The Development of Nickel Catalyst for Ammonia Decomposition

THE DEVELOPMENT OF NICKEL CATALYST FOR

AMMONIA DECOMPOSITION

BY

SUPAROEK HENPRASERTTAE

A DISSERTATION SUBMITTED IN PARTIAL FULFILLMENT OF

THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF

PHILOSOPHY (ENGINEERING AND TECHNOLOGY)

SIRINDHORN INTERNATIONAL INSTITUTE OF TECHNOLOGY

THAMMASAT UNIVERSITY

ACADEMIC YEAR 2018

Ref. code: 25615622300134IGC

THE DEVELOPMENT OF NICKEL CATALYST FOR

AMMONIA DECOMPOSITION

BY


A DISSERTATION SUBMITTED IN PARTIAL FULFILLMENT OF

THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF

PHILOSOPHY (ENGINEERING AND TECHNOLOGY)

SIRINDHORN INTERNATIONAL INSTITUTE OF TECHNOLOGY

THAMMASAT UNIVERSITY

ACADEMIC YEAR 2018

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ii

Abstract

THE DEVELOPMENT OF NICKEL CATALYST FOR AMMONIA

DECOMPOSITION

by


Bachelor of Science, Silpakorn University, 2000

Master of Engineering, Sirindhorn International Institute of Technology Thammasat

University, 2009

Doctor of Philosophy, Sirindhorn International Institute of Technology Thammasat

University, 2018

Ammonia decomposition can be a potential route for clean H2 production and a

solution for H2 transportation. This reaction requires a catalyst for the enhancement of

H2 production where Ni/Al2O3 catalysts are widely used. However, Ni/Al2O3 catalysts

still need developments to improve their catalytic activity. Modification of Al2O3

supports is a method to improve the activity of Ni catalysts. This study shows that the

catalytic activity of Ni/Al2O3 catalyst in ammonia decomposition was enhanced by

partial doping the Al2O3 framework with Sr, Y, Zr, or Ce for the formation of defects

in Al2O3. The improvements of catalyst properties in terms of Ni dispersion as well as

surface acidity and basicity are observed. The catalysts were characterized using BET,

XRD, XANES, SEM, TEM, Chemisorption, NH3-TPRx and activation energy

measurements. The lattice constant of Al2O3 from the XRD technique and the local

structure of Al2O3 from the XANES technique confirm the achievement of the partial

doping of Al2O3 with Sr, Y, Zr, or Ce as supports for Ni catalysts. The equivalent Ni

contents (19.61–21.69 wt%) in the catalysts analyzed by the ICP-OES technique ensure

the same Ni mass basis for evaluation of the catalyst activities in this study. When

compared to Ni/γ-Al2O3, Ni on the doped Al2O3 exhibits enhanced Ni dispersion that

results in higher catalytic activity and stability. The enhanced Ni dispersion of the

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Ni/doped Al2O3 catalysts corresponds to the small crystallize sizes and cluster sizes of

Ni in these catalysts, evidenced by the XRD and TEM techniques, respectively. The

NH3-TPRx results imply that desorption of surface nitrogen and recombine to form N2

could be the rate-determining step in the catalysis of NH3 decomposition. The

decomposition of low-concentration NH3 originated from synthetic urine was also

demonstrated over the Ni/Ce-doped Al2O3 catalyst. The results indicate that it is

potentially feasible to utilize the catalyst for H2 production as well as for wastewater

treatment. The role of dopants in the Al2O3 frameworks for the enhancement of catalyst

activity was discussed to understand the mechanism of NH3 decomposition over the

catalysts.

Keywords: Ammonia decomposition, Dopants in Al2O3 supports, Hydrogen

production from urine and wastewater, Ni/doped Al2O3, Support modification.

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Acknowledgements

Firstly, I would like to express my sincere gratitude to my advisor Assoc. Prof.

Dr. Pisanu Toochinda for the continuous support of my Ph.D. study and related

research, for his patience, motivation, and distinguished knowledge. His guidance

helped me in all the time of research and writing of this thesis. I could not have imagined

having a better advisor and mentor for my Ph.D. study.

Besides my advisor, I would like to thank my co-advisor Dr. Sumittra

Charojrochkul for her valuable guidance and giving an opportunity for my Ph.D. study.

I would like to thank the rest of my thesis committee: Assoc. Prof. Dr. Luckhana

Lawtrakul, Asst. Prof. Dr. Wanwipa Siriwatwechakul, and Assoc. Prof. Dr. Siwarutt

Boonyarattanakalin, for their insightful comments and encouragement, and also for the

hard question, which incented me to widen my research from various perspectives. I

would like to thank Dr. Pimpa Limthongkul for her trust on me, which encouraged me

to study in higher education.

My sincere thanks also goes to National Metal and Materials Technology Center

(MTEC), which provided me the access to the laboratory and research facilities. I have

to thank Dr. Wantana Klysubun and her team, for their kind supports in X-ray

absorption experiment at Synchrotron Light Research Institute (Thailand).

I would like to acknowledge the Bangchak Graduate Scholarship program of

Sirindhorn International Institute of Technology (SIIT), which financially supported for

my Ph.D. study. In addition, my Ph.D. study was also supported by the National

Research University Project of Thailand Office of Higher Education Commission

(NRU-15/2559) and Thammasat University Grant (TU-2/23/2558).

I thank my fellow lab mates: Dr. Chaicharn, Mr. Pumiwat, Ms. Sirintra, Mr.

Nathasak, Mr. Krit, Mr. Pawan and others for their helps in my experiments, and for all

the fun we have had in my Ph.D. study. I have to thank my buddy Mr. Mahinsasa

Rathnayake, who helped me a lot to improve my listening, speaking, and writing in

English as well as helping me in English proof reading and MATLAB codings.

Last but not the least, I would like to thank my family: my parents, my sister,

my brother, my wife, and my son for supporting me spiritually throughout writing this

thesis, my Ph.D. study, and my life in general.

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Table of Contents

Chapter Title Page

Signature Page i

Abstract ii

Acknowledgements iv

Table of Contents v

List of Tables viii

List of Figures ix

1 Introduction 12

1.1 Introduction 12

1.2 Objectives of study 15

1.3 Scopes of study 15

2 Literature Review 17

2.1 Ammonia as a hydrogen carrier 17

2.2 Catalysts for ammonia decomposition 20

2.2.1 Active component for NH3 decomposition 30

2.2.2 Promoter for NH3 decomposition 31

2.2.3 Support for NH3 decomposition 34

2.3 Defect structure and oxygen vacancy 38

3 Methodology 42

3.1 Materials 42

3.2 Supports preparation 43

3.3 Catalysts preparation 44

3.4 NH3 decomposition from pure NH3 46

3.5 NH3 decomposition from synthetic urine 47

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3.6 Supports and catalysts characterization 48

3.6.1 Nitrogen physisorption technique 48

3.6.2 X-ray diffraction technique 49

3.6.3 X-ray absorption near edge structure technique 50

3.6.4 Differential scanning calorimetry technique 51

3.6.5 Scanning electron microscope technique 52

3.6.6 Transmission electron microscope technique 53

3.6.7 Inductively coupled plasma optical emission

spectrometry technique 54

3.6.8 Chemisorption techniques 55

3.7 Determination of activation energy 57

4 Results and Discussions 58

4.1 Evaluation of Ni/Zr-doped Al2O3 catalysts in NH3

decomposition 58

4.1.1 Preparation of Zr-doped Al2O3 supports and their

properties 58

4.1.2 Characterization of Ni/Zr-doped Al2O3 62

4.1.3 Activities of Ni/Zr-doped Al2O3 for H2 production

from NH3 decomposition 65

4.1.4 Role of Zr in Al2O3 supports for NH3 decomposition 66

4.2 Effect of valency and ionic size of dopant in Al2O3 support

toward Ni catalyzed NH3 decomposition 69

4.2.1 Partial doping of Sr, Y, Zr, or Ce into Al2O3

frameworks and their properties 70

4.2.2 Evaluation of Ni-support interaction and crystal

structure of Ni catalysts 74 4.2.3 Ni active sites for activity and stability in NH3

decomposition 78

4.2.4 Evaluation of activation energy of NH3 decomposition 82

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4.2.5 Role of dopants in Al2O3 frameworks to reaction

mechanism and rate-determining steps of NH3

decomposition 84

4.2.6 Effect of dopant amount in Al2O3 support to Ni

catalyzed NH3 decomposition 93

4.2.7 Applications for H2 production from NH3

decomposition over Ni/Ce-doped Al2O3 using urine

and wastewater 96

5 Conclusions and Suggestions for Future Studies 99

References 101

Appendices 110

Appendix A 111

Appendix B 113

Appendix C 115

Appendix D 116

Appendix E 117

Appendix F 120

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List of Tables

Tables Page

2.1 Ammonia properties [17]. 18

2.2 Acute health effects of NH3. 19

2.3 Conditions and activities of various catalysts in NH3 decomposition. 21

2.4 Properties of 20 wt% Ni catalysts and H2 formation rates. 33

2.5 NH3 decomposition over supported 4.8 wt%-Ru catalysts. 34

2.6 H2 yields of Ni catalysts over supports. 36

2.7 Physicochemical properties of Ni catalysts supported on various metals [5].

36

2.8 Interaction of Pd on supports and CO on Pd [70]. 40

3.1 Chemical used in this study. 42

4.1 Physical properties of Al2O3 and Zr-doped Al2O3 supports. 61

4.2 Catalytic activities of Ni over Al2O3 and Zr-doped Al2O3 supports. 65

4.3 Surface acidity of catalysts over Al2O3 and Zr-doped Al2O3 supports. 67

4.4 Ni dispersion of catalysts over Al2O3 and Zr-doped Al2O3 supports. 68

4.5 Physical properties of supports. 71

4.6 Ni contents of catalysts. 78

4.7 NH3 conversion of Ni catalysts. 78

4.8 H2 formation rate of Ni catalysts. 79

4.9 Ni surface area of catalysts. 80

4.10 Characteristic properties of reduced catalysts with NH3 conversion. 85

4.11 Ni crystallize size of reduced catalysts. 87

4.12 Catalytic activities of Ni/Ce-doped Al2O3. 93

4.13 Ni dispersion and Ni surface area of catalysts. 94

A.1 Calculation of metal used for 15 g support preparation 111

A.2 Calculation of compound used for 0.5 M solution preparation. 111

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List of Figures

Figures Page

2.1 Energy densities of various energy carriers. 17

2.2 NH3 decomposition using CNTs-supported metal catalysts. 30

2.3 NH3 decomposition over Ru/CNTs modified by different metal nitrates. 32

2.4 Improvement of Cu-Zn dispersion using urea as a promoter [61]. 33

2.5 Ammonia conversion of 10 wt% Ni catalysts [7]. 35

2.6 Spinel structure of MgAl2O4. 37

2.7 Oxygen vacancy in crystal lattice. 38

2.8 Optimized structures of (A) Ni/regular MgO and (B) Ni/defective MgO [8].

39

2.9 Optimized structures of (A) CO-bound on Pd/regular MgO and (B) CO-

bound on Pd/defective MgO [70]. 39

3.1 Schematic of supports preparation. 43

3.2 Schematic of catalysts preparation. 44

3.3 H2 formation rate as function of residence time. 45

3.4 Schematic of NH3 decomposition testing system from pure NH3. 46

3.5 Schematic of NH3 decomposition from synthetic urine. 47

3.6 N2 physisorption instrument. 48

3.7 X-ray diffractometer. 49

3.8 Schematic of XANES experiment [76]. 50

3.9 Differential scanning calorimetry. 51

3.10 Scanning electron microscopy. 52

3.11 Transmission electron microscopy. 53

3.12 Inductively coupled plasma optical emission spectrometry. 54

3.13 Chemisorption catalyst analyzer. 55

3.14 Temperature programmed reactor and Mass Spectrometer. 56

4.1 XRD pattern of Zr-doped Al2O3 precursor dried at 110 °C. 59

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4.2 XRD patterns of the calcined Zr-doped Al2O3 supports; : γ-Al2O3;

: -Al2O3; : α-Al2O3; : tetragonal-ZrO2; : monoclinic-ZrO2. 60

4.3 DSC profiles of impregnated catalysts. 62

4.4 XRD patterns of catalysts over Al2O3 and Zr-doped Al2O3 supports. 63

4.5 SEM image and EDS profile of Ni/α-Al2O3 catalyst. 64

4.6 NH3-TPD profiles of catalysts over Al2O3 and Zr-doped Al2O3 supports. 66

4.7 CO-pulse profiles of catalysts over Al2O3 and Zr-doped Al2O3 supports. 67

4.8 XRD patterns of supports, : face center cubic-Al2O3;

: simple cubic-Al2O3; : tetragonal-ZrO2; : face center cubic-CeO2.71

4.9 XANES spectra of supports. 72

4.10 H2-TPR profiles of catalysts. 74

4.11 Ni-O interaction over regular site and oxygen vacancy site. 75

4.12 XRD patterns of catalysts, : NiO; : Ni; : γ-Al2O3; : ZrO2;

: CeO2; : CeAlO3. 76

4.13 Stability test of Ni catalysts, : Ni/γ-Al2O3; : Ni/Sr-doped Al2O3;

: Ni/Y-doped Al2O3; : Ni/Zr-doped Al2O3; : Ni/Ce-doped Al2O3. 81

4.14 Arrhenius plots of catalysts. 82

4.15 Correlation between experimental and calculated reaction rates,

: Ni/γ-Al2O3; : Ni/Sr-doped Al2O3; : Ni/Y-doped Al2O3;

: Ni/Zr-doped Al2O3; : Ni/Ce-doped Al2O3. 83

4.16 Ammonia decomposition mechanisms. 84

4.17 CO chemisorption profiles of catalysts. 85

4.18 NH3-TPD profiles of catalysts. 87

4.19 TEM images of catalysts (brightness adjusted). 88

4.20 CO2-TPD profiles of catalysts. 89

4.21 Role of oxygen vacancies in doped Al2O3 support for Ni catalyst. 90

4.22 N2 formation over catalysts from NH3-TPRx (A) with

their first derivatives (B). 91

4.23 CO chemisorption profiles of catalysts (10 wt% Ni and 20 wt% Ni). 94

4.24 XRD patterns of Ce-doped Al2O3 supports, : γ-Al2O3; : CeO2. 95

4.25 NH3 decomposition from synthetic urine, : Ni/γ-Al2O3;

: Ni/Ce-doped Al2O3. 97

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4.26 Schematic of NH3 decomposition from urea plant wastewater. 98

C.1 Lattice constants of unit cell. 115

D.2 X-ray absorption near edge structure (XANES). 116

E.3 CO-pulse injection technique and CO-pulse profile. 117

E.4 Illustration of TPD technique. 118

E.5 Illustration of TPR and TPRx techniques. 119

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Chapter 1

Introduction

1.1 Introduction

Nowadays, energy demand has been exponentially increased along with many

environmental problems. Researchers have tried to develop alternative energies, which

can lower greenhouse gas emissions into the environment. There are many renewable

energy sources, such as biomass, solar, wind, etc., but hydrogen can be considered as

an interesting choice for clean energy. Fuel cells generate electricity from

electrochemical reaction of hydrogen with atmospheric oxygen, which produces water

as an environmental benign byproduct. Hence, hydrogen is the major fuel for electricity

generation from fuel cells.

The major problems associated with the hydrogen fuel cell technology are the

high cost of hydrogen production, and greenhouse gas emissions from the conventional

hydrogen production processes, i.e., hydrocarbons reforming, water-gas shift, and

pyrolysis of hydrocarbons. The hydrogen production cost depends on the used catalyst,

the energy consumption in the production process, and the hydrogen purification

process [1]. The future success of hydrogen fuel cells not only depends on the fuel cell

efficiency and the hydrogen production cost, but also depends on low carbon emissions

from the hydrogen production process. Recently, most of the hydrogen fuel is produced

from the thermo-chemical process of natural gas, which emits greenhouse gases as

byproducts. Besides CO2 emissions, this process can also yield CO as a by-product,

which causes poisoning of electrodes in proton exchange membrane fuel cells

(PEMFCs) [2-4]. In addition, catalyst used in hydrogen production from hydrocarbons

can be deactivated by coking during the reaction process. Therefore, hydrogen

production from a carbon-free source, such as ammonia is an attractive choice for clean

hydrogen production.

Being carbon free, ammonia is also considered as a good H2 carrier for on-site

H2 generation, which has high H2 contents per unit volume. One mole of NH3 contains

1.5 mol of H2, which is 17.65 wt% or 108 kg of H2/m3 of NH3, embedded in liquid NH3

at 20 °C and 8.5 atm. Ammonia decomposition is shown in chemical reaction-1.

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2NH3 ↔ N2 + 3H2 ∆Hrxn = 46 kJ/mol reaction-1

Ruthenium (Ru) has been reported as the most active catalyst for ammonia

decomposition. However, Ru catalysts suffer from high cost and limited availability.

Iron (Fe) catalysts have a lower cost with abundant resources, but their catalytic

activities are very low, compared to Ru catalysts. Nickel over alumina catalysts

(Ni/Al2O3) has been reported as a replacement for expensive Ru catalysts for

economical hydrogen production. However, Ni/Al2O3 catalysts have a lower catalytic

activities for H2 production, compared to Ru catalysts. In general, Ni/Al2O3 catalysts

require reaction temperatures at 600–700 °C for high NH3 conversion while Ru

catalysts require only 450–550 °C. Thus, Ni/Al2O3 catalysts require some developments

to improve its catalytic activities for hydrogen production. Studies have provided many

approaches to improve the catalytic activities of Ni catalysts, such as using bimetallic

active sites, adding promoters in catalyst preparation, and using different supports. The

use of a new support other than conventional Al2O3 or activated carbon may alter the

catalytic activity, but the large surface area of these supports is essential for providing

a large number of Ni active sites and an improvement of Ni dispersion. Besides a high

surface area of Al2O3, thermal stability and the price of Al2O3 are important factors for

using Al2O3 as a catalyst support. The high thermal stability of Al2O3 allows the use of

a catalyst at high temperatures without support deformation, which can change the

catalytic activity. The price of Al2O3 is also economically attractive for low-cost H2

production. The literature reported that Ni dispersions of Ni/Y2O3, Ni/ZrO2, and

Ni/CeO2 were decreased at a higher reduction temperature of 600 °C, compared to that

of Ni/Al2O3 [5]. The main reason is the low surface areas of these supports, i.e., Y2O3

(43.0 m2/g or 7 m2/g), ZrO2 (85.7 m2/g), CeO2 (88.2 m2/g or 4 m2/g), and SrO (1.05

m2/g) [5-7]. Thus, modification of an Al2O3 support is an interesting approach to

enhance the Ni catalyst activity while maintaining a high surface area of the support.

Alumina supports can be modified using partial doping of atoms with different

valencies in the Al2O3 frameworks. This method may cause defect formation in the

Al2O3 framework due to the differences in atomic valency and/or ionic size to those of

the native Al atom [8]. Verwey reported that the crystal structure of γ-Al2O3 is similar

to the defect spinel structure, which contains cation vacancies in the structure [9]. These

vacancies increase the opportunity to insert dopant atoms and partially replace the Al

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atoms by dopant atoms in the Al2O3 framework. The insertion or replacement forms

oxygen vacancies, which alter the Ni-support interactions of our Ni catalysts and

improve their properties of Ni dispersion, acidic sites, and basic sites [8]. These

improvements facilitate the dehydrogenation of ammonia and desorption of nitrogen

adsorbates from the Ni active sites, which are the possible rate determining steps in

ammonia decomposition [10-12].

