High surface area mesoporous perovskites for …...HIGH SURFACE AREA MESOPOROUS PEROVSKITES FOR...

HIGH SURFACE AREA MESOPOROUS

PEROVSKITES FOR CATALYTIC APPLICATIONS

Thèse

Mahesh Muraleedharan Nair

Doctorat en Chimie

Philosophiae Doctor (PhD)

Québec, Canada

© Mahesh Muraleedharan Nair, 2014

III

RÉSUMÉ

Les pérovskites sont des oxydes métalliques mixtes qui peuvent être

représentés par la formule générale ABO3. Depuis la première revue mettant en

évidence leur activité catalytique, ces matériaux ont attiré l‟attention des

chercheurs dans le monde entier. Il a été confirmé que les pérovskites peuvent

être considérées comme des alternatives rentables et efficaces aux métaux nobles

pour plusieurs applications (les réactions de synthèse à titre d‟exemple). En outre,

ces oxydes métalliques mixtes sont bien connus pour leur stabilité à haute

température, leur grande mobilité d'oxygène ainsi que la stabilisation des

inhabituels états d'oxydation des cations. Pour ces raisons, plusieurs stratégies ont

été développées pour la synthèse de ces matériaux. Cependant, les méthodes

conventionnelles de synthèse des pérovskites permettent d‟obtenir seulement des

matériaux ayant une faible surface spécifique, ce qui constitue un inconvénient

majeur du fait que des applications catalytiques sont mis en jeux. La faible surface

spécifique est due à un traitement thermique de haute température appliqué au

cours de la synthèse de ces matériaux. Le premier objectif de ce présent travail est

donc l‟obtention d‟oxydes métalliques mixtes structurés de type pérovskite avec

une grande surface spécifique. Le “Nanocasting”, une méthode de gabarits solides

récemment développée, a montré son efficacité pour la synthèse de diverses

compositions chimiques ayant des valeurs extrêmement élevées de surface

spécifique. En se basant sur cette méthode, plusieurs pérovskites LaBO3 (B = Mn ,

Ni , Co, Fe) ont été synthétisées. Ces matériaux se caractérisent par leur grande

surface spécifique qui peut atteindre 150 m2 g-1.

Les premiers essais de l'oxydation totale du méthanol, une molécule sonde,

out confirmé que ces nouveaux matériaux sont des catalyseurs très actifs, en

particulier les LaMnO3. De plus, d'autres études ont confirmé que l'augmentation

de l‟activité catalytique est évidemment liée à la plus grande surface spécifique et

a la plus grande quantité d‟oxygène adsorbée des pérovskites développées. Les

résultats ont montré une proportionnalité entre les vitesses des réactions et la

IV

surface spécifique du catalyseur. Dans une étude suivante, l‟intérêt de la

recherche est porté sur reformage du méthane à sec, comme cette réaction est

très pertinente pour l‟industrie du fait qu‟elle consiste en la conversion de deux gaz

à effet serre (CH4 et CO2) en gaz de synthèse (CO + H2). Des résultats

prometteurs ont été obtenus dans ce cas aussi en utilisant les matériaux

développés de type LaNiO3 comme un pré-catalyseur. De meilleures efficacité et

stabilité ont été observées pour Ni/La2O3, catalyseurs dérivés des LaNiO3, par

rapport à son homologue en vrac.

V

ABSTRACT

Perovskites are mixed metal oxides that can be represented by the general

formula ABO3. Since the initial report regarding their catalytic activity, these

materials have received immense research attention worldwide. Perovskites are

proven to be cost effective and efficient alternatives to noble metals for various

total/partial oxidation as well as synthetic chemical reactions. Additionally these

mixed metal oxides are well known for their high temperature stability, high mobility

of oxygen and the stabilization of unusual cation oxidation states. For these

reasons various strategies were developed for the synthesis of these materials.

However perovskites synthesized using conventional methods generally result in

low specific surface area materials, which is a major drawback as far as catalytic

applications are concerned. This pertinent lower value of surface area is resulting

from the high temperature treatment involved in the synthesis of these materials.

This issue was taken up and in the present project the first goal was to obtain

perovskite structured mixed metal oxides with high specific surface area.

Nanocasting is a recently developed solid templating method that is proven to be

efficient for the synthesis of various chemical compositions with extremely high

values of specific surface area. By applying this method a series of LaBO3 (B = Mn,

Ni, Co, Fe) perovskites were synthesized and these materials were found to

posses extremely high values of specific surface areas (up to 150 m2g-1).

Initial tests for the total oxidation of methanol as a probe molecule confirmed

that these novel materials are highly active catalysts, especially LaMnO3. Further

studies confirmed that the enhanced activity was obviously related to the higher

specific surface areas and higher amount of adsorbed oxygen species obtained for

the nanocast perovskites in comparison with the bulk. Our results demonstrated

the proportionality of reaction rates to the specific surface area of the catalyst. In a

following study, we chose dry reforming of methane, since this reaction involves

the conversion of two green house gases (CH4 and CO2) into syngas (CO + H2),

which is more industrially relevant. Promising results were obtained in this case

also using nanocast LaNiO3 as a pre-catalyst. Enhanced efficiency and stability

VI

were observed for Ni/La2O3 catalysts derived from nanocast LaNiO3 in comparison

to its bulk counterpart. In particular, these materials were found to be coke

resistant for 48 hours under the conditions of dry reforming.

VII

Table of contents

RÉSUMÉ ................................................................................................................ III

ABSTRACT ............................................................................................................. V

LIST OF TABLES ................................................................................................... XI

LIST OF FIGURES ............................................................................................... XIII

ABBREVIATIONS ............................................................................................... XVII

ACKNOWLEDGEMENTS .................................................................................... XIX

PREFACE ............................................................................................................ XXI

Chapter 1– Introduction .......................................................................................... 1

1.1 Perovskites ................................................................................................ 1

1.1.1 Structure and properties ...................................................................... 1

1.1.2 Synthesis and catalytic properties ....................................................... 3

1.2 Nanocasting ............................................................................................... 5

1.2.1 Principles of nanocasting ..................................................................... 5

1.2.2 Ordered mesoporous silica templates ................................................. 8

1.2.3 Overview nanocasting ....................................................................... 12

1.3 Catalytic tests ........................................................................................... 18

1.3.1 Total oxidation of methanol ............................................................... 18

1.3.2 Dry reforming of methane .................................................................. 21

Chapter 2 – Experimental methods ....................................................................... 25

2.1 Synthesis of ordered mesoporous silica templates .................................. 25

2.2 Synthesis of mesoporous perovskites ...................................................... 26

2.3 Characterization ....................................................................................... 27

2.3.1 Powder X-Ray Diffraction (XRD) ....................................................... 27

2.3.2 N2 – physisorption ............................................................................. 29

2.3.3 Elemental analysis ............................................................................. 31

2.3.4 Electron Microscopy (TEM and SEM) ................................................ 31

2.3.5 X-ray photoelectron spectroscopy (XPS) ........................................... 32

2.3.6 Temperature programmed reduction (TPR-H2) ................................. 32

2.3.7 Temperature programmed desorption (TPD-O2,) .............................. 33

2.3.8 Thermogravimetric / Differential thermal analysis (TG/DTA) ............. 33

2.4 Catalytic tests ........................................................................................... 34

2.4.1 Total oxidation of methanol ............................................................... 34

2.4.2 Dry reforming of methane .................................................................. 36

VIII

Chapter 3 - Kinetics of methanol oxidation over mesoporous perovskite catalysts 39

3.1 Résumé .................................................................................................... 40

3.2 Abstract .................................................................................................... 41

3.3 Introduction .............................................................................................. 42

3.4 Experimental ............................................................................................ 43

3.4.1 Synthesis of ordered mesoporous KIT-6 silica .................................. 43

3.4.2 Nanocasting of mesoporous perovskites ........................................... 43

3.4.3 Characterization ................................................................................ 44

3.4.4 TPR/TPD ........................................................................................... 44

3.4.5 Catalytic tests .................................................................................... 45

3.4.6 Kinetic studies ................................................................................... 45

3.5 Results and discussion ............................................................................ 45

3.5.1 Synthesis and characterization of mesoporous perovskites .............. 45

3.5.2 Temperature programmed reduction by hydrogen ............................ 47

3.5.3 Temperature programmed desorption of oxygen .............................. 49

3.5.4 Catalytic tests .................................................................................... 51

3.5.5 Stability .............................................................................................. 52

3.5.6 Kinetic studies ................................................................................... 53

3.6 Conclusions ............................................................................................. 56

3.7 References ............................................................................................... 57

Chapter 4 - Pore structure effects on the kinetics of methanol oxidation over nanocast mesoporous perovskites ........................................................................ 59

4.1 Résumé .................................................................................................... 60

4.2 Abstract .................................................................................................... 61

4.3 Introduction .............................................................................................. 62

4.4 Experimental ............................................................................................ 63

4.4.1 Synthesis of ordered mesoporous SBA-15 silica ............................... 63



4.4.4 Temperature programmed reduction (TPR) and desorption (TPD) ... 65

4.4.5 Catalytic tests .................................................................................... 65

4.4.6 Kinetic studies ................................................................................... 66



4.5.2 Temperature programmed reduction of hydrogen ............................. 72

IX

4.5.3 Temperature programmed desorption of oxygen .............................. 73

4.5.4 Catalytic tests .................................................................................... 74

4.5.5 Kinetic studies ................................................................................... 75

4.6 Conclusions ............................................................................................. 78

4.7 References ............................................................................................... 80

Chapter 5- Surface properties of nanocast mesoporous perovskites .................... 83

5.1 Résumé .................................................................................................... 84

5.2 Abstract .................................................................................................... 85

5.3 Introduction .............................................................................................. 86

5.4 Experimental ............................................................................................ 87

5.4.1 Synthesis of ordered mesoporous KIT-6 silica .................................. 87





5.5.2 SEM-EDS analysis ............................................................................ 95

5.5.3 XPS analysis ..................................................................................... 96

5.6 Conclusions ........................................................................................... 101

5.7 References ............................................................................................. 103

Chapter 6 - Coke resistant nanostructured Ni/La2O3 catalyst for dry reforming of methane .............................................................................................................. 105

6.1 Résumé .................................................................................................. 106

6.2 Abstract .................................................................................................. 107

6.3 Introduction ............................................................................................ 108

6.4 Experimental .......................................................................................... 110

6.4.1 Synthesis of ordered mesoporous SBA-15 silica ............................. 110

6.4.2 Synthesis of LaNiO3 perovskites ..................................................... 110

6.4.3 Characterization .............................................................................. 111

6.4.4 Catalytic tests .................................................................................. 112

6.5 Results and discussion .......................................................................... 113

6.5.1 Physicochemical characterization.................................................... 113

6.5.2 Catalytic studies .............................................................................. 119

6.5.3 Stability tests ................................................................................... 121

6.6 Conclusion ............................................................................................. 125

6.7 References ............................................................................................. 127

X

Chapter 7 – General conclusions and perspectives ............................................ 129

Bibliographic references ...................................................................................... 133

APPENDIX .......................................................................................................... 137

XI

LIST OF TABLES

Table 1.1 General methods used for the synthesis of perovskite structured mixed metal oxides. SBET is the specific surface area determined by the BET method …........................4

Table 3.1 Structural parameters of the KIT-6 template and nanocast perovskites obtained by performing N2 physisorption analysis at -196 ºC………….……………..........................48

Table 3.2 Amount of H2 consumed during TPR-H2……………...........................................50

Table 3.3 Amount of O2 desorbed during TPD-O2…………...............................................51

Table 4.1 Structural parameters of the SBA-15 templates and nanocast perovskites obtained by performing N2 physisorption analysis at -196 ºC…………...............................68

Table 4.2 Amount of H2 consumed during TPR-H2……………………................................72

Table 4.3 Amount of O2 desorbed during TPD-O2.…………………....................................74

Table 4.4 Kinetic parameters obtained for total oxidation of methanol over nanocast perovskites………………………………………………………………………………………..78

Table 5.1 Structural parameters of the KIT-6 template and nanocast perovskites obtained by performing physisorption analysis at -196 ºC.……….....................................................92

Table 5.2 Surface and bulk elemental composition of the nanocast mesoporous perovskites.…………………..............................................................................................101

Table 6.1 Structural parameters obtained for LaNiO3 perovskites and Ni/La2O3 obtained by performing reduction at 700 ºC……..……………..............................................................114

Table 6.2 Amount of H2 consumed during TPR-H2……………........................................118

Table 6.3 Structural parameters obtained for Ni/La2O3 catalysts after performing the stability tests.....................................................................................................................125

XII

XIII

LIST OF FIGURES

Figure 1.1 Representations of the ideal perovskite structure with BX6 octahedron forming the centre of the cube (left) and BX6 octahedron occupying the corners of the cube with A site atom at the centre. (right)….............................................................……………………..1

Figure 1.2 Schematic representation of the steps involved in the mesostructure evolution in the nanocasting technique using ordered mesoporous silica as hard template…................................................................................................................………..6

Figure 1.3 Mechanistic pathway showing the self assembly of formation of mesoporous materials ……………..…………………….............................................................................9

Figure 1.4 Pore models of mesostructures with symmetries (A) p6mm, (B) Ia3d, (C) Pm3n, (D) Im3n, (E) Fd3m, (F) Fm3m…...................................................................……..10

Figure 1.5 The effect of the hydrothermal aging temperature on the pore structure of SBA-15-type materials: (a) low emperature aging (35-60 ºC), main mesopores 5-6 nm, wall thickness 4 nm, micropore volume = 0.2 mLg-1; (b) aging at 80-100 °C, main mesopores 7-9 nm, wall thickness 3.2 nm, micropore volume 0.1 mLg-1; (c) high temperature aging (>120 °C), main mesopores >9 nm, wall thickness 2 nm, no micropores.……...…............11

Figure 1.6 TEM image showing hexagonal arrangement of carbon rods in CMK-3 (left) and CMK-5 carbon tubes templated from SBA-15.….....……….........................................14

Figure 1.7 TEM images of (1) LiCoO2 (2) MgO and (3) Co3O4 synthesized using hard templating method………………………..............................................................................15

Figure 1.8 Schematic representation of the main reaction products obtained from methanol oxidation as a function of acidic-basic character of the catalyst surface.…........19

Figure 2.1 Wide angle X-Ray diffraction patterns of various perovskite oxides (left) and small angle XRD patterns of mesoporous MgO along with parent templates. RG represents samples synthesized by reactive grinding method and CIT represents sample synthesized using citrate method.…........................................................................………28

Figure 2.2 IUPAC classifications of adsorption isotherms (left) and hysteresis loops (right)...............................................................................................................……………..30

Figure 2.3 TEM images of mesoporous hematite, α-Fe2O3, templated using ordered mesoporous silica KIT-6…..............................................................................…………..…32

Figure 2.4 Schematic representation of the experimental set up used for performing the total oxidation of methanol ……………...............................................................................35

Figure 2.5 Schematic representation of the experimental set up used for the dry reforming of methane……...................................................................................................................37

Figure 3.1 N2 physisorption isotherm and pore size distribution (inset) of the parent KIT-6 silica hard template (left). Wide-angle powder XRD patterns of mesoporous perovskite oxides synthesized by use of ordered mesoporous KIT-6 silica aged at 100 ºC as a hard template. The lowest curve corresponds to the LaMnO3 perovskite synthesized without using citric acid (right)……………....................................................................................…46

Figure 3.2 TEM images of nanocast LaMnO3 synthesized by use of ordered mesoporous KIT-6 silica aged at 100 ºC as a hard template…………….................................................47

XIV

Figure 3.3 (A) N2 physisorption isotherms and b) the corresponding pore size distributions of nanocast perovskite oxides synthesized by use of ordered mesoporous KIT-6 silica aged at 100 ºC as a hard template. The isotherms of LaCoO3 and LaFeO3 are plotted with an offset of 60 and 110 cm3g-1, respectively, for clarity. (B) Pore size distributions were calculated from the adsorption branch of the isotherm by using the NLDFT method……………………...........................……………………………………………..……48

Figure 3.4 TPR-H2 (left) and TPD-O2 (right) profiles of nanocast mesoporous perovskite oxides synthesized by use of ordered mesoporous KIT-6 silica aged at 100 ºC as a hard template……………............................................................................................................49

Figure 3.5 Methanol conversion profiles as a function of temperature over LaMnO3, LaCoO3, and LaFeO3 perovskites synthesized by using the nanocasting method (left). Methanol conversion profiles as a function of temperature over LaMnO3 perovskites synthesized by using the nanocasting method, the reactive grinding method (RG), and the conventional citrate process (C). The empty symbols represent the recalculated values of conversion for LaMnO3 synthesized by using the nanocasting method (Calculated KIT-6), the reactive grinding method (Calculated-RG), and the conventional citrate process (Calculated-C)…......................................................................................................………52

Figure 3.6 A comparison of the N2 physisorption isotherms (A) and wide angle XRD patterns (B) of the nanocast LaMnO3 perovskite before and after the catalytic test……...........................................................................................................................…53

Figure 3.7 Comparison between fresh catalyst and used catalyst for methanol conversion over LaMnO3 perovskites synthesised using nanocasting. The open symbols represent the calculated value of conversion…….....................................................................................53

Figure 3.8 (A) Methanol conversion over nanocast LaMnO3 at different space velocities. The values of the space velocities increase at rates of 19500, 39100, 58600, and 78200 h-

1from left to right. (B) Cross-plotting the values of experimental conversions obtained for nanocast LaMnO3 at selected temperatures as a function of pseudo-contact time. Points represent experimental data, and lines are calculated by using Equation (4.1). (C) The numerical values of rates obtained are represented as a function of the partial pressure of methanol; 1 atm=101325 Pa and (D) Arrhenius plot of the rate constant k obtained for nanocast LaMnO3………….................................................................................................55

Figure 4.1 N2 physisorption isotherms (-196 °C) and the corresponding pore size distributions (right) of ordered mesoporous SBA-15 silica hard templates. Pore size distributions were calculated from the adsorption branch of the isotherm using the NLDFT method.………………………………………………………………………………….…….......67

Figure 4.2 Wide angle powder XRD patterns of mesoporous LaMnO3 perovskite oxides synthesized by use of ordered mesoporous SBA-15 as the hard template. The numbers denote the aging temperature of the template……….……………………………………….68

Figure 4.3 TEM images of LaMnO3-35 (a, b), LaMnO3-100 (c, d) and LaMnO3-140 (e, f)…………………………………………………………………………………………………..69

Figure 4.4 N2 physisorption isotherms (-196°) and the corresponding pore size distributions (right) of nanocast perovskite oxides synthesized by use of ordered mesoporous SBA-15 silica hard templates. Pore size distributions were calculated from the adsorption branch of the isotherm using the NLDFT method.……………………………….70

XV

Figure 4.5 N2 physisorption isotherms (-196 °C) and the corresponding NLDFT theoretical isotherms (colour) of nanocast LaMnO3 perovskite oxides synthesized by use of ordered mesoporous SBA-15 silica hard templates. …....................................................................71

Figure 4.6 (A) TPR-H2 and (B) TPD-O2 profiles of nanocast mesoporous perovskite oxides synthesized by use of ordered mesoporousSBA-15 silica hard templates aged at different temperatures…………….....................................................................................................73

Figure 4.7 Methanol conversion profiles as a function of temperature over nanocast LaMnO3 perovskites synthesized using SBA-15 silica hard template aged at different temperatures (GHSV = 39100 h-1).……………...................................................................75

Figure 4.8 Temperature dependent methanol conversion profiles for the total oxidation of methanol over nanocast LaMnO3-35 (left) LaMnO3-100 (middle) and LaMnO3-140 (right) at different space velocities ……………..................................................................................76

Figure 4.9 Cross-plotting the values of experimental conversions at selected temperatures as a function of pseudo-contact time obtained for nanocast LaMnO3. Points represent experimental data, and lines are calculated by using Equation (6.1). The numerical values of rates obtained in each case are represented as a function of the partial pressure of methanol (right).…………………………..............................................................................77

Figure 4.10 Arrhenius plots for the rate constant k obtained for nanocast LaMnO3-35, LaMnO3-100 and LaMnO3-140. The linear correlation between the pre-exponential factor and the specific surface area shown on the right.….……...................................................78

Figure 5.1 N2 physisorption isotherms (-196 °C) and the corresponding pore size distributions (bottom) of ordered mesoporous KIT-6 silica hard templates. Pore size distributions (inset) were calculated from the desorption branch of the isotherm using the NLDFT method.…………………………………………………………………………………90

Figure 5.2 Wide-angle powder XRD patterns of mesoporous perovskite oxides synthesized by use of ordered mesoporous KIT-6 silica aged at 100 ºC as a hard template. A comparison of the wide angle XRD pattern of nanocast LaMnO3 with its bulk counterpart synthesized by the citrate process is given on the right.………………...............................90

Figure 5.3 TEM images of nanocast LaFeO3, LaCoO3 and LaMnO3 perovskites synthesized using ordered mesoporous KIT-6 aged at 100 ºC as hard tem…………….....91

Figure 5.4 (A) N2 physisorption isotherms and b) pore size distributions of nanocast perovskite oxides synthesized using ordered mesoporous KIT-6 silica aged at 100 ºC as a hard template. The isotherms of LaCoO3 and LaFeO3 are plotted with an offset of 60 and 110 cm3g-1, respectively, for clarity. (B) Pore size distributions were calculated from the adsorption branch of the isotherm using the NLDFT method.…………..............................93

Figure 5.5 N2 physisorption isotherms (left) and pore size distributions (right) LaMnO3-KIT-6 composite after each step of impregnation. The materials were calcined at 500 and 700 ºC after the first and second impregnation respectively. Pore size distributions were calculated from the adsorption branch of the isotherm using the NLDFT method.…………………......................................................................................................94

Figure 5.6 N2 physisorption isotherms (left) and pore size distributions (right) of nanocast LaMnO3 during each step of NaOH treatment. The isotherm of LaMnO3-NaOH-1 is plotted with an offset of 30 cm3g-1, for clarity. Pore size distributions were calculated from the adsorption branch of the isotherm using NLDFT method.……...........................................94

Figure 5.7 29Si MAS NMR spectra of LaMnO3-KIT-6 composite calcined at 700 ºC.……95

XVI

Figure 5.8 Representative SEM images of the nanocast perovskites. LaMnO3-C represents the bulk sample synthesized using the citrate process.…………………...........95

Figure 5.9 Representative SEM images of the nanocast LaMnO3 perovskites during each step of template removal …………………….......................................................................96

Figure 5.10 O 1s, La 3d and Si 2s XPS spectra of nanocast mesoporous perovskites after removal of the KIT-6 hard template……………………………………………………………97

Figure 5.11 Fe 2p, Co 2p and Mn 2p core level XPS spectra of nanocast mesoporous perovskites after removal of the KIT-6 hard template………………………………………..98

Figure 5.12 Mn 2p, La 3d, O 1s and Si 2s core level XPS spectra of nanocast mesoporous perovskites during each step of template removal step using 2M NaOH…………………100

Figure 6.1 N2 physisorption isotherm and the corresponding pore size distribution (inset) of ordered mesoporous silica SBA-15 aged at 100 ºC. The pore size distribution was calculated from the adsorption branch of the isotherm using NLDFT method (left). Wide angle XRD patterns of LaNiO3 and Ni/La2O3 obtained by performing reduction at 700 ºC (right). (a) LN-NCR, (b) LN-NC, (c) LN-CR and (d) LN-C.................................................114

Figure 6.2 TEM images of LN-C (a,b), LN-CR (c,d), LN-NC (e,f) and LN-NCR (g,h)...................................................................................................................................115

Figure 6.3 N2 physisorption isotherms of as synthesized and reduced forms of nanocast and bulk LaNiO3 perovskites (left). Pore size distributions calculated from the adsorption branch using NLDFT method for as synthesized and reduced nanocast LaNiO3 are given on the right........................................................................................................................116

Figure 6.4 TPR-H2 profiles of nanocast and bulk LaNiO3 perovskites (left). Ni (3p) XPS spectra of nanocast LaNiO3 and Ni/La2O3 catalysts obtained from nanocast LaNiO3 are given on the right……………………………………………………………….………………117

Figure 6.5 Temperature dependent conversion profiles of CH4 and CO2 over nanocast and bulk LaNiO3 perovskites (left) and Ni/La2O3 catalysts obtained from nanocast and bulk LaNiO3 perovskites are given on the right (GHSV = 2.1 x 105 h-1)....................................120

Figure 6.6 Temperature dependent variation of experimental product ratios obtained for methane dry reforming over nanocast and bulk LaNiO3 perovskites (inset) (GHSV = 2.1 x 105 h-1)...............................................................................................................................120

Figure 6.7 CH4 and CO2 conversions as a function of time on stream at 700 ºC over Ni/La2O3 catalysts derived from nanocast and bulk LaNiO3 (GHSV = 2.1 x 105 h-1). Variation of experimental product ratios under the same conditions are shown on the right……………………………………………………………………………………………..122

Figure 6.8 Thermogravimetric analysis and Raman spectra (inset) of the catalysts after

stability tests for 48 hours (left). N2 physisorption isotherms and wide angle XRD patterns

(inset) of Ni/La2O3 cataysts obtained from nanocast and bulk LaNiO3 after 48 hours on

stream at 700 ºC (right). (* represents La silicates and ● represents La(OH)3)…………..123

XVII

ABBREVIATIONS

BET – Brunauer Emmett Teller

CMC – Critical Miscelle Concentration

CMT – Critical Miscelle Tempearture

CMK – Carbon Mesostructures from Korea

KIT-6 – Korea Institute of Technology No.6

MCM-41 – Mobil Composition of Matter No. 41

NLDFT – Non-Local Density Functional Theory

PEO – Poly Ethylene Oxide

PPO – Poly Propylene Oxide

SBA-15 – Santa Barbara No. 15

SEM – Scanning Electron Microscopy

TEM – Transmission Electron Microscopy

TPR – Temperature Programmed Reduction

TPD – Temperature Programmed Desorption

XPS – X-ray Photoelectron Spectroscopy

XRD – X-Ray Diffraction

XVIII

XIX

ACKNOWLEDGEMENTS

First of all I would like to express my gratitude towards my supervisor Prof.

Freddy Kleitz for giving me the opportunity to work in his research group. Not least,

I would like to express my gratefulness towards my co-supervisor Prof. Serge

Kaliaguine for giving me the occasion to explore the exciting theme of catalysis. I

am deeply grateful towards both my supervisors for all the advices, motivation and

guidance I received during the entire period of my research. I am indebted to them

for the freedom I received, to explore various aspects of material science, apart

from my dissertation topic.

I would like to thank Mr. Gilles Lemay, Dr. Bendaoud Nohair and Dr. Zahra

Sarshar for the assistance I received from them on various laboratory aspects in

Chemical Engineering, especially for the training to handle various experimental

setups.

Further I would like to express my gratefulness to Dr, Pascale Chevallier,

Mr. Jean Frenette, Mr. Richard Janvier, Mr. Alain Adnot and Mr. André Ferland for

their assistance in various characterization techniques such as XPS, XRD and

electron microscopy.

Also I would like to gratefully acknowledge Dr. Francois Berubé, Dr. Benoit

Levasseur and Mr. Remy Guillet-Nicolas for their kind friendship, stimulating

discussions and laboratory training during the initial period of my stay in Québec.

I wish to thank all my colleagues belonging to Kleitz group and Kaliaguine

group for all the help that I received.

Finally, I express my sincere gratitude to all the Professors, research

professionals, technicians, administrative staff and all other members of the

Department of Chemistry and Department of Chemical Engineering, for all the

valuable help that I received, during the entire period of my dissertation in

Université Laval.

I wish to thank my family and friends for their valuable support,

understanding and patience.

XX

XXI

PREFACE

For the present time the major issues and challenges that we face are the

ones concerning environment and energy. Most of our energy demands are

currently dealt with using fossil fuels. On one hand the progressive depletion of the

fossil fuel reserves puts forward the need for alternative renewable and sustainable

energy resources. On the other hand the harmful effects produced from the

expulsion of green house gases and volatile organic compounds inherent with the

burning of hydrocarbons contained in the fossil fuels is taking place on a large

scale. Technologies that can tackle this pollution resulting from the industrial and

automobile emissions are highly demanding and the need of the hour. The

development of such technologies depends on high throughput catalytic

conversions. Since catalysts can accelerate the degradation or transformation

reactions, a wide range of studies were performed in the past regarding the

synthesis of durable, cost effective and efficient catalytic systems for the

development of sustainable chemical processes. The objective of this doctoral

thesis will be to develop novel catalysts for various end-of-the-stream processes

with an environmental perspective as well as synthetic chemical reactions leading

to important feedstock which can be further processed to value added chemicals.

Perovskite-structured mixed metal oxides are well known cost effective and

efficient alternatives to noble metals for various catalytic processes, resulting from

their high temperature stability and interesting physical and surface properties. As

of now, most of the existing synthetic methods for this class of materials rely on

high temperature treatments for the formation of the crystalline phase. These high

temperature exposures lead to particle sintering, resulting in materials with lower

values of specific surface area. Since specific surface area is one of the most

important parameters influencing the efficiency of a catalyst, it is challenging to

develop novel strategies to synthesize perovskite oxides with enhanced values of

specific surface area. This issue was taken up and in the present project the first

goal was to obtain mesoporous perovskite structured mixed metal oxides with high

specific surface area.

