Lanthanum oxide promoted cobalt catalyst supported on mesoporous alumina for syngas production via methane dry reforming

LANTHANUM OXIDE-PROMOTED COBALT CATALYST SUPPORTED ON MESOPOROUS ALUMINA FOR SYNGAS PRODUCTION VIA METHANE DRY REFORMING TRAN NGOC THANG DOCTOR OF PHILOSOPHY UNIVERSITI MALAYSIA PAHANG

LANTHANUM OXIDE-PROMOTED COBALT CATALYST SUPPORTED ON MESOPOROUS ALUMINA FOR SYNGAS PRODUCTION VIA METHANE DRY

REFORMING

TRAN NGOC THANG

DOCTOR OF PHILOSOPHY UNIVERSITI MALAYSIA PAHANG

UNIVERSITI MALAYSIA PAHANG

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DECLARATION OF THESIS AND COPYRIGHT

Author’s Full Name : TRAN NGOC THANG

Date of Birth : 20 th NOVEMBER 1982

CATALYST SUPPORTED ON MESOPOROUS ALUMINA

DRY REFORMING Academic Session : SEMESTER 1 2021/2022

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SUPERVISOR’S DECLARATION

We hereby declare that we have checked this thesis and in our opinion, this thesis is adequate in terms of scope and quality for the award of the degree of Doctor of Philosophy

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Full Name : TS DR SUMAIYA BT ZAINAL ABIDIN @ MURAD

Position : ASSOCIATE PROFESSOR

Date : 04JANUARY 2022

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(Co-supervisor’s Signature)

Full Name : DR NURUL AINI BINTI MOHAMED RAZALI

Position : ASSOCIATE PROFESSOR

Date : 04JANUARY 2022

STUDENT’S DECLARATION

I hereby declare that the work in this thesis is based on my original work except for quotations and citations which have been duly acknowledged I also declare that it has not been previously or concurrently submitted for any other degree at Universiti Malaysia Pahang or any other institutions

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Full Name : TRAN NGOC THANG

ID Number : PKC18003

Date : 04JANUARY 2022

LANTHANUM OXIDE-PROMOTED COBALT CATALYST SUPPORTED ON MESOPOROUS ALUMINA FOR SYNGAS PRODUCTION VIA METHANE DRY

REFORMING

TRAN NGOC THANG

Thesis submitted in fulfillment of the requirements

for the award of the degree of Doctor of Philosophy

College of Engineering UNIVERSITI MALAYSIA PAHANG

JANUARY 2022

ACKNOWLEDGEMENTS

I would like to express my honest gratefulness to my supervisor, Assoc Prof Dr Sumaiya bt Zainal Abidin @ Murad for her meaningful guidance and support throughout the difficult time in the COVID-19 pandemic condition She always encouraged me with her outstanding experience and valuable awareness I also would like to thank my ex-main supervisor Dr Vo Nguyen Dai Viet for his constant support He has imparted me with professional knowledge and unique insights as well as useful research skills in the reaction engineering and heterogeneous catalysis field for nurturing me as a qualified academician I also wish to acknowledge my co-supervisor, Assoc Prof Dr Nurul Aini Mohamed Razali for her suggestions and cooperation throughout the study

I appreciate the co-operation and information sharing in this research from all my colleagues in GTL group including Mahadi Bahari, Attili Ramkiran, Fahim Fayaz, Tan

Ji Siang, Lau Ngie Jun, Shafiqah Nasir, and Sharanjit Singh Additionally, I would like

to acknowledge all my lab mates, friends, and teaching staff of the Department of Chemical Engineering, College of Engineering and the Faculty of Chemical and Process Engineering Technology, Universiti Malaysia Pahang for their collaboration and friendship

Finally, I dedicate this thesis to my family for their endless love, support, and the source

of my motivation in pursuing the studies

ABSTRAK

Tindak balas pembaharuan kering metana (MDR) baru-baru ini muncul sebagai antara pendekatan pelbagai guna yang terbaik untuk menukar dua gas rumah hijau, karbon dioksida (CO2) dan metana (CH4), kepada bahan mentah yang berharga untuk proses hiliran petrokimia Pada masa ini, masih terdapat cabaran dalam membangunkan pemangkin yang sangat stabil dan aktif untuk tindak balas MDR di samping rintangan yang lebih baik terhadap pemendapan karbon Baru0baru ini, pemangkin berasaskan kobalt yang disokong mesopori alumina muncul sebagai pemangkin yang berpotensi Walau bagaimanapun, bahan-bahan konvensional yang digunakan untuk menyediakan sokongan pemangkin mesopori ini ialah prekursor organik dan etanol yang agak mahal dan berbahaya kepada alam sekitar Oleh itu, dalam kajian ini, penggunaan mesopori alumina (Al2O3), yang direka menggunakan prekursor aluminium bukan organik yang murah dan tersedia dalam pelarut binari etanol-air, telah dikaji sebagai sokongan untuk pemangkin kobalt Penyiasatan ini bertujuan untuk mereka bentuk sistem pemangkin berasaskan kobalt yang berkesan untuk tindak balas MDR, yang mengatasi halangan penyahaktifan yang disebabkan oleh karbon Kesan promosi La2O3 pada ciri fizikokimia pemangkin kobalt yang disokong Al2O3 dan prestasi pemangkinnya juga telah dijelaskan Penilaian mangkin dalam tindak balas MDR telah dijalankan untuk mangkin 10%Co/Al2O3 dan 10%Co/Al2O3 yang digalakkan La2O3 (pemuatan La adalah dalam 1% – 8%) dalam reaktor katil tetap pada julat suhu 923 – 1073 K dan tekanan separa bagi bahan tindak balas dari 10 hingga 40 kPa Sokongan Al2O3 mempunyai luas permukaan BET 173.4 m2 g-1 dan nanopartikel kobalt tersebar dengan halus diatas sokongan dengan saiz kristal yang dikehendaki berjulat dari 5.2 - 9.2 nm Interaksi kuat antara CoO dan

