Luận án tiến sĩ: Nanoporous zeolite and solid-state electrochemical devices for nitrogen-oxide sensing

The sensors composed of tungsten/H2O2 deposited sensing electrodes and more hydrothermal stable Pt-loaded siliceous zeolite Y PtSY reference electrodes have stable NO2 signal at 5-10% wa

NANOPOROUS ZEOLITE AND SOLID-STATE ELECTROCHEMICAL DEVICES

FOR NITROGEN-OXIDE SENSING

DISSERTATION

Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University

UMI Number: 3241702

3241702 2007

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ABSTRACT

Solid-state electrochemical gas sensing devices composed of stabilized-zirconia electrolyte have used extensively in the automobile and chemical industry Two types of electrochemical devices, potentiometric and amperometric, were developed in this thesis for total NOx (NO + NO2) detection in harsh environments In potentiometric devices, Pt covered with Pt containing zeolite Y (PtY) and WO3 were examined as the two electrode materials Significant reactivity differences toward NOx between PtY and WO3 led to the difference in non-electrochemical reactions and resulted in a electrode potential With gases passing through a PtY filter, it was possible to remove interferences from 2000 ppm CO, 800 ppm propane, 10 ppm NH3, as well as to minimize effects of 1~13% O2,

CO2, and H2O Total NOx concentration was measured by maintaining a temperature difference between the filter and the sensor The sensitivity was significantly improved

by connecting sensors in series

ppm), high NOx selectivity relative to CO and oxygen, and linear dependence on NOxconcentration

The non-electrochemical reactions around the triple-phase boundary were studied

to understand the origin of the superior performance of WO3 on potentiometric NOxsensing From TPD, DRIFTS, XRD, Raman, and catalytic activity measurements, the interfacial reactions between WO3 and YSZ were found to dramatically reduce the NOx

catalytic activity of YSZ WO3 reacted with surface Y2O3 on YSZ and formed less catalytically active yttrium tungsten oxides and monoclinic ZrO2, which suppressed the non-electrochemical reactions around the triple-phase boundary These two products also decreased the oxygen vacancy density around the triple-phase boundary, slowed down the electrochemical oxygen reduction reaction, and in turn increased the NOx signal

The surface nanostructure of electrodes was modified by wet chemical processes

to change the non-electrochemical NOx reactions A thin WO3 coating prepared from the peroxytungstate solution with well-defined triple-phase boundaries resulted in higher sensitivity and better response times than the electrode fabricated from commercial WO3powders The electrodeposited porous Pt layer greatly increased the surface area and led

to a similar catalytic activity with PtY on NOx sensing The modified electrodes demonstrated the importance of the surface nanostructure and interfacial species for potentiometric NOx sensing The sensors composed of tungsten/H2O2 deposited sensing electrodes and more hydrothermal stable Pt-loaded siliceous zeolite Y (PtSY) reference electrodes have stable NO2 signal at 5-10% water in 600°C

Dedicated to my family

The current and past members of Dutta group, including Dr Nick Szabo, Dr Marla De Lucia, Dr Joe Trimboli, Dr Bob Kristovich, Dr John Doolittle, Dr Kefa Onchoke, Dr Dipankar Sukul, Dr Radha Vippagunta, Dr Joe Obirai, Dr Cheruvallil Rajesh, Toni Ruda, John Spirig, Haoyu Zhang, Mariela Oyola, Bill Schumacher, Brian Peebles, Jeremy White and Dedun Adeyemo are greatly acknowledged for their cooperation and friendship

I must also thank Professor Henk Verweij, Professor Umit Ozkan, Professor Nitin Padture, Professor Richard McCreery, Professor Patrick Woodward, as well as the members in their groups and CISM: Dr Jing-Jong Shyue, Dr Sehoon Yoo, Dr Krenar Shqau, Dr Jingyu Shi, Dr Di Yu, Dr Rick Watson, Matt Yung, Lanlin Zhang, Matt Mottern, Mike Rauscher, Inhee Lee, Jin Wang, Hong Tian, Pengbei Zhang, and Haihe Liang, for their fruitful discussion and instrumental support

Finally, I would like to thank my parents, my wife Ju-Ya, my son Eli, and my daughter Erica, for being my solid support through this process.