The active Ni catalyst in this study was further investigated for hydrogen

production from urine. The catalyst must be invulnerable to water vapor and active for

a low concentration of NH3 (e.g. urine). Urine contains 0.31–0.33 M urea, which can

be further converted to NH3 by urea dissociation [13, 14]. In addition, wastewater from

ammonia and urea plants contains ammoniacal nitrogen in the range of 400–1000 mg

of N/L, which can be a great source for H2 production from NH3 decomposition [15].

This process can also decrease the toxic ammonia in wastewater prior to environmental

release. Ammonia in water can be harmful to the aquatic life [13]. Therefore, NH3

decomposition using an active and water-resistant Ni over doped Al2O3 catalyst can

serve as a simultaneous solution for energy production and environmental problems.

This study provides an understanding of the reaction mechanism, Ni-support

interactions, and effects of doped Al2O3 supports for the H2 production process from

NH3 decomposition. Thus, the active catalysts of this study may lead to the

development of a novel clean hydrogen production process from NH3 and to other

related applications.

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1.2 Objectives of study

The main objective of this study is to develop the Ni catalysts for hydrogen

production from ammonia decomposition. The Ni catalysts were developed via the

modification of Al2O3 supports using the partial doping of heteroatoms in the Al2O3

frameworks. The main objective can be clarified as follows.

1. To evaluate the activities of Ni/doped Al2O3 catalysts, compared to Ni over

commercial γ-Al2O3 catalyst.

2. To in-depth study the role of dopants in the Al2O3 frameworks toward the

Ni catalysts properties.

3. To evaluate the activation energies of NH3 decomposition over the

catalysts.

4. To study the rate determining step in the NH3 decomposition mechanism.

5. To investigate the feasibility of the H2 production process from urine.

1.3 Scopes of study

The partial doping of heteroatoms in the Al2O3 frameworks leads to change the

supports properties, which affect Ni-support interaction and enhance the Ni catalysts

activities. In this study, the same range of ionic sizes with variation of valencies in Sr2+

(170 pm), Y3+ (160 pm), or Zr4+ (160 pm) were selected as the dopants to evaluate the

effects of dopant valency in the Al2O3 framework for the Ni catalyst activities. The

same valency with different ionic sizes in Ce4+ (200 pm) in comparison to Zr4+ was also

selected as the dopant to evaluate the effects of dopant ionic size in Al2O3 framework

for the Ni catalyst activities [16]. The Ni catalysts were prepared using the incipient

wetness impregnation method over the doped Al2O3 supports. The Ni catalyst over γ-

Al2O3 was also prepared using the same preparation method and used as the reference

in this study. The embedded dopants in the Al2O3 frameworks were investigated using

the XRD and XANES techniques. Interaction of Ni on each support was evaluated

using the H2-TPR technique to investigate the effect of dopants in the Al2O3

frameworks to the Ni active sites. The catalytic activity tests in ammonia decomposition

were conducted in a quartz reactor at a temperature range of 500–600 °C and

atmospheric pressure. The performances of the catalyst were evaluated in terms of the

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ammonia conversion and the hydrogen formation rate. The role of dopants in the Al2O3

frameworks for the Ni catalysts was studied using the chemisorption technique, i.e.,

CO-pulse, CO2-TPD, NH3-TPD, and NH3-TPRx to understand the NH3 decomposition

mechanism over the catalysts in this study. The decomposition of low-concentration

NH3, originated from synthetic urine was also evaluated over the active Ni/Ce-doped

Al2O3 catalyst to investigate the feasibility of catalyst utilization for a simultaneous

solution of H2 production and environmental problems.

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Chapter 2

Literature Review

2.1 Ammonia as a hydrogen carrier

The main obstacles in the H2 technology are H2 storage and transportation. Till

now, hydrogen storage materials, such as metal hydride, complex hydrides, and

organic-inorganic framework cannot satisfy the practical requirements of fuel cell

application. Moreover, transportation of compressed H2 by truck or rail is very

expensive [17, 18]. Ammonia is considered as a good hydrogen carrier for on-site H2

generation due to its several desirable characteristics. One mole of NH3 contains 1.5

mol of H2, which is 17.65 wt% or 108 kg of H2/m3 of NH3, embedded in liquid NH3 at

20 °C and 8.5 atm [3, 19-21]. This means that NH3 can be stored in liquid phase under

mild conditions in pressure vessels, and NH3 has a H2 density around 45 % higher than

that of liquid H2. Figure 2.1 shows energy densities of various energy carriers [3].

Figure 2.1: Energy densities of various energy carriers [3].

Ammonia also exhibits an energy density higher than that of compressed

hydrogen and close to fossil fuel, i.e., coal and oil. Ammonia can be decomposed to

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produce H2 for non-alkaline fuel cells or directly fed in an alkaline fuel cell to generate

electricity without carbon emission. Ammonia could also be an excellent fuel as it can

be fed directly in an internal combustion engine. Thus, ammonia can be one of the best

potential options for H2 carrier to produce H2 at on-site generation without CO and CO2

emissions. Ammonia distribution and transportation can be more attractive than

hydrogen distribution because, it is routinely transported in large tonnage by rail, truck,

barge, and pipeline [3, 17]. The physical and chemical properties of NH3 are listed in

Table 2.1 [17].

Table 2.1: Ammonia properties [17].

Ammonia is normally produced from the Haber-Bosch process by the reaction

of N2 and H2. The Haber-Bosch process was discovered in 1909 by German scientists,

Fritz Haber and Carl Bosch. This process is generally operated over Fe-based catalysts

at a temperature of 400–600 °C and a pressure of 200–400 atm. The hydrogen is

normally produced from various sources, such as natural gas, petroleum coke, and

Hydrogen content

H2 weight fraction 17.65 wt% H2 volume density 0.105 kg/liter

Solid phase

Melting point -78 °C Latent heat of fusion (1 atm at triple point)

-337.37 kJ/kg

Liquid phase

Vapor pressure

(20°C) 8.6 bar

Liquid density (1 atm at boiling point)

682 kg/m3

Boiling point (1 atm) 33.5 °C Latent heat of

vaporization (1 atm at boiling point)

1371.2 kJ/kg

Critical temperature 132.4 °C Critical pressure 112.8 bar

Gas phase

Gas density (1 atm at boiling point)

0.86 kg/m3 Compressibility (Z)

(1 atm at 15 °C) 0.9929

Specific gravity

(air=1) (1 atm at 20 °C) 0.597

Specific volume (1 atm at 20 °C)

1.411 m3/kg

Critical density 0.24 g/mL Thermal

conductivity 22.19 mW/mK

Miscellaneous

Water solubility (1 atm at 0 °C)

862 vol/vol Auto ignition

temperature 630 °C

Lower flammable

limit in air 15 vol%

Upper flammable

limit in air 28 vol%

Molecular weight 17.03 g/mol

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biomass. These feedstocks are thermally reformed to obtain hydrogen and other gases,

which can be reacted with water and nitrogen to produce ammonia. The majority of

NH3 is produced from hydrocarbon sources that emit carbon into the atmosphere, but

opportunities for carbon sequestration are available in NH3 production plants. World

ammonia production has increased over time as ammonia is used in both agricultural

and industrial sectors. In 2017, the United States Geological Survey (USGS) reported

that the world ammonia production was around 150 million metric tons (Mt) of

ammonia [22].

Safety, toxicity, and social acceptance must be considered for the use of

ammonia. Ammonia is a poisonous gas with exposure limits of 50 ppm in atmospheric

air for 8-hour exposure, reported by the Occupational Safety and Health Administration

(OSHA). However, ammonia is commonly used as an agricultural and industrial

chemical, which can be handled safely. The health effects from ammonia exposure and

acute effects from short exposure are listed in Table 2.2 [17].

Table 2.2: Acute health effects of NH3 [17].

Only a few documents have reported the health effects of NH3 from long term

and low-level exposures. Coon et al. studied animal inhalation on ammonia over rats,

guinea pigs, New Zealand albino rabbits, squirrel monkeys, and beagle dogs. They

Effect NH3 concentration in air

(by volume)

Least perceptible odor 5 ppm

Readily detectable odor 20–50 ppm

No discomfort or impairment of health for prolonged

exposure 50–100 ppm

General discomfort and eye tearing; no lasting effect

on short exposure 150–200 ppm

Severe irritation of eyes, ears, nose and throat, no

lasting effect on short exposure 400–700 ppm

Coughing, bronchial spasms 1700 ppm

Dangerous, less than half hour exposure may be fatal 2,000–3,000 ppm

Serious edema, strangulation, asphyxia, rapidly fatal 5,000–10,000 ppm

Immediately fatal 10,000 ppm (1 %)

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reported that mild nasal irritation was observed in 12 of 49 rats at 262 mg/m3 of NH3

exposure. An NH3 exposure at 455 mg/m3 made 32 of 51 rats dead at day 25, and 50 of

51 rats died after 65 days at the same NH3 exposure. At 470 mg/m3 of NH3 exposure,

13 of 15 rats and 4 of 15 guinea pigs died after 65 days, and eye irritation was found in

dogs and rabbits. These dogs also had nasal discharge. However, the NH3 exposure of

40 mg/m3 for 114 days and 127 mg/m3 for 90 days did not affect to any organs or tissues

of animals [23]. Therefore, NH3 can be the potential source for H2 production according

to the good properties and safety issues, and NH3 decomposition is worth to be

investigated for on-site H2 production.

2.2 Catalysts for ammonia decomposition

Ammonia decomposition is the reverse reaction of ammonia synthesis, and a

mildly endothermic process, which yields hydrogen and nitrogen. The temperature

required for efficient cracking depends on the catalyst, i.e., 450–550 °C for Ru catalysts

and 600–700 °C for Ni catalysts. The drawback of ammonia decomposition is

unconverted ammonia that can poison fuel cell electrodes even at a trace amount (0.1

ppm). However, unconverted ammonia can be reduced below 200 ppb using a suitable

absorber [18].

There are many catalysts used for ammonia decomposition, reported in the

literature. The performance of catalysts depends on active component, promoter, and

support as well as reaction conditions. Table 2.3 shows the catalytic system of ammonia

decomposition, including the catalytic performance and the reaction conditions.

Ref. code: 25615622300134IGC

21

Table 2.3: Conditions and activities of various catalysts in NH3 decomposition.

Ref./ Year Catalysts

Reaction

temperature

(°C)

Ammonia

conversion

(%)

Hydrogen rate

(mmol/min·gcat)1

(mL/min·gcat)2

Remarks

[24] / 1999 Fused Fe 500 20–80 - - Use the industrial catalyst PS3 INS.

- NH3 conversion depend on space velocity of NH3.

[25] / 2000

Nitrided MoNx/α-Al2O3

600–750

75–99.7

-

- The Mo and NiMo catalysts were prepared using the

impregnation method.

- The catalysts were further nitrided using temperature

programmed reaction with NH3. Nitrided NiMoNy/α-Al2O3 78–99.9

NiO/MgO 69.9–99.9 - Commercial catalyst.

[26] / 2001

10 % Ni/SiO2 400–650 1.4–70.0 0.44–21.11

- The catalysts were prepared using the wet

impregnation method. 10 % Ir/SiO2 400–700 3.9–98.0 1.2–30.61

10 % Ru/SiO2 400–650 14.3–99.0 4.5–30.91

65 % Ni/SiO2/Al2O3 400–650 3.5–97.0 1.1–30.31 - Commercial catalyst from Aldrich chemicals.

[27] / 2002

Ni-Pt/Al2O3 500–600 14.5–78.1

-

- Commercial catalyst (G43-A, United Catalyst).

Raney Ni 600–700 18.8–81.6 - Commercial catalyst (2800, Grace Davison).

Ru/Al2O3 500–700 6.5–84.5 - Commercial catalyst (146, Johnson Matthey).

[28] / 2002

Ru/active carbon

400 - -

- The catalysts were prepared over the commercial

active carbon (RO-08, Norit Company) using the


- The catalysts activities were reported as the reaction

rate, which can be ranked as Cs-Ru > Ba-Ru > Ru.

Ba-Ru/active carbon

Cs-Ru/active carbon

Ref. code: 25615622300134IGC

22


Reaction

temperature

(°C)

Ammonia

conversion

(%)

Hydrogen rate

(mmol/min·gcat)1

(mL/min·gcat)2

Remarks

[29] / 2003

0.5 wt% Ru/Al2O3

580 - -

- The Pd, Pt, Ir, Rh, and Ru are commercial catalysts

from Alfa Aesar.

- The remaining catalysts were prepared using the wet


- The catalysts activities were reported as Turn over

frequency of NH3 (TOF), which can be ranked as Ru

> Ni > Rh > Co > Ir > Fe > Pt > Cr > Pd > Cu > Te.

- The rate determining step of Ru, Ni, Co, Fe, and Cr

catalysts is the recombination of nitrogen adsorbate

on the active sites.

1.0 wt% Ni/Al2O3

0.5 wt% Rh/Al2O3

1.0 wt% Co/Al2O3

1.0 wt% Ir/Al2O3

1.0 wt% Fe/Al2O3

1.0 wt% Pt/Al2O3

1.0 wt% Cr/Al2O3

0.5 wt% Pd/Al2O3

1.0 wt% Cu/Al2O3

1.0 wt% Te/Al2O3

[30] / 2004

2 % Fe/charcoal 550–850 10–100

-

- The Fe over charcoal catalyst was prepared using the

pyrolysis method.

- The Fe over activated carbon was prepared using the


6 % Fe/charcoal 550–850 20–100

8 % Fe/activated carbon 750 25

6 % Ca/charcoal 750–850 40–100

Ref. code: 25615622300134IGC

23


Reaction

temperature

(°C)

Ammonia

conversion

(%)

Hydrogen rate

(mmol/min·gcat)1

(mL/min·gcat)2

Remarks

[31] / 2004

Ba-Ru/MgAl2O4

430 - -

- The catalysts were prepared using the incipient

wetness impregnation method.

- The catalytic activities were reported in terms of

TOF, which can be ranked as Cs-Ru/C > Ba-Ru/C >

Cs-Ru/MgAl2O4 > Ba-Ru/MgAl2O4.

Cs-Ru/MgAl2O4

Ba-Ru/C (carbon)

Cs-Ru/C (carbon)

[32] / 2004

Ru/CNTs (carbon nanotubes)

350–500

5–45

581

- This study reported that the best catalytic activity

was obtained at 1:1 wt. ratio for MgO-CNTs support.

- Hydrogen production is 58 mmol/min·gcat at 80 %

NH3 conversion.


wetness impregnation method with 4.8 wt% Ru.

K-Ru/CNTs 30–100

Ru/MgO-CNTs 5–55

K-Ru/MgO-CNTs 40–100

Ru/MgO 5–35

K-Ru/MgO 10–70

[33] / 2004

Ru/CNTs

450–550

43.7–100 14.6–33.51

- Using acetone as the solvent for catalyst preparation.


wetness impregnation method with 5 wt% Ru.

- The Ru/K molar ratio of catalyst is 1.

Ru/MgO 30.9–91.8 10.4–30.71

Ru/AC 28.7–78.9 9.6–26.41

Ru/ZrO2 24.8–77.0 8.3–25.81

Ru/Al2O3 23.4–73.7 7.8–23.51

K-Ru/CNTs 400–450 59.8–97.3 20.0–32.61

[34] / 2004 Rh/CNTs

350–500 1–11 0.2–4.191

Ni/CNTs 1–8 0.10–2.901

Ref. code: 25615622300134IGC

24


Reaction

temperature

(°C)

Ammonia

conversion

(%)

Hydrogen rate

(mmol/min·gcat)1

(mL/min·gcat)2

Remarks

[34] / 2004

(continue)

Pd/CNTs

350–500

1–3 0.03–1.431 - The catalysts were prepared using the incipient

wetness impregnation method with the metal loading

of 4.75 × 10-4 mol/gcat.

- Using acetone as the solvent for catalyst preparation.

- GHSV of NH3 = 30,000 mL/(h∙gcat)

Fe/CNTs 1–2 0.02–0.651

Pt/CNTs 1–3 0.05–1.411

Ru/CNTs 8–85 2.11–28.351

[35] / 2005

Ni/Fumed SiO2

400–700

4.6–93.4 1.5–31.31

- The catalysts were prepared with 5 wt% Ni.

- IMP = incipient wetness impregnation method.

- TIE = template ion-exchange method.

- The molar ratio of K-Ni is 2:1.

- KOH was used as promoter.

Ni/MCM-41 (IMP) 8.0–95.4 2.7–32.01

Ni/MCM-41 (TIE) 6.3–98.8 2.1–33.11

K-Ni/SBA-15 8.2–93.6 2.8–31.31

K-Ni/Fumed SiO2 6.8–95.7 2.3–32.11

K-Ni/MCM-41 (IMP) 8.6–97.8 2.9–32.81

K-Ni/MCM-41 (TIE) 9.5–100 3.2–33.51

K-Ni/SBA-15 9.5–94.9 3.2–31.81

[36] / 2007

10 % Ni/γ-Al2O3

400

32

- - The catalysts were prepared using the wet


10 % Ni-0.5 % Ir/γ-Al2O3 36

10 % Ni-0.7 % Ir/γ-Al2O3 44

10% Ni-1 % Ir/γ-Al2O3 34

[37] / 2007 5 wt% Ru/fly ash

550 6.9 4.21 - The supports (fly ash and red mud) were obtained

from industrial wastes. 5 wt% Ru/red mud 11 7.361

Ref. code: 25615622300134IGC

25


Reaction

temperature

(°C)

Ammonia

conversion

(%)

Hydrogen rate

(mmol/min·gcat)1

(mL/min·gcat)2

Remarks

[38] / 2007

Ni

600–650

39.0–65.7 87.8–147.8* - The catalysts were prepared using the wet-lay

papermaking process to form the thin-sheet micro-

fibrous nickel.

- * Unit of the H2 production rate is mL/min.

71.43 % Ni/28.57 % Al 62.6–88.7 140.9–199.6*

99.86 % Ni/0.14 % Ce 64.6–89.6 145.4–201.6*

95.49 % Ni/0.04 % Ce/4.49 % Al 81.0–99.4 182.3–223.7*

[39] / 2007

Ru/AC (activated carbon)

550

14.4 4.41




- This work investigated the importance of carbon

structure as the support and found that graphite is the

most suitable support for Ru catalyst.

Ru/CMK-3 (mesoporous carbon) 22.7 7.01

Ru/CB-C (synthesized carbon black) 26.1 8.01

Ru/CB-S (commercial carbon black) 52.7 16.21

Ru/CNTs 84.7 26.01

Ru/GC (graphite carbon) 95.0 29.11

[40] / 2007

Ru/CMK-3

550

22.7 7.01



- The CMK-3 supports were modified with various

acid and alkali solutions.

Ru-Cl/CMK-3 37.4 11.51

Ru-SO4/CMK-3 35.6 10.91

Ru-PO4/CMK-3 33.9 10.41

Ru-Li/CMK-3 15.2 4.71

Ru-Na/CMK-3 50.8 15.61

Ru-K/CMK-3 78.9 24.21

Ru-Ca/CMK-3 48.5 14.91

Ref. code: 25615622300134IGC

26


Reaction

temperature

(°C)

Ammonia

conversion

(%)

Hydrogen rate

(mmol/min·gcat)1

(mL/min·gcat)2

Remarks

[41] / 2008

Ni1.2/Al2O3

450–600

16–98 -

- The catalysts were prepared using the co-

precipitation method.


