*} XS

Esaryarain< Cyerde « Nitrite N-oxide

Aeorniz nitrogen » Nirosen

on the other hand, increases the rate of NO, releases under the rich-apike ‘Ihe effect of water is tare controversial in thal promotion of NOx adsorption was obsorved belaw 250

°C by addition of small amounts of water (1%), whereas at higher temperature an

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inhibition effect was observed, However, such promoting effect was not seen when both

water CO: were co-fed [67]

NG + 20,

Ne + CO,

Figure 1.14 Principle of operation of an NSR catalyst; NO, are stored under oxidising conditions (1) and then

reduced on a TWC when the A/F is temporarily switched to rich conditions (2) [67]

1.4.4 Reaction mechanism of three-way catalysts

Figure 1.15 describes schematically the seven main steps involved in the conversion of

the exhaust gas pollutants ina channel of a TWC, including mass transfer between the bulk

gas and washcoat surface, pore diffusion, adsorption/desorption and chemical reaction In brief, step 1 represents the transport of reactants from the bulk gas to the gas-solid interface (external mass transfer); step 2 represents the internal transport of reactants into the porous washcoat (internal mass transfer); step 3 represents the adsorption of reactants

at the interior of catalyst particle, step 4 represents the chemical reaction of adsorbed

reactants to adsorbed products, step 5 represents the desorption of adsorbed products, step

6 represents the transport of products from the interior sites to the interface gas— solid of

the washcoat and, finally, step 7 represents the transport of products fiom the gas-solid

interface to the bulk fluid stream [100]

1 - External mass transter

5 - Desorption

6 - Internal mass transfer

7 - External mace trancter

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‘A modeling and simulation study on Pt-catalyzed conversion of automotive exhaust gases is presented by J Koop The model is based on a newly developed surfave reaction

mechanism consisting of 73 elementary-step like reactions among 22 surface and 11 gas-

the major pollutants CO, CHa, CsHs, and NO, are included ‘The mechanism is

implemented in a two-dimensional flow field description of a single channel of the

catalytic monolith The model is cvaluated by comparison with data derived fiom

isothermal laboratory experiments in a flat bed reactor with platimum-coated monoliths

using synthetic lean/rich eycling exhaust gas mixtures The influence of CO and C3He at

lean and I], al rich conditions on NO conversion is investigated, both al sleady

conditions Furthermore, the modzt is also applied for the simulation of emis

hydrocarbons, CO, and NO from a gasoline snginc (stoichiometric cxhaust gas) in a

dynamic engine test bench |101|

Table 1.2 Adsorption‘desorption reactions an Pt catalyst {101}

Gilg PIG) | PS) > CH CO ' Pits) + COWS)

Colles) — Pls) 1 PUGS) 1 CTs COa() —› CÓ; L PIS)

CH, + PIG) + PíS) — CH:G) + Ha) NO+ Pi) + NOG)

‘O: + Pt(s) + PS) > OG) + OG) NOG) > NO + Pits)

‘O(s) + Ofs) > O2+ Pt(s) + Pt(s) NO + Pi(s) + NOx(s)

C3Hes) + CsHs(s) + H(s) C:H;(s) + O(s) — CH3COXs) — Pi(s) C;H;G) + H(s) > Cals) CH;CO(s) + Pt(s) > C.Hs(s) + O(s) C3Hs(s) — Pts)» C2Ha(s) — CH)

CiH36) — CHAS) + CsHs(9) + PL) CHA) +.CO(s)_> CHsCO(S) — PS)

e › C:H;(s) + PIG)

CIG) L Pls) > CTI) | THs) CT1) ! ÖG) —> OIIG) | CHaG)

Ch@) 1 1ø) —¬ CIs) | Pt(s) OHG) ! CI:@) —> CIk@) ! OG)

CIF) 1 Pt) = CG) UG) CH4G) OG) > OU® | CIs)

CIMs) ! Tự) — CIL@) L PLS) GIs) ! CHặ) > CIE) _ O68)

Chis) | Pts) > CAs) 1 Ts) CLG) 1 Ofs) > Olle) (CE) (6) + Hí6) —> CHÍS) + Pt(s) OHis) + Cls) > CHO) +O

CsHe(s) + fs) > C His) + OH(s)

‘The applied elementary-step mechanism includes dissociative adsorption of CHa, Qs, H:

and non-dissociative adsorption of NO, NO», N-O, CO, CO», Clg, 11,0 and desorption of

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all species except CIl, Gas-phase reactions can be neglected due to the low pressure and lemperalure in automotive calalylic converters All reactions on platinum are modeled as reversible reactions The developed mechanism can be subdivided into four parts:

Phe oxidation of carbon monoxide to carbon dioxide,

- ‘The formation of water via an adsorbed hydroxyd species (OLD,

- Reactions for the conversion of nitrogen oxides [101]

