Synthesis and Characterization of Novel Titanium Oxide Nanotubes – Applications As Catalyst Support for the Selective Catalytic Reduction of Nitrogen Oxides

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Synthesis and Characterization of Novel Titanium Oxide Nanotubes – Applications as Catalyst Support for the Selective Catalytic Reduction of Nitrogen Oxides

A thesis submitted to the

Division of Research and Advanced Studies

of the University of Cincinnati

in partial fulfillment of the

requirements for the degree

Master of Science (M.S.)

in the Department of Chemical Engineering

of the College of Engineering and Applied Science

2014

Dimitrios Pappas

Diploma in Chemical Engineering. Aristotle University of Thessaloniki, Greece

Committee Chair: Dr. Panagiotis G. Smirniotis

Abstract

Novel titanium oxide nanotubes were synthesized by adopting the alkaline hydrothermal treatment of TiO2 nanoparticles. TiO2 with different specific surface area

(m2/g), crystallographic phases and particle size were used for the synthesis of the nanotubular structures. The resulting titania nanotubes possess different morphological features with rutile TiO2 producing well-defined long uniform nanotubes while anatase

TiO2 produces non uniform fragmented nanotubes. The synthesized nanotubes were characterized using Raman, HR-TEM and XRD. The produced materials were used as supports for manganese confined catalytic formulations utilized for the low temperature selective catalytic reduction of NOx, exhibiting remarkable deNOx potential. Manganese oxide supported on titania nanotubes prepared from anatase TiO2 (UV-100 Hombikat) possess superior catalytic activity exhibit almost complete conversion of NOx to N2 at

o 100 C. The remarkable catalytic activity of the catalyst is attributed to the promotion of

Mn4+ species on the surface, the high surface area, the large amounts of Lewis acidic sites and the fine dispersion of manganese oxide on the titania nanotubes.

After the optimization of the titania nanotube support different metal oxides supported on titania nanotubes were investigated regarding their deNOx potential. Copper oxide, chromium oxide and vanadia exhibit remarkable catalytic activity at low temperature, with vanadia confined on titania nanotube exhibiting a large operation window attributed to the acidity and the existence of V2O5 species on the support.

Bimetallic combinations of ceria and manganese oxide supported on titania nanotubes were also investigated, exhibiting a synergistic effect as the presence of ceria enhances the high temperature activity of the manganese oxide based catalyst.

Another important aspect of catalytic formulations utilized in the SCR of NOx by NH3 is the resistance of the material to high temperatures. Titania nanotubes have been reported to lose their nanotubular structure and high specific area at temperature above

550 oC; converting to nanorods and to aggregates. Ion exchanged titania nanotube where prepared by exchanging different metals (Ce, La, Zr, Sb and Y) on the structure and were evaluated regarding their behavior at high temperatures. The effects of thermal aging

(550 oC or 650 oC for 12 hrs.) on the materials were investigated through XRD, BET and

TEM. Yttrium exchanged nanotubes appear to preserve the high surface area and the nanotubular geometry after thermal aging at 650 oC for 12 hrs. The pristine and ion exchanged titania nanotubes were used as supports for manganese oxide based catalytic formulations prepared adopting the deposition precipitation method. The resulting catalysts were thermally aged at different temperature and then evaluated regarding their catalytic activity for the low temperature SCR. The manganese supported on pristine titania nanotube exhibits a complete deactivation after thermal aging at 650 oC attributed to the collapse of the nanotubular structure and the agglomeration of active sites. Manganese oxide loaded on lanthanum and yttrium exchanged titania nanotubes exhibit high deNOx potential despite the thermal aging; especially the yttrium sample possesses remarkable activity for the low temperature SCR attributed to the preservation of the nanotubular structure which provides accessibility to the reactants to manganese active sites.

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I dedicate this work to my beloved fiancée Paraskevi for her love, patience and above everything for being in my life.

Acknowledgements

First of all I would like to thank my advisor Dr. Panagiotis G. Smirniotis for providing me with the opportunity to work in his research group and for his suggestions and help during the course of my research as well as his support. I also wish to thank my thesis committee members Dr. Anastasios Angelopoulos and Dr. Vesselin Shanov for their time and input.

I had the opportunity to work with a number of very bright people while I pursued my M.Sc. degree. First of all I would like to thank Dr. Thirupathi Boningari for all his help that ranges from the operation of various equipment to the fruitful and enlightening discussions on research topics. I would like also to thank Dr. Krishna Reddy Gunugunuri

Ephraim Sheerin, Siva Inturi for the interaction while I was working in the laboratory and for the knowledge shared during our discussions. I would particularly want to thank Dr.

Punit Boolchand for giving me the opportunity to use the Raman equipment from his lab.

I want to thank my parents for the encouragement and support during the course of my studies. Finally I want to express my gratitude to my loving fiancée Paraskevi for her unconditional encouragement, support and love.

Table of Contents

List of Tables 6

List of Figures 8

Chapter 1 – Introduction

1.1. Selective Catalytic Reduction of Nitrogen Oxides by Ammonia 18

1.2. Titanium Oxide Nanotubes 21

Chapter 2 – Novel Interweaved Titanium Oxide Nanotubes Confined Manganese

Oxide Catalytic Formulations with Remarkable Low-Temperature Activity: Effect of

Morphology on Selective Catalytic Reduction (SCR) of NOx by NH3.

2.1. Introduction 36

2.2. Experimental 39

2.2.1. Materials Synthesis 39

2.2.1.1. Titania Nanotubes Synthesis 39

2.2.1.2. Synthesis of Mn(0.25)/TNT-X catalysts 40

2.2.1.3. Synthesis of Mn(x)/TNT-H catalysts 41

2.2.2. Materials Characterization 42

2.2.3. Catalytic Activity Evaluation Experiments 43

2.3. Results and Discussion 45

2.3.1. Specific Surface Area and Pore Size Distribution 45

2.3.2. X-ray Diffraction 52

2.3.3. Transmission Electron Microscopy 60

2.3.4. Raman Spectroscopy 65

2.3.5. Hydrogen-Temperature Programmed Reduction (H2-TPR) 68

2.3.6. Ammonia-Temperature Programmed Desorption (NH3-TPD) 75

2.3.7. X-Ray Photoelectron Spectroscopy (XPS) 81

2.3.8. Catalytic Activity Evaluation 85

2.4. Conclusions 92

Chapter 3 – Novel Titania Nanotube Confined Metal Oxide Catalytic Formulations

M/TNT (M = Mn, Cu, Ce, Fe, V, Cr, Co) for the Selective Catalytic Reduction of NOx:

Evaluation of the Catalytic Activity of Different Metal Oxides Supported on Titania

Nanotubes

3.1. Introduction 95

3.2. Experimental 97

3.2.1. Material Synthesis 97

3.2.1.1. Titania Nanotubes Synthesis 97

3.2.1.2. Synthesis of M/TNT (M = Mn, Co, Cr, Ce, Fe, V and Cu) catalytic

formulations 97

3.2.1.3. Synthesis of Mn–Ce(x)/TNT-H catalysts 98

3.2.2. Materials Characterization 99

3.2.3. Catalytic activity evaluation experiments 100

3.3. Results and Discussion 102

3.3.1. Specific surface area (m2/g) and Pore Volume Measurements 102

3.3.2. X-Ray Diffraction (XRD) 104

3.3.3. H2-Temperature Programmed Reduction (H2-TPR) 106

3.3.4. Ammonia – Temperature Programmed Desorption (NH3-TPD) 111

3.3.5. X-Ray Photoelectron Spectroscopy 115

3.3.6. Catalytic Activity Evaluation 120

3.4. Conclusions 124

Chapter 4 – Systematic Control of the Tubular Structure of Titania Nanotubes and

Superior Catalytic Performance for the Selective Catalytic Reduction of NOx at Low

Temperatures.

4.1. Introduction 127

4.2. Experimental 130

4.2.1. Materials Synthesis 130

4.2.1.1. Titania Nanotubes Synthesis 130

4.2.1.2. Ion Exchanged Titania Nanotubes (X-TNT) synthesis 130

4.2.1.3. Synthesis of Mn/X-TNT Catalytic Formulations 131

4.2.1.4. Thermal Treatment of the Materials 132

4.2.2. Materials Characterization 133

4.2.3. Catalytic Activity Evaluation Experiments 133

4.3. Results and Discussion 135

4.3.1. Specific Surface Area and Pore Size Distribution 135

4.3.2. X-ray Diffraction 142

4.3.3. Transmission Electron Microscopy 146

4.3.4. Ammonia – Temperature Programmed Desorption (NH3-TPD) 151

4.3.5. Catalytic activity evaluation 154

4.4. Conclusions 159

Summary and Future Work 161

Bibliography 163

Appendices 177

Appendix 1 178

Appendix 2 179

Appendix 3 181

Appendix 4 182

Appendix 5 183

Appendix 6 194

List of Tables

Chapter 2

2 Table 2.1.: Specific surface area (m /g) of TiO2 and titania nanotubes.

Table 2.2.: Isotherm type, hysteresis loop type, specific surface area (m2/g), pore volume (cm3/g) and pore diameter (nm) of synthesized titania nanotubes.

Table 2.3.: Specific surface area (m2/g), pore volume (cm3/g) and pore diameter (nm) of titania nanotube confined manganese oxide catalysts.

Table 2.4.: Specific surface area (m2/g), pore volume (cm3/g) and pore diameter (nm) of titania nanotube confined manganese oxide catalysts with different Mn/Ti ratio.

o -1 Table 2.5.: Reduction peaks temperature ( C) and H2 consumption (μm∙g ) for the titania nanotube confined manganese oxide catalytic formulations with Mn/Ti = 0.25 Mn(0.25)/TNT-X.

o -1 Table 2.6.: Reduction peaks temperature ( C) and H2 consumption (μm∙g ) for the titania nanotube confined manganese oxide catalytic formulations with different Mn/Ti Mn(x)/TNT-H.

o -1 Table 2.7.: Reduction peaks temperature ( C) and H2 consumption (μm∙g ) for the manganese oxide supported on titania nanotubes Mn(0.25)/TNT-H and on TiO2 UV-100

Hombikat Mn(0.25)/TiO2.

Table 2.8.: Binding energies and surface atomic ratios of active Mn species for the selected catalysts determined from deconvoluted XPS spectra.

-3 -1 Table 2.9.: Mn/Ti atomic ratio, NOx conversion (%) and turn over frequency (×10 s ) of the Mn(0.25)/TNT family of catalytic formulations.

-3 -1 Table 2.10.: Mn/Ti atomic ratio, NOx conversion (%) and turn over frequency (×10 s ) of the Mn(x)/TNT-H family of catalytic formulations.

Chapter 3

Table 3.1.: Specific surface area (m2/g), pore volume (cm3/g) and pore diameter (nm) of pristine titania nanotube and titania nanotube confined metal oxide catalytic formulations.

Table 3.2.: Binding energies and surface atomic ratios of active components for the selected potential (V/TNT-H, Mn/TNT-H, Fe/TNT-H, Co/TNT-H, Cu/TNT-H and Ce/TNT-H) catalysts determined from deconvoluted XPS spectra.

Chapter 4

Table 4.1.: Specific surface area (m2/g), pore volume (cm3/g) and pore diameter of pristine and ion exchanged titania nanotubes calcined at 400 oC for 2 hrs. (TNT-F) and thermally aged at 550 for 12 hrs. (X-TNT-550).

Table 4.2.: Specific surface area (m2/g), pore volume (cm3/g) and pore diameter of pristine and ion exchanged titania nanotubes calcined at 400 oC for 2 hrs. (X-TNT-F) and thermally aged at 650 for 12 hrs. (X-TNT-650).

Apendices

Table A.1.: Literature Survey of X-Ray Diffraction for titania nanotubes.

Table A.2.: Literature Survey of Raman spectroscopy for titania nanotubes.

List of Figures

Chapter 1

Figure 1.1.: Possible mechanisms for the formation of titania nanotubes from nanosheets. (a) The helical scrolling, (b) the curving of stacked nanosheets (c) direct production of a multi-walled nanotube.

Figure 1.2.: TEM images of titania nanotube prepared by the treatment with 7.5 M NaOH o aqueous solution at 140 C a. from anatase phase TiO2 b. from rutile phase TiO2 [48].

Figure 1.3.: TEM images of (a) pure titania nanotubes, (b) antimony modified, (c) bismuth modified, (d) antimony and bismuth modified calcined at 600 OC.

Chapter 2

Figure 2.1.: Adsorption – desorption isotherms of pure titania nanotubes and titania nanotube confined manganese oxide catalytic formulations.

Figure 2.2.: Pore volume distribution (BJH desorption) of titania nanotubes and titania nanotube confined manganese oxide catalytic formulations.

Figure 2.3.: X-Ray Diffraction (XRD) spectra of (a) TiO2 UV-100 Hombikat (100 %

Anatase), (b) TiO2 after treatment with 70 mL of 10 M NaOH aqueous solution (c) TNT-H uncalcined (d) TNT-H calcined at 400 oC for 2 hrs.

Figure 2.4.: X-Ray Diffraction (XRD) spectra of (a) TiO2 P25 Degussa (80 % Anatase, 20 %

Rutile), (b) TiO2 after treatment with 70 mL of 10 M NaOH aqueous solution (c) TNT-P25 uncalcined (d) TNT-P25 calcined at 400 oC for 2 hrs.

Figure 2.5: X-Ray Diffraction (XRD) spectra of (a) TiO2 Sigma Aldrich (5 % Anatase, 95 %

Rutile), (b) TiO2 after treatment with 70 mL of 10 M NaOH aqueous solution (c) TNT-SA uncalcined (d) TNT-SA calcined at 400 oC for 2 hrs.

Figure 2.6.: XRD patterns of titania nanotube confined manganese oxide catalytic formulations (a) Mn(0.25)/TNT-H, (b) Mn(0.25)/TNT-I, (c) Mn(0.25)/TNT-P25, (d) Mn(0.25)/TNT-TOS, (e) Mn(0.25)/TNT-K and (f) Mn(0.25)/TNT-SA.

Figure 2.7.: XRD patterns of manganese oxide supported on TiO2 and TNT-H with different

Mn/Ti ratios catalytic formulations (a) Mn(0.25)/TNT-H/TiO2, (b) Mn(0.15)/TNT-H, (c) Mn(0.25)/TNT-H, (d) Mn(0.35)/TNT-H.

Figure2.8.: Transmission Electron Microscopy (TEM) images of pristine titania nanotube and titania nanotube confined manganese oxide catalytic formulations (a), (b) general and close view of Mn(0.25)/TNT-SA, (c), (d) general view and tubular structure of Mn(0.25)/TNT-H, (e), (f) evidence of foil structure and open ended nanotubes, (g) spherical nanoparticles of conventional TiO2 UV-100 Hombikat, (h) general view of TNT-H and (i) TNT-P25.

Figure 2.9.: (a) High resolution TEM image of the pristine titania nanotubes synthesized from TiO2 UV-100 Hombikat (TNT-H); inset is the selected-area electron diffraction (SAED) pattern, (b) HRTEM images of Mn(0.25)/TNT-SA catalyst.

Figure 2.10.: The Raman spectra of (a) TiO2 UV-100 Hombikat, (b) TNT-H calcined at 400 oC for 2hrs. and (c) titania nanotube confined manganese oxide catalyst Mn(0.25)/TNT-H.

Figure 2.11.: Reduction profiles of titania nanotube confined manganese oxide catalytic formulations with Mn/Ti = 0.25 Mn(0.25)/TNT-X, obtained from Hydrogen-Temperature

Programmed Reduction (H2-TPR).

Figure 2.12.: Reduction profiles of titania nanotube confined manganese oxide catalytic formulations with different Mn/Ti ratio Mn(x)/TNT-H obtained from Hydrogen-

Temperature Programmed Reduction (H2-TPR).

Figure 2.13.: Reduction profiles of manganese oxide supported on titania nanotubes

Mn(0.25)/TNT-H and on TiO2 UV-100 Hombikat Mn(0.25)/TiO2 obtained from Hydrogen-

Temperature Programmed Reduction (H2-TPR).

Figure 2.14.: Ammonia – Temperature Programmed Desorption (NH3 – TPD) patterns of titania nanotube confined manganese oxide catalytic formulations with Mn/Ti = 0.25 Mn(0.25)/TNT-X.

Figure 2.15.: Ammonia – Temperature Programmed Desorption (NH3 – TPD) patterns of Reduction profiles of titania nanotube confined manganese oxide catalytic formulations with different Mn/Ti ratio Mn(x)/TNT-H.

Figure 2.16.: Ammonia – Temperature Programmed Desorption (NH3 – TPD) patterns of manganese oxide supported on titania nanotubes Mn(0.25)/TNT-H and on TiO2 UV-100

Hombikat Mn(0.25)/TiO2.

Figure 2.17.: Deconvoluted Mn 2p (XPS) spectra of (a) Mn(0.25)/TNT, (b) Mn(0.25)/TNT, (c) Mn(0.25)/TNT and (d) Mn(0.25)/TNT catalytic formulations.

Figure 2.18.: Catalytic evaluation of manganese confined on different types of titania nanotubes (Mn(0.25)/TNT-X) family of catalyst for the SCR of NOx by NH3, in the presence of 900 ppm NO, 100 ppm NO2, 1000 ppm NH3, 10 vol. % O2 with He balance under a GHSV of 50,000 h-1 in the temperature range from 100 oC to 300 oC.

Figure 2.19.: Effect of Mn/Ti ratio on the performance of Mn(x)/TNT-H family of catalysts for the SCR of NOx by NH3, in the presence of 900 ppm NO, 100 ppm NO2, 1000 ppm NH3, 10 -1 o vol. % O2 with He balance under a GHSV of 50,000 h at 100, 120, 140 and 160 C.

Figure 2.20.: NOx conversion of Mn(0.25)/TNT and Mn(0.25)/TNT-H catalysts, in the presence of 900 ppm NO, 100 ppm NO2, 1000 ppm NH3, 10 vol. % O2 with He balance under a GHSV of 50,000 h-1 in the temperature range from 100 oC to 300 oC.

Chapter 3

Figure 3.1.: XRD patterns of pristine titania nanotubes and titania nanotube confined metal oxide catalysts (a) TNT uncalcined, (b) TNT calcined, (c) Mn/TNT, (d) Cu/TNT, (e) Ce/TNT, (f) Fe/TNT, (g) V/TNT, (h) Cr/TNT and (i) Co/TNT.

Figure 3.2.: Reduction profiles from Hydrogen-Temperature Programmed Reduction (H2- TPR) for titania nanotube confined metal oxide catalytic formulations M/TNT where M = Mn, Cu, Ce, Fe, V, C and Co.

Figure 3.4.: Reduction profiles from Hydrogen-Temperature Programmed Reduction (H2- TPR) for bimetallic catalytic formulations supported on titania nanotubes, Mn/TNT, Ce/TNT, Mn-Ce(1.27)/TNT, Mn-Ce(2.55)/TNT and Mn-Ce(5.10)/TNT.

Figure 3.5.: Ammonia – Temperature Programmed Desorption (NH3-TPD) patterns of the titania nanotube confined metal oxide catalytic formulations, Mn/TNT where M = Mn, Cu, Ce, Fe, V, Cr and Co.

Figure 3.6.: Ammonia – Temperature Programmed Desorption (NH3-TPD) patterns of the bimetallic catalytic formulations supported on titania nanotubes, Mn/TNT, Ce/TNT, Mn- Ce(1.27)/TNT, Mn-Ce(2.55)/TNT and Mn-Ce(5.10)/TNT.

Figure 3.7.: Deconvoluted (XPS) spectra of Co 2p in Co/TNT, Cu 2p in Cu/TNT, Fe 2p in

Fe/TNT, V 2p in V/TNT, Ce 3d in Ce/TNT and Mn 2p in Mn/TNT catalytic formulations.

Figure 3.8.: Catalytic activity evaluation of titania nanotube confined metal oxides catalytic formulations M/TNT, where M = Mn, Cu, Ce, Fe, V, Cr and Co, in the presence of 900 ppm -1 NO, 100 ppm NO2, 1000 ppm NH3, 10 vol. % O2 with He balance under a GHSV of 50,000 h in the temperature range from 100 to 400 oC with 50 oC increments.

Chapter 4

Figure 4.1.: Pore volume distribution curves (BJH Adsorption) of pristine and ion exchanged titania nanotubes calcined at 400 oC for 2 hrs. (X-TNT-F).

Figure 4.2.: Pore volume distribution curves (BJH Adsorption) of pristine and ion exchanged titania nanotubes thermal aged at 550 oC for 12 hrs. (X-TNT-550).

Figure 4.3.: Pore volume distribution curves (BJH Adsorption) of pristine and ion exchanged titania nanotubes thermal aged at 650 oC for 12 hrs. (X-TNT-650).

Figure 4.4.: XRD patterns of pristine and cerium exchanged titania nanotubes fresh and thermal aged at different temperatures (a) TNT-F, (b) TNT-550, (c) TNT-650, (d) Ce-TNT-F, (e) Ce-TNT-550 and (f) Ce-TNT-650

Figure 4.5.: XRD patterns of antimony, lanthanum and antimony exchanged titania nanotubes fresh and thermal aged at different temperatures (a) Sb-TNT-F, (b) Sb-TNT-550, (c) Sb-TNT-650, (d) La-TNT-F, (e) La-TNT-550, (f) La-TNT-650, (g) Y-TNT-F, (h) Y-TNT- 550 and (i) La-TNT-650.

Figure 4.6.: TEM images of pristine titanina nanotubes a) TNT-F (Calcined at 400 oC for 2 hrs.) b) TNT-550 (Thermally Aged at 550 oC for 12 hrs.) and c) TNT-650 (Thermally Aged 650 oC for 12 hrs.).

Figure 4.7.: TEM images of ceria exchanged titanina nanotubes (Ce-TNT) a) Ce-TNT-F (Calcined at 400 oC for 2 hrs.) b) Ce-TNT-550 (Thermally Aged at 550 oC for 12 hrs.) and c) Ce-TNT-650 (Thermally Aged 650 oC for 12 hrs.).

Figure 4.8.: TEM images of antimony exchanged titanina nanotubes (Sb-TNT) a) Sb-TNT-F (Calcined at 400 oC for 2 hrs.) b) Sb-TNT-550 (Thermally Aged at 550 oC for 12 hrs.) and c) Sb-TNT-650 (Thermally Aged 650 oC for 12 hrs.).

Figure 4.9.: TEM images of lanthanum exchanged titanina nanotubes (La-TNT) a) La-TNT- F (Calcined at 400 oC for 2 hrs.) b) La-TNT-550 (Thermally Aged at 550 oC for 12 hrs.) and c) La-TNT-650 (Thermally Aged 650 oC for 12 hrs.).

Figure 4.10.: TEM images of yttrium exchanged titanina nanotubes (Y-TNT) a) Y-TNT-F (Calcined at 400 oC for 2 hrs.) b) Y-TNT-550 (Thermally Aged at 550 oC for 12 hrs.), c) Y-TNT-650 (Thermally Aged 650 oC for 12 hrs.), d) Y-TNT-700 (Thermally Aged 700 oC for 12 hrs.), e) Y-TNT-750 (Thermally Aged 750 oC for 12 hrs.) and f) Y-TNT-800 (Thermally Aged 800 oC for 12 hrs.).

Figure 4.11.: Ammonia – Temperature Programmed Desorption (NH3-TPD) patterns of pristine and ion exchanged titania nanotubes X-TNT-F where X = Ce, Sb, Sb, La and Y.

Figure 4.12.: Ammonia – Temperature Programmed Desorption (NH3-TPD) patterns of manganese oxide supported on pristine and ion exchanged titania nanotubes Mn/X-TNT-F where X = Ce, Sb, Sb, La and Y.

Figure 4.13.: Catalytic evaluation of Mn/X-TNT-F family of catalyst calcined at 400 oC for 2hrs. (Mn/TNT-F, Mn/Y-TNT-F, Mn/Ce-TNT-F, Mn/Sb-TNT-F and Mn/La-TNT-F) in the presence of 900 ppm NO, 100 ppm NO2, 1000 ppm NH3, 10 vol. % and He balance under GHSV of 50,000 h-1 in the temperature range 100 – 350 oC.

Figure 4.14.: Catalytic evaluation of Mn/X-TNT-550 family of catalyst calcined at 400 oC for 2hrs. and then thermally aged for 12 hrs. at 550 (Mn/TNT-550, Mn/Y-TNT-550, Mn/Ce- TNT-550, Mn/Sb-TNT-550 and Mn/La-TNT-550) in the presence of 900 ppm NO, 100 ppm -1 NO2, 1000 ppm NH3, 10 vol. % and He balance under GHSV of 50,000 h in the temperature range 100 – 350 oC.

Figure 4.15.: Catalytic evaluation of Mn/X-TNT-650 family of catalyst calcined at 400 oC for 2hrs. and then thermally aged for 12 hrs. at 650 (Mn/TNT-650, Mn/Y-TNT-650, Mn/Ce- TNT-650, Mn/Sb-TNT-650 and Mn/La-TNT-650) in the presence of 900 ppm NO, 100 ppm -1 NO2, 1000 ppm NH3, 10 vol. % and He balance under GHSV of 50,000 h in the temperature range 100 – 350 oC.

Appendices

Figure A.1.: Schematic of the experimental setup for the catalytic activity evaluation of the synthesized catalytic formulations.

Figure A.2.: Picture of the Eco Physics CLD 70S NO/NOx Analyzer

Figure A.3.: NO/NOx chemiluminescence detector signal response (ppm) after the

o -1 introduction of 1000 ppm NOx at 250 C under a GHSV of 50,000 h with 100 mg TIO2 loaded in the reactor.

Figure A.4.: Calibration of calcination furnace.

Figure A.5.: Sample histogram graph for 1000 ppm NO.

Figure A.6.: Sample histogram graph for 1000 ppm NH3.

Figure A.7.: Sample histogram graph for 1000 ppm N2.

