Nghiên cứu cơ chế của phản ứng khử chọn lọc xúc tác NOx với CH4 và NH3 trên xúc tác Co Fe và Mn ZSM 5

Nghiên cứu cơ chế của phản ứng khử chọn lọc xúc tác NOx với CH4 và NH3 trên xúc tác Co Fe và Mn ZSM 5 Nghiên cứu cơ chế của phản ứng khử chọn lọc xúc tác NOx với CH4 và NH3 trên xúc tác Co Fe và Mn ZSM 5 luận văn tốt nghiệp thạc sĩ

TÊN ĐỀ TÀI LUẬN VĂN

MECHANISTIC STUDIES OF CH4- AND NH3-SCR OVER ZSM-5

ZEOLITES WITH Co, Fe, Mn

LUẬN VĂN THẠC SĨ KHOA HỌC

KỸ THUẬT HÓA HỌC

BỘ GIÁO DỤC VÀ ĐÀO TẠO TRƯỜNG ĐẠI HỌC BÁCH KHOA HÀ NỘI

- Nguyễn Quang Minh

TÊN ĐỀ TÀI LUẬN VĂN

MECHANISTIC STUDIES OF CH4- AND NH3-SCR OVER ZSM-5

ZEOLITES WITH Co, Fe, Mn

Chuyên ngành : Kỹ thuật Hóa học

LUẬN VĂN THẠC SĨ KHOA HỌC

KỸ THUẬT HÓA HỌC

NGƯỜI HƯỚNG DẪN KHOA HỌC:

Dr Đào Quốc Tùy

Dr Ursula Bentrup

Hà Nội - 2018

Mechanistic studies of CH4- and NH3-SCR over ZSM-5 zeolites with Co, Fe, Mn

Minh, Nguyen

Mechanistic studies of CH4- and NH3-SCR over ZSM-5 zeolites with Co, Fe, Mn

A Dissertation Presented to The Academic Faculty

Hanoi University of Science and Technology

X Leibniz Institut für Katalyse

[12/2018]

COPYRIGHT © 2018 BY NGUYEN QUANG MINH

Statement

I assure that all the results in this Thesis are written by myself that I have personally done in Leibniz Institute for Catalysis - Leibniz Institut für Katalyse, Rostock, Germany

Hanoi, 07.12.2018

Nguyen Quang Minh

Acknowledgment

First of all, I want to give my thanks and appreciation to my mentor Dr Ursula

Bentrup, who gave me this opportunity to join in this fantastic group for the short

time of my Master Term, her great support during my time, helpful guidance and very nice smile, my huge respectation that I also want to express to my supervisor in

Vietnam Dr Dao Quoc Tuy

I am grateful to Dr Vuong Thanh Huyen for her kind help Under her instruction

about literature, studying method and suggestions

Many thanks to Ms Sonja Keller with her incredibly valuable guidance, helping me

to improve the laboratory skill, working with technical equipment and catalytic system

I also want to thank Dr Henrik Junge for experimental help in XRD, Dr Hanan

Mrs Christine Rautenberg for py-FTIR measurements, and all other members of the

analytic group for their help

I want to give my appreciation of ROHAN DAAD Sustainable Development Goals Graduate school

I would like to acknowledge Assoc Prof Dr Le Minh Thang for the cooperation

that provides for us the chance for exchange study in LIKAT, so that we will have huge advantage for the future

Abstract

In this work, 3 categories of catalysts namely different Co-ZSM-5 catalysts which were prepared by solid ion-exchange (SE) and liquid exchange in methanolic solution (LE-MeOH); commercial Fe-ZSM-5 from Clariant and Zeolyst International, and synthetic Mn-ZSM-5 system were investigated All catalysts were, the catalysts were first characterized by ICP-OES, XRD, H2-TPR and pyridine adsorption Then, the nature and stability of adsorbed species on the catalyst surface formed under NO and NO+O2 co-adsorption conditions were performed by in situ FTIR spectroscopy

The results show that, the introduction of NO causes the formation of: NO+ occupying cationic zeolite positions [υ(NO) at 2133 cm-1], Co2+(NO)2 dinitrosyls [υs(NO) = 1895

cm-1 and υas(NO) = 1812 cm-1], and Co3+-NO linear species [υ(NO) at 1937 cm-1] was observed In the presence of gaseous O2 the formation of nitrate and nitrite species is facilitated (1300 to 1650 cm-1), the extent of which depends on the metal content as well as the nature of Co-, Fe- and Mn- species formed by the different synthesis methods The adsorption process in NO+O2 co-adsorption was exposed at Room Temperature (RT), 150⁰C, 250⁰C and 350⁰C to see the change of thermal behavior and state of adsorbed surface species

