Catalytic activity and reaction mechanism of autio2 for the reduction of nox by propene

75 Figure 6-1 DRIFTS spectra of adsorbed species over CeO2 after exposing in NO/O2 a and C3H6/O2 b for 40 min at various temperatures.. 80 Figure 6-4 DRIFTS spectra of adsorbed species o

Catalytic activity and reaction mechanism of

TOKYO INSTITUTE OF TECHNOLOGY

DEPARTMENT OF INTERNATIONAL DEVELOPMENT ENGINEERING

Nguyen Quang Long

Dissertation submitted in partial fulfillment of requirements

for a doctoral degree of engineering

Catalytic activity and reaction mechanism of

GRADUATE SCHOOL OF SCIENCE AND ENGINEERING TOKYO INSTITUTE OF TECHNOLOGY

2009

Table of Content

CHAPTER 1 INTRODUCTION 1

1.1 Outline 1

1.2 Background 1

1.2.1 Automobile exhaust 1

1.2.2 Emission limitations 2

1.2.3 Lean-burn engine and effectiveness of three way catalysts 4

1.3 Treatment of NOx from lean-burn exhaust 5

1.3.1 NOx storage reduction 5

1.3.2 Selective catalytic reduction of NOx 6

1.4 Gold as a catalyst 7

1.4.1 A brief history of gold catalysis 7

1.4.2 Potential uses of gold as a catalyst 9

1.5 Objectives of the research and the structure of this thesis 10

References 12

CHAPTER 2 REVIEW OF RELATED LITERATURE 13

2.1 Outline 13

2.2 Reduction of NOx by hydrocarbons 13

2.2.1 Zeolite-based as catalysts 13

2.2.2 Non-zeolitic catalysts 15

2.3 Mechanisms of the reduction of NOx by hydrocarbons 22

2.3.1 Adsorption-Dissociation mechanism 22

2.3.2 Oxidation-reduction mechanism 24

2.4 Gold catalysts for NOx reduction by hydrocarbons 25

References 28

CHAPTER 3 CATALYTIC ACTIVITY OF Au/TiO2 30

3.1 Outline 30

3.2 Experimental 30

3.2.1 Preparation and characterization of the catalysts 30

3.2.2 Measurement of the catalytic activity 31

3.3 Results and discussion 32

3.3.1 Influence of preparation conditions 32

3.3.2 Influence of TiO2 crystalline type on NOx reduction 36

3.3.3 Influence of gold loading levels 38

3.3.4 Influence of the feed concentrations 40

3.4 Conclusions 43

References 44

CHAPTER 4 EFFECT OF CeO2, Mn2O3 ON THE CATALYTIC ACTIVITY OF Au/TiO2 45

4.1 Outline 45

4.2 Experimental 45

4.2.1 Preparation and characterization of the catalysts 45

4.2.2 Measurements of the catalytic activity 46

4.2.3 Temperature program desorption experiments 46

4.3 Results and discussion 47

4.3.1 Effect of CeO2 addition 47

4.3.2 Effect of Mn2O3 addition 49

4.3.3 Catalytic activity for NO oxidation to NO2 50

4.3.4 Catalyst characteristics and NOx desorbed in TPD measurement 51

4.4 Conclusion 53

References 54

CHAPTER 5 REACTION MECHANISM OVER Au/TiO2 55

5.1 Outline 55

5.2 Experimental 55

5.3 Results 57

5.3.1 Formation of adsobed species during co-adsorption of reactants 57

5.3.2 Formation of adsorbed species during SCR reaction 63

5.3.3 Consumption of adsorbed species 69

5.4 Discussion of reaction mechanism 73

5.5 Conclusions 75

References 76

CHAPTER 6 MECHANISTIC STUDY ON THE EFFECT OF CeO2 AND Mn2O3 77

6.1 Outline 77

6.2 DRIFTS results and discussion on CeO2-added Au/TiO2 catalyst 77

6.2.1 Co-adsorption of reactants on CeO2 77

6.2.2 Effect of CeO2 on the formation of adsorbed species 80

6.2.3 Effect of CeO2 on the consumption of adsorbed species 86

6.2.4 Conclusions of the influence of CeO2 on the reaction mechanism 88

6.3 DRIFTS results and discussion on Mn2O3-added Au/TiO2 catalyst 89

6.3.1 Co-adsorption of reactants on Mn2O3 89

6.3.2 Effect of Mn2O3 on the formation of adsorbed species 91

6.3.3 Effect of Mn2O3 on the consumption of adsorbed species 95

6.3.4 Conclusions of the influence of Mn2O3 on the reaction mechanism 97

6.4 Summary of the effect of CeO2 and Mn2O3 98

References 98

CHAPTER 7 GENERAL CONCLUSIONS 99

ACKNOWLEDGEMENTS 101

LIST OF ORIGINAL PUBLICATIONS 102

List of Figures

Figure 1-1 The emissions regulation for heavy-duty diesel vehicle [7] 3 Figure 1-2 Fuel consumption and 3-way performance of a gasoline engine as a function

of air-fuel (A/F) ratio [10] 5

Figure 1-3 Possible mechanism of the NOx storage-reduction on NSR catalyst [10] 6

Figure 1-4 The numbers of published articles concerning gold catalysts in recent years

(Data from ISI) 8

Figure 2-1 Effect of H2O on NO conversion to N2 and N2O on Pt/Na–ZSM-5 [7] 14

Figure 2-2 NO conversion and selectivity for Ga2O3/Al2O3 and Ga-ZSM-5 in the absence

Figure 2-5 Classification of the cooperation effect of catalytic species (a) Multiple-stage

catalysts, (b) mechanical or physical mixture catalysts, and (c) multifunctional catalyst [21] 19

Figure 2-6 NO reduction conversion as a function of temperature mixtures of Ni–Ga

oxide with different amounts of Mn2O3 [27] 20

Figure 2-7 Conversion of NO to N2 over Au/Al2O3, CeO2, mechanical mixture, and dual bed Au/Al2O3-CeO2 or CeO2-Au/Al2O3 [29] 21

Figure 2-8 Proposed mechanism for C3H6-SCR over Pt-containing catalysts [39] 23

Figure 2-9 Proposed mechanism of C3H6-SCR over Cu/Al2O3 [43] 24

Figure 2-10 Temperature dependence of NO conversion to N2 over Al2O3 and gold supported on a variety of metal oxides [47] 26

Figure 3-1 Schematic diagram of the catalytic activity test set-up 32 Figure 3-2 XRD patterns of rutile support Ti6 and Au(1wt.%)/Ti6 with various mass

ratios PVA/Au 33

Figure 3-3 XRD patterns of Au/TiO2 with different TiO2 types and Au loading levels 34

Figure 3-4 TEM images of Au/TiO2 : Au(1wt.%)/Ti4 (a), Au(1wt.%)/Ti6 (b), Au(1wt.%)/Ti7 (c) 35

Figure 3-5 Size distribution of Au particles on different titania supports 35 Figure 3-6 The effect of titania type on the HC-SCR activity of Au/TiO2 37

Figure 3-7 The effect of Au content on the HC-SCR activity as a function of temperature.