Figure A.8.: Sample histogram graph for 1000 ppm N2O.

Figure A.9.: NO calibration for mass spectrometer.

Figure A.10.: NO calibration for mass spectrometer in presence of 1100 ppm of NH3.

Figure A.11.: NH3 calibration for mass spectrometer.

Figure A.12.: NH3 calibration for mass spectrometer in the presence of 900 ppm NO.

Figure A.13.: N2 calibration for mass spectrometer.

Figure A.14.: N2O calibration for mass spectrometer.

Figure A.15.: O2 calibration for mass spectrometer.

Figure A.16.: N2 Selectivity (SN2 (%)) of manganese oxide confined on different titania nanotubes Mn(0.25)/TNT-X catalytic formulations in the presence of 900 ppm NO, 100

-1 ppm NOx, 1000 ppm NH3, 10 % O2 in He balance under a GHSV of 50,000 h in the

o temperature range from 100 to 300 C.

Figure A.17.: N2 Selectivity (SN2 (%)) of manganese oxide confined on titania nanotube prepared form TiO2 UV-100 Hombikat with different Mn/Ti ratios Mn(x)/TNT-H catalytic formulations in the presence of 900 ppm NO, 100 ppm NOx, 1000 ppm NH3, 10 % O2 in He

-1 o balance under a GHSV of 50,000 h in the temperature range from 100 to 300 C.

Figure A.18.: N2 Selectivity (SN2 (%)) of manganese oxide supported on titania nanotube prepared form TiO2 UV-100 Hombikat (Mn(0.25)/TNT-H) and on conventional TiO2

(Mn(0.25)/ TiO2) in the presence of 900 ppm NO, 100 ppm NOx, 1000 ppm NH3, 10 % O2 in

-1 o He balance under a GHSV of 50,000 h in the temperature range from 100 to 300 C..

Figure A.19.: N2 Selectivity (SN2 (%)) of titania nanotube confined metal oxide catalytic formulation M/TNT where (M = Mn, Cu, Ce, Fe, V, Cr and Co) in the presence of 900 ppm

-1 NO, 100 ppm NOx, 1000 ppm NH3, 10 % O2 in He balance under a GHSV of 50,000 h in the

o temperature range from 100 to 300 C.

Figure A.20.: N2 Selectivity (SN2 (%)) of bimetallic combination of manganese oxide and ceria confined on titania nanotube catalytic formulations Mn-Ce(x)/TNT (where x is Mn/Ce atomic ratio) in the presence of 900 ppm NO, 100 ppm NOx, 1000 ppm NH3, 10 % O2 in He

-1 o balance under a GHSV of 50,000 h in the temperature range from 100 to 300 C.

Figure A.21.: N2 Selectivity (SN2 (%)) of fresh pristine and ion exchange titania nanotubes confined manganese oxide catalytic formulations Mn/X-TNT (where X = Ce, Sb, La and Y) in the presence of 900 ppm NO, 100 ppm NOx, 1000 ppm NH3, 10 % O2 in He balance under a

-1 o GHSV of 50,000 h in the temperature range from 100 to 300 C.

Figure A.22.: N2 Selectivity (SN2 (%)) of pristine and ion exchange titania nanotubes confined manganese oxide catalytic formulations thermal aged at 550 oC for 12 hrs. Mn/X-

TNT-550 (where X = Ce, Sb, La and Y) in the presence of 900 ppm NO, 100 ppm NOx, 1000

-1 ppm NH3, 10 % O2 in He balance under a GHSV of 50,000 h in the temperature range from

o 100 to 300 C.

Figure A.23.: N2 Selectivity (SN2 (%)) of pristine and ion exchange titania nanotubes confined manganese oxide catalytic formulations thermal aged at 650 oC for 12 hrs. Mn/X-

TNT-650 (where X = Ce, Sb, La and Y) in the presence of 900 ppm NO, 100 ppm NOx, 1000

-1 ppm NH3, 10 % O2 in He balance under a GHSV of 50,000 h in the temperature range from

o 100 to 300 C..

Chapter 1

Introduction

1.1. Selective Catalytic Reduction of Nitrogen Oxides by Ammonia

The utilization of fossil fuels in order to meet the modern industrialized society’s energy requirements has tremendous effects on the environment as vast amounts of pollutants are emitted in the atmosphere. The main pollutants include carbon monoxide

(CO), carbon dioxide (CO2), sulfur dioxide (SO2), particulate matter (PM) and nitrogen oxides (NOx).

Nitrogen oxides (NOx) consist of nitric oxide (NO) and nitrogen dioxide (NO2). NOx are responsible for major environmental issues such as photochemical smog, acid rain, ozone layer depletion and tropospheric ozone formation [1-1]. The majority of the nitrogen oxides emissions originates in anthropogenic sources, which can be divided in two categories stationary and mobile. The term stationary includes power generations plants, industrial heaters, cogeneration plants and general manufacturing and industrial processes. The mobile sources include mostly automobiles which are the main contributor to the NOx emissions.

In the past decades the academia and the industry have focused large efforts in order to reduce the NOx emissions. These efforts have contributed into developing a wide range of NOx abatement and control technologies. The current available technologies can be divided into two big categories: combustion modification and post-combustion processes.

Combustion modifications include such technologies as low-NOx burners, overfire air, coal reburning, and gas reburning [1-1]. Some combustion modification processes incorporate flue gas recirculation [1-1]. Post-combustion technologies include the Selective

Catalytic Reduction (SCR) and the Selective Non-catalytic Reduction (SNCR). Some systems can be a combination of the above processes which involve control of both NOx and SO2

[1-1].

Selective Catalytic Reduction (SCR) units are mainly utilized in stationary applications due to the need of an additional feed of reducing agents such as ammonia and urea.

Environment governing laws which stringent the NOx emissions oppose a challenge which can be met by the utilization of SCR units for automotive applications.

The selective catalytic reduction of NOx is a well-established process were the nitrogen oxides are reduced to nitrogen over a catalytic formulation using ammonia or urea as a reducing agent. The process where nitrogen oxides from gaseous streams are reduced to nitrogen using a catalyst using ammonia as reducing was discovered by Cohn et.al. [1-2]. The catalytic formulation introduced by Cohn et.al. was a microporous non- zeolitic molecular sieve composition.

The main chemical reactions occurring during the SCR of NOx by NH3 [1-3] are shown below:

(1.1)

(1.2)

Along with these desired reactions to reduce ammonia certain undesired reactions may occur [1-3] that compromise the effectiveness of the SCR unit.

(1.3)

(1.4)

Among the products of the undesired reactions is the nitrous oxide (N2O), which is identified as an atmospheric pollutant. N2O is one of the most dangerous greenhouse gases which can lead to global warming. Another issue that can occur during the operation of an

SCR unit is ammonia slip, which refers to the exit of unreacted ammonia during the operation of the unit. Consequently for the optimum process of an SCR unit the catalytic formulation must yield high N2 selectivity as well as high NH3 conversion.

Different catalytic formulations for the SCR of NOx to N2 by ammonia were suggested since the introduction of the process. The current commercial catalytic formulations used consist of V2O5 supported on TiO2 (anatase), frequently promoted by

o WO3 or MoO3, having an operating temperature window from 300 to 400 C [1-4 – 1-7].

The high temperatures required for the operation of the catalytic formulation place the SCR unit upstream of the desulfurization unit or the particulate filter, as a result SO2 and soot presence in the flue gases affects the activity of the catalysts resulting to a short lifetime.

Issues such as the catalyst deactivation and SO2 oxidation could be solved by placing the SCR unit downstream of desulfurization unit or the particulate filter. For the achievement of this configuration active catalysts at the low temperature region are required. The requirement for such formulations is also made by the automotive industry in order to achieve the abatement of NOx emitted by automotive sources. The development of low temperature SCR catalysts in an upcoming research conducted by various research groups globally.

As mentioned active catalytic formulations for the low temperature SCR of NOx by ammonia are required. Manganese oxide based catalysts according to literature are one of the most efficient low temperature catalysts, researchers from the early 90s report alumina supported manganese oxides as catalyst for the low temperature SCR [1-8]. Manganese oxides supported on TiO2 [1-9 – 1-13], Al2O3 [1-8, 1-12, 1-14], activated carbon [1-16 –

1-17] SiO2 [1-12], CexZryO2 [1-18 – 1-19] have been reported to have high deNOx potential and yield high N2 selectivity at low temperatures. In order to enhance the catalytic properties of supported manganese oxides researchers have reported metal oxides as dopants. Manganese oxide co-doped with CeO2 [1-20 – 1-22], Fe2O3 [1-23 – 1-25], Ni [1-26

– 1-27] are reported by researchers to yield high NOx conversion in the low temperature region. Except manganese oxide catalysts other metal oxides have been reported to be active for the SCR of NOx at low temperature, these metal oxides include copper [1-28 –

1-30] and vanadium [1-31 – 1-32]. Metal oxides like ceria, iron oxide, tungsten oxide, chromia and nickel oxide are commonly used to enhance the catalytic activity and characteristics by the co-doping on active metal oxides.

As it can be observed through the literature TiO2 is the most common support for catalytic formulations used in the SCR of NOx by NH3. Different dopants are introduced to

TiO2 in order to enhance the catalytic activity, the physical properties and the durability of the particular support. Among the different dopants used for TiO2 are ZrO2 [1-33], CeO2

[1-34], WO3 [1-35], MoO3 [1-36] and SiO2 [1-36 – 1-39]. It can be easily comprehended that the support is a crucial component of the catalytic formulation; supports with high surface area and morphologies that can provide good active metal dispersion such as titania nanotubes can enhance the deNOx potential of catalytic formulations.

1.2. Titanium Oxide Nanotubes

Titanium oxide nanotubes are a novel material in terms of structural and physicochemical characteristics. The preparation of titania nanotubes by alkaline hydrothermal treatment was introduced by Kasuga et.al [1-40] where the group prepared nanotubes with diameter of 8 nm and length around 100 nm by treatment of TiO2 with 10M

NaOH aqueous solution in Teflon – lined stainless steel autoclave for 20 hrs. at 110 oC

[1-40]. By the time titania nanotube synthesis by alkaline hydrothermal treatment was introduced, numerous groups have studied the different parameters during the synthesis, the structural characteristics of the product, the physicochemical properties as well as the formation mechanism.

One of the first reports on the structure of titania nanotubes was by Du et.al. [1-41] where they report that the nanotubes are constructed by multiple layers of titania nanosheets with 0.75 nm interlayer spacing [1-41]. The same group also reports that protons are present in the titania nanotube structure [1-41]. The titanium oxide nanotubes are a result of scrolling (100) trititanate sheets in the [001] direction according to Chen et.al. [1-42]. The composition of the titania nanotubes after the acid washing is H2Ti3O7

[1-42], the group also reports the nanotubes to be scroll type multiwalled with 0.78 nm interlayer spacing and consists of H – Ti – O bonds [1-42].

Yao et.al. in their paper [1-43] studied the formation mechanism of TiO2 nanotubes and they report the sequence of events that lead to the nanotubes formation, at first TiO2 nanopowder is delaminated into nanosheets during alkaline treatment then stacked nanosheets of 3D anatase phase of single layer nanosheets roll up resulting in multiwalled,

22 open ended nanotubes [1-43]. In accordance with the previous studies [1-43] Yang et.al. proposed a mechanism which starts with the formation of planar fragments from anatase

TiO2 particles, the fragments are converted into nanotubes during alkaline treatment then further treatment with HCl aqueous solution removes the sodium from the structure resulting in H2Ti2O4(OH)2 structures [1-44]. Zhang et.al. in [1-45] report the formation of

H2Ti3O7 sheets due to the alkaline environment from disordered phase of TiO2. The driving force of the nanosheet rolling as discussed in [1-45] is proposed to be the asymmetry caused by H – deficiency of the titania nanosheets.

One of the later studies on the titania nanotube formation by Bavykin et.al. explains step by step through equations the formation mechanism [1-46]:

1. Dissolution of TiO2:

(1.5)

2. Dissolution crystallization of nanosheets:

(1.6)

3. Curving of nanosheets:

(1.7)

4. Washing of nanotubes:

(1.8)

The formation of the nanotubes from titania nanosheet is also discussed in [1-46] and can be a result of the helical scrolling of a single titania nanosheet or the result

23 of curving of several stacked nanosheets and finally the direct production of multiwalled nanotubes by stacked nanosheets [1-46].

Figure 1.1.: Possible mechanisms for the formation of titania nanotubes from nanosheets. (a) The helical scrolling, (b) the curving of stacked nanosheets, (c) direct production of a multi-walled nanotube [1-46].

The properties of titanium oxide nanoparticles used for the titania nanotube synthesis have been reported to effect the final product. It is reported that ultrafine rutile

TiO2 through alkaline hydrothermal treatment can produce titania nanotubes with length

24 up to 500 nm [1-47]. When anatase TiO2 is used to prepare nanotubes the result is non uniform nanotubes with shorter length compared with the ones prepared from rutile TiO2

[1-47]. Large crystallite sizes are more difficult to prepare nanotubes as larger driving force is required for the phase transformation [1-48]. Morgan et.al. comparing rutile and anatase phases concludes that rutile phase TiO2 requires harsher condition for nanotube formation while resulting in more uniform nanotubes [1-48] which is in accordance with the findings of Zhang et.al. [1-47].

Figure 1.2.: TEM images of titania nanotube prepared by the treatment with 7.5 M NaOH

o aqueous solution at 140 C a. from anatase phase TiO2 b. from rutile phase TiO2 [1-48].

Controlling the length of the nanotubes as discussed earlier can be achieved by selecting the proper crystalline phase of titania [1-47 – 1-48]. Viriya-empikul et.al. [1-49] proposed that the sonication of the precursor solution of TiO2 and NaOH effects the length

25 of the nanotubes. Sonication of the precursor solution results to longer nanotubes due to deagglomeration of the TiO2 nanoparticles which results in larger and more uniform nanosheets which then are scrolled into longer nanotubes [1-49].

Sodium is present in the nanotube structure after the alkaline hydrothermal treatment and later is removed by acid washing where the Na+ are replaced by protons.

The presence of sodium in the nanotube structure can affect the adsorption properties and surface area of the synthesized nanotubes [1-50 – 1-51]. The removal of sodium from the titania nanotubes increases their specific surface area [1-50 – 1-51] and their pore size distribution. The presence of sodium in the nanotube structure increases Lewis acids sites of the product. [1-50].

Tsai et.al. have systematically studied the effects of different parameters during the alkaline hydrothermal synthesis of titania nanotubes in order to regulate their physical characteristics [1-52]. Their studies reveal that the concentration of the alkaline solution, the thermal treatment temperature and time as well as acid concentration during washing step impact the physical characteristics of the nanotubes such as the specific surface area

[1-52]. In the same direction Poudel et.al. studied the different parameters during titania nanotube synthesis, their studies concluded that the alkaline hydrothermal treatment in autoclave controls the crystallinity of the sample, while the washing with HCl solution controls the impurities, the group also proposed that washing with 1M HCl aqueous solution leads to nanotube with no sodium impurities [1-53].

Different characterization techniques such as NMR and Raman spectroscopy have been applied in order to study the composition and bonds that are present in the titania

26 nanotubes. Raman studies reveal a new peak in the Raman spectra corresponding to

Ti – OH bonds at 266 cm-1 [1-54] The Ti – OH bond is reported to be crucial for the stability of the structure [1-54], annealing the nanotubes at 400 oC transforms the Ti – OH bond to

Ti – O – Ti bonds thus altering the morphology [1-54]. The formation of the titania nanotubes results in the strengthening of the Ti – O bonds [1-55].

Magic Angle Spinning NMR was applied in order to study the protons present in the protonated titanate nanotubes; Bavykin et.al. report that except from the physisorbed H2O protons are present in the form of crystallographic H2O and of ion-exchangeable protons of structural OH groups [1-56].

The presence of hydroxyl groups and crystallographic water in titania nanotubes have lead many researchers to study the dehydration and the thermal stability of the nanotubes. Dehydration of the titania nanotubes by calcination is leading to the following sequence of events: H2Ti3O7∙xH2O → H2Ti3O7 → H2Ti6O13 → TiO2(B) → TiO2(anatase) as the temperature increases from 140 to 500 oC [1-56]. The presence of sodium in the structure can be represented as NaxH2-xTi3O7nH2O where 0

27 thermal treatment [1-59]. Generally the nanotubes are unstable at high temperatures

[1-60] and is reported that after treatment at 550 oC they can convert to nanowires thus losing their hollow nanotubular structure [1-53].

The titania nanotubes except their unique tubular structure, enhanced specific surface are and variable composition possess a high ion exchange potential which can lead to improvement of certain properties and characteristics [1-61 – 1-67]. Cation exchange of titanate nanotubes with different metals such as Co, Cu, Ni and Ag was investigated by Sun et.al. proving the high ion exchangeability potential of the nanotubes due to their composition and high specific area [1-61]. The resulting materials are complex metal oxides with enhanced properties and great potential to be utilized in catalysis and adsorption [1-61]. Cobalt modified 1-D nanostructured trititanes where synthesized via ion exchange of Co into sodium rich trititanes [1-62] resulting into NaxCoy/2H2-x-yTi3O7nH2O structures [1-62]. The introduction of cobalt enhanced the specific surface area of the nanotubes and reduced the interlayer spacing of the layers; the synthesized material is unstable at temperatures above 400 oC [1-62].

(NH4)2Ti3O7 nanotubes were synthesized by ion exchange of titanate nanotubes

+ + (Na2Ti3O7) with NH4 by Chang et.al. [1-63]; the ion exchange took place between NH4 and

Na+ cations [1-63]. Fe modified nanostructure titanates where synthesized by the ion exchange of 1D sodium trititanate nanotubes with iron at room temperature [1-64]. The introduction of iron in the structure does not affect the morphology of the nanotubes, although it affects the material’s optical properties and the interlayer spacing [1-64]. Iron

28 in the Fe modified nanostructure titanates is present in the Fe3+ oxidation state as proven by the XPS results and the material is thermally stable up to 400 oC [1-64].

The acid washing of the titania nanotubes that follows the alkaline hydrothermal treatment is basically an ion exchange where the Na+ are replace by protons removing the sodium from the structure and can affect certain properties of the nanotubes [1-65], the removal of sodium from the structure results to materials that possess better photocatalytic activity and possesses higher specific surface area [1-65].

Trititanate nanotubes prepared via alkaline hydrothermal treatment method using anatase TiO2 were ion exchanged with bismuth and antimony by Rónavári et.al. [1-66], the group studied the effects of calcination temperature synthesized materials, their studies conclude that the presence of antimony in the nanotube structure can help preserve the tubular structure even at the calcination temperature of 600 oC inhibiting also the phase transformation from trititanate to rutile [1-66]. As it can observed in Figure 1.3 below pristine nanotubes convert into nanorods through calcination at 600 oC while bismuth modified samples form nanoparticles due to the thermal treatment on the other hand antimony is presented to help preserve the nanotubular structure dispite the high temperature [1-66].

Figure 1.3.: TEM images of (a) pure titania nanotubes, (b) antimony modified, (c) bismuth modified, (d) antimony and bismuth modified calcined at 600 OC [1-67].

As it clear titania nanotubes are systems that possess certain properties which make them very attractive for a wide range of applications from catalysis to adsorption. The utilization of titania nanotubes as support for catalytic formulations derives from the fact that they exhibit enhanced physical properties compared to TiO2 nanoparticles as they possess high surface area and a hollow structure

Titania nanotubes with diameter of 5 nm prepared form anatase TiO2 have been used as catalyst for the liquid phase Friedel–Crafts alkylation of aromatics with benzyl chloride or benzyl alcohol and exhibited high activity compared to titania nanorods [1-67].

The titania nanotube possess coordinated unsaturated Ti4+ sites on the surface and exhibit high amounts of active Lewis and Brónsted acid sites on their surface [1-67].

Titanium oxide nanotubes have been also used as a support for precious metal base catalysts, Pt and Au particles supported on titania nanotubes exhibit uniform dispersion and the resulting catalysts are highly active for CO2 hydrogenation and CO oxidation at room temperature respectively [1-68]. The activity of the catalytic formulation was attributed to the high surface area of the nanotube and the uniform dispersion of the active sites [1-68]. Gold based catalyst supported on titania nanotubes with diameter ranging from 8 to 10 nm and with varying length has been utilized for the low temperature water gas shift reaction and provided promising results [1-69]. Idakiev et.al. concluded that the regulation of the physical characteristic of the nanotubes such as the diameter can further increase the performance of the catalyst [1-69].

V2O5 supported on titania nanotubes were utilized in the chlorobenzene total oxidation [1-70], sulfation of the TiO2 nanotubes leads more efficient catalytic formulations attributed to higher acidity and to the redox sites [1-70].

Titania nanotubes have also been utilized for the adsorption of CO at low temperatures (-173 oC) [1-71]. Through FTIR it is proposed that CO adsorbed on OH groups

2- present in the titania nanotubes; as the temperature increases CO converts to CO2 and CO3 and finally the OH surface groups are restored at 200 oC [1-71].

Copper based catalysts supported on titania nanotubes have been utilized for the selective catalytic reduction of nitrogen oxides by ammonia [1-52, 1-72], the nanotube supported catalysts exhibit superior activity compared to copper supported on conventional titania [1-52, 1-72]. The higher activity of the nanotube supported catalyst is attributed to the higher specific area of the formulation and the higher dispersion of Cu on the nanotube support resulting to higher accessibility of the reactants [1-52, 1-72]. Nian et.al. correlates the superior catalytic activity of the catalyst to the high degree of Cu dispersion on the nanotube support, to the structural similarity of the nanotubes to anatase

TiO2 and to the fact that nanotube supported catalyst is less prone to copper agglomeration

[1-72].

Manganese oxide supported on titanate nanotubes catalytic formulations for the selective catalytic oxide of nitric oxide by ammonia exhibit superior activity compared to catalysts supported on conventional TiO2 in the presence of 600 ppm NO, 720 ppm NH3 and

-1 3% O2 under GHSV of 23600 h , [1-73]. Yao et.al. studied also the optimum manganese loading on the nanotubes concluding that 10wt.% and 15wt.% exhibit the highest DeNOx potential [1-73].

Titania nanotube confined CeO2 catalysts have been evaluated on their catalytic performance for the selective catalytic reduction of nitric oxide by ammonia [1-74 – 1-77], comparing the nanotube supported catalyst with one supported on conventional TiO2, with the same Ce:Ti (1/19) ratio, the titania nanotube confined CeO2 catalyst exhibits over 95%

o conversion in the range from 250 C to 500 in the presence of 600 ppm NO, 600 ppm NH3

-1 and 3.5% O2 under a GHSV of 100,000 h , superior compared to the one supported on

32 conventional titania [1-74]. The enhanced catalytic activity of the titania nanotube confined catalyst was attributed to the redox ability of Ce on the support and to the NH3 adsorption of the catalyst [1-74]. Comparing the performance on the selective catalytic reduction on nitric oxide by ammonia of ceria based catalyst supported on different types of titanate nanostructures (nanoparticles, nanotubes, nanowires, nanorods and fragment) proves that the nature of titanate supports effects the performance of the catalysts [1-75]. Certain nanostructures such as nanotubes possess high surface area, ammonia adsorption properties as well as they promote to chemical states of CeO2 which enhances the activity of the titania nanotube confined CeO2 catalysts [1-75]. The washing process as discussed above can affect the structure, composition and properties of titanate nanotubes, acidic, neutral (H2Ti3O7) and alkaline titanate nanotubes were used as supports for CeO2 based catalysts [1-76]. The catalytic performance for the selective catalytic reduction of nitric oxide by ammonia indicate that the catalyst supported on H2Ti3O7 nanotubes exhibits superior activity attributed to the high surface area, uniform tubular structure and the low sodium content [1-76].

Titania nanotube confined ceria catalysts exhibit remarkable activity for the selective catalytic reduction of nitric oxide by ammonia [1-74 – 1-76], Chen et.al. further studied the alkali metal poisoning of the catalysts and the effect on its performance, the catalysts exhibited excellent resistance to poisoning as the tubular structure protected

CeO2 [1-77]. Ceria is protected inside the nanotubular structure while the alkali metals are present in the outer surface of the nanotubes or they are ion exchanged between the nanotube layers [1-77].

V2O5 catalysts supported on titania nanotubes calcined at different temperatures temperature for the low selective catalytic reduction of nitric oxide by ammonia and were compare to catalyst supported on conventional TiO2 exhibiting higher activity when calcined at 350 oC and 400 oC [1-78]. The high activity was attributed to the better dispersion of the vanadium species on the support which leads to increased acidity resulting in more NH3 adsorption [1-78].

Chapter 2

Novel Interweaved Titanium Oxide Nanotubes Confined

Manganese Oxide Catalytic Formulations with Remarkable

Low-Temperature Activity: Effect of Morphology on Selective

Catalytic Reduction (SCR) of NOx by NH3.

2.1. Introduction

The Selective Catalytic Reduction (SCR) of NOx using ammonia (NH3) as the reducing agent is a well-established and efficient process for the abatement of nitrogen oxide (NOx) emissions. The prevailing commercial catalytic formulation consists of V2O5 supported on

TiO2 (anatase), frequently promoted by WO3 or MoO3, having an operating temperature window from 300 to 400 oC [2-1 – 2-2 ]. The high temperature required for the operation of the catalyst influences its performance due to the presence of SO2 and soot [2-3 – 2-5]. The development of catalytic formulations active in low temperature region will help resolve issues caused by SO2 oxidation and poisoning, moreover will assist the widespread utilization of SCR units by the automotive industry in order to achieve the abatement of

NOx emitted by automobile sources.

Manganese oxides supported on TiO2 exhibit promising DeNOx potential at the low temperature region as it can be observed through the literature [2-1, 2-4, 2-6 – 2-11].