Table 1.4 Stuface reactions of CO axidation {101}

COlS) + Ofs) = CO.Ks) + Pris) Cls) + OG) — COG) + Pt@)

COs) + Pils) — COGS) — OS) COs) + PLS) — CEs) + Of)

‘Table 1.5 Surface reactions of hydroxyl spices, NO and NO; [101]

HG) + 0) > OH) + Pls) NOG) + Pts) + N@) + OG) GHG) + Pts) —> H@) + OG) NG) + OG) > NOG) + Ptis) OH(s) + H@) > H2O(s) + Pi(s) 0) +NO > NOG)

HO(s) + PU(s)_> OH(s) + Hệ) NOS)» O() + NO

OHG) + OH(s)_ > H:06) + OG) NG) — NOG)» NOG) + PG)

H;OG)+OG) + OHG) + OH() N;OG) + PLS)» Nes) + NOES

x OMS) + TICOOKS) | PUGS) fs) NOW) > NO-(s) 1 PIG)

TICOOG) | Pts) — COG) | OTs) NOS) | PKS) —> OG) I NOG)

HCOO() + Of) > OH(s) + COG) H@® + NO(s) > OHis) + NG)

OII(s) ! Ca) —›HCOOG) OG) OG) | Xs) > HG) ! NOG)

ICOOG) PIG) UG | CO-G) NOs) US) > OIG) | NO’

HG) + CO.) > HCOOG) + Pt) OG) + NOG) > NO) + HE)

1.6 Aims of the thesis

Today TWCs are based on combinations of Pt and/or Pd and Rh, alumina and ceria,

together with a variety of support stabilizers, activity promoters, and selectivity improvers

The choice and loading of the NM is a compromise between the required efficiency of the

converter and the market price of the NM Jowever, the application of noble metals for the irzatment of exhaust gas in Vietnam is duc lo the high price and facility to poisoning by

sulfu-containing compounds Perovskites have also been widely investigated as the oxidation and NO removal catalysts Perovskites were even proposed as an alternative to

NM supported catalysts since they present similar activities in oxidation, However, the

The aim of this thesis is to find out the valalylic systems with low cost, casy lo apply Therefore, metallic oxides were selected Although aim to focus on low cost catalyst from

metallic oxides, the thesis still has the purpose to obtain effective catalyst as comparable

with that of noble catalysts Therefore, the catalyst must be multiple oxides since the emission gases have many different, components, the treatment o| laicd to

both oxidation and reduction reaction Among mclallic oxides investigated im literature,

CeO:, Cos, and MnO; were reported.to be the best catalysts for pollutant treatment

owever, there is no consensus about the composition of the catalyst that exhibits the highest activity and application Catalysts based on MnO-, CoxOx, CeO» was applied for (raating CO and hydrocarbon duc to high OSC and mobile exygen (O, 07) in the Taltiee

Therefore, MnO Co3O4, CeO; are component te be chosen to focus on Other metal

oxides such as NiO, ZrO», BaQ, CuO, VsO¿, ZnO, SnOs, and WO; would also be studied

since they can help to increase the activity or the thermal resistant ‘I'he catalytic activity of

different single metallic oxides would be studied for the complete oxidation of Nguyen The ‘Tien

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hydrocarbon and CO in the deficient oxygen since it was believed that if a catalyst is good

in Oy deficient condition, il may be belter in O» sufficient condition Mixtures of good imetallic oxides would also tested The obtained catalysts would be further investigated for the oxidation of hydrocarbon, CO, soot and simultaneously treatment of pollutants These tests would lead to find the optimal composition of the highest active catalysis

Jurthermore, the influence of composition of reactants, aging process with steam and

SO, and the activation to activity of the catalyst would also be aumed to be examined The

comparison of catalytic activity of the obtained mixed oxide catalysts with the noble

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2 EXPERIMENTAL

2.1 Synthesis of the catalysts

2.1.1 Sol-gel synthesis of mixed catalysts

In this thesis, some single and nmlti-oxides used for active phases were synthesized by

sol-gel method Different ftom precipitation, solid-state reaction or spray drying, this method is based on the addition of an organic complexation agent (here citric acid) into the precursors The presenee of the organic complexation agont distinguishes this

complexation method from the other methods owing lo the complexation and gelation

steps These steps are influenced mainly by the atomic ratio of citric acid to metal cations and pl of the solution It was shown that this sol-gel method leads to the farmation of very pure and homogeneous catalyst powders exhibiting high surface area [105]

Different salts af Mn{NO;), (solution with the concentration 50% wi-Sigma Aldrich),

