Synthesis and catalytic properties of catalyst system based on ceo2 zro2 for the complete oxidation of hydrocarbon to treat motorcycles exhaust gases

TABLES IN THE THESIS 2 Annual Combustion-Generated Emissions of Selected Pollutants by Stationary Source Category in USA 23 5 Emission Reduction from Different NO x Control Technologie

MINISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF TECHNOLOGY

-

NGUYEN THE TIEN

SYNTHESIS AND CATALYTIC PROPERTIES OF CATALYST SYSTEM

BASED ON CeO2-ZrO2 FOR THE COMPLETE OXIDATION OF

HYDROCARBON TO TREAT MOTORCYCLE’S EXHAUST GASES

SPECIALITY: ORGANIC AND PETROCHEMICAL TECHNOLOGY

CONTENTS

Page

Acknowledgements 5

Introduction, aims and outline of the thesis 13

I.1.2 Air pollution problem in the world and in Vietnam 21

I.3 International and Vietnam researches on catalyst for exhaust gas

treatment

33

I.3.3 The imperative task, the aim and the research direction of the

thesis

39

I.4 The catalysts based on Cerium and Zirconium oxide 41

I.4.1 Role of CeO2 in the three-way catalyst 41

I.4.2 Phase diagram of CeO2-ZrO2 mixed oxide 42

I.4.4 Characteristic of CeO2-ZrO2 mixed oxide 45

a Oxygen storage capacity (OSC) of CeO2–ZrO2 mixed oxides 45

b Thermal stability of CeO2–ZrO2 mixed oxides 45

I 5.Completed hydrocarbon oxidation of hydrocarbon 47

II.1.2 Synthesis of several single oxides, CeO2-ZrO2 and CeO2-Co3O4

catalysts by sol-gel method

52

II.1.4 Synthesis of Co3O4/CeO2-ZrO2 catalysts by impregnation 54

II.2 Methods to determine pollutant concentration 55

II.3.Physico-Chemical Experimental Techniques 59

II.3.3 BET method for the determination of surface area 62

III.1 Composition of motorcycle exhausts gases 68

III.1.2 O2 volume concentration in the exhaust gas 69

III.1.3 Hydrocarbon concentrations in the exhaust gas analyzed by

GC-MS and GC- FID

70

III.2 Characterization of several single metallic oxides for the

hydrocarbon completed oxidation

73

III.2.1 Surface properties of the investigated oxides 73

III.2.2 Phase composition of investigated oxides 73

III.2.3 Catalytic activity of investigated oxides 74

III.3 Characterization of CeO2-SnO2 mechanical mixtures 76

III.3.1 Catalytic activity of CeO2-SnO2 mechanical mixtures for

complete oxidation reaction of propylene

77

III.3.2 Phase composition and surface properties of CeO2-SnO2

mechanical mixtures

78

III.4 Characterization of CeO2-ZrO2 mixtures 80

III.4.1 Phase composition and surface properties of CeO 2 -ZrO 2

III.5 Catalytic activity of CeO2-Co3O4 catalysts 87

III.6 Catalytic activities of Co3O4/CeO2-ZrO2 89

ACKNOWLEDGEMENTS

This Master thesis has been carried out at the Department of Organic and

Petrochemical Technology and Laboratory of Petrochemistry and Catalysis Material,

Faculty of Chemical Technology, Hanoi University of Technology during the

period February 2010 to August 2010 The work has been completed under

supervision of Associate Prof Dr Le Minh Thang

Firstly, I would like to thank Associate Prof Dr Le Minh Thang She helped me a

lot in the scientific work with her thorough guidance, her encouragement and kind

help

I want to thank all teachers of Department of Organic and Petrochemical

Technology and the technicians of Laboratory of Petrochemistry and Catalysis

Material, Faculty of Chemical Technology for their guidance, and their helps in my

PROTESTATION IN THE THESIS

I assure that my scientific results are righteous They haven’t been published in

any scientific document I have responsibilities for my protestation and my research

results in the thesis

SYMBOLS IN THE THESIS

HUT: Hanoi University of Technology

PM10: particulate matter less than 10 nm in diameter

NOx: oxides of nitrogen

VOCs: volatile organic compounds

PAHs: polycyclic aromatic hydrocarbons

HAPs: hazardous air pollutants

USA: United States of America

HCMC: Ho Chi Minh City

LEA: Low excess air

OFA: Overfire air

FRG: Flue gas recirculation

LNB: Low NOx burner

SNCR: Selective noncatalytic reduction

SCR: Selective catalytic reduction

A/F: air/fuel ratio

TWC: three-way catalyst

Cpsi: cell per square inch

SULEV: super ultra low level vehicle

ULEV: ultra low level vehicle

CZ: mixtures of Cerium oxide and Zirconium oxide

CZS: mixtures ofCerium oxide, Zirconium oxide, Strontium oxide

λ: the theoretical stoichiometric value

T50: temperature for 50% conversion

in.: inch

OSC: oxygen storage capacity

CZALa: mixtures of Cerium oxide, Zirconium oxide, Aluminum oxide, Lanthanum

oxide

NGVs: Natural Gas Vehicles

LPG: Liquefied Petroleum Gas

ECE R40: Economic Commission for Euro Regulation 40- Emission of gaseous

pollutants of motorcycles)

HMDC: Hanoi Motorcycle Driving Cycle

GC-MS: Gas Chromatography – Mass Spectroscopy

GC-FID: Gas Chromatography- Flame Ionization Detector

XRD: X-ray diffraction

SEM: Scanning Electron Microscopy

BET equation: Brunauer- Emmett-Teller

r,w, C3H6 cons: Reaction rate of propylene consumption

TABLES IN THE THESIS

2 Annual Combustion-Generated Emissions of Selected Pollutants

by Stationary Source Category in USA

23

5 Emission Reduction from Different NO x Control Technologies 27

9 Measurement conditions using a GC-Thermo Electron with FID 58

10 Retention time of some organic compound detected by GC Thermo

Electron with FID detector and the condition mentioned in table 9

58

17 Composition of Organic compounds in the motorcycle’s exhaust

gases with measurement time up to 40 minutes

71

19 CO 2 selectivity of investigated metal oxides at different reaction

temperatures

76

21 BET surface area of some Ce-Zr oxides depend on temperature 82

22 CO 2 selectivity of CeO 2 -Co 3 O 4 catalysts at different reaction

temperatures

89

23 CO 2 selectivity of CeO 2 -ZrO 2 supports, Co 3 O 4 active phase,

Co 3 O 4 /CeO 2 -ZrO 2 samples

91

FIGURES IN THE THESIS

2 Schematic drawing, causes and effects of air pollution: (1)

greenhouse effect, (2) particulate contamination, (3) increased UV

radiation, (4) acid rain, (5) increased ozone concentration, (6)