VITA

June 5, 1973 Born – Taichung, Taiwan

1995 B.S Chemistry National Chiao-Tung

University, Hsinchu, Taiwan

1997 M.S Chemistry National Taiwan University,

Taipei, Taiwan

1997 – 1999 Second Lieutenant, Military Police

1999 – 2001 .Process Engineer, Taiwan Semiconductor

Manufacturing Company, Tainan, Taiwan

2002 – present Graduate Teaching and Research Associate,

The Ohio State University

PUBLICATIONS

Research Publications

1 Jiun-Chan Yang, Hsin-Yen Hwang, Che-Chen Chang “Surface Microchemistry

Associated with Particle Bombardment on Ni(111)” Mat Res Soc Sym Proc

1997, V438, P.671

2 Jiun-Chan Yang, Prabir Dutta, “High temperature amperometric total NOx

sensors with platinum-load zeolite Y modified electrodes”, Sensors and Actuators

B, 2007, in press

FIELDS OF STUDY

Major Field: Chemistry

TABLE OF CONTENTS

P a g e

Abstract ii

Dedication .iv

Acknowledgments v

Vita vii

List of Tables xvi

List of Figures xvii

Chapters 1 Introduction .1

1.1 Nitrogen oxides chemistry .2

1.1.1 Oxidation state +1: N2O 2

1.1.2 Oxidation state +2: NO .2

1.1.3 Oxidation state +4: NO2, N2O4 .4

1.1.4 Oxidation state +3 and +5: N2O3, N2O5 .4

1.1.5 Total NOx 5

1.2 The impact and sources of NOx 5

1.2.1 The sources .5

1.2.1.1 Thermal NOx 6

1.2.1.2 Fuel NOx 6

1.2.1.3 Prompt NOx 7

1.2.2 The impact 7

1.2.2.1 Health hazards .7

1.2.2.2 Acid deposition 8

1.3 NOx emission control with NOx sensors 9

1.3.1 Vehicles 9

1.3.2 Power plants 11

1.4 The existing techniques of NOx sensing 12

1.4.1 Optical adsorption and emission 12

1.4.2 Chemiluminescence 12

1.4.3 Semiconductor 14

1.4.4 Electrochemical 15

1.5 High temperature electrochemical NOx sensors 15

1.6 Selectivity of electrochemical type sensors .25

References 28

2 Promoting selectivity and sensitivity for a high temperature YSZ-based potentiometric total NOx sensor by using a Pt-loaded zeolite Y filter 57