Content/Outline

ACKNOWLEDGMENT III ABSTRACT IV LIST OF ABBREVIATIONS VII LIST OF TABLES VIII LIST OF FIGURES IX AIM OF THESIS X

CHAPTER 1 OVERVIEW 1

1.1 Motivation 1

1.2 State of the Art 3

1.2.1 Ammonia-SCR 9

1.2.2 Methane-SCR 13

1.3 Synthesis of Catalysts 20

1.3.1 Solid ion exchange 21

1.3.2 Liquid ion-exchange 22

1.3.3 Special-liquid ion exchange 24

1.4 Characterization techniques 25

1.4.1 ICP-OES 25

1.4.2 XRD 26

1.4.3 H2-TPR 26

1.4.4 Pyridine-adsorption 26

1.5 In situ-FTIR (Fourier Transformed Infrared Spectroscopy) 27

1.5.1 Introduction 27

1.5.2 Infrared absorption spectrum 28

CHAPTER 2 EXPERIMENTAL 30

2.1 Synthesis of catalysts 30

2.1.1 Solid ion exchange 30

2.1.2 Liquid ion exchange 31

2.1.3 Special-liquid ion exchange 31

2.2 Catalytic characterization method 33

2.2.1 ICP-OES Measurement 33

2.2.2 XRD Measurement 33

2.2.3 Hydrogen - Temperature-Programmed Reduction measurement 33

2.2.4 Pyridine-adsorption measurement 33

2.3 In situ-FTIR procedure 34

CHAPTER 3 RESULTS AND DISCUSSION 36

3.1 Characterization 36

3.1.1 ICP-OES 36

3.1.2 X-ray Diffraction Result 37

3.1.3 H2-TPR 40

3.1.4 Acidity (Pyridine-IR) 44

3.2 Catalytic Study of Co-ZSM-5 47

3.2.1 The Nature and Stability of NxOy species in NO adsorption 47

3.2.2 NO+O2 co-adsorption 52

3.2.3 In summary 54

3.3 Catalytic Study of Mn-ZSM-5 56

3.3.1 The formation and stability of NxOy species NO adsorption 56

3.3.2 NO+O2 co-adsorption 58

3.3.3 In summary 60

3.4 Catalytic Study of Fe-ZSM-5 61

3.4.1 The formation and stability of NxOy species NO adsorption 61

3.4.2 NO+O2 co-adsorption 66

3.4.3 In summary 67

CONCLUSIONS 69

OUTLOOK 71

PUBLICATIONS 72

REFERENCES 73

APPENDIX A - SCR REACTION 84

FTIR Fourier transform infrared

GHSV Gas hourly space velocity

H 2 -TPR Hydrogen temperature-programmed reduction

ICP-OES Inductively coupled plasma optical emission spectrometry

NH 3 -SCR Selective catalytic reduction of NOx with NH3

CH 4 -SCR Selective catalytic reduction of NOx with CH4

XRD X-Ray Diffraction

List of Tables

Table 1 Nitrogen Oxides (NO x ) 3

Table 2 NOx control Methods 7

Table 3 Catalyst systems and compositions 36

Table 4 Consumption H/Co 41

Table 5 Percentage of reduced Mn and Fe 43

Table 6 Acidity intensity of Co sample 45

Table 7 Acidity intensity of Mn and Fe sample 45

Table 8 The assignment of the bands observed during NO adsorption on different Fe-containing catalysts 63

List of Figures

Figure 1 Exhaust gas composition in representative Engine [3, 4] 5

Figure 2 D3 Standard 6

Figure 3 D4 Standard 6

Figure 4 Mechanism of SCR over vanadium oxide catalysts [23] 10

Figure 5 The mechanism of NO-SCR reaction by methane over Co-oxide promoted 18

Figure 6 Necessary equipments forSE 30

Figure 7 SE procedure 30

Figure 8 LE procedure 31

Figure 9 LE-MeOH procedure 32

Figure 10 LE-MeOH Setup 32

Figure 11 FTIR system (Thermo Scientific Nicolet 6700 spectrometer) 34

Figure 12 Cell (a) and sample holder (b) 35

Figure 13 XRD result of parent ZSM-5 38

Figure 14 XRD pattern of the samples belong to section 1 (Co) 38

Figure 15 XRD pattern of the samples belong to section 2 (Mn and Fe) 39

Figure 16 TPR profile of Co system 40

Figure 17 TPR profile of M07, M11 and M12 42

Figure 18 Acidity of parent zeolite 44

Figure 19 Pyridine adsorption on Co 45

Figure 20 Pyridine adsorption on Mn and Fe 46

Figure 21 FTIR results of NO adsorption on Co after 5 min 48

Figure 22 The peak trend of adsorbed bands in M01 (a) and M02 (b) 50

Figure 23 The peak trend of adsorbed bands in M08 (a) and M09 (b) 50

Figure 24 The peak trend of adsorbed bands in M03 (a) and M04 (b) 51

Figure 25 The formation of nitrate and nitrite species onto Co system 53

Figure 26 NO+O 2 co-adsorption on Co system 54

Figure 27 NO adsorption of Mn System 56

Figure 28 The peak trend of adsorbed bands in M05 (a) and M06 (b) 57

Figure 29 The peak trend of adsorbed bands in M07 58

Figure 30 NO+O 2 co-adsorption on Mn-system 60

Figure 31 OH characterization of NO adsorption on Fe system 61

Figure 32 The NO adsorption at RT after 5min 62

Figure 33 The evolution of NO adspecies for increasing adsorption times of M11 65 Figure 34 NO+O 2 co-adsorption at RT, 150, 250 and 350⁰C 66

Aim of Thesis

The selective catalytic reduction (SCR) of NOx is an important reaction to transform toxic NO+NO2 from exhaust gases of mobile and stationary sources It is a contemporary issue The different commercial catalysts in Industry has recently been Fe-ZSM-5, V2O5-WO3/TiO2 and different Ceria-based catalysts were studied in NH3-SCR

It is known that Fe-ZSM-5 is active in NH3-SCR, but not in CH4-SCR, while ZSM-5 shows a reverse behavior The reasons are not really understood and under debate, but it seems that the different reactivity of the formed nitrates against the reducing agent plays a role Therefore, the aim of this Thesis is to elucidate the role

Co-of nitrates (nature and stability) formed over Co-ZSM-5, Fe-ZSM-5 and Mn-ZSM-5

in the respective SCR with CH4 as well as NH3 to get mechanistic insights

For this purpose, various Co-, Mn-ZSM-5 catalysts were synthesized by exchange into ZSM-5 zeolite framework, and Fe-ZSM-5 as commercial catalyst will be used The work comprises the following topics: In situ and operando FTIR investigations,

to study:

(i) Catalysts behaviors

(ii) NO and NO+O2 co-adsorption

(iii) Reactivity of preadsorbed nitrates and their stability

Chapter 1 OVERVIEW

1.1 Motivation

NOx emission from motor vehicle exhaust and power plants is becoming a serious problem in recent two decades The main source of NOx emissions in populated areas

is combustion of fuels Stationary source emissions can be controlled by modification

of the combustion process or by post combustion treatment Conventional combustion modification techniques can achieve up to 50% reduction of NOx, while advanced techniques have the potential for over 80 % reduction Post combustion treatment techniques can solve up to 90 % control, however, higher costs may be conquerable problem compare to its of combustion modification techniques The main components of these exhaust gases are non-absolutely burned hydrocarbons, carbon oxides, oxides of nitrogen and particle matter (PM) These components also appear in Power Plant and have their own criteria represented, not only in the Federal Republic but also throughout Europe and all over the World

Researchers have been reported numerous studies for solving this issue, unfortunately, it is not potential for vanishing total amount because of many kinds of nitrous compounds, such as, mono (NO), dinitrogen oxide NO2, N2O, N2Ox (whereas

x = 3, 4, 5) Furthermore, there are not only N-compound, but also other components exist in exhaust gas as mentioned

