Development of novel deNPAC catalysts for treatment of diesel engine exhaust

It has been found that compared with SBA-15 impregnated with CuO particles, Cu/SBA-15 has improved the catalytic activity of pyridine oxidation with a lower NOx yield, which may be due t

DEVELOPMENT OF NOVEL deNPAC CATALYSTS FOR

TREATMENT OF DIESEL ENGINE EXHAUST

ZENG HOUXU (M ENG, RIPP)

A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING

DEPARTMENT OF CHEMICAL & BIOMOLECULAR ENGINEERING

NATIONAL UNIVERSITY OF SINGAPORE

2005

Acknowledgement

First of all, I would like to express my sincerest thanks to my supervisors, Prof Sibudjing Kawi and Dr Yu Liya, for their unselfish help throughout all my master candidate period I appreciate their constant encouragement and invaluable guidance Their profound understanding of catalysis and environment helped me a lot whenever I met problems during my research Besides, I would like to express my thanks to Prof Hidajat, who gave me many constructive suggestions

I also want to take this chance to thank all our group members who share the laboratories and gave me a lot of help, Dr Shen Shoucang, Yong Siekting, Zhang Sheng, Tang Yunpeng, Luan Deyan, Li Peng, Song Shiwei, Yang Jun, Sun Gebiao They are all my best friends and teachers From them, I learned not only technical knowledge but also personality The time I spent with them will give me indelible good memory

Special mentions should go to Mdm Chiang Hock Joo, Mdm Siew Woon Chee and Mr Ng Kim Poi for all the help they have offered throughout my research My thanks also should give to Mdm Fam Hwee Koong, Dr Yuan Zeliang, Mr Chia Phai Ann and Mr Shang Zhenhua

The last, I also wish to thank National University of Singapore for providing

me excellent environment and abundant resources of research

This thesis is dedicated to my family for their encouragement and support

Table of Contents

Acknowledgement i

Table of Contents ii

Summary v

Nomenclature vii

List of Figures ix

List of Tables and Scheme xii

Chapter 1 Introduction 1

1.1 Background 1

1.2 Nitrogen-containing polycyclic aromatic compounds (NPACs) 2

1.2.1 NPACs in the atmosphere 2

1.2.2 NPACs in diesel exhaust 3

1.3 Research objectives 4

1.4 Organization of thesis 5

Chapter 2 Literature Review 7

2.1 Introduction to catalytic combustion of NPACs 7

2.1.1 Oxidation of ammonia 8

2.1.2 Oxidation of hydrogen cyanide 9

2.1.3 Oxidation of organic nitrogen-containing compounds 11

2.2 Introduction to the catalytic oxidation of pyridine 12

2.2.1 Pyridine oxidation over metal oxide catalysts 12

2.2.2 Catalytic supercritical water oxidation of pyridine (SCWO) 15

2.2.3 Kinetics of catalytic oxidation of pyridine 17

2.2.4 Intermediates and mechanism of pyridine oxidation 18

2.3 Ordered mesoporous SBA-15 material 19

2.3.1 Synthesis and formation mechanism of SBA-15 21

2.3.2 Modification of SBA-15 24

2.3.3 Highly dispersed nanoparticles on SBA-15 26

Chapter 3 Synthesis, Characterization and Application of SBA-15 Modified with nano Cu Particles 28

3.1 Preface 28

3.2 Introduction 28

3.3 Experimental techniques 30

3.3.1 Chemicals 30

3.3.2 Synthesis of pure SBA-15 and modified SBA-15 30

3.3.3 Characterization 31

3.3.4 Catalytic activity 32

3.4 Results and discussion 33

3.5 Conclusions 45

Chapter 4 Design Cu-containing Catalysts for deNPAC 46

4.1 Preface 46

4.2 Introduction 46

4.3 Experimental techniques 48

4.3.1 Synthesis of CuO/SBA-15, CuAl/SBA-15 and CuOAl/SBA-15 48

4.3.2 Characterization 49

4.3.3 Catalytic activity test 50

4.4 Results and discussion 51

4.5 Conclusions 64

Chapter 5 Synthesis, Characterization and Application of Cu-Zn-Al Spinel-Structured Catalysts for deNPAC Reaction 65

5.1 Preface 65

5.2 Introduction 65

5.3 Experimental techniques 67

5.3.1 Catalyst preparation 67

5.3.2 Characterization 68

5.3.3 Catalytic activity test 69

5.4 Results and discussion 70

5.4.1 Effects of catalysts preparation methods 70

5.4.2 Effects of molar ratio 73

5.4.3 Effects of hydrothermal treatment temperature 77

5.5 Conclusions 88

Chapter 6 SBA-15 Embedded with Spinel: Synthesis and Application 89

6.1 Preface 89

6.2 Introduction 89

6.3 Experimental techniques 91

6.3.1 Chemicals 91

6.3.2 Synthesis of modified SBA-15 91

6.3.3 Characterization 92

6.3.4 Catalytic activity test 93

6.4 Results and discussion 93

6.5 Conclusions 103

Chapter 7 Conclusions and Future Work 104

7.1 Conclusions 104

7.2 Future Work 106

References 108

Summary

This thesis reports the study of catalytic oxidation of pyridine in the presence of excess oxygen Four catalysts: SBA-15 modified with nano Cu (Cu/SBA-15), SBA-15 modified with Cu and Al (CuAl/SBA-15), Cu-Zn-Al spinel and SBA-15 modified with Cu-Zn-Al spinel (SBA/SP-x), have been synthesized

It has been found that compared with SBA-15 impregnated with CuO particles, Cu/SBA-15 has improved the catalytic activity of pyridine oxidation with a lower NOx yield, which may be due to nano Cu particles on Cu/SBA-15 have a smaller particle size and a better distribution, yet the performance of Cu/SBA-15 catalyst at high temperatures still needs to be improved

In order to investigate the influence of acidic property and active component on the catalytic activity and NOx yield, CuAl/SBA-15 catalyst has been designed For the pyridine oxidation reaction, Al provides acidic sites to adsorb the reactant and Cu is the source of active component to control the yield of NOx By improving the acidic properties of the catalyst, the pyridine adsorption ability of catalyst can be enhanced The NOx yield is controlled by Cu ion loaded by an ion-exchange method Compared with CuO/SBA-15 and CuOAl/SBA-15, CuAl/SBA-15 has a better catalytic activity of pyridine oxidation and a lower NOx yield In addition, CuAl/SBA-15 is more easily prepared than Cu/SBA-15 But its NOx control ability at high temperatures is still not good enough

