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

Synthesis and catalytic properties of catalyst system based on CeO2 – ZrO2 for the
complete oxidation of hydrocarbon to treat motorcycle’s exhaust gases

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

SCIENCE MASTER THESIS
ORGANIC AND PETROCHEMICAL TECHNOLOGY

SUPERVISOR:
Associate Professor, Doctor LE MINH THANG

HANOI, 2010
Nguyen The Tien

Organic and Petrochemical
Technology
1


Synthesis and catalytic properties of catalyst system based on CeO2 – ZrO2 for the
complete oxidation of hydrocarbon to treat motorcycle’s exhaust gases

CONTENTS
Assistant cover page
Acknowledgements
Protestation in the thesis
Symbols in the thesis
Tables in the thesis
Figures in the thesis
Abstract
Introduction, aims and outline of the thesis
Chapter I: Literature review
I.1. Air pollution problem
I.1.1. Air pollutants
I.1.2. Air pollution problem in the world and in Vietnam
I.2. Air pollution treatments
I.2.1. Original pollutant treatments
a. CO treatments
b. VOC treatments
c. NOx treatments
I.2.2. Treatments of simultaneous three pollutants – three-way
catalysts
a. Method 1
b. Method 2
I.2.3. Three-way catalyst characteristic
I.3. International and Vietnam researches on catalyst for exhaust gas
treatment
I.3.1. International researches
a. Noble metallic catalysts
b. Perovskite catalysts
c. Base metallic catalysts

d. Metallic oxide catalysts
e. Other catalysts
I.3.2. Researches in Vietnam
I.3.3. The imperative task, the aim and the research direction of the
thesis
I.4. The catalysts based on Cerium and Zirconium oxide
I.4.1. Role of CeO2 in the three-way catalyst
I.4.2. Phase diagram of CeO2-ZrO2 mixed oxide
I.4.3. Synthesis of CeO2-ZrO2 mixed oxide
I.4.4. Characteristic of CeO2-ZrO2 mixed oxide
a. Oxygen storage capacity (OSC) of CeO2–ZrO2 mixed oxides
Nguyen The Tien

Page
1
5
6
7
9
10
12
13
15
15
15
21
24
24
24
25

27
28
28
28
29
33
33
33
36
37
37
38
38
39
41
41
42
44
45
45

Organic and Petrochemical
Technology
2


Synthesis and catalytic properties of catalyst system based on CeO2 – ZrO2 for the
complete oxidation of hydrocarbon to treat motorcycle’s exhaust gases

b. Thermal stability of CeO2–ZrO2 mixed oxides

I. 5.Completed hydrocarbon oxidation of hydrocarbon
I.5.1. Hydrocarbon oxidation mechanism
I.5.2. Kinetic of hydrocarbon oxidation

45
47
47
51

CHAPTER II: EXPERIMENTAL
II.1. Chemicals and synthesizes method

52
52

II.1.1. Chemicals
II.1.2. Synthesis of several single oxides, CeO2-ZrO2 and CeO2-Co3O4

52
52

catalysts by sol-gel method
a. Introduction
b. Application in the thesis
II.1.3. Mechanical mixtures of oxides
II.1.4. Synthesis of Co3O4/CeO2-ZrO2 catalysts by impregnation
II.2. Methods to determine pollutant concentration
II.2.1. Driving cycle methods
II.2.2. GC methods
II.3.Physico-Chemical Experimental Techniques

II.3.1. X-ray diffraction
a. Principles
b. Application in the thesis
II.3.2. Scanning electron microscopy (SEM)
a. Principles
b. Application in the thesis
II.3.3. BET method for the determination of surface area
a. Principles
b. Application in the thesis
II.4.Catalytic test
II.4.1. Equipment description
II.4.2. The loading of the catalyst
II.4.3. The analysis of the results

52
52
54
54
55
55
58
59
59
59
61
61
61
62
62
62

64
64
64
66
66

CHAPTER III: RESULTS AND DISCUSSIONS
III.1. Composition of motorcycle exhausts gases
III.1.1. Pollutant concentrations

68
68
68

III.1.2. O2 volume concentration in the exhaust gas

69

III.1.3. Hydrocarbon concentrations in the exhaust gas analyzed by

70

GC-MS and GC- FID
III.2. Characterization of several single metallic oxides for the
hydrocarbon completed oxidation
Nguyen The Tien

73

Organic and Petrochemical

Technology
3


Synthesis and catalytic properties of catalyst system based on CeO2 – ZrO2 for the
complete oxidation of hydrocarbon to treat motorcycle’s exhaust gases

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

77

complete oxidation reaction of propylene
III.3.2. Phase composition and surface properties of CeO2-SnO2

78

mechanical mixtures
III.4. Characterization of CeO2-ZrO2 mixtures

80

III.4.1.