Al2O3 telah disahkan dengan kehadiran spinel kobalt-aluminat dan struktur tekstur pemangkin adalah stabil terhadap suhu tindak balas Tingkah laku promosi La2O3

memudahkan pengurangan H2 dengan menyediakan ketumpatan elektron yang lebih tinggi dan meningkatkan kekosongan oksigen dalam 10%Co/Al2O3 Penambahan La2O3

boleh mengurangkan tenaga pengaktifan ketara bagi penggunaan CH4; lalu, meningkatkan penukaran CH4 sehingga 93.7% pada 1073 K Pembentukan lanthanum dioksikarbonat secara terus semasa MDR bertanggungjawab dalam pengurangan karbon termendap melalui kitaran redoks sebanyak 17-30% bergantung pada suhu tindak balas Selain itu, tahap kekosongan oksigen meningkat kepada 73.3% dengan promosi La2O3 Pemuatan 5%La ialah kandungan penggalak yang optimum untuk penukaran bahan tindak balas serta penghasilan H2 dan CO 5%La-10%Co/Al2O3 juga mempamerkan rintangan tertinggi terhadap pemendapan karbon kerana sifat asas, ciri redoks penggalak

La2O3 Tindak balas MDR ke atas pemangkin 5%La-10%Co/Al2O3 telah diyakini mengikuti mod penjerapan bersekutu CH4 dan CO2 pada dwi tapak zarah aktif atau berbeza dan pemangkin menunjukkan kestabilan yang baik semasa tindak balas 48 jam pada 1023 K Nisbah H2/CO 0.84-0.98 yang terhasil adalah sesuai untuk tindak balas Fischer-Tropsch di hiliran untuk menjana bahan api hidrokarbon cecair Akibatnya, penggunaan sokongan mesopori alumina dan penggalakk La2O3 meningkatkan aktiviti

Co dengan efektif dalam tindak balas MDR disamping menahan pemendapan karbon pada permukaan pemangkin

ABSTRACT

Methane dry reforming reaction (MDR) has recently emerged as a promising multipurpose approach for converting two greenhouse gasses, included carbon dioxide (CO2) and methane (CH4), into valuable feedstock for downstream petrochemical processes At present, there is still a challenge in developing the highly stable and active catalysts for MDR reaction as well as better resistance to carbon deposition Though the mesoporous alumina supported Co-based catalysts have recently appeared to be the potential catalysts However, the common starting materials for preparing these well-ordered mesoporous catalyst supports are organic precursors and anhydrous ethanol which are quite expensive and harmful to the environment Therefore, in this study, mesoporous alumina (Al2O3), fabricated using a cheap and available inorganic aluminium precursor in binary water-ethanol solvent, was implemented as support for cobalt catalyst This investigation aimed to design an effective cobalt-based catalyst system for MDR reaction, which overcomes coke-related deactivation barriers The promotional effect of

La2O3 on the physicochemical features of Al2O3 supported cobalt catalyst and its catalytic performance were also elucidated The catalyst evaluations in MDR reaction were conducted for 10%Co/Al2O3 and La2O3-promoted 10%Co/Al2O3 catalysts (La loading was in 1% – 8%) in a fixed-bed reactor at temperature range of 923 – 1073 K and partial pressure of individual reactant from 10 to 40 kPa The Al2O3 support has BET surface area of 173.4 m2 g-1 and cobalt nanoparticles were finely dispersed on the support with desired crystallite size ranged from 5.2 - 9.2 nm The strong interaction of CoO and Al2O3

phases was confirmed by the presence of cobalt-aluminate spinel and the textural structure of catalysts was stable with reaction temperature The promotion behavior of

La2O3 facilitated H2-reduction by providing higher electron density and enhanced oxygen vacancy in 10%Co/Al2O3 The addition of La2O3 could reduce the apparent activation energy of CH4 consumption; hence, increasing CH4 conversion up to 93.7% at 1073 K

Lanthanum dioxycarbonate transitional phase formed in situ during MDR was

accountable for mitigating deposited carbon via redox cycle for 17-30% relying on reaction temperature Additionally, the oxygen vacancy degree increased to 73.3% with

La2O3 promotion 5%La loading was an optimal promoter content for reactant conversions as well as yield of H2 and CO 5%La-10%Co/Al2O3 also exhibited the highest resistance to carbon deposition owing to the basic nature, redox feature of La2O3

dopant The MDR reaction over 5%La-10%Co/Al2O3 catalyst was convinced to follow

an associative adsorption mode of CH4 and CO2 on dual or different sites of active particles and the catalyst exhibited a good stability during 48 h reaction at 1023 K The resulting H2/CO ratios of 0.84-0.98 are suitable for Fischer-Tropsch reaction in downstream to generate liquid hydrocarbon fuels As a result, the employment of mesoporous alumina support and La2O3 promoter efficiently boosted the Co activity in MDR reaction along with suppressing the carbon deposition on the catalyst surface

2.3 Methane Dry Reforming Reaction 11

2.3.2 Catalyst Deactivation 182.3.3 The Catalyst Development 22

CHAPTER 4 EVALUATION OF MESOPOROUS ALUMINA-SUPPORTED COBALT NANOCATALYST FOR METHANE DRY REFORMING

4.2.4 Surface Morphology Analyses 61

4.4 Spent Catalysts Characterization 65

CHAPTER 5 LA-DOPED COBALT SUPPORTED ON MESOPOROUS

ALUMINA CATALYSTS FOR IMPROVED METHANE DRY

5.3.1 Influence Reaction Temperature 77

5.3.2 Influence of Promoter Loading 805.3.3 Influence of CH4 and CO2 Partial Pressure 835.3.4 Mechanistic Study of The MDR over 5%La-10%Co/Al2O3 86

LIST OF TABLES

Table 2.1 List of involved reactions in the MDR reaction 15Table 2.2 List of LH rate expressions proposed for MDR reaction 17Table 2.3 Summary of MDR performance over different supported catalysts

Table 2.4 Summary of textural properties of alumina and alumina supported

catalysts reported in literature 39Table 3.1 List of purchased chemicals and gases 44Table 3.2 List of experimental equipments 44Table 3.3 Summary of used chemicals for catalysts preparation 46Table 3.4 Information of standard gas from GC analysis 52Table 3.5 Properties used in the calculation of transport resistances 53Table 4.1 Summary of textural attributes of Al2O3 and 10%Co/Al2O3 58Table 4.2 Summary of peak assignment during H2 reduction of