- The molar ratio of Ni/Al is 1.20.

- Ce was used as promoter with the molar ratio of

Ce/Ni from 0.07 to 0.5.

Ni1.2Ce0.07/Al2O3 21–97

Ni1.2Ce0.10/Al2O3 29–100 24.11

Ni1.2Ce0.20/Al2O3 27–99 -

Ni1.2Ce0.50/Al2O3 22–95

[42] / 2010

CNT

500–600

1–10

-

- Nitrogen doping was done in the plasma treatment

process.

- The catalysts were prepared using the dry

impregnation method with 0.8 wt% Ru.

Ru/CNT-undoped N 22–72

Ru/CNT (N-doped at 200 W) 32–98

Ru/CNT (N-doped at 400 W) 29–92

[4] / 2010

Nano-Fe@SiO2

400–550

0–59 20.31 at 550 °C

- The catalysts were prepared with TEOS at Si/metal

ratio equal to 0.2 to form nano-metal encapsulated

by SiO2 shell.

Nano-Ni@SiO2 8–61 20.71 at 550 °C

Nano-Co@SiO2 1–39 -

Nano-Ru@SiO2 35–100 33.51 at 550 °C

[43] / 2011

3.5 wt% Fe/CNFs

450–600

2–48

- - GHSV of NH3 = 6,500 mL/(h∙gcat). 3.5 wt% Fe/mica 6–88

3.5 wt% Fe-CNFs/mica 10–98

Ref. code: 25615622300134IGC

27


Reaction

temperature

(°C)

Ammonia

conversion

(%)

Hydrogen rate

(mmol/min·gcat)1

(mL/min·gcat)2

Remarks

[44] / 2012

Ni/Al2O3

550

32.7 35.02

- The catalysts were prepared using the impregnation

method with 3.6 wt% Ni, except 2Ni/Ce0.8Zr0.2O2

(7 wt% Ni).

- The Ce0.8Zr0.2O2 supports were prepared using sol

gel, co-precipitation, and surfactant-assisted method.

- SA = surfactant-assisted

Ni/CeO2 60.8 65.12

Ni/Ce0.8Zr0.2O2-sol gel 80.2 85.92

Ni/Ce0.8Zr0.2O2-co-precipitation 84.5 90.52

Ni/Ce0.8Zr0.2O2-SA 91.0 97.52

2Ni/Ce0.8Zr0.2O2-SA 91.6 93.72

[45] / 2012

10 wt% Ni/Al2O3

550

70

-


method.

- This work also studied effect of calcination of the

Ni/La2O3 catalysts and reported that the calcination

at 600 °C yields the highest catalytic activity.

10 wt% Ni/SiO2 60

10 wt% Ni/MgO 45

10 wt% Ni/CeO2 45

10 wt% Ni/TiO2 35

10 wt% Ni/ZrO2 10

10 wt% Ni/La2O3 63

20 wt% Ni/La2O3 65

40 wt% Ni/La2O3 79

Ref. code: 25615622300134IGC

28


Reaction

temperature

(°C)

Ammonia

conversion

(%)

Hydrogen rate

(mmol/min·gcat)1

(mL/min·gcat)2

Remarks

[46] / 2013

Co

500

4.5 1.21 - The Co catalyst was prepared using the precipitation

method.

- The modified Co catalysts were prepared using the

co-impregnation with the solutions of calcium,

potassium, and aluminum nitrates.

- GHSV of NH3 = 24,000 mL/(h∙gcat).

Modified Co 40.1 8.31

Modified Co with 0.25 wt% Mn 23.0 6.21

Modified Co with 0.16 wt% Cr 35.0 7.31

Modified Co with 0.28 wt% Cr 32.0 6.81

[47] / 2014

MRM

700

63.4 21.21

- The catalysts were prepared using the homogeneous


- MRM = modified red mud.

3 wt% Ni/MRM 69.3 23.21

6 wt% Ni/MRM 72.0 24.11

9 wt% Ni/MRM 75.9 25.41

12 wt% Ni/MRM 93.2 31.21

15 wt% Ni/MRM 97.9 32.81

18 wt% Ni/MRM 82.0 27.51

[48] / 2015

Ni/Al2O3

550

60.0

-


method.

- Y, La, Ce, Pr, Nd, Sm, Eu, and Gd were used as

promoter.

Y-Ni/Al2O3 87.5

La-Ni/Al2O3 92

Ce-Ni/Al2O3 81

Pr-Ni/Al2O3 90

Ref. code: 25615622300134IGC

29


Reaction

temperature

(°C)

Ammonia

conversion

(%)

Hydrogen rate

(mmol/min·gcat)1

(mL/min·gcat)2

Remarks

[48] / 2015

(continue)

Nd-Ni/Al2O3

550

91

-


method.

- Y, La, Ce, Pr, Nd, Sm, Eu, and Gd were used as

promoter.

Sm-Ni/Al2O3 85

Eu-Ni/Al2O3 83

Gd-Ni/Al2O3 83

[5] / 2016

Ni

450–600 -

0.3–12.4*

- The catalysts were prepared using the co-


- * = H2 yield (%).

Ni/Y2O3 23.4–99.9*

Ni/CeO2 28.6–99.0*

Ni/MgO 20.9–98.2*

Ni/La2O3 21.2–98.0*

Ni/Al2O3 16.9–98.0*

Ni/ZrO2 14.2–95.0*

[7] / 2016

Ni/Al2O3

550

70

-


method.

- The Ni loading was fixed at 10 wt%.

Ni/La2O3 62

Ni/CeO2 23

Ni/Sm2O3 80

Ni/Gd2O3 80

Ni/Y2O3 87

Ref. code: 25615622300134IGC

30

2.2.1 Active component for Catalyst in NH3 decomposition

The component of catalysts must be investigated to improve the catalytic

activities. In general, heterogeneous catalysts are composed of three major parts, i.e.,

active component, support, and promoter. According to Table 2.3, there are many

metals, alloys, and compounds, which have been tested for NH3 decomposition. Most

of the studies in the literature reported the use of ruthenium (Ru), Iron (Fe), and Nickel

(Ni) as the active components in catalysts. A few studies reported the use of iridium

(Ir), rhodium (Rh), platinum (Pt), and palladium (Pd) as the active components in the

catalysts. Some metal nitrides and carbides were also reported as the active components

in catalysts. Figure 2.2 shows the activities of metals over CNTs supports with the same

metal loading at reaction temperature of 400 °C [18].

Figure 2.2: NH3 decomposition using CNTs-supported metal catalysts [18].

Figure 2.2 shows that Ru catalyst is the most active catalyst for NH3 decomposition.

Goodman et al. also reported that the catalytic activity of NH3 decomposition can be

ranked as Ru > Ir > Ni with the same metal loading, as shown in Table 2.3. The activity

of Ru catalyst surpassing other catalysts can be explained by the turn over frequency

(TOF) of H2 production over the catalysts, which shows the same trend as the catalytic

activity ranking. The re-combinative desorption of nitrogen from the metal surfaces has

been proposed as the rate-limiting step in NH3 decomposition reaction, which implies

Ref. code: 25615622300134IGC

31

that nitrogen has a stronger bound on Ni than that of Ir and Ru [26]. The drawback of

Ru and Ir catalysts is their high cost and limited availability. Even though the activity

of Ni catalyst cannot compete with that of Ru and Ir catalysts, Ni catalyst exhibits the

same range of the catalytic activity to other noble metals, i.e. rhodium (Rh), platinum

(Pt), and palladium (Pd). However, a high reaction temperature of 600–700 °C was

required for a high catalytic activity of Ni catalysts to increase the rate in re-

combinative desorption of nitrogen from the Ni active sites [26, 27, 35, 38]. One

approach of Ni development is using bimetallic catalyst. Han et al. reported the use of

bimetallic Ni-Ir/γ-Al2O3 catalysts, which exhibit a bit higher of NH3 conversion than

Ni/γ-Al2O3 catalyst [36]. In terms of cost, Ni can be a potential active component in

catalyst for NH3 decomposition, but it still needs an improvement of catalytic

performance.

2.2.2 Promoter for Catalyst in NH3 decomposition

Another approach to increase the catalyst activity is the use of a promoter. A

promoter is a substance added to a solid catalyst to improve the catalyst performance

of a chemical reaction. Normally, a promoter has little or no catalytic effect by itself,

but it can enhance the catalyst activity. Some promoters interact with active components

in catalysts, and thereby alter their chemical effect on a catalyzed substance. Alkali,

alkaline earth, or rare earth metal ions have been reported as effective promoters for

supported Ru or Fe catalysts in NH3 synthesis. A promoter can prevent the sintering of

Ru or Fe clusters during the thermal treatment process in NH3 synthesis [49-57].

Ref. code: 25615622300134IGC

32

Figure 2.3: NH3 decomposition over Ru/CNTs modified by different metal nitrates

[58].

For NH3 decomposition, Wang et al. reported the use of alkali, alkaline earth,

or rare earth metal ions as promoters for Ru catalysts. The catalysts activities in NH3

decomposition were conducted at reaction temperature of 400 °C as shown in Figure

2.3 [58]. Considering in the same group (K, Na, and Li; Ba and Ca), a high

electronegativity of the promoter results in a low catalytic activity for NH3

decomposition. Li et al. [35] reported the use of KOH as a promoter in Ni catalysts for

NH3 decomposition. The KOH promoter added in Ni catalysts resulted in a little

improvement of the catalytic activities, as shown in Table 2.3. Lanthanum and cerium

were also reported as promoters in Ni catalysts with little improvements in the catalysts

activities for NH3 decomposition [41, 59]. Okura et al. investigated the Ni catalyst

activities using various promoters for NH3 decomposition. The catalytic activities were

improved significantly for the Ni catalysts using promoters, such as lanthanum (La),

praseodymium (Pr), and Neodymium (Nd), as reported in Table 2.3. Okura et al.

reported that high activities of these catalysts could come from depletion of the

hydrogen inhibition phenomenon. The use of promoters facilitated desorption of

hydrogen from the Ni active sites of catalysts, evidenced by the H2 desorption data [48].

However, the use of rare elements as promoters suffers from their prices and

H2 formation rate

Ref. code: 25615622300134IGC

33

availabilities, which increases the cost of catalyst production. Thus, the use of basic

chemical as a promoter can be economically attractive for catalyst production.

Urea has been reported as a promoter in the precipitated Cu-Zn catalysts, which

can increase the homogeneity and increase surface area of catalysts [60]. Our previous

work found that, the use of urea as a promoter in the impregnated Cu-Zn/Al2O3 catalysts

can increase the dispersion of Cu-Zn clusters on the Al2O3 support, which enhance the

catalytic activity for the methanol steam reforming, as shown in Figure 2.4 [61].

Figure 2.4: Improvement of Cu-Zn dispersion using urea as a promoter [61].

The use of urea as a promoter in Ni catalysts was also investigated for H2 production

from NH3 decomposition in this study. The catalysts activities were tested using 30

vol% NH3 solution with 30 mL/min Ar as a carrier gas at 550 °C. The preliminary data

show that the use of urea in Ni catalyst can also improve Ni dispersion and Ni surface

area of the catalyst, resulted in a higher activity of Ni catalyst for NH3 decomposition,

as shown in Table 2.4. Therefore, the Ni catalysts were prepared using urea as a

promoter to improve Ni dispersion and Ni surface area of the catalysts in this study.

Table 2.4: Properties of 20 wt% Ni catalysts and H2 formation rates.

Catalysts Ni dispersion Ni surface area H2 formation rate

(%) (m2/g of Ni) (μmol/min·gcat)

Ni/γ-Al2O3 1.14 7.58 1590

Ni/γ-Al2O3-2 mol urea 1.58 10.54 1856

Cu-Zn/Al2O3 Cu-Zn/Al2O3 with 1 mol urea

10 μm

10 μm 10 μm

Cu-Zn/Al2O3 with 2 mol urea

Ref. code: 25615622300134IGC

34

2.2.3 Support for Catalyst in NH3 decomposition

Supports are generally used to provide an area for depositing active

components. High surface area of supports is essential for the improvement in

dispersion and surface area of active component as well as high thermal stability is also

important for good supports. However, supports often involve in a reaction, which

enhances the catalysts activities. Such phenomena have been observed in many studies.

Goodman et al. reported that the Al2O3-supported Ru and Ir catalysts exhibit lower

activities than the SiO2-supported Ru and Ir catalysts due to a lower TOF of H2 from

the Al2O3-supported catalysts, compared to that of SiO2-supported catalysts [26]. Yin

et al. reported that the Ru catalysts activities depend on the supports, as shown in

Table 2.5, which indicate that carbon nanotubes (CNTs) is the best support for Ru

catalysts in NH3 decomposition. The high surface area, high degree of graphitization,

and tube structure of CNTs support are beneficial for an increased Ru dispersion, which

can enhance the catalytic activity for NH3 decomposition [62].

Table 2.5: NH3 decomposition over supported 4.8 wt%-Ru catalysts [62].

Temperature

(°C)

NH3 conversion (%)

Ru/CNTs Ru/AC Ru/Al2O3 Ru/MgO Ru/TiO2

400 2.3 0.8 2 2.4 1.9

450 6.4 2.7 5.9 6.3 5.3

500 20.5 7.2 17.6 12.2 11.5

550 40.6 14.4 35.9 19.5 21.6

Chen et al. reported that the doping of nitrogen in CNTs using the microwave plasma

technique can enhance the activity of Ru/N-doped CNTs catalyst, compared to the

Ru/undoped CNTs catalyst [42]. Two types of nitrogen species were formed in the N-

doped CNTs supports, which are pyridinic nitrogen at the edge of a graphene sheet on

the surface of CNTs and quaternary nitrogen within a graphene sheet. The average size

of Ru particles decreased with the increase of pyridinic nitrogen content on CNTs, and

the dispersion of Ru particles on CNTs were improved due to strong interaction of Ru

particles with pyridinic nitrogen on the surface of CNTs.

Ref. code: 25615622300134IGC

35

The Ni catalysts activities also depend on the supports [5, 7, 44, 45].

Deng et al. reported the doping of Zr in CeO2 support to form the solid solution of

Ce0.8Zr0.2O2 using different preparation method. The Ni/Ce0.8Zr0.2O2 catalysts exhibit

higher catalytic activities than the Ni/CeO2 and the Ni/Al2O3. The doping of Zr4+ into

CeO2 lattice increases the surface area of Ce0.8Zr0.2O2 and improves Ni dispersion of

the Ni/Ce0.8Zr0.2O2 catalyst. This doping also creates plentiful oxygen vacancies on the

Ce0.8Zr0.2O2 surface, which can enhance the Ni catalyst activity for NH3 decomposition

by increasing the Ni(e-) content over the oxygen vacancy sites [44]. Okura et al.

reported the activities of Ni catalysts using various supports, as shown in Figure 2.5.

The catalytic activities can be ranked as Y2O3 > Sm2O3 ≈ Gd2O3 > Al2O3 > La2O3 >

CeO2 [7].

Figure 2.5: Ammonia conversion of 10 wt% Ni catalysts [7].

Nakamura et al. also reported the H2 yields of Ni over various supports in Table 2.6.

The order of catalytic activities is different from the Okura’s work. The Ni/CeO2

catalyst exhibits the highest H2 yield at 450 °C while the H2 yield at 600 °C is close to

the Ni/Y2O3. The Ni/Al2O3 catalysts exhibited a fair catalytic activity in both studies,

but a high surface area of Al2O3 supports is essential for Ni dispersion in both studies.

The Nakamura’s work also reported that the high surface area of Al2O3 can prevent the

sintering of Ni clusters at a high reduction of 600 °C, as shown in Table 2.7 [5].

Ref. code: 25615622300134IGC

36

Table 2.6: H2 yields of Ni catalysts over supports [5].

Catalysts H2 yield (%)

450 °C 500 °C 550 °C 600 °C

Ni 0.3 1.0 4.7 12.4

Ni/Y2O3 23.4 60.8 98.3 99.9

Ni/CeO2 28.6 60.0 92.9 99.0

Ni/MgO 20.9 51.2 87.2 98.2

Ni/La2O3 21.2 48.6 84.3 98.0

Ni/Al2O3 16.9 42.3 84.0 98.0

Ni/ZrO2 14.2 31.4 71.8 95.0

Table 2.7: Physicochemical properties of Ni catalysts supported on various metals [5].

Catalysts

Ni dispersion (%) Specific

surface

area

(m2/g)

Pore size

(nm)

Pore

volume

(cm3/g) 400 °C

reduction

600 °C

reduction

NiO 0.66 0.16 18.7 21.2 0.112

Ni/Y2O3 11.3 6.4 43.0 17.8 0.192

Ni/CeO2 9.3 4.0 88.2 17.1 0.413

Ni/MgO 1.9 4.9 177.8 5.1 0.311

Ni/La2O3 8.9 3.5 65.6 20.6 0.377

Ni/Al2O3 0.03 4.7 289.5 4.5 0.467

Ni/ZrO2 4.3 1.7 85.7 4.6 0.138

However, Muroyama et al. reported the ranking of Ni catalysts activities over various

supports as Al2O3 > La2O3 > SiO2 > CeO2 ≈ MgO > TiO2 > ZrO2. In addition, the

apparent activation energies of the supported Ni catalysts (80–90 kJ/mol) are lower than

those of Ni films (180 kJ/mol) and Ni wires (209 kJ/mol) [45]. Similar evidences are

also observed in the supported Ru catalysts, compared to Ru films [26]. Unlike the Ru

and Ni catalysts, little differences between the supported and unsupported catalysts

were observed in Ir, Pt, Rh, and Pd catalysts [63]. Thus, the supports play an important

role in the activities of Ni catalysts. A high surface area of support is needed for Ni

dispersion, which can also prevent the sintering of Ni clusters. The oxygen vacancies

in the supports provide positive effects on the catalytic activities for NH3

decomposition. Therefore, modification of high surface area Al2O3 for Ni catalysts can

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be the potential approach to promote the Ni catalysts activities and maintain a high

surface area of Al2O3 support as well as the creation of oxygen vacancies in the support.

There are many phases of Al2O3, i.e., α-Al2O3 and γ-Al2O3. A support, α-Al2O3, is rarely

used due to its low surface area (25 m2/g) [64]. On the other hand, γ-Al2O3 is widely

used as a support due to its high surface area (176 m2/g), which is beneficial for metal

dispersion. Verwey reported that the crystal structure of γ-Al2O3 is similar to the spinel

structure of MgAl2O4 with two differences, i.e., the number of atoms in a unit cell and

the cation vacancies [9].

Figure 2.6: Spinel structure of MgAl2O4.

(Source: http://www.chemtube3d.com/solidstate/_spinel(final).htm)

The spinel structure of MgAl2O4 has 56 atoms in the unit cell, including 32 oxygen

atoms, 16 aluminum atoms in octahedral sites, and 8 magnesium atoms in tetrahedral

sites, as illustrated in Figure 2.6. The structure of γ-Al2O3 contains 40 atoms in the unit

cell, which includes 24 oxygen atoms, 16 aluminum atoms, and 2 cation vacancies

randomly distributed in octahedral/tetrahedral/both sites. The literature reported the

calculation of all possible configurations using the density functional theory, and found

that the two cation vacancies energetically prefer to distribute in octahedral sites [65,

66]. These vacancies increase the opportunity to insert the dopant atoms along with the

partial replacement of Al atoms by dopant atoms in the Al2O3 framework. The insertion

or replacement may cause defect formation in the doped Al2O3 supports, resulted by the

differences of atomic valency and/or ionic size, compared to those of the native Al

Octahedral site

Tetrahedral site

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atoms. The defects lead to form oxygen vacancies in the supports, which can alter the

Ni-support interaction and improve the Ni catalysts activities in this study.