Cu-Zn-Al spinel has been chosen in this experiment because of its excellent thermal and hydrothermal stability The influence of preparation method, molar ratio

of precursors and hydrothermal treatment temperature on the morphology of spinel has been investigated Spinel prepared by hydrothermal method at pH=8 with a molar ratio

of Cu:Zn:Al=1:1:4 has the optimal catalytic activity and a lower NOx yield Calcination is a necessary step for Cu-Zn-Al spinel prepared by the hydrothermal method, but too high a calcination temperature will decompose the spinel Spinel prepared at 180°С has the best reactivity

Finally, a novel catalyst combining the advantages of SBA-15 and spinel has been prepared successfully Spinel was embedded into SBA-15 by a precipitation method Two diameter Cu-Zn-Al spinel particles have been observed: one at 15 nm, the other below 7 nm The best modified catalyst with the lowest NOx yield even at high temperatures is SBA/SP-5, which achieves 100% pyridine conversion at 450°C with zero NOx formation

Keywords: Cu/SBA-15, CuAl/SBA-15, Cu-Zn-Al spinel, SABA/SP, pyridine oxidation, NOx yield

Nomenclature

°C Degree Centigrade

Å angstrom

DTA differential thermal analysis

FESEM Field Emission Scanning Electron Microscopy

FETEM Field Emission Transmission Electron Microscopy FTIR Fourier Transform Infrared Spectroscopy

ppm part per million

TEM Transmission Electron Microscopy

vol Volume

wt Weight

XPS X-ray Photoelectron Spectroscopy

Si/Al atom ratio of Si and Al

Cu/SBA-15 SBA-15 material modified with nano Cu particles

CuO/SBA-15 SBA-15 material modified with CuO particle

Al/SBA-15 SBA-15 material incorporated with Al

CuAl/SBA-15 AlSBA-15 material ion-exchanged with Cu

CuOAl/SBA-15 AlSBA-15 material modified with CuO

Cu-Zn-Al spinel CuO, ZnO and Al2O3 oxide complex

SBA/SP-x SBA-15 modified with Cu-Zn-Al spinel, here x is the

weight ratio of CuO in spinel to SBA-15

Fig 3.9 Pyridine conversion on SBA-15, Cu/SBA-15 and CuO/SBA-15 44 Fig 3.10 NOx yield on SBA-15, Cu/SBA-15 and CuO/SBA-15 45 Fig 4.1 Low angle XRD patterns of SBA-15, CuO/SBA-15, CuOAl/SBA-

Fig 4.3 TEM images of SBA-15,CuAl/SBA-15 and CuO/SBA-15 55

Fig 4.4 N2 adsorption-desorption isotherms of SBA-15, CuO/SBA-15,

Fig 4.5 Pore size distribution curves of SBA-15, CuO/SBA-15,

Fig 4.6 XPS spectra of Cu for CuO/SBA-15, CuOAl/SBA-15 60 Fig 4.7 FTIR spectra of pyridine adsorption on CuO/SBA-15,

CuOAl/SBA-15 and CuAl/SBA-15

61

Fig 4.8 FTIR spectra of pyridine adsorption at different temperature on

Fig 4.10 NOx yield on Cu-containing SBA-15 catalysts during pyridine

oxidation at different temperatures

64

Fig 5.1 XRD patterns of Cu-Zn-Al catalysts prepared by different

methods: sample(1) with wet impregnation; sample(2) with

hydrothermal method; sample(3) with precipitation

71

Fig 5.2 The effect of preparation methods on pyridine conversion 72 Fig 5.3 The effect of preparation methods on NOx yield 73

Fig 5.4 XRD patterns of Cu-Zn-Al spinel prepared with different molar

ratio: (a) spinel(0.5:1.5:4); (b) spinel(1:1:4); (c) spinel(1.5:0.5:4) 74 Fig 5.5 The effect of molar ratio on pyridine conversion 76 Fig 5.6 The effect of molar ratio on NOx yield 76

Fig 5.7 XRD patterns of Cu-Zn-Al spinel synthesized by the hydrothermal

method at different reaction temperatures: (1) Zn-Al hydroxides;

(2) γ-AlO(OH); (3) Cu-Zn-Al spinel

78

Fig 5.8 TEM images of the Cu-Zn-Al spinel synthesized by the

hydrothermal method at different temperatures: (a)25°C;

(b)100°C;(c) 180°C; (d)225°C; (e)275°C

80

Fig 5.9 DTA curves of the Cu-Zn-Al spinel synthesized by the

hydrothermal method at different temperatures

81

Fig 5.10 TGA curves of the Cu-Zn-Al spinel synthesized by the

hydrothermal method at different temperatures

81

Fig 5.11 In-situ XRD patterns of the Cu-Zn-Al spinel prepared by the

hydrothermal method at 25°C Here (1) is Zn-Al hydroxides; (3) is

Cu-Zn-Al spinel; (4) is Al2O3

82

Fig 5.12 In-situ XRD patterns of the Cu-Zn-Al spinel prepared by the

hydrothermal method at 100°C

83

Fig 5.13 In-situ XRD patterns of the Cu-Zn-Al spinel prepared by the

hydrothermal method at 180°C Here (2) is γ-AlO(OH)

83

Fig 5.14 In-situ XRD patterns of the Cu-Zn-Al spinel prepared by the

hydrothermal method at 225°C

84

Fig 5.15 In-situ XRD patterns of the Cu-Zn-Al spinel prepared by the

hydrothermal method at 275°C

84

Fig 5.16 XRD patterns of the Cu-Zn-Al spinel prepared by the hydrothermal

method and calcinated at 750°С

Fig 6.1 Small angle XRD patterns of (A)SBA-15, (B) SBA/SP-50, (C)

SBA/SP-10, (D) SBA/SP-5, (E) SBA/SP-1

94

Fig 6.2 Large angle XRD patterns of (A) SBA/SP-50, (B) SBA/SP-10, (C)