Table 5.6 Summary of MDR performance over different catalysts reported in

(b) dissociative adsorption of CO2, (c) migration, (d) intermediates

Figure 2.4 Langmuir-Hinshelwood mechanism 16Figure 2.5 The mechanism of carbonaceous deposit formation on Ni-based

Figure 2.8 TPO profiles and summary of intergrated peak area of spent

catalysts (1) Ni/Al2O3, (2) Ni–Mg/Al2O3, (3) Ni–Ca/Al2O3, and (4)

Figure 2.13 Schematic representation of basic processes involved during

impregnation of precursors on porous support 37Figure 3.1 Overall flowchart of this study 42Figure 3.2 The Bragg Law for XRD analysis 48Figure 3.3 Schematic diagram for MDR set-up 51Figure 4.1 N2 physisorption and pore size distribution plots of Al2O3, fresh

Figure 4.2 XRD spectra of Al2O3, calcined and reduced 10%Co/Al2O3 59Figure 4.3 H2-TPR profiles of (a) Al2O3 and(b)10%Co/Al2O3 60Figure 4.4 HRTEM images of fresh (a) Al2O3 and (b) 10%Co/Al2O3 61Figure 4.5 Blank test of MDR at stoichiometric feed composition and 1023 K

Figure 4.6 CH4 conversion with TOS on 10%Co/Al2O3 at stoichiometric feed

composition and temperature of 973-1073 K 63

Figure 4.7 CO2 conversion with TOS on 10%Co/Al2O3 at stoichiometric feed

composition and temperature of 973-1073 K 63Figure 4.8 Influence of reaction temperature on gaseous product yields and

H2/CO ratio for 10%Co/Al2O3 at stoichiometric feed ratio 64Figure 4.9 N2 physisorption and pore size distribution plots of Al2O3 and spent

10%Co/Al2O3 catalysts after MDR at different reaction temperature

Figure 4.10 TPO weight and derivative weight profiles of spent Co/Al2O3 after

MDR at feed ratio of 1 and reactions at 973 K, 1023 K, and

Figure 4.11 Raman spectra for (a) fresh 10%Co/Al2O3 and (b) spent

10%Co/Al2O3 after MDR at 1023 K with stoichiometric reactant

Figure 5.1 N2 adsorption-desorption isotherms of Al2O3, 10%Co/Al2O3,and

La2O3-promoted 10%Co/Al2O3 catalysts with different La loadings 72Figure 5.2 XRD patterns of Al2O3 support and La2O3-promoted 10%Co/Al2O3

Figure 5.3 H2-TPR profiles of (a) 10%Co/Al2O3, (b) 3%La-10%Co/Al2O3, (c)

4%La-10%Co/Al2O3, (d) 5%La-10%Co/Al2O3, and (e)

Figure 5.4 CO2-TPD results of Al2O3, 10%Co/Al2O3, 3%La-10%Co/Al2O3,

5%La-10%Co/Al2O3 and 8%La-10%Co/Al2O3 samples 76Figure 5.5 Effect of temperature on CO2 and CH4 conversions in MDR over

10%Co/Al2O3 and 3%La-10%Co/Al2O3 at stoichiometry ratio 78Figure 5.6 Effect of temperature on CO and H2 yields in MDR over

10%Co/Al2O and 3%La-10%Co/Al2O at stoichiometry ratio 78Figure 5.7 Arrhenius plots for activation energy estimation of MDR over

10%Co/Al2O3 and 3%La-10%Co/Al2O3 79Figure 5.8 Effect of temperature on H2/COratio in MDR over 10%Co/Al2O3

and 3%La-10%Co/Al2O3 at stoichiometry ratio 80Figure 5.9 The time-on-stream conversion of CH4 in MDR over La2O3-

promoted 10%Co/Al2O3 catalysts with different La loadings at 1023

Figure 5.10 The time-on-stream conversion of CO2 in MDR over La2O3

-promoted 10%Co/Al2O3 catalysts with different La loadings at 1023

Figure 5.11 Effect of La loading on gaseous product yields and H2/CO ratio of

MDR over La2O3-promoted catalyst system at 1023 K and feed ratio

Figure 5.12 Effects of CH4 partial pressure on the conversion on MDR

performance over 5%La-10%Co/Al2O3 catalyst at 923, 973, 1023

Figure 5.13 Effects of CO2 partial pressure on the reactant conversion in MDR

over 5%La-10%Co/Al2O3 catalyst at 923, 973, 1023 and 1073 K 85Figure 5.14 Parity plots for the reaction rates by power-law model 88Figure 5.15 Estimation of activation energy from Model 2 90Figure 5.16 Parity plot for the reaction rate of CH4 by Langmuir-Hinshelwood

Figure 5.17 Time-on-stream profile for reactant conversions attained from

longevity test of 5%La-10%Co/Al2O3 at 1023 K and feed ratio of 1 92Figure 5.18 XRD patterns for spent (a) 10%Co/Al2O3,(b) 3%La-10%Co/Al2O3,

(c) 5%La-10%Co/Al2O3, (d) 8%La-10%Co/Al2O3 catalysts after

Figure 5.19 Raman spectra of (a) fresh 10%Co/Al2O3, (b) fresh

5%La-10%Co/Al2O3, (c) spent 10%Co/Al2O3, and (d) spent 10%Co/Al2O3 after MDR at 1023 K 95Figure 5.20 Raman spectra of spent catalsyts with various promoter loadings

5%La-after MDR at 1023 K and feed ratio of 1 96Figure 5.21 Carbon formation on spent 10%Co/Al2O3 and 3%La-10%Co/Al2O3

after MDR at 973 K, 1023 K, and 1073 K and feed ratio of 1 97Figure 5.22 La2O3 redox cycle for surface carbon removal during MDR 98Figure 5.23 Derivative weight TPO profiles of selected spent 10%Co/Al2O3,

3%La-10%Co/Al2O3, 5%La-10%Co/Al2O3, and 10%Co/Al2O3 after MDR at 1023 K and feed ratio of 1 99Figure 5.24 Mechanism for carbonaceous deposition removal from catalyst

8%La-surface with the assistance of La2O3 promoter 99Figure 5.25 The correlation between catalyst performance and carbon formation

Figure 5.26 Co 2p3/2 XPS spectra for spent (a) 10%Co/Al2O3 and (b)