2.1 Experimental .60

2.1.1 Preparation and characterization of sensor materials .60

2.1.2 Catalytic NOx conversion measurements .61

2.1.3 Temperature programmed desorption 61

2.1.4 Sensor fabrication .62

2.1.5 Gas sensing measurements .63

2.2 Results 64

2.2.1 Electrode materials .64

2.2.2 Sensor characteristics .65

2.2.3 Interferences .66

2.2.3.1 CO2, CO interference .66

2.2.3.2 NH3 interference 67

2.2.3.3 Propane interference 67

2.2.3.4 Oxygen interference 68

2.2.3.5 Water interference .68

2.2.4 Stability .69

2.3 Discussion 69

2.3.1 Choice of electrodes 69

2.3.2 Role of PtY Filter 71

2.3.2.1 Interference from oxidizing gases 71

2.3.2.2 Oxygen interference 73

2.3.3 Strategies to increase sensitivity 74

2.4 Conclusion 75

References 76

3 Amperometric total NOx sensors with integrated Pt-loaded Zeolite catalytic

layers 95

3.1 Experimental 96

3.1.1 Pt-loaded zeolite Y preparation and characterization 96

3.1.2 Sensor fabrication .97

3.1.3 Sensor testing setup 98

3.2 Results 99

3.2.4 Stability .104

3.3 Discussion 104

3.4 Conclusion 108

References 110

4 The influence of the interfacial reactions at the electrode-solid electrolyte

interface to NOx adsorption and potentiometric NOx sensing 128

4.1 Experimental 130

4.1.1 Preparation and characterization of materials 130

4.1.2 Sensor fabrication and electrical measurements .131

4.1.3 Catalytic NOx conversion measurements .132

4.1.4 Temperature programmed desorption measurements .133

4.1.5 Diffuse reflectance infrared Fourier transform spectroscopy .133

4.2 Results 134

4.2.1 NO2 and O2 sensing behavior 134

4.2.2 Electrode impedance .135

4.2.3 Catalytic NOx conversion measurements .136

4.2.4 Temperature programmed desorption 137

4.2.5 X-Ray diffraction and Raman scattering 138

4.2.5.1 WO3-YSZ samples 138

4.2.5.2 WO3-Y2O3 samples 139

4.2.6 Diffuse reflectance infrared Fourier transform spectroscopy 140

4.2.6.1 NO2/O2 co-adsorption on YSZ and ZrO2 .140

4.2.6.2 NO2/O2 co-adsorption on Y2O3 and WO3-Y2O3 142

4.2.6.3 NO2/O2 co-adsorption on WO3-YSZ .142

4.3 Discussion 143

4.3.1 NOx adsorption and conversion on WO3, ZrO2, and YSZ 143

4.3.2 The interfacial reactions between WO3 and YSZ 144

4.3.3 The influence of interfacial reactions to NOx adsorption at the

triple-phase boundary 146

4.3.4 The influence of interfacial reactions to NOx sensing 146

4.4 Conclusion 150

References 151

5 Nanostructured Pt / WO3 electrodes and siliceous zeolite Y for sensor

optimization 173

5.1 Experimental 174

5.1.1.6 Pt-loaded Zeolite Y (PtY) reference electrode 177

5.1.1.7 Pt-loaded siliceous Zeolite Y (PtSY) reference electrode .177

5.1.2 Electrode characterization 178

5.1.3 Sensing measurements 178

5.2 Results 178

5.2.1 Crystal structures and surface nanostructure of WO3 electrodes 178

5.2.2 Surface nanostructure of electrodeposited Pt electrodes 180

5.2.3 Pt-loaded zeolite Y and siliceous zeolite Y characterization 181

5.2.4 NO2 sensing behavior 181

5.2.4.1 Pt sensing / PtY reference (Sensor D, E) 181

5.2.4.2 WO3 sensing / PtY reference (Sensor A, B, C) 182

5.2.4.3 Pt sensing / Pt reference (Sensor G) 182

5.2.4.4 WO3 sensing / PtSY reference (Sensor F) 183

5.3 Discussion 183

5.3.1 WO3 electrodes on YSZ from proxy-tungstate solutions .183

5.3.2 PtY and PtSY 186

5.3.3 Surface modified Pt electrodes .187

5.4 Conclusion .188

References 190

Bibliography 208

LIST OF TABLES

1.1 Typical concentrations of exhaust gas compositions 41 1.2 Examples of the two main types of potentiometric based sensors .45

2.1 The relative changes due to the presence of CO2, CO, propane,

NH3, oxygen, and water on NO signal with and without a 400oC PtY filter 79 4.1 Samples prepared in Chapter 4 155

LIST OF FIGURES

1.1 The Lewis structures of various nitrogen oxides 36

1.2 The equilibrium constants vs temperatures for reaction 1.4 and the ratio

of NO or NO2 over total NOx (NO+NO2) in 3% O2 37

1.3 Chemical pathway of NOx formation and destruction 38

1.4 The acid deposition process .39

1.5 NOx caps under the clear sky initiative .40

1.6 The conversion efficiency of three-way catalysts 42

1.7 Conventional automotive engine control system 43

1.8 Control system for new gasoline direction inject engines 43

1.9 Illustration of depleted grain boundaries and the effects of a reducing gas

on the conduction process 44

1.10 Illustration of a potentiometric gas sensor with an air reference electrode 46

1.11 Diagram of a equilibrium type NOx sensor 47

1.12(a) Mixed-potential when two electrochemical reactions have comparable kinetics

48

(b) Mixed-potential with faster oxygen kinetics 48

(c) Mixed-potential with slower NOx kinetics .49

(d) Mixed-potential with NO/NO2 in equilibrium 49

1.13 Mixed-potential signal of NO and NO2 at different temperatures 50 1.14 Mixed-potential type NOx sensor from Ceramatec Inc 51

1.15 Schematic cross-section of a typical amperometric oxygen sensor with a

channel-type diffusion barrier 52

1.16 Principle of an amperometric two-stage cell for the simultaneous detection of

oxygen and NO .53 1.17 A commercial NOx sensor from Siemens VDO / NGK 54 1.18 Design of commercial NOx sensors from NGK 55

1.19 Standard electrode potentials of various reactions at 900K with reference to

reaction 1.12 56 2.1 Sensor testing setup (PtY = Pt-Zeolite Y) 80

2.2 Potentiometric sensors composed of YSZ, WO3 sensing electrodes, and

PtY/Pt reference electrodes .81

2.3 TEM micrographs and TPD profile of NO peak (m/z =30) from a sample

of PtY following adsorption of 2500 ppm NO and 5% O2 at room

temperature .82 2.4 Measure of NOx equilibration as a function of temperature with 600 ppm