XXII

Initial investigations regarding the catalytic efficiency of these materials were

performed for the removal of volatile organic compounds. In this study, methanol, a

comparatively simple molecule was used as the model compound. Efficiency of

these mesoporous perovskites as catalysts will be discussed for the total oxidation

of methanol along with the deeper insights obtained from the kinetic data

processing. Correlation of the observed catalytic activity was made with respect to

the specific surface area and amount of oxygen species. Comparisons were

performed by varying the B site metallic cation in the perovskite structure (Mn, Fe

and Co) as well as with similar bulk compositions synthesized using the

conventional citrate process and reactive grinding method. Further a more

industrially relevant reaction designated as dry reforming (also called CO2

reforming) of methane was chosen for the second stage. This reaction involves the

conversion of two green house gases (CH4 and CO2) into syngas (CO + H2), which

is a valuable feedstock that can be further processed into various value added

chemicals. Ni based perovskite compositions were chosen for this study. In this

case mesoporous perovskite is presented more as a catalyst precursor than as a

catalyst itself. Also, comparisons were performed with the bulk counterpart

synthesized by the conventional citrate process regarding reactant gas conversion

and long term stability.

Briefly, besides the present introductory chapter, this thesis includes the following

ones:

Chapter 1 gives an introduction about perovskite structured mixed metal oxides,

an overview of the nanocasting strategy which is the synthetic protocol used in this

study and a short review of the two catalytic reactions performed, i.e., total

oxidation of methanol and dry reforming of methane.

Chapter 2 discusses the specific experimental methods employed for the

synthesis of mesoporous perovskites by nanocasting followed by a presentation of

the characterization methods used in this study. Also details of the specific

experimental set up regarding the catalytic reactions used for this study are

depicted.

XXIII

Chapter 3 gives the summary of initial results obtained for total oxidation of

methanol over nanocast mesoporous perovskites synthesized using ordered

mesoporous silica KIT-6 as hard template, along with the results obtained from

kinetic data processing

Chapter 4 describes the effects of variation of the template (SBA-15) parameters

on the nanocast perovskites with emphasis on catalytic and kinetic results

Chapter 5 gives a brief account of the surface characterization of mesoporous

perovskite compositions synthesized by nanocasting.

Chapter 6 illustrates the utility of the Ni/La2O3 catalyst obtained from mesoporous

LaNiO3 perovskite synthesized using ordered mesoporous silica SBA-15 as hard

template, as catalyst precursor for dry reforming of methane.

Chapter 7 summarizes the general conclusions obtained followed by some

suggestions as future perspectives

XXIV

1

Chapter 1– Introduction

1.1 Perovskites

1.1.1 Structure and properties

Perovskites are mixed metallic compounds represented by the general

formula ABX3, where A and B represent a large cation and a medium sized cation

respectively whereas X represents an anion.1 Based on the anion in the structure,

the final compound could be an oxide, a sulphide, a hydride or a halide (mostly

fluoride). Anions can also be a mixture of elements; oxynitrides being the most well

known examples. Figure 1.1 depicts the structure of an ideal ABX3 perovskite.

Figure 1.1 Two different representations of the ideal perovskite structure with BX6 octahedron forming the

centre of the cube (left) and BX6 octahedron occupying the corners of the cube with A site atom at the centre of the cube (right).

The perovskite structure can be viewed as the BX6 octahedron forming the

centre of a cube with the larger A atoms occupying the corners or as a cube

consisting of corner sharing BX6 octahedron with A atoms incorporated at the

centre. The coordination number of A and B cations are 12 and 6 respectively. For

2

an ideal perovskite structure, the following relation between ionic radii holds where

rN is the ionic radii of atoms.2

rA+rX = √2 (rB+rX) (1.1)

Ideal structure, however, was found to be retained in very few cases. Orthorhombic

and rhombohedral distortions are commonly found along with less common

distortions – tetragonal, monoclinic and triclinic. The measure of deviation from the

ideal structure can be represented by the “Tolerance factor” as introduced by

Goldschmidt.3 For an ideal perovskite structure, the „t‟ value is found to be unity.

However those with lower t values (0.75 < t ≤ 1) are found to exist.4

t = (rA+ rX) / √2 (rB+rX) (1.2)

Oxides form the most common and interesting compounds with perovskite

structure. Almost all the metallic natural elements in the periodic table are found to

be stable in the perovskite structure. The wide range of properties shown by

perovskite type oxides finds applications in catalysis, magnetism, solid oxide fuel

cells, superconductivity, etc. Proper combination or partial substitution of the A

and/or B site atoms introduces abnormal valences or lattice defects which in turn

gives rise to modifications in their chemical properties. The ready availability of a

family of isomorphic solids with controllable physical properties makes them

attractive for basic research in catalysis.5

Apart from the requirements imposed by the tolerance factor,

electroneutrality is another important property that needs to be maintained for a

perovskite structure. As a rule, the total cationic charge must be balanced by the

total anionic charge. However cationic or anionic nonstoichiometry is often

encountered in this class of materials leading to the formation of lattice defects in

the structure. Oxygen excess nonstoichiometry in perovskite oxides is not as

common as oxygen deficiency because of thermodynamic limitations. The best-

characterized perovskite showing oxidative nonstoichiometry is LaMnO3+λ.6,7

Neutron diffraction studies revealed that oxygen excess in LaMnO3+λ could be

accommodated by the formation of cationic vacancies with partial elimination of La

3

(as La2O3) or by the presence of active metal in an unusual oxidation state (Mn4+

for example).8 On the other hand an example for oxygen deficient perovskite is the

brownmillerite structured LaNiO2.5. In this case, octahedral layers alternate with

one square planar layer to achieve oxygen vacancy ordering.9 Also, tetragonal,

orthorhombic and monoclinic forms of LaCuO3-λ can be obtained by performing

calcinations under inert atmosphere.10 Small size and large charge of the B site

cations induces thermodynamic limitations for the formation of B site vacancies.

However since BO3 array forms a stable network, the structure can tolerate the

absence a minor percentage of A site cations.

1.1.2 Synthesis and catalytic properties

Various strategies were developed for the synthesis of perovskite-structured

oxides. Out of these the choice of a particular method depends on the type of

application expected. Generally, perovskite oxides for catalytic applications contain

a lanthanide ion in the A-site and a transition metal cation in the B-site. For

catalytic applications specific surface area and crystal structure play crucial roles.

For this reason, the synthesis of these materials for catalytic applications always

focused on obtaining crystalline materials with high values of specific surface area.

Broadly, the synthesis methods can be classified into the solid state routes and

liquid – solid routes (Table 1.1).

1.1.2.1 Solid state routes

The oldest method developed for the synthesis of perovskite structured

mixed metal oxides is the ceramic method. In this method, thoroughly mixed

precursors (oxides, hydroxides or carbonates) of the metals are calcined at

elevated temperature required for the crystallization of the perovskite phase (1000

⁰C) for many hours. However, the surface areas of thus synthesized perovskites

were found to be less than 5 m2g-1.11,12 The high temperature used in solid state

reactions, for perovskite crystallization, results in the sintering of particles which in

turn leads to a large grain size and low surface area. Microwave method is another

solid state method developed in the last decade by Rao et al. This method involves

microwave irradiation of the precursors for a few minutes (10 minutes). Even

4

though nanometer sized crystalline perovskites were obtained by this method; no

information regarding the specific surface area was reported by the authors.13

Recently another method, which involves grinding additives developed by

Kaliaguine et al. produced perovskite oxides with surface areas >100 m2/g. Here

thermal energy required for the crystallization of the perovskite structure is

replaced by mechanical energy and thereby limiting the sintering of particles.14,15

High surface area materials were obtained when low temperature was used for the

formation of the perovskite phase. High surface concentrations of OH groups were

observed in perovskites prepared by the reactive grinding method. This method

has the advantage of providing high density of catalytically active sites. However,

in this case also the values of specific surface area are found to decrease at higher

calcination temperatures.

Table 1.1 General methods used for the synthesis of perovskite structured mixed metal oxides. SBET is the

specific surface area determined by the BET method.

Method Composition SBET (m2g-1)

Ceramic11, 12 LaAMnO3

(A = Na, K, Rb)

< 3

Microwave13 LaBO3

(B = Cr, Co, Ni)

-

Reactive

grinding14,15

LaCoFeO3 10 – 105

Coprecipitation16 LaMnO3 15

Complexation17,18 LaCoO3 20

Dispersion on a

support20

LaCoO3/MCM-41 340

5

1.1.2.2 Liquid – solid routes

Since high surface area is a requirement for perovskite oxides synthesized

for catalytic applications, various low temperature synthesis methods were

developed. Among these, co-precipitation16 is a simple method for preparing

agglomeration free powders with an average particle size of 80 nm using

hydroxides as precursors. Complexation and precipitation is another commonly

used method developed to synthesize perovskites with enhanced surface area.

Citric acid is the most common complexing agent used to obtain a highly

homogeneous precursor which on calcination at comparatively low temperature

leads to crystalline perovskite oxides.17,18 Freeze drying and spray drying are other

common liquid – solid routes followed to synthesize perovskites.19 However in all

these cases the specific surface areas of the final products were found to be less

than 25 m2g-1. Attempts were also made to disperse the perovskites on a support

matrix having high specific surface area. For this a high surface area support is

chosen (most commonly silica or alumina) and impregnated with the precursor

solution. Here, the sintering of the particles during high temperature calcination

process is restricted by the support. Kaliaguine et al. reported the synthesis of

LaCoO3 perovskites supported on mesoporous silica MCM-41.20 The sample with

47.5% LaCoO3 possessed a BET surface area of 340 m2g-1. The supported

catalysts showed higher catalytic activity and resistance to SO2 poisoning in the

complete methane oxidation compared to bulk LaCoO3 catalysts. However mass

transfer limitations were observed. Another concern that needs to be taken care of

while using a support is the compatibility of the support with the desired phase.

1.2 Nanocasting

1.2.1 Principles of nanocasting

Nanocasting21 is a procedure in which a mold with relevant structures in the

nanometer scale is filled with another material and the mold is afterwards removed.

In this technique the structure and properties of the templates play a crucial role.

Relatively precise negative replica of the template is created. An advantage of

using the hard template is the fact that the synthesis is relatively easy to control,

6

since the template structures are fixed. Inorganic porous solids such as ordered

mesoporous silica or carbon are mostly used as nanoscale hard templates. The

pore structure of these parent materials is transferred into the solid structure of the

generated porous materials while the walls of the parent become voids. One of the

most versatile materials to be used as hard templates is ordered mesoporous silica

which can be prepared in diverse pore structures and particle morphologies. Since

a three dimensional structure is necessary in the mold to maintain a stable replica,

only silicates having interconnected pores are successful, leading to nanowire

arrays or 3-D frameworks with tunable mesostructures.

Figure 1.2 Schematic representation of the steps involved in the mesostructure evolution in the nanocasting

technique using ordered mesoporous silica as hard template.

Choice of the precursor is of particular importance in nanocasting. A

precursor which does not chemically react with the template needs to be selected.

The conversion of the precursors into a material of desired composition inside the

pores should be simple and with as little volume shrinkage as possible. In order to

facilitate easy diffusion into the mesopores, the precursor needs to be in the

gaseous or liquid form under ambient conditions. The degree of precursor loading

also affects a faithful replication. The interaction between the pore surface and the

precursor generally includes weak interactions like hydrogen bonding, coulombic or

van der Waals interactions.22 Higher pore filling can be achieved by enhancing the

hydrophilicity of the surface. The presence of an OH group in the precursor can be

necessary, since hydrogen bonds can be easily generated with the silanol groups

of the templates. Also it needs to be noted that a very strong interaction between

7

the precursor and the template results in deposition and accumulation of the

precursors near the pore entrances resulting in pore blocking. Much attention

should be paid for selecting the solvent. For silica pore surface with free silanol

groups present, a polar solvent is suitable. This will enhance the diffusion of the

precursor through the pores. The need for a polar solvent is inevitable since the

solubility of the metal salts is high in polar solvents. Ethanol serves as one of the

preferred solvent in the nanocasting process because of its low boiling point and

high solubility of most of the inorganic precursors. Ethanol also has the amphiphilic

property compatible to the silica pore wall surface, which enhances the capillary

force. For the replica to be homogeneous, decomposition products should leave

the samples preferentially via gas phase.

Four different methods of impregnation have been used for the synthesis of

ordered mesoporous non siliceous materials.

1. Wet impregnation - In this method, the template is dispersed as a powder in

a dilute solution (water or ethanol) solvent. The precursor species dissolved

in the solvent is added with stirring and will get diffused into the pores where

they are adsorbed to the pore walls. This often results in a limited loading of

the precursor solution inside the pores and hence requires several

impregnation cycles. After subsequent removal of the solvent, the formation

of the desired phase takes place at high temperature.23

2. Incipient wetness - A saturated solution of the precursor of the same volume

as the pore volume of the template, is used for impregnation. The solution is

drawn into the pores by capillary forces. Since the amount of the precursor

is determined by the pore volume of the template, no precursor material is

expected to be deposited on the outer surface. Incipient wetness technique

usually leads to higher loadings than wet impregnation24.

3. Dual-solvent - Here two different solvents are to be used to disperse the

silica mold and to dissolve the metal precursors. Generally, the silica

template is suspended in a non polar solvent and an aqueous solution of the

metal precursor with respect to the pore volume of the template has to be

8

slowly added. The precursor is expected to move into the pores during

stirring. This method helped to improve the filling efficiency of the porous

channels and homogeneity of the products. Anne Davidson and coworkers

utilized this method for the nanocasted synthesis of Co3O4 and β-MnO2

using water and dry hexane as solvents.25,26

4. Solid-liquid - In this method, a mixture of the template and the precursor in

the solid phase, without using any solvents, is subjected to heat treatment.

The essential condition for this method is that the melting point of the

precursor should be lower than its decomposition temperature. On reaching

the melting point, the precursor gets converted to the liquid phase and

enters the pores through capillary action. As the temperature is raised

further, the precursor will start decomposing to reach the final state. Yue

and Zhou synthesized porous single crystals of Co3O4, NiO, CeO2, and

Cr2O3 using this method.27

In order to obtain the final replica, the template needs to be removed without

affecting the cast. In the case of mesoporous silica templates, leaching with

different agents (HF or NaOH) depending on the stability of the final product can be

used. Using HF will facilitate the complete removal of silica. However because of

the severe hazardous nature of HF, NaOH is preferred for those situations where

both these leaching agents can be applied. However a small amount of residual

silicon is sometimes observed when NaOH is used as the leaching agent. When

carbon is used as the hard template, simple heat treatment under an oxidizing

atmosphere is used to remove the template.

1.2.2 Ordered mesoporous silica templates

Considerable milestones were achieved regarding the synthesis,

characterization and textural parameter control of ordered mesoporous silica (pore

size: 2 – 50 nm according to IUPAC28) during the past two decades, starting from

the initial discovery of MCM-41 in early nineties.29 The syntheses of these

materials rely on the formation of miscellar aggregates of molecular surfactants or

block copolymers. This self assembly process is followed by the deposition of

9

inorganic phase where a cooperative self assembly takes place between the

template and the inorganic precursor. Finally, selective removal of the template

results in the formation of ordered mesoporous silica materials.30

A wide variety of surfactants including ionic (cationic and anionic), neutral

and non-ionic block polymers can be used. Non-ionic surfactants which are

available in a wide variety of chemical structures will be a suitable choice owing to

their biodegradability, low price and non toxicity. Also, it is much easier to obtain

ordered mesoporous silica with interconnected pore systems when non-ionic

surfactants such as triblock copolymers are used as soft templates. Generally, the

synthesis of mesoporous silica under non-ionic surfactants takes place under

acidic conditions. The mesophase formation can be controlled by varying the

hydrophilic/hydrophobic ratio of the structure directing agent, solution pH and

temperature. Moreover, the use of non- ionic surfactants results in improved

hydrothermal stability and enables more straightforward tailoring of the pore

parameters. A silica source is added to a dilute micellar solution and

polymerization of the silica is induced by an acid. The van der Waals or hydrogen

bonding interactions between the polymerizing silica and the amphiphile result in

the precipitation of silica-amphiphile mesophase composite. Porous structure

results when the structure directing agent is removed by calcination, solvent

extraction or microwave treatment.31 Calcination is the most common method in

which the material is treated at high temperature (generally 550 ºC) to burn away

the organic species.

Figure 1.3 A simplistic mechanistic pathway showing the self assembly of formation of mesoporous materials

As a result of tremendous research efforts, a large variety of ordered structures

were developed by the soft templating method through the non-covalent

10

intermolecular interactions, including 2D hexagonal32,33 (p6mm), 3D cubic34-37

(Im3m, Ia3d, Fm3m), etc (Figure 1.4). Another advantage of this method is that the

resulting periodic structure is restricted to the nanometer scale. The pore structure,

pore width and wall thickness can be fine tuned by varying one or more of the

synthesis parameters. Varying the concentration of the templating agent or

changing the composition of the copolymers are found successful to fine tune the

pore size and wall thickness of the material.32,38

Changes in the molecular geometry and the polymer chain length also allow

the fine tuning of the pore size. Pore size control of mesoporous materials can be

achieved by varying the time and temperature of the synthesis and hydrothermal

treatment. An increase in the synthesis temperature leads to an increase in

hydrophobicity of the micelle core volume which leads to an increase in pore size

and decrease in wall thickness.39 An increase in aging temperature also leads to

an increase in pore diameter and pore volume and modifies the intra-wall porosity.

Figure 1.4 Pore models of mesostructures with symmetries (A) p6mm, (B) Ia3d, (C) Pm3n, (D) Im3n, (E) Fd3m, (F) Fm3m

32

A longer period of hydrothermal treatment also induces a similar effect.40

The ratio between the silica source and surfactant can be varied to change the

degree of pore connectivity. Increasing the silica content in the reaction mixture

has been found to decrease the pore size, pore volume and surface area. The

11

variation in network connectivity with respect to the change in silica/surfactant ratio

has been evidenced by imaging platinum replicas of SBA-15, with TEM.33 The

thickening of the pore walls of SBA-15 and KIT-6 has been observed as a result of

increasing the silica/surfactant ratio.41

Addition of acid or base during the synthesis leads to the modification of the

pH of the solution. Since the degree of condensation and polymerization of the

inorganic species is pH dependent, the modification of the pore dimensions is

obvious.42 Salt addition also affects the properties of the resulting mesoporous

solids. Both Critical Micelle Concentration (CMC) and Critical Micelle Temperature

(CMT) of the block polymer can be decreased on salt addition.30 The solubilization

of hydrophobic additives inside the core of the micellar assembly can be employed

to increase the pore size. Mesitylene (trimethylbenzene) is the most widely used

additive. Variations of cohesive properties of the solvent by performing the

synthesis in mixed solvents can also be used to fine-tune the pore size. The

addition of butanol as cosurfactant coupled with a low HCl concentration leads to

the formation of cubic Ia3d KIT-6 mesophase.34

Figure 1.5 The effects of the hydrothermal aging temperature on the pore structure of SBA-15-type materials:

(a) low temperature aging (35-60 ºC), main mesopores 5-6 nm, wall thickness 4 nm, micropore volume = 0.2 mL/g; (b) aging at 80-100 °C, main mesopores 7-9 nm, wall thickness 3.2 nm, micropore volume = 0.1 mL/g; (c) high temperature aging (>120 °C), main mesopores >9 nm, wall thickness 2 nm, no micropores.

36

12

As mentioned above, calcination is the most common method used to

remove the organic surfactant templates. As-synthesized materials are heated

under oxygen or air at a temperature above 550 ⁰C to decompose the organic

surfactants. However the condensation of surface silanol groups that occur during

calcination results in the lowering of the unit cell parameter and an increase in the

surface hydrophobicity of the final material. The framework contraction has direct

influence on mesopore size and the complementary intra-wall porosity.43 Extraction

is another method to remove the surfactants. This can be achieved by liquid

extraction, acid treatment, oxygen plasma treatment or supercritical fluid extraction.

The framework-surfactant interactions are weak in SBA-type materials. Such weak

interactions are favorable for solvent extractions. The advantages of this method

are that the physicochemical properties are not modified and the framework

contraction is limited. However solvent extraction does not remove the templating

species completely and as a result the intrawall microporosity is not often

completely developed.30 Methods were also developed to liberate the main

mesopores first and then the intrawall porosity.44 Ether cleavage with sulfuric acid

solution removes the accessible polymer located in the large mesopore system,

where as the poly ethylene oxide (PEO) groups should remain essentially

unaffected. A mild calcination at 200⁰C then allows the removal of the remaining

PEO groups in the walls and makes the micropores accessible. This acid treatment

prevents any pronounced shrinkage of the mesostructure which in turn affords

materials with larger pore dimensions than the regularly calcined SBA-15. More

importantly, the strongly acidic medium not only induces a more pronounced

condensation of the silica but also permits the generation of a high density of

silanol groups on the surface.

1.2.3 Overview nanocasting

One of the major applications of ordered mesoporous silica is their use as

hard templates for the synthesis of various other mesostructured compositions.

This method designated as nanocasting or hard templating is widely used by

research groups around the world for the synthesis of ordered mesoporous non

13

silicious materials as an alternative to the soft templating strategies. This approach

enables to overcome limitations of the direct templating processes with soft

micellar surfactant aggregates especially in the case of carbons or transition metal-

based materials that are very sensitive to thermal treatment conditions and redox

reactions. Nanocasting is comparatively easier and predictable since the

nanoscale pore structure is fixed.

Knox et al. were the first to synthesize porous carbon via nanocasting

method, using silica gel or porous glass as templates.45 However, the templates as

well as replicas contained a disordered pore structure. The first successful

synthesis of nanocast mesoporous carbon with an ordered pore structure was

reported in 1999, by Ryoo et al., where MCM-48 was used as a template. CMK-1

thus obtained was not an exact replica of MCM-48 since its symmetry was

somewhat lower.46 This is because the pore architecture of the MCM-48 template

consists of two different channel systems which are not interconnected. After

template removal, the carbon sub networks formed inside the disconnected silica

channels are joined by displacement with one another. Similarly when MCM-41,

consisting of linear mesopores arranged parallel to each other was replicated,

bundles of linear rods of the respective product were obtained.

Many studies were carried out to synthesize mesoporous carbons with

ordered structures designated as CMK-n series, using different silicas. Generally,

the synthetic procedure for nanocast ordered mesoporous carbon involves

impregnation of mesoporous silica as a template with carbon precursor.

Subsequent polymerization followed by carbonization of the precursor in the pore

system results in a carbon-silica composite. Finally, the mesoporous replica can be

obtained after removal of the silica template by HF or NaOH leaching. One should

keep in mind that carbon precursors that have high carbon yield and do not just

decompose during the carbonization step should be selected. Suitable carbon

precursors were found to be sucrose, furfuryl alcohol, phenolic resin, mesophase

pitch, etc.41,47-49 Sucrose is a convenient precursor, but its polymerization is difficult

to control with H2SO4 as catalyst. Also sucrose based carbon materials have

14

systematically smaller lattice parameters and less dense frameworks. Mesophase

pitch consisting of well stacked layers of carbon rings is interesting since it results

in the formation of graphitized carbon materials. Furfuryl alcohol is attractive

because it is a liquid at room temperature and is miscible in many organic solvents.

It can be easily polymerized especially in presence of an acid catalyst.

Figure 1.6 TEM image showing hexagonal arrangement of carbon rods in CMK-3 (left) and CMK-5 carbon

tubes templated from SBA-15.21,50

The symmetry of the templates was retained when silica materials with

interconnected pore system were used. The hexagonal p6mm symmetry of SBA-

15 was preserved in its replica CMK-3 and the cubic Ia3d symmetry of mesoporous

silica, KIT-6 was found to be preserved in CMK-8.46 CMK-3 consists of hexagonal

arrangement of 1D carbon rods as shown in Figure 1.6. Interestingly, by varying

the filling degree of the carbon precursor, the structure of the mesoporous carbon

replica can easily be varied. If the pore system of the Al-containing SBA-15 is only

coated by the carbon precursor, rather than being completely filled, a surface-

templated mesoporous carbon, named CMK-5, with an array of hollow carbon

tubes can be obtained.50 A TEM image and structure model of CMK-5 is presented

in Figure 1.6. Two different types of pores are observed in CMK-5, one generated

in the inner part of the channels which are not filled with carbon precursor and the

other one is obtained from where the silica walls of the SBA-15 template had

previously been. For this reason CMK-5 shows extremely large surface areas and

large pore volumes, which gives this material great potential in adsorption and as a

catalyst support.51

The successful synthesis of a large variety of ordered mesoporous carbon

encouraged the extension of this method for the synthesis of other non siliceous

15

compositions. In particular, a broad range of scientific endeavor reflects a growing

interest in ordered mesoporous metal oxides. As in the case of carbon the choice

of a hard template is essential for nanocast mesoporous oxides. So far KIT-6 and

SBA-15 mesoporous silica are frequently used as the templates and metal salts

are used as the precursors. There are few reports on nanocast mesoporous metal

oxides applying cage-type cubic mesoporous silica SBA-16. The use of a single

metal precursor will result in the formation of a single metal oxide where as the use

of different metal salts as precursors in the correct stoichiometry can result in the

formation of high surface area mixed metal oxides such as spinels or perovskites.

A major factor which determines the resulting structure is the filling degree of the

voids and template removal conditions. Materials with higher surface areas are

obtained for sufficient loadings of the precursors.

Crystalline mesoporous CeO2 with high thermal stability was synthesized

using SBA-15 or KIT-6 as the hard templates and inorganic salt CeCl3.7H2O as the

precursor. The products have very narrow pore-size distribution, large surface area

and pore volume.52 Zhao et al. reported the synthesis of nanoarrays of In2O3,

Cr2O3, Fe2O3, Co3O4, CeO2 and NiO using microwave digested SBA-15, SBA-16

and FDU-1 silica materials as hard templates.53 All oxides synthesized at a

temperature of 650⁰C, exhibited long range order except Cr2O3 and NiO. Very high

surface areas up to 137 m2g-1, determined using the BET method and pore

volumes up to 0.4 cm3g-1 were obtained. Wang et al. reported the synthesis of

Co3O4 using vinyl functionalized KIT-6 as hard template with BET surface area 122

m2g-1 and pore volume 0.2 cm3g-1.54

Figure 1.7 TEM images of (1) LiCoO256

(2) MgO64

and (3) Co3O458

synthesized using the hard templating

method.

16

The influence of the network interconnectivities of the parent SBA-15 and KIT-6

silica materials and the loading of the precursors for the tuning of the nanocast

materials were studied by Schuth and coworkers.55 One of the limitations of the

nanocasting method is that the precursor should be selected such that it should not

react with the silica template, a problem which renders it difficult to synthesize

compounds containing alkali metals. Jiao et al. has reported that post synthesis

reactions can be applied to incorporate alkali metals while retaining the

nanostructured morphology.56 Mesostructured Co3O4 has been synthesized using

SBA-15 and KIT-6 as hard templates. After template removal, Co3O4 was treated

with LiOH to obtain mesostructured LiCoO2 (Figure 1.7). The BET surface areas of

the samples were found to be 70 and 92 m2g-1 for the samples templated from

SBA-15 and KIT-6 respectively. It was also shown that by varying the calcination

temperature during the synthesis of hard templates, the pore sizes and wall

thickness of the mesoporous oxides can be tuned.

Regarding the synthesis of mixed metal oxides using multiple precursors,

various compositions with spinel and perovskite structure are reported. Cabo et al.

synthesized NiCo2O4 spinels using ordered mesoporous silica SBA-15 and KIT-6

as hard templates.57 Hoang et al. reported the synthesis of NiFe2O4 and CuFe2O4

spinels by the method of nanocasting using ordered mesoporous silica KIT-6 and

SBA-15 as hard templates.58 These authors also reported the morphology

controlled synthesis of spherical mesostructures using MCM-48 nanospheres as

hard templates. The first synthesis of mesoporous perovskites were reported by

Schwickardi et al.59 These authors reported the synthesis of LaFeO3 with

disordered pore structure using carbon as a hard template. More recently ordered

mesostructures of LaCoO3 perovskites were reported by Wang et al.60 using vinyl

functionalized KIT-6 as hard template.

Nanocast materials are found to be highly efficient for a large number of

catalytic applications, either as catalysts or as supports. Regarding catalytic

oxidation reactions, CO oxidation is the most studied one using nanocast catalysts.

Shen et al. synthesized mesoporous CeO2 using ordered mesoporous KIT-6 as

17

hard template. These materials with a specific surface area 112 m2g-1 showed

excellent CO conversion efficiency in comparison with conventional CeO2

catalysts. These authors also showed that by loading 20 % CuO on these nanocast

CeO2 replicas, T50 (the temperature required to attain 50 % conversion) for the

reaction was decreased to 115 ºC.61 Among other ordered mesoporous oxides

studied for CO oxidation, Co3O4, β-MnO2 and NiO showed considerable extent of

conversion below 0 ºC.62 Zhu et al. used SBA-15 as hard template to synthesize

mesoporous CuCo2O4, MnCo2O4 and NiCo2O4. In their study it was observed that

CuCo2O4 and MnCo2O4 exhibit high activities and good stability in CO oxidation.63

Tuysuz et al. synthesized mesoporous Co3O4, with highly ordered pore structure

and enhanced specific surface area using ordered mesoporous KIT-6 silica as hard

template. 100 % conversion of CO to CO2 at a space velocity of 18 000 mL gcat-1 s-

1, was observed around room temperature. The activity was found to be dependent

on the textural properties of the catalysts in which the catalyst with the highest

specific surface area and the most open pore system was found to be the best.