increased levels of nitrogen oxides

21

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

engine exhaust control

29

5 Three way catalyst performance determined by engine air to fuel 30

6 Wash-coats on automotive catalyst can have different surface

structures as shown with SEM micrographs

31

9 Scheme of catalytic hydrocarbon oxidation; H-hydrocarbon,

C-catalyst, R 1 to R 5 -labile intermediate, probably of the peroxide type

50

11 Scheme of the synthesis of Co 3 O 4 /CeO 2 -ZrO 2 catalysts by

15 Illustrates how diffraction of X-ray by crystal planes allows one

to derive lattice by using Bragg relation

60

16 The interaction between the primary electron beam and the sample in

an electron microscope leads to a number of detectable signal

61

20 X-ray pattern of several single oxides synthesized using sol-gel

method

73

21 Reaction rate of propylene conversion (r,w, C 3 H 6 conv) of several

oxides at different reaction temperatures

75

22 Reaction rate of propylene consumption (r,w, C 3 H 6 cons) of

CeO 2 -SnO 2 mechanical mixtures

77

23 CO 2 selectivity of CeO 2 -SnO 2 mechanical mixtures (in low

temperatures, CO and CO 2 peaks were not detected)

26 X-ray pattern of CeO 2 -ZrO 2 solid solution with different Ce/Zr

atomic ratios before reaction

82

27 Reaction rate of propylene consumption of CeO 2 -ZrO 2 solid

solutions (Ce 1-x Zr x O 2 ) at different reaction temperatures

83

28 CO 2 selectivity of CeO 2 -ZrO 2 solid solutions (Ce 1-x Zr x O 2 ) at

different reaction temperatures (x: optional of Zr atom in solid

solution) (in low temperatures, CO and CO 2 peaks were not

detected)

84

29 Reaction rate of propylene conversion of CeO 2 /ZrO 2 =8/2 prepared

by mechanical mixed and sol-gel synthesis at different reaction

temperatures

85

30 CO 2 selectivity of CeO 2 /ZrO 2 =8/2 synthesized by mechanical

mixture and sol-gel method at different reaction temperatures (at

low temperatures, CO or CO 2 peaks were not detected)

86

31 XRD patterns of mechanical sample before reaction (a), sol-gel

samples before and after reaction (b) with molar ratio

33 Reaction rate of propylene consumption of CeO 2 -Co 3 O 4 catalysts at

different reaction temperatures

88

34 Reaction rate of propylene consumption of CeO 2 -ZrO 2 support,

Co 3 O 4 active phase, Co 3 O 4 supported on Ce 0.9 Zr 0.1 O 2 samples

89

35 Reaction rate of propylene consumption of CeO 2 -ZrO 2

supports, Co 3 O 4 active phase, Co 3 O 4 supported on Ce 0.8 Zr 0.2 O 2

ABSTRACT

In Vietnam, a developing country, motorcycles are the main way of transportation for the

moment The number of motorcycles is about 90% of all vehicles in Vietnam Emission

from fuel combustion contributes essentially in pollution Hydrocarbons in exhaust

gases enhance the greenhouse effect, global warming…

The aim of this work is synthesis and determination of catalytic activity of catalyst

systems based on CeO2 and ZrO2 for the complete oxidation of hydrocarbon (propylene) to

treat motorcycle’s exhaust gases Furthermore, other metal oxides such as SnO2, TiO2,

Al2O3, V2O5, Co3O4, NiO, catalytic activity of some of their mixtures and impregnation

samples were tested The physicochemical properties of these investigated catalysts were

also characterized carefully

The following main findings have been obtained from the results:

- The main composition of the exhaust gases are 0.5÷4.5 vol.% hydrocarbon, 0.5÷ 8 vol.%

CO, 4÷12 vol.% CO2, 0.05 to 0.4 vol.% NOx and 1.8÷12 vol.% O2 Amongst

hydrocarbons in the exhaust gases, C3H6 are one of the main components

- Amongst investigated catalysts (Co3O4, CeO2, SnO2, TiO2, NiO, ZrO2, V2O5 and Al2O3),

Co3O4 and CeO2 exhibited the highest catalytic activity

- Catalytic activity of mixtures of CeO2 and SnO2, CeO2 and ZrO2 were investigated

CeO2-SnO2 mixtures exhibited rather high activity for the complete oxidation of propylene

but CeO2-ZrO2 sample has even higher activity

- Catalytic activity of some cobalt oxide supported on CeO2-ZrO2 catalysts was studied It

can be seen that the samples has high activity at wide reaction temperature range since at

low temperature due to the combination of Cobalt and Cerium active site acts The

impregnation limit should be 10%wt on Ce0.9Zr0.1O2 and 5%wt on Ce0.8Zr0.2O2

- Amongst investigated catalysts, the sample with 5% wt Co3O4 impregnated on

Ce0.8Zr0.2O2 exhibited highest conversion and selectivity at wide temperature range from

2500C to 5000C This catalyst was able to convert 33 % propylene in the reactant with the