SBA/SP-5, (D) SBA/SP-1, (E) spinel

95

Fig 6.3 N2 adsorption-desorption isotherms for SBA/SP-50, SBA/SP-10,

SBA/SP-5 and SBA/SP-1

97

Fig 6.4 Pore size distribution curves for 50, 10,

Fig 6.5 FETEM and XRD results of SBA/SP-5 after the complete removal

of the silica framework (a) A TEM image of spinel nanoparticles; (b) A FETEM image of the crystalline nanoparticle structure with a

SAED pattern in the inset; (c) A XRD pattern of spinel nanoparticles

List of Tables and Scheme

Table 1.1 The exhaust composition of diesel and Otto engines 1 Table 2.1 Preparation of mesoporous silica supported metal and metal

carbonyl nanoparticles and their application for catalysis

27

Table 3.1 Structural parameters of SBA-15, CuO/SBA-15 and

Cu/SBA-15 derived from XRD and BET

39

Table 4.1 Structural parameters of Cu-containing SBA-15 catalysts

derived from XRD and BET

Table 6.1 Structural parameters of SBA-15, SBA/SP-50, SBA/SP-10,

SBA/SP-5 and SBA/SP-1 derived from XRD and BET 98

Scheme 3.1 The protocol for the formation of Cu nanoparticles in the

channels of SBA-15 mesoporous material

42

Chapter 1 Introduction

At present, air pollution has been one of the main problems, because it influences the daily life of everyone As a major source of air pollution, vehicle exhaust is the problem of most concern of government and society Table 1.1 shows the exhaust composition of diesel engine and Otto engines, which are the main types of gasoline engines From Table 1.1, we can see that although the concentration of nitrogen oxide, hydrocarbon, and carbon monoxide from diesel engines are comparably less than those from Otto engines, diesel engines are the major source of particulate emissions

Table 1.1 The exhaust composition of diesel and Otto engines*

Catalytic automotive exhaust after treatment (Koltsakis and Stamatelos, 1997).

In 1977, the EPA (Environment Protection Association of USA) first identified diesel exhaust particulates as mutagens Since Nitrogen-containing Polycyclic

Aromatic Compounds (NPACs) can easily deposit on exhaust particulates, they are responsible for up to 90% of the total mutagenicity caused by diesel particulates and as these particulates can be easily inhaled to deeper areas of the lungs, they are considered to be a cause of increasing levels of lung cancer (Wei and Shu, 1983; Rosenkranz et al., 1986)

Reducing nitrogen oxides and particulate matters to nontoxic materials, such as N2, CO2, H2O, is the hottest topic The most convenient method at present is the selective catalytic reduction utilized to treat the nitrogen oxides in diesel exhausts, while particulate matter is treated via oxidation reactions However, little effort has been given to reduce NPACs in diesel exhausts

1.2 Nitrogen-containing polycyclic aromatic compounds (NPACs)

1.2.1 NPACs in the atmosphere

The main reason why NPACs caused attention is due to their capacity to induce direct mutagenic activity and their contribution for 10% to the total mutagenicity of inhaled suspended particles in polluted areas (Arey et al., 1988) The source of NPACs

in the atmosphere comes from two ways: one is through a reaction between polycyclic aromatic compounds and nitric oxides in air (Pitts et al., 1985), the other is from engine or plant exhaust Until now, researchers have found NPACs in diesel exhaust particles (Newton et al., 1982; Schuetzle et al., 1983), gasoline exhaust (Rosenkranz et al., 1982; Alsberg et al., 1985), coal-burning power plants (White et al., 1985), cigarette smoke (White et al., 1985) and air particulate matter (Wang et al., 1978; Oehme et al., 1982)

1.2.2 NPACs in diesel exhaust

So far, at least 60 specific NPACs, such as nitropyrene, nitronaphthalene, nitrobiphenyl, nitrofluorene, nitrophenanthrene, nitroanthracene and nitrofluoranthene have been identified in diesel exhaust (Schuetzle et al., 1983) Although NPACs have been identified as potential mutagens and possible carcinogens, they are not listed in the emission regulations due to their low concentration in the diesel engine exhausts The concentration of NPACs in diesel exhaust is dependent on the fuel type, the engine type and the engine working conditions For example, the concentration of 1-nitropyrene, the most abundant NPAC in diesel emission extracts, is between 14 and

2280 ppm (Schuetzle et al., 1982; Schuetzle et al., 1983)

Although some research has been done to determine the concentration of NPACs, little effort has been done on deNPAC But we can say that, to meet the increasing demand of air quality and environmental protection, it is necessary to perform some studies about how to remove NPACs from diesel emissions

To understand the catalytic decomposition of NPACs from the exhaust of diesel engines, pyridine has been chosen as the model component to be analyzed The reason is that not only the catalytic oxidation of pyridine has been investigated extensively, but also that it is easier to be handled Furthermore, pyridine is also the main form of fuel-nitrogen in liquid fuels made from coal If future liquid fuels are to

be made from coal, an understanding of fuel-nitrogen oxidation is a fundamental necessity for the development of strategies to control nitric oxide emission The experiments to be conducted in our studies will be carried out under excess oxygen and

the reaction temperature will be changed from room temperature to 700°C to simulate the operation of diesel engines

1.3 Research objectives

The purpose of this research is to explore the behavior of catalytic oxidation of NPACs and design novel catalysts for deNPAC reaction to adapt more stringent automobile emission legislation This research aims to design novel catalysts having a high pyridine oxidation and a low NOx yield under the operational conditions with excess oxygen from room temperature to 650°C In this study, Cu was selected as the basic active component, because Cu-based catalysts were found to be the most promising catalysts for catalytic oxidation of nitrogen containing compounds (Luo et al., 1985; Zhou et al., 2004) In order to obtain the best activity of catalysts with the lowest amount of the active component, SBA-15 was chosen as the support of catalyst due to its outstanding properties, such as large and uniform pores, thick pore walls and

a high surface area

Some research has been performed to study the catalytic combustion of pyridine in order to remove pyridine from the exhaust of coal or fuel combustion The combustion of pyridine was studied in the temperature range of 675 to 775 °C (Houser

et al., 1982) The nitrogen products present in the reactor effluent include N2, N2O, NOx, and HCN These results showed the presence of undesirable toxic reaction by-products, such as NOx and HCN The catalytic oxidation of pyridine on several catalysts, including Pt, CuCr2O4, NiCr2O4, and CuO supported on γ-Al2O3, has also been reported (Ismagilov et al., 1983a; Ismagilov et al., 1983b; Ismagilov et al., 1990)