NO2 in 3% O2 83

2.8 Response curves and EMF–log ([NO]) plots for a 3-sensor array and a single

sensor 87

2.9 Schematic representation of gas composition during testing of interferences

from CO, CO2, propane, and NH3 88

2.10 Response transient of 1-13 ppm NO in the presence of 3% O2 and different levels of CO2 or CO 89

2.11 Response transients from 1-13 ppm NO in the presence of 3% O2 and NH3 90

2.12 Response curves of 1-13 ppm NO in the presence of 3% O2 and propane .91

2.13(a) Response curves of 10 ppm NO in different oxygen levels 92

(b) EMF–oxygen level plots for NOx with or without a PtY filter at 400°C 92

2.14 EMF vs water level in 10% O2 with a PtY filter at 400°C 93

2.15 Stability of 1-13 ppm NO sensing signal over a 7-day test period in 3% O2

with a PtY filter at 400°C 94

3.1 Schematic representation of sensors composed of YSZ, PtY, and

Pt electrodes 113

3.2 Sensor test setup 114

3.3 Homemade portable potentiostat powered by a 9V battery or

an AC/DC adapter .115

3.4 The connection of sensors and electrical measuring instruments .116

3.5 I-V curves acquired in 600 ppm NO or NO2 with 3% O2 117

3.6 I-V curves acquired on Pt and LSCFO electrodes in 3% O2 118

3.7 Calibration curves for 100-800 ppm NOx and 250-1000 ppm CO in 3% O2 119

3.8 Comparison of responses for 1-110 ppm total NOx between a type B sensor

and a chemiluminescent NOx analyzer 120

3.9 Calibration curves for a type B sensor with 1-120 ppm NOx in 3% O2 121

3.10 Comparison of responses for 1-3 ppm total NOx between a type D sensor

and a chemiluminescent NOx analyzer 122

3.11 Oxygen interference test on a type B sensor .123

3.12 Oxygen interference test on a type C sensor 124

3.13 Electrode interfacial impedance in difference oxygen partial pressure 125

3.14 Long term stability test for a type B sensor at 500°C over a 30-day

testing period 126

3.15 Calibration curves during the 30-day test period obtained from the data in Figure 3.13 127

4.1 Potentiometric sensors composed of YSZ, WO3, and PtY 156

4.2 EMF – log ([NOx]) plots acquired from the same sensor heated at 700°C

and 950°C 157

4.3 Response transient of a sensor in different oxygen concentrations after

heated at 700°C and 950°C 158

4.4 SEM micrographs for WO3 electrodes heated for 2 hours in air at 700°C

and 950°C 159

4.9 Raman spectra of the surface of a sensor heated at 950ºC, WYSZ26B,

ZrO2, and WO3 164

4.10 Room temperature XRD of 1:1 mole ratio WO3-Y2O3 treated at different

temperatures in air 165

4.11 Raman spectra of pure Y2O3 and 1:1 mole ratio WO3-Y2O3 treated at

different temperatures in air 166

4.12 DRIFTS spectra acquired after 1000 ppm NO2 and 10% O2 adsorption on the

300°C YSZ surface .167

4.13 DRIFTS spectra acquired after 1000 ppm NO2 and 10% O2 adsorption on the

500°C YSZ surface .168

4.14 DRIFTS spectra acquired after 1000 ppm NO2 and 10% O2 adsorption on the

300°C and 500°C ZrO2 surface .169

4.15 DRIFTS spectra acquired after 1000 ppm NO2 and 10% O2 adsorption on the

300°C Y2O3 and WY50 surface 170

4.16 DRIFTS spectra acquired at 300°C after exposing at 1000 ppm NO2/10% O2

for 20min and evacuating for 10 min 171

4.17 The overall influence on the reactions that affect the measured EMF from

using WO3 as the sensing electrode with YSZ 172

5.1 Potentiometric sensors composed of an YSZ electrolyte, Pt or WO3 sensing

electrodes, and zeolite Y coated reference electrodes 193 5.2 Apparatus for electrodeposition used on Sensor (E) and (G) 194 5.3 SEM micrographs of Pt electrodes and WO3 coating .195 5.4 Room temperature XRD of the WO3-coated platinum electrode 196 5.5 SEM micrographs of 700°C treated commercial WO3 powder and WO3

deposited on YSZ from the tungsten/H2O2 solution .197

5.6 Room temperature XRD of WO3 deposited on YSZ from the tungsten/H2O2

solution after heat treatment 198

5.7 Raman spectra of 700°C treated commercial WO3 powder, tetragonal YSZ,

monoclinic ZrO2, and WO3 deposited on YSZ from the tungsten/H2O2

solution after 700°C treatment 199

5.8 Raman spectra and XRD of the mixture of tungsten/H2O2 solution and

tetragonal YSZ powder after treated at 700°C for 2 hours 200 5.9 SEM micrographs of electrodeposited porous Pt .201 5.10 SEM micrographs of Pt-loaded siliceous zeolite Y and Pt-loaded zeolite Y 202 5.11 TEM micrographs of PtSY / PtY before and after the steam treatment 203 5.12 EMF – log[NO2] plots of sensors illustrated in Figure 5.1 .204

5.13 Signal transient in 3% oxygen and 40-800 ppm NO2 at 600°C from

Sensor (A) and Sensor (B) 205

5.14 Signal transient of Sensor (G) in 3% oxygen and 40-800 ppm NO2

at 500°C (a) before electrodeposition, (b) after electrodeposition 206 5.15 EMF – log[NO2] plots of Sensor (F) and Sensor (B) during the

steam treatment .207

CHAPTER 1

INTRODUCTION

This dissertation concentrates on the development and study of solid-state

electrochemical NOx (NO and NO2) gas sensors with an application towards based systems such as automobiles and coal-fired boilers in power plants Chapter 1 discusses nitrogen oxide chemistry as well as the general principles and issues of high temperature NOx sensors Chapters 2 and 3 focus on the development of a highly

combustion-selective and sensitive potentiometric and amperometric total NOx sensing system

employing Pt-loaded zeolite catalytic filters In Chapter 4, the fundamental study of heterogeneous catalytic activities of the electrode and electrolyte, as well as the

interfacial reactions is discussed in detail The influence of these factors to NOx

adsorption and potentiometric NOx sensing is also addressed Chapter 5 concentrates on the modification of the surface nanostructure of sensing and reference electrodes with wet chemical processes, peroxy-complex-deposition and electrodeposition, for performance enhancement and electrode nanostructure study Siliceous zeolite is also used to improve the hydrothermal stability of the reference electrode