Different TiO2 supports for manganese oxide based catalysts exhibit different catalytic activities. Especially anatase TiO2 (UV-100 Hombikat) has been reported to be a superior support compared to other types of commercially available TiO2 [2-11 – 2-12]. Manganese oxides exhibit high dispersion and remarkable low temperature activity when supported on TiO2 anatase. [2-12]. Novel titania nanotubes prepared by alkaline hydrothermal treatment provide a high surface area and a unique nanotubular structure which can be beneficial for the dispersion of the active species and can improve the DeNOx potential of manganese oxide based catalytic formulations.

The physicochemical and structural properties of the titania nanotubes have attracted substantial attention for their utilization in catalysis [2-13 – 2-15]. Metal oxide based catalysts supported on titania nanotubes utilized for the low temperature SCR of NOx by NH3 is a research in primary stages. Copper oxide [2-16 – 2-17], ceria [2-18 – 2-22], vanadia [2-23] and manganese oxide [2-24] supported on titania nanotubes have been investigated for the SCR of NO by NH3 and exhibit higher activities compared to catalyst supported on conventional TiO2 [2-16 – 2-24]. The superior activity of nanotube supported catalytic formulations is attributed to the high surface area [2-16 – 2-24], the dispersion of active species [2-16 – 2-17, 2-22 – 2-23], the promotion of certain chemical states of the metal oxides [2-19 – 2-20] and to the ammonia adsorption properties of the material [2-18,

2-21].

In the present work TiO2 nanoparticles with different surface area, particle size and crystallographic phases were used to prepare titania nanotubes via the alkaline hydrothermal treatment method. The nanotubes where synthesized following the optimized hydrothermal synthesis reported in [2-16] in order to yield high surface area nanotubes. The resulting titania nanotubes possess different morphological features such as specific surface area, length and diameter due to the different characteristics of TiO2 nanoparticles used for their synthesis [2-25 – 2-26]. Wet impregnation of manganese oxide on the titania nanotubes was used to prepare a series of titania nanotube confined manganese oxide catalytic formulations. The catalytic activity of the materials was investigated for the low temperature SCR of NOx by NH3 with excess O2 under a GHSV of

-1 50,000 h . The catalysts exhibit remarkable DeNOx potential at temperatures as low as 100 oC and in a wide temperature range up to 300 oC. Compared to manganese oxide supported

37 on conventional TiO2 UV-100 Hombikat the synthesized catalysts exhibit superior activity in the whole temperature range. The activity of the titania nanotube confined catalysts is attributed to the high surface area of the support, to the promotion of certain manganese oxidation states which are active for the low temperature SCR and the high amount of

Lewis acid sites that have been reported to promote the low temperature activity. The

Mn/Ti atomic ratio of the catalytic formulations was also investigated in order to find the optimum amount of manganese oxide on the titania nanotubes.

2.2. Experimental

2.2.1. Materials Synthesis

2.2.1.1. Titania Nanotubes Synthesis

Different types of TiO2 nanoparticles (TiO2 Hombikat UV-100 from Sachtleben, Ultra

High Purity Titanium Oxide PT-101 from Ishihara Sangyo Kaisha LTD., Titanium (IV) Oxide

99.9+ % from Aldrich Chemical Company, Titandioxid P25 from Degussa – Hüls, Ultrafine

Titanium Dioxide from Kemira and TiO2 prepared using titanium oxysulfate from GFS

Chemicals as the precursor) were used for the titania nanotube synthesis.

To prepare TiO2 from titanium oxysulfate, a required amount of the precursor was dissolved in deionized water then stirring and heating were applied. The resulting solution was washed with deionized water in order to remove sulfates and the precipitant was dried in over at 80 oC for 18 hrs., finally grinded in mortar and then sieved using a mesh with 300μm openings.

The synthesis of the titania nanotubes was achieved by dissolving 2 g of TiO2 in

70 mL of 10 M NaOH aqueous solution and then hydrothermal treated at 130 oC for 24 hrs. in a Teflon-lined stainless steel autoclave. After the thermal treatment, the content of the autoclaves was filtered and washed initially with deionized water until the PH became neutral then the material was treated with 2 L of 0.2 M HCl aqueous solution until the PH reached approximately 1-2 and finally washed again with deionized water until the PH was resorted to neutral. The resulting nanotubes where dried at 80 oC for 18 hrs. in order to avoid agglomeration of the structure. Finally the material was grinded in mortar and sieved

39 in a mesh with 300 μm openings. The resulting titania nanotubes are denoted as TNT-X where X indicates the TiO2 used for the preparation as can be observed in Table 2.1 along with the corresponding specific surface area (m2/g) of the materials.

2.2.1.2. Synthesis of Mn(0.25)/TNT-X catalysts

A series of titania nanotube confined manganese oxide catalysts with manganese to titanium atomic ratio (Mn/Ti) equal to 0.25 were prepared by adopting the wet impregnation method. The different synthesized nanotubes were utilized as the support of the catalytic formulations, using manganese(II) nitrate hydrate (MnN2O6∙xH2O 99.99% trace metal basis from Sigma-Aldrich) as the manganese oxide precursor. The Mn/Ti ratio was kept constant for the series of the catalysts and equal to 0.25 in order to investigate the effect of different titania nanotube morphology on the catalytic activity for the low temperature SCR of NOx by NH3. The required amount of the manganese precursor was added in a 200 mL beaker containing 2 g of the support along with 100 mL deionized water. The excess water was slowly evaporated in a water bath with continuous stirring at

70 oC. The resulting materials were oven dried at 80 °C for 18 hrs. Finally the catalysts were ground in mortar and sieved in mesh with 300 μm openings in order to obtain homogeneous powder. The material was calcined in a tubular oven at 400 °C with 5 °C per minute temperature increments for 2 hours in open-airm before the catalytic activity evaluation experiment. The resulting catalytic formulations are denoted as Mn(x)/TNT-X where x is the Mn/Ti atomic ratio and X indicates the different titania nanotube type. The

40 catalyst denotation, Mn/Ti ratio, types of TNT along with BET surface area, pore diameter and pore volume of the materials are summarized in Table 2.3.

2.2.1.3. Synthesis of Mn(x)/TNT-H catalysts

After the evaluation of the Mn(0.25)/TNT-X series of catalytic formulations for the low temperature SCR of NOx by NH3 with excess oxygen, manganese oxide catalysts supported on titania nanotubes prepared from TiO2 (UV-100 Hombikat) with different

Mn/Ti ratios were prepared, in order to investigate the effect of manganese oxide amount on the catalytic activity. The titania nanotubes were prepared from TiO2 anatase (Hombikat

UV-100) using the synthesis method described above and denoted as TNT-H.

Manganese(II) nitrate hydrate (MnN2O6∙xH2O 99.99% trace metal basis from Sigma-

Aldrich) was used as manganese oxide precursor. The Mn/Ti ratios varied from 0.15 to

0.35. For this purpose the required amount of manganese(II) nitrate hydrate was added to a 200mL beaker containing the required amount of TNT-H, in order to yield 2 g of catalyst, in 100 mL deionized water. The excess water was slowly evaporated on a water bath with continuous stirring at 70 °C. The resulting materials were oven dried at 80 °C for 18 hrs., and then were ground in mortar and sieved in mesh with 300 μm openings to obtain homogeneous powder. Prior to the reaction studies, the powder was calcined in a tubular oven at 400 °C with 5 °C per minute temperature increments for 2 hrs. in open-air.

Table 2.3 presents the synthesized catalysts, Mn/Ti ratio, along with specific surface area, pore diameter and pore volume.

2.2.2. Materials Characterization

The specific surface area (m2/g), pore volume (cm3/g), average pore diameter (nm), adsorption desorption isotherms and pore volume distribution of the synthesized titania nanotubes and the resulting manganese oxide based catalytic formulations were obtained from N2 adsorption isotherms at liquid nitrogen temperature (77 K) using a surface area and porosimetry analyzer (Micromeritics Tristar 3000) analyzer.

XRD patterns of the materials were obtained using a Phillips Xpert diffractometer occupied with nickelfiltered CuK radiation source. The intensity data were collected over a 2 range of 10°  80° with a step size of 0.025° and a step time of 0.25 seconds. The crystalline phases of the materials were determined by comparing the XRD patterns to the reference data from International Center for Diffraction Data (ICDD) files.

The Raman spectra of the synthesized titania nanotubes and the corresponding catalytic formulations were obtained using a triple stage Raman spectrometer (T64000

Horiba Jobin Yvon), equipped with a microscope (Olympus BX-41) and optical multichannel detector (CCD array) using He-He laser. The spectral resolution of the analysis was 0.631 cm-1 and ranging from 10 to 1000 cm-1.

In order to determine the morphological features of the nanotubes and titania nanotube confined manganese oxide catalytic formulations high resolution transmission electron microscopic (HR – TEM) images were taken with a FEI Tecnai F30 transmission electron microscope (TEM) operated at 200 KV.

Hydrogen – Temperature Programmed Reduction (H2 – TPR) of the titania nanotube confined manganese oxide catalysts was carried out in a catalyst characterization system

(Micromeritics, model AutoChem II 2910). The hydrogen consumption of the materials in in the temperature range from 75 to 700 oC are obtained by integrating the thermal conductivity detector (TCD) signal.

Ammonia – Temperature Programmed Desorption (NH3 – TPD) data were collected using the same automated catalyst characterization system (Micromeritics Autochem II

2910). The samples were saturated with anhydrous NH3 and then flushed with helium in order to remove weakly bound (physisorbed) NH3. Finally the ammonia desorption patterns were obtained by gradually increasing the temperature from 75 to 700 oC.

X-Ray Photoelectron Spectroscopy (XPS) was applied to study the atomic surface concentration on the titania nanotube confined manganese oxide catalysts. The experiments were carried out on a Pyris-VG thermo scientific X-Ray photoelectron spectrometer system equipped with a monochromatic AlK as a radiation source at 300 W under ultra-high vacuum (UHV = 6.7  10-8 Pa). The binging energies for C 1s, O 1s, Ti 2p and Mn 2p were measured.

2.2.3. Catalytic Activity Evaluation Experiments

The catalytic activity evaluation of the titania nanotube confined manganese oxide catalysts for the low-temperature Selective Catalytic Reduction (SCR) of NOx by NH3 with excess oxygen was carried out in a fixed bed quartz reactor under atmospheric pressure.

Due to the different density of the catalytic formulations the amount of catalyst loaded into the reactor varied in order to evaluate all the catalytic formulation under constant Gas

Hour Space Velocity (GHSV) equal to 50,000 h-1. The GHSV was calculated according to equation 2.1.

( ) (2.1)

The corresponding amount of each catalytic formulation was arranged between two glass wool plugs in the center of the reactor. The reaction gas mixture consisted of 900 ppm

NO, 100 ppm NO2, 1000 ppm NH3 and 10 vol.% O2 in He. The NO and NO2 concentrations were constantly monitored by a chemiluminescence NO/NOx detector (Eco Physics CLD

70S). To avoid errors caused by the oxidation of ammonia in the converter of the analyzer, an ammonia trap containing phosphoric acid solution was installed prior to the chemiluminescence detector inlet. The reactor was heated externally via a tubular furnace regulated by a temperature controller. The fresh catalysts were evaluated starting at 100 to

300 °C with 20 °C increments. The reactants and products were analyzed using the chemiluminescence detector (Eco Physics CLD 70S) and an on-line quadrapole mass spectrometer (MKS PPT-RGA). The reactant and product contents were recorded only after steady state was achieved at each temperature step. The NOx conversion and N2 selectivity were calculated using the following equations (equation 2.2 and 2.3).

( ) [ ] (2.2)

( ) [ ] (2.3) ( )

2.3. Results and Discussion

2.3.1. Specific Surface Area and Pore Size Distribution

2 The specific surface area (m /g) of commercial and synthesized TiO2 nanoparticles and the corresponding titania nanotubes, obtained by N2 physisorption at liquid nitrogen temperature, are presented in Table 2.1 below. As the results indicate the alkaline hydrothermal treatment of TiO2 results in structures with higher specific surface area compared to the primal TiO2, which is in accordance with earlier findings [2-14, 2-25 –

2-26]. From the results in Table 2.1 it is clear that TiO2 UV-100 Hombikat possesses the

2 highest specific surface area among the TiO2 nanoparticles (SSA = 309 m /g) and also produces the nanotubes (TNT-H) with the highest specific surface area as well (SSA = 421

2 m /g). The titania nanotubes prepared from TiO2 P25 Degussa (TNT-P25) have a very large specific surface area (SSA = 418 m2/g) almost equal to TNT-H and also present the largest increase in specific surface area through the alkaline hydrothermal treatment. The TNT-SA sample exhibits the lowest surface area (SSA = 82 m2/g) among the synthesized titania nanotubes.

2 Table 2.1.: Specific surface area (m /g) of TiO2 and titania nanotubes.

Titania TiO SSA (m2/g) SSA (m2/g) 2 Nanotubes

TiO2 UV-100 Hombikat 309 TNT-H 421

27 TiO2 Ishihara PT-101 25 TNT-I 236

27 TiO2 Degussa P25 57 TNT-P25 418

28 TiO2 Kemira 54 TNT-K 315

TiO2 Sigma Aldrich 51 TNT-SA 82

TiO2 (TOS) 274 TNT-TOS 334

Tables 2.2 and 2.3 present information on the isotherm and hysteresis loop types as well as the specific surface area (m2/g), pore volume (cm3/g) and pore diameter (nm) of synthesized titania nanotubes and the corresponding manganese oxide based catalytic formulations. It can be observed from the results that there is a decrease in the specific surface area after the impregnation of manganese oxide on the support and the calcination of the catalytic formulations; especially the specific surface area of TNT-H and TNT-P25 samples, reduces to almost half from 421 to 221 m2/g and from 419 to 197 m2/g respectively. The specific surface area (m2/g) of titania nanotubes was measured for as synthesized samples so the decrease is also attributed to the calcination of the material at

400 oC for 2 hrs. which causes dehydration of titanates present on the wall resulting in the formation of anatase TiO2 [2-17]. According to the literature the calcination also causes the increase of the nanotube diameter as well as the reduce of the nanotubes length [2-24,

2-29].

Table 2.2.: Isotherm type, hysteresis loop type, specific surface area (m2/g), pore volume (cm3/g) and pore diameter (nm) of synthesized titania nanotubes.

Titania Isotherm Hysteresis SSA Pore Volume Pore Diameter Nanotube Type Loop Type (m2/g) (cm3/g) (nm) TNT-H V H3 421 0.75 7.1 TNT-I V H1 236 0.71 11.9 TNT-P25 V H1 419 1.56 15.0 TNT-K V H1 315 1.13 14.4 TNT-SA V H1 82 0.29 14.2

TNT-TOS VI H3 334 0.40 4.8

Table 2.3.: Specific surface area (m2/g), pore volume (cm3/g) and pore diameter (nm) of titania nanotube confined manganese oxide catalysts.

Catalyst SSA Pore Volume Pore Diameter (m2/g) (cm3/g) (nm) Mn(0.25)/TNT-H 221 0.47 8.5 Mn(0.25)/TNT-I 161 0.46 11.3 Mn(0.25)/TNT-P25 197 0.76 15.4 Mn(0.25)/TNT-K 196 0.80 16.4 Mn(0.25)/TNT-SA 55 0.23 16.7 Mn(0.25)/TNT-TOS 161 0.22 5.5

The adsorption desorption isotherms for as synthesized titania nanotubes and the corresponding manganese oxide based catalytic formulations have also been investigated.

As it is indicated in Table 2.2 and can be observed in Figure 2.1 most of the titania nanotubes (TNT-H, TNT-I, TNT-SA, TNT-P25 and TNT-K) according to the IUPAC classification of adsorption - desorption isotherms belong to V type of isotherms, while the isotherm of TNT-TOS sample belongs to IV type. The IV type isotherm reveals information on the mesopore structure and it is very close to the V type isotherm which shows weak absorbent – absorbate interaction [2-30]. Information on the hysteresis loops can be derived from the adsorption - desorption isotherms, the hysteresis effects exhibited by all the materials indicate that the investigated titania nanotubes are mainly mesoporous

[2-16].

As it is indicated from the results most of the titania nanotubes (TNT-I, TNT-SA,

TNT-K, and TNT-P25) exhibit H1 type of hysteresis loop, which corresponds to uniform

47 mesopores inside aggregates of particles having a tubular geometry and are open at both ends [2-16, 2-31]. The morphology of the synthesized titania nanotubes is presented in the transmission electron microscopy images and in conjunction with these data it can be proposed that the small pores are pores inside the nanotubular structure while the larger pores are the ones between the titania nanotubes [2-31]. The broad hysteresis loop of the samples such as TNT-H and TNT-I indicates a wide pore distribution which can also be observed in Figure 2.2 [2-31]. The impregnation of manganese oxide on the titania nanotubes and the calcination affect the quantity of N2 adsorbed on the material as it decreases for all the samples. On the other hand the type of the isotherms is the same indicating that the nanotube structure stays intact despite the calcination at 400 oC for

2 hrs. and the impregnation of manganese oxide on the support which correlates with the

TEM images presented in later section. The hysteresis of the adsorption-desorption isotherms decreases for the titania nanotube confined manganese oxide catalytic formulation which can be related to the decrease in pore volume (cm3/g).

Figure 2.2 depicts the pore volume distribution of titania nanotubes and titania nanotube confined manganese oxide catalytic formulations. It is clear from Figure 2.2 that upon the impregnation of manganese oxide and the calcination of titania nanotubes the pore volume decreases, however no major effect on the distribution of the pores is present.

In the range of 2 to 10 nm the TNT-H sample exhibits the largest pore volume compared to the rest of the samples. All the materials exhibit pores in the mesoporous region (2 – 50 nm) which is with accordance with the adsorption – desorption isotherms which reveal the mesoporous structure of the materials. The samples TNT-I, TNT-K and TNT-P25 exhibit a small amount of pores in the macropore region (pore sizes larger than 50 nm) as can it be

observed in Figure 2.2. From the pore size distribution and the adsorption desorption

isotherms we can conclude that the synthesized titania nanotubes are mainly mesoporous

materials.

600 800 TNT-H TNT-K

) 400 Mn(0.25)/TNT-H 600 Mn(0.25)/TNT-K

400 200

200 /gSTP

3 0 0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0

600

cm 200 ( TNT-I TNT-SA Mn(0.25)/TNT-I 400 Mn(0.25)/TNT-SA

100 200

0 0.0 0.2 0.4 0.6 0.8 1.0 0 0.0 0.2 0.4 0.6 0.8 1.0

400 1000 TNT-P25 TNT-TOS 800 Mn(0.25)/TNT-P25 Mn(0.25)/TNT-TOS 600 200

400 QuantityAdsorbed 200

0 0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0

Relative Pressure P/P o

Figure 2.1.: Adsorption – desorption isotherms of pure titania nanotubes and titania nanotube confined manganese oxide catalytic formulations.

0.008 0.008 TNT-H TNT-K Mn(0.25)/TNT-K

) 0.006 Mn(0.25)/TNT-H 0.006 -1 0.004

Å 0.004 -1

g 0.002 0.002 3

0.000 0.000

1 10 100 1 10 100 cm

( 0.008 0.008 TNT-I TNT-SA 0.006 Mn(0.25)/TNT-I 0.006 Mn(0.25)/TNT-SA

0.004 0.004

0.002 0.002

0.000 0.000 1 10 100 1 10 100

0.008 0.008 TNT-P25 TNT-TOS 0.006 Mn(0.25)/TNT-P25 0.006 Mn(0.25)/TNT-TOS

0.004 0.004 Pore Volume, Pore dV/dD 0.002 0.002

0.000 0.000 1 10 100 1 10 100

Pore Diameter (nm)

Figure 2.2.: Pore volume distribution (BJH desorption) of titania nanotubes and titania nanotube confined manganese oxide catalytic formulations.

Table 2.4 presents the specific surface area (m2/g), pore volume (cm3/g) and

average pore diameter (nm) of the manganese oxide confined on titania nanotubes with

different Mn/Ti ratios family of catalytic formulations, Mn(x)/TNT-H (where x = 0.15, 0.20,

0.25, 0.30, 0.35). As it is clear from the results the catalysts having 0.15 and 0.20

50 manganese to titanium ratio (Mn/Ti) present the highest BET surface area while the

Mn(0.35)/TNT-H sample exhibits the lowest surface area among the prepared catalysts.

From these results it can concluded that increasing the manganese oxide loading decreases the specific surface area of the catalytic formulation. The same phenomenon is also present for the pore volume which decreases as the manganese loading is increased. The

Mn(0.25)/TNT-H sample exhibits the lowest median pore diameter (nm) of 8.45 among the synthesized family of catalysts.

Table 2.4.: Specific surface area (m2/g), pore volume (cm3/g) and pore diameter (nm) of titania nanotube confined manganese oxide catalysts with different Mn/Ti ratio.

SSA Pore Volume Pore Diameter Catalyst (cm2/g) (cm3/g) (nm) Mn(0.15)/TNT-H 238 0.546 9.15 Mn(0.20)/TNT-H 252 0.576 9.15 Mn(0.25)/TNT-H 221 0.467 8.45 Mn(0.30)/TNT-H 200 0.465 9.31 Mn(0.35)/TNT-H 192 0.450 9.39

2.3.2. X-ray Diffraction

The XRD patterns of TiO2 UV-100 Hombikat, TiO2 P25 Degussa and TiO2 Sigma

Aldrich during different stages of the titania nanotube synthesis are presented in the

Figures 2.3, 2.4 and 2.5 below. As demonstrated in the Figure 2.3. the characteristic crystallographic peaks at d = 3.54, 1.90 and 2.40 Å (JPCD no. 21-1272) ascribed to anatase titania are present for the TiO2 UV-100 Hombikat and no peaks ascribed to rutile phase are present indicating that the sample is pure anatase. TiO2 P25 exhibits the peaks corresponding to anatase but also peaks ascribed to rutile at d = 3.24, 2.49, 1.69 Å (JPCD

21-1276), according to equation 2.4. the sample consists of 80 % anatase and 20 % rutile phase. The XRD patterns of TiO2 Sigma Aldrich sample indicate that the sample consists mostly of rutile phase titania (95 %), although a small peak d = 3.54 Å ascribed to anatase

(5 %) is present.

After the alkaline hydrothermal treatment of the different TiO2 samples peaks at d =

2.93, 2.24 and 1.66 Å [2-32 – 2-33] are present which area scribed to sodium titanates

(Na2Ti3O7). The peaks corresponding to Na2Ti3O7 are a result of the treatment of TiO2 with

10M NaOH aqueous solution at 130 oC where Ti – O bonds brake and sodium ions are introduced to the structure.

The washing of the material with deionized water and 0.2 M HCl aqueous solution removes Na+ from the titanates replacing them with H+ through ion exchange. Peaks ascribed to H2Ti3O7 are present at d = 2.37 Å and 2.61 Å for the uncalcined synthesized titania nanotubes [2-31 – 2-32]. The calcination of the prepared titania nanotubes, at 400 oC for 2hrs., increases the crystallinity of the samples and the peaks corresponding to

52 titanates shift to lower 2θ. It is reported that during the thermal treatment at temperatures

o around 400 C the titanates presents in the wall convert to anatase TiO2 [2-17].

The hydrothermal treatment of the TiO2 P25 Degussa results to the transformation of rutile phase to anatase, the peaks corresponding to rutile phase vanish after the alkaline hydrothermal treatment of TiO2. Figure 2.4. exhibits the different XRD patterns during the

TNT-P25 synthesis the uncalcined and calcined titania nanotube do not present peaks ascribed to rutile phase. The extend of phase transformation of the TNT-P25 sample is not present for the titania nanotubes prepared from TiO2 from Sigma Aldrich (5 % Anatase,

95 % Rutile) as illustrated in Figure 2.5. the sample exhibits sharp peaks corresponding to rutile TiO2 before and after the nanotube formation, the final product presents an increase in the percent of anatase phase from 5 to 11 %.

(2.4)

Where: IR : XRD intensity of rutile (110)

IA : XRD intensity of rutile (101)

A (204) A

7 A (200) A

A(211)

A(105)

A (101) A

Nanotubes

2 7

A (004) A

2 Na (d)

(c)

(b) Intensity (a.u.) Intensity

(a)

10 20 30 40 50 60 70 80

()

Figure 2.3.: X-Ray Diffraction (XRD) spectra of (a) TiO2 UV-100 Hombikat (100 %

Anatase), (b) TiO2 after treatment with 70 mL of 10 M NaOH aqueous solution (c) TNT-H uncalcined (d) TNT-H calcined at 400 oC for 2 hrs.

O 3

A (200) A

R(110)

R (220) R

R (301) R

A (101) A (204) A

A (105) A

Nanotubes 2

Nanotubes

H Na A (004) A (d)

(c)

(b) Intensity (a.u.) Intensity

(a)

10 20 30 40 50 60 70 80

 ( )

Figure 2.4.: X-Ray Diffraction (XRD) spectra of (a) TiO2 P25 Degussa (80 % Anatase, 20 %

Rutile), (b) TiO2 after treatment with 70 mL of 10 M NaOH aqueous solution (c) TNT-P25 uncalcined (d) TNT-P25 calcined at 400 oC for 2 hrs.

Ti R(110)

R (301) R

R (211) R

R (220) R

R(101)

R(111)

A (101) A 7

Nanotubes

2 H

(d)

(c)

(b) Intensity (a.u.) Intensity

(a) 10 20 30 40 50 60 70 80

 ( )

Figure 2.5: X-Ray Diffraction (XRD) spectra of (a) TiO2 Sigma Aldrich (5 % Anatase, 95 %

Rutile), (b) TiO2 after treatment with 70 mL of 10 M NaOH aqueous solution (c) TNT-SA uncalcined (d) TNT-SA calcined at 400 oC for 2 hrs.