However about 50 % of activity loss was observed over 4 h during the reaction.64

Roggenbuck et al. synthesized mesoporous CeO2 using CMK-3 carbon as a

hard template. This material with a specific surface area of 148 m2g-1 when used

as a catalyst for methanol decomposition showed substantially higher performance

than that of a commercial CeO2 with a low surface area of 6 m2g-1.65 Wang et al.

synthesized a series of mesoporous metal oxides by using vinyl-functionalized KIT-

6 as a hard template and demonstrated that mesoporous Cr2O3 (surface area 113

m2g–1) exhibits higher activity than commercial Cr2O3 (10 m2g–1) in toluene

combustion.66 Xia et al. demonstrated the application of mesoporous Cr2O3 in the

combustion of toluene, ethyl acetate, formaldehyde, acetone, and methanol,

mesoporous α- Fe2O3 in the oxidation of acetone and methanol and mesoporous

Co3O4 in the oxidation of toluene and methanol.67 Also, LaCoO3 perovskites

synthesized using vinyl functionalized KIT-6 as hard template was found to show

extremely high value of specific surface area (96 m2g-1) and excellent activity for

the total oxidation of methane in comparison with the bulk catalyst synthesized

using the citrate process.60 Nanocast mesoporous oxides were found to be efficient

18

for various photocatalytic applications also. Crystalline mesoporous anatase was

prepared using KIT-6 and SBA-15 as hard templates and exhibited superior

photocatalytic activity using the oxidation of toluene to benzaldehyde in liquid

phase as an example68

Efficiency of these nanocast oxides was also proven in various energy

conversion and storage applications. SnO2 with a 3D bicontinuous cubic

mesostructure and high surface area was synthesized using KIT-6 as a hard

template and was employed as a photo anode for Dye Sensitized Solar Cell

(DSSC).69 This improved the open-circuit voltage, short-circuit current, and fill

factor, leading to more than a 3-fold improvement in the energy conversion

efficiency. A large number of ordered mesoporous oxides were found to be highly

efficient as Li ion battery cathodes. Ordered mesoporous β-MnO2 was prepared

using KIT-6 as a hard template, and used as cathode for Li-ion batteries.70,71 These

materials with a highly ordered pore structure was capable of reversibly

accommodating lithium, up to a composition of Li0.92MnO2. LiCoO2 spinel prepared

by the lithiation of nanocast mesoporous Co3O4 exhibited better properties as a

cathode compared with low temperature LiCoO2 nanoparticulates.56 The

mesoporous Li1.12Mn1.88O4 retained the ability to cycle, storing 50 % more Li than

the equivalent bulk phase at a charge–discharge rate of 3000 mAg-1.71 Ordered

mesoporous Co3O4 nanoarrays prepared with SBA-15 as a hard template provided

a capacitance of about 250 Fg-1, approximately four times larger than that of Co3O4

prepared by direct calcination of the nitrate precursor.72 This was further improved

when mesoporous Co3O4 templated from KIT-6 was used.73

1.3 Catalytic tests

1.3.1 Total oxidation of methanol

Oxidation of volatile organic compounds (VOC) and in particular, of

methanol is a highly interesting reaction from an industrial point of view. Depending

on the reaction conditions and the catalyst properties, different products can be

obtained such as formaldehyde (HCHO), formic acid (HCOOH), carbon dioxide

(CO2) etc. In general, the total oxidation of methanol is favored by the increased

19

nucleophilic character of the catalyst.74 On the other hand, amphoteric active sites

favor partial oxidation products such as formaldehyde or methyl formate. For this

reason methanol oxidation can also be considered as a characterization technique

for the acidity/basicity of the active sites. In this section, emphasis will be laid on

metallic catalysts especially those with perovskite structure for the oxidation of

methanol.

CH3OH + ½ O2 HCHO + H2O (1.3)

CH3OH + O2 HCOOH + H2O (1.4)

CH3OH + 3/2 O2 CO2 + H2O (1.5)

Figure 1.8 Schematic representation of the main reaction products obtained from methanol oxidation as a

function of acidic-basic character of the catalyst surface.74

As early as in 1953, perovskite structured mixed metal oxides were used as

catalysts for the oxidation of CO and other hydrocarbons.75 However studies

concerning the gas phase oxidation of methanol are very little. The initial use of

perovskites for the oxidation of methanol appeared in the 1980‟s. Arakawa et al.

synthesized a series of LnFeO3 (Ln = La, Pr, Nd, Sm, Eu and Gd) perovskites by

varying the A site metal cation and showed that the change of rare earth in the

structure induces some effect in the catalytic activity for the oxidation of

20

methanol.76 The same authors later obtained similar results for LnCoO3 (Ln = La,

Nd, Sm and Eu).77 Based on conductivity measurements and kinetic calculations

they showed that the adsorption of methanol on the catalysts surface is

dissociative and takes place in an ionic manner. A correlation of specific surface

area with catalytic activity was established by Kim et al.78 These authors observed

that an increase in calcination temperature led to a decrease in specific surface

area which in turn influenced the catalytic activity. Further, Nitadori et al. observed

better efficiency of Mn-based perovskites in comparison with Co in the B site79 and

for Ni-based compositions full conversions were observed only at 425 ºC with 90 %

selectivity to CO2.80

More recently, the promoting effect of various metals loaded on the

perovskite structure were investigated. Wang et al. showed that the deposition of 6

% Ag on the surface of LaSrMnO3 led to the reduction of 50 ºC in the temperature

required for the total oxidation of methanol. The authors claimed that the deposition

of Ag resulted in the enhancement of Schottky defects leading to an increased

amount of Mn4+ in the lattice. The presence of these Mn4+ species that can be

reduced at a lower temperature accounts for the reduction of conversion

temperature for the total oxidation of methanol.81 The influence on catalytic activity

of the ionic character of the bond between the metal cation and the surface oxygen

in the perovskite structure was highlighted independently for LaSrMnO3 by Wang

et al. and LaSrCoFeO3 by Galenda et al.82 For obtaining materials with enhanced

values of specific surface area Makshina et al. synthesized LaCoO3 supported on

MCM-41. Their work showed that full conversion occurred at temperature as low as

200 ºC with a CO2 selectivity of 95 %. This higher efficiency was attributed to the

very large surface area of the material ( 1000 m2g-1).83

Recently, studies were performed for the total oxidation of methanol by

Levasseur and Kaliaguine over perovskite-structured oxides synthesized by

reactive grinding. These results indicate that the highest efficiency observed for the

LaMnO3 catalyst in comparison with LaCoO3 and LaFeO3 synthesized by the

reactive grinding method and this variation in activity is correlated to the high

21

surface density of α-O2.84 Further, these authors performed methanol oxidation by

varying the A site metal in the AMnO3 structure. The effects induced by the nature

of the A cation were indirectly linked to the amount of oxygen desorbed.85

1.3.2 Dry reforming of methane

There has been an increasing global concern for the increase in

atmospheric concentration of anthropogenic CO2. Although the precise CO2

emission flux is uncertain, there are several indicators which raise the possibility

that anthropogenic greenhouse gas emissions are causing a global problem.

Consequently, there has been an enhanced interest in a better understanding of

CH4 and CO2 removal / utilization as well as the influence of these gases in the

atmosphere. Of the various strategies available to control the CO2 and CH4 in

atmosphere, dry reforming (otherwise called CO2 reforming) is unique since it

interconnects the sequestration of these greenhouse gases CO2 in the atmosphere

with their chemical utilization as a feed stock for the production of value added

chemicals. CH4 and CO2 are relatively inexpensive due to their natural abundance

and this enhances the importance for the development of a strategy for conversion

of these two molecules to higher-value compounds.

Fischer and Tropsch have reported on CO2 reforming of CH4 on various

base metals as early as in 1928. However, extensive investigation started from the

1990‟s because of the increasing environmental and energy concerns. It is well

known that this reaction can be catalyzed by noble metals or by much more cost

effective transition metals, especially Ni. However, there is a serious problem of

catalyst deactivation associated with these catalysts. According to Hu et al. carbon

formation rather than sintering is the main cause of deactivation for platinum

containing catalysts.86 A large number of investigations have been conducted to

develop catalysts overcoming the deactivation problems.87-89 Of particular

importance is the Ni based catalyst systems since this composition shows high

activity comparable to that of noble metals and a large number of strategies were

reported to minimize the formation of coke on the catalyst surface. However,

22

severe deactivation is still the major barrier preventing this technology from large-

scale industrial application.

Various strategies were developed either to avoid or to minimize the coke

deposition on the catalyst surface during dry reforming conditions. Rostrup-Nielsen

proposed that the coke resistance of the Ni-based catalysts can be improved by

enhancing the adsorption of CO2, by enhancing the rate of the surface reaction, or

by decreasing the rate and degree of methane activation and dissociation.90 One

way to achieve this is by using a promoter. Basic promoters like alkali metals can

significantly enhance the CO2 adsorption on the catalyst. Small amounts of noble

metals can also be used as promoters in order to enhance the coke resistance of

Ni-based catalysts. Liu et al. reported that Pt addition on the Ni/MCM-41 improved

the coke resistance for CO2 reforming.91 However investigations were much

focused on developing cheaper alternatives. Lanthanum and praseodymium were

applied as promoters for CO2 reforming of methane over Ni catalysts.92,93 La2O3 is

found to activate CO2 leading to the formation of CO and O, which makes the

removal of surface carbon species much easier. The high oxygen storage capacity

and the excellent redox properties between Ce3+/Ce4+ of CeO2 significantly helps

the oxidation of the surface carbon species which is highly desired for the

regeneration of the catalyst.94 Moreover, Ceria-based Ni catalyst presents a strong

metal support interaction that helps the stabilization of the catalyst.95

Apart from using a promoter or a support, the coke resistance properties of

the Ni catalyst can be significantly enhanced if the active catalyst is located in a

well defined structure. Many recent investigations have been made to confine Ni

particles by using nickel precursors with defined structures like spinels, perovskites

or solid solutions. With such defined structures, the temperature for metal reduction

had to be increased to near the reaction temperature. High dispersion of the metal

could be achieved with enhanced coke resistance for reforming reactions. The

crystalline oxide precursors are normally prepared by conventional methods such

as sol–gel or co-precipitation. After calcination, the material contains active metal

species, homogeneously dispersed inside the bulk. Subsequent reduction at

23

elevated temperature leads to the migration of some of the metal atoms to the

surface, forming homogeneously dispersed nanostructured metal catalysts well

dispersed on the support. It has been indicated that the metal-support interaction is

stronger than that obtained by conventional impregnation methods. The thermal

stability of the catalyst can be also improved by this way. Studies were performed

employing spinels (AB2O4) as catalyst supports for reforming reactions. Guo et al.

reported the MgAl2O4 supported Ni catalyst shows enhanced reactivity for CO2

reforming of methane.96 Here dehydrogenation of methane is inhibited and leads to

an improved coke resistance. NiAl2O4 supported Ni catalysts were also reported

with better stability compared to other supporting materials.97 Ribeiro et al.

prepared homogeneous nanocrystallites of Ni by the postsynthetic reduction of

NiAl2O4, with an average particle size of 4 nm. The catalyst produced this way

shows an excellent stability for CO2 reforming of methane (>40 h). The CO2

conversion was reported to reach 100 % at 800 ºC.98

Alternatively, perovskite type oxides can be used as the precursors for

highly dispersed Ni catalysts by the post synthetic reduction. The B site metal

serves as the primary active site, while the A site metal has a strong effect on the

stability and potentially improves the catalytic performance via the interaction with

the B site metal.99 Gallego et al. compared the activity and stability of Ni catalysts,

obtained from the reduction of LaNiO3, LaNiMgO3 and LaNiCoO3 perovskites for

CO2 reforming of methane.100 During the reduction using hydrogen at elevated

temperatures, the perovskites are destroyed partially or completely and Ni

nanoparticles are generated as catalysts. They found that the catalysts from

LaNiO3 and LaNiMgO3 show the better activity, compared with those containing

cobalt. The increasing amount of Mg leads to a decrease in the coke formation.

The coke resistance of the catalysts made from LaNiO3 and LaNiMgO3 can be

further improved using Pr and Ce as promoters.101 Similar preparation of Ni

catalysts from the LaNiO3 perovskite showed remarkable stability for CO2

reforming. In general, the perovskite-based Ni catalysts normally show sufficiently

good catalytic properties for CO2 reforming of methane in terms of activity, coke

resistance, and thermal stability.102 However, the low specific surface area of the

24

perovskite-based Ni catalysts limits their applications in industry. Considering this,

Rivas et al. attempted to incorporate a series of Ni-based perovskites (LaNiO3,

LaCaNiO3, and LaCaNiCoO3) into the highly ordered mesoporous SBA-15 silica-

host for CO2 reforming of methane. The activity and selectivity are found to be

improved and the catalysts show a good low temperature activity with an enhanced

metal-support interaction.103

In summary, perovskite-structured mixed metal oxides can be considered as

cost effective alternatives to noble metals for a wide range of catalytic applications.

A major milestone that needs to be achieved to explore the applicability of these

materials is to develop novel strategies to synthesize these materials with

enhanced values of specific surface area. The recently developed solid templating

method called nanocasting is proven to be versatile for synthesizing various

compositions with very high values of surface areas. These materials are found to

be highly efficient for a variety of applications including catalysis. The goal of this

thesis is to synthesize perovskite oxides using the nanocasting method and test

the efficiency of these materials for various catalytic reactions.

25

Chapter 2 – Experimental methods

2.1 Synthesis of ordered mesoporous silica templates

One of the major objectives of this thesis was the synthesis of perovskite-

structured mixed metal oxides with high values of specific surface area. For

reaching this goal, the nanocasting strategy was used. The initial requirement in

this method is a solid mold that can be used as the hard template. Two different

ordered mesoporous silica materials with interconnected porous structure were

chosen for this purpose. The synthetic strategies of the template silicas used in this

work are outlined below.

SBA-15 (2D hexagonal p6mm symmetry)

SBA-15 (Santa Barbara No.15) is synthesized according to an acidic route

using pluronic type triblock copolymer PEO-PPO-PEO (P123, molecular

mass=5800 gmol-1) as the structure directing agent and tetraethyl orthosilicate

(TEOS,) as the silicon source, following the procedure previously reported by Choi

et al.33 The synthesis was performed with the following initial molar gel

composition: 0.99 TEOS/0.54 HCl/0.016 P123/100 H2O. In a typical synthesis, 4.0

g of P123 was dissolved in 76 g of deionized water and 2.3 g of hydrochloric acid

(37%) at 35 °C under magnetic stirring. To the obtained homogeneous solution, 8.6

g of TEOS was rapidly added with continued stirring for 24 h at 35 °C and

subsequently subjected to hydrothermal treatment at desired temperature for an

additional 24 h to ensure further framework condensation. After cooling, the

resulting solution was filtered and the solid products were dried at 100 °C for 24 h.

Finally, the powders were calcined at 550 °C for 3 h in order to remove the organic

copolymer template. Hydrothermal treatment temperatures were varied from 35 –

140 °C in order to vary the textural parameters.

KIT-6 (3D cubic Ia3d symmetry)

KIT-6 (Korea Institute of Technology No-6) was synthesized according to

the protocol reported by Kleitz et al.34,35 In this case also the synthetic strategy was

26

similar to that of SBA-15 except for the use of butanol as a co-surfactant. The

synthesis was performed with the following initial molar gel composition: 1

TEOS/1.9 HCl/0.017 P123/195 H2O/1.31 Butanol. In a typical synthesis, P123 (6 g)

was dissolved in distilled water (217 g) containing 35% HCl (11.8 g). To this

solution, n-butanol (6 g) was added at 35 ºC. After 1 h, tetraethoxysilane (12.9 g)

was added and stirred for 24 h at 35 ºC. The resulting mixture was hydrothermally

treated at 100 ºC for another 24 h, and the solid product obtained was filtered,

dried at 100 ºC for 24 h, and calcined at 550 ºC for 3 h in order to remove the

surfactant template.

2.2 Synthesis of mesoporous perovskites

Perovskite materials for catalytic applications generally involve lanthanum in

the A site and first row transition metals in the B site. In this study, a series of

perovskite oxides were synthesized by varying the metal at the B site of the ABO3

structure. LaCoO3, LaFeO3 LaMnO3 and LaNiO3 which are well known for their

catalytic activity in various gas phase reactions, were synthesized for applications

performed in this work. Metal nitrates in appropriate stoichiometric ratios are first

treated with citric acid, which was used as a complexing agent. The citrate complex

thus formed was impregnated into the template pores by the method of wet

impregnation. High temperature calcination under air results in the formation of

perovskite-silica composite. Removal of the silica template by treating with

aqueous NaOH solution results in the formation of ordered mesoporous perovskite

oxides with high surface area. Multiple impregnations were performed in order to

enhance the loading. Conventional perovskites were also synthesized using the

citrate process. In this case syntheses were performed in the absence of the

template silica while all the other conditions remained the same. In some cases,

comparison of catalytic activities was also performed using perovskites

synthesized by the reactive grinding method. The materials were provided by the

group of Prof. Kaliaguine, Department of Chemical Engineering, Université Laval,

Canada

27

A typical synthesis of mesoporous perovskites used in this study can be

summarized as follows. 3 mmol of metal precursors corresponding to the A site

(La) and B site (Fe, Co, Mn and Ni) were dissolved in an ethanolic solution of citric

acid (10 mL) to obtain an equimolar solution, which was added slowly to KIT-6 /

SBA-15 (1 g) dispersed in water (10 mL). The molar ratio of total metal ions and

citric acid was kept at 2:1. The mixture was stirred for a few hours at room

temperature, and then the solvent was evaporated under vacuum with a rotary

evaporator. The powder thus obtained was further dried at 80 ºC for 24 h, ground

well in a mortar, and calcined at 500 ºC for 4h to remove the organic part.

Impregnation was repeated twice, using for the second time one half of the amount

of the precursor, to achieve higher loadings. The final powder was calcined at 700

ºC for 6h, and the silica template was then removed by treating the composite 3

times with NaOH (2M) at room temperature.

2.3 Characterization

The full potential of the development of novel porous materials for catalytic

applications can be realized only by the comprehensive characterization with

respect to crystallinity, phase purity, textural properties (pore size, specific surface

area and pore volume) and surface and redox properties. For the complete

characterization of all these parameters information has to be gathered from

corresponding techniques. Each method has a certain length scale of applicability.

In the present thesis, the following methods were used for the complete

characterization of nanocast mesoporous perovskite oxides.

2.3.1 Powder X-Ray Diffraction (XRD)

The rapid and reliable identification of crystalline solids can be made using

powder X-Ray Diffraction. When an X-ray beam strikes a crystalline solid

constructive interference occurs and selective reflections of intensity I for (h,k,l)

planes will be observed as a diffractogram when the glancing angle satisfies the

Bragg law,

nλ = 2dsinθ (2.1)

28

where n is an integer determined by the order of diffraction, λ is the wave length, d

is the spacing between the planes and θ is the angle between the incident ray and

scattering plane.

Figure 2.1 Wide angle X-Ray diffraction patterns of various perovskite oxides(left) and small angle XRD

patterns of mesoporous MgO along with parent templates. RG represents samples synthesized by the reactive grinding method and CIT represents sample synthesized by the citrate method.

84,104

Information about the size and symmetry of the lattice can be obtained from

the diffractogram. Comparison of the position and intensity of the reflections with

those of known substances enables a qualitative identification. Phase

determination relies mainly on the position of the peaks. Also the size of the crystal

domains (D) can be analyzed from the width of the peaks using Scherrer equation:

D = Kλ / (βcosθ) (2.2)

where K is a constant equal to 0.86, λ is the wavelength of X-ray used, β is the

effective line width of the X-ray reflection and θ is the angle of reflection. Typical

XRD patterns corresponding to perovskite structured mixed metal oxides4 are

shown in Figure 2.1. Further, careful examination of the small angle region of the

X-Ray diffraction pattern gives information on structural periodicity in the

nanometer range. From these reflections detected at 2θ values less than 10⁰,

information about the symmetry of the mesoscopic lattice is obtained. Reflections

observed in this region are due to ordered array of aligned pore walls and can be

indexed to a specific space group.5

29

In the present study, wide angle powder XRD analyses were performed with

a Siemens Model D5000 diffractometer using Cu Kα radiation (λ = 0.15496 nm).

2.3.2 N2 – physisorption

The surface area, pore size distribution and pore volume can be measured

using physical adsorption of a gas probe (adsorptive, usually nitrogen or argon) on

the porous solid (adsorbant). Adsorption isotherm of a gas on a porous solid

represents the amount of the gas adsorbed as a function of pressure. The first step

of interpretation is the inspection of the isotherm. The shape of the isotherms

contains information about the interactions between the adsorbant and adsorptive.

IUPAC conventions have been proposed for classifying pore sizes and gas

adsorption isotherms28 (Figure 2.2). The six types of isotherms are characteristic of

adsorbents that are microporous (type I), nonporous or macroporous (type II and

III) or mesoporous (types IV and V). Type VI is the stepwise adsorption of layers.

Type IV isotherms are associated with the occurrence of H1 or H2 hysteresis loops

(Figure 2.2), which results from the filling and emptying of the gas on the

mesopores by capillary condensation and evaporation. The H1 hysteresis loop is

indicative of a narrow distribution of rather uniform mesopores with cylindrical-like

pore geometry. The more frequent H2 hysteresis loop is typically attributed to

percolation effects in complex pore networks, pore blocking effects or cavitation

effects in ink-bottle shaped pores. The specific surface area is usually determined

by the method of Brunauer, Emmett and Teller (BET).105 From the BET model the

monolayer coverage of the surface Nm and from this quantity the surface area SBET

is calculated using the equation:

SBET = Nm L σ (2.3)

where L is the Avogadro number and σ is the average area occupied by each

molecule in a completed monolayer. Usually the validity of BET equation holds

within a relative pressure range p/po = 0.05-0.30 which is the range of linearity of

the BET plot used to extract Nm.

30

The most accurate way to determine pore size distributions is by applying

density functional theory methods (DFT).36,106-107 The DFT methods were

developed by taking into account the particular characteristics of the hysteresis like

pore shape. Non-intersecting pores of different size are assumed to be of the same

regular shape (cylinders, slits or spheres). Correspondingly, pore size distributions

are calculated for a given pore geometry, using a series of theoretical isotherms

(kernels) for pores of a given chemical composition of the respective geometry with

different diameters. In principle, the non local density functional theory (NLDFT)

method may be applied over the complete range of nanopore sizes when suitable

kernels are available.

Figure 2.2 IUPAC classifications of adsorption isotherms (left) and hysteresis loops (right).28

In this study N2 physisorption analyses were performed using a

Micromeritics ASAP 2010 sorption analyzer. All the ordered mesoporous silica

templates were degassed at 200 ºC overnight before analysis. For mesoporous

perovskites, the degassing temperature was fixed at 150 ºC. The adsorption

measurements were performed at -196 ºC.

31

2.3.3 Elemental analysis

Elemental analysis by atomic absorption spectroscopy (AAS) can be used to

confirm the complete removal of the template and the elemental composition of the

perovskite oxides. In this technique a known flux of light energy is passed through

the atomized sample, and by then measuring the flux of light remaining after

absorption it is possible to determine the concentration of the element being

measured.

In this study, analyses were performed on an M1100B Perkin-Elmer atomic

absorption spectrophotometer. An appropriate amount of the sample was dissolved

in 50 ml of 10 % HCl and 1 ml of concentrated HF in a polypropylene bottle at 60

ºC for 48 hours before the analysis.

2.3.4 Electron Microscopy (TEM and SEM)

Information regarding surface features, morphology, texture and pore

structure can be obtained from electron microscopy. Direct images of the structural

features of the samples can be obtained by Transmission Electron Microscopy

(TEM). In this method, a beam of electrons is focused on to the sample and parts

of it are transmitted and projected on a fluorescent screen. Scattered or diffracted

electrons from the samples will form the image contrast. The wavelength of the

electron beam applied limits the resolution of the microscope. The high energy

electron beam provided in high voltage microscope systems allows reaching the

domain of the structural resolution, where the resolution distance is in the range of

the unit cell size. The degradation of the sample is one disadvantage of this

method. TEM analysis is often conducted to visualize the pore structure of ordered

mesoporous materials. In this method, ionization is induced by an electron beam

on the surface of the sample and the backscattered electrons can be examined.

The samples were first dispersed in ethanol and were deposited on carbon grids

before analysis. Scanning electron microscopy (SEM) can be used to examine the

surface of the sample and thus, information regarding the shape, morphology and

size can be obtained. For SEM analysis, the samples were first dispersed on an

aluminium stub and coated with Au/Pd film.

32

Figure 2.3 TEM images of mesoporous hematite, α-Fe2O3, templated using ordered mesoporous silica KIT-

6108

In this study TEM analyses were performed on a JEOL JEM 1230

microscope and SEM images and EDS spectra were recorded using a JEOL JSM-

840 instrument.

2.3.5 X-ray photoelectron spectroscopy (XPS)

XPS is a technique which is widely used to get information about the surface

chemical composition of a material. In this technique, X-rays are used to illuminate

a region of the sample under analysis. From the energy of the photoelectrons thus

emitted, information regarding the elements present and their chemical state can

be obtained.

In this study the analyses were performed using a PHI 5600-ci spectrometer

(Physical Electronics, Eden Prairie, MN). The main XPS chamber was maintained

at a base pressure of < 8*10-9 Torr. A monochromatic aluminium X-ray source (Al

k = 1486.6 eV) at 300W was used to record survey spectra (1400-0 eV, 10min)

with charge neutralization. The detection angle was set at 45º with respect to the

normal of the surface and the analyzed area was 0.8*2 mm2 (aperture 5). The high

resolution spectra were recorded with the standard magnesium X-ray source (Mg

k = 1253.6 eV) at 300W without charge neutralization.

2.3.6 Temperature programmed reduction (TPR-H2)

Temperature Programmed Reduction (TPR) can be used to study the

reduction properties of the material under investigation. The method is based on

33

the monitoring of the amount of hydrogen consumed when the catalyst is subjected

to an increase in temperature under a flow of hydrogen containing gas. Based on

this information, the degree of reduction of the sample can be calculated. Often a

pre-oxidation treatment is required so as to remove the adsorbed impurities on the

material surface. Information about the dispersion of the metallic component as

well as the metal-metal interactions in the catalyst can also be obtained.

In this study a RXM-100 multicatalyst testing and characterization system

(Advanced Scientific Design Inc.) was used to perform TPR-H2. Appropriate

amount of the catalyst was placed in a quartz reactor and pre-treated under a flow

of 20 mLmin-1 (20 % O2 in He) at 500 ºC for 1 h. The TPR was performed under a

flow of 10 mLmin-1 (5 % H2 in Ar) with a temperature ramp of 5 ºCmin-1 from 25 to

900 ºC. The consumption of hydrogen was monitored and quantified with a thermal

conductivity detector (TCD).

2.3.7 Temperature programmed desorption (TPD-O2,)

TPD-O2 is a technique widely used to collect information about the nature of

oxygen species on oxides including perovskites. Typically for perovskites two types

of oxygen can be distinguished. The oxygen desorbed at lower temperature is

designated as α-O2 which originates from the surface. The oxygen desorbed from

the lattice of the perovskite structure is termed as the β-O2. Desorption of β-O2

takes place at higher temperature than that of α-O2. This β-O2 desorption involves

the formation of anionic vacancies and is often regarded as a measure of oxygen

mobility of the material.

TPD-O2, experiments were performed in a similar manner as mentioned in

the previous section for TPR-H2. The same pre-treatment was performed and a

TCD was used for the quantification of the oxygen desorbed.

2.3.8 Thermogravimetric / Differential thermal analysis (TG/DTA)

Thermogravimetry is a method used to monitor the changes in weight with

respect to temperature. Differential Thermal Analysis records the changes in the

sample with respect to an inert reference due to difference in temperature. Both

34

methods in combination is a powerful tool to investigate the physical and chemical

processes in solids. Thermogravimetry enables to calculate the mass loss as a

function of temperature and DTA enables the evaluation of heat exchanges.

TG/DTA analysis can be used to trail the decomposition processes occurring to the

metal nitrate precursor during calcination.

In this study, thermogravimetric studies were performed to determine the

amount of deposited carbon after dry reforming of methane. The analyses were

performed using a NETZSCH STA 449C thermogravimetric analyzer under an air

flow of 20 ml min-1 with a heating rate of 10 K min-1.

2.4 Catalytic tests

In this thesis two gas phase reactions were chosen to verify the catalytic

efficiency of mesoporous perovskites synthesized by the nanocasting method. The

experimental conditions used in both these cases are summarized below.

2.4.1 Total oxidation of methanol

Initial studies were performed on the total oxidation of methanol. This

reaction is known to occur at comparatively low temperatures, much less than the

calcination temperature required for the crystallization of the perovskite structure.

Stoichiometric equation representing the total oxidation of methanol is given as

equation 2.474,84 and the schematic representation of the experimental set up is

shown in Figure 2.4.

CH3OH + 3/2 O2 CO2 + 2H2O (2.4)

The feed, composed of 0.5 % CH3OH and 5 % O2 in He, was passed

through the U-shaped quartz reactor (internal diameter = 5 mm) where a catalytic

bed with appropriate amount of the catalyst was placed. To purge the catalytic

system, the catalysts were first flushed with He for 1 h at room temperature and

then a pre-treatment was performed for 1 h at 200 ºC before the catalytic tests.