1.1 Nitrogen oxides chemistry

Most of the world's nitrogen occurs naturally in the atmosphere as an inert gas contained in air, which consists of approximately 78% N2 by volume Nitrogen and

oxygen can form a series of compounds with oxidation states from +1 to +4:

1.1.1 Oxidation state +1: N 2 O

Nitrous oxide (N2O), also known as dinitrogen oxide or laughing gas, is a

colorless non-flammable gas with a pleasing odor and slightly sweet taste [1] N2O

molecule is stable at room temperature and has about 150-year lifetime in the atmosphere

It is produced mainly from adipic acid production, nitric acid manufacture, fossil fuels, biomass combustion and land cultivation [2]

N2O is isoelectronic with CO2 As shown in Figure 1.1a, the structure of N2O is linear (N–N–O) with the N–N bond order of 2.7 and N–O of 1.6 The activation energy needed to break the N–O bond is 250-270 kJ/mole [3] Temperatures above 900K are required to reach measurable decomposition:

Catalysts like metal loaded zeolite and perovskite oxides are helpful to accelerate

the 5σ antibonding ortibal makes NO an electron donor Coordination of NO to a Lewis

acid via the nitrogen atom is accompanied with a partial charge transfer from the 5σ

orbital and increases the bond order The nitrosonism ion (NO+) has the N–O bond order

around 3 and is more stable in the cationic vacancy on a solid surface [5]

The direct formation of NO from N2 and O2 at room temperature is difficult

because it is not thermodynamically favored:

The reaction has large positive Gibbs free energy (+83.7 kJ/mole) and thus very

small equilibrium constant at 25°C (K= 4.6x10-31) at room temperatures [3] At 2000°C,

about 0.4% NO is formed at equilibrium Therefore, the temperatures within an internal

combustion engine are high enough to make reaction 1.2 an important source of NO, as

will be discussed briefly in section 1.2.1.1

The back reaction of reaction 1.2 is the decomposition of NO:

In contrast, the reaction has large negative Gibbs free energy and hence a very

large equilibrium constant (K= 2.2x1030) at room temperatures However, the rate of

dissociation is very slow due to high activation energy of 364 kJ/mol [6] Catalysts can be

a great help to lower the activation energy and facilitate the decomposition [7] The

deNOx catalysis is an active research field and will be discussed in detail in section 1.3.1

and Chapter 2

At moderate temperatures (i.e < 700°C) NO can react with oxygen to form NO2:

A specific characteristic of this reaction is that the rate constant has a negative

temperature coefficient in the range of 400–900°C [8] In the presence of oxygen, NO

and NO2 are in equilibrium with each other and the NO/NO2 ratio is determined by the

temperature and oxygen concentration Figure 1.2 shows the graph of equilibrium

constants of reaction 1.4 and the calculated thermodynamic conversion of NO/NO2 in

3%O2 over a range of temperatures It can be observed that NO is more stable at high

temperatures and NO2 is more stable at low temperatures This reaction is the basis of

using catalytic filters to achieve total NOx detection, which will be addressed more

closely in Chapter 2 and 3

1.1.3 Oxidation state +4: NO 2 , N 2 O 4

NO2 is a highly toxic and reddish brown gas with choking odor [1] The molecule

has a V-shaped structure with an N–O bond distance of 1.19Å and a bond angle of 134°

An unpaired electron on nitrogen leads to paramagnetic characteristics At lower

temperatures, NO2 is in equilibrium with colorless/diamagnetic N2O4 by dimerization; at

temperatures above 140°C, equilibrium lies favorably toward NO2:

antisymmetric (O=N–NO2) and symmetric (O=N–O–N=O) [5] They can be

distinguished by spectroscopic methods

N2O5 is also an unstable compound and can be observed at low temperatures only

At room temperatures, it loses oxygen or converts to NO2+–NO3– ionic species [5] It is also called the acid anhydride of nitric acid since nitrogen has the same oxidation state (+5) in both compounds [1]

1.1.5 Total NO x

The term “NOx” or “total NOx” is a general term for the various nitrogen oxides produced during combustion Since N2O3, N2O5, and N2O4 are not stable at temperatures higher than 200°C, only NO and NO2 are considered as “NOx” Normally, nitrous oxide

is also not included because of the different origin and environmental impact [5] Only 2% of the total N2O emission in the United States is from vehicles [9] The U.S