CO2 selectivity of 100%

INTRODUCTION, AIMS AND OUTLINE OF THE THESIS

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

Motorcycles are the main way of transportation for the moment The number of

motorcycles is about 90% of all vehicles in Vietnam Thus, the environmental

pollution is very heavy by emission from exhaust gases of motorcycles

Hydrocarbons in exhaust gases contribute to enhance the greenhouse effect, global

warming or hazard to human health…

The most popular catalysts for the treatment of automobile exhaust gases are noble

metals such as Pt, Pd The studies attracted attentions of many international

researchers since 1980s [4-7] Some other groups focused on the preparation of

perovskites catalysts [8, 9, 58] Recently, some authors paid attention on the use of

ceria – zirconia mixed oxides based on their oxygen storage capacity and their

enhanced stability against thermal sintering [11, 17] In Vietnam, there has been

several research groups (Faculty of Chemistry, Hanoi National University of

Science, Institute of Chemistry, Vietnamese Academy of Science and Technology)

studied on designing new catalytic systems for the treatment of automobile exhaust

gases [28] However, the results were still primitive and difficult to be applied

In this thesis, the composition of motorcycle exhaust gas was studied in detail by

different methods The aim of this work is synthesis and determination of catalytic

activity of catalyst systems based on CeO2 and ZrO2 Furthermore, catalytic activity

of other metal oxides such as SnO2, TiO2, Al2O3, V2O5, Co3O4, NiO, some of their

mixtures and Co3O4 supported on CeO2-ZrO2 catalysts were also tested The

properties of these investigated catalysts were also characterized

The main content of the thesis included three chapters The first chapter

summarizes the aspects about air pollution problem, immediate consequences, main

pollutants, pollutant treatments, researches in the world and in Vietnam, the catalyst

based on Ceria-Zirconia and complete hydrocarbon oxidation using metal catalyst

in the literature

The second chapter introduces methods to synthesize catalysts (sol-gel,

impregnation), methods to determine the pollutant concentrations (ECE R40,

HMDC, GC- FID and GC-MS), physicochemical techniques (XRD, SEM and BET)

and catalytic test used in the thesis

The most important chapter of this thesis (chapter III) focused on results and

discussions on composition of motorcycle exhaust gases, characterization and

catalytic activity of investigated catalysts

CHAPTER I: LITERATURE REVIEW I.1 Air pollution problem

An air pollutant is known as a substance in the air that can cause harm to humans

and the environment Pollutants can be in the form of solid particles, liquid droplets,

or gases In addition, they may be natural or man-made [18]

Now a day, air pollution is one of serious problems in the world and immediate

consequences are hazards such as: acid rain, the greenhouse effect, ozone hole…

Emission from fuel combustion contributes essentially in pollution In Vietnam,

environment, especially in big city, is so bad due to the rapid increase of automobile

[60]

Air pollution from World War II production Smog over Santiago

Fig 1 Images of air pollution over the world [18]

I.1.1 Air pollutants

Pollutants that are of primary concern are those that, in sufficient ambient

concentrations, adversely impact human health and/or the quality of the

environment Those pollutants for which health criteria define specific acceptable

levels of ambient concentrations are known in the United States as "criteria

pollutants." The major criteria pollutants are carbon monoxide (CO), nitrogen

dioxide (NO2), ozone, particulate matter less than 10 nm in diameter (PM10), sulfur

dioxide (SO2), and lead (Pb) Ambient concentrations of NO2 are usually controlled

by limiting emissions of both nitrogen oxide (NO) and NO2, which combined are

referred to as oxides of nitrogen (NOx) NOx and SO2 are important in the formation

of acid precipitation, and NOx 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 [3]

Other compounds, such as benzene, polycyclic aromatic hydrocarbons (PAHs),

other trace organics, and mercury and other metals, are emitted in much smaller

quantities, but are more toxic and in some cases accumulate in biological tissue over

time These compounds have been grouped together as hazardous air pollutants

(HAPs) or "air toxics," and have recently been the subject of increased regulatory

control Also of increasing interest are emissions of compounds such as carbon

dioxide (CO2), methane (CH4), or nitrous oxide (N2O) that have the potential to

affect the global climate by increasing the level of solar radiation trapped in the

Earth's atmosphere, and compounds such as chlorofluorocarbons (CFCs) that react

with and destroy ozone in the stratosphere, reducing the atmosphere's ability to

screen out harmful ultraviolet radiation from the sun [40]

The year 2000 had seen over 500 million passenger cars in use worldwide with an

annual worldwide production of new cars approaching 60 million In addition, there

are about 40% more passenger vehicles represented by trucks The majority of these

vehicles (automobiles and trucks) use a spark ignited gasoline engine to provide

power and this has become the most frequent form of transportation Gasoline blend

still remains a mixture of paraffins and aromatic hydrocarbons which combust in air

at a very high efficiency [14]

The simplified reaction is

Gasoline + O2 (in air) → CO2 + H2O + heat Due to incomplete combustion in the engine, there are a number of incomplete

combustion products Typical exhaust gas composition at the normal engine

• Hydrogen (H2, 0.17 vol %);

• Water (H2O, 10 vol %);

• Carbon dioxide (CO2, 10 vol %);

• Oxygen (O2, 0.5 vol %)

HC, CO and NOx 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 The NOx is formed during the very

high temperatures (>1500 ◦C) of the combustion process resulting in thermal

fixation of the nitrogen in the air which forms NOx [66]

Because of the large vehicle population, significant amounts of HC, CO and NOx

are emitted to the atmosphere The formation of ground level ozone occurs as a

result of a chemical reaction between HC and NOx and sunlight When stagnant air

masses linger over urban areas, the pollutants are held in place for long periods of

time Sunlight interacts with these pollutants, transforming them into ground level

ozone Ozone is a major component of smog Of course, CO is a direct poison to

humans The benefits of catalytic controls have been documented and it is now

estimated that by the year 2000, over 800 million tons of combined pollutants of

HC, CO and NOx will have been abated using auto exhaust catalyst and prevented

from entering the atmosphere [15]

• Particle matter (PM10): Particulates, alternatively 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 Sources of

particulate matter can be man made or natural Some particulates occur

naturally, originating from volcanoes, dust storms, forest and grassland fires,

living vegetation, and sea spray Human activities, such as the burning of

fossil fuels in vehicles, power plants and various industrial processes also

generate significant amounts of aerosols Averaged over the globe,

anthropogenic aerosols—those made by human activities—currently account

for about 10 percent of the total amount of aerosols in our atmosphere

Increased levels of fine particles in the air are linked to health hazards such

as heart diseases, altered lung function and lung cancer [28, 60]

• Sulfur oxides: (SOx) - especially sulfur dioxide, a chemical compound with

the formula SO2 SO2 is produced by volcanoes and in various industrial

processes Since coal and petroleum often contain sulfur compounds, their

combustion generates sulfur dioxide Further oxidation of SO2, usually in the

presence of a catalyst such as NO2, forms H2SO4, and thus acid rain This is

one of the causes for concern over the environmental impact of the use of

these fuels as power sources [18, 28, 60]