Selective catalytic reduction with NH3 (NH3-SCR), in which, ammonia is used as the reducing agent of NOx in the presence of a catalyst, is one of the most popular post combustion techniques for depletion of NOx emission by Commercial catalysts based

on V2O5-WO3/TiO2 oxides in the suitable temperature range of 300-500⁰C However, due to the secondary pollution of ammonia, a different direction was discovered in the past, which is selective catalytic reduction with methane (CH4-SCR) Furthermore, methane is also an excess gas in non-absolute burned of fuel and exist

in exhaust gases A variety of metals were studied with these two NH3 and CH4-SCR reactions to investigate the formation, stability, interaction of intermediates, the effect

and consequence of them to the formation of product However, the mechanism is varied by the catalyst system, the condition and the concentration of feed gases, therefore, it is still not clear and need further investigation

In this study, cobalt, manganese and iron will be chosen and loaded in zeolite framework by ion-exchange method for mechanistic study Besides, the redox behaviors of catalysts for SCR reaction with the incoporation of metals by solid ion-exchange and ion-exchange in the absolute methanol also were investigated

1.2 State of the Art

Nitrogen oxides (NOx) are represented as a family of seven compounds that the main source of nitrogen oxides which comes from burning of fossil fuels The burning process of fuels inside the engines and combustion the materials in the electric power plants Moreover, the sources of emission include chemical plants producing nitric acid, sulfuric acid and fertilizers, also in organic synthesis using nitric acid; iron and steel mills, cement manufacture, glass manufacture, petroleum refineries Biogenic

or natural sources of nitrogen oxide include lightning, forest fire, grass fires, trees, bushes, grasses, and yeasts These different sources define different kinds and amounts of each emission oxide The group of NOx compounds and some their properties are shown in Table 1

Table 1 Nitrogen Oxides (NOx)

Formula Name

Nitrogen Valence Properties

N 2 O Nitrous oxide 1

Colorless gas Water soluble

N 2 O 3 Dinitrogen trioxide 3 Black solid Water soluble, decompose in water

N 2 O 5

Dinitrogen

White solid Very water soluble, decompose in water

N2O, NO and NO2 are the most abundant nitrogen oxides in the air, however, NOx

emission are primarily in NO form (about 90-95% in total emitted amount) These

NOx compounds may cause environmental damages such as photochemical smog, depletion of ozone, acid rain and serious problems to human health likes damage to

lung, and so on Bosch and Janssen [1] categories three types of NOx formed through the combustion process (NOx from engine exhaust typically consists of a mixture of 95% of NO and 5% of NO2)

Thermal NO x - is formed by the oxidation of N2 at high temperature

N2 + O2 → 2NO, ∆H0298 = 180.6 kJ/mole (1) This reaction is exposed above 1300K and followed by the Zeldovich mechanism of chain reactions involving N* and O* activated atoms:

N2 + O* → NO + N* (2)

N* + O2 → NO + O* (3) The rate of NO formation is essentially controlled by reaction (2) and increase exponentially with temperature The Zeldovich mechanism dominates NO formation under most engine conditions [2] The NOx emission from the engine can be controlled by lowering the combustion temperature by operating the engine under excess air (fuel-lean) conditions, but most these approaches based on high temperature air combustion (HiTAC) are effective

Fuel NO x - is formed from the oxidation of nitrogen in fuels such as coal and heavy

oils In contrast to thermal NOx, fuel, NOx formation is relatively independent of temperatures [2]

Prompt NO x - (also termed as Fenimore NO) which is formed by the reaction of

hydrocarbon fragments with atmospheric nitrogen to yield products such as HCN and

H2CN These can be subsequently oxidized to NO in the lean zone of the flame NO can further react with oxygen to NO2 or N2O

NO + 1/2O2 ↔ NO2, ∆H0298 = -113 kJ/mole (4) 2NO ↔ N2O + 1/2O2, ∆H0298 = -99 kJ/mole (5) Prompt NOx formation is proportional to the number of carbon atoms present per unit volume and independent of the identity of the parent hydrocarbon Increasing the concentration of hydrocarbon radicals increases the quantity of HCN formation

Significant amount of prompt NOx can be formed at low-temperature, in fuel-rich conditions and where residence times are short

Note that, the formation of NO can be via nitrous oxide In this mechanism, O-atom attacks molecular nitrogen in the presence of a third molecule that results in the formation of N2O This subsequently reacts with an O-atom to form NO: N2O + O → 2NO (6) with an activation energy of 97 kJ/mole This reaction route is overlooked because the total NO formed by this reaction is not significant However, lean conditions suppress Fenimore NO and low temperatures suppress Zeldovich NO High pressures promote this reaction, the formation of NO by this route occurs primarily in lean premixed combustion in high pressure gas turbine engines

All the NOx compounds and remaining part after the combustion process in engine: carbon dioxide, particulate matter (PM) and non-absolutely burned hydrocarbons are under legislation Strict norm has been set to reduce vehicular emissions and their impact on environment The description of individual exhaust gas components is shown:

Figure 1 Exhaust gas composition in representative Engine [3, 4]

(Petrol engines may also emit small quantities of sulfur dioxide SO2)

Electric power plants produce about 40% of NOx emission from stationary sources [3] These components also appear in Power plant and have their own criterias In recent years, resolution and laws aimed at curbing the emission of air pollutants have been passed Not only in the Federal Republic but also throughout Europe and the

world And all of vehicle’s transportation at the particular market must get the satisfaction with regional legislation For example, the US federal vehicle emission standard effective in 2007 requires tight control of NOx For light duty vehicles, the current standard of Tier 2 Bin 5 is 0.07 g/mile NOx for 120000 miles However, the proposed future standard is 0.03 g/mile for NMOG (non-methane organic gases) + NOx (SULEV30 ) at 150000 miles [4, 5] The German standards were introduced on

a voluntary basis in order to promote the fulfilment of limit values which exceed the

EU standards This means that, when the customers buy a new vehicle which meets not only the current EU III standard but also the D4 emission standard

D3 Standard: The D3 Standard has expired on 31.12.1999, tightens up the EU II

standard at a national level (NECC - New European Driving Cycle)

Figure 2 D3 Standard

D4 Standard: The D4 is valid up until 31.12.2004 It stipulates more stringent limit

value than EU III standard and makes tax assistance possible:

In general, the methods were used for depletion of NOx can be described in Table 2 [6]

Table 2 NOx control Methods

Essentially, there are 6 main methods, the 7th being an international combination from the six others