The XRD patterns of titania nanotubes confined manganese oxide catalytic formulations are presented in Figure 2.6. The peak observed at d = 3.08 Å for the samples is ascribed to the (110) phase of monoclinic H2Ti3O7 [2-21, 2-34]. Anatase crystalline phase is present for samples supported TNT-H, TNT-I, TNT-P25, TNT-TOS as the corresponding characteristic peaks at d = 3.54, 1.90 and 2.40 Å (JPCD no. 21-1272) ascribed to anatase are present. The catalytic formulations prepared from TiO2 with high percent of rutile crystalline phase (TNT-K, TNT-SA) exhibit peaks corresponding to rutile at d = 3.24, 2.49,

1.69 Å (JPCD 21-1276).

The XRD patterns of manganese supported on titania nanotubes and TiO2 with different Mn/Ti ratio are illustrated in Figure 2.7. The catalytic formulations exhibit strong characteristic peaks at d = 3.54, 1.90 and 2.40 Å (JPCD no. 21-1272) ascribed to anatase phase TiO2. The titania nanotube supported catalytic formulations exhibit peaks at d = 3.65, 2.37, and 3.08 Å ascribed to H2Ti3O7 and at d = 2.61 and 2.32 ascribed to the nanotubular structure [2-31], the center of the peaks is shifted towards anatase phase due to calcination of the material. As the amount of manganese oxide supported on the titania nanotube increases no peaks ascribed to crystalline MnO2 are present as can be observed in Figure 2.7, indicating that MnOx is well dispersed on the nanotube surface. As illustrated in the XRD patterns the catalyst supported on TiO2 exhibits peaks with higher intensity ascribed to anatase crystalline phase compared to the titania nanotube supported samples. The catalytic formulations where titania nanotubes are used as the support exhibit broader peaks with higher Full Width at Half Maximum (FWHM) which through Sherrer’s equation corresponds to smaller crystallite size. The smaller particle

57 size of TNT-H compared to TiO2 UV-100 Hombikat is consistent with the higher specific area exhibited by the titania nanotubes.

R (110) R

A (004) A

R (111) R

R (220) R

A (200) A (301) R

Nanotubes R (211) R

A (101) A

Nanotubes

R (101) R

2 H (f) (e) (d)

Intensity (a.u.) Intensity (c)

(b) (a)

10 20 30 40 50 60 70 80

()

A (200) A

A (004) A

H (204) A

A (105) A (211) A

A (101) A

7 Nanotubes

Nanotubes

H H

(d)

(c)

(b) Intensity (a.u.) Intensity

(a) 10 20 30 40 50 60 70 80

 ( )

Figure 2.7.: XRD patterns of manganese oxide supported on TiO2 and TNT-H with different

Mn/Ti ratios catalytic formulations (a) Mn(0.25)/TNT-H/TiO2, (b) Mn(0.15)/TNT-H, (c) Mn(0.25)/TNT-H, (d) Mn(0.35)/TNT-H.

2.3.3. Transmission Electron Microscopy

A series of titania nanotubes with two curled layers are successfully prepared by the alkaline hydrothermal synthesis method and subsequent calcination. The hierarchical two curled layers yield a thin wall, and thus the attained multi-walled structure results in short distances of electrons and enhances ion transport [2-35]. The synthesized materials were investigated by using High Resolution Transmission Electron Microscopy HR-TEM/TEM for an adequate determination of the morphology and textural properties of the pristine and manganese oxide confined titania nanotubes illustrated in Figure 2.8. As shown in Figure

2.8, a large quantity of pristine titania nanotubes with nearly uniform outer diameters approximately 9 nm can be observed, a few isolated particles and plenty of intact nanotubes with inner diameter of 3 – 6 nm, and length of several hundred nanometers

(100 – 300 nm) were observed in the samples. The inner diameter of nanotubes fitted well with their pore volume distribution curves illustrated in Figure 2.2. It can be clearly seen that all the spherical nanoparticles of titania transformed to nanotubes under the current experimental conditions, and all the nanotubes are open-ended with multiwall structure as illustrated in Figure 2.8. and was earlier suggested by the adsorption desorption isotherms presented in Figure 2.1. It has been well established in the literature and found in our current studies that the formation of the nanotubular structure certainly depends on the titania phase composition [2-13, 2-25 – 2-26]. Our HR-TEM studies illustrate that the well- defined tubular structures presented in Figure 2.8.a and 2.8.b are generated from rutile

TiO2 precursor (TiO2 Sigma Alrdich, 95 % rutile 5 % anatase), whereas hierarchical two curled layers nanotubes formed with 100% anatase TiO2 precursor (TiO2 UV-100

Hombikat) presented in Figure 2.8.h. The TiO2 (Degussa, P-25) consisting of anatase and

60 rutile crystalline phases (80 % anatase, 20 % rutile) also produced well-defined nanotubes as can be observed in Figure 2.8.i. It can be observed from Figure 2.8. that the rutile titania leads to the formation of uniform long length titania nanotubes (Figure 2.8.a and 2.8.b) whereas the anatase phase leads to non-uniform fragmented nanotubes (Figure 2.8.c and

2.8.d). Moreover, it has been reported that upon calcination the gradual interlayer dehydration leads to a crystal phase transformation of H2Ti3O7 (protonated titanate) to anatase TiO2 accompanied by the conversion of the tubular structure into nanorods. Our high-resolution transmission electron microscopic (HR-TEM) investigations proved that the nanotubes still retain the tubular structure even after calcination at 400 °C for 2 hrs.

[2-36].

The inset in Figure 2.9.a demonstrates the selected-area electron diffraction (SAED) pattern recorded on randomly oriented pristine titania nanotubes from TiO2 UV-100

Hombikat, showed reflections characteristic of a tubular structure. Two spots at the edge of the zero spot were in proximity with the layered structure of nanotubes [2-37]. Two high crystalline diffraction rings can be identified as (101) and (200) planes of the titania anatase phase. These results illustrate that the anatase TiO2 formed under such conditions comprises turbostratic stacking of the (101) faces with a defective alignment [2-37]. Other diffractions in this pattern are very feeble. In particular, some strong diffractions such as

(004) and (105) planes can hardly be identified, suggesting that the shells would consist of a quasi-two dimensional lattice [2-38]. Moreover, HR-TEM revealed that the nanotubes were multilayered and their interlayer spacing was observed at about 0.67 nm as illustrated in Figure 2.9.b which is in agreement with the previous reports [2-39]. The d- spacing of the lattice fringes was measured as 0.35 nm, which is in proximity to the (101)

61 plane of anatase phase titania. Conversely, it is difficult to establish from TEM images whether the tiny manganese particles deposited inside the TiO2 nanotube or outside the nanotubes, we could assume that the MnO2 particles mainly exist inside the tubular channels of titania nanotubes [2-34]. Indeed, neither XRD nor TEM measurements find segregated manganese rich phases in the studied materials. However, these techniques are limited to recognizing about the percent concentration minority phases [2-34]. As can be seen from Figure 2.8.b, the left inset is an enlarged picture of the tube wall for the

Mn(0.25)/TNT-SA sample. The periodicity of the fine fringes was 0.35 nm and the interspacing of the tube layers (walls) was 0.67 nm. We also observed a typical foil structure of titania nanotube in the HR-TEM image of Mn(0.25)/TNT-SA sample (Figure

2.8.f and 2.8.e). The above results imply that the titania nanotubes were formed by rolling up the anatase single-layer sheets. Our HR-TEM results are in good agreement with our

Raman spectroscopy analysis where the formation of Ti-O-Ti bonding and creation of oxygen vacancies have been established.

a c

d e f

g h i

TNT-P25.

2.3.4. Raman Spectroscopy

Figure 2.10 illustrates the Raman spectra of the pristine titania nanotubes prepared

o from TiO2 UV-100 Hombikat (TNT-H), calcined titania nanotubes at 400 C for 2 hrs. and the manganese oxide confined on titania nanotube (Mn(0.25)/TNT-H). Compared to the

TiO2 UV-100 Hombikat, manganese oxide loaded titania nanotubes catalytic formulation

(Mn(0.25)/TNT-H) exhibits the relative weak intensity of Raman peak because of the poor crystallinity. Anatase has tetragonal symmetry with two TiO2 formula units (six atoms) per

19 primitive cell, the space group is D 4h (I4/amd), number 141 in the standard listing [2-40 –

2-41]. Out of the 15 optical modes (1A1g + 1A2u + 2B1g+ 1B2u + 3Eg+ 2Eu) only the A1g, B1g along with Eg modes are Raman active while one A2u mode as well as two Eu modes are infrared active whereas the B2u mode is both Raman and infrared inactive [2-42]. All the

-1 -1 -1 -1 Raman active modes identified at 146 cm (Eg), 396 cm (B1g), 517 cm (A1g), 640 cm (Eg) can be ascribed to the anatase phase of TiO2 [2-43 – 2-45]. No characteristic peak at 442

-1 cm belongs to Eg photon mode of rutile phase TiO2 was found in our Raman spectrum

[2-45] of manganese oxide confined titania nanotubes. It is interesting to note that the hydrothermal synthesis technique, heat treatment of the nanotubes at 400 oC, and the promoter oxide do not transform anatase phase to rutile. Furthermore, in the Raman spectrum of manganese oxide loaded titania nanotubes (Mn(0.25)/TNT-H) , the lowest frequency Eg phonon mode of anatase which demonstrated that the tetrahedron structure had been formed in nanotube, shifted from 146 to 155 cm-1. The blueshift of 9 cm-1 is due to the creation of oxygen vacancies [2-46]. Two newly evolved peaks at 121 and 268 cm-1 in the Raman spectrum of Mn(0.25)/TNT-H and TNT-H samples are ascribed to the presence of tubular structure [2-45]. The second order harmonics or radial breathing oscillations

65 inherent to nanotubular structures seem to be the origin of these Raman peaks [2-47]. In particular, the Raman peak at around 268 cm-1 is due to the Ti–O bonds in titania nanotubes with layered structure [2-48]. The overlapped new peak at 121 cm-1 could be

-1 -1 instigated from left over Na–O bonds [2-49]. The redshift of 14 cm from 640 cm (TiO2

UV-100 Hombikat) to 626 cm-1 in Mn(0.25)/TNT-H sample indicated the formation of a new phase or structural difference [2-37]. On the other hand, low intensity broad bands of

-1 -1 B2g and second order of B1g at 396 cm phonon mode of anatase and A1g peak at 517 cm observed in pure TNT-H and Mn(0.25)/TNT-H samples. The broadening of second order phonon mode suggests that the crystallinity of titania nanotubes prepared form TiO2

UV-100 Hombikat is lower than that of the primal TiO2 nanoparticles, which is in

-1 accordance with our XRD findings. Peaks for MnO2 at 650, 576 and 523 cm and for Mn2O3 at 581, 509 and 630 cm-1 [2-50] could not be observed in Mn(0.25)/TNT-H sample illustrated in Figure 2.10. It is also clearly evident by the low intensity of the Raman peaks of Mn(0.25)/TNT-H sample compared to pure TiO2 UV-100 Hombikat and to pristine titania nanotubes (TNT-H). Our XRD results are in consistent with the Raman results where the diffraction patterns of various titania nanotubes confined manganese oxide catalysts do not show specific manganese crystalline phases.

Mn(0.25)/TNT-H

TNT-H

(a.u.) Intensity

TiO UV-100 Hombikat

100 200 300 400 500 600 700 800 Wavenumber (cm-1)

Figure 2.10.: The Raman spectra of (a) TiO2 UV-100 Hombikat, (b) TNT-H calcined at 400 oC for 2hrs. and (c) titania nanotube confined manganese oxide catalyst Mn(0.25)/TNT-H.

2.3.5. Hydrogen-Temperature Programmed Reduction (H2-TPR)

For an adequate determination of the effect of different titania nanotube supports on the reduction profile of manganese oxide, Hydrogen-Temperature Programmed

Reduction (H2-TPR) experiments were carried out. The reduction profiles for the series of

Mn(0.25)/TNT catalytic formulations are illustrated in Figure 2.11, while the reduction

-1 peak temperatures and the H2 consumption in μm∙g are presented in Table 2.5. As it can be observed from the results the Mn(0.25)/TNT catalytic formulations exhibit two or three distinct peaks in the temperature range from 75 to 700 oC. The low temperature reduction peak which is the most intense for all the samples corresponds to the reduction of MnO2 to

o Mn2O3 [2-9, 2-51 – 2-52] and appears in the temperature range from 306 to 342 C. The medium temperature peak centers in the range from 306 to 342 oC and is attributed to the reduction of Mn2O3 to Mn3O4 [2-9, 2-52]. The third peak which is considerably smaller from the other two and can only be observed in the individual spectra is exhibited by only three samples and appears at high temperatures from 386 to 430 oC corresponding to the reduction of Mn3O4 to MnO [2-9]. Comparing the intensities of the peaks it can easily be concluded that the peak corresponding to the reduction of MnO2 to Mn2O3 has higher intensity, indicating that Mn4+ oxidation state is dominant in the catalytic formulations compared to the Mn3+ and Mn2+ oxidation states. The peak corresponding to the reduction of Mn3O4 to MnO is present in only three samples (Mn(0.25)/TNT-I, Mn(0.25)/TNT-P25 and Mn(0.25)/TNT-K), the intensity of the peak is low compared to the other two indicating that Mn2+ is the less favorable oxidation state.

The presence of only two intense peaks in the reduction profile of the catalytic formulations shows that manganese oxide species are majorly present as MnO2 and Mn2O3 oxides which correspond to Mn4+ and Mn3+ oxidation states respectively. Our earlier studies revealed that the different manganese oxides have different activities for the selective catalytic reduction of nitrogen oxides by ammonia following the sequence: MnO2

4+ 3+ >> Mn2O3 <<< Mn3O4 ≈ MnO [2-10]. The presence of only Mn and Mn manganese species in the catalytic formulations is correlated with the remarkable catalytic activity of the

Mn(0.25)/TNT catalytic formulations in the whole temperature range.

The samples where titania nanotubes prepared from TiO2 Ishihara PT-101 and TiO2

Hombikat are used as the support exhibit the peak corresponding to the reduction of MnO2

o o to Mn2O3 at relatively lower temperatures at 306 C and 309 C compared to the other samples, which is an indication of the high reducibility of those catalysts [2-6].

o -1 Table 2.5.: Reduction peaks temperature ( C) and H2 consumption (μm∙g ) for the titania nanotube confined manganese oxide catalytic formulations with Mn/Ti = 0.25 Mn(0.25)/TNT-X.

o Catalyst T ( C) H2 Consumption

-1 T1 T2 T3 μm∙g

Mn(0.25)/TNT-H 309 388 - 2212.2 Mn(0.25)/TNT-I 306 386 547 2447.6 Mn(0.25)/TNT-P25 332 406 542 2988.9 Mn(0.25)/TNT-K 342 430 594 2706.4 Mn(0.25)/TNT-SA 340 411 - 2518.2 Mn(0.25)/TNT-TOS 329 410 - 2306.3

Mn(0.25)/TNT-TOS

Mn(0.25)/TNT-SA

Mn(0.25)/TNT-K

Mn(0.25)/TNT-P25

TCD Signal (a.u.) Signal TCD Mn(0.25)/TNT-I

Mn(0.25)/TNT-H

100 200 300 400 500 600 700 o Temperature ( C)

Figure 2.11.: Reduction profiles of titania nanotube confined manganese oxide catalytic formulations with Mn/Ti = 0.25 Mn(0.25)/TNT-X, obtained from Hydrogen-Temperature

Programmed Reduction (H2-TPR).

As is the case for the Mn(0.25)/TNT-H catalytic formulation that only two distinct reduction peaks corresponding to the reduction MnO2 to Mn2O3 and of Mn2O3 to Mn3O4 exist, varying the manganese oxide loading on the TNT-H support does not alter this profile. In Figure 2.12 the reduction patterns of Mn(x)/TNT-H samples are presented. As

70 mentioned all the samples exhibit two distinct peaks, the intensity of those peaks and the temperature differ along the Mn/Ti ratio of the samples. It is clear that as the Mn/Ti increases from 0.15 to 0.35 the intensity of the reduction peaks increases which is also correlated with the H2 consumption of the sample presented in Table 2.6. The

Mn(0.35)/TNT-H exhibits the largest peaks corresponding to the reduction MnO2 to Mn2O3

o o and of Mn2O3 to Mn3O4 at 340 C and 405 C respectively which is related to the higher amount of manganese oxides loaded on the support and the highest H2 consumption of

3035.9 μm∙g-1. The reduction peaks for the Mn(0.25)/TNT-H and Mn(0.3)/TNT-H samples are relatively smaller than the ones of the Mn(0.35)/TiNT-H sample. Both reduction peaks of Mn(0.25)/TNT-H sample appear in lower temperatures compared to the other samples indicating that the reducibility of the catalyst is higher, this can be attributed to the better dispersion of manganese species on the support and also can be correlated to the highest

DeNOx potential of Mn(0.25)/TNT-H [2-10].

o -1 Table 2.6.: Reduction peaks temperature ( C) and H2 consumption (μm∙g ) for the titania nanotube confined manganese oxide catalytic formulations with different Mn/Ti Mn(x)/TNT-H.

o Catalyst T ( C) H2 Consumption

-1 T1 T2 T3 μm∙g

Mn(0.15)/TNT-H 344 415 - 1270.9 Mn(0.20)/TNT-H 336 413 - 1812. Mn(0.25)/TNT-H 309 386 - 2212.2 Mn(0.30)/TNT-H 357 449 - 2682.9 Mn(0.35)/TNT-H 340 405 - 3035.9

Mn(0.35)/TNT-H

Mn(0.3)/TNT-H

Mn(0.25)/TNT-H

Intensity (a.u.) Intensity

Mn(0.2)/TNT-H Mn(0.15)/TNT-H

100 200 300 400 500 600 700

Temperature (oC)

Figure 2.12.: Reduction profiles of titania nanotube confined manganese oxide catalytic formulations with different Mn/Ti ratio Mn(x)/TNT-H obtained from Hydrogen-

Temperature Programmed Reduction (H2-TPR).

The H2-TPR profiles of the Mn(0.25)/TNT-H and the Mn(0.25)/TiO2 (UV-100

Hombikat) catalytic formulations are presented in Figure 2.13 below. The study will provide insights in the effect of the different TiO2 supports on the reduction profile of the manganese based catalysts. As described before the Mn(0.25)/TNT-H sample exhibits only

72 two reduction peaks corresponding to the reduction MnO2 to Mn2O3 and of Mn2O3 to Mn3O4

o at 309 and 386 C respectively. In contrast the Mn(0.25)/TiO2 sample exhibits three peaks corresponding to the reduction MnO2 to Mn2O3, Mn2O3 to Mn3O4 and Mn3O4 to MnO at 308,

o 404 and 482 C respectively. As discussed earlier according to our previous studies MnO2 and Mn2O3 are the most active manganese oxides for the selective catalytic reduction of nitrogen oxides. The presence of the third peak for the Mn(0.25)/TiO2 sample indicates the presence of Mn3O4 which is less active compared to the other two manganese oxides species correlated with the lower activity of the TiO2 supported sample compared to the nanotube supported catalyst. The presence of the second peak corresponding to the reduction of Mn2O3 to Mn3O4 at lower temperature for the nanotube supported sample indicates the higher reducibility of the sample which can be correlated to the superior activity of the sample.

o -1 Table 2.7.: Reduction peaks temperature ( C) and H2 consumption (μm∙g ) for the manganese oxide supported on titania nanotubes Mn(0.25)/TNT-H and on TiO2 UV-100

Hombikat Mn(0.25)/TiO2.

o Catalyst T ( C) H2 Consumption

-1 T1 T2 T3 μm∙g

Mn(0.25)/TNT-H 309 386 - 2212.2 Mn(0.25)/TiO 308 404 482 2 2471.1

Mn(0.25)/TNT-H

Intensity (a.u.) Intensity

Mn(0.25)/TiO 2

100 200 300 400 500 600 700

Temperature (oC)

Figure 2.13.: Reduction profiles of manganese oxide supported on titania nanotubes

Mn(0.25)/TNT-H and on TiO2 UV-100 Hombikat Mn(0.25)/TiO2 obtained from Hydrogen-

Temperature Programmed Reduction (H2-TPR).

2.3.6. Ammonia-Temperature Programmed Desorption (NH3-TPD)

The presence of acid sites is a profound aspect of the catalytic formulations utilized in the SCR of NOx by NH3. The identification of these sites as well as their distribution are highly important and can be directly correlated to the activity of the catalysts. The acid sites distribution for the titania nanotube confined manganese oxide catalytic formulations were determined using ammonia – temperature programmed desorption (NH3-TPD); the corresponding desorption patterns are illustrated in Figure 2.14 below. Ammonia can be bound on weak, medium and strong acid sites, depending on the strength of the acid sites ammonia desorbs at different temperatures. Ammonia bounded on strong acid sites desorbs in the temperature range from 400 to 650 oC as indicated by the desorption patterns presented. All the catalysts present intense desorption peak in the mentioned temperature range indicating that strong acid sites are the prevalent type. Desorption of

NH3 bound on weak and medium acid sites takes place in the temperature range from 200 to 400 oC, and can possible be ascribed to Lewis acidity. As it can be seen from the figure

Mn(0.25)/TNT-H, Mn(0.25)/TNT-K, Mn(0.25)/TNT-I and Mn(0.25)/TNT-TOS samples exhibit intense peaks in the temperature region from 200 to 400 oC possible corresponding to ammonia desorption from Lewis acid sites. The presence of the NH3 bounded on Lewis acid sites has been correlated to the good low temperature activity of catalytic formulations [2-9, 2-11 – 2-12, 2-53]. From the catalytic activity evaluation presented in

Figure 2.18 it can be observed that Mn(0.25)/TNT-H possesses remarkable low temperature activity which can be attributed to the high desorption of ammonia at low temperatures (300 oC). As it is clear from Figure 2.14 Mn(0.25)/TNT-SA sample exhibits a low intensity peak in the temperature range from 250 to 400 oC, indicating that the amount

75 of adsorbed ammonia is less compared to the rest of the catalytic formulations, the small peak can be correlated to the poor catalytic activity of the sample for the SCR of NOx by

NH3.

Mn(0.25)/TNT-TOS

Mn(0.25)/TNT-SA

Mn(0.25)/TNT-K

Mn(0.25)/TNT-P25

Mn(0.25)/TNT-I TCD Signal (a.u.) Signal TCD

Mn(0.25)/TNT-H

100 200 300 400 500 600 700

Temperature oC

Figure 2.14.: Ammonia – Temperature Programmed Desorption (NH3 – TPD) patterns of titania nanotube confined manganese oxide catalytic formulations with Mn/Ti = 0.25 Mn(0.25)/TNT-X.

The ammonia temperature programmed desorption (NH3–TPD) patterns for the

Mn(x)/TNT-H series of catalysts, where x is the Mn/Ti atomic ratio (x = 0.15, 0.20, 0.25,

0.30, 0.35), are presented in Figure 2.15 below. Increasing the loading of manganese oxide on the catalytic formulation results in the increase of peak intensity indicating a direct correlation of the amount of acid sites and the manganese oxide loading on the support.

The amount of ammonia desorbing in the temperature range from 250 to 400 oC, which is possibly attributed to desorption of ammonia bounded on Lewis acidic sites increases along with the Mn/Ti ratio. The same effect is present for the high temperature desorption peaks, with Mn(0.35)/TNT-H presenting the more intense peak, while the amount of the particular acid sites seems to be suppressed for the Mn(0.25)/TNT-H sample. The temperature of the peaks for the Mn(0.25)/TNT-H sample display a shift to lower temperatures compared to catalytic formulation with higher or lower Mn/Ti atomic ratio.

The lower temperature of the peaks attributed to weak and medium acid sites of the

Mn(0.25)/TNT-H could be correlated to the superior catalytic activity of the sample as exhibited in Figure 2.18.

Mn(0.35)/TNT-H

Mn(0.30)/TNT-H

Mn(0.25)/TNT-H TCD SIgnal (a.u.) SIgnal TCD Mn(0.20)/TNT-H

Mn(0.15)/TNT-H

100 200 300 400 500 600 700

Temperature (oC)

In order to compare the acidity of the titania nanotube confined manganese oxide catalytic formulation to manganese oxide loaded on conventional TiO2 UV-100 Hombikat ammonia – temperature programmed desorption (NH3 – TPD) experiments were carried out and the corresponding desorption patterns are presented in Figure 2.16 below.

Desorption patterns indicate that both catalysts exhibit medium and weak acid sites. The titania nanotube supported sample (Mn(0.25)/TNT-H) exhibits a shoulder before the medium temperature peak. The temperature of the peak is shifted at lower temperature for the the titania nanotube supported sample compared to the TiO2 one. The peak corresponding to the desorption of ammonia from medium temperature acid sites appears to be larger for the Mn(0.25)/TNT-H sample indicating a higher amount of acid sites. The weak and medium acid sites possibly corresponds to Lewis acid sites which can directly correlate to the high DeNOx potential of the Mn(0.25)/TNT-H compared to the

Mn(0.25)/TiO2. The higher amount of acid sites for the nanotube supported catalyst reveals the better dispersion of the manganese oxide on the support as proposed in earlier studies

[2-23].

Mn(0.25)/TiO

Mn(0.25)/TNT-H TCD Signal (a.u.) Signal TCD

100 200 300 400 500 600 700

Temperature (oC)

Figure 2.16.: Ammonia – Temperature Programmed Desorption (NH3 – TPD) patterns of manganese oxide supported on titania nanotubes Mn(0.25)/TNT-H and on TiO2 UV-100

Hombikat Mn(0.25)/TiO2.