2.3 Defect structure and oxygen vacancy

An oxygen vacancy is a defective site in a support, which loses oxygen atoms.

In other words, it is formed when the support has less oxygen atoms than the oxygen

atoms in the perfect crystal lattice. Since, an oxygen atom needs two electrons to satisfy

an octet in a crystal lattice. The loss of an oxygen atom in a crystal lattice of a support

leads to gain two electrons, as shown in Figure 2.7. Hence, an oxygen vacancy contains

two trapped electrons, which can be transferred to active metal atom deposited on this

site. Localization of trapped electrons in the oxygen vacancies to active metal atoms

increases the electron density of the metal atoms.

Figure 2.7: Oxygen vacancy in crystal lattice.

A defective surface with oxygen vacancy is more reactive than a normal surface. The

presence of oxygen vacancies on a defective surface is important in determining the

catalytic and chemical properties of a metal cluster [67-69]. Abbet et al. reported that

the metal atoms deposited on a regular surface are totally inert, while a specific

chemical activity occurred from the metal atoms deposited on a defective surface [67].

There are at least two important roles of the oxygen vacancies on a defective surface in

the metal-support interaction: they act as nucleation centers for the growth of a metal

cluster on a surface [70-73]; they modify electronic interactions and the chemical

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properties of a deposited metal cluster [67-69]. Giordano et al. reported that oxygen

vacancies in the MgO crystal lattice can alter interaction of Ni on the MgO support [8].

Figure 2.8: Optimized structures of (A) Ni/regular MgO and (B) Ni/defective MgO [8].

Figure 2.8 shows that the bond length of Ni-Ni and Ni-O on the lattice plane (1 0 0) of

regular MgO support are 2.311 Å and 2.880 Å, respectively [8]. These bond lengths on

the same lattice plane with an oxygen vacancy of defective MgO support are shorter as

2.298 Å and 2.595 Å, respectively. The bond strength of Ni deposited on the defective

MgO support (2.71 eV) is also stronger than that of Ni deposited on the regular MgO

support (1.14 eV). The deposition of Pd atoms on the defective crystal lattice of MgO

was also investigated to study the role of oxygen vacancies in the MgO support, as

shown in Figure 2.9 [70].

Figure 2.9: Optimized structures of (A) CO-bound on Pd/regular MgO and (B) CO-

bound on Pd/defective MgO [70].

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Table 2.8: Interaction of Pd on supports and CO on Pd [70].

Interaction Bond length

(Å)

Bond strength

(eV)

Pd supported on regular MgO 2.210 0.96

Pd supported on defective MgO 1.524 3.42

CO on unsupported Pd 1.871 1.71

CO-Pd/regular MgO 1.847 2.23

CO-Pd/defective MgO 1.969 0.52

According to Table 2.8, the shorter bond length of supported Pd on defective MgO is

observed with a stronger bond strength than those of supported Pd on regular MgO,

which similar to those of Ni atom. Considering the interaction of CO on Pd atoms, the

bond strength of CO on an unsupported Pd is 1.71 eV. The bond strength of CO on a

supported Pd/regular MgO is 2.23 eV, while the bond strength of CO on a supported

Pd/defective MgO decreases to 0.52 eV [70]. The oxygen vacancies in a defective

support not only act as nucleation centers for the growth of the metal clusters, but also

strengthens the bond of metal clusters on the defective support. The strengthening bond

of a metal atom on a defective support comes from the trapped electrons in oxygen

vacancies, which can be localized to the metal atoms on a defective support. In addition,

the bond strength of CO on Pd/MgO support is a direct effect from the theory of bond-

order conservation in an A-B-C system [74]. The strengthening bond of Pd on a

defective MgO support results in the weakening bond of CO on Pd/defective MgO

support. On the other hand, the weakening bond of Pd on the regular MgO yields the

strengthening bond of CO on Pd/regular MgO. Therefore, metal-support interaction can

be altered by the oxygen vacancies in a defective support.

In this study, nickel was selected as the active component to reduce the catalyst

cost. Urea promoter was used to improve Ni dispersion from explosive decomposition

during the calcination process. The spinel-like structure of γ-Al2O3 was selected to serve

as the catalyst support because it has a high surface area and cation vacancies in the

framework. It is envisaged that the partial doping of heteroatoms to substitute in the

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cation vacancies of Al2O3 frameworks can form the defects in the form of oxygen

vacancies, which alter Ni-support interaction. Surface acidity of a catalyst is one of the

important parameter, which enhance the NH3 adsorption step in the NH3 decomposition

mechanism. Thus, zirconium was selected as the first dopant due to its acid properties,

which could increase the surface acidity of Ni/Zr-doped Al2O3 catalyst. Cerium, same

valency as zirconium, was selected as a dopant to form Ce-doped Al2O3 to investigate

the effect of dopant ionic size on Ni-catalyzed NH3 decomposition. For a

comprehensive study, dopant atoms from the same period as zirconium were also used

to form Sr-doped Al2O3 and Y-doped Al2O3 to investigate the effect of dopant valency

on Ni-catalyzed NH3 decomposition. The partial doping of heteroatoms in the Al2O3

frameworks not only alters Ni-support interaction but also maintains the surface area of

doped Al2O3 supports close to that of γ-Al2O3 support. The change in Ni-support

interaction could improve the catalyst properties and the catalytic activities for NH3

decomposition.

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Chapter 3

Methodology

3.1 Materials

The purities and brands of the chemicals and gases in this study are listed in

Table 3.1.

Table 3.1: Chemical used in this study.

Chemical Purity Brand Purpose

Urea 99 % CARLO ERBA

Reagents Catalyst preparation

Ammonia solution 30 vol% Panreac AppliChem Support preparation

α-Al2O3 99 % Reference support

γ-Al2O3 99 % Sigma-Aldrich

Sr(NO3)2 99 %

Support preparation

Y(NO3)3·6H2O 99 % Alfa Aesar

Zr(NO3)2·6H2O 99 % ACROS

Ce(NO3)3·6H2O 99 % Fluka

Al(NO3)3·9H2O 98 % LOBA Chemie

Ni(NO3)2·6H2O 99 % Catalyst preparation

Fine quartz powder 99 % Sigma-Aldrich

Catalytic test H2 (gas) 99.99 %

Linde Thailand Pub

Co., Ltd.

Ar (gas) 99.999 %

NH3 (gas) 99.5 %

CO2 (gas) 99.999 %

Catalyst

characterization H2 in Ar (gas) 5 %

NH3 in He (gas) 5 %

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3.2 Supports preparation

The new supports, Sr-doped Al2O3, Y-doped Al2O3, Zr-doped Al2O3, and Ce-

doped Al2O3, were prepared with 5 mol% of dopants using the so-gel method according

to the following steps. Sr(NO3)2, Y(NO3)3·6H2O, Zr(NO3)2·6H2O, Ce(NO3)3·6H2O,

and Al(NO3)3·9H2O were dissolved in deionized water, separately, to form the salt

solutions with concentration of 0.5 M. The salt solutions of Sr, Y, Zr, and Ce were

mixed with Al salt solution to form the Sr-Al, Y-Al, Zr-Al, and Ce-Al salt solutions

with a mole ratio of 0.05:0.95. The ammonia solution (30 vol%, Panreac AppliChem)

was dropped into the mixed salt solutions to form gels until the pH of solutions reached

9 within 60 min. The gels of Sr-Al, Y-Al, Zr-Al, and Ce salt were kept at room

temperature for 48 h and dried at 110 ºC for 24 h. After that, the dried gels were calcined

at 800 °C for 4 h to obtain Sr-doped Al2O3, Y-doped Al2O3, Zr-doped Al2O3, and Ce-

doped Al2O3 supports. The Zr-doped Al2O3 supports calcined at 600, 1100, and

1200 °C were also prepared to investigate the effect of calcination temperature on the

support properties. In addition, the Ce-doped Al2O3 support was also prepared with 1

mol% of Ce to investigate the effect of dopant amount on the catalytic activity

compared with 5 mol% of Ce doped in Al2O3 support. The Alumina, α-Al2O3 and γ-

Al2O3, were used as the reference supports in this study. All supports were ground and

sieved using 106 µm sieve to control the uniformity of support particles. Figure 3.1

shows the schematic of supports preparation.

Figure 3.1: Schematic of supports preparation.

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3.3 Catalysts preparation

The 10 wt% and 20 wt% Ni catalysts were prepared using the incipient wetness

impregnation method. Ni(NO3)2·6H2O was dissolved in deionized water to obtain a

solution concentration of 1.25 M. Urea was added and dissolved in the Ni solution prior

to the impregnation process. The molar ratio of urea to Ni was kept at 2:1 in this study.

The Ni solutions were impregnated drop by drop on the supports until the supports get

saturated, then the supports were dried in the hood at room temperature. The

impregnation steps were repeated until the Ni solutions were completely introduced to

impregnate all the Ni solutions over the supports. After impregnation, the impregnated

catalysts were dried at 110 °C for 12 h, and then calcined at 500 °C for 4 h. Figure 3.2

shows the schematic of catalyst preparation.

Figure 3.2: Schematic of catalysts preparation.

The literature reported that the catalysts with particle size less than 125 µm have

negligible mass transfer effects [27]. Hence, the catalysts were sieved using 106 µm

sieve to ensure that the mass transfer limitation can be neglected in this study. In

addition, experiments were conducted by varying catalyst weights, dilution ratios

between catalyst and fine quartz powder, and NH3 flow rates at 40, 50, and 60 mL/min.

Figure 3.3 shows the H2 formation rate as a function of the residence time (W/F). The

change in hydrogen formation rates indicates that the catalytic activitiy tests in this

study were not performed under mass tranfer limitation. The Ni/γ-Al2O3 using Zr as the

promoter was also prepared to compare the catalytic activities with the Ni catalyst using

Zr as the dopant in Al2O3 framework. Zr(NO3)2·6H2O solution was added into the Ni

solution with the same preparation process to prepare the catalyst. The composition of

Zr as the promoter is kept at the same amount as Zr in the Ni/Zr-doped Al2O3 catalyst.

The calculation of supports and catalysts preparation was shown in Appendix-A.

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Figure 3.3: H2 formation rate as function of residence time.

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3.4 Ammonia decomposition from pure ammonia

The catalysts were tested using a quartz tubular reactor with an inner diameter

of 1 cm to evaluate the H2 production rate from NH3 decomposition. To minimize the

temperature gradients inside the catalyst bed, the catalyst (0.4 g) was mixed with fine

quartz powder (1 g) then packed between quartz wool in the reactor. Figure 3.4 shows

the schematic of the NH3 decomposition testing system from pure NH3.

Figure 3.4: Schematic of NH3 decomposition testing system from pure NH3.

The reduction of the catalysts was conducted using 50 vol% H2 balanced in Ar

at 50 mL/min of total flow rate at 600 °C for 1 h. The reactor was purged with 50 ml/min

Ar at 600 °C for 30 min to remove excess H2 out of the reactor. The catalyst activity

tests were performed at 500–600 °C in a continuous operation with 50 mL/min NH3,

corresponding to 7500 mL/(h·gcat) space velocity. The product gas was directly sent to

an auto sampling unit of the Gas Chromatography with Thermal Conductivity Detector

(GCMS-2010 Ultra, Shimadzu Corporation, Japan) to determine the concentration of

product gas compositions. The stability of the catalysts was evaluated using the

accelerated deactivation process at 800 °C with 50 mL/min NH3 flow for 5 h and 5

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cycles. The NH3 conversion at 550 °C of catalysts was evaluated with 50 mL/min NH3

flow before and after each cycle of the accelerated deactivation.

3.5 Ammonia decomposition from synthetic urine

The feasibility of H2 production from NH3 in urine was evaluated over

Ni/Ce-doped Al2O3 using synthetic urine, as shown in Figure 3.5. The NH3

concentration in synthetic urine was calculated to equivalent urea concentration in urine

[13]. The ammonia solution was diluted to 1.83 vol% NH3 in deionized water, which is

equivalent to 0.33 M urea in urine. The synthetic urine was loaded into a saturator and

heated at 60 °C. The vapors of NH3 and water was carried out at 30 mL/min Ar and sent

to the reactor at 550 °C in the continuous mode. The product gas was trapped to get rid

of water and directly sent to an auto sampling unit of the Gas Chromatography with

Thermal Conductivity Detector (GCMS-2010 Ultra, Shimadzu Corporation, Japan) to

determine H2 concentration in product gas.

Figure 3.5: Schematic of NH3 decomposition from synthetic urine.

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3.6 Supports and catalysts characterization

3.6.1 Nitrogen physisorption technique

Physical adsorption of N2 over the supports was evaluated using the N2

physisorption instrument (Autosorp-1, Quantachrome, USA), as shown in Figure 3.6.

The support was packed in a sample tube then attached in the instrument. A blank

sample tube was also attached in the instrument as a reference. The sample tube was

heated at 300 °C under vacuum condition for 12 h to degas from the support surface.

The sample tube was cooled down to room temperature under He flow. Both sample

and blank tubes were vacuumed to get rid of any gases in the tubes, then immersed in

the liquid nitrogen dewar. Nitrogen gas was introduced into the sample and blank tubes,

then the pressure of the sample and blank tube was measured as P and P0, respectively.

The data of N2 adsorption were calculated using Brunauer, Emmett, and Teller (BET)

equation (Appendix-B) to determine the surface area, pore volume, and pore diameter

of the supports.

Figure 3.6: N2 physisorption instrument.

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3.6.2 X-ray diffraction technique

The structures of supports were characterized using the X-ray diffraction

technique with Cu Kα1 radiation at 40 kV and 45 mA (XRD, X’Pert PRO

diffractometer, Panalytical, Almelo, the Netherlands), as shown in Figure 3.7. Each

support was packed in a glass sample holder then the sample holder was attached in the

X-ray diffractometer. The XRD patterns of supports were recorded using scintillation

detector with 23°–73° 2-theta, 0.02° step size, and 0.5 sec step time. JADE software

(Materials Data, Inc., Livermore, California) was utilized to identify phases and crystal

structures of the supports. The XRD patterns of supports were searched and matched

with the reference patterns from the International Centre for Diffraction Data (ICDD).

In addition, the lattice constant of the Al2O3 structure was calculated using the two

highest intensity peaks from diffraction planes of (4 0 0) and (4 4 0), corresponding to

the diffraction angles at around 46° and 67° in the diffractrogram, respectively [75].

Peak deconvolution was performed on the XRD pattern of supports prior to calculate

the lattice constant using JADE software. The details of the lattice constant calculation

is provided in Appendix-C.

Figure 3.7: X-ray diffractometer.

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The structures of catalysts were also characterized using the XRD technique

with Cu Kα1 radiation at 40 kV and 45 mA. The XRD patterns of catalysts were

recorded with the same method as the XRD patterns of supports. JADE software and

the reference patterns from the ICDD were also utilized to identify phases and crystal

structures of the catalysts. In addition, the Ni crystallite sizes of reduced catalysts were

calculated based on the Scherrer’s equation using the peak broadening at the diffraction

angle around 76 degree, which correspond to the diffraction plane of (2 2 0). The peak

around 76 degree was selected to determine the Ni crystallize size as it is free from peak

overlapping.

3.6.3 X-ray absorption near edge structure technique

The local structure of Al2O3 framework in the supports was investigated using

the X-ray absorption near edge structure spectroscopy (XANES, Appendix-D) at

Beamline-8 [76] of Synchrotron Light Research Institute (Thailand), as shown in

Figure 3.8 (http://www.slri.or.th).

Figure 3.8: Schematic of XANES experiment [76].

Each support was spread on a conductive copper tape for electron-yield mode in

XANES measurements. The X-ray absorption of Al K-edge was determined using Ie/I0,

where Ie and I0 were electron-yield current intensity and incident X-ray intensity,

respectively. An aluminum foil was used to calibrate photon energy in the range of

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1545–1560 eV. The XANES spectra were measured with 1560–1600 eV photon

energy, 0.2 eV step size, and 1 sec step time and analyzed using Athena software [77].

3.6.4 Differential scanning calorimetry technique

The calcination temperature of catalyst was determined using the Differential

scanning calorimeter (DSC, DSC-1, Mettler Toledo, USA), as shown in Figure 3.9. The

impregnated catalyst was filled in the aluminum pan, then the sample pan and the blank

aluminum pan were placed at the measurement position in the furnace of DSC

instrument. The measurement was conducted from 100–600 °C with the temperature

programmed at 10 °C/min. The differences in temperatures between the sample pan and

the blank pan were plotted with the increase of furnace temperature to investigate the

exothermic reaction of catalysts during the calcination process. The DSC plots of

catalysts were also utilized to determine the proper calcination temperature of catalysts

in this study.

Figure 3.9: Differential scanning calorimetry.

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3.6.5 Scanning electron microscope technique

The surface morphology of catalyst was evaluated using the Scanning electron

microscopy (SEM, JSM-5410, Jeol, Japan). The elemental composition of catalyst was

also determined using an Energy dispersive Spectrometry (EDS, INCA-300, Oxford

Instruments, UK), which is coupled with the SEM, as shown in Figure 3.10. The

catalyst powders were spilled over the conductive carbon tape, which was pasted on the

copper stub. The catalyst powders were coated with gold to make them conductive for

the SEM photography.

Figure 3.10: Scanning electron microscopy.

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3.6.6 Transmission electron microscope technique

The transmitted morphology of catalysts was evaluated using the Transmission

electron microscopy, as shown in Figure 3.11 (TEM, JEM-2100 Plus, Jeol, Japan). The

catalyst powders were suspended in ethanol solution to distribute the catalyst powders

over the copper grid and dried at room temperature for 24 h prior to TEM photography.

The Ni cluster sizes and Ni distribution of catalysts were investigated at 150,000 times

of magnification.

Figure 3.11: Transmission electron microscopy.

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3.6.7 Inductively coupled plasma optical emission spectrometry technique

The Ni contents on the catalysts were determined using the Inductively coupled

plasma optical emission spectrometry (ICP-OES, OptimaTM 8300, PerkinElmer, USA,

Figure 3.12) to verify the same range of the Ni contents in each catalyst. The catalysts

were digested with 65 vol% HNO3 solution to obtain the Ni catalyst solutions using the

Microwave digestion technique. The Ni catalyst solutions were diluted 50 times with

deionized water to obtain the Ni contents in the catalyst solutions in the range of mg/L.

The Ni contents in the catalyst solutions were determined using the Multi-Element

Calibration Standard 2 (PerkinElmer Plus, USA), working range for Ni: 0.1–100 mg/L.

Figure 3.12: Inductively coupled plasma optical emission spectrometry.

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3.6.8 Chemisorption techniques

The CO chemisorption by pulse injection technique using 10 vol% CO in He

(Linde Thailand Pub Co., Ltd.) was carried out to determine Ni surface area of the

catalysts using the Chemisorption catalyst analyzer (BELCAT-B, BEL Japan Inc.,

Japan), as shown in Figure 3.13.

Figure 3.13: Chemisorption catalyst analyzer.

