FEDERAL UNIVERSITY OF RIO GRANDE DO NORTE

TECHNOLOGY CENTER

EXACT AND EARTH SCIENCES CENTER

POST-GRADUATE PROGRAM IN MATERIALS SCIENCE

AND ENGINEERING

DOCTORAL THESIS

Ni-Containing Hybrid Ceramics Derived from Polysiloxanes

for Production of CH4 via Hydrogenation of CO2

Heloísa Pimenta de Macedo

Supervisor: Prof. Dr. Dulce M. de Araújo Melo

Co-supervisor: Dr. Michaela Wilhelm

Natal-Brazil

2018

FEDERAL UNIVERSITY OF RIO GRANDE DO NORTE

TECHNOLOGY CENTER

EXACT AND EARTH SCIENCES CENTER

POST-GRADUATE PROGRAM IN MATERIAIS SCIENCE AND

ENGINEERING

Heloísa Pimenta de Macedo

Developed jointly with the University of Bremen

Ni-Containing Hybrid Ceramics Derived from Polysiloxanes

for Production of CH4 via Hydrogenation of CO2

Thesis presented to the Post-Graduate

Program in Materials Science and

Engineering of the Federal University of Rio

Grande do Norte, as requisite to obtain the

title of Doctor in Materials Science and

Engineering.

Supervisor: Prof. Dr. Dulce M. de Araújo Melo

Co-supervisor: Dr. Michaela Wilhelm

Natal-Brazil

2018

FEDERAL UNIVERSITY OF RIO GRANDE DO NORTE

TECHNOLOGY CENTER

EXACT AND EARTH SCIENCES CENTER

POST-GRADUATE PROGRAM IN MATERIAIS SCIENCE AND

ENGINEERING

Heloísa Pimenta de Macedo

Ni-Containing Hybrid Ceramics Derived from Polysiloxanes

for Production of CH4 via Hydrogenation of CO2

This work was evaluated as adequate to obtain the title of "Doctor in Materials Science

and Engineer" and approved by the Post-Graduate Program in Materials Science and

Engineering from the Federal University of Rio Grande do Norte, Brazil.

Examination Board:

_______________________________________ Prof. Dr. Dulce M. de Araújo Melo - Supervisor

(Federal University of Rio Grande do Norte)

_______________________________________ Dr. Michaela Wilhelm – Co-supervisor

(University of Bremen)

_______________________________________ Prof. Dr. Maurício R. Bomio Delmonte – Coordinator

(Federal University of Rio Grande do Norte)

_______________________________________ Prof. Dr. Bráulio Silva Barros

(Federal University of Pernambuco)

_______________________________________ Prof. Dr. Renata Martins Braga

(Federal University of Rio Grande do Norte)

Universidade Federal do Rio Grande do Norte - UFRN

Sistema de Bibliotecas - SISBI

Catalogação de Publicação na Fonte. UFRN - Biblioteca Central Zila Mamede

Macedo, Heloisa Pimenta de.

Ni-Containing Hybrid Ceramics Derived from Polysiloxanes for

Production of CH4 via Hydrogenation of CO2 / Heloisa Pimenta de

Macedo. - 2018.

109 f.: il.

Tese (doutorado) - Universidade Federal do Rio Grande do

Norte, Centro de Ciências Exatas e da Terra, Programa de Pós-

Graduação em Ciência e Engenharia de Materiais. Natal, RN, 2018.

Orientadora: Profª. Drª. Dulce Maria de Araújo Melo.

Coorientadora: Profª. Drª. Michaela Wilhelm.

1. Hybrid ceramics - Thesis. 2. Polysiloxane - Thesis. 3.

Polymer-derived ceramic - Thesis. 4. Complexing agent - Thesis.

5. Nickel catalyst - Thesis. 6. CO2 methanation - Thesis. I.

Melo, Dulce Maria de Araújo. II. Wilhelm, Michaela. III. Título.

RN/UF/BCZM CDU 666.3/.7(043.2)

To my beloved parents for

the unconditional support and love.

Ackwnowledgements

To Prof. Dulce M. de Araújo Melo for all the opportunities offered me over the past 4

years of working together. Her guidance, dedication and encouragement were of great

importance for this work;

To my co-supervisor, Dr. Michaela Wilhelm, for the deposited trust and for giving me

the opportunity to develop the present work in Germany. Her help, commitment and

experience were essential to this researcher;

To the Federal University of Rio Grande do Norte (UFRN) and the Post-Graduate

Program in Materials Science and Engineering (PPGCEM);

To the brazilian federal agency “Coordenadoria de Aperfeiçoamento de Pessoal de Nível

Superior” (CAPES) for the financial support;

To the Advanced Ceramic Group and the University of Bremen, represented by Prof.

Kurosh Rezwan, for the infrastructure and working facilities offered;

To the members of the board for their considerations and suggestions;

To the professors Marcus Antônio de Freitas Melo, Renata Martins Braga and Eledir

Vitor Sobrinho, for their friendship and collaborations during the past years;

To Jan Ilsemann (Institute of Applied and Physical Chemistry, University of Bremen) for

the catalytic tests performed;

To Dr. Rodolfo Luiz Medeiros (UFRN) for the in-situ XRD measurements (in LNLS -

Brazil), and for the help in difficult times, excellent company and for always trust in my

work;

To my friends from the Environmental Technology Laboratory (LABTAM-UFRN) and

also the ones from the Advanced Ceramic Group (University of Bremen) for the

friendship and collaborations;

To my family for the unconditional support and for always believing in me;

To all who contributed somehow to this work.

“Ninguém conhece as suas próprias capacidades

enquanto não as colocar à prova”.

Públio Siro

Abstract

Preceramic polymers are Si-based organic-inorganic materials that can be converted

under inert atmospheres in hybrid ceramics with adjustable characteristics and interesting

physicochemical properties. In this work, Ni-containing hybrid ceramics were prepared

by pyrolytic conversion of polysiloxanes (with either methyl or methyl-phenyl groups)

combined with nickel salt in order to evaluate their applicability in the hydrogenation of

CO2 toward the production of CH4 (CO2 methanation). Bis(trimethoxysilylpropylamine)

(BisA), an amino-siloxane was used as complexing agent in order to homogeneously

disperse the metal nanoparticles through the Si matrix. Materials with tailorable

characteristics were generated by varying the pyrolysis temperature from 400 to 600℃,

leading to the formation of porous materials in a hybrid state where the polymer is just

partly converted to ceramic, so-called ceramer. The materials were characterized by

thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), N2

adsorption-desorption isotherms (BET-BJH), water and n-heptane adsorption, X-ray

diffraction (XRD), scanning and transmission electron microscopy (SEM-TEM). CO2

methanation tests were performed between 200 – 400℃ at atmospheric pressure. In-situ

X-ray diffraction analysis (in-situ XRD) was used to evaluate the Ni particle structure and

size during a simulated CO2 methanation reaction. Porous ceramers with high specific

surface areas (100-550 m2 g-1), hydrophobic or hydrophilic surfaces and different Ni

particle sizes (4-7 nm) were obtained by varying the pyrolysis temperature and

polysiloxane composition. The addition of the complexing agent (BisA) not only

permitted a good dispersion of the Ni nanoparticles but also enabled the formation of

hierarchical porosity with micro-, meso- and macropores. Regarding the catalytic

performance, ceramers prepared from methyl polysiloxane exhibited a more hydrophobic

surface and, thus, improved catalytic performance compared to the ones preparade from

methyl-phenyl polysiloxane. A negative effect on the catalytic performance of ceramers

was observed with increasing pyrolysis temperatures, which led to an increase in the Ni

particle size (from 4 to 7 nm), and consequently, lower levels of conversion and

selectivity. The ceramers pyrolyzed at 400℃ exhibited the best catalytic performance,

showing selectivity up to ~77% and good stability over a 10 h reaction, during which the

Ni particle size was preserved. Therefore, Ni-containing hybrid ceramics are promising

materials for catalytic applications as the CO2 methanation, presenting high and stable

activity.

Keywords: Hybrid ceramics, polysiloxane, polymer-derived ceramic, complexing

agent, nickel catalyst, CO2 methanation

Resumo

Polímeros pré-cerâmicos são materiais orgânico-inorgânicos à base de Si que podem ser

convertidos sob atmosferas inertes em cerâmicas híbridas com características ajustáveis

e propriedades físico-químicas interessantes. Neste trabalho, cerâmicas híbridas contendo

Ni foram preparadas através da pirólise de polissiloxanos (com grupos metil ou metil-

fenil) combinados com sal de níquel a fim de avaliar a aplicabilidade desses materiais

para a produção de CH4 via hidrogenação do CO2 (metanação do CO2).

Bis(trimetoxisililpropilamina) (BisA), um amino-siloxano foi utilizado como agente

complexante para dispersar homogeneamente as nanopartículas metálicas através da

matriz de Si. Materiais com características específicas foram sintetizados variando-se a

temperatura de pirólise entre 400 a 600℃, levando à formação de materiais porosos em

um estado híbrido onde o polímero é apenas parcialmente convertido em cerâmica,

conhecido por ceramer. Os materiais foram caracterizados por análise termogravimétrica

(TGA), espectroscopia de infravermelho com transformada de Fourier (FTIR), isotermas

de adsorção-dessorção de N2 (BET-BJH), adsorção de água e n-heptano, difração de

raios-X (DRX) e por microscopia eletrônica de varredura e transmissão (MEV-MET).

Testes catalíticos de metanação foram realizados entre 200 - 400℃ à pressão atmosférica.

A análise de difração de raios-X in-situ (DRX in-situ) foi utilizada para avaliar a estrutura

e o tamanho das partículas de Ni durante uma reação de metanação simulada. Materiais

porosos com altas áreas superficiais (100-550 m2 g-1), superfícies hidrofóbicas ou

hidrofílicas e diferentes tamanhos de partículas de Ni (4-7 nm) foram obtidos variando-

se a temperatura de pirólise e a composição do polissiloxano. A adição do agente

complexante (BisA) não só permitiu uma boa dispersão das nanopartículas de Ni como

também possibilitou a formação de uma porosidade hierárquica com micro-, meso- e

macroporos. Em relação ao desempenho catalítico, ceramers preparados a partir de

polissiloxanos contendo grupos metil exibiram superfícies mais hidrofóbicas e, assim,

melhor desempenho catalítico em comparação àqueles preparados a partir de

polissiloxanos contendo grupos metil-fenil. Um efeito negativo no desempenho catalítico

dos ceramers foi observado com o aumento da temperatura de pirólise, o que levou a um

aumento no tamanho das partículas de Ni (de 4 para 7 nm) e, consequentemente, menores

níveis de conversão e seletividade. Ceramers pirolisados a 400ºC apresentaram melhor

performance catalítica, com seletividade de até ~ 77% e boa estabilidade por 10 h,

mantendo o tamanho das partículas de Ni estável. Portanto, cerâmicas híbridas contendo

Ni são materiais promissores para aplicações catalíticas como a metanção do CO2,

apresentando alta atividade e estabilidade.

Palavras-chave: Cerâmica híbrida, polissiloxano, cerâmica derivada de polímero, agente

complexante, catalisador de níquel, metanação do CO2

List of Figures

Fig 2.1: Conversion of CO2 to chemicals and fuels through hydrogenation……....

23

Fig. 2.2: Schematic illustration of the “Power-to-Gas” process….……………….

25

Fig 2.3: (a) equilibrium constants (K) of the reactions involved in the methanation

process, and (b) product compositions for CO2 methanation at equilibrium (0.1

MPa)……………………………………………………………………………...

27

Fig. 2.4: General simplified formula of silicon-based preceramic polymers……...

32

Fig. 2.5: Main classes of silicon-based preceramic polymers in the Si-O-C-N-B

system…………………………………………………………………………….

32

Fig. 2.6: General structures of polysilsesquioxanes………………………………

35

Fig. 2.7: Sol-gel hydrolysis and condensation polymerization reaction…………..

37

Fig. 2.8: Polymer-to-ceramic conversion and microstructure evolution with

pyrolysis temperature…………………….……………………………………….

40

Fig 2.9: Overview of the molecular structure of silicon-based polymers during

thermal decomposition……………… …………………………………….……...

41

Fig. 2.10: General overview of a complexing metal precursor and the resultant

cross-linked polysiloxane………………………………………………………

44

Fig. 3.1: Molecular structure of the siloxanes used: (a) methyl-phenyl siloxane

(H44), (b) methyl siloxane (MK) e (c) Bis(trimethoxysilylpropyl)amine (BisA)…

48

Fig. 3.2: Schematic overview of the processing route…………………………….

50

Fig. 3.3: (a) Schematic flowchart of the synthesis route and (b) temperature ramps

applied during pyrolysis…………………………………………………………..

53

Fig. 3.4: Representation of the catalytic system used……………………………..

58

Fig. 4.1: Thermogravimetric analysis of cross-linked precursors under N2

atmosphere………………………………………………………………………...

61

Fig. 4.2: FT-IR spectra of ceramers (a) Ni/H44+BisA and (b) Ni/MK+BisA cross-

linked at 200°C and pyrolyzed at 400-600°C……………………………………...

63

Fig. 4.3: N2-adsorption/desorption isotherms for (a) H44+BisA, (b) MK+BisA,

(c) Ni/H44+BisA and (d) Ni/MK+BisA ceramers pyrolyzed at 400-600°C……….

66

Fig. 4.4: Pore size distribution (inset from 2 to 11 nm) of (a) H44+BisA, (b)

MK+BisA, (c) Ni/H44+BisA and (d) Ni/MK+BisA ceramers pyrolyzed at 400-

600°C……………………………………………………………………………...

67

Fig 4.5: (a) N2-adsorption/desorption isotherm and (b) pore size distribution (inset

from 2 to 11 nm) of Ni/H44+BisA and Ni/MK+BisA cross-linked at 200°C……...

68

Fig. 4.6: Specific BET surface areas of metal-free and Ni-containing ceramers

pyrolyzed at 400, 500 and 600°C………………………………………………….

71

Fig 4.7: Ratio of maximum water and n-heptane adsorption for ceramers

pyrolyzed at 400, 500 and 600°C………………………………………………….

73

Fig. 4.8: SEM micrographs of Ni/H44+BisA (a-c) and Ni/MK+BisA (d-f)

pyrolyzed at 400, 500 and 600°C………………………………………………….

74

Fig. 4.9: TEM images of the fresh Ni-containing ceramers pyrolyzed at 400°C

(a,d), 600°C (b,e) and the spent catalysts after 10 h stability test (c,f)……………..

77

Fig 4.10: Particle size distributions of Ni-containing ceramers pyrolyzed at 400°C

(a,d), 600°C (b,e) and the spent catalysts after 10 h stability test (c,f)……………..

78

Fig. 4.11: TEM image of CNT formed in Ni/H44+BisA-600……………………..

79

Fig. 4.12: XRD patterns of the Ni-containing ceramers pyrolyzed at 400, 500 and

600 °C……………………………………………………………………………..

80

Fig. 4.13: (a) CO2 conversion and (b) CH4 selectivity for Ni-containing ceramers

pyrolyzed at 400, 500 and 600°C and standard catalyst Ni/SiO2 pyrolyzed at

400°C……………………………………………………………………………...

82

Fig. 4.14: CO2 conversion (black) and CH4 selectivity (blue) for catalysts

pyrolyzed at 400°C during test at 350°C for 10 h………………………………….

85

Fig. 4.15: In-situ XRD during simulated CO2 methanation reaction at 400°C for

(a) Ni-H44+BisA-400, (b) Ni-MK+BisA-400, (c) Ni-H44+BisA-600 and (d) Ni-

MK+BisA-600 ceramers; and (e) crystallite size versus time on stream…………

87

List of Tables

Table 2.1: Main reactions involved in methanation of carbon oxides…………….

26

Table 3.1: Siloxanes used in the experimental procedure……………………….

48

Table 3.2: Chemical composition and pyrolysis temperature of the hybrid

ceramics……………………………………………………………………….….

51

Table 4.1: Average pore size and pore volume, Ni average crystallite and particle

size, and Ni content of ceramers cross-linked and pyrolyzed at 400, 500 and

600°C………………………………………………………..…………………….

69

List of Acronyms, Symbols and Abbreviations

APTES 3- Aminopropyltriethoxysilane

BET Brunauer, Emmet and Teller

BisA Bis(trimethoxysilylpropyl) amine

BJH Barrett-Joyner-Halenda

CH4 Methane

CNTs Carbon Nanotubes

CO Carbon Monoxide

CO2 Carbon Dioxide

FTIR Fourier-transform infrared spectroscopy

GHG Greenhouse Gas

H2O Water

H44 Methylphenyl polysiloxane

MK MethylPolysiloxane

Ni Nickel

NH3 Ammonia

PDCs Polymer-derived Ceramics

PTG Power-to-Gas

SEM Scanning Electron Microscopy

SGN Synthetic Natural Gas

SSA Specific Surface Area

SiC Silicon Carbide

SiO2 Silicon Dioxide

SiOxCy Silicon Oxicarbide

TEM Transmission Electron Microscopy

THF Tetrahydrofuran

TGA Thermogravimetry Analysis

XRD X-Ray Diffraction

∆G Entropy

∆H Enthalpy

Content

I – INTRODUCTION................................................................................................. 17

II – LITERATURE REVIEW................................................................................... 21

2. CO2 Conversion…………………………………………………………………… 22

2.1 CO2 Hydrogenation……………………………………………………………… 22

2.1.1 CO2 Hydrogenation to Methane (CO2 Methanation)……………………….... 24

2.1.2 Heterogeneous Catalysts for CO2 Methanation………………………………... 28

2.1.3 Deactivation……………………………………………………………………. 30

2.2 Polymer-derived ceramics……………………………………………………….. 31

2.2.1 Preceramic precursors………………………………………………………….. 31

2.2.2 Polysiloxanes…………………………………………………………………... 34

2.2.3 Processing of Preceramic Polymers……………………………………………. 36

2.2.3.1 Synthesis of Polysiloxanes………….……………………………………….. 36

2.2.3.2 Shaping and Cross-linking…………………………………………………… 38

2.2.3 Pyrolysis (Polymer-to-ceramic conversion)…………………………………… 39

2.2.4 Metal-Containing PDCs………………………………………………………... 42

III – METHODOLOGY............................................................................................. 46

3. Materials & Methods……………………………………………………………… 47

3.1 Materials................................................................................................................. 47

3.2 Approach of the Work…………………………………………………………… 49

3.3 Composition……………………………………………………………………… 50

3.4 Preparation route…………………………………………………………………. 52

3.5 Characterization Techniques…………………………………………………….. 54

3.5.1 Thermogravimetric Analysis (TGA)…………………………………………... 54

3.5.2 Fourier-transform infrared spectroscopy………………………………………. 54

3.5.3 Adsorption/desorption of Nitrogen…………………………………………….. 54

3.5.4 Surface Characteristic Analysis………………………………………………... 55

3.5.5 Scanning Electron Microscopy (SEM)………………………………………… 56

3.5.6 Transmission Electron Microscopy (TEM)……………………………………. 56

3.5.7 X-Ray Diffraction (XRD)……………………………………………………… 56

3.5.8 Catalytic Test…………………………………………………………………... 57

3.6.9 In-situ X-Ray Diffraction (In-situ XRD)………………………………………. 58

IV – RESULTS & DISCUSSIONS............................................................................ 59

4.1 Pyrolytic Conversion…………………………………………………………….. 60

4.2 Porosity…………………………………………………………………………... 64

4.3 Surface Characteristics…………………………………………………………... 71

4.4 Surface Morphology……………………………………………………………... 73

4.5 Metallic Nanoparticles…………………………………………………………… 75

4.6 Crystalline Structure……………………………………………………………... 79

4.7 Catalytic Activity………………………………………………………………… 81

4.8 In-situ Analysis…………………………………………………………………... 86

V – CONCLUSIONS & OUTLOOK........................................................................ 89

5. Conclusions………………………………………………………………………... 90

6. Outlook……………………………………………………………………………. 91

REFERENCES……………………………………………………………………... 92

SUPPLEMENTARY DATA...................................................................................... 103

Appendix A................................................................................................................... 104

Appendix B................................................................................................................... 107

I - Introduction

17

1- INTRODUCTION

Over the past century, growing human demands and the dependence on fossil fuels

to produce energy have changed the global energetic scenario and induced environmental

issues by the emission of greenhouse gases (GHG) into the atmosphere. Carbon dioxide

(CO2) has potentially contributed to this scenario of global warming and climate changes

corresponding for proximally 78% of the total GHG emissions in the last decades [1].