Environmental Protection Agency (EPA) also defines nitrogen oxides as “all oxides of nitrogen except nitrous oxide” [10]

1.2 The impact and sources of NO x

1.2.1 The sources

The major source of NOx is the combustion of fossil fuels from power plants, vehicles, and airplanes NO accounts for 95% of all nitrogen oxide emissions Figure 1.3 shows that NOx formation in combustion processes is a complex process with many reaction intermediates [11] The amount of NOx is highly dependent on fuel type, air/fuel

ratio, and temperature Generally, three primary sources of NOx formation in combustion processes are reported [11-13]:

1.2.1.1 Thermal NO x

Thermal NOx refers to NOx formed through high temperature oxidation of the N2

from atmosphere following reaction 1.2 At high temperatures, N2 and O2 in the

combustion air disassociate into their atomic states and participate in a series of reactions described by Zeldovich mechanism [14] The rate of NO formation increases

exponentially with temperature Above 1100°C, it is generally the predominant NOx

formation mechanism in combustion processes

1.2.1.2 Fuel NO x

Fuel NOx is produced from the pyrolysis and oxidation of heterocyclic nitrogen compounds in fuel during combustion In the combustion process, the nitrogen bound in the fuel is released as a free radical and eventually forms free N2 or NO Fuel NOx is not

a concern for high-quality gaseous fuels like natural gas or propane, which normally have

no heterocyclic nitrogen compounds However, fuel NOx can contribute as much as 50%

1.2.1.3 Prompt NO x

NOx formed at the initial stage of combustion, also known as “prompt NOx” is from relatively fast and complex reactions between nitrogen, oxygen, and hydrocarbon radicals The radicals such as C, CH, and CH2 fragments derived from fuel react with oxygen and nitrogen in the combustion air, resulting in the formation of species

containing nitrogen and carbon such as NH, HCN, H2CN and CN, that can then oxidize

to NO Prompt NOx is an important mechanism in lower temperature combustion

processes Generally, it is much less important compared to thermal NOx formation at the higher temperatures in industrial combustion processes

1.2.2 The impact

1.2.2.1 Health hazards

Although NO has been well recognized as a biological messenger molecule to mediate blood vessel relaxation [15, 16], inhalation of larger amounts of NO can cause tightness of the chest, nausea, vomiting, and headache Prolonged exposure to NO can result in irritation of the eyes, violent coughing, difficulty in breathing, and cyanosis

NO2 is a deep lung irritant that can produce pulmonary edema and is more toxic than NO Exposure to 50 ppm NO2 may produce cough, hemoptysis, dyspnea, and chest pain Exposure to higher concentrations of NO2 (>100 ppm) can produce pulmonary edema that may be fatal or may lead to bronchiolitis obliterans [17, 18]

NO also contributes to ground level ozone formation by reacting with volatile organic compounds in the presence of ultraviolet light [19, 20] The mix of hydrocarbons, nitrogen oxides, and ozone are the major components of smog that frequently occurs in

urban and suburban areas Ozone in the lower atmosphere is undesirable since it can irritate the lungs, causing coughing and shortness of breath Higher exposures can lead to headaches, upset stomach, vomiting, and pulmonary edema

of building materials and paints

Dry deposition refers to acidic gases and particles The wind blows these acidic particles and gases onto buildings, cars, homes, and trees When the gases and particles are washed from trees and surfaces by rainstorms, the runoff water adds those acids to acid rain, making the combination more acidic than the falling rain alone Along with

NOx, SO2 is also another source of acid deposition

1.3 NO x emission control with NO x sensors

1.3.1 Vehicles

Exhaust gases in a typical spark ignition engine during high power use can reach temperatures as high as 900°C The engine exhaust temperature generally falls in the 400

to 600°C [13] The NOx concentration in exhaust gas is typically about 1000 ppm,

depending on the combustion condition and air/fuel ratio [22] The highest concentration occurs under high load conditions In addition to NOx, CO, CO2, H2, water, and

hydrocarbons are also formed Table 1.1 lists the typical concentration of these gases in automotive exhaust [22]

To reduce emissions of CO, hydrocarbon, and NOx, from 1975, catalytic

converters were widely introduced for automobiles in the U.S market The converters, also known as three-way catalysts (TWC), composed of a mixture of Pt, Rh, Pd and metal oxides dispersed onto a monolith to eliminate more than 90% of these three pollutants [22-24] The TWC has its highest conversion efficiency at the oxygen level near the stoichiometric air/fuel ratio (A/F) of 14.6 (14.6 g of air for each gram of fuel), as shown

in Figure 1.6 An oxygen sensor, also known as lambda sensor, is employed in a feedback loop with the engine computer to control the A/F in the combustion process in

accordance with the TWC requirements Figure 1.7 illustrates the engine control system composed of an oxygen sensor, control unit, and TWC [25]