Temperature Programmed Reduction with Hydrogen (H2-TPR) using 5 vol% H2 in Ar,

Temperature Programmed Desorption of NH3 (NH3-TPD) using 5 vol% NH3 in He, and

Temperature Programmed Desorption of CO2 (CO2-TPD) using pure CO2 were also

measured using the Chemisorption catalyst analyzer. Each catalyst was packed into a

quartz tube, then the quartz tube was attached in the instrument. The catalyst was

reduced with H2 gas at 600 °C for 30 min then cooled down to room temperature prior

to the measurements of NH3-TPD and CO2-TPD. The H2-TPR was applied to determine

the reduction temperatures of catalysts and to investigate the Ni-O interaction over the

supports. The acidic sites of reduced catalysts were calculated from the peak area of the

NH3-TPD profiles to investigate the NH3 adsorption step in the ammonia

decomposition mechanism. The basic sites of reduced catalysts were calculated from

the peak area of CO2-TPD profiles to investigate the basicity of reduced catalysts

towards the catalytic performance in ammonia decomposition. The details of the

chemisorption techniques are provided in Appendix-E.

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Figure 3.14: Temperature programmed reactor and Mass Spectrometer.

The temperature Programmed Reaction of NH3 (NH3-TPRx) was conducted in

a temperature programmed reactor (Shimax Co., Ltd.) and using Mass Spectrometer

(GCMS-2010 Ultra, Shimadzu Corporation, Japan) as a detector, shown in Figure 3.14.

Each catalyst was packed into the quartz reactor then placed in the temperature

programmed reactor. The catalyst was reduced with 50 vol% H2 in Ar for 1 h, then the

reactor was cooled down to 100 °C. Pure NH3 gas was fed at 50 mL/min into the reactor

with the temperature programmed at 10 °C/min from 100–600 °C. The N2 formation

rate from the recombination of nitrogen adsorbates was determined using an online

Mass Spectrometer from a mass spectrum of N2 (M/Z = 28). The maximum N2

formation rate of Ni catalysts was obtained from the first derivative of MS profiles,

which indicate the N2 desorption temperature of each catalyst.

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3.7 Determination of activation energy

Equation 1 indicates the power law model, where partial pressures of N2 does

not affect the reaction rate of NH3 decomposition (𝑟𝑁𝐻3) over Ni catalysts [12]. The

reaction rates were determined under 60 mL/min of the reactant with different NH3 and

H2 concentrations at 375–475 °C. The reaction rate constant (k) and the reaction rate

orders (a and b) in the power law model for NH3 decomposition were calculated via a

computer coding on MATLAB software (version R2016a). The codes were written to

perform the iterations for initial values of k, a, and b within a given range following the

error minimization method [78]. The error between the calculated reaction rates and the

experimental reaction rates was calculated at each iteration and the iterations sequence

was stopped when the least square error is less than a predefined tolerance. The k, a,

and b values at the last iteration were displayed as the optimal values. The k, a, and b

values at different reaction temperatures (375–475 °C) were calculated using the same

calculation procedure. The apparent activation energies (Ea) of NH3 decomposition

over the catalysts were calculated from the Arrhenius relationship between the reaction

rate constant (k) and the temperature (1/T), which can be described in equation 2–3

[12].

𝑟𝑁𝐻3 = 𝑘𝑃𝑁𝐻3𝑎 𝑃𝐻2

𝑏 Equation 1

𝑘 = 𝑘0𝑒−𝐸𝑎𝑅𝑇 Equation 2

𝑙𝑛 𝑘 = −𝐸𝑎

𝑅

1

𝑇+ 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 Equation 3

Where 𝑃𝑁𝐻3 and 𝑃𝐻2 are the partial pressures of NH3 and H2, respectively, 𝑘0 is the

pre-exponential factor and R is the gas constant.

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Chapter 4

Results and Discussion

4.1 Evaluation of Ni/Zr-doped Al2O3 catalysts in NH3 decomposition

The catalytic activities of Ni/Al2O3 catalysts in NH3 decomposition depend on

the properties of catalyst, i.e., Ni dispersion, surface acidity, and basicity. Supports play

an important role to improve these catalyst properties and promote the activities of the

Ni catalysts. Alumina in the forms of γ-Al2O3 and α-Al2O3 are generally used as

supports for Ni catalysts because γ-Al2O3 provides a high surface area for Ni deposition,

and α-Al2O3 provides a very high thermal stability for Ni catalysts. In this study, NH3

decomposition over Ni catalysts requires the high support surface area for the

improvement of Ni dispersion and no deformation at the high reaction temperature of

600–700 °C for high NH3 conversion. Modification of Al2O3 by the partial doping of

Zr in the Al2O3 framework can change the catalyst properties, such as surface acidity

and Ni-support interaction, which boost the catalyst activity and maintain the properties

of Al2O3 support. The Zr-doped Al2O3 supports were prepared to obtain the α-Al2O3

and γ-Al2O3 phases, separately, in each Zr-doped Al2O3 support. The catalytic activities

of Ni/Zr-doped Al2O3 catalysts were evaluated in comparison to Ni/α-Al2O3 and Ni/γ-

Al2O3 to investigate the enhanced activities of Ni catalysts by Zr doping for NH3

decomposition.

4.1.1 Preparation of Zr-doped Al2O3 supports and their properties

The crystal structure or phase of Al2O3 is a key parameter, which directly affects

to the surface area and thermal properties of Al2O3. The Al2O3 supports can be

synthesized in different phases by varying calcination temperatures of aluminum

hydroxide precursors. The γ-Al2O3 phase can be synthesized from the aluminum

hydroxide of Gibbsite, Boehmite, or Bayerite with the calcination temperature range

around 500–800 °C and 1100–1200 °C for α-Al2O3 phase [79]. In this study, the Zr-

doped Al2O3 support was also synthesized from the aluminum hydroxide precursor. An

initial phase of aluminum hydroxide of the support was investigated using the XRD

technique, as shown in Figure 4.1.

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Figure 4.1: XRD pattern of Zr-doped Al2O3 precursor dried at 110 °C.

The XRD pattern of Zr-doped Al2O3 precursor shows that the initial phase of

aluminum hydroxide is Bayerite, which can form the Zr-doped Al2O3 in both γ-Al2O3

and α-Al2O3 phases. Thus, the Zr-doped Al2O3 precursors were calcined at 600, 800,

1100, and 1200 °C and characterized using the XRD technique to investigate the Al2O3

phase in each Zr-doped Al2O3 support after calcination. The XRD patterns of the Zr-

doped Al2O3 supports were compared with those of the γ-Al2O3 and α-Al2O3 supports

to evaluate the effect of calcination temperature to the structure of Zr-doped Al2O3.

Figure 4.2 shows the XRD patterns of the calcined Zr-doped Al2O3 supports at different

calcination temperatures.

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Figure 4.2: XRD patterns of calcined Zr-doped Al2O3 supports; : γ-Al2O3;

: -Al2O3; : α-Al2O3; : tetragonal-ZrO2; : monoclinic-ZrO2.

From Figure 4.2, the XRD pattern of the Zr-doped Al2O3 calcined at 600 °C

shows an amorphous phase as majority of the support with a low crystallinity of γ-

Al2O3 phase. Calcination of the Zr-doped Al2O3 at 800 °C yields the γ-Al2O3 phase

(JCPDS: 10-0425, face center cubic structure) similar to the γ-Al2O3 support with a

presence of ZrO2 (JCPDS: 50-1089, tetragonal structure). For the Zr-doped Al2O3

calcined at 1100 °C, the γ-Al2O3 phase was transformed to -Al2O3 phase (JCPDS: 35-

0121, monoclinic structure) with higher crystallinity of the ZrO2 tetragonal structure.

Small amount of Baddeleyite ZrO2 (JCPDS: 37-1484, monoclinic structure) and

Corundum α-Al2O3 (JCPDS: 10-0173, hexagonal structure) were also observed in this

support. The α-Al2O3 phase was clearly observed in the XRD pattern of the Zr-doped

Al2O3 calcined at 1200 °C, which is similar to that of the α-Al2O3 support. Some of -

Al2O3 peaks were still observed in this XRD pattern as well as the tetragonal-ZrO2 and

monoclinic-ZrO2 were also observed at the calcination temperature of 1200 °C.

23 33 43 53 63 73

Norm

ali

zed

in

ten

sity

(a.u

.)

2-theta (degree)

Zr Al-800 °C

γ-Al2O

3

α-Al2O

3

Zr-Al-1100 °C

Zr-Al-1200 °C

Zr Al-600 °C

Ref. code: 25615622300134IGC

61

Therefore, the crystal structure of Al2O3 in the Zr-doped Al2O3 supports calcined at 800

and 1200 °C are similar to γ-Al2O3 and α-Al2O3 supports, respectively.

The surface area of the Zr-doped Al2O3 supports in this study were determined

and compared with that of the α-Al2O3 and γ-Al2O3 supports. Table 4.1 lists the values

of the surface area, pore volume, and average pore diameter.

Table 4.1: Physical properties of Al2O3 and Zr-doped Al2O3 supports.

Supports Surface area

(m2/g)

Pore volume

(cm3/g)

Average pore

diameter

(Å)

α-Al2O3 3.3 0.0035 43.3

γ-Al2O3 175.6 0.29 66.7

Zr-doped Al2O3-800 °C 145.1 0.29 81.1

Zr-doped Al2O3-1100 °C 44.2 0.22 195.9

Zr-doped Al2O3-1200 °C 15.4 0.07 183.0

The results show that a high calcination temperature of the supports yields low surface

area and pore volume of the supports. The Zr-doped Al2O3 calcined at 800 °C exhibits

the surface area and pore volume close to the γ-Al2O3 support while the surface area

and pore volume of the Zr-doped Al2O3 calcined at 1200 °C are close to those of the α-

Al2O3 support. Hence, the Zr-doped Al2O3 calcined at 800 and 1200 °C were used as

supports for Ni catalysts. The Ni catalysts activities using the Zr-doped Al2O3 calcined

at 800 and 1200 °C supports were investigated and compared to those of Ni over

conventional α-Al2O3 and γ-Al2O3 supports to evaluate the effect of support

modification by the partial doping of Zr in the Al2O3 framework.

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62

4.1.2 Characterization of Ni/Zr-doped Al2O3

The Ni over Zr-doped Al2O3 catalyst, calcined at 800 and 1200 °C were

prepared by incipient wetness impregnation with 20 wt% of Ni loading. Calcination

temperature of the catalysts after the impregnation process is a crucial factor, where

high calcination temperatures can affect the size of Ni clusters on the supports. The

DSC technique was used to investigate the Ni nitrate precursor and urea promoter under

the calcination process with air to form NiO over the catalysts, as shown in Figure 4.3.

Evaluation of the calcination temperature of catalysts is required to ensure that none of

the nitrate, hydroxyl, and ammonium compounds remain after the calcination process.

Figure 4.3: DSC profiles of impregnated catalysts.

The DSC profiles show that the exothermic reaction of Ni nitrate precursor and urea

promoter in all catalysts occurred at around 280 °C, and a small peak was also observed

at around 380–400 °C for the Ni/α-Al2O3 catalyst. Based on the literature, the

calcination temperature at 500–600 °C is usually selected as the calcination temperature

of Ni catalysts [5, 7, 12, 44, 59]. Therefore, all catalysts in this study were calcined at

100 200 300 400 500 600

Norm

ali

zed

hea

t fl

ow

(a.u

.)

Temperature (°C)

↑ Exothermic

Ni/Zr-Al-800 °C

Ni/Zr-Al-1200 °C

Ni/γ-Al2O

3

Ni/α-Al2O

3

Ref. code: 25615622300134IGC

63

500 °C to get rid of any unwanted compounds and prevent the agglomeration of Ni

clusters after the calcination process.

The catalysts were then characterized using the XRD technique to identify the

phase of Ni and ensure that there are no remaining unwanted compounds in the catalysts

after the calcination process. Figure 4.4 shows the XRD patterns of the catalysts in this

section, which exhibit the same characteristic peaks of NiO phase (JCPDS: 71-1179,

cubic structure) on all supports. The same phase of NiO over these supports catalyzes

the same reaction mechanism of ammonia decomposition. In addition, the same range

of the support surface area and a similar crystal structure of Al2O3 could allow fair

comparison for all catalysts to study solely on the effect of the support surface area and

Zr doped in Al2O3 supports.

Figure 4.4: XRD patterns of catalysts over Al2O3 and Zr-doped Al2O3 supports.

Ni/Zr-Al-1200 °C

Ni/α-Al2O

3

23 33 43 53 63 73

Norm

ali

zed

in

ten

sity

(a.u

.)

2-theta (degree)

Ni/γ-Al2O

3

Ni/Zr-Al-800 °C

= NiO

Ref. code: 25615622300134IGC

64

Incipient wetness impregnation was used to prepare the catalysts in this study.

The catalysts after the impregnation process were investigated the Ni clusters on the

support using the SEM-EDS technique. The Ni/α-Al2O3, a non-porous support, was

used as a representative catalyst for surface investigation to clearly observe the

distribution of Ni clusters on the support, as shown in Figure 4.5.

Figure 4.5: SEM image and EDS profile of Ni/α-Al2O3 catalyst.

The SEM-EDS image shows the distribution of NiO clusters over the α-Al2O3 support,

which are confirmed by the Ni peaks in the EDS profile of small clusters on the sheets

of α-Al2O3. This can be pointed out that the incipient wetness impregnation method can

serve as the facile method for catalyst preparation in this study.

Ref. code: 25615622300134IGC

65

4.1.3 Activities of Ni/Zr-doped Al2O3 for H2 production from NH3 decomposition

The performance of catalysts in NH3 decomposition were evaluated at 500 and

550 °C in the continuous mode, and the activities of the catalysts are listed in

Table 4.2. The performance of catalysts was compared in terms of NH3 conversion and

the H2 formation rate to evaluate the effect of Zr doped in Al2O3 and support surface

area towards the Ni catalysts activities.

Table 4.2: Catalytic activities of Ni over Al2O3 and Zr-doped Al2O3 supports.

Catalysts

NH3 conversion

(%)

H2 formation rate

(µmol/min·gcat)

500 °C 550 °C 500 °C 550 °C

Ni/α-Al2O3 22.3 ± 0.5 43.2 ± 0.7 1593 ± 21 2949 ± 27

Ni/Zr-doped Al2O3

(Calcined at 1200 °C) 33.8 ± 0.7 64.3 ± 0.4 2239 ± 51 4209 ± 8

Ni/γ-Al2O3 28.4 ± 0.2 59.2 ± 0.2 2019 ± 11 3982 ± 16

Ni/Zr-doped Al2O3

(Calcined at 800 °C) 48.8 ± 0.3 87.0 ± 0.3 3341 ± 18 5836 ± 18

Ni/γ-Al2O3

(Zr as promoter) 28.3 ± 0.1 58.3 ± 0.1 2005 ± 10 3904 ± 10

The Ni/γ-Al2O3 catalyst exhibits higher catalytic activities than Ni/α-Al2O3, and the

Ni/Zr-doped Al2O3-800 °C catalyst also exhibits higher catalytic activities than the

Ni/Zr-doped Al2O3-1200 °C. The results point out that a larger surface area of support

can increase the catalyst activity. At the same range of support surface area, Zr doped

in the Al2O3 frameworks can increase the catalyst activities, evidenced by higher

activities of Ni/Zr-doped Al2O3 catalysts than those of the Ni/Al2O3 catalysts. In

addition, the use of Zr as the promoter in Ni catalyst was investigated and compared

with the use of Zr as the dopant in Ni catalysts. The activities of Ni/γ-Al2O3 with and

without Zr promoter in Table 4.2 exhibit insignificant differences among catalytic

activities. Therefore, introduction of Zr as a dopant in the Al2O3 framework is more

favorable to improve the Ni catalyst activity for H2 production from NH3 decomposition

than using as a promoter.

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66

4.1.4 Role of Zr in Al2O3 supports for NH3 decomposition

In this study, it was expected that partial doping of Zr in the Al2O3 framework

increases the surface acidity of Ni/Zr-doped Al2O3 catalysts, which could promote the

NH3 adsorption step. The NH3-TPD technique was used to determine the surface acidity

of the catalysts in this section, as shown in Figure 4.6.

Figure 4.6: NH3-TPD profiles of catalysts over Al2O3 and Zr-doped Al2O3 supports.

The acidic sites of the catalysts were determined from area under the peaks of the NH3

profiles, and the values are listed in Table 4.3. The data show that a higher surface area

of the support exhibits a higher acidic sites on the support. The partial doping of Zr in

the Al2O3 frameworks can increase the acidic sites on both of the supports calcined at

800 and 1200 °C, compared to those of γ-Al2O3 and α-Al2O3, respectively. Hence, the

increased surface acidity of Zr-doped Al2O3 supports promote the NH3 adsorption step,

which is beneficial for H2 production from NH3 decomposition.

0

100

200

300

400

500

600

700

800

900

80 130 180 230 280 330 380 430 480 530 580

Des

orp

tion

sig

nal

(μv, a.u

.)

Desorption temperature (°C)

Ni/γ-Al2O3

Ni/Zr-Al-800 °C

Ni/Zr-Al-1200 °C

Ni/α-Al2O3

Ref. code: 25615622300134IGC

67

Table 4.3: Surface acidity of catalysts over Al2O3 and Zr-doped Al2O3 supports.

Catalysts Acidic sites

(µmol/gcat)

Ni/α-Al2O3 11

Ni/Zr-doped Al2O3-1200 °C 55

Ni/γ-Al2O3 210

Ni/Zr-doped Al2O3-800 °C 251

The CO chemisorption technique was utilized to evaluate the effect of Zr doped

in the Al2O3 framework to Ni dispersion and the Ni surface area of the catalyst.

Figure 4.7 shows the CO-pulse profiles of the Ni catalysts in this section. The

adsorption volume of CO on each catalyst was determined from the CO-pulse profiles

to calculate Ni dispersion and the Ni surface area of each catalyst, and the values are

listed in Table 4.4.

Figure 4.7: CO-pulse profiles of catalysts over Al2O3 and Zr-doped Al2O3 supports.

0 200 400 600 800 1000 1200 1400

Norm

ali

zed

CO

sig

nal

from

TC

D (

a.u

.)

Time (sec)

Ni/Zr-doped Al2O3-800 C

Ni/γ-Al2O3

Ni/Zr-doped Al2O3-1200 C

Ni/α-Al2O3

Ref. code: 25615622300134IGC

68

Table 4.4: Ni dispersion of catalysts over Al2O3 and Zr-doped Al2O3 supports.

Catalysts

Ni

dispersion

(%)

Ni surface area

(m2/g of Ni)

Low surface

area

Ni/Zr-dopedAl2O3

(1200 °C) 1.4 9.6

Ni/α-Al2O3 0.3 2.1

High surface

area

Ni/Zr-dopedAl2O3

(800 °C) 3.3 22.0

Ni/γ-Al2O3 1.6 10.5

Ni/γ-Al2O3

(Zr as promoter) 1.4 9.53

Table 4.4 show that Ni dispersion on the catalysts can be enhanced by partial doping of

Zr in the Al2O3 frameworks. The Ni/Zr-doped Al2O3 catalysts, calcined at 800 and

1200 °C exhibit higher Ni dispersions and Ni surface areas than those of Ni/γ-Al2O3

and Ni/α-Al2O3, respectively. In addition, the use of Zr as promoter in Ni/γ-Al2O3

catalyst cannot enhance its Ni dispersion and Ni surface area, compared to Ni/γ-Al2O3

without Zr promoter. Therefore, the roles of Zr as a dopant in this study are as follows;

1) increasing the surface acidity of Ni catalyst, which promotes the NH3 adsorption

step; 2) improvement of Ni dispersion, which promotes the dehydrogenation step. The

activity data in Table 4.2 correspond to these statements, since the Ni/Zr-doped Al2O3-

800 °C exhibits the highest catalytic activities among the catalysts in this section.

