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(1)COPYRIGHT AND CITATION CONSIDERATIONS FOR THIS THESIS/ DISSERTATION. o Attribution — You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use. o NonCommercial — You may not use the material for commercial purposes.. o ShareAlike — If you remix, transform, or build upon the material, you must distribute your contributions under the same license as the original.. How to cite this thesis Surname, Initial(s). (2012). Title of the thesis or dissertation (Doctoral Thesis / Master’s Dissertation). Johannesburg: University of Johannesburg. Available from: http://hdl.handle.net/102000/0002 (Accessed: 22 August 2017)..

(2) Catalytic Applications of Mesoporous Transition Metal Oxides and Supported Au and Pt Nanoparticles for Oxidation and Hydroformylation Reactions. Ph.D. Thesis Morena Xaba November 2018.

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(4) Catalytic Applications of Mesoporous Transition Metal Oxides and Supported Au and Pt Nanoparticles for Oxidation and Hydroformylation Reactions. By. Morena Samuel Xaba (M.Sc., North-West University, 2012). Thesis Submitted in fulfillment of the requirements for the degree Philosophiae Doctor In Chemistry Department of Chemistry, Faculty of Science University of Johannesburg, Auckland Park 1st November 2018 Supervisor: Professor. Reinout Meijboom. Cover illustration: Transmission electron microscopy image of mesoporous Co3O4-150 and Langmuir-Hinshelwood mechanism in morin oxidation.. i.

(5) Dedications. I dedicate this PhD thesis to the memory of my late sister, Miss. Mwelase Joanna Xaba. iii.

(6) Preface and acknowledgements I want to praise the Almighty Tlatlamatjholo for giving me strength, perseverance, guidance, protection, and wisdom since my birth till this age. I am grateful to Prof. Reinout Meijboom for sharing his immense knowledge in chemistry and for his friendly supervision. I want to thank my parents, Mrs. Mangose Xaba and Mr. Mokete Xaba for all their prayers of success during my studies. I thank my siblings, Elvis Xaba and Moleboheng Xaba for support and patience they gave me. My lovely fiancée Mmanini Mohlamme for being supportive of this journey, you were my smile in difficult times. Your courage is unbelievable, your love for our family is breathtaking and to our beautiful girls, Mbali, Bongiwe and Rethabile I hope I will continue to encourage you. I acknowledge Keabetswe Mokgadi, Oluwatayo Onisuru, Mpendulo Vilakazi for helping with UV. The following are acknowledged; Joe Ndolomingo, Batsile Mogudi, Sibusiso Hlatshwayo, Yonas Belay, Teboho Sebogodi, Louis-Charl Coetzee, Lerato Zikitani, Rapelang Patala, Ngonidzashe Masunga, Jeremy Legolie, and Dr. Ji-Hyang Noh for many and long discussions. I acknowledge Solomon and Themba for keeping the labs running daily, Siyasanga Mpelane for help with TEM and Thabo Mahlaka for a cool lab mate. I also wish to recognize many scientists who passed on while we were studying at universities, the late Mpho Motoboli, ‘Lilly’ Mokoena and Gabriel Maboe for being the Champions of African Scientists. My late grandfather, Mr. Thinyane Xaba who passed on in 2017 at the age of 99 years, Nonkosi, Mwelase, Shwabade. I acknowledge the University of Johannesburg for financial assistance. The financial assistance of the National Research Foundation (NRF) towards this research is hereby acknowledged. Opinions expressed, and conclusions arrived at, are those of the author and are not necessarily to be attributed to the NRF.. iv.

(7) Table of Contents. Page. Affidavit............................................................................................................................................. ii Dedications .......................................................................................................................................iii Preface and ackonwledgements ........................................................................................................ iv Publications and conferences presentations ...................................................................................... x Keywords ......................................................................................................................................... xii Abstract ...........................................................................................................................................xiii Abbreviations and nomenclature ..................................................................................................... xv List of Figures ................................................................................................................................ xvii List of Tables ................................................................................................................................. xxii List of Schemes ............................................................................................................................xxiii List of Figures (Supplementary information) ...............................................................................xxiii List of Tables (Supplementary information) ................................................................................. xxv Chapter 1: Synopsis ......................................................................................................................... 1 1.1. Background ............................................................................................................................. 1 1.2. Properties of mesoporous transition metal oxides .................................................................. 2 1.2.1. Synthetic methods of mesoporous transition metal oxides .............................................. 3 1.2.2. Mesoporous transition metal oxides in catalysis .............................................................. 4 1.3. Supported gold and platinum nanoparticles in catalysis ......................................................... 5 1.3.1. Synthesis of dendrimer encapsulated Pt and Au