• Nitrous oxides: (NOx) - especially nitrogen dioxide are emitted from high

temperature combustion Can be seen as the brown haze dome above or

plume downwind of cities Nitrogen dioxide is the chemical compound with

the formula NO2 It is one of the several nitrogen oxides This reddish-brown

toxic gas has a characteristic sharp, biting odor NO2 is one of the most

prominent air pollutants Nitrous oxides can be formed by some reactions:

NO + ½ O2 NO2

3NO2 + H2O 2 HNO3 + NO [28]

There are three major pathways to form NO in combustion systems: thermal NOx,

fuel NOx, and prompt NOx Thermal NOx is created when the oxygen (O2) and

nitrogen (N2) present in the air are exposed to the high temperatures of a flame,

leading to a dissociation of O2 and N2 molecules and their recombination into NO

The rate of this reaction is highly temperature-dependent; therefore, a reduction in

peak flame temperature can significantly reduce the level of NOx emissions

Thermal NOx is important in all combustion processes that rely on air as the

oxidizer [40]

Fuel NOx is due to the presence of nitrogen in the fuel and is the greatest

contributor to total NOx emissions in uncontrolled coal flames By limiting the

presence of O2 in the region where the nitrogen devolatilizes from the solid fuel, the

formation of fuel NOx, can be greatly diminished NO formation reactions depend

upon the presence of hydrocarbon radicals and O2, and since the

hydrocarbon-oxygen reactions are much faster than the nitrogen-hydrocarbon-oxygen reactions, a controlled

introduction of air into the devolatilization zone leads to the oxygen preferentially

reacting with the hydrocarbon radicals (rather than with the nitrogen) to form water

and CO Finally, the combustion of CO is completed, and since this reaction does

not promote NO production, the total rate of NOx production is reduced in

comparison with uncontrolled flames This staged combustion can be designed to

take place within a single burner flame or within the entire furnace, depending on

the type of control applied (see below) Fuel NOx is important primarily in coal

combustion systems, although it is important in systems that use heavy oils, since

both fuels contain significant amounts of fuel nitrogen [40]

Prompt NOx forms at a rate faster than equilibrium would predict for thermal NOx

formation Prompt NOx forms from nonequilibrium levels of oxide (O) and

hydroxide (OH) radicals, through reactions initiated by hydrocarbon radicals with

molecular nitrogen, and the reactions of O atoms with N2 to form N2O and finally

the subsequent reaction of N2O with O to form NO Prompt NOx can account for

more than 50% of NOx formed in fuel-rich hydrocarbon flames However, prompt

NO was not done typically account for a significant portion of the total NO

emissions from combustion sources [40] Like SOx, nitrous oxides damage the

respiratory system and they are causes of acid rain, photochemical smog [28]

• Carbon monoxide (CO): is a colorless, odorless, non-irritating but very

poisonous gas [18] Carbon monoxide emissions are typically the result 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

ensure that adequate combustion air is available in the combustion zone and

that the air and fuel are well mixed at high temperatures Where large

amounts of CO are emitted in relatively high concentration streams,

dedicated CO boilers or thermal oxidation systems may be used to burn out

the CO to CO2 CO boilers use the waste CO as the primary fuel and extract

useful heat from the combustion of the waste gas An auxiliary fuel, usually

natural gas, is used to maintain combustion temperatures and as a start-up

fuel [40]

• 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 categories of methane (CH4) and

non-methane (NMVOCs) Methane is an extremely efficient greenhouse gas

which contributes to enhance 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 First, VOCs react in the atmosphere in the

presence of sunlight to form photochemical oxidants (including ozone) that

are harmful to human health Second, many of these compounds are harmful

to human health at relatively low concentrations This second group of VOCs

is referred to as hazardous air pollutants (HAPs) and is included for potential

regulation under Title III of the Clean Air Act Amendments of 1990 Total

VOC emissions in the U.S have been declining over the past 10 years,

primarily due to significant improvements in vehicle emission levels During

the same period, VOC emissions from industrial sources, solvent utilization,

and chemical manufacturing have increased slightly, making these sources

more important from a control perspective In addition to VOCs, heavier

organic compounds, such as polycyclic aromatic hydrocarbons (PAHs),

nitrogenated PAHs, polychlorinated biphenyls (PCBs), and polychlorinated

dibenzodioxins (PCDDs), are also important HAPs that may be emitted from

a variety of sources Combustion processes in general can form PAHs;

however, proper equipment operation and maintenance typically results in

PAH emissions from combustion sources on the order of parts per billion or

less Within the NMVOCs, the aromatic compounds benzene, toluene and

xylene are suspected carcinogens and may lead to leukemia through

prolonged exposure 1,3-butadiene is another dangerous compound which is

often associated with industrial uses [18, 40]

I.1.2 Air pollution problem in the world and in Vietnam

Combustion processes are a major anthropogenic source of air pollution in the

United States, responsible for 24% of the total emissions of CO, NOx, SO2, VOCs,

and particulates In 1992, 146 million tonnes (161 million tons) of these pollutants

were emitted in the United States Of these pollutants, stationary combustion

processes emit 91% of the total U.S SO2 emissions, and 50% of the total U.S NOx

emissions The major combustion-generated pollutants (not including CO2) by

tonnage are CO, NOx, PM, SO2, and VOCs [40]

Fig 2.Schematic drawing, causes and effects of air pollution: (1) greenhouse effect,

(2) particulate contamination, (3) increased UV radiation, (4) acid rain, (5)

increased ozone concentration, (6) increased levels of nitrogen oxides

Table 1 presents total estimated anthropogenic and combustion-generated emissions

of selected air pollutants in the United States Combustion-generated air pollution

can be viewed as originating through two major methods, although some overlap

occurs between the two The first of these methods is origination of pollution

primarily from constituents in the fuel Examples of these "fuel-borne" pollutants

are SO2 and trace metals The second is the origination of pollutants through

modification or reaction of constituents that are normally nonpolluting CO, NOx,

and volatile organics are examples of "process-derived" pollutants

Table1 Anthropogenic Emissions of Selected Air Pollutants in USA [40]

Emissions, Tons /Year

Combustion Emissions, Tons /Year

In the case of NOx, fuel-borne nitrogen such as that in coal plays a major role in the

formation of the pollutant; however, even such clean fuels as natural gas (which

contains no appreciable nitrogen) can emit NOx when combusted in

nitrogen-containing air Major stationary sources of combustion-generated air pollution

include steam electric generating stations, metal processing facilities, industrial

boilers, and refinery and other process heaters Table 2 shows the total U.S

emissions of criteria pollutants from these and other sources [40]

