Sep 16, 2005 - Zainab AL-Kassab, Dr. Abbas M. Bashi, Dr. Salah Mahdi, Dr. Jallal ... of TiO2 or metalized TiO2 varying from 50 to 300 mg. ..... 3-20. Rate of Formaldehyde Formation in Photocatalytic. Reaction with Different ...... Volume of aqueous solution. /mL. 1.000. 2.000. 0.500. 1.730. 1.000 ..... were added to 2.5 ml of.
Ministry of Higher Education and Scientific Research University of Babylon -College of Science Chemistry Department
Enhanced Photocatalytic Activity of Titanium Dioxide Nanoparticles by Metal Deposition A Thesis Submitted to the Council of the College of Science, University of Babylon as a Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Chemistry
By Luma Majeed Ahmed Al Rihaymee B. Sc. 1998- 1999
M. Sc 2002-2003
Supervisor Prof. Dr. Falah Hassan Hussein
2013 A
1434 H 1
2
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Declaration I, Luma Majeed Ahmed, the researcher, who conducted this study entitled" Enhanced Photocatalytic Activity of TiO2 Nanoparticles by Metal Deposition" and submitted it as a partial fulfillment of the requirements of philosophy of chemistry degree award for Ph.D. The research was carried out during the period 14-04-2010 to 13-07-2011 at the Department of Chemistry, College of Science, University of Babylon and from 19-07-2011 to 1-10-2011 at School of the BEST Institute, Built Environment, Liverpool John Moor University, England, then from 3-102011 to 23-12-2012 at the Department of Chemistry, College of Science, Babylon University, Iraq. The study was carried out under the supervision of Prof. Dr. Falah H. Hussein at the Department of Chemistry, College of Science, University of Babylon. I confirm that the work presented in this thesis has not been previously submitted for a degree or diploma at any higher educational institute. Also I declare that all the information in this document has been obtained and presented in accordance with academic rules and ethical conduct, I have fully cited and referenced all material and results that are not original to this work, or where information has been derived from other sources, I confirm that this has been indicated in this thesis.
Luma M. Ahmed
4
Acknowledgments First of all, I would like to express my great appreciation to the almighty Allah for his support enabling me to complete this work. Special thanks are due to Prof. Dr. Falah Hassan Hussein, my supervisor who greatly enriched my knowledge. Thanks are also due to the late Prof. Dr. Ali Abed Al Sahab Al-Fatlawy. Also, I would like to thank all people who introduced direct or indirect assistance for me in Leibniz University-Hannover-Germany, School of the BEST Institute- Built Environment,- Liverpool John Moor University-Liverpool-England and College of Education for Pure Science/ Ibn Al-Haitham– University of Baghdad -Baghdad. My gratitude is due to all faculty members of Department of Chemistry– College of Science in Babylon University and in Karbala University, for their constant support. I would like to express my deep and special grateful to friends who help and encourage me a lot during 4 years and 6 months ago in Babylon University, thank you for Miss Maryam, Miss Luma Al-Nakash, Dr. Ayad, Mr. Ahmed Fozy, Mr. Faiq, Mr. Majeed, Mr. Feras , Mr. Ahmed Moslum, Mrs. Asyel, Mrs. Fatma (Um Zaid), Dr. Dirgham Al-Khafaji, Dr. Sadeq, Dr. Sadoun, Dr. Abbas Al-Sharify, Dr. Abbas A. Drea, Mr. Adnan Al-Khafaji, Dr. Raheem, Mr. Ali (Abo Shahad) and Mr. Wessam. In Karbala Univesity, thank you for Dr. Jafer Hussein, Miss Zainab AL-Kassab, Dr. Abbas M. Bashi, Dr. Salah Mahdi, Dr. Jallal Jafer, Mrs. Samar and Mrs. Huda. In Techology University, thank you for Dr. Evan Tarek. In Liverpool John Moor University, thank you for Dr. Ahmed Al-Shamma'a, Dr. Rafid Al-khaddar, Dr. Hassan Al-Nageim, Dr. Shakir Al- Bustan and Dr. Abbas. In England thank you for my uncle's family, and my friends Mrs. Ragaa, Dr. Ahmed Salam and Mr. Adeeb. In Karkuk University, thank you for Dr. Omeed Al-Muhandis. In University of Baghdad, thank you for Dr. Issam Abd AlKareem, Mrs. Tagreed, Dr. Kalid Fahad, Dr. Tarek and Dr. Abed AlKareem. In Salah Al-Deen University, thank you for Dr. Sharwan and Mr. Karwain.
Luma Majeed Ahmed 2013 5
Dedication To my late father..... To my mother…... To my family who make my dreams possible...... To my classmates ...... To my friends...... who enhance me all time
_âÅtBECDF
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Abstract This work consists of five parts. The first part is concerned with the preparation of metalized TiO2 nanoparticales. Metalized TiO2 was prepared by photodeposition of different percentage of platinum (Pt) or gold (Au) on the surface of TiO2 Hombikat (UV 100) surface. The properties of prepared photocatalysts were investigated by Atomic absorption (A.A) analysis, Fourier transform infrared (FT-IR) analysis, X-ray diffraction (XRD) analysis and Atomic force microscope (AFM) analysis. Scherrer equation and modified Scherrer equation were used to calculate the mean crystallite sizes and crystallite sizes of bare and metalized TiO2 via XRD data. The calculated mean crystallite sizes and crystallite sizes of bare TiO2 decreased with the increasing of metal percentage. The AFM images indicated that the shape of bare and metalized TiO2 is spherical. The particle size was found ranged between 9 and 11 crystallite size. The band gap energy values for bare TiO2, Pt(0.5)/TiO2 and Au(0.5)/TiO2 were calculated after applying the Kubelka-Munk transformation. The results show shifting ultra violet absorption to visible light absorption (red shift) and band gap narrowing. The band gap of bare TiO2 was reduced from 3.289 eV to 3.263 eV for Pt(0.5)/TiO2 and to 3.246 eV for Au(0.5)/TiO2 too. The second part deals with the studying the effect of different parameters on photocatalytic oxidation of methanol by using bare and metal loaded on TiO2 surface. The parameters include: weight of catalyst, types of metal, percentage of loaded metal, methanol concentration, pH of solution and temperature. However the third part deals with the studying the effect of the same parameters in the second part on photocatalytic dehydrogenation of methanol by using bare and metal loaded on TiO2 surface. The photocatalytic activities of bare and metalized nanoparticles have been assessed by formaldehyde formation and hydrogen evolution from aqueous methanol solution. The effect of catalyst mass was studied by employing different masses of TiO2 or metalized TiO2 varying from 50 to 300 mg. The results 7
indicate the increasing of photocatalytic activity with increasing in catalysts mass and then it becomes constant at 175 mg/100ml from catalyst under purged Oxygen and nitrogen gases. It is clear from consideration of the catalyst concentrations at which the activity plateau was achieved that the mass affect does not depended upon the type of reaction, type of catalyst, temperature and source of irradiation. Moreover, plateau region was achieved and then the photocatalytic activity decreased with increasing catalyst concentration for all types of catalysts used in this work. The photocatalytic activity of oxidation of aqueous methanol solution of bare and metalized TiO2 was found attributed to the sequence Pt(0.5)/TiO2 > TiO2 > Au(0.5)/TiO2. However, the photocatalytic activity of dehydrogenation of aqueous methanol solution of bare and metalized TiO2 was found attributed to the sequence Pt(0.5)/TiO2 > Au(0.5)/TiO2 > TiO2. Highest photoactivity was observed for metalized TiO2 with platinum 0.5% and gold 2% under purged nitrogen gas. Pt/TiO2 was still conformed to be better photoreaction than Au/TiO2, in agreement with their function work values, where ФPt = 5.93 eV and ФAu = 5.31 eV. The rate of reaction increased with increasing the concentration of methanol, that will increase the formaldehyde formation with using bare TiO2 and Pt(0.5)/TiO2. Optimum pH values for bare TiO2 and Pt(0.5)/TiO2 in the existence of O2 showing maximum rate of reaction are 7.06 and 7.08 respectively. However, the optimum pH value for platinized titanium dioxide in the existence of nitrogen gas in maximum rate of reaction is 7.15. The rate of reaction for photocatalytic oxidation and photocatalytic dehydrogenation of aqueous solution of methanol with bare and Pt(0.5)/TiO2 increased with increasing temperature. Arrhenius plot shows that the activation energy is equal to ∼22 kJ mol-1 with using bare TiO2, and reduced the value to ∼10 kJ mol-1 with using Pt(0.5)/TiO2 under purged O2. However, the activation energy value is equal to ∼14 kJ mol-1 with using Pt(0.5)/TiO2 under purged N2. Part four is concerned with the hydrogen production by photocatalytic dehydrogenation of methanol with using 0.5% of platinum and 0.5% of gold loaded on TiO2 surface. The Pt/TiO2 was more active 8
than Au/TiO2 in hydrogen production reaction from aqueous solution of methanol. Part five is focused on quantum yield measurements as function of different in weight of catalyst, pH of solution and temperature on photocatalytic oxidation and photocatalytic dehydrogenation of methanol by using bare and platinum (0.5) loaded on TiO2 surface. The optimum conditions to get a better photooxidation of methanol are 175 mg/ 100 mL of TiO2 and Pt(0.5)/ TiO2, initial pH equal ∼ 7, and temperature around 298.15 K. mechanisms for photocatalytic oxidation and . Two dehydrogenation of methanol with a limiting quantum yield of 0.5 are discussed.
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Contents Contents
Page
Acknowledgements Abstract Contents List of Tables List of Figures List of Schemes List of Abbreviations and Symbols
III V-VI VII-IX XII-XIV XV-XVIII XIX XXX
CHAPTER ONE: INTRODUCTION 1.1 1.2 1.3 1.4 1.5 1.5.1 1.5.2 1.6 1.6.1 1.6.2 1.6.3 1.7 1.8 1.9 1.9.1 1.9.2 1.10 1.10.1 1.10.2 1.10.3 1.10.4 1.10.5 1.10.6 1.11 1.12 1.13 2.1
General Introduction Semiconductors Crystal Morphology of TiO2 Advanced Oxidation Processes (AOPs) Photocatalysis Homogenous Photocatalysis Heterogeneous Photocatalysis Modification of Photocatalyst Surface Surface Sensitization Composite Semiconductor Metal - Semiconductor Modification (Metalized TiO2 Surfaces) Schottky Barrier Adsorption Adsorption on Bare TiO2 and Metalized TiO2 Surfaces Adsorption of Oxygen Adsorption of Alcohols Photocatalytic Reaction Parameters Mass of Catalyst Initial Concentration of Substrate Initial pH of Solution Temperature Light Intensity (Photon Flux) Quantum Yield Hydrogen Production TiO2 Nanoparticales Aims of the Present Work CHAPTER TWO: EXPERIMENTAL Photocatalytic Reactor Units 10
1 1 3 5 7 7 8 10 10 10 11 13 14 14 16 18 19 21 21 22 23 24 25 27 28 31 32
2.2
Chemicals
35
2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11
Instruments Preparation of Metal Loaded on TiO2 Atomic absorption Spectrophotometry (A.A) Fourier Transform Infrared Spectroscopy (FTIR) X-Ray Diffraction Spectroscopy (XRD) Atomic Force Microscopy (AFM) Band Gap Energy Measurements Photocatalytic Oxidation of Methanol Detection of Formaldehyde by Spectrophotometric Method
36 37 39 41 41 42 43 44 44
2.12 2.12.1 2.12.2 2.13 2.14
Light Intensity Measurements Preparation of Calibration Graph for Ferrous Iron Theoretical Calculations of Actinometer Solution Reaction Quantum yield Measurements Photocatalytic Hydrogen Production CHAPTER THREE: RESULTS Physical Characterizations of Catalysts Atomic absorption Spectrophotometry (A.A) Fourier Transform Infrared Spectroscopy (FTIR) X-Ray Diffraction Spectroscopy (XRD) Atomic Force Microscopy (AFM) Band Gap Energy Measurements Photocatalytic Oxidation of Methanol Preliminary Experiments Dark Reaction(Adsorption Reaction) Photolysis Reaction Photocatalytic Reaction under purged N2 Photocatalytic Reaction under purged O2 Effect of Type of Irradiation The Effect of Different Parameters on Photocatalytic Oxidation of Methanol by Using Bare and Metalized TiO2 Effect of Catalyst Mass Effect the Percentage Loaded Metals Effect for Methanol Concentration Effect the pH of Solution Effect of Temperature
46 47 48 50 50
3.1 3.1.1 3.1.2 3.1.3 3.1.4 3.1.5 3.2 3.2.1 3.2.1.1 3.2.1.2 3.2.1.3 3.2.1.4 3.2.2 3.2.3 3.2.3.1 3.2.3.2 3.2.3.3 3.2.3.4 3.2.3.5 3.2.4 3.2.4.1
The Effect of Different Parameters on Photocatalytic Dehydrogenation of Methanol by Using Bare and Metalized TiO2
Effect of Mass of Catalyst
11
52 52 52 55 59 69 70 70 70 70 72 72 73 75 75 79 84 87 91 95 95
3.2.4.2
Effect the Percentage of Pt and Au Loaded in Presence of N2
3.2.4.3 3.2.4.4 3.2.4.5 3.2.5
Effect for Methanol Concentration Effect the pH of Solution Effect of Temperature Hydrogen Production by Photocatalytic Dehydrogenation of Methanol Using Metalized TiO2 CHAPTER FOUR: DISCUSSION 4.1 Physical Characterizations of Catalysts 4.1.1 Atomic absorption Spectrophotometry (A.A. ) 4.1.2 Fourier Transform Infrared Spectroscopy (FTIR) 4.1.3 X-Ray Diffraction Spectroscopy (XRD) 4.1.4 Atomic Force Microscopy (AFM) 4.1.5 Band Gap Energy Measurements 4.2 Photocatalytic Oxidation of Methanol 4.2.1 Preliminary Experiments 4.3 The Effect of Different Parameters on Photocatalytic Oxidation of Methanol by Using Bare and Metalized TiO2 4.3.1 Effect of Catalyst Mass 4.3.2 Effect the Percentage Loaded Metals 4.3.3 Effect of Methanol Concentration 4.3.4 Effect the pH of Solution 4.3.5 Effect of Temperature 4.4 The Mechanism of Photooxidation of Methanol in the Presence of Oxygen 4.5 The Mechanism of Photooxidation of Methanol in the Presence of Inert Gas 4.6 Hydrogen Production by Photocatalytic Dehydrogenation of Methanol Using Metalized TiO2 4.7 The Quantum Yield Measurements 4.7.1 Effect of Mass of Catalyst 4.7.2 Effect the pH of Solution 4.7.3 Effect of Temperature CHAPTER FIVE: CONCLUSIONS AND RECOMMENDATION 5.1 Conclusions 5.2 Recommendations References
12
98 102 104 106 108 110 110 110 111 112 113 113 113 114 115 116 117 118 119 121 122 122 123 123 125 127 129 131 132
List of Tables
1-1 1-2 1-3 2-1 2-2 2-3 2-4 2-5 2-6 2-7 2-8 2-9 2-10 2-11 3-1 3-2 3-3 3-4 3-5 3-6 3-7 3-8 3-9 3-10 3-11
Titles of Tables
Page
Some Main Properties of TiO2 Crystal Common Sources for Generation of Hydroxyl Radical in Different AOPs Work Functions of Some Metals Chemicals Instruments Loaded Calculations of Pt on TiO2 Surface Loaded Calculations of Au on TiO2 Surface Calibration Curve Data of Pt Concentrations Calibration Curve Data of Au Concentrations Calibration Curve of Formaldehyde Measured Instrumental Photon Flux as Function of the Lamp Height Calibration curve of Fe(II) Light Intensity Calculated by Chemical Actinometer Calibration Curve of Hydrogen Gas Loaded Calculations of Pt on TiO2 Surface Loaded Calculations of Au on TiO2 Surface Mean Crystallite Sizes and Crystallite Sizes of Bare TiO2 and Pt Loaded on TiO2 Mean Crystallite Sizes and Crystallite Sizes of Bare TiO2 and Au Loaded on TiO2 Particle Size Measured by AFM and Crystallinity Values of Bare TiO2 and |Metalized TiO2. Band Gap Measured by UV-Visible Diffuse Reflectance Spectra of Bare TiO2 and Metalized TiO2 Concentration of Formaldehyde Formation in Dark Reaction with O2 and N2 Concentration of Formaldehyde Formation in Photolysis Reaction with Purged O2 Concentration of Formaldehyde Formation in Photocatalytic Reaction with Purged N2. Concentration of Formaldehyde Formation in Photocatalytic Reaction under Purged O2 Rate of Formaldehyde Formation in Photocatalytic Reaction with UV- A Light and Visible Light
4 5
13
12 35 36 38 39 40 41 45 46 47 49 51 52 52 57 59 69 70 71 71 72 72 74
3-12
3-13
3-14
3-15
3-16
3-17
3-18
3-19
3-20
3-21
3-22
3-23
3-24
3-25
Rate of Formaldehyde Formation in Photocatalytic Reaction with Different Masses of Bare TiO2 under Purged O2 Rate of Formaldehyde Formation in Photocatalytic Reaction with Different Masses of Pt(0.5)/ Tio2 under Purged O2 Rate of Formaldehyde Formation in Photocatalytic Reaction with Bare and Different Percentage of Pt Loaded on TiO2 Surface under Purged O2 Rate of Formaldehyde Formation in Photocatalytic Reaction with Bare and Different Percentage of Au Loaded on TiO2 Surface under Purged O2 Rate of Formaldehyde Formation in Photocatalytic Reaction with Different Concentrations of Methanol and Bare TiO2 under Purged O2 Rate of Formaldehyde Formation in Photocatalytic Reaction with Different Concentrations of Methanol and Pt(0.5)/ TiO2 under Purged O2 Rate of Formaldehyde Formation in Photocatalytic Reaction with Different Values of Initial pH Solution with Bare TiO2 under Purged O2 Rate of Formaldehyde Formation in Photocatalytic Reaction with Different Values of Initial pH Solution with Pt(0.5)/ TiO2 Surface under Purged O2 Rate of Formaldehyde Formation in Photocatalytic Reaction with Different Temperatures with Bare TiO2 under Purged O2 Rate of Formaldehyde Formation in Photocatalytic Reaction with Different Temperatures with Pt(0.5)/ TiO2 under Purged O2 Rate of Formaldehyde Formation in Photocatalytic Reaction with Bare and Different Percentage of Pt Loaded on TiO2 Surface under Purged N2 Rate of Formaldehyde Formation in Photocatalytic Reaction with Different Concentrations of Methanol and Pt(0.5)/ TiO2 under Purged N2 Rate of Formaldehyde Formation in Photocatalytic Reaction with Bare TiO2 and Different Percentage of Au Loaded on TiO2 Surface under Purged N2 Rate of Formaldehyde Formation in Photocatalytic Reaction with Different concentration of Pt(0.5)/ TiO2 under purged N2 14
76
78
80
82 84 86 88 90 92
94
96 98 100 102
3-26
3-27
3-28
4-1
4-2
4-3
Rate of Formaldehyde Formation in Photocatalytic Reaction with Different Values of Initial pH Solution with Pt(0.5)/ TiO2 under Purged N2 Rate of Formaldehyde Formation in Photocatalytic Reaction with Different Temperatures with Pt(0.5)/ TiO2 under Purged N2 Amounts of Hydrogen Production in Photocatalytic Dehydrogenation of Methanol with Pt(0.5)/ TiO2 and Au(0.5)/TiO2,under Purged Ar Quantum yield of formaldehyde formation in photocatalytic reaction of methanol with different doses of bare TiO2 and Pt(0.5)/TiO2, under O2 and N2. Quantum Yield of Formaldehyde Formation in Photocatalytic Reaction of Methanol with Bare TiO2 and Pt(0.5)/TiO2, with Different Initial pH, under O2 and N2. Quantum Yield of Formaldehyde Formation in Photocatalytic Reaction of Methanol with Bare TiO2 and Pt(0.5)/TiO2, with Different Ranged of Temperatures, under O2 and N2.