NO conversion to N2 and (b) C3H6 conversion to CO2 48

Figure 4-2 Catalytic performance of the mechanical mixtures of Au/TiO2 and Mn2O3: (a)

NO conversion to N2 and (b) C3H6 conversion to CO2 49

Figure 4-3 Activity of the catalysts for the oxidation of NO to NO2 50

Figure 4-4 XRD patterns of singles and mixtures of 1%Au/TiO2 and MOx (M=Ce, Mn).

Figure 5-4 DRIFTS spectra of adsorbed species over Au/TiO2 after exposing in flow of NO/O2/He for 40 min at different temperatures 59

Figure 5-5 Comparison spectra of surface adsorbed species between Au/TiO2 and TiO2

after exposing to NO/O2/He for 40 min at 200 oC and 300 oC 60

Figure 5-6 DRIFTS spectra of adsorbed species over Au/TiO2 after exposing in flow of

C3H6/O2/He for 40 min at different temperatures 61

Figure 5-7 DRIFTS spectra of adsorbed species over TiO2 (a) and Au/TiO2 (b) after exposing in C3H6/O2/He and Au/TiO2 after exposing inC3H6/He (c) for 40 min at

200 oC 63

Figure 5-8 DRIFTS spectra of adsorbed species after 40 min in SCR reaction at different

temperatures 64

Figure 5-9 DRIFTS spectra of adsorbed species after 40 min in reaction condition over

TiO2 and Au/TiO2 66

Figure 5-10 DRIFTS spectra of adsorbed species during the SCR reaction at 200oC as a function of time 67

Figure 5-11 DRIFTS spectra of adsorbed species during the SCR reaction at 300 oC over TiO2, 0.1%Au/TiO2, and 1%Au/TiO2 for 5’ (a), 20’ (b), and 40’ (c) 68

Figure 5-12 DRIFTS spectra recorded over Au/TiO2 after flowing of C3H6/O2/He for 40 min followed by purging He for 20 min (a), then flowing of NO/O2/He 69

Figure 5-13 DRIFTS spectra recorded over Au/TiO2 after flowing of C3H6/O2/He for 40 min followed by purging He for 20 min (a), then flowing of NO/O2/He 70

Figure 5-14 DRIFTS spectra recorded over Au/TiO2 at 300 oC after flowing of

C3H6/O2/He for 40 min followed by purging He for 20 min then flowing of He (a),

O2/He (b), and NO2/He (c) 71

Figure 5-15 DRIFTS spectra recorded over Au/TiO2 after flowing of NO/O2/He for 40’ followed by purging He for 20’, then flowing of C3H6/O2/He 72

Figure 5-16 Schematic diagram of reaction mechanism over Au/TiO2 catalyst 75

Figure 6-1 DRIFTS spectra of adsorbed species over CeO2 after exposing in NO/O2 (a) and C3H6/O2 (b) for 40 min at various temperatures 78

Figure 6-2 DRIFTS spectra of adsorbed species over Au/Ti-Ce after exposing in NO/O2

(a) and C3H6/O2 (b) for 40 min at various temperatures 80

Figure 6-3 Comparison of DRIFTS spectra of adsorbed species over different samples

after exposing in NO/O2 (a) and C3H6/O2 (b) for 40 min at 200 0C 80

Figure 6-4 DRIFTS spectra of adsorbed species over Au-Ti-Ce after exposure to reaction

condition NO/C3H6/O2 for 40 mins at various temperatures (a) and at 200 oC in various reaction times (b) 82

Figure 6-5 Comparison of DRIFTS spectra of adsorbed species over different samples

after exposure to reaction condition NO/C3H6/O2 for 40 mins at 200 0C 83

Figure 6-6 Comparison of (-NCO) peak’s area on Au/TiO2 and Au/Ti-Ce at 250oC as a function of reaction time Reaction condition: NO: 1500 ppm, C3H6: 1500ppm, O2: 10% in He 84

Figure 6-7 Change of (-NCO) peak’s area on Au/TiO2 under streams of NO+O2or NO2 The sample was pre-exposed to the reaction mixture NO/C3H6/O2for 40 minutes followed by purging He for 20 minutes 85

Figure 6-8 DRIFTS spectra in (C-H) stretching region of adsorbed species over (a)

Au/TiO2, (b) CeO2 and (c)Au/Ti-Ce after exposing in C3H6/O2 for 40 mins followed

by purging He for 20 min, and then under flowing of NO/O2 for 5 mins or 40 mins

at 250 oC 87

Figure 6-9 DRIFTS spectra of adsorbed species over (a) Au/TiO2, (b) CeO2 and (c)Au/Ti-Ce after exposing in NO/O2 for 40 mins followed by purging He for 20 min, and then under flowing of C3H6/O2 for 5 mins or 40 mins at 250 oC 88

Figure 6-10 DRIFTS spectra of adsorbed species over Mn2O3 after exposing in NO/O2

(a) and C3H6/O2 (b) for 40 min at various temperatures 90

Figure 6-11 DRIFTS spectra of adsorbed species over Au/Ti-Mn after exposing in

NO/O2 (a) and C3H6/O2 (b) for 40 min at various temperatures 91

Figure 6-12 Comparison of DRIFTS spectra of adsorbed species over different samples

after exposing in (a) NO/O2 and (b) C3H6/O2 for 40 min at 200 0C 92

Figure 6-13 DRIFTS spectra of adsorbed species over Au-Ti-Mn after exposure to

reaction condition NO/C3H6/O2 for 40 mins at various temperatures (a) and at 200

0C for various reaction times (b) 93

Figure 6-14 Comparison of DRIFTS spectra of adsorbed species over over different

samples after exposure to reaction condition NO/C3H6/O2 for 40 mins at 200 0C 94

Figure 6-15 DRIFTS spectra of adsorbed species over (a) Au/TiO2, (b) Mn2O3 and (c)Au/Ti-Mn after exposing in C3H6/O2 for 40 mins followed by purging He for 20 min, and then under flowing of NO/O2 for 5 mins or 40 mins at 200 oC 95