3%La-10%Co/Al2O3 after MDR at 1023 K and feed ratio of 1 102Figure 5.27 C 1 s XPS spectra for spent (a) 10%Co/Al2O3 and (b) 3%La-

10%Co/Al2O3 after MDR at 1023 K and feed ratio of 1 103Figure 5.28 La 3d XPS spectrum for spent 3%La-10%Co/Al2O3 after MDR at

1023 K and feed ratio of 1 104Figure 5.29 O 1s XPS spectra for spent (a) 10%Co/Al2O3 and (b) 3%La-

10%Co/Al2O3 after MDR at 1023 K and feed ratio of 1 105Figure 5.30 HRTEM images of (a) fresh 10%Co/Al2O3, (b) fresh 3%La-

10%Co/Al2O3, (c) spent 10%Co/Al2O3, and (d) spent 10%Co/Al2O3 after MDR at 1023 K and feed ratio of 1 106

 The changes in the standard Gibbs free energy, kJ mol-1

n Number of moles, mol

k Mass transfer coefficient, cm s-1

h Heat transfer coefficient, J m-2 s-1 K-1

R Ideal gas constant, J mol-1 K-1

 Heat of reaction, kJ mol-1

r Rate of reaction, mol gcat-1 s-1

U Superficial gas velocity

n Molar flow rates, mol h-1

cat

W Weight of catalyst, g

 Full width at half maximum of peaks, nm

XRD wavelength, nm Bragg angle, o





LIST OF ABBREVIATIONS

MDR Methane dry reforming

EISA Evaporation-induced self-assembly

TPO Temperature-programmed oxidation

IWI Incipient wetness impregnation

GISS Goddard Institute for Space Studies

AAGR Average Annual Growth Rate

TPOX Thermal partial oxidation

CPOX Catalytic partial oxidation

MREs Mixed rare earth metals

JCPDS Joint Committee on Powder Diffraction Standards TCD Thermal conductivity detector

LH Langmuir-Hinshelwood

BMV Boudart-Mears-Vannice

BJH Barret–Joyner–Halenda

PSD Pore size distribution

FWHM Full width at half-maximum

TOS Time-on-stream

RWGS Reverse water-gas shift

I.D Inner diameter

MCM-41 Mobil Composition of Matter No 41

SBA-15 Santa Barbara Amorphous-15

US United States

Though the catalytic performance of Co-based catalyst is neither superior to nickel nor to the noble metal catalysts, its potential in large-scale applications has been convinced since it owns the thermal stability during reforming reaction (Bahari, Setiabudi, Nguyen, Jalil, et al., 2020; Budiman, Song, Chang, Shin, & Choi, 2012) The degree of carbon formation on cobalt-based catalysts mainly originates from methane disproportion on catalyst surface (Xin, Cui, Cheng, & Zhou, 2018) In fact, carbon formation was reportedly associated

with metal particle size larger than 10 nm (S Chen, Zaffran, & Yang, 2020; Juan-Juan, Roman-Martinez, & Illan-Gomez, 2009) Therefore, high surface area bearing materials have widely been employed as support for facilitating the active metal dispersion and stabilization (Ma et al., 2016; S Singh, Kumar, Setiabudi, Nanda, & Vo, 2018; Taherian, Yousefpour, Tajally, & Khoshandam, 2017)

Mesoporous alumina has appeared to be a promising carrier due to its abundant availability and excellent stability against temperature changes Ma reported that Al2O3-supported Pd-Ni catalysts for MDR showed good thermal stability and reduced the thermal sintering effect while the high surface area of Al2O3 support is beneficial for nanoparticle dispersion (Ma et al., 2016) Essentially, the controllable pore size of alumina material is the key factor to produce catalyst support with desired structure (W Wu, Wan, Chen, Zhu, & Zhang, 2015; W Wu, Zhu, & Zhang, 2018) The conventional evaporation-induced self-assembly (EISA) method used in mesoporous alumina preparation normally employs structure-directing polymer template, aluminum alkoxides and anhydrous alcohol solvent (W Wu et al., 2015; Yuan et al., 2008) However, from the economic and environmental perspectives, the organic aluminum precursors used in EISA approach are quite expensive and harmful to the environment (Cai, Yu, Anand, Vinu, & Jaroniec, 2011) Thus, implementing less expensive and highly available inorganic aluminum precursor is gaining

an alluring interest The fabrication of alumina at low cost by sol-gel using inorganic aluminum precursor such as aluminum nitrate has been intensely reported recently (Abdelkader, Osman, Halawy, & Mohamed, 2018; Moghadam, Alizadeh, Ghamari, & Mousavi, 2021; Roque-Ruiz, Medellín-Castillo, & Reyes-López, 2019) The structural configuration of mesoporous Al2O3 is strongly affected by selecting the alumina precursor

In particular, the reaction rate of alumina-template fabricating process greatly depends on the charge and the diameter of anions in the precursor (D Zhao, Huo, Feng, Chmelka, & Stucky, 1998)

The basic character of rare-earth metal oxides reportedly improved the CO2

adsorption, thereby increasing carbon gasification from catalyst surface (Bahari et al., 2017) Lanthanum oxide owning basic properties and high oxygen storage capacity was recently found to enhance carbon resistance in various reforming processes (Aramouni et al., 2018; Bahari et al., 2017; Fayaz et al., 2019) The dispersion of Co active metal on mesoporous

Al2O3 with La2O3 promotion could exhibit high reforming activity and coke resilience but

this potential metal oxide combination has not been previously examined for MDR to the best of our knowledge Since the above-mentioned catalytic recipe could possess outstanding performance and carbon resistance and was not explored before in literature, it is crucial to examine its catalytic behavior in MDR

1.2 Motivation

MDR is a multipurpose pathway for converting two major greenhouse gasses, viz.,

carbon dioxide and methane to syngas Additionally, the H2/CO ratio in MDR products is about unity which is preferred for downstream generation of long-chain olefinic hydrocarbons via Fischer-Tropsch process

Co-based catalysts have been verified as the effective catalyst for the industrial reforming processes owing to their high thermal stability, abundant resources, and great activity while mesoporous Al2O3 induce several benefits when implemented as the support for cobalt catalyst under harsh operating conditions