At temperatures between 240 and 550 °C, reaction products were found to include

carbon dioxide, water, nitrogen, and NOx, with the yield of NOx increasing with an increase in the reaction temperature They also reported the formation of hydrogen cyanide and complete conversion of pyridine The yield of NOx decreased when metal oxide catalysts were used instead of Pt catalyst

Although most studies mentioned above were carried out over γ-Al2O3 or metal oxide supported catalysts, one of the main problems with the conventional γ-Al2O3 is that it can not disperse the active component efficiently due to the relatively low surface area of γ-Al2O3 However, SBA-15, which is a mesoporous silicate material, has been shown to be a suitable support for metals and metal oxides This mesoporous material possesses a high specific surface, a large specific pore volume and a highly ordered pore structure with a narrow size distribution In addition to having these advantages as the catalyst support, the mesoporous framework may also control the particle size of active component by limiting the growth of the clusters introduced into the confined space of the channels The main objective of this thesis is to investigate whether one can combine Cu active component and SBA-15 support together in order

to obtain active catalysts for pyridine oxidation In this experiment, some nitrogen oxides (NOx, including N2, NO, and NO2) are expected to be present as the reaction products during pyridine oxidation Since NOx is poisonous, it must be removed from the product gas as well Thus the catalysts that we need to develop in this thesis should fulfill two important tasks: oxidize pyridine completely at low reaction temperatures and control NOx emissions from the reaction to be as low as possible

Chapter 2 Literature Review

2.1 Introduction to catalytic combustion of NPACs

A particular characteristic of oxidation of nitrogen-containing compounds is

that both molecular nitrogen and nitrogen oxides may be formed (Luo et al., 1998a):

RN + O2 → CO2 + H2O + N2 (2.1)

RN + O2 → CO2 + H2O + NOx (2.2)

It is well known that nitrogen oxides – NOx (NO and NO2) are the most common and

dangerous pollutants in the atmosphere These compounds have overall toxic and lung

irritating effects In most industrially developed countries, very strict ambient air

quality standards have been established to eliminate NOx Therefore, for catalytic

combustion, it is important to achieve a low yield of NOx formed via reaction 2.2 at the

expense of reaction 2.1 To understand this reaction and achieve this target, many

studies have been performed on the catalytic oxidation of various types of

nitrogen-containing compounds, such as NH3, HCN and some organic nitrogen-nitrogen-containing

compounds In general, NOx yield in the oxidation of nitrogen containing compound is

determined by the nature of catalysts, the concentration and the nature of nitrogen-

containing compounds, the concentration of oxygen and the reaction temperature (Luo

et al., 1998a)

2.1.1 Oxidation of ammonia

Catalytic oxidation of ammonia has been intensively studied, due to its great

importance for industrial applications (Golodets et al., 1971; Karaiev et al., 1976) The

reaction may proceed via three main steps:

2NH3 + 1.5O2 → N2 + 3H2O (2.3) 2NH3 + 2O2 → N2O + 3H2O (2.4) 2NH3 + 2.5O2 → 2NO + 3H2O (2.5) All these reactions are practically irreversible For many catalysts in the temperature

range of 400-500°C, all three nitrogen-containing products (i.e., N2, N2O and NO) are

formed in various ratios, whereas on selective catalysts under certain conditions, only

one of the three processes predominates Catalytic ammonia oxidation has been carried

out over metal and metal oxide catalysts For metal catalysts, taking platinum as an

example, when a stoichiometric mixture of ammonia and oxygen were added,

molecular nitrogen is the predominant product at low temperatures (i.e at 150 to 400

℃) When the temperature is increased, N2O appears as the reaction product, its yield

passing through a maximum as the temperature is increased, while NO is the major

product above 400 ℃ (Gland and Korchak, 1978; Bradley et al., 1995)

A reaction mechanism of ammonia oxidation has been proposed by Mieher and

Ho (1995) and Sobczyk et al (2004) and The dissociation of ammonia is promoted by

the presence of atomic oxygen on the surface of the catalyst (* is the active site on the

NH2(a) + O(a) → NH(a) + OH(a) (2.9)

NH(a) O(a) → N(a) + OH(a) (2.10)

Conditions of excess oxygen yield more NO than N2 The N atoms are found to be the mobile species for the formation of NO and N2 The main nitrogen-containing products production mechanism is proposed by Kilpinen and Hupa (1991), and Mieher and Ho (1995):

N + O → NO (2.11)

instead of

NH + O → NO + H (2.12) Meanwhile, the formation of molecular nitrogen is described as follows:

2N → N2 (2.13) The concentration of nitrous oxide was much less and it was formed via the following steps:

NH + NO → N2O + H (2.14)

Although some parts of ammonia oxidation are still not very clear, the general conclusion is that the products depend on the reaction temperature, the concentration

of ammonia and oxygen, and the properties of the catalysts

2.1.2 Oxidation of hydrogen cyanide

Compared to ammonia oxidation, the catalytic oxidation of hydrogen cyanide has been studied to a much lesser extent Neumann et al (1929) studied the catalytic oxidation of HCN with excess oxygen and reported some data Depending on the

reaction conditions, the products of the reaction are either cyanic acid, HCNO, or

molecular nitrogen, or nitrogen oxides:

2HCN + 2.5O2 → N2 + 2CO2 + H2O (2.16) 2HCN + 3.5O2 → 2NO + 2CO2 + H2O (2.17)

Hydrogen cyanide oxidation over platinum gauze started at 450°C and

achieved a complete conversion at 500°C, when the formation of N2 and NOx occurred

As the temperature was increased up to 900°C, the NO yield went through a maximum

that corresponded to 700°C Neumann et al (1929) explained this phenomenon by the

following reaction:

2HCN + 5NO → 2CO2 + 2H2O + 3.5N2 (2.18)

Kilpinen and Hupa (1991) and Wargadalam et al (2000) pointed out that at

high temperatures, the main reaction route of HCN oxidation was:

For the oxidation of HCN, the influence of temperature is very obvious It seems that

high temperatures can enhance the formation of N2

2.1.3 Oxidation of organic nitrogen-containing compounds

Untill now, the published studies dealing with the oxidation of organic nitrogen-containing compounds focus on the choice of catalysts and the conditions for pollutant abatement by catalytic oxidation Ismagilov and Kerzhentsev (1990 and 1999) performed some study on the catalytic oxidation of various types of nitrogen- containing compounds (i.e pyridine, nitromethane acetonitrile and dimethylformamide) The kinetic parameters of the nitrogen-containing compound oxidation reactions and those of NOx formation have been investigated They found that the nature of the catalyst and its quantitative and qualitative composition have an important effect on the yield of NOx They concluded that oxide metal catalysts produced much less NOx during the oxidative reaction of nitrogen-containing compounds as compared to noble metal catalysts They thought that the parameters of the process also have a strong effect on NOx formation The concentration of fuel NOx can be decreased by operating at a low excess of air and a low temperature

The kinetics of catalytic oxidation of n-butylamine has been studied over Pt/HM and Fe-Mn/HM catalysts by Luo et al (1996) The reaction was zero order in the presence of excess oxygen The catalytic activity of Fe-Mn/HM was higher than that of Pt/HM For both catalysts, these were a maximum value for the selectivity of nitrogen oxide, 410℃ over Pt/HM and 460℃ over Fe-Mn/HM

Kantak et al (1997) summarized the mechanism and modeling of methylamine oxidation in a flow reactor According to their data, the mechanism of CH3NH2 oxidation was comprised of 350 elementary reactions and 65 reactive species They also used different models to predict the products For example, when CH3NH2 was

oxidized in the presence of atomic O, the concentrations of CH3NH2, O, O2, CO, H, H2, CH4, NO and HCN at the outlet were relevant to the inlet mole fraction of CH3NH2

A study on the oxidation of dimethylhydrazine over heterogeneous catalysts was reported by Ismagilov et al (2002) Three catalysts: CuxMg1-xCr2O4/γ-Al2O3, 32.9%Ir/ γ-Al2O3 and β-Si3N4, were compared in a temperature range of 150-400℃ The catalyst, CuxMg1-xCr2O4/γ-Al2O3, was found to have high yields of CO and a low yield of NOx They concluded that a noticeable increase of conversion to CO2 begins at 200℃ At temperatures above 300℃, a more completed oxidation happened with CO2, H2O and N2 observed

As a conclusion, the oxidation of organic nitrogen-containing compounds depends on the nature of the compounds, process parameters and the nature of the catalysts

2.2 Introduction to the catalytic oxidation of pyridine

2.2.1 Pyridine oxidation over metal oxide catalysts

Pyridine oxidation over γ-Al2O3 based metal oxide catalysts such as Al2O3, Cu/γ-Al2O3, Mn/γ-Al2O3, Cr/γ-Al2O3, Fe/γ-Al2O3, Co/γ-Al2O3, Ni/γ-Al2O3, V/γ-Al2O3 and Ce/γ-Al2O3 was studied by Luo et al (1996a) and Luo et al (1998b) The oxidation ability and NOx control ability of the above catalysts decrease in the order: Ag/γ-Al2O3 > Cu/γ-Al2O3 > Mn/γ-Al2O3 > Cr/γ-Al2O3 > Fe/γ-Al2O3 ≈ Co/γ-Al2O3 > Ni/γ-Al2O3, V/γ-Al2O3 > Ce/γ-Al2O3 (Luo et al., 1996a) They also studied the relationship between nitrogen oxides generated by pyridine oxidation and reaction

Ag/γ-temperature and found that the NOx yield went through a maximum value when the temperature was increased They thought this phenomenon was due to the different reaction rate changes with an increase in reaction temperature (Luo et al., 1996a) In another study (Luo et al., 1985), it was also found that by increasing the metal loading amount, the oxidation ability and NOx control ability was improved, but there existed a threshold value A loading amount above that value would not improve the oxidation ability and NOx control ability would also not improve any more (Luo et al., 1998b)

To study the effect of supports’ acidity on the pyridine oxidation, pyridine oxidation was tested over Cu-O/γ-Al2O3, Cu-O/SiO2 and Cu-K-O/γ-Al2O3 (prepared by immersing Cu/γ-Al2O3 in a KOH solution) (Luo et al., 1985) The result of this experiment showed that CuO/γ-Al2O3 exhibited the highest oxidation ability but the poorest NOx control ability They related this phenomenon to the acidity of supports Pyridine prefers to adsorb on an acid, surface because it is a basic compound As γ-Al2O3 occupied the strongest acidity among the three supports and showed the best oxidation ability and poorest NOx control ability, they concluded that the acidic sites of the catalysts were good for pyridine oxidation, but not good for NOx control In another study performed by Luo et al (1998a), different metal oxides supported on Al2O3 catalysts were prepared and used They found that the activity of binary oxide catalysts for pyridine oxidation was higher than that of individual catalysts The NOx control ability of binary oxide catalysts was better than that of individual ones They believed the reason may be because that there was a synergistic interaction in binary oxide catalysts

The formation of nitrogen oxides in the oxidation of pyridine over 0.64% Al2O3, 26% CuO/γ-Al2O3, 5% CuCr2O4/γ-Al2O3 and 30% CuCr2O4/γ-Al2O3 catalysts

Pt/γ-was tested by Ismagilov et al (1983) The kinetics of the total oxidation of pyridine was studied in the temperature range of 260-360°C The initial concentration of pyridine was varied from 0.05 to 2 vol.%, with the concentration of oxygen being 50 vol.% As a result, in the whole range of the steady state concentration of pyridine, both the NOx formation reaction and the total oxidation of pyridine are zero order with respect to pyridine Based on the results of their experiments, all the catalysts mentioned above showed a similar value of specific activity towards the oxidation of pyridine while their abilities to control the formation of nitrogen oxides differed much Among these catalysts, 0.64% Pt/γ-Al2O3 produced the largest amount of nitrogen oxides with a 10-40% conversion of bound nitrogen to oxides On the contrary, a lesser amount of nitrogen oxides was formed over metal oxide catalysts under otherwise identical conditions For metal oxide catalysts, the yield of NOx did not usually exceed 10% They also tested the catalysts at higher temperatures (350-520°C) in the range of total conversion of pyridine to oxidation products of CO2, H2O, N2 and NOx and found that the conversion of bound nitrogen to oxides decreased sharply as the initial concentration of pyridine was increased At a constant pyridine concentration, the temperature increase lead to an increase in the content of nitrogen oxides The values

of CNOx and XNOx depended little on oxygen concentration at a more than 1.5-fold excess of oxygen and sharply decreased, as the composition of the mixture approached stoichiometry They also found that nitrogen oxides concentration and tended to decrease with a decrease in the content of the active component in copper chromite catalysts: 30% CuCr2O4/γ-Al2O3 > 11%CuO/γ-Al2O3 > 5% CuCr2O4/γ-Al2O3