Recent studies estimate that global temperature might increase by more than 2°C by the

year 2050 and more than 4°C by 2100 if no firm global action is taken against climate

change. In order to overcome this issue, GHG emissions must be reduced by at least 50%

by 2050 [2]. Hence, there has been increasing efforts from countries and scientists to limit

the emissions and develop more efficient systems for CO2 utilization.

In principle, three strategies for reducing the CO2 concentrations are possible: (i)

control of the CO2 emissions, (ii) CO2 capture and storage, and (iii) chemical conversion

and utilization of CO2. The first approach is based on the intensive use of renewable

energy sources such as wind or solar, and on energy efficient improvements. Carbon

capture and storage is a relatively well-established process for cutting CO2 emissions

quickly but has an issue of potential leakage of CO2. Yet the utilization of CO2 as an

economical, safe, and renewable carbon source, arise as an attractive strategy for

producing chemicals while reducing the emissions [2, 3]. Hydrogenation of CO2 to value-

added products is one of the most intensively investigated areas of CO2 conversion.

Catalytic hydrogenation of CO2 to methane, also called Sabatier reaction (CO2 +

4H2 → CH4 + 2H2O, ∆H0 = −164 kJ mol-1) has attracted attention over the last decades

by converting a greenhouse gas such CO2 into a valuable-added gas (CH4), that can be

used as a fuel or raw material for the production of other chemicals [4]. Noble (Rh, Ru,

and Pd) and transition (Ni, Fe, and Co) metal-based catalysts are the most investigated

catalysts for the CO2 methanation. Nobel metals are known to be very active at low

temperatures, however, Ni-based catalysts are most commonly employed and promising

for industrial applications, owing to the low-cost, abundance and high activity [5].

Developing highly active and stable catalysts is a great challenge in current catalysis. The

support plays an important role in the performance of a heterogeneous catalyst due to the

metal-support or reactant-support interactions, and adsorption capability [6]. Supports

with high specific surface area are desired to improve the dispersion of the active phase

18

and to facilitate internal mass transfer. Moreover, the preparation methodology has a

significant effect on the catalytic performance, influencing the dispersion of the active

metal as well as the metal-support interaction [7]. It is important to ensure highly

dispersed and easily accessible nanoparticles in order to improve the utilization of the

metal catalyst. Furthermore, especially in reactions where water is formed as a product, a

hydrophobic support is favorable to avoid the hindrance of the further catalytic reaction

as observed in the CO2 methanation [8].

A relatively new concept is the application of metal-containing polymer-derived

ceramics (PDCs) for heterogeneous catalytic reactions. In general, PDCs were proposed

over 40 years ago for the production of Si-based advanced materials. The conversion

process of polymers precursors such as polysiloxanes, polycarbosilanes or polysilazanes

into ceramic enabled significant technological breakthroughs in ceramic science, offering

many advantages in terms of high thermal and chemical stability, outstanding oxidation

and corrosion resistance, low density, high permeability, high thermal shock resistance

and good mechanical strength [9]. Therefore, they have been used in many applications

such as for sensors [10], adsorbents [11], anodes [12], coatings [13] and membranes [14].

The pyrolytic conversion of polymeric precursors under inert atmospheres at

temperatures of 400-1500°C is a versatile processing route as compared to the much more

energy consuming traditional ceramic routes. Pyrolysis up to 1000°C leads to the

formation of amorphous materials, whereas pyrolysis at higher temperatures can generate

crystalline materials. [15]. The pyrolytic conversion can be used to adjust the ceramic

characteristics, resulting in interesting physicochemical properties. Pyrolysis at lower

temperatures (400-800°C) leads to incomplete polymer-to-ceramic conversion, resulting

in highly porous hybrid ceramics, so-called ceramers, that presents properties related to

both ceramic and polymeric characteristics [16]. Furthermore, the properties and

characteristics of the PDCs can also be tailored by the introduction of fillers or precursors

with different compositions.

In order to add a catalytic function to the PDCs, metal nanoparticles can be

incorporated or in-situ generated, leading to catalytically active ceramics. One advantage

of the PDC route, in comparison to the conventional preparation methods, is the in-situ

reduction of the metal during the pyrolysis [17], by which the formation of hardly

reducible mixed oxides can be avoided. With manufacturing of porous hybrid materials

containing in-situ generated metallic nanoparticles, some enhanced properties are to be

expected. For instance, the long-term stability should be increased due to a better

19

dispersion of the particles and stronger particle-matrix interaction that avoids extensive

sintering of the metal particles. At the same time, the adjusted porosity should provide a

good accessibility of the catalyst particles and facilitate the mass transfer, leading to an

increased selectivity towards the desired product. Moreover, specific modification of the

surfaces by changing the chemical nature of the precursor and thereby the hydrophobicity

behavior of the surface may influence the interaction with gas reagent and products

formed, avoiding block of the pores.

Therefore, metal-containing PDCs have been applied in many different reactions

such as CO oxidation, Fischer-Tropsch, hydrogenation of acetylenes, etc [18-20].

Specifically, Ni-containing PDC has been investigated for the CO2 methanation [19],

however, catalytic measurements were only performed with microporous cerames which

may present a mass transport limitation due to their microporosity. Besides, investigations

were based on only one type of polysiloxane precursor.

Thus, the present work aims at the preparation of porous Ni-containing hybrid

ceramics derived from polysiloxanes and their applicability for use as catalyst in the CO2

methanation reaction. The effect of the pyrolysis temperature and precursor composition

on the material properties and catalytic activity is investigated. The purpose was to adjust

the properties of the ceramers, resulting in improved characteristics such as high specific

surface area with micro- and mesopores, enhanced hydrophobicity behavior of the surface

and homogeneous dispersion of the metal particles in the polysiloxane matrix. In order to

achieve these aims, the following specific objectives were defined:

1. Synthesis of hybrid ceramics from different polysiloxanes precursors in order to

modify the surface chemical behavior (hydrophobicity/hydrophilicity) by varying

the organic side groups (methyl or methyl/phenyl groups);

2. Addition of an amine-containing siloxane, bistrimethoxysilylpropylamine (BisA),

in order to homogenously distribute the metal particles through the silicon matrix

by complexing the metal ions and anchoring them to the polysiloxane. BisA was

selected since it can also induce the formation of mesoporous when combined

with preceramic polymers;

3. Investigation of the influence of the pyrolysis temperature (400, 500, 600°C) on

the structural, chemical and catalytic properties. This range of temperature is

known to favor the development of high specific surface areas;

20

4. Characterization of the materials by thermogravimetric analysis (TGA), Fourier-

transform infrared spectroscopy (FTIR), N2 adsorption-desorption isotherms

(BET-BJH), water and n-heptane adsorption, X-ray diffraction (XRD), scanning

and transmission electron microscopy (SEM, TEM) and in-situ X-ray diffraction

(in-situ XRD);

5. Catalytic and stability tests for CO2 methanation (200– 400°C).

21

II - Literature Review

22

2. CO2 Conversion

Transformation and utilization of carbon dioxide is considered as a promising

pathway to reduce CO2 environmental impact (greenhouse gas effect) and generate

valuable chemicals as raw materials or energy rich compounds, that are not derived from

fossil sources. The hydrogenation of CO2 has gained attention as an attractive route due

to the range of products that can be obtained. The present section comprehensively

discusses some important aspects concerning the hydrogenation of CO2 toward CH4.

Reaction mechanisms, effects of catalyst and deactivation issues are presented.

2.1 CO2 Hydrogenation

The present technologies for utilization of carbon dioxide have emerged as a

suitable pathway to reduce CO2 emissions by developing advantageous uses of this GHG.

The prospect to employ CO2 as a feedstock for synthetic applications in chemical and fuel

industries is a current challenge that has received great attention. The conversion of CO2

into valuable compounds, for instance, is of great interest to space agencies like NASA

(National Aeronautics and Space Administration) due to the possibility of using CO2

present in the atmosphere of Mars to produce CH4 and H2O (through CO2 methanation

reaction) for fuel and astronaut life-support systems [21]. Moreover, the German car

manufacturer Audi, reported the development of the “fuel of future” using catalytic

chemical conversion of CO₂ into synthetic “e-diesel”, which is currently in the midst of

demonstration for high scale production [22].

The major concern in the chemical transformation of CO2 lies in the energy

limitation to overcome the great thermodynamic stability of this gas and its low chemical

reactivity due to the strong double bond between carbon and oxygen atoms (C=O=C)

[23]. In general, CO2 conversion reactions into other C1 chemicals such as carbon

monoxide, formic acid, formaldehyde, dimethyl ether, methanol and methane, commonly

require a considerable amount of external energy. A commonly alternative to lower the

energy demand in such reactions is by adding a substance with relatively higher Gibbs

energy, such as hydrogen (H2), making the CO2 conversion more favorable

thermodynamically. In addition, the use of an effective catalyst is necessary for further

lowering the energy barrier and increasing the conversion levels [23].

23

Hence, catalytic conversion of CO2 using H2 is considered as a potential approach

for the production of varied chemicals. Indeed, several routes are currently available and

make use of catalytic hydrogenation reactions for converting the CO2 [24]. The main

products of CO2 hydrogenation can fall into two categories -fuels and chemicals - as

depicted in Fig. 2.1. Methanol, dimethyl ether and hydrocarbons, for example, are

excellent fuels in internal combustion engines and are easy to store and transport. Besides,

methanol and formic acid are raw materials and intermediates for many chemical

industries [3].

The range of products obtained from hydrogenation reactions is associated with

the properties of the catalyst and the process conditions. Particularly, Ni and nobel-based

catalysts were found to be very selective to CH4 production over a H2/CO2 ratio of 4:1.

By employing Cu-based catalysts, instead, and H2/CO2 ratio of 3:1 methanol is mainly

produced. Co-based catalysts, however, are most popular for reducing CO2 to CO through

Water Gas Shift Reaction (WGSR) [25].

Fig 2.1: Conversion of CO2 to chemicals and fuels through hydrogenation. Adapted

from [2].

24

2.1.1 CO2 Hydrogenation to Methane (CO2 Methanation)

Methanation was first reported by the French chemist Paul Sabatier in 1902 who

proposed the conversion of CO2 and CO with hydrogen to form methane (CH4) and water

using metal-based catalysts (equation 2.1 and 2.2) [6].

CO2 + 4H2 → CH4 + 2H2O, ∆H0 = −164 kJ mol-1 (2.1)

CO + 3H2 → CH4 + H2O, ∆H0 = −206.1 kJ mol-1 (2.2)

The methane produced from this process presents several advantages over other

chemicals because it can be used directly as a fuel (Synthetic Natural Gas - SGN), as raw

material for the production of other chemicals or can be stored in tanks and, when needed,

used for heat or electricity production. In addition, methanation reaction happens under

atmospheric pressure and at relatively low temperatures [26].

Based on the so-called “power-to-gas” (PTG) technology, CO2 captured from

biomass, industries or other carbon-containing resources can be converted with hydrogen

(obtained from renewable sources as water electrolysis) into methane, establishing then a

renewable and sustainable cycle [27]. Fig. 2.2 shows an overview of the PTG process.

This technology is a particularly promising system solution, which supports the

integration of renewable energies and also contributes to reducing the emissions of GHG.

Since 2009, projects have been conducted in Europe, mainly in Germany, involving the

production of methane from CO2 at commercial scale or pilot plant with capacity ranging

from 25 kW to 6300 kW [28]. One important challenge in the search for efficient

hydrogenation processes is the hydrogen source. In order to be considered a sustainable

process, H2 must come from non-fossil sources. Possible alternatives are water

electrolysis to produce H2 using electricity generated by solar, wind or other renewable

energy, and water splitting using photocatalytic, photoelectrochemical or other

photochemical processes [29].

25

Fig. 2.2: Schematic illustration of the “Power-to-Gas” process. Adapted from [2].

The hydrogenation of CO2 to methane (CO2 methanation) is thermodynamic

favorable and considerably faster than other reactions which form hydrocarbons or

alcohols [26]. However, during methanation, some side-reactions may occur which

affects the purity of the CH4 produced. Table 2.1 lists the main possible reactions involved

in the methanation process. Besides reaction R2, carbon monoxide methanation reaction

can also occur at lower H2/CO ratio (R3). The Boudouard reaction (R4) is of great

importance since carbon on the catalyst surface is considered as a necessary intermediate

during the methanation reaction. In addition, water plays an important role through the

water–gas shift reaction (R5), which would modify the surface and catalytic chemistry of

methanation catalysts. Among these reactions, it has to be noticed that R1, R2, and R4

can be regarded as three independent reactions. The other reactions can be described as a

linear combination of these three reactions [30].

26

Table 2.1: Main reactions involved in methanation of carbon oxides. Adapted from [6].

Reação ∆H298 K (kJ mol-1) ∆G298 K (kJ mol-1)

R1 CO2 + 4H2 ↔ CH4 + 2H2O - 164.0 - 113.2

R2 CO + 3H2 ↔ CH4 + H2O - 206.1 - 141.9

R3 2CO + 2H2 ↔ CH4 + CO2 - 247.3 - 170.4

R4 2CO ↔ C + CO2 - 172.4 - 119.7

R5 CO + H2O ↔ CO2 + H2 - 41.2 - 28.6

R6 2H2 + C ↔ CH4 - 74.8 - 50.7

R7 CO + H2 ↔ C + H2O - 131.3 - 91.1

R8 CO2 + 2H2 ↔ C + 2H2O - 90.1 - 62.5

The equilibrium constants of the eight reactions involved in the methanation are

depicted in Fig 2.3 (a) at different temperatures. It can be seen that all the reactions are

favorable at low temperatures (<400°C) due to their exothermic characteristics.

Additionally, the product composition distributions are presented in Fig 2.3 (b) and show

high equilibrium conversions up to temperatures of 400°C, where methane and water are

the main products at low temperatures (200–250°C). Raising the reaction temperature

above 450°C results in the increase of the carbon monoxide by-product, due to the reverse

water–gas shift reaction, and meanwhile, unreacted carbon dioxide and hydrogen also

increase, along with a decrease in the methane yield [30].

27

Fig 2.3: (a) equilibrium constants (K) of the reactions involved in the methanation

process, and (b) product compositions for CO2 methanation at equilibrium (0.1 MPa).

Adapted from [6].

Two different mechanisms are proposed for the CO2 methanation. The first one

involves the dissociation of CO2 to CO prior to methanation, and in the subsequent

reaction, CO is converted to CH4 by reacting with H2. The second one does not involve

the formation of CO as intermediate steps and direct hydrogenation of CO2 to CH4 takes

place [31].

The proposed first path is further suggested to be completed by two different

mechanisms. The first mechanism involves the associative adsorption of CO2 and H2

followed by the formation of formiate (CHOO−) species as intermediate, where these

species are decomposed in CO. The second mechanism suggests the dissociation of CO2

into CO and O adsorbed species without the formation of formiate [32]. In both

mechanisms, the intermediate formed is the adsorbed CO, the only difference is the way

in which this intermediate is formed. Most researchers believe that CO2 methanation

follows the model involving a CO intermediate without the formation of formiate.

In general, it can be concluded that CO2 methanation is an exothermic and

thermodynamically favorable reaction; however, owing to its high stability, the reduction

of the fully oxidized carbon to CH4 has significant kinetic limitations that require a

substantial energy input, catalysts with high stability and activity, and optimized reaction

conditions [33].

28

2.1.2 Heterogeneous Catalysts for CO2 Methanation

A fundamental aspect of the efficiency of the methanation process is related to the

catalyst itself, which strongly influences conversion, selectivity and deactivation process.

Hydrogenation of CO2 toward methane has been investigated using a number of catalytic

systems based on noble metals such as Pt, Pd, Ru and Rh or transition metals as Ni, Co

and Fe [34-37]. Noble metals such as Pd, Pt, Ru, and Rh are known to give better activity

and selectivity as a catalyst, however, the low availability and high price of these metals

have restricted their uses. Nickel-based catalysts are most commonly employed for CO2

methanation and promising for industrial applications, owing to the low-cost, abundance

and high activity [38].

The addition of a second metal on the surface of monometallic catalysts drastically

change the electronic, physical and chemical properties of the materials. Ren et al. [39]

prepared Ni/ZrO2 catalysts with the addition of a second metal (Fe, Co or Cu) by

impregnation. They concluded that Fe could efficiently enhance the catalytic activity of

Ni/ZrO2 in CO2 methanation at low temperatures (240°C). The addition of Fe (optimal

amount of 3 wt.%) not only improved the dispersion and degree of reduction of Ni but

also enhanced the partial reduction of zirconia.

The support plays an important role in the performance of a heterogeneous

catalyst, with the major role to disperse the active component. Porous materials are often

used as support in order to achieve a larger surface area and thus improve metal

dispersion, as well as to facilitate internal mass transfer. Commonly, oxides as SiO2,

Al2O3, CeO2 or zeolites have been extensively studied for the preparation of well-

dispersed catalysts [40]. The support usually affects the morphology of the active phase,

adsorption capability, metal-support interaction and catalytic properties [41, 42]. Le et al.

[43] investigated Ni-based catalysts supported on different materials (Ni/SiO2 and

Ni/SiC) for the CO and CO2 methanation. They concluded that the high Ni dispersion

coupled with the high thermal conductivity enabled by the SiC was beneficial both for

CO and CO2 methanation. According to Yan et al. [44], catalysts with moderate Ni-

support interactions show superior activity for CO2 methanation, whereas inferior activity

is obtained with an overly strong metal-support interaction.

The surface properties are also a significant factor in addition to the metal– support

interaction. Especially in methanation reactions where water is formed as a product, a

29

hydrophobic support is favorable to avoid the blocking of the adsorption sites by water

[45]. Furthermore, to activate the CO2 molecules, it is important to adjust the catalyst’s

surface chemistry to improve the adsorption capability towards CO2, which can be

achieved by introducing rare earth or other alkaline metals. Guo et al. [46] prepared Ni-

based catalysts supported in different oxides and test them for the CO2 methanation at

200-450°C. The differences in catalytic activity could be explained by the basic property

of the catalysts, indicating that the basic sites should have a positive effect on the catalytic

activity for CO2 methanation.

For structure-sensitive reactions such as the CO2 methanation, a serious problem

is often the sintering of metallic particles at higher temperatures, resulting in a decrease

in the activity and long-term stability of the catalyst [47]. Thus, an effective highly

dispersed metal nanoparticles with low or no particle growth at the same time are of great

interest. Vogt et al [48] reported a series of SiO2-supported Ni catalysts with diameters

ranging from 1 to 7 nm in size for CO2 methanation reaction. They demonstrated the

presence of a distinct particle-size effect revealing different activities within a narrow

particle size distribution. A maximum activity was found for catalysts possessing Ni

particles with an average size of 2.5 nm. In another work, Kesavan et al. [49] reported the

catalytic conversion of CO2 to CH4 over five Ni/YSZ samples, with the same nickel

loading (10 wt%) but different Ni particle size. The results showed that catalytic activity

and CH4 selectivity depended on size and morphology of Ni particles. Moreover, the type

and amount of deposited carbon strongly depended on the Ni morphology. The most

efficient catalyst presented smaller and well-dispersed Ni particles. According to Wu et

al. [50], a large difference in Ni particle size strongly affects the kinetic parameters of the

CO2 hydrogenation and the reaction selectivity. At low Ni loading (0.5 wt%), the catalyst

showed a comparatively higher catalytic activity for CO2 hydrogenation with high CO

selectivity. With Ni loading increased to 10 wt% (ca. 9 nm particles), the selectivity was

switched to favor CH4 formation. It becomes clear, that for each specific catalytic system an

optimal particle size is aimed in order to improve the activity and selectivity toward the desired

product.

Moreover, the preparation methodology has a significant effect on the catalytic

performance, influencing the metal-support interaction as well as the dispersion of the

active metal [7]. It is important to ensure highly dispersed and easily accessible

nanoparticles in order to improve the utilization of the metal catalyst. Various

methodologies have been investigated for preparing the catalysts, including

30

impregnation, co-precipitation, sol-gel, hydrothermal and combustion method [51-54].