I Reducing temperature - Basically, this technique using of fuels, air, flue gas or

steam for diluting calories The different kinds of this technique are applied for both low and high nitrogen content Controlling NOx from combustion at high nitrogen content (for fuels from coal) can be understood by the net stoichiometric ratios and for low nitrogen content fuels (such as gas and oil) can be seen versus rich fuel/air ratios Low-NOx burners are based partially on this principle [7, 8] The basic technique is to reduce the temperature of combustion products with an excess of fuel, air, flue gas and/or steam

III Chemical Reduction of NO x - This technique requires chemically reducing

agent with Selective Catalytic Reduction (SCR) and Non-Selective Catalytic Reduction (NSCR) which use ammonia or urea, and Fuel Re-burning (FR) Due to the strict criteria of NOx emission to the environment and more and more extended context SCR technique is much more concerned and focused on

IV Oxidation of NO x - This principle can be described by increasing the valence of

nitrogen to absorb in solution (water or alkaline/solution) based on the greater solubility of NOx at higher valence

V Removal of Nitrogen from combustion - (1) Using oxygen instead of air in the

combustion process or (2) using ultra-low nitrogen content fuel to form less fuel NOx These methods will provide one of the most important ways for industry of NOx

1.2.1 Ammonia-SCR

Catalysts

The industrial catalysts for SCR from stationary sources are based on TiO2 supported

V2O5-WO3 and/or V2O5-MoO3/TiO2 catalysts [9, 10] while mobile systems use zeolite based catalysts Anatase form of TiO2 is used as a the support mainly because

SO2 poisoning does not take place on TiO2 Generally, the active sites on the V2O5

-WO3/TiO2 and V2O5-MoO3/TiO2 industrial SCR catalysts are vanadium oxide species [11-12] It is interesting to note that V2O5/TiO2 (anatase) is unstable where TiO2 (anatase) is a metastable polymorph that converts into thermodynamically stable form rutile at higher temperature and pressure V2O5 favors this transformation and the anatase sintering and decreasing of surface area However, with WO3 and MoO3, both the reduction of surface area and transformation of anatase to rutile are lower [13] SCR reaction is a redox process that occurs with a redox or Mars-van Krevelen-type mechanism on vanadium-based catalysts NH3 adsorbs on pure V2O5, on V2O5-TiO2, on V2O5/SiO2-TiO2, on V2O5-WO3/TiO2 and on V2O5-MoO3/TiO2 in two different strongly held species: (i) molecularly adsorbed ammonia, through a Lewis-type interaction and (ii) ammonia observed as ammonium ions, over Bronsted acidic -OH surface hydroxyl groups [14-16] The TiO2-anatase supports, only shows Lewis acidity [17], whereas ammonium ions are formed on V-OH sites The adsorption of

NO (the other SCR reactant) has also been extensively investigated in the literature

It has been shown that the interaction of NO is very weak over many V2O5-based catalysts By adsorption of NO over V2O5-TiO2, Ramis et al observed the formation

of a surface nitrosyl species, coordinated to Ti4+ sites [18] However, NO does not adsorb on an ammonia covered surface because NH3 has greater basicity and blocks the Ti4+ adsorption sites This data is in good agreement with the kinetic observations

on V2O5-based catalysts where zero and first order kinetics concerning NH3 and NO, respectively [19-21] is observed In vanadia-based catalysts, NH3 adsorbs mainly on the Bronsted acid sites while insignificant NO adsorption is observed Thus an Eley-

Rideal mechanism has been suggested for SCR Takagi et al [22] and M Inomata et

al [23] proposed one of the first reaction schemes for SCR over V2O5-based catalysts (c.f Fig 4)

Figure 4 Mechanism of SCR over vanadium oxide catalysts [23]

In the 1990s, Ramis et al [18] proposed the reaction pathway in which ammonia is adsorbed over a Lewis acid site that activates ammonia to an amide NH2 species resulting in catalyst reduction This amide species, then reacts with gas-phase NO to nitrosamide as an intermediate, which decomposes to nitrogen and water

It was also found that Cu2+-exchanged ZSM-5 zeolites are active catalysts for the reduction of nitric oxide with ammonia in the presence of oxygen It was shown that

NO reduction by NH3 over Cu(II) ion-exchanged Y-type zeolites [Cu(II)NaY] followed Langmuir-Hinshelwood kinetics [24] Many studies suggest that the NH3-SCR

molecularly adsorbed ammonia species interacts with NO from the gas phase or from

a weakly adsorbed state through an Eley-Rideal mechanism [25]

It has been reported that Fe2O3 [26], Fe containing mixed oxides [27] and exchanged materials [28], and in particular Fe-ZSM-5 [29-32] show significant SCR activity A comparison of different metal-exchanged zeolites showed that iron beta has good activity, high N2 selectivity and aging characteristics, however, studies of low-temperature NH3-SCR of NO over zeolite based catalysts are few Besides using zeolites ZSM-5 and zeolite beta (BEA) in these reactions, different synthetic and natural zeolitic materials such as mordenite (MOR), heulandite-clinoptilolite (HEU), ferrierite (FER) and chabazite (CHA) have also been investigated Among them, Fe-mordenite and Fe-clinoptilolite have shown catalytic properties for the SCR of NO with ammonia, similar to that of Fe-ZSM-5 [33] Natural zeolites have also been used because they have a good combination of their crystalline structures and exhibit interesting physicochemical properties with various metal oxy-hydroxide phases that are naturally embedded inside their pores However, potential commercialization of natural zeolites as catalysts is not feasible due to the cost of homogenization and purification of the zeolite-bearing tuffs [34] Unreacted ammonia can escape in the exhaust (called slip) and thus the emission controls also restrict ammonia in the exhaust Further, the transportation and storage of ammonia is not very cost-effective Therefore the research on the NOx reduction by hydrocarbons as reducing agents

Fe-Mechanism/Chemistry

For reasons of safety and toxicity, urea is the preferred selective reducing agent for mobile SCR applications When urea is used as the reductant, a detailed understanding of its roles in the NOx reduction process is critical for optimizing the SCR process [35]

In the SCR catalyst, NO and NO2 react selectively with NH3 as a reducing agent The overall reactions taking place in the reactor are the adsorption-desorption equilibrium

of ammonia and the reactions between NH3 and the NxOy species, expressed in

equation 1.1-1.4, where 1.3 designates the ammonia adsorbed on a catalytic site Eq 1.1-1.4 represent desirable reactions which reduce NOx to elemental nitrogen in the SCR-reactor:

NH3 ↔ NH3* (1.1) 4NO + 4NH3 + O2 → 4N2 + 6H2O (1.2) 3NO2 + 4NH3 → 7/2N2 + 3H2O (1.3)

NO + NO2 + 2NH3 → 2N2 +3H2O (1.4) The equation (1.2) is called the standard SCR reaction which represents the dominant reaction mechanism; the equation (1.3) is the slow SCR reaction, and the reaction in equation (1.4) is referred to the fast SCR reaction The standard SCR reaction occurs

at temperatures above 200°C However, the reaction rate is significantly faster at higher temperatures [36]

The fast SCR reaction is favored by an equimolar mixture of NO and NO2, and the reaction occurs at temperatures as low as 140-170⁰C Thus, the effect of NO2 in the exhaust gas is significantly higher at low temperatures due to the low reaction rate of the standard SCR reaction According to [35] the NO comprise above 90% of the NOx in the exhaust gas, and therefore, actions have to be taken if the optimal 1:1 of NO:NO2 ratio is desired, favoring the fast SCR reaction Thus, one way to improve the reaction rate at low temperatures is by increasing the amount of NO2 in the exhaust gas This can be done by the use of an oxidation catalyst placed upstream the SCR catalyst Furthermore, the oxidation of NO to NO2 has been reported to occur over iron exchanged zeolites following the reaction is given in equation (1.5) [37]

2NO + O2 → 2NO2 (1.5) The slow SCR reaction occurs at temperatures above 275⁰C if excess NO2 is present Besides the reduction reactions, there can be several unwanted side reactions

occurring which are presented in Appendix A

1.2.2 Methane-SCR

Nowadays, the NH3-SCR most common and recent way and also widely applied in this field This technology is widely used due to its high selectivity of N2, and the presence of oxygen will accelerate the reaction However, there are still issues with this reductant:

(1) The secondary pollution problem because of the excess of ammonia

(2) Storage and transportation are inconvenient because since under normal temperature and pressure NH3 is gaseous

(3) The risk and toxicity of catalyst systems containing vanadium (the most common and popular industrial catalysts are V2O5-WO3-TiO2)

Concerning all of that problems, a potential approach was developed is that using hydrocarbons as reductant starting from 1970’s However, the hydrocarbons under the condition of the engine will undergo the combustion process in the presence of oxygen, therefore limit their abilities

Catalysts

Since Iwamoto et al [38] first found that Cu-ZSM-5 can be used as a potential catalyst

to directly decompose of NO and the reduction of NO with non-methane hydrocarbons (NMHC), there were significant progresses have been discovered for this field A series of metallic ion exchange-zeolite, such as Cu-, Fe-, Pt-, Co-, Ga-, Ce- have been found to catalyze this reaction

In previous stages, the study of NO reduction by hydrocarbons was generally focused

on Non-methane hydrocarbons (NMHC) by using ethane, propylene, propane… The reason for that is the difficulty to active methane However, further studies revealed that some of the catalysts such as Pt-catalyst on SiO2 or La2O3/Al2O3, Co-ZSM-5, Co-FER were effective catalysts when CH4 was used as a reductant in the presence

of oxygen The reasons that CH4 is getting the reputation can be:

(1) Methane is present in the exhausted gas along with hydrocarbons from both gas of the engine (around 20% of hydrocarbons, this amount depends on fuel, the engine and engine technology) and stationary from power plants

out-(2) Methane is the main gas of natural gas, which is abundant and low price

Therefore, this technology will not only help to decrease environmental pollution, but also increase the utilization efficiency of resources [39]

There are some routes to apply this technique:

a The direct reduction of NO by CH 4 without a catalyst (SNCR)

The direct reduction of NO by CH4 without a catalyst should be conducted in the absence of oxygen But since oxygen is always present in the real exhausts, the commercial application of this process is very limited In the reaction system containing oxygen, there are two competing reactions:

2NO + CH4 + O2 → N2 + CO2 + 2H2O (1.6)

CH4 + 2O2 → CO2 + 2H2O (1.7) The rate of reaction (1.7) is much greater than that of reaction (1.6) Therefore the rate of entire conversion is characterized by reaction (1.6) It can be implied that in the presence of oxygen, the reduction of NO occurs only after the reaction between

O2 and CH4 has proceeded to a certain extent Thus, an excess amount of CH4 should

be added into the reaction Otherwise, the temperature will increase up to1200⁰C due

to the combustion of CH4 resulting in the formation of CO, the byproduct of reaction (1.6) and then will further be completely oxidized to form CO2 Besides, if this conversion happens, it will also cause the formation of NO, and consequently lower the efficiency of NO removal [39] In summary, the direct reduction of NO by CH4

without a catalyst will not be widely commercialized because of the high cost and low efficiency

b Selective catalytic reduction (SCR)

Several catalysts have been investigated for SCR including zeolites catalysts, metal oxides, and noble metal

Concerning the metal oxide catalyst, the thermal and hydrothermal stability

properties is much better than its of molecular sieve materials [40] For instant, SnO2

was more active than Cu-ZSM-5 for the reaction of NO by C2H4, because of its greater hydrothermal stability, but SnO2 was much less active for the reaction of NO/CH4, as compared to the Co-ZSM-5 In other case, Li/MgO and MgO had certain activities for the reaction of NOx/CH4, and the activity of Li/MgO was slightly better than that of the MgO [39] In general, the catalytic activities of Group IIIB nano-crystalline metal oxides for the reduction of nitric oxide with methane were found to

be comparable to that of Co-ZSM-5 La2O3 is more active than Li/MgO At the similar conversion of NO, the reaction temperature over La2O3 catalyst is about 100⁰C higher than that of the reaction over Co-ZSM-5 La2O3 in the difference with Li/MgO, only exhibits activity in the presence of oxygen When the reaction is exhibited at a higher temperature (800-1000⁰C), La2O3 with 4% has a better activity in the reaction NO/CH4 La2O3 catalyst when loaded on δ- or γ-Al2O3 is more active than that of the pure form and the conversion of NO at 700⁰C over La2O3 got 60% higher than that of Cu-ZSM-5 catalyst [41] Group IIA, Group IIIB, and lanthanide oxide catalysts, however, are not as active as certain zeolitic catalysts for the selective catalytic reduction of NOx with methane, these materials show exhibit significantly better hydrothermal and high-temperature stabilities than many of the zeolitic systems that have been studied [42]