The temperature was controlled by using a K-type thermocouple placed in the

reactor. Gas samples were collected in the steady-state regime after an interval of

35

2 h of constant conversion, and the products were analyzed with a gas

chromatograph (HP 6890 series) equipped with a TCD. Reactants and products

were separated using a Haye-Sep T column (internal diameter=1 mm, L = 2 m x 5

m). Comparisons were performed at constant space velocities. For obtaining the

data required for kinetic processing, flow rates were adjusted to obtain the desired

space velocity under ambient conditions of temperature and pressure.

Figure 2.4 Schematic representation of the experimental set up used for performing the total oxidation of

methanol

In order to obtain the kinetic data, methanol steady-state conversions were

monitored at the reactor outlet at different flow rates (typically, 10–40 mLmin-1

corresponding to respective gas hourly space velocity of 19500–78200 h-1). From

the values of experimental conversions obtained, the one‟s corresponding to a few

selected temperatures was cross-plotted against the pseudo-contact time W/F (in

which W is the weight of the catalyst and F is the molar flow rate of methanol). This

yields a series of isothermal curves. The resultant data were then fitted by using an

appropriate mathematical function which depicts the path of the reaction (in the

present study the sigmoid represented by equation (2.5) served the purpose). The

analytical derivatization of this function gives the values of rates.

(

) (2.5)

36

in which Y is the conversion of methanol and a, b, and c are nonlinear regression

parameters.

To calculate the rate constant, the numerical values of the obtained reaction

rates were fitted by using the simplified Equation (3.6) proposed by Arai et al,

which assumes the participation of only β-O2 in the catalytic process.11

r = K.PMethanol (2.6)

The linear regression of the above equation gives the values of rate constant. The

values of pre-exponential factor (A) and activation energy (Ea) were determined

from the Arrhenius plot by performing linear regression analysis.

2.4.2 Dry reforming of methane

This reaction takes place at higher temperatures approximately in the range

of that required for the crystallization of the perovskite structure. Tests were

performed using as synthesized perovskites as well as catalysts obtained by

performing reduction of the perovskites. In the latter case, the reduced form was

obtained by treating the as synthesized perovskite at 700 ºC for 2 hours under a

flow of H2. Stoichiometric equation representing the dry reforming of methane is

given as equation 2.7109 and the schematic representation of the experimental set

up is shown in figure 2.5

The catalytic activities of the materials were tested under steady state

conditions. 100 mg of the catalyst was inserted between two quartz wool plugs

placed in a U-shaped quartz reactor (internal diameter = 5 mm). The temperature

was controlled using a K-type thermocouple placed in the reactor without direct

contact with the catalyst. To purge the catalytic system, the catalysts were first

flushed with Ar for 1 h at room temperature. The feed, composed of CH4 (10 ml

min-1), CO2 (10 ml min-1) and Ar as diluent with a total flow of 50 ml min-1 (GHSV =

2.1 x 105 h-1) was passed through the reactor and the temperature was increased.

Gas samples were collected in the steady-state regime after a period of 2 h of

constant conversion, and the products were analyzed using a gas chromatograph

37

(HP 6890 series) equipped with a TCD. Reactants and products were separated

with a Haye-Sep T column (internal diameter=1 mm, L = 2 m x 5 m). Stability tests

were performed at 700 °C for 48 hours with all other conditions remaining the

same.

CH4 + CO2 2CO + 2H2 (2.7)

Figure 2.5 Schematic representation of the experimental set up used for the dry reforming of methane.

38

39

Chapter 3 - Kinetics of methanol oxidation over mesoporous

perovskite catalysts

Mahesh Muraleedharan Nair,a Freddy Kleitz *a and Serge Kaliaguine*b

aDepartment of Chemistry and Centre de Recherche sur les Matériaux Avancés

(CERMA), 1045, Avenue de la Médecine, Université Laval, Quebec city, G1V 0A6,

Canada.

bDepartment of Chemical Engineering, Université Laval, Quebec city, G1V 0A6,

Canada.

Published in ChemCatChem 2012, 4, 387

40

3.1 Résumé

Une série d‟oxydes métalliques mixtes pérovskites ayant la formule générale LaBO3 (B = Mn, Co, Fe) a été synthétise grâce à l‟utilisation de la méthode de nanomoulage, en utilisant la silice mésoporeuse ordonnée KIT-6 comme matrice solide. Bien que les matériaux obtenus ne soient pas les répliques exactes de la silice ayant servi de moule, leurs surfaces spécifiques BET (Brunauer–Emmett–Teller) se sont avérées être extrêmement élevées (110–155 m2g-1). Les propriétés rédox des pérovskites mésoporeuses nanomoulées ont été détermines par réduction thermo-programmée ainsi que par désorption d‟oxygène thermo-programmée. L‟activité catalytique des répliques LaMnO3 mésoporeuses a été testée en utilisant l‟oxydation du méthanol comme réaction modèle, permettant aussi d‟établir le premier modèle cinétique pour ce systém. Les catalyseurs nanomoulés LaMnO3 mésoporeux ont démontré la meilleure efficacité de conversion pour le méthanol en comparaison avec les répliques nanomoulées de types LaCoO3 et LaFeO3 ainsi qu‟avec des échantillons LaMnO3 préparés selon d‟autres méthodes. Ce résultat est sans aucun doute associé avec la plus haute surface spécifique de cette pérovskite nanomoulée. De plus, les matériaux se sont montrés stables lorsque soumis aux conditions reactionelle. Les vitesses de réaction obtenues à partir des conversions expérimentales à différentes vélocités spatiales (19500 – 78200 h-1) pour les LaMnO3 nanomoulées se sont avérées suivre une équation de vitesse qui dépend de la pression partielle de méthanol. En utilisant les constantes de vitesse obtenues, l‟énergie d‟activation et le facteur pré-exponentiel ont été déterminés en traçant un graphique d‟Arrhenius. Les valeurs de conversions calculées à partir des vitesses normalisées par les surfaces spécifiques se sont montrées en accord avec les conversions observées expérimentalement, ce qui reflète bien la proportionnalité des vitesses à la surface spécifique.

41

3.2 Abstract

By using the nanocasting method, a series of mixed metal perovskite oxides with general formula LaBO3 (B = Mn, Co, Fe) were synthesized with use of ordered mesoporous KIT-6 silica as a hard template. Even though the resulting materials were not found to be exact replicas of the template, extremely high values of Brunauer–Emmett–Teller specific surface areas (110–155 m2g-1) were obtained for the materials. The redox properties of nanocast mesoporous perovskites were determined by performing temperature-programmed reduction and temperature-programmed desorption of oxygen. Catalytic activity was monitored by using methanol oxidation as a model reaction over mesoporous LaMnO3, and the first kinetic model was developed for the same. Nanocast mesoporous LaMnO3 catalysts were found to show the highest conversion efficiency for methanol under steady-state conditions as compared with both LaCoO3 and LaFeO3 nanocasts and with LaMnO3 samples prepared by using other methods. This result is clearly associated with the higher specific surface area of this nanocast perovskite. Furthermore, these materials were found to be stable under conditions prevailing in the reactor. Reaction rates obtained from the experimental conversions at various space velocities (19500 – 78200 h-1) for nanocast LaMnO3 were found to follow a rate equation that depends on the partial pressure of methanol. Using the obtained rate constants the values of activation energy and pre-exponential factor were determined from the Arrhenius plot. The calculated values of conversions from the rates modified with surface areas were found to agree with the experimental conversions, which in turn reflect the proportionality of rates to the specific surface area.

42

3.3 Introduction

The efficiency of perovskite structured oxides (ABO3) for the catalytic

elimination of hydrocarbons and volatile organic compounds has made them a

topic of intense interest.1,2 Apart from these being an alternative to expensive noble

metal catalysts, the fact that a variety of metallic cations fits into the perovskite

structure with only some requirements on the tolerance factor makes it possible to

fine-tune their physical and chemical properties.3 As regards the catalytic

applications, their specific surface area and crystal structure play important roles.

For this reason, various solid-state4 and wet chemical methods5-7 were developed

to synthesize well-crystallized, perovskite-structured mixed metal oxides with

enhanced specific surface areas. In all the early methods, the formation of the

crystalline phase resulted from high-temperature treatment. Consequently, the

specific surface areas were found to be low (< 30 m2g-1). Kaliaguine et al.

developed a method known as “reactive grinding” that enabled the preparation of

perovskite oxides with higher surface areas (> 100 m2g-1) where calcination is

performed at temperatures of around 200 ºC;8 the specific surface area was,

however, also found to decrease at higher calcination temperatures. Nanocasting,

a solid templating method developed during the last decade, has proven to be a

very versatile method for the synthesis of mesoporous oxides with high surface

areas.9–21 Various authors used this method for developing binary and ternary

oxides with high specific surface areas, which cannot be obtained from other

methods. In this method, crystallization takes place inside the pores of a solid

template, usually silica or carbon; hence, the confined space inside the template

pores restricts the crystal growth.9–21 The resulting framework structure, after the

template removal, leads to the formation of crystalline mesoporous materials with

very high specific surface areas. Such materials are proven to be useful not only as

efficient catalysts22,23 but also for various other applications (e.g., batteries and

sensors).24–31 Even though a large number of binary metal oxides with high surface

areas were synthesized by using this method, only limited attempts have been

made until now for developing multi-metal oxides, such as perovskites.32–35 To our

knowledge, there is no report yet that describes the kinetics of catalytic reactions

43

performed over nanocast mesoporous catalysts. In this chapter, we report on the

nanocasting synthesis of mesoporous perovskites with the general formula LaBO3

(B=Mn, Co, Fe) and describe in detail catalytic and kinetic studies of methanol

oxidation over high surface area nanocast LaMnO3 perovskites.

3.4 Experimental

3.4.1 Synthesis of ordered mesoporous KIT-6 silica

The synthesis of ordered mesoporous KIT-6 silica, which was used as a

hard template, was performed by using the method reported elsewhere by Kleitz et

al.36–38 with use of Pluronic P123 (EO20PO70EO20, molecular mass=5800 gmol-1)

as the surfactant and tetraethoxysilane as the silica source. In a typical synthesis,

P123 (6 g) was dissolved in distilled water (217 g) containing 35% HCl (11.8 g). To

this solution, n-butanol (6 g) was added at 35 ºC. After 1 h, tetraethoxysilane (12.9

g) was added and stirred for 24 h at 35 ºC. The resulting mixture was

hydrothermally treated at 100 ºC for another 24 h, and the solid product obtained

was filtered, dried at 100 ºC for 24 h, and calcined at 550 ºC for 3 h.

3.4.2 Nanocasting of mesoporous perovskites

For the nanocasting method, a modified protocol of that previously reported

by Wang et al. was used.34 Typically, a citrate complex of metal cations was used

as the perovskite precursor. The precursor was impregnated into the template by

using the wet impregnation method. In a typical synthesis for LaMnO3,

La(NO3)3·6H2O and Mn(NO3)2·xH2O (3 mmol each) were dissolved in an ethanolic

solution of citric acid (10 mL) to obtain an equimolar solution, which was added

slowly to KIT-6 (1 g) dispersed in water (10 mL). The molar ratio of total metal ions

and citric acid was kept at 2:1. The mixture was stirred overnight at room

temperature, and then the solvent was evaporated under vacuum with a rotary

evaporator. The powder thus obtained was further dried at 80 ºC for 24 h, ground

well in a mortar, and calcined at 500 ºC for 4h to remove the organic part.

Impregnation was repeated twice, using for the second time one half of the amount

of the precursor, to achieve higher loadings. The final powder was calcined at 700

44

ºC for 6h, and the silica template was then removed by treating the composite 3

times with NaOH (2M) at room temperature. A fourth treatment of the samples with

NaOH (2M) did not lead to further decrease in the amount of residual Si. The

obtained product was washed with water and ethanol and dried overnight at 80 ºC.

For comparison, nanocasting of one sample was also performed by using metal

nitrates as precursors without the presence of citric acid and keeping all other

conditions the same.

3.4.3 Characterization

Wide-angle powder XRD was performed with a Siemens Model D5000

diffractometer using Cu Kα radiation (λ = 1.5496 Aº). Elemental analysis was

performed with an M1100B Perkin-Elmer atomic absorption spectrophotometer. N2

physisorption analyses were performed at -196 ºC with an ASAP 2010 sorption

analyzer. Prior to analysis, samples were degassed overnight at 150 ºC. Specific

surface areas of nanocast perovskites were calculated by using the BET method

on the lower relative pressure region of the isotherm (0.05–0.2). Pore size

distributions were obtained by using the NLDFT method38,49 assuming cylindrical

pore geometry (applying the kernel of metastable NLDFT adsorption isotherm, i.e.,

adsorption branch) supplied by the Autosorb-1 1.55 software from Quantachrome

Instruments. The total pore volume was calculated from the nitrogen sorption

capacity at P/P0=0.95. For TEM images, the samples were first dispersed in

ethanol and deposited on carbon grids and analyzed on a JEOL JEM 1230

microscope.

3.4.4 TPR/TPD

A RXM-100 multicatalyst testing and characterization system (Advanced Scientific

Design Inc.) was used to perform TPD-O2 and TPR-H2. For TPR experiments, the catalyst

(50 mg) was placed in a quartz reactor, which was pretreated under a flow of 20 mLmin-1

(20 % O2 in He) at 500 ºC for 1 h. The TPR was performed under a flow of 10 mLmin-1 (5

% H2 in Ar) with a temperature ramp of 5 ºCmin-1 from 25 to 900 ºC. The consumption of

hydrogen was monitored and quantified with a thermal conductivity detector (TCD). For

TPD-O2, the same pretreatment was performed as for the TPR experiments and the same

45

amount of the catalyst was used under a flow of 10 mLmin-1 of He. A TCD was used for

the quantification of the oxygen desorbed.

3.4.5 Catalytic tests

The catalytic bed was set up with the catalyst (200 mg) inserted between

two quartz wool plugs in a U-shaped quartz reactor (internal diameter = 5 mm).

The temperature was controlled by using a K-type thermocouple placed in the

reactor. To purge the catalytic system, the catalysts were first flushed with He for 1

h at room temperature and then pretreated for 1 h at 200 ºC before performing the

catalytic tests. The feed, composed of 0.5 % CH3OH and 5 % O2 in He, was

passed through the reactor and the temperature was increased. Gas samples were

collected in the steady-state regime at an interval of 2 h of constant conversion,

and the products were analyzed with a gas chromatograph (HP 6890 series)

equipped with a TCD. Reactants and products were separated with a Haye-Sep T

column (internal diameter = 1 mm, L=2 m x 5 m).

3.4.6 Kinetic studies

To obtain kinetic data, methanol steady-state conversions were monitored

for the nanocast mesoporous LaMnO3 catalyst at the reactor outlet at different flow

rates (10 – 40 mLmin-1 corresponding to a gas hourly space velocity of 19500 –

78200 h-1). The obtained experimental conversions were cross-plotted as a

function of pseudo-contact time over selected reaction temperatures. The values of

reaction rates obtained by the analytical derivatization of the equation

corresponding to the fitted curves were evaluated by the simplified linear equation

proposed originally by Arai et al. for the total oxidation of methane.46

3.5 Results and discussion

3.5.1 Synthesis and characterization of mesoporous perovskites

The ordered mesoporous KIT-6 silica (3D porous network)36–38 was chosen

as a suitable hard template and characterized through N2 physisorption performed

at -196 ºC (Figure 3.1). By using the nanocasting method, a series of mesoporous

46

perovskites was synthesized with lanthanum in the A site and manganese, cobalt,

or iron in the B site of the ABO3 structure. Initially, the presence of the perovskite

structure was verified in each case. Characteristic peaks corresponding to the

perovskite structure were observed in the XRD patterns shown in Figure 3.1 for all

three compositions if citric acid was used.

Figure 3.1 Nitrogen physisorption isotherm and pore size distribution (inset) of the parent KIT-6 silica hard

template (left). Wide-angle powder XRD patterns of mesoporous perovskite oxides synthesized by use of ordered mesoporous KIT-6 silica aged at 100 ºC as a hard template. The lowest curve corresponds to the LaMnO3 perovskite synthesized without using citric acid (right).

Without citric acid an X-ray amorphous material was obtained, which was not

characterized further. No impurity peaks were detected except for a low-intensity

broad peak centered at 2θ = 28 º in the case of LaMnO3, which indicates the

presence of an X-ray amorphous phase, most likely residual silica or silicates. For

LaFeO3 and LaCoO3, atomic absorption analysis indicates the presence of 3 % Si,

whereas for LaMnO3 the value was approximately 10 %. (These amounts could not

be reduced any further by supplementary treatments with 2M NaOH.) Here, the

amorphous phase visible in the XRD pattern of LaMnO3 could thus be owing to the

presence of some insoluble rare-earth silicates that might be formed because of

the strong interaction between silica and rare-earth elements, as reported in recent

studies of similar catalysts for chemical looping combustion.39

Information regarding mesoscopic order was obtained by means of TEM,

and the images obtained are shown in Figure 3.2. Even though well-ordered

mesostructure domains were clearly observed, less defined and disordered

nanoporous regions were also present. N2 physisorption analysis was performed at

47

-196 ºC on mesoporous LaMnO3, LaCoO3, and LaFeO3 perovskites, and the

adsorption–desorption isotherms and the corresponding pore size distributions are

shown in Figure 3.3.

Figure 3.2 TEM images of nanocast LaMnO3 synthesized by use of ordered mesoporous KIT-6 silica aged at

100 ºC as a hard template.

All these isotherms exhibit a type IV behavior. An hysteresis loop appears in

the relative pressure range from 0.5 to 1.0, which is typical of such mesoporous

metal oxides obtained from nanocasting.9,10,16,20,21 Brunauer–Emmett–Teller (BET)

specific surface areas obtained for nanocast perovskites are exceptionally high (up

to 155 m2g-1) as compared with those of materials obtained by using conventional

methods, especially considering the high calcination temperature of 700 ºC (see

Table 3.1). The total pore volumes of perovskites were calculated from the volume

adsorbed at the relative pressure of 0.95, and the pore sizes were derived from the

adsorption branch by using the nonlocal density functional theory (NLDFT)

method.38 The resulting data are compiled in Table 3.1.

3.5.2 Temperature programmed reduction by hydrogen

The reduction behavior of metal cations in nanocast mesoporous

perovskites was examined by use of the hydrogen temperature-programmed

reduction (TPR-H2) analysis. Because the A-site metal is non reducible under the

present conditions of TPR-H2, the observed H2 consumption peaks is due to the

reduction of B-site metal cations, and the observed profiles are shown in Figure

3.4. For LaMnO3, a broad peak with a main peak centered at approximately 380

48

ºC along with shoulders at 310 and 530 ºC were observed. Previous reports

suggest that complete reduction to Mn0 does not occur for LaMnO3.40,41 The main

peak therefore corresponds to the reduction of Mn3+ to Mn2+. The low-temperature

shoulder could result from the presence of Mn4+, whereas the high-temperature

shoulder suggests the presence of manganese ions that are not easily

Table 3.1 Structural parameters of the KIT-6 template and nanocast perovskites obtained by performing N2

physisorption analysis at -196 ºC.

Sample SBET

(m2g-1)a

Dp (nm)b Vp (cm3g-1)c

KIT-6 template 948 8.4 1.2

LaMnO3 155 4.8 0.2

LaMnO3-Ud 140 5.0 0.2

LaCoO3 125 4.8 0.1

LaFeO3 110 4.8 0.1

LaMnO3-RGe 40 - -

LaMnO3-Cf 15 - -

aCalculated by using the BET method on the relatively low-pressure region (0.05–0.2);

bNLDFT pore size;

cPore volume;

dNanocast LaMnO3 after one catalytic run;

eSynthesized by using the reactive grinding

method;44

fSynthesized by using the amorphous citrate route.

Figure 3.3 (A) N2 physisorption isotherms and b) the corresponding pore size distributions of nanocast

perovskite oxides synthesized by use of ordered mesoporous KIT-6 silica aged at 100 ºC as a hard template. The isotherms of LaCoO3 and LaFeO3 are plotted with an offset of 60 and 110 cm

3g

-1, respectively, for clarity.

49

(B) Pore size distributions were calculated from the adsorption branch of the isotherm by using the NLDFT method.

38, 49

accessible. In the case of LaCoO3, the first peak in the low-temperature peak (370

ºC) corresponds to the reduction of Co3+ to Co2+ and the high-temperature peak

(560 – 580 ºC) corresponds to the reduction of Co2+ to metallic Co. In this case, a

small shoulder is also present at approximately 280 ºC, which could be due to the

presence of adsorbed oxygen species that are weakly bound to the surface. Note

that the areas of the two reduction peaks in the LaCoO3 trace are in a 1:2 ratio, as

required by the stoichiometry. This indicates negligible interaction with the residual

silica because very significant changes in the reducibility would occur if nanosized

LaCoO3 particles interacted with the silica lattice of MCM-41.42,43 Two peaks were

also observed in the case of LaFeO3. The first peak at 410 ºC indicates the

reduction of Fe3+ to Fe2+, and the second one at 660ºC shows its complete

reduction to the metallic state. The amounts of hydrogen consumed during the

reduction of nanocast perovskites are given in Table 3.2. Compared to reactive

grinded samples, the reduction steps occurred at much lower temperatures in the

case of nanocast perovskites and the amount of hydrogen consumed per mole of B

atom seems to be slightly lower for nanocast perovskites.44

Figure 3.4 TPR-H2 (left) and TPD-O2 (right) profiles of nanocast mesoporous perovskite oxides synthesized by

use of ordered mesoporous KIT-6 silica aged at 100 ºC as a hard template.

3.5.3 Temperature programmed desorption of oxygen

Information regarding the state of oxygen atoms on nanocast perovskites

was obtained from the temperature-programmed desorption of oxygen (TPD-O2)

50

Table 3.2 Amount of H2 consumed during TPR-H2.

Perovskite H2 consumed [molH2 atomB-1]

First step Second step

LaMnO3 0.524 -

LaCoO3 0.498 0.978

LaFeO3 0.496 1.01

profiles. Two types of oxygen species are generally found to be desorbed from

perovskite oxides.8,45 Desorption of oxygen weakly bound to the surface takes

place below 700 ºC (designated as α-O2), and desorption of lattice oxygen takes

place at a still higher temperature (designated as β-O2). For LaMnO3 and LaCoO3,

two peaks were observed, whereas for LaFeO3 only a broad signal was observed

(see Figure 3.4). The amounts of desorbed oxygen were calculated from the

deconvolution of experimental peaks by using a Lorentzian function and are listed

in Table 3.3. Compared with the samples synthesized by using the reactive

grinding method, nanocast perovskites show a higher value for desorbed α-O2 and

a lower value for desorbed β-O2. The higher values for α-O2 are in line with the

higher specific surface areas.

Furthermore, the numbers of desorbed monolayers were calculated by

assuming 4 µmolm-2 per monolayer of oxygen. The number of monolayers was

found to be much lower than that observed in the case of similar compositions

synthesized by using the reactive grinding method (Table 4.3).44 These differences

are likely associated with the higher volume of grain boundaries in the reactive-

grinded materials, in which oxygen mobility is higher.40,41,44

51

Table 3.3 Amount of O2 desorbed during TPD-O2.

Perovskite α-O2 desorbed

(µmolg-1)

β-O2 desorbed

(µmolg-1)

α-O2

monolayers

β-O2

monolayers

LaMnO3 394 258 0.63 0.42

LaCoO3 170 120 0.34 0.24

LaFeO3 91 47 0.20 0.11


The catalytic oxidation efficiency of the mesoporous perovskites was

examined with use of the model compound methanol, in flow conditions

corresponding to a gas hourly space velocity of 39100 h-1. In the steady-state

conversion profiles shown in Figure 3.5, LaMnO3 clearly exhibits higher conversion

efficiency than that of LaCoO3 and LaFeO3. Furthermore, comparisons of the

mesoporous catalysts were made with LaMnO3 samples synthesized by using the

conventional citrate process (LaMnO3-C) and the reactive grinding method

(LaMnO3-RG).44 In the conversion curves shown in Figure 3.5, nanocast LaMnO3

(LaMnO3-KIT-6) clearly exhibits a higher conversion efficiency, especially in

comparison to the catalyst synthesized by using the conventional citrate process.

In the case of the nanocast mesoporous catalyst, full conversion was observed at

150 ºC, which is much lower than the temperatures of full conversion for catalysts

synthesized by using the reactive grinding method (185 ºC) or the conventional

citrate process (220 ºC). The decrease in temperatures for the complete

elimination of methanol could be attributed to the enhanced specific surface area

afforded by a well-developed mesoporous framework obtained for nanocast

LaMnO3. Note that some conversion, even though rather low, was already

observed near to room temperature for nanocast LaMnO3. Also, CO2 was the only

product detected, without any presence of formaldehyde or CO, which is another

indication of the efficiency of the catalyst.

52

3.5.5 Stability

After performing a catalytic run up to 200 ºC, the nanocast LaMnO3 catalyst

was recovered and analyzed by using XRD and N2 physisorption to examine its

stability. The results are displayed in Figure 3.6 . Figure 3.6 A represents the N2

physisorption isotherms of nanocast LaMnO3 after one catalytic run. The shape of

the isotherm is retained, and the variation in the BET specific surface area is found

to be very small (Table 3.1). Well-resolved peaks corresponding to the perovskite

structure are observed in the XRD pattern (Figure 3.6 B), indicating that the

perovskite structure is retained. Also, no impurity peaks were observed

corresponding to the binary oxides or hydroxides of lanthanum and/or manganese.

Figure 3.5 Methanol conversion profiles as a function of temperature over LaMnO3, LaCoO3, and LaFeO3

perovskites synthesized by using the nanocasting method (left). Methanol conversion profiles as a function of temperature over LaMnO3 perovskites synthesized by using the nanocasting method, the reactive grinding method (RG),

44 and the conventional citrate process (C). The empty symbols represent the calculated values

of conversion for LaMnO3 synthesized by using the nanocasting method (Calculated KIT-6), the reactive grinding method (Calculated-RG), and the conventional citrate process (Calculated-C).

Also, a second catalytic run was performed with use of nanocast LaMnO3

recovered after one complete catalytic run, and the results are shown in Figure 3.7.

The corresponding conversion curve was found to be reproducible, similar to that

of the fresh catalyst although the low-temperature conversion was found to be

diminished.

53

Figure 3.6 A comparison of the N2 physisorption isotherms (A) and wide angle XRD patterns (B) of the

nanocast LaMnO3 perovskite before and after the catalytic test.

Figure 3.7 Comparison between fresh catalyst and used catalyst for methanol conversion over LaMnO3

perovskites synthesised using nanocasting. The open symbols represent the calculated values of conversion.


As mesoporous LaMnO3 is found to be the most efficient catalyst among the

three perovskites, this catalyst was considered for kinetic data processing. For

determining the catalytic oxidation rates, the total flow rate of the gas feed was

varied, which resulted in gas hourly space velocities ranging from 19500 to 78200

h-1. The corresponding conversion curves are shown in Figure 3.8. The

experimental conversions thus obtained at several selected temperatures were

cross-plotted against the pseudo-contact time W/F (in which W is the weight of the

catalyst and F is the molar flow rate of methanol), which yielded a series of

54

isothermal curves as shown in Figure 3.8 B. The resultant data were then fitted by

using the sigmoidal Equation (3.1), the analytical derivatization of which gave the

values of rates:

(

) (3.1)

in which y is the conversion of methanol and a, b, and c are nonlinear regression

parameters.

To calculate the rate constant, the numerical values of the obtained reaction

rates were fitted by using the simplified Equation (3.2) proposed by Arai et al,46

which assumes the participation of only β-O2 in the catalytic process and the

corresponding fit is shown in Figure 3.8.

r = K.PMethanol (3.2)

The pre-exponential factor (A) and activation energy (Ea) were determined from

the Arrhenius plot by performing linear regression analysis (see Figure 3.8 D) and

are found to be 2.07 molg-1mPa-1 and 35.5 kJmol-1, respectively. The rate data

obtained in this study were found to follow Equation (3.2). Thus, the reaction rates

obtained for the total oxidation of methanol over nanocast LaMnO3 depend only on

the partial pressure of methanol and temperature.

55

Figure 3.8 (A) Methanol conversion over nanocast LaMnO3 at different space velocities. The values of the

space velocities increase at rates of 19500, 39100, 58600, and 78200 h-1

from left to right. (B) Cross-plotting the values of experimental conversions obtained for nanocast LaMnO3 at selected temperatures as a function of pseudo-contact time. Points represent experimental data, and lines are calculated by using Equation (4.1). (C) The numerical values of rates obtained are represented as a function of the partial pressure of methanol; 1 atm=101325 Pa and (D) Arrhenius plot of the rate constant k obtained for nanocast LaMnO3.

Even though the model used to describe the reaction considers the

participation of only β-O2 in the catalytic process, the temperature range observed

for the conversion of methanol is notably rather low (< 200 ºC). The above value of

Ea is found to be much lower, less than half, than that observed for similar

catalysts if used in various partial and total oxidation reactions.47,48 The variations

in reaction conditions should, however, be kept in mind when making this

comparison. The mechanism for the total oxidation of methanol over LaMnO3

catalysts was established in our recent paper.44 It involves methanol deprotonation

forming adsorbed methoxide and adsorbed OH over a transient vacancy–O- pair.