The composition of nitrogen oxides formed at a complete oxidation of nitrogen-containing compounds was determined by the nature of the compound, temperature, and oxygen concentration

2.2.2 Catalytic supercritical water oxidation of pyridine (SCWO)

Supercritical water oxidation (SCWO) is an emerging technology that has been developed to treat hazardous wastewater streams During the SCWO process, organic compounds are oxidized by an oxidizing agent, such as oxygen or hydrogen peroxide,

in the presence of water above its critical point (i.e.374°C and 22.13 MPa) Aqueous water contaminated with pyridine have been treated using different methods, including incineration, catalytic combustion, biological oxidation, carbon adsorption, and photocatalytic degradation (Shukla and Kaul, 1974; Shukla and Kaul, 1975; Watson and Cain, 1975;Shukla and Kaul, 1986; Low et al., 1991; Pahari et al., 1991; Maillard-Dupuy et al., 1997; Pandey and Sandya, 1997; Pichat, 1997)

Aki et al (1997) reported the effect of MnO2/CeO2 catalyst on the SCWO of pyridine Their results were obtained in a batch reactor system and under the influence

of mass-transfer limitations These results indicate that aqueous waste contaminated with pyridine and its derivatives can be treated using several methods Most of these methods have been developed for a specific application and may not be generally useful For example, if the concentration of the organic pollutant is less than 25 ppm, a treatment process based on photocatalysis or biological oxidation can be used These processes may not be effective in the presence of other pollutants Other treatment alternatives (i.e incineration and catalytic combustion) are limited by the generation of undesirable partial oxidation products, such as hydrogen cyanide and NOx

Aki et al (1999) also compared the SCWO over several catalysts, such as Al2O3, MnO2/γ-Al2O3 and MnO2/CeO2 and found that the addition of the heterogeneous catalyst increased the efficiency of the SCWO process in oxidizing pyridine Some experiments conducted with α-Al2O3 and γ-Al2O3 confirmed the inertness of these catalysts toward the oxidation of pyridine Among the catalysts studied, the platinum catalyst was found to be the most effective with a complete conversion of pyridine at temperatures as low as 370 °C Ammonia and nitrogen oxides were not observed over a platinum catalyst The platinum catalyst favored the formation of nitrous oxide and nitrate ions as the nitrogen-containing products, while the manganese catalyst favored the formation of nitrogen and nitrate ions The stability

Ptof the catalysts was verified experimentally Their results indicate that the MnO2 Al2O3 catalyst was unstable at these extreme conditions as a result of agglomeration of the γ-Al2O3 particles However, the Pt/γ-Al2O3 catalyst did not suffer from such a problem, presumably due to the high diluent concentration and the ability of platinum

/γ-to redistribute on the support during oxidation

In addition, Aki et al (1999) modeled the results by assuming power-law kinetics As a result, it was found that the order of the reaction and the activation energy were dependent on the type of catalyst used The rate of pyridine oxidation on Pt/γ-Al2O3 catalyst was described by a second-order reaction, whereas it was found as first order in the presence of a manganese catalyst These observations revealed that the reaction mechanism for the oxidation of pyridine in SCW was also dependent on the type of catalyst The redox mechanism was able to describe the oxidation of pyridine on the MnO2 catalyst However, the formation of a dimerization product on the catalyst surface as the first step was proposed to describe the second-order behavior

of the pyridine oxidation in the presence of a platinum catalyst The proposed mechanisms are similar to those used to describe catalytic oxidation in the gas phase, indicating the potential to extrapolate gas-phase catalytic data to supercritical conditions

2.2.3 Kinetics of catalytic oxidation of pyridine

So far, some studies on the kinetics of pyridine catalytic oxidation have been performed Ismagilov et al (1983 and 1990) reported that, at temperatures below 350°C, both the NOx formation reaction and the total oxidation of pyridine are zero-order with respect to pyridine The reaction order with respect to oxygen is close to unity The concentration of nitrogen oxides is much lower on oxide catalysts than on the platinum catalysts under otherwise identical conditions, that is, a high selectivity of oxide catalysts toward molecular nitrogen formation is observed Thus, at 300°C, the conversion of the fixed nitrogen to oxides (XNOx) does not, as a rule, exceed a few percent The rate of NOx formation on copper chromite catalysts increases with increasing content of the active component in a catalyst

At temperatures above 350°C, the complete conversion of nitrogen-containing compounds to oxidation products has been studied The regularities of NOx formation under these conditions are common for these compounds, and the conversion of fixed nitrogen into NOx is determined by (i) the nature of an active component of the catalyst used, (ii) the concentration of an organic compound and oxygen, and (iii) temperature When pyridine, acetonitrile, and dimethylformamide are oxidized over oxide catalysts, the major part of the fixed nitrogen converts to N2, and NOx yield does not exceed several percent The platinum catalyst is characterized by a much higher NOx yield (up

to 40% for pyridine and 90% for acetonitrile) At constant temperature and in excess oxygen, the fixed nitrogen conversion to NOx decreases, as the initial concentration of

an organic compound is increased The values of CNOx and XNOx depend little on oxygen concentration at a more than 1.5-fold excess of oxygen and sharply decrease,

as the composition in the mixture approaches stoichiometry A temperature increase leads to an increase of the fixed nitrogen conversion to NOx It has been found that nitrogen oxide concentration and XNOx tend to decreaing the order of decrease in the content of the active component in copper chromite catalysts: 30% CuCr2O4/γ-Al2O3 > 11% CuCr2O4/γ-Al2O3 > 5% CuCr2O4/γ-Al2O3 The NOx formation is favored by the increase in temperature and oxygen concentration

2.2.4 Intermediates and mechanism of pyridine oxidation

It is very important for pyridine oxidation to understand the mechanism of this reaction Some work has been conducted on this topic Sloan et al (1980) studied the pyridine oxidation and found that during pyridine oxidation, nitriles were formed as an intermediate