2.3.7. X-Ray Photoelectron Spectroscopy (XPS)

Titania nanotube confined manganese oxide catalytic formulations were investigated by X-Ray Photoelectron Spectroscopy (XPS) in order to identify the different species atomic concentration and to obtain information on the oxidation states of manganese oxide on the surface layer. The center of the peaks corresponds to Mn 2p2/3 and

Mn 2p1/2 binding energies of the reference samples MnO2, Mn2O3, Mn3O4 and MnO are illustrated in Table 2.8 [2-10]. Two peaks ascribed to Mn 2p3/2 and Mn 2p1/2 binding energies are illustrated in Figure 2.17. It is established in the literature that the peaks ascribed to Mn4+, Mn3+ and Mn nitrate appear at 642.1 ± 0.2, 641.3 ± 0.2, and 644.2±0.4 eV respectively for Mn 2p3/2 [2-10, 2-53]. For the identification of the surface manganese oxide phases and the relative percentages of Mn4+, Mn3+ and Mn nitrate species, the overlapped Mn 2p peaks were deconvoluted into several peaks by searching for the optimal combination of Gaussian bands. The deconvoluted peaks are ascribed to specific phases of manganese (a) MnO2, (b) Mn2O3 and (c) to Mn nitrate species as exhibited in

Figure 2.18. Through the area of the corresponding characteristic peaks the relative percentages of the manganese and titania species were calculated and the results are illustrated in Table 2.8. As illustrated in Figure 2.18 and Table 2.8, the manganese species

4+ 3+ (M and Mn ) can be characterized by Mn 2p1/2 and Mn 2p3/2 peaks which are located in

3+ the range from 639.0 to 658 eV. The 2p3/2 binding energies of Mn is 641.3 ± 1 eV and of

Mn4+ is at 642.1 ± 1 eV as it is established in the literature and our earlier studies [2-4,

2-10, 2-53]. The relative atomic percentage of Mn4+/Mn3+ as was calculated from the XPS results; Mn(0.25)/TNT-H sample exhibits a relatively high amount of Mn4+ species as the

Mn4+/Mn3+ = 2.15, the same ratio is relatively smaller for Mn(0.25)/TNT-P25 and

Mn(0.25)/TNT-K, whereas for the Mn(0.25)/TNT-SA sample it is low Mn4+/Mn3+ = 0.36 as presented in Table 2.8. Among the characterized catalytic formulations Mn(0.25)/TNT-H exhibits high Mn4+ concentration (Mn4+/ Ti = 0.225), which can ascribed to the high surface coverage by MnO2, the better dispersion on the TiO2 support and to the high specific surface area of the TNT-H support. The rest of the catalytic formulations exhibit relatively lower Mn4+ surface concentration especially Mn(0.25)/TNT-SA sample exhibits Mn4+/ Ti =

0.059. The high surface concentration of MnO2 manganese oxide species of the

Mn(0.25)/TNT-H can be directly correlated to the remarkable low temperature activity of the catalysts especially at the low temperature regions, it has been established in our earlier studies that MnO2 species are higly active for the SCR of NOx by NH3.

Figure 2.17.: Deconvoluted Mn 2p (XPS) spectra of Mn(0.25)/TNT-SA, Mn(0.25)/TNT-K,

Mn(0.25)/TNT-P25 and Mn(0.25)/TNT-H catalysts: (a) MnO2, (b) Mn2O3 and (c) Mn-nitrate

Table 2.8.: Binding energies and surface atomic ratios of active Mn species for the selected catalysts determined from deconvoluted XPS spectra.

Catalyst B. E. (eV) (Mn4+/Mn3+ )b (Mn4+/Mnn+ )b Mn/Tib Mn4+/Tib

Ti 2p3/2 Ti 2p1/2 O 1s Mn 2p3/2 Mn 2p1/2

a MnO2    642.1 653.6   

a Mn2O3    641.3 653.0   

a Mn3O4    641.4 652.9   

MnOa    641.5 653.1   

Mn(0.25)/TNT-SA 459.9 465.1 530.1 642.1± 0.2 653.6± 0.2 0.36 0.12 0.30 5.95E-02 641.3± 0.2 653.0± 0.2 Mn(0.25)/TNT-K 460.3 465.7 530.3 642.1± 0.2 653.6± 0.2 1.34 0.29 0.25 7.82E-02 641.3± 0.2 653.0± 0.2 Mn(0.25)/TNT-P25 460.5 465.9 530.1 642.1± 0.2 653.6± 0.2 1.18 0.29 0.36 1.10E-01 641.3± 0.2 653.0± 0.2 Mn(0.25)/TNT-H 459.4 464.8 530.8 642.1± 0.2 653.6± 0.2 2.15 0.38 0.67 2.25E-01 641.3± 0.2 653.0± 0.2 a Our earlier studies (Boningari et al. Journal of Catalysis 2012, 288, 74–83) [2-10] b Relative amounts are according to the metal atomic ratio determined from deconvoluted XPS spectra.

2.3.8. Catalytic Activity Evaluation

The catalytic activity of the titania nanotube confined manganese oxide catalytic formulations with Mn/Ti atomic ratio equal to 0.25 has been evaluated under a Gas Hour

-1 Space Velocity (GHSV) of 50,000 h and in the presence of 900 ppm NO, 100 ppm NO2,

1000 ppm NH3, 10 vol. % O2 in balance of He. The DeNOx potential of the Mn(0.25)/TNT-X catalysts in the temperature range from 100 to 300 oC is illustrated in Figure 2.18. As one can observe from the results Mn(0.25)/TNT-SA exhibits the poorest catalytic activity in the entire temperature range. At low temperatures Mn(0.25)/TNT-K sample exhibits poor activity but as the temperature increases the activity increases as well, exhibiting 93% NOx conversion at 300 oC where all other sample’s activity seems to decline. Mn(0.25)/TNT-P25 and Mn(0.25)/TNT-I exhibit almost the same activity in the whole temperature range. The

Mn(0.25)/TNT-H sample exhibits superior low temperature activity as it shows over 95%

o NOx conversion in a wide temperature range from 100 to 260 C, the superior activity of the sample declines slightly as the temperature increase above 260 oC.

The relative turnover frequency (×10-3 s-1) was also calculated at 100 oC using

o equation 2.5 [2-54] and presented in Table 2.8 along with the NOx conversion at 100 C. As the results in Table 2.9 indicate the Mn(0.25)/TNT-H sample exhibits the highest relative

TOF of 2.31 (×10-3 s-1) compared to the other catalytic formulations.

As indicated by the catalytic evaluation results and the relative turnover frequency calculations Mn(0.25)/TNT-H sample exhibits superior activity compared to the rest of the samples. The superior activity of the Mn(0.25)/TNT-H sample can be attributed to the

85 higher specific surface area (SSA = 421 m2/g) of the sample and also to the fact that the sample possesses higher reducibility and a high amount of Lewis acidic sites.

100

(%)

NOx Mn(0.25)/TNT-H

X 60 Mn(0.25)/TNT-I Mn(0.25)/TNT-P25 50 Mn(0.25)/TNT-K Mn(0.25)/TNT-SA Mn(0.25)/TNT-TOS 40 100 150 200 250 300

Temperature (oC)

( ) ( )

( )

Where:

( )

( should be calculated from XPS results)

( )

-3 -1 Table 2.9.: Mn/Ti atomic ratio, NOx conversion (%) and turn over frequency (×10 s ) of the Mn(0.25)/TNT family of catalytic formulations.

-3 -1 Catalyst Mn/Ti NOx Conversion TOF (×10 s )

Mn(0.25)/TNT-H 0.25 99.4 2.31

Mn(0.25)/TNT-P25 0.25 67.1 1.56

Mn(0.25)/TNT-K 0.25 54.4 1.26

Mn(0.25)/TNT-SA 0.25 56.2 1.31

Mn(0.25)/TNT-I 0.25 70.1 1.63

Mn(0.25)/TNT-TOS 0.25 73.6 1.71

From the first set of catalytic activity evaluation experiments we concluded that the sample Mn(0.25)/TNT-H shows superior low temperature activity so the optimization of the manganese content of the catalysts will provide insights in terms on how the amount of manganese oxide supported on the TNT-H affects the activity. For these reasons a second family of catalysts with different manganese to titanium atomic ratio was synthesized and denoted as Mn(x)/TNT-H where x is the Mn/Ti ratio (x = 0.15, 0.20, 0.25, 0.30, 0.35). In

Figure 2.19 below the NOx conversion as a function of the Mn/Ti at four different temperatures (100 oC, 120 oC, 140 oC, 160 oC) is presented. As it is clear from Figure 2.19 at the temperature of 160 oC the catalytic activity in the whole Mn/Ti range is over 95%. The catalyst with Mn/Ti = 0.25 exhibits NOx conversion over 95% at every temperature. The largest differences in NOx conversion between the catalysts with different Mn/Ti ratios are exhibited in the lowest temperature of 100 oC, this difference between the conversions is minimized as the temperature increases. The sample with the lowest Mn/Ti ratio exhibits the poorest activity at all temperatures as the Mn/Ti ratio increases the activity increases as well reaching a maximum at Mn/Ti = 0.25. The poor activity of the catalysts with low

Mn/Ti ratio is possibly attributed to the lack of active catalytic sites due to the low loading of manganese oxide. Further increment of the Mn content beyond Mn/Ti = 0.25 has an inhibiting effect on low temperature activity as the catalysts with Mn/Ti = 0.30 and

Mn/Ti = 0.35 exhibit almost 70% and 63 % conversion at 100 oC respectively, in contrast to the sample with Mn/Ti = 0.25. The low activity of the samples with Mn/Ti ratios more than 0.25 is attributed to the low specific surface area and the agglomeration of Mn species as reported by Yao et.al. [2-24]. The catalytic activity results are also confirmed by the relative TOF at 100 oC which are presented in Table 2.10 along with the Mn/Ti ratios and

88 the NOx conversion at the same temperature. From the relative TOF results and the catalytic activity evaluation it can be concluded that the optimum Mn/Ti ratio is 0.25.

100

(%)

NOx 60

X 100 oC o 50 120 C o 140 C 40 160 oC

0.15 0.20 0.25 0.30 0.35

Mn/Ti Atomic Ratio

Figure 2.19.: Effect of Mn/Ti ratio on the performance of Mn(x)/TNT-H family of catalysts for the SCR of NOx by NH3, in the presence of 900 ppm NO, 100 ppm NO2, 1000 ppm NH3, 10

-1 o vol. % O2 with He balance under a GHSV of 50,000 h at 100, 120, 140 and 160 C.

-3 -1 Table 2.10.: Mn/Ti atomic ratio, NOx conversion (%) and turn over frequency (×10 s ) of the Mn(x)/TNT-H family of catalytic formulations.

-3 -1 Catalyst Mn/Ti NOx Conversion TOF (×10 s )

Mn(0.15)/TNT-H 0.15 43.8 1.02

Mn(0.2)/TNT-P25 0.20 50.6 1.18

Mn(0.25)/TNT-K 0.25 99.4 2.31

Mn(0.30)/TNT-SA 0.30 70.0 1.63

Mn(0.35)/TNT-I 0.35 62.4 1.45

Finally in order to confirm the superiority of titania nanotubes compared to TiO2 nanoparticles as a catalyst support for the SCR of NOx by NH3 the sample Mn(0.25)/TNT-H was compared with manganese oxide supported on conventional TiO2 UV-100 Hombikat

o with Mn/Ti = 0.25 denoted as Mn(0.25)/TiO2, in the temperature range from 100 to 300 C in the presence of 900 ppm NO, 100 ppm NO2, 1000 ppm NH3 and 10 vol.% O2 under a

GHSV of 50,000 h-1. The results presented in Figure 2.20 confirm that between the samples with the same Mn/Ti ratio titania nanotubes exhibit higher NOx conversion in the whole

o temperature region. Especially at the low temperature of 100 C the Mn(0.25)/TiO2 exhibits 67 % NOx conversion compared to the Mn(0.25)/TNT-H which exhibits almost

100 % conversion. The higher activity of the Mn(0.25)/TNT-H sample can be attributed to the higher accessibility of the reactants to active sites compared to the conventional TiO2 which is an outcome of the higher specific surface area of the TNT-H (SSA = 421 m2/g)

2 compared to TiO2 (SSA = 309 m /g) and the nanotubular structure of the TNT-H. As discussed earlier the higher catalytic activity of the Mn(0.25)/TNT-H sample is a result of

90 the presence of only MnO2 and Mn2O3 manganese oxide species as exhibited in the H2-TPR results, those results also show that the peak corresponding to the reduction of MnO2 to

Mn2O3 is larger in area for the TNT-H supported sample to the TiO2 one indicating a better dispersion of manganese oxide on the TNT sample [2-16.].

100

(%)

NOx X

20 Mn(0.25)/TiO2

Mn(0.25)/TNT-H

0 100 150 200 250 300 o Temperature ( C)

2.4. Conclusions

TiO2 nanoparticles with different crystallographic phases, surface area and particle size were used to prepare titanium oxide nanotubes via the alkaline hydrothermal treatment synthesis method. The resulting materials have different morphological features with anatase TiO2 producing short non-uniform nanotubes with high specific surface area while the structures produced from rutile TiO2 have a well-defined tubular geometry. The synthesized titanium oxide nanotubes were characterized with different techniques such as

Raman and XRD the results present clear evidence of the nanotubular structure. The synthesized titania nanotubes were used to prepare a series of titania nanotube confined manganese oxide catalytic formulations which are evaluated regarding their catalytic activity for the low temperature Selective Catalytic Reduction (SCR) of NOx by NH3 with excess oxygen under a GHSV of 50,000 h-1. The synthesized catalytic formulations exhibit remarkable catalytic activity at low temperatures, especially manganese supported on titania nanotubes prepared from UV-100 Hombikat possess superior activity at low

o temperatures as they exhibit almost 100 % NOx conversion at 100 C. The different morphological characteristics of the titania nanotubes impact the deNOx potential of the catalytic formulations. Titania nanotube supported catalysts exhibit higher activity than manganese oxide supported on conventional TiO2 in the whole temperature region of the catalytic evaluation. Hydrogen-Programmed Reduction (H2-TPR) profiles and data derived from X-Ray Photoelectron Spectroscopy (XPS) for the titania nanotube confined catalytic formulations indicate that the manganese oxide is present in Mn4+ and Mn3+ oxidation states on the support, the Mn(0.25)/TNT-H catalyst exhibits the highest amount of Mn4+ species (Mn4+/Ti = 0.225) correlated with the remarkable low temperature activity. The

92 ammonia desorption patterns collected through Ammonia-Temperature Programmed

Desorption (NH3-TPD) suggest that the manganese oxide supported on titania nanotube catalysts exhibit a high amount of Lewis acidic sites responsible for the low temperature activity of the catalyst. The optimum manganese oxide content on the titania nanotubes from TiO2 UV-100 Hombikat was also investigated, reveling that Mn/Ti = 0.25 is the optimum ratio to provide active sites and promote the activity. Titania nanotubes provide a superior support for manganese oxide as the catalyst exhibit remarkable activity attribute to the high surface area, the dispersion of active species and to the promotion of certain chemical states of manganese oxide.

Chapter 3

Novel Titania Nanotube Confined Metal Oxide Catalytic

Formulations M/TNT (M = Mn, Cu, Ce, Fe, V, Cr, Co) for the

Selective Catalytic Reduction of NOx: Evaluation of the Catalytic

Activity of Different Metal Oxides Supported on Titania

Nanotubes

3.1. Introduction

Nitrogen oxide (NOx) emissions, originated in mobile and stationary sources, are major pollutants as they are associated with various environmental issues such as ozone layer depletion, smog and global warming. There are different NOx abatement technologies; amongst them the Selective Catalytic Reduction (SCR) of NOx by ammonia (NH3) as the reducing agent is the most pre-eminent technology utilized by the industry. Different metal oxide based catalytic formulation have been proposed for the process; V2O5 supported on

TiO2 (anatase) is one of the most prevailing commercial catalysts [3-1, 3-2].

Metal oxide based catalytic formulations such as Mn/TiO2 [3-1, 3-3 – 3-8], CeO2/ACF

[3-9 – 3-10], CeO2/TiO2 [3-11 – 3-12], Cr/TiO2 [3-8, 3-13], V2O5/TiO2 [3-14], FexTiOy [3-15],

Cu/TiO2 [3-8, 3-16] and Co/TiO2 [3-16] have been reported to exhibit promising DeNOx potential in a wide temperature range. Another important aspect of the catalytic formulations is the support; through the literature different kinds of supports for metal oxide based catalyst have been proposed. Titania nanotube supported catalysts have also been investigated as potential supports for metal oxide based catalytic formulations and have been reported to exhibit higher DeNOx efficiency than conventional TiO2.

Up to date the reports on metal oxide based catalytic formulations supported on titania nanotubes for the low temperature SCR on NOx are few. Titania nanotube confined ceria [3-17 – 3-21] copper oxide [3-22 – 3-23] manganese oxide [3-24] and vanadia [3-25] exhibit promising DeNOx potential. The superiority of the titania nanotube confined metal oxides is attributed to the high surface area of the support, ammonia adsorption properties

[3-22 – 3-23] and the high dispersion of metal oxides on the titania nanotubes [3-20, 3-22 –

3-23, 3-25]. In the previous chapter titania nanotubes synthesized using TiO2 nanoparticles with different crystallographic properties, specific surface area and particle size were used as support for manganese oxide based catalytic formulations and were evaluated regarding their DeNOx potential. Among the synthesized supports titania nanotube prepared from

TiO2 UV-100 Hombikat exhibited the highest surface area, and a superior low temperature activity.

In the present work TiO2 UV-100 Hombikat have been used to prepared titania nanotubes via the alkaline hydrothermal treatment synthesis method. The resulting material was used as the support for a series of metal oxide based catalytic formulations.

Different metal oxides were investigated regarding their catalytic activity for the SCR of

-1 NOx by NH3 with excess O2 under a GHSV of 50,000 h . Manganese oxide, copper oxide, chromium oxide and vanadium oxide supported on titania nanotubes possess remarkable

o low temperature activity as they exhibit complete conversion of NOx at 150 C; while

o vanadia based sample exhibits complete NOx conversion up to 300 C. Titania nanotubes loaded with ceria and iron oxide exhibit stable conversions at high temperature were the activity of the rest of the catalysts declines due to non-selective ammonia oxidation.

Bimetallic combinations of manganese oxide and ceria with different Mn/Ce atomic ratios were also investigated in order to study the synergistic effects of combining two different metal oxides, resulting in the improvement of low temperature activity compared to the ceria based sample and to more stable conversion at higher temperature were mangansese oxide based catalyst seems to decline.

3.2. Experimental

3.2.1. Material Synthesis

3.2.1.1. Titania Nanotubes Synthesis

Titania nanotubes were synthesized via the alkaline hydrothermal treatment method using TiO2 anatase Hombikat UV-100 from Sachtleben. The synthesis of the titania nanotubes was achieved by dissolving 2 g of TiO2 in 70 mL of 20 M NaOH solution and then hydrothermal treated at 130 oC for 24 hrs. in a Teflon-lined stainless steel autoclave. After the thermal treatment, the content of the autoclaves was filtered and washed initially with deionized water until the PH became neutral, then the material was treated with 2 L of 0.2

M HCl aqueous solution until the PH reached approximately 1-2 and finally washed with deionized water until the PH was resorted to neutral. The resulting nanotubes where dried at 80 oC for 18 hrs in an oven. Finally the material was ground in mortar and sieved in a mesh with 300 μm opening. The resulting titania nanotubes were denoted as TNT.

3.2.1.2. Synthesis of M/TNT (M = Mn, Co, Cr, Ce, Fe, V and Cu) catalytic formulations

A series of titania nanotube confined metal oxide catalysts (Mn, Co, Cr, Ce, Fe, V, Cu) with constant loading of 15wt.% of metal on the support were prepared by adopting the wet impregnation method. The as synthesized nanotubes (TNT) were used as the support of the catalytic formulations, using manganese(II) nitrate hydrate (MnN2O6∙xH2O 99.99% trace metal basis from Sigma-Aldrich), cerium(III) nitrate hexahydrate (CeN3O9∙6H2O

≥99.0% from Fluka Analytical), chromium(III) nitrate nonahydrate (Cr(NO3)3∙9H2O, 99%

97 from Sigma Aldrich), iron (III) nitrate nonahydrate. The Mn/Ti (Fe(NO3)3∙9H2O 98+% from

Sigma Aldrich), copper(III) nitrate hemipentahydrate (Cu(NO3)2∙2.5H2O 99.99% from

Aldrich), cobalt(II) nitrate hexahydrate (Co(NO3)2∙6H2O 98% from Sigma Aldrich) and ammonium metavanadate (H3NO3V ≥99.0% from Sigma Aldrich) as the metal oxide precursors. The required amount of the precursor, in order to achieve the desired metal loading, was added in a 200 mL beaker containing 2 g of the support along with 100 mL of deionized water. The excess water was slowly evaporated in a water bath with continuous stirring at 70 oC. The resulting materials were oven dried at 80 °C for 18 hrs. Finally the powders were ground in mortar and sieved in mesh with 300 μm openings in order to obtain homogeneous powder. Prior to the catalytic activity evaluation experiments, the materials were calcined in a tubular oven at 400 °C with 5 °C per minute temperature increments for 2 hrs. in open-air. The resulting catalytic formulations are denoted as

M/TNT where (M = Mn, Co, Cr, Ce, Fe, V, Cu).

3.2.1.3. Synthesis of Mn–Ce(x)/TNT-H catalysts

After the evaluation of the titania nanotube confined metal oxide series catalysts, bimetallic combinations of manganese oxide and ceria supported on titania nanotubes, with 15 wt.% metal loading, were prepared. As synthesized titania nanotubes prepared from TiO2 (UV-100 Hombikat) were used as the support; manganese oxide along with cerium oxide were loaded by adopting the wet impregnation method, with different Mn/Ce atomic ratios. Manganese(II) nitrate hydrate (MnN2O6∙xH2O 99.99% trace metal basis from

Sigma-Aldrich) and cerium(III) nitrate hexahydrate (CeN3O9∙6H2O ≥99.0% from Fluka

Analytical) were used as the metal oxide precursors. In order to prepare the catalytic formulations the required amount of manganese(II) nitrate hydrate and cerium(III) nitrate hexahydrate were added to a 200 mL beaker containing 2 g of TNT and 100 mL of deionized water. Stirring was applied in the resulting solution which was placed in a water bath at 70 °C in order to remove excess water via evaporation. The resulting materials were oven dried at 80 °C for 18 hrs., and then were ground in mortar and sieved in mesh with 300 μm openings to obtain homogeneous powder. Prior to the catalytic evaluation experiments, the powders were calcined in a tubular oven at 400 °C with 5 °C per minute temperature increments for 2 hrs. in open-air. The catalytic formulations are denoted as

Mn-Ce(x)/TNT where x is the Mn/Ce atomic ratio (x = 1.27, 2.55 and 5.10).

3.2.2. Materials Characterization

The specific surface area (m2/g), pore volume (cm3/g) and pore diameter (nm), of the TNT support and the titania nanotube confined metal oxide catalytic formulations were obtained from N2 adsorption isotherms at liquid nitrogen temperature (77 K) using a surface area and porosimetry analyzer (Micromeritics Tristar 3000) analyzer.

The X-Ray Diffraction patterns of the synthesized catalytic formulations were recorded using a Phillips Xpert diffractometer occupied with a nickelfiltered CuK radiation source. The intensity data were collected over a 2 range of 10°  80° with a step size of 0.025° and a step time of 0.25 seconds. The crystalline phases of the materials were finally determined by the comparison of the received XRD patterns to the reference data from International Center for Diffraction Data (ICDD) files.

Hydrogen – Temperature Programmed Reduction (H2-TPR) of the titania nanotube confined metal oxide catalysts was carried out in a catalyst characterization system

(Micromeritics, model AutoChem II 2910). The hydrogen consumption of the catalysts in the temperature range from 75 to 700 oC is obtained by integrating the thermal conductivity detector (TCD) signal intensities.

Ammonia – Temperature Programmed Desorption (NH3-TPD) data were collected using the same automated catalyst characterization system (Micromeritics Autochem II

X-Ray Photoelectron Spectroscopy (XPS) was applied to study the atomic surface concentration on the titania nanotube confined metal oxide catalysts. The experiments were carried out on a Pyris-VG thermo scientific X-Ray photoelectron spectrometer system equipped with a monochromatic AlK as a radiation source at 300 W under ultra-high vacuum (UHV = 6.7  10-8 Pa).

3.2.3. Catalytic activity evaluation experiments

The low-temperature selective catalytic reduction of NOx by ammonia with excess oxygen catalytic activity evaluations were carried in a fixed bed continuous flow quartz reactor under atmospheric pressure. The Gas Hour Space Velocity was kept constant and equal to 50,000 h-1. The catalyst was placed in the center of reactor between two glass wool

100 plugs. The gas flows were measured and calibrated using a digital flow meter (Humonics

Hewlett Packard Optiflow 520). The reaction gas mixture consisted of 900 ppm NO, 100 ppm NO2, 1000 ppm NH3, 10 vol.% O2, and in helium balance. The fixed bed reactor was heated using a tubular furnace regulated by a temperature controller (Omega CN 2041).

The catalytic formulations were evaluated in the temperature range from 100 to 400 °C with 50 °C increments. The reactants and products were measured by a chemiluminescence NO/NOx analyzer (Eco Physics CLD 70S) and on-line Quadrapole mass spectrometer (MKS PPT-RGA). In order to avoid errors in the concentration measurements an ammonia trap was installed prior to the inlet of the chemiluminescence analyzer. The concentration of the reactants and products were recorded only after steady state was achieved at each temperature step. The NOx conversion and N2 selectivity were calculated using the following equations.

( ) [ ] ( )

( ) [ ] ( ) ( )

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3.3. Results and Discussion

3.3.1. Specific surface area (m2/g) and Pore Volume Measurements

The specific surface area (SSA m2/g), pore volume (cm3/g) and pore diameter (nm) of titania nanotube confined metal oxide catalytic formulations are illustrated in Table 3.1 below. The specific surface area of the as prepared titania nanotubes measured by N2

2 physisorption at liquid N2 temperature was calculated as 421 m /g. The calcination of the nanotubes at 400 oC for 2 hrs. is reducing the specific surface area to 223 m2/g while increasing the pore diameter. The decrease in surface area is attributed to the dehydration of the titanate on the nanotube walls to anatase TiO2 [3-23] and the formation of shorter nanotubes with larger diameter [3-26 – 3-27]. The impregnation of the metal oxides onto the as prepared titania nanotube support followed by the calcination of the catalysts at

400 oC for 2hr results in catalysts with surface area close to the calcined nanotubes. The pore volume and pore diameter decrease by the introduction of the metal oxide on the support as the metal oxides deposit on the titania nanotube walls. The specific surface area of the titania nanotube confined metal oxide catalysts ranges from 190 to 252 m2/g. The lowest specific surface area is exhibited by the vanadia loaded sample (V/TNT) which is with accordance with the literature that vanadia causes the sintering of anatase phase

[3-28]. Generally all the prepared catalysts exhibit relatively high specific surface area attributed to the high surface area and the morphology of the titania nanotube support.