The results from section 4.1 show that the partial doping of different valencies

and ionic sizes of Zr4+ can increase the Ni catalyst activities, compared to Al3+ in the

Al2O3 framework. The effect of valences and ionic sizes of different dopants were also

studied in section 4.2 to investigate the roles of dopants in the Ni catalyst activities for

NH3 decomposition.

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69

4.2 Effect of valency and ionic size of dopant in Al2O3 support toward Ni

catalyzed NH3 decomposition

In section 4.1, it was discovered that doping of Zr4+ in the Al2O3 framework

enhanced the activity of the Ni/Zr-doped Al2O3 catalyst by increasing Ni dispersion and

acidic sites of the catalyst. Sr2+ and Y3+, from the same period of Zr4+, were also selected

to use as the dopants in this section. The same range of ionic sizes in Sr2+ (170 pm), Y3+

(160 pm), and Zr4+ (160 pm) were doped in the Al2O3 frameworks to investigate the

effect of dopant valency on the Ni catalyst activity [16]. In addition, Ce4+ (200 pm) with

a larger ionic size, compared to Zr4+ (160 pm) was also doped in the Al2O3 framework

to investigate the effect of dopant ionic size on the Ni catalyst activity [16]. The amount

of all dopants in this section were kept at 5 mol% equal to that of the Zr doped Al2O3

in section 4.1. In addition some Al3+ can also be doped in a segregated phase of each

dopant (SrO, Y2O3, ZrO2, and CeO2) when it is present in the crystalline form. The

reaction of this doping may be written using the Kröger-Vink notation (Appendix F) as

follows [80].

2𝑆𝑟𝑂𝐴𝑙2𝑂3→ 2𝑆𝑟𝐴𝑙

′ + 𝑉𝑂∙∙ + 2𝑂𝑂

𝑥 𝑜𝑟 𝐴𝑙2𝑂3𝑆𝑟𝑂→ 2𝐴𝑙𝑆𝑟

∙ + 𝑂𝑖′′ + 2𝑂𝑂

𝑥

𝑌2𝑂3𝐴𝑙2𝑂3→ 2𝑌𝐴𝑙

𝑥 + 3𝑂𝑂𝑥 𝑜𝑟 𝐴𝑙2𝑂3

𝑌2𝑂3→ 2𝐴𝑙𝑌

𝑥 + 3𝑂𝑂𝑥

2𝑍𝑟𝑂2𝐴𝑙2𝑂3→ 2𝑍𝑟𝐴𝑙

∙ + 𝑂𝑖′′ + 3𝑂𝑂

𝑥 𝑜𝑟 𝐴𝑙2𝑂3𝑍𝑟𝑂2→ 2𝐴𝑙𝑍𝑟

′ + 𝑉𝑂∙∙ + 2𝑂𝑂

𝑥

2𝐶𝑒𝑂2𝐴𝑙2𝑂3→ 2𝐶𝑒𝐴𝑙

∙ + 𝑂𝑖′′ + 3𝑂𝑂

𝑥 𝑜𝑟 𝐴𝑙2𝑂3𝐶𝑒𝑂2→ 2𝐴𝑙𝐶𝑒

′ + 𝑉𝑂∙∙ + 2𝑂𝑂

𝑥

The oxygen vacancy and oxygen interstitial in the doped Al2O3 supports can alter their

electronic properties and change interaction of Ni on each doped Al2O3. The doping of

Sr, Y, Zr, or Ce in the Al2O3 frameworks was investigated using the XRD and XANES

techniques in section 4.2.1. The differences in Ni-support interactions were evaluated

using the H2-TPR technique. The change in Ni-support interactions affects the

properties of catalysts, resulting in the enhancement of activities for Ni/doped Al2O3

catalysts in section 4.23

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4.2.1 Partial doping of Sr, Y, Zr, or Ce into Al2O3 frameworks and their

properties

From section 4.1, the Ni catalyst activity partially depends on the support

surface area. Thus, the doped supports in this section were calcined at 800 °C to obtain

a high surface area of the γ-Al2O3 phase. The macrostructure of supports were evaluated

from the XRD patterns to investigate the replacement and/or insertion of dopants into

the Al2O3 frameworks. Figure 4.8 shows the XRD patterns and peak assignment of the

γ-Al2O3 and doped Al2O3 supports in this section. The commercial γ-Al2O3 shows two

crystal structures, i.e., face center cubic (JCPDS: 10-0425, 2-theta about 37°, 40°, 46°

and 67°) and simple cubic structure (JCPDS: 04-0880, 2-theta about 37°, 40°, 43°, 46°

and 67°). The doped Al2O3 supports show different diffractograms from the γ-Al2O3.

Absence of the simple cubic structure peak at 43° indicates that there are only face

center cubic structures in the doped Al2O3 supports. The different intensity ratio of the

doped Al2O3 supports and γ-Al2O3 indicates different compositions of the face center

and simple cubic structures on the supports. A segregated ZrO2, tetragonal structure

(JCPDS: 50-1089) and segregated CeO2, face center cubic structure (JCPDS: 43-1002),

were observed in the XRD pattern of Zr-doped Al2O3 and Ce-doped Al2O3,

respectively. Segregated phases of SrO and Y2O3 were not observed in the XRD pattern

of Sr-doped Al2O3 and Y-doped Al2O3, respectively. However, they may be present in

an amorphous form, which cannot be detected by the XRD technique.

Ref. code: 25615622300134IGC

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Figure 4.8: XRD patterns of supports, : face center cubic-Al2O3; : simple cubic-

Al2O3; : tetragonal-ZrO2; ▲: face center cubic-CeO2.

Table 4.5: Physical properties of supports.

Supports

Surface

area

(m2/g)

Pore

volume

(cm3/g)

Average pore

diameter

(Å)

Lattice constant

(Å)

a

γ-Al2O3 176 0.29 66.7 7.8856

Sr-doped Al2O3 172 0.39 91.7 7.9264

Y-doped Al2O3 167 0.39 92.8 7.9206

Zr-doped Al2O3 145 0.29 81.1 7.9189

Ce-doped Al2O3 146 0.34 93.7 7.9302

In addition, the XRD patterns were used to determine the lattice constants of γ-Al2O3

and doped Al2O3 supports, to evaluate the changes in unit cells of Al2O3 in the supports.

Table 4.5 shows a larger lattice constant of the Al2O3 structure in the doped Al2O3

Ref. code: 25615622300134IGC

72

supports than that of γ-Al2O3. The partial doping of Sr, Y, Zr, or Ce in the Al2O3

frameworks causes a change in the unit cell due to different ionic sizes, which results

in a larger lattice constant. Table 4.5 also lists the physical properties of the Sr-doped

Al2O3, Y-doped Al2O3, Zr-doped Al2O3, Ce-doped Al2O3, and γ-Al2O3 supports. The

doped Al2O3 supports exhibit the surface areas in the range of 145–172 m2/g, which are

close to that of γ-Al2O3 (176 m2/g). Using the same range of support surface area allows

a fair comparison, to evaluate the catalytic activity of Ni for each doped Al2O3 and

γ-Al2O3 supports.

Figure 4.9: XANES spectra of supports.

According to the cation vacancies in the γ-Al2O3 framework, these vacancies

may provide a chance for dopant atoms to occupy the Al2O3 framework. The local

structure of the doped Al2O3 and γ-Al2O3 supports were investigated using the

X-ray absorption near edge structure technique (XANES) to confirm the replacement

and/or insertion of dopant atoms into the Al2O3 framework. Figure 4.9 illustrates the

Al K-edge XANES spectra of the doped Al2O3 and γ-Al2O 3 supports. The XANES

spectra of the supports show the peaks at 1566 eV, 1568 eV, and 1572 eV, which

γ-Al2O

3

Al K-edge

Sr-doped Al2O

3

Y-doped Al2O

3

Zr-doped Al2O

3

Ce-doped Al2O

3

Ref. code: 25615622300134IGC

73

correspond to the electrons transition from the 1s to antibonding a1g (s-like) state, 1s to

antibonding t1u (p-like) state, and 1s to t2g (d-like) state, respectively [81]. The relative

intensity of these peaks corresponds to the neighboring atoms around the Al atoms in

the Al2O3 framework. The peak intensity at 1566 eV relates to the local symmetry of

Al, which is influenced by the distortion of the octahedral sites in the Al2O3 framework,

where the dopant atoms could occupy the cation vacancies. This result agrees well with

the computational results, which reported that the cation vacancies in octahedral sites

of the Al2O3 framework have the lowest total energy [65, 82, 83]. The peak intensity at

1568 eV represents the electron transition from 1s to unoccupied valence in p-state

called white line. The peak intensity at 1572 eV is contributed by the multiple scattering

from the distant atomic shells called the symmetry-forbidden shape resonance [84, 85].

The XANES spectrum of γ-Al2O3 shows the highest peak at 1568 eV, which is different

from the doped Al2O3 supports. The differences in the relative peak intensities indicate

that the neighboring atoms are different around the Al atoms in the doped Al2O3

supports, compared to the γ-Al2O3 support. This shows that the doped Al2O3 supports

contain the doped atoms inside the Al2O3 frameworks, which may cause defect

formation in terms of oxygen vacancies in the doped Al2O3 supports. The oxygen

vacancies in the doped Al2O3 supports can alter their electronic properties, resulting in

different Ni-support interactions between the Ni/doped Al2O3 and the Ni/γ-Al2O3

catalysts. Therefore, the results of the lattice constant from the XRD measurements and

the local structure from the XANES measurements confirm the partial doping of Sr, Y,

Zr, or Ce in the Al2O3 frameworks in this study.

Ref. code: 25615622300134IGC

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4.2.2 Evaluation of Ni-support interaction and crystal structure of Ni catalysts

The Ni catalysts were prepared by incipient wetness impregnation and calcined

at 500 °C, which is the same procedure as the catalysts reported in section 4.1. The

Hydrogen temperature programmed reduction technique (H2-TPR) was used to

determine the reduction temperatures of catalysts. Different reduction temperatures of

the catalysts show that different energies are required to reduce NiO to Ni0, which can

be used to investigate the differences of Ni-support interactions among the catalysts in

this section. Figure 4.10 shows the H2-TPR profiles of Ni/γ-Al2O3, Ni/Sr-doped Al2O3,

Ni/Y-doped Al2O3, Ni/Zr-doped Al2O3, and Ni/Ce-doped Al2O3 catalysts.

Figure 4.10: H2-TPR profiles of catalysts.

Figure 4.10 shows that the Ni/γ-Al2O3 exhibits a single major peak of H2

consumption at 570 °C. The Ni/doped Al2O3 catalysts exhibit two major peaks of H2

consumption at 420 °C and 590 °C (570 °C for the Ni/Y-doped Al2O3), which

correspond to the weak and strong interactions of the O to Ni active sites on the

supports, respectively. The different valencies of dopants (Sr2+, Zr4+, and Ce4+) in the

Al2O3 frameworks yield a higher peak intensity at 420 °C than Y3+, which the same

Ref. code: 25615622300134IGC

75

valency as native Al3+ in the Al2O3 framework. Higher peak intensities at 420 °C for

the catalysts imply higher amounts of the weak interactions of O to Ni active sites on

these supports, which were reduced easily to Ni0. Therefore, the differences in H2-TPR

profiles of these catalysts indicate different Ni-support interactions for the Ni/doped

Al2O3 and Ni/γ-Al2O3 catalysts in this study. This phenomenon may come from the

concept of oxygen vacancy sites [70] and the theory of bond-order conservation in an

A-B-C system [74], as shown in Figure 4.11.

Figure 4.11: Ni-O interaction over regular site and oxygen vacancy site.

The partial doping of Sr, Y, Zr, or Ce in the Al2O3 frameworks cause defect

formations in terms of oxygen vacancies in the doped Al2O3 supports. The defects in

the doped Al2O3 supports lead to the changes in support properties, resulted in different

Ni-support interactions between the Ni/doped Al2O3 and the Ni/γ-Al2O3 catalysts. The

oxygen vacancy is caused by the loss of an oxygen atom in a crystal lattice of support

and provides two trapped electrons on the site. These trapped electrons can be

Ref. code: 25615622300134IGC

76

transferred to the Ni site, which increase the electron density of the Ni site. The high

electron density at Ni site facilitates reduction of NiO to be Ni0, which partly lower the

reduction temperature of Ni/doped Al2O3 catalysts, compared to that of Ni/γ-Al2O3.

Another theory to support this phenomenon is the theory of bond order conservation in

an A-B-C system [74]. The strengthening bond of Ni over defective support results in

the weakening bond of adsorbed species on Ni relative to that of Ni over regular

support.

Figure 4.12: XRD patterns of catalysts, : NiO; : Ni; : γ-Al2O3; : ZrO2; :

CeO2; : CeAlO3.

Figure 4.12 shows the XRD patterns of the Ni catalysts before and after

reduction at 600 °C for 1 h, to identify the Ni crystal structure of catalysts. The calcined

catalysts show the peaks of NiO, cubic structure (JCPDS: 71-1179, 2-theta about 37°,

Ref. code: 25615622300134IGC

77

43°, and 63°). After reduction, the XRD patterns of the reduced catalysts exhibit the

same crystal structure of Ni, a cubic structure (JCPDS: 04-0850, 2-theta about 44°, 52°,

and 76°) without any characteristic peaks of NiO. The results indicate that a reduction

temperature of 600 °C is sufficient to reduce the Ni catalysts in this study. There is no

change in the Ni cubic structure over each support. In addition, the crystal structure of

each support in the reduced catalysts is similar to that of each support in the calcined

catalysts, which indicates that there is no support deformation after reduction at 600 °C.

Figure 4.12 also shows the XRD patterns of the Ni catalysts after H2-TPR

measurements, to investigate the crystal structure of Ni. The catalysts after H2-TPR

exhibit the same characteristic peaks of Ni as in the reduced catalysts at 600 °C, which

correspond to the cubic structure of Ni (JCPDS: 04-0850). The results show that there

is only one Ni structure available in the catalysts, even when the catalysts were reduced

at a higher temperature of 900 °C. The XRD patterns also show that the characteristic

peaks of Al2O3 in the catalysts remain almost the same after H2-TPR at 900 °C, as

evidenced by the major peak of Al2O3 about 67°. However, the segregated CeO2 in the

Ni/Ce-doped Al2O3 was transformed to CeAlO3 (JCPDS: 48-0051, 2-theta about 24°,

34°, 41°, 48°, and 60°) after H2-TPR measurement at 900 °C. The results imply that all

catalysts in this study can be used under the reaction temperature up to 900 °C without

support deformation, except the Ni/Ce-doped Al2O3. Therefore, the differences in the

H2-TPR profiles of catalysts imply different interactions of Ni over each support,

resulting from partial doping of Sr, Y, Zr, or Ce in the Al2O3 frameworks in this section.

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4.2.3 Ni active sites for activity and stability in NH3 decomposition

The Ni catalysts in this section were prepared by incipient wetness impregnation

using the same 20 wt% of Ni loading. The Ni contents of catalysts were determined by

the ICP-OES technique, as shown in Table 4.6. The results show the same range of Ni

contents in each catalyst (from 19.61–21.69 wt%). This ensures that the evaluation of

Ni catalysts ativities is based on the same Ni mass basis in this section.

Table 4.6: Ni contents of catalysts.

Catalysts Ni contents[a] Catalyst weight[b] Ni contents

mg/L mg wt%

Ni/γ-Al2O3 10.18 ± 0.05 50.2 20.28 ± 0.11

Ni/Sr-doped Al2O3 9.84 ± 0.06 49.8 19.76 ± 0.12

Ni/Y-doped Al2O3 10.66 ± 0.09 50.1 21.28 ± 0.19

Ni/Zr-doped Al2O3 9.75 ± 0.10 49.7 19.61 ± 0.20

Ni/Ce-doped Al2O3 10.91 ± 0.09 50.3 21.69 ± 0.18

[a] determined from ICP-OES, [b] weight of catalyst used in the digestion process

Table 4.7: NH3 conversion of Ni catalysts.

Catalysts NH3 conversion (%)

500 °C 550 °C 575 °C 600 °C

Ni/γ-Al2O3 28.4 ± 0.2 59.2 ± 0.2 76.6 ± 0.3 90.3 ± 0.3

Ni/Sr-doped Al2O3 45.4 ± 0.1 84.4 ± 0.3 93.1 ± 0.3 96.7 ± 0.3

Ni/Y-doped Al2O3 42.6 ± 0.3 79.2 ± 0.4 91.1 ± 0.4 95.0 ± 0.2

Ni/Zr-doped Al2O3 48.8 ± 0.3 87.0 ± 0.3 96.3 ± 0.1 98.9 ± 0.1

Ni/Ce-doped Al2O3 53.0 ± 0.3 89.9 ± 0.4 97.0 ± 0.2 99.0 ± 0.3

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The catalytic activities of NH3 decomposition over the Ni catalysts were studied

at 500–600 °C, 1 atm in the continuous mode.

Table 4.7 lists the NH3 conversion using the catalysts in this section. The

Ni/doped Al2O3 catalysts exhibit higher NH3 conversion than that of the Ni/γ-Al2O3 at

all reaction temperatures. Dopants with different valencies compared to Al3+ (Sr2+, Zr4+,

and Ce4+) exhibit higher NH3 conversion than the dopant with the same valency (Y3+).

The dopants with higher valencies (Zr4+ and Ce4+) can enhance the Ni catalysts

activities better than that of the dopant with a lower valency (Sr2+). The dopant with a

larger ionic size (Ce-doped in Al2O3) exhibits higher NH3 conversion than a smaller

dopant (Zr-doped in Al2O3) with the same valency. High NH3 conversion can be

achieved at 575 °C on Ni/Zr-doped Al2O3 (96.3 %) and Ni/Ce-doped Al2O3 (97.0 %),

which correspond to a H2 production rate of 6453 and 6520 μmol/min·gcat, respectively,

as shown in Table 4.8. Thus, the partial doping of Sr, Y, Zr, or Ce in the Al2O3

frameworks can enhance the Ni catalysts activities for ammonia decomposition.

Table 4.8: H2 formation rate of Ni catalysts.

Catalysts H2 formation rate (μmol/min·gcat)

500 °C 550 °C 575 °C 600 °C

Ni/γ-Al2O3 2,019 ± 11 3,982 ± 16 5,155 ± 52 6,051 ± 36

Ni/Sr-doped Al2O3 3,125 ± 9 5,696 ± 30 6,289 ± 34 6,556 ± 29

Ni/Y-doped Al2O3 2,916 ± 40 5,308 ± 24 6,081 ± 22 6,411 ± 27

Ni/Zr-doped Al2O3 3,341 ± 18 5,836 ± 18 6,453 ± 44 6,656 ± 6

Ni/Ce-doped Al2O3 3,659 ± 5 6,027 ± 42 6,520 ± 27 6,657 ± 58

The number of Ni active sites plays a key role in catalyzed NH3 decomposition.

The reduced catalysts were evaluated for their Ni surface area using the CO pulse

technique. The values of the Ni surface area are listed in Table 4.9.

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Table 4.9: Ni surface area of catalysts.