This initial step is followed by successive deprotonation and oxidation steps, which

56

may involve α-O2. The linear rate relationship suggests that the initial step is rate

determining. The rather lower activation energy determined by this work would

then be the energy barrier associated with methanol adsorption.To check the

proportionality of the rate constant to the specific surface area, the experimental

values of conversions were calculated by use of the rate values modified

proportionally to the specific surface area (Figure 3.5 B). In these calculations, the

temperature is assumed to be constant over the length of the reactor. The good

agreement observed between the experimental curves and the calculated data

points confirms the proportionality of rate constants to the specific surface area.

3.6 Conclusions

In conclusion, we have synthesized a series of mesoporous perovskite

oxides with lanthanum in the A site and manganese, cobalt, or iron in the B site by

using the nanocasting method. Even though the resulting materials were not found

to be the exact replicas of the template, these materials were shown to comprise

extremely high specific surface area. Nanocast LaMnO3 clearly exhibited higher

catalytic activity than did similar materials synthesized by using other methods,

especially compared to the ones synthesized by using the conventional citrate

process. This result is clearly associated with the higher specific surface area of

the nanocast perovskite. The nanocast catalysts are shown to be stable as

evidenced by XRD and N2 physisorption, which is expected because the

temperature range at which the conversion of methanol takes place is less than

200 ºC. Furthermore, the rate data obtained for methanol oxidation over nanocast

LaMnO3 depend on the partial pressure of methanol. The work is in progress to

elucidate the nature of the surface species in nanocast perovskites, especially

regarding the amount of Si remaining in LaMnO3. This is expected to provide

information toward the effect of the remaining Si species on the catalytic activity of

nanocast LaMnO3. In addition, it will be interesting to use ordered mesoporous

silica materials with different pore topologies as templates or as supports and to

determine whether there is an effect on the catalytic properties.

57

3.7 References

1. Meadowcroft, D. B. Nature 1970, 226, 847. 2. Pedersen, L. A.; Libby, W. F. Science 1972, 176, 1355. 3. Goldschmidt, M. Acad. Oslo J. Mater. Nat.1926, 2, 7. 4. Voorhoeve, R. J. H.; Remeika, J. P.; Johnson, D. W. Science 1973, 180,

62. 5. Baythoun, M. S. G.; Sale, F. R. J. Mater. Sci. 1982, 17, 2757. 6. Taguchi, H.; Yamada, S.; Nagao, M.; Ichikawa, Y.; Tabata, K. Mater. Res.

Bull. 2002, 37, 69. 7. Johnson, D. W.; Gallagher, P. K.; Wertheim, G. K.; Vogel, E. M. J. Catal.

1977, 48, 87. 8. Kaliaguine, S.; Van Neste, A.; Szabo, V.; Gallot, J. E.; Bassir, M.;

Muzychuk, R. Appl. Catal. A 2001, 209, 345. 9. Tian, B. Z.; Liu, X.; Solovyov, L.; Liu, Z.; Yang. H.; Zhang, Z.; Xie, S.;

Zhang, F.; Tu, B.; Yu, C.; Terasaki, O.; Zhao, D. J. Am. Chem. Soc. 2004, 126, 865.

10. Laha, S. C.; Ryoo, R. Chem. Commun. 2003, 2138. 11. Yang, H.; Zhao, D. J. Mater. Chem. 2005, 15, 1217. 12. Jiao, F.; Shaju, K. M.; Bruce, P. G. Angew. Chem. 2005, 117, 6708;

Angew. Chem. Int. Ed. 2005, 44, 6550. 13. Lu, A-H.; Schuth, F. Adv. Mater. 2006, 18, 1793. 14. Jiao, F.; Harrison, A.; Jumas, J. C.; Chadwick, A. V.; Kockelmann, W.;

Bruce, P. G. J. Am. Chem. Soc. 2006, 128, 5468. 15. Dickinson, C.; Zhou, W.; Hodgkins, R. P.; Shi, Y.; Zhao, D.; He, H. Chem.

Mater. 2006, 18, 3088. 16. Rumplecker, A.; Kleitz, F.; Salabas, E.; Schuth, F. Chem. Mater. 2007, 19,

485. 17. Shi, Y.; Wan, Y.; Liu, R.; Tu, B.; Zhao, D. J. Am. Chem. Soc. 2007, 129,

9522. 18. Tuysuz, H.; Lehmann, C. W.; Bongard, H.; Schmidt, R.; Tesche, B.;

Schuth, F. J. Am. Chem. Soc. 2008, 130, 11510. 19. Tiemann, M. Chem. Mater. 2008, 20, 961. 20. Gu, X.; Zhu, W.; Jia, C.; Zhao, R.; Schmidt, W.; Wang, Y. Chem. Commun.

2011, 47, 5337. 21. Yen, H.; Seo, Y.; Guillet-Nicolas, R.; Kaliaguine, S.; Kleitz, F. Chem.

Commun. 2011, 47, 10473. 22. Tuysuz, H.; Comotti, M.; Schuth, F. Chem. Commun. 2008, 4022. 23. Garcia, T.; Agouram, S.; Sanchez-Royo, J. F.; Murillo, R.; Mastral, A. M.;

Aranda, A.; Vasquez, I.; Dejoz, A.; Solsona, B. Appl. Catal. A 2010, 386, 16.

24. Salabas, E.; Rumplecker, A.; Kleitz, F.; Radu, E.; Schuth, F. Nano Lett. 2006, 6, 2977.

25. Jiao, F.; Harrison, A.; Hill, A. H.; Bruce, P. G.; Adv. Mater. 2007, 19, 4063. 26. Jiao, F.; Bao, J.; Hill, A. H.; Bruce, P. G. Angew. Chem. 2008, 120, 9857;

Angew. Chem. Int. Ed. 2008, 47, 9711.

58

27. Rossinyol, E.; Prim, A.; Pellicer, E.; Arbiol, J.; Ramirez, F.; Peiro, F.; Cornet, A. Morante, J.; Solovyov, L. A.; Tian, B. Z.; Tu, B.; Zhao, D. Adv. Funct. Mater. 2007, 17, 1801.

28. Tiemann, M. Chem. Eur. J. 2007, 13, 8376. 29. Cheng, F.; Tao, Z.; Liang, J.; Chen, J. Chem. Mater. 2008, 20, 667. 30. Ren, Y.; Hardwick, L. J.; Bruce, P. G. Angew. Chem. 2010, 122, 2624;

Angew. Chem. Int. Ed. 2010, 49, 2570. 31. Hill, A. H.; Harrison, A.; Ritter, C.; Yue, W.; Zhou, W. J. Magn. Magn.

Mater. 2011, 323, 226. 32. Schwickardi, M.; Johann, T.; Schmidt, W.; Schuth, F. Chem. Mater. 2002,

14, 3913. 33. Valdes-Solis, T.; Marben, G.; Fuertes, A. B. Chem. Mater. 2005, 17, 1919. 34. Wang, Y.; Ren, J.; Wang, Y.; Zhang, F.; Liu, X.; Guo, Y.; Lu, G. J. Phys.

Chem. C 2008, 112, 15293. 35. de Lima, R. K. C.; Batista, M. S.; Wallau, M.; Sanches, E. A.;

Mascarenhas, Y. P.; Urquita-Gonzalez, E. A.; Appl. Catal. B 2009, 90, 441. 36. Kleitz, F.; Choi, S. H.; Ryoo, R. Chem. Commun. 2003, 2136. 37. Kim, T. W.; Kleitz, F.; Paul, B.; Ryoo, R. J. Am. Chem. Soc. 2005, 127,

7601. 38. Kleitz, F.; Bérubé, F.; Guillet-Nicolas, R.; Yang, C.; Thommes, M. J. Phys.

Chem. C 2010, 114, 9344. 39. Sarshar, Z.; Kleitz, F.; Kaliaguine, S. Energy Environ. Sci. 2011, 4, 4258. 40. Royer, S.; Alamdari, H.; Duprez, D.; Kaliaguine, S. Appl. Catal. B 2005, 58,

273. 41. Baiker, A.; Martin, P. E.; Keusch, P.; Fritsch, E.; Reller, A. J. Catal. 1994,

146, 268. 42. Makshina, E. V.; Sirotin, S. V.; van den Berg, M. W. E.; Klementiev, K. V.;

Yushchenko, V. V.; Mazo, G. N.; Grunert, W.; Romanovsky, B. V.; Appl. Catal. A 2006, 312, 59.

43. Makshina, E. V.; Sirotin, S. V.; Yushchenko, V. V.; Mazo, G. N.; van den Berg, M. W. E.; Klementiev, K. V.; Grunert, W.; Romanovsky, B. V. Kinet. Catal. 2006, 47, 49.

44. Levasseur, B.; Kaliaguine, S. Appl. Catal. A 2008, 343, 29. 45. Ciambelli, P.; Cimino, S.; Lisi, L.; Faticanti, M.; Minelli, G.; Pettiti, I.; Porta,

P. Appl. Catal. B 2001, 33, 193. 46. Arai, H.; Yamada, T.; Eguchi, K.; Seiyama, T. Appl. Catal. 1986, 26, 265. 47. Seiyama, T. Catal. Rev. Sci. Eng. 1992, 34, 281. 48. Szabo, V.; Bassir, M.; Van Neste, A.; Kaliaguine, S. Appl. Catal. B 2003,

43, 81. 49. Ravikovitch, P. I.; Neimark, A. V. J. Phys. Chem. B 2001, 105, 6817.

59

Chapter 4 - Pore structure effects on the kinetics of methanol

oxidation over nanocast mesoporous perovskites

Mahesh Muraleedharan Nair,a Freddy Kleitz *a and Serge Kaliaguine*b



Canada.


Canada.

Submitted (2013)

60

4.1 Résumé

Des catalyseurs pérovskites mésoporeux LaMnO3 ayant des grandes surfaces spécifiques ont été synthétisés en utilisant une méthode récente de réplication solide connue sous le nom de « nanomoulage ». La silice mésoporeuse ordonnée dénommée SBA-15 a été utilisée comme matrice solide. Il a été découvert que la surface spécifique des pérovskites nanomoulées pouvait être contrôlée (80 – 190 m2g-1) en variant la température de traitement hydrothermal de la silice SBA-15 utilisée comme moule. Les catalyseurs LaMnO3 nanomoulés ont démontré de grandes efficacités de conversion pour l‟oxydation totale du méthanol en regime stationaire. Le catalyseur ayant la plus grande surface spécifique étan t, comme attendu, le meilleur catalyseur. Des études cinétiques ont été faites pour tous les catalyseurs synthétisés. La variation observée dans les constantes de vitesse résultantes venir linéairement avec la surface spécifique des catalyseurs nanomoulés, qui dépendent de la température de traitement hydrotermique de la silice utilisée comme moule. En utilisant les constantes de vitesse obtenues à partir des conversions observées expérimentalement à différentes vélocités spatiales (19000 à 78000 h-1), l‟énergie d‟activation et le facteur pré-exponentiel ont été déterminés pour les trois LaMnO3 nanomoulés grâce à une régression linéraire du graphique d‟Arrhenius. Il s‟est avéré que l‟énergie d‟activation de tous les catalyseurs reste constante, indépendamment de la variation de la surface spécifique des matériaux. De plus, une relation linéaire a été obtenue entre le facteur pré-exponentiel et la surface spécifique des catalyseurs, indiquant que les vitesses normalisées par la surface spécifique restent les mêmes pour tous les catalyseurs.

61

4.2 Abstract

Mesoporous LaMnO3 perovskite catalysts with high surface area were synthesized by using the recently developed hard templating method designated as “nanocasting.” Ordered mesoporous silica designated as SBA-15 was used as the hard template. It was found that the surface area of the nanocast perovskites can be tuned (80 – 190 m2g-1) by varying the aging temperature of the SBA-15 template. Nanocast LaMnO3 catalysts showed high conversion efficiencies for the total oxidation of methanol under steady state conditions, the one with the highest value of surface area being the best catalysts, as expected. Kinetic studies were performed for all of the synthesized catalysts. Rate constants were found to vary in accordance with the specific surface area of the nanocast catalyst which depends on the aging temperature of the parent template. From the rate constants obtained from experimental conversions at various space velocities (19000 to 78000 h-1), values of activation energy and pre-exponential factor for the three nanocast LaMnO3 catalysts were determined by the linear regression of the Arrhenius plot. It is observed that the activation energy for all the catalysts remain constant irrespective of the variation in specific surface area. Further, a linear relationship was found to exist between the pre-exponential factor and specific surface areas of the catalysts indicating that the rate per unit surface area remains the same for all the catalysts.

62

4.3 Introduction

Development of strategies in order to limit the emission of toxic gases such

as volatile organic compounds (VOC) from industrial processes is one of the major

challenges of the present time. It is desirable to develop environmentally friendly

technologies to eliminate pollutants without resulting in further toxic by-products.

Thermal combustion is effective; however, using a catalyst can result in much

better conversion rate at a comparatively lower temperature by altering the

kinetics. For this reason, various scientific efforts were made to develop novel

catalytic materials for that purpose.1,2 Even though noble metals were found to be

effective in most of the reactions concerned, it is not at all a cost effective solution

from an industrial perspective. Interestingly, perovskite structured mixed metal

oxides (ABO3) were found to be as effective as noble metals for various catalytic

partial or total oxidation reactions especially those of hydrocarbons and volatile

organic compounds.3,4 However, the applicability of these materials is not yet fully

exploited since the high temperature ( 700 ºC) conditions used in the synthesis of

these materials result in very low specific surface area (< 30 m2g-1).4-8 Hence a

major milestone that needs to be achieved for the effective utilization of these

materials on an industrial scale is the development of a synthesis strategy that

helps to achieve higher surface areas. On this regard, Kaliaguine et al.

successfully synthesized perovskite oxides with higher surface areas (100 m2g-1)

where calcination is performed at temperatures around 200 °C.9 However, in this

case also, the specific surface area was found to decrease at higher calcination

temperatures.

The discovery of ordered mesoporous silica and the developments that

followed on the research focusing on various mesoporous materials in the past two

decades have made it possible to also synthesize various non-siliceous

composition materials (carbon, metal oxides, carbides, etc) with extremely high

values of specific surface areas.10,11 Out of the methods available for the synthesis

of mesoporous materials, nanocasting enjoys a unique position. This method is

found to be efficient for developing mono-metallic and/or mixed oxides with high

63

specific surface areas,12-15 which cannot be obtained using other methods. Various

studies were performed utilizing the nanocasting approach for the synthesis of a

variety of compositions which were successfully utilized for a wide range of

applications.16-20 Even though these nanocast oxides were examined for a large

number of catalytic reactions, most of such studies focused on the measurement of

temperature dependent conversions as a function of catalyst composition or

surface area.14,21-25 For the successful employment of these materials in the

industry advanced knowledge on the surface reactions as well as the reaction

kinetics is required.

We have recently reported the synthesis and catalytic studies of

mesoporous perovskite oxides with high specific surface area synthesized using

the method of nanocasting.15 Our studies clearly demonstrated the higher catalytic

efficiency of the nanocast perovskites compared to their bulk counterparts

synthesized using reactive grinding method and conventional citrate method for

various gas phase reactions. To supplement our previous study,15 we report here

on the synthesis of high surface area LaMnO3 materials using SBA-15 silica aged

at different temperatures (35, 100 and 140 ºC) as the hard templates, and discuss

the influence of the porosity parameters of the thus-obtained perovskites on their

catalytic activity and kinetics. The catalytic properties of these high surface area

materials were studied for the total oxidation of methanol. The surface and redox

properties of the materials were analyzed using temperature-programmed

characterization methods. Detailed kinetic data processing was performed for

these materials to achieve a better understanding of the high catalytic efficiencies

observed for these new materials.

4.4 Experimental

4.4.1 Synthesis of ordered mesoporous SBA-15 silica

Ordered mesoporous silica SBA-15 hard templates were synthesized

according to the previously reported procedure, using Pluronic P123 as the

structure-directing agent and tetraethylorthosilicate (TEOS) as the silicon source.26

64

In a typical synthesis, 4.0 g of P123 was dissolved in 76 g of deionized water and

2.3 g of hydrochloric acid (37%) at 35 °C under magnetic stirring. To the obtained

homogeneous solution, 8.6 g of TEOS was rapidly added with continued stirring for

24 h at 35 °C and subsequently subjected to hydrothermal treatment at a desired

temperature (35, 100 and 140 ºC) for an additional 24 h to ensure further

framework condensation. After cooling, the resulting solution was filtered and the

solid products were dried at 100 °C for 24 h. Finally, the powders were calcined at

550 °C in order to remove the organic copolymer template.


Nanocasting of the mesoporous perovskites was performed by the

previously reported procedure using a citrate complex of metal cations as the

perovskite precursor and ordered mesoporous silica SBA-15 as the hard

template.15 The precursor was impregnated into the template by using the wet

impregnation method. In a typical synthesis, La(NO3)3·6H2O and Mn(NO3)2·xH2O

(3 mmol each) were dissolved in an ethanolic solution of citric acid (10 mL) to

obtain an equimolar solution, which was added slowly to SBA-15 (1 g) dispersed in

water (10 mL). The molar ratio of total metal ions and citric acid was kept at 2:1.

The mixture was stirred for a few hours at room temperature, and then the solvent

was evaporated under vacuum with a rotary evaporator. The powder thus obtained

was further dried at 80 ºC for 24 h, ground well in a mortar, and calcined at 500 ºC

for 4h to remove the organic part. Impregnation was repeated twice, using for the

second time one half of the amount of the precursor, to achieve higher loadings.

The final powder was calcined at 700 ºC for 6h, and the silica template was then

removed by treating the composite 3 times with NaOH (2M) at room temperature.

The obtained product was washed with water and ethanol and dried overnight at

80 ºC. Three syntheses were performed using template SBA-15 aged at 35, 100

and 140 ºC and here after, these samples will be denoted as LaMnO3-35, LaMnO3-

100 and LaMnO3-140, where the numbers indicate the aging temperature of the

SBA-15 template used.

65



diffractometer using Cu Kα radiation (λ = 1.5496 Aº). N2 physisorption analyses

were performed at -196 ºC with an ASAP 2010 sorption analyzer. Prior to analysis,

the samples were degassed overnight at 150 ºC. Specific surface areas of

nanocast perovskites were calculated using the BET method on the lower relative

pressure region of the isotherm (0.05–0.2). Pore size distributions were obtained

by using the NLDFT method assuming cylindrical pore geometry (applying the

kernel of metastable NLDFT adsorption isotherm, i.e., adsorption branch) supplied

by the Autosorb-1 1.55 software from Quantachrome Instruments.27-29 The total

pore volume was calculated from the nitrogen sorption capacity at P/P0=0.95.

Elemental analysis was performed using an M1100 B Perkin-Elmer atomic

absorption spectrophotometer. For TEM images, the samples were first dispersed

in ethanol, deposited on carbon grids and analyzed on a JEOL JEM 1230

microscope.

4.4.4 Temperature programmed reduction (TPR) and desorption (TPD)

A RXM-100 multicatalyst testing and characterization system (Advanced

Scientific Design Inc.) was used to perform TPD-O2 and TPR-H2. For TPR

experiments, the catalyst (50 mg) was placed in a quartz reactor and pretreated

under a flow of 20 mLmin-1 (20 % O2 in He) at 500 ºC for 1 h. The TPR was

performed under a flow of 10 mL.min-1 (5 % H2 in Ar) with a temperature ramp of 5

ºCmin-1 from 25 to 900 ºC. The consumption of hydrogen was monitored and

quantified with a thermal conductivity detector (TCD). For TPD-O2, the same pre-

treatment was performed as for the TPR experiments and the same amount of the

catalyst was used under a flow of 10 mL.min-1 of He. A TCD was used for the

quantification of the oxygen desorbed.


The catalytic bed was set up with the catalyst (200 mg) inserted between

two quartz wool plugs in a U-shaped quartz reactor (internal diameter = 5 mm).

66

The temperature was controlled using a K-type thermocouple placed in the reactor.

To purge the catalytic system, the catalysts were first flushed with He for 1 h at

room temperature and then pretreated for 1 h at 200 ºC before performing the

catalytic tests. The feed, composed of 0.5 % CH3OH and 5 % O2 in He, was

passed through the reactor and the temperature was increased. Gas samples were

collected in the steady-state regime after an interval of 2 h at constant conversion,

and the products were analyzed using a gas chromatograph (HP 6890 series)

equipped with a TCD. Reactants and products were separated using a Haye-Sep T

column (internal diameter = 1 mm, L=2 m X 5 m).


To obtain kinetic data, methanol steady-state conversions were monitored

for the nanocast mesoporous LaMnO3 catalyst at the reactor outlet at different flow

rates (10 – 40 mLmin-1 corresponding to a gas hourly space velocity of 19500 –

78200 h-1). The obtained experimental conversions were cross-plotted as a

function of pseudo-contact time over selected reaction temperatures. The values of

reaction rates obtained by the analytical derivatization of the equation

corresponding to the fitted curves were evaluated using a simplified linear equation

proposed originally by Arai et al. for the total oxidation of methane.4



The ordered mesoporous SBA-15 silica synthesized at different aging

temperatures used as the hard templates were characterized using N2

physisorption performed at -196 ºC (Figure 4.1). Clear indication of variations in

pore size, pore volume and surface area were observed in close agreement with

the literature.26,29-31 Using these silica materials as hard templates, a series of

LaMnO3 mesoporous perovskites were synthesized. After the removal of the silica

template, the presence of the perovskite structure was confirmed in all these

materials from the wide angle XRD patterns shown in Figure 4.2. No peaks

corresponding to crystalline impurities such as mono-metallic oxides of lanthanum

67

or manganese were observed in the wide angle region. Intensity of reflections was

higher for LaMnO3-35, most probably because this sample was dominated by non

porous bulk particles. A low intensity broad peak around 2 θ = 28 º was observed

for all the samples indicating the presence of an X-ray amorphous phase; most

probably silicates resulting from the incomplete removal of the SBA-15 template.

Figure 4.1 N2 physisorption isotherms (-196 °C) and the corresponding pore size distributions (right) of

ordered mesoporous SBA-15 silica hard templates. Pore size distributions were calculated from the adsorption branch of the isotherm using the NLDFT method.

Atomic absorption analysis confirmed the presence of approximately 10 % Si. This

might have occurred due to enhanced interactions between the template silica and

the rare earth metal in the perovskite structure. The presence of such residual

silicate species in nanocast perovskites were documented previously.14,15

Information regarding mesostructural order was obtained from the TEM images,

and representative images for all samples are shown in Figure 4.3. Well-ordered

mesostructural domains were clearly observed for LaMnO3-100 and LaMnO3-140

along with some less defined and disordered nanoporous regions. When SBA -15

aged at 35 ºC was used as the hard template, the presence of ordered regions

were comparatively less than the other nanocast perovskites.

68

Figure 4.2 Wide angle powder XRD patterns of mesoporous LaMnO3 perovskite oxides synthesized by use of

ordered mesoporous SBA-15 as the hard template. The numbers denote the aging temperature of the template.

Table 4.1 Structural parameters of the SBA-15 templates and nanocast perovskites obtained by performing N2


Sample SBET (m2g-1)a Dp (nm)b Vp (cm3g-1)c

SBA-15-35 738 5.8 0.66

SBA-15-100 960 7.3 1.1

SBA-15-140 852 9.0 1.7

LaMnO3-35 80 4.8 0.15

LaMnO3-100 140 5.1 0.21

LaMnO3-140 190 6.0 0.29


bNLDFT pore size;

cPore volume.

69

Figure 4.3 TEM images of LaMnO3-35 (a, b), LaMnO3-100 (c, d) and LaMnO3-140 (e, f).

Further information regarding the porosity and textural parameters were

obtained by performing N2 physisorption analysis at -196 ºC on mesoporous

LaMnO3 perovskites synthesized using SBA-15 hard templates aged at different

temperatures. The adsorption–desorption isotherms and the corresponding pore

size distributions for all these materials are shown in Figure 4.4. A type IV behavior

was observed for the isotherms.

70

Figure 4.4 N2 physisorption isotherms (-196°) and the corresponding pore size distributions (right) of nanocast

perovskite oxides synthesized by use of ordered mesoporous SBA-15 silica hard templates. Pore size distributions were calculated from the adsorption branch of the isotherm using the NLDFT method.

Hysteresis loops appear in the relative pressure range from 0.5 to 1.0 for

LaMnO3-100 and LaMnO3-140, which is typical of such mesoporous metal oxides

obtained from nanocasting.12,32 For LaMnO3-35 capillary condensation occurs at

comparatively higher relative pressure ie.; P/P0 = 0.6. Brunauer–Emmett–Teller

(BET) specific surface areas obtained for nanocast perovskites are exceptionally

high when compared with those of materials obtained by using conventional

methods, especially considering the high calcination temperature of 700 ºC (Table

4.1). Variations in specific surface area were observed in the case of nanocast

LaMnO3 with respect to the aging temperature of the hard template. A similar trend

was observed in the case of NLDFT average pore sizes and pore volumes

calculated from the volume adsorbed at the relative pressure of 0.95. An excellent

agreement was observed between the theoretically derived NLDFT isotherms and

the experimental isotherms for all the nanocast perovskites in the present study,

which validates the use of this method for the pore size determination of the

nanocast perovskites (Figure 4.5).

71

Figure 4.5 N2 physisorption isotherms (-196 °C) and the corresponding NLDFT theoretical isotherms (colour)

of nanocast LaMnO3 perovskite oxides synthesized by use of ordered mesoporous SBA-15 silica hard templates.

Studies examining the effect of template pore structure on the final nanocast

replica were performed by various authors. Rumplecker et al. 32 found that the

structure of Co3O4 replica can be tuned from randomly arranged rods to highly

ordered mesoporous network structures depending on interconnectivity of the

SBA-15 template, loading of the precursor and impregnation procedure. These

authors also confirmed that better replicas were observed by using microwave

digested silica template owing to the presence of higher fraction of interconnecting

mesopores in the same. Further, Jiao et al. synthesized ordered mesoporous NiO

with a bimodal pore size distribution consisting of a series of small (3.3 nm) and

large pores (11 nm). In this case, the bimodal porosity in nanocast NiO replica was

achieved by varying the degree of microporous bridging between the two sets of

mesopores in the ordered mesoporous KIT-6 hard template.33 Tuysuz et al.

performed studies on nanocasting of Co3O4 using KIT-6 as the hard template.34

These authors have shown that the textural parameters of the final replica strongly

depend on the structure parameters of the parent template used. In short, an

inverse correlation between the template aging temperatures with the BET surface

72

area, pore volume and pore size was observed. More recently, similar results were

obtained by Yen et al. for Cu-CeO2 mixed oxides.35 In all these studies the authors

used metal nitrates as the precursors for respective oxides. We believe that the

opposite trends obtained for perovskites in the present study is probably due to the

variation in pore size, pore volume and enhanced interconnectivity of the SBA-15

template, resulting from the increase in aging temperature. Larger pore volume and

interconnectivity seems to be needed to facilitate a more adequate loading of the

complexed precursor which consists of a bulky large organic molecule (e.g., metal

cations chelated with citric acid). This results in a comparatively better structural

order after the removal of the template and hence the highest value of surface area

when SBA-15 aged at 140 ºC was used as the hard template. Note that the

isotherm of LaMnO3-140 shows a particularly well-developed mesoporosity,

expressed by a sharp capillary condensation step and hysteresis loop,

characteristic of large ordered cylindrical-like mesopores.

4.5.2 Temperature programmed reduction of hydrogen

The reduction behavior of metal cations in nanocast mesoporous perovskites was

examined by using temperature-programmed reduction by hydrogen (TPR-H2).

Because the A-site metal is non reducible under the present conditions of TPR-H2,

the observed H2 consumption peaks correspond to the reduction behaviour of Mn

ions in the perovskite structure and the observed profiles are shown in Figure 4.6A.

Different from other perovskite compositions, complete reduction to Mn0 does not

occur for LaMnO3, under the present analysis conditions.36,37

Table 4.2 Amount of H2 consumed during TPR-H2.

Perovskite H2 consumed

[molH2 atomB-1]

Mn4+ (%)

LaMnO3-35 0.516 3

LaMnO3-100 0.536 7

LaMnO3-140 0.576 15

73

For all the three samples a broad main peak with a high temperature and a

low temperature shoulder were observed. The main peak can be assigned to the

reduction of Mn3+ to Mn2+ (0.5 mol H2 per atom of Mn). The low temperature

shoulder clearly indicates the presence of Mn4+ resulting from the presence of

over-stoichiometric oxygen as observed previously38 where as the high

temperature shoulder suggests the presence of Mn ions that are not easily

accessible. Interestingly, it was also observed that the Mn4+ content was higher for

LaMnO3-140. Absence of other noticeable peaks in the TPR pattern, especially at

higher temperatures indicates that a very negligible interaction, if any, exists

between the reducible metal (Mn in this case) and the residual silicon species. Also

significant changes in the reduction behaviour of Mn were hardly observed for any

of the samples in the present study. The amounts of hydrogen consumed during

the reduction of nanocast perovskites are given in Table 4.2. The values of Mn4+

show a clear variation from 3 to 15 %, which reflects the variations in the amount of

excess oxygen in the structure.