Global warming and energy crisis are always the biggest concerns of the

development of automotive combustion control Efforts are currently underway to

decrease the amount of emission and minimize fuel consumption Manufacturers are now focusing on diesel engines and lean-burn gasoline (Gasoline Direct Inject, GDI) that have

higher fuel burning efficiency [25] However, the compression-ignited diesel engine has

relatively higher NOx emission than conventional gasoline engines equipped with TWC

GDI engines inject gasoline right into the combustion chamber of each cylinder, as

opposed to conventional multi-point fuel injection that happens in the intake manifold

[13] The problem of GDI is the oxygen excess in the exhaust of GDI engines operating

in the lean mode (more air) prevents the NOx reduction with existing TWC

To solve the problem of TWC and the high NOx emission from diesel engines,

two alternative methods are being developed for NOx abatement: NH3 or hydrocarbon

selective catalytic reduction (SCR) and NOx storage SCR involves reducing NOx over a

catalyst with NH3 or hydrocarbons to form N2 and water [7, 24, 26] In the case of

ammonia, typical SCR reactions occur as:

4NO + 4NH3 + O2 ↔ 4N2 + 6H2O (1.6) 2NO2 + 4NH3 + O2 → 3N2 + 6H2O (1.7)

In reality, storing NH3 on vehicles can be dangerous and costly Safer and less

expensive alkanes and alkenes have been evaluated as reductants in SCR with zeolites,

and noble metal catalysts [27, 28] However, hydrocarbons are less effective reductants

and sufficiently active catalysts have yet been discovered [26] In both SCR methods, a

Another promising method for NOx abatement is NOx storage NOx adsorption catalysts (NOx traps) are composed of noble metals, alkali, alkaline earth, or rare earth oxides [29, 30] At the lean condition, NOx is trapped on adsorption catalysts in the form

of nitrate (NO3–) Meanwhile, hydrocarbons, H2, and CO can be oxidized easily by the abundant oxygen When the engine is switched to the oxygen deficient fuel-rich

condition, instead of getting oxidized, hydrocarbons, H2, and CO can reduce adsorbed nitrates into N2 The adsorption catalyst is also regenerated in this step In addition to the

NOx trap, an oxidation catalyst is usually used to achieve higher hydrocarbons and CO conversion rate As can be seen for the scheme shown in Figure 1.8, a reliable and

accurate NOx sensor is the key to monitor the NOx breakthrough and trigger the

regeneration The estimated annual OEM (original equipment manufacturer) market in North America for NOx sensing elements is $32 million for diesel trucks and $400 million for passenger vehicles, which includes light-duty diesel trucks, automobiles and sports utility vehicles [31]

1.3.2 Power plants

Similar technology as mentioned above is employed for reducing NOx emission from boilers used in power generation and industry The major difference is the size and cost of the installation Both aqueous and anhydrous ammonia as well as urea are used as

a reductant [7, 32] In recent years, low-NOx burner technology has been developed to reduce NOx formation at the source [11] A NOx sensor is also needed to monitor the NOx

content to optimize the coal-burning process and control SCR

1.4 The existing techniques of NO x sensing

Several types of techniques have been developed to detect NOx in combustion environment:

1.4.1 Optical adsorption and emission

NOx has intense and characteristic adsorption in the UV and Infrared region Many commercial UV instruments can detect low-ppm level NOx with diode arrays Portable analyzers utilizing the non-dispersive infrared adsorption technique are also capable of detecting ppm level NOx in combustion environment Laser-induced

fluorescence detection has excellent sensitivity and provides ppt level detection limit [33] Generally, optical detection features good selectivity and is non-intrusive However, at least one optical window is required, which is difficult to incorporate in a practical device for on-board-diagnosis or closed-loop control Hence, optical detection is mainly used on external detection

1.4.2 Chemiluminescence

Chemiluminescence has long been recognized as the most sensitive method for

excited NO2 returns to the ground state by emitting radiation in the wavelength between

600-2800 nm, which can be monitored by a photomultiplier tube [35]:

The radiation has intensity maximum at approximately 1200 nm When ozone is

present in excess, the signal is proportional to the NO concentration of the sample gas In

fact, a large portion of NO2* is quenched by colliding with other molecules like H2O,

CO2, and O2 The pressure in the reaction chamber has to be effectively reduced to

achieve better detection limit

NO2 needs to be converted to NO in order to be measured by chemiluminescence

To detect NO2, the sample gas is passed through a converter heated to a specific

temperature between 350°C and 650°C Carbon is an inexpensive and popular material

to reduce NO2 to NO:

Although chemiluminescence technique has great performance for NOx detection,

delicate optics components, high voltage power supplies, ozone generators, vacuum

pumps impede its application for on-board-diagnosis and closed-loop control of vehicles