Catalysts

Ni surface area (m2/gNi)

After reduction After stability test

Ni/γ-Al2O3 10.5 0.8

Ni/Sr-doped Al2O3 22.4 12.9

Ni/Y-doped Al2O3 13.6 11.2

Ni/Zr-doped Al2O3 22.0 11.6

Ni/Ce-doped Al2O3 26.4 16.4

Table 4.9 shows that the Ni/doped Al2O3 catalysts exhibit higher Ni surface area

than the Ni/γ-Al2O3. The dopants with different valencies (Sr2+and Zr4+) increase the Ni

surface area, larger than that of the dopant with the same valency (Y3+) where these

dopants have the same range of ionic size. The larger ionic size of Ce4+ also increases

the Ni surface area more than Zr4+ with the same valency. The Ni/Ce-doped Al2O3

exhibits the largest Ni surface area from a combination of the differences in the dopant

valency and ionic size. Thus, the partial doping of Sr, Y, Zr, or Ce in the Al2O3

frameworks can increase the number of Ni active sites for catalyzed NH3

decomposition. The catalysts in this section were further evaluated for stability in

catalyzed NH3 decomposition.

Figure 4.13 shows the H2 formation rates of the Ni/doped Al2O3 catalysts at

550 °C before and after the accelerated deactivation process under 50 mL/min NH3 at

800 °C for 5 h and 5 cycles in the stability test. Deactivation of the catalysts occurs up

to the 3rd cycle, and deactivation of the catalysts slightly decreases to the 5th cycle. The

drop in the H2 formation rate of the catalysts (µmol/min·gcat) can be ranked in the order

of Ni/γ-Al2O3 (1115) > Ni/Zr-doped Al2O3 (1020) > Ni/Sr-doped Al2O3 (938) ≈ Ni/Ce-

doped Al2O3 (938) > Ni/Y-doped Al2O3 (236). In contrast to the activity results, Ni/Y-

doped Al2O3 exhibits the best catalytic stability for NH3 decomposition. Table 4.9

shows a decrease in the Ni surface area of the catalysts after the stability test, compared

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to that of the catalysts after reduction. The decrease in the Ni surface area after the

stability test of Ni/Y-doped Al2O3 is only 2.4 m2/gNi, which is lower than that of Ni/γ-

Al2O3 (9.7 m2/gNi) Ni/Sr-doped Al2O3 (9.5 m2/gNi), Ni/Zr-doped Al2O3 (10.4 m2/gNi),

and Ni/Ce-doped Al2O3 (10.0 m2/gNi). The results show that partial doping of the dopant

with same valency (Y3+) in the Al2O3 framework for the Ni catalyst prevents the

agglomeration of Ni active sites at high reaction temperatures, which enhances the

catalytic stability better than that of different valencies (Sr2+, Zr4+, and Ce4+) in the

Al2O3 frameworks for Ni catalysts. The catalyst activity and stability results can lead

to the development of a new active Ni catalyst with high catalytic stability for NH3

decomposition.

Figure 4.13: Stability test of Ni catalysts, : Ni/γ-Al2O3; : Ni/Sr-doped Al2O3; :

Ni/Y-doped Al2O3; : Ni/Zr-doped Al2O3; : Ni/Ce-doped Al2O3.

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4.2.4 Evaluation of activation energy of NH3 decomposition

Figure 4.14 shows the Arrhenius plots of NH3 decomposition over the Ni/γ-

Al2O3 and Ni/doped Al2O3 catalysts and lists the apparent activation energies of the

reaction (Ea) in this section. The Ni/doped Al2O3 catalysts have a significant decrease

of the apparent activation energies, compared to the Ni/γ-Al2O3. The apparent activation

energies can be ranked as Ni/Ce-doped Al2O3 ≈ Ni/Zr-doped Al2O3 < Ni/Sr-doped Al2O3

≈ Ni/Y-doped Al2O3 < Ni/γ-Al2O3. The Ea values of each catalyst agree well with the

catalytic activity ranking in Table 4.7. Figure 4.15 shows a good correlation between

the experimental and calculated reaction rates. This validates the reliability of the

kinetics parameters in this section. Therefore, the partial doping of Sr, Y, Zr, or Ce in

the Al2O3 frameworks facilitates the improvements of the catalytic activities over Ni

catalysts.

Figure 4.14: Arrhenius plots of catalysts.

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Figure 4.15: Correlation between experimental and calculated reaction rates, : Ni/γ-

Al2O3; : Ni/Sr-doped Al2O3; : Ni/Y-doped Al2O3; : Ni/Zr-doped Al2O3; :

Ni/Ce-doped Al2O3.

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4.2.5 Role of dopants in Al2O3 frameworks in reaction mechanism and rate-

determining steps of NH3 decomposition

According to the existing studies, NH3 decomposition consists of three major

steps in the reaction mechanism, i.e., adsorption, surface reaction, and desorption, as

indicated in steps (I) to (VI) in Figure 4.16 [12]:

Figure 4.16: Ammonia decomposition mechanisms.

The literature reported that either step (II) or step (VI) or both steps could be the

rate-determining steps in NH3 decomposition [12, 27]. Evaluation of step (II) or step

(VI) of the reaction mechanism is required to understand the role of the dopants in the

Al2O3 supports that can affect the activation energy in this section. There are many

factors that can alter the activation energy, including Ni dispersion, acidic sites, and

basic sites evaluated from CO chemisorption, NH3-TPD, and CO2-TPD results,

respectively, as listed in Table 4.10. The Ni dispersion values were calculated from the

same data of CO adsorption as the Ni surface area values in Table 4.9. The first possible

rate-determining step, i.e., dehydrogenation of NH3 in step (II), depends on the number

of NH3 adsorbates and the number of available Ni active sites. This step is related to Ni

dispersion over the support and acidic sites of the catalyst to promote NH3

dehydrogenation. According to the oxygen vacancies in a defective support act as

nucleation centers for the growth of the metal clusters on the support. In this study, Ni

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dispersion of the Ni/doped Al2O3 catalysts were increased, compared to those of Ni/γ-

Al2O3, as evidenced in Figure 4.17 and Table 4.10. The increase of Ni dispersion

improved NH3 conversion, accordingly.

Figure 4.17: CO chemisorption profiles of catalysts.

Table 4.10: Characteristic properties of reduced catalysts with NH3 conversion.

Catalysts Ni

dispersion (%)

Acidic

sites (µmol/gcat)

Basic

sites (µmol/gcat)

NH3 conversion

at 550 °C (%)

Ni/γ-Al2O3 1.6 210 41 59.2 ± 0.2

Ni/Sr-doped Al2O3 3.4 209 105 84.4 ± 0.3

Ni/Y-doped Al2O3 2.0 209 67 79.2 ± 0.4

Ni/Zr-doped Al2O3 3.3 251 49 87.0 ± 0.3

Ni/Ce-doped Al2O3 4.0 212 67 89.9 ± 0.4

0 200 400 600 800 1000 1200

Norm

ali

zed

CO

sig

nal

from

TC

D (

a.u

.)

Time (sec)

Ni/CeO2-Al2O3

Ni/SrO-Al2O3

Ni/Y2O3-Al2O3

Ni/ZrO2-Al2O3

Ni/γ-Al2O3

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Greater Ni dispersion of Ni/Sr-doped Al2O3, Ni/Y-doped Al2O3, and Ni/Ce-

doped Al2O3 results in higher NH3 conversion than Ni/γ-Al2O3, where these catalysts

have the same range of acidic sites. The number of acidic sites on a reduced catalyst

corresponds to the capability of NH3 adsorption, evidenced by the peak areas of the

NH3-TPD profiles in Figure 4.18. The higher number of the acidic sites on Ni/Zr-doped

Al2O3 also yields higher NH3 conversion than Ni/Sr-doped Al2O3 where these catalysts

have the same range of Ni dispersion. Therefore, greater Ni dispersion and acidic sites

of a catalyst can provide more available Ni active sites and more ammonia adsorbates,

which can promote the dehydrogenation of ammonia in step (II). In addition, Table 4.11

lists that the catalysts with a smaller Ni crystallize size have a higher Ni dispersion in

Table 4.10. The Ni particle size of the catalysts over doped Al2O3 supports are smaller

than that of the catalyst over γ-Al2O3 and corresponds to the Ni crystallize size in this

section. The catalysts were also conducted TEM to investigate the Ni cluster sites in

each catalyst. The TEM images of the catalysts were shown in Figure 4.19. The black

dots in the TEM images are the Ni clusters in the catalysts. The catalysts with a high

Ni dispersion also exhibit a small Ni clusters in the TEM images. The TEM image of

Ni/Ce-doped Al2O3 exhibits the smallest Ni cluster sizes among the catalysts in this

section. The size of Ni clusters from the TEM images can be ranked as Ni/γ-Al2O3 >

Ni/Y-doped Al2O3 > Ni/Sr-doped Al2O3 > Ni/Zr-doped Al2O3 > Ni/Ce-doped Al2O3,

which agrees well with the results from the CO chemisorption technique.

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Figure 4.18: NH3-TPD profiles of catalysts.

Table 4.11: Ni crystallize size of reduced catalysts.

Catalysts

Ni crystallize size

from XRD

Ni particle size

from CO chemisorption

nm nm

Ni/γ-Al2O3 15.3 64

Ni/Sr-doped Al2O3 10.4 30

Ni/Y-doped Al2O3 12.2 50

Ni/Zr-doped Al2O3 10.1 31

Ni/Ce-doped Al2O3 8.4 26

3500

3600

3700

3800

3900

4000

4100

4200

4300

4400

80 180 280 380 480 580

Des

orp

tion

sig

nal

(µv)

Desorption temperature (°C)

Ni/γ-Al2O3

Ni/Sr-doped Al2O3

Ni/Y-doped Al2O3

Ni/Zr-doped Al2O3

Ni/Ce-doped Al2O3

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Figure 4.19: TEM images of catalysts (brightness adjusted).

Ni/γ-Al2O

3

50 nm

Ni/Sr-doped Al2O

3

50 nm

Ni/Y-doped Al2O

3

50 nm

Ni/Zr-doped Al2O

3

50 nm

Ni/Ce-doped Al2O

3

50 nm

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The basicity of catalysts can alter the activity of the catalysts for NH3

decomposition [34]. Figure 4.20 shows the CO2-TPD profiles of the reduced catalysts,

which were used to evaluate the number of basic sites on the catalysts, as listed in

Table 4.10. The Ni/doped Al2O3 catalysts exhibit higher number of basic sites than the

Ni/γ-Al2O3 catalyst, which can enhance their catalytic activities. However, the highest

number of basic sites for Ni/Sr-doped Al2O3 cannot yield the highest NH3 conversion.

This shows that the basicity of catalysts could enhance the catalytic activity, but it is

not the main factor for NH3 decomposition in this study.

Figure 4.20: CO2-TPD profiles of catalysts.

Another possible rate-determining step for NH3 decomposition is the desorption

of the nitrogen adsorbates from the Ni active sites (step VI). The recombination of

nitrogen adsorbates after losing all hydrogen and leaving the Ni active sites as gaseous

N2 in step (VI) is essential. This step can be enhanced by good dispersion and a large

number of Ni active sites on the support. A large number of Ni active sites allows

nitrogen adsorbates to be close to each other, promoting recombination to form gaseous

N2. The bond strength of NH3 on the Ni active site also plays an important role to

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promote this step, as shown in Figure 4.21. The weaker bond of NH3 on the Ni active

sites can yield easier desorption of nitrogen adsorbates on the Ni active sites after the

dehydrogenation process. Hence, the gaseous N2 formation from NH3 decomposition

over the catalysts was investigated using NH3-TPRx coupled with a mass spectrometer

(MS) to evaluate the support effects on N2 desorption in step (VI). Figure 4.22 shows

the gaseous N2 formation from the MS profiles of the catalysts, which indicate the rate

of nitrogen recombination in the desorption step.

Figure 4.21: Role of oxygen vacancies in doped Al2O3 support for Ni catalyst.

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Figure 4.22: N2 formation over catalysts from NH3-TPRx (A) with their first derivatives

(B).

300 400 500 600

Norm

ali

zed

N2

form

ati

on

(a.u

.)

Temperature (°C)

Ni/Zr-doped Al2O

3

Ni/γ-Al2O

3

Ni/Ce-doped Al2O

3

Ni/Sr-doped Al2O

3

Ni/Y-doped Al2O

3

(A)

300 400 500 600

1st

der

ivati

ve

N2

form

ati

on

(a.u

.)

Temperature (°C)

520 °C

500 °C

510 °C

Ni/Zr-doped Al2O

3

Ni/γ-Al2O

3

Ni/Ce-doped Al2O

3

Ni/Sr-doped Al2O

3

Ni/Y-doped Al2O

3

(B)

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The maximum N2 formation rate of Ni catalysts can be obtained from the first

derivative of the MS profile of N2, which indicates the N2 desorption temperature of

each catalyst. Ni/Ce-doped Al2O3 and Ni/Zr-doped Al2O3 exhibit the lowest N2

desorption temperature at 500 °C. The Ni/Sr-doped Al2O3 and Ni/Y-doped Al2O3

catalysts exhibit the N2 desorption temperature at 510 °C while N2 desorption over Ni/γ-

Al2O3 occurs at a higher temperature of 520 °C. A lower desorption temperatures of the

Ni/doped Al2O3 catalysts allows a lower energy requirement for nitrogen desorption

from the Ni/doped Al2O3 catalysts. These phenomena may be caused by the strong bond

of Ni on the doped Al2O3 supports, which results in the weak bond of nitrogen adsorbate

on the Ni over doped Al2O3 supports after the dehydrogenation process, as shown in

Figure 4.21. In addition, the N2 desorption temperatures of the catalysts correlate well

with the activation energies of NH3 decomposition over the catalysts in section 4.2.4.

The results imply that the N2 desorption in step (VI) could be the rate-determining step

of NH3 decomposition over the catalysts in this study.

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4.2.6 Effect of dopant amount in Al2O3 support toward Ni catalyzed NH3

decomposition

From the results in section 4.2.3, the Ni/Ce-doped Al2O3 exhibits the best

catalytic activity for H2 production from NH3 decomposition. To investigate the effect

of the dopant amount, the Ni/Ce-doped Al2O3 was selected as the representative catalyst

in this section. The catalyst activity was evaluated, based on two factors, which were

the Ni loadings and Ce dopant amounts. The catalytic activity of Ni/Ce-doped Al2O3

for NH3 decomposition were evaluated in terms of NH3 conversion and the H2

formation rate at 500 °C and 550 °C in the continuous mode, and the activities of

catalysts are listed in Table 4.12.

Table 4.12: Catalytic activities of Ni/Ce-doped Al2O3.

Catalysts

NH3 conversion

(%)

H2 formation rate

(µmol/min·gcat)

500 °C 550 °C 500 °C 550 °C

10 wt% Ni/1 mol% Ce-

doped Al2O3 36.6 ± 0.1 73.7 ± 0.1 2643 ± 13 5185 ± 10


doped Al2O3 40.9 ± 0.1 80.2 ± 0.2 2899 ± 5 5653 ± 15


doped Al2O3 53.0 ± 0.3 89.9 ± 0.4 3659 ± 5 6027 ± 42

The data show that the activities of Ni/Ce-doped Al2O3 catalysts depend on the

amount of dopant, as evidenced by a higher NH3 conversion and H2 formation rate of

10 wt% Ni over 5 mol% Ce-doped Al2O3, compared to those of 10 wt% Ni over 1 mol%

Ce-doped Al2O3. In addition, the effect of Ni loadings was also investigated at 10 wt%

and 20 wt% of Ni loadings on the 5 mol% Ce-doped Al2O3 supports, as listed in Table

4.12. The 20 wt% Ni over 5 mol% Ce-doped Al2O3 exhibits a higher catalytic activity

than 10 wt% Ni on 5 mol% Ce-doped Al2O3 due to higher Ni loading that provides

more Ni active sites. The catalysts were characterized using the CO chemisorption

technique to investigate the Ni dispersion and Ni surface area of the catalysts in this

section, as shown in Figure 4.23, and the values were listed in Table 4.13.

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Figure 4.23: CO chemisorption profiles of catalysts (10 wt% Ni and 20 wt% Ni).

Table 4.13: Ni dispersion and Ni surface area of catalysts.

Catalysts Ni dispersion Ni surface area

% m2/g of Ni

10 wt% Ni/1 mol% Ce-doped Al2O3 1.9 12.5



The 20 wt% Ni catalyst exhibits a higher Ni dispersion and Ni surface area than

the 10 wt% Ni catalyst, which indicates the adequate space of the support for the

distribution of 20 wt% Ni loading. For the 10 wt% Ni catalysts, higher amount of Ce

doped in the Al2O3 framework can increase the Ni active sites, as evidenced by higher

Ni dispersion and Ni surface area of 10 wt% Ni over 5 mol% Ce-doped Al2O3,

compared to those of 10 wt% Ni over 1 mol% Ce-doped Al2O3. This improvement

could come from the degree of Ce doped in the Al2O3 framework, which can be

investigated by the lattice constant of Al2O3 unit cells in both supports. The supports

were characterized using the XRD technique to investigate the crystal structures and

0 200 400 600 800 1000 1200 1400

Norm

ali

zed

CO

sig

nal

from

TC

D (

a.u

.)

Time (sec)

20Ni/5Ce-doped Al2O3



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determine the lattice constant of Al2O3 unit cells in 1 mol% Ce-doped Al2O3 and 5

mol% Ce-doped Al2O3, as shown in Figure 4.24.

Figure 4.24: XRD patterns of Ce-doped Al2O3 supports, : γ-Al2O3; ▲: CeO2.

The Ce-doped Al2O3 supports exhibit the same characteristic peaks of Al2O3,

but the 5 mol% Ce-doped Al2O3 support exhibits higher characteristic peaks of CeO2

than the 1 mol% Ce-doped Al2O3 support. Even though a higher amount of Ce yields

an improved amount of segregated CeO2, it increases a chance to dope more Ce in the

Al2O3 framework, which can be investigated from the lattice constant of Al2O3 unit

cells in the support. The 5 mol% Ce-doped Al2O3 exhibits a larger lattice constant than

the 1 mol% Ce-doped Al2O3, which represents larger Al2O3 unit cells in 5 mol% Ce-

doped Al2O3 than that in 1 mol% Ce-doped Al2O3. Larger Al2O3 unit cells in 5 mol%

Ce-doped Al2O3 can imply more Ce atoms doped in the Al2O3 framework, which create

more defects in terms of oxygen vacancies in this support. The oxygen vacancy in the

support acts as a nucleation center for a growth of Ni cluster, resulted in the enhanced

Ni dispersion and Ni surface area of the 10 wt% Ni/5 mol% Ce-doped Al2O3. This can

1Ce-doped Al2O

3

23 33 43 53 63 73

Norm

ali

zed

in

ten

sity

(a.u

.)

2-theta (degree)

5Ce-doped Al2O

3

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be the reason for a higher activity of the 10 wt% Ni/5 mol% Ce-doped Al2O3 than that

of the 10 wt% Ni/1 mol% Ce-doped Al2O3 in this section.

4.2.7 Applications for H2 production from NH3 decomposition over Ni/Ce-doped

Al2O3 using urine and wastewater

For Hydrogen production from ammonia decomposition can utilize the free

ammonia sources, such as urine and industrial wastewater. The urea (0.31–0.33 M) in

urine can be dissociated to form NH3 [13, 14] for H2 production from NH3

decomposition. High ammonical nitrogen (400–1000 mg of N/L) from ammonia and

urea plants is also a great source for H2 production from NH3 decomposition [15].