The most common intermediate that forms during the oxidation of containing compounds is HCN Houser et al.(1980) studied the kinetics of formation of HCN during pyridine pyrolysis and claimed that HCN was formed through a sequence

nitrogen-of reactions rather than during the initial step(s) involving the disappearance nitrogen-of pyridine The following global reaction scheme was proposed by Houser et al (1980):

A → HCN + hydrocarbon and nitrogenous products (2.24)

They assumed that a fraction α of the decomposed pyridine formed an intermediate, a

fraction of which was capable of reacting further to form HCN and hydrocarbons in

Reaction (2.12)

Ismagilov and Kerzhentsev (1990) proposed a mechanism for pyridine catalytic

oxidation based on the kinetic regularities observed and on the known forms of

pyridine adsorption:

C5H5N + ( ) → (C5H5N) (2.25) (C5H5N) + O2 → (HNC5H4OO) (2.26) (HNC5H4OO) + O2 → (HN) + CO2 + (C4H4OO) (2.27)

The mechanism of transformation of adsorbed NH species probably occurs as below:

where the parentheses indicate an adsorbed state

2.3 Ordered mesoporous SBA-15 material

In this experiment, SBA-15 mesoporous material is used as the catalyst support

due to its unique properties, such as a high surface area, large and uniform pores and a

thick wall SBA-15 is a kind of honey-comb structure material with very uniform pore

size distribution In this study, the role of SBA-15 is the same as the support of

industrial automotive exhaust catalysts A typical three-way catalysis (the name came

from the demands of simultaneously catalyzing three types of reactions: CO oxidation, hydrocarbon oxidation, and NOx reduction) is relatively simple in formulation: novel metals (Pt and Rh) dispersed at a ratio of 5:1 on an alumina support (washcoat), and a cordierite monolith (or alumina pellets) with a honey-comb structure From the point

of view of application, honey-comb materials have many advantages over conventional pellet-shaped catalysts and are widely used as the catalyst support The honey-comb materials used in industry are uniform structure composed of inorganic oxides or metals on the walls of honey-comb structure, where the honey-comb materials have equal sized and parallel channels For example, a square-channel extruded ceramic with 400 cells per square inch is commonly used for automotive applications As the commercial ceramic monoliths have large pores and relatively low surface areas (i.e., 3m2/g), it is necessary to coat some high surface area carriers on the wall of support The coating layer (i.e., catalyzed washcoat layer) is composed of a high surface area carrier, such as Al2O3, and active component, such as novel metals

SBA-15 is a kind of micro-scale honey-comb material, with pore size varying from 5-30 nm SBA-15 has much larger pore size than those of microporous materials, such as zeolite, breaking through the limitation of pore size and making the loading of active components much easier Meanwhile, SBA-15 also has a very high surface area,

as high as 1000 m2/g Therefore, the active components can be highly and uniformly dispersed on its surface In addition, SBA-15 has a thicker wall as compared to another mesoporous material MCM-41, which will assure that SBA-15 has a better thermal and hydrothermal stability In addition, the mesoporeous structure can be modified according the requirements The surface can be loaded with various metal oxides, such

as Al2O3, TiO2, and PdO, which is very similar to the wash-coat layer of commercial honey-comb catalysts

2.3.1 Synthesis and formation mechanism of SBA-15

SBA-15 was firstly synthesized by Stucky and his coworker (Zhao et al., 1998) From then on, it has attracted much attention, because it has some excellent properties, which represents an interesting catalyst support for metal oxides and metals due to the combination of good accessibility, a uniform pore size and a high surface area Since siliceous SBA-15 has no active sites, much research has been done to incorporate a variety of active components on SBA-15

Ordered mesoporous materials, such as MCM-41 and SBA-15, are synthesized using amphiphilic molecules as structure directors, which are different from single, organic, cationic species used in zeolite synthesis When the surfactant is removed, a porous material remained with a similar mesostructure as the liquid crystalline of a surfactant-water system Different polymers and synthesis conditions can induce lamellar, hexagonal and cubic structures For SBA-15, Pluronic P123 (EO20-PO70-EO20)

is employed as the structure-directing polymer to produce two dimensional hexagonal products Flodström et al (2004) stated the steps forming SBA-15 mesoporous silica particles They are:

They thought that the polymerization of the silica lead to the introduction of an attractive force between micelles due to a bridging mechanism After that, flocs of micelle formed by this attraction, and precipitated when their size was large enough There was a continuous structural change within these clusters, involving the coalescence of micelles that tended to form cylindrical aggregates, which formed the two dimensional structure

In a typical reaction (Zhao et al., 1998), a homogeneous mixture, comprising of

4 g P123 (EO20-PO70-EO20), 0.24 mol 37% hydrochloric acid, and 6.67 mol distilled water, is first heated 308 K during stirring, and then 0.041 mol tetraethyl orthosilicate (TEOS) is added into the solution The resulting gel is finally kept at 308 K for 24 h As-synthesized SBA-15 is separated by filtration, followed by washing The residues is dried and calcined at 773 K for 8 hours SBA-15 possesses regular hexagonal arrays of uniform channels X-ray diffraction patterns of SBA-15 always show three diffraction peaks at 2θ angle 0.5~2º Its BET surface area is around 600~1000 m2/g and the pore size is around 8 nm The amount of the surfactant is very important When the concentration of polymer is higher than 6 weight %, either silica gel will occur or no

precipitation will be yielded When the concentration is below 0.5 weight %, only amorphous silica will be formed Another important factor is pH value SBA-15 can only be obtained at acidic environments Kim and Stucky (2000) also proposed another method to synthesize SBA-15 with sodium silicate Because the TEOS is not easily available, it limits the application of SBA-15 By using sodium metasilicate, they prepared a highly ordered SBA-15 The replacement of TEOS by sodium silicate leads to the low cost production of mesoporous silica materials for practical application Most important, other substituents such as aluminum, titanium, can be introduced into the silica framework to extend the applications of SBA-15, such as catalysis and ion exchange

According to Zhao et al (1998), the pore size of hexagonal mesoporous

SBA-15 can be increased to more than 300 Å by increasing the hydrophobic volume of assembled aggregates They mentioned that this can be obtained by changing the copolymer composition or block sizes, or by adding cosolvent organic molecules, such

self-as trimethylbenzene (TMB) Jana et al (2004) tested the influence of different auxiliary chemicals on the pore size of mesoporous materials, such as SBA-15 and MCM-41 They found that methyl-substituted benzene and isopropyl- substituted benzene were effective for the large increase in the BJH pore diameter Yamada and his co-workers summarized the main three pore size controlling methods: one is the introduction of a swelling agent into the structure-directing agent, the next one is the framework condensation control method, and the other is the micelle control method that is directly related with assembled condition of the template surfactant