Chemiluminescence analyzers are now primarily used for research and industrial

detection

1.4.3 Semiconductor

Solid-state gas sensors based on semiconducting oxides have been widely studied since the discovery in 1953 that adsorption of gases onto the surface of a semiconductor produced a large change in its electrical resistance [36] In n-type semiconductors,

oxygen lattice defects can act as electron donors When oxygen is adsorbed on the crystal surface with a negative charge, the donor electrons in the crystal surface are transferred to the adsorbed oxygen, resulting in a positively charged charge As shown in Figure 1.9, the space charge layer formed in the process serves as a potential barrier against electron flow and the magnitude varies with the amount of adsorbed oxygen The depth of this layer is known as Debye length When the oxide crystallite size is larger than twice of the Debye length, the potential barrier at the grain boundary becomes a major factor in

determining the overall resistance [37] In the presence of an oxidizing or reducing gas, the surface density change of the negatively charged oxygen leads to a dramatic change

of electrical resistance The magnitude of change in electrical resistance is a direct

measure of the concentration of the target gas

WO3 [38-40] and SnO2 [41-43] are the two most popular materials for

semiconductor NOx sensors Ppb level of NOx can be easily detected by using

the response by pattern-recognition algorithm is another way to achieve better selectivity [44, 45]

The electrochemical NOx sensor is essentially an electrochemical cell composed

of a solid electrolyte and two or more electrodes The solid electrolyte is an ionic

conductor and usually needs to be heated to a certain temperature for obtaining sufficient conductivity [46] There has been a variety of ionic conductors used in gas sensor

applications such as yttria-stabilized zirconia (O2–)[47], Ce0.9Gd 0.2O1.9 (O2–)[48, 49], Na–β–alumina (Na+)[50], NASICON (Na+)[51], Ag–β″–alumina (Ag+)[52], NO+–β″–alumina (NO+)[53], and Ga11O17 (NO+)[54] In practice the solid electrolyte can be formed into any shape for the desired application

Various noble metal and metal oxides have been utilized as the electrode

materials Au, Pt, and Ag are widely used since they have excellent chemical and

mechanical stability and do not easily delaminate from the electrolyte Pt has excellent high temperature stability and is used in current oxygen sensors Metal-oxide electrodes and cermet electrodes with either electronic conductivity or mixed ionic/electronic

conductivity are getting more attention in recent years, because of the potential to offer

better performance in sensing and cheaper in cost than noble metal electrodes However, the mechanical stability of these electrodes is not as good as noble metal electrodes

1.5.2 Potentiometric type

1.5.2.1 Classification

For a potentiometric gas sensor, the potential difference between the two

electrodes is measured versus time in a changing gas condition The potential difference arises because of the asymmetry in the chemical reactions of the two electrodes towards the analyte gas [55-57] Electrochemical reactions that affect the electrochemical

potential are believed to take place at the triple-phase boundary (TPB) between the electrode, gaseous phase and the electrolyte In the case of a mixed conducting electrode, the TPB can be extended into three dimensions above the surface of the electrolyte Depending on the interaction between the target gas and the solid electrolyte, these gas sensors can be classified into three types [58-60] The solid electrolytes of Type I sensors have mobile ions in common with the target gases Type II employs a solid electrolyte with immobile ions related to the sensing gas Type III sensors utilize auxiliary phases to provide sensing properties and have no direct relation between the solid electrolyte and

1.5.2.2 Equilibrium-Potential Type

For equilibrium-potential type sensor, the measured potential can be simply

interpreted by the Nernst Equation It is useful to start with the equilibrium-potential type

oxygen sensor as it is one of the first commercial gas sensors and many sensor designs

used today are based on it [62] As shown in Figure 1.10, the oxygen sensor is composed

of yttria-stabilized zirconia (YSZ) as the solid electrolyte and two porous Pt electrodes,

one being an air reference and the other a sensing electrode exposed to the sensing gas

The concentration electrochemical cell structure can be represented as:

pO2 (air reference), Pt | YSZ | Pt, pO2(sensing) (1.11) Two half-cell reactions occur at the triple-phase boundary of each electrode,

which involves equilibrium between gaseous O2 and O2– ions from the YSZ lattice:

Assuming no electronic conduction in YSZ, the resulting cell voltage between the

two electrodes can be represented by the Nernst equation:

)ln(

)4(

sin , 2 , 2

g sen ref

pO pO F

RT

By knowing the cell voltage, temperature, and the reference pO2 (21% from air)

one can calculate the pO2 of the sensing environment Exhaust oxygen levels can be

around 1–5% for typical spark ignition engines and higher than 10% for diesel and

lean-burn gasoline

As oxygen-ion-conducting YSZ is used for equilibrium-potential type oxygen

sensors, equilibrium-potential type NO sensors can also be fabricated by using NO