4.5.3 Temperature programmed desorption of oxygen

Figure 4.6 (A) TPR-H2 and (B) TPD-O2 profiles of nanocast mesoporous perovskite oxides synthesized by use

of ordered mesoporousSBA-15 silica hard templates aged at different temperatures.

Information regarding the nature of oxygen atoms on nanocast LaMnO3

perovskites was obtained by recording the temperature-programmed desorption of

oxygen (TPD-O2) profiles. Two types of oxygen species are generally found to be

desorbed from perovskite oxides. Desorption of oxygen bound to the surface

74

takesplace below 700 ºC (designated as α-O2), and desorption of lattice oxygen

takes place at a higher temperature (designated as β-O2). In the present work two

peaks were observed for LaMnO3-100 and LaMnO3-35, whereas in the case of

LaMnO3-140, the high temperature peak corresponding to the β-O2 was negligible.

Very clear increase in the intensity of the low temperature peaks (α-O2) was

observed for the nanocast LaMnO3 with respect to the increase in surface area of

the material (Figure 4.6B). The amounts of desorbed oxygen were calculated and

are listed in Table 4.3. Assuming 4 µmolm-2 of oxygen amounts to one monolayer,

the number of desorbed monolayers was also calculated.

Table 4.3 Amount of O2 desorbed during TPD-O2.

Perovskite α-O2 desorbed (µmolg-1)

β-O2 desorbed (µmolg-1)

α-O2 monolayers

β-O2 monolayers

LaMnO3-35 264 372 0.82 1.16

LaMnO3-100 506 220 0.82 0.38

LaMnO3-140 638 - 0.83 -


In our previous work we clearly showed that the nanocast perovskite

materials were better oxidation catalysts than similar compositions synthesized by

other methods.15 The catalytic oxidation efficiency of the mesoporous perovskites

was examined using methanol as the model compound, under the flow conditions

corresponding to a gas hourly space velocity of 39100 h-1. The comparison of

mesoporous LaMnO3 catalysts synthesized by varying the aging temperature of

the hard template, and thus varying porosity and surface area parameters, was

performed and the obtained temperature dependent conversion curves are given in

Figure 4.7. Clear variations in the conversion efficiencies with the values of the

catalyst specific surface area were observed. For the catalyst with largest specific

surface area full conversion was observed at 145 ºC. As expected, an increase in

full conversion temperature was observed for lower value of specific surface area.

75

Even though some conversion was already observed near to room temperature for

nanocast LaMnO3-100 and LaMnO3-140 ºC, the values of conversion was slightly

higher for the latter under same conditions of temperature. Also, CO2 was the only

product detected for all the catalysts, without any presence of formaldehyde or CO,

which indicates that all of these nanocast materials are highly efficient methanol

oxidation catalysts.

Figure 4.7 Methanol conversion profiles as a function of temperature over nanocast LaMnO3 perovskites

synthesized using SBA-15 silica hard template aged at different temperatures (GHSV = 39100 h-1

).


Kinetic data processing was performed for the total oxidation of methanol

over all three nanocast LaMnO3 catalysts. For this purpose the temperature

dependent conversions were monitored for these catalyst materials at different flow

rates which resulted in gas hourly space velocities ranging from 19500 to 78200 h-

1. The corresponding conversion curves obtained in the case of LaMnO3-35,

LaMnO3-100 and LaMnO3-140 are shown in Figure 4.8.

The conversion values at several selected temperatures were cross-plotted

against the pseudo-contact time W/F (in which W is the weight of the catalyst and

F is the molar flow rate of methanol). This yielded a series of isothermal curves as

shown in Figure 4.9. The resultant data were then fitted using the sigmoidal

equation 4.1, the analytical derivatization of which gave the values of reaction

rates:

76

(

) (4.1)

in which X is the conversion of methanol and a, b, and c are nonlinear regression

parameters.

Figure 4.8 Temperature dependent methanol conversion profiles for the total oxidation of methanol over

nanocast LaMnO3-35 (left) LaMnO3-100 (middle) and LaMnO3-140 (right) at different space velocities

The numerical values of the reaction rates (r) thus obtained were further

fitted using equation 4.2 and the values of rate constants (k) were obtained. This

equation which was previously proposed by Arai et al. assumes the participation of

only lattice oxygen in the catalytic process4 and the corresponding fit is shown in

Figure 4.9.

r = dX / d(W/F) = k.PMethanol (4.2)

The values of pre-exponential factor (A) and activation energy (Ea) for the

nanocast catalysts were determined from the Arrhenius plot by performing linear

regression analysis as shown in Figure 4.10. These values are reported in Table

4.4. The values of Ea were found to remain essentially constant irrespective of the

surface area of the catalyst used. Interestingly, a linear correlation was found to

exist between the pre exponential factors and the values of specific surface area of

the catalysts as shown in Figure 4.10. This indicates that the specific activity per

unit surface area remains the same for all the catalysts used in the present study.

77

Figure 4.9 Cross-plotting the values of experimental conversions at selected temperatures as a function of

pseudo-contact time obtained for nanocast LaMnO3. Points represent experimental data, and lines are calculated by using Equation (6.1). The numerical values of rates obtained in each case are represented as a function of the partial pressure of methanol (right).

The residual Si species remaining in these nanocast perovskites, that

cannot be completely removed after the multiple template removal steps (3 times

using 2M NaOH), can cause several effects towards the efficiency of these

materials. Previous studies on such residual species indicated a negative

contribution on the value of specific surface area.39 On the other hand, these

residual Si species could as well

78

Table 4.4. Kinetic parameters obtained for total oxidation of methanol over nanocast perovskites.

Perovskite aEa (kcal.mol-1) bA (molg-1atm-1) x 104

LaMnO3-35 13.49 9.7

LaMnO3-100 13.52 12.3

LaMnO3-140 13.51 24.2

Figure 4.10 Arrhenius plots for the rate constant k obtained for nanocast LaMnO3-35, LaMnO3-100 and

LaMnO3-140 (A). The linear correlation between the pre-exponential factor and the specific surface area is shown on the right (B).

induce a positive contribution by providing improved stability to these nanoporous

framework structures. Also, they could interact with the active phase and thereby

affect the efficiency of these materials. However, to confirm the existence of either

of these aforementioned effects on the catalytic activity of the nanocast

perovskites, more information regarding the exact nature of the residual Si species

are necessary. Studies are being in progress in this direction.

4.6 Conclusions

In conclusion, we have synthesized a series of mesoporous perovskite

oxides with lanthanum in the A site and manganese in the B site by using the

nanocasting method and SBA-15 aged at three different temperatures as hard

templates. These materials were found to display extremely high specific surface

area. Also a correlation between the aging temperature of the template and the

specific surface area was observed. The observed values of activation energy for

the catalysts were low and remained constant for all the catalysts under the

79

present conditions of study. Furthermore, apart from the variations in the textural

parameters of these materials, the specific activity for methanol oxidation per unit

surface area remains the same for all these nanocast perovskites. Further work is

in progress to determine the nature of residual Si species on these nanocast

perovskites that cannot be removed during the template removal step. We believe

that this is necessary to determine the role of these residual species on the

efficiency of such nanocast materials.

80

4.7 References

1. O„Malley, A.; Hodnett, B. K. Catal. Today 1999, 54, 31. 2. Garcia, T.; Solsona, B.; Cazorla-Amoros, D.; Linares-Solano, A.; Taylor, S.

H. Appl. Catal. B 2006, 62, 66. 3. Kim, C. H.; Qi, G.; Dahlberg, K.; Li, W. Science 2010, 327, 1624. 4. Arai, H.; Yamada, T.; Eguchi, K.; Seiyama, T. Appl. Catal. 1986, 26, 265. 5. Belessi, V. C.; Trikalitis, P. N.; Ladavos, A. K.; Bakas, T. V.; Pomonis, P. J.

Appl. Catal. A 1999, 177, 53. 6. Taguchi, H.; Yamada, S.; Nagao, M.; Ichikawa, Y.; Tabata, K. Mater.

Research Bull. 2002, 37, 69. 7. O‟Brien, S.; Brus, L.; Murray, C. B. J. Am. Chem. Soc. 2001, 123, 12085. 8. Kirchnerova, J.; Klvana, D. Solid State Ionics 1999, 123, 307. 9. Kaliaguine, S.; Van Neste, A.; Szabo, V.; Gallot, J. E.; Bassir, M.;

Muzychuk, R. Appl. Catal. A 2001, 209, 345. 10. Yang, H.; Zhao, D. J. Mater. Chem. 2005, 15, 1217. 11. Lu, A.H.; Schuth, F. Adv. Mater. 2006, 18, 1793. 12. Yen, H.; Seo, Y.; Guillet-Nicolas, R.; Kaliaguine, S.; Kleitz, F. Chem.

Commun. 2011, 47, 10473. 13. Jiao, F.; Harrison, A.; Hill, A. H.; Bruce, P. G. Adv. Mater. 2007, 19, 4063. 14. Wang, Y.; Ren, J.; Wang, Y.; Zhang, F.; Liu, X.; Guo, Y.; Lu, G. J. Phys.

Chem. C 2008, 112, 15293. 15. Nair. M. M.; Kleitz, F.; Kaliaguine, S. ChemCatChem 2012, 4, 387. 16. Tuysuz, H.; Lehmann, C. W.; Bongard, H.; Schmidt, R.; Tesche, B.;

Schuth, F. J. Am. Chem. Soc. 2008, 130, 11510. 17. Tiemann, M. Chem. Mater. 2008, 20, 961. 18. Jiao, F.; Shaju, K. M.; Bruce, P. G. Angew. Chem. Int. Ed. 2005, 44, 6550. 19. Tian, B. Z.; Liu, X.; Solovyov, L.; Liu, Z.; Yang. H.; Zhang, Z.; Xie, S.;

Zhang, F.; Tu, B.; Yu, C.; Terasaki, O.; Zhao, D. J. Am. Chem. Soc. 2004, 126, 865.

20. Jiao, F.; Harrison, A.; Jumas, J. C.; Chadwick, A. V.; Kockelmann, W.; Bruce, P. G. J. Am. Chem. Soc. 2006, 128, 5468.

21. Li, W-C.; Comotti, M.; Lu, A. H.; Schuth, F. Chem. Commun. 2006, 1772. 22. de Lima, R. K. C.; Batista, M. S.; Wallau, M.; Sanches, E. A.;

Mascarenhas, Y. P.; Urquita-Gonzalez, E. A. Appl. Catal. B 2009, 90, 441. 23. Du, Y.; Meng, Q.; Wang, J.; Yan, J.; Fan, H.; Liu, Y.; Dai, H. Microporous

Mesoporous Mater. 2012, 162, 199. 24. Marques, S. P. D.; Pinheiro, A. L.; Braga, T. P.; Valentini, A.; Filho, J. M.;

Oliveira, J. C. J. Mol. Catal. A 2011, 348, 1. 25. Sarshar, Z.; Kleitz, F.; Kaliaguine, S. Energy Environ. Sci., 2011, 4, 4258. 26. Choi, M.; Heo, W.; Kleitz, F.; Ryoo, R. Chem. Commun. 2003, 10, 1340. 27. Neimark, A. V.; Ravikovitch, P. I. Microporous Mesoporous Mater. 2001,

44, 697. 28. Landers, J.; Yu.Gor, G.; Neimark, A. V. Colloids Surf. A: Physicochem.

Eng. Aspects 2013, http://dx.doi.org/10.1016/j.colsurfa.2013.01.007. 29. Kleitz, F.; Bérubé, F.; Guillet-Nicolas, R.; Yang, C. M.; Thommes, M. J.

Phys. Chem. C 2010, 114, 9344.

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30. Galarneau, A.; Cambon, H,; Di Renzo, F.; Ryoo, R.; Choi, M.; Fajula, F. New J. Chem. 2003, 27, 73.

31. Kruk, M.; Jaroniec, M.; Ko, C. H.; Ryoo, R. Chem. Mater. 2000, 12, 1961. 32. Rumplecker, A.; Kleitz, F.; Salabas, E.; Schuth, F. Chem. Mater. 2007, 19,

485. 33. Jiao, F.; Hill, A. H.; Harrison, A.; Berko, A.; Chadwick, A. V.; Bruce, P.G. J.

Am. Chem. Soc. 2008, 130, 5262. 34. Tuysuz, H.; Comotti, M.; Schuth, F. Chem. Commun. 2008, 34, 4022. 35. Yen, H.; Seo, Y.; Kaliaguine, S.; Kleitz, F. Angew. Chem. Int. Ed. 2012, 51,

12032. 36. Royer, S.; Alamdari, H.; Duprez, D.; Kaliaguine, S. Appl. Catal. B 2005, 58,

273. 37. Baiker, A.; Martin, P. E.; Keusch, P.; Fritsch, E.; Reller, A. J. Catal. 1994,

146, 268. 38. Levasseur, B.; Kaliaguine, S. Appl. Catal. A 2008, 343, 29. 39. Marbam, G.; Fuertes, A. B.; Valdés-Solis, T. Microporous Mesoporous

Mater. 2008, 112, 291.

82

83

Chapter 5- Surface properties of nanocast mesoporous

perovskites

Mahesh Muraleedharan Nair,a Justyna Agata Florek,a Remy Guillet-Nicolas,a

Serge Kaliaguineb and Freddy Kleitz *a



Canada.


Canada.

Submitted (2013)

84

5.1 Résumé

Des oxydes pérovskites mésoporeux LaBO3 (B = Fe, Co and Mn) ayant des grandes surfaces spécifiques ont été synthétisés en utilisant une méthode récente de réplication solide connue sous le nom de « nanomoulage ». La silice mésoporeuse ordonnée dénommée KIT-6 a été utilisée comme matrice solide. Etant donné que la surface spécifique joue un rôle majeur pour une multitude d‟applications en catalyse, divers efforts ont été fais pour obtenir des informations quant à la structure de surface de ces pérovskites nanomoulées grâce à l‟utilisation de la 29Si RMN, du MEB couplé avec de l‟EDS et de la spectrométrie de photoélectrons induits par rayons X. Nos résultats indiquent clairement qu‟un faible pourcentage du moule de silice utilisé pour la synthèse de ces pérovskites mésoporeuses subsiste, et ce même après plusieurs traitements basiques (2M NaOH) censés dissoudre la matrice silicique. Ces impuretés restantes à la surface des pérovskites nanomoulées se présentent sous la forme de silicates de terres rares, résultant de la forte intéraction du moule de silice avec la terre rare métallique (La) utilisée lors de la synthèse des pérovskites. Nous avons aussi observé que la quantité de silicates résiduelle variait en fonction du métal présent dans les sites B de la structure pérovskite. De plus, l‟évolution de la structure surfacique de ces catalyseurs lors de chaque étape du processus de dissolution de la matrice de silice a été suivi en détails pour LaMnO3, étant donné que la quantité résiduelle d‟espèces contenant du Si après dissolution du moule était légèrement plus importante pour ce matériaux que pour les autres compositions synthétisées par nanomoulage.

85

5.2 Abstract

Mesoporous LaBO3 (B = Fe, Co and Mn) perovskite oxides with high surface areas were synthesized by using the recently developed hard templating method designated as “nanocasting”, using ordered mesoporous silica KIT-6 as the hard template. Since, for a multitude of applications as in catalysis, the surface composition plays a major role, attempts were made to obtain some information regarding the surface structure of these nanocast perovskites using 29Si–NMR, SEM-EDS and X-ray photoelectron spectroscopy. Our studies clearly indicated that a minor percentage of silica template used for the synthesis of these mesoporous perovskites remained even after multiple template removal treatments under alkaline conditions (2M NaOH). Clearly, this impurity species remaining on the surface of nanocast perovskites were found to exist in the form of rare earth silicates resulting from the enhanced interaction of the template with the rare earth metal (La) used for the perovskite synthesis. We also observed that the amount of these remaining silicates varied based on the B site metal of the perovskite structure. Further, we followed the evolution of the surface structure of these catalysts during each step of the template removal process for LaMnO3 since the amount of residual Si-species after the template removal was found to be slightly higher compared to the other compositions synthesized by nanocasting.

86

5.3 Introduction

Perovskite-type mixed metal oxides (ABO3) are well known for their

applications in catalysis and advanced technologies.1-5 The physical and chemical

properties of these materials, particularly for catalytic applications, are strongly

dependent on their structural composition, especially on the surface. The relation

between the surface structure and catalytic properties has raised substantial

interest in the past.6,7 These studies have revealed that the composition and the

method used for synthesis of the materials play an important role in determining

the surface properties of these mixed metal oxides. For example, the defect

concentration, reactive oxygen species or the presence of active metal in unusual

oxidation states strongly influences the redox processes that occur on the catalyst

surface.8-10 The formation of these surface species depends on the precursors and

the conditions used for the synthesis. Also, the surface properties of perovskite

catalysts can be varied by the partial substitution of either or both of the cations in

the A or B site in the structure.11,12 On the other hand, in the case of anion excess

perovskites, cation vacancies are formed which leads to the formation of inactive

species in the form of monometallic oxides of individual cations on the perovskite

surface, thereby altering the catalytic activity of the material.8 Various synthesis

strategies were developed in order to obtain perovskites with enhanced specific

surface area and modified surface properties.13-16 All this studies revealed that

depending on the difference in the synthesis steps, precursors used and the

temperature of crystallization, significant variations in the surface composition were

possible for these materials. Hence in order to fully utilize the enormous potential

inherent to these materials for advanced applications, a thorough investigation of

the surface properties is still required.

Nanocasting is one of the most recent methods developed for the synthesis

of high surface area metal oxides.17-19 Due to the versatility in producing a wide

variety of compositions with high values of specific surface area, this method has

gained considerable attention in the past decade. The materials synthesized by

this method are proven to exhibit excellent performance in a variety of

87

applications.20-22 In this method desired replica structures can be obtained by filling

the pores of a solid template with a precursor material, followed by subsequent

thermal treatment and template removal. Ordered mesoporous silica is the most

preferred template for nanocasting because of their comparatively easy and cost

effective synthesis. Among other factors, the complete removal of this template

silica is crucial for obtaining faithful replicas of the desired phase with high degree

of purity. This is generally achieved by selectively leaching the template either in

the presence of concentrated NaOH or HF. Although treatment with HF ensures

the complete removal of silica template, NaOH is preferred in most of the oxide

compositions because of the compatibility and safety issues. In this case, a small

amount of residual Si species is observed on the final nanocast material as

reported by various authors.23-29

Recently, we reported the synthesis and catalytic properties of various

nanocast mesoporous perovskite oxides.25,28 In our studies, we observed a certain

amount of residual Si after the complete removal of the template, especially for

manganese based perovskites. On this regard, we believe that it is particularly

important to examine the nature of residual Si and its effect on the properties of

these materials. Since the amount of residual Si species in nanocast LaMnO3 was

found to be higher than in other nanocast perovskites, the evolution of Si species in

LaMnO3 was carefully monitored during each step of template removal using 29Si-

NMR, SEM-EDX and X-ray photoelectron spectroscopy (XPS).

5.4 Experimental

5.4.1 Synthesis of ordered mesoporous KIT-6 silica

The synthesis of ordered mesoporous KIT-6 silica, which was used as a

hard template, was performed by using the method reported elsewhere by Kleitz et

al. using Pluronic P123 (EO20PO70EO20, MW = 5800 gmol-1) as the surfactant and

tetraethoxysilane as the silica source.30-32 In a typical synthesis, P123 (6 g) was

dissolved in distilled water (217 g) containing 35% HCl (11.8 g). To this solution, n-

butanol (6 g) was added at 35 ºC. After 1 h, tetraethoxysilane (12.9 g) was added

88

and stirred for 24 h at 35 ºC. The resulting mixture was hydrothermally treated at

100 ºC for another 24 h, and the solid product obtained was filtered, dried at 100

ºC for 24 h, and calcined at 550 ºC for 3 h.


Nanocasting of the mesoporous perovskites were performed by the

previously reported procedure using a citrate complex of metal cations as the

perovskite precursor and ordered mesoporous silica KIT-6 as the hard

template.25,28 The template was impregnated with the precursor by using the wet

impregnation method. In a typical synthesis for LaMnO3, La(NO3)3·6H2O and

Mn(NO3)2·xH2O (3 mmol each) were dissolved in an ethanolic solution of citric acid

(10 mL) to obtain an equimolar solution, which was added slowly to KIT-6 (1 g)

dispersed in water (10 mL). The molar ratio of total metal ions and citric acid was

kept at 2:1. The mixture was stirred for a few hours at room temperature, and then

the solvent was evaporated under vacuum in a rotary evaporator. The powder

thus-obtained was further dried at 80 ºC for 24 h, ground well in a mortar, and

calcined at 500 ºC for 4h. Impregnation was repeated twice, using for the second

time one half of the amount of the precursor, to achieve higher loadings. The final

powder was calcined at 700 ºC for 6h, and the silica template was then removed

by treating the composite 3 times with NaOH (2M) at room temperature. The

obtained product was washed with water and ethanol and dried overnight at 80 ºC.

In order to monitor the evolution of the surface structure and composition of

nanocast LaMnO3, a small amount of material was collected during each step of

NaOH treatment and dried at 80 ºC. The as made LaMnO3-silica composite after

calcination is designated below as LaMnO3-KIT-6. The materials after first, second

and third time NaOH treatment are designated as LaMnO3-NaOH-1, LaMnO3-

NaOH-2 and LaMnO3, respectively. Similarly, LaCoO3 and LaFeO3 represent

nanocast perovskites after 3 times NaOH treatment. For the synthesis of these

materials Mn(NO3)2·xH2O, was replaced by Co(NO3)3.6H2O and Fe(NO3)3. 9H2O

respectively, by keeping all the synthesis conditions the same.

89



diffractometer using Cu Kα radiation (λ = 1.5496 Aº). Elemental analysis was

performed with an M1100B Perkin-Elmer atomic absorption spectrophotometer. N2

physisorption analyses were performed at -196 ºC with an ASAP 2010 sorption

analyzer. Prior to analysis, samples were degassed overnight at 150 ºC. Specific

surface areas of nanocast perovskites were calculated using the BET method on

the lower relative pressure region of the isotherm (0.05–0.2). Pore size

distributions were obtained using the NLDFT method32,33 assuming cylindrical pore

geometry (applying the kernel of metastable NLDFT adsorption isotherm) supplied

by the Autosorb-1 1.55 software from Quantachrome Instruments. The total pore

volume was calculated from the nitrogen sorption capacity at P/P0 = 0.95.

Elemental analysis was performed using a M100B Perkin Elmer atomic absorption

spectrophotometer. For SEM-EDS analysis, the samples were first dispersed on an

aluminium stub and coated with Au/Pd film. Solid-state magic-angle spinning

(MAS) 29Si nuclear magnetic resonance (NMR) spectra were obtained on a Bruker

DRX300 MHz NMR spectrometer. The spectra were measured at 59.60 MHz using

7 mm rotors spinning at 4 kHz. For investigating the surface composition a PHI

5600-ci X-ray spectrometer (Physical Electronics, Eden Prairie, MN) was used.

The main XPS chamber was maintained at a base pressure of < 8*10-9 Torr. A

monochromatic aluminium X-ray source (Al k = 1486.6 eV) at 300 W was used to

record survey spectra (1400 eV, 10 min) with charge neutralization.



The synthesis and preliminary structural analysis of the nanocast

mesoporous perovskites synthesized using ordered mesoporous silica hard

templates (SBA-15 or KIT-6) and their applications in various gas phase catalytic

reactions were reported in our previous contributions.25, 28 For the present study,

we chose ordered mesoporous KIT-6 silica (3D porous network) as a suitable hard

90

Figure 5.1 N2 physisorption isotherms (-196 °C) and the corresponding pore size distributions (bottom) of

ordered mesoporous KIT-6 silica hard templates. Pore size distributions (inset) were calculated from the desorption branch of the isotherm using the NLDFT method.

32, 33

template and characterized using N2 physisorption performed at -196 ºC (Figure

5.1). By using the nanocasting method, a series of mesoporous perovskites was

synthesized with lanthanum in the A site and manganese, cobalt, or iron in the B

site of the ABO3 structure. Initially, the formation of the perovskite structure was

confirmed in each case from the characteristic peaks observed in the wide angle

XRD patterns shown in Figure 5.2

Figure 5.2 Wide-angle powder XRD patterns of mesoporous perovskite oxides synthesized by use of ordered

mesoporous KIT-6 silica aged at 100 ºC as a hard template. A comparison of the wide angle XRD pattern of nanocast LaMnO3 with its bulk counterpart synthesized by the citrate process is given on the right.

91

Figure 5.3 TEM images of nanocast LaFeO3, LaCoO3 and LaMnO3 perovskites synthesized using ordered

mesoporous KIT-6 aged at 100 ºC as hard template.

No crystalline impurity peaks corresponding to the monometallic oxide phases

resulting from the A or B site cations in the perovskite structure was found.

However in the case of LaMnO3 a low-intensity very broad peak centered at 2θ =

28 º was observed, indicating the presence of X-ray amorphous phase. As shown

in Figure 5.2, this broad peak observed in the case of nanocast LaMnO3 was

absent in the wide-angle XRD pattern of its bulk counterpart synthesized using the

amorphous citrate process under the same conditions. Further, results obtained

from the atomic absorption analysis indicate the presence of 3 % Si in the case of

LaFeO3 and LaCoO3, whereas for LaMnO3 this amount was found to be

approximately 10 %. Coupling these two facts it can be confirmed that this

92

amorphous species resulted from the residual Si species indicating incomplete

removal of template silica. Possibly the silica template might have undergone very

strong interaction with the metallic species (La and/or B = Mn, Co or Fe) during the

high temperature treatment involved in the synthesis of perovskites leading to the

formation of corresponding silicates.

Table 5.1 Structural parameters of the KIT-6 template and nanocast perovskites obtained by performing N2



bNLDFT pore size;

cPore volume;

dNanocast LaMnO3 after one catalytic run;

eSynthesized by using the reactive grinding

method;44

fSynthesized by using the amorphous citrate route.

In order to obtain information about the mesostructural order, TEM analysis

was performed and the representative images obtained are shown in Figures 5.3.

Well-ordered mesostructural domains were clearly observed, however, along with

the presence of some less defined and disordered nanoporous regions. Further, N2

physisorption analysis was performed at -196 ºC on mesoporous LaMnO3,

LaCoO3, and LaFeO3 perovskites. The adsorption–desorption isotherms and the

corresponding pore size distributions are shown in Figure 5.4. All these isotherms

exhibit a type IV behavior. An hysteresis loop appears in the relative pressure

range from 0.5 to 1.0, which is typical of such nanocast mesoporous metal

Sample SBET (m2g-1)a Dp (nm)b Vp (cm3g-1)c

KIT-6 948 8.4 1.2

LaCoO3 125 4.8 0.1

LaFeO3 110 4.8 0.1

LaMnO3-Cd 15 - -

LaMnO3-500 ºC 392 5.8 0.4

LaMnO3-700 ºC 115 5.8 0.1

LaMnO3-NaOH-1 132 6.0 0.2

LaMnO3-NaOH-2 163 6.0 0.2

LaMnO3 163 5.8 0.2

93

oxides.23-26,29 Brunauer– Emmett–Teller (BET) specific surface areas obtained for

nanocast perovskites are exceptionally high (up to 163 m2g-1) as compared with

those of materials obtained by using conventional methods, especially considering

the high calcination temperature of 700 ºC. The total pore volumes of perovskites

were calculated from the volume adsorbed at the relative pressure of 0.95, and the

pore sizes were derived from the adsorption branch by using the nonlocal density

functional theory (NLDFT) method. The resulting data are compiled in Table 5.1.

Figure 5.4 (A) N2 physisorption isotherms and b) pore size distributions of nanocast perovskite oxides

synthesized using ordered mesoporous KIT-6 silica aged at 100 ºC as a hard template. The isotherms of LaCoO3 and LaFeO3 are plotted with an offset of 60 and 110 cm

3g

-1, respectively, for clarity. (B) Pore size

distributions were calculated from the adsorption branch of the isotherm using the NLDFT method.32, 33

In the case of LaMnO3, N2 physisorption analysis was performed on the

calcined samples after each step of impregnation. The thus obtained isotherms

and pore size distributions are given in Figure 5.5. Considerable decrease in the

volume adsorbed was observed after each step of impregnation. Also the values of

BET surface area, pore size and pore volume are reduced in comparison with the

pure KIT-6 silica. This indicates that the template pores are being occupied. The

value of BET surface area of the LaMnO3-KIT-6 composite calcined at 700 ºC

agrees well with the previously reported value for LaCoO3-KIT-6 composite.24 The

NLDFT pore size distribution was found to be narrow for both the samples with a

shift towards lower pore size region compared to what is observed for the KIT-6

template.

94

Figure 5.5 N2 physisorption isotherms (left) and pore size distributions (right) LaMnO3-KIT-6 composite after

each step of impregnation. The materials were calcined at 500 and 700 ºC after the first and second impregnation respectively. Pore size distributions were calculated from the adsorption branch of the isotherm using the NLDFT method.

32,33

Further, during the first two NaOH treatments an increase in specific surface

area was observed which remained constant after the third NaOH treatment.

Eventhough an increase in pore volume was observed after the first NaOH

treatment, no further variation was observed. The data are compiled in Table 5.1.

Figure 5.6 N2 physisorption isotherms (left) and pore size distributions (right) of nanocast LaMnO3 during each

step of NaOH treatment. The isotherm of LaMnO3-NaOH-1 is plotted with an offset of 30 cm3g

-1, for clarity.