Nobel metal catalyst: There are three sections that Ohtsuka found when study the

effects of noble metals toward NOx reduction by methane: (1) low activity for NO oxidation to NO2, and high activity for NO2 reduction to N2 (Pd, Rh); (2) high activity for NO oxidation to NO2, and low activity for NO2 reduction to N2 (Ru, Ir, Pt); (3) low activity for both reactions (Ag, Au) [43] When studied of Platinum, Palladium and Rhodium, carried on Al or Si separately, it can catalyze the CH4 reaction to a certain extent The activity sequence follows: Pt/Si > Pt/Al > Pd/Al > Pd/Si > Rh/Al

> Rh/Si In other hand, Pd shows the highest catalytic reactivity for the CH4 oxidation reaction, while for the NO/CH4 reaction, Pt exhibits the highest activity Furthermore,

it can be seen that Si is a good support for Pt, while Al is a good support for Pd or

Rh

As mentioned, Iwamoto et al was first found that Cu-ZSM-5 could selectively catalyze the reduction of NO This opened the subsequent researches by using ion-exchanged molecular sieves as catalysts for NO reduction However, in the presence

of oxygen, the reactivity of Cu-ZSM-5 for the reduction of NO by methane is very low, thus, Iwamoto et at classified hydrocarbons as selective (C2H4, C3H6 and C3H8) and non-selective (such as CH4 and C2H6) At the same time, some researchers found that Co-ZSM-5 exhibited much better reactivity for CH4/NO system, and the reaction would be promoted when the oxygen concentration was in the range of 1-21% With Co-FER catalyst, the highest conversion can get at above 500⁰C And when Co-KL and Co-Y were used, the conversion rate was very low As for CoO/Al2O3, CoO/TiO2, Co/TiO2, CoO/Silicate, Co/Al2O3-SiO2 and Co3O4 no catalytic reactivity at all If there were no Oxygen in the reaction, the reactivity was followed by the order: Rh-ZSM-5 > Pt-ZSM-5 > Co-ZSM-5 > Cu-ZSM-5 and Rh-ZSM-5 > Co-ZSM-5 > Cu-ZSM-5 > Pt-ZSM-5 [44] in the presence of oxygen

Mechanism

First of all, the mechanism of CH4-SCR is variable, depends on the catalyst (active sites, synthesized conditions) and applied technology However, no matter what the reductant is, no matter the mechanisms can be, the key questions are:

(1) How are the hydrocarbons activated? Or question for how is C-H bond broken? (2) How is nitrogen formed? Or how is N-N bond formed?

Numerous studies claimed some proposed mechanisms as follow:

 Iwamoto and Mizuno [38]: The key step is the production of the intermediate oxidation product The mechanism is as follows: (1.8)

Hydrocarbons are partially oxidized by oxygen, forming a certain kind of intermediate product which will further react with NO for producing nitrogen

 Another mechanism includes the formation of NO2 (1.9)

NO is oxidized homogeneously into NO2, which further oxidizes hydrocarbons into intermediate oxidation products, and finally produces N2 The mechanism is similar

to the first one, but the intermediate product CxHyOz is formed via the NO2 formation

 Armor et al [39] put forward a mechanism on how Co exchanged molecular sieves catalytically reduce the reaction CH4-SCR of NO as below: (1.10)

In this mechanism NO is adsorbed on the active sites of Co, and reacts with O2 to form Z-Co-NO2 It is the Z-Co-NO2 intermediate that activates CH4 to form Z|Co|NO2CH3, which reacts with NO to form N2

 Cant et al [45] explained selective catalytic reduction with methane on MFI as follows: (1.11)

Co-This mechanism can be explained that NO must first be oxidized to NO2 to provide a site which can get hydrogen from the hydrocarbons The same or another NOx then can generate a nitroso or nitro as intermediates of reaction, which follows through a series of rearrangement and degradation steps a reduced nitrogen centre

 F Lónyi et al with their research study claimed a mechanism on Co-ZSM-5:

Figure 5 The mechanism of NO-SCR reaction by methane over Co-oxide promoted The mechanism, outlined by Scheme 1 (c.f Fig 5), provides a plausible explanation

of how the charge balance of the system can be maintained in the catalytic cycle Note that the reactions according to Eq 1.12: It could be nitromethane as it is often suggested [45, 46] Accepting this suggestion, the activation of methane is envisioned

as follows:

[Co-NO3]+Z− + CH4 ↔ [Co-OH]+Z− + CH3NO2 (1.12) and (1.13)

NO+Z− + CH3NO2 ↔ N2 + H+Z− + H2O + CO2 (1.13) proposed by [47, 48] proceed on NO3 −/NO+ ion pairs formed on Co2+/[Co-OH]+ sites (Eq (1.14) and (1.15): [49, 50]

Co2+Z2− + 2NO2 ↔ [Co-NO3]+Z− + [NO]+Z− (1.14) [Co-OH]+Z− + H+Z− + 2NO2 ↔ [Co-NO3]+Z− + [NO]+Z− + H2O (1.15)

The reaction of Eq (1.16):

CH4 + O2 + 2NO → CO2 + 2H2O + N2 (1.16) implies the formation of water that may come from the reaction of NO2 and [Co-OH]+sites (Eq 1.15) or by water desorption The water is released that would be in excess

to the actual equilibrium coverage of the Co2+ sites by heterolytically dissociated H2O molecules [51, 52] Higher temperature and higher concentration of Co2+ sites favor water desorption This process may have importance, because the NO2 activation (Eq 1.14) proceeds in the electrostatic field of Co2+, which is stronger than the field of [Co-OH]+ sites Thus, the formation of NO3 −/NO+ ion pairs and the N2-forming reaction might be limited by water desorption and require relatively high temperature (> 400⁰C) to proceed at a rate comparable to the rate of the NO2-forming NO-COX reaction

Scheme 1 shows the interplay of the CH4/NO-SCR and the NO-COX activities If the activation of methane by the surface nitrate is the rate determining step of the NO-SCR reaction, the higher rate of NO3 −/NO+ formation leads to a higher rate of the