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

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

motorbikes is about 90% of all vehicles in Vietnam In 2006, there were eighteen

million operating motorbikes; the average increase of motorbikes is 15-30% each

year Thus, the environmental pollution is very heavy, the emitted exhaust gases

each year are six million tons of CO2, sixty one thousand tons of CO, thirty five

thousand tons of NO2 , which excess the standard 1.5 – 5 times [63] In big cities,

the air pollution is becoming serious The air in Hanoi and Ho Chi Minh City

contains dangerous levels of benzene and sulfur dioxide Levels of one of the most

dangerous pollutants, microscopic dust known as PM10, are moderate compared

with other developing Asian cities but could worsen if Vietnam chooses to build

coal-fired power plants to satiate demand for electricity, which is growing at

double-digit annual rates [19] The most recent check of the level of dust and

other pollutants in the air shows that air pollution is increasing at an alarming rate

over many residential areas and main streets in HCMC, according to the (HCMC)

Environmental Protection Agency

Table 2 Annual Combustion-Generated Emissions of Selected Pollutants by

Stationary Source Category in USA [40]

Stationary Fuel Combustion Emissions

According to the agency, over the first seven months of year 2010, dust and

pollutants in the air exceeded the permitted levels at 90% of the monitoring stations

The monitoring station at An Suong Intersection in District 12 showed that the

concentration of dust exceeded the acceptable standard by 5.6 times Go Vap

Crossroads, Dinh Tien Hoang-Dien Bien Phu Intersection and areas along Hanoi

Highway show the highest concentrations of air pollution in the city The agency’s

monitoring also showed that levels of lead, benzene, nitrogen dioxide and noise

around the city have been increasing at a faster rate Compared to year 2009, lead

content has increased by 2.2 times and benzene by 1.4 times

Table 3 Percents of pollutants in Euro in 1994 (%) [60]

*COVNM: compound organic volatile not counting methane

Highway and on many city streets have contributed to the air pollution According

to the city’s Transport Department, the city has 3.8 million motorbikes, 300,000

cars and 30,000 industrial manufacturers discharging large amounts of smoke into

the air Some 60% of the motorbikes do not meet smog standards and some 80% of

the industrial smoke is still untreated [20]

Emission standards for passenger cars and light commercial vehicles are

summarized in the following tables

Table 4 EU emission standard for passenger car, g/km

I.2 Air pollution treatments

I.2.1 Original pollutant treatments

a CO treatments:

Method 1: Carbon monoxide can be converted by oxidation:

CO + O2 CO2 The catalysts base on noble metals [10, 11] Moreover, some transition metal

oxides (Co, Ce, Cu, Fe,W, Mn…) can be used for treating CO [12, 52, 53]

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

CO + H2O CO2 + H2 ∆H0

298K= -41.1 kJ/mol This reaction was catalyzed by catalysts base on precious metal [35, 49]

Method 3: NO elimination:

NO + CO CO2 + ½ N2 The most active catalyst is Rd [60] Besides, Pd catalysts were applied [23, 33]

b VOC treatments:

Volatile organic compounds (VOCs) are emitted from a broad variety of stationary

sources, primarily manufacturing processes, and are of concern for two primary

reasons Firstly, VOCs react in the atmosphere in the presence of sunlight to form

photochemical oxidants (including ozone) that are harmful to human health

Secondly, many of these compounds are harmful to human health at relatively low

concentrations This second group of VOCs is referred to as hazardous air

pollutants (HAPs) and is included for potential regulation of the Clean Air Act

Amendments of 1990

Some control technologies were used:

Thermal oxidizers: destroy organic compounds by passing them through

high-temperature environments in the presence of oxygen In practice, thermal oxidizers

or incinerators typically operate by directing the pollutant stream into the

combustion air stream, which is then mixed with a supplementary fuel (usually

natural gas or fuel oil) and burned

Boilers or industrial furnaces that are already present on a plant site can also be

used as thermal incineration systems for appropriate streams of VOCs and organic

HAPs

Flares are a simple form of thermal oxidation that does not use a confined

combustion chamber As with other forms of thermal oxidation, flares often require

supplemental fuel Flares are often used when the emission stream is intermittent or

uncertain, such as the result of a process upset or emergency

Catalytic oxidizers use a catalyst to promote the reaction of the organic

compounds with oxygen, thereby requiring lower operating temperatures and

reducing the need for supplemental fuel Destruction efficiencies are typically near

95%, but can be increased by using additional catalyst or higher temperatures (and

thus more supplemental fuel) The catalyst may be either fixed or mobile (fluid

bed) Because catalysts may be poisoned by contacting 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

Adsorption systems rely on a packed bed containing an adsorbent material to

capture the VOC or organic HAP compound(s) Activated carbon is the most

common adsorbent material for these systems, but alumina, silica gel, and polymers

are also used Adsorbers can achieve removal efficiencies of up to 99 %, and in

many cases allow for the recovery of the emitted compound Organic compounds

such as benzene, methyl ethyl ketone, and toluene are examples of compounds that

are effectively captured by carbon bed adsorption systems

Condensers are used to reduce the concentrations of VOCs and organic HAPs by

lowering the temperature of the emission stream, thereby condensing these

compounds Condensers are most often used to reduce pollutant concentrations

before the emission stream passes into other emission reduction systems such as

thermal or catalytic oxidizers, adsorbers, or absorbers

Biofilters rely on microorganisms to feed on and thus destroy the VOCs and

organic HAPs In these systems, the emission stream must come into direct contact

with a filter containing the microorganism for sufficient time for the bioreaction to

occur Although biofilters can have lower overall costs than other technologies,

technical problems, such as proper matching of the emission stream and the

microorganisms, long-term operational stability, and disposal of the resulting solid

wastes, may prevent their use in particular situations [40]

c NOx treatments

NOx formed by the combustion of fuel in air is typically composed of greater than

90% NO, with NO2 making up the remainder Unfortunately, NO is not amenable to

flue gas scrubbing processes, as SO2 is An understanding of the chemistry of NOx

formation and destruction is helpful in understanding emission-control technologies

for NOx

Because the rate of NOx formation is so highly dependent upon temperature as well

as local chemistry within the combustion environment, NOx is ideally suited to

control by means of modifying the combustion conditions There are several

methods of applying these combustion modification NOx controls, ranging from

reducing the overall excess air levels in the combustor to burners specifically

designed for low NOx emissions [40]