Figure 6-16 DRIFTS spectra of adsorbed species over (a) Au/TiO2, (b) Mn2O3 and (c)Au/Ti-Mn after exposing in NO/O2 for 40 mins followed by purging He for 20 min, and then under flowing of C3H6/O2 for 5 mins or 40 mins at 200 oC 96

List of Tables

Table 3-1 The effect of PVA and pH in preparation of Au(1wt.%)/Ti6 by metal sol

method 33

Table 3-2 The actual Au loading by ICP and Au particle size of 1%Au/TiO2 by TEM 36

Table 4-1 Catalytic behaviors of the mechanical mixtures of Au/TiO2 and MOx 47

Table 4-2 Specific surface areas and amount of NOx desorbed from the catalysts 52

Table 5-1 Band assignment for adsorbed species on Au/TiO2 during adsorption NO/O258

Table 5-2 Band assignment for adsorbed species on Au/TiO2 during adsorption C3H6/O2

and reaction 62

Table 6-1 Band assignment for adsorbed species on CeO2 79

Table 6-2 Band assignment for adsorbed species on Mn2O3 90

CHAPTER 1 INTRODUCTION

1.1 Outline

This chapter describes the arms of the research presented in this thesis The information about background is presented in section 1.2 while brief introductions about treatment of NOx and gold catalysis are reported in section 1.2 and 1.3 Finally, objectives of the research and structure of this thesis are decribed in section 1.4

1.2 Background

1.2.1 Automobile exhaust

Treatment of air pollutants generated from mobile sources has been raised as a serious problem for urban air quality control nowadays The concern is due to the fact that the majority of engines employ combustion of crude oil derived fuels as a source of energy The non-perfect oxidation of the fuels into CO2 and H2O leads to the presence in

a significant amount of unburn hydrocarbon and partially combusted products such as aldehydes, ketones, and carboxylic acids, referred as HC thereafter, together with large amount of CO in the exhaust gas Additionally, at the high temperature in the combustion chamber N2 and O2 react to form nitric oxide (NO) The combination of NO and its oxidized form nitrogen dioxide (NO2) is refered to as NOx Therefore, three major primary pollutants in the automobile’s exhaust are NOx, HC, and CO [1]

The pollutants cause several environment and human-being related serious issues such as photochemical smogs and acid rains The origin of these smogs was two of the primary pollutants from cars (HC and NOx) They underwent photochemical reactions to generate ozone, a strong irritant, and other more noxious compounds, peroxyacetylnitrate (PAN) for instance [1] The other important ecological problem is the deforestation of the northern hemisphere by contamination Among various factors causing the deforestation, acid rain remains as a major contributor Although rain is generally slightly acidic, and its

pH value is around 5–6, acid rain pH has lower values, between 4 and 4.5 In this process,

nitrogen oxides play influential roles in the photochemistry of both troposphere and stratosphere [2] Additionally, automotive exhaust pollutants, such as CO, NOx, as well

as sulfur dioxide (SO2), volatile organic compounds (VOCs), and particulate matter (PM), have both acute and chronic effects on human health, affecting a number of different systems and organs It ranges from minor upper respiratory irritation to chronic respiratory and heart disease, lung cancer, acute respiratory infections in children and chronic bronchitis in adults, aggravating pre-existing heart and lung disease, or asthmatic attacks [3]

The environmental problems originated from the transportation have been gaining attention recently in Vietnam Transportation is the main contributor of carbon monoxide (CO) in Hanoi’s atmosphere, accounting for 95 % of total CO It also responsible to 10%

of sulfur dioxide (SO2) and about 35 % of NOx emission found in the city’s ambient air Air pollution from mobile sources has become an increasingly urgent problem and seriously affected since concentration of harmful pollutants (NOx, SO2, VOCs) was found many times higher than the national standards in some street intersections in 2004 [4] A similar situation was reported in Ho Chi Minh city Consequently, there was an increase

in the number of diseases that are usually related to air pollution, particularly in children such as asthma, pneumonia and otitis media according to a admission survey from 1996

to 2005 in a center children hospital [5]

1.2.2 Emission limitations

Starting from early 1970s, policies has gradually involved in various countries, especially Japan, in order to control the emission levels of the pollutants from the automobile’s exhaust The earliest exhaust gas regulation was implemented in Japan in

1978 In the European Union and the United States, the emission standard systems (named “Euro” and “Tier”, respectively) started from 1992 and 1994, correspondingly Step-by-step the regulations have become more severe For instance, in the European Union, the NOx emission limitation for the heavy-duty diesel vehicles has been reduced from 5 g/kWh in 2000 (EuroIII) to 2 g/kWh in 2008 (EuroV) The stricter NOx regulations, which started also from 2009 and 2010 for the corresponding vehicles, are 1

g/kWh and 0.27 g/kWh in Japan and the United States, respectively [6] The Figure 1-1

shows the emission regulations of NOx, and particulate matter (PM) in European Union,

Japan, and the United States In addition, the other stricter standards are in the progress

to be implemented in the near future in the developed countries such as EuroVI which

will mainly reduce NOx to 0.4 g/kWh in 2013 as proposed In developing countries,

however, lower NOx emission standards are currently applied In Vietnam, since the

number of vehicles has increased rapidly especially motorbikes, the use of EuroII

standard, which is also commonly applied in other Asian countries, for vehicle emission

control has been implemented from July 2007

Figure 1-1 The emissions regulation for heavy-duty diesel vehicle [7]

5 4 3 2 1 1 2 3 4 5

0.20 0.16 0.12 0.08 0.04

0.04 0.08 0.12 0.16 0.20

NOx

(g/kWh)

NOx(g/kWh)

USA EUROPE

JAPAN

1.2.3 Lean-burn engine and effectiveness of three way catalysts

The emission amounts of HC, CO, and NOx depends on the engine air-to-fuel (A/F) ratio, defined as: A/F= mass of air consumed by the engine/mass of fuel consumed by the engine For gasoline engine it can be calculated that the air-to-fuel ratio at stoichiometry, i.e where there is just enough air for complete combustion of all hydrocarbon in the fuel,

is 14.7 If the A/F ratio is below this value, and the engine operates under excess fuel conditions, giving raise to incomplete fuel combustion The exhaust gas will then contain more reducing reactant (HC, CO) than oxidizing reactants (O2, NO), and is called rich If the A/F ratio exceed 14.7, then the engine operates under excess air condition, giving raise to an exhaust gas that contains more oxidizing reactants than reducing reactants, and the exhaust gas composition is call lean [8]