CH4/NO- SCR reaction The NO2-forming NO-COX reaction was significantly faster

in the presence of Co-oxo species than on Brønsted acid sites The fast NO2

generation promoted the formation of NO3 −/NO+ pairs, thereby, the rate of methane activation and the rate of the whole NO-SCR process

They are all proposed mechanism and still under debate Therefore, this work is a small part will contribute to the understanding of adsorbed species of NO and NO+O2

co-adsorption on Co-, Mn- and Fe-ZSM-5 and mechanism as well

1.3 Synthesis of Catalysts

The removal of nitrogen oxides from mobile and stationary sources remains an important technological task for solving pollution problems More than a thousand catalyst compositions have been tested for application of reduction and/or decomposition of nitrogen oxides [53] Transition metal-exchanged MFI zeolites (average pore diam 0.55 nm), particularly Cu-ZSM-5, were proven to be active in catalytic reduction of NO by NH3 [54] or hydrocarbons [55, 56] as well as in the direct decomposition of NO to molecular nitrogen and oxygen in “lean-burn” engine conditions [57] Numerous sophisticated measurements were carried out to identify the oxidation states and coordinations of transition metals in ZSM-5 zeolite structure and to clarify the possible mechanism of the reactions [58] Some researches compared the catalyst’s features: loading-ability metal content, specific area, pore volume, acidic property and also the incorporation of metal into zeolites via different ways

Steric constraints can limit ion exchange in the liquid phase (LE) due to the formation

of bulky hydration shells of the exchangeable cations Intermediate calcination is required to facilitate cation migration Furthermore, the degree of exchange is limited

by the thermodynamic equilibrium, which makes it necessary to repeat the exchange procedure several times to reach high exchange levels Solid-state ion exchange (SE), reported in 1973 by Rabo [59] and Clearfield [60] is a highly efficient procedure Therefore, this method has attracted much attention in recent years [61, 62] In this method, a mechanical mixture of the zeolite and a cation precursor, which is often applied as chloride salt, acetate, was prepared and followed by calcination and washing step The primary advantage over conventional exchange is the significantly higher degree of exchange reached in a one-step treatment compared to the procedure

in aqueous solution, which has to be repeated two to five times

1.3.1 Solid ion exchange

As mentioned before, the classical method to modify zeolite for their compensating cations was, for a long time, ion-exchange in aqueous medium The study of the potential of solid state ion-exchange commenced only in the mid-1980s, when two research groups began independently with aiming to introduce compounds

charge-of ions into the variety charge-of mesoporous zeolite [63-66] These studies were also independent earlier of previous observations reported during the 1970s by Rabo et al [59] and Clearfield et al [60], who used solid-state ion-exchange to eliminate Bronsted acid-based catalytic properties from zeolite, and react mixture of ammonium-containing zeolites with halides, respectively Group of Slinkin [62-65] focused on the solid-state introduction of transition metal cations into high-silica zeolites, such as, ZSM-5 and Mordenite with using mainly electron spin resonance (ESR) spectroscopy for monitoring the reaction Whereas, the present author’s group began with quantitive investigation on solid-state exchange reactions between alkaline and alkaline earth halides, and hydrogen or ammonium forms of zeolites, such as, NH4-Y, H-ZSM-5 and H-MOR, for zeolite structure such as A, X, Y, MOR,

L, BETA, SAPOs, MCM-41 and acronyms [67] In these latter studies, infrared (IR) spectroscopy and chemical analysis were mainly employed During the subsequent decades, research into solid ion exchange was extended to as-synthesized sodium forms of zeolites as starting materials, and the incorporation of cations of possibly catalytic importance such as lanthanum, iron, copper, manganese, and noble metals Similarly, the number of techniques suitable for monitoring solid-state ion exchange considerably increased In this chapter the potential of the various methods will be demonstrated by appropriate examples

Procedure: An efficient solid-ion exchange reaction between the starting zeolite and

the salt, which contains the desired in-going cation, requires an intimate mixture of the solids This mixture can be achieved, for instance, by careful milling or grinding the two components together In cases where interesting milling or grinding of the mixture may affect the integrity of the zeolite structure, it is preferable to prepare a

suspension of the powdered salt and the zeolite in an inert solvent such as hexane When the components have been thoroughly mixed by moving the suspension, the solvent may easily be removed volatile products such as hydrogen halides, ammonia, water, etc A reaction temperature of 400-550⁰C and a reaction time of a few hours are usually sufficient to bring the process of solid-state ion exchange to its maximum

In some cases, SE takes place at a lower temperatures (e.g at 400⁰C), however, in other cases more severe conditions are required In any case, the crystallinity of the zeolites exchange via SE after the reaction should be checked, although, in the absence of water vapor, the evolution of hydrogen halides (HCl, HF) does not affect the zeolite structure The possible effect of water vapor can be reliably avoided of the mixture of the solid-state reaction It turns out that, in fact, chlorides are the most suitable precursors for SE, while flouries and bromides seem to be less reactive In the case of iodides, elemental iodine is easily formed, and more complex cations such

as a CO32-, and SO42- frequently decompose upon SE In some instances, the reaction between the solids (salts and zeolites) can be facilitated in the presence of an oxidizing agent In the former case, the ion exchange may be mediated through the gas phase as a result of the formation of volatile reactants

1.3.2 Liquid ion-exchange

Liquid ion-exchange - LE (conventional exchange) is usually carried out by suspending the zeolite powder in aqueous solutions of salts which contain the desired in-going cation The suspension is stirred, frequently at temperatures higher than ambient Basically, conventional ion exchange is described by Eq 1.17 where, for the sake of simplicity, the chloride anion (Cl) is chosen as the counter ion of the in-going cation, Mn+, and the sodium form of the zeolite (Z) as the solid, is suspended

in m moles of water:

a[M(H2O)w]n+ + n.a.Cl- + b.Na+Z- + mH2O ↔ xMn+Z- + (a-x)[M(H2O)w]n+ + n.a.Cl

-+ n.x.Na+ + (b-nx)Na+Z- + mH2O (1.17)