Phase composition and surface properties of CeO2-ZrO2

81

III.4.2. Catalytic activity of CeO2-ZrO2 for complete oxidation

82

mixtures

reaction of propylene
III.5. Catalytic activity of CeO2-Co3O4 catalysts

87

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

89

CONCLUSIONS

92

REFERENCES

94

Nguyen The Tien

Organic and Petrochemical
Technology
4


Synthesis and catalytic properties of catalyst system based on CeO2 – ZrO2 for the
complete oxidation of hydrocarbon to treat motorcycle’s exhaust gases

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
work.
I acknowledge to all members in my research group for their friendly attitude and

their assistances.
Finally, I want to thank my family for their love and encouragement during the
whole period.
Nguyen The Tien
August 2010

Nguyen The Tien

Organic and Petrochemical
Technology
5


Synthesis and catalytic properties of catalyst system based on CeO2 – ZrO2 for the
complete oxidation of hydrocarbon to treat motorcycle’s exhaust gases

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.

Nguyen The Tien

Organic and Petrochemical
Technology
6


Synthesis and catalytic properties of catalyst system based on CeO2 – ZrO2 for the
complete oxidation of hydrocarbon to treat motorcycle’s exhaust gases

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
CFCs: chlorofluorocarbons
HC: hydrocarbon
SOx: sulfur oxides
COVNM: compound organic volatile not counting methane
PCBs: polychlorinated biphenyls
PCDDs: polychlorinated dibenzodioxins
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 of Cerium oxide, Zirconium oxide, Strontium oxide
λ: the theoretical stoichiometric value

Nguyen The Tien

Organic and Petrochemical
Technology
7


Synthesis and catalytic properties of catalyst system based on CeO2 – ZrO2 for the
complete oxidation of hydrocarbon to treat motorcycle’s exhaust gases

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

Nguyen The Tien

Organic and Petrochemical

Technology
8


Synthesis and catalytic properties of catalyst system based on CeO2 – ZrO2 for the
complete oxidation of hydrocarbon to treat motorcycle’s exhaust gases

TABLES IN THE THESIS
Number
1
2

Table Name
Anthropogenic Emissions of Selected Air Pollutants in USA
Annual Combustion-Generated Emissions of Selected Pollutants
by Stationary Source Category in USA

Page
22
23

3
4
5
6
7
8
9
10

Percents of pollutants in Euro in 1994
EU emission standard for passenger car
Emission Reduction from Different NOx Control Technologies
Classification of the phases in the CeO2-ZrO2 binary system
Chemicals used in the thesis
Characteristic of ECE R40 and HMDC driving cycles
Measurement conditions using a GC-Thermo Electron with FID
Retention time of some organic compound detected by GC Thermo
Electron with FID detector and the condition mentioned in table 9

24
24
27
43
52
56
58
58

11
12
13
14
15
16
17

Measurement conditions using the GC-MS
Pollutant concentration of some motorcycle types
Pollutant concentrations analyzed by ECE R40 and HMDC

Oxygen concentrations at different operating condition
Composition of exhaust gas in different operating conditions
Hydrocarbon concentrations analyzed by GC-MS
Composition of Organic compounds in the motorcycle’s exhaust
gases with measurement time up to 40 minutes

59
68
69
69
70
70
71

18
19

BET surface area of some metal oxides
CO2 selectivity of investigated metal oxides at different reaction
temperatures

72
76

20
21
22

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

CO2 selectivity of CeO2-Co3O4 catalysts at different reaction

81
82
89

temperatures
23

CO2 selectivity of CeO2-ZrO2 supports, Co3O4 active phase,

91

Co3O4/CeO2-ZrO2 samples

Nguyen The Tien

Organic and Petrochemical
Technology
9


Synthesis and catalytic properties of catalyst system based on CeO2 – ZrO2 for the
complete oxidation of hydrocarbon to treat motorcycle’s exhaust gases

Number
1
2

FIGURES IN THE THESIS

Figure name
Images of air pollution in the world
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

Page
15
21

3
4

Scheme of successive two converter model
Diagram of a modern TWC/engine/oxygen sensor control loop for
engine exhaust control

28
29

5
6

Three way catalyst performance determined by engine air to fuel
Wash-coats on automotive catalyst can have different surface
structures as shown with SEM micrographs

30
31

7
8
9

Improvement trend of catalytic converter
Phase diagram of the CeO2-ZrO2 system
Scheme of catalytic hydrocarbon oxidation; H-hydrocarbon, Ccatalyst, R1 to R5-labile intermediate, probably of the peroxide type