A successful technology for automotive pollutant removal has been developed and commercialized as the automotive three-way catalysts (TWCs) They consist of platinum-group metals, especially platinum, palladium, and rhodium The TWCs are able to eliminate HC, CO, and NOx simultaneously, as the origin of their name However, the effectiveness of TWCs is limited to the four-stroke spark ignited-engine running at stoichiometry value of A/F ratio (A/F= 14.7), as shown in Fig.1-2 [9,10]

In automobile industry, in the concern of energy crisis and global warming, a higher fuel-efficiency engine that emits less CO2 is desired Lean-burn engine technology, which has been featured in vehicles since 1984, forces engine combustion to occur at high A/F ratios The normal 14.7:1 (stoichiometric) ratio produces exhaust gas that contains the right balance of CO, H2 and HC to reduce nitrogen oxide (NOx) and O2 Lean-burn engines, which operate at A/F ratios of 25:1 and above, effectively improve the fuel efficiency of the vehicles Three types of engines can effectively run under lean-burn, i.e diesel, four-stroke/lean-burn and two-stroke engine The use of diesel engines and gasoline engines at lean-burn condition has become the alternative to the conventional gasoline engine because the fuel burns more efficiently and its consumption is lower up

to 30% compared to the stoichiometric combustion [9,10] Another advantage of

lean-bun engines is the fact that the highest exhaust temperatures, which affect the catalyst durability, are typically lower compared to the stoichiometric engines [9]

Therefore, the developments of lean-burn gasoline and diesel engines are increasingly carried out in all over the world However, the presence of excess oxygen (5-15%) in the exhaust of lean-burn engine makes the application of TWCs for NOx removal impossible [9] Hence, NOx removal from lean exhaust stream remains one of the mayor challenges in environmental research problems, particularly to satisfy the stringent regulation for NOx emission

Figure 1-2 Fuel consumption and 3-way performance of a gasoline engine as a

function of air-fuel (A/F) ratio [10]

1.3 Treatment of NO x from lean-burn exhaust

There are two practical methods for converting NOx to N2 under lean conditions:

NOx storage reduction and selective catalytic reduction of NOx

1.3.1 NO x storage reduction

The NOx storage reduction method (NSR) was developed by Toyota researchers for the control of NOx emission under lean-burn conditions In this approach, the catalyst functions with alternatively lean and rich conditions Under lean conditions, excess NOx

is oxidized over Pt catalyst and then is stored in a form of nitrate (NO3-) at the surface of

Lean combustion

Ba-based storage material When the engine is switched to operation with the fuel-rich condition, the resulting exhaust becomes comparatively oxygen-deficient and HC, H2, and CO remains un-oxidized Thus, the three components react with the NO3- stored in the catalyst to form harmless nitrogen, water, and carbon dioxide as illustrated in Fig 1-3

However, the NSR method possesses some disadvantages [11,12] One serious problem of NSR catalyst is the deactivation caused by SO2 Reaction of SO2 and Ba-containing material forms undesirable stable barium sulfate Sulfur is contained in the fuel and in the engine lubricant Thus, although a low sulfur content fuel (< 10 ppm) can

be supplied by oil companies, the sulfur concentration in the lubricant (obligatory for its lubricating property) is large enough for a noticeable degradation of the system Another drawback of NSR system is the thermal deterioration which causes Pt sintering and more platinum oxide formed Additionally, in order to generate the alternatively lean and rich cycles, the car manufacturers much deal with specific engine control strategies

1.3.2 Selective catalytic reduction of NO x

The second lean NOx control technology is the selective catalytic reduction (SCR),

in which, reduction of NOx successfully competes with the reduction of O2, which presents in large excess

One approach to SCR method is using additional NH3 (SCR-NH3) as a reductant since the SCR of NOx with NH3 under excess oxygen condition is widely commercialized

technology for NOx removal from stationary sources For the removal of NOx from stationary sources, NOx is converted to N2 by reaction with NH3 over the V2O5/TiO2

catalyst

The application of SCR-NH3 for heavy-duty vehicles has been commercially available since 2006 in Europe For safety reasons, NH3 are provided in the vehicles on two precursor forms Solution of urea, which hydrolyzes to NH3, is known commercially

as “Adblue” containing 32.5% urea “Adblue” is corrosive and not suitable for round use in the Northern countries because the freezing point is at -11 oC Urea hydrolysis requires at least 180oC, and undesirable urea crystals or polymers may form if the exhause line temperature is lower than 300 oC [11] The other NH3 precursor is solid ammonium carbamate which leads to NH3 by decomposition The sublimation of solid blocks of ammonium carbamate is achieved by a local heater up to 60 oC By now, no car manufacturers have chosen this precursor perhaps due to the energy requested to reach the sublimation point, and the system response time

year-The supply and control the amount of NH3 in SCR-NH3 system are the most serious hindrances for broadly applying this technology Pure ammonia is an irritating and toxic gas which cannot be released in the exhaust line

Another approach to SCR method is SCR using hydrocarbons as reductants HC) Since Iwamoto et al.[13] and Held et al [14] reported that selective reduction of NOx by hydrocarbons proceeded under lean-conditions over Cu-ZSM-5 catalyst in early 1990s, the selective catalytic reduction of NOx by hydrocarbon has been studied extensively using metal oxide, noble metals, and also ion-exchange zeolite catalysts However, an efficient and commercially applicable catalyst has not been found

(SCR-1.4 Gold as a catalyst

1.4.1 A brief history of gold catalysis

Historically, gold was considered as an inert element and chemically inactive Gold has been used by jewelers to create some of the most beautiful objects throughout history

http://apps.isiknowledge.com), through the searching target as: Title= (gold OR Au) AND Topic= (catalyst OR catalysts OR catalysis OR catalytic OR catalytical OR catalytically

OR catalyse OR catalyze) The articles that contain the word “gold” or “Au” in the title

and at lease one of these catalyst-related words in the title, abstract, or keyword list are automatically counted in the numbers

Figure 1-4 The numbers of published articles concerning gold catalysts in recent

years (Data from ISI)

Figure 1-4 shows that the annual numbers of research papers related to gold catalyst increased after the discoveries in the late 1980s, especially very rapidly rose since 2002