The basic and redox properties of La2O3 promoter in supported cobalt catalyst is widely reported improving the adsorption of CO2 reactant and enhancing carbon resistance

in various reforming processes

The EISA is an effective method for producing mesoporous alumina with the controllable pore size distribution by adjusting synthesis parameters Essentially, the structural properties of the resulting mesoporous alumina depended notably on the hydrolysis

of aluminum precursor From the economic and environmental perspectives, the implementation less expensive and highly available inorganic aluminum precursor is gaining

an alluring interest

Kinetic study in the catalytic reaction is an indispensable step to fully understand reaction mechanism and catalyst behaviour for heterogeneous catalyst involved during reforming reaction The kinetics and mechanistic analysis on MDR have been extensively reported over noble and non-noble catalysts

Secondly, the coke formation leading to the rapid catalyst deactivation at high reaction temperature during MDR is the main obstacle hindering Co-based catalyst in the industrial-scale application Appropriate promoters with basic or redox properties are widely reported as an efficient approach for impeding carbon deposition Though lanthanum oxide with basic properties and high oxygen storage capacity was recently found to enhance carbon resistance in various reforming processes, less attention has been given to the function of this promoter and its loading for the performance of Co-based catalyst in MDR

Finally, the bibliographic knowledge about MDR reaction over La2O3-promoted cobalt catalyst is still little-known as reported in the literature due to the complexity of this reaction Therefore, the kinetics modelling investigation is crucial for further applications of this catalyst

1.4 Objectives of Study

The objectives of this research are

1 To evaluate the effects of La2O3 promoter on the physicochemical properties of cobalt

catalyst supported on a novel mesoporous alumina which is prepared from an inorganic aluminum precursor

2 To assess the performance of mesoporous Al2O3-supported cobalt catalyst and

La2O3-promoted 10%Co/Al2O3 catalysts for MDR reaction at various operation conditions and promoter loading

3 To determine kinetic model of MDR reaction over La2O3-promoted cobalt catalyst

1.5 Scope of Study

The planned activities to obtain the objective 1

1 Synthesis of 10%Co/Al2O3, x%La-10%Co/Al2O3 catalysts (x: 1-8) which employing

a mesoporous Al2O3 support prepared through EISA method using an inorganic aluminum nitrate nonahydrate precursor and a mixed solvent of water-ethanol solution

2 Characterization of the physicochemical properties of the Al2O3 support, unpromoted

10%Co/Al2O3 and La2O3-promoted 10%Co/Al2O3 catalysts such as N2

adsorption/desorption behavior, specific BET surface area, total pore volume and average pore diameter, H2 reduction property, crystalline phase, the basicity, surface morphology and surface chemical composition

The planned activities to obtain the objective 2

1 Evaluation of the catalytic performance of 10%Co/Al2O3, 3%La-10%Co/Al2O3

catalysts for MDR reaction at feed ratio of 1 and reaction temperature range of 973 –

1073 K

2 Evaluation of La2O3-promoted 10%Co/Al2O3 catalysts with La loadings from 1 – 8%

for MDR reaction at feed ratio of 1 and reaction temperature of 1023 K

3 Investigation on the effect of reactant partial pressure in MDR reaction over the

selected La2O3-promoted 10%Co/Al2O3 catalyst at reaction temperatures range of

923 – 1073 K The reactant partial pressure was chosen in the range from 10 to 40 kPa

4 Evaluation of the catalyst stability in MDR reaction at 1023 K and feed ratio of 1 for

48 h

5 Characterization of the spent catalysts through the X-ray diffraction analysis, N2

adsorption/desorption study, temperature-programmed oxidation and Raman spectroscopy measurements, X-ray photoelectron spectroscopy analysis, and HRTEM spectroscopy

The planned activities to obtain the objective 3

1 Kinetic parameters determination via a Levenberg–Marquardt nonlinear least‐square

regression technique using Polymath software

2 Investigation of the MDR kinetic modelling over selected La2O3-promoted

10%Co/Al2O3 through the Langmuir-Hinshelwood mechanisms in Polymath software

1.6 Thesis Organization

This research has been prepared in 6 chapters and outlined as follows:

Chapter 1 briefly introduces the background of MDR reaction, the motivation,

problem statements, research objectives and scopes of the study The organization of this thesis is also illustrated in this chapter

Chapter 2 contains a detailed literature review of syngas, the existing literature on

reforming technologies for syngas production, reaction thermodynamic study, reaction mechanism, catalyst development and catalyst deactivation during reforming process

Chapter 3 describes the details of employed materials and methods in this research

for producingsupport and catalysts The characterization techniques and the experimental set-up for the catalyst activity evaluation in MDR reaction were detailed in this chapter Additionally, the MDR kinetics modelling approach was also mentioned

Chapter 4 focuses on evaluating mesoporous Al2O3 as a support for cobalt catalyst

in MDR reaction at different temperatures The supported catalyst physiochemical properties before and after MDR reaction were elucidated using various analytical techniques

Chapter 5 reveals the promotional effects of La2O3 on the physiochemical properties

of promoted 10%Co/Al2O3 catalyst as well as its catalytic activity for MDR reaction at

different temperature The investigation was focused on the improvement of catalyst activity and coke mitigation in which the effect of La loading and the initial reactant partial pressure

on the catalyst performance was detail studied Additionally, this chapter also discusses the kinetics modelling of MDR over the optimal La2O3-promoted 10%Co/Al2O3 catalyst and the catalyst stability

Chapter 6 summarizes and points out the general conclusions obtained in this study

and the suggestions for upcoming studies

2.2 Overview of Syngas

Syngas primarily consisted of hydrogen (H2) and carbon monoxide (CO) is an important feedstock for downstream petrochemical processes such as synthetic hydrocarbon fuels, ammonia and methanol synthesis (Hernández et al., 2017; K Liu, Song, & Subramani, 2009)