Emission Reduction,

%

Stoker-fired coal boilers

5-20

Natural gas reburn fuel with pulverized-coal main fuel

50-60 Reburning

Coal reburn fuel with coal main fuel

I.2.2 Treatments of simultaneous three pollutants – three-way catalysts

a Method 1: It can be treated simultaneous three pollutants (NOx, CO, HC) by

designing successive oxidation and reduction converters The main reactions

in treatment process are:

In this method, reduction converter only operates well in excess fuel condition

Furthermore, NH3 can be formed in reduction condition This pollutant will be

converted into NO-another pollutant in oxidation media [28]

Fig 3 Scheme of successive two converter model

b Method 2: The basic reactions for CO and HC in the exhaust are oxidation

with the desired product being CO2, while the NOx reaction is a reduction

with the desired product being N2 and H2O A catalyst promotes these

reactions at lower temperatures than a thermal process giving the following

desired reactions for HC, CO and NOx:

Reduction converter Oxidation converter

I.2.3 Three-way catalyst characteristic

There are some common components, which represent the state-of-art of the

wash-coating composition:

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

• CeO2–ZrO2 mixed oxides, principally added as oxygen storage promoters

• Noble metals (NM = Rh, Pt and Pd) as active phases

• Barium and/or lanthanum oxides as stabilizers of the alumina surface area [65]

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, i.e 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 (Fig 4) 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 (Fig 4) [25]

All the above reactions required some heat or temperature on the catalyst surface

for the reaction to occur When the automobile first starts, both the engine and

catalyst are cold After startup, the heat of combustion is transferred from the

engine and the exhaust 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; i.e depends on the

chemistry of the catalyst since the transport reactions are fast Typically, the CO

reaction begins first followed by the HC and NOx 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 the overall conversions are controlled

by pore diffusion and/or bulk mass transfer Figure 5 shows a typical response of a

TWC catalyst as a function of the engine air to fuel ratio

Fig 5 Three way catalyst performance determined by engine air to fuel ratio

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 of three-way catalyst [22] The monolith can be thought of as a series of

parallel tubes with a cell density ranging from 300 to 1200 cpsi Advances in

monolith technology, catalyst-mounting methods, flexibility in reactor design, low

pressure drop and high heat and mass transfer rates, are the main reasons the

monolithic support dominates the entire market as the preferred catalyst support

Ceramic materials were chosen for the initial monolith and they still dominate the

market The preferred material is called cordierite (synthetic cordierite has a

composition approximating 2MgO, 5SiO2 and 2Al2O3 and a softening point

>1300◦C) [10] In preparing the catalyst, this desirable property has to be matched

by the thermal expansion properties of the catalyst carrier or wash-coat to prevent a

mismatch in thermal properties Figure 6 shows the surface coating on a modern

TWC

Fig 6 Wash-coats on automotive catalyst can have different surface structures as

shown with SEM micrographs

Today, cordierite monoliths of 400 cpsi and 0.004 in wall thickness and 600 cpsi

and 0.004 in wall thickness are available [10] Monoliths of 900 and 1200 cpsi

have been made and tested in road-simulated aging and offer benefits for the super

ultra low level vehicle (SULEV) type vehicles Recently, metallic monolith

structures are being used in certain niche markets for exhaust control because they

can be made with thinner walls and have open frontal areas close to 90%, allowing

lower pressure drop Cell densities >400 cpsi can be used which permits smaller

catalyst volumes and smaller converters The base material of construction is a

ferritic stainless steel alloy having iron/chrome/aluminum/rare earths Typical

monoliths have 400 and 600 cpsi with 0.002 in wall thickness By the year 2000,

approximately 30 years of catalyst technology development have been devoted to

the automotive exhaust catalyst Figure 6 shows a typical auto catalyst design

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

developing new engine platforms and new sensor and control technology 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 During this

time period the auto catalyst has progressed through the following development

phases

Fig 7 Improvement trend of catalytic converter

Additional background information on the other technology developments from

- Approaching 950 0 C

- Stabilized Ce with Zr

- Pt/Rh, Pd/Rh and Pt/Rh/Pd

All Palladium three way catalyst

Layered coating

- Stabilized Ce with Zr

Low emission Vehicles

- High temperature close couple catalyst approaching 1050 0 C

underfloor catalyst, high

precious metal loading

- Optional trap

I.3.International and Vietnam researches on catalyst for exhaust gas treatment

I.3.1 International researches

Three-way catalyst (TWC) is one of new methods for exhaust treating now It can

be transformed polluted agents approximately 100% in large temperature range to

reach Euro III and IV standards Catalysts are classified to some groups beyond

metal characteristic:

a Noble metallic catalyst: Noble metal catalysts have received considerable

attention for more than 20 years for used in automotive emission control systems,

[37] essentially base on Pt group ex: Pt, Pd, Rh on supports Supports can be CeO2-

ZrO2, Al2O3, mixtures of some oxides Catalyst based on noble metal exhibited high

catalytic activity in pollutant treatment and these catalysts were used extensively

Containing Pd catalyst was researched by Jianqiang Wang et al.[66] For fresh

catalyst it can be observed that both Pd/CZ (Ce-Zr) and Pd/CZS (Ce-Zr-Sr) show

the almost same oxidation activity for CO, the conversion of which can reach

almost 100% under λ > 1 conditions, but descend as decreasing λ -value under λ < 1

conditions (λ: the theoretical stoichiometric value and λ can be calculated λ=

(2O2+NO)/ (10C3H8+CO)

Pd supported on Ce-Zr-La-Al2O3 was used for transforming CO, C3H8, NO With

these fresh catalytic systems, the conversions are 100% at above 240, 300, 3400C for

CO, NO, C3H8 respectively And operating temperatures for aged catalysts are higher

[34] Furthermore, Palladium catalysts were prepared by impregnation on CeO2

-ZrO2-Al2O3 (CZA) and CeO2-ZrO2-Al2O3-La2O3 (CZALa) for CH4, CO and NOx

treatment in the mixture gas simulated the exhaust from natural gas vehicles

(NGVs) operated under stoichiometric condition was investigated by Xiaoyu Zhang

[67]