When adding (NO OR NO x OR nitrogen oxide OR nitric oxide OR nitrogen monoxide OR

target, the total articles published from 1987 to 2008 were only 44, indicating very few researches on using gold catalyst for NOx reduction

1.4.2 Potential uses of gold as a catalyst

Supported Au catalysts can also catalyze many reactions other than CO oxidation The characteristic features of supported gold catalysts are: active at low temperatures, activated by moisture and often very selective [16,17] It has now been demonstrated that heterogeneous gold catalysts, when prepared in an appropriate manner, are highly active and selective for a number of reactions, often at lower temperatures than existing commercial catalysts

There is clearly the potential to apply catalysis by gold in practical commercial uses, most likely within four broad application areas: (1) pollution and emission control technologies; (2) chemical processing; (3) clean hydrogen production and fuel cell systems; (4) sensors to detect poisonous or flammable gases or substances in solution [17,18]

For automotive pollution control, compared to research focused on the use of platinum group metals (PGMs, such as Pt, Pd, Rh, etc), the science of gold catalysis is still very new and relatively underdeveloped One of the potential advantages that the use

of gold catalysts offer compared to other precious metal catalysts is lower cost and greater price stability, gold being substantially cheaper (on a weight for weight basis) and considerably more plentiful than platinum In research on SCR of NOx from lean exhaust,

Al2O3 supported gold catalysts were attempted in some studies for the reduction of NOx

by hydrocarbons [19-22] These studies have almost focused on propene as a reductant The advantages of gold-based catalysts for propene-SCR are their more excellent than PGMs based catalysts in the selectivity for N2 and the tolerance to the presence of water [23] However, the high activity of these catalysts was observed at considerably high temperatures (above 400 oC) In fact, the temperatures in diesel exhausts can be very low For example, it is known that under a European urban driving cycle, temperatures average 80–180 oC (maximum at 230 oC) while averages of 180–230 oC (maximum at

440 oC) in the extra-urban part of the cycle have been observed [9] Additionally, the sulfur poisoning is a serious problem of using Al2O3 support Therefore, further researches should be studied in order to understand the catalytic activity of gold-based catalysts on the SCR of NOx by hydrocarbons

1.5 Objectives of the research and the structure of this thesis

Two important conclusions that can be drawn from the above background are (1) the necessary of developing catalysts for NOx treatment of lean-burn engine exhaust and (2) the real potential of using gold based catalyst for the reduction of NOx by hydrocarbons

It is also evidenced that the more studies should be conducted in order to understand the catalytic activity of Au-based catalyst

This study, in general, aims to investigate the catalytic activity and the reaction mechanism of Au-based catalysts for the reduction of NOx by hydrocarbons over gold supported on a sulfur-resistant material, TiO2 Specifically, the objectives of the present research are as follows:

1 To prepare and test the catalytic activity of nano-sized Au/TiO2 for SCR of

NOx by propene

2 To investigate the effect of mechanical additions of CeO2 and Mn2O3 to

Au/TiO2 on the catalytic activity

3 To investigate the reaction mechanism of the SCR by propene over Au/TiO2

4 To understand the mechanistic effect of the additional CeO2 and Mn2O3

This thesis consists of seven chapters, and the summary of each chapter is given as follows:

Chapter 1

This chapter provides the background about the necessary of catalyst development for NOx removal from lean-burn exhaust and the potentials of using gold catalysts for the reduction of NOx by hydrocarbons The structure of this thesis is also given in chapter 1

Chapter 2

In this chapter, a survey of literature studies on the NOx treatment by SCR using hydrocarbons, the proposed reaction mechanism, and the application of Au-based catalysts are provided

Chapter 3

In this chapter, the preparation, characterization, and detail of catalytic activity test for SCR over Au/TiO2 are reported The chapter contains the results, discussion and conclusions about the reactivity of Au/TiO2

Chapter 4

This chapter describes the preparation, characterization, and detail of catalytic activity test for SCR over CeO2-added Au/TiO2 and Mn2O3-added Au/TiO2 catalyst are reported The chapter contains the results, discussion and conclusions about reactivity of these catalysts

Chapter 5

In this chapter, the reaction mechanism studied by Diffuse Reflectance Fourier Transform Infrared Spectroscopy (DRIFTS) over Au/TiO2 for SCR by propene is extensively reported The formation and consumption of surface adsorbed species is studied The reaction mechanism is discussed

Chapter 6

In this chapter, the explanations for the promotion in catalytic activity after the additions of CeO2 and Mn2O3 to Au/TiO2 are discussed based on the results of DRIFTS Some conclusions are drawn at the end of the chapter

Chapter 7

This final chapter provides general conclusions of the research

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[6] http://www.dieselnet.com, “emission standards”, accessed on March 16th, 2009

[7] Fischer, S., Rusch, K., and Amon, B Proceedings of “Asian Vehicle Emission Control Conference”, Beijing, China, (2004) 27-29

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[9] J Kaspar, P Fornasiero, N Hickey, Catal Today, 77 (2003) 419-449

[10] S Matsumoto, Cattech 4 (2000) 102-109

[11] P Granger (ed.), Past and present in DeNOx catalysis, Elsevier, 2007

[12] Z Liu, S I Woo, Catal Rev Sci Eng 48 (2006) 43-49

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CHAPTER 2 REVIEW OF RELATED LITERATURE

2.1 Outline

This chapter presents a literature survey related to this research work The survey consists of information about catalysts, which have been used for reduction of NOx by hydrocarbons (section 2.2), the proposed reaction mechanisms for this process (section 2.3), as well as gold related catalysts for this process (section 2.4)

2.2 Reduction of NO x by hydrocarbons

2.2.1 Zeolite-based as catalysts

It was independently reported in 1990s by Iwamoto et al [1] and Held et al [2] that the selective catalytic reduction of NOx with hydrocarbons in an excess of oxygen (HC-SCR) can be achieved over Cu-ZSM-5 Iwamoto and co-workers also classified the hydrocarbon reductants over Cu-ZSM-5 as selective (i.e., C2H4, C3H6, C3H8, and C4H8) and non-selective (CH4 and C2H6); and that the rate of reduction increases at higher hydrocarbon concentrations, and that unsaturated hydrocarbons are better reducing agents than the corresponding saturated species (i.e., C3H6 versus C3H8, etc.) [3]

Since then a vast number of papers dealing with HC-SCR on Cu-containing zeolites, mainly Cu on ZSM-5, mordenite and beta, have been published However, deactivation

of copper zeolite catalysts tends to be a problem due to the much longer lifetimes needed

in a real operation in a vehicle Another drawback is that the temperature range of high