2.3.2 Modification of SBA-15

Since SBA-15 was synthesized successfully, it has caught much scientific attention for its potential application in catalysis, separation and biosynthesis This kind material has some excellent properties, such as high thermal and hydrothermal stability due to its thick wall; large pore size which can be tuned from 3 to 30 nm; a high surface area and a large pore volume, providing a large amount of accessible sites

So far, the main methods to modified SBA-15 includes: incorporating some metals on SBA-15, introducing some nanoparticle into the channels, and functionalizing the surface of SBA-15 with organic groups

The silanol groups on the surface of SBA-15 are non-acidic or very weakly acidic The introduction of aluminum into the framework of SBA-15 forms both acid and ion exchange sites Generally, the process of alumination can be done in two ways: direct synthesis: adding aluminum precursor into the solution directly and post-synthesis: introducing aluminum after SBA-15 has been synthesized For direct synthesis, organic aluminum is normally used as the aluminum source, such as aluminum tri-tert-butoide (Yue et al., 1999) Luan et al (1999b) have listed three different ways to prepare aluminum-containing SBA-15 by postsynthesis They used AlCl3, aluminum iospropoxide and sodium aluminate as the aluminum precursor, respectively They all obtained the conclusion that alumination could increased the stability of AlSBA-15 compared with that of SBA-15, besides an improvement in the catalytic activity Meanwhile, AlSBA-15 also became more fragile (Springuel-Huel et al., 2001) However, according to the literature, acidity and catalytic activity were still not good enough for a number of acid-catalyzed reactions for Al-containing materials One of the most popular methods to enhance the acidity is modifying SBA-15

mesoporous material with SO42-/ZrO2 (Hua et al., 2001) The SO42-/ZrO2 modified sample was prepared by introducing zirconium firstly through wet impregnation The Zr-containing sample was then treated with H2SO4 It was proven that SO42-/ZrO2 modified SBA-15 had a higher acidity than Al-containing SBA-15 Many studies have been done on the introduction of different metals to SBA-15, such as V (Liu et al., 2004); Ti (Grieken et al., 2002; Chen et al., 2004); Ru (Hartmann et al., 2001) Although the active components are different, the methods are almost the same as the alumination of SBA-15

SBA-15 is also used as the matrix of drug delivery system, such as gentamicin (Doadrio et al., 2004) and amoxicillin (Vallet-Regí et al., 2004) In general, the presence of micropores (pore diameter <2.0 nm) should be avoided in matrices designed for efficient and controlled drug delivery, as they produce generally undesirable low diffusion rates and prevent the incorporation of interesting molecules due to size discrimination The pore size and pore volume of SBA-15 make it suitable potential matrix for hosting and for a further release of a large variety of molecules having therapeutic activity Therefore, SBA-15 is expected to have less restriction for the delivery of bulky molecules The main functionalization methods include direct synthesis and post-synthesis which are almost the same as the alumination of SBA-15

Another very important application of SBA-15 is to be used as a template to synthesize nanostructure materials due to its highly ordered and uniform pore structure These nanostructure materials are catching more and more attention due to their potential applications in many areas, e.g., electronics, optics, and catalysis In general, the synthesis procedure is as follows: (a) the preparation of porous materials (template);

(b) the formation the expected materials inside the pores of template; (c) the removal

of template We will state this part in detail in the next section, because we take advantage of SBA-15 as a support of nanoparticles in our experiment

2.3.3 Highly dispersed nanoparticles on SBA-15

The deposition of catalytically active nanoparticles on support materials with high dispersion is an important and effective strategy for the design of catalysts (Taguchi and Schüth, 2005) Compared to other support materials, the ordered mesoporous solids have the advantage of stabilizing metal or metal oxide particles, since they cannot grow to sizes larger than the pore size unless they move to the external surface

The application of metal nanoparticles in catalytic reactions has been studied for decades (Chen et al., 2004) These materials have a great potential as catalysts due

to the large surface area and the high fraction of atoms present at the surface of particles, which are the reaction sites in heterogeneous catalysis (Roucoux et al., 2002) Unfortunately, small particles are unstable with respect to agglomeration to bulk materials due to their high surface energies, which leads to the loss of the properties associated with particle size (Hakim et al., 2005; Lee et al., 2005) Mesoporous materials can stabilize the dispersed nanoparticles by confining the particles in their uniform pores

Some methods for depositing the metal and metal carbonyl nanoparticles are listed in Table 2.1 (Taguchi and Schüth, 2005) Direct inclusion of metal particles in synthetic gel of mesoporous materials has been applied widely, such as Pd (Yuranov et

al., 2003) and Pt (Aramendía et al., 1999) Because the silica surface is negatively charged at pH values above approximately 2, cationic precursors are normally used during the incorporation reaction with silica After changing the surface charge of silica by attaching positively charged materials, such as a tetraalkylammonium ion, to the silanol groups, anionic precursors can also be anchored Yao et al (2001) introduced a new technique designated as vacuum evaporation impregnation The MCM-41 was suspended in an aqueous solution of [Pt(NH3)4](NO3)2 at pH 5, heated at

70 °C and the water was evaporated in vacuum The obtained material shows a very high Pt dispersion (1.03) at a loading level of 1.3 wt%

Table 2.1 Preparation of mesoporous silica supported metal and metal carbonyl

nanoparticles and their application for catalysis

Preparation method Catalysis

Pt Inclusion in synthetic gel CO oxidation

Vacuum evaporation Super critical CO2 Ion exchange to positively charged trimethylalkyl

ammonium functionalized surface, and reduction

Ship-in-the-Bottle Water–gas shift reaction,

with organic counter cations

Pd Vapor phase diffusion–deposition Heck reaction

of Pd(g-C 5 H 5 )(g 3-C 3 H 5 )

Deposition–precipitation Phenol hydrogenation

Pd, Au Inclusion in synthetic gel/incipient

wetness

CO oxidation

Co Vapor phase diffusion–deposition of

Co 2 (CO) 8

Co, Co–Ru Incipient wetness CO hydrogenation, Fischer–Tropsch