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Table 3.1.: Specific surface area (m2/g), pore volume (cm3/g) and pore diameter (nm) of pristine titania nanotube and titania nanotube confined metal oxide catalytic formulations.

SSA Pore Volume Pore Diameter Catalyst (m2/g) (cm3/g) (nm)

a TNT 421 0.75 7.1

TNTb 223 0.64 11.5

Mn/ TNTb 221 0.47 8.5

Ce/ TNTb 229 0.52 9.0

V/ TNTb 190 0.42 8.8

Fe/ TNTb 204 0.49 9.6

Cu/ TNTb 199 0.48 9.5

Cr/TNT 252 0.50 7.9

Co/ TNTb 219 0.58 10.5 a Uncalcined b Calcined at 400 oC for 2 hrs.

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3.3.2. X-Ray Diffraction (XRD)

The X-Ray Diffraction patterns for the series of M/TNT catalytic formulations where

X = Mn, Cu, Ce, Fe, V, Cr and CO along with the calcined and pristine titania nanotubes prepared from TiO2 UV-100 Hombikat are illustrated in Figure 3.1. The calcined nanotubes exhibit intense peaks corresponding to anatase TiO2, at d = 3.54, 1.90 and 2.40 Å (JCPD no.

21-1272) the same is the case for the titania nanotube confined metal oxide catalysts. The peak corresponding to 101 anatase at d = 3.54 Å varies in intensity which indicates that some samples are more crystalline than others, specifically Ce/TNT exhibits a weak peak indicating the sample is more amorphous. The characteristic peaks corresponding to

H2Ti3O7 can be observed in the XRD spectra, small peaks at d = 3.65, 2.37 and 3.08 Å are evident for the samples [3-29 – 3-30]. The peaks at d = 2.61 and 3.32 Å that can be observed in the XRD spectra are ascribed to the nanotubular structure of the materials

[3-29 – 3-30]. In earlier studies it has been reported that the trititanates convert to anatase at high temperature, which is evident in the spectra as the peaks corresponding to H2Ti3O7 and to nanotubes shift toward peaks corresponding to the anatase phase of TiO2. For the titania nanotube confined metal oxide catalytic formulations no peaks corresponding to crystalline phases of the metal oxides are present; which indicates that the metal oxides are finely dispersed on the titania nanotube surface.

104

A(200)

A(004)

A(101)

A(105) A(211)

Nanotubes H Nanotubes (i) (h) (g) (f) (e) (d)

10 20 30 40 50 60 70 80

()

105

3.3.3. H2-Temperature Programmed Reduction (H2-TPR)

The reduction profiles of the M/TNT series of catalytic formulations, where M = V,

Cr, Mn, Cu, Ce, Co and Fe, were determined by Hydrogen – Temperature Programmed

Reduction (H2-TPR) and are presented in Figure 3.2 below. As it is illustrated the V/TNT catalysts exhibits only one broad peak centered at 533 oC, the peak is ascribed to the reduction of monomeric isolated vanadia species [3-31]. The chromium oxide based catalyst, Cr/TNT, exhibits one intense peak attributed to the reduction of Cr2O3 to CrO centered at 294 oC [3-32 – 3-33]. The reduction profile of the titania nanotube confined manganese oxide catalyst (Mn/TNT) as it can be observed from the results exhibits two distinctive peaks in the whole temperature range from 75 to 700 oC. The low temperature reduction peak centered at 309 oC which is the more intense one corresponds to the

o reduction of MnO2 to Mn2O3 [3-6, 3-34 – 3-35] while the second peak at 388 C is attributed to the reduction of Mn2O3 to Mn3O4 [3-6, 3-35] as discussed earlier the presence of only

Mn4+ and Mn3+ manganese species in the Mn/TNT is correlated with the remarkable catalytic activity of sample at low temperatures [3-7]. The reduction profile of Cu/TNT exhibits three narrow peaks at low temperatures centered at 125 , 148 , 175 oC and one broad peak at 222 oC initially Cu2+ is reduced to Cu1+ and then Cu1+ to Cu0. The low temperature peaks are ascribed to the reduction of isolated copper atoms which on the

TiO2 surface are coordinated with oxygen and to small copper oxide clusters present on the surface while the high temperature peak to larger copper oxide agglomerates which are not easily reduced [3-36 – 3-37]. The titania nanotube confined cerium oxide catalytic formulation (Ce/TNT) presents two peaks centered at 408 and 596 oC, the high temperature peak is relatively smaller and can only be observed in the individual spectra.

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The peak at 408 oC is ascribed to the reduction of surface oxygen and amorphous ceria on the TNT surface, Ce4+ - O - Ce4+, while the high temperature peak to the reduction Ce3+ - O -

4+ Ce of bulk CeO2. The low intensity of the second peak can reveal a good dispersion of ceria on the support avoiding large agglomerates which is in accordance with the XRD pattern of the Ce/TNT sample where no CeO2 phase can be observed [3-38 – 3-39]. The Co/TNT

o sample exhibits one small peak at 356 C corresponding to the reduction of Co3O4 to CoO as the temperature increases a wide peak is present at 576 oC ascribed to the reduction of CoO to metallic cobalt [3-40 – 3-42]. The presence of the shoulder in the high temperature peak indicates a non-uniform distribution of cobalt oxides on the TNT [3-40 – 3-42]. The reduction profile of the iron oxide based catalyst exhibits two distinct peaks at 287 and

o 379 C respectively, the low temperature peak corresponds to the reduction of Fe2O3 to

3+ Fe3O4 where the Fe located on the octahedral sites are reduced while the peak present at

o 379 C to the further reduction of Fe3O4 to metallic iron [3-43 – 3-42].

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Co/TNT

Cr/TNT

V/TNT

Fe/TNT

Ce/TNT Cu/TNT

TCDSignal (a.u.) Mn/TNT

100 200 300 400 500 600 700

o Temperature ( C)

Figure 3.2.: Reduction profiles from Hydrogen-Temperature Programmed Reduction (H2- TPR) for titania nanotube confined metal oxide catalytic formulations M/TNT where M = Mn, Cu, Ce, Fe, V, C and Co.

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The reduction pattern of the bimetallic combinations of ceria and manganese oxide supported on titania nanotubes were received using Hydrogen-Temperature Programmed

Reduction (H2-TPR) in order to investigate the effect on the reduction profile. In Figure 3.4 below the reduction profiles of Mn/TNT and Ce/TNT are presented for comparison reasons along with the reduction profiles of Mn-Ce(x)/TNT catalysts, where x is the Mn/Ce atomic ratio (x = 1.27, 2.55 and 5.10). As discussed above Mn/TNT presents two peaks

corresponding to the reduction of MnO2 to Mn2O3 and of Mn2O3 to Mn3O4 [3-6, 3-34 –

3-35]. Two peaks are also present for the Ce/TNT sample ascribed to the reduction of surface oxygen and amorphous ceria on the titania nanotubes surface, Ce4+ - O - Ce4+, and to

3+ 4+ the reduction Ce - O - Ce from bulk CeO2. The Mn-Ce(x)/TNT samples mainly exhibit one broad peak located in the temperature range from 250 to 500 oC. The presence of wide peaks for the bimetallic samples indicate that the reduction of Ce4+ to Ce3+ takes place along with the reduction of manganese oxide [3-45]. As the Mn/Ce atomic ratio increases the intensity of the peak increases as well indicating that the amount of manganese oxide mainly contributes to the intensity of the peak. The Mn-Ce(5.10)/TNT sample exhibits a small shoulder following the main peak at 400 oC, which can possible be ascribed to the reduction of Mn2O3 species. Thirupathi et.al. proposed that for Mn-Ce/TNT catalytic formulations the low temperature peak is ascribed to surface manganese oxide species reduction and the peak at 350 oC, to the reduction of surface oxygen of ceria [3-6]. As proposed in earlier studies the wide broad peak present in the reduction patterns of Mn-Ce catalytic formulations indicates a new center of O2 storage where reduction peaks of manganese oxide and ceria emerge [3-45].

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Mn-Ce(5.10)/TNT

Mn-Ce(2.55)/TNT

Mn-Ce(1.27)/TNT

TCD Signal (a.u.) Signal TCD Mn/TNT

Ce/TNT

100 200 300 400 500 600 700

o Temperature ( C)

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3.3.4. Ammonia – Temperature Programmed Desorption (NH3-TPD)

In order to get information on the amount and strength of different acid sites of the

M/TNT family of catalytic formulations, where M = Mn, Cu, Ce, Fe, V, Cr and Co, Ammonia –

Temperature Programmed Desorption (NH3-TPD) experiments were carried out. The ammonia desorption patterns collected are illustrated in Figure 3.5 below. All the catalytic formulations exhibit primarily two peaks in the temperature range from 75 to 700 oC. The peaks that appear at low temperatures are ascribed ammonia bound on weak and medium acid sites while the high temperature peaks to strongly bound ammonia. Cr/TNT catalyst exhibits a large peak centered at 538 oC ascribed to desorbing of ammonia bonded on strong acid sites. The peaks present at low temperatures below 400 oC as discussed are ascribe to weak and medium acid sites possible Lewis acid sites which through literature are correlated to the low temperature activity of the catalytic formulations [3-6, 3-8, 3-46 –

3-47]. Samples Cu/TNT and Mn/TNT exhibit intense peaks centering at 334 and 299 oC respectively, the intensity of the peaks and the low temperature can be correlated with the low temperature DeNOx potential of the catalytic formulations. The vanadia and cobalt loaded titania nanotubes also exhibit broad peaks at temperatures below 400 oC and as well exhibit good low temperature activity. It can be stated that the ammonia bounded on weak and medium acid sites contributes to the low temperature activity of the catalytic formulations. The Ce/TNT sample exhibits one broad peak in the temperature range from

200 to 450 oC corresponding to desorption of ammonia from Lewis acidic sites the low temperature part of the peak can be attributed to weak Lewis acid sites while the peak at

389 oC corresponds to strong Lewis acid sites. The special adsorption properties of ammonia on the metal oxide confined nanotubes can be attributed to the higher binding

111 energies of ammonia in the inner surface of the nanotubes, which result the enriched adsorption of ammonia in the channels compared to the exterior surface of the nanotube

[3-17, 3-48].

Co/TNT

Cr/TNT V/TNT

Fe/TNT

Ce/TNT

Cu/TNT TPD Signal (a.u.) Signal TPD

Mn/TNT

100 200 300 400 500 600 700 o Temperature ( C)

Figure 3.5.: Ammonia – Temperature Programmed Desorption (NH3-TPD) patterns of the titania nanotube confined metal oxide catalytic formulations, Mn/TNT where M = Mn, Cu, Ce, Fe, V, Cr and Co.

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The ammonia desorption profiles of the Mn-Ce(x)/TNT catalytic formulations along with the Mn/TNT and Ce/TNT samples are presented in Figure 3.6 below. The NH3 desorption patterns of manganese oxide (Mn/TNT) and ceria (Ce/TNT) confined on titania nanotubes are discussed above; Mn/TNT sample exhibits an intense peak corresponding to the desorption of ammonia from strong acid sites that peak is suppressed for the Ce/TNT sample; Both monometallic catalytic formulations exhibit broad and intense peaks corresponding to the desorption of ammonia from weak and medium acid sites; ceria confined sample exhibits peaks at higher temperature combined with Mn/TNT indicating the stronger bonds of ammonia on the sample. The combination of manganese oxide and ceria in the catalytic formulation results in catalysts with wider and less intense peaks corresponding to the desorption of ammonia from weak and medium acid sites. As the

Mn/Ce atomic ratio increases there is an increase in the intensity of the peaks corresponding to ammonia desorbing from strong acid sites which indicates that manganese oxide is responsible for the strong acid sites which ceria does not possess.

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Mn-Ce(5.10)/TNT

Mn-Ce(2.55)/TNT

Mn-Ce(1.27)/TNT

Ce/TNT TCD Signal (a.u.) Signal TCD Mn/TNT

100 200 300 400 500 600 700

Temperature (oC)

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3.3.5. X-Ray Photoelectron Spectroscopy

X-Ray Photoelectron Spectroscopy was applied for the titania nanotube confined metal oxide catalytic formulations in order to study the oxidation states of different metal oxides on the titania nanotube and the surface atomic ratios of the active components. The deconvoluted XPS spectra are illustrated in Figure 3.7 below and the binding energies along with the atomic ratios of active components are exhibited in Table 3.2.

The peaks ascribed to Co 2p were deconvoluted and shown in Figure 3.7.a below.

The Co 2p with binding energies of 779.5 and 794.5 eV are characteristic of Co3+ with a satellite signal at 788.5 eV, likewise the Co2+ exhibits characteristic peaks at 782.1 and

796.8 eV with a satellite signal at 803.5 eV [3-49]. From the ratio of Co2+/Co3+ which is presented in Table 3.2 it can be stated that the cobalt oxide species are present mainly as

CoO on the titania nanotube surface; which can be possible correlated with the poor activity of the cobalt oxide on titania nanotube for the SCR of NOx.

The deconvoluted peaks corresponding to Cu 2p1/2 and to Cu 2p3/2 are also presented in Figure 3.7.b for the copper oxide confined titania nanotube catalyst (Cu/TNT).

As indicated from the results copper oxide exist as a mixture of C2+ and Cu+1 oxidation

+1 +1 states. The peaks corresponding to Cu 2p3/2 and Cu 2p1/2 appear at 936.3 and 955.8 eV

2+ +1 respectively; while the peaks ascribed to Cu 2p3/2 and Cu 2p1/2 at 933.4 and 954.4 eV respectively and the satellite signal at 994.2 eV [3-50]. The deconvolution of the peaks reveals that copper species exist mostly as Cu2+ as indicated by the Cu2+/ Cu1+ exhibited in

Table 3.2. which is equal to 1.28.

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The Fe 2p spectra from the iron oxide confined on titania nanotube were also investigated through XPS; the binding energies of Fe 2p3/2 and Fe2p1/2 are 712.2 and

725.6 eV respectively [3-51]. The deconvolution of the peaks separates each peak into two

2+ 3+ new one corresponding to Fe and Fe . It is stated in the literature that Fe2O3 presents characteristic peak corresponding to Fe 2p3/2 from 711.0 to 711.2 eV [3-51]. The peaks for

2+ Fe 2p3/2 at 409.6 – 710.1 eV are ascribed to Fe oxidation stated [3-52]. From the deconvoluted spectra presented in Figure 3.7.c it is easily observed that Fe3+ oxidation

2+ state is dominating towards Fe indicating that iron is mostly present as Fe2O3 oxide on the titania nanotube surface which is also stated in Table 3.2 were the atomic ratio of different oxidation states of iron are presented.

To gain additional insight about the oxidation state of vanadia species confined on the titanium oxide nanotubes XPS spectra of the samples were recorded. The V 2p spectra

5+ 4+ 3+ of the V2O5, VO2 and V2O3 oxides corresponding to the V , V and V oxidation states respectively were measured by Demeter et.al. [3-53]. The V/TNT catalytic formulation exhibits two main peaks due to V 2p3/2 and V 2p1/2 which are illustrated in Figure 3.7.d The

5+ 4+ 3+ peaks ascribed to V 2p3/2, V 2p3/2, and V 2p3/2 appear at 517.5 ± 0.2, 516.2 ± 0.2, and

515.8 ± 0.2 eV, respectively [3-54]. The V 2p3/2 and V 2p 1/2 peaks were deconvoluted in to peaks by finding the optimal combination of Gaussian bands and the resulting peaks are ascribed to specific oxidation states of vanadia species as illustrated in Figure 3.7.d. From the results illustrated in the figure and the atomic surface concentrations presented in

Table 3.2 it can be concluded that V5+ species is the favorable oxidation state of vanadium so it can be proposed that vanadia exists mainly as V2O5 on the titania nanotube surface layer.

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The deconvoluted XPS spectra of Ce 3d on ceria confined on titania nanotube catalyst, Ce/TNT, are presented in Figure 3.7. As illustrated in the figure the peaks denoted as v are ascribed to Ce 3d5/2 and the ones denoted as u to Ce 3d3/2 spin orbit components.

The peaks denoted as v, v’’, v’’’, u, u’’ and u’’’ are attri uted to Ce4+ species on the surface while v’ and u’ peaks to surface Ce3+ [3-55]. From the peaks it can be easily observed that

Ce4+ is the dominant species and the peaks for Ce3+ are weak. The sub-bands denoted as u’ and v’ are ascri ed to the 3d10 4f1 which corresponds to Ce3+ surface species, whereas the v, v’’, v’’’, u, u’’ and u’’’ indicate the presence of 3d10 4f0 electronic state which corresponds to

Ce4+ species [3-55]. In table 3.2 the Ce3+/ Ce4+ratio equal to 0.79 indicate a high surface concentration of Ce3+ species known for enhancing the redox cycle of ceria based catalytic formulations.

The XPS spectra of the titania nanotube confined manganese oxide catalytic formulation exhibits two peaks ascribed to Mn 2p1/2 and to Mn 2p3/2 at 654.3 and 643.3 eV respectively. The deconvolution of those peaks results to peaks that are ascribe to specific manganese species MnO2, Mn2O3 and to Mn nitrate species at 642.1 ± 0.2, 641.3 ± 0.2, and

644.2±0.4 eV respectively [3-3, 3-7, 3-16]. As indicated from the results in the table manganese oxide is mostly present as MnO2 in the surface layer of the catalytic formulation as Mn4+/Mn3+ ratio is equal to 2.15. It has been suggested in our earlier studies [3-7] than

MnO2 are the most active species for the low temperature SCR of NOx by NH3.

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4+ Mn2p 3/2 (f) Ce Ce-TNT-HCe/TNT (e) Mn/TNT-HMn/TNT Ce3+ Ce3d Ce3d 3/2 5/2

u' 4+ v' Mn u'' Mn2p u''' u v 1/2

v'' 4+

v''' 3+ 3+ Mn

Intensity (a.u.) Intensity Intensity (a.u.) Intensity

940 930 920 910 900 890 880 870 665 660 655 650 645 640 635

Binding Energy (eV) Binding Energy (eV)

Fe/TNT-H (c) V-TNT-H V2p (d) Fe/TNT Fe2p V/TNT 3/2 3/2

3+ Fe V5+ V4+

Fe2p

1/2

3+ 2+ Fe Fe 2+ Fe Sat. 3+ V2p

1/2 V Intensity (a.u.) Intensity (a.u.) Intensity

735 730 725 720 715 710 705 700 524 522 520 518 516 514 512

Binding Energy (eV) Binding Energy (eV)

Co2p (a) Cu/TNT-H Cu2p (b) Co/TNT-HCo/TNT 3/2 Cu/TNT 3/2 2+ Co Cu2+

Co2p Co3+ 1/2 Sat. Sat.

Co2+ Cu1+

Cu2p Co3+ 1/2 Sat. Cu2+ Sat. 1+

Intensity (a.u.) Intensity Intensity (a.u.) Intensity

812 805 798 791 784 777 770 960 955 950 945 940 935 930 925 Binding Energy (eV) Binding Energy (eV)

Figure 3.7.: Deconvoluted (XPS) spectra of Co 2p in Co/TNT, Cu 2p in Cu/TNT, Fe 2p in

Fe/TNT, V 2p in V/TNT, Ce 3d in Ce/TNT and Mn 2p in Mn/TNT catalytic formulations.

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Table 3.2. Binding energies and surface atomic ratios of active components for the selected potential (V/TNT-H, Mn/TNT-H, Fe/TNT-H, Co/TNT-H, Cu/TNT-H and Ce/TNT-H) catalysts determined from deconvoluted XPS spectra.

Binding Energy (eV)a (Fe3+/Fe2+)b (Fe2+/Fe3+)b Fe/Ti b Fe3+/Ti b Fe2+/Ti b Fe3+/(Fe3++Fe2+)b Catalyst Fe

Ti2p O1s Fe2p3/2 Fe2p1/2 Fe/TNT-H 459.2 530.8 712.2 725.6 3.81 0.26 0.36 0.22 0.05 0.79 464.8

Binding Energy (eV)a (Co3+/Co2+)b (Co2+/Co3+)b Co/Ti b Co3+/Ti b Co2+/Ti b Co3+/(Co3++Co2+)b Catalyst Co Ti2p O1s Co2p Co2p 3/2 1/2 Co/TNT-H 459.7 531.5 782.1 797.5 0.65 1.52 0.44 0.09 0.13 0.39 465.2

Binding Energy (eV)a (Cu2+/Cu1+)b (Cu1+/Cu2+)b Cu/Ti b Cu2+/Ti b Cu1+/Ti b Cu1+/(Cu1++Cu2+)b Catalyst Cu Ti2p O1s Cu2p3/2 Cu2p1/2 Cu/TNT-H 460.5 532.3 936.3 955.8 1.28 0.77 0.34 0.12 0.09 0.43

466.0

Binding Energy (eV)a (Ce4+/Ce3+)b (Ce3+/Ce4+)b Ce/Ti b Ce4+/Ti b Ce3+/Ti b Ce3+/(Ce3++Ce4+)b

Catalyst Ce Ti2p O1s C3d5/2 Ce3d3/2 Ce/TNT-H 461.2 532.6 886.2 902.1 1.26 0.79 0.31 0.17 0.14 0.44 466.6

Binding Energy (eV)a (V5+/V4+)b (V4+/V5+)b V/Ti b V5+/Ti b V4+/Ti b V5+/(Vn+)b

Catalyst V Ti2p O1s V2p3/2 V2p1/2 V/TNT-H 457.4 529.0 515.8 521.6 1.75 0.57 1.21 0.65 0.37 0.54 462.8

Binding Energy (eV)a (Mn4+/Mn3+)b (Mn3+/Mn4+)b Mn/Ti b Mn4+/Ti Mn3+/Ti Mn4+/(Mn3++Mn4+)b

Catalyst b b Mn

Ti2p O1s Mn2p3/2 Mn2p1/2 Mn/TNT-H 459.4 530.8 643.3 654.3 2.15 0.46 0.67 0.22 0.10 0.68 464.8 a Binding energy of various components present in the metal-confined titania nanotubes b Relative amounts are according to the metal atomic ratio determined from deconvoluted XPS spectra.

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3.3.6. Catalytic Activity Evaluation

The catalytic performance of the titania nanotube confined metal oxide catalytic formulations M/TNT, where M = Mn, Cu, Ce, Fe, V, Cr, Co, was evaluated in a fixed bed quartz reactor in the temperature range from 100 to 400 oC with 50 oC in the presence of

-1 900 ppm NO, 100 ppm NO2, 1000 ppm NH3, 10 vol. % O2 and under GHSV of 50,000 h . In

o Figure 3.8 it is clear that Mn/TNT catalyst exhibits the highest DeNOx potential at 100 C as it exhibits almost complete NOx conversion at that temperature. As the temperature increases to 150 oC the catalytic activity of the titania nanotubes confined with copper oxide, vanadium oxide and chromium oxide increase as well exhibiting complete conversion of NOx, having remarkable low temperature activity. The vanadium oxide based

o catalytic formulation exhibits almost 100 % conversion of NOx up to 300 C, indicating that the catalyst possesses a wide temperature operation window. All the catalytic formulations exhibit conversions more than 85 % at 250 oC with Co/TNT sample exhibiting the lowest conversion at this temperature and the poorest catalytic activity among all the samples.

The catalytic formulations exhibiting very good low temperature activity as the temperature increases above 300 oC show a decline in their activity caused by non- selective ammonia oxidation. Titania nanotube confined ceria catalysts exhibits almost

o complete conversion of NOx in the temperature range from 200 to 300 C, the catalyst also exhibits very good stability at temperatures above 300 oC.

120

100

(%) Mn/TNT 40

NOx Cu/TNT

X Ce/TNT Fe/TNT 20 V/TNT Cr/TNT Co/TNT

0 100 150 200 250 300 350 400

Temperature (oC)

Figure 3.8.: Catalytic activity evaluation of titania nanotube confined metal oxides catalytic formulations M/TNT, where M = Mn, Cu, Ce, Fe, V, Cr and Co, in the presence of 900 ppm

-1 NO, 100 ppm NO2, 1000 ppm NH3, 10 vol. % O2 with He balance under a GHSV of 50,000 h in the temperature range from 100 to 400 oC with 50 oC increments.

121

After the catalytic evaluation of the M/TNT family of catalytic formulation and considering the results presented in Figure 3.8 a series of bimetallic catalytic formulations containing manganese and cerium oxide were prepared. The weight percentage of the metals supported on the titania nanotubes was kept constant and the manganese to cerium atomic ratio differed (Mn/Ce = 5.1, 2.5 and 1.3). In Figure 3.9 the catalytic activities of the

Mn/TNT, Ce/TNT and Mn-Ce(x)/TNT catalytic formulations are presented, the evaluation of the catalysts was done in the presence of 900 ppm NO, 100 ppm NO2, 1000 ppm NH3,

-1 10 vol.% O2 in He balance and under GHSV of 50,000 h . It is clear from the figure that the introduction of ceria on the catalytic formulation results in the decline of the catalytic

o performance at 100 C as the samples exhibit NOx conversion around 75 – 8 0 %. Increasing the temperature increases the deNOx potential of the bimetallic catalytic formulations

o which exhibit almost complete conversion of NOx from 150 to 250 C. The sample with

Mn/Ce atomic ratio of 1.3 exhibits increased stability of the conversion at high temperatures compared to the samples with higher Mn/Ce, indicating that the presence of cerium oxide increases the high temperature activity of the catalyst. The results illustrate that the combination of manganese oxide and ceria improves the conversion of Mn/TNT catalysts at higher temperatures indicating a synergistic effect between the metal oxides while it compromises the low temperature activity.