Liu et al. [51] reported the synthesis of NiO/SBA-15 catalysts with a three-dimensional

(3D) network structure by a one-pot hydrothermal method for CO2 methanation and

compared the results with a catalyst prepared by traditional impregnation. The results

showed that the catalyst prepared by one-pot method showed higher catalytic activity and

sintering resistance than the one synthesized by traditional impregnation method, due to

the higher Ni dispersion in the meso-channels.

2.1.3 Deactivation

Catalyst deactivation is a complex process that combines many mechanisms

together, among which poisoning, coking and sintering are important concerns in the

methanation process.

Methanation catalysts are sensitive to different gas impurities containing, for

instance, chlorine compounds, tars, ammonia, sulfur compounds, or alkalis. Poisoning

occurs due to the chemisorption of impurities present in the feed stream on catalytically

active sites, thereby blocking the sites for catalytic reaction. These impurities can also

modify the chemical nature of the catalysts and result in the formation of new compounds

that definitively alter the catalyst performance [55].

For catalytic reactions involving hydrocarbons (or even carbon oxides) side

reactions occur on the catalyst surface leading to the formation of carbonaceous residues

(usually referred to as coke or carbon) which tend to physically cover the active surface

deactivating the catalyst either by covering of the active sites and by pore blocking.

Thermodynamically, carbon formation is favorable at high-temperatures (above 450 °C),

however, due to some mechanisms carbon can be observed at much lower temperatures

[56]. The formation of such species depends on the operating conditions and catalyst

formulation. In practice, the coke deposition may be controlled to a certain extent by using

an optimal catalyst composition (with the addition of suitable promoters) and an

appropriate combination of process conditions by adding steam or increasing the H2/CO2

ratio because hydrogen reacts with the carbon deposits and prevents catalyst deactivation

[55].

31

Another possible deactivation process is the sintering. High reaction temperatures

can rapidly promote agglomeration and coalescence of small metal crystallites into larger

ones with lower surface-to-volume ratios, causing a reduction of the activity and catalyst

lifetime. Sintering usually refers to the loss of active surface via structural modification

of the catalyst [57]. To mitigate the metal sintering, common strategies used are

increasing the metal dispersion through strong metal–support interaction or adding

catalyst promoters.

2.2 Polymer-derived ceramics

Polymer-Derived Ceramics (PDCs) are a relatively new concept of advanced

ceramics with unique properties and characteristics that have gained great interest in

many areas of science. The transformation process (polymer-to-ceramic) enabled

significant technological breakthroughs, offering many advantages compared to the

traditional ceramics. In this section, some fundamental aspects related to the PDCs are

described including the structure characteristics, synthesis, processing and conversion

process.

2.2.1 Preceramic precursors

Preceramic polymers are a class of organosilicon polymers (e.g., polymers based

on a backbone of Si atoms containing also Caron (C), Oxygen (O), Nitrogen (N), Boron

(B) and Hydrogen (H) atoms), which can be directly converted into a ceramic material by

pyrolysis under controlled conditions. The molecular structure and type of the preceramic

polymer influence not only the composition but also the phases and microstructure of the

final ceramic produced. Thus, the chemical and physical properties of PDCs can be varied

and adjusted by the design of the molecular precursor [9]. A simplified representation of

the molecular structure of a preceramic organosilicon polymer is shown in Fig. 2.4.

32

Fig. 2.4: General simplified formula of silicon-based preceramic polymers. Adapted

from [9].

The classification of the organosilicon polymers is based on the (X) group bonding

to the silicon atoms in the polymer backbone. The main classes are the polysilanes (X =

Si), polycarbosilanes (X = carbon), the polysiloxanes (X = oxygen), the polysilazanes (X

= nitrogen), the polyborosilanes (X = boron) or combinations of two of them [9]. Each

class will derive ceramics with compositions based on the corresponding (X) group. Fig.

2.5 represents the main classes of silicon-based preceramic polymers.

Fig. 2.5: Main classes of silicon-based preceramic polymers in the Si-O-C-N-B system.

Adapted from [9].

33

Regarding the side chain functional groups (R1 and R2), mainly hydrogen or

organic groups such as methyl, vinyl and/or phenyl are used. The nature of these side

groups is fundamental in determining the global properties and characteristics of the

polymer, especially during their processing before the pyrolysis step. By modifying the

R groups, properties like solubility, thermal stability and viscosity as a function of the

temperature could be easily tailored [9, 15].

Moreover, suitable functionalities are necessary to achieve the polymer setting

through cross-linking reactions before the pyrolysis step, which is strictly related to the

nature of the side groups involved (e.g. condensation reactions in the case of –OH

functionalities, addition reactions in the case of –vinyl groups). In addition, suitable

organic groups can be used to control or to incorporate relatively large amounts of carbon

content on the final ceramic composition [58]. Depending on the carbon content, different

structural and functional properties can be obtained. Carbon-rich PDCs were shown to

have a higher resistance against crystallization and a higher ceramic yield than their low-

carbon containing analogs [59]. The direct application of this knowledge is an approach

to tailor the composition and nanostructure of the resulting ceramic by the manipulation

of chemistry and molecular architecture of the preceramic polymers.

In order to be appropriate for the production of polymer-derived ceramics some

requirements should be fulfilled by the preceramic polymers [9, 60]: (i) the polymers

should possess a sufficiently high molecular weight in order to avoid volatilization of low

molecular components; (ii) they should have appropriate rheological properties and

solubility for the shaping process, and (iii) latent reactivity (presence of reactive,

functional groups) for the cross-linking step. Furthermore, the possibility to modify the

molecular structure of the preceramic polymers and the application of shaping

technologies well known from plastic forming combined with relatively low synthesis

temperatures, address great advantages to the use of these precursors.

Among the silicon-based precursors, polysiloxanes represent the most studied

class of preceramic polymers and will be further discussed in this literature review.

34

2.2.2 Polysiloxanes

Polysiloxanes are versatile materials showing excellent chemical, physical,

electrical and optical properties. They exhibit outstanding thermo-mechanical stability

owing to the combination of relatively unique features such as high stability at elevated

temperatures and in oxidative environments, as well as a pronounced elasticity at low

temperatures [61]. Therefore, polysiloxanes are widely used in many fields as for coating,

optical, electronic, high temperature and biomedical applications [62-65].

The high-temperature stability of polysiloxanes is related to the inherent strength

of the siloxane bond as well as to the pronounced chain flexibility (large Si-O-Si angles

from 140 to 180°). The partial ionic and double bond character of the Si–O bond respond

to its exceptional strength since both effects increase the binding force between the

participating silicon and oxygen atoms [66]. Thus, the Si–O bond withstands exposure to

higher temperatures than the bonds normally found in organic polymers, leading to a

significantly higher thermal stability of polysiloxanes than that of their organic (C–C)

counterparts.

The main reasons for the polysiloxanes widespread utilization are related to their

generally low cost (the lowest among all Si-based polymers) and their easy and cheap

synthesis route, which makes this class of precursors very versatile, easy to handle and

processable under normal conditions without any particular precaution.

Most commonly, polysiloxanes are synthesized starting from functionalized

silanes with the general formula: RxSiX4-x, (with X = Cl, OR, OC(=O) or NR2 and R is

an alkyl or aryl group [60]. One of the most used functionalized silanes for the industrial

synthesis of polysiloxanes is the dimethyl dichloro silane. The general route used consists

of two steps: (i) hydrolysis of the dichloro dimethyl silane, which leads to the formation

of a mixture of linear and cyclic oligosiloxanes (equation 2.3); and (ii) polycondensation

of hydroxyl-functionalized short-chain polysiloxanes (equation 2.4) or ring-opening

polymerization process of the cyclic oligomers (equation 2.5), which leads to high

molecular weight polymers [66].

(m+n) R2SiCl2 + (m+n+1) H2O → HO(R2SiO)mH + (R2SiO)n + 2(m+n) HCl (2.3)

35

HO(R2SiO)mH → [R2SiO]n (Polycondensation) (2.4)

(R2SiO)n → [R2SiO]n (Ring opening polymerization) (2.5)

Besides linear or cyclic polymers, another important sub-class of polysiloxanes

are the polysilsesquioxanes, characterized by the general formula –[RSi–O1.5]n– (with R

being H or an organic aryl or alkyl group) and the high degree of cross-linking [67]. This

particular class of siloxanes is characterized by a highly-branched molecular structure due

to the trifunctional nature of the silsesquioxane moieties, which allows for a variety of

many possible configurations, such as random, “ladder” or “cage” structure [66], as

depicted in Fig. 2.6. Due to their high branching level, this class of polymers is often

referred to as silicon resins. They are generally solid at room temperature and

characterized by very high ceramic yields upon pyrolysis.

Fig. 2.6: General structures of polysilsesquioxanes. Adapted from [9].

36

2.2.3 Processing of Preceramic Polymers

Preceramic polymers are synthesized from tailor-made molecular precursors,

enabling to control and adjust their chemical composition as well as their architecture.

The processing of ceramics using preceramic polymers offers many advantages compared

to the conventional (powder) synthesis procedures, including [68]: (i) low processing

temperatures, (ii) no additives required for densification, (iii) low-cost plastic-forming

techniques can be applied with easy control over rheological properties, (iv) utilization of

unique polymeric properties that cannot be found in ceramic powders, such as appreciable

plasticity and solubility of precursors in organic solvents, (v) possibility of adding

different types of fillers, and (vi) ceramic products containing unique combination of

polymer and ceramic properties (hardness, creep resistance and oxidation resistance) can

be obtained.

Manufacturing of engineered ceramic materials from polymer precursor typically

involves the following steps: (i) polymer synthesis from preceramic precursor compounds

(silanes); (ii) shaping and thermally cross-linking the polymer (generally at 150-250°C)

to generate a thermoset material; and (iii) pyrolysis under inert atmosphere between 400-

1500°C, resulting in an amorphous or crystalline ceramic depending on the temperature

applied.

2.2.3.1 Synthesis of Polysiloxanes

Cross-linked polysiloxanes or silicon resins can be synthesized through sol–gel

process via hydrolysis and condensation of hybrid silicon alkoxides. In this method,

colloidal systems in the sol state are converted into gels by the addition of controlled

amounts of water to a solution of metal-organic monomers (mostly metal alkoxides)

dissolved in anhydrous organic solvents. The hydrolysis of the metallic species triggers

the polymerization of those by condensation, leading to the formation of a polymeric gel

[69].

The precursors are organically modified silicon alkoxides of the general formula

R’xSi(OR)4−x (R’ = alkyl, allyl, aryl; R = methyl, ethyl), which are converted into silicone

resins of composition R’xSiO(4−x)/2 [70]. The reaction is generally divided into two steps:

hydrolysis of metal alkoxides to produce hydroxyl groups in the presence of water

37

(usually in the addition of acid or base catalyst), followed by polycondensation of the

resulting hydroxyl groups and residual alkoxyl groups to form a three-dimensional

network [71] as shown in Fig. 2.7. The hydrolysis and condensation reactions are

presented in more detail by equations 2.6 – 2.8 [66].

R’SiOR3 + 3H2O → RSi(OH)3 + 3HOR (hydrolysis) (2.6)

3R’Si(OH)3 → 3R’SiO3/2 + 1.5 H2O (condensation) (2.7)

R’Si(OH)3 + R’SiOR3 → RSiO3/2 + 3HOR (condensation) (2.8)

As seen above, Si-OR bonds are converted into Si-O-Si via the hydrolysis (2.6) of

Si–OR into Si-OH, followed by the condensation reactions (2.7) and (2.8) leading to Si-

O-Si networks.

Fig. 2.7: Sol-gel hydrolysis and condensation polymerization reaction. Adapted

from [71].

Factors such as the nature of the organic group, solvent, temperature, water to

alkoxide molar ratio, the presence of acid or base catalysts, etc., are known to affect the

38

hydrolysis reaction. In the presence of a base catalyst, for example, the reaction takes

place through a nucleophilic substitution and the rate of condensation is fast compared to

hydrolysis, resulting in the formation of higher molecular weight polymers with branched

chains. On the other hand, if hydrolysis is catalyzed by acids, an electrophilic reaction

takes place, and the rate of condensation is slow relative to the rate of hydrolysis leading

to low molecular weight polymers with longer and less branched chains [72].

2.2.3.2 Shaping and Cross-linking

The shaping of preceramic polymer is one of the most advantageous aspects of

the PDCs route. When compared to common powder processing routes, polymeric

systems have the potential advantage of adopting plastic-forming techniques to generate

shaped components. A variety of ceramic products can be straightforwardly produced

such as powders, whiskers, fibers, foams and coatings. The techniques applied to

preceramic polymers manufacturing include: casting [73], tape-casting [74], warm

pressing [75], coating [76], injection molding [77], extrusion [78] and fiber drawing [79].

The possibility of obtaining a machinable component prior to the polymer-to-ceramic

conversion is a remarkable advantage since it permits a more precise shape control

without the problems related to ceramic brittleness and tool wear [80]. Moreover,

processing techniques such as sacrificial template, replica technique, foaming, blowing,

etching, freeze casting or emulsification combined with controlled pyrolysis conditions

can be applied to produce porous ceramics with a variety of pore sizes (micro-, meso –

and macropores) [81, 82]. Another strategy for producing highly porous ceramics from

preceramic polymers is the mixing of precursors with different molecular architecture

[83].

A distinctive drawback of the PDC technology is the poor control of shrinkage

and structural integrity of the products during the polymer-to-ceramic transformation. As

a consequence of thermally induced gas releasing reactions, high volume shrinkage is

observed during pyrolysis by the loss of volatile species, resulting in an increase of the

density [84]. Moreover, the formation of cracks and/or pores is likely to occur. The

introduction of fillers has been one of the main strategies followed enabling the

fabrication of dense, crack-free and strong monoliths with a single ceramization step.

39

Fillers of various nature (polymeric, metallic, ceramic) and shape/dimension (nano- or

microsized powders, nanotubes, nano- or long fibers) can be added to a preceramic

polymer before shaping. Generally, the fillers can be inert or active [85]: (a) inert or

passive fillers do not react with the ceramic residue from the preceramic polymer, the

decomposition gases or the heating atmosphere (typical examples are SiC or Si3N4

powders); (b) active fillers, i.e., metallic or intermetallic powders that react with the

decomposition gases generated during pyrolysis, or the heating atmosphere or (less

frequently) with the ceramic residue from the preceramic polymer.

After the shaping process, the cross-linking of the preceramic polymer is

fundamental for obtaining a thermoset material capable of retaining its shape during the

pyrolysis step, when the polymer converts into the final ceramic material. Thermal cross-

linking up to 200°C represents one of the most common techniques, which could be easily

achieved if suitable functionalities are present inside the polymer structure, such as –OH,

–H or –vinyl groups, that could enable condensation or addition reactions [68]. For

instance, for polysiloxanes having functional groups in the structure, e.g., hydroxy or

alkoxy groups, the cross-linking process occurs upon condensation of the silanol groups.

Furthermore, thermal cross-linking temperatures could be lowered in the presence of an

appropriate catalyst or radical initiator [86]. Polymer cross-linking is important for

obtaining high ceramic yields upon pyrolysis. Effective cross-linking reactions lead to

highly branched and higher molecular weight polymeric molecules, and so to a lower

content of oligomers and low molecular weight chains that volatilize at higher

temperatures, thus increasing the final ceramic yield. The degree of cross-linking strongly

affects the rheological behavior of a preceramic polymer, and therefore it has to be

carefully controlled especially when plastic forming technologies are used [87].

2.2.3 Pyrolysis (Polymer-to-ceramic conversion)

The polymer-to-ceramic conversion is a complex process in which a number of

simultaneous physical and chemical transformations take place for the formation of the

ceramic component. Once most of these transformations occur within a given temperature

range (400-1500°C), a general profile of the polymer-to-ceramic conversion can be drawn

as a function of the temperature, as seen in Fig. 2.8. The thermal conversion happens

mainly through structural rearrangements that result in the cleavage of chemical bonds

40

and the release of organic functional groups, which yields a mixture of volatile

compounds and free (hydro)carbon with the formation of an inorganic network.

At temperatures up to 400°C the elimination of mainly water, solvent and low

molecular-weight products from the polymeric matrix occurs while cross-linking of

siloxanes is accomplished by addition or condensation reactions, accompanied by the

cleavage of weak bonds (e.g. Si-H, Si-Si and N-H side groups bonds) [88]. In the range

from 400°C to about 800°C, cleavage of strong bonds (e.g. C-C, C-N, Si-N, Si-C and C-

H side groups bonds) happens. The exact nature of these reactions depends on the type of

the functional groups bonded to the precursor backbone and the previous handling of the

system. In the case of polysiloxane resins for example, it has been verified that in the

600–800°C range, methane is the main volatile species released, while at higher

temperatures (600–1100°C) the release of hydrogen is observed, resulting from the

cleavage of Si-H, Si-C and C-H bonds [89]. From 800°C to approximately 1000°C mass

losses are minimal indicating that organic-inorganic conversion is completed, although

bonds rearrangements in the amorphous covalent ceramics and consequently

compositional changes befall [88]. By further increasing the temperature, the

crystallization process starts, leading to phase separation and formation of crystals.

Fig. 2.8: Polymer-to-ceramic conversion and microstructure evolution with pyrolysis

temperature. Adapted from [9].

41

PDCs experience a profound microstructural change when exposed to varied

temperatures. At temperatures between 400–1000°C, the microstructure is composed by

homogeneous and amorphous phase, consisting of a random mixture of different covalent

bonds (e.g. Si–O, Si–C, Si–N, etc.) directly related to the molecular structure of the

polymer as seen in Fig 2.9. For instance, polysiloxanes lead to the formation of silicon

oxycarbide (SiOxCy), whereas polysilanes and polycarbosilanes generate silicon carbide

(SiCx). In this phase, although part of the carbon is bonded to silicone, excessive carbon

(the carbon that is not linked to silicon atoms) is present as carbon clusters (basic

structural units, BSU) [90].

Fig 2.9: Overview of the molecular structure of silicon-based polymers during thermal

decomposition. Adapted from [9].

At higher temperatures, the devitrification process begins and the gradual

redistribution of chemical bonds generates a progressive phase separation process and

42

formation of crystalline structures. Carbon clusters redistribute as well, to generate a

turbostratic (“quasi-graphitic”) carbon network [91]. The extensive phase separation

leads to the formation of crystals of SiC, Si3N4, SiO2, B4C, BN, etc., which can increase

their size by a further increase of temperature or of dwell time [92]. Depending on the

polymer composition, precipitation of excessive carbon and nucleation and growth of

crystalline phases occur at different temperatures: at 800-1000°C for Si-C system and

above 1600°C for Si-B-N-C [93].

Furthermore, the pyrolytic conversion can be used to adjust the ceramic

characteristics, resulting in interesting physicochemical properties. Pyrolysis at lower

temperatures between 400 and 800°C can lead to very high specific surface areas

materials (up to 600 m2 g-1) with micro- and/or mesopores, as a result of the

decomposition reactions and the subsequent release of gaseous products [83]. The

evolution of volatile species reaches its maximum at approximately 500–700°C and by

interrupting the pyrolysis at this stage of conversion, the formation of highly porous and

surface-rich materials in a hybrid state where the polymer is just partly converted to

ceramics is possible. The derived material, so-called ceramer, presents properties related

to both ceramic and polymeric characteristics [16]. The well-controlled pyrolytic

conversion at intermediate temperatures is decisive to change the chemical nature of the

surface and to develop high porous materials. This porosity generated is however

transient, once it is eliminated when the pyrolysis temperature is increased leading to the

completion of the ceramization process. The specific surface area is stable up to the

pyrolysis temperature reached, even after prolonged heating at the same temperature [94].

2.2.4 Metal-Containing PDCs

Catalytic functions can be generated in polymer-derived ceramics by

incorporation of metal precursors during processing. Introduction of such functionalities

is possible either by (i) integrating the metal precursor to the polymer main chain, or (ii)

by attaching the metal precursor to the polymer backbone through adequate use of

complexing precursors with organic moieties attached, normally amino-containing

siloxane. Such approaches can lead to a homogeneous distribution of the metal through

the silicon matrix [60]. One advantage of the PDC route, in comparison to conventional

impregnation methods, is the simultaneous reduction of the metal during the pyrolysis

43

process due to the reductive atmosphere formed by the hydrocarbons and hydrogen

released during thermal decomposition [17].