In high temperature (above 3000C), Pd is suitable whereas Pt is better in low

temperature [60] F Dong and colleagues research investigation of the OSC

performance of Pt/CeO2.ZrO2.Y2O3 catalysts by CO oxidation and 18O/16O isotopic

exchange reaction and obtained good results They indicated that the development

of a more efficient oxygen storage material is a very important approach for the

optimization of automotive catalysts [6]

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

nature of ageing atmosphere and temperature.These catalysts reach their maximum

conversions by the temperature of 400 ◦C[32]

Sudhanshu Sharma showed that 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 from 0.02 wt% to 2 wt% which is

sufficient but can be loaded even up to 12 wt% by repeating dip dry combustion

Adhesion of catalyst to cordierite surface is via oxide growth, which is very strong

[54]

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

added to promote activity or reduce pride but properties preserving or increase

activity

Author Guo Jiaxiu and co-worker investigated influence of Ce0.35Zr0.55Y0.10 solid

solution performance Pt-Rh three-way catalyst The results revealed that

Ce0.35Zr0.55Y0.10 had cubic structure similar to Ce0.5Zr0.5O2 and its specific surface

area can maintain higher than Ce0.5Zr0.5O2 after 10000C calcinations for 5h Being

hydrothermal aged at 10000C for 5h, the catalyst containing Ce0.35Zr0.55Y0.10 still

exhibited higher conversion of C3H8, CO and NO and lower light-off temperature

in comparison with Ce0.5Zr0.5O2 TWC [22]

Ana Iglesias et al [23] showed that the behaviors of a series of Pd–M 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 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 PdCuCZ (Ce-Zr)

Li-Ping Ma et al.[37] investigated “Kinetic study of three-way catalyst of

automotive exhaust gas: modeling and application” The results show the catalytic

activity of Pd-Rh (1.6% noble metal, Pd: Rh=5:1) supported by alumina system is

very good for treating exhaust gas

Hyuk Jae Kwon investigated the light-off temperature of the oxidations of CO and

C3H6 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 course of the reactions The catalysts are

containing Pd only and Pt-Rh/Ce catalysts [31]

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 T50 (convert 50% amount of gas) values, were consistent

with ageing temperature and time In spite of the severe thermal impacts caused by

ageing, evidenced by the characterization results, the commercial catalyst could still

convert 100% of CO at 450◦C [7]

HU Chunming et al [4] showed that the Pt/Pd/Rh three-ways catalyst was

prepared using a high-performance Ce0.55Zr0.35Y0.05La0.05O2 solid solution and high

surface area La-stabilized alumina (La/Al2O3) as a wash-coat layer The activity and

durability of the catalysts under simulated conditions and actual vehicle test

conditions were studied The results revealed that Ce0.55Zr0.35Y0.05La0.05O2 solid solution

maintains superior textual and oxygen storage properties, and La/Al2O3 has superior

textual properties The catalyst had high low-temperature activity, wide air-to-fuel

ratio windows, and good thermal stability The results from the emission test of a

motorcycle showed that the catalyst can meet Euro III emission requirements

Some added metals not only increase thermal resistant of supports but also

increase oxygen storage capacity, redox properties and catalytic activity of catalyst

The effect of La2O3 on the physicochemical properties of supports and catalysts

was characterized by various techniques La2O3 restrained effectively the sintering

of crystallite particles, maintained the crystallite particles in nanoscale and

stabilized the crystal phase after calcinations at 1000◦C La2O3 improved the

textural properties, reducibility and OSC of composite supports Activity testing

results showed that the catalysts exhibit excellent activities for the simultaneous

removal of methane, CO and NOx in the simulated exhaust gas The catalysts

supported on CZALa showed remarkable thermal stability and catalytic activity for

the three pollutants, especially for NOx The prepared palladium catalysts have high

ability to remove NOx, CH4 and CO, and they can be used as excellent catalysts for

the purification of exhaust from NGVs operated under stoichiometric condition

The catalysts reported in this work also have significant potential in industrial

application because of their high performance and low cost

A Papavasiliou and his colleague idenfied that ceria–zirconia solid solution yields

an improvement in ceria’s oxygen storage capacity (OSC), redox properties,

thermal resistance and catalytic activity at low temperatures These improved

properties originate from the structural defects induced by Zr4+ incorporation in

ceria lattice which enhances mobility of bulk oxygen ions Trivalent cations such as

: La3+, Y3+, Ga3+ favour phase homogeneity of the CeO2–ZrO2 solid solution and

improve OSC even at low temperatures [50]

b Perovskite catalysts

D Fino and colleague realized that the LaMn0.9Fe0.1O3 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 CeO2 (1:1 molar ratio) and with 1 wt% Pd This promoted catalyst

was lined on cordierite monoliths in a γ-Al2O3-supported form [8]

Following L Forni’s investigation, series of La1-xCexCoO3+δ perovskite-type

catalysts, with x ranging from 0 to 0.20, showed to be quite active for reduction of

NO by CO and for oxidation of CO by air oxygen at temperatures ranging from 373

to 723 K [9]

Hirohisa Tanaka et al.[58] showed that one of the most important issues of

automotive catalysts is the endurance of fluctuations between reductive and

oxidative (redox) atmospheres at high temperatures exceeding 1173 K The

catalytic activity and structural stability of La0.9Ce0.1Co1−xFex O3 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 real and simulated “model” automotive

exhaust gases

c Base metallic catalysts

Some authors used base metals for exhaust gas treatment Sander Stegenga and

colleague realized that containing Cu-Cr catalytic activity as well as noble catalyst

for redox reaction The support is Al2O3 (dp from 105 to 145 µm, surface area =

195m2/g) [56]

d Metallic oxide catalysts

JIA Liwei showed that redox performances of ceria-based materials could be

enhanced by synergetic effects between Mn-O and Ce-O Fresh and aged samples

were characterized with the fluorite-type cubic structure similar to CeO2, and

furthermore, the thermal stability of Mn0.1 Ce0.9Ox materials was improved by the

introduction of some Zr atoms [36]