122

100

(%)

NOx 40

X Mn/TNT Ce/TNT Mn-Ce(1.27)/TNT 20 Mn-Ce(2.55)/TNT Mn-Ce(5.10)/TNT

0 100 150 200 250 300 350 400

Temperature (oC)

Figure 3.8.: Catalytic activity evaluation of manganese oxide and ceria confined on titania nanotube catalytic formulations and their bimetallic combination Mn-Ce(x)/TNT, where x is the Mn/Ce atomic ratio (x = 1,27, 2.55, 5.10) in the presence of 900 ppm NO, 100 ppm

-1 NO2, 1000 ppm NH3, 10 vol. % O2 with He balance under a GHSV of 50,000 h in the temperature range from 100 to 400 oC with 50 oC increments.

123

3.4. Conclusions

A series of titania nanotube confined metal oxide catalytic formulations M/TNT (M = Mn, Cu,

Ce, Fe, V, Cr, Co) were synthesized using incipient wetness technique and evaluated for the low temperature Selective Catalytic Reduction (SCR) of NOx with NH3 in presence of excess oxygen under a GHSV of 50,000 h-1. The different metal oxides were supported on novel titania nanotubes synthesized via the alkaline hydrothermal treatment using TiO2 UV-100 Hombikat which in the previous chapter were proven to be a superior support for catalytic formulations.

The synthesized support possesses a high surface area which helps the dispersion of active metal species and results to catalyst with high surface area as well. XRD patterns reveal the nanotubular structure of the catalytic formulation as peaks at d = 2.37, 3.08 and 3.08 Å corresponding to H2Ti3O7 and at d = 2.32 and 2.61 Å ascribed to nanotubes are present, while no peaks corresponding to metal oxides are present indicating the fine dispersion of metal oxides.

The acidity of the catalytic formulations was studied through ammonia-temperature programmed desorption, which reveals a direct correlation of the low temperature activity of Cu/TNT,

Mn/TNT, V/TNT and Co/TNT to high amount to weak and medium acidic sites possibly ascribed to Lewis acidity. XPS and H2-TPR results indicate that manganese is present as MnO2 and vanadia exhibits monolayer isolated species where V2O5 is dominant; which can be correlated with the remarkable activity of the catalytic formulations. Ce3+ oxidation state of cerium is promoted for the ceria confined metal oxide catalytic formulations thus improving the redox cycle of the oxide. The catalytic performance of the catalysts was initially evaluated keeping the weight percentage of metal supported on the titania nanotubes equal to 15, under a

GHSV of 50,000 h-1. Titania nanotubes confined manganese oxide, vanadia, chromia and copper oxide provide remarkable low temperature activity as they exhibit complete conversion of NOx

124

o to N2 at 150 C. Titania nanotubes confined vanadium oxide catalytic formulation exhibits a broad operation temperature window attributed to the dispersion of active species on the support.

Ceria based catalytic formulation exhibits high conversion at temperature beyond 250oC. After the evaluation of monometallic catalytic formulations bimetallic combinations of ceria and manganese oxide where synthesized with different Mn/Ce atomic ration (Mn-Ce(x)/TNT, x =

1.27, 2.55 and 5.10). The presence of ceria improves the activity of manganese oxide at high temperature revealing a synergistic effect for the catalytic formulations.

125

Chapter 4

Systematic Control of the Tubular Structure of Titania

Nanotubes and Superior Catalytic Performance for the Selective

Catalytic Reduction of NOx at Low Temperatures.

126

4.1. Introduction

The alkaline hydrothermal treatment for the synthesis of titania nanotubes was introduced by Kasuga et.al. [4-1]. The straightforward and cost effective synthesis method has triggered research on the nanotubes synthesis parameters, characteristics and applications [4-2 – 4-3]. Titania nanotubes have been proposed as supports for catalysts utilized in numerous heterogeneous catalysis applications [4-3 – 4-7]. Metal oxides supported on titania nanotubes have been reported through the literature to exhibit high

DeNOx potential, attributed to the high surface area of the support, the promotion of certain active chemical states of the metal oxides as well as their high dispersion on the nanotube surface [4-2, 4-8 – 4-14]. In the previous chapters the effects of the morphology of titania nanotube confined manganese oxide catalytic formulations and the activity of different metal oxides supported on titania nanotubes for the low temperature SCR of NOx by NH3 were studied. The findings suggest that titania nanotubes provide a superior support for catalytic formulations. Except from the catalytic performance, the stability at high temperature is another aspect of the catalytic formulations. The high temperatures of the stack gasses during the operation of the SCR units can affect the catalytic performance and lead to the deactivation of the catalyst.

High temperature leads to the dehydration of the titania nanotubes following sequence of events: H2Ti3O7∙xH2O → H2Ti3O7 → H2Ti6O13 → TiO2(B) → TiO2(anatase) [4-15].

The thermal treatment of proton rich titania nanotubes results to TiO2 (B) and then to rutile phase titania [4-16]. Generally the nanotubes are unstable at high temperatures

[4-17] and is reported that after treatment at 550 oC they can convert to nanowires thus

127 losing their hollow nanotubular structure [4-18]. The titania nanotubes except their unique tubular structure and high specific surface area, possess a high ion exchange potential which can lead to improvement of certain properties and characteristics [4-19 – 4-22].

Metal exchanged of titania nanotubes such as Co [4-19 – 4-20] Cu [4-19], Ni [4-19], Ag

[4-19], Fe [4-21], Bi [4-22] and Sb [4-22] have been reported in the literature, the resulting materials are complex metal oxides with enhanced properties and great potential for catalytic and adsorption applications [4-19]. The introduction of cobalt enhances the specific surface area of the nanotubes and reduces the interlayer spacing [4-19 – 4-20]; however the synthesized material is unstable at temperatures above 400 oC [4-20].

Exchanging iron on the titania nanotubes does not affect their morphology, although it enhances the material’s optical properties and the interlayer spacing; iron is present in Fe3+ oxidation state and the material is thermally stable up to 400 oC [4-21]. Rónavári et.al.

[4-22] studied the effects of calcination temperature on ion exchanged titania nanotubes, their studies conclude that the presence of antimony in the structure can help preserve the tubular structure of the material after the calcination at 600 oC for 1 hr. inhibiting also the phase transformation from trititanate to rutile [4-22].

In this chapter novel pristine and ion exchanged titania nanotube were synthesized and their thermal stability at different temperatures was evaluated. Titania nanotubes were synthesized using TiO2 UV-100 Hombikat as the precursor via alkaline hydrothermal treatment. Different metals (Zr, Ce, Sb, La and Y) have been exchanged on the titania nanotubes via ion exchange. The specific surface area and morphology of the synthesized materials was studied through BET, X-Ray Diffraction (XRD) and Transmission Electron

Microscopy (TEM). As the results indicate the ion exchange improves the resistance of the

128 structure at high temperatures especially for yttrium exchanged titania nanotubes where the tubular structure is preserved after aging at 650 oC for 12 hrs.. The pristine and ion exchanged titania nanotubes were then used as support for manganese oxide based catalytic formulations. The ammonia adsorption properties and the catalytic activity for the low temperature SCR of NOx by NH3 with excess oxygen were evaluated. Manganese oxide supported on yttrium and lanthanum exchanged titania nanotube exhibit remarkable

o deNOx potential despite the thermal aging of the catalysts at 650 C for 12hrs.

129

4.2. Experimental

4.2.1. Materials Synthesis

4.2.1.1. Titania Nanotubes Synthesis

The preparation of the titania nanotubes was achieved by adopting the alkaline hydrothermal treatment using anatase TiO2 UV-100 Hombikat from Sachtleben, as the starting material. TiO2 (2 g) was dissolved in 70 mL of 10 M NaOH aqueous solution, the resulting material was hydrothermally treated at 130 oC for 24 hrs. in a Teflon-lined stainless steel autoclave. After 24 hrs. the content of the autoclave was filtered and washed initially with deionized water until the PH became equal to 7, then the material was treated with 2 L of 0.2 M HCl aqueous solution until the PH reached approximately 1-2 and finally washed with deionized water until the PH was resorted to neutral. The resulting nanotubes were dried at 80 oC for 18 hrs in oven. Finally the material was grinded in a mortar and sieved in a mesh with 300 μm openings. The resulting titania nanotubes are denoted as

TNT.

4.2.1.2. Ion Exchanged Titania Nanotubes (X-TNT) synthesis

The metal exchanged titania nanotubes (X-TNT where X = Y, Sb, Ce, La and Zr) were prepared by adopting the ion exchange synthesis method. The as prepared titania nanotubes from TiO2 UV-100 Hombikat (TNT) were dispersed in 700 mL deionized water along with the required amount of the corresponding precursors, yttrium(III) chloride hexahydrate (99.99% YCl3·6H2O from Sigma Aldrich), antimony(III) acetate ((CH3CO2)3Sb,

130

99.99% from Sigma Aldrich), cerium(III) acetate hydrate (C6H9CeO6∙xH2O 99.9% from

Sigma Aldrich) lanthanum(III) chloride hydrate (99.9% LaCl3·xH2O from Sigma Aldrich) and zirconium chloride (99.99% ZrCl4 from Sigma Aldrich), in order to achieve a nominal composition of 10 wt.% of metal on the titania nanotube. As the nanotubes and the metal precursors along with 700 mL deionized water were added in a 1000 mL beaker stirring was applied. Diluted ammonia was added dropwise until the PH reached 8 and the resulting solution was stirred for 24 hrs. Then the material was filtered and washed with deionized water in order to remove the remaining acetates or chlorides. Finally the precipitant was oven dried at 80 oC for 18 hrs, ground in mortar and sieved in a mesh with

300 μm opening.

4.2.1.3. Synthesis of Mn/X-TNT Catalytic Formulations.

The synthesis of the Mn/X-TNT series of catalysts (where X = Y, Sb, Ce, La and Zr) was achieved by adopting the deposition precipitation method. The as synthesized metal exchanged nanotubes (X-TNT) and pristine titania nanotubes (TNT) were dispersed 700 mL of deionized water along with the required amount of manganese(II) nitrate hydrate

(MnN2O6∙xH2O 99.99% from Sigma-Aldrich) which was used as the manganese precursor in order to achieve the required nominal composition of 5 wt.% of manganese. Stirring was applied to the resulting solution followed by the addition of diluted ammonia dropwise until the PH become 8. The solution was kept stirring for 4 hrs. and then was aged for another 18 hrs.. After the aging step the material was filtered and washed with deionized

131 water to remove nitrates. Finally the precipitate was oven dried at 80 oC for 18 hrs. ground in mortar and sieved in a mesh with 300 μm openings.

4.2.1.4. Thermal Treatment of the Materials

The synthesized families of materials (X-TNT, Mn/X-TNT) prior to their characterization and catalytic activity evaluation were calcined at 400 oC for 2 hrs. with

5 oC per minute temperature increments in a conventional tubular oven in the presence of air. The calcined materials are denoted as X-TNT-F and Mn/X-TNT-F accordingly, which indicates that the materials are fresh and not thermally aged. In order to study the effects of thermal aging on the structure, physicochemical properties and the catalytic activity of the materials, they were thermally aged at 550 oC and 650 oC accordingly for 12 hrs. with

5 oC per minute temperature increments in a conventional tubular oven in the presence of air. The thermally aged materials are denoted according to the thermal aging temperature,

X-TNT-550 and Mn/X-TNT-550 for thermally aged materials at 550 oC and X-TNT-650 and

Mn/X-TNT-650 for the materials aged at 650 oC

4.2.2. Materials Characterization

The specific surface area (m2/g), pore volume (cm3/g), average pore diameter (nm), and the pore size distribution of the fresh and aged pristine and ion exchanged titania nanotubes were obtained from N2 adsorption isotherms at liquid nitrogen temperature (77

K) using a surface area and porosimetry analyzer (Micromeritics Tristar 3000) analyzer.

132

The diffraction patterns of the fresh and aged materials were obtained using a

Phillips Xpert diffractometer occupied with a nickelfiltered CuK radiation source. The intensity data were collected over a 2 range of 10° 80° with a step size of 0.025° and a step time of 0.25 seconds. The crystalline phases of the materials were determined by comparing the XRD patterns to the reference data from International Center for Diffraction

Data (ICDD) files.

Ammonia – Temperature Programmed Desorption (NH3 – TPD) patterns were collected using an automated catalyst characterization system (Micromeritics Autochem II

Transmission Electron Microscopy (TEM) images were collected for the fresh and thermally aged pristine and ion exchanged titania nanotubes using FEI CM-20 instrument operated on 200 kV. The materials were dispersed in isopropyl alcohol and then place on lacei carbon supports for the analysis.

4.2.3. Catalytic Activity Evaluation Experiments

The catalytic activity of the Mn/X-TNT series of catalysts for the low temperature

SCR of NOx by NH3 with excess oxygen was evaluated in fixed bed continuous flow quartz reactor under atmospheric pressure. The GHSV was kept constant at 50,000 h-1 calculated according to equation 4.1.

133

( ) ( )

The catalyst powder was placed in the reactor between two quartz wool plugs. The gas flows were measured and calibrated using a digital flow meter (Humonics Hewlett

Packard Optiflow 520). The reaction gas mixture consisted of 900 ppm NO, 100 ppm NO2,

1000 ppm NH3, 10 vol. % O2, and helium in balance. The NO and NO2 concentrations were continually monitored using a chemiluminescence NO/NOx detector (Eco Physics CLD 70S).

An ammonia trap was placed prior to the inlet of the detector to avoid errors caused by ammonia oxidation. The reactor temperature was controlled using a tubular furnace regulated by a temperature controller (Omega CN 2041). The catalytic activity of the samples was evaluated starting at 100 to 350 °C with 50 °C increments. The reactants and products concentrations were analyzed using the chemiluminescence detector (Eco Physics

CLD 70S) and on-line Quadrapole mass spectrometer (MKS PPT-RGA). The measurements were recorded only after steady state was achieved at each temperature step. The NOx conversion and N2 selectivity were calculated according to the following equations.

( ) [ ] ( )

( ) [ ] ( ) ( )

134

4.3. Results and Discussion

4.3.1. Specific Surface Area and Pore Size Distribution

The specific surface area (m2/g), pore volume (cm3/g), the pore diameter (nm) and pore volume distribution of pristine and ion exchanged titania nanotubes calcined at 400 oC for 2 hrs. (X-TNT-F) and the thermally aged materials at 550 oC for 12 hrs. (X-TNT-550) obtained by N2 physisorption at liquid nitrogen temperature are summarized in Table 4.1 and Figures 4.1, 4.2. The synthesized titania nanotubes after calcination (TNT-F) possess a specific surface are of 223 m2/g. The titania nanotube exchanged with zirconium (Zr-TNT-

F) and cerium (Ce-TNT-F) samples exhibit a specific surface area of 202 m2/g and 235 m2/g respectively close to the one of the pristine sample. These samples also exhibit a similar pore volume distribution to the pristine titania nanotubes as illustrated in Figure 4.1, having pores in the mesoporous region (2 – 20 nm). The thermal aging of the samples at

550 oC for 12 hrs. sharply decreases the specific surface area and shifts the pore volume distribution to higher pore diameters. The exchange of lanthanum on the titania nanotubes appears to protect the high surface area of the sample as after the thermal aging at 550 oC it exhibits SSA equal to 136 m2/g, increasing the aging temperature further decreases the surface area as illustrated in Table 4.2. The thermal aging of the samples results in a sharp decrease in the surface area as indicated by the results which can be attributed to the collapse of the nanotubes and the formation of nanorods and aggregates as illustrated in

TEM images presented in later section. The antimony exchanged titania nanotube exhibits a high specific surface area after calcination equal to 356 m2/g which decreases after the thermal aging but remaining higher than the rest of the samples. The Sb-TNT-650 sample

135 exhibits the wider pore distribution among the samples aged at 650 oC as illustrated in

Figure 4.3. The titania nanotubes exchanged with yttrium and thermal aged at 550 oC and

650 oC exhibit a high specific surface area of 217 and 80 m2/g respectively. The Y-TNT-550 sample exhibits the highest surface area among the catalysts thermally aged at 550 oC, as illustrated in Table 4.1, indicating that the introduction of yttrium preserves the high surface area of the titania nanotubes. The latter sample exhibits a pore volume distribution with the majority of pores having diameters from 5 - 6 nm as illustrated in Figure 4.2.

Generally the exchange of different metals on the pristine titania nanotubes enhances the specific surface area as has been reported in earlier studies [4-21]. The presence of yttrium and antimony in the titania nanotubes appears to preserve the high specific area of the nanotubes, especially the yttrium exchanged sample prevents the conversion of the nanotubes to nanorods and helps protect the structure from sintering.

136

SSA Pore Volume Pore Diameter Material (m2/g) (cm3/g) (nm)

TNT-F 223 0.64 11.5

TNT-550 44 0.35 32.1

Zr-TNT-F 202 0.55 10.9

Zr-TNT-550 89 0.43 19.2

Ce-TNT-F 235 0.69 11.3

Ce-TNT-550 91 0.57 25.0

Sb-TNT-F 356 0.65 7.3

Sb-TNT-550 149 0.52 14.0

La-TNT-F 308 0.69 8.9

La-TNT-550 136 0.62 17.9

Y-TNT-F 307 0.67 8.7

Y-TNT-550 217 0.63 11.6

137

)

-1 0.006 TNT-F Å

-1 Zr-TNT-F g 3 0.005 Ce-TNT-F

Sb-TNT-F cm ( La-TNT-F 0.004 Y-TNT-F

0.003

0.002

0.001

Pore Volume,dV/dD 0.000 1 10 Pore Diameter (nm)

Figure 4.1.: Pore volume distribution curves (BJH Adsorption) of pristine and ion exchanged titania nanotubes calcined at 400 oC for 2 hrs. (X-TNT-F).

138

) 0.007

-1 TNT-550 Å

-1 0.006 Zr-TNT-550 g 3 Ce-TNT-550 Sb-TNT-550 cm 0.005 ( La-TNT-550 0.004 Y-TNT-550

0.003

0.002

0.001

Pore Volume,dV/dD 0.000 1 10 100 Pore Diameter (nm)

Figure 4.2.: Pore volume distribution curves (BJH Adsorption) of pristine and ion exchanged titania nanotubes thermal aged at 550 oC for 12 hrs. (X-TNT-550).

139

SSA Pore Diameter Material Pore Volume (cm3/g) (m2/g) (nm)

Y-TNT-F 307 0.67 8.7

Y-TNT-650 88 0.51 20.7

Sb-TNT-F 356 0.65 7.3

Sb-TNT-650 100 0.47 16.0

La-TNT-F 308 0.69 8.9

La-TNT-650 80 0.50 25

140

0.007

)

-1 Å

-1 0.006 Sb-TNT-550 g 3 La-TNT-550

0.005 Y-TNT-550

cm (

0.004

0.003

0.002

0.001

Pore Volume,dV/dD 0.000 1 10 100

Pore Diameter (nm)

Figure 4.3.: Pore volume distribution curves (BJH Adsorption) of pristine and ion exchanged titania nanotubes thermal aged at 650 oC for 12 hrs. (X-TNT-650).

141

4.3.2. X-ray Diffraction

The XRD patterns of pristine titania nanotubes and cerium exchanged titania nanotubes calcined and thermal aged at different temperatures are illustrated in Figure 4.4 below. As it can be observed from the spectra, the peaks exhibited by TNT-F and Ce-TNT-F at d = 3.65 and 1.88 Å are ascribed to H2Ti3O7 [4-23]. The cerium exchanged titania nanotubes (Ce-TNT-F) exhibit a characteristic peak at d = 3.15 Å (JCPDS 78-0694) which is

o o ascribed to CeO2. The thermal aging of the materials at 550 C and 650 C results to the transformation of the titanate structure to anatase as it can be observed by the XRD patterns as the peaks corresponding to H2Ti3O7 disappear and only peaks at d = 3.54, 1.90 and 2.40 Å (JPCD no. 21-1272) ascribed to the anatase phase are present. The thermal aging of pristine titania sample at 650 oC for 12 hrs. (TNT-650) results to the transformation of anatase to rutile, as a weak peak ascribed to rutile (110) appears at d = 3.24 Å (JPCD 21-1276). The transformation of anatase phase to rutile has been reported to take place at temperatures higher than 700 oC [4-24]. Generally the thermal aging of the sample increases the crystallinity as illustrated by the diffraction peaks especially for the pristine titania nanotubes. The disappearance of the titania nanotube ascribed peaks can be correlated with the TEM images which present a clear collapse of the tubular structure and the formation of nanorods and aggregates. The peaks corresponding to anatase become narrower as the temperature increases suggesting the growth of anatase crystallites [4-24].

142

A (101) A 7

2 A (105) A

(204) A

R (110) R

A (200) A

A (004) A (211) A

2 CeO (f)

(e)

(d)

(a)

10 20 30 40 50 60 70 80

()

143

In Figure 4.5 the XRD patterns of the titania nanotubes exchanged with antimony, lanthanum and yttrium are presented. As illustrated in the figure the fresh samples after calcination exhibit peaks at d = 3.65 and 2.37 Å ascribed to H2Ti3O7 [4-23] same as the previously described samples. The peaks ascribed to the H2Ti3O7 for the Sb-TNT and La-

TNT samples disappear after the thermal aging indicating the transformation to anatase phase. The intensity of the peaks corresponding to anatase phase increases as the samples are treated at higher temperature indicating the increase of their crystallinity especially for the Sb-TNT sample. Yttrium exchanged titania nanotubes present the wider peak

o corresponding to H2Ti3O7, despite the calcination of the material at 400 C for 2 hrs., compared to the rest of the pristine and ion exchanged titania nanotubes at d = 3.65 and

1.88 Å. The clear peaks ascribe to H2Ti3O7 of the Y-TNT-F sample are correlated with the

TEM images presented in Figure 4.10, where long, uniform and well defined nanotubes are illustrated. After the thermal aging of the yttrium exchanged sample at 550 oC the peak at d = 1.88 Å shifts to lower 2θ (o) towards the anatase (200) peak. Comparing the Full Width at Half Maximum of the samples it can be concluded that the thermal aging results to bigger crystallites according to Scherrer’s equation. The ion exchanged titania nanotubes with yttrium and lanthanum despite the thermal aging exhibit the formation of smaller crystallites compared to the rest of the pristine and ion exchanged titania nanotubes indicating that the introduction of these metals protects the samples from sintering towards to the formation of large aggregates.

144

7 7

Ti (101) A

A (004) A

A (024) A H

(211) A A (200) A (i)

(h) (g)

(f)

(e) (d)

(b) (a)

10 20 30 40 50 60 70 80

()

145

4.3.3. Transmission Electron Microscopy

Transmission Electron Microscopy (TEM) images of the synthesized pristine and ion exchanged titania nanotubes calcined 400 oC for 2 hrs. (X-TNT-F) and the corresponding thermally aged materials at 550 oC and 650 oC for 12 hrs. were collected and are presented in the figures below. The TEM images of the fresh and thermally aged pristine titania nanotubes are presented in Figure 4.6, as it can be observed from the figure the titania a b c nanotubes with length from 60 to 140 nm after the thermal aging at 550 oC for 12 hrs. convert to rod like structures with varying length from 60 to 160 nm. Harsher aging conditions at 650 oC decreases the length of the rods and aggregates with 25 nm to 40 nm diameters are formed. As discussed earlier the thermal aging of the pristine titania nanotubes reduces the specific surface area which is a result of the collapse of the tubular structure and the sintering of the material. The ion exchange of cerium on the titania nanotubes does not significantly affect the tubular structure of the fresh material, after thermal aging at 550 oC for 12 hrs. a few nanotubes are present but most of the material converts to rods and aggregates which are more clear after increasing the thermal aging of the material at 650 oC. It has been reported in earlier studies that the ion exchange of titania nanotubes with antimony can help preserve the nanotubular structure when the material was calcined at 600 oC for 1 hr. [4-22] as exhibited in Figure 4.8. the calcination of the Sb-TNT at 400 oC for 2 hrs. does not impact the nanotubular structure of the material.

The thermal aging of Sb-TNT-F at 550 oC results to fractured nanotubes with length around

40 nm and nanorods of almost equal length. The results of thermal aging are more evident when the material is treated at 650 oC for 12 hrs. small nanorods and agglomerates with diameter around 17 nm are present. Clear nanotube structure is present for the lanthanum

146 exchanged titania nanotubes calcined at 400 oC for 2 hrs. (La-TNT-F) the thermal aging results into the sintering of the material as can be seen in Figure 4.9 especially for the La-

TNT-650 were clear rodlike structures with length around 50 nm and aggregates are present. Finally in Figure 4.10 the TEM images of fresh and aged yttrium exchanged titania nanotubes are present. The calcined sample Y-TNT-F exhibits clear nanotubular structure; the structure of the nanotubes appears to be more uniform compared to the rest of the sample with lengths varying from 110 nm to 160 nm which also can be observed by the

XRD results were distinct peaks corresponding to H2Ti3O7 are clearly present. The introduction of yttrium to the titania nanotubes as exhibited by the TEM images seems to protect the nanotubular structure, Y-TNT-550 exhibits mainly nanotube structure along with some small aggregates of the material. Thermal aging of the material at 650 oC for

12 hrs. converts some of the nanotubes to nanords and agglomerates but the majority of the material preserves its structure (Figure 4.10.3.). Yttrium exchanged titania nanotubes with diameters from 60 – 120nm with some metal agglomerates on their surface are easily distinguishable. In order to investigate the response to the material at harsher thermal aging we heated the sample at 700, 750 and 800 oC for 12 hrs., as the TEM images presented indicate exposing the sample at 700 oC results to further collapsing of the nanotubular structure. Thermal aging at 800 oC for 12 hrs. results to collapse of the nanotubes converting to bulky particles.