Some research groups have investigated the modification of preceramic polymers,

such as polycarbosilanes and polycarbosilazanes, by incorporating metal compounds into

their main chain. Such polymer precursors exhibit reactive bonds within their structure

such as Si–H, N–H, Si-vinyl, etc., which allow their reaction with coordination

compounds and thus the incorporation of late transition metals within the polymeric

architecture. After cross-linking, the metal compound becomes part of the precursor, and

the following pyrolysis results in metal-containing polymers which exhibit promising

catalytic properties [95-99]. Modified SiC materials with tailored porosity and integrated

Ni nanoparticles were prepared from polycarbosilane-block-polyethylene precursors

followed by pyrolysis at 700°C, resulting in materials with selectivity for hydrogenolysis

of aryl ethers [98]. Zaheer et al. [20] reported the synthesis of microporous polysilazanes

modified by a Ni complex compound under pyrolysis at 600°C, which lead to materials

selectively active for the semi-hydrogenation of acetylenes.

Polysiloxanes have also been used as precursors for the preparation of metal-

containing materials. By using copper salts as the metal source, ceramics were generated

at pyrolysis temperature of 1000-1200°C with the aim to produce electroconductive

materials which possessed metal particles of 30–80 nm [100]. In other approaches, iron,

cobalt or platinum-containing materials were produced via inert gas pyrolysis at 1250–

1400°C in which, depending on the atmosphere, SiC or Si3N4 nanowires were generated

exhibiting an iron, cobalt or platinum tip, respectively [101-103].

Recent studies have demonstrated that metal-containing ceramers with controlled

size and homogenous distribution of nanoparticles (Ni, Pt) can be generated by using an

amine-containing siloxane as complexing precursor at varied pyrolysis temperatures

[104-107]. In the work reported by Sonström et al. [18], Pt-containing polysiloxanes were

prepared by mixing the preceramic precursor with a complexing agent (3-

aminopropyltriethoxysilane - APTES) and a solution of H2PtCl6 or preformed Pt,

followed by pyrolysis at temperatures of 500–1000°C. The results showed that the use of

APTES enabled to obtain a homogeneous distribution of Pt nanoparticles (particle size

between 2 – 3.75 nm) in the ceramer matrix up to 1000°C. The catalytic tests showed an

enhanced CO oxidation activity due to the effective anchoring of Pt particles to the

precursor matrix. Another related study investigated the formation of Ni and Co-

containing ceramers for CO2 methanation and Fisher-Troopship reactions, respectively

44

[19]. The materials were prepared from a polysiloxane based route using APTES (3-

aminopropyltriethoxysilane) as complexing agent and pyrolysis temperatures of 400 to

600°C. Microporous ceramers with high SSA were obtained displaying well-dispersed

particles of ~3 nm (Ni) and 5-10 nm (Co). In contrast to the Ni catalysts, for the Co-

containing ceramers particle formation was dependent on the pyrolysis temperature. The

catalysts with the lowest tendency to adsorb water showed the highest activity.

The amine-siloxane agent stabilizes the metal particles during the cross-linking

process through the complexation of the metal ions, preventing particle from sintering

and agglomeration. The key to controlling the metal particle size and distribution is the

atomic dispersion of the metal ions by the amino groups from the complexing siloxane in

order to coordinate the metallic ions and to anchor the resulting metal complexes to the

silicon matrix [108], as seen in Fig. 2.10. After cross-linking, the metal complexing

siloxane is bond to the precursor polymer, which ensures the homogenous distribution

and thermally stable embedding of the metal particles throughout the matrix. Then,

pyrolysis under inert atmosphere leads to the formation of a metal-containing PDC

material with in-situ reduced metal particles [106, 107].

Fig. 2.10: General overview of a complexing metal precursor and the resultant cross-

linked polysiloxane.

The size of the metal particles can be adjustable by some parameters such as the

ratio of metal ions and complexing agent, chemical structure of the complexing agent,

type of metal salt precursor, etc. According to Adam et al. [107], with increasing the

molar ratio between complexing agent and metal ion (Pt), the particle size decreased

(from 6.3 to 3.4 nm) while homogeneity was improved. An optimal ratio between the

complexing agent (APTES) and Pt ions was found to be 8:1. It was also demonstrated

45

that the absence of the complexing agent resulted in a rapid in situ-formation of the metal

particles, leading to a pronounced heterogeneity of both size and distribution of the

particles. In the case of Ni-containing siloxanes [106], broader distribution of particle

sizes in the range of 40–60 nm were observed when increasing APTES:Ni ratio from 1:1

to 4:1. Higher molar ratio (APTES:Ni 6:1) lead to smaller particles (20–30 nm), but these

particles were agglomerated to aggregates of around 150 nm in diameter. The addition of

further phenyl siloxane precursor (PhT) resulted in the formation of smaller nanoparticles

of 35–45 nm (PhT:APTES:Ni 80:10:10) and 15–20 nm (PhT:APTES:Ni 10:45:45). In

another correlated work [109], it was found that for nickel-containing silica the particle

size was nearly independent of the type and fraction of the complexing siloxane, whereas

a stronger dependency on the nickel salt used as metal precursor was observed. Particles

in the range of 3 to 82 nm were detected with smaller particles found for nickel nitrate

and acetate, and bigger particles for nickel chloride and acetylacetonate.

Cecci [110] demonstrated that the structure of the amino-siloxane precursor has a

great influence on the formation of pores and metal particles. Micro- and mesoporous Pt-

containing hybrid ceramics with nanoparticles in the range of 3 to 11 nm were prepared

using different complexing agents such as: [3-(2-aminoethylamino) propyl]

trimethoxysilane (AEAPTM), 3- aminopropyltriethoxysilane (APTES), (triethoxysilyl)

ethane (Bis) and (trimethoxysilylpropyl) amine (BisA). The selection of these siloxanes

was defined mainly to improve the creation of a mesoporous structure around the

platinum particles by using precursor without amino groups (Bis), with one (APTES,

BisA) or two (AEAPTM) amino groups in the molecular structure. Materials preparade

with Bis/APTES displayed the highest amount of mesopores and pore volume. All

materials showed particles well-dispersed and no agglomeration. Smaller Pt particles

were found for the ceramers preparade with a siloxane containing two amino groups

(AEAPTM), whereas bigger particles were identified for the ones prepared with BisA and

Bis/APTES.

Moreover, the combination of complexing siloxanes with preceramic polymers

can be used for fine-tuning the surface properties of metal-free ceramers, what is

important for adjusting materials to particular applications. For instance, hybrid ceramic

materials with amine functionalization were prepared from amino-containing siloxanes

and polysiloxane precursors for CO2 gas separation. By using BisA as complexing

siloxane mesoporous materials with hydrophobic surfaces were obtained, whereas

APTES lead to microporous materials with higher tendency of adsorbing water [111].

46

III – Methodology

47

3 – MATERIALS & METHODS

In this chapter, the materials used in the experimental procedure are presented as

well as the preparation route of the hybrid materials. In addition, the characterization

techniques and the catalytic system used are described.

3.1 Materials

All chemicals used in the present study were of analytical grade and used as

received without further purification. For the synthesis of the hybrid ceramics, the

following commercially available silicon resins (polysilsesquioxane) with hydroxyl and

ethoxy functional groups (OH, OC2H5) were used as starting materials: 1) methyl-phenyl

polysiloxane, known as Silres® H44; and 2) methyl polysiloxane, denoted as Silres®

MK, both purchased from Wacker Chemie. In addition, an amine-containing siloxane,

Bis(trimethoxysilylpropyl)amine (BisA), purchased from ABCR was also used. The

molecular structures and some important characteristics of these materials are shown in

Figure 3.1 and Table 3.1, respectively.

As solvent, tetrahydrofuran (THF, VWR Chemicals), with a molecular formula of

C4H8O, molar mass of 72.11 g mol-1, density of 0.889 g cm-3 and a boiling point of 66°C

was chosen. As metal source, nickel nitrate hexahydrate (Ni, MERCK), with molecular

formula of Ni(NO3)2·6H2O, molar mass of 290.81 g mol-1 and density of 2.05 g cm-3 was

used as precursor to generate nickel particles for catalytic activity. An aqueous ammonia

solution (25% NH3, MERCK) with molecular formula of NH3, molar mass of 17.03 g

mol-1 and density of 0.903 g cm-3 was used as a catalyst for the hydrolysis and

condensation reactions during the synthesis.

48

Fig. 3.1: Molecular structure of the siloxanes used: (a) methyl-phenyl siloxane (H44),

(b) methyl siloxane (MK) e (c) Bis(trimethoxysilylpropyl)amine (BisA).

Table 3.1: Siloxanes used in the experimental procedure.

*Composition in the cross-linked state based on active cross-linking groups [112].

H44 MK BisA

Molecular Formula (C6H5)0,62(CH3)0,31R0,07SiO1,5

R = -OH, OC2H5

(CH3)0,96R0,04SiO1,5

R = -OH, OC2H5 C12H31NO6Si2

Molar Mass (g

mol-1) 108,15* 70,29* 341,55

Density (g cm-3) 0,45 0,5 1,04

Melting Point (°C) 45 - 60 35 - 55 -

Boiling Point (°C) - - 152

Appearance White powder White powder Colorless to

Yellow liquid

49

3.2 Approach of the Work

In order to develop improved hybrid ceramics with adjustable properties different

approaches were established for conducting this work:

1. By using different polysiloxane precursors (H44 or MK), interesting possibilities

arise in order to modify the properties such as porosity and surface chemical

behavior (hydrophobicity/hydrophilicity) by varying the organic side groups

(methyl or methyl-phenyl groups);

2. Addition of an amine-containing siloxane is proposed in order to homogenously

distribute the metal particles through the silicon matrix by complexing the metal

ions and bonding to the polysiloxane. BisA was selected since it can induce the

formation of mesoporous when combined with preceramic polymers;

3. By varying the pyrolysis temperature (400, 500, 600°C) modifications on the

structural and chemical characteristics are expected and may influence the final

properties of the ceramers. This range of temperature was selected since it is

known to favor the development of high specific surface areas.

Therefore, by establishing such approaches is expected to develop tailored

ceramics with enhanced properties such as chemical and thermal stability, high specific

surface area and hierarchical porosity, hydrophobic surface, homogeneous distribution of

metal particles, and, consequently, improved catalytic performance.

The process of development and application of the hybrid ceramics consists of

forth general steps: (i) synthesis from preceramic polymer precursors, (ii) polymer-to-

ceramic transformation by pyrolytic conversion, (iii) characterization of the ceramics, and

(iv) catalytic testing. Fig. 3.2 shows a schematic overview of the steps adopted in the

process of this work.

50

Fig. 3.2: Schematic overview of the processing route.

3.3 Composition

Hybrid ceramics were prepared by using either methyl-phenyl polysiloxane (H44)

or methyl polysiloxane (MK) as polymeric precursors and

Bis(trimethoxysilylpropylamine) (BisA) as complexing agent. A molar ratio of nickel

ions to the complexing siloxane was adjusted to 1:4, to ensure the complete complexation

of metal ions. Furthermore, samples without any metal content were also produced using

a ratio of polysiloxane to BisA of 1:0.4. The hydrolysis and condensation reactions were

catalyzed under basic conditions by the addition of aqueous 0.2 M-NH3 with 7.5 times

molar excess of water per hydrolyzable group of the precursor. The compositions of the

materials are given in Table 3.2 including the samples nomenclature.

51

Table 3.2: Chemical composition and pyrolysis temperature of the hybrid ceramics.

Samples

Nomenclature

Polysiloxane

Type

Polysiloxane

(mol)

Complexing Agent

BisA (mol)

Ni Nitrate

(mol)

Molar ratio (mol)

BisA:Ni

Pyrolysis

Temperature (°C)

H44+BisA-400

H44

1 0.4 - - 400

H44+BisA-500 1 0.4 - - 500

H44+BisA-600 1 0.4 - - 600

MK+BisA-400

MK

1 0.4 - - 400

MK+BisA-500 1 0.4 - - 500

MK+BisA-600 1 0.4 - - 600

Ni/H44+BisA-400

H44

1 1.2 0.3 4 400

Ni/H44+BisA-500 1 1.2 0.3 4 500

Ni/H44+BisA-600 1 1.2 0.3 4 600

Ni/MK+BisA-400

MK

1 1.2 0.3 4 400

Ni/MK+BisA-500 1 1.2 0.3 4 500

Ni/MK+BisA-600 1 1.2 0.3 4 600

52

3.4 Preparation route

The synthesis procedure carried out is similar to previous works [19]. Initially, the

polysiloxane precursor (H44 or MK) is dissolved in THF until the solution is completely

homogenous (solution A). Simultaneously, nickel (II) nitrate hexahydrate and BisA are

separately dissolved in THF. Afterward, the nickel solution is added to the amino-

siloxane solution very slowly to form a complex (solution B). Subsequently, both

solutions (A and B) are slowly mixed (solution C) and then a 0.2 M NH3 solution is added

dropwise. The whole solution is stirred at 85°C for 72 hours under reflux. The solvent is

removed under reduced pressure at moderate temperature (ca. 350 mbar and 40 °C) in a

rotary evaporator (Büchirotavapor R-124, Büchi Corporation) and the solid obtained is

dried completely at 70°C for 12 hours. Thereafter, the samples are further cross-linked in

a preheated oven at 200°C for 2 hours (air atmosphere) and cooled in the oven to room

temperature. Next, the samples are crushed and sieved (< 300 μm) and then pyrolyzed at

400, 500 or 600°C under N2-atmosphere (flow of 100 L/h). The heating rate for the

pyrolysis process is 120°C h-1 to 100°C below the final temperature and 30 °C h-1 to reach

the maximum temperature with a dwelling time of 4 hours, followed by a cooling ramp

of 120°C h-1 until room temperature. Ceramers with ~ 5 wt% of Ni after pyrolysis were

prepared for both polysiloxanes precursors.

Figure 3.3 (a) and (b) shows the schematic flowchart of the synthesis route and

the pyrolysis ramps applied in the heat treatment of the specimens, respectively. For

comparison of catalytic performance, a standard catalyst with ~5 wt% of Ni was prepared

by wet impregnation of nickel (II) nitrate hexahydrate (Merck) on a commercial fumed

silica support (Aerosil-380, Evonik Industries AG), followed by drying at 70°C for 12 h

and pyrolysis at 400°C using the same conditions described above. The characterization

results for this catalyst, denoted as Ni/SiO2-400, can be found in the supplementary data.

53

Fig. 3.3: (a) Schematic flowchart of the synthesis route and (b) temperature ramps applied during pyrolysis.

(a) (b)

54

3.5 Characterization Techniques

3.5.1 Thermogravimetric Analysis (TGA)

The decomposition and thermal behavior of the cross-linked ceramers were

analyzed by thermogravimetric analysis in a STA 503 thermal analysis system from

BÄHR Thermoanalyse GmbH. This technique is of great importance to evaluate the

ceramic yield of the product formed after pyrolysis and to help to understand the polymer-

to-ceramic conversion and decomposition process. The analysis was carried out from

room temperature up to 1000°C with a heating rate of 120ºC h-1 and flow rate of 2 L h-1

of N2. About 10 milligrams of each sample was used. In order to calculate the metal

content, firstly, the amount of inserted metal was calculated from the amount of metal salt

added. The metal content could be calculated by determining the weight loss during

pyrolytic conversion according to equation 3.1. For this purpose, isotherms were carried

out for 4 hours at each pyrolysis temperature (i.e. 400, 500 and 600°C) in order to simulate

the pyrolysis conditions used in the preparation of the materials.

𝑁𝑖 (𝑤𝑡%) =𝑀𝑎𝑠𝑠𝑁𝑖 𝑛𝑖𝑡𝑟𝑎𝑡𝑒 ∗ (1 − 𝑊𝑒𝑖𝑔ℎ𝑡 𝑙𝑜𝑠𝑠𝑁𝑖 𝑛𝑖𝑡𝑟𝑎𝑡𝑒)

(𝑀𝑎𝑠𝑠𝑃𝑜𝑙𝑦𝑠𝑖𝑙𝑜𝑥𝑎𝑛𝑒 + 𝑀𝑎𝑠𝑠𝑁𝑖 𝑛𝑖𝑡𝑟𝑎𝑡𝑒) ∗ (1 − 𝑊𝑒𝑖𝑔ℎ𝑡 𝑙𝑜𝑠𝑠𝑠𝑎𝑚𝑝𝑙𝑒)∗ 100 (3.1)

3.5.2 Fourier-Transform Infrared Spectroscopy (FTIR)

FT-IR spectroscopy was used for structural analysis of the cross-linked samples

and pyrolyzed ceramers. The IR measurements were conducted on a Shimadzu

IRPrestige-21 spectrophotometer using the standard KBr pellet technique in the range

from 4000 to 400 cm-1.

3.5.3 Adsorption/desorption of Nitrogen (BET/BJH)

The specific surface area, pore volume, and pore size distributions were

determined by nitrogen adsorption-desorption isotherms in a Belsorp-Mini device from

Bel Japan, Inc. The isotherms were measured under relative P/P0 pressures between 0.1

and 0.99 at 77K. Surface areas were calculated from adsorption data using BET

55

(Brunauer-Emmett-Teller) method at P/P0 ranging from 0.1 to 0.3, while pore volume and

pore size were determined from desorption data based on BJH (Barrett-Joyner-Halenda)

method. Previously, the samples were outgassed for 12 h at 120°C under vacuum, for the

removal of gases and moisture from the surface of the solids.

3.5.4 Surface Characteristic Analysis

In order to investigate the surface sorption properties (hydrophilicity and

hydrophobicity), water and n-heptane vapor adsorption measurements were determined.

First, ca. 0.5 g of each sample (particle sizes ≤300 μm) were dried at 70°C for 24 hours

and then stored inside closed Erlenmeyer flasks at room temperature under water or n-

heptane vapor atmosphere (~ 20 mL). After 24 hours, the weight of the samples was

measured and the amount of adsorbate was determined. The values of the specific surface

areas were used to calculate the maximum amount of water and n-heptane adsorbed per

square meter according to equations 3.2 and 3.3, respectively. The amount of water and

n-heptane adsorbed at ambient pressure displays the maximum adsorption capacity, and

the ratio of the two adsorbates gives a first (non-quantitative) idea of the degree of

hydrophilicity according to equation 3.4.

𝐴𝑑𝑠𝑜𝑟𝑏𝑒𝑑𝐻2𝑂 (𝑚𝑚𝑜𝑙

𝑚2) =

1000 ∗ 𝐴𝑑𝑠𝑜𝑟𝑏𝑎𝑡𝑒 𝑤𝑒𝑖𝑔ℎ𝑡 (𝑔)

𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝐴𝑟𝑒𝑎𝑠𝑎𝑚𝑝𝑙𝑒 (𝑚2

𝑔) ∗ 𝑆𝑎𝑚𝑝𝑙𝑒 𝑤𝑒𝑖𝑔ℎ𝑡 (𝑔) ∗ 𝑀𝑜𝑙𝑎𝑟 𝑀𝑎𝑠𝑠 𝐻2𝑂 (

𝑔𝑚𝑜𝑙

) (3.2)

𝐴𝑑𝑠𝑜𝑟𝑏𝑒𝑑𝐻𝑒𝑝𝑡. (𝑚𝑚𝑜𝑙

𝑚2 ) = 1000 ∗ 𝐴𝑑𝑠𝑜𝑟𝑏𝑎𝑡𝑒 𝑤𝑒𝑖𝑔ℎ𝑡 (𝑔)

𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝐴𝑟𝑒𝑎𝑠𝑎𝑚𝑝𝑙𝑒 (𝑚2

𝑔) ∗ 𝑆𝑎𝑚𝑝𝑙𝑒 𝑤𝑒𝑖𝑔ℎ𝑡 (𝑔) ∗ 𝑀𝑜𝑙𝑎𝑟 𝑀𝑎𝑠𝑠𝐻𝑒𝑝𝑡. (

𝑔𝑚𝑜𝑙

) (3.3)

𝑅𝑎𝑡𝑖𝑜 = 𝐴𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝐴𝑑𝑠𝑜𝑟𝑏𝑒𝑑𝐻2𝑂

𝐴𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝐴𝑑𝑠𝑜𝑟𝑏𝑒𝑑𝐻𝑒𝑝𝑡𝑎𝑛𝑒 (3.4)

56

3.5.5 Scanning Electron Microscopy (SEM)

In order to investigate the texture, shape and morphology of the samples scanning

electron microscopy images were taken. The micrographs were obtained using a SSX 550

SuperScan Shimadzu microscope with tungsten filament operating at 15 kV. Previously

to the analysis, the samples were covered with a thin layer of gold by sputtering.