NOx conversion starting at about 300 °C or above is relatively high as comparing with the average temperature of diesel exhaust gases [4]

The investigations of HC-SCR over Pt-containing zeolites have been studied by several groups [5,6] Iwamoto et al conducted a comparative investigation of Pt-ZSM-5, Cu-ZSM-5 and also Fe-mordenite for their performances in the NO reduction by C2H4 [5] Pt-ZSM-5 was found to be more active than the other two zeolites at low temperatures

However, the main disadvantage of using Pt-containing zeolites is the undesirable

N2O which was formed with very high selectivity instead of N2 during the NO reduction

by hydrocarbons as illustrated in Fig 2-1 [6,7] For example, selectivity towards N2O over Pt-ZSM-5 was very high and increased with increasing temperature, reaching nearly 100% at temperatures above 350 °C [7]

A remarkable catalyst for SCR by CH4 is Co-ZSM-5, which was firstly reported by

Li and Armor [8,9] They observed at a reaction temperature of 400 oC conversion of NO

to N2 reached 95% at cCH4/CNO =2.4 at a gas hourly space velocity 7500 h-1 In contrast to Pt-containing catalysts, where the main reduction product is N2O, on Co-containing catalyst neither N2O nor CO was detected The ability of CH4 activation of Co-containing zeolites makes them promising catalysts However, the sensitivity to H2O and SO2 and

the high temperatures necessary for reaction make them become difficult for the diesel exhaust application [10]

Moreover, many other metal-exchanged zeolites were attempted for HC-SCR such as

Pd, Ce, Ga, Ir, Fe, Ag, Ni, Mn, Rh containing zeolites or even combination of two or more metals However, the main obstacle to limiting the zeolite-based catalysts to practical application is their poor hydrothermal stability, although some preparation methods and pretreatment have been applied [11]

2.2.2 Non-zeolitic catalysts

2.2.2.1 Metal oxide catalysts

Metal oxides are another class of catalyst for SCR by hydrocarbons, although their activities are generally lower than zeolite-containing catalysts However, since metal oxides are more stable than zeolites in hydrothermal conditions, this class of catalysts can

be attractive candidates for practical use if further improvement of the activity is possible Many investigations, particularly in Japan, have been devoted to screening of metal oxide based catalysts A research in 1994 of Hamada et al [12] compared the activity of various metal oxides for this reaction and showed that Al2O3 is one of the most active metal oxides Low temperature activity of Al2O3 was reported to be enhanced by the addition of a small amount of transition metals, such as Co, Cu, Fe, Ag, In, Ga, Zn, and

Sn [13] However, this activity enhancement generally accompanies an increase in hydrocarbon oxidation activity, and hence the decrease of selectivity

The use of Ga2O3/Al2O3 exhibited higher NO conversion than Ga-ZSM-5 above

550 oC for SCR by CH4 It performed the highest activity and selectivity among the gallium oxide catalysts tested [14] This finding implied that the role of zeolite can be replaced by thermally stable oxide without loss of catalytic activity Furthermore,

Ga2O3/Al2O3 exhibits higher tolerance against water than Ga-ZSM-5 (Fig 2-2) This implies that the effect of water vapor depends not only on the nature of gallium itself but also on the nature of the support

○,‘: Before hydrothermal treatment

●,¡: After hydrothermal treatment

Cu-Transition metal-aluminate catalysts (Cu-, Ni- and Co-Al2O3) showed high activity and selectivity of SCR by C3H6 in dry conditions [15] These catalysts showed moderate activity even in the presence of 10% water Among the catalysts tested, Cu-Al2O3 with a rather high Cu content (16 wt%) exhibited the highest NO conversion at low temperatures It is remarkable that 16 wt% Cu-Al2O3 exhibited higher activity than Cu-ZSM-5 below 350 oC at a low level of O2 concentration (1%), as shown in Fig 2-3 After hydrothermal treatment, a decrease in crystallinity and a significant activity loss were observed for Cu-ZSM-5, while the activity decrease was much less for Cu-Al2O3 after the same treatment

Another metal oxide catalyst is Ag/Al2O3 which was reported as an attractive material for SCR with higher hydrocarbons As carbon number increased, the activity window of Ag/Al2O3 catalyst shifted to lower temperature region [16] For propane and n-butane, inhibition of water was observed over Ag/Al2O3 catalyst However, for n-hexane, the depression of water lessened Promoting the effect of water was even observed when n-octane was used In this case, the additional water vapor significantly reduced the surface coke which is one of major reason for deactivation, thus showing promoting effect The activity of Ag/Al2O3 catalyst, however, was usually inhibited by the presence of SO2 [17]

2.2.2.2 Platinum group metals (PGMs)

The poor activity at low temperature for SCR with hydrocarbons of metal oxide catalysts and zeolite-based catalysts can be improved by the use of platinum group metals (PGMs) since comparing with the two types of catalyst, noble metal catalyst is active at low temperature regions and relatively better resistance against SO2 compared to the metal oxide catalyst [18]

As can be seen in Fig.2-4, Pt/Al2O3 catalyst was active at about 250oC with a narrow temperature window However, the temperature window was shifted to higher temperatures and become wider over Ru/Al2O3 and Rh/Al2O3 catalysts Additionally, the activities of Pd/Al2O3 and Ir/Al2O3 were very poor [19] Therefore, among the PGM

Figure 2-4 Activities of different noble metal catalysts for the selective reduction of

NO x [19]

2.2.2.3 Combined catalysts

In order to satisfy the practical demand of catalysts for SCR with hydrocarbon, several researchers have studied on the combined catalysts in which the NOx reduction activity was enhanced by combination of two active catalytic species They could be classified as multiple-stage catalysts, mechanical or physical mixtures, or multifunctional catalysts as shown in Fig.2-5 [21,11]

(a) (b) (c)

Figure 2-5 Classification of the cooperation effect of catalytic species (a)

Multiple-stage catalysts, (b) mechanical or physical mixture catalysts, and (c) multifunctional catalyst [21]

In multiple-stage catalysts, the catalysts are placed in series to improve the activity for SCR Improving low-temperature activity and widening the reactive temperature range are the most important targets in catalyst development for diesel exhaust treatment Obuchi et al [22] found that Pt/Al2O3 showed a good NO reduction activity at low reaction temperatures around 250 °C, whereas Rh/Al2O3 was active at higher temperatures of about 350 °C Thus, they used a two-stage catalyst composed of a Rh/Al2O3 layer followed by a Pt/Al2O3 layer and found that a wider temperature range for