Masakuni Ozawa [48] described as the examples of the high potential of rare earth

modification in alumina-based catalyst as well as support The surface modification

using rare earths to alumina lead better heat-stable catalytic support with nanometer

order particles The effect of the La pre-modification on the thermal stability of

transition metal catalysts was evident Using this effect, the La-modified, transition

metals-promoted alumina catalyst, e.g CuOx–La–Al2O3 is developed as an

inexpensive de-NOx catalyst with heat stability

K.A Bethke and co-worker indicated that a number of metal oxides are effective

lean NOx reduction catalysts Incorporation of reducible transition metal ions into

the metal oxides, such that the transition metal ions are highly dispersed, greatly

increased the activity of the catalysts such that lean NOx reduction occurs readily at

temperatures below 573 K Mixed metal oxide catalysts of ZrO2 containing Cu, Co,

Fe, and Ni, which are as active as the corresponding ZSM-5-based catalysts [1]

e Other catalysts

Some researchers are interested in some kind catalysts like that : Cu-ZSM-5,

complexes catalyst MAX (M: transition metals such as: Cu, Fe, Co, Ni; A: SO42-,

SO32- anion, X: 3-amino-1,2,4 triazol) was condensed with formaldehyde on porous

supporter ( silica gel, Al2O3, bentonite, zeolite); Ag catalyst or Ag compound (

halogen, oxide, sulfate, phosphate) on Sn oxide; and may be added Al2O3, TiO2…

[30, 55]

L Keiski showed that metal substrate ZSM-5 zeolites ion-exchanged with copper

are effective catalysts in the elimination of nitrogen oxides from lean automotive

exhaust gases when propene works as a reductant Some co-cations improve the

catalytic activity of Cu-ZSM5 [27]

I.3.2 Researches in Vietnam

In Vietnam, exhaust gas treatment was researched in recent years However, it was

not studied as much as in the world Authors introduced essentially some catalysts

for single pollutant treatment

Le Thi Hoai Nam studied on Au-ZSM5 catalysts for carbon monoxide oxidation to

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

temperature Catalytic activity increases when temperature increases and it is more

preeminent than some systems (Au/α-Fe2O3 Au:Fe=1:19), Pd/ γ-Al2O3) [44]

Tran Thi Nhu Mai and co-worker used of V2O5-TiO2/Me2Ox (Me= Mo, Cu, Ce)

catalyst supported on honeycomb-texture ceramic Catalysts properties were

estimated by LPG advanced oxidation reaction The reaction temperature range was

from 350 to 4000C to reach 100% conversion [39]

Author Nguyen Van Quy researched Ag-Co system for selective catalytic reduction

of NOx by propylene in the presence of excess oxygen At about 2400C, the oxidation

of propylene observed with the strong formation of CO2 The consumption of NOx

was nearly 100% below temperature range The strong formation of CO2 shows that

the catalyst is a good oxidation one [51]

Tran Thi Minh Nguyet studied deNOx properties of La1-xSrxCoO3 perovskite/complex

oxides The results showed that catalyst with molar ratios La:Sr:Co=0.4:0.6:1; a single

phase perovskite exhibited only an oxidation function, while the product with three

phases realized three functions of DeNOx reaction The conversion was 40% [46]

Author Le Minh Thang synthesized some catalysts such as: Ni/γ-Al2O3/Cordierite

(5% Ni, 10% γ-Al2O3), Co/γ-Al2O3/Cordierite (5% Co, 10% γ-Al2O3),

Ni-Co/γ-Al2O3/Cordierite (2.5% Ni, 2.5% Co, 10% γ-Al2O3) for complete oxidation of

hydrocarbon They have suitable operation temperature is from 350 to 4000C

Containing Co catalysts are better than Ni-catalysts in n-hexane oxidation and

equivalent with noble catalyst The maximum conversion was 80 % [60]

I.3.3 The imperative task, the aim and the research direction of the thesis

The imperative task of the thesis:

As mentioned previously, environmental pollution from motorcycle in Vietnam is

significant since the number of motorcycle used in Vietnam is very high However,

expensive and complex catalytic converters may not be welcome in a developing

country like Vietnam Thus, the catalysts should not be noble catalysts (Pt, Pd ) as

normal used and studied in the world

The aim of the thesis

Therefore, the objective of this project is to study and find out optimal catalysts for

treatment of motorcycle exhaust gases The expected catalysts should have better

activity to treat exhaust gases, longer lifetime, low cost, be able to operate at low

temperature and require very little modification of motorcycle’s design, which can

help to apply it easier (even with being used motor bikes) in a developing country

like Vietnam The catalysts should be found from metal oxides like cerium,

vanadium, titanium To make the converters more simple, catalysts should work at

low temperature, thus the heat of the exhaust gas flow is enough for the reaction

Catalysts should also work under the condition of lacking oxygen

The new direction of the research

In the world, noble metallic catalysts such as Pt, Rh and Pd were used for treating

gaseous emission from fuel combustion The catalytic activity is high but decrease

rapidly at high temperature These systems are poisoned easily in harsh condition

and very expensive They aren’t suitable in developing country like Vietnam

Meanwhile, some catalysts base on metal such as Co, Ni, Fe may also have high

activity (because these metals are in VIII and IB group as noble metals) but cheaper

than noble metals

Our proposal is to synthesize catalysts from inexpensive components Since these

metals are easier to be oxidized than well-known used noble metals, the oxide forms

of these components should be used instead of the metal forms To make the

converters more simple, catalysts should work at low temperatures, the temperature

of the exhaust gas flow The catalysts should also work under the condition of

lacking oxygen since the normal composition of exhaust gases in motor engines

only contains a small amount of oxygen Catalysts should accelerate many different

reactions from complete oxidation to reduction; therefore a multi component system

with multi functions is necessary Based on the desired properties described above,

this project will focus on the synthesis and characterization of catalyst: CeO2 for its

high capacity to store oxygen, ZrO2 for its high conductivity and possibility to

convert NOx, SnO2 for its ability to break carbon-carbon bonds, V2O5 for its ability

to break oxygen – oxygen bonds, Ni to convert hydrocarbons,…

In these metallic oxides, CeO2 and ZrO2 were paid attention because of their

advanced characteristics CeO2 has been widely used in automotive emission

control because it can store or release oxygen under oxygen-lean or –rich conditions

and promoted the dispersion and catalytic activity of noble metal However, CeO2

will deactivate significantly at high temperature due to the loss of surface area and

oxygen storage capacity It has been shown that the introduction of foreign cation

with smaller ionic radius than Ce4+ can favorably modify the structure of ceria–

zirconia so as to improve its properties [66]