3.5.6 Transmission Electron Microscopy (TEM)

The distribution and size of the nickel particles were determined by transmission

electron microscopy. The images were acquired using a FEI Titan 80/300 microscope

coupled with an image filter operating at a voltage of 300 keV. The samples were

suspended in ethanol and left for 5 min in an ultrasonic bath. Subsequently, a drop of this

solution was placed on copper-coated disk-shaped grids (200 mesh, Ø 3,05 mm) and

allowed to dry. The size of the nickel particles was calculated based on 100 particles and

then statistically analyzed. The samples were analyzed after different pyrolysis

temperatures (400 and 600°C).

3.5.7 X-Ray Diffraction (XRD)

The structure of the Ni-containing ceramics was analyzed by X-ray diffraction

(XRD) in a Shimadzu diffractometer model XRD 7000. Measurements were obtained

using Cu-Kα radiation (λ = 1.5409 Å) operating at 40 kV and 30 mA. The scans were

determined in the 2θ range from 10 to 80° in steps of 0.02° and a counting time per step

of 2° min-1. The positions and peak intensities observed were compared with data

provided by the Joint Committee on Powder Diffraction Standards (JCPDS) contained in

the 2003 International Center for Diffraction Data (ICDD) database, thus allowing the

correct identification of the phases formed. The crystallite size (Tc) of the nickel was

estimated based on the most intense peak (111) according to the Scherrer’s equation (Eq.

3.5).

𝑇𝐶 = 0,89 𝜆

𝛽𝑐𝑜𝑠𝜃 (3.5)

57

where: λ is the wavelength of X-ray; β is the width at half height of the diffraction angle

(FWHM - Full Width at Half Maximum) after subtracting the instrumental influence

(alumina was used as reference); and θ is the diffraction angle.

3.5.8 Catalytic Test

In order to compare the activity and selectivity of each sample, catalytic tests were

carried out at atmospheric pressure in a tubular fixed-bed reactor as represented in Fig.

3.4. Accordingly, 50 mg of the powder catalyst (sieve fraction of 100-200 µm) were

diluted in 300 mg of an inert material (Al2O3) to facilitate heat transfer and avoid hotspots

during reaction. The powders were fixated with quartz wool in a quartz-glass tube reactor

(inner diameter of 6 mm). The total flow rate was set at 50 mLN min-1 composed of

H2/CO2/Ar = 4/1/5, which gives a weight hourly space velocity (WHSV) of 60 L gcat-1 h-

1. The catalysts were tested between 200-400°C in intervals of 50°C. Each temperature

was held for 42 minutes and the outflowing gas was continuously monitored with an on-

line compact gas chromatograph (Global Analyser Solution) equipped with a thermal

conductivity detector. Two columns were used, a RT-Molsieve 5 Å column (15 m) to

detect CO and CH4, and a RT-Porabond column (30 m) for the detection of CO2. The

CO2 conversion, as well as CH4 yield and selectivity, were determined according to the

following equations:

𝑋𝐶𝑂2= 1 −

𝑐𝐶𝑂2,𝑜𝑢𝑡

𝑐𝐶𝑂2,𝑜𝑢𝑡+ 𝑐𝐶𝐻4,𝑜𝑢𝑡

+ 𝑐𝐶𝑂,𝑜𝑢𝑡

(3.6)

𝑌𝐶𝐻4=

𝑐𝐶𝐻4,𝑜𝑢𝑡

𝑐𝐶𝑂2,𝑜𝑢𝑡+ 𝑐𝐶𝐻4,𝑜𝑢𝑡

+ 𝑐𝐶𝑂,𝑜𝑢𝑡

(3.7)

𝑆𝐶𝐻4=

𝑌𝐶𝐻4

𝑋𝐶𝑂2

(3.8)

In pre-experiments, no higher hydrocarbons were found. The catalysts were

activated in-situ by flowing H2 at 430°C for 10 h (heating ramp 1 °C/min). Afterward, the

reactor was cooled down in inert gas atmosphere. All tubing was heated to prevent water

from condensing in the experimental set-up.

58

Fig. 3.4: Representation of the catalytic system used.

3.6.9 In-situ X-Ray Diffraction (In-situ XRD)

In-situ X-ray diffraction analysis was performed in order to evaluate possible

phase changes and the evolution of nickel crystallite size during a simulated CO2

methanation reaction at 400°C for 1 h. The measurements were performed on a XPD-10B

beamline from the Brazilian Synchrotron Light Laboratory - LNLS. The samples were

placed in a furnace installed in the goniometer (Hubber model) operating in Bragg-

Brentano geometry (θ-2θ) and equipped with a Mythen-1 K detector (Detris) located on

a meter of the oven. Scans were performed in the 2θ range from 35 to 65º to verify the

formation of the main peaks, using a radiation with a wavelength of 1.7712 Å and energy

of 7000 eV. The crystallite size was calculated from the Scherrer’s equation based on the

main peak (111). The experiment was conducted under the following conditions: (i)

activation using a mixture of 5% H2/He with a flow rate of 75 ml min-1 up to 430°C for 1

h; (ii) cool down to 400°C while purging with He for 30 min; and (iii) start of reaction by

flowing a mixture of 40% of H2, 10% of CO2 and 50% of He (flow rate of 50 ml min-1)

at 400°C for 1 h.

59

IV - Results & Discussions

60

4.1 Pyrolytic Conversion

The high thermal stability of polysiloxanes is one of the most important properties

of this class of polymers and is directly related to the high energy and ionic character of

the Si-O bond. The thermal degradation happens mainly through the cleavage and

reorganization of the organic groups and several bonds (Si-CH3, Si-C6H5, Si-H, Si-CH2-

Si, etc), which yields a mixture of volatile compounds and free (hydro)carbon

accompanied by a weight loss [113, 114]. The decomposition behavior of the cross-linked

polysiloxanes and ceramers was investigated by thermogravimetric analysis up to 1000°C

as shown in Fig. 4.1.

At temperatures below 400°C the evaporation of mainly cross-linked products

(water and alcohols), oligomers and solvent are observed, which are released while the

cross-linking of siloxanes is accomplished [115]. At higher temperatures (~450-800°C)

the decomposition of the organic groups takes place by breaking the Si-H, Si-C and C-H

bonds with the release of typical hydrocarbons, CH4, H2 or other volatile compounds. At

temperatures beyond 800°C mainly H2 is eliminated [116]. Regarding the pure

commercially available polysiloxanes H44 and MK, a higher overall thermal stability is

observed for MK. The onset of significant decomposition of H44 and MK starts around

500°C and 600°C, respectively, with H44 presenting a higher weight loss at 1000°C

(~24%) compared to MK (~17%). Phenyl groups have a higher carbon content and,

although more carbon will remain in the structure, the overall weight loss for H44 is

higher. The residue after pyrolysis for H44 corresponds to the atomic proportions of

18.7% Si–28.7% O–52.6% C, while that for MK is 31.6% Si–48.1% O–20.2% C [115].

This could be explained by the higher decomposition of the phenyl groups present in H44

as well to a decrease in molecular weight and increased loss of small oligomers formed

by depolymerization [116].

By analyzing the metal-free ceramers (H44+BisA and MK+BisA) a decreased

thermal resistance is observed with decomposition starting at approximately 400°C,

characterized by a strong weight loss. This indicates a reduced thermal stability of the

complexing agent compared to the groups of the polysiloxanes. At about 600°C the

weight loss levels off, indicating that the majority of the polymer-to-ceramic conversion

has occurred. The ceramers prepared from methyl polysiloxane (MK) present a higher

61

ceramic yield when compared to the ones prepared with methyl-phenyl polysiloxane

(H44).

By adding nickel, the decomposition during the pyrolytic conversion is slightly

shifted to lower temperatures indicating that nickel catalyzes the decomposition process

to a certain extent. After the initial weight loss, the decomposition continues with

increasing the temperature. The overall weight loss for the Ni-containing ceramers is

about 15-23% at 400°C and increases to 25-30% at 600°C, resulting in a slightly higher

Ni content (wt%) at 600°C, as seen in Table 4.1. This behavior has also been observed

for polysiloxanes containing others metal as reported in the literature [104, 106, 107].

Fig. 4.1: Thermogravimetric analysis of cross-linked precursors under N2 atmosphere.

62

Further investigation on the structure of the polysiloxanes was performed by

Fourier-transform infrared spectroscopy. FT-IR spectra of ceramers after cross-linking

at 200°C (denoted as Ni/H44+BisA-CL and Ni/MK+BisA-CL) and at different pyrolysis

temperatures (400-600°C) are displayed in Fig. 4.2 (a,b). The FT-IR spectra presented

similar peaks and comparable intensities. The broad intense peak centered at 1100 cm-1

is attributed to Si–O–Si asymmetric vibration which is typical of any siloxane [117]. For

ceramers prepared with methyl-phenyl polysiloxane (H44), the peaks observed at 1430

cm-1, 740 cm-1 and 695 cm-1 are attributed to Si–C6H5 vibrations as seen in Fig 4.2 (a).

The peaks located at 1272 and 885 cm-1 are related to Si-CH3 vibrations present in both

polysiloxanes precursors (H44 and MK) [118]. Moreover, Si-C and C–H stretching

vibrations are observed at 789 and 2960 cm-1, respectively. The peak at 1622 cm−1 is

attributed to N-H vibration from the amino-siloxane precursor (BisA) [119].

It can be seen that by increasing the pyrolysis temperature the intensity of the

peaks attributed to Si-C6H5 at 1439 cm-1; 740 cm-1; 695 cm-1, and the peaks at 2960 cm-1

related to C-H vibrations in C6H5 decrease, indicating a higher decomposition of the

organic groups with increasing the temperature. At 600°C, these peaks disappear,

revealing that most of the phenyl groups have been decomposed. The situation is different

for the methyl groups, as the peaks associated with Si-CH3 vibrations remain nearly

constant up to 600°C, demonstrating their higher thermal stability, in agreement with the

TGA results.

Therefore, the thermal stability and decomposition behavior are intrinsically

influenced by the pyrolysis temperature and composition of the material.

63

Fig. 4.2: FT-IR spectra of ceramers (a) Ni/H44+BisA and (b) Ni/MK+BisA cross-

linked at 200°C and pyrolyzed at 400-600°C.

64

4.2 Porosity

For catalytic applications, a high specific surface area material with hierarchical

porosity is desired to improve the dispersion of the active phase and to facilitate internal

mass transfer, leading to increased catalytic activity. The porosity in polymer-derived

ceramics can be further engineered by developing micro- and mesopores through a

controlled pyrolytic treatment as a result of the decomposition of organic groups and

release of volatiles. N2-adsorption/desorption isotherms were measured to evaluate the

pore structure and the specific surface area (SSA). Fig. 4.3 (a-d) shows the obtained N2-

isotherms. The corresponding BJH plots, illustrating the respective pore size distribution,

are shown in Fig. 4.4(a-d).

The isotherms for both metal-free and Ni-containing ceramers reveal an

intermediate state of type II behavior but also partly of type IV at all pyrolysis

temperatures, which are typical for macro- and mesoporous materials, respectively [120].

Additionally, a considerable amount of micropores is expected as indicated by the

significant adsorption at relatively lower pressures (p/p0). The hysteresis can be correlated

with a type H3 found for materials with aggregates of plate-like particles giving rise to

slit-shaped pores [103]. Therefore, the resulting materials show hierarchical porosity

formed by micro- (d < 2 nm), meso- (2-50 nm) and macropores (d > 50 nm), which is

important for many applications such as catalysis and adsorption [121].

The pure polysiloxanes H44 and MK are known to present isotherms of type I

after pyrolysis [16] (not shown), which is related to the predominat presence of

micropores. The addition of the complexing siloxane BisA, though, leads to the formation

of further meso- and macropores at all temperatures of 400-600°C as seen in Fig 4.3.

Furthermore, cross-linked samples (Ni/H44+BisA-CL and Ni/Mk+BisA-CL) also show

meso- and macropores as evidenced by the isotherms and pore size distributions in Fig.

4.5. The isotherms present the same characteristics as for the pyrolyzed samples,

indicating an intermediary type II and type IV behaviors, and pore sizes with average

diameters of 5.47 and 9.25 nm for Ni/H44+BisA-CL and Ni/Mk+BisA-CL, respectively.

These results reveal that the introduction of BisA is responsible for inducing the evolution

of meso- and macroporosity even before pyrolysis. During pyrolytic conversion, such

porosity is increased by the formation of new pores in the micro and mesopore range,

whereas maintaining the pores already formed in the cross-linked stage. Prenzel et al.

65

[111] also reported the preparation of ceramers with mesoporosity by combining MK and

BisA as polysiloxane precursors and pyrolyzing the samples at temperatures of 350-

600°C.

The corresponding BJH plots, illustrating the respective pore size distribution for

the pyrolyzed ceramers, are shown in Fig. 4.4 (a-d). For ceramers prepared with methyl

polysiloxane (MK+BisA and Ni/MK+BisA), a higher pore volume with the presence of

bigger pores is observed, as indicated by the strongly increased adsorption in the range of

p/p0 ~ 1 (Fig. 4.3 (b and d)). The addition of Ni led to an increase in pore size and pore

volume as shown in Table 4.1 for both types of ceramers. This could be caused by an

enhanced decomposition of the siloxanes around the Ni nanoparticles. Considering that

the nickel ions are surrounded primarily by the complexing siloxane chains and that an

increased decomposition is observed when BisA is present, the formation of a higher

porosity directly around the nickel can be expected, what explains the development of

larger pores and improved metal accessibility with the presence of the complexing

siloxane. Broader pore size distribution was observed for Ni/MK+BisA ceramers with

bigger pores up to 90 nm, an average pore size of 18.37 nm and pore volume up to 1.1

cm3 g-1 (see Table 4.1). On the other hand, a narrow distribution was observed for

Ni/H44+BisA ceramers consisting of pores up to 35 nm, an average pore size of 5.46 nm

and pore volume up to 0.59 cm3 g-1. Regarding the metal-free ceramers, MK+BisA

materials presented larger pore sizes and pore volumes compared to H44+BisA ceramers,

both revealing narrow distributions. The BJH profiles for the metal-free and Ni-

containing ceramers confirm the presence of mesoporous, revealing the same overall

trend at temperatures of 400-600°C. The average pore size was not affected by the

increased temperature, however, the pore volume increased from 400 to 500°C, and

decreased from 500 to 600°C, following the same trend as the specific surface area (see

Fig. 4.6).

66

Fig. 4.3: N2-adsorption/desorption isotherms for (a) H44+BisA, (b) MK+BisA, (c) Ni/H44+BisA and (d) Ni/MK+BisA ceramers

pyrolyzed at 400-600°C.

67

Fig. 4.4: Pore size distribution (inset from 2 to 11 nm) of (a) H44+BisA, (b) MK+BisA, (c) Ni/H44+BisA and (d) Ni/MK+BisA ceramers pyrolyzed at 400-600°C.

68

Fig 4.5: (a) N2-adsorption/desorption isotherm and (b) pore size distribution (inset from

2 to 11 nm) of Ni/H44+BisA and Ni/MK+BisA cross-linked at 200°C.

69

Table 4.1: Average pore size and pore volume, Ni average crystallite and particle size,

and Ni content of ceramers cross-linked and pyrolyzed at 400, 500 and 600°C.

Sample

Specific

Surface Area

(m2 g-1) a

Average

pore

size

(nm)b

Average

pore

volume

(cm3/g)b

Ni average

crystallite

size (nm)c

Ni

average

particle

size (nm)d

Ni

content

(wt%)e

H44+BisA-400 99.06 3.75 0.1792 - - -

H44+BisA-500 558.95 3.75 0.5365 - - -

H44+BisA-600 546.11 3.75 0.1951 - - -

MK+BisA-400 374.39 9.23 0.8374 - - -

MK+BisA-500 527.77 9.23 0.9428 - - -

MK+BisA-600 470.88 9.23 0.8019 - - -

Ni/H44+BisA-CL 146.01 5.47 0.3496 - - -

Ni/MK+BisA-CL 124.91 9.25 0.4826 - - -

Ni/H44+BisA-400 273.19 5.46 0.4580 4.9 4.3 4.41

Ni/H44+BisA-500 435.17 5.46 0.5959 7.8 - 4.81

Ni/H44+BisA-600 457.62 5.46 0.4081 13.5 7.3 5.01

Ni/MK+BisA-400 329.92 18.37 0.9479 4.3 4.0 5.08

Ni/MK+BisA-500 335.40 18.37 1.1411 7.6 - 5.36

Ni/MK+BisA-600 381.79 18.37 1.1240 11.5 7.0 5.51

[a] calculated by BET method from N2-adsorption [b] calculated by BJH method from N2-desorption [c] calculated from XRD based on Scherrer’s equation [d] calculated from TEM images based on 100 particles for materials pyrolyzed at 400 and 600°C [e] calculated from TGA at 400, 500 and 600°C for 4 hours

70

The specific BET surface areas calculated from the nitrogen adsorption isotherms

are shown in Fig. 4.6. Surface areas of approximately 100–550 m2 g-1 and 330–530 m2 g-

1 were generated for ceramers prepared using H44 and MK as polysiloxanes, respectively.

Porosity is generated through the gas released during the polymer-to-ceramic

transformation due to the decomposition of the CH3, C3H6-NH2 and C6H5 organic groups

present in the polysiloxanes, resulting in a porous structure with high SSA. This porosity

is however transient since the pores are only stable at temperatures as high as their initial

pyrolysis temperature [82, 83]. By analyzing the metal-free samples, a dependency of the

SSA over the pyrolysis temperature was observed. With increasing pyrolysis temperature

from 400°C to 500°C the SSA increases, what can be explained by the decomposition of

the organic groups, leading to the development of porosity and high SSA. The highest

surface areas are detected for the metal-free samples after pyrolysis at 500°C in the range

of 500 m2 g-1. However, a further increase in the pyrolysis temperature leads to a decrease

in the surface areas, since the pores start to collapse [81]. Regarding the Ni-containing

ceramers, the same trend is observed, with increasing pyrolysis temperature the SSA

increases. However, lower surface areas are observed for the ceramers pyrolyzed at 500

and 600°C compared to the metal-free samples. This can be explained by the fact that Ni-

containing materials show a higher material density, and also by the influence of Ni on

the decomposition behavior of the precursors during pyrolysis, as observed in the

thermogravimetric measurements. This behavior also has been observed for

polysiloxanes containing others metal as reported in the literature [104, 105].

Summarizing, these results demonstrate that the microstructure of the material can

be tailored by varying the pyrolysis temperature, by the addition of metal salt and the

polysiloxane composition. Therefore, materials with high surface area and hierarchical

porosity can be developed by pyrolyzing polysiloxanes combined with amino siloxanes.

71

Fig. 4.6: Specific BET surface areas of metal-free and Ni-containing ceramers

pyrolyzed at 400, 500 and 600°C.

4.3 Surface Characteristics

In catalysis, especially for methanation reactions where water is formed as a

product, a hydrophobic support is favorable to avoid the blocking of adsorption sites by

water. Therefore, it is of great interest to investigate how the precursor composition and

the pyrolysis temperatures influence the surface characteristics of the materials. Thus, the

surface hydrophilicity and hydrophobicity were analyzed by water (which is mainly

polar) and n-heptane (which shows mainly dispersive interaction [121]) adsorption

measurements. This method provides a simple way to obtain initial information about the

hydrophobicity/hydrophilicity of porous media. The ratio of adsorbed water/n-heptane

for all prepared ceramers is shown in Fig. 4.7. The values of the specific BET surface

areas were used to calculate the amount of solvent adsorbed per square meter of the

ceramers surface area.