NO reduction was attained [23] Iwamoto et al used (Pt-MFI) as the first layer for oxidation of NO to NO2, subsequently NO2 reacted with intermediate added C2H4 over (In-MFI) of the second layer It was also reported by Li et al that if Pt/Al2O3 was packed

in the first reactor and In/Al2O3 was packed in the latter reactor, the NO conversion was significantly enhanced over the entire temperature region, especially in the low temperature range (200–400 oC) [24]

5 and CuO show higher catalytic activity for NO reduction with hydrocarbons than that

of each component catalyst under lean conditions [25] Misono et al [26] added mechanically various metal oxides to Ce-ZSM-5 to improve the catalytic activity for SCR

by propene They found out that mixture of Ce-ZSM-5 and Mn2O3 (and also CeO2) showed higher NO reduction activity than Ce-ZSM-5 Whereas, mechanical mixing with CuO or Cr2O3 decreased the activity of Ce-ZSM-5 The group proposed that Mn2O3 and CeO2 are served as NO oxidation catalysts, while CuO and Cr2O3 accelerate the undesirable oxidation reaction of C3H6

Figure 2-6 NO reduction conversion as a function of temperature mixtures of Ni–Ga

) ● Mechanical mixture CeO 2 -Au/Al 2 O 3

U Dual bed Au/Al 2 O 3 – CeO 2

‘ Dual bed CeO 2 -Au/Al 2 O 3

○ Au/Al 2 O 3

† CeO 2

The mixture of Mn2O3 and spinel Ni-Ga oxide resulted in a significant enhancement

of NO reduction activity in the low and medium temperature ranges as shown in Fig 2-6 [27] Ueda and Haruta [28] promoted the activity of Au/Al2O3 by following Misono’s procedure It was discovered that the mechanical mixture of Au/Al2O3 and Mn2O3 was much more active than Au/Al2O3 over the entire temperature range Recently, promotive effects of mechanically added CeO2 (and Mn2O3) on the NO reduction activity of Au/Al2O3 were reported by Niakolas et al [29] who attempted to improve the catalytic performance of Au/Al2O3 by using mechanical mixing method The combination of CeO2

and Au/Al2O3 by dual-bed (multiple-stage method), however, did not improve the catalytic activity as illustrated in Fig 2-7

dual bed Au/Al 2 O 3 -CeO 2 or CeO 2 -Au/Al 2 O 3 [29]

In multifunctional catalysts, two or more active species with different functions are presented in a catalyst, although it is sometimes difficult to differentiate the role of the species The addition of Rh to Ag/Al2O3 remarkably promoted the catalytic activity of

Ag/Al2O3 for NO reduction by decane as reported by Sato et al [17] The role of Rh is to form Agnδ+ clusters which contribute to the formation of NCO intermediates In another case, adding 0.01% Pd to Ag/Al2O3 improved the formation of enolic species which was very active to react with NO2 and NO3- to form intermediate compounds [30] Additional

Sn to Co/Al2O3 improved not only the thermal stability but also the water tolerance of the catalyst [31] The presence of Sn in Pt-Sn/Al2O3 prevented the sintering of Pt particles during the high temperature reaction [32] For Ga/Al2O3 catalyst, addition of ZnO by hydrothermal method contributed to the increase of acid sites over which hydrocarbons is activated Consequently, the catalytic activity was improved [33] This combination method was also able to broaden activity temperature window For example Pt/V/MCM-

41 exhibited wider temperature window than Pt/MCM-41[34]

2.3 Mechanisms of the reduction of NO x by hydrocarbons

The overall reaction mechanism of SCR by hydrocarbons is complicated and has not been fully elucidated for any given SCR system [35] However, a general figure of the most significant steps likely to occur during SCR can be drawn from a large amount of data published For understanding the reaction mechanism, mostly studied catalysts are Cu-ZSM-5 and Al2O3-based catalysts with propene as a reductant In general, the reaction mechanism of the SCR by hydrocarbons can be classified into two categories which are adsorption-dissociation and oxidation-reduction The reaction over Cu-ZSM-5 and also noble catalysts are usually regarded as happening by the adsorption-dissociation mechanism [11], while the second one was usually proposed for Al2O3-based catalyst [13]

2.3.1 Adsorption-Dissociation mechanism

In this mechanism, the adsorption of NO on the active metal sites and then dissociates into N(ads) and O(ads) The O(ads) reacts with hydrocarbon to form CO2 and two N(ads) combine to produce N2 Undissociated NO combines with N(ads) to form

N2O Since the dissociation occurs on Cu+ or Pt0 active sites [35,36], Cu-ZSM-5 and containing catalysts likely follow this route

be explained by the stability of dinitrosyl Cu(NO)2+ reaction intermediate, which will decrease as the temperature rises The role of zeolite in Cu-ZSM-5 catalyst is to provide appropriate chemical and physical environment for Cu active site Moreover, acidic sites

on zeolite may also activate hydrocarbons to generate partially oxidized hydrocarbon species [37] Burch and Millington [38] propose that the NO decomposition reaction occurs over Cu+ sites, and that the hydrocarbon reductant is responsible for maintaining a reasonable concentration of the active Cu+ sites during the course of the reaction

This mechanism firstly proposed for Pt/Al2O3 by Burch et al [39] The reactions on

Pt surface of Pt-contaning catalysts during NO reduction with propene are illustrated in Fig 2-8 In this scheme, N2 is formed by the coupling of two N(ads) species, while the O(ads) species react with adsorbed hydrocarbon-derived species and leave active sites for the following adsorption step This mechanism explains the fact that there is a large amount of N2O generated during NO reduction using Pt-containing catalysts The importance of the reducing agent is rapidly remove O(ads) formed during the dissociations of NO and also O2 to liberate active sites for other NO to be adsorbed and dissociated The mechanism for N2 formation under lean deNOx conditions has also been proposed by Cho [40] for Pt/ZSM-5

This mechanism firstly proposed for Al2O3 by Shimizu’s group [41] The reaction mechanism did not change for Ga/Al2O3 or Cu/Al2O3 as illustrated in Figure 2-9 [42,43] Among the oxygenates, acetate was the most active compounds to proceed the (NCO) or (CN) intermediates which then react with adsorbed NO3- to produce N2 and other products The kinetic data showed that the presence of Cu2+ ions promote the rate of actate formation, nitrate formation, nitrate reaction with C3H6 and acetate reaction with NO+O2 The acetate adsorbed on Cu2+-O site because the position of acetate bands was different with that on Al2O3 Furthermore, Cu2+ was found to be the adsorption sites for (NCO) compounds