The global syngas market was valued at $43.6 billion in 2019, and is projected to reach $66.5 billion by 2027, growing at a CAGR of 6.1% from 2020 to 2027 and the syngas applications were detailly presented in Figure 2.1 One of the key factors driving the growth

of the global syngas market is rise in demand for synthetic gas from the chemical industry Syngas is further used to manufacture SNG that is used in the rail, marine, and road transport industries in the form of liquified natural gas (LNG) and compressed natural gas (CNG) Syngas has gained significant traction in the global market, owing to its advantages such as low energy costs, improved stability, and its use to fuel gas engines for power supply In addition, increase in environmental awareness and enforcement of stringent government regulations on the use of renewable fuels significantly propel the development of the market Furthermore, syngas is critical in reducing atmospheric waste emissions in landfills and greenhouse gases, thereby augmenting its demand globally However, high capital spending and financing hinder the growth of the global market

Figure 2.1 Syngas derivatives with reference to its composition (*H2/CO molar ratio) Source: Hernández et al (2017)

The production pathways of syngas include natural gas or liquid hydrocarbons steam reforming, the coal or biomass gasification, and in some types of waste-to-energy gasification facilities (Al-Fatesh, Fakeeha, Ibrahim, & Abasaeed, 2021)

Steam reforming is the most prevalent approach for syngas production from hydrocarbon fuels such as natural gas The reaction was conducted at 700 – 1100°C over the nickel-based catalyst, which is commonly employed to generate H2 from natural gas as described in Equation 2.1

CH  H O  CO 3H  2.1

About 50% of hydrogen production in the world today is based on methane steam reforming and H2 demand is vigorously increasing because of its significant role in energy sources and chemical reactants (Carrara, Perdichizzi, & Barigozzi, 2010) Despite of its efficacy, methane steam reforming still suffers from several drawbacks such as: (1) the requirement of excessive over-heated steam to curb coke formation, (2) the high endothermic nature requiring high energy for manufacturing, (3) the high operating cost (Pakhare & Spivey, 2014) and (4) the unsuitable H2/COratio for Fischer-Tropsch synthesis (D Li, Li, &

Gong, 2016) Therefore, many other reforming techniques have been studied in the current years

Partial oxidation is a method by which sub-stoichiometric feed-air mixture reacts in

an exothermic process to yield the hydrogen-rich syngas as shown in Equation 2.2

Today's advanced production industry faces challenges of growing the earth’s temperature caused by greenhouse gases Therefore, the utilization of CO2 instead of steam

as reactant in reforming process was considered as an emerging potential technique for syngas production (Lavoie, 2014) Thus, dry reforming is a reaction of CO2 with a hydrocarbon such as methane to produce syngas, a valuable building block for downstream petrochemical processes, as shown in Equation 2.3

Dry reforming methane (MDR) has gained a lot of attention, especially regarding raw material alteration, gas supply, availability of bio‐based gas and the demand of technologies with the potential of CO2 import (Lavoie, 2014) The term “dry” in dry reforming is based

on the substitution of water in steam reforming by CO2 Compared to steam reforming, MDR discloses several benefits regarding energy proficiency because there is less required energy

for water evaporation Additionally, the product mixture has low H2/CO ratio which can be directly used as the feedstock for Fischer-Tropsch synthesis (D Zeng et al., 2021) Although this method can provide several environmental and economic incentives, the drawbacks relating to its catalytic usage, such as expensiveness and/or low operational stability have impeded its large-scale applications Thus, designing a cost-effective catalyst with high activity and coke mitigation is the main concern in MDR industry and numerous research on MDR have been conducted and published in recent years

2.3 Methane Dry Reforming Reaction

2.3.1 The Kinetics Studies

In general, it is believed that the entire surface of the solid catalyst is not responsible for catalysing any reaction Only certain sites on the catalyst surface participate in the reaction and these sites are called active sites on the catalyst These sites may be the unsaturated atoms resulting from surface irregularities or atoms with chemical properties that enable the interaction with the adsorbed reactant atoms or molecules Activity of the catalyst is directly proportional to the number of these active sites available on the surface Thus, a solid catalytic reaction A → B goes through 7 steps illustrated in Figure 2.2

Step 1: Transportation of reactant (A) from bulk fluid to pore mouth on the external surface

of catalysts pellets

Step 2: Diffusion of the reactant (A) from the pore mouth through the catalyst pores to the

immediate vicinity of internal catalytic surface

Step 3: Adsorption of reactant (A) onto the catalyst surface

Step 4: Reaction of (A) on the catalyst surface producing product (B)

Step 5: Desorption of the product (B) from the surface

Step 6: Diffusion of the product (B) from interior part of the pores to the pore mouth on the

external surface

Step 7: Transfer of the product (B) from pore mouth on the external surface to the bulk fluid

Figure 2.2 Steps in solid catalytic reactions

The adsorption, surface reaction and desorption are the sequential steps for a catalytic reaction The chemical kinetics are represented in steps of 3, 4 and 5 The steps of 1 and 7 are representing the external mass transfer processes whilst steps 2 and 6 are associated with the internal mass transfer processes The overall rate of reaction is equal to the rate of the slowest step in the mechanism

2.3.1.1 External Mass Transfer Resistance

The external mass transfer resistance could be neglected if the Mears criterion given

in Equation 2.4 is fulfilled (Levenspiel, 1999; Manokaran, Saiprasad, & Srinath, 2015)

 (−𝑟𝑒𝑥𝑝) is reaction rate (mol gcat-1 s-1)

 𝑏 is the bulk density of catalyst bed (g cm-3)

 𝑅𝑝 is catalyst particle radius (m)

 𝑛 is the reaction order of MDR

 𝐶𝐴𝑏 is the bulk gas phase concentration (mol cm-3)

 𝑘𝑐 is the mass transfer coefficient (cm s-1)

2.3.1.2 Internal Mass Transfer Resistance

The internal mass transfer resistance could be neglected if the expression (Equation 2.5), Weisz-Prater criterion is met (Mears, 1971b; Weisz & Prater, 1954)

  2 exp

 𝜌𝐶 is the density of catalyst pellet (g cm-3)

 𝐷𝑒𝑓𝑓 isthe effective diffusivity of CH4 into mixture of N2 and CO2 (m2 s-1)

2.3.1.3 External Heat Transfer Resistance

The external heat transfer resistance could be ignored if the expression (Equation 2.6), Mears criterion is satisfied (Mears, 1971a)

 𝑇𝑏 is the reactant gas bulk temperature (K)

 𝐸𝐴 is the activation energy (J mol-1)

 𝑅 is ideal gas constant (J mol-1 K-1)