72

The hydrophobicity can be adjusted by varying the pyrolysis temperature. At

400°C, the ceramers are strongly hydrophobic showing a predominant adsorption of n-

heptane in contrast to the lower adsorption of water. Decomposition of the organic

compounds starts at around 450°C, converting the organic part into free (hydro)carbon,

with a still hydrophobic matrix. During pyrolysis of phenyl and methyl-group containing

polysiloxanes, the phenyl groups are easily converted at lower temperatures than the

methyl groups. Therefore, the conversion from strongly hydrophobic to hydrophilic starts

at different pyrolysis temperatures, for H44 at around 600°C and for MK above 630°C

[122]. Thus, the metal-free and Ni-containing ceramers prepared with MK show a more

hydrophobic behavior compared to the ones with H44. For the metal-free ceramers, there

is a low uptake of water but an increased uptake for n-heptane. The situation is different

for the Ni-containing ceramers where the ratio of water to n-heptane adsorption is

inverted, revealing more hydrophilic materials. Ni catalyzes the decomposition reaction

of the hydrophobic organic groups, as shown in the thermogravimetric measurements,

leaving behind a higher amount of inorganic parts remaining in the material. Regarding

the pyrolysis temperature, with increasing the temperature a significant increase in water

and decrease in n-heptane adsorption is found, since less hydrophobic organic groups are

present due to the loss of the methyl and phenyl groups, resulting in a more hydrophilic

surface. This is sustained by comparing the weight losses at 400 and 600°C (Fig. 4.1)

taken from the TGA profile, where the weight loss for all samples is about 5-17% at

400°C and increases to 12-30% at 600°C.

Accordingly, these results demonstrate that the surface properties are influenced

by the precursor composition, by the metal addition and strongly by the pyrolysis

temperature. Therefore, this preparation route is suitable for generating hydrophobic as

well as hydrophilic catalytic materials.

73

Fig 4.7: Ratio of maximum water and n-heptane adsorption for ceramers pyrolyzed at

400, 500 and 600°C.

4.4 Surface Morphology

The morphology of the Ni-containing ceramers was analyzed by scanning electron

microscopy (SEM). The micrographs are presented in Fig. 4.8 (a-f) and reveal powders

with aggregates of irregular sizes and shapes. The pyrolysis temperature and polysiloxane

composition did not show a clear effect on the morphology of the powders.

74

Fig. 4.8: SEM micrographs of Ni/H44+BisA (a-c) and Ni/MK+BisA (d-f) pyrolyzed at

400, 500 and 600°C.

75

4.5 Metallic Nanoparticles

For heterogeneous catalytic applications, it is important to ensure highly dispersed

and easily accessible nanoparticles with low or no particle growth at the same time in

order to improve the utilization of the metal catalyst and increase the catalytic activity.

The key to controlling the metal distribution is the atomic dispersion and complexation

of the metal ions by the amino groups of the complexing agent, which after cross-linking

are anchored throughout the silicon matrix, preventing the particles from sintering and

agglomeration [108].

The formation and distribution of Ni nanoparticles were investigated by TEM

analysis of the fresh ceramers pyrolyzed at 400 and 600°C, and after 10 h of reaction as

shown in Fig. 4.9 (a–f). The particle size distributions are presented in Fig 4.10 (a-f). The

effect of the pyrolysis temperature on the size and distribution of nanoparticles is clearly

demonstrated. Similar Ni particle sizes are found for both types of ceramers, whereas

MK-based ceramers present narrower particle size distribution. The complexing siloxane

BisA seems to effectively stabilize and homogeneously disperse the nickel nanoparticles

at low pyrolysis temperatures of 400°C, resulting in an average particle size of ~4 nm. At

higher pyrolysis temperature of 600°C, a still homogeneous distribution is observed,

however with the formation of bigger particles with an average size of ~7 nm. This is an

unexpected result as in similar siloxane-based catalysts the use of a complexing agent

with amino groups was essential for the generation of well-distributed metal nanoparticles

(Ni, Pt) and for the suppression of particle clustering during varied pyrolysis temperatures

[104-107].

This unforeseen result could be addressed probably due to the higher local

temperature caused by the pyrolysis of a greater number of organic groups. Harms et al.

[123] reported in their work the preparation of siloxane-based electrocatalysts with

platinum nanoparticles (2.5–3.4 nm) homogeneously distributed on the support at

pyrolysis temperatures of 500°C, whereas a higher pyrolysis temperature of 600°C led to

agglomeration of the metal particles. Complexing agent free samples showed larger

particles (4.7 nm) and agglomerates.

Images of ceramers (pyrolyzed at 400°C) which were obtained after testing the

samples during 10 hours for the CO2 methanation at a temperature of 350°C are presented

in Fig. 4.9 (c,f). No sign of sintering of the Ni particles was found, demonstrating a good

76

stability of the ceramers, which displayed an average particle size of 4.6 and 4.3 nm for

Ni/H44+BisA and Ni/MK+BisA, respectively.

A further investigation allowed to observe that the increase of the pyrolysis

temperature to 600°C led to the formation of carbon nanotubes (CNTs) for both ceramers

types (more prominently for H44-based ceramers). The diameters of the CNTs are in the

range between 3 and 7 nm which corresponds to the size of the nickel particles, as

depicted in Fig. 4.11 for Ni/H44+BisA-600.

As reported in previous investigations, Ni acts as a catalytic active phase for the

formation of CNTs during pyrolysis of polysiloxanes. The hydrocarbons released during

the thermal decomposition allow the CNT formation under chemical vapor deposition

conditions [124, 125]. The temperature of the maximum hydrocarbon release rate

depends on the organic substituent and is located at ~750°C for methyl polysiloxanes and

at ~600°C for methyl-phenyl polysiloxanes [115]. By adding nickel acetate to a methyl-

phenyl polysiloxane, Scheffler et al. [91] obtained metallic Ni particles of 40–60 nm

during the pyrolytic conversion at temperatures of 700–1000°C, which led to the

formation of carbon nanotubes and turbostratic carbon inside the material. Terry et al.

[126] investigated the in-situ formation of CNT in Ni-containing polysiloxanes over

temperatures of 500-1200°C. Higher pyrolysis temperature (starting at ~800°C) were

found to increase the formation of CNTs, which were ascribed to be end-capped, thus

limiting the access to the interior.

Two types of growth mechanisms for the formation of nanotubes on metal-

containing substrates have been reported. Firstly, when the interaction between the metal

particle and the substrate is strong, a base-growth model of CNTs is favored, resulting in

the formation of closed-carbon structures. In this type of model, nanotube growths above

the particle that remains attached to the substrate. Secondly, when the interaction metal-

substrate is weak another mechanism overcomes, the tip-growth model. Here, the

mechanism pushes the catalyst particle that resides at the growing tip further away from

the substrate [127, 128].

In sum, the results demonstrate that Ni nanoparticles were homogeneously

dispersed through the matrix by using BisA as complexing precursor. Moreover, a

temperature-dependence was observed for the particle size, with smaller particles formed

at lower pyrolysis temperatures.

77

Fig. 4.9: TEM images of the fresh Ni-containing ceramers pyrolyzed at 400°C (a,d), 600°C (b,e) and the spent catalysts after 10 h stability test (c,f).

78

Fig 4.10: Particle size distributions of Ni-containing ceramers pyrolyzed at 400°C (a,d), 600°C (b,e) and the spent catalysts after 10 h stability test (c,f).

79

Fig. 4.11: TEM image of CNT formed in Ni/H44+BisA-600.

4.6 Crystalline Structure

X-ray diffraction analysis was performed in order to further investigate the

formation of Ni particles and to quantify their crystallite sizes. XRD patterns of the fresh

Ni-containing ceramers pyrolyzed at temperatures of 400, 500 and 600°C are shown in

Fig. 4.12. The XRD patterns presented broad reflections between 15-30° 2θ indicating

the formation of Si-based amorphous phase for all ceramers [129]. Moreover, the

materials presented characteristic diffraction peaks corresponding to the crystallographic

planes of cubic Ni structure with Fm-3m symmetry (JCPDS No. 89-7128) around 45°,

52° and 77° 2θ, indicating the formation of metallic Ni crystallites. No diffraction peaks

related to secondary phases (such as Ni2Si) were detected, revealing the efficiency for

generating in-situ metallic particles. The pyrolytic conversion of metal-containing

polysiloxanes under an inert atmosphere is found to lead to an in-situ reduction of the

metal precursor to metal nanoparticles since hydrocarbons and hydrogen released during

pyrolysis constitute a reductive atmosphere [17].

By increasing the temperature, the intensity of the Ni peaks increases and the

width decreases, which is associated with an increase in the crystallite size as seen in

80

Table 4.1, indicating a remarkable increase of nearly 60% in crystallite size at higher

temperatures. The smallest crystallites were found for the samples pyrolyzed at 400°C

with 4.9 and 4.3 nm for Ni/H44+BisA and Ni/Mk+BisA, respectively. With increasing

pyrolysis temperature to 600°C, the crystallite sizes increase to 13.5 and 11.5 nm,

respectively. These results are in agreement with the TEM images presented in Fig. 4.9.

The crystallite size calculated for ceramers pyrolyzed at 400°C is almost the same as the

particle size calculated from TEM, indicating the formation of single crystallite Ni

particles. However, the crystallite size for ceramers pyrolyzed at 600°C differs from TEM

data, possibly due to the formation of Ni agglomerates consisting of several Ni

crystallites.

Thus, these results demonstrate that Ni nanoparticles can be in-situ generated

during pyrolysis, which represents a significant advantage of the PDC route in

comparison to the conventional preparation methods.

Fig. 4.12: XRD patterns of the Ni-containing ceramers pyrolyzed at 400, 500 and 600

°C.

81

4.7 Catalytic Activity

CO2 methanation measurements were performed in order to analyze the

applicability of the Ni-containing ceramers for catalytic purposes, considering the

influence of the polysiloxane composition and the pyrolysis temperature on the catalytic

activity. According to the characterization results, the ceramer’s properties are influenced

by the polysiloxane composition, and further, strongly dependent on the pyrolysis

temperature. Therefore, different behaviors and catalytic performances are expected. The

CO2 conversion and CH4 selectivity of the ceramers are displayed as a function of

temperature (200−400°C) in Fig. 4.13, along with results for the standard catalyst

Ni/SiO2-400.

It is clearly observed that CO2 conversion and CH4 selectivity increase with

increasing the temperature, reaching the maximum rate at a reaction temperature of

400°C. In thermodynamic equilibrium, CO2 conversion increases as the reaction

temperature increases, while the selectivity of CH4 increases by increasing reaction

temperature to a certain limit and then starts decreasing at elevated temperatures [130].

Regarding the ceramers, the highest conversion (~52%) and selectivity (~77%) are

observed for the ceramers pyrolyzed at 400°C. The situation is different for ceramers

pyrolyzed at 600°C showing the lowest activity with a decrease of ca. 20% and 50% in

conversion and selectivity, respectively. Meanwhile, intermediate performance was

found for ceramers pyrolyzed at medium temperatures (500°C). The explanation for such

behavior is ascribed, mainly, to the Ni particle size and dispersion. Due to the structure-

sensitivity of the reaction, the metal particle size greatly influences the catalytic activity,

in which an improved performance is achieved with an optimum particle size [48, 49].

According to the XRD and TEM results, smaller (~4 nm) and better dispersed Ni particles

were found for ceramers pyrolyzed at 400°C, thus, justifying the highest conversion and

selectivity, once smaller particles offer a large number of surface atoms [131]. At higher

pyrolysis temperatures (600°C) larger particles were formed (~7 nm), leading to a lower

metal surface, possibly explaining the lower conversion and selectivity levels.

82

Fig. 4.13: (a) CO2 conversion and (b) CH4 selectivity for Ni-containing ceramers

pyrolyzed at 400, 500 and 600°C and standard catalyst Ni/SiO2 pyrolyzed at 400°C.

83

Analyzing the ceramers pyrolyzed at the same temperature, a clear trend is

evident, characterized by the higher CO2 conversion and CH4 selectivity found for

ceramers prepared from methyl polysiloxane (MK). For instance, comparing the ceramers

pyrolyzed at 600°C at a reaction temperature of 300°C, Ni/MK+BisA exhibits 10% higher

conversion and 18% higher selectivity than Ni/H44+BisA. The same trend is found for

ceramers pyrolyzed at 400 and 500°C. When analyzing the characteristics of such

ceramers, similar Ni particle size and comparable surface areas (see Table 4.1) are found

at each pyrolysis temperature. One first explanation for the better results of the MK-based

ceramers is attributed to the larger pore sizes and pore volumes of these materials, which

possibly improved the nickel particles accessibility, resulting in a higher catalytic

performance. Moreover, the surface hydrophilicity plays a crucial role in the catalytic

activity. It is known that water produced during the reaction has an inhibiting effect on

the methanation activity, hindering the accessibility of the active sites and, thus,

decreasing the reaction rate and selectivity [45]. By using a catalyst with a less

hydrophilic surface this effect can be repressed and the amount of blocked active sites

can be reduced. Hence, the more hydrophobic MK-based ceramers show a better catalytic

activity and selectivity. The organic groups present in the polysiloxane precursors favor

the hydrophobic behavior of the ceramers, therefore, since the phenyl groups in H44 are

more easily decomposed then the methyl groups, more hydrophilic surfaces are obtained.

This behavior was also verified by Schubert et al. when testing Ni and Co-containing

ceramers for CO2 methanation and Fisher-Tropsch reactions, respectively [19]. The

difference in catalytic activity was related to the different surface hydrophilicity. The

highest activity was found for the catalysts with the lowest tendency to adsorb water

(hydrophobic behavior).

Therefore, the order of activity and selectivity for CO2 methanation can be

assigned as Ni/MK+BisA-400> Ni/H44+BisA-400 > Ni/MK+BisA-500 > Ni/H44+BisA-

500 > Ni/MK+BisA-600 > Ni/H44+BisA-600. Moreover, in order to compare the

performance of the ceramers with those of existing catalysts, a standard catalyst Ni/SiO2-

400 was prepared and analyzed. Hence, it was possible to properly compare both

materials under the same reaction conditions. With respect to the Ni/SiO2-400 catalyst,

the same dependency on the reaction temperature is observed, with maximum conversion

and selectivity at 400°C. The CO2 conversion is low up to 300°C, whereas from 350°C

on, an increase in conversion is observed revealing a better activity than ceramers

pyrolyzed at 600°C, however lower than ceramers pyrolyzed at 400 and 500°C. In relation

84

to the selectivity, Ni/SiO2-400 showed comparable performance to the ceramers

pyrolyzed at 500°C and improved selectivity than ceramers pyrolyzed at 600°C.

Although, selectivity up to 18% lower than ceramers pyrolyzed at 400°C was obtained.

This behavior may be intrinsically related to the accessibility and size of Ni particles as

well as to the surface properties. Despite the fact of showing a hydrophilic surface and

agglomerated Ni particles of about 8 nm (see supplementary data, Fig. S1 – S3), the

impregnation method used to prepare the standard catalyst produces Ni enrichment on the

surface of the support [132], which possibly favors the performance, leading to better

results than ceramers pyrolyzed at 600°C. However, the more hydrophobic surface of

ceramers pyrolyzed at 400°C associated with well-distributed and smaller Ni particles (4

nm), led to an improved performance due to the structure-sensitivity of the CO2

methanation.

Ni catalyst can deactivate even at low temperatures due to sintering of Ni particles

or carbon deposition, affecting its performance and stability [133]. Therefore, in order to

investigate the stability during the methanation reaction, catalysts pyrolyzed at 400°C

(Ni/H44+BisA-400, Ni/MK+BisA-400 and Ni/SiO2-400) were tested at 350°C for 10

hours as depicted in Fig. 4.14. The catalysts exhibited performance fairly close to the

measurements in Fig. 4.13, indicating a good reproducibility within the experimental

error. The standard catalyst showed continued conversion and selectivity, revealing a

good stability of its Ni particles. In the same way, virtually constant conversion and

selectivity during the entire test were displayed by the ceramers. The long-term

performance of ceramers can be related to the improved dispersion of Ni particles and

better particle-matrix interactions. As shown by the TEM images (Fig. 4.9c and 4.9f) after

the 10 h test, no sintering of the Ni particles was observed indicating their good stability.

85

Fig. 4.14: CO2 conversion (black) and CH4 selectivity (blue) for catalysts

pyrolyzed at 400°C during test at 350°C for 10 h.

In summary, it could be seen for the catalytic activity that the polysiloxane

composition and pyrolysis temperature influence the catalysts behavior. Small and finely

dispersed nickel particles in hydrophobic ceramers are more suitable for CO2

methanation, showing good stability and improved conversion and selectivity when

compared to a standard catalyst. A decrease in pyrolysis temperature from 600 to 400°C,

decreases the Ni particle size and increases the catalytic performance. A further

optimization of the ceramers based e.g. on the formation of smaller and well-distributed

Ni particles is expected to further improve the catalytic activity.

86

4.8 In-situ Analysis

In-situ diffraction analysis was conducted during a simulated methanation

reaction in order to verify possible phase changes and the evolution of the Ni crystallite

sizes over the catalytic test. This investigation allows a better understanding on the

stability of the metal particle during the reaction. The measurements were conducted

under the same conditions of the catalytic test during a shorter period. Fig. 4.15 (a-d)

shows the in-situ XRD patterns of the ceramers pyrolyzed at 400 and 600°C during the

first hour of CO2 methanation reaction.

The diffraction peaks of Ni are shifted to higher angles located at approximately

51 and 60° 2θ, once a radiation with a higher wavelength of 1.7712 Å was applied in the

diffractometer. The peaks did not suffer substantial changes and no undesired phase

formation was observed during the reaction, demonstrating that the structure is preserved.

The utmost interesting aspect observed from the in-situ XRD experiments is the stability

of the Ni crystallites during the reaction as depicted in Fig. 4.15(e), revealing constant

values with no further growth during the entire test, corroborating with the TEM analysis

of the spent ceramers. The crystallites are stable at temperatures as high as their initial

pyrolysis temperature. For both H44 and MK-based ceramers at 400 and 600°C, the

crystallite sizes of Ni are maintained constant. At 400°C both ceramers present similar

crystallite sizes (5.8 nm), whereas at 600°C, Ni/MK+BisA-600 shows smaller crystallites

(~10.8 nm) compared to Ni/H44+BisA-600 (~13.3 nm). These results are in good

agreement with the diffraction data presented early (see Table 4.1 and Fig 4.12). A small

difference in crystallite sizes comparing both XRD and in-situ XRD is to be expected

once different devices were used to measure the samples. The influence of the pyrolysis

temperature is also clearly observed, revealing peaks more intense and with narrower

width at pyrolysis temperature of 600°C (Fig. 4.15 (c,d)), indicating the presence of

bigger particles and/or the formation of agglomerates.

Accordingly, it could be shown that the structure and crystallite sizes of Ni are

stable during the catalytic reaction, indicating no sintering effects.

87

Fig. 4.15: In-situ XRD during simulated CO2 methanation reaction at 400°C for (a) Ni-

H44+BisA-400, (b) Ni-MK+BisA-400, (c) Ni-H44+BisA-600 and (d) Ni-MK+BisA-

600 ceramers; and (e) crystallite size versus time on stream.

88

In this work, it could be demonstrated that it is possible to generate a material with

tailored hydrophilicity, hierarchical porosity and high specific surface area containing

nanodispersed nickel particles. The easily adjust of the materials characteristics provides

a good control of the catalysts’ properties with increased activity. Long-term stability and

improved catalytic performance could be achieved due to the high porosity, well-

dispersion of the small particles and solid particle-matrix interaction, which provided a

good accessibility of the catalyst particles, facilitating the internal mass transfer and

leading to an increased activity, besides avoiding extensive sintering of the metal

particles. Also, specific modification of the surfaces by changing the hydrophobicity

behavior of the surface influenced the levels of conversion and selectivity. Therefore, the

metal-containing ceramers are promising materials for catalytic applications.

89

V – Conclusions & Outlook

90

5. Conclusions

In this work, Ni-containing hybrid ceramics were prepared by a simple route using

commercially available and low-cost polysiloxane under low pyrolysis temperatures. By

using different precursors (either methyl or methyl-phenyl polysiloxanes) and varied

pyrolysis temperature (400-600°C), it was possible to adjust the ceramers properties,

leading to porous materials with high specific surface areas, finely dispersed metal

particles and tunable surface characteristics, resulting in improved catalytic activity

toward the CO2 methanation.

Hierarchical porosity (formed by micro-, meso- and macropores) and high specific

surface areas (100 – 550 m2 g-1) could be induced through the pyrolytic conversion of

polysiloxanes with BisA, providing a good accessibility of the catalyst particles and

facilitating the internal mass transfer. Due to the use of BisA as complexing siloxane, Ni

particles were homogeneously distributed, showing average sizes of 4–7 nm. Their stable

embedding avoided extensive sintering of the particles even during long hours of reaction.