32
43
50

10
11

Scheme of CeO2-ZrO2 synthesis by sol-gel method
Scheme of the synthesis of Co3O4/CeO2-ZrO2 catalysts by
impregnation method
Velocity profile of ECE R40 driving cycle
Velocity profile of HMDC
Scheme of CVS (constant volume sampling) system to determine
pollutant concentrations

53
55

12
13
14

56
56
57

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

17
18
19
20

Constitution and operating principle of SEM instrument
The BET plot
Schematic diagram of the micro-reactor setup
X-ray pattern of several single oxides synthesized using sol-gel
method

61
63
65
73

21

Reaction rate of propylene conversion (r,w, C3H6 conv) of several
oxides at different reaction temperatures

75

Nguyen The Tien

Organic and Petrochemical
Technology
10


Synthesis and catalytic properties of catalyst system based on CeO2 – ZrO2 for the
complete oxidation of hydrocarbon to treat motorcycle’s exhaust gases

22
23

Reaction rate of propylene consumption (r,w, C3H6 cons) of
CeO2-SnO2 mechanical mixtures
CO2 selectivity of CeO2-SnO2 mechanical mixtures (in low
temperatures, CO and CO2 peaks were not detected)

77
78

24

X-ray patterns of pure CeO2, pure SnO2, mechanical mixture of
CeO2-SnO2

79

25

SEM images of CeO2/SnO2=53.28/46.72 mechanical mixture before

80

26

after reaction
X-ray pattern of CeO2-ZrO2 solid solution with different Ce/Zr
atomic ratios before reaction

82

27

Reaction rate of propylene consumption of CeO2-ZrO2 solid
solutions (Ce1-xZrxO2) at different reaction temperatures

83

28

CO2 selectivity of CeO2-ZrO2 solid solutions (Ce1-xZrxO2) at
different reaction temperatures (x: optional of Zr atom in solid
solution) (in low temperatures, CO and CO2 peaks were not
detected)
Reaction rate of propylene conversion of CeO2/ZrO2=8/2 prepared
by mechanical mixed and sol-gel synthesis at different reaction
temperatures
CO2 selectivity of CeO2/ZrO2=8/2 synthesized by mechanical
mixture and sol-gel method at different reaction temperatures (at
low temperatures, CO or CO2 peaks were not detected)

84

29
30

31

85
86

XRD patterns of mechanical sample before reaction (a), sol-gel
samples before and after reaction (b) with molar ratio
CeO2/ZrO2=8/2
SEM images of CeO2/ZrO2=8/2 sol-gel sample before and after
reaction

86

33

Reaction rate of propylene consumption of CeO2-Co3O4 catalysts at
different reaction temperatures

88

34

Reaction rate of propylene consumption of CeO2-ZrO2 support,

89

32

87

Co3O4 active phase, Co3O4 supported on Ce0.9Zr0.1O2 samples
35

Reaction rate of propylene consumption of CeO2-ZrO2

90

supports, Co3O4 active phase, Co3O4 supported on Ce0.8Zr0.2O2
36

sample

SEM images of pure Ce0.8Zr0.2O2 and 5%wt Co3O4/ Ce0.8Zr0.2O2
before reaction

Nguyen The Tien

91

Organic and Petrochemical
Technology
11


Synthesis and catalytic properties of catalyst system based on CeO2 – ZrO2 for the
complete oxidation of hydrocarbon to treat motorcycle’s exhaust gases

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%.

Nguyen The Tien

Organic and Petrochemical
Technology
12


Synthesis and catalytic properties of catalyst system based on CeO2 – ZrO2 for the
complete oxidation of hydrocarbon to treat motorcycle’s exhaust gases

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.

Nguyen The Tien

Organic and Petrochemical
Technology
13

Synthesis and catalytic properties of catalyst system based on CeO2 – ZrO2 for the
complete oxidation of hydrocarbon to treat motorcycle’s exhaust gases

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.

Nguyen The Tien

Organic and Petrochemical
Technology
14


Synthesis and catalytic properties of catalyst system based on CeO2 – ZrO2 for the
complete oxidation of hydrocarbon to treat motorcycle’s exhaust gases

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
Nguyen The Tien

Organic and Petrochemical
Technology
15


Synthesis and catalytic properties of catalyst system based on CeO2 – ZrO2 for the
complete oxidation of hydrocarbon to treat motorcycle’s exhaust gases

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
operating conditions is
• Carbon monoxide (CO, 0.5 vol. %);
• Unburned hydrocarbons (HC, 350 vppm);
• Nitrogen oxides (NOx , 900 vppm);
Nguyen The Tien