 (−∆𝐻𝑟)1073 𝐾 is the heat of MDR (J mol-1)

 h is the heat transfer coefficient (J m-2 s-1 K-1)

2.3.1.4 Intraparticle Heat Transfer Resistance

The internal heat transfer resistance could be neglected if the Anderson criterion (Equation 2.7) is achieved (Anderson, 1963; Theron, Dry, Van Steen, & Fletcher, 1997)

exp 2

Where 𝑝 is particle thermal conductivity

The MDR general mechanism is detailed into four steps as depicted in Figure 2.3 (Papadopoulou, Matralis, & Verykios, 2012):

The first step is the dissociative adsorption of CH4 as shown in Figure 2.3a where the

CH4 component was adsorbed and dissociated into CHx speciesand hydrogen atoms Each

CHx species tends to bond with the active metal at a proper site to complete its tetra-valence

For example, the CH3 species prefers to adsorb to the tip of active metal fragment while the

CH2 segment deposits between two metal atoms

Figure 2.3 Reaction steps for MDR reaction (a) dissociative adsorption of CH4, (b) dissociative adsorption of CO2, (c) migration, (d) intermediates oxidation and elimination Source: Papadopoulou et al (2012)

The second step, illustrated in Figure 2.3b, is the dissociative adsorption of CO2

which can be followed through three models In particular, the CO2 adsorption can happen via (i) C-only coordination, (ii) both C and O coordination, or (iii) O-only coordination The MDR reaction is first order in the pressure of CO2, suggesting that CO2 dissociation is the rate-determining step (Sarusi et al., 2011)

The third step includes the migration of deposited hydrogen atoms from CH4

cleavage to the support for forming OH- groups at temperatures below 1073 K as depicted in Figure 2.3c

The last step is the intermediates oxidation and elimination which the oxygen reacts with the CHx segments linked on catalyst surface to form the Cat-CHxO or Cat-CO intermediates These intermediates are then decomposed to yield CO and H2 as illustrated in Figure 2.3d In many circumstances, this intermediates formation and/or disintegration process is recognized as rate-limiting steps and the desorption of CO and hydrogen are known

to be fast Possible reactions involved in the MDR reaction are displayed in Table 2.1 (Nikoo

& Amin, 2011)

Table 2.1 List of involved reactions in the MDR reaction

No Reaction Reaction equation H 298K

(kJ/mol)

Major reaction

1 CH4 dry reforming CH4 + CO2  2CO + 2H 2 247

Reactions cause the decline in H 2 /CO ratio to <1

2 Reverse water-gas-shift (RWGS) CO2 + H2  CO + H2O 41

Reactions that produce coke (carbon)

CH4 decomposition (reaction no 3), CO disproportionation (reaction no 4), CO2

hydrogenation (reaction no 5), and CO hydrogenation (reaction no 6) are accountable for coke formation during MDR while RWGS (reaction no 2) made H2/CO proportion lower than 1

2.3.1.5 Power Law Model

Power law, as expressed in Equation 2.8, is commonly used to estimate the kinetic parameters such as activation energy (Ea), the reaction of order (m and n), the rate of constants (k), and pre-exponential factor (A) in reforming processes due to the simplicity in application and determination The reaction rate constant (k) in power law equation can be calculated via the Arrhenius model and shown in Equation 2.9 This model is only suitable within the small partial pressure range (Iyer, Norcio, Kugler, & Dadyburjor, 2003)

where r is the CO2 or CH4 consumption rate and R is the gas constant, 8.314 (J mol

-1 K-1) The root mean squared deviation (R msd) was employed to explain the difference

between the observed and predicted values

Several studies have employed power law model for determining the Ea of reforming processes Akpan reported the Ea of51 kJ mol-1 in ethanol steam reforming reaction over Ni-

based catalyst at temperature of 673-863 K, while Pichas determined the Ea of CO2 and CH4

at 6.4 kJ mol-1and 41.2 kJ mol-1, respectively in MDR reaction over La2-xSrxNiO4 perovskite type oxide at the reaction temperature of 723 to 1073 K (Akpan, Akande, Aboudheir, Ibrahim, & Idem, 2007; Pichas, Pomonis, Petrakis, & Ladavos, 2010) The activation energy

of ethanol dry reforming over NiO/SiO2-Al2O3 was also calculated via power law model and was at about 97.87 kJ mol-1 (Bej, Bepari, Pradhan, & Neogi, 2017)

2.3.1.6 Langmuir-Hinshelwood Model

In this mechanism, suggested by Irving Langmuir in 1921 and further developed by Cyril Hinshelwood in 1926, two molecules adsorb on neighbouring sites and the adsorbed molecules undergo a bimolecular reaction as shown in Figure 2.4:

Figure 2.4 Langmuir-Hinshelwood mechanism

Source: Liang et al (2014)

Langmuir-Hinshelwood model is commonly employed for description of catalytic reactions on single or dual site (Cheng, Foo, & Adesina, 2010b) As seen in Figure 2.4, two reactants(A and B) are proposed to be first adsorbed on the catalyst The reaction occurs on the catalyst surface followed by the elimination of product from the catalyst

In some specific kinetic studies, the MDR reaction prefers to proceed via Hinshelwood (LH) mechanisms (Múnera et al., 2007; Oemar, Kathiraser, Mo, Ho, & Kawi, 2016; Sierra Gallego, Batiot-Dupeyrat, Barrault, & Mondragón, 2008) and CH4, CO2

Langmuir-adsorption on the catalyst surface could either be on a similar active site or dual active site

In the single-site mechanism for MDR reaction, the CH4 and CO2 adsorption take place in the only site and have been proposed 2 step-single site rate-determining step (RDS)

as shown in Equations 2.10-2.13 (Moradi, Rahmanzadeh, & Sharifnia, 2010; Tsipouriari & Verykios, 2001)

Table 2.2 List of LH rate expressions proposed for MDR reaction

No Rate equation model Description Reference

(Richardson & Paripatyadar, 1990)

(Pichas et al., 2010)

(Cheng, Foo, & Adesina, 2010a)

(Cheng et al., 2010a)

(Foo, Cheng, Nguyen, & Adesina, 2011b)