A decrease of pyrolysis temperature lowered the Ni particle size and changed the

hydrophobicity behavior of the surface, increasing the catalytic performance. Ceramers

prepared with MK as polysiloxanes showed a more hydrophobic surface owing to the

better thermal resistance of the methyl groups, which improved the catalytic activity since

water presents an inhibiting effect during the methanation reaction.

CO2 conversion and CH4 selectivity could be improved by increasing the reaction

temperature and decreasing the pyrolysis temperature of ceramers. Therefore, ceramers

pyrolyzed at 400℃ exhibited the highest CO2 conversion with selectivity up to ~77% and

an overall good stability during 10 h of reaction. For structure-sensitive reactions, as e.g.

CO2 methanation, the catalytic performance is closely related to the intrinsic properties

of the catalyst. Thus, a less hydrophilic surface with smaller and finely dispersed Ni

particles showed a positive effect on the stability and activity. The opposite situation was

found for ceramers with bigger Ni particles and more hydrophilic surfaces, leading to a

significantly lower conversion and selectivity.

In summary, it could be demonstrated that metal-containing ceramers with highly

dispersed metal particles and tunable properties are promising materials for catalytic

applications, especially for low-temperature reactions, in which the particle size may

affect the catalytic performance and where a hydrophobic material is preferable to avoid

the blocking of the adsorption sites by water.

91

6. Outlook

Although the first results showed promising application of the hybrid ceramics as

catalysts, further investigations may be conducted in order to optimize the ceramers

properties and improve their catalytic performance.

One first approach is centered on investigating different complexing siloxane

precursors. By varying the composition of the complexing agent and/or its molar ration

to metal ions, it is expected to change the interaction metal-polymer and therefore

improve the stabilization and anchoring of the metal ions, leading to a controlled metal

size and distribution at even higher temperatures. Siloxanes containing one (3-

aminopropyltriethoxysilane) or two (3-(2-aminoethylamino)propyltrimethoxysilane)

amino groups could be further considered.

A second approach consists on mixing precursors with different molecular

architecture in order to enhance the porosity and surface area of ceramers. When mixing

two precursors with different weight loss and structures, pores of different sizes can be

formed during pyrolysis, because of the different behavior that the two polymers display.

A third approach involves the development of monoliths structures since for

industrial applications, monolithic catalysts offer several advantages (e.g., lower energy

input, controllable mass transport, easy catalyst separation) and are in many cases

preferable compared to classical powder materials. Processing techniques such as

sacrificial template, replica technique, foaming, blowing, self-assembly and freeze

casting combined with controlled pyrolysis conditions can be applied to produce monolith

porous ceramics with a variety of pore sizes.

92

REFERENCES

[1] IPCC: Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II

and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change,

Geneva, Switzerland, (2014), 151.

[2] W. Li, H. Wang, X. Jiang, J. Zhu, Z. Liu, X. Guo, C. Song, A short review of recent

advances in CO2 hydrogenation to hydrocarbons over heterogeneous catalysts, RSC

Advances 8 (2018) 7651-7669.

[3] W. Wang, S. Wang, X. Ma, J. Gong, Recent advances in catalytic hydrogenation of

carbon dioxide, Chemical Society reviews 40 (2011) 3703-3727.

[4] S. Saeidi, N.A.S. Amin, M.R. Rahimpour, Hydrogenation of CO2 to value-added

products—A review and potential future developments, Journal of CO2 Utilization 5

(2014) 66-81.

[5] H. Muroyama, Y. Tsuda, T. Asakoshi, H. Masitah, T. Okanishi, T. Matsui, K. Eguchi,

Carbon dioxide methanation over Ni catalysts supported on various metal oxides, Journal

of Catalysis 343 (2016) 178-184.

[6] J. Gao, Q. Liu, F. Gu, B. Liu, Z. Zhong, F. Su, Recent advances in methanation

catalysts for the production of synthetic natural gas, RSC Advances 5 (2015) 22759-

22776.

[7] X. Yan, Y. Liu, B. Zhao, Z. Wang, Y. Wang, C.-j. Liu, Methanation over Ni/SiO2:

Effect of the catalyst preparation methodologies, International Journal of Hydrogen

Energy 38 (2013) 2283-2291.

[8] X. Bai, S. Wang, T. Sun, S. Wang, The sintering of Ni/Al2O3 methanation catalyst for

substitute natural gas production, Reaction Kinetics, Mechanisms and Catalysis 112

(2014) 437-451.

[9] P. Colombo, G. Mera, R. Riedel, G.D. Sorarù, Polymer-Derived Ceramics: 40 Years

of Research and Innovation in Advanced Ceramics, Journal of the American Ceramic

Society 93 (2010) 1805-1837.

[10] N. Li, Y. Cao, R. Zhao, Y. Xu, L. An, Polymer-derived SiAlOC ceramic pressure

sensor with potential for high-temperature application, Sensors and Actuators A: Physical

263 (2017) 174-178.

[11] G. Simões dos Reis, C.H. Sampaio, E.C. Lima, M. Wilhelm, Preparation of novel

adsorbents based on combinations of polysiloxanes and sewage sludge to remove

93

pharmaceuticals from aqueous solutions, Colloids and Surfaces A: Physicochemical and

Engineering Aspects 497 (2016) 304-315.

[12] M. Wilamowska, M. Graczyk-Zajac, R. Riedel, Composite materials based on

polymer-derived SiCN ceramic and disordered hard carbons as anodes for lithium-ion

batteries, Journal of Power Sources 244 (2013) 80-86.

[13] K. Wang, J. Unger, J.D. Torrey, B.D. Flinn, R.K. Bordia, Corrosion resistant

polymer derived ceramic composite environmental barrier coatings, Journal of the

European Ceramic Society 34 (2014) 3597-3606.

[14] F. Guo, D. Su, Y. Liu, J. Wang, X. Yan, J. Chen, S. Chen, High acid resistant SiOC

ceramic membranes for wastewater treatment, Ceramics International 44 (2018) 13444-

13448.

[15] R. Riedel, G. Mera, R. Hauser, A. Klonczynski, Silicon-Based Polymer-Derived

Ceramics: Synthesis Properties and Applications-A Review, Journal of the Ceramic

Society of Japan 114 (2006) 425-444.

[16] M. Wilhelm, C. Soltmann, D. Koch, G. Grathwohl, Ceramers—functional materials

for adsorption techniques, Journal of the European Ceramic Society 25 (2005) 271-276.

[17] T. Schmalz, T. Kraus, M. Günthner, C. Liebscher, U. Glatzel, R. Kempe, G. Motz,

Catalytic formation of carbon phases in metal modified, porous polymer derived SiCN

ceramics, Carbon 49 (2011) 3065-3072.

[18] P. Sonström, M. Adam, X. Wang, M. Wilhelm, G. Grathwohl, M. Bäumer, Colloidal

Nanoparticles Embedded in Ceramers: Toward Structurally Designed Catalysts, The

Journal of Physical Chemistry C 114 (2010) 14224-14232.

[19] M. Schubert, M. Wilhelm, S. Bragulla, C. Sun, S. Neumann, T.M. Gesing, P. Pfeifer,

K. Rezwan, M. Bäumer, The Influence of the Pyrolysis Temperature on the Material

Properties of Cobalt and Nickel Containing Precursor Derived Ceramics and their

Catalytic Use for CO2 Methanation and Fischer–Tropsch Synthesis, Catalysis Letters 147

(2016) 472-482.

[20] M. Zaheer, C.D. Keenan, J. Hermannsdörfer, E. Roessler, G. Motz, J. Senker, R.

Kempe, Robust Microporous Monoliths with Integrated Catalytically Active Metal Sites

Investigated by Hyperpolarized 129Xe NMR, Chemistry of Materials 24 (2012) 3952-

3963.

[21] P. J. Lunde and F. L. Kester, Carbon dioxide Methanation on a Ruthenium Catalyst,

Industrial & Engineering Chemistry Process Design and Development 13 (1974) 27-33.

94

[22] F. Macdonald, Audi Has Successfully Made Diesel Fuel From Carbon Dioxide And

Water. Science Alert, 27 April 2015. Available at: https://www.sciencealert.com/audi-

have-successfully-made-diesel-fuel-from-air-and-water. Acessed in: 09 November 2018.

[23] E. Alper, O. Yuksel Orhan, CO2 utilization: Developments in conversion processes,

Petroleum 3 (2017) 109-126.

[24] I. Dimitriou, P. García-Gutiérrez, R.H. Elder, R.M. Cuéllar-Franca, A. Azapagic,

R.W.K. Allen, Carbon dioxide utilisation for production of transport fuels: process and

economic analysis, Energy & Environmental Science 8 (2015) 1775-1789.

[25] Y. Li, S.H. Chan, Q. Sun, Heterogeneous catalytic conversion of CO2: a

comprehensive theoretical review, Nanoscale 7 (2015) 8663-8683.

[26] M.A.A. Aziz, A.A. Jalil, S. Triwahyono, A. Ahmad, CO2 methanation over

heterogeneous catalysts: recent progress and future prospects, Green Chemistry 17 (2015)

2647-2663.

[27] M. Götz, J. Lefebvre, F. Mörs, A. McDaniel Koch, F. Graf, S. Bajohr, R. Reimert,

T. Kolb, Renewable Power-to-Gas: A technological and economic review, Renewable

Energy 85 (2016) 1371-1390.

[28] S. Rönsch, J. Schneider, S. Matthischke, M. Schlüter, M. Götz, J. Lefebvre, P.

Prabhakaran, S. Bajohr, Review on methanation – From fundamentals to current projects,

Fuel 166 (2016) 276-296.

[29] C. Song, Global challenges and strategies for control, conversion and utilization of

CO2 for sustainable development involving energy, catalysis, adsorption and chemical

processing, Catalysis Today 115 (2006) 2-32.

[30] G. Jiajian, Y. Wang, Y. Ping, D. Hu, G. Xu, F. Gu, F. Su, A thermodynamic analysis

of methanation reactions of carbon oxides for the production of synthetic natural gas,

2012.

[31] B. Miao, S.S.K. Ma, X. Wang, H. Su, S.H. Chan, Catalysis mechanisms of CO2 and

CO methanation, Catalysis Science & Technology 6 (2016) 4048-4058.

[32] M. Jacquemin, A. Beuls, P. Ruiz, Catalytic production of methane from CO2 and H2

at low temperature: Insight on the reaction mechanism, Catalysis Today 157 (2010) 462-

466.

[33] W. Wei, G. Jinlong, Methanation of carbon dioxide: an overview, Frontiers of

Chemical Science and Engineering 5 (2010) 2-10.

[34] P. Panagiotopoulou, Hydrogenation of CO2 over supported noble metal catalysts,

Applied Catalysis A: General 542 (2017) 63-70.

95

[35] M. Younas, L. Loong Kong, M.J.K. Bashir, H. Nadeem, A. Shehzad, S. Sethupathi,

Recent Advancements, Fundamental Challenges, and Opportunities in Catalytic

Methanation of CO2, Energy & Fuels 30 (2016) 8815-8831.

[36] G.D. Weatherbee, C.H. Bartholomew, Hydrogenation of CO2 on group VIII metals:

I. Specific activity of NiSiO2, Journal of Catalysis 68 (1981) 67-76.

[37] G.D. Weatherbee, C.H. Bartholomew, Hydrogenation of CO2 on group VIII metals:

IV. Specific activities and selectivities of silica-supported Co, Fe, and Ru, Journal of

Catalysis 87 (1984) 352-362.

[38] P. Frontera, A. Macario, M. Ferraro, P. Antonucci, Supported Catalysts for CO2

Methanation: A Review, Catalysts 7 (2017) 59.

[39] J. Ren, X. Qin, J.-Z. Yang, Z.-F. Qin, H.-L. Guo, J.-Y. Lin, Z. Li, Methanation of

carbon dioxide over Ni–M/ZrO2 (M = Fe, Co, Cu) catalysts: Effect of addition of a second

metal, 2015.

[40] T.A. Le, M.S. Kim, S.H. Lee, T.W. Kim, E.D. Park, CO and CO2 methanation over

supported Ni catalysts, Catalysis Today 293-294 (2017) 89-96.

[41] S. Kattel, W. Yu, X. Yang, B. Yan, Y. Huang, W. Wan, P. Liu, J.G. Chen, CO2

Hydrogenation over Oxide-Supported PtCo Catalysts: The Role of the Oxide Support in

Determining the Product Selectivity, Angewandte Chemie 55 (2016) 7968-7973.

[42] C.K. Vance, C.H. Bartholomew, Hydrogenation of carbon dioxide on group viii

metals: III, Effects of support on activity/selectivity and adsorption properties of nickel,

Applied Catalysis 7 (1983) 169-177.

[43] T.A. Le, J.K. Kang, E.D. Park, CO and CO2 Methanation Over Ni/SiC and Ni/SiO2

Catalysts, Topics in Catalysis 61 (2018) 1537-1544.

[44] Y. Yan, Y. Dai, Y. Yang, A.A. Lapkin, Improved stability of Y2O3 supported Ni

catalysts for CO2 methanation by precursor-determined metal-support interaction,

Applied Catalysis B: Environmental 237 (2018) 504-512.

[45] M.A.A. Aziz, A.A. Jalil, S. Triwahyono, M.W.A. Saad, CO2 methanation over Ni-

promoted mesostructured silica nanoparticles: Influence of Ni loading and water vapor

on activity and response surface methodology studies, Chemical Engineering Journal 260

(2015) 757-764.

[46] X. Guo, A. Traitangwong, M. Hu, C. Zuo, V. Meeyoo, Z. Peng, C. Li, Carbon

Dioxide Methanation over Nickel-Based Catalysts Supported on Various Mesoporous

Material, Energy & Fuels 32 (2018) 3681-3689.

[47] F. Pinna, Supported metal catalysts preparation, Catalysis Today 41 (1998) 129-137.

96

[48] C. Vogt, E. Groeneveld, G. Kamsma, M. Nachtegaal, L. Lu, C.J. Kiely, P.H. Berben,

F. Meirer, B.M. Weckhuysen, Unravelling structure sensitivity in CO2 hydrogenation

over nickel, Nature Catalysis 1 (2018) 127-134.

[49] J.K. Kesavan, I. Luisetto, S. Tuti, C. Meneghini, G. Iucci, C. Battocchio, S. Mobilio,

S. Casciardi, R. Sisto, Nickel supported on YSZ: The effect of Ni particle size on the

catalytic activity for CO2 methanation, Journal of CO2 Utilization 23 (2018) 200-211.

[50] H.C. Wu, Y.C. Chang, J.H. Wu, J.H. Lin, I.K. Lin, C.S. Chen, Methanation of CO2

and reverse water gas shift reactions on Ni/SiO2 catalysts: the influence of particle size

on selectivity and reaction pathway, Catalysis Science & Technology 5 (2015) 4154-

4163.

[51] Q. Liu, Y. Tian, One-pot synthesis of NiO/SBA-15 monolith catalyst with a three-

dimensional framework for CO2 methanation, International Journal of Hydrogen Energy

42 (2017) 12295-12300.

[52] Y.-H. Huang, J.-J. Wang, Z.-M. Liu, G.-D. Lin, H.-B. Zhang, Highly efficient Ni-

ZrO2 catalyst doped with Yb2O3 for co-methanation of CO and CO2, Applied Catalysis

A: General 466 (2013) 300-306.

[53] S. Hwang, J. Lee, U.G. Hong, J.G. Seo, J.C. Jung, D.J. Koh, H. Lim, C. Byun, I.K.

Song, Methane production from carbon monoxide and hydrogen over nickel–alumina

xerogel catalyst: Effect of nickel content, Journal of Industrial and Engineering Chemistry

17 (2011) 154-157.

[54] A. Zhao, W. Ying, H. Zhang, H. Ma, D. Fang, Ni–Al2O3 catalysts prepared by

solution combustion method for syngas methanation, Catalysis Communications 17

(2012) 34-38.

[55] P. Forzatti, L. Lietti, Catalyst deactivation, Catalysis Today 52 (1999) 165-181.

[56] J.G. McCarty, H. Wise, Hydrogenation of surface carbon on alumina-supported

nickel, Journal of Catalysis 57 (1979) 406-416.

[57] M. Agnelli, H.M. Swaan, C. Marquez-Alvarez, G.A. Martin, C. Mirodatos, CO

Hydrogenation on a Nickel Catalyst: II. A Mechanistic Study by Transient Kinetics and

Infrared Spectroscopy, Journal of Catalysis 175 (1998) 117-128.

[58] E. Ionescu, G. Mera, R. Riedel, Polymer-Derived Ceramics: Materials Design

towards Applications at Ultrahigh-Temperatures and in Extreme Environments, 2013.

[59] G. Mera, R. Riedel, F. Poli, K. Müller, Carbon-rich SiCN ceramics derived from

phenyl-containing poly(silylcarbodiimides), Journal of the European Ceramic Society 29

(2009) 2873-2883.

97

[60] G. Mera, M. Gallei, S. Bernard, E. Ionescu, Ceramic Nanocomposites from Tailor-

Made Preceramic Polymers, Nanomaterials 5 (2015) 468-540.

[61] Y. Abe, T. Gunji, Oligo- and polysiloxanes, Progress in Polymer Science 29 (2004)

149-182.

[62] Z. Ren, S. Yan, Polysiloxanes for optoelectronic applications, Progress in Materials

Science 83 (2016) 383-416.

[63] J. Cordelair, P. Greil, Electrical conductivity measurements as a microprobe for

structure transitions in polysiloxane derived Si–O–C ceramics, Journal of the European

Ceramic Society 20 (2000) 1947-1957.

[64] A. Farhang, M. Hamid, K. Ali-Asgar, Modification of polysiloxane polymers for

biomedical applications: a review, Polymer International 50 (2001) 1279-1287.

[65] N. R Mowrer, Polysiloxane Coatings Innovations, (2018).

[66] R. G. Jones, W. Ando, J. Chojnowski, Silicon-Containing Polymers, Springer

(2000).

[67] J. Zeschky, T. Höfner, C. Arnold, R. Weißmann, D. Bahloul-Hourlier, M. Scheffler,

P. Greil, Polysilsesquioxane derived ceramic foams with gradient porosity, Acta

Materialia 53 (2005) 927-937.

[68] B.V. Manoj Kumar, Y.W. Kim, Processing of polysiloxane-derived porous ceramics:

a review, Science and technology of advanced materials 11 (2010) 044303.

[69] B.J.J. Zelinski, D.R. Uhlmann, Gel technology in ceramics, Journal of Physics and

Chemistry of Solids 45 (1984) 1069-1090.

[70] G. Christel, B. Florence, D. Nicola, S. GianDomenico, Sol-Gel-Derived Silicon-

Boron Oxycarbide Glasses Containing Mixed Silicon Oxycarbide (SiCxO4−x) and Boron

Oxycarbide (BCyO3−y) Units, Journal of the American Ceramic Society 84 (2001) 2160-

2164.

[71] D.K. Chattopadhyay, K.V.S.N. Raju, Structural engineering of polyurethane

coatings for high performance applications, Progress in Polymer Science 32 (2007) 352-

418.

[72] J. Wen, G.L. Wilkes, Organic/Inorganic Hybrid Network Materials by the Sol−Gel

Approach, Chemistry of Materials 8 (1996) 1667-1681.

[73] X. Liu, Y.-L. Li, F. Hou, Fabrication of SiOC Ceramic Microparts and Patterned

Structures from Polysiloxanes via Liquid Cast and Pyrolysis, Journal of the American

Ceramic Society 92 (2009) 49-53.

98

[74] P. Cromme, M. Scheffler, P. Greil, Ceramic Tapes from Preceramic Polymers,

Advanced Engineering Materials 4 (2002) 873-877.

[75] D. Galusek, J. Sedláček, R. Riedel, Al2O3–SiC composites prepared by warm

pressing and sintering of an organosilicon polymer-coated alumina powder, Journal of

the European Ceramic Society 27 (2007) 2385-2392.

[76] O. Goerke, E. Feike, T. Heine, A. Trampert, H. Schubert, Ceramic coatings

processed by spraying of siloxane precursors (polymer-spraying), Journal of the

European Ceramic Society 24 (2004) 2141-2147.

[77] S. Walter, D. Suttor, T. Erny, B. Hahn, P. Greil, Injection moulding of

polysiloxane/filler mixtures for oxycarbide ceramic composites, Journal of the European

Ceramic Society 16 (1996) 387-393.