Pore size distributions were calculated from the adsorption branch of the isotherm using the NLDFT method.

32,33

Attempts were made to elucidate the nature of the residual silicates using

29Si MAS NMR spectra. Even though we observed a broad peak corresponding to

the Qn resonances for the calcined (700 ºC) LaMnO3-KIT-6 composite (Figure 5.7),

no detectable signals were observed for any of the samples after NaOH treatment.

95

Figure 5.7 29

Si MAS NMR spectra of LaMnO3-KIT-6 composite calcined at 700 ºC.

5.5.2 SEM-EDS analysis

Figure 5.8 Representative SEM images of the nanocast perovskites. LaMnO3-C represents the bulk sample

synthesized using the citrate process.

Representative SEM images of the nanocast LaCoO3, LaFeO3 and LaMnO3

samples are given in Figure 5.8. For all of the present samples, random

agglomeration of fine particles with a sponge-like appearance was observed. For

comparison SEM analysis of the bulk LaMnO3 synthesized by the citrate method

was also performed. From the figure, it is clearly observed that the extent of

agglomeration is much lower for the nanocast perovskites in comparison to the

bulk.

96

SEM analysis was also performed for LaMnO3 during each step of template

removal (Figure 5.9). The elemental compositions of the materials during each step

of template removal, as determined by performing EDS mapping confirmed that the

surface layer consisted of a homogeneous distribution of La, Si and the

corresponding transition metal of the perovskite composition. The La/M ratio was

found to be approximately 1 which is expected for the perovskite stoichiometry.

Slight deviations were observed in the case of LaMnO3. This indicates that the

surface layer is dominated by La in comparison with the transition metal most

probably because of the formation of silicate species.

Figure 5.9 Representative SEM images of the nanocast LaMnO3 perovskites during each step of template

removal.

5.5.3 XPS analysis

XPS analysis of all the nanocast mesoporous perovskites was performed. In

all these materials after the removal of the template, the oxygen photoelectron

peaks (O 1s) was found to be deconvoluted into two peaks, as shown in Figure

5.10. The lower binding energy peak (529.1 eV) could be attributed to the

presence of lattice oxygen species (O2-) and the higher binding energy peak 530.8

eV) to less electron rich oxygen species (OH-, surface adsorbed oxygen or

97

resulting from the water or CO32- adsorbed on the surface).34-36 The intensity of the

higher binding energy peak was found to be slightly higher for LaMnO3.

For La 3d peaks a doublet is observed for each component La 3d3/2 and La

3d5/2.37 The position of the La 3d peaks is dependent on the B site atom in the

perovskite structure with respect to the relative electron density around the La

atom. Generally, the values of binding energy decrease in the order LaMnO3 <

LaFeO3 < LaCoO3.38 In the present case the position of La 3d5/2 appeared at a

binding energy of 832.9 eV, 833.9 eV and 834.1 eV for LaMnO3, LaFeO3 and

LaCoO3 respectively (Figure 5.10). The peaks observed for the transition metal

ions (Mn 2p, Co 2p and Fe 2p) in the respective perovskite oxides are given in

Figure 5.11. The values of binding energies match well with the respective bulk

compositions as reported earlier.34,39-42 For Mn 2p, two peaks were observed at

641.2 eV and at 652.9 eV.39 Shifting of the Mn 2p peaks towards higher values of

binding energy (above 642 eV in the case of Mn 2p3/2 and above 653.5 eV for Mn

2p1/2) generally indicates the presence of lower valence state manganese.40

Figure 5.10 O 1s, La 3d and Si 2s XPS spectra of nanocast mesoporous perovskites after removal of the KIT-

6 hard template.

98

However in the present case it is confirmed that all the Mn atoms are in 3+

or 4+ oxidation state as observed previously by TPR-H2.25 For the LaFeO3, the

peak positions of Fe 2p3/2 and Fe 2p1/2 are at 709.8 eV and 723.9 eV,

respectively.41 The value of Fe 2p3/2 binding energy is much lower than the value

reported for the same peak in the case of Fe2O3 (710. 8 eV).41 This confirms that

the Fe species in the surface of the sample consists exclusively of Fe3+ in the

perovskite structure. For LaCoO3, two peaks were observed which also correspond

to Co 2p3/2 – Co 2p1/2 energy splitting at 778.1 eV and 793.1 eV respectively.

Absence of shake up peaks at 783 eV and 802 eV confirms the absence of lower

valence Co. The experimental energy splitting of 15 eV is in good agreement with

what is reported previously in the case of LaCoO3.34,42 The value of Co 2p3/2

obtained in the present work matches well with what was previously reported for

mesoporous LaCoO3.24

Figure 5.11 Fe 2p, Co 2p and Mn 2p core level XPS spectra of nanocast mesoporous perovskites after

removal of the KIT-6 hard template.

99

Irrespective of the composition, peaks corresponding to Si are detected on

all the samples analyzed; only differences in the intensity were observed.

Generally quantification of Si species is based on Si 2p peaks. However in this

study we observed a very strong overlapping between the Si 2p and La 4d peaks

which made the interpretation difficult. Hence, Si 2s was used for determining the

nature of Si in the nanocast perovskites.44 The binding energy of Si 2s in pure SiO2

is approximately at 157 eV.45 As shown in Figure 5.10, the Si 2s region

encompasses a single peak centered on 154.1 eV in the case of both LaCoO3 and

LaFeO3, whereas for LaMnO3, the peak position shifted slightly to 154.6 eV along

with a slight enhancement in the peak intensity. Previous studies have shown the

occurrence of Si peaks belonging to rare earth silicates in this region.45 In the

present case, it is believed that this peak corresponds to the presence of X ray

amorphous La silicates on the surface of these nanocast perovskites which might

have resulted from the strong interaction between the rare earth precursor and the

parent silica template. In a previous study using nanocast LaMnO3, the formation

of lanthanum silicate and manganese silicate was observed after prolonged

exposure to the reaction stream.29

Further we performed the XPS analysis of LaMnO3 sample collected during

each step of template removal to understand the evolution of the surface structure

during the template removal step. Peaks corresponding to O2, La, Mn and Si were

observed for all the samples and are shown in Figure 5.12. The intensity of Mn 2p

peaks were very small for LaMnO3-KIT-6 composite and were found to increase

during each step of template removal. A similar trend was observed for La 3d. In

the case of oxygen photoelectron peaks (O 1s) for LaMnO3-KIT-6, the intensity of

the higher binding energy peak was found to be much higher indicating that the

major contribution from this peak could be attributed to the O2 from SiO2. Even

though the intensity of this peak decreased after one time NaOH treatment,

indicating that the contribution from silicates is reduced, it again become enhanced

on further NaOH treatments which can be attributed to the presence of less

electron rich oxygen species (OH-, surface adsorbed oxygen or resulting from the

water or CO32- adsorbed on the surface).34-36 The Si 2s peak of the LaMnO3-KIT-6

100

was found to be deconvoluted into two peaks; a low intensity peak at 154.5 eV and

a high intensity peak at 157.3 eV. The intensity of the higher binding energy peak

decreased during each step of template removal and finally almost disappears for

the sample after 3 times NaOH treatment.

Figure 5.12 Mn 2p, La 3d, O 1s and Si 2s core level XPS spectra of nanocast mesoporous perovskites during

each step of template removal step using 2M NaOH.

Surface compositions were obtained from the XPS analysis. For all the

nanocast perovskites, the La/M atomic ratios reach the expected stoichiometric

values, close to 1 and slightly higher in the case of LaMnO3. The surface

compositions of the materials are given in Table 5.2. The highest La/M ratio is

observed for as synthesized LaMnO3-KIT-6 composite. This could be possibly

because of the enhanced affinity of the rare earth (La) with the template silica. Also

it is to be noted here that the Mn/Si ratio is negligible in the composite. After each

step of template removal, it is observed that the La/Si and M/Si ratios tend to

increase. The presence of rare earth silicates after these nanocast perovskites

being subjected to various catalytic reactions was previously reported.19,20

101

Variations in La/Si and M/Si ratio was observed with respect to the exchange of the

transition metal in the perovskite structure. In general, for anion excess perovskites

vacancies are formed in the cation sub lattice leading to the formation of separate

La2O3 or La(OH)3, in order to maintain the electroneutrality. We believe that in the

case of nanocast perovskites, this excess La, resulting from the formation of

vacancy sites are reacting with the SiO2 template, leading to the formation of La

silicates.

Table 5.2 Surface elemental composition of the nanocast mesoporous perovskites.

Sample La/M La/Si M/Si

LaFeO3 1.01 0.89 0.88

LaCoO3 1.02 1.08 1.05

LaMnO3 1.23 1.15 0.94

LaMnO3-KIT-6 3.4 0.13 0.04

LaMnO3-NaOH-1 1.7 0.66 0.39

LaMnO3-NaOH-2 1.3 0.95 0.74

5.6 Conclusions

In conclusion, high surface area mesoporous perovskites were obtained by

nanocasting using ordered mesoporous silica as hard templates. After NaOH

treatment, generally applied for the removal of template silica, it was observed that

a small amount of residual silica species remain on the thus obtained perovskites,

especially in the case of LaMnO3. Initial indication of the formation of this

amorphous silicate species was obtained from the wide angle XRD analysis, which

was further attributed to the presence of Si species from the results obtained from

atomic absorption spectroscopy. Further, by performing SEM-EDS analysis, a

homogeneous distribution of this residual silicate species throughout the surface

layer of these nanocast perovskites was observed. The presence of amorphous

rare earth silicates were also observed from wide angle XRD and XPS analyses. In

general, for anion excess perovskites vacancies are formed in the cation sub lattice

102

leading to the formation of separate La2O3 or La(OH)3, in order to maintain the

electroneutrality. We believe that in the case of nanocast perovskites, this excess

La, resulted from the formation of vacancy sites are reacting with the SiO2

template, leading to the formation of La silicates.

103

5.7 References

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2. Suntivich, J.; Gasteiger, H. A.; Yabuuchi, N.; Nakanishi, H.; Goodenough, J. B.; Shao-Horn, Y. Nat. Chem. 2011, 3, 546.

3. Wakabayashi, Y.; Upton, M. H.; Grenier, S.; Hill, J. P.; Nelson, C. S.; Kim, J.-W.; Ryan, P. J.; Goldman, A. I.; Zheng, H.; Mitchell, J. F. Nat. Chem. 2007, 6, 972.

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273. 10. Levasseur, B.; Kaliaguine, Appl. Catal. A 2008, 343, 29. 11. Spinicci, R.; Marini, P.; De Rossib, S.; Faticantib, M.; Portab, P. J. Mol.

Catal. 2001, 176, 253. 12. Pecchi, G.; Reyes, P.; Zamora, R.; Campos, C.; Cadus, L. E.; Barberob, B.

P. Catal. Today 2008, 133, 420. 13. Baythoun, M. S. G.; Sale, F. R. J. Mater. Sci. 1982, 17, 2757. 14. Taguchi, H.; Yamada, S.; Nagao, M.; Ichikawa, Y.; Tabata, K. Mater. Res.

Bull. 2002, 37, 69. 15. Johnson, D. W.; Gallagher, P. K.; Wertheim, G. K.; Vogel, E. M. J. Catal.

1977, 48, 87. 16. Kaliaguine, S.; Van Neste, A.; Szabo, V.; Gallot, J. E.; Bassir, M.;

Muzychuk, R. Appl. Catal. A 2001, 209, 345. 17. Yang, H.; Zhao, D. J. Mater. Chem. 2005, 15, 1217. 18. Lu, A.H.; Schuth, F. Adv. Mater. 2006, 18, 1793. 19. Tiemann, M. Chem. Mater. 2008, 20, 961. 20. Jiao, F.; Harrison, A.; Hill, A. H.; Bruce, P. G. Adv. Mater. 2007, 19, 4063. 21. Tiemann, M. Chem. Eur. J. 2007, 13, 8376. 22. Tuysuz, H.; Comotti, M.; Schuth, F. Chem. Commun. 2008, 4022. 23. Yen, H.; Seo, Y.; Guillet-Nicolas, R.; Kaliaguine, S.; Kleitz, F. Chem.

Commun. 2011, 47, 10473. 24. Wang, Y.; Ren, J.; Wang, Y.; Zhang, F.; Liu, X.; Guo, Y.; Lu, G. J. Phys.

Chem. C 2008, 112, 15293. 25. Nair. M. M.; Kleitz, F.; Kaliaguine, S. ChemCatChem 2012, 4, 387. 26. Yen, H.; Seo, Y.; Kaliaguine, S.; Kleitz, F. Angew. Chem. Int. Ed. 2012, 51,

12032. 27. Wang, Y.; Yang, C. M.; Schimdt, W.; Spleithoff, B.; Schuth, F. Adv. Mater.

2005, 17, 53. 28. Nair. M. M.; Kleitz, F.; Kaliaguine, S. submitted. 29. Sarshar, Z.; Kleitz, F.; Kaliaguine, S. Energy Environ. Sci. 2011, 4, 4258. 30. Kleitz, F.; Choi, S. H.; Ryoo, R. Chem. Commun. 2003, 2136.

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31. Kim, T. W.; Kleitz, F.; Paul, B.; Ryoo, R. J. Am. Chem. Soc. 2005, 127, 7601.

32. Kleitz, F.; Bérubé, F.; Guillet-Nicolas, R.; Yang, C.; Thommes, M. J. Phys. Chem. C 2010, 114, 9344.

33. Ravikovitch, P. I.; Neimark, A. V.; J. Phys. Chem. B 2001, 105, 6817. 34. Natile, M. M.; Ugel, E.; Maccato, C.; Glisenti, A. Appl. Catal. B 2007, 72,

351. 35. Kaliaguine, S.; Van Neste, A.; Szabo, V.; Gallot, J. E.; Bassir, M.;

Muzychuk, R. Appl.Catal. A 2001, 209, 345. 36. Dacquin, J. P.; Lancelot, C.; Dujardin, C.; Da Costa, p.; Djega-

Mariadassou, G.; Beaunier, p.; Kaliaguine, S.; Vaudreuil, S.; Royer, S.; Granger, P. Appl. Catal. B 2009, 91, 596.

37. Milt, V. G.; Spretz, R.; Ulla, M. A.; Lambardo, E. A.; Fierro, J. L. G. Catal. Letters 1996, 42, 57.

38. Xin, Z.; Qiuhua, Y.; Jinjin, C. J. Rare Earth 2008, 26, 511. 39. Sinquin, G.; Petit, C.; Hindermann, J. P.; Kinnemann, A. Catal. Today

2001, 70, 183. 40. Shannigrahi, S. R.; Tan, S. Y. Mater. Chem. Phys. 2011, 129, 15. 41. Petrovic,; Terlecki-Baricevic, A.; Karanovic, L.J.; Kirilov-Stefanov, P.;

Zdujic, M.; Dondur, V.; Paneva, D.; Mitov, I.; Rakic, V. Appl. Catal. B 2008, 79, 186.

42. Seim, H.; Nieminem, M.; Niinisto, L.; Fjellvag, H.; Johansson, L. S. Appl. Surf. Sci. 1997, 112, 243.

43. Natile, M. M.; Ugel, E.; Maccato, C.; Glisenti, A. Appl. Catal. B 2007, 72, 351.

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45. Guillet-Nicolas, R.; Bridot, J-L.; Seo, Y.; Fortin, M-A.; Kleitz, F. Adv. Funct. Mater. 2011, 21, 4653.

105

Chapter 6 - Coke resistant nanostructured Ni/La2O3 catalyst for

dry reforming of methane

Mahesh Muraleedharan Nair,a Serge Kaliaguine*b and Freddy Kleitz *a



Canada.


Canada.

Submitted (2013)

106

6.1 Résumé

Le reformage à sec du méthane suscite un intérêt croissant, notamment grâce au fait que ce procédé converti efficacement deux gaz à effets de serre (CH4 et CO2) en gaz de synthèse (CO + H2) qui peut ensuite être utilisé pour fabriquer des combustibles liquides ou des produits chimiques. Dans cette contribution, un matériau nanostructuré Ni/La2O3, dérivé d‟une pérovskite est proposé comme catalyseur efficace et stable pour cette réaction. Des précurseurs de pérovskites LaNiO3 à haute surface spécifique sont d‟abord synthétisés grâce à la méthode du nanomoulage en utilisant une silice ordonnée mésoporeuse de type SBA-15 comme matrice solide. La pérovskite nanostructurée ainsi obtenue possède une grande surface spécifique BET (150 m2g-1). Le comportement de la pérovskite nanomoulée lors de sa réduction a été suivi par réduction thermo-programmée par l‟hydrogène (TPR-H2). Il a été observé que la destruction complète de la structure pérovskite survenait en dessous de 700 °C, conduisant à la formation de Ni0 hautement dispersé dans La2O3, tel que montré par l‟analyse DRX du matériau après réduction. Un comportement similaire a été observé pour une pérovskite LaNiO3 synthétisée en utilisant le procédé citrate conventionnel. Cependant, la surface spécifique du matériau nanomoulé s‟est avéré plus imortante que celle du matériau citrate (50 m2g-1). De manière évidente, cette différence est liée à l‟architecture mésoporeuse du LaNiO3 obtenu par nanomoulage. Dans les conditions de réactions utilisés dans cette étude, le Ni/La2O3 nanostructuré obtenu après réduction du LaNiO3 nanomoulé a démontré une activité plus importante dans la conversion du CH4 et CO2 que le catalyseur pérovskite conventionnel. En particulier, la formation de coke n‟a pas été observée lors de l‟utilisation du catalyseur mésoporeux dans les conditions d‟opération décrites ici, ce qui démontre la stabilité accrue du catalyseur obtenu à partir du LaNiO3 nanomoulé. La meilleure performance du catalyseur nanostructuré est attribuée à la meilleure accessibilité des sites actifs du à une grande surface spécifique ainsi qu‟à l‟effet de confinement conduisant à la stabilisation des nanoparticules de Ni.

107

6.2 Abstract

Dry reforming of methane is gaining great interest owing to the fact that this process efficiently converts two green house gases (CH4 and CO2) into synthesis gas (CO + H2) which can be further processed into liquid fuels and chemicals. Herein, a perovskite-derived nanostructured Ni/La2O3 material is reported as an efficient and stable catalyst for this reaction. High-surface-area LaNiO3 perovskite precursor is first synthesized by the method of nanocasting using ordered mesoporous silica SBA-15 as a hard template. The resulting nanostructured perovskite was found to possess high specific surface area as obtained from the BET method (150 m2g-1). The reduction behaviour of the nanocast perovskite was monitored by performing temperature programmed reduction by hydrogen (TPR-H2). It has been found that the complete destruction of perovskite structure occurs below 700 °C, leading to the formation of highly dispersed Ni0 in La2O3, as observed in the XRD pattern of the material after reduction. Similar behavior was observed for the LaNiO3 perovskite synthesized using the conventional citrate process. However, the specific surface area of the former material was found to be much higher than that of the latter (50 m2g-1), which obviously resulted from the mesoporous architecture of the nanocast LaNiO3. It was found that the nanostructured Ni/La2O3 obtained from the reduction of the nanocast LaNiO3 exhibited higher activity for the conversion of the reactant gases (CH4 and CO2) compared to the catalyst obtained from conventional perovskite, under the reaction conditions used in the present study. Particularly, no coke formation was observed for the mesoporous catalyst under the present conditions of operation, which in turn reflects the enhanced stability of the catalyst obtained from the nanocast LaNiO3. The improved performance of the nanostructured catalyst is attributed to the accessibility of the active sites resulting from the high specific surface area and the confinement effect leading to the stabilization of Ni nanoparticles.

108

6.3 Introduction

Clean and energy efficient technologies mostly rely on high temperature

catalytic conversions, prominent examples being the hydrocarbon reforming

reactions either in presence of H2O (steam reforming)1-3 or in presence of CO2 (dry

reforming).4-6 Steam reforming (Equation 6.1 ) is used commercially on a large

scale as an effective method for producing hydrogen and synthesis gas (CO + H2).

Alternatively, dry reforming of methane (Equation 6.2) is gaining attention since it

consumes two green house gases (CH4 and CO2) and converts them into

synthesis gas, which can further be processed into liquid fuels or chemicals.

Indeed, the produced synthesis gas (H2/CO ratio close to unity) is highly desirable

for the industrial production of many valuable chemicals.7,8 The dry reforming

reaction shows also great potential in energy transfer and storage systems due to

its high endothermicity.9

CH4 + H2O CO + 3H2 ΔH0 = 206 kJmol-1 (6.1)

CH4 + CO2 2CO + 2H2 ΔH0 = 247 kJmol-1 (6.2)

Thus far, noble metals were found to be the most active catalysts for dry

reforming.10-13 However, previous studies showed that Ni metal could possibly

replace the expensive noble metal catalysts.14-17 However, these studies revealed

that the major problem associated with Ni-based catalysts is that they also

catalyzes the formation of coke (carbon). Such carbon formation on the surface of

the catalyst impedes the activity and thereby affects the long term stability, which in

turn hinders industrial applications of Ni catalysts. Various strategies were

developed either to avoid or to minimize the coke deposition on the catalyst

surface during dry reforming, e.g., reducing the particle size, use of a promoter,

using a basic support etc.18 Alternatively, resistance to coke formation can be

significantly enhanced if the nickel catalyst is located within a well defined

structure. For this, preformed crystalline oxides (such as spinels or perovskites)

that contain the active metal homogeneously dispersed inside the bulk can be used

as catalyst precursors.19,20 Upon reduction, these oxides lead to the migration of

109

some of the active metals to the surface, resulting in homogenous distribution of

active metal sites on the support. In this case, the metal-support interaction and

hence the thermal stability can also be enhanced. A well-known example is the

formation of Ni/La2O3 by performing the reduction of a LaNiO3 perovskite

precursor. Previous reports suggested that these materials are active for dry

reforming.20-24 However, multiple heat treatment steps involved in the synthesis of

these materials generally result in the formation of catalysts with extremely low

values of specific surface area. Therefore, efforts are still needed to produce such

catalysts with high specific surface area and to substantiate the influence of Ni

dispersion on the performance of these materials in the dry reforming reaction.

Nanocasting is a highly versatile method which enables the synthesis of various

non-siliceous mesoporous materials with extremely high values of specific surface

area (oxides, carbon, etc).25,26 These nanostructured materials are found to be

proficient for a multitude of applications such as catalysts, sensors, batteries, and

so forth.27-29 Recently, we reported the synthesis of high surface area perovskite-

structured mixed metal oxides by using the nanocasting method.30 These materials

exhibited high catalytic activity for the total oxidation of methanol. Although the

nanocasting method is widely preferred for the synthesis of a range of oxide and

non oxide materials, these nanocast materials are still rarely used for applications

involving high temperatures. Moreover, the high temperature structural

modifications which can take place in the nanocast materials are rarely studied,31

especially for mixed metal oxides.

Here, we describe the synthesis of nanocast mesoporous LaNiO3, prepared

using ordered mesoporous SBA- 15 silica as the hard template and the resulting

nanostructured Ni/La2O3 catalyst obtained by post-synthesis reduction of the

nanocast perovskite. Furthermore, the activity and stability of the high surface area

materials in the catalytic dry reforming of methane is demonstrated. Comparisons

were made using Ni/La2O3 obtained from conventional bulk LaNiO3 perovskites

synthesized using the citrate method. Our work shows that the materials obtained

by nanocasting are greatly superior to those of similar composition obtained from

the conventional process. Moreover, the exceptional resistance of the nanocast

110

materials to coke formation is revealed, which is particularly significant for catalytic

applications.

6.4 Experimental

6.4.1 Synthesis of ordered mesoporous SBA-15 silica

SBA-15 material was synthesized using Pluronic P123 as the structure-

directing agent and tetraethylorthosilicate (TEOS) as the silicon source.32 In a

typical synthesis, 4.0 g of P123 was dissolved in 76 g of de-ionized water and 2.3 g

of hydrochloric acid (37%) at 35 °C under magnetic stirring. To the obtained

homogeneous solution, 8.6 g of TEOS was rapidly added with continued stirring for

24 h at 35 °C and subsequently subjected to hydrothermal treatment at 100 ºC for

an additional 24 h, to ensure further framework condensation. After cooling, the

resulting solution was filtered and the solid products were dried at 100 °C for 24 h.

Finally, the powders were calcined at 550 °C in order to remove the organic

copolymer template.

6.4.2 Synthesis of LaNiO3 perovskites

Nanocasting was performed using a citrate complex of lanthanum and nickel

as the precursor for perovskite.30 In a typical synthesis, 3 mmol each of

La(NO3)3.6H2O and Ni(NO3)3.6H2O were dissolved in an ethanolic solution of citric

acid (10 ml) to obtain an equimolar solution, which was then added slowly to 1g

SBA-15 dispersed in 10 ml water. The mixture was stirred for 4 h at room

temperature and, subsequently, the solvent was evaporated under vacuum using a

rotary evaporator. The thus-obtained powder was further dried at 80 °C for 24 h,

ground well in a mortar and calcined at 500 °C for 4 hours at the rate of 2 ºC/h with

an intermediate dwell at 170 ºC. The molar ratio of metal ions and citric acid was

kept at 2 : 1. Impregnation was repeated once for achieving higher loading. The

final powder was calcined at 700 °C for 6 h with the same heating ramp as

mentioned before. Finally, the silica template was removed by treating the

composite 3 times with 2M NaOH at room temperature. The final LaNiO3 product

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(designated as LN-NC hereafter) was washed with water and ethanol and dried

overnight at 80 °C.

For the synthesis of bulk perovskites,24 10 mmol each of La(NO3)3.6H2O and

Ni(NO3)3.6H2O were dissolved in an aqueous solution of citric acid (10 ml). The

mixture was stirred overnight at room temperature and then the solvent was

evaporated at 80 °C. The LaNiO3 powder thus-obtained (designated as LN-C

hereafter) was ground carefully in a mortar and calcined at 700 °C for 6 h.

For obtaining Ni/La2O3, both mesoporous and bulk perovskites were

reduced at 700 °C for 2 hours under a flow of 5% H2 in argon. These materials are

designated as LN-CR and LN-NCR to represent bulk or nanocast Ni/La2O3

samples, respectively.


Wide angle powder XRD analysis was performed with a Siemens 80 Model

D5000 diffractometer using Cu Kα radiation (λ = 0.15496 nm). N2 physisorption

analyses were performed at -196 °C with an ASAP 2010 sorption analyzer. Prior to

analysis, samples were degassed overnight at 150 °C. Specific surface areas of

nanocast perovskites were determined using the BET method on the lower relative

pressure region of the adsorption isotherm (0.05–0.2). Pore size distributions were

obtained by using the NLDFT method33,34 assuming cylindrical pore geometry

(applying the kernel of metastable NLDFT adsorption of N2 in cylindrical pores on

an oxide surface i.e., adsorption branch) supplied by the Autosorb-1 1.55 software

from Quantachrome Instruments. The total pore volume was calculated from the

nitrogen sorption capacity at P/P0=0.95. For TEM images, the samples were first

dispersed in ethanol and deposited on carbon grids and analyzed on a JEOL JEM

1230 microscope.

X-ray photoelectron spectroscopy (XPS) spectra were collected on a Kratos

Axis-Ultra electron spectrometer (UK) using a monochromatic Al Kα X-ray source

at a power of 300 W and operated with a base pressure of 6.66 x 10-8 Pa (5 x 10−10

Torr). Charge compensation was required using a low-energy electron beam

112

perpendicular to the surface of the samples. Survey spectra used for determining

the elemental composition were collected at pass energy of 160 eV and a step size

of 1 eV. Ni is usually quantified with Ni(2p) or with Ni(3s). However in the present

case strong interference were observed Ni(2p) and Ni(3s) with La(3d) and La(4d)

respectively. The interference free Ni(3p) signal is used in the present study. TPR-

H2 analysis was performed using an RXM-100 multicatalyst testing and

characterization system. The catalyst (60 mg) was placed in a quartz reactor,

which was pretreated under a flow of 20 ml min-1 (20 % O2 in He) at 500 °C for 1 h.

The TPR was performed under a flow of 10 mL min-1 (5% H2 in Ar) with a

temperature ramp of 5 °C min-1 from 25 to 900 °C. The consumption of hydrogen

was monitored with a thermal conductivity detector (TCD) and quantified.


The catalytic activities of the materials were tested under steady state

conditions. 100 mg of the catalyst was inserted between two quartz wool plugs

placed in a U-shaped quartz reactor (internal diameter = 5 mm). The temperature

was controlled using a K-type thermocouple placed in the reactor without direct

contact with the catalyst. To purge the catalytic system, the catalysts were 10 first

flushed with Ar for 1 h at room temperature. The feed, composed of CH4 (10 ml

min-1), CO2 (10 ml min-1) and Ar as diluent with a total flow of 50 ml min-1 (GHSV =

2.1 x 105 h-1) was passed through the reactor and the temperature was increased.

Gas samples were collected in the steady-state regime at an interval of 2 h of

constant conversion, and the products were analyzed with a gas chromatograph

(HP 6890 series) equipped with a TCD. Reactants and products were separated

with a Haye-Sep T column (internal diameter=1 mm, L = 2 m x 5 m). Stability tests

were performed at 700 °C for 48 hours with all other conditions remaining the

same.

The used catalyst was purged with He for one hour and then treated at 800

°C in a flow of 5 % O2 in He. The CO2 evolved was monitored using a gas

chromatograph as a method to estimate the amount of residual carbon.