Where M is the n-valent cation; Z is the mono valent negatively charged zeolite fragment; a, b, and x are stoichiometric numbers; m is the number of solvent (H2O) moles; and w is the number of the solvating H2O in the solvate shell of cation M Usually, the equilibrium does not lie heavily on the right hand side of Eq.1.17 Thus, the (partially exchanged) solid and the solution must be separated In order to achieve

a high degree of exchange, the procedure must typically be repeated several times, and consequently ion exchange in aqueous solution is a time consuming process Moreover, the procedure produces large amounts of waste solutions which must either be regenerated or discarded in an environmentally friendly manner Likewise,

in several instances it is very difficult or even impossible to introduce all particular cations in to a given zeolite using of conventional ion exchange-that is, in aqueous solution Examples are encountered where the cations are strongly solvated or available only in complexes, which are too bulky to enter the narrow pores of the particular zeolites

Comparison of LE and SE:

In some cases, LE, notably those with highly siliceous, acid-resistant zeolites, the cation of the as-synthesized form (Na+, K+) may be exchanged for protonts via a careful treatment with a mineral acid such as diluted HCl or HNO3 However, LE is usually carried out by suspending the zeolite powder in aqueous solutions of salts which contain desired on-going cation The suspension is stirred for some time, at the ambient condition or a little bit higher

Procedure: Basically, the LE was carried out by dissolving the metallic salt in desired

solution (or water with conventional) liquid/solid ratio, then, a calculated amount of zeolite is added into solution before adjusting pH = 6.5 and continue stirring for a time for guarantee the exchange level The powder is separated by filtering and washing with deionized water step Drying overnight and final step is calcination to get a product

1.3.3 Special-liquid ion exchange

In the presence of absolute methanol, the formation of surface -OH group would be prevented In this case, salt and absolute methanol were mixed and stirred until get homogeneous solution (about 2-3 hours) Then, the amount of CaSO4 (about 60g) was added and continuously stirred for about 2-3 hours

The concentration is: 0.05M (liquid/solid: = 100)

250ml Co(ac)-salt/250ml methanol/5g zeolite

1.4 Characterization techniques

1.4.1 ICP-OES

ICP OES has inductively coupled plasma optical emission spectrometry It is a laboratory technique which is used to determine the composition of elements in a sample using of a plasma and a spectrophotometer Therefore, the ICP-OES is composed of two components: ICP and optical spectrophotometer

In liquid phase, ICP-OES system can be used for analyzing samples The sample is often dissolved in water, this mixture is then driven through a nebulizer using a peristaltic pump Nebulizer conducts the solution into a spray chamber in which an aerosol is formed from the sample solution Then this aerosol enters an Argon plasma (plasma is one of the states of matter)

The atoms can be move to excited level when plasma energy is given to the sample due to the absorption behavior of energy by the atoms This excited state is unstable due to the high energy level, therefore, the excited atoms tend to go back to a lower energy level (ground level) Then, the energy is released The energy is released in the form of photons in emission rays/spectrum rays We can get the information of the photons wavelength of emitted ray by detecting these rays From these wavelengths and their intensities data, we may know the elements and their composition of samples This is because each element has their own characteristic emission spectrum Formation of the plasma phase includes supplying argon gas to the coil of a torch (made of quartz), followed by applying a high voltage to that coil Resulting the formation of an electromagnetic field inside the torch tube, which is created in ionization of Argon Eventually, the plasma phase of Argon can be obtained This plasma has a high density of electrons and a high temperature Therefore, the energy can be applied to catalysts to excite atoms in the sample Basic features of ICP-OES includes the ability to analyze several elements simultaneously, minimum chemical interferences, high sensitivity, wide linear final curve

1.4.2 XRD

Basically, there is about 95% of all solid materials can be described as crystalline

When x-rays interact with a crystalline substance (Phase), one gets a diffraction

pattern When an electron is in an alternating electromagnetic field, it will oscillate with the same frequency as the field Which means that, if an x-ray beam hits an random atom, the electrons around the atom start oscillating with exactly the same frequency as the incoming beam We will have destructive interference, which means that in almost cases, the combining waves are out of phase and we get no resultant energy leaving the solid catalyst samples The atoms in a crystal, however, are arranged in a regular pattern, and in a very few directions we will have constructive interference Therefore, by detecting and measuring the x-ray beams leaving the sample, we can well defined the waves will be in phase at various directions Consequently, a diffracted beam can be described as a beam composed of a large number of scattered rays mutually reinforcing one another

1.4.3 H 2 -TPR

The redox behaviors can cause the difference in De-NOx reaction TPR-hydrogen is

a useful technique for detection of metal oxides, mixed metal oxides, and metal oxides dispersed on a support The TPR method yields quantitative information of the reducibility of the oxide’s surface, as well as the heterogeneity of the reducible surface

1.4.4 Pyridine-adsorption

Pyridine-FTIR is commonly used to study the acidic properties of catalysts Pyridine

is a basic molecule (pKB = 8.75) and is often preferred to the other molecules, since

it is more selective and less reactive than NH3, it shows stronger adsorption on acidic surfaces than CO and it is relatively more sensitive to Lewis acidic sites than NO To achieve an accurate measure, in pyridine-FTIR, the catalyst surface has to be free from other adsorbed molecules (e.g water and carbon dioxide) by heating the catalyst

those molecules which would hinder pyridine adsorption on the catalyst surface In the OH-region (3500-3800 cm-1) several peaks can be seen upon drying due to dissociative adsorption of residual H2O molecules, forming hydroxyl groups on the surface Initially, a broad band can be observed due to the presence of H2O molecules However, temperature increase favors H2O desorption, thus allowing the observation

of one or more peaks due to hydroxyl groups present on the investigated materials

These peaks are characteristic of the nature of the surface hydroxyl groups

1.5 In situ-FTIR (Fourier Transformed Infrared Spectroscopy)

1.5.1 Introduction

There are 4 factorials, called degrees of freedom that can be: electronic, translational, rotational and vibrational, that atoms within a molecule are constrained by molecular bonds to move together in a certain specified directions In electronic motion, the electrons change energy levels or directions of spins The translational motion is characterized by a shift of an entire molecule to a new position while the rotational motion is revealed as a rotation of the molecule around its center of mass The vibration of molecule is described when the individual atoms within a molecule change their relative position

The wavelength-adsorption of molecule can happen when a molecule is exposed to wide-spectrum radiation, when some distinct parts of it are absorbed by the molecule There are only suitable wavelengths just can be adsorbed, and the absorbed wavelengths are the ones that match the transitions between the different energy levels of the corresponding degrees of freedom of that molecule The vibrational transitions are the most important transitions for IR spectroscopy because IR radiation

is too low to affect the electrons within the individual atoms and too powerful for rotational and translational transitions [74]