[78] K. Young-Wook, E. Jung-Hye, W. Chunmin, P.C. B., Processing of Porous Silicon

Carbide Ceramics from Carbon-Filled Polysiloxane by Extrusion and Carbothermal

Reduction, Journal of the American Ceramic Society 91 (2008) 1361-1364.

[79] K. Okamura, T. Shimoo, K. Suzuya, K. Suzuki, SiC-Based Ceramic Fibers Prepared

via Organic-to-Inorganic Conversion Process-A Review, Journal of Ceramic Society of

Japan 144 (2006) 445-454.

[80] R.M.d. Rocha, P. Greil, J.C. Bressiani, A.H.d.A. Bressiani, Complex-shaped ceramic

composites obtained by machining compact polymer-filler mixtures, Materials Research

8 (2005) 191-196.

[81] S. Harald, K. Dietmar, G. Georg, C. Paolo, Micro‐/Macroporous Ceramics from

Preceramic Precursors, Journal of the American Ceramic Society 84 (2001) 2252-2255.

[82] C. Vakifahmetoglu, D. Zeydanli, P. Colombo, Porous polymer derived ceramics,

Materials Science and Engineering: R: Reports 106 (2016) 1-30.

[83] P. Colombo, Engineering porosity in polymer-derived ceramics, Journal of the

European Ceramic Society 28 (2008) 1389-1395.

[84] P. Greil, Near Net Shape Manufacturing of Polymer Derived Ceramics, Journal of

the European Ceramic Society 18 (1998) 1905-1914.

[85] E. Bernardo, L. Fiocco, G. Parcianello, E. Storti, P. Colombo, Advanced Ceramics

from Preceramic Polymers Modified at the Nano-Scale: A Review, Materials 7 (2014)

1927-1956.

[86] R. Harshe, C. Balan, R. Riedel, Amorphous Si(Al)OC ceramic from polysiloxanes:

bulk ceramic processing, crystallization behavior and applications, Journal of the

European Ceramic Society 24 (2004) 3471-3482.

99

[87] C. Wang, J. Wang, C.B. Park, Y.-W. Kim, Cross-Linking Behavior of a Polysiloxane

in Preceramic Foam Processing, Journal of Materials Science 39 (2004) 4913-4915.

[88] Y. Pang, K. Feng, Y.H. Mariam, Pyrolyzability of Preceramic Polymers, Physical

Properties of Polymers Handbook, Springer (2007).

[89] O. Schneider, Thermoanalytical investigations on curing and decomposition of

methyl silicone resin, Thermochimica Acta 134 (1988) 269-274.

[90] P. Greil, Polymer Derived Engineering Ceramics, Advanced Engineering Materials

2 (2000) 339-348.

[91] M. Scheffler, P. Greil, A. Berger, E. Pippel, J. Woltersdorf, Nickel-catalyzed in situ

formation of carbon nanotubes and turbostratic carbon in polymer-derived ceramics,

Materials Chemistry and Physics 84 (2004) 131-139.

[92] P. Greil, Active-Filler-Controlled Pyrolysis of Preceramic Polymers, Journal of the

American Ceramic Society 78 (1995) 835-848.

[93] R. Kumar, Y. Cai, P. Gerstel, G. Rixecker, F. Aldinger, Processing, crystallization

and characterization of polymer derived nano-crystalline Si–B–C–N ceramics, Journal of

Materials Science 41 (2006) 7088-7095.

[94] W. Julin, G.M. J., M.A. K., InSitu Densification Behavior in the Pyrolysis

Consolidation of Amorphous Si-N-C Bulk Ceramics from Polymer Precursors, Journal

of the American Ceramic Society 84 (2001) 2165-2169.

[95] M. Seifollahi Bazarjani, H.-J. Kleebe, M.M. Müller, C. Fasel, M. Baghaie Yazdi, A.

Gurlo, R. Riedel, Nanoporous Silicon Oxycarbonitride Ceramics Derived from

Polysilazanes In situ Modified with Nickel Nanoparticles, Chemistry of Materials 23

(2011) 4112-4123.

[96] M. Kamperman, A. Burns, R. Weissgraeber, N. van Vegten, S.C. Warren, S.M.

Gruner, A. Baiker, U. Wiesner, Integrating Structure Control over Multiple Length Scales

in Porous High Temperature Ceramics with Functional Platinum Nanoparticles, Nano

Letters 9 (2009) 2756-2762.

[97] G. Germund, S. Thomas, K. Tobias, H. Frank, M. Günter, K. Rhett, Copper-

Containing SiCN Precursor Ceramics (Cu@SiCN) as Selective Hydrocarbon Oxidation

Catalysts Using Air as an Oxidant, Chemistry – A European Journal 16 (2010) 4231-

4238.

[98] M. Zaheer, J. Hermannsdörfer, W.P. Kretschmer, G. Motz, R. Kempe, Robust

Heterogeneous Nickel Catalysts with Tailored Porosity for the Selective Hydrogenolysis

of Aryl Ethers, ChemCatChem 6 (2014) 91-95.

100

[99] M. Zaheer, T. Schmalz, G. Motz, R. Kempe, Polymer derived non-oxide ceramics

modified with late transition metals, Chemical Society reviews 41 (2012) 5102-5116.

[100] P. Colombo, T. Gambaryan-Roisman, M. Scheffler, P. Buhler, P. Greil, Conductive

Ceramic Foams from Preceramic Polymers, Journal of the American Ceramic Society 84

(2001) 2265-2268.

[101] C. Vakifahmetoglu, E. Pippel, J. Woltersdorf, P. Colombo, Growth of One-

Dimensional Nanostructures in Porous Polymer-Derived Ceramics by Catalyst-Assisted

Pyrolysis. Part I: Iron Catalyst, Journal of the American Ceramic Society 93 (2010) 959-

968.

[102] C. Vakifahmetoglu, P. Colombo, S.M. Carturan, E. Pippel, J. Woltersdorf, Growth

of One-Dimensional Nanostructures in Porous Polymer-Derived Ceramics by Catalyst-

Assisted Pyrolysis. Part II: Cobalt Catalyst, Journal of the American Ceramic Society 93

(2010) 3709-3719.

[103] M. Adam, C. Vakifahmetoglu, P. Colombo, M. Wilhelm, G. Grathwohl, G. Soraru,

Polysiloxane-Derived Ceramics Containing Nanowires with Catalytically Active Tips,

Journal of the American Ceramic Society 97 (2014) 959-966.

[104] M. Adam, M. Bäumer, M. Schowalter, J. Birkenstock, M. Wilhelm, G. Grathwohl,

Generation of Pt- and Pt/Zn-containing ceramers and their structuring as

macro/microporous foams, Chemical Engineering Journal 247 (2014) 205-215.

[105] M. Adam, S. Kocanis, T. Fey, M. Wilhelm, G. Grathwohl, Hierarchically ordered

foams derived from polysiloxanes with catalytically active coatings, Journal of the

European Ceramic Society 34 (2014) 1715-1725.

[106] M. Wilhelm, M. Adam, M. Bäumer, G. Grathwohl, Synthesis and Properties of

Porous Hybrid Materials containing Metallic Nanoparticles, Advanced Engineering

Materials 10 (2008) 241-245.

[107] M. Adam, M. Wilhelm, G. Grathwohl, Polysiloxane derived hybrid ceramics with

nanodispersed Pt, Microporous and Mesoporous Materials 151 (2012) 195-200.

[108] A. Kaiser, C. Görsmann, U. Schubert, Influence of the metal complexation on size

and composition of Cu/Ni nano-particles prepared by sol-gel processing, Journal of Sol-

Gel Science and Technology 8 (1997) 795-799.

[109] G. Trimmel, C. Lembacher, G. Kickelbick, U. Schubert, Sol-gel processing of

alkoxysilyl-substituted nickel complexes for the preparation of highly dispersed nickel in

silica, New Journal of Chemistry 26 (2002) 759-765.

101

[110] R. Cecci. Development of Catalytic Hybrid Materials Derived from Polysiloxane

Precursors. Diploma Thesis, Advanced Ceramics, University of Bremen (2009).

[111] T. Prenzel, M. Wilhelm, K. Rezwan, Tailoring amine functionalized hybrid

ceramics to control CO2 adsorption, Chemical Engineering Journal 235 (2014) 198-206.

[112] M. Scheffler, T. Gambaryan-Roisman, T. Takahashi, J. Kaschta, H. Muenstedt, P.

Buhler, P. Greil, Pyrolytic Decomposition of Preceramic Organo Polysiloxanes, Ceramic

Transaction 115 (2000) 239-250.

[113] J.D. Jovanovic, M.N. Govedarica, P.R. Dvornic, I.G. Popovic, The

thermogravimetric analysis of some polysiloxanes, Polymer Degradation and Stability 61

(1998) 87-93.

[114] C. Vakifahmetoglu, P. Colombo, A Direct Method for the Fabrication of Macro-

Porous SiOC Ceramics from Preceramic Polymers, Advanced Engineering Materials 10

(2008) 256-259.

[115] M. Scheffler, T. Gambaryan-Roisman, T. Takahashi, J. Kaschta, H. Muenstedt, P.

Buhler, P. Greil, Pyrolytic decomposition of preceramic organo polysiloxanes, 115

(2000) 239-250.

[116] F.I. Hurwitz, P. Heimann, S.C. Farmer, D.M. Hembree, Characterization of the

pyrolytic conversion of polysilsesquioxanes to silicon oxycarbides, Journal of Materials

Science 28 (1993) 6622-6630.

[117] S. Nedunchezhian, R. Sujith, R. Kumar, Processing and characterization of polymer

precursor derived silicon oxycarbide ceramic foams and compacts, Journal of Advanced

Ceramics 2 (2013) 318-324.

[118] W. Zhou, H. Yang, X. Guo, J. Lu, Thermal degradation behaviors of some branched

and linear polysiloxanes, Polymer Degradation and Stability 91 (2006) 1471-1475.

[119] D. Zhu, W.J. van Ooij, Corrosion protection of metals by water-based silane

mixtures of bis-[trimethoxysilylpropyl]amine and vinyltriacetoxysilane, Progress in

Organic Coatings 49 (2004) 42-53.

[120] International Union of Pure and Applied Chemistry, Reporting Physisorption Data

for Gas/Solid Systems with Special Rederence to the Determination of Surface Area and

Porosity, Pure Applied Chemistry, 57 (1985) 603-619.

[121] T. Prenzel, T.L.M. Guedes, F. Schlüter, M. Wilhelm, K. Rezwan, Tailoring surfaces

of hybrid ceramics for gas adsorption – From alkanes to CO2, Separation and Purification

Technology 129 (2014) 80-89.

102

[122] T. Prenzel, M. Wilhelm, K. Rezwan, Pyrolyzed polysiloxane membranes with

tailorable hydrophobicity, porosity and high specific surface area, Microporous and

Mesoporous Materials 169 (2013) 160-167.

[123] C. Harms, M. Adam, K.A. Soliman, M. Wilhelm, L.A. Kibler, T. Jacob, G.

Grathwohl, New Electrocatalysts with Pyrolyzed Siloxane Matrix, Electrocatalysis 5

(2014) 301-309.

[124] Y. Peng, K. Wang, M. Yu, A. Li, R.K. Bordia, An optimized process for in situ

formation of multi-walled carbon nanotubes in templated pores of polymer-derived

silicon oxycarbide, Ceramics International 43 (2017) 3854-3860.

[125] N. Mantzel, S. Rannabauer, E.C. Bucharsky, K.G. Schell, M.J. Hoffmann, M.

Scheffler, A Novel Approach for the Processing of Advanced Polymer Derived Ceramics

with Carbon Nanotubes with the Help of Pores, Advanced Engineering Materials 16

(2014) 295-300.

[126] C.S. Terry, F. Scheffler, J.D. Torrey, R.K. Bordia, M. Scheffler, In Situ Carbon

Nanotube Formation in Templated Pores of Polymer-Derived Ceramics, Advanced

Engineering Materials 13 (2011) 906-912.

[127] M. Kumar, Carbon Nanotube Synthesis and Growth Mechanism, Carbon

Nanotubes - Synthesis, Characterization, Applications, InTech (2011).

[128] M.V. Kharlamova, Investigation of growth dynamics of carbon nanotubes,

Beilstein Journal of Nanotechnology 8 (2017) 826-856.

[129] T. Takahashi, J. Kaschta, H. Münstedt, Melt rheology and structure of silicone

resins, Rheologica Acta 40 (2001) 490-498.

[130] Z. Fan, K. Sun, N. Rui, B. Zhao, C.-j. Liu, Improved activity of Ni/MgAl2O4 for

CO2 methanation by the plasma decomposition, Journal of Energy Chemistry 24 (2015)

655-659.

[131] N. Toshima, Metal Nanoparticles for Catalysis, in: L.M. Liz-Marzán, P.V. Kamat

(Eds.) Nanoscale Materials, Springer US, Boston, MA, 2003, pp. 79-96.

[132] W. Cai, Q. Zhong, Y. Zhao, Fractional-hydrolysis-driven formation of non-uniform

dopant concentration catalyst nanoparticles of Ni/CexZr1−xO2 and its catalysis in

methanation of CO2, Catalysis Communications 39 (2013) 30-34.

[133] C.H. Bartholomew, Mechanisms of catalyst deactivation, Applied Catalysis A:

General 212 (2001) 17-60.

103

Supplementary Data

104

Appendix A

Characterization results for the standard catalyst Ni/SiO2 pyrolyzed at

400°C are presented below in Fig. S1 to S3.

Fig. S1: Ratio of maximum water and n-heptane adsorption for Ni/SiO2 catalyst

pyrolyzed at 400°C. (Data from ceramers pyrolyzed at 400, 500 and 600°C are shown

for comparison).

105

Fig S2: (a) N2-adsorption/desorption isotherm and (b) pore size distribution (inset from

2 to 11 nm) of Ni/SiO2-400 catalyst.

106

Fig. S3: (a) TEM image and (b) particle size distribution of Ni/SiO2-400 catalyst.

(a)

(b)

107

Appendix B - Curriculum Vitae

______________________________________________________________________

Personal Data

Name Heloísa Pimenta de Macedo

Adress Federal University of Rio Grande do Norte, Chemistry Institute -

Environmental Technology Laboratory

59072970, Natal-RN, Brazil

Email macedo.p.heloisa@gmail.com

______________________________________________________________________

Background

2014 - 2018 PhD in Materials Science and Engineering.

Federal University of Rio Grande do Norte, Natal, Brazil, with

sandwich period at the University of Bremen - Germany

Title: Ni-Containing Hybrid Ceramics Derived from Polysiloxanes for

Production of CH4 via Hydrogenation of CO2.

Surpervisor: Dulce Maria de Araújo Melo

Co-surpervisor: Michaela Wilhelm

2012 - 2014 Master in Materials Science and Engineering.

Federal University of Rio Grande do Norte, Natal, Brazil

Title: Nickel, Iron and Cobalt-based Solid Oxygen Carriers Supported

in Olivina for Chemical Looping Technology.

Surpervisor: Dulce Maria de Araújo Melo

2007 - 2011 Bachelor in Materials Engineering.

Federal University of Rio Grande do Norte, Natal, Brazil

Title: Synthesis and Characterization of Solid Solutions of Neodymium

Doped Ceria

Surpervisor: Carlson Pereira de Souza

_____________________________________________________________________

Publications

1. Nickel-containing hybrid ceramics derived from polysiloxanes with hierarchical

porosity for CO2 methanation. Macedo, H. P.; Medeiros, R. L. B.A.; Ilsemann, J.;

Melo, D. M. A.; Rezwan, K.; Wilhelm, M., Microporous and Mesoporous Materials,

2018. https://doi.org/10.1016/j.micromeso.2018.11.006

108

2. A comparative study of dry reforming of methane over nickel catalysts supported

on perovskite-type LaAlO3 and commercial α-Al2O3. Figueredo, G. P.; Medeiros, R.

L. B. A.; Macedo, H. P.; Oliveira, A. A. S.; Braga, R. M.; Mercury, J. M. R.; Melo, M.

A. F.; Melo, D. M. A., International Journal of Hydrogen Energy, v.43, p.11022-11037,

2018.

3. One-step synthesis of LaNiO3 with chitosan for dry reforming of methane.

Oliveira, A. A. S.; Medeiros, R. L. B. A.; Figueredo, G. P.; Macedo, H. P.; Braga, R. M.;

Maziviero, F. V.; Melo, M. A. F.; Melo, D. M. A.; Vieira, M. M., International Journal

of Hydrogen Energy, v.43, p.9696 - 9704, 2018.

4. Study of the reactivity of Double-perovskite type oxide La1-xMxNiO4 (M= Ca or

Sr) for chemical looping hydrogen production. Melo, V. R. M.; Medeiros, R. L. B. A.;

Braga, R. M.; Macedo, H. P.; Ruiz, J. A. C.; Moure, G. T.; Melo, M. A. F.; Melo, D. M.

A., International Journal of Hydrogen Energy, v.43, p.1406 - 1414, 2018.

5. Use of Retorted Shale as Sulfur Adsorbent. Medeiros, R. L. B. A.; Silva, M. F. D.;

Santiago, R. C.; Figueredo, G. P.; Macedo, H. P.; Melo, M. A. F.; Melo, D. M. A.,

Materials Science Forum (Online), v.930, p.562 - 567, 2018.

6. Characterization of ZnAl2O4 Spinel Obtained by Hydrothermal and Microwave

Assisted Combustion Method: A Comparative Study. Macedo, H. P.; Medeiros, R.

L. B. A.; Medeiros, A. L.; Oliveira, A. A. S.; Figueredo, G. P.; Melo, M. A. F.; Melo, D.

M. A., Materials Research-Ibero-american Journal of Materials, v.20, p.29 - 33, 2017.

7. Estudo da influência da razão combustível/oxidante e da potência do micro-ondas

na formação da α-Al2O3 via reação de combustão. Medeiros, R. L. B. A.; Oliveira, A.

A. S.; Melo, V. R. M.; Macedo, H. P.; Carvalho, A. F. M.; Melo, D. M. A.; Melo, M. A.

F., Cerâmica, v.63, p.272 - 280, 2017.

8. Study of the reactivity by pulse of CH4 over NiO/Fe-doped MgAl2O4 oxygen

carriers for hydrogen production. Medeiros, R. L. B. A.; Macedo, H. P.; Figueredo,

G. P.; Costa, T. R.; Braga, R. M.; Melo, M. A. F.; Melo, D. M. A., International Journal

of Hydrogen Energy, v.42, p.24823 - 24829, 2017.

9. Synthesis of MgAl2O4 by Gelatin Method: Effect of Temperature and Time of

Calcination in Crystalline Structure. Figueredo, G. P.; Carvalho, A. F. M.; Medeiros,

R. L. B. A.; Silva, F. M.; Macedo, H. P.; Melo, M. A. F.; Melo, D. M. A., Materials

Research-Ibero-american Journal of Materials, v.20, p.254 - 259, 2017.

10. Ni supported on Fe-doped MgAl2O4 for dry reforming of methane: Use of

factorial design to optimize H2 yield. Medeiros, R. L. B. A.; Macedo, H. P.; Melo, V.

R. M.; Oliveira, A. A. S.; Barros, J. M. F.; Melo, M. A. F.; Melo, D. M. A., International

Journal of Hydrogen Energy, v.41, p.14047, 2016.

11. Síntese de MgAl2O4 por combustão assistida por micro-ondas: influência dos

parâmetros de síntese na formação e na estrutura cristalina. Medeiros, R. L. B. A.;

Macedo, H. P.; Oliveira, A. A. S.; Melo, V. R. M.; Carvalho, A. F. M.; Melo, M. A. F.;

Melo, D. M. A., Cerâmica, v.62, p.191 - 197, 2016.

109

12. Application of Design of Experiments to the Alkaline Treatment in Mordenite

Zeolite: Influence on Si/Al Ratio. Macedo, H.P.; Oliveira Felipe, L. C.; Silva, L.B.;

Garcia, L. M. P.; Medeiros, R.L.B.A.; Costa, T.R., Materials Science Forum (Online),

v.798-799, p.435 - 440, 2014.

13. High temperature conduction and methane conversion capability of BaCeO3

perovskite. Macedo, H. P.; Bezerra Lopes, F. W.; Arab, M.; Souza, C. P.; Gavarri, J. R.;

Souza, J. F., Powder Technology (Print), v.219, p.186 - 192, 2012.