Thermogravimetric analyses (TGA) of the used catalysts were performed using a

113

NETZSCH STA 449C thermogravimetric analyzer under an air flow of 20 ml min-1

with a heating rate of 10 °C min-1. The Raman spectra were recorded at 22.0 ± 0.5

°C using a LABRAM 800HR Raman spectrometer (Horiba Jobin Yvon,Villeneuve

d‟Ascq, France) coupled to an Olympus BX 30 fixed stage microscope. 514.5 nm

line of an Ar laser (Coherent, INNOVA 70C Series Ion Laser, Santa Clara, CA) was

used as the excitation light source. The laser beam was focused on the sample at

an intensity of approximately 5 mW.


6.5.1 Physicochemical characterization

The ordered mesoporous SBA-15 silica used as the hard templates were

characterized using N2 physisorption performed at -196 ºC (Figure 6.1). First, wide-

angle X-ray diffraction analysis was performed to confirm the formation of the

perovskite structure. The XRD patterns obtained for the synthesized perovskites

(i.e., LN-NC and LN-C samples) are shown in Figure 6.1. As observed, both the

nanocast and the bulk perovskites exhibit reflections characteristic of the single

phase orthorhombic LaNiO3 structure. No peaks representing possible impurity,

e.g. nickel or lanthanum oxide phases, were observed. For both LN-C and LN-NC,

diffraction peaks were observed in the same regions, however with different

intensities. The wide angle XRD patterns of the perovskites after performing

reduction at 700 °C for 2 hours (Figure 6.1) show that the perovskite structure is

completely destroyed for both the nanocast and bulk LaNiO3. All the observed

peaks can be indexed either to La2O3 or to Ni0. The sizes of the Ni particles were

estimated from the X-ray line broadening by applying the Scherrer equation (see

Table 6.1). Obviously, much smaller sized Ni particles dispersed on La2O3 were

obtained when performing reduction of the nanocast mesoporous LaNiO3

compared to its bulk counterpart.

114

Figure 6.1 N2 physisorption isotherm and the corresponding pore size distribution (inset) of ordered

mesoporous silica SBA-15 aged at 100 ºC. The pore size distribution was calculated from the adsorption branch of the isotherm using NLDFT method (left). Wide angle XRD patterns of LaNiO3 and Ni/La2O3 obtained by performing reduction at 700 ºC (right). (a) LN-C, (b) LN-CR, (c) LN-NC and (d) LN-NCR.

Table 6.1 Structural parameters obtained for LaNiO3 perovskites and Ni/La2O3 obtained by performing

reduction at 700 ºC.

Sample SBET

(m2g-1)a

Dp (nm)b Vp (cm3g-1)c SXRD

(nm)d

LN-C 10 - - -

LN-CR 18 - - 21

LN-NC 150 5.8 0.2 -

LN-NCR 50 6.5 0.1 11


bNLDFT average pore

size; cPore volume;

d Ni particle size calculated by the Scherrer method.

Information regarding the formation of the mesostructure of nanocast

LaNiO3 was obtained by transmission electron microscopy (TEM). Representative

images of all the samples are given in Figure 6.2. Well-ordered domains were

observed along with the presence of minor fractions of disordered regions in the

case of LN-NC. After performing reduction treatment of nanocast LaNiO3 at 700 ºC,

porosity was still persistent but to a lower extent as evident from Figure 6.2g-h. For

LN-C, large agglomerates constituting particles in the size range 50-100 nm were

115

observed. This agglomeration resulted from sintering occurring at the high

temperature necessarily used for the synthesis of these materials. Generally, high

Figure 6.2 TEM images of LN-C (a,b), LN-CR (c,d), LN-NC (e,f) and LN-NCR (g,h)

temperature reduction of perovskites leads to the formation of nanoparticles of B

site metal (e.g., Ni) dispersed on the oxide formed by the A site metal (e.g., La).21-

24 Here, the results obtained from wide angle XRD analysis (Figure 6.1) clearly

shows diffraction peaks corresponding to Ni0 and La2O3, without the presence of

116

any impurity phase. Even though particle agglomeration still persists after

reduction, it has been found that the extent of agglomeration seems to be much

lower compared to the parent perovskite, which might have resulted from the

structural changes occurring during reduction (evidenced from XRD). The particle

sizes of Ni estimated from the TEM images were found to vary between 5-20 nm,

with the larger fraction centered approximately around 20 nm in agreement with the

XRD results (Table 6.1).

Further, N2 physisorption analysis was performed at -196 ºC on both

mesoporous and conventional LaNiO3 perovskites and Ni/La2O3 obtained after the

post-synthetic reduction treatment at 700 ºC. The adsorption-desorption isotherms

and the corresponding pore size distributions are given in Figure 6.3. The isotherm

obtained for the nanocast LaNiO3 exhibits type IV behaviour, with a well-developed

hysteresis loop appearing in the relative pressure range from 0.4 to 0.7. This

observation agrees with previous reports on nanocast transition metal oxides.27-30

Figure 6.3 N2 physisorption isotherms of as synthesized and reduced forms of nanocast and bulk LaNiO3

perovskites (left). Pore size distributions calculated from the adsorption branch using NLDFT method for as synthesized and reduced nanocast LaNiO3 are given on the right.

Also, a small hysteresis loop was observed in the high relative pressure region,

indicating the presence of inter-particle pores.35 A similar isotherm was observed in

the case of Ni/La2O3 obtained from the reduction of nanocast LaNiO3. However,

the hysteresis loop becomes smaller in the latter case. Also, the volume adsorbed

was found to be lower for the reduced material. All these observations indicate that

a significant percentage of porosity was lost for the nanocast material after post-

synthesis reduction at 700 ºC. For LaNiO3 synthesized using the conventional

117

citrate process, the isotherm was found to match well with those generally

observed for non porous bulk materials. In this case, the shape of the isotherm

remains roughly the same after performing reduction treatment, with an

enlargement of the hysteresis loop in the higher relative pressure region (Figure

6.3). As reported in Table 6.1, the specific BET surface area of the nanocast

LaNiO3 was exceptionally high (150 m2g-1) compared to that obtained for the

conventional LaNiO3 (10 m2g-1), especially considering the high calcination

temperature of 700 ºC. Interestingly, this enhancement in the specific surface area

persists in the case of nanocast material (50 m2g-1) in comparison to the bulk

perovskite, even after performing the post-synthesis reduction treatment at 700 ºC.

Here, it is to be noted that, the reduction treatment is performed after the removal

of the silica template and it is thus different from the initial calcination step. The

pore sizes of the materials were derived from the adsorption branch by applying

the non-local density functional theory (NLDFT) method.33,34 The average pore size

distributions are found to be narrow for both as-synthesized and post-reduced

nanocast materials. The pore size of the nanocast materials was found to increase

slightly after reduction (Table 6.1). Also, it has been found that the total pore

volume of the bulk perovskite, even though very low, increased slightly after

performing the reduction treatment at 700 ºC.

Figure 6.4 TPR-H2 profiles of nanocast and bulk LaNiO3 perovskites (left). Ni (3p) XPS spectra of nanocast LaNiO3 and Ni/La2O3 catalysts obtained from nanocast LaNiO3 are given on the right.

This implies that the collapse of the perovskite structure, leading to the formation of

Ni/La2O3, resulted in the introduction of porosity to some extent, which also

118

explains the slight enhancement of specific surface area of this bulk perovskite

after reduction.

XPS analysis was performed for the nanocast LaNiO3 and its derived

Ni/La2O3 samples. For both these samples, strong interference between Ni (2p)

and La (3d) and Ni (3s) and La (4d) were observed. Hence, interference free Ni

(3p) signal was used for the analysis of the surface Ni species (Figure 6.4). For

LaNiO3, a single peak centered at 65.9 eV was observed. This indicates that all the

Ni species present at the surface are included in the perovskite structure. After

reduction, it was found that the peak maxima shifted towards lower binding energy

(64.9 eV) which is due to the fact that reduction of the perovskite structure

occurred, leading to a change of chemical state of surface Ni species. In addition, a

significant decrease in Ni/La atomic ratio was observed after the reduction

treatment (0.30) in comparison to LaNiO3 (1.68). These results point to the

formation of small Ni particles situated most likely inside the pores of the La2O3

support.

Table 6.2 Amount of H2 consumed during TPR-H2

Perovskite H2 consumed [molH2 atomB-1]

First step Second step

LN-C 0.511 1.03

LN-NC 0.525 1.09

In order to obtain information about the reduction behavior of as-synthesized

perovskites, temperature-programmed reduction (TPR-H2) treatment was

performed. In general, the observed H2 consumption peaks in the case of

perovskites arise from the reduction of B site metal cation in the ABO3 structure.

The reduction profiles obtained are shown in Figure 6.4. Typically, two main

reduction steps, one at lower and one at higher temperature, were observed for the

bulk LaNiO3, which is in line with previous reports.36,37 The first peak centered

around 330 ºC could be attributed to the reduction of Ni3+ in the perovskite

119

structure to Ni2+ leading to the formation of La2Ni2O5. The shoulder visible around

380 ºC for LN-C indicates further reduction of some nickel in the grain boundaries,

as observed previously for LaCoO3 synthesized by reactive grinding.38 In the case

of nanocast LaNiO3, the reduction profile appears in a similar way, however with

the absence of the shoulder. This may correspond to the absence of grain

boundaries in the nanocast materials. Also, from the peak position of the second

reduction step, it can be implied that the complete reduction of the nanocast

perovskite took place at a slightly lower temperature in comparison with the one

prepared by the citrate method. The values of hydrogen consumption were

determined for the nanocast and the bulk LaNiO3 materials (see Table 6.2). In

general the ratio of peak area of the second peak to that of the first being equal to

2 indicates that the first step of reduction corresponds to the formation of Ni2+

which in turn gets reduced to Ni0 during the second step. For both LN-C and LN-

NC, the values of hydrogen consumed during each step correspond perfectly with

the formation of Ni2+ and Ni0 during the corresponding stages of reduction.

6.5.2 Catalytic studies

Initial monitoring of the catalytic activity was performed using as-synthesized

perovskites without any reduction step. The steady state conversion profiles for

CH4 and CO2 obtained as a function of temperature are shown in Figure 6.5. It is

well known that the reduction of perovskite will take place under the conditions

used for dry reforming although higher temperatures are required for complete

reduction compared to that necessary in the presence of H2.39 We found that in the

case of LN-C, very low values of conversions were observed for both CH4 and CO2

until 800 ºC. For LN-NC, comparatively higher conversions (approximately 40 %)

were observed under the same conditions. Incomplete reduction of the perovskite

structure under the present reaction conditions could be the reason for the

observed rather low values of conversion. In turn, this could lead to the lower

exposure of the active Ni0 phase to the reactants. Figure 6.5 also depicts the

temperature dependent conversion profiles for the bulk and nanocast catalysts,

after performing the post-reduction step. In this case, we observed almost 90 %

120

conversion for both CH4 and CO2 when LN-NCR was used as the catalyst, at 800

ºC.

Figure 6.5 Temperature dependent conversion profiles of CH4 and CO2 over nanocast and bulk LaNiO3

perovskites (left) and Ni/La2O3 catalysts obtained from nanocast and bulk LaNiO3 perovskites are given on the right (GHSV = 2.1 x 10

5 h

-1).

In comparison, when LN-C was used after reduction, the values of conversion were

found to be still less than 60 %. Thus, it is clear that the performance of the LN-

NCR catalyst obtained from nanocast LaNiO3 is exceedingly better than that of LN-

CR

Figure 6.6 Temperature dependent variation of experimental product ratios obtained for methane dry

reforming over nanocast and bulk (inset) LaNiO3 perovskites (GHSV = 2.1 x 105 h

-1).

obtained from LaNiO3 synthesized using the citrate process. Moreover, for the

nanocast catalyst, the H2/CO ratio remained closer to unity compared to what is

observed for the citrate counterpart (Figure 6.6). Generally, the H2/CO ratio is

expected to be slightly lower than the stoichiometric value of one because the H2

121

produced is partly consumed by the reverse water gas shift reaction (RWGS)

(Equation 6.3) during the dry reforming of methane.17

CO2 + H2 CO + H2O (6.3)

However, under the present reaction conditions, the product ratio was always

found to be slightly above unity, which indicates the suppression of RWGS when

Ni/La2O3 derived from nanocast mesoporous LaNiO3 was used as the catalyst. A

slight increase in the ratio with the increase in temperature, indicative of an

increased selectivity towards hydrogen, was observed for the nanocast catalysts.

This may be due to the fact that at higher temperatures the rate of methane

decomposition is increased. Indeed for the entire temperature range studied,

methane conversion was found to be slightly higher than CO2 conversion. In the

case of bulk LN-CR, the H2/CO ratio was found to be highly dependent on the

reactant temperature (Figure 6.6). In this case, since this ratio remains lower than

one, RWGS seems to operate, especially at lower temperatures. Also, the CH4

conversion was found to be slightly lower than that of CO2 at lower temperatures

until 750 ºC, where it got reversed. At higher temperature, the H2/CO ratio almost

reached one.

6.5.3 Stability tests

A major concern in the development of Ni-based catalysts for dry reforming

is their lack of durability, which is caused both by sintering and carbon deposition

on the surface. Our initial experiments demonstrated that nanocast materials either

post-reduced or not, are clearly efficient for the conversion of CO2 and CH4 in

comparison to their bulk counterparts. Our results also indicate that Ni/La2O3

obtained by performing a reduction treatment exhibits greatly enhanced catalytic

activity. These materials were thus analysed to verify their long term stability under

reforming conditions. For this, after the reduction step, the catalyst was exposed to

the flow of the reactant gases (CH4 and CO2) at 700 ºC. In this experiment,

essentially constant values of conversions were observed for both CH4 and CO2

during the entire evaluation period, as shown in Figure 6.7.

122

Figure 6.7 CH4 and CO2 conversions as a function of time on stream at 700 ºC over Ni/La2O3 catalysts derived

from nanocast and bulk LaNiO3 (GHSV = 2.1 x 105 h

-1). Variation of experimental product ratios under the

same conditions are shown on the right.

For the catalysts derived from bulk LaNiO3, constant conversions were

observed during the first few hours. However, after 20 hours, CO2 conversion

started to decline while CH4 conversion almost remained constant. It then declined

too after 40 hours. The outlet products were analysed at regular intervals

continuously over 48 hours. The experimental H2/CO ratio obtained for nanocast

Ni/La2O3 during this long term run was found to remain constant near unity (Figure

6.7). Note that, the ratio remained slightly above one indicating the absence of

RWGS during the entire period of the present investigation. For the catalysts

synthesized using the citrate process, the H2/CO ratio only remained close to and

slightly above one during the initial hours of investigation. Then, a steady increase

in the H2/CO ratio was observed with respect to the exposure time so that, after 48

hours this ratio reached 2.

After monitoring the conversion for 48 hours, the catalyst was purged with

He for one hour and then treated at 800 °C at a ramp of 5 ºC mim-1 in a flow of 5 %

O2 in He. The amount of carbon was determined from the amount of CO2 evolved.

Estimate of the amount of carbon deposited on the surface of the catalyst showed

that initially both the catalysts obtained either by the citrate process or by

nanocasting were found to be highly coke resistant for short term runs. No

detectable amount of carbon was found on both catalysts after a single run. But

most importantly, for the catalyst derived from nanocast LaNiO3, no detectable

amount of carbon was found after prolonged exposure to the reactant gases even

123

after 48 hours at 700 ºC. This is in line with the fact that the extent of conversion

remained constant throughout the period and no noticeable variation was found on

the H2/CO ratio. On the other hand, for the citrate-based catalyst, it has been found

that 16 % carbon formed on the catalyst surface after 48 hours run, which would

obviously be the reason for the decline of activity and the increase in the H2/CO

ratio with respect to time.

Prolonged coke resistance was discussed previously for Ni/La2O3 obtained

from bulk LaNiO3.23 However, in these studies the experiments were performed in

pulses which implies a shorter reaction time to form coke deposits on the catalyst

surface compared to that performed over a continuous flow of reactants.

Thermogravimetric analysis (TGA) of the catalysts after 48 hours of run were

performed under oxidising atmosphere (air) to probe the amount of carbon

deposited, as depicted in Figure 6.8. A slight initial increase in weight was

observed for both LN-NCR and LN-CR after the stability tests of 48 hours. This

weight gain most likely occurs due to (re)oxidation of reduced metallic nickel. No

weight loss was observed for LN-NCR confirming the absence of carbon on this

catalyst after prolonged exposure to the reactant stream. In contrast, for LN-CR, a

large weight loss step approximately between 300 ºC and 750 ºC

Figure 6.8 Thermogravimetric analysis and Raman spectra (inset) of the catalysts after stability tests for 48

hours (left). N2 physisorption isotherms and wide angle XRD patterns (inset) of Ni/La2O3 cataysts obtained from nanocast and bulk LaNiO3 after 48 hours on stream at 700 ºC (right). (* represents La silicates and

●

represents La(OH)3).

124

was observed indicating the presence of amorphous carbon, followed by a much

smaller one above 600 ºC indicating the burning of more graphitic-like carbon.

Together, these two steps account for 20 % of carbon deposited during the dry

reforming reaction on LN-CR. Further, evidence for carbon deposition can also

obtained from the Raman spectra of the used catalysts after the 48 hours run. As

viewed from Figure 6.8 (inset), the Raman spectrum of LN-CR shows two bands:

the disorder induced band (D band) centered around 1370 cm-1 and the graphitic

band (G band) centered around 1593 cm-1. The first one indicates the presence of

defective polycrystalline carbonaceous material and the second peak indicated the

presence of carbonaceous material of graphitic nature. Differently, for LN-NCR no

noticeable peaks were observed in this region, which confirmed the absence of

carbon formation for this nanocast catalyst.

Moreover, wide-angle XRD analysis of the spent catalysts after 48 hours of

run, reveal reflections corresponding to La2O2CO3, Ni0, La(OH)3 and some

comparatively low intensity peaks corresponding to rare earth silicates in the case

of catalysts derived from nanocast LaNiO3, as shown in Figure 6.8. Since the size

of Ni0 particles is one of the most important parameters that controls the coke

formation on the surface,41,42 we compared the particle sizes of the used catalysts

with the fresh ones. For LN-CR, prolonged exposure to the reactant stream

resulted in the sintering of Ni0 particles leading to marked size enhancement which

in turn facilitated coke formation. In the case of LN-NCR under the same testing

conditions, the Ni0 size remained much closer to that observed for the fresh

catalyst. Also, for the used catalysts, a decrease in BET surface area was

observed for both the materials derived from the bulk and the nanocast perovskites

(see Table 6.3).

The observed formation of rare earth silicates is resulting from interactions

with residual parts of the SiO2 template (SBA-15) which cannot be completely

removed during the leaching step with NaOH.43,44 Since we did not observe the

presence of rare earth silicates in the XRD patterns of as-synthesized or reduced

perovskites, it is clear that these initially amorphous silicates were crystallized over

125

the long term exposure to the reactant gases.43 This suggests that the contribution

of these residual silicates towards the surface area of the nanocast LaNiO3 is

almost negligible since a decline in specific surface area was observed after

reduction at 700 ºC. Also, since no reflections corresponding to nickel silicates

were observed in the wide angle region of XRD, it is clear that the active phase

remained unaffected.

The coke resistance of Ni/La2O3 is generally attributed to the presence of

La2O2CO3.21-24 Since we clearly observed the presence of La2O2CO3 in the XRD

patterns, it is likely that a similar effect is taking place in the case of the nanocast

catalyst as well.

Table 6.3 Structural parameters obtained for Ni/La2O3 catalysts after performing the stability tests.

Sample SBET (m2g-1)a Dp (nm)b Vp (cm3g-1)c SXRD (nm)d

LN-CR 3 - - 47

LN-NCR 30 8.1 0.1 16


bNLDFT average pore

size; cPore volume;

d Ni particle size calculated by the Scherrer method

Finally, another important phenomenon responsible for the enhanced

conversion efficiency and high stability of the nanocast catalyst is the confinement

effect occurring owing to the mesoporous architecture. The reduction treatment of

the mesoporous perovskite resulted in the formation of Ni0 particles well dispersed

on the La2O3 matrix which thus played an important role in controlling particle

growth. Additionally, it cannot be excluded that the crystallization of rare earth

silicate species during the reaction could also contribute in controlling the sintering

of the particles.

6.6 Conclusion

To conclude, we have synthesized high surface area mesoporous LaNiO3

by the nanocasting method, and the structural and phase changes occurring upon

exposure of the material to reducing environment at high temperature (700 ºC)

126

were studied. The catalyst obtained from nanocast LaNiO3 was found to be highly

promising towards applications in dry reforming process. Much higher conversions

and enhanced stability towards coke formation were observed for mesoporous

Ni/La2O3 in comparison to similar compositions synthesized by conventional

processes. In the case of the nanocast-derived catalysts, the mesoporous

architecture, high surface area, higher pore volume and large pore size could

supply more catalytically active Ni0 sites accessible to the reactants, and hence

superior conversions for both of the reactants. Most critically, no coke formation

was observed for the mesoporous catalyst under the present conditions, which in

turn reflects a substantially enhanced stability of the nanocast catalyst. Although

negligible interference with impurity silicate phases (produced over long term

operation, e.g. 48h) was observed on the efficiency of the nanocast catalyst,

further studies are in progress to elucidate a role of these silicates on the surface

properties of the catalysts.

127

6.7 References

1. Besenbache, F.; Chorkendorf, I.; Clausen, B. S.; Hammer, B.; Molenbroek, A. M.; Norskov, J. K.; Stensgaard, I. Science 1998, 279, 1913.

2. Tada, M.; Zhang, S.; Malwadkar, S.; Ishiguro, N.; Soga, J.; Nagai, Y.; Tezuka, K.; Imoto, H.; Otsuka, S.; Ohkoshi, S.; Iwasawa, Y. Angew. Chem. Int. Ed. 2012, 51, 9361.

3. Wu. X.; Kawi, X. Energy Environ. Sci. 2010, 3, 334. 4. Sun, N.; Wen, X.; Wang, F.; Wei. W.; Sun, Y. Energy Environ. Sci. 2010, 3,

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129

Chapter 7 – General conclusions and perspectives

The initial objective of this doctoral thesis was to develop novel catalyst

compositions that can meet the demands put forward globally for the development

of more environment friendly, sustainable and cost-effective processes. Initially it

was shown that the recently developed hard templating method, which is

designated as “nanocasting”, can successfully be utilized for the synthesis of

mesoporous mixed metal oxides showing extremely high values of specific surface

area. Specifically, well crystallized and phase pure perovskite oxides were

synthesized using this method. For the preparation, ordered mesoporous silica

materials with different pore topologies (SBA-15 and KIT-6) were used as hard

templates. By keeping the A site metal the same (A = La), a variety of perovskite

structured oxides were synthesized by varying the B site metal (B = Mn, Fe, Co or

Ni) and this clearly demonstrates the versatility of this method. Nanocast LaBO3

perovskites exhibited specific surface areas exceeding 150 m2g-1 as determined by

the BET method. Further, a variation in the values of specific BET surface areas

with respect to the aging temperature of the parent SBA-15 template was

observed. This study also proved that these nanocast mixed oxides retained high

values of specific surface area even after performing post-synthetic high

temperature treatments, as observed in the case of the reduction of LaNiO3.

All these nanocast perovskites were characterized thoroughly with regard to

their phase purity, textural parameters, porosity, redox properties and surface

composition. In all cases, wide-angle XRD analysis confirmed the formation of

phase pure perovskite compositions without the formation of any crystalline

impurities, other than an amorphous phase observed in the case of LaMnO3,

illustrated by the presence of a broad peak around 2θ = 28 º. This amorphous

phase was found to represent the formation of rare earth silicates on the surface

resulting from the interaction between the metal oxide and the silica template, as

confirmed by XPS. High degree of mesoporosity was confirmed by performing N2

physisorption analysis at -196 ºC and transmission electron microscopy; however

130

with the presence of some less defined disordered nanoporous regions. NLDFT

average pore sizes obtained were found to be in the mesopore region

(approximately 5 nm). The accuracy of NLDFT method for the pore size analysis of

these nanocast perovskites was verified.

By performing temperature programmed reduction under H2, complete

reduction of the perovskite structure leading to the formation of transition metal

dispersed on La2O3 was observed below 700 ºC, except for Mn-based composition

in which the metal remained in the 2+ oxidation state. Further high degree of

surface oxygen species was revealed in these nanocast perovskites, especially for

LaMnO3, by TPD-O2

First, the catalytic activity of these materials was monitored for the total

oxidation of methanol as a model reaction. Nanocast mesoporous LaMnO3

catalysts were found to show the highest conversion efficiency for methanol under

steady-state conditions, compared to both LaCoO3 and LaFeO3 nanocasts and to

LaMnO3 samples prepared by using other methods. The higher catalytic activity

was obviously related to the higher specific surface area and higher amount of

adsorbed oxygen species available on the surface of the nanocast perovskites. No

changes either in the phase or the structure of these nanocast materials were

observed under the reaction conditions.

Furthermore, the first kinetic studies were performed, which demonstrated

the proportionality of reaction rates to the specific surface area of the catalyst.

Here, extremely low values of activation energies were obtained. Also, these

studies derived a linear correlation between the specific surface area and the pre-

exponential factor for the total oxidation of methanol in the case of these nanocast

LaMnO3 catalysts.

Further studies were performed to check the effectiveness of these

nanocast perovskites for the dry reforming of methane. This reaction involves the

conversion of two green house gases (CH4 and CO2) into syngas (CO + H2), which

is more industrially demanding and can be converted into useful products (for

131

instance via Fischer-Tropsch process). In this case, a nanostructured Ni/La2O3

material derived from the nanocast LaNiO3 is demonstrated to be most efficient

and stable catalyst. This material was found to show better conversion efficiencies

(up to 90 %) in comparison with similar compositions derived from the bulk

counterparts (up to 60 %), for both the reactant gases (CH4 and CO2). Further,

constant conversions were observed for this nanostructured catalyst for up to 48

hours. Most interestingly, no coke formation was observed for the mesoporous

catalyst under the present conditions of operation ensuring an enhanced stability of

this new nanostructured catalyst. The improved performance is attributed to the

accessibility of the active sites resulting from the high specific surface area and the

confinement effect leading to the stabilization of Ni nanoparticles.

In the future, it will be highly interesting to utilize these nanocast

mesoporous perovskites as supports for active metals or oxides for various

applications. The combined effects of mesoporosity and the synergy between

support and the dispersant can lead to very promising results for various

applications. Also, the synthesis of A site and/or B site substituted perovskites with

high specific surface area has a huge potential for a variety of catalytic reactions,

including electronchemical reactions such as OER and ORR.110,111 Currently

studies are progressing in this direction in our group and the initial results that we

obtained are promising. Alternatively, very few studies were performed until now

for synthesizing hybrid perovskite-carbon meso/nanostructures. Developing such

synthesis strategies for dispersing perovskite nanoparticles on ordered

mesoporous carbon which could possibly combine the advantages of both

materials will be highly challenging. Such hybrid nanostructures may be highly

demanded particularly for energy-specific applications.

Some perovskites (e.g. Sr-La-Mn-O) are well known for their charge

ordering and presence of colossal magnetoresistence. Thus far, applications of

mesoporous and mesostructured materials are not being explored on this regard;

especially for spintronics. It will be interesting to study the effect of mesoporous

perovskites for such applications. For instance, BiFeO3 is the only material that

132

exhibits ferroelectricity and antiferromagnetism (known as multiferroics) at room

temperature. Synthesizing in the form of thin films, nanotubes etc. are found to

considerably fine tune the properties of this material.112 However, unfortunately no

studies were conducted so far to synthesize mesoporous BiFeO3 for such

applications. Possibly, such a study could provide interesting information leading to

the development of novel improved memory devices that can be addressed

electrically and magnetically. It is to be noted that for nanocast materials to be

used for such applications, new methods that ensure the complete removal of

template silica is inevitable.

133

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137

APPENDIX

For performing the kinetic data processing for the total oxidation of

methanol, non linear curve fitting procedure was executed using the ORIGINPRO 8

software.

Experimental conversions were initially determined by varying flow rates.

Then the values of conversions at different selected temperatures were cross

plotted as a function of pseudo contact times. This yielded a sigmoidal distribution

of data points. These data points were then fitted using the sigmoidal equation:

(

)

where Y is the value of conversion, (W/F) is the pseudo contact time and a, b and k

are the non linear regression parameters.

The function was generated as follows:

Function Name = Oxidation

Brief Description = Sigmoidal function

Function Type = Built-in

Source type = built-in, user-defined

Number of parameters=3

Number of independent Variables=1

Number of dependent Variables=1

[Fitting Parameters]

Naming Method = user defined

Names = a, b, c

Lower bounds = 0.0(X,ON),0.0(X,ON),0.0(X,ON)

Upper bounds = not defined

Meanings = amplitude, coefficient, coefficient

138

[Formula]

y = a/(1 + b*exp(-c*x))

[Parameters Initialization]

sort( x_y_curve );

//smooth( x_y_curve, 2 );

a = max( y_data );

k = 4.0 / fwhm( x_y_curve );

b = exp( k * xaty50( x_y_curve ) )

Iterations were performed automatically until the fit converged completely.

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High surface area mesoporous perovskites for …...HIGH SURFACE AREA MESOPOROUS PEROVSKITES FOR...

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