Co(NO;);6H:O-Bigma AIldich, Ce(XO,);.6H:O-Merck, Ni(MO,):.6H.O-Merck,

Cu(NOs):,.6H:0-Merck, Zn(XO5):.6H:O-Merck, ZrOCh.6H.0-Merck, NH-VO3-Sigma

Aldrich, SnCL.5iLO-Merck, Ba(XOk)-.2113O-Merek, (NIL):aW¡aOai.1IO-Sigma Aldrich

were dissolved in water in order to obtain the solution with the concentration of 0.125M

10%wt citric acid solutions prepared from citric acid monohydrate - C;HạO; H;O (99.5%, Merck)

MnO:-Co;0,-CeO: catalyst was synthesized by dropping a suitable amount of Mn(NO:)> and Ce(NO:); solutions info a suitable volume of Co(NO;) solution conesponding Lo different MnO-/Co;0./CeO, molar ratios If precipilalion occurred, concentrated HNO; solution was added until the precipitates disappear A suitable amount

of citric acid solution was dropped into the obtained solution with the molar ratio of citric

to metals of 2 he obtained pink solution was stabled within 30 minutes and evaporated at 60-80 °C unlit the gel was obtained The get was thon dricd at 120 °C for 3 hours The obtained solid were calcinated af 550°C for 3 hours with the healing rae is 3 °C/min

Single metallic oxides (CosOa, NiO, CeO, CuO, ZnO, V20s, SO», ZrO2), bi-metallic

oxides (Co;Q4-CeO», MO:-SnO., MnOrZnO, MnO~Co;0.), other triple inetallic oxides (MnO CozO¿-NiO) and tetra metallic oxides (MnO Co,0,-CeO, added V.0:, 210», Ba

WOs, NiO) were synthesized simitarly to MnO.-Co;0,-CoO, samples The calalyst was labeled correponding to the oxide uobw composition For example, MuCoCe 1-3-0.75 catalyst contains MnO», CosO,, and CeO: with the ratio MnO+/Co;04/CeO; 1/3/0.75

2.1.2 Catalysts supported on y-Al,0;

In order to compare the activity of metallic oxide and precious metal, the metallic oxide catalysts were impregnated on commercial y-AlOs A suitable amount of Mn(NO3)-, Co(NOs)p, CefNOos, citric acid solutions were dropped ina beaker and wixed for | hour

The support was dried at 120 °C for 1 hour and then cooled down at room temperature After that, a suitable amount of the mixed solution was unpregrated on 7-ALO; to obtain different loading content (10-50%) ‘he supported samples were dried at 80 °C until drying

completely and then calcinated at 550 °C for 3 hours

Trccious metal Pd was impregnated on Al:O; fiom a solution 0.125M with precusors PdCNO,),.2H.0 (Merck) The sample was also dried at 80 °C until drying completely and

then calcinated at 550 °C for 3 hours The weight loading content is 0.1% and 0.5%, The

noble catalysts were reduced in 35% H./Ar flow (80 ml/min) at 300 °C for 5 hours

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2.1.3 Aging process

Ta order to study the influence of sore faclors such as lomperaturc, slam, SO to

catalytic activity, the catalyst was aged in some specific conditions (detail in Table 2.1)

‘The aging process were shown in ignte 2.1 Caloinated catalyst was loaded in a quartz

tube with an immer diameter of 30 mm This tube was put in a tube fiace 6 The catalyst

were calcinaled al 800 °C for 24 hours with heating rats of 10 °CAnin in all of conditions

The temperature program was controlled and displayed by screen controller 5 Air was

blown to aging tube by pump 1 via 2 lines One line was connected to small quartz tube

located in furnace 2 with inner diameter of 6 mm in order to form SO, from l’eS at 100 °C

The velocity of the gas was 20 ml/min Meanwhile, the other was plugged in water tank 3

that was heated up by heater 4 with gas velocity of 440 l/h The content of steam in air was

57% and 27% volume correspond to water temperature was 65 °C and 28 °C Valve 1 and

2 on 2 gas lines were open ot close depending on each condition

Aging process of the catalyst ‘I: air pump: 2,6: tube furnace, 3: water tank, 4: heater, 5,7: screen

controller, VI,V2: gus valve)

2.2 Physico-Chemistry Experiment Techniques

2.2.1 X-ray Diffraction

‘Xuay diffraction is onc of the oldest and mmost Ñcqusntly applicd techniques in catalyst characterization, It is used to identify crystallin: inside catalyst by means of lattice structural parameters, and to obtain an indication of particle size

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Tiêu đề	Synthesize and investigate the catalytic activity of three-way catalysts based on mixed metal oxides for the treatment of exhaust gases from internal combustion engine
Tác giả	Nguyen The Tien
Người hướng dẫn	Associate Prof. Dr. Le Minh Thang
Trường học	Hanoi University of Science and Technology
Chuyên ngành	Chemical Engineering
Thể loại	Luận án
Năm xuất bản	2014
Thành phố	Hanoi

Định dạng
Số trang	117
Dung lượng	2,96 MB