Ammonia in water is one of the enviromental problems, because it can be harmful to

the aquatic life [13]. Thus, the decomposition of NH3 from urine and industrial

wastewater can serve as a simultaneous solution for energy production and

environmental problems. The catalyst in this process requires active performance under

water vapor environment for a low-concentrations of NH3 in urine and industrial

wastewater. According to the result from section 4.2.3, Ni/Ce-doped Al2O3 exhibits the

best catalytic activity in NH3 decomposition. The Ni/Ce-doped Al2O3 catalyst was

tested for active behavior under these conditions, to evaluate the feasibility of the

catalyst for NH3 decomposition from urine and industrial wastewater. The activities of

Ni/Ce-doped Al2O3 were also compared to that of Ni/γ-Al2O3. Figure 4.25 shows NH3

conversion from synthetic urine using the Ni/Ce-doped Al2O3 and Ni/γ-Al2O3 catalysts

at 550 °C in the continuous mode. The NH3 conversions from synthetic urine were

achieved at around 72 % and 58 % for the Ni/Ce-doped Al2O3 and Ni/γ-Al2O3 catalysts,

respectively. The H2 production rates are in the range of 169–188 and 134–152

µmol/(min·gcat) for the Ni/Ce-doped Al2O3 and Ni/γ-Al2O3 catalysts, respectively. In

comparison to Ni/γ-Al2O3, Ni/Ce-doped Al2O3 showed high catalytic activities in the

decomposition of high-concentration NH3, and Ni/Ce-doped Al2O3 still exhibits high

catalytic activities under water vapor environment for the decomposition of low-

concentration NH3. The results show that NH3 decomposition from urine using the

Ni/Ce-doped Al2O3 catalyst is possible for H2 production and NH3 reduction in

wastewater sources, before releasing it back into the environment.

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Figure 4.25: NH3 decomposition from synthetic urine, : Ni/γ-Al2O3; : Ni/Ce-doped

Al2O3.

For the NH3 sources from wastewater, the potential industry could be the urea

production plants. Urea has been widely used in fertilizers for agricultural activities,

which require more than 50 metric tons of annual urea production. The average

wastewater generation rate is 450–480 kg for a metric ton of urea production, which

typically composed of 3–8 % NH3, 1.5–2 % CO2, and 0.2–5.1 % urea [86, 87]. High

amounts of NH3 and urea in the wastewater from urea plants can be a great source for

H2 production from NH3 decomposition simultaneous to the wastewater treatment. The

H2 production process using the wastewater from urea plants was proposed in Figure

4.26. Urea can be produced from the reaction of NH3 and CO2, which is an exothermic

reaction [87]. The heat from this reaction can also be recovered as an energy source for

the proposed H2 production process. The pretreatment of wastewater is needed to

remove oil, solid particles, and etc., which can damage pumps. After pretreatment, the

liquid effluents are pumped and preheated before feeding into a distillation tower using

heat recovery from the urea production process. The different volatilities of NH3, urea,

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and water are separated in the distillation tower, where water is condensed to the bottom

of the tower, and the NH3 and urea vapors are floated up and collected from the top of

the tower. The vapor of urea can then be hydrolyzed to NH3 vapor in the top section.

The NH3 vapor from the urea plant wastewater is flowed into the reactor with the

packed catalyst for NH3 decomposition. The proposed process can serve as a solution

for simultaneous H2 production and treatment of urea plant wastewater.

Figure 4.26: Schematic of NH3 decomposition from urea plant wastewater.

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Chapter 5

Conclusions and Suggestions for Future Studies

It is concluded that the catalytic activity of Ni/Al2O3 catalysts for ammonia

decomposition can be enhanced by partially doping the Al2O3 frameworks with Sr, Y,

Zr, or Ce atoms. The enhancement was attributed to defects in the form of oxygen

vacancies in the doped Al2O3 supports. The oxygen vacancies act as nucleation centers

for the growth of Ni clusters, and as a consequence, altering the bond lenghts and bond

strengths of Ni over these sites. The improved properties of the catalysts were observed,

such as increasing dispersion of Ni on the supports (Ce-doped Al2O3 > Sr-doped Al2O3

≈ Zr-doped Al2O3 > Y-doped Al2O3 > γ-Al2O3). Partilly doping also changes the acidic

and basic sites of the catalysts, evidenced by CO-pulse, NH3-TPD, and CO2-TPD,

respectively. Hence, the enhanced Ni dispersion and the enrichment of surface acidic

sites enlarge the availability of Ni sites for NH3 adsorption, resulting in higher NH3

conversion as a result of the promoted NH3 dehydrogenation step. High basicity of the

Ni catalysts may also enhance the performance of catalysts. The recombination of

nitrogen in the NH3 decomposition mechanism could be the rate limiting step in this

study, evidenced by the N2 formation profiles in the NH3-TPRx experiments. The

Ni/Ce-doped Al2O3 catalyst exhibits the highest catalytic activities while the Ni/Y-

doped Al2O3 catalyst exhibits the best catalytic stability in this study. The

decomposition of low-concentration NH3 originated from urine was further evaluated

over the active Ni/Ce-doped Al2O3 catalyst, revealing the H2 formation rate of 169–188

µmol/(min·gcat) with 72.1 % NH3 conversion. The H2 production process from the urea

plant wastewater was also proposed as a solution for simultaneous H2 production and

treatment of urea plant wastewater. A simple preparation method of doped Al2O3

supports provides cost-effective way to synthesize effective Ni catalysts for H2

production from NH3 decomposition, and maintain a high surface area and high thermal

stability of doped Al2O3 at a level similar to that of conventional γ-Al2O3. The results

of this study demonstrate a facile way for the development of active Ni catalysts for

clean hydrogen production from NH3 sources, including NH3 from urine and

wastewater.

Ref. code: 25615622300134IGC

100

This study discovered that the size and valency of dopants in Al2O3 frameworks

affect the properties of Ni catalysts. The dopants with different valencies (Sr2+, Zr4+,

and Ce4+) relative to Al3+ significantly enhance the activities of Ni catalysts while in

the case of Y3+ with valency equal to that of Al3+, there is also an improvement of the

catalytic stability. The larger size of Ce4+ in comparison to Zr4+ (same valency) results

in a further enhancement of activity, making the Ni/Ce-doped Al2O3 catalyst the most

active among the prepared catalysts in this study.

It is envisaged that a combination of doping Y and Ce into the Al2O3 framework

for a Ni catalyst could result in improvement of both the catalytic activity and stability.

These finding would be a research direction for future investigation. The sol gel method

used in support preparation could also be modified, such as using magnetic field

inducement to improve atomic distribution of dopants in Al2O3. Preliminary data

indicated that the magnetic field inducement during support preparation can indeed

improve Ni dispersion on the support, which is an important property to enhance the Ni

catalyst activity for NH3 decomposition.

Ref. code: 25615622300134IGC

101

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Appendices

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Appendix A

Supports and catalysts preparation

For 15 g of the doped Al2O3 supports, the amount of dopants used was calculated

and listed in Table A.1. However, the amount of Al for 15 g of the doped Al2O3 supports

depends on the total weight of each doped Al2O3 support.

Table A.1: Calculation of metal used for 15 g support preparation.

Element Mole fraction MW (g/mol) Weight (g) Weight (g)

(15 g of support)

Weight of Al (g)

(15 g of support)

Al 0.95 26.982 25.633 - -

Sr 0.05 87.620 4.381 2.189 12.811

Y 0.05 88.906 4.445 2.217 12.783

Zr 0.05 91.224 4.561 2.266 12.734

Ce 0.05 140.116 7.006 3.220 11.780

Since, the interested elements are available in the nitrate form. To prepare the solution

of 0.5 M, the amount of metal nitrate used was calculated as follows to prepare the

solution of 0.5 M. The values are shown in Table A.2.

Table A.2: Calculation of compound used for 0.5 M solution preparation.

Compound MW

(g/mol)

Weight

(g)

Water

(cm3)

Weight of

Al(NO3)2∙9H2O

(g)

Water

(cm3)

Sr(NO3)2 211.630 5.287 50 178.111 950

Y(NO3)3∙6H2O 383.012 9.551 50 177.722 948

ZrO(NO3)2∙xH2O 339.320 8.429 50 177.040 944

Ce(NO3)3∙6H2O 434.220 9.979 46 163.781 873

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112

Example calculation: amount of Ce(NO3)3∙6H2O and Al(NO3)3∙9H2O as well as water

for Ce-doped Al2O3

𝐶𝑒(𝑁𝑂3)3 ∙ 6𝐻2𝑂 =3.220 𝑔 × 434.220

𝑔𝑚𝑜𝑙

1 × 140.116 𝑔𝑚𝑜𝑙

= 9.979 𝑔

𝐻2𝑂 =9.979 𝑔

434.220 𝑔𝑚𝑜𝑙 × 0.5

𝑚𝑜𝑙𝐿

×1000 𝑐𝑚3

1 𝐿= 46 𝑐𝑚3

𝐴𝑙(𝑁𝑂3)3 ∙ 9𝐻2𝑂 =11.780 𝑔 × 375.130

𝑔𝑚𝑜𝑙

1 × 26.982 𝑔𝑚𝑜𝑙

= 163.781 𝑔

𝐻2𝑂 =163.781 𝑔

375.13 𝑔𝑚𝑜𝑙 × 0.5

𝑚𝑜𝑙𝐿

×1000 𝑐𝑚3

1 𝐿= 873 𝑐𝑚3

For 10 g of catalysts, the catalysts were prepared with 20 wt% of Ni over the supports.

The amount of Ni nitrate and water used in the preparation process were calculated as

follows. The Ni nitrate solutions were prepared with 1.25 M and impregnated over the

supports.

𝑁𝑖(𝑁𝑂3)2 ∙ 6𝐻2𝑂 =2 𝑔 × 290.810

𝑔𝑚𝑜𝑙

1 × 58.693 𝑔𝑚𝑜𝑙

= 9.910 𝑔

𝐻2𝑂 =2 𝑔

58.6934 𝑔𝑚𝑜𝑙 × 1.25

𝑚𝑜𝑙𝐿

×1000 𝑐𝑚3

1 𝐿= 27.260 𝑐𝑚3

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Appendix B

Brunauer-Emmett-Teller (BET) theory

The Brunauer-Emmett-Teller (BET) theory provides an explanation on the

physical adsorption of gas molecules over a solid surface. The BET theory also serves

as the basis for the specific surface area of material measurements. The BET theory was

first published as an article in the Journal of the American Chemical Society in 1938

by Stephen Brunauer, Paul Hugh Emmett, and Edward Teller. This theory can apply

for the multilayer adsorption of gas molecules over a solid surface. The adsorption of

gas molecules is measured using a gas, which does not chemically react with material

surfaces. Nitrogen is commonly used as the probing gas to investigate the properties of

material surfaces using the BET method. The concept of the BET theory is an extension

of the Langmuir theory, which is a theory for monolayer molecular adsorption, to

multilayer adsorption with the following hypotheses:

1. The gas molecules physically adsorb on a solid in layers infinitely.

2. The gas molecules only interact with adjacent layers.

3. The Langmuir theory can be applied to each layer.

The resulting BET equation is

1

𝑣 [(𝑝0𝑝) − 1]

=𝑐 − 1

𝑣𝑚𝑐(𝑝

𝑝0) +

1

𝑣𝑚𝑐

Where, p and p0 are the equilibrium and the saturation pressure of adsorbates at the

temperature of adsorption, respectively. v is the adsorbed gas quantity (for example, in

volume units), and vm is the monolayer adsorbed gas quantity. c is the BET constant.

𝑐 = 𝑒(𝐸1−𝐸𝐿)𝑅𝑇

Where, E1 is the heat of adsorption for the first layer, and EL is the heat of adsorption

for the second and upper layers, which is equivalent to the heat of liquefaction. A BET

plot can be plotted between 1

𝑣[(𝑝0𝑝)−1]

as the y-axis and (𝑝

𝑝0) as the x-axis, which

maintains the (𝑝

𝑝0) value in the range of 0.05–0.35. The slope and y-intercept of this

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114

plot are used to calculate the values of vm and c. These values are further used to

calculate the total surface area (Stotal) and the specific surface area (SBET) of a material,

as indicated in the following equation.

𝑣𝑚 =1

𝑠𝑙𝑜𝑝𝑒 + 𝑦𝑖𝑛𝑡𝑒𝑟𝑐𝑒𝑝𝑡

𝑣𝑚 = 1 +𝑠𝑙𝑜𝑝𝑒

𝑦𝑖𝑛𝑡𝑒𝑟𝑐𝑒𝑝𝑡

𝑆𝑡𝑜𝑡𝑎𝑙 =(𝑣𝑚𝑁𝑠)

𝑉

𝑆𝐵𝐸𝑇 =(𝑆𝑡𝑜𝑡𝑎𝑙)

𝑎

Where, N is Avogadro's number, and s is the cross section of the adsorbing species. V

is the molar volume of the adsorbate gas, and 𝑎 is the mass of the solid sample or

adsorbent.

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Appendix C

Lattice constant calculation

The lattice constant, or lattice parameter, can be used to explain the unit cells

dimension in a crystal lattice. There are three lattice constants, i.e., a, b, and c, which

are referred to the dimensions in x, y, and z directions, respectively. Three angles are

also used to explain the shape of unit cells, which are α, β, and γ, as shown in

Figure C.1.

Figure C.1: Lattice constants of unit cell.

For cubic crystal structures, the lattice constant (a) and angle (α = 90°) can be used only

to explain the unit cell dimensions in the crystal lattice due to the symmetry of the

crystal structure. The lattice constant (a) can be calculated from the diffraction angle of

a particular peak in an X-ray diffractogram. The diffraction angle of a particular peak

represents the distance between its lattice planes (d h k l), which is calculated from the

Bragg’s equation as follows.

2𝑑ℎ 𝑘 𝑙 sin 𝜃 = 𝑛𝜆

The distance between lattice planes is further used to calculate the lattice constant of

unit cells in the cubic crystal structure using the equation below.

𝑎2 = 𝑑ℎ 𝑘 𝑙2 (ℎ2 + 𝑘2 + 𝑙2)

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Appendix D

X-ray absorption near edge structure spectroscopy

X-ray Absorption Near Edge Structure (XANES) is one of the techniques to

analyze the X-ray absorption spectra (XAS) of a substance. Synchrotron radiation is

normally used to generate X-ray photon energy, which is used in XAS experiment. In

this study, the XAS experiment of the Al atoms in the supports was conducted in the

X-ray energy range of 1560-1600 eV to obtain the XAS spectrum of the doped Al2O3

supports and γ-Al2O3 in the XANES region. A rapid increase of X-ray absorption was

occurred when the energy of X-ray was scanning through the binding energy regime of

Al K-edge. This phenomenon corresponds to absorption of the X-ray photon by

electrons in the K-shell of Al atoms, which is ejected as the photoelectron. The

oscillatory structure in the XANES region was appeared from the interference between

the outgoing photoelectron wave and the back-scattered photo electron wave from

neighboring atoms, shown in Figure D.2. Therefore, the oscillatory pattern and

amplitude in the Al K-edge XANES profile of support provides information on the local

structure of probed Al atoms in the support.

Figure D.2: X-ray absorption near edge structure (XANES).

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Appendix E

Chemisorption Techniques

The characteristic properties of catalysts can be determined via chemisorption

techniques, i.e., CO-pulse, NH3-TPD, and CO2-TPD. The Ni dispersion and Ni surface

area can be obtained from the CO-pulse injection technique. The acidic sites and basic

sites can be obtained from NH3-TPD, and CO2-TPD techniques, respectively. The

principle of each chemisorption technique is illustrated as follows. The figures in this

appendix were copied from the operation manual of Catalyst Analyzer (BELCAT-B,

BEL Japan Inc., Japan).

CO-pulse injection technique

The CO-pulse injection technique is generally used to evaluate the metal

dispersion and metal surface area of a catalyst. The catalyst has to be reduced to remove

oxygen out from NiO. The known volume of CO is injected as a pulse into the reduced

catalyst for chemical adsorption until the reduced catalyst is saturated. The volume of

CO chemisorption on the reduced catalyst can be determined from the CO-pulse profile,

as shown in Figure E.3.

Figure E.3: CO-pulse injection technique and CO-pulse profile.

The adsorption volume is calculated from the comparison between the adsorbed peak

and saturated peak. The Ni dispersion and Ni surface area are calculated via the

adsorption volume using the equations below.

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Ni dispersion (%) 𝐷𝑚 =𝑉𝑐ℎ𝑒𝑚×𝑀𝑊

22414×(𝑚×𝑝

100)× 100

Ni surface area (m2/g of Ni) 𝐴𝑚 =𝑉𝑐ℎ𝑒𝑚×6.02×10

23×𝜎𝑚

22414×(𝑚×𝑝

100)

Where Vchem = adsorption volume (cm3)

MW = atomic weight of Ni (g/mol)

m = catalyst weight (g)

p = Ni content in the catalyst (wt%)

σm = crossectional area of Ni atom (nm2)

Temperature programmed desorption techniques

Temperature Programmed Desorption (TPD) is generally used to study the

physical adsorption and chemical adsorption of a gas on a catalyst. A TPD profile can

be obtained from the measurement of a desorbed gas during the ramping-up of

temperature, continuously. The type of adsorption and number of adsorption sites on

the catalyst can be determined from the desorption peaks and peak area in the TPD

profile, respectively. The illustration of the TPD techniques is shown in Figure E.4.

Figure E.4: Illustration of TPD technique.

For the evaluation of acidic sites, NH3 is used as the probe molecule for

adsorption on the catalyst while CO2 is used as the probe molecule for the evaluation

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of basic sites. After the adsorption process, the NH3-TPD and CO2-TPD profiles are

recorded using Thermal Conductivity Detector (TCD) under the He carrier gas flow

during the ramping up of temperature, continuously.

Temperature Programmed Reduction and Temperature Programmed Reaction

Temperature programmed reduction (TPR) is typically used to study the

characteristics of a catalyst during the reduction process. The reaction products with

respect to the temperature are studied using Temperature programmed reaction (TPRx).

The illustration of the TPR and TPRx techniques is shown in Figure E.5.

Figure E.5: Illustration of TPR and TPRx techniques.

For the TPR technique, H2 is used as the reactive gas to remove the O2 out from NiO

on the catalyst. The H2-TPR profile is recorded using TCD under the 5 vol% H2/He

flow during the ramping-up of temperature, continuously. For the TPRx technique, NH3

is used as the reactant to evaluate the production rate of N2 gas with respect to the

temperature. The TPRx profile is recorded using a Mass Spectrometer under a pure

NH3 flow during the ramping up of temperature, continuously.

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Appendix F

Kröger-Vink notation

Kröger–Vink notation is a set of symbols, which are used to explain lattice

position, ionic vacancy, and electrical charge for a point defect in crystals. This notation

was proposed by F. A. Kröger and H. J. Vink, and it is useful for explanation of defect

reactions. The Kröger–Vink notation is shown as follows.

MSC

M can be to atoms (Al, Sr, Y, Zr, or Ce), vacancy (V), electron (e), and electron

holes (h).

S is the lattice site of native atom or interstitial site that the M species occupies. For

example, SrAl and Sri indicate the Sr atom on the Al lattice site and the Sr atom on

the interstitial site, respectively.

C indicates the electrical charge of the species relative to its occupied site. It is

calculated by the charge on the original site minus the charge on the occupied site.

The symbols, , , and indicates a null charge, positive charge, and negative

charge, respectively. For example, the Sr atom on the Al lattice site in the Al2O3

framework can be written as SrAl ′ . The Y atom or Zr atom on the Al lattice site in

the Al2O3 framework can be written as YAl × or ZrAl

, respectively.

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Documents

The Development of Nickel Catalyst for Ammonia Decomposition