147

a b

20nm 20 nm

Figure 4.6.: TEM images of pristine c titanina nanotubes a) TNT-F (Calcined at 400 oC for 2 hrs.) b) TNT-550

(Thermally Aged at 550 oC for 12 hrs.)

o and c) TNT-650 (Thermally Aged 650 C for 12 hrs.).

20nm

a b

20nm 20nm rs.). c Figure 4.7.: TEM images of ceria exchanged titanina nanotubes (Ce-TNT) a) Ce-TNT-F alcined at 400 oC for 2 hrs.)

b) Ce-TNT-550 (Thermally Aged at 550 oC for 12 hrs.) and c) Ce-TNT-650 (Thermally Aged 650 oC for 12 hrs.).

20nm

148

a b

20nm 20nm

c Figure 4.8.: TEM images of antimony exchanged titanina nanotubes (Sb-TNT) a) Sb-TNT-F (Calcined at 400 oC for 2 hrs.) b) Sb-TNT-550 (Thermally Aged at 550 oC for 12 hrs.) and c) Sb-TNT-650 (Thermally Aged 650 oC for 12 hrs.).

20nm

a b

20nm 20nm

c Figure 4.9.: TEM images of lanthanum exchanged titanina nanotubes (La-TNT) a) La-TNT-F (Calcined at 400 oC for 2 hrs.) b) La-TNT-550 (Thermally Aged at 550 oC for 12 hrs.) and c) La-TNT-650 (Thermally Aged 650 oC for 12 hrs.).

20nm

149

a b

20nm 20nm c d

20nm 20nm

e f

20nm 20nm

150

4.3.4. Ammonia – Temperature Programmed Desorption (NH3-TPD)

The presence of acidic sites and their distribution for the synthesized families of materials were investigated through Ammonia – Temperature Programmed Desorption

NH3-TPD. The ammonia desorption patters for the pristine and ion exchanged titania nanotubes are presented in Figure 4.11 below. As illustrated in the results pristine titania nanotubes exhibit a broad peak in the temperature range from 250 to 500 oC, which can be ascribed to the desorption of ammonia from weak Brónsted acidic sites [4-25]. The ion exchange of cerium on the titania nanotube (Ce-TNT) does not significantly alter the desorption pattern of the material with both materials not exhibiting complete ammonia desorption up to 700 oC, a possible indication the presence of ammonia bound on strong acid sites. The introduction of antimony in the titania nanotube structure increases the acidity of the catalysts as two sharp peaks are emerging at 487 and 600 oC indicating the presence of a large amount of strong acid sites. The acidity of the titania nanotubes increases as well when lanthanum (La-TNT-F) and yttrium (Y-TNT-F) are exchanged, as sharp and strong peak is emerging at 465 oC for the La-TNT-F sample. The Y-TNT-F sample exhibits a broad distribution of acidic sites, the desorption of ammonia starts at 349 oC but does not fully desorbs in the whole temperature range as indicated by the desorption patterns.

The ammonia desorption patterns of the pristine and ion exchanged nanotubes loaded with manganese oxide are presented in Figure 4.12. From the results it can be observed that the loading of manganese oxide increases the weakly and medium acid sites of the catalytic formulations as the samples exhibit a desorption peak located in the temperature range from 250 oC to 400 oC possibly ascribe to Lewis acid sites. The Lewis

151 acid sites are correlated with the low temperature activity for the SCR of NOx by NH3

[4-26 – 4-29] especially the Mn/Y-TNT-F sample exhibits a strong desorption peak centered 323 oC, lower than the other samples. After the deposition of manganese oxide on the Sb-TNT-F sample the peak ascribed to the desorption from medium acid sites is shifted to lower temperature and increases in intensity compared to the antimony exchanged titania nanotubes.

Y-TNT-F

La-TNT-F

Sb-TNT-F

Ce-TNT-F

TCD Signal (a.u.) Signal TCD

TNT-F

100 200 300 400 500 600 700

Temperature (oC)

igure 4.11.: Ammonia – Temperature Programmed Desorption (NH3-TPD) patterns of pristine and ion exchanged titania nanotubes X-TNT-F where X = Ce, Sb, Sb, La and Y.

152

Mn/Y-TNT-F

Mn/La-TNT-F

Mn/Sb-TNT-F

TCD Signal (a.u.) Signal TCD Mn/Ce-TNT-F

Mn/TNT-F

100 200 300 400 500 600 700 o Temperature ( C)

Figure 4.12.: Ammonia – Temperature Programmed Desorption (NH3-TPD) patterns of manganese oxide supported on pristine and ion exchanged titania nanotubes Mn/X-TNT-F where X = Ce, Sb, Sb, La and Y.

153

4.3.5. Catalytic activity evaluation

The synthesized catalytic formulations were evaluated regarding their catalytic performance for the low temperature Selective Catalytic Reduction (SCR) of NOx by NH3. In

Figure 4.13 below the performance of pristine titania nanotube loaded with manganese oxide fresh and aged catalytic formulations is presented. The fresh sample Mn/TNT-F exhibits superior catalytic performance in the whole temperature region from 100 – 350 oC.

Thermal aging of the sample seems to have tremendous impact on its catalytic performance. The sample aged at 550 oC for 12 hrs. exhibits poor activity at 100 oC and it

o reaches almost 100 % NOx conversion at 200 C. As the temperature further increases the

o catalytic activity declines and the catalyst exhibits NOx conversion of 33.1 % at 350 C. The sample thermal aged at 650 oC for 12 hrs. exhibits complete deactivation as the NOx conversion is zero in the whole temperature range. The deactivation of the catalyst is attributed to the decrease of the specific surface area of titania nanotube support through thermal aging and the complete destruction of the nanotubular structure as can be observed in the Figure 4.15.

In order to prevent the deactivation of the catalytic formulations through thermal aging a series of ion exchanged titania nanotubes where used as the support, as discussed earlier ion exchange of different metals affects the behavior of the titania nanotubes at high temperatures. In Figure 4.13 the catalytic activity of the fresh catalytic formulations is illustrated. As it can be observed Mn/TNT-F, Mn/Sb-TNT and Mn/Y-TNT samples exhibit conversion over 90 % at low temperatures in contrast to Mn/La-TNT-F and Mn/Ce-TNT-F sample which exhibit low activity at 100 oC. In the temperature range from 150 to 300 oC

154 all the catalysts exhibit NOx conversions over 90 %. All the fresh catalytic formulations exhibit remarkable DeNOx potential which is attributed to the manganese oxide active species, the high specific surface area and the nanotubular structure of the support.

In order to study the effects of temperature on the catalytic formulations the samples were thermally aged at 550 oC for 12 hrs. and were evaluated regarding to the

DeNOx potential which is illustrate in Figure 4.14. As it can be observed in the figure most of the samples exhibit poor NOx conversion at 100 oC with the exception of the manganese supported on yttrium exchange titania nanotubes (Mn/Y-TNT-550). As the temperature increases the catalytic activity of the samples increases as well Mn/TNT exhibits the

o highest DeNOx potential at 250 C although it fails to convert NOx to N2 at lower and higher temperatures. The manganese oxide supported on Sb-TNT sample exhibitits the poorest conversion in the whole temperature region. Noteworthy is the catalytic activity of the

Mn/La-TNT-550 catalysts which a exhibits a promosing conversion at temperature above

150 oC. The thermal aging seems to have less effect on the Mn/Y-TNT-550 sample which

o exhibits superior NOx conversion in the range of 90 – 95 % at temperatures up to 300 C.

Figure 4.15 is a comparison of the catalytic activity of the samples thermally aged at

650 oC for 12 hrs. in the temperature range from 100 – 350 oC. The manganese supported on pristine titania nanotubes aged at 650 oC (Mn/TNT-650) exhibit complete deactivation as it shows zero NOx conversion at the whole temperature range as a result of the thermal aging. The thermal aging seems to have great effect on the Mn/Sb-TNT sample as it reaches maximum NOx conversion of 70 % at 200 oC. Samples Mn/Ce-TNT and Mn/La-TNT exhibit very poor low temperature activity which increases with the temperature. The later

155

o o exhibits high deNOx potential at temperatures above 150 C which is stable up to 300 C.

The Mn/Y-TNT sample exhibits remarkable low temperature activity compared to the rest of the samples as it shows 90 % NOx conversion at 100 oC, which is stable up to 250 oC. The remarkable catalytic activity of the samples is attributed to the fact that despite the harsh thermal aging conditions it preserves is nanotubular structure making the active sites accessible to the reactants.

100

(%)

NOx 40 X Mn/TNT-F Mn/Y-TNT-F 20 Mn/Ce-TNT-F Mn/Sb-TNT-F Mn/La-TNT-F 0 100 150 200 250 300 350 Temperature (oC)

156

100

(%)

NOx

X 40 Mn/TNT-550 Mn/Y-TNT-550 20 Mn/Ce-TNT-550 Mn/Sb-TNT-550 Mn/La-TNT-550 0 100 150 200 250 300 350 Temperature (oC)

-1 NO2, 1000 ppm NH3, 10 vol. % and He balance under GHSV of 50,000 h in the temperature range 100 – 350 oC.

157

100

(%)

NOx

X 40 Mn/TNT-650 Mn/Y-TNT-650 20 Mn/Ce-TNT-650 Mn/Sb-TNT-650 Mn/La-TNT-650 0 100 150 200 250 300 350 Temperature (oC)

-1 NO2, 1000 ppm NH3, 10 vol. % and He balance under GHSV of 50,000 h in the temperature range 100 – 350 oC.

158

4.4. Conclusions

Different metals such as Ce, Zr, Sb, La and Y were exchanged in pristine titania nanotubes by adopting the ion exchange method. The resulting materials were studied regarding their properties before and after the thermal aging at 550 oC and 650 oC for 12 hrs. Specific surface area measurements indicate that yttrium and antimony exchanged titania nanotube exhibit high surface area despite the treatment at high temperatures; especially antimony exchanged titania nanotubes aged at 650 oC for 12 hrs. exhibit a specific surface area of 100 m2/g. The X-Ray Diffraction (XRD) patterns illustrate that yttrium exchanged titania nanotubes exhibits clear peaks corresponding to H2Ti3O7 indicating the presence of well-defined titania nanotubes which can be correlated with the

Transmission Electron Microscopy (TEM) images presented. The thermal aging of the pristine titanium oxide nanotubes results to the collapse of the nanotubular structure and the formation of rods and aggregates; the introduction of yttrium in the nanotubes preserves their tubular structure even after the thermal aging at 650 oC which can be observed in the TEM images. The different ion exchanged titania nanotubes were used as supports for manganese based catalytic formulations. Through ammonia – temperature programmed desorption (NH3-TPD) of the pristine and ion exchanged titania nanotubes it is indicated that the exchange of metals on the titania nanotubes enhances the acidity of the pristine material, as the amount of ammonia adsorbed on the sample increases. The deposition of manganese oxide on the supports increases the ammonia bound on weak and medium acidic site, ascribed to increased Lewis acidity. The resulting catalytic formulations were evaluated on their performance for the low temperature SCR of NOx by NH3 with

-1 excess oxygen under a GHSV of 50,000 h . The fresh catalysts exhibit remarkable NOx

159 conversion with the Mn/TNT-F sample exhibiting over 90 % NOx conversion in the whole temperature range from 100 – 350 oC. Thermal aging impacts the performance of the catalytic formulations, as it results to the collapse on nanotubular structure and sintering which reduce the accessibility of reactant to active manganese oxide sites. Manganese oxide supported on lanthanum and yttrium exchange exhibit promising deNOx potential despite the thermal aging. The remarkable performance of the thermally aged manganese oxide deposited on yttrium exchanged titania nanotubes is attribute to the preservation of the nanotubular structure of the material and the high surface area of the support after thermal aging.

160

Summary and Future Work

The current work focuses on the synthesis and characterization of novel titanium oxide nanotubes and their utilization as catalytic support for the selective catalytic reduction of nitrogen oxide by ammonia. Titania nanotubes were prepared using TiO2 with different crystallographic phases, surface area and particle size. Manganese oxide confined on titania nanotubes prepared from TiO2 anatase UV-100 Hombikat exhibit remarkable catalytic activity for the low temperature SCR attributed to the high surface area, the promotion of certain manganese oxide species, the high dispersion of active species and the high amount of acidic sites.

The catalytic activity of different metal oxide supported on titania nanotubes was also investigated with various metal oxides such as copper oxide and vanadia exhibiting impressive deNOx potential and wide temperature window. Ceria confined on titania nanotubes showed good activity at high temperatures. Bimetallic combinations of ceria and manganese oxide supported on titania nanotube exhibit a synergistic effect on the activity.

Titania nanotubes provide a superior support for metal oxide based catalytic formulation as they enhance the dispersion of active species, promote certain oxidation states active for the reaction and exhibit special ammonia adsorption properties.

The stability of the titania nanotube structure at high temperatures has been studied in the literature and it is stated that they are unstable at temperature beyond

500 oC. In order to enhance the thermal stability of the titania nanotubes different metals were ion exchanged in the structure. Yttrium exchanged titania nanotubes exhibit thermal stability after the thermal aging of the material at 650 oC for 12 hrs. The ion exchanged

161 titania nanotubes were also used as support for manganese oxide based catalytic formulations exhibiting remarkable deNOx potential. Manganese oxide confined on yttrium exchanged titania nanotubes exhibits remarkable resistance to thermal aging as it exhibits impressive catalytic performance at low temperature despite the aging.

Reaction engineering experiments in order to find the intrinsic properties followed by mechanistic studies along with transient isotopic labeling studies on the reduction of

NOx over titania nanotube confined catalytic formulation will provide insights on the reaction pathways. In order to consider titania nanotube confined metal oxide catalytic formulation as potential commercial catalysts their deposition on monoliths and the bench testing of the catalyst under realistic conditions is required.

Advanced characterization techniques like Extended X-ray Absorption Fine

Structure (EXAFS) will provide information on why Mn4+ oxidation state is promoted for the manganese confined on titania nanotube catalytic formulations and will help investigate the role of yttrium exchange on the preservation of the nanotubular structure of the material after harsh thermal aging.

In the present work utilizing the high ion exchange capacity of titania nanotubes we successfully synthesized materials with enhanced properties. Especially yttrium exchange titania nanotubes exhibit remarkable thermal stability. The systematic study of different metals exchanged and bimetallic combinations of them on the titania nanotubes can provide materials with enhanced properties and high stability at high temperature which can be utilized in various catalytic and other applications.

162

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Appendices

177

Appendix 1

Figure A.1.: Schematic of the experimental setup for the catalytic activity evaluation of the synthesized catalytic formulations.

178

Appendix 2

The Nitric Oxide (NO) and Nitrogen Dioxide (NO2) emissions where measured using the the Eco Physics CLD 70S NO/NOx detector. The operation of the equipment is based on the chemiluminescence technique which according to the Environmental Protection

Agency (EPA Method 7E) is a reliable method for monitoring NO and NOx emissions.

The basic principle behind the operation of the instrument is that excited molecules when they revert back to ground state emit electromagnetic radiation which is detected photo-electrically by the device. The sample gas that enters the analyzer is initially mixed with Ozone (O3), which is produced on site. Ozone oxidizes nitric oxide to nitrogen dioxide

* and about 20% percent of the NO2 produced is found in excited state NO2 . The process can

’ be described by the following reactions A.1 and A.2. The excited NO2 goes back to the

* ground state while emitting electromagnetic radiation (Equation A.3). Excited NO2 can also convert to ground state due to collisions with other molecules (Equation A.4), in order to avoid that phenomenon the reactor chamber in the analyzer in kept in a reduced pressure to enhance the emmited light yield. The radiation emited has a wavelength from 600 –

3000 nm and with an intensity of 1200 nm. The presence of excess ozone makes the signal proportional to the concentration of NO in the analyzed sample.

The measurement of NO2 present in the sample gas is achieved by its reduction over

o a metal catalyst with high surface area in a heated at 650 C. Through this process NO2 is converted to NO and then measured as described above.

179

(A.1)

(A.2)

(A.3)

(A.4)

Figure A.2.: Picture of the Eco Physics CLD 70S NO/NOx Analyzer

180

Appendix 3

1000

800

600

400 Analyzer Singal (ppm) Singal Analyzer

x 200

0 NO/NO 0 2 4 6 8 10 12 14 16

Time (min)

Figure A.3.: NO/NOx chemiluminescence detector signal response (ppm) after the

o -1 introduction of 1000 ppm NOx at 250 C under a GHSV of 50,000 h with 100 mg TIO2 loaded in the reactor.

181

Appendix 4

800

700

600

500

400

Furnace Indication Furnace 300

y=0.9996x+32.588 200 200 300 400 500 600 700 800 Actual Temperature (oC)

Figure A.4.: Calibration of calcination furnace.

182

Appendix 5

1.00E-008 1000 ppm NO

8.00E-009

6.00E-009

4.00E-009

2.00E-009 Pressure(Torr)

0.00E+000 9 12 15 18 21 24 27 30 33 36 39 42 45 48

m/e

Figure A.5.: Sample histogram graph for 1000 ppm NO

183

2.00E-008 1000 ppm NH3

1.50E-008

1.00E-008

5.00E-009 Pressure(Torr)

0.00E+000 10 15 20 25 30 35 40 45 50

m/e

Figure A.6.: Sample histogram graph for 1000 ppm NH3.

184

3.00E-008 1000 ppm N 2 2.50E-008

2.00E-008

1.50E-008

1.00E-008

Pressure(Torr) 5.00E-009

0.00E+000 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50

m/e

Figure A.7.: Sample histogram graph for 1000 ppm N2.

185

5.00E-009

100 ppm N O 2

4.00E-009

3.00E-009

2.00E-009

1.00E-009 Pressure(Torr)

0.00E+000 9 12 15 18 21 24 27 30 33 36 39 42 45 48

m/e

Figure A.8.: Sample histogram graph for 1000 ppm N2O.

186

1000 y = 1.30E+11x + 82.616

900

800

700

600

NO (ppm) NO 500

400

300 0.00E+000 3.00E-009 6.00E-009 9.00E-009

m/e = 30 (Torr)

Figure A.9.: NO calibration for mass spectrometer.

187

1000 y = 9.688E+10x + 32.2

800

600

400 NO (ppm) NO

200

0 2.00E-009 4.00E-009 6.00E-009 8.00E-009 1.00E-008 m/e = 30 (Torr)

Figure A.10.: NO calibration for mass spectrometer in presence of 1100 ppm of NH3.

188

1000 y = 5.747E+10x + 92.19 900

800

700

600

(ppm)

3 500

NH 400

300

200 0.00E+000 5.00E-009 1.00E-008 1.50E-008

m/e = 17 (Torr)

Figure A.11.: NH3 calibration for mass spectrometer.

189

1000 y = 9.667E+10x + 1.67E+02

800

600

(ppm) 3 400

NH 200

0 0.00E+000 2.00E-009 4.00E-009 6.00E-009 8.00E-009 1.00E-008

m/e = 17 (Torr)

Figure A.12.: NH3 calibration for mass spectrometer in the presence of 900 ppm NO.

190

1200 y = 5.929E+10x-264

1000

800

600

(ppm)

2 400 N

200

5.00E-009 1.00E-008 1.50E-008 2.00E-008 2.50E-008

m/e = 28 (Torr)

Figure A.13.: N2 calibration for mass spectrometer.

191

500 y = 5.733E+10x-148.5

400

300

200

O (ppm) O

2 N 100

2.00E-009 4.00E-009 6.00E-009 8.00E-009 1.00E-008 1.20E-008

m/e = 44 (Torr)

Figure A.14.: N2O calibration for mass spectrometer.

192

120000

y = 7.492E10x - 7626 100000

80000

60000

(ppm)

2 O

40000

20000 0.0000000 0.0000005 0.0000010 0.0000015 0.0000020 m/e = 32 (Torr)

Figure A.15.: O2 calibration for mass spectrometer.

193

Appendix 6

100

(%)

2 90

S Mn(0.25)/TNT-H

Mn(0.25)/TNT-I Mn(0.25)/TNT-P25 85 Mn(0.25)/TNT-K Mn(0.25)/TNT-SA Mn(0.25)/TNT-TOS

80 100 150 200 250 300 o Temperature ( C)

Figure A.16.: N2 Selectivity (SN2 (%)) of manganese oxide confined on different titania nanotubes Mn(0.25)/TNT-X catalytic formulations in the presence of 900 ppm NO, 100

-1 ppm NOx, 1000 ppm NH3, 10 % O2 in He balance under a GHSV of 50,000 h in the

o temperature range from 100 to 300 C. (TOS = 1 hr.)

194

100

(%)

N S 100 (oC) 85 o 120 ( C) 140 (oC) o 160 ( C) 80 0.15 0.20 0.25 0.30 0.35

Temperature (oC)

-1 o balance under a GHSV of 50,000 h in the temperature range from 100 to 300 C. (TOS = 1 hr.)

195

100

(%) 2

90 N

Mn(0.25)/TNT-H Mn(0.25)/TiO 2 80 100 150 200 250 300

o Temperature ( C)

Figure A.18.: N2 Selectivity (SN2 (%)) of manganese oxide supported on titania nanotube prepared form TiO2 UV-100 Hombikat (Mn(0.25)/TNT-H) and on conventional TiO2

(Mn(0.25)/ TiO2) in the presence of 900 ppm NO, 100 ppm NOx, 1000 ppm NH3, 10 % O2 in

-1 o He balance under a GHSV of 50,000 h in the temperature range from 100 to 300 C. (TOS =

1 hr.)

196

100

(%) 2

90 N

Mn/TNT S Cu/TNT

Ce/TNT 85 Fe/TNT V/TNT Cr/TNT Co/TNT 80 100 150 200 250 300 350 400 Temperature (oC)

Figure A.19.: N2 Selectivity (SN2 (%)) of titania nanotube confined metal oxide catalytic formulation M/TNT where (M = Mn, Cu, Ce, Fe, V, Cr and Co) in the presence of 900 ppm

-1 NO, 100 ppm NOx, 1000 ppm NH3, 10 % O2 in He balance under a GHSV of 50,000 h in the

o temperature range from 100 to 300 C. (TOS = 1 hr.)

197

100

(%) 90

N S Mn/TNT Ce/TNT 85 Mn-Ce(1.27)/TNT Mn-Ce(2.55)/TNT

Mn-Ce(5.10)/TNT

80 100 150 200 250 300 350 400 o Temperature ( C)

-1 o balance under a GHSV of 50,000 h in the temperature range from 100 to 300 C. (TOS = 1 hr.)

198

100

(%)

N S Mn/TNT-F 85 Mn/Ce-TNT-F Mn/Sb-TNT-F Mn/La-TNT-F Mn/Y-TNT-F 80

100 150 200 250 300 350

o Temperature ( C)

-1 o GHSV of 50,000 h in the temperature range from 100 to 300 C. (TOS = 1 hr.)

199

100

(%)

N S Mn/TNT-550 85 Mn/Ce-TNT-550 Mn/Sb-TNT-550 Mn/La-TNT-550

Mn/Y-TNT-550 80 100 150 200 250 300 350

Temperature (oC)

Figure A.22.: N2 Selectivity (SN2 (%)) of pristine and ion exchange titania nanotubes confined manganese oxide catalytic formulations thermal aged at 550 oC for 12 hrs. Mn/X-

TNT-550 (where X = Ce, Sb, La and Y) in the presence of 900 ppm NO, 100 ppm NOx, 1000

-1 ppm NH3, 10 % O2 in He balance under a GHSV of 50,000 h in the temperature range from

o 100 to 300 C. (TOS = 1 hr.)

200

100

(%) 2

S Mn/TNT-650 Mn/Ce-TNT-650 85 Mn/Sb-TNT-650

Mn/La-TNT-650 Mn/Y-TNT-650

80 100 150 200 250 300 350

Temperature (oC)

Figure A.23.: N2 Selectivity (SN2 (%)) of pristine and ion exchange titania nanotubes confined manganese oxide catalytic formulations thermal aged at 650 oC for 12 hrs. Mn/X-

TNT-650 (where X = Ce, Sb, La and Y) in the presence of 900 ppm NO, 100 ppm NOx, 1000

-1 ppm NH3, 10 % O2 in He balance under a GHSV of 50,000 h in the temperature range from

o 100 to 300 C. (TOS = 1 hr.)

201

Appendix 6

Table A.1.: Literature Survey of X-Ray Diffraction for titania nanotubes.

Crystallographic 2θ Reference Phase Thorne, A.; Kruth, A.; Tunstall, D.; Irvine, J. T. S.; Zhou, W. J. Phys. Chem. B 2005, 109, 30.48 5439. Na2Ti3O7 40.22 55.29 Cao, X.; Xue, X.; Zhu, L.; Chen, P.; Song, Y.; Chen, M. J. Mater. Chem. 2010, 20, 2322.

Bavykin, D. V; Parmon, V. N.; Lapkin, A.; Walsh, F. C. J. Mater. Chem. 2004, 14, 3370. 24.37 H Ti O 28.97 2 3 7 Thorne, A.; Kruth, A.; Tunstall, D.; Irvine, J. 48.38 T. S.; Zhou, W. J. Phys. Chem. B 2005, 109, 5439.

38.78 Bavykin, D. V; Parmon, V. N.; Lapkin, A.; Titania Nanotubes 44.60 Walsh, F. C. J. Mater. Chem. 2004, 14, 3370

202

Table A.2.: Literature Survey of Raman spectroscopy for titania nanotubes.

Wavelength (cm-1) Reference

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Ti – O with layered 268 cm-1 Byeon, S.H.; Lee, S.O.; Kim, H.; J. Solid State structure Chem. 1997, 130, 110.

Miyaji, F.; Yoko, T.; Kozuka, H.; Sakka, S.; J. 121 cm-1 Na – O Mater. Sci. 1991, 26, 248.

203