Organic and Petrochemical
Technology

16


Synthesis and catalytic properties of catalyst system based on CeO2 – ZrO2 for the
complete oxidation of hydrocarbon to treat motorcycle’s exhaust gases

• 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.
Nguyen The Tien

Organic and Petrochemical
Technology
17


Synthesis and catalytic properties of catalyst system based on CeO2 – ZrO2 for the
complete oxidation of hydrocarbon to treat motorcycle’s exhaust gases

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:

N2 + O2

2NO
NO2

NO + ½ O2

2 HNO3 + NO [28]
3NO2 + H2O
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
Nguyen The Tien

Organic and Petrochemical
Technology
18


Synthesis and catalytic properties of catalyst system based on CeO2 – ZrO2 for the
complete oxidation of hydrocarbon to treat motorcycle’s exhaust gases

formation of fuel NOx, can be greatly diminished. NO formation reactions depend
upon the presence of hydrocarbon radicals and O2, and since the hydrocarbonoxygen reactions are much faster than the nitrogen-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,
Nguyen The Tien

Organic and Petrochemical
Technology
19


Synthesis and catalytic properties of catalyst system based on CeO2 – ZrO2 for the
complete oxidation of hydrocarbon to treat motorcycle’s exhaust gases

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
Nguyen The Tien

Organic and Petrochemical
Technology
20


Synthesis and catalytic properties of catalyst system based on CeO2 – ZrO2 for the
complete oxidation of hydrocarbon to treat motorcycle’s exhaust gases

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
Nguyen The Tien

Organic and Petrochemical
Technology
21


Synthesis and catalytic properties of catalyst system based on CeO2 – ZrO2 for the
complete oxidation of hydrocarbon to treat motorcycle’s exhaust gases

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]
Pollutant

Anthropogenic
Emissions,
Tons /Year

Combustion

Emissions,
Tons /Year

NOx
2.3 x 107
2.2 x 107
N2O
7.7 x 106
2.6 x 106
SO2
2.2 x 107
2.0 x 107
Total PM
1.1 x 107
2.1 x 106
Metal PM
1.4 x 105
7.0 x 102
Hg
3.3 x 102
2.1 x 102
CO2
5.5 x 109
5.4 x 109
CO
9.7 x 107
9.2 x 107
PAH
3.6 x 104
1.8 x 104

CH4
3.0 x 107
7.0 x 105
VOC
2.3 x 107
1.2 x 107
Organic HAPs
9.4 x 106
N /A
3
Pb
4.9 x 10
2.6 x 103
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 nitrogencontaining 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
Nguyen The Tien

Organic and Petrochemical
Technology
22

Synthesis and catalytic properties of catalyst system based on CeO2 – ZrO2 for the
complete oxidation of hydrocarbon to treat motorcycle’s exhaust gases

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]
Pollutant

Stationary Fuel Combustion Emissions
Utility

Industrial

Other

% of total

CO
311
714
5154
6.4
NOx
7468
3523
734
50.7
Total particulates
454
1030
493
18
SO2
15841
3090
589
88.7
VOC
32
279
394
3.1
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.

Nguyen The Tien

Organic and Petrochemical
Technology
23


Synthesis and catalytic properties of catalyst system based on CeO2 – ZrO2 for the
complete oxidation of hydrocarbon to treat motorcycle’s exhaust gases

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

Energy

Industry

Transport

Agriculture

SO2
60
25

9
0
NOx
20
13
63
1
*
COCNM
8
37
47
0
NH3
0
2.5
0.5
97
N2O
7
37
5
48
CO2
33
24
24
4
CO
3

15
69
3
CH4
0
52
1
45
40÷55
15÷30
10÷25
Particle
matter
*
COVNM: compound organic volatile not counting methane

Civil
6
3
8
0
3
15
10
2
-

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
Tier

Date

CO

HC

HC+NOx

NOx

PM

Euro 1
Euro 2
Euro 3
Euro 4
Euro 5
Euro 6

1992.07
1996.01
2000.01
2005.01

2009.09
2014.09

2.72(3.16)
2.2
2.3
1.0
1.0
1.0

0.2
0.10
0.10
0.10

0.97(1.13)
0.5
-

0.15
0.08
0.06
0.06

0.005
0.005

I.2. Air pollution treatments
I.2.1. Original pollutant treatments
a. CO treatments:

Method 1: Carbon monoxide can be converted by oxidation:
Nguyen The Tien

Organic and Petrochemical
Technology
24


Synthesis and catalytic properties of catalyst system based on CeO2 – ZrO2 for the
complete oxidation of hydrocarbon to treat motorcycle’s exhaust gases

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

∆H0298K= -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 hightemperature 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
Nguyen The Tien

Organic and Petrochemical
Technology
25