15
104
106
108 124 126
127
List of Figures
1-1 1-2 1-3 1-4 1-5 1-6 1-7 1-8 1-9 1-10 1-11 2-1 2-2 2-3 2-4 2-5 2-6 2-7 2-8 2-9 2-10 2-11 3-1 3-2 3-3 3-4
Titles of Figures
Pages
Band Gap Energy and Band Edge Positions of Different Semiconductor Oxides at pH=1 Optical Transitions in Direct and Indirect Semiconductors Crystal Structure of TiO2 Phases Essential Processes under Illumination of Semiconductor Particles Types of Recombination in Semiconductor Photocatalyst Excitation steps using dye molecule sensitizer Photo-excitation in composite semiconductor photocatalyst Metal-Modification Semiconductor Photocatalyst Particle Band Diagram of a Metal and a Semiconductor (a) before and (b) after Being Brought into Contact Rate of Reaction as Function of Common Different Parameters: (a) Mass of Catalyst, (b) Initial Concentration of Substrate, (c) Initial pH of Solution, (d) Temperature and (e) Light Intensity Change in the Electronic Structure of a Semiconductor Compound as the Number N of Monomeric Units Present Increases from Unity to Clusters of More than 2000 Photocatalytic Reactor Type 1 Photocatalytic Reactor Type 2 Photocatalytic Reactor Type 3 Image of the Photodeposition of Pt Loaded on TiO2 Image of the Photodeposition of Au Loaded on TiO2 Calibration Curve at Different Concentration of Platinum Calibration Curve at Different Concentration of Gold Calibration Curve for Formaldehyde Calibration curve for Fe(II) as Complex with 1,10-Phenanthroline Image for the Chemical Actinometry Experiment. (a)Hg Lamp Setup Reactor and (b) W Lamp Setup Reactor Calibration Ccurve at Different Concentration of Hydrogen gas FT-IR Spectra for Bare and Different Percentage of Pt Loaded on TiO2, at a)Bare TiO2 , b)Pt(0.25)/ TiO2, c)Pt(0.50)/ TiO2, d)Pt(0.75)/ TiO2 and e)Pt(1.00)/ TiO2 FT-IR Spectra for Bare and Different Percentage of Au Loaded on TiO2, at a) Bare TiO2, b)Au(0.50)/ TiO2, c)Au(1.00)/ TiO2, d)Au(2.00)/ TiO2 and e)Au(4.00)/TiO2
2
XRD Patterns of Bare and Different Percentage of Pt Loaded on TiO2 Surface
Modified Scherrer equation of bare and Pt loaded on TiO2 plot, at a)Bare 16
3 3 8 9 10 11 12 13 20 29 33 33 43 38 39 40 41 46 47 48 15 53 54 55 56
3-5 3-6 3-7 3-8 3-9 3-10 3-11 3-12 3-13 3-14 3-15 3-16 3-17 3-18 3-19 3-20 3-21
TiO2, b)Pt(0.25)/ TiO2, c)Pt(0.50)/ TiO2, d)Pt(0.75)/ TiO2 and e)Pt(1.00)/ TiO2 XRD patterns of bare and different percentage of Au loaded on TiO2 surface Modified Scherrer equation of bare and Au loaded on TiO2 plot, at a) Bare TiO2, b)Au(0.50)/ TiO2, c)Au(1.00)/ TiO2, d)Au(2.00)/ TiO2 and e)Au(4.00)/TiO2 AFM Image of Bare TiO2, a) 2- Dimensions Image b) 3- Dimensions Image AFM Image of 0.25 % Pt Loaded on TiO2, a) 2- Dimensions Image b) 3- Dimensions Image AFM Image of 0.50 % Pt Loaded on TiO2, a) 2- Dimensions Image b) 3- Dimensions Image. AFM Image of 0. 75 % Pt Loaded on TiO2, a) 2- Dimensions Image b) 3- Dimensions Image AFM Iimage of 1.00 % Pt Loaded on TiO2, a) 2- Dimensions image b) 3- Dimensions image AFM Image of 0.50 % Au Loaded on TiO2, a) 2- Dimensions Image b) 3- Dimensions Image. AFM Image of 1.00 % Au Loaded on TiO2, a) 2- Dimensions Image b) 3- Dimensions Image AFM Image of 2.00 % Au Loaded on TiO2, a) 2- Dimensions Image b) 3- Dimensions Image AFM Image of 4.00 % Au Loaded on TiO2, a) 2- Dimensions Image b) 3- Dimensions Image UV-Visible Diffuse Reflectance Spectra of Bare , Pt and Au Loaded on TiO2 Surface UV-Visible Kubelka -Munk Transformed Diffuse Reflectance Spectra of Bare and Metalized TiO2 Photocatalytic Oxidation of Methanol on Bare TiO2, Pt(0.5)/TiO2 and Au(0.5)/TiO2 Surfaces under Different Reaction Conditions Photocatalytic Oxidation of Methanol on Bare TiO2 and Pt(0.5)/TiO2 Surfaces under Different Types of Lamps Photocatalytic Oxidation of Methanol on Bare TiO2 and Pt(0.5)/TiO2 Surface under Different Types of Lamps Photocatalytic Oxidation of Methanol on Different Masses of Bare TiO2 Surface
Rate of Reaction as Function of Different Masses of Bare TiO2 Surface, under Purged O2 Photocatalytic Oxidation of Methanol on Different Masses of Pt(0.5)/ 3-23 TiO2 Surface 3-24 Rate of Reaction as Function of Different Masses of Pt(0.5)/TiO2 Surface,
3-22
17
57 58 60 61 62 63 64 65 66 67 68 69 70 73 74 75 77 77 78 79
3-25 3-26 3-27 3-28 3-29 3-30 3-31 3-32 3-33 3-34 3-35 3-36 3-37 3-38 3-39 3-40 3-41 3-42 3-43 3-44
under Purged O2 Photocatalytic Oxidation of Methanol on Bare TiO2 and Different Percentage of Pt Loaded on TiO2 Surface Rate of Reaction as Function of Bare TiO2 and Different Percentage of Pt on TiO2 Surface, under Purged O2 Photocatalytic Oxidation of Methanol on Bare TiO2 and Different Percentage of Au Loaded on TiO2 Surface Rate of Reaction as Function of Bare TiO2and Different Percentage of Au on TiO2 Surface, under Purged O2 Photocatalytic Oxidation of Different Concentration of Methanol on Bare TiO2 Surface Rate of Reaction as Function of Different Concentration of Methanol with Bare TiO2 Surface, under Purged O2 Photocatalytic Oxidation of Different Concentration of Methanol on Pt(0.5)/ TiO2 Surface Rate of Reaction as Function of Different Concentration of Methanol with Pt(0.5)/ TiO2 Surface, under Purged O2 Photocatalytic Oxidation of Methanol on Bare TiO2 Surface, at Different Initial pH of Solution Rate of Reaction as Function of Initial pH of Solution on TiO2 Surface, under Purged O2 Photocatalytic Oxidation of Methanol on Pt(0.5)/TiO2 Surface, at Different Initial pH of Solution Rate of Reaction as Function of Initial pH of Solution on Pt(0.5)/TiO2 Surface, under Purged O2 Photocatalytic Oxidation of Methanol on Bare TiO2 Surface, at Different Temperatures Temperature Dependence for the Photocatalytic Oxidation of Methanol on Bare TiO2 Surface Photocatalytic Oxidation of Methanol on Pt(0.5)/ TiO2 Surface, at Different Temperatures Temperature Dependence for the Photocatalytic Oxidation of Methanol on Pt(0.5)/TiO2 Surface Photocatalytic Dehydrogenation of Methanol on Different Mass of Pt(0.5)/ TiO2 Surface Rate of Reaction as Function of Different Masses of Pt(0.5)/TiO2, under Purged N2 Photocatalytic Dehydrogenation of Methanol on Bare TiO2 and Different Percentage of Pt Loaded on TiO2 Surface Rate of Reaction as Function of Bare and Different Percentage of Pt on TiO2 Surface, under Purged N2
18
81 81 83 83 85 85 86 87 89 89 90 91 93 93 94 95 97 97 99 99
3-45 3-46 3-47 3-48 3-49 3-50 3-51 3-52 3-53 4-1 4-2 4-3 4-4
4-5 4-6 4-7
4-8
Photocatalytic Dehydrogenation of Methanol on Bare TiO2 and Different Percentage of Au Loaded on TiO2 Surface
101
Rate of Reaction as Function of Bare TiO2 and Different Percentage of Au on TiO2 Surface, under Purged N2 Photocatalytic Dehydrogenation of Different Concentrations of Methanol on Pt(0.5)/ TiO2 Surface Rate of Reaction as Function of Different Concentration of Methanol with Pt(0.5)/ TiO2 Surface, under Purged N2 Photocatalytic Dehydrogenation of Methanol on Pt (0.5)/ TiO2, at Different Initial pH of Solution Rate of Reaction as Function of Initial pH of Solution on Pt(0.5)/TiO2, under Purged N2 Photocatalytic Dehydrogenation of Methanol on Pt(0.5)/ TiO2 Surface, at Different Temperatures Temperature Dependence for the Photocatalytic Dehydrogenation of Methanol on Pt(0.5)/TiO2 Surface Hydrogen Production In Photocatalytic Dehydrogenation of Methanol with Pt(0.5) and Au(0.5) Loaded on TiO2 Relationship Between Calculated sizes from XRD Analysis and Different Percentage of (a) Pt Loaded on TiO2 Surface and (b) Au Loaded on TiO2 Surface Plot. Rate of Reaction as Function of Different Masses of Bare TiO2 and Pt(0.5)/TiO2 Surface, under Purged O2 and N2.
101
Relationship Between Rate of Reaction and Methanol Concentration with Pt(0.5)/TiO2 Surface under Different Types Carrier Gases Effect of Initial pH of Solution on the Rate of Reaction with Using Bear TiO2 with Purged O2, Pt(0.5)/ TiO2 with Purged O2 and Pt(0.5)/ TiO2 with Purged N2
117
Effect of Temperature on Rate of Reaction with Using Bear TiO2 with Purged O2, Pt(0.5)/ TiO2 with Purged O2 and Pt(0.5)/ TiO2 with Purged N2 Energy Level Variation at Pt-TiO2 Interface: (a) before Contact and (b) after Contact. Energy Level Variation at Au-TiO2 Interface: (c) before Contact and (d) after Contact. Weight Effect of Catalyst on Quantum Yield of Formaldehyde Formation for Bear TiO2 with Purged O2, Pt(0.5)/ TiO2 with Purged O2 and Pt(0.5)/ TiO2 with Purged N2, at 278.15 K, with a Light Intensity 1.570 x 10-7 Ens s-1. 19
103 103 105 105 107 107 109 112
115
118
120 123 123
125
Effect of Initial pH of Solution on Quantum Yield of Formaldehyde Formation for Bear TiO2 with Purged O2 , Pt(0.5)/ TiO2 with Purged O2 4-9 and Pt(0.5)/ TiO2 with Purged N2, at 278.15 K, with a Light Intensity 1.570 x 10-7 Ens s-1. Effect of Temperature on Quantum Yield of Formaldehyde Formation for Bear TiO2 with Purged O2, Pt(0.5)/ TiO2 with Purged O2 and Pt(0.5)/ 4-10 TiO2 with Purged N2, with a Light Intensity 1.570 x 10-7 Ens s-1.
126
128
List of Schemes
1-1 1-2 1-3 1-4 1-5 1-6 1-7 2-1 2-2
Titles of Schemes
Pages
Main Sources of .OH in Advanced Oxidation Processes The Electron Transfer Process between the Adsorbed Oxygen and the TiO2 Surface The Potential Diagram for Oxygen at pH 7 Forms of Adsorption for (R-O-) Over TiO2 Surface, (a) Monodentate and (b) Bidentate Bonding Geometries Adsorption of Methanol over TiO2 Surface (a) in the Absence O2 and (b)in the Presence of O2 Some Possible Methods to Produce Hydrogen Gas from Water Schematic for Main Methods for Preparation of TiO2 Nanoparticales Schematic Diagram of experimental set-up (a) Photocatalytic Vessel (b). Nash Reaction
5
20
16 17 18 19 28 30 32 45
List of Abbreviations and Symbols Abbreviation The Meaning and Symbols A.A
Atomic absorption
AFM
Atomic force microscope
AOPs
Advanced Oxidation Processes
CB
Conductive Band
DHP
Dihydropyridine
Dp
Particle Size
e-
Photoelectron
Ea
Activation Energy
fcc
Face centered cubic
fs
Femto second
Ef
Fermi Level
Eg
Band Gap Energy
F(R)
The Kubelka-Munk Function
FLM
Fermi Energy Level of Metal
FLS
Fermi Energy Level of Semiconductor
FT-IR
Fourier transform infrared
hν
Light Energy
GC
Gas Chromatography
h+
Photohole
Io
light intensity
L
Mean Crystallite Size
21
Ĺ
Crystallite Size
ms
Milli second
ns
Nano second
pHzpc
Zero Point Charge
ps
Pico second
Ptn
Pt Nucleus
R
The Reflectance Data
SCL
Space Charge Layer
VB
Valance Band
XRD
X-Ray Diffraction
ε
Molar Absoptivity
Φ
Quantum Yield
Φb
Schottky Barrier
Φm
Work Function of Metal
Φs
Work Function of Semiconductor
QSE
Quantum Size Effect
22
CHAPTER ONE INTRODUCTION
23
1.1 General Introduction: Photocatalysis is defined as acceleration of a photoreaction in the presence of a catalyst [1]. Semiconductor photocatalysis is rapidly expanding technology applied for energy generation and environmental applications. One of the most common photocatalysts is titanium dioxide (TiO2). The advantages of using titania photocatalysts are strong resistance to chemical and photocorrosion, strong oxidation capability, low operational temperature, low cost, and non-toxic nature [2]. These parameters make TiO2 photocatalytic materials perfect candidate for photocatalytic processes. TiO2 has been extensively studied and demonstrated to be suitable for numerous applications such as, destruction of microorganisms [3,4], inactivation of cancer cells [5,6], protection of the skin from the sun[7], photosplitting of water to produce hydrogen gas [8,9] and mineralization of toxic organic pollutants in water [10,11]. Even though TiO2 is widely used as a semiconductor, it has some disadvantages like low surface area, fast recombination between photogenerated holes and electrons and wavelength maximum lies in the ultraviolet region. Different attempts were made to improve the efficiency of TiO2. These include sensitizing of TiO2 with colored inorganic or organic compounds to improve its optical absorption in the visible light region [12-16], modification of the TiO2 surface with other semiconductors to alter the charge-transfer properties between TiO2 and the surrounding environment [17] and bulk modification by cation and anion doping [1820]. TiO2 is more active as a photocatalyst in the form of nanoparticles than in bulk powder. The origin of prefix ‘nano’ is Greek language. It means a dwarf. The range of size in nanoparticales is 1-100 nm. The nanoparticles have two principle factors that favored them on other materials, increasing the relative surface area and quantum effects. Mostly these properties enhance other characteristic parameters such as reactivity, electrical properties and strength [21]. TiO2 nanoparticales can be prepared in various morphologies such as spheres, wires, and tubes, depending on the method of preparation like sol method, sol-gel method, electrodeposition, hydrothermal method and microwave method [22]. 1.2 Semiconductors: Semiconductors are crystalline or amorphous solid materials, which have electrical conductivity values. The intermediate values lies between a metal and an insulator, and can be changed by altering the impurity, or the temperature, or size in quantum dot or by illumination with light [23]. To understand the semiconductor properties, one must 24
know the Band Theory for electrical conductivity. At zero Kelvin a perfect crystal of semiconductor materials consists of group of very close and filled electronic states, all electrons fill the valence band of semiconductor and no electrons in the conductive band, the region between valence band and conductive band is called band gap[24]. Semiconductors are particularly useful as photocatalysts (such as TiO2, CdS, ZnO, and SrTiO2) because the semiconductors absorb the photon which has energy equal to or, greater than the band gap (energy gap). Figure 1-1 shows the band gap energy and band edge positions of different semiconductor oxides at pH=1. The upper edge (black colour) refers to conduction band and the lower edge (red colour) refers to valence band. The energy scale is measured by electron volts using either the vacuum level or the normal hydrogen electrode (NHE) at pH=1.
Figure 1-1: Band Gap Energy and Band Edge Positions of Different Semiconductor Oxides at pH=1. (Picture Adapted from References [2428]). The band gap of semiconductors is divided into direct and indirect, depending on the symmetry of band structure of crystal lattice. The direct transition occurs when the transition of electron from the maximum of the valance band to the minimum of conductance band at the same values of the wave factor k, thereby ∆k = 0 and both the energy and electron momentum must be conserved. If the transition of electron from the maximum of the valance band to the minimum of conductance band 25
occur at the different values of the wave factor k, and ∆k 0. These electron transitions are forbidden therefore the momentums can't be conserved, but the electron transition occurs indirectly, when the extra particle, especially a phonon (is the quanta of crystal lattice vibration energy) allows to conserve the momentums. Where: (ν: photon frequency, Ω: phonon frequency, ε: energy) in Figure 1-2, as reported before [29].
∆k
∆k = 0
0
Figure 1-2 : Optical Transitions in Direct and Indirect Semiconductors. (Modified from Reference[29]). 1.3 Crystal Morphology of TiO2 Titanium dioxide is n-type semiconductor, it is also known as titania. There are three types of crystal structures in natural titanium oxide: rutile, anatase, and brookite. The crystal structure of TiO2 phases is shown in Figure 1-3. O Ti
Rutile
Anatase
Brookite
Figure 1-3: Crystal Structure of TiO2 Phases. Picture Taken from Reference [30]. 26
The most common form is rutile, it has a tetragonal structure, contains 6 oxygen atoms per unit cell, TiO6 as octahedron which slightly distorted. (Rutile is the most thermodynamically stable phase of TiO2, i.e, rutile has a low value of Gibbs free energy at atmospheric pressure compared with those values for anatase and brookite that attitude to transform irreversibly of anatase and brookite to rutile with calcinations, starts appearance at 600 oC, and completes at 900 oC [31, 32]). Anatase has a tetragonal structure too, while the TiO6 octahedron is more distorted than in rutile phase. At low temperatures, anatase is in more stable form and commonly, rutile and anatase are used in the photocatalysis processes. Anatase is more photoactive than rutile [33] because the conductive band of it, is closer to the negative position and this will increase the reducing power. Moreover, the Fermi level is slightly higher, high electron mobility, low density and lower surface energy all that leads to manufacturing of the solar cell of anatase [31, 34, 35]. Brookite has orthorhombic crystal system, unite cell consists of eight formula units of TiO2 which is formed by edge-sharing TiO6 octahedron [31]. Table 1-1 is displaying some main properties of TiO2 crystal. Table 1-1: Some Main Properties of TiO2 Crystal [26, 36-39]. Properties
Rutile
Anatase
Brookite
Discovery
Major ore, 1803 by Werner stable tetragonal 4.13 a = 4.5936 c = 2.9587 Low 1.949 (4) 1.980 (2) 81.2 90.0
1801 by R.J. Hauy metastable tetragonal 3.79 a = 3.784 c = 9.515 high 1.937 (4) 1.965 (2) 77.7 92.6
1825 by A. Levy metastable Orthorhombic 3.99 a = 9.184 b = 5.447 c = 5.145 sometimes 1.87-2.04 77.0-105 -
2.98
3.05
3.26
1825
Transformation to rutile
Transformation to rutile
Stability Crystal structures Density(g.cm-3) Lattice constant (Å) Photocatalytic activity Ti-O bond length (Å) O-Ti-O bond angle (deg) Experimental band gap (eV), at pH=7 Melting point (°C)
27
1.4 Advanced Oxidation Processes (AOPs) Advanced oxidation processes are deemed one of the most important methods to break down the pollutants and form a friendly .
products. The driving force of these processes is hydroxyl radical ( OH) which regards a second strong oxidation species and has an oxidation .
potential equal to 2.8 eV. The main sources for generation of OH are explained in Scheme 1-1 and Table 1-2. UV/TiO2 /O3
Sonolysis
UV/TiO2
.
(Photo-ozonation) UV/O3
(Photo - Fenton) H2O2+Fe2++UV
OH
UV/H2O2
(Fenton) H2O2+Fe2+
UV/TiO2 /H2O2
Ozone sonolysis .
Scheme 1-1: Main Sources of OH in Advanced Oxidation Processes [40-43]. Table1-2: Common Sources for Generation of Hydroxyl Radical in Different AOPs. Name
Key reaction 2+
3+
Fe +H2O2
Fenton Process
Summary
.
-
Fe + OH + OH .
Pollutants+ OH
(degradation products)
PhotoFenton Process
Fe3++H2O+hν .
Pollutants+ OH
.
Fe2++ OH + H+ CO2+ H2O
-Have a high rate constant for [43-45] oxidation Fe2+to Fe3+. -Used at λ> 300 nm to be active for breaking down the pollutants. -Needed a environment(pH=3)
28
acidic [41,43,46]
-Completed the mineralization of organic pollutants.
(mineralization)
Ref.
[43,46] .
O3 + hν
O2 +O
H2O +O.
Photoozonation
-No sludge remain.
H2O2 .
Pollutants+ OH
2.OH (degradation products)
O3 + Ultrasonic irradiation
Ozonation
-Needed one step to decolorization and degradation of organic pollutants.
-Perfomed easy and with [43,46] little space. - All residual O3 transformed to oxygen and water.
O2 +O.
sonolysis H2O +O.
H2O2 .
Pollutants+ OH
.
2 OH (degradation products)
2 OH .
Pollutants+ OH
-O3 with Ultrasonic irradiation is more active to killing of bacteria. -Both methods have a large molar absorptive value.
-Has a small molar absorptive
.
H2O2+ hν
Hydrogen peroxide
-Minimum danger.
value.
(degradation products)
[41,43]
-Used at λ < 300 nm to be active for breaking down the pollutants.
AOPs have advantages and disadvantages, and fortunately the advantages are more than those of disadvantages. The advantages of AOPs are summarized as [43] follows: 1. Reduced the reaction time. 2. Have low economic cost. 3. Have a suitable potential to reduce the toxicity of organic pollutant compounds by completing the mineralization of them. But, AOP is quenching some application by increasing the amount of peroxide.
29
1.5 Photocatalysis: Photocatalysis is a Greek word originally, which consists of two parts: the prefix "photo" means (Phos: light) and "catalysis" deals with (katalyo: break apart, decompose)[47]. This process was defined as the ability to accelerate the photoreaction in presence of a catalyst. The photocatalysis requirements are summarized as [48]: 1. The thermochemical reaction must be an endothermic reaction. 2. The photocatalysis process must be cyclic, and without side reactions to avoid the degradation of photochemical reactants. 3. The photochemical reaction should employ in a large range of light, i.e., UV, visible, and part of IR, hence, the solar spectrum is suitable economically. 4. The back reaction must be very slow to aid the storage of the products, while, to recover the energy content, the reaction must be rapid. 5. The products of the photochemical reaction must be easy to save and transport. During the last few years, there has been wide interest with employing the semiconductor material as photocatalysts especially after the photocatalytic splitting of water on TiO2 electrode by Fujishima and Honda in 1972[33].Generally, photocatalysis process is classified into two basic types according to the kind of phase for both the catalyst and the reactants; the first is called homogenous photocatalysis and the other is heterogeneous photocatalysis, both processes can produce a hydroxyl . radical ( OH)[42, 43]. 1.5.1 Homogenous Photocatalysis: This process performs with catalyst and reactants have similar phases (single-phase) under near UV irradiation, and generation .OH radical which plays a crucial role in destruction of organic pollutants mostly in water such as textile dyes. The first applications interested with using UV/ozone and UV/H2O2 systems, which produced .OH, see table 12. The usages of UV light via photodegradation of organic pollutants can be classified into two principal areas [48]: 1. Photooxidation, that deals with the use of UV light plus an oxidant to generate hydroxyl radicals, which attack the organic pollutants. 30
2. Direct photodegradation depends on direct excitation of the organic pollutant by UV light. 1.5.2 Heterogeneous Photocatalysis: Heterogeneous photocatalysis is a discipline which regards a worldwide, depends on finding a semiconductor catalyst and reactants in multi-phase states under atmosphere O2, and works in high range of spectra (UV and visible light), or solar light [44, 49]. The irradiation process of a semiconductor catalyst is an essential process to photo excitation it, which promoted photoelectron from the highest occupied energy band (valence band (VB)), to the lowest unoccupied band at zero Kelvin (conductance band (CB)), that leaves a positive photohole in the valence band. The generation of photoelectronhole pair separates by an energy distance referred to as the band gap (Ebg). At the beginning, the photon energy of illuminated light is equal or more than the energy of band gap. Photoelectron-hole pair may recombine and generate heat. The photo-reduction process occurs on the conductance band by electron-acceptor species such as O2, while the photo-oxidation process occurs on the valence band by electron- donor species such as -OH which the H2O originally, and produces the hydroxyl radical used for degradation and mineralization of pollutants [1, 50-52]. The essential processes under illumination of semiconductor particles are shown in the Figure 1-4. Photo-reduction
e
hv 1 VB
4 Recombination
2
Eg
Photo excitation
CB
-
h+
e
6
-
9
O 2 -.
H+
H+
O2
H+
.
2 OH
3 8 5
7 h+
.
OH + H+
Photo-oxidation H 2O
Figure 1-4: Essential Processes under Illumination of Semiconductor Particles.
31
The advantages of heterogeneous photocatalysis process are low cost, have a high conversion efficiency and quantum yield, high stability, high activity, widely used fields such as industry and environment, and works in high range of spectra (UV and visible light), or solar light [49, 53], but the recombination process regards a disadvantage of heterogeneous photocatalysis process, due to loss in the energy as heat. In photocatalysis process, the efficiency of photo-catalyst rises with increase in the surface species which acts as traps by adsorbing them on the photocatalyst surface, while the major factor which depresses the efficiency is photo electron- hole recombination. There are three important mechanisms of recombination [52, 54, 55]: 1. Direct recombination, the photo electron in conductive band drops directly into an unoccupied state in the valance band and combines with the photo hole by electrostatic attraction. 2. Surface recombination is lower probability, due to the surface species which can capture the photogenerated charge carriers (photo electronhole) and undergo the chemical reaction. 3. Recombination at recombination centres (volume recombination) is of high probability, because the recombination centres lies at lattice sites transition within the bulk of the crystal and the transition beyond to initial ground state. These types of recombination are pictorially shown in Figure 1-5.
Energy
e-
CB
hv Ebg
h+
VB
hv
Volume recombination
g > Eb
5
1 1 2
3 4
6
Photo-reduction
Direct recombination 7
+
A
D Photo-oxidation
8
D
where:
is e-
and
-
A
Surface recombination
is h+
Figure 1-5: Types of Recombination in Semiconductor Photocatalyst. (Modified from References [51,54 ,56]). 32
Generally, most metal oxides photocatalysts are wide band-gap, thereby, they will decrease the efficiency of photo-catalyst. However, the essential prerequisite to increase the efficiency of photo-catalyst is modified the surface of photo-semiconductor [48].
1.6 Modification of Photocatalyst Surface: There are three essential methods for modification of photocatalytic surface to depress recombination process by increasing the charge separation and the life time of photo hole, thereby that will increase the efficiency of photoreaction and increase the ability to absorb the large range of the wavelengths [50] and are: 1.6.1 Surface Sensitization: The surface sensitization favours for a wide band gap semiconductor via physical or chemical adsorption of coloured materials like dye, which absorbs the visible or solar light after irradiation, to excite it either singlet or triplet excited state, then injected the electron via the conductive band of semiconductor which more negative than dye, this modification reports in references [57-60]. All of these can be displayed by Figure 1-6. 3 6
4
*
Dye
e
+
Dye
Donor + Donor
7
2
-
e
-
e
-5
Photo-reduction
CB
Dye
VB
hv 1
Figure 1-6: Excitation Steps Using Dye Molecule Sensitizer. 1.6.2 Composite Semiconductor: This method is used, when the energy of the irradiated light is not enough to irradiate the semiconductor because it has a big band gap, while the other semiconductor has a small band gap, thereby the coupled 33
process of two semiconductors will increase the efficiency with use near UV or visible or utilise from solar light. This process has two advantages: 1. Increasing the response of semiconductor has a large band gap exists in UV by coupling within other that has a small band gap exists in visible light [57]. 2. Suppressing the recombination of photo electron-hole by injecting the electrons from the higher laying of conductive band which beyond to semiconductor has a small band gap into the lower laying of conductive band of large band gap semiconductor [60]. Several semiconductors have been studied thoroughly in combination with TiO2: TiO2-CdS [61,62], TiO2- SnO2[63], TiO2SiO2 [64].The idea of this modification is shown in Figure 1-7. 5
Photo-reduction
e e
-
6
-
-
e
2
CB
3 eExcitation
7
hv 1
8 +
h
+
4 h
Photo-oxidation
VB
Figure 1-7: Photo-excitation in Composite Semiconductor Photocatalysts.
1.6.3Metal-Semiconductor Modification (Metalized TiO2 Surfaces): The metal is deposited on the surface of semiconductor, this modification is shown in Figure 1-8, which raises the selectivity and the efficiency of photoreaction. This is attributed to the use of the metal as sink of electron, moreover, the lifetime of photo hole increases too. Most of metals in the periodic table can be deposited on semiconductor such as, Pt, Fe, V, and W [18, 19, 65, 66]. That depends on electric properties, so, the most important electric properties are work functions of both. Table 1-3 shows some values of work functions for metals at fcc(111), which determined. The Miller indices (111) has most compact atomic arrangement and is most stable. This indicates, is Schottky barrier formed or no. 34
5 -
e
6 e M
e 3 Excitation
CB
hv 1 8 Photo-oxidation
h+
VB
4
7 Photo-reduction
2
h+
Schottky barrier
Figure 1-8: Metal-modification Semiconductor Photocatalyst Particle. Several metal-semiconductors have been studied thoroughly in combination metals with TiO2, and explained the Pt-TiO2 is more active comparative with Pd, Rh and Au –TiO2 [67]. Table 1-3: Work Functions of Some Metals [68,69]. Metals
Surface
Work functions (eV)
Pt
fcc(111)
5.93
Pd
fcc(111)
5.6
Au
fcc(111)
5.31
Co
fcc(111)
5.0
Rh
fcc(111)
4.98
Ag
fcc(111)
4.74
Al
fcc(111)
4.54
Cr
fcc(111)
4.50
Ti
fcc(111)
4.6-4.7
35
1.7 Schottky Barrier Schottky barrier was named after Walter H. Schottky in 1938, it is a potential barrier generated at a metal–semiconductor junction formed. This barrier is formed when the work function of metal (Φm) is higher than the function of semiconductor (Φs) and the Fermi energy level of metal (FLM) is lower than that for semiconductor (FLS). After semiconductor illuminates by light, the photoelectron – hole pair created, and the photoelectrons in semiconductor conductive band will spontaneously flow from semiconductor to the surface of metal (metal acts as an electron trap) until the Fermi levels of both are closer to each other and becomes equal. The excess positive charge is accumulated on the surface of the semiconductor, thereby the energy band of semiconductor bends upwards, and forming Schottky barrier (Φb). The band bending is called space charge layer (SCL) or depletion layer [70,71]. The height of Schottky barrier is equal to (Φm – Ex) where Ex is electron affinity of semiconductor [56], As shown in Figure 1-9. V acuum E nergy
Φs
a
Ex
Φm
CB FL
S
FLM
Metal
Eg VB
N-type semiconductor
V a c u u m E n e r gy
Ex
b
e
Φm
-
Φs Φb
FL
CB FLS
M
Eg
VB
SC L
Figure 1-9: Band Diagram of a Metal and a Semiconductor (a) Before and (b) after Being Brought into Contact. (Modified and Redrawn from Reference[56]). 36
During exothermic catalytic reaction, hot electron generates in metal-semiconductor nanodiodes, and can flow from metal surface at energy 1-3 eV into the bulk of metal catalyst. This energy is high thereby the hot electron is overcoming on the height of Schottky barrier and the electron moves to semiconductor [72]. 1.8 Adsorption: Adsorption is a surface phenomenon which depends on the concentration of a chemical species (adsorbate) as vapor phase or solution over the surfaces of solid catalyst (adsorbent). The reverse process of adsorption is called desorption [73]. Adsorption is deeming one of the important factors, that affected in heterogeneous photocatalysis. However, the adsorption efficiency of photocatalyst enhanced by physical structure of photocatalyst such as surface area, distribution of pore sizes, and stability [74]. According to Langmuir-Hinshelwood (L-H) relationship, the initial concentration of reactant (adsorbate) is linearly proportional with rate of photochemical, and the adsorption- desorption kinetics is faster than the photochemical reaction [75,76]. As any process, adsorption has some important advantages such as, the high ability of removal the toxic organic compounds from wastewater, and finding different types of catalyst. While the disadvantages are summarized as, the catalyst losses the activity gradually, the active sites of catalyst are blockage with finding a high content of macromolecular compounds in solution and some types of catalyst are high cost[75]. 1.9 Adsorption on Bare TiO2 and Metalized TiO2 Surfaces: In heterogeneous photocatalysis, the adsorption phenomenon of reactant species on active sites of the photocatalysts surface is of prime importance for the photoreactions, and formed an intermediate species which are dissociated to generate the final product. These processes are needed to various five independent steps as follows [77, 78]. The five processes explained below are necessary for the photocatalysis: 1. Transfer of the mass of the reactants (or contaminated organic) in the fluid (liquid or gas) phase to the bare or metalized TiO2 surfaces. 2. Adsorption of the reactants (or organic contamination) on the bare or metalized TiO2 surfaces. 3. Reaction of the adsorbed phase with the bare or metalized TiO2 surfaces by photocatalytic reaction. 37
4. Desorption of intermediates or products from the bare or metalized TiO2 surface. 5. Removal of the mass of the intermediates or products from the bare or metalized TiO2 surface. The transfer of mass in step (1) and the removal of the mass in step (5) depend on the dose and the particle size of photocatalysts, also on the concentration of each reactants and products. All steps from (2) to (4) count on the chemical rivalry of reactant and product molecules with the active site of the photocatalysts, while at step (3), the photocatalytic reaction by absorption the photons of light to produce reactive radical .
(mainly OH) which regards a driver power of photo reaction. The mechanism of photocatalytic oxidation on the bare or metalized TiO2 surface consists of the four essential following processes[1, 37, 79]: 1. Charge-carrier generation (e- -h+) pair by photo excitation. TiO 2 or M / TiO 2 + hv → ( h + − e − ) (exciton ) → h + + e −
(fs )
1−1
2. Charge- carrier trapping. − Ti IV OH + e CB ↔
Mn /
− ≡ e TR
Ti III OH
− Ti IV OH + eCB ↔ M n− /
Ti IV OH + h +VB →
(100 ps ) − ≡ eTR
Ti IV OH
Ti IV . OH
+
+ ≡ h TR
1− 2
( ps )
1− 3
(10 ns )
1− 4
3. Charge-carrier recombination, like (surface recombination) and produced a heat. − e CB + + h VB +
Ti IV .OH
+
→
Ti III OH →
Ti IV OH Ti IV OH
(100 ns )
1− 5
(10 ns )
1− 6
4. Photocatalytic degradation, by either decolorization (broken the chromophore groups in compound) or mineralization process (transferred the organic compound to CO2 and H2O). a-Photoexcited e- scavenging − (O 2 ) ads + eTR
→
Ti IV OH + (O 2−. ) ads
38
( ms )
1− 7
.
b-Photogeneration of OH −
+ OH ads + hTR →
+ .OH ads
Ti IV OH
1− 8
(100 ns)
+
H (O −2. )ads + H + ( H 2 O) →. OOH → H 2 O 2 → 2 .OH
1− 9
.
c-Photodegradation by OH R − H + . OH ads
→ R . → Intermediate → CO 2 + H 2 O
1 − 10
Dye + . OH ads → Dye+. → Intermediate → CO 2 + H 2 O
1 − 11
The adsorption process is an important step to enhance the light absorption via the photocatalytic reaction. 1.9.1 Adsorption of Oxygen: The presence of oxygen is one of the essential prerequisites to take place in the photoreaction processes. Oxygen molecule is chemisorbed on the electron rich sites, and acts as traps for conduction band electrons of TiO2, then produced an active species after illumination by light, such as superoxide anion (O2.-) and hydrogen peroxide H2O2 …etc. These species play a vital role in acceleration of photoreaction. However, the formation of superoxide anion will increase the splitting between the photoelectron-hole pairs and decrease the recombination process [80]. Scheme 1-2 shows the adsorption of oxygen on TiO2 and produced other species.
TiO2
hν
O2
TiO2
e
e e e e e e e e e e e e e
TiO2 +++++++++++++ H, H+
is O2 molecular e is O2.+ is hole is H2O2 molecular
TiO2 +++++++++++++
Scheme 1-2: The Electron Transfer Process between the Adsorbed Oxygen and the TiO2 Surface. (Modified from Reference [80]).
39
The reduction process of oxygen molecular to form O2.- is faster than that in the formation of H2O2, because it has a lower redox potential than that of formed H2O2. Scheme 1-3 shows two redox potentials for (O2/ O2.-), for the standard potential of Eo (O2/ O2.-) equal -0.33 V, and for the (1 mole of O2/L) Eo (O2(aq.)/ O2.-) equal -0.16 V, while Eo (O2./H2O2) has higher redox potential equal + 0.89 V, so the process of conversion for H2O2 to O2.- or O2.- to O2 needed a strong oxidation agent like Fenton Fe2+ or Fe3+ due to Eo (FeIII/FeII) near 0 V at pH =7 [81]. O2−. + Fe3+ → O2 + Fe2+ ←
1 −12
H2O2 + Fe2+ → .OH+ HO−+ Fe3+
1−13
H2O2 + Fe3+ → O2−. + H++ Fe2+
1−14
O2
O2.-
H2O + .OH
H 2O 2
-0.33 (-0.16)V + 0.89V
+0.38V
+ 0.281(0.81)V
2H2O
+2.3V
+1.349V
Scheme 1-3: The Potential Diagram for Oxygen at pH 7. (Redrawn and Modified from Reference[81]). .
The adsorption of oxygen gas on TiO2 surface and the formation of OH can occur according to the following mechanism [52, 82]: O2 (gas) → O2 (ads)
1−15
O2 (ads) + e− → O.2− (ads)
1−16
O.2−( ads) + e− → 2 O− (
1 − 17
O−(ads) + e− → O2
ads)
−
1−18
( ads)
O.2−(ads) + H+ → .OOH
1−19
O.2−(ads) + .OOH+ H+ → H2O2 + O2
1− 20
H2O2 → 2 .OH
1− 21 40
1.9.2 Adsorption of Alcohols: In view of the fact that adsorption process of aliphatic or aromatic alcohol molecules on the surface of photocatalyst plays a crucial role in the photoreaction of alcohols. The interactions between the adsorbed alcohol molecules and the surface adsorption Lewis acid sites heterogeneity generate aldehydes or ketones and H2 as a renew energy throughout dehydrogenation process [19, 67, 83]. The adsorption of different alcohols on TiO2 surface has been reported by many pieces of researches. Lin and coworkers [84,85] had reported the adsorption of methanol and ethanol over TiO2 surface, CH3O- and C2H5O- are adsorbed in two forms: monodentate and bidentate bonding geometries, as shown in Scheme 1-4, which was confirmed by IR spectra. Moreover, the rate of photoxidation of monodentate adsorption form is more than that in bidentate adsorption. The suggested mechanism for adsorption and photoxidation of alcohols is shown in Scheme 1-5, (a) in the absence and (b) in the presence of oxygen.
O
R
R
O
O
Ti
O
Ti
O
O
OH
OH
Ti OH
O
Ti OH
O
Ti
O
OH
b
a
Scheme 1-4: Forms of Adsorption for (R-O-) Over TiO2 Surface, (a) Monodentate and (b) Bidentate Bonding Geometries. (Redrawn and Modified from Reference [85]).
41
H R
C.
R
CH 2
O
O
H+ hv
(a )
T iO 2
electron injectio n into T iO 2
RCHO
T iO 2
(g) +
H
ho le capture
H R
R
CH 2
C
OO .
O
O hv
(b ) T iO 2
co m bin a tio n w ith
.
OOH
T iO 2
h o le ca p tu re and O 2 in co rp o ratio n
O R H
H
C
O
O
+
O
C O
O O2
R
O H
HO
d ec o mb in a tio n T iO 2
T iO 2
Scheme 1-5: Adsorption of Methanol over TiO2 Surface (a) in the Absence O2 and (b)in the Presence of O2.( Redrawn and Modified from Reference [84]). Nuhu [86] has studied the monolayer adsorption and reaction of ethanol over TiO2 (P 25) and over Au-TiO2 in using pulse flow reactor, and found that the products of ethanol oxidation on Au- TiO2 were ethane with carbon monoxide, and hydrogen at (300 oC). At high temperature and by employing temperature programmed pulse flow reaction, ethanol was the product. However, the products that completed the oxidation in presence of TiO2 and Au-TiO2 were carbon dioxide and water. The irreversibly of adsorption of different derivatives of tri-carbon-ol, i.e., 1,2- propanediol, 2-propanol and 1-propanol, and, propene oxide over TiO2 (P25) support was detected, using FTIR spectroscopy [87]. Robert and coworkers [88] employed FTIR spectroscopy to study and discussed the adsorption isotherms of phenol, different substituted phenolic compounds and organic diacids in aqueous phase over TiO2 surface. The chemisorption of phenols is poorly adsorbed via Tiphenolate adsorbates, while the chemisorption of an organic acid is very fast due to titanium carboxylate formation. 1.10 Photocatalytic Reaction Parameters: The photocatalysis process on catalyst surface utilizes by arising the redox capability via formed the photoelectrons - photoholes, and 42
controlled on the recombination of the photoelectrons and photoholes. This also increased the degradation of organic pollutant substrates, by counting on different parameters such as pH of solution, concentration of substrate..etc., as displayed in the sub-sections below. Figure 1-10 shows the relation between the rate of reaction and common different parameters. (b)
Middle and High Temp.
1/T
Initial pH of solution
Rate of reaction
Low and Middle Temp.
M-OH +-O H
M-OH + H
(d) Very High Temp.
ln (Rate of reaction)
-
( c)
M-O + H2O
pHZPC
Order of reaction = 0
Initial concentration of substrate
+
Rate of reaction
M-OH2
+
Mass of catalyst
Order of reaction = 1
Rate of reaction
Rate of reaction
(a)
(e ) Rate α (I)1/2
Rate α I0
Rate α I M.L.I
H.L.I
L.L.I
Light intensity (I) Figure 1-10: Rate of Reaction as Function of Common Different Parameters: (a) Mass of Catalyst, (b) Initial Concentration of Substrate, (c) Initial pH of Solution, (d) Temperature and (e) Light Intensity, (Redrawn and Modified from References [18,19,67,77]).
43
1.10.1 Mass of Catalyst In order to shun the unnecessary excess of catalyst and to sponsor a wholly absorption of light without loss, the mass of catalyst must be determined. Figure 1-10 (a) shows the rate of reaction is directly proportional with the mass of catalyst; this behavior reflects the increment of the numbers of the active sites of catalyst. However, above a certain level of catalyst mass, the reaction rate becomes flat and is not dependent on the mass; this limit relies on the geometry and the conditions of photoreactor and the type of the UV lamp[77]. At high mass of catalyst, the rate of reaction and the penetration of light depresses that attitude to scattering effect [16, 19]. 1.10.2 Initial Concentration of Substrate In photocatalytic or photoadsorption reactions, the degradation rate of reaction counts on the substrate concentration. Moreover, the active species, i.e., .OH and O2.- over the catalyst surface enhance this process. The degradation rate in photocatalytic reaction of substrate or organic pollutants over illuminated catalyst fit the Langmuir-Hinshelwood (L-H) kinetics model, which can be approximated by the equation (1-22), Langmuir-Hinshelwood equation assuming that adsorption desorption kinetics is faster than the photochemical reaction [75,76]: dC r = - dt
k KC
=
1-22
(1+ KC)
where: k is the rate constants, K is the Langmuir constant reflecting the adsorption/desorption equilibrium between the substrate and the photocatalyst surface, r is the rate of reaction, C is concentration of substrate and t is time of illumination. When the concentration of substrate tends to be zero (low concentration), the equation 1-22 must be modified to give an apparent first order equation [15,77]: ln
Co
= k K .t = kapp. t
1-23
C
While, with the large concentrations of substrate, the rate of reaction is maximum and of the zero order (Figure 1-10 (b)).
44
The Langmuir-Hinshelwood (L-H) kinetics model has four possible [77, 89, 90]: 1. The reaction occurs between two adsorbed substances (substrate(ads.) and .OH(ads.)). 2. A nonbound radical (radical in solution) reacts with an adsorbed substance molecular (substrate(ads.) and .OH(sol.)). 3. The reaction occurs between a radical linked to the surface and substance molecular in solution (substrate(sol.) and .OH(ads.)). 4. The reaction obtains between two free species in solution (substrate(sol.) and .OH(sol.)). In all cases, the express of reaction rate equation is similar and the process happens either on the surface in solution or at the interface. 1.10.3 Initial pH of Solution The pH is considered one of the most benefit parameters that impacts on the surface charge of catalyst, and enhancements the photodegradation of the organic pollutants in presence of photocatalyst, thereby, there are two interpretations for understanding the effect of pH on the rate of reaction: a) It is related to the ionization state of surface of catalyst, due to the metal oxide catalyst such as TiO2 is amphoteric behaviour in aqueous media, and so, the electronic charge of catalyst surface is determined by the zero point charge (zpc), which is called point of zero charge (pzc) also. At zpc, the surface charge density is zero and gives a maximum adsorption and increment of the rate of reaction around it (Figure 1-10(c)). At pH values lower than the pHzpc, the surface of catalyst is in protonated form such as TiOH2+, and TiO2 particle agglomeration that due to depress the absorption of photon and reduce the substrate adsorption on surface, that beyond to compete between Cl- anion of HCl and substrate molecule on the adsorption process over catalyst surface. However, at higher pH values than pHzpc, the surface is undergoing deprotonation becoming negatively charged (TiO- ), and the Na+ ion adsorption of NaOH comparative with substrate molecules over catalyst surface[1,77, 91-94].
45
TiOH + H+ TiOH + -OH
TiOH2+
(Ka1)
TiO- + H2O (Ka2)
1-24 1-25
where, Ka1 and Ka2 are formation constants in acidic and basic mediums respectively. By utilizing from equations (1-24) and (1-25), the pHzpc of TiO2 can be calculated as pHzpc = (pKa1+ pKa2)/2. The pHzpc of photocatalyst value alters with increasing the metalized over surface, so, the pHzpc of the samples containing Cr, Mo, V and W loading on TiO2 moves to a value more acidic than that of the bare TiO2, this behavior attitudes to generate acidic species such as CrO3, MoO3, V2O3 or WO3, but, for Co, Cu, Fe loading on TiO2, the pHzpc moves to a higher value, that indicated to reduce into a metallic state (M0) to act as co-catalyst [95]. b) Hydroxyl radical concentrations are considered a power of photoreaction, they can be formed by the reaction between -OH and positive holes. At low pH, the positive hole deems as the major oxidation species. However, at neutral or high pH levels, the hydroxyl radical are considered as the predominant species, although it should be noted, but at very high pH, the Columbic repulsion force between -OH and the negatively charged surface is increased, that will depress the formation of .OH and reduce the rate of reaction [77, 93]. However, there are three possible mechanisms which can contribute to substrate degradation which deemed on the natural of substrate and the pH of solution [96]: 1. Hydroxyl radical attack. 2. Direct oxidation by the positive photohole in valance band of photocatalyst. 3. Direct reduction by the photoelectron in conductive band of photocatalyst. 1.10.4 Temperature In photocatalytic system, the room temperature is enough to active the photoreaction, so, the true activation energy Et is nil, while, the apparent activation energy Ea is defined in as a minimum amount of energy which required to promote the photoelectron from trapping centers to conductive band of photocatalyst [67,77, 97]. 46
The apparent activation energy is calculated by Van't HoffArrhenius plot employing the ln rate constant (k) verse (1/ temperatures of reaction) [98]. -Ea ln k =
1-26
+ ln A RT
where: k is the rate constant, Ea is an apparent activation energy, R is gas constant, T is a temperature of reaction, and A is a Pre-exponential (frequency) factor. At medium temperature range, which approximately have values more than 20 oC and less than 80 oC, the apparent activation energy has very small values, i.e., a few kJ/mol and near zero, this behavior reflects the closed in the rate of reaction and the reaction is independent of temperature [77, 98]. However, Ahmed et al in 2012 reported that the rate of photocatalytic oxidation of methanol in aqueous solution on bare and platinized TiO2 is incremented linearly with rising temperature in the ranges of 5-25 oC, and the apparent activation energy has small values [19]. At low temperatures (lower than 0oC), the apparent activation energy increases. The rate limiting step becomes desorption of final product and tends to the heat of adsorption of product, this reaction occurs by dehydrogenation of alcohol. Whereas, when temperature above 80 oC, the system temperature is approaching to the boiling point of water, and the exothermic adsorption of substrate becomes disfavored, and tends to be a rate limiting step. That will give a negative value of apparent activation energy and depress the activity of photoreaction. Moreover, the desorption process of adsorbed species increases, and the recombination process of charge carriers increases also [77, 92, 99] (Figure 1-10(d)). 1.10.5 Light Intensity (Photon Flux) The power of incident irradiation per unit area (mW/ cm2) is called photon flux, which is measured directly by radiometer or light intensity instrument. The rate of photocatalytic reaction depends largely on photon flux, there is given a linear relationship between rate of reaction and the light intensity (Figure 1-10 (e)). The effect of light intensity on the kinetics for photoreaction has been reported by Ollis and coworkers, which consist of three reigns [100-102]: a) At low light intensities range (0-20) mW.cm-2, the rate of reaction is increased directly with rising light intensity (first order).The formation of electron–hole is predominant, and the recombination process of electron–hole is negligible. 47
b) At middle light intensities approximately (25-40) mW.cm-2, the rate of reaction is increased directly with increasing of square root of light intensity (half order).This referees to the photocatalytic process is still found, and the electron–hole recombination process becomes predominant. This indicates that part of light absorption and losses other part by recombination of electron–hole pairs. c) At high light intensities, the rate of photoreaction is independent of light intensity (zero order). However, Singh and coworkers[103], found that the rate of photocatalytic decolorization of Direct Yellow 12 dye on TiO2 is incremented linearly with rising light intensity of incident UVA radiation in the range of 25-50 W.cm-2, this range closes to the average light intensity of sunlight, there is important in degradation of high concentration of substrate. 1.10.6 Quantum Yield The quantum yield (Φ) is defined as the number of moles for molecules (Nmol), which undergoes a photoreaction relative to the number of quanta(Einstein of photons) (Nphoton), which are absorbed by the photocatalyst, according to the following equations [104-106]: Nmol (mol/s)
Φ=
= Nphoton (Einstein/s)
Φ =
Rate (mol/s. L) .V(L)
1-27
Iabsorption (Einstein/s)
Rate of reaction
1-28 Rate of absorption of photons
In heterogeneous photocatalytic reaction, the calculated quantum yield of metal oxides such as TiO2 is regarded very difficult due to the catalyst particles absorb, scatter, or transmit the incident light by photocatalyst, this called true quantum efficiency (Φ), while the apparent quantum efficiency (Φs) is determined as a function of the total incident light intensity from the source. However, the apparent quantum efficiency is smaller than that in true quantum efficiency because the incident light is more than that in absorption of light, the incident light is measured by radiometer [54,106,107]. Φs =
Nmol (mol/s)
=
Nphoton (Einstein/s)
Rate (mol/s. L) .V(L) Iincident (Einstein/s)
48
1-29
Φs =
Rate of reaction
1-30
Rate of incident of photons
The optimum value of quantum yield is unity (Φλ=1) that depends on the ability of the absorbing species or chromospheres to absorb the photon that generates a product. However, most photoreaction have quantum yield less than one (Φλ>1) that beyond to the found the photoinduced chain reaction which needed a light only for the initiation step [108]. The values of quantum yield alter depending on the photoreaction conditions, such as, type and concentration of substrate, type of photocatalyst, metal doped photocatalyst, light intensity, the wavelength of radiation absorbed, kind of carrier gas and mechanism of reaction [18, 77, 109, 110]. Serpone in 1997 [111] explaines several causes which affect the quantum yield like: 1. Differences in crystalline structure, sizes, shape of the photocatalyst particles such as TiO2. 2. Differences in the density of –OH groups on the photocatalyst surface, and the number of adsorbed water molecules on the surface also. 3. Differences in the number and nature of trap sites over the surface and in the lattice of photocatalyst. 4. The adsorption-desorption characteristics of surface. Hussein and Rudham [18] found that the photoxidation of high concentration of methanol with metalized TiO2 leads to half value of quantum yield compared with presence bare TiO2 approximately 0.45 and 0.8 respectively. Vulliet and coworkers noted that the quantum efficiency is more important for the low light intensity, compared with the high light intensity, that is limited by: (i) increasing the electron-hole pairs recombination, and (ii) increasing the desorption process, at increasing 49
the flux of light. Thus, the quantum efficiency depresses with raises in flux [102]. This is in agreement with what is reported by Rabani and coworker, and the maximum value of quantum yield for formation product depended on the pH of solution, so, the best value of quantum yield for generating formaldehyde become at pH equal 7 [109]. Bahnemann and coworkers[110] studied the effect of catalyst loading for two types of TiO2: TiO2 (P 25) and TiO2 (Hombikat UV 100) on the quantum yield of formation of formaldehyde, undergoing photoxidation of methanol, and found the used the Hombikat UV 100 is best at dose more than 2.5 g L-1, due to have a high surface area. However, the results vise verse at below 2.5 g L-1. This effect depended on pH, so, in the case of P25 and Hombikat UV 100, the best pH equal 7.7 and 10.4 respectively. Wang and coworkers [112] explained the quantum yield of formation formaldehyde via photoxidation of methanol increases with using platinized TiO2 and suggested two different mechanisms, first in O2saturated suspension and other in N2-saturated suspension. 1.11 Hydrogen Production Hydrogen gas is the third most common element in earth and deemed as the ideal energy carrier of the future because it is clean and very flexible in various forms of energy. The hydrogen gas comes from either a renewable resource (such as agricultural waste or water), or nonrenewable (fossil fuel or nuclear plant) so, it is useful economically and environmentally [113,114]. In 1972, Fujishima and Honda [33], first found the hydrogen production from splitting of water molecule, when using TiO2, which can generate a photocurrent in an electrochemical cell, which employed Pt as the counter-electrode, the essential of photocurrent beyond to produce hole-electron pairs, then, revolutionary research studies have been published in prestigious journals[115-118]. Bockris and coworkers in 1985[119] reviewed the different possible routes for hydrogen production from water. These routes can be summarized in Scheme as shown in Scheme1- 6 below:
50
Electrolysis Magnetolysis
Used of Light
Hydrogen Production
Radiolysis
Plasmolysis
Bio-catalytic Decomposition of water
Scheme 1-6: Some Possible Methods to Produce Hydrogen Gas from Water [119].
Recently, the process of hydrogen production from water was deemed as an important step in photocatalytic oxidation of methanol, since H2 is regarded as a renewable resource and natural energy source [31]. From other studies, Chiarello and coworker [120] found, that hydrogen can be produced by photocatalytic process of methanol steam reforming over a series of noble metal such as (Ag, Au, Au–Ag alloy and Pt) loading on TiO2 photocatalyst. However, methanol oxidized to CO2 via the formation of formaldehyde and formic acid. Kwak and coworkers [121] reported results about the photocatalyst of aqueous methanol solution at optimum operational conditions, which lead to high photocatalytic hydrogen evolution activity by increasing the percentage of 0.1 wt % pd loading TiO2. Wu and Lee [122] supported the TiO2 (P 25) by Cu to utilize from photoxidation of methanol to produce a hydrogen, and given the high activity at the optimum loading of nearly 1.2 wt% Cu. 1.12 TiO2 Nanoparticales TiO2 photocatalyst in form of nanoparticale is more active than that in the form of bulk powder, this beyond to alter of physical properties. Nanoparticale has low melting point, low phase transition temperature, low particle sizes, increased the surface area, increased the porosity, increased the number of active sites, reduced the lattice constants, 51
increased the crystallinity, and reduced the no. of defects in the inner grains [123, 124]. Besides, the optical absorption peak of nanoparticale shifts toward the blue shift (shorter wavelengths) as the particle size is reduced, that conformed to what was reported by Mill and Le Hunte in 1997. The authors referred to the increase of the redox potentials of the photoelectron -photoholes generation. Moreover, the decreasing particle sizes from bulk semiconductor to nanoparticale can be ascribed to break up the band structure of bulk semiconductor into quantum levels, i.e., transition from bulk semiconductor to molecular properties [54], as shown in Figure 1-11.
Bulk semiconductor
Nan crystal
Molecule
Energy
LUMO (CB)
Eg HOMO (VB)
N> >> 2000
Big
Small
N= 2
Figure 1-11: Change in the Electronic Structure of a Semiconductor Compound as the Number (N) of Monomeric Units Present Increases from Unity to Clusters of More than 2000. (Modified from References [50, 54 ]. At particle sizes below 10 nm, these sizes are counted as critical radius, and the quantization (quantum size effect) (QSE) yields, so, the band gap increases as the particle size decreases, and the band edges shift to obtain a large redox potential, thus improvement in the photocatalytic activity, in spite of the optical absorption peak of this particle shifts toward the blue shift [50, 125]. However many studies reported the optimum particle sizes lied between (10-20) nm which gave a high photoactivity [124]. On the other hand, the small particle size is associated with a higher number of defects over the surface, while the number of defects in the inner grain is decreased, so, the defect sites act
52
as trap sites, which increased with increasing the surface area of catalyst [26, 124]. TiO2 nanoparticales have different shapes, according to methods of preparing them such as nanorode, nanotube or nanowier, that depend on the method of preparation of nanoparticales [22]. Scheme 1-7 shows the main method for that. Hydrothermal Method
Electrodeposition
Solvothermal Method
Direct oxidation
Micelle and inverse
Method
Micelle Method
Sol Method
TiO2 nanopartical
Sol-Gel Method
Sonochemical
Microwave Method
Method Chemical Vapour
Physical Vapour
Deposition
Deposition
Scheme 1-7: Schematic for Main Methods for Preparation of TiO2 Nanoparticales. (Modified from Reference [22]).
In general, nanoparticales tend to agglomerate, that will decrease the ratio of surface to volume. The agglomeration process depends on the Van der Waals attraction and the pH of solution, so, the agglomeration obtained at pH close to pHZPC. This behavior enhanced the increment of the photocurrent via TiO2 nanoparticale, that depressed the recombination, while, the electrostatic and steric repulsions with the surfaces of nanoparticales suppressed the agglomeration [126,127]. TiO2 nanoparticales can be employed for a different applications including hydrogen production [127], treatment of tumor cell [6], waste water treatment [128], self cleaning textile [129], remove the organic pollutants [130, 131], increasing the photocatalytic reaction [19]..etc. 53
1.13 Aims of the Present Work This work reports an investigation into the performance of TiO2 (Hombikat UV 100) supported platinum and gold catalysts, prepared by photodeposition. This study was set to achieve the following: 1- Studying the properties of bare, (0.25, 0.50, 0.75, and 1.0) wt platinized TiO2 and (0.50, 1.00, 2.00, and 4.00) wt gold TiO2 by Atomic absorption (A.A) analysis, Fourier transform infrared (FT-IR) analysis, X-ray diffraction (XRD) and Atomic force microscope (AFM) analysis. 2- Studying the effect of different conditions on photocatalytic oxidation of methanol by using bare and metalized TiO2. 1- Weight of catalyst. 2- Percentage of loaded metals (Pt or Au). 3- Methanol concentration. 4- pH of solution . 5- Temperature. 6- Quantum yield. 3- Studying the effect of different conditions on photocatalytic dehydrogenation of methanol using bare and metalized TiO2. 1- Weight of catalyst. 2- Percentage of loaded metals (Pt or Au). 3- Methanol concentration. 4- pH of solution . 5- Temperature. 6- Quantum yield. 7- Hydrogen production.
54
CHAPTER TWO EXPERIMENTAL
55
2.1 Photocatalytic Reactor Units A general diagram of the experimental set-up is shown in Scheme 2-1. The essential components of photocatalytic reactor was irradiation source (lamp), photoctalytic vessel (pyrex reactor), and magnetic stirrer. (a) (13)
(4)
(15) (11)
(4) (4) (12)
(1)
(14)
(5)
(2)
(6) (10)
(9) (7) (3) (8)
(1)O2 Gas. (2)N2 Gas. (3)Ar Gas. (4)Valve. (5)Gas Inlet. (6)Gas Outer. (7) Photocatalytic Vessel. (8) Magnetic Stirrer.
(9)Water Inlet. (10)Water Outer. (11) Irradiation Source. (12)Sampling. (13)UV-Visible Spectroscopy Analysis. (14) Gas Chromatography Analysis. (15) Analysis of Results by Computer. Sample
gas outer gas inlet
(b) 3 cm 4 cm
5.5 cm
Water outer
7 cm
2 cm 8.5 cm
Water inlet
2 cm 10 cm
Scheme 2-1: Schematic Diagram of Experimental Set-up (a) Photocatalytic Vessel (b).
56
There are three types of photocatalytic reactors were used in this work, which depended on the type of irradiation sources. These reactors are shown in Figures 2-1, 2 2-2 and 2-3. The photocatalytic reactor type 1 was consisted on irradiation source of UV- A light, Philips (efbe-Schon) (efbe Schon) low pressure mercury lamp, containing 6 lamps with 15W for each as UVUV A light at wave length 365 nm. While the second type of photocatalytic reactor was consisted on tungsten lamp (LITE-WAY WAY TM lighting) with 100 W at wavelength waveleng 3502500 nm. The lamps in photocatalytic reactor type 1 and 2 were positioned perpendicularly above the pyrex reactors. See Figures 2-1 2 and 2-2. Mercury lamp
.
Fan Gas inlet Gas outer Tap water out Reaction vessel Tap water w in Magnetic stirrer Figure 2-1: 2 Photocatalytic Reactor Type 1.
Tungsten lamp Gas inlet Gas outer Tap water w out Reaction vessel Tap water w in Magnetic stirrer
Figure 2-2: 2 Photocatalytic Reactor Type 2. 57
Vacuum fan Xenon lamp Gas inlet Gas outer Reaction vessel Magnetic stirrer
Figure 2-3: 2 Photocatalytic Reactor Type 3.
The third type of photocatalytic reactors were consisted on xenon lamp (Osram XBO) with 1000 watt as UV-B UV B light at wavelength 240240 1000 nm. Position of lamp in photocatalytic reactor was positioned horizontal next to the pyrex reactors. See Figure 2-3. 2 3. All the lamps were switched on 5 min before irradiation to ensure ensure the stability of intensity of lamps. In all experimental, photocatalytic vessel consisted of pyrex glass cylinder, that has (7 cm diameter, 5.5 cm length) with outer glass jacket (10 cm diameter, 8.5 cm length). In upper part of vessel, a small side-arm side (4 cm length) is used to pull the samples in the certain period of time, and two small side-arms arms (3 cm lengths), which are connected with elastic adapters to purge the current gases (oxygen, nitrogen or argon) to reaction via the solutions. Insides sides of the outer glass jacket, two small side-arms side (2cm length) are employed to connect with water bath recirculation by elastic adapters to control the temperature of reaction. See Scheme 2-1(b). 2 Magnetic stirrer was used to control the rate of mixing, the rate of mixing (250 rpm) was used for photodeposition of metals on TiO2, while the rate of mixing (750 50 rpm) was employed to mix of solutions for all photoreactions. All components of photoreactors were insulated in black wooden boxes or between two black wooden slides to prevent the escape of harmful radiation, and using fan or vacuum fan to minimize the temperature fluctuations. The photocatalytic reactor type 1 was most employed apparatus for preparation of metals loaded on TiO2 surface via photodeposition and for photo-dehydrogenation photo dehydrogenation from methanol. 58
2.2 Chemicals The employed chemicals in this work are listed in Table 2-1. All the used chemicals were employed without further purification.
Table 2-1: Chemicals No
Chemicals
Company supplied
1
Formaldehyde (40%)
Chemanol, Arabia Sudia Kingdom.
2
Methanol (HPLC grade, 99.8 %)
3
Methanol (A.R quality , 99.85 %)
Sd fine-CHEM limited, Mumbai, India. Hayman, England.
4
Titanium dioxide(Hombikat UV100), crystal size 5-10 nm Hexa chloro platinic (IV)acid hexa hydrate (H2PtCl6.6H2O) Tetra chloro auric(III) acid tri hydrate (HAuCl4.3H2O)
Sachtleben Chemie GmbH, Germany.
7
Silica gel (2-20 mesh)
BDH, England.
8
Ammonium acetate
BDH- Analar, England.
9
Acetyl acetone (98%)
Fluka AG, Switzerland.
5 6
Riedel-De-Haen AG, Seelze, Hannover, Germany. Riedel-De-Haen AG, Seelze, Hannover, Germany.
10 Acetic acid (96%)
Merck, Germany.
11 Iron (II) sulfate hepta hydrate (FeSO4. 7 H2O) 12 Iron (III) sulfate hepta hydrate (Fe2(SO4)3.7 H2O) 13 Potassium oxalate (K2(C2O4))
Merck, Germany.
14 Sulphuric acid (99 %)
Fluka AG, Switzerland.
15 1,10- Phenonethroline
BDH, England.
16 Hydrochloric acid (35.4%)
BDH- Analar, England.
17 Sodium hydroxide
BDH, England.
Merck, Germany. BDH, England.
59
18 O2 gas cylinder, 99.5 %
Emirates Industrial gasses /Dubai.
19 N2 gas cylinder, 99.995 %
Emirates Industrial gasses /Dubai.
20 Ar gas cylinder, 99.999 %
BOC Gases (NZ) Ltd, Canada.
2.3 Instruments The instruments used in this study with its companies are shown in Table 2-2.
Table 2-2: Instruments No.
Instrument
Company
1
Sensitive balance.
BL 210 S, Sartorius- Germany.
2
UV-Visible spectrophotometer
T80+ -PG instruments limitedEngland.
3
UV-Visible spectrophotometer with Varian Cary 100 Scan, Labsphere diffuse reflectance Laposphere- 99-010, Maryland accessory. United States.
4
Atomic absorption spectrophotometer AA-6300, Shimadzu-Japan.
5
Fourier Transform spectrophotometer.
6
X-Ray Diffraction Spectroscopy.
Lab X XRD 6000, Shimadzu-Japan.
7
Scan Probe Microscope.
AFM model, AA 3000, Advanced Angstrom Inc.- USA.
8
Gas Chromatography.
GC-8A, Shimadzu - Japan.
9
Ultrasound.
FALC-Italy.
10
Mercury lamp -UV (A).
Philips – Germany
11
Xenon lamp -UV (B)
Osram XBO-Germany
Infrared 8400S, Shimadzu- Japan.
60
12
Tingestun lamp (Visible).
LITE –WAY TM- B22 ClearChina.
13
Electrical Magnetic Stirrer.
HeidolphGermany.
14
pH meter.
Hanna- Rommana.
15
Water bath recirculation.
Stuart- England.
16
Centrifuge.
Hettich- Universall II- Germany.
17
Vacuum Pressure Station.
Barnant company-USA.
18
Oven.
Memmert-Germany.
19
UV meter.
Honel UV technology- Germany.
Mr
Hei-Standard-
2.4 Preparation of Metal Loaded on TiO2 Different percentages of platinum and gold loaded on TiO2 were prepared by photodeposition method analogous [67, 132]. In all experiments, 2 gm of titanium dioxide UV 100, in the presence 40 ml of 40% formaldehyde and 10 ml of absolute methanol were mixed, followed by the addition of desired amount of as-prepared solution from Pt (1% H2PtCl6. 6H2O/ 0.1M HCl) or Au (1% HAuCl4. 3H2O/ 0.1 M HCl) under continuous magnetic stirring at 250 rpm. The reaction vessels were then irradiated by UV-A light at light intensity equal 3.49 mW/ cm2 by a Philips Hg lamp (Efbe-Schon 6 lamps90 W), under inert environment by purging with N2 gas for 4h and 8 h for loaded each Pt and Au respectively. The produced solutions were pale grey colour for Pt loaded and pale purple colour for Au loaded on TiO2, as shown in Figures 2-4 and 2-5. These suspension solutions were filtered by using two filtration papers together (Chm - CHEMLAB GROUP, size 150 mm) under vacuum, repeated the filtration process until the filter solution was become colourless, then washed by absolute methanol, and threw overnight in desecrater that contained silica gel to remove the formaldehyde and methanol. At the end the product was dried by employed oven at 100 oC for 2h to remove a humidity. 61
Table 2-3: Loaded Calculations of Pt on TiO2 Surface. H2PtCl6.6 H2O concentrations /(gm/100 mL)% 1.000 1.000 1.000 1.000
Wt. of catalyst /gm 2.000 2.000 2.000 2.000
Pt % 0.250 0.500 0.750 1.000
Volume of aqueous solution /mL 1.350 2.700 4.050 5.400
Figure 2-4: Image of the Photodeposition of Pt Loaded on TiO2.
Table 2-4: Loaded Calculations of Au on TiO2 Surface. HAuCl4. 3H2O concentrations /(gm/100 mL)% 1.000 1.000 1.000 1.000
Wt. of catalyst /gm 2.000 2.000 2.000 2.000
62
Au % 0.500 1.000 2.000 4.000
Volume of aqueous solution /mL 1.730 3.460 6.920 13.840
Figure 2-5: Image of the Photodeposition of Au Loaded on TiO2.
2.5 Atomic Absorption Spectrophotometry (A.A) Atomic absorption instrument was employed to find the amount of Pt or Au in prepared samples with passing mixture of air and acetylene via the flame using (Shimadzu-AA-6300) instrument. The analysed samples were measured before and after 4h or 8h of irradiation for Pt or Au respectively. The calibration curves data are shown in Tables 2-5 and 2-6, and plotted in Figures 2-6 and 2-7 for Pt and Au respectively. Table 2-5: Calibration Curve Data of Pt Concentrations. Pt concentrations /ppm 0 10 20 30 40 60 80 100
Intensity 0.000 0.057 0.092 0.151 0.203 0.317 0.411 0.496
63
0.5 y = 0.005x R² = 0.998
Inensity
0.4
0.3
0.2
0.1
0 0
10
20
30
40
50
60
70
80
90
100
Figure 2-6: Calibration Curve at Different Concentration of Platinum. Conc. of Pt/ (ppm)
Figure 2-6: Calibration Curve at Different Concentration of Platinum. Table 2-6: Calibration Curve Data of Au Concentrations. Au concentrations /ppm 0 5 15 25 35 50 75
Intensity 0.000 0.071 0.249 0.368 0.485 0.804 1.1645
64
y = 0.015x R² = 0.995
1.2 1
Inensity
0.8 0.6 0.4 0.2 0 0
10
20
30
40
50
60
70
80
90
100
Figure 2-7: CalibrationConc. Curve Different ofatAu/ (ppm) Concentration of Gold.
Figure 2-7: Calibration Curve at Different Concentration of Gold.
2.6 Fourier Transform Infrared Spectroscopy (FTIR) Fourier Transform Infrared spectra for bare and Pt or Au loaded on TiO2 were recorded using Shimadzu-8400S instrument. The analysed samples were measured in the range of 4000-400 cm-1 on palletised with KBr dose at room temperature.
2.7 X-Ray Diffraction Spectroscopy (XRD) X-Ray diffraction (XRD) data were analysed by Lab X XRD 6000 instrument equipped. This instrument was employing Cukα 1 as a target source (wave length 1.54060 Å, voltage 40.0 kV and current 30 mA), slit (divergence 1.00000o, scatter 1.00000o and receiving 0.30000o), 2θ range from 10 to 80o, speed 12.0000 (deg/min) and preset time 0.10 sec. XRD data was employed to calculate the mean crystallite sizes (L) by Scherrer's formula in the following equation [133, 134]. 65
k λ L =
2-1 β Cos θ
where: L is the mean crystallite size, k is the Scherrer’s constant (0.94) which depends on the shape of the crystal, λ is wavelength of the x-ray radiation (0.15406 nm for Cukα), β is the full width of half-maximum (FWHM) intensity expressed in radians (originally, β is measured in degrees then multiply by (π/180) to convert to radians), and θ is a diffraction (Bragg) angle. Modified Scherrer's equation by Monshi and co-workers [135] were used to estimate the more accurate crystallite size (Ĺ) by employing XRD data. (k λ) β=
1 .
Ĺ
cos θ 2-2
By making logarithm on both sides: (k λ) ln β = ln
1 2-3
+ ln
Ĺ
cos θ
By plotting lnβ against ln(1/cosθ), the slope equal 1 and the intercept is ln(kλ/ Ĺ). The exponential of the intercept is obtained:
exp
ln
(k λ) Ĺ
k λ =
Ĺ
2-4
where: k and λ are substituted 0.94 and 0.15406 nm respectively.
66
2.8 Atomic Force Microscopy (AFM) Atomic Force Microscopy image was recorded with Scanning Probe Microscopy (SPM-AA3000) instrument, employing software WSxM (nanotech). The glass slides were cut to 1x2 cm and cleaned by putting them in (1:1) (ethanol: deionised water) solution and treated by ultrasonic (Ultrasound, FALC) instrument at 3 min in power equal 50 kHz. The sample solutions of bare and Pt and Au loaded on TiO2 were prepared by adding very small amounts (about 0.001g) of Pt and Au to absolute ethanol, then used ultrasonic at the same time and power to get on colloidal solutions and threw them 1 h to give a fine colloidal solutions. The prepared fine colloidal solutions poured on the glass slides as drop, then threw to dry, this step continuously repeated until a good spot occurred. The Crystallinity Index was calculated by using the following equation[136]. Dp
Crystallinity Index =
2-5
L or Ĺ
where: Dp is the particle size which is measured by AFM analysis and L is the mean crystallite size or Ĺ is crystallite size that is calculated by Scherrer equation and modified Scherrer equation of XRD data respectively.
2.9 Band Gap Energy Measurements Band gap energies of bare, Pt(0.5) and Au(0.5) loaded on TiO2 surface were determined, via the measurment of reflectance data R by (Cary 100 Scan) UV-visible spectrophotometer system. It is equipped, with using a Labsphere integrating sphere diffuse reflectance accessory for diffuse reflectance spectra over a range of 300-500 nm by employing BaSO4 as reference material. The measured reflectance data (R ) was transformed to the Kubelka-Munk function F(R) from the following equation[137, 138]. (1-R)2 F(R) =
2-6
2R 67
F(R). E1/2 =
(1-R)2
1/2
.E
2-7
2R
The band gap energy for all samples was measured from the plot of (F(R).E)1/2 versus (E) energy of light (hν) in eV. That depended on the intersection of tangent via the point of inflection in the absorption band and the photon energy axis.
2.10 Photocatalytic Oxidation of Methanol The photocatalytic oxidation of methanol was performed in the reaction vessel with glass jacket to control the temperature of reaction by turning the water passing a recycle water bath. The pH of reaction was controlled by addition of 0.1 M HCl or 0.1 M NaOH as required. In an exemplary run, 175 mg of the photocatalyst (bare or metals loaded TiO2) was suspended in 100 ml of aqueous solution of 40 mM methanol. The suspension was stirred at 750 rpm without UV light (dark reaction) for 30 minutes to reach for adsorption equilibrium of methanol molecules on the photocatalyst surface within slowly purged of O2 or N2 as required of experiment. After ending the adsorption time, the reaction mixture was irradiation from upon – outside by UV-A light supplied by Philips Hg lamp (90 W). Different intensities of light were obtained with changing the height of it about the reactor and measured by UV intensity. The rate of photooxidation of methanol was determined by measuring the amount of producing formaldehyde by employing spectrophotometric method [138,139].
2.11 Detection of Formaldehyde by Spectrophotometric Method The concentration of producing formaldehyde was measured by the measurement of absorption spectra by spectroscopic method (Nash method) [138-140]. This method is based on the Hantzch reaction. Scheme 2-2 explains the Nash reaction. 68
Nash method was performed by mixing of 0.5 ml of formed formaldehyde with 0.5 ml of Nash reagent (0.1 ml acetylaceton, 7 gm of ammonium acetate and 0.14 ml of acetic acid in final volume equal 50 ml from deionised water) then heated at 60 oC for 15 min by using water bath. The yellow coloured solution was obtained, which refers to the produce Dihydropyridine (DHP), then diluted to 5mL by deionised water. This solution has a maximum absorption at 412 nm. The absorption of all solutions were determined against blank solution at room temperature by employing (T80+) UV-visible spectrophotometer system. The molar absorptive of dihydropyridine formation was 7104 L. mol-1. cm-1, it is good in agreement with the previously reported value 7700 L. mol-1. cm-1 [138]. The calibration curve is shown in Table 2-7 and plotted in Figure 2-8.
O
O
C CH3
O
C
NH4 +
CH3
OH
+
O
C
C
CH3
Acetic acid
Ammonium acetate
O
-
C
4 H 2O +
OH
C
+
N
by water bath
C
C
C CH 3
C H
H
Heating at 60 oC
H
C
+
Formaldehyde
O H
CH 3
acetyl acetone
Nash Reagent
O
O
CH 3
C CH 3
H
Scheme 2-2: Nash Reaction. Dihydropyridine (DHP) Yellow coloured
Scheme 2-2: Nash Reaction.
69
at 15 min
Table 2-7: Calibration Curve of Formaldehyde. Formaldehyde concentrations x 103/ M 0.000 0.001 0.005 0.010 0.020 0.030 0.060 0.080 0.100
Absorbance at 412 nm 0.000 0.008 0.028 0.066 0.136 0.199 0.418 0.568 0.722
0.8 0.7
R² = 0.999
Absorbance
0.6 0.5 0.4 0.3 0.2 0.1 0 0
0.02
0.04
0.06
0.08
0.1
0.12
Figure 2-8: Calibration Curve for Formaldehyde. 3 Conc. of formaldehyde x E 03(M) / (M) 10 x Conc. of formaldehyde/
Figure 2-8: Calibration Curve for Formaldehyde.
2.12 Light Intensity Measurements The energy of incident photon flux Philips Hg lamp, Tungsten W lamp and xenon lamp were determined by two methods. The first method 70
is based on direct measured for the power of irradiation per unit area (mW/ cm2) [141] by UV-meter (honle), and the measured values were displayed in Table 2-8. The second is the ferrioxalate actinometer (Hatchard- Parker actinometer) method was used to calculate light intensity [142,143]. Table 2-8: Measured Instrumental Photon Flux as Function of the Lamp Height. Height /cm Hg Lamp(90 W) 12.5 W Lamp(100 W) 12.5 Xe Lamp (1000 W) Type of lamps
Instrumental photon flux /mW cm-2 3.49 0.17 22.00
2.12.1 Preparation of Calibration Graph for Ferrous Iron The calibration curve was performed by series of solutions over ranged (1-5) x 10-4 M, using stock solution (1 x 10-3 M) FeSO4. 7 H2O dissolved in 0.1 M H2SO4, 0.5 ml of these solutions mixed within 2.5 ml of 0.1% from 1,10-phenanethrolene, that will produce a trisphenanthroline complex. The absorbance of complex was measured at 510 nm by employing (T80+) UV-visible spectrophotometer system. The molar absorptivity of the complex at 510 nm was 1.045 x 104 L. mol-1. cm-1. It is in good agreement with the previously reported value 1.390 x104 L. mol-1.cm-1 [143]. The calibration data is listed in Table 2-9 and plotted in Figure 2-9.
Table 2-9: Calibration Curve of Fe(II). Fe(II) concentrations x 104 / M 0.000 0.200 0.400 0.600 0.800 1.000
71
Absorbance at 510 nm 0.000 0.218 0.403 0.609 0.828 1.067
1.2 R² = 0.998 1
Absorbance
0.8 0.6 0.4 0.2 0 0
0.2
0.4
0.6
0.8
1
1.2
Conc. of Fe (II) x E 04/ M
\
Figure 2-9: Calibration Curve for Fe(II) as Complex with 1,10Phenanthroline
2.12.2 Theoretical Calculations of Actinometer Solution Reaction The light flux density for Philips Hg lamp (efbe-Schon) and Tungsten W lamp (B22 Clear) were determined after switching on for five minutes to ensure the stability of intensity of lamp. The actinometric method was used to measure the light flux density by using the same photocatalytic reactor with same volume of reaction mixture (100 ml) of actinometric solution. The ferrioxalate actinometric solution was prepared by mixing 40 ml from 0.15 M of Fe2(SO4)3.7 H2O with 50 ml from 0.45 M of K2(C2O4) and 10 ml from 0.05 M of H2SO4 in photocatalytic reactor, then irradiation under atmospheric oxygen. The colour of solution was changed to yellowish green to indicate the production of K3[Fe(C2O4)3].3H2O, as in shown in Figure 2-10.
72
(a)
(b)
Figure 2-10: 10: Image for the Chemical Actinometry Experiment. (a)Hg Lamp Setup Reactor and (b) W Lamp Setup Reactor A 3 cm3 of irradiated solution was collected in regular intervals (5, 5, 10 and 15) min by test tubes and centrifuged (4000 rpm, 5 min) in 800 B centrifuge, 0.5 ml of filtered solutions were added to 2.5 ml of 1,10-phenonethroline phenonethroline to produce redish orange complex comp which absorbed at 510 nm. The photolysis process of ferrioxalate solution produces, according to the following equations [143-146]: hν
[Fe3+(C2O4)3]3-
hν
[Fe3+(C2O4)3]3[Fe3+(C2O4)3]3-+C2O4
. .-
.-
Fe2+ + 2(C2O4) 2- + C2O4
.--
[Fe3+(C2O4)3]3- +C2O4 2[Fe3+(C2O4)3]3-
[Fe2+(C2O4)2]2- + C2O4
[Fe2+(C2O4)2]2- +C2O4 Δ
2-8 .-
.-
Fe2+ + 2CO + 3C2O4
2-9 + 2CO2
2-
2[Fe2+(C2O4)2]2- +C2O4 2- + 2CO2
2-10 2-11 2-12
The light intensity (Io) was calculated by depending on the calculation of the amount of quantity of produced ferrous ions during an irradiation time by using the following equations [143]:
73
2+
moles of Fe
V1 x V3 x A(510 nm) =
2-13
V2 x l x ε(510 nm) x 103 mole of Fe2+
Io =
2-14
Φ λxt
where:V1 is total of irradiation volume (100 cm3), V3 is volume of irradiation solution mixed with 1,10-phenonethroline (3 cm3), V2 is volume of irradiation solution (0.5 cm3), l is optical path length (1 cm), ε is molar absoptivity 1.045 x 104 L. mol-1. cm-1 (which represented a slope in figure 2-9), A510 is average absorbance of solution after irradiation in different internals time with 1,10-phenonethroline, (t) is average of irradiation time and Фλ is quantum yield (1.2) [143]. The measured results of chemical actinometer were displayed in Table 2-10. Table 2-10: Light Intensity Calculated by Chemical Actinometer . Type of lamps
Height /cm
Hg Lamp(90 W) W Lamp(100 W) Xe Lamp (1000 W)
12.5 12.5 -
Light intensity calculated by chemical actinometer x 107 Ens s-1 1.570 1.770 6.134
2.13 Quantum Yield Measurements Quantum yield (Φ) of formaldehyde formation in photocatalysis of methanol was calculated by the following relations [104, 105].
Φ=
Nmol (mol/s)
Rate (mol/s. L) .V(L)
Nphoton (Einstein/s)
Iabsorption (Einstein/s)
=
2-15
where: Iabsorption was calculated depending on light intensity that calculation in section 2.12.2 and equations 2-13 and 2-14.
74
Rate of reaction Φ =
2-16
Rate of absorption of photons
2.14 Photocatalytic Hydrogen Production The photo-dehydrogenation of aqueous solution of methanol was performed in a double jacket Duran glass reactor. A 50 % methanol with 175 mg/100 ml of Pt(0.5) or Au(0.5) loaded on TiO2 were irradiation by xenon lamp (Osram XBO-1000 watt) as UV-B light at light intensity -2
22.00 mW.cm , under Ar environment for 30 min, and temperature equal 298.15 K. Before the irradiation process of this solution, the librated hydrogen gas was determined by using gas chromatography (GCShimadzu 8 A, TCD detector) at zero time, and then irradiation the solution with passing inert gas (Ar). The librated hydrogen gas was determined by GC instrument in series of different times using Ar gas as carrier gas and molecular sieve 5 A packed column. Typical calibration values of hydrogen gas are given in Table 2-11 and plotted in Figure 211. Table 2-11: Calibration Curve of Hydrogen Gas. Hydrogen gas concentrations x 10 6 /mol 0.000 1.022 2.044 3.067 4.089 5.111
75
Area 0.000 0.003 0.007 0.010 0.014 0.017
0.02
y = 0.003x R² = 0.999
0.018 0.016 0.014
Area
0.012 0.01 0.008 0.006 0.004 0.002 0 0
1
2
3
4
5
106 x Conc. of H2 / (mol)
Figure 2-11: Calibration Curve at Different Concentration of Hydrogen Gas.
76
6
CHAPTER THREE RESULTS
77
3.1 Physical Characterizations of Catalysts 3.1.1 Atomic ِAbsorption Spectrophotometry (A.A) The atomic absorption spectroscopy was used to deduce the presence of platinum ions (Pt4+) and gold ions (Au3+) on the surface of TiO2 as Pt atoms at percentage ranged (0.25 - 1.00), and as Au atoms at percentage ranged (0.50 - 4.00). The Pt and Au amounts were measured before and after irradiation with light intensity 1.570 x 10-7 Ens s-1 at 4 h and 8 h respectively. These results were shown in Tables 31 and 3-2. Table 3-1: Loaded Calculations of Pt on TiO2 Surface. Pt % added as H2PtCl6.6 H2O
[ Pt] added /ppm
[ Pt] measured by A.A before irradiation / ppm
[ Pt] measured by A.A after 4 h from irradiation / ppm
0.250 0.500 0.750 1.000
100.238 202.940 301.158 402.790
101.628 203.256 304.884 406.572
nil nil nil nil
Table 3-2: Loaded Calculations of Au on TiO2 Surface. Au % added as HAuCl4.3 H2O
[Au] added /ppm
[ Au] measured by A.A before irradiation /ppm
[ Au] measured by A.A after 8 h from irradiation /ppm
0.50 1.00 2.00 4.00
194.148 375.171 704.732 883.545
192.188 378.354 706.042 881.897
nil nil nil nil
3.1.2 Fourier Transform Infrared Spectroscopy (FTIR) Fourier Transform Infrared spectra were used to detect the changes in the intensity of peaks with the increasing in the amount of Pt and Au loaded on TiO2 surface. The spectra are displayed in Figures 3-1 and 3-2.
78
a
b
c
d
e
Figure 3-1: FT-IR Spectra for Bare and Different Percentage of Pt Loaded on TiO2, at a)Bare TiO2 , b)Pt(0.25)/ TiO2, c)Pt(0.50)/ TiO2 , d)Pt(0.75)/ TiO2 and e)Pt(1.00)/ TiO2
79
a
b
c
d
e
Figure 3-2: FT-IR IR Spectra for Bare and Different Percentage of Au Loaded on TiO2, at a) Bare TiO2, b)Au(0.50)/ TiO2, c)Au(1.00)/ TiO2, d)Au(2.00)/ TiO2 and e)Au(4.00)/TiO2. 80
3.1.3 X-Ray Diffraction Spectroscopy (XRD) XRD was employed to study the phase stability and transformation phase of bare and metallised TiO2 as percentage ranged from 0.25 to 1.00 for Pt and from 0.50 to 4.00 for Au respectively. Figures 3-3 and 3-5, show the XRD patterns of bare and metallised TiO2 at different percentage of loading. The mean crystallite sizes (L) in nm were calculated by using Scherrer's formula (see equation 2-1), and crystallite sizes (Ĺ) in nm by plotting the modified Scherrer's equation (see equations 2-2 and 2-3) and Figures 3-4 and 3-6 that depends on the full width at half maximum (FWHM) for the peak and diffraction (Bragg) angles [133-135]. The calculated results are illustrated by Tables 3-3 and 3-4. 700
Pt(1.0)-TiO2 Pt(0.75)-TiO2 Pt(0.5)-TiO2 Pt(0.25)-TiO2 Bare TiO2
600
Relative intensity (a.u)
500
400
300
200
100
0 20
30
40
50
60
2θ/ (Degrees)
Figure 3-3: XRD Patterns of Bare and Different Percentage of Pt Loaded on TiO2 Surface.
81
3-
3-
a
y = 8.655x - 4.248 R² = 0.968
3.5-
ln β
ln β
3.54-
b
y = 5.536x - 4.237 R² = 0.828
4-
4.5-
4.50
0.02
0.04
0.06
0.08
0
0.1
0.02 0.04 0.06 0.08
ln (1/cos θ) 3-
0.1
ln (1/cos θ) 3-
c
y = 3.576x - 4.184 R² = 0.817
3.5-
y = 8.380x - 4.193 R² = 0.991
d
ln β
ln β
3.5-
4-
4-
4.5-
4.50
0.02
0.04
0.06
0.08
0
0.1
0.02 0.04 0.06 0.08
ln (1/cos θ) 3-
ln β
0.1
ln (1/cos θ) e
y = 1.707x - 4.044 R² = 0.696
3.544.50
0.02
0.04
0.06
0.08
0.1
ln (1/cos θ) Figure 3-4: Modified Scherrer Equation of Bare and Pt Loaded on TiO2 Plot, at a)Bare TiO2, b)Pt(0.25)/ TiO2, c)Pt(0.50)/ TiO2, d)Pt(0.75)/ TiO2 and e)Pt(1.00)/ TiO2.
82
Table 3-3: Mean Crystallite Sizes and Crystallite Sizes of Bare TiO2 and Pt Loaded on TiO2 .
Crystal components
Pt %
Mean Crystallite sizes (L)/nm
TiO2 Hombikate (UV 100)
0.000
11.487
10.132
Pt-TiO2
0.250
10.799
10.021
Pt-TiO2
0.500
9.355
9.503
Pt-TiO2
0.750
10.221
9.589
Pt-TiO2
1.000
10.475
8.262
Au(4.0)-TiO2 Au(2.0)-TiO2 Au(1.0)-TiO2 Au(0.5)-TiO2 Bare TiO2
2000
Relative intensity (a.u)
Crystallite sizes (Ĺ)/nm
1500
1000
500
0 20
30
40
50
60
2θ/ (Degrees)
Figure 3-5: XRD Patterns of Bare and Different Percentage of Au Loaded on TiO2 Surface.
83
3-
3-
a y = 8.655x - 4.248 R² = 0.968
3.5-
ln β
ln β
3.5-
b y = 5.890x - 4.165 R² = 1
4-
4-
4.5-
4.50
0.02
0.04
0.06
0.08
0.1
0
0.02
ln (1/cos θ) 3-
0.06
0.08
0.1
ln (1/cos θ) -3
c
y = 6.245x - 4.056 R² = 0.877
ln β
3.5-
ln β
0.04
4-
y = 0.420x - 4.019 R² = 0.866
d
-3.5 -4
4.5-
-4.5 0
0.02 0.04 0.06 0.08
0.1
0
0.02
0.04
0.06
0.08
0.1
ln (1/cos θ)
ln (1/cos θ) y = 9.969x - 4.400 R² = 0.752
3-
e
ln β
3.544.50
0.02
0.04
0.06
0.08
0.1
ln (1/cos θ)
Figure 3-6: Modified Scherrer Equation of Bare and Au Loaded on TiO2 Plot, at a) Bare TiO2, b)Au(0.50)/ TiO2, c)Au(1.00)/ TiO2, d)Au(2.00)/ TiO2 and e)Au(4.00)/TiO2.
84
Table 3-4: Mean Crystallite Sizes and Crystallite Sizes of Bare TiO2 and Au Loaded on TiO2. Crystal components
Au %
Mean crystallite sizes(L)/nm
Crystallite sizes(Ĺ)/nm
TiO2 Hombikate (UV 100)
0.000
11.487
10.132
Au-TiO2
0.500
10.998
9.324
Au-TiO2
1.000
10.451
8.361
Au-TiO2
2.000
10.175
8.057
Au-TiO2
4.000
12.028
11.794
3.1.4 Atomic Force Microscopy (AFM) AFM images were used to measure particle sizes of bare TiO2, Pt and Au loaded on TiO2 surface. Crystallinity index was calculated by depending particle size and mean crystallite size or crystallite size as indicated in equation 2-5.
85
a
b
c Figure 3-7: AFM Image of Bare TiO2, a) 2- Dimensions Image b) 3- Dimensions image and c) The Histogram.
86
a
b
c Figure 3-8: AFM Image of 0.25 % Pt Loaded on TiO2, a) 2- Dimensions Image b) 3- Dimensions Image and c) The Histogram. 87
a
b
c Figure 3-9: AFM Image of 0.50 % Pt Loaded on TiO2, a) 2- Dimensions Image b) 3- Dimensions Image and c) The Histogram.
88
a
b
c Figure 3-10: AFM Image of 0. 75 % Pt Loaded on TiO2, a) 2- Dimensions Image b) 3- Dimensions Image and c) The Histogram. 89
a
b
c Figure 3-11: AFM Image of 1.00 % Pt Loaded on TiO2, a) 2- Dimensions Image b) 3- Dimensions Image and c) The Histogram. 90
a
b
c Figure 3-12: AFM Image of 0.50 % Au Loaded on TiO2, a) 2- Dimensions Image b) 3- Dimensions Image and c) The Histogram. 91
a
b
c Figure 3-13: AFM Image of 1.00 % Au Loaded on TiO2, a) 2- Dimensions Image b) 3- Dimensions Image and c) The Histogram.
92
a
b
c Figure 3-14: AFM Image of 2.00 % Au Loaded on TiO2, a) 2- Dimensions Image b) 3- Dimensions Image and c) The Histogram. 93
a
b
c Figure 3-15: AFM Image of 4.00 % Au Loaded on TiO2, a) 2- Dimensions Image b) 3- Dimensions Image and c) The Histogram. 94
Table 3-5: Particle Size Measured by AFM and Crystallinity Values of Bare TiO2 and Metalized TiO2. Particle size / nm
*Crystallinity Index
**Crystallinity Index
Average of Crystallinity Index
TiO
80.940
7.046
7.988
7.517
Pt(0.25)/TiO2
63.600
5.889
6.346
6.117
Pt(0. 50)/TiO
2
77.020
8.233
8.104
8.168
Pt(0.75)/TiO2
54.890
5.370
5.724
5.547
Pt(1.00)/TiO2
73.130
6.981
8.851
7.916
Au(0.50)/TiO2
83.490
7.591
8.954
8.272
Au(1.00)/TiO2
71.390
6.830
6.538
6.684
Au(2.00)/TiO2
84.980
8.351
10.547
9.449
Au(4.00)/TiO
75.100
6.243
6.367
6.305
Samples
2
2
*Crystallinity index calculated by divided particle size on mean crystallite size. **Crystallinity index calculated by divided particle size on crystallite size.
3.1.5 Band Gap Energy Measurements UV-Visible diffuse reflectance spectra of bare, Pt(0.5)/TiO2 and Au(0.5)/TiO2 were recorded to investigate the optical band gap energy. The spectra are shown in Figures 3-16 and 3-17 and the results are listed in Table 3-6.
95
5 Pt(0.5)-TiO2 Au(0.5)-TiO2 TiO2
4.5 4 3.5
F(R)
3 2.5 2 1.5 1 0.5 0 300
320
340
360
380
400
420
440
460
480
500
λ/ (nm)
Figure 3-16:UV-Visible Diffuse Reflectance Spectra of Bare , Pt and Au Loaded on TiO2 Surface. 12
Au(0.5)-TiO2 Pt(0.5)-TiO2 TiO2
10
(F(R). E)1/2 /(eV1/2)
8
6
4
2
0 2
2.2 2.4 2.6 2.8
3
3.2 3.4 3.6 3.8
4
4.2 4.4 4.6 4.8
5
E/ (eV)
Figure 3-17:UV-Visible Kubelka -Munk Transformed Diffuse Reflectance Spectra of Bare and Metalized TiO2.
96
Table 3-6: Band Gap Measured by UV-Visible Diffuse Reflectance Spectra of Bare TiO2 and Metalized TiO2. Parameters
TiO2
Pt(0.5)/TiO2
Au(0.5)/TiO2
λ/ nm
377
380
382
Eg/ eV
3.289
3.263
3.246
3.2 Photocatalytic Oxidation of Methanol 3.2.1 Preliminary Experiments A series of experiments had been done at light intensity 1.570x 10-7 Ens s-1. All results are listed in Tables 3-7 , 3-8 , 3-9 and 3-10 and plotted in Figure 3-18 to maintain the essential conditions for photocatalytic reaction.
3.2.1.1 Dark Reaction(Adsorption Reaction) The dark reaction was performed in the absence of UV light under O2 and N2 for adsorbing 40 mM CH3OH using 175 mg of bare TiO2 in 100 ml at 298.15 K and adsorbed time equal 60 min. The results are listed in Table 3-7.
Table 3-7: Concentration of Formaldehyde Formation in Dark Reaction with O2 and N2. Adsorption time/min 0 5 10 20 30 40 50 60
[formaldehyde] x105 / mol L-1 CH3OH +TiO2 + N2 CH3OH +TiO2 + O2 nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil
97
3.2.1.2 Photolysis Reaction The photolysis reaction was done in the absence of catalyst. Table 3-8 shows the results of photolysis for 100 ml of 40 mM CH3OH at 298.15 K, under purged O2 and at irradiation time equal 60 min. Table 3-8: Concentration of Formaldehyde Formation in Photolysis Reaction with Purged O2 . Irradiation time/min 0 5 10 20 30 40 50 60
[formaldehyde] x105 / mol L-1 CH3OH +UV-A light + O2 0.000 0.563 0.784 1.469 1.831 1.972 2.113 2.254
3.2.1.3 Photocatalytic reaction under purged N2 The reaction was carried out in the existence of radiation of 100 ml from 40 mM CH3OH in the presence of 175 mg of bare, Pt (0.5) and Au(0.5) loaded on TiO2 by UV-A light under purged N2 at 298.15 K and at adsorbed time equal 30 min. The results are shown in Table 3-9. Table 3-9: Concentration of Formaldehyde Formation in Photocatalytic Reaction with Purged N2. [formaldehyde] x105 / mol L-1 Irradiation time/min 0 5 10 20 30 40 50 60
CH3OH + TiO2 + UV-A light + N2
CH3OH+Pt(0.5)/TiO2 + UV-A light +N2
CH3OH+Au(0.5)/TiO2 + UV-A light +N2
0.000 0.141 2.958 3.662 5.070 5.352 5.634 6.620
0.000 17.042 34.084 72.535 111.549 149.859 170.140 213.098
0.000 11.550 18.733 20.845 31.127 39.860 45.493
98
3.2.1.4 Photocatalytic Reaction under Purged O2 The results in Table 3-10 indicate that the photocatalytic reaction took place with the same conditions of photocatalytic reaction in the existence of O2 gas. Table 3-10: Concentration of Formaldehyde Formation in Photocatalytic Reaction under Purged O2. [formaldehyde] x105 / mol L-1 Irradiation time/min 0 5 10 20 30 40 50 60
CH3OH+Pt(0.5)/TiO2 + UV-A light +O2
CH3OH+Au(0.5)/TiO2 + UV-A light +O2
0.000 16.338 24.367 40.423 56.620 99.719
0.000 14.789 23.381 48.874 60.986 78.592 107.324 117.606
0.000 10.564 25.775 28.451 44.507 63.381 85.212 99.860
-
160
105 x [Formaldehyde] / (mol L-1 min-1)
CH3OH + TiO2 + UV-A light +O2
Photocatalysis(CH3OH+ Pt(0.5)-TiO2+ hv+N2) Photocatalysis(CH3OH+ Pt(0.5)-TiO2+ hv+ O2)
140
Photocatalysis( CH3OH+ TiO2+hv+ O2) Photocatalysis(CH3OH+ Au(0.5)-TiO2 +hv+ N2)
120
Photocatalysis( CH3OH+Au(0.5)- TiO2+hv+ O2) Photocatalysis( CH3OH+ TiO2+hv+ N2)
100
Photolysis(CH3OH+ hv+O2 ) dark reaction( CH3OH+ TiO2+O2 or N2)
80 60 40 20 0 0
10
20
30
40
50
60
Time /(min)
Figure 3-18: Photocatalytic Oxidation of Methanol on Bare TiO2, Pt(0.5)/TiO2 and Au(0.5)/TiO2 Surfaces under Different Reaction Conditions.
99
3.2.2 Effect of Type of Irradiation In order to investigate the effect of lamp type on the photocatalytic reaction, the Hg-lamp (UV-A) and W-lamp (Visible) in photocatalytic reactors (type 1 and 2) were used to irradiate of 40 mM of CH3OH over 175 mg of bare and Pt (0.5) loaded on TiO2 under purged O2 and N2 at 278.15 K and adsorbed time 30 min. The results were illustrated in Table 3-11 and drawn in Figures 3-19 and 3-20.
Table 3-11: Rate of Formaldehyde Formation in Photocatalytic Reaction with UV- A Light and Visible Light. [formaldehyde] x105 / mol L-1 Irradiation time/min
CH3OH + TiO2 +light + O2
CH3OH+Pt(0.5)/TiO2 + light + O2
CH3OH+Pt(0.5)/TiO2 +light +N2
UV A- light
Visible light
UV A- light
Visible light
UV A- light
Visible light
0
0.000
0.000
0.000
0.000
0.000
0.000
5
14.504
0.422
20.000
0.563
17.605
0.704
10
22.957
0.704
25.633
0.845
26.901
1.408
20
35.070
0.985
38.169
1.267
-
2.535
30
47.183
1.408
51.408
1.690
77.323
3.943
40
56.478
1.549
65.633
1.830
94.225
5.352
50
66.901
1.690
88.169
1.971
106.901
6.760
60
75.774
1.971
104.085
2.253
131.126
7.746
Rate of 5 reaction x 10 /
1.377
0.073
1.868
0.090
2.253
0.131
mol.L1.min-1
100
160
Pt(0.5)-TiO TiO2 +UV-A light+ N2
10 5 x [Formaldehyde ] / (mol L-1)
Pt(0.5)-TiO TiO2 +UV-A light+ O2
140
TiO2 +UV--A light+ O2 Pt(0.5)-TiO TiO2 +Visible light+ N2
120
Pt(0.5)-TiO TiO2 +Visible light+ O2
100
TiO2 + Visible light+ O O2
80 60 40 20 0 0
10
20
30
40
50
60
Time/ (min)
Figure 3-19:: Photocatalytic Oxidation of Methanol on Bare TiO2 and Pt(0.5)/TiO2 Surfaces under Different Types of Lamps amps.
Figure 3-20: Photocatalytic Oxidation of Methanol on Bare TiO2 and Pt(0.5)/TiO2 Surface under Different Types of Lamps.
101
3.2.3 The Effect of Different Parameters on Photocatalytic Oxidation of Methanol by Using Bare and Metalized TiO2 The photocatalytic oxidation of methanol was carried out under purged O2, using bare, Pt and Au loaded on TiO2 surface, with exposure time 30 min and using UV-A light that has light intensity equal 1.570 x 10-7 Ens s-1.
3.2.3.1 Effect of Catalyst Mass Different mass of bare TiO2 and Pt(0.5)/ TiO2 were done in the range (50-300)mg with 40 mM of methanol, at 278.15 K. The experimental results are displayed in Tables 3-12 and 3-13, and drawn in Figures 3-21, 3-22, 3-23 and 3-24.
Time of irradiation/ min
Catalyst mass/ mg
Table 3-12: Rate of Formaldehyde Formation in Photocatalytic Reaction with Different Masses of Bare TiO2 under Purged O2. [formaldehyde] x105 / mol L-1 50
100
150
175
200
250
300
0
0.000
0.000
0.000
0.000
0.000
0.000
0.000
5
6.619
6.901
6.760
14.504
6.760
6.478
4.225
10
10.422
10.422
13.521
22.957
13.661
12.253
8.450
15.211
19.577
19.718
35.070
22.676
22.394
19.774
30
23.380
32.394
31.830
47.183
31.830
30.042
28.169
40
31.690
39.154
46.901
56.478
46.056
40.000
40.704
34.647
49.859
54.678
66.901
52.112
50.422
45.492
46.760
61.126
62.253
75.774
61.549
58.450
54.929
0.761
1.109
1.352
1.377
1.363
1.148
0.954
20
50 60 Rate of reaction x 105/ mol L-1 min-1
102
10 5 x [Formaldehyde ] / (mol L-1)
140 120 100 80 60 40 20 0 0
10
20
30
40
50
60
Figure 3-21: Photocatalytic Oxidation of Methanol on Different Masses of Bare TiO2 Surface.
10 5 x Rate of reaction/ (M min-1)
2.5
2
1.5
1
0.5
0 0
50
100
150
200
250
300
350
Mass of catalyst/ (mg/100 mL)
Figure 3-22: Rate of Reaction as Function of Different Masses of Bare TiO2 Surface, under Purged O2.
103
Catalyst mass/ mg
Table 3-13: Rate of Formaldehyde Formation in Photocatalytic Reaction with Different Masses of Pt(0.5)/ TiO2 under Purged O2. Time of irradiation / min
0 5 10 20 30 40 50 60 Rate of reaction
[formaldehyde] x105 / mol L-1 50
100
150
175
200
250
300
0.000 7.605 13.239 20.281 27.042 31.549 38.591 48.450 0.811
0.000 10.140 22.253 31.126 40.422 45.633 58.028 1.182
0.000 15.633 19.577 34.788 47.887 64.366 77.042 88.169 1.847
0.000 20.000 25.633 38.169 51.408 65.633 88.169 104.084 1.868
0.000 14.788 24.084 32.535 44.788 56.338 67.887 75.070 1.839
0.000 12.957 16.619 30.140 43.521 54.647 60.704 68.873 1.429
0.000 14.366 18.450 28.873 38.028 49.295 60.563 73.239 1.245
5
x 10 /
mol L-1 min-1
10 5 x [Formaldehyde ] / (mol L-1)
140
175 mg 150 mg 200 mg 250 mg 300 mg 100 mg 50 mg
120 100 80 60 40 20 0 0
10
20
30
40
50
60
Time/( min)
Figure 3-23: Photocatalytic Oxidation of Methanol on Different Masses of Pt(0.5)/ TiO2 Surface. 104
10 5 x Rate of reaction/ (M min-1)
2.5
2
1.5
1
0.5
0 0
50
100
150
200
250
300
350
Mass of catalyst/ (mg/100 mL)
Figure 3-24: Rate of Reaction as Function of Different Masses of Pt(0.5)/TiO2 Surface, under Purged O2.
3.2.3.2 Effect the percentage Loaded Metals Metals loaded on TiO2 as Pt% (0.25-1.00) and Au% (0.50-4.00) were prepared in order to increase the photocatalytic activity of 40 mM of methanol in 100 ml. Catalyst masses of 175 mg were used for all prepared catalysts at 278.15 K throughout the experiments. The results are listed in Tables 3-14 and 3-15, and plotted in Figures 3-25, 3-26, 3-27 and 3-28.
105
Table 3-14: Rate of Formaldehyde Formation in Photocatalytic Reaction with Bare and Different Percentage of Pt Loaded on TiO2 Surface under Purged O2.
[formaldehyde] x105 / mol L-1 Pt %
Time of irradiation/ min
0.00
0.25
0.50
0.75
1.00
0
0.000
0.000
0.000
0.000
0.000
5
14.504
7.464
20.000
9.710
7.887
10
22.957
17.323
25.633
16.760
15.492
20
35.070
-
38.169
25.211
29.014
30
47.183
62.676
51.408
42.535
38.450
40
56.478
82.253
65.633
50.563
42.394
50
66.901
85.915
88.169
58.591
54.225
60
75.774
94.647
104.085
74.507
57.605
1.377
1.684
1.868
1.729
1.475
Rate of reaction 5
x 10 /
mol L-1 min-1
106
Pt(0.50)-TiO2 Pt(0.25)-TiO2 TiO2 Pt(0.75)-TiO2 Pt(1.00)-TiO2
10 5 x [Formaldehyde ] / (mol L-1)
120 100 80 60 40 20 0 0
10
20
30
40
50
60
70
Time/( min)
Figure 3-25: Photocatalytic Oxidation of Methanol on Bare TiO2 and Different Percentage of Pt Loaded on TiO2 Surface.
10 5 x Rate of reaction/ (M min-1)
2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 0
0.25
0.5
0.75
1
Pt % loaded on TiO2 surface
Figure 3-26: Rate of Reaction as Function of Bare TiO2 and Different Percentage of Pt on TiO2 Surface, under Purged O2. 107
Table 3-15: Rate of Formaldehyde Formation in Photocatalytic Reaction with Bare and Different Percentage of Au Loaded on TiO2 Surface under Purged O2. [formaldehyde] x105 / mol L-1 Au%
Time of irradiation/ min
0.00
0.50
1.00
2.00
4.00
0
0.000
0.000
0.000
0.000
0.000
5
14.504
3.662
4.648
4.648
3.535
10
22.957
8.592
9.015
11.831
8.028
20
35.070
17.888
16.057
24.084
18.591
30
47.183
24.930
23.381
34.507
28.028
40
56.478
31.831
34.226
45.070
37.000
50
66.901
39.71
41.409
52.535
43.521
60
75.774
45.77
56.339
60.704
53.380
1.377
0.833
0.907
1.132
0.783
Rate of reaction 5
x 10 / -1
mol L min
-1
108
120
10 5 x [Formaldehyde ] / (mol L-1)
TiO2 Au(2.0)-TiO2
100
Au(4.0)-TiO2 Au(1.0)-TiO2
80
Au(0.5)-TiO2 60
40
20
0 0
10
20
30
40
50
60
Time/( min)
Figure 3-27: Photocatalytic Oxidation of Methanol on Bare TiO2 and Different Percentage of Au Loaded on TiO2 Surface.
10 5 x Rate of reaction/ (M min-1)
2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 0
1
2
3
4
5
Au % loaded on TiO2 surface
Figure 3-28: Rate of Reaction as Function of Bare TiO2 and Different Percentage of Au on TiO2 Surface, under Purged O2.
109
3.2.3.3 Effect for Methanol Concentration These experiments were carried out at different concentrations of methanol in the ranged (10- 40) mM, 175 mg of bare TiO2 and Pt (0.5)/TiO2, pH 5.3 and 6.5 respectively. The temperature was 278.15 K. The results are given in Tables 3-16 and 3-17, and plotted in Figures 329, 3-30, 3-31 and 3-32.
[CH3OH] / mM
Table 3-16: Rate of Formaldehyde Formation in Photocatalytic Reaction with Different Concentrations of Methanol and Bare TiO2 under Purged O2. Time of irradiation / min
[formaldehyde] x105 / mol L-1 10
20
30
40
0
0.000
0.000
0.000
0.000
5
2.676
4.507
5.774
14.504
10
5.211
9.169
12.394
22.957
20
9.859
16.056
20.704
35.070
30
15.915
25.492
28.591
47.183
40
20.563
34.225
36.338
56.478
50
25.492
42.112
45.633
66.901
60
30.140
49.154
58.591
75.774
0.523
0.913
1.222
1.377
Rate of reaction 5
x 10 /
mol L-1 min-1
110
10 5 x [Formaldehyde ] / (mol L-1)
120
40 mM CH3OH 30 mM CH3OH 20 mM CH3OH 10 mM CH3OH
100
80
60
40
20
0 0
10
20
30
40
50
60
Time/ (min)
Figure 3-29: Photocatalytic Oxidation of Different Concentration of Methanol on Bare TiO2 Surface.
10 5 x Rate of reaction/ (M min-1)
2.5
2
1.5
1
0.5
0 0
10
20
30
40
50
10 3 x Conc. of Methanol / (M)
Figure 3-30: Rate of Reaction as Function of Different Concentration of Methanol with Bare TiO2 Surface, under Purged O2.
111
Table 3-17: Rate of Formaldehyde Formation in Photocatalytic Reaction with Different Concentrations of Methanol and Pt(0.5)/ TiO2 under Purged O2. [formaldehyde] x105 / mol L-1
[CH3OH] / mM
Time of irradiation / min
0 5 10 20 30 40 50 60
10
20
30
40
0.000 3.380 6.478 10.563 17.042 22.676 28.169 35.07
0.000 4.647 10.281 18.309 28.732 36.478 47.605 58.309
0.000 7.746 13.943 22.53 35.070 44.084 57.323 69.577
0.000 20.000 25.633 38.169 51.408 65.633 88.169 104.084
0.653
1.008
1.425
1.868
Rate of reaction 5
x 10 /
mol L-1 min-1
10 5 x [Formaldehyde ] / (mol L-1)
120 40 mM CH3OH 30 mM CH3OH
100
20 mM CH3OH 10 mM CH3OH
80 60 40 20 0 0
10
20
30
40
50
60
Time/ (min)
Figure 3-31: Photocatalytic Oxidation of Different Concentration of Methanol on Pt(0.5)/ TiO2 Surface.
112
10 5 x Rate of reaction/ (M min-1)
2.5
2
1.5
1
0.5
0 0
10
20
30
40
50
10 3 x Conc. of Methanol / (M)
Figure 3-32: Rate of Reaction as Function of Different Concentration of Methanol with Pt(0.5)/ TiO2 Surface, under Purged O2.
3.2.3.4 Effect the pH of Solution Initial pH of 40 mM solution of methanol was investigated over broad pH range of 1-12, using 175 mg of bare TiO2 and Pt(0.5)/TiO2, at 278.15 K, under purged O2 with a light intensity 1.570 x 10-7 Ens s-1. The experimental results are shown in Tables 3-18 and 3-19, and drawn in Figures 3-33, 3-34, 3-35 and 3-36.
113
Table 3-18: Rate of Formaldehyde Formation in Photocatalytic Reaction with Different Values of Initial pH Solution with Bare TiO2 under Purged O2. [formaldehyde] x105 / mol L-1
pH
Time of irradiation / min
1.04 0.000
2.05 0.000
4.18 0.000
5.30 0.000
6.04 0.000
7.06 0.000
8.12 0.000
9.17 0.000
12.03 0.000
5
1.830
4.366
6.619
14.504
17.183
15.492
16.760
13.239
7.323
10
3.098
7.464
12.253
22.957
24.225
29.436
21.267
17.183
11.267
20
3.661
11.549
31.408
35.070
32.535
44.140
30.985
27.042
16.197
30
5.915
14.225
42.253
47.183
49.295
64.239
39.295
36.901
20.704
40
6.338
18.309
55.352
56.478
63.239
73.802
47.183
43.521
25.492
50
9.295
22.957
62.816
66.901
69.014
95.211
55.211
51.971
29.577
60
10.563
26.760
77.323
75.774
79.154
115.070
62.957
61.408
39.436
0.321
0.771
1.245
1.377
2.625
2.974
2.371
1.904
1.194
0
Rate of reaction 5
x 10 /
mol L-1 min-1
114
200
pH=7.06 pH=6.04 pH= 5.3 pH= 4.18 pH= 8.12 pH= 9.17 pH= 12.03 pH= 2.05 pH= 1.04
10 5 x [Formaldehyde ] / (mol L-1)
180 160 140 120 100 80 60 40 20 0 0
10
20
30
40
60
50
Time/( min)
Figure 3-33: Photocatalytic Oxidation of Methanol on Bare TiO2 Surface, at Different Initial pH of Solution.
10 5 x Rate of reaction/ (M.min-1)
4 3.5 3 2.5 2 1.5 1 0.5 0 0
2
4
6
8
10
12
14
Initial pH of solution
Figure 3-34: Rate of Reaction as Function of Initial pH of Solution on TiO2 Surface, under Purged O2. 115
Table 3-19: Rate of Formaldehyde Formation in Photocatalytic Reaction with Different Values of Initial pH Solution with Pt(0.5)/ TiO2 Surface under Purged O2. [formaldehyde] x105 / mol L-1 pH
Time of irradiation / min 0
1.09
2.09
3.09
4.12
6.50
7.08
9.10
10.16
11.20
12.15
0.000
0.000
0.000
0.000
0.000
0.000
0.000
5
4.929
12.676
13.661
10.422
20.000
16.056
13.521
0.000 5.9154
0.000 6.901
0.000 4.225
10
6.901
19.859
25.070
17.822
25.633
32.112
20.845
11.830
12.676
6.197
20
12.253
29.577
32.253
33.943
38.169
56.478
34.788
21.267
18.873
13.661
30
16.056
42.253
41.408
49.859
51.408
70.98
40.985
35.211
20.140
16.338
40
18.591
53.380
55.915
67.323
65.633
84.647
58.169
27.464
20.000
50
26.478
57.887
69.014
82.535
88.169
96.478
62.394
45.633
32.676
26.760
60
30.704
-
78.169
107.042
104.084
107.746
71.408
57.323
39.859
32.816
0.749
1.625
1.836
1.842
1.868
3.211
1.851
1.183
1.026
0.664
42.816
Rate of reaction 5
x 10 /
mol L-1 min-1
10 5 x [Formaldehyde ] / (mol L-1)
200
pH= 7.08 pH= 6.5 pH= 4.12 pH= 3.09 pH= 9.10 pH= 2.09 pH= 10.16 pH= 11.20 pH= 12.15 pH= 1.09
180 160 140 120 100 80 60 40 20 0 0
10
20
30
40
50
60
Time/( min)
Figure 3-35: Photocatalytic Oxidation of Methanol on Pt(0.5)/TiO2 Surface, at Different Initial pH of Solution. 116
10 5 x Rate of reaction/ (M min-1)
4 3.5 3 2.5 2 1.5 1 0.5 0 0
2
4
6
8
10
12
14
Initial pH of solution
Figure 3-36: Rate of Reaction as Function of Initial pH of Solution on Pt(0.5)/TiO2 Surface, under Purged O2.
3.2.3.5 Effect of Temperature Reaction was followed at five temperatures in the range of (278.15298.15) K, 40 mM of methanol, 175 mg of bare TiO2 and Pt(0.5)/TiO2, at pH equal 5.3 and 6.5 respectively. The order of reaction is zero order, thereby the rate of reaction is equal the rate of constant. The results are listed in Tables 3-20 and 3-21, and shown in Figures 3-37, 3-38, 3-39 and 3-40.
117
Table 3-20: Rate of Formaldehyde Formation in Photocatalytic Reaction with Different Temperatures with Bare TiO2 under Purged O2.
Time of irradiation / min
T/ K
[formaldehyde] x105 / mol L-1
278.15
283.15
288.15
293.15
298.15
0
0.000
0.000
0.000
0.000
0.000
5
14.504
8.169
14.789
14.366
16.338
10
22.957
18.169
23.099
23.803
24.367
20
35.070
29.014
39.296
43.662
40.423
30
47.183
39.859
53.381
55.352
56.62
40
56.478
52.535
63.944
71.268
82.676
50
66.901
59.718
76.338
80.704
99.719
60
75.774
67.324
92.254
97.606
124.93
1.377
1.780
2.077
2.478
2.602
Rate of reaction 5
x 10 /
mol L-1 min-1
118
10 5 x [Formaldehyde ] / (mol L-1)
140 298.15 K 293.15 K 288.15 K 278.15 K 283.15 K
120 100 80 60 40 20 0 0
10
20
30
40
50
60
Time/( min)
Figure 3-37: Photocatalytic Oxidation of Methanol on Bare TiO2 Surface, at Different Temperatures. ln (10 5 x Rate of reaction)/ (M min-1)
12.8
12.6
y = -2.672x + 21.48
12.4
12.2
12
11.8
11.6 3.25
3.3
3.35
3.4
3.45
3.5
3.55
3.6
(103 K/ T )
Figure 3-38: Temperature Dependence for the Photocatalytic Oxidation of Methanol on Bare TiO2 Surface. 119
3.65
Table 3-21: Rate of Formaldehyde Formation in Photocatalytic Reaction with Different Temperatures with Pt(0.5)/ TiO2 under Purged O2.
Time of irradiation / min
T/K
[formaldehyde] x105 / mol L-1
0 5 10 20 30 40 50 60
278.25
283.15
288.15
293.15
298.15
0.000 20.000 25.633 38.169 51.408 65.633 88.169 104.084
0.000 10.282 19.282 30.141 39.437 49.578 65.493 81.409
0.000 10.141 22.113 31.831 41.690 59.859 71.690 81.831
0.000 18.451 32.395 55.071 61.831 70.845 74.507 103.24
0.000 14.789 23.381 48.874 60.986 78.592 107.324 117.606
1.868
1.953
2.174
2.366
2.448
Rate of reaction 5
x 10 /
mol L-1 min-1
140.000
10 5 x [Formaldehyde ] / (mol L-1)
298.15 K 120.000
293.15 K 278.15 K
100.000
288.15 K 80.000
282.15 K
60.000 40.000 20.000 0.000 0
10
30
20
40
50
60
Time/( min)
Figure 3-39: Photocatalytic Oxidation of Methanol on Pt(0.5)/ TiO2 Surface, at Different Temperatures.
120
ln (10 5 x Rate of reaction)/ (M min-1)
12.8 y = -1.221x + 16.51 12.6 12.4 12.2 12 11.8 11.6 3.25
3.3
3.35
3.4
3.45
3.5
3.55
3.6
3.65
(103 K/ T )
Figure 3-40: Temperature Dependence for the Photocatalytic Oxidation of Methanol on Pt(0.5)/TiO2 Surface.
3.2.4 The Effect of Different Parameters on Photocatalytic Dehydrogenation of Methanol by Using Bare and Metalized TiO2 The photocatalytic dehydrogenation of methanol was performed by employing methanol, with bare and metals loaded on TiO2 surface, at 278.15 K under purged N2 with light intensity 1.570 x 10-7 Ens s-1.
3.2.4.1 Effect of mass of catalyst The action mass of Pt(0.5)/TiO2 was investigated over range (50300) mg in 40 mM of methanol, at 278.15 K. The results are displayed in Table 3-22, and shown in Figures 3-41 and 3-42.
121
Catalyst mass/ mg
Table 3-22: Rate of Formaldehyde Formation in Photocatalytic Reaction with Different Concentrations of Pt(0.5)/ TiO2 under Purged N2. Time of irradiation / min
[formaldehyde] x105 / mol L-1 50
100
150
175
200
250
300
0
0.000
0.000
0.000
0.000
0.000
0.000
0.000
5
6.971
8.732
8.450
17.605
19.718
11.69
15.211
10
15.633
14.366
-
26.901
31.408
18.450
21.54
20
25.070
28.028
40.985
54.084
50.563
38.169
32.676
30
31.830
43.943
63.661
77.323
72.112
48.591
42.253
40
40.985
61.690
85.492
94.225
79.577
63.098
56.056
50
58.309
78.873
93.380
106.901
99.014
86.619
62.394
60
67.887
80.563
110.985 131.126 109.577
97.746
77.042
1.086
1.534
1.664
1.336
Rate of reaction 5
x 10 /
2.116
mol L-1 min-1
122
2.253
2.238
10 5 x Rate of reaction/ (M min-1)
160 140 120 100 80 60 40 20 0 0
10
20
30
40
50
60
Mass of catalysts/ (mg/100 mL)
10 5 x Rate of reaction/ (M min-1)
Figure 3-41: Photocatalytic Dehydrogenation of Methanol on Different Mass of Pt(0.5)/ TiO2 Surface. 2.5
2
1.5
1
0.5
0 0
50
100
150
200
250
300
Mass of catalyst/ (mg/100 mL)
Figure 3-42: Rate of Reaction as Function of Different Masses of Pt(0.5)/TiO2, under Purged N2.
123
3.2.4.2 Effect the Percentage of Pt and Au Loaded in Presence of N2 In order to depress the recombination process in the photocatalytic activity of methanol by metalized TiO2 as Pt% (0.25-1.00) and Au% (0.50-4.00). The results are listed in Tables 3-23 and 3-24, and plotted in Figures 3-43, 3-44, 3-45 and 3-46.
Table 3-23: Rate of Formaldehyde Formation in Photocatalytic Reaction with Bare and Different Percentage of Pt Loaded on TiO2 Surface under Purged N2.
Pt %
Time of irradiation / min
[formaldehyde] x105 / mol L-1 0.00
0.25
0.50
0.75
1.00
0
0.000
0.000
0.000
0.000
0.000
5
0.704
10.985
17.605
12.816
9.014
10
2.113
21.126
26.901
24.929
19.577
20
3.239
39.436
-
43.239
34.647
30
4.225
56.478
77.323
66.901
56.619
40
4.789
79.295
94.225
91.830
72.957
50
5.070
95.492
106.901
113.239
87.971
60
5.352
113.380
131.126
126.197
107.887
0.107
1.917
2.412
2.201
1.799
Rate of reaction 5
x 10 /
mol L-1 min-1
124
Pt(0.50)-TiO2
10 5 x [Formaldehyde ] / (mol L-1)
160
Pt(0.75)-TiO2
140
Pt (0.25)- TiO2 120
Pt(1.00)-TiO2 TiO2
100 80 60 40 20 0 0
10
20
30
40
60
50
Time/( min)
Figure 3-43: Photocatalytic Dehydrogenation of Methanol on Bare TiO2 and Different Percentage of Pt Loaded on TiO2 Surface.
10 5 x Rate of reaction/ (M min-1)
3 2.5 2 1.5 1 0.5 0 0
0.25
0.5
0.75
Pt % loaded on TiO2 surface
Figure 3-44: Rate of Reaction as Function of Bare and Different Percentage of Pt on TiO2 Surface, under Purged N2. 125
1
Table 3-24: Rate of Formaldehyde Formation in Photocatalytic Reaction with Bare and Different Percentage of Au Loaded on TiO2 Surface under Purged N2.
Time of irradiation / min
Au%
[formaldehyde] x105 / mol L-1
0.00
0.50
1.00
2.00
4.00
0
0.000
0.000
0.000
0.000
0.000
5
0.704
4.789
10.141
9.578
9.296
10
2.113
8.733
15.353
22.536
13.521
20
3.239
20.704
26.902
35.493
30.704
30
4.225
33.521
42.817
52.958
45.493
40
4.789
44.366
53.662
69.014
60.563
50
5.070
54.789
70.000
85.212
73.662
60
5.352
61.690
84.789
99.860
89.859
0.107
1.068
1.398
1.707
1.496
Rate of reaction 5
x 10 / -1
mol L min
-1
126
10 5 x [Formaldehyde ] / (mol. L-1)
120.000
Au(2.0)-TiO2 Au(4.0)-TiO2
100.000
Au(1.0)-TiO2 Au(0.5)-TO2
80.000 TiO2 60.000
40.000
20.000
0.000 0
10
20
30
40
50
60
Time/( min)
Figure 3-45: Photocatalytic Dehydrogenation of Methanol on Bare TiO2 and Different Percentage of Au Loaded on TiO2 Surface.
10 5 x Rate of reaction/ (M min-1)
3 2.5 2 1.5 1 0.5 0 0
1
2
3
4
Au% loaded on TiO2 surface
Figure 3-46: Rate of Reaction as Function of Bare TiO2 and Different Percentage of Au on TiO2 Surface, under Purged N2.
127
5
3.2.4.3 Effect for methanol concentration Different concentrations of methanol in ranged (10-40) mM were supplied in volume 100 ml, with 175 mg of Pt (0.5)/TiO2, at pH 6.5, 278.15 K. All results occurred in Table 3-25, and shown in Figures 347and 3-48.
[formaldehyde] x105 / mol L-1 mM
Time of irradiation/ min
[CH3OH]/
Table 3-25: Rate of Formaldehyde Formation in Photocatalytic Reaction with Different Concentrations of Methanol and Pt(0.5)/ TiO2 under Purged N2.
10
20
30
40
0
0.000
0.000
0.000
0.000
5
4.225
8.873
10.140
17.605
10
7.042
11.267
18.028
26.901
20
14.084
24.647
34.045
-
30
21.126
39.154
53.239
77.323
40
28.169
51.690
67.323
94.225
50
38.028
59.154
85.774
106.901
60
43.661
71.830
98.450
131.126
0.729
1.223
1.687
2.253
Rate of reaction 5
x 10 /
mol L-1 min-1
128
160
40 mM CH3OH 30 mM CH3OH 20 mM CH3OH 10 mM CH3OH
10 5 x [Formaldehyde ] / (mol L-1)
140 120 100 80 60 40 20 0 0
10
20
30
40
50
60
Time/ (min)
Figure 3-47: Photocatalytic Dehydrogenation of Different Concentrations of Methanol on Pt(0.5)/ TiO2 Surface.
10 5 x Rate of reaction/ (M min-1)
2.5
2
1.5
1
0.5
0 0
10
20
30
40
50
10 3 x Conc. of Methanol / (M)
Figure 3-48: Rate of Reaction as Function of Different Concentration of Methanol with Pt(0.5)/ TiO2 Surface, under Purged N2. 129
3.2.4.4 Effect the pH of solution The initial pH of solution was determined, at broad pH range of 111 employed 40 mM methanol, with 175 mg of Pt(0.5)/TiO2, at 278.15 K. The results are displayed in Table 3-26, and plotted in Figures 3-49 and 3-50.
Table 3-26: Rate of Formaldehyde Formation in Photocatalytic Reaction with Different Values of Initial pH Solution with Pt(0.5)/ TiO2 under Purged N2. [formaldehyde] x105 / mol L-1 pH
Time of irradiation / min
0 5 10 20 30 40 50 60
1.16 0.000
2.17 0.000
3.07 0.000
4.05 0.000
6.50 0.000
7.15 0.000
9.04 0.000
10.09 0.000
11.1 0.000
1.830
12.253
10.845
15.492
17.605
23.802
17.605
16.197
12.676
3.943
18.591
19.014
28.450
26.901
40.281
26.901
26.765
23.802
8.591
31.971
36.901
48.873
54.084
77.464
54.084
46.901
49.295
12.816
47.183
550.000
59.577
77.323
111.408
77.323
72.957
68.732
16.760
60.704
70.704
78.028
94.225
132.394
108.309
95.633
87.183
20.140
71.830
92.676
95.070
106.901
169.436
120.985
120.563
110.985
24.380
84.647
108.59
117.746
131.126
186.056
137.887
144.647
144.507
2.194
2.253
3.540
2.674
2.412
2.260
Rate of reaction 5
x 10 /
0.421
1.400
1.755
mol L-1 min-1
130
pH= 7.15
250
10 5 x [Formaldehyde ] / (mol L-1)
pH= 9.04 pH =10.09
200
pH= 11.1 pH= 6.5 pH= 4.05
150
pH= 3.07 pH= 2.17 pH= 1.16
100
50
0 0
10
20
30
40
50
60
Time/( min)
Figure 3-49: Photocatalytic Dehydrogenation of Methanol on Pt (0.5)/ TiO2, at Different Initial pH of Solution. 4
10 5 x Rate of reaction/ (M min-1)
3.5 3 2.5 2 1.5 1 0.5 0 0
2
4
6
8
10
12
Initial pH of solution
Figure 3-50: Rate of Reaction as Function of Initial pH of Solution on Pt(0.5)/TiO2, under Purged N2. 131
3.2.4.5 Effect of Temperature The effect of temperature in the photocatalytic dehydrogenation of 40 mM of methanol solution was studied in the ranged (278.15- 298.15) K, with 175 mg Pt(0.5)/TiO2, at pH 6.5. The order of reaction is zero order, thereby the rate of reaction is equal the rate of constant. The results are listed in Table 3-27, and plotted in Figures 3-51 and 3-52 as ln(105 rate of reaction) against (1000/T).
Table 3-27: Rate of Formaldehyde Formation in Photocatalytic Reaction with Different Temperatures with Pt(0.5)/ TiO2 under Purged N2.
T/K
Time of irradiation/ min
[formaldehyde] x105 / mol L-1
278.15
283.15
288.15
293.15
298.15
0
0.000
0.000
0.000
0.000
0.000
5
17.605
20.422
16.901
18.450
17.042
10
26.901
34.084
27.605
35.352
34.084
20
54.084
56.619
54.084
60.140
72.535
30
77.323
77.887
78.873
86.478
111.549
40
94.225
119.014
109.718
119.436
149.859
50
106.901
134.507
138.169
155.493
170.140
60
131.126
157.183
166.478
186.338
213.0986
2.253
2.721
2.750
3.065
3.545
Rate of reaction x 105/ mol L-1 min-1
132
250
298.15 K
10 5 x [Formaldehyde ] / (mol L-1)
293.15 K 283.15 k
200
288.15 K 278.15 K
150
100
50
0 0
10
20
30
40
50
60
Time/( min)
Figure 3-51: Photocatalytic Dehydrogenation of Methanol on Pt(0.5)/ TiO2 Surface, at Different Temperatures.
ln (10 5 x Rate of reaction)/ (M min-1)
12.8 y = -1.709x + 18.48 12.6
12.4
12.2
12
11.8
11.6 3.3
3.35
3.4
3.45
3.5
3.55
3.6
(103 K/ T )
Figure 3-52: Temperature Dependence for the Photocatalytic Dehydrogenation of Methanol on Pt(0.5)/TiO2 Surface. 133
3.65
3.2.5 Hydrogen production by Photocatalytic Dehydrogenation of Methanol using Metalized TiO2 The results of evaluating hydrogen gas were measured for photocatalytic dehydrogenation of 100 ml from 3% of methanol, with 175 mg of pt(0.5)/TiO2 and Au(0.5)/TiO2, at 298.15 K, purged Ar, by using photocatalytic reactor type 3, that contained on Xe lamp which have light intensity 6.134 x 10-7 7 Ens s-1. The experimental results are displayed in Table 3-28 and plotted in Figure 3-53.
Table 3-28: Amounts of Hydrogen Production in Photocatalytic Dehydrogenation of Methanol with Pt(0.5)/ TiO2 and Au(0.5)/TiO2,under Purged Ar.
Time of irradiation
Hydrogen production x106 / (mol)
/ min
Pt(0.5)/TiO2
Au(0.5)/TiO2
0
0.000
0.000
10
1.714
0.685
15
2.571
0.857
30
-
1.285
45
6.285
2.714
60
8.857
3.571
134
10 Pt(0.5)-TiO2 9
Au(0.5)-TiO2
10 6x H2 production/ (mol)
8 7 6 5 4 3 2 1 0 0
10
20
30
40
50
Time/(min)
Figure 3-53: Hydrogen Production in Photocatalytic Dehydrogenation of Methanol with Pt(0.5) and Au(0.5) Loaded on TiO2 .
135
60
CHAPTER FOUR DISCUSSION
136
4.1 Physical Characterizations of Catalysts 4.1.1 Atomic Absorption Spectrophotometry (A.A. ) The required time for complete photodeposition of gold on TiO2 is double than that required time for complete photodeposition of platinum on TiO2. These effects can be related to the difference in their work function values (Ф).The work function for the 111 crystal plane for gold is 5.31eV and for platinum is 5.93 eV [68, 69], where the work function values for titanium dioxide are in the range 4.6- 4.7 eV[68]. In photodeposition process of Pt and Au on TiO2 surface was performed in four and three separated steps respectively, that was described in the following mechanisms[147, 148]. TiO2 + hν PtIV + 4e-CB AuIII + 3e-CB
e-CB + h+VB Pt0 Au0
4-1 4-2 4-3
4.1.2 Fourier Transform Infrared Spectroscopy (FTIR) The Fourier Transform Infrared spectra of bare and metalized TiO2 are depicted in Figures 3-1 and 3-2. That illustrates peaks corresponding to stretching vibration of O-H bond of free water molecules around 33503450 cm-1, and bending vibration of O-H bond of chemisorbed water molecules around 1620-1630 cm-1. The absorption intensity of surface OH in TiO2 was regularly increased with the increasing of the percentage of metals content. These findings are in good agreement with previous findings [149-151]. The broad intense band below 1200 cm-1 is due to Ti-O-Ti bridging stretching mode in crystal. This peak appears unsymmetrical valley with
137
increasing metal (M) loaded on TiO2 and has centred around 580 cm-1. These changes the attitude to form Ti-O-M vibrations [152,153]. The strong band at 3621, 3645 and 3696 cm-1 in all spectra, characteristic of tetrahedral coordinate vacancies and designated as 4Ti4+OH, besides two bands at 3765 and 3840 cm-1. These attitudes to octahedral vacancies and designated 6Ti3+-OH. In finding metals loaded on TiO2, the peaks of
6Ti
3+
-OH are not observed, due to metal acts as
collector of electrons, which leads to decrease the ability to form Ti3+OH[125,154]. Ti4+-OH + eM + e-
Ti3+-OH M-
4-4 4-5
4.1.3 X-Ray Diffraction Spectroscopy (XRD) From the XRD spectra inset in Figures 3-3 and 3-5, the XRD data were used in plotted Figures 3-4 and 3-6. No peaks correspond to Pt and Au were detected by XRD spectra, due to the low loading percentage and high dispersion of metals present in the prepared Pt/TiO2 and Au/TiO2 [151,154]. Based on the Figure 4-1 (a) and (b), the calculated values of mean crystallite sizes and crystallite sizes of bare TiO2 were more than those values for metalized TiO2. This is due to the location and incorporation of Pt(IV) and Au(III) with Ti(III) in TiO2 lattice. Moreover, the ionic radius of Pt(IV) (0.63 Å) is relatively smaller than that of Ti(III) (0.67Å). While the Au (III) incorporated with Ti (III) in TiO2 lattice, although it has high an ionic radius (0.85 Å) [155-158].
138
Calculated Sizes from XRD analysis /(nm)
the Pt/TiO2 catalyst. 14
Mean crystallite sizes
(a)
12
crystallite sizes
10 8 6 4 2 0
0.00 1
0.25 2
0.50 3
0.75 4
1.00 5
Calculated Sizes from XRD analysis /(nm)
Pt % loaded on TiO2 surface
14 Mean crystallite sizes Crystallite sizes
(b) 12 10 8 6 4 2 0
0.00 1
0.50 2
1.00 3
2.00 4
4.00 5
Au % loaded on TiO2 surface
Figure 4-1: Relationship Between Calculated Sizes from XRD Analysis and Different Percentage of (a) Pt Loaded on TiO2 Surface and (b) Au Loaded on TiO2 Surface Plot. 4.1.4 Atomic Force Microscopy (AFM) AFM images indicate that the shape of bare and metalized TiO2 are spherical. The values of particle sizes for all samples are found to be more than values for mean crystallite size and crystallite size, because particles could be formed of several crystallite sizes (grains)[159]. (grain The results in Table 3-5 refer to the particle sizes contain from 9 to 11 crystals. 139
The maximum values of average crystallinity index for Pt(0.5)/TiO2 and Au(2.0)/TiO2 are found equal to 8.168 and 9.449 respectively. That referred to suppress a number of crystal defects through decreasing the amorphous phase present in the TiO2 and rising the photocatalytic activity of TiO2[124]. 4.1.5 Band Gap Energy Measurements The results of UV-Visible diffuse reflectance spectra of bare and metalized TiO2 and UV-Visible Kubelka - Munk transformed diffuse reflectance spectra of bare and metalized TiO2 that indicate that the band gap of bare TiO2 is larger than that of metalized TiO2. The effective band gap of TiO2 3.289 eV is reduced to 3.263 eV and 3.246 eV for Pt(0.5)/TiO2 and Au(2.0)/TiO2 respectively. This is related to admix d orbital of Pt and Au with TiO2 to produce a new intermediate energy levels inside TiO2 band gap. These results were caused a decreasing in Fermi level of TiO2 and depressing the charge transfer transition from the valence band (mainly formed by 2p orbital of the oxide anions) to the conduction band (mainly formed by 3d t2g orbital of Ti4+cations), thereby, the excitation of metalized TiO2 occurs with lower energy radiation (red shift)[160]. 4.2 Photocatalytic Oxidation of Methanol 4.2.1 Preliminary Experiments Control experiments indicate that the existence of light, electron scavenger (O2 or metal) and titanium dioxide are essential for effective photooxidation or photodehydrogenation of methanol. When the photocatalytic reaction of methanol was occurred in existence of N2 gas, the obtained grey colloid solution, indicates the consumption of the lattice oxygen of TiO2 and no reaction was produced[18,19, 67, 160-163]. Equations 4-6, 4-7 and 4-8 show that their is no reaction (N.R) occurred)[19]. TiO2 or M/TiO2+ CH3OH+ O2 or N2
N.R
4-6
hν + CH3OH+ O2
N.R
4-7
140
TiO2+ hν + CH3OH+ N2
N.R
4-8
The photocatalytic reaction with employing methanol, TiO2, oxygen and UV light occured because they are essential parameters for the formation electron-hole pairs which play a vital role for pushing the photoreaction[19,162], as in equation 4-9. TiO2+ hν + CH3OH+ O2
Oxidation reaction
4-9
It was proved that the existance of metal on the surface of TiO2 could enhance the photocatalytic reaction by inhibition of the electronhole recombination. The Pt loaded on TiO2 enhanced the effeciency of photo-oxidation of methanol, however, Au loaded on TiO2 decreased the effeciency. Pt is more active than Au in collecting electrons and incresing the life time of photoholes. Moreover, Pt formed a hight schottky barrier with TiO2 than that for Au. However, the photocatalytic dehydrogenation of methanol occured under N2 with metal loaded on TiO2, due to metals act as charge scavenger hindering charge recombination, moreover, no further oxidized products i.e. formaldehyde formed only. The maximum efficiency of photoraction was obtained with loading Pt on TiO2 surface [19,67,163]. Equations 4-10 and 4-11 explain the effect of different parameters on the photocatalytic oxidation and photocatalytic dehydrogenation of methanol[19]. M/TiO2+ hν + CH3OH+ O2
Oxidation reaction
M/TiO2+ hν + CH3OH+ N2
Dehydrogenation reaction
4-10 4-11
4.3 The Effect of Different Parameters on Photocatalytic Oxidation and Photocatalytic Dehydrogenation of Methanol by Using Bare and Metalized TiO2 The effect of different parameters on photocatalytic reactivity of methanol was studied. The photocatalytic reaction of methanol was performed under purged O2 with using bare and metalized TiO2, with 278.15 K, adsorbed time 30 min and light intensity 1.570 x 10-7 Ens s-1.
141
4.3.1 Effect of Catalyst Mass The results in Figure 4-2 indicate that the photocatalytic oxidation and photocatalytic dehydrogenation of aqueous solutions of methanol increase with increasing catalyst concentration for all types of catalysts (bare TiO2 and Pt/TiO2) and become constant above 1500 mg L-1 till 2000 mg L-1. The increasing of photocatalytic activity with the increasing of catalyst masses are explained due to the increasing availability of photocatalyst sites[16,164,165]. After this level photocatalytic activity decreases with increasing catalyst concentration. At high concentration of catalyst screening effect [165,166] increase and aggregation of nanocatalysts formed, causing a decrease in the number of active sites on the surface of catalyst[167]. The decreasing of photocatalytic activity with the increasing of catalysts concentration above the plateau value can also be explained due to the deactivation of activated photocatalytic particles by collision with ground state particles[168,169]. Particle # + Particle
Particle + Particle
4-12
Where particle refers to particle with active species adsorbed on its surface and Particle# the deactivated from of the photocatalyst particles.
105 x Rate of reaction / (mol L-1 min-1)
2.5
Pt(0.5)/TiO2 + N2 Pt(0.5)/TiO2 + O2 TiO2 + O2
2
1.5
1
0.5
0 0
50
100
150
200
250
300
350
Mass of catalysts/ (mg/100 mL)
Figure 4-2: Rate of Reaction as Function of Different Masses of Bare TiO2 and Pt(0.5)/TiO2 Surface, under Purged O2 and N2.
142
4.3.2 Effect of the Percentage Loaded Metals Among all tested Pt(x)/TiO2 samples, Pt(0.5)/TiO2 showed the best photocatalytic activity. The existence of Pt particles on TiO2 surface will increase the charge carrier space distance (CCSD), and produce Schottky barrier by transforming the electrons from TiO2 conduction band to Pt conduction band. Thereby the recombination process depresses, according to the following equations[19,67,170]: Pt/TiO2+ hν
h+VB + e-CB
4-13
Ptn + e-CB
Pt -n
4-14
O2 + Pt –n
O2-. + Ptn
4-15
Platinum acts as charge scavenger hindering charge recombination and ultimately causing the enhancement of the photoreactivity as in the following equation [19, 48, 171].
Pt –n + h+VB
Ptn
4-16
The optimum content of Pt loading for photocatalytic oxidation and photocatalytic dehydrogenation is 0.5%, while the optimum content of Au loading for photocatalytic dehydrogenation is 2%. The photocatalytic activity of platinized TiO2 increased with platinum content until 0.5 % and then decreased. This behavior may be related to two reasons. The first, is the decreasing of metal content. Moreover, the existence of metal on the surface of TiO2 reduces surface hydroxyl groups and as a result reducing the photoreactivity [166], i.e. the metal on the surface of TiO2 behaves in two opposite directions, as an efficient trap site and as a recombination center [97]. However, the Au loaded on TiO2 surface decreases the photocatalytic reaction of methanol under O2, because Au has low work function (Ф = 5.31) and low Schottky barrier height thereby the activity is less than that activity for Pt [68,69]. Hence the rate of formaldehyde formation is slow and fast convert to formic acid, Thereby the photocatalytic experiments with Au loaded on TiO2 were excluded, in further experiments. On the other hand, the increasing of the amount of metal which gives a depth of grey color for Pt samples and purple color for Au samples, that will shield the light to reach of TiO2 and the excitation of electrons to conductive band was depressed [19,172]. 143
4.3.3 Effect of Methanol Concentration The photocatalytic reactivity noticeably increased with the increasing of methanol concentration. Methanol is a quenching agent of radicals as in the following equation[19, 173]: CH3OH + .OH
HCHO + H2O + 1/2 H2
4-17
Equation 4-17 explains the increasing of formaldehyde formation with the increasing of methanol concentration. Moreover, the proportionality between the rate of reaction and methanol rate of reaction, and methanol concentration was not exactly linear in the existence of O2. The deviation from linearity is related to the further oxidation of formed formaldehyde to formic acid. HCHO +1/2 O2
HCOOH
4-18
This was proved by measurement of the pH of final solution and found to be 5.86 and 4.82 at the end of the reaction, with using bare TiO2 and Pt(0.5)/TiO2 respectively. Figure 4-3 indicates the relationship between the rate of reaction with methanol concentration was more linear when nitrogen gas was used as a carrier gas instead of oxygen.
10 5 x Rate of reaction/ (M min-1)
2.5 Pt(0.5)/TiO2 + N2 Pt(0.5)/TiO2 + O2
2
1.5
1
0.5
0 0
10
20
30
40
10 3 x Conc. of Methanol/ (M)
Figure 4-3: Relationship Between Rate of Reaction and Methanol Concentration with Pt(0.5)/TiO2 Surface under Different Types Carrier Gases. 144
4.3.4 Effect of the pH of Solution pH of the solution plays an important role in photocatalytic reaction on the surface of catalyst because photocatalysis takes place on the surface of catalyst. The effect of pH is dependent on the type of reactant and on properties of TiO2 surface and for that reason pH value
10 5 x Rate of reaction/ (M min-1)
must be checked before any photocatalytic application[101].
4
Pt(0.5)/TiO2 +N2
3.5
Pt(0.5)/TiO2 + O2 TiO2 + O2
3 2.5 2 1.5 1 0.5 0 0
2
4
6
8
10
12
Initial pH of solution
Figure 4-4: Effect of Initial pH of Solution on the Rate of Reaction with Using Bare TiO2 with Purged O2, Pt(0.5)/ TiO2 with Purged O2 and Pt(0.5)/ TiO2 with Purged N2. Figure 4-4 shows that the higher photocatalytic activity of bare TiO2 and platinized TiO2 at neutral pH (7.06 -7.15). These results indicate the photocatalytic reaction of methanol to formaldehyde is strongly dependent on the pH value of aqueous TiO2 and Pt(0.5)/TiO2 suspension solutions. This effect is relevant for the adsorption and dissociation of reactant (methanol), and adsorption of HO- ions over the surface of used catalysts [104].
However, the initial pH of suspension solution
dominants on the ionization state of the surface hydroxyl groups over TiO2 for the bare or platinized. This behavior is explained by the zero 145
point charge of TiO2. Zero Point Charge (pHzpc), is a concept relating to adsorption phenomenon and defined as the pH at which the surface of an oxide is uncharged [52, 174]. In aqueous solution, at pH higher than pHzpc, the oxide surface is negatively charged and then the adsorption of cations is favoured and as a consequence, oxidation of cationic electron donors and acceptors are favoured. TiVI - OH + -OH
TiVI - O– + H2O, (pK2)
pH > pHzpc 4-19
At pH lower than pHzpc, the adsorbent surface is positively charged and then the adsorption of anions is favoured and as a consequence, the acidic water donates more protons than hydroxide groups. TiVI - OH + H+
TiVI – OH2+, (pK1)
pH < pHzpc
4-20
The optimum pH values of reaction in O2 purged suspension of TiO2 and Pt(0.5)/TiO2 were 7.06 and 7.08 respectively. This reaction was produced formaldehyde and further materials (formic acid or CO2 and H2O).However, The optimum pH values of reaction in N2 purged suspension of Pt(0.5)/TiO2 was 7.15, this reaction was produced formaldehyde only.
4.3.5 Effect of Temperature It is known that there is a slight effect of temperature on photocatalytic activity of photoreaction. However, consecutive redox reaction may be affected largely by temperature and as a result that will effect on adsorption equilibrium and collision frequency of the molecules [175] .
146
Pt(0.5)/TiO2 +N2 Pt(0.5)/TiO2 +O2 TiO2+ O2
ln (10 5 x Rate of reaction)/ (M min-1)
13
12.8 y = -1.708x + 18.48 12.6
12.4 y = -1.223x + 16.52 12.2
12
11.8 y = -2.675x + 21.49 11.6 3.3
3.35
3.4
3.45
3.5
3.55
3.6
3.65
(103 K/ T )
Figure 4-5: Effect of Temperature on Rate of Reaction with Using Bare TiO2 with Purged O2, Pt(0.5)/ TiO2 with Purged O2 and Pt(0.5)/ TiO2 with Purged N2. Figure 4-5 shows that the results obeyed Arrhenius law. The activation energy for bare TiO2 , Pt/TiO2 with purged O2 and Pt/TiO2 with purged N2 are equal to 22.215 kJ mol-1, 10.151 kJ mol-1 and 14.208 kJ mol-1 respectively. The results refer to increase the rate of reaction with increasing of temperature in the range (278.15-298.15) K, beyond to be the photoreaction of generating a formaldehyde and further oxidize compounds is endothermic. The activation energy of the photocatalytic oxidation of methanol under purged O2 is equal ∼ 22 kJ mol-1 with the bare TiO2, but decreased to ∼10 kJ mol-1 with using Pt(0.5)/TiO2. This is related to use the metal which acts as sink of electrons and finding O2 which captures of electrons that flow from conductive band of catalysts[18, 161].
147
4.4 The Mechanism of Photooxidation of Methanol in the Presence of Oxygen: When the photon fills on the suspension solution of methanol with catalyst either TiO2 or platinized TiO2 (Pt/TiO2), the photoelectron is promoted from valance band to conductive band and creates a (e-- h+) pair, then the photohole and photoelectron input in series of reactions according to the following equations[19,83, 112, 173]. (e- - h+)
TiO2 or Pt/TiO2 + hν
e- + h +
4-21
Ptn + e-
Pt-n
4-22
O2 + e-
O2-.
4-23
H+ + -OH
4-24
O2-. + H+
.
OOH
4-25
OOH + H+
H2O2
4-26
H2O2
2.OH
4-27
-
.
4-28
H2O
.
OH + h+
CH3OH + .OH
OH .
CH2OH + H2O
4-29
The formed .CH2OH in equation 4-29 could react either with itself or with hydroxyl radical, which is formed in equations 4-27 and 4-28. The limiting quantum yield for the formation of formaldehyde is 0.5 in equation 4-30.
.
2.CH2OH
HCHO + CH3OH
4-30
CH2OH +.OH
HCHO + H2O
4-31
The formed of .CH2OH in equation 4-29 could react with O2 or O2-. and produce formic acid. .
CH2OH +O2
HCOOH + .OH
4-32
.
CH2OH + O2-.
HCOOH + -OH
4-33
The formed of HCHO in equations 4-30 and 4-31 could react with O2 and produce formic acid also. 148
2HCHO +O2
2 HCOOH
4-34
4.5 The Mechanism of Photooxidation of Methanol in the Presence of Inert Gases (N2 or Ar): In the photocatalytic reaction with purged a N2 or Ar gases in solution, methanol could react with hydroxyl radical which is formed in equation 4-27 and 4-28 to produce a formaldehyde [19, 173]. CH3OH +.OH
.
CH2OH + H2O
4-35
CH2OH+.OH
HCHO + H2O
4-36
.
Formaldehyde formation with a limiting quantum yield of 0.5 could occur in the possibility: 2.CH2OH
HCHO + CH3OH
4-37
The formed H2O in equations 4-29 and 4-31 interacts with Pt-n, and the hydroge gas produces by essentially process is called reverse of hydrogen spillover [19, 67]. Pt-n + H2O
Ptn-H+ -OH
Ptn +1/2 H2 + -OH 4-38
The last equation indicates that hydrogen gas is released and -OH is regarded which adsorbs on the catalyst surface. 4.6 Hydrogen Production by Photocatalytic Dehydrogenation of Methanol Using Metalized TiO2 Hydrogen production is an important factor that is regarded as a renewable resource and natural energy source. Methanol is a good hydrogen resource. The metals are different in the ability to produce a hydrogen gas from photocatalytic dehydrogenation process of methanol. The catalyst which consisted of Pt is more active to produce hydrogen gas than catalyst that consisted of Au. That attitude is to differentiate the height of Schottky barrier formed that depends on the values of work functions. The work function of Pt (5.93) is more than that value of the work function of Au (5.31) that will produce a high Schottky barrier height for Pt-TiO2 contact compared with that for Au-TiO2 contact therefore the electrons flowing from TiO2 to Pt must have energy levels 149
higher than Schottky barrier for Au-TiO2 contact (see Figures 4-6 and 47), and the ability for storing the electrons in the Schottky barrier near Pt is the best and increase the separation of photo-generated carriers[52, 68,69]. Vacuum E nergy
V acuum E nergy
ΦTiO2
ΦPt
Ex
ΦTiO2
ΦAu
Ex CB
CB
FL
T iO
FL Pt
2
FL TiO2
FL A u
(c)
(a)
VB
VB
V acuum E nergy V acuum E nergy
ΦPt FL Pt
Ex
e-
ΦAu
Φb
FL Au
CB
FL
e-
Φb
CB
FL TiO
2
T iO2
Eg
Eg VB
VB
(b)
(d)
SCL
Figure 4-6: Energy level variation at Pt-TiO2 interface: (a) before contact and (b) after contact.
SCL
Figure 4-7: Energy level variation at Au-TiO2 interface: (c) before contact and (d) after contact.
4.7 The Quantum Yield Measurements The effect of different parameters such as weight of catalyst, initial pH and temperature were studied on photocatalytic oxidation and photocatalytic dehydrogenation of 40 mM of methanol by using bare TiO2 and Pt(0.5)/ TiO2 surface, at 278.15 K, adsorbed time 30 min under purged O2 and N2, under purged O2 and N2, with a light intensity 1.570 x 10-7 Ens s-1.
4.7.1 Effect of Mass of Catalyst The effect of mass of bare TiO2 and Pt(0.5)/TiO2 was found over range (50-300) mg/ 100 mL. The experimental results are listed in Table 4-1, and plotted in Figures 4-8. 150
Table 4-1: Quantum Yield of Formaldehyde Formation in Photocatalytic Reaction of Methanol with Different Doses of Bare TiO2 and Pt(0.5)/TiO2, under O2 and N2.
Catalyst doses
Quantum yield of formaldehyde formation
0
TiO2 + O2 0.000
Pt(0.5)/TiO2+ O2 0.000
Pt(0.5)/TiO2+ N2 0.000
50 mg
0.080
0.086
0.115
100 mg
0.117
0.125
0.162
150 mg
0.143
0.196
0.224
175 mg
0.145
0.198
0.239
200 mg
0.143
0.195
0.237
250 mg
0.123
0.151
0.176
300 mg
0.104
0.132
0.141
151
Pt(0.5)/TiO2 + N2
0.3
Quantum yield of formaldehyde formation
Pt(0.5)/TiO2 + O2 0.25
TiO2 + O2
0.2 0.15 0.1 0.05 0 0
50
100
150
200
250
300
Dose of catalysts/ (mg/100 mL)
Figure 4-8: Weight Effect of Catalyst on Quantum Yield of Formaldehyde Formation for Bare TiO2 with Purged O2, Pt(0.5)/ TiO2 with Purged O2 and Pt(0.5)/ TiO2 with Purged N2, at 278.15 K, with a Light Intensity 1.570 x 10-7 Ens s-1. The values of quantum yield in the presence of aqueous TiO2 and platinized TiO2 solutions were found to be directly proportional to catalyst dose at ranged (50-200) mg/100 mL, that beyond to increase of active sites on surface of catalyst, while at higher catalyst mass, the values of quantum yield of formaldehyde formation decline. This behavior is due to the screening effect which is attributed to increase the turbidity of the solution and ultraviolet rays start getting scattered [173,176]. The maximum quantum yield value for photooxidation of methanol solution to formaldehyde was noted at a plateau region where catalyst mass equal to 175 mg/ 100 mL in present O2 and N2. The photoreaction for producing a formaldehyde was faster in the presence of Pt(0.5)/TiO2 with purged N2 in methanol solution, this issue emphasizes that only formaldehyde was produced and no further oxidized products i.e. formic acid or CO2 and H2O occurred [112,173]. The maximum value of quantum yield was 0.375 for Pt(0.5)/TiO2 in the existence of nitrogen gas.
4.7.2 Effect the pH of Solution The optimum initial pH of solution of formed formaldehyde was investigated, at wide pH range of 1-12 employed 100 mL of 40 mM 152
methanol with 175 mg of Pt(0.5)/TiO2, at 278.15 K, adsorbed time 30 min under purged O2 and N2 with a light intensity 1.570 x 10-7 Ens s-1. Table 4-2: Quantum Yield of Formaldehyde Formation in Photocatalytic Reaction of |Methanol with Bare TiO2 and Pt(0.5)/TiO2, with Different Initial pH, under O2 and N2.
Quantum yield of formaldehyde formation
Parameters
pH TiO2 + O2 pH Pt(0.5)/TiO2 + O2 pH Pt(0.5)/TiO2 + N2
1.04
2.05
4.18
5.30
6.04
7.06
8.12
9.17
12.03
-
0.034
0.081
0.132
0.145
0.278
0.315
0.251
0.202
0.126
-
10.16
11.20
12.15
1.09
2.09
3.09
4.12
6.50
7.08
9.10
0.079
0.172
0.194
0.195
0.198
0.340
0.196
0.125
0.108
0.070
1.16
2.17
3.07
4.05
6.50
7.15
9.04
10.09
11.1
-
0.044
0.148
0.186
0.232
0.239
0.375
0.283
0.256
0.239
-
0.4
Pt(0.5)/TiO2 + N2 Pt(0.5)/TiO2 + O2
Quantum yied of formahdehyde formation
0.35
TiO2 +O2
0.3 0.25 0.2 0.15 0.1 0.05 0 0
2
4
6
8
10
12
Initial pH of solution
Figure 4-9: Effect of Initial pH of Solution on Quantum Yield of Formaldehyde Formation for bare TiO2 with Purged O2, Pt(0.5)/ TiO2 with Purged O2 and Pt(0.5)/ TiO2 with Purged N2, at 278.15 K, with a Light Intensity 1.570 x 10-7 Ens s-1. 153
The optimum quantum yield values for reaction were obtained at zero point charge (pHZPC) of bare TiO2 and Pt(0.5)/TiO2 in O2 purged suspension that equal to 7.06 and 7.08 respectively. However, the maximum value of quantum yield was obtained for Pt(0.5)/TiO2 in nitrogen purged. This implies that the reaction was achieved by two mechanisms, either by purged O2 suspension, that produced formaldehyde and further materials (formic acid or CO2 and H2O), or by purged N2 suspension which removed all O2 from suspension solution and only produced formaldehyde [173, 177].
4.7.3 Effect of Temperature The results in Table 4-3 show the quantum yield values increased with increasing of temperature. However, the value of quantum yield does not exceed 0.5 even if an extrapolation was done to proportionality line. This is in a good agreement with the proposed mechanisms which are mentioned before (see section 4.4 and 4.5 ).
Table 4-3: Quantum Yield of Formaldehyde Formation in Photocatalytic Reaction of Methanol with Bare TiO2 and Pt(0.5)/TiO2, with Different Ranged of Temperatures, under O2 and N2. Temperatures /K
Quantum yield of formaldehyde formation
278.15
TiO2 + O2 0.146
Pt(0.5)/TiO2+ O2 0.198
Pt(0.5)/TiO2+ N2 0.239
283.15
0.188
0.207
0.288
288.15
0.22
0.230
0.291
293.15
0.263
0.251
0.325
298.15
0.276
0.259
0.376
154
0.4
Pt(0.5)/TiO2 + N2 Pt(0.5)/TiO2 + O2
Quantum yield of formaldehyde formation
0.35
TiO2 +O2
0.3 0.25 0.2 0.15 0.1 0.05 0 3.3
3.35
3.4
3.45
3.5
3.55
3.6
3.65
(103 K/ T )
Figure 4-10: Effect of Temperature on Quantum Yield of Formaldehyde Formation for Bare TiO2 with Purged O2, Pt(0.5)/ TiO2 with Purged O2 and Pt(0.5)/ TiO2 with Purged N2, with a Light Intensity 1.570 x 10-7 Ens s-1.
155
CHAPTER FIVE CONCLUSIONS AND RECOMMENDATIONS
156
5.1 Conclusions This study focused on the formation of formaldehyde or hydrogen production by photocatalytic oxidation or photocatalytic dehydrogenation of aqueous methanol solution with bare and metalized TiO2. The main conclusions can be summarized as follows: 1. The atomic absorption measurements prove that the added metal (Pt and Au) was completely photodeposited on TiO2 surface after 4 h and 8 h respectively. 2. The FT-IR spectra show that the pecks of surface O-H in TiO2 increased with the increasing of the metals percentage (Pt or Au ) loaded on TiO2 surface. That is confirmed by the increasing of the corresponding peaks at 3450 cm-1 and 1630 cm-1. Moreover, the two bands at 3765 and 3840 cm-1 which are related to 6Ti3+-OH, disappearance with the increasing of the percentage of metalized TiO2. 3. The mean crystallite sizes and crystallite sizes of bare TiO2 and metals percentage (Pt or Au) loaded on TiO2 surface were calculated by depending on the XRD data. The mean crystallite sizes and crystallite sizes of bare TiO2 decreased with the increasing of metal percentage on TiO2. 4. AFM images indicate that the shapes of bare and metalized TiO2 are spherical. 5. The particle size is ranged between 9 to 11 crystals. 6. The band gap values decreased with the metal photodeposited on TiO2. 7. In photoreaction, no reaction occurred with using bare TiO2 under inert gas (N2), however in the existence of metal the photoreaction occurred i.e., the existence of the metal substituted the needed for the O2. 8. In the existence of O2, the reaction was carried on to form formic acid as a result of further oxidation of formaldehyde. While in the absence of the O2, the dehydrogenation of methanol occurred and no further photooxidation was observed. 157
9. The existence of light, catalyst and electron scavenger are essential for the dehydrogenation of methanol. 10. The activity of the catalyst changed when the pH changed. The maximum activity was achieved at zero point charge point. 11. The activation energy of the photocatalytic reaction in the existence of O2 with the bare TiO2 is equal to ∼ 22 kJ/mol, this value was reduced to ∼10 kJ/mol in platinum TiO2 with the existence of O2. This is related to the function of the metal as trap of electrons in addition to the activity of O2 as electron scavenger. 12. In existence of N2, the value of activation energy with Pt(0.5)/TiO2 is slightly more than that value in the absence of N2. This related to find metal only as trap of electrons. 13. Pt(0.5)/TiO2 was confirmed to be more active than Au(0.5)/ TiO2 in hydrogen production reaction from aqueous solution of methanol. This is in agreement with their work function values. 14. The quantum yield of formaldehyde depends on the catalyst dose, pH of solution and temperature. The limiting quantum yield for the formation of formaldehyde is 0.5. 15. The optimum conditions to get a better photooxidation of methanol are 175 mg/ 100 mL of TiO2 and Pt(0.5)/ TiO2, initial pH equal ∼ 7, and temperature around 298.15 K.
158
5.2 Recommendations After having arrived at the results of this study, the researcher thinks that the following are several recommendations for further studies: 1. Studing the different parameters on the photocatalytic oxidation and photocatalytic dehydrogenation for different types of aliphatic alcohol such as ethanol, 1-propanol, 2-propanol with bear TiO2 and Pt or Au loaded on TiO2 surface. 2. Loaded mixtures of different percentage of Pt and Au together on TiO2 surface, then studied the nanoparticles characterizations and the ability to produce a hydrogen from oxidation of alcohols. 3. Improved the propertied of TiO2 by loading different types of metals on the surface of TiO2 such as Pd, Ag and Co, then studied the nanoparticles characterizations and compared the hydrogen production efficiency. 4. Focusing on the preparation of TiO2 by sol gel method, then loaded the metals on the surface of it by photodeposition, microwave method and physical vapor deposition. 5. TEM analysis could be useful to investigate the exact amounts of metals that loaded on TiO2 surface, shapes of nanoparticles, and some properties such as thermal and mechanical properties. 6. EPR measurements could be used for the analysis of radicals produced. 7. Designing a new photo- reactor systems for producing the hydrogen gas from the wastewaters, agriculture and industry wastes.
159
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Publications [1] Luma M. Ahmed, Falah H. Hussein, and Ali A. Mahdi, "Photocatalytic Dehydrogen-ation of Aqueous Methanol Solution by Bare and Platinized TiO2 Nanoparticles", Asian Journal of Chemistry; vol. 24, no. 12, pp. 5564-5568 , 2012. [2] Luma M. Ahmed and Falah H. Hussein, "Quantum yield of formaldehyde formation from methanol in the presence of TiO2 and platinized TiO2 photocatalysts", Iraqi National Journal of Chemistry, vol.6, ISSN 2075-7328, 2013. [3] Luma M. Ahmed and Falah H. Hussein,"Synthesis, Characterization, and Applications of Platinized Titanium Dioxide", submitted for publication by Journal of solid state chemistry, 2013. [4] Luma M. Ahmed, Ahmed F. Halbous, Ayad F. Al-Kaim and Falah H. Hussein, "The photocatalytic hydrogen production from aqueous methanol solution over metalized TiO2", submitted for publication by International Journal of hydrogen energy, 2013.
Presentations 1- Oral Presentations [1] Luma M. Ahmed, Falah H. Hussein, and A. A. Mahdi, "Photocatalytic Dehydrogenation of Aqueous Methanol Solution by Bare and Platinized TiO2 Nanoparticles", International Conference On Global Trends In Pure And Applied Chemical Sciences(ICGTIACS-2012), 34 March 2012, Udaipur (Rajasthan) India. [2] Luma M. Ahmed and Falah H. Hussein, "Quantum yield of formaldehyde formation from methanol in the presence of TiO2 and platinized TiO2 photocatalysts", College of Science 6th Annual Scientific Conference (CSASC 2012), 6-7th November 2012.
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2- Poster Presentations [1] Luma M. Ahmed, Ali Abdul Saheb and Falah H. Hussein, "Photocatalytic Oxidation of CH3 OH by TiO2 Suspension", December 2010, College of Science, Chemistry Department, Babylon University-Iraq. [2] Luma M. Ahmed, Ali Abdul Saheb and Falah H. Hussein, "Photocatalytic Oxidation of Aqueous Solution of Methanol by Titanium Dioxide And Platinized Titanium Dioxide", April 2011, College of Science, Chemistry Department, Babylon University-Iraq. [3] Luma M. Ahmed, Ali Abdul Saheb and Falah H. Hussein, "Enhancement of photocatalytic activity of methanol solution by Naked and paltanized TiO2 catalyst", August 2011, College of Science, Chemistry Department, Babylon University-Iraq. [4] Luma M. Ahmed, Ali Abdul Saheb and Falah H. Hussein, "Photocatalytic Dehydrogenation of aqueous methanol solution by naked and Platinized TiO2 nanoparticles", International Conference On Global Trends In Pure And Applied Chemical Sciences(ICGTIACS-2012), 3-4 March 2012, Udaipur (Rajasthan) India. [5] Luma M. Ahmed, Ali Abdul Saheb and Falah H. Hussein, "Quntum Yield of Produced Formaldehyde from Poto-oxidation of methanol solution by Naked and pt doped TiO2 under O2 and N2", June 2012, College of Science, Chemistry Department, Babylon University-Iraq. [6] Luma M. Ahmed, Ali Abdul Saheb and Falah H. Hussein, "Quantum yield of formaldehyde formation from methanol in the presence of TiO2 and platinized TiO2 photocatalysts", September 2012, College of Science, Chemistry Department, Babylon University-Iraq.
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