Figure 2-9 Proposed mechanism of C 3 H 6 -SCR over Cu/Al 2 O 3 [43]

In the case of Ag/Al2O3, one of the roles of Ag is to promote the oxidation of NO to adsorbed NOx [44] which then resign on both Al2O3 and Ag The effect is so strong that it may cover overall catalyst surface leading to deactivation if it is saturated by NO [45] This deactivation did not observed in the case of Co/Al2O3 probably due to the formation

of strong bonded adsorbed NOx which do not readily migrate to the Al2O3 support Moreover, nitrates on Ag/Al2O3 do not all have the same activity The most reactive are monodentate species [46]

2.4 Gold catalysts for NO x reduction by hydrocarbons

Recently, gold-based catalysts have been the hotspot in the heterogeneous catalysis field due to its exceptionally high activities for low temperature CO oxidation However,

in contrast with the extensively studied on the low-temperature CO oxidation, NOx

reduction by hydrocarbons under excess oxygen conditions has been less studied over Au catalysts The first study using Au catalyst for NO reduction by propene was reported by Ueda et al in 1997 [47] Au supported on several metal oxides was prepared by deposition-precipitation methods to obtain Au nano particles highly dispersing on the supports The results were redrawn in Figure 2-10 in which the highest conversion to N2

obtained over Au/Al2O3 at 400 oC Changing the Au loading, the authors obtained the optimal gold loading was 0.17 wt% Over Au/Al2O3 the presence of water vapor did not inhibit the NO reduction but slightly increased the activity, although it was markedly depressed over Al2O3 alone

The activity of Au/Al2O3 depended on the Au particle size Deposition-precipitation method produced Au/Al2O3 at 0.82 wt.% Au with 4.9 nm Au particle size Whereas, the sample (1.1% Au/Al2O3) prepared by impregnation possessed bigger Au particle size, about 32 nm The former exhibited significantly higher the latter in NO reduction by propene as shown in Fig 2-10 Au supported on MgO,TiO2 α-Fe2O3 and ZnO were active at lower temperatures than that on Al2O3, especially Au supported on ZnO However, N2O was significantly over Au/ZnO catalyst and ZnO support is less thermally stable compared with other supports

supported on a variety of metal oxides [47]

Seker and Gulari studied the C3H6-SCR over Au/Al2O3 made by single step sol-gel method [48] The optimal gold loading in this study was 0.8 wt.% Moreover, the gold precusor in sol-gel method affected the NO reduction activity of the Au/Al2O3 and highest acitivity was obtained when using gold acetate as the gold precusor The research reported that the presence of 2 % water vapor in the feed reduced the N2 selectivity from 100% to 58% but increased the total NOx conversion Over Au/CeO2 prepared by deposition-precipitaiton with urea at 0.7 wt% Au, the maximum NOx conversion obtained

at 210 0C but with only 50% N2 selectivity [49] Over an acidic support (ZSM-5) and Au presented in particle size of 3-5 nm by deposition-presipitation method, the maximum

NO conversion obtained at very high temperature, 450 oC [50]

Addition of Mn2O3 to Au/Al2O3 by mechanical mixing appreciably improved the catalytic activity at low temperatures The best ratio was Mn2O3:Au (0.17 wt%)/Al2O3 = 1:19 in weight [28] With similar method, Niakolas et al investigated the effect of several metal oxides on the catalytic activity of 0.3 wt% Au/Al O for CH -SCR They found

out that activity enhancement when mechanically adding CeO2 to Au/Al2O3 was better than adding Mn2O3 [51] Wang et al studied the influence of CeO2 for NO reduction of Au/Al2O3 by depositing Au on a composite support CeO2/Al2O3 [52] The improvement

of activity at low temperatures was also observed on the Au/CeO2/Al2O3 catalyst

Mechanisms of C3H6-SCR by Al2O3 supported nano-sized Au catalysts were proposed in some publications [47,53] Ueda’s group suggested that the formation of NO2

by the oxidation of NO with O2 may be the first and slowest step followed by the reaction

of NO2 with C3H6 In-situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) was used in research of Bamwenda et al over Au/γ-Al2O3 catalyst [53] They reported that the oxidation of NO to NO2 is a prerequisite step followed by coupling of the NO2 or its adspecies (NOx-) with activated C3H6 on active site on Al2O3 to form

CnHmNxOy species, such as –NCO or -CN, which are responsible for the propagation step The important of NO2 was also reported by the observation of a significant enhancement

in NO reduction when adding Mn2O3 to Au/Al2O3 [28]

On the other hand, the mechanism of promotive effects of additional CeO2, which is much less active for the oxidation of NO to NO2, was proposed by Niakolas et al Based

on their experimental results, the absence of an intimate contact of CeO2 (and also

Mn2O3) with Au/Al2O3 made the synergistic bahaviors impossible Indeed, the contact is needed for surface mobile (‘spillover’) species from one phase to another phase, and control the number or kind of it catalytically active sites In this case, the key intermediate compounds should be unstable, short-lived, and not NO2 molecule in the gas phase [51]

In summary, one advantage of Au-based catalysts for NO reduction by hydrocarbons

is their higher N2 selectivity than counterpart Pt catalysts Whereas, the reactive temperatures of Au/Al2O3 which has been focused by many researchers was quite high, above 400 oC, while the application for lean-burn exhaust, especially diesel engine exhaust, require much lower temperatures In order to improve the low temperature activity, metal oxides, typically Mn2O3 and CeO2, were added to the Au/Al2O3, and enhancement in catalytic activity of the catalysts was obtained

However, using Al2O3 may cause deactivation by SO2, which presents also in the engine exhaust, because Al2O3 can adsorb irreversibly SO2 which might reduce catalytically active centers of the catalyst On the other hand, the desorption of SO2

adsorbed on TiO2 is easier making TiO2 a sulfur-resistant support [54] Moreover, the activity for NO reduction over Au/TiO2 was higher than that of Au/Al2O3 at temperatures lower than 350 oC with similar wt% Au loading (Fig 2-10) However, an intensive study

of Au/TiO2 for SCR has not been reported yet Therefore, the investigation of catalytic activity and reaction mechanism over Au/TiO2 for NO reduction by propene will be studied in this research

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