2.3.2 Catalyst Deactivation

Long catalyst lifetime is an important factor for realizing the application of advanced and robust catalyst systems for MDR reaction in the industrial scale The disadvantages of cobalt- and Ni-based catalysts in MDR reaction connecting to the deactivation are accumulated carbon, active metals sintering and/or oxidation during the operation (Ochoa et al., 2017)

There have been many affords to understand the deactivation process mechanism and design highly stable catalyst as well as to optimize for slower catalyst deactivation in MDR reaction However, this issue is still the obstacle hindering the MDR application for syngas manufacture

2.3.2.1 The formation of carbon/coke

The distinguishing between carbon and coke is classified from the route of their formation Carbon is generated from the CO disproportionation process and coke is formed via hydrocarbon depositing on the catalyst The coke is mainly polymerized long-chain hydrocarbons (C Wang et al., 2015) The deposited carbonaceous material forming greatly depends on the O/C and H/C proportion in the feed of the reaction Compared to other methods such as methane steam reforming (SRM) and methane partial oxidation, MDR reaction has the lowest H/C ratio of feeding input, which prompts the deposit formation

(Usman, Daud, & Abbas, 2015) The blocking of catalyst pores by polymeric components, especially coke, is another widely encountered cause of catalyst deactivation If these are deposited near the pore openings, catalyst activity and selectivity can be influenced due to impaired mass transport into and out of the pores

The thermodynamic study of CH4 decomposition and the Boudouard’s reaction which cause the carbon deposition in catalyst have been conducted using Gibbs free energy calculation (S Wang, Lu, Millar, & fuels, 1996) The CH4 decomposition proceed at the temperature higher 553 oC while the Boudouard reactions appear in the temperature below

700 oC In the range 553–700 oC, both above-mentioned conversions are accounted for the carbon formation The CH4 decomposition on the active metal site is known to be one of the most important paths in MDR reaction because it generates the reactive carbon species then are oxidized to CO by the oxygen-containing species (Budiman et al., 2012) No carbon deposition would occur when the reactions between these methane decomposition and carbon oxidation on the surface of the catalyst are kinetically balanced The balance between reduction and oxidation can strongly be reached to provide better catalytic performance by selection of optimum cobalt loading

Reaction temperature, nature of support and promoters have been found to determine the type of carbon formed and their resultant effect on deactivation Deactivation by the formed carbon can be happened through (1) encapsulation of metal particles, (2) monolayer chemisorption or multilayer physical adsorption blocking accessibility of reactant to active sites, (3) continuous build-up of filamentous carbon in pores to the point of stressing and subsequently fracturing the support

The mechanisms of deposit formation on the catalyst are elucidated in Figure 2.5

CO was adsorbed and dissociated on the active sites of metal to generate atomic carbon species, Cα These atomic carbons are followed to react together to give the polymeric carbon filaments, Cβ The Cα and Cβ tend to transform to less active graphitic carbon, Cγ at high temperature

Figure 2.5 The mechanism of carbonaceous deposit formation on Ni-based catalyst Source: Argyle et al (2015)

The basicity of catalyst surface has been found to play a crucial part on the performance of catalysts for reforming process (Ni, Chen, Lin, & Kawi, 2012) Acidity of catalyst was supposed to inhibit dissociative chemisorption of CO2 onto catalyst surface due

to accretion of dehydrogenated carbon deposits on catalyst surface resulting to aging and graphitization of carbon deposits making it inactive These inactive carbon residues cover catalyst sites responsible for CO2 adsorption and activation leading to swift catalyst loss of activity (Phan et al., 2018) Basicity of catalyst however was found to enhance adsorption of

CO2 which accelerates gasification of carbon deposited avoiding any chance for intensification and graphitization The basic nature of catalyst attracts the acidic CO2

molecules adsorption hence increasing the CO2 surface coverage and reduction in carbon deposition through Boudouard reaction However, the surplus of basic sites on catalyst provokes the occurrence of RWGS reaction and formation of metallic oxides via oxidation which are also causative to catalyst deactivation It was concluded that a balance must be established on a moderate acid-base sites providing minimum equivalent activation energy for CH4 and CO2 towards MDR at the least chance of catalyst deactivation

Carbon/coke formation is also known to be sensitive to nature of metal crystallite and support structure since the formation of a carbon type involves the formation of C-C bonds

on the number of available sites (Tsakoumis, Rønning, Borg, Rytter, & Holmen, 2010)

2.3.2.2 Metal sintering

Sintering is the process of compressing and forming larger solid particles due to the heat or pressure without melting at the point of liquefaction The sintering phenomena is a result of surface energy reduction of kinetically favoured crystallites and their size-dependent movement on the catalyst support

This process is sensitive to the temperature, metal loading, metal – support interaction, support nature, particle distribution and size, (Tsakoumis et al., 2010) The catalyst support should have resistance toward thermal limitation caused by the Hu¨ttig and Tamman temperature In the Hu¨ttig temperature, atoms at defects will become mobile then later when the Tamman temperature is reached, atoms from the bulk will exhibit mobility The mobility of atoms forms larger aggregates resulting in the loss of catalyst active surface

by crystal growth of either the bulk material or the active phase In case of supported metal catalyst, it reduces the active surface area via agglomeration and coalescence of small metal crystallites into larger ones

Metal sintering in MDR usually happened at temperatures higher than 700°C, commonly boosted by the existence H2O generated from RWSG side-reaction The mechanism of metal crystallites growth in MDR could be described in Figure 2.6 via two principal pathways on support (Argyle & Bartholomew, 2015): (1) Atomic movement by coarsening and Ostwald ripening; (2) Crystallite relocation in the form of coalescence Both mechanisms could take place concurrently through certain complicated physicochemical periods including: (a) detachment of metal particles from its crystallite, (b) adsorption and setup of the metal particles in the pores and external surfaces of support, (c) diffusion of entrapped particles across support pores and surfaces, (d) wetting of support pores and surfaces by metal particles, (e) nucleation of metal particles, (f) coalescence between metal particles, (g) capture of metal particles by larger metal particles

Figure 2.6 The sintering phenomena (A) atomic movement, (B) crystallite relocation Source: Argyle et al (2015)

The metal sintering is a slow process, especially at moderate temperatures, however, these reactions are normally hard to reverse so that the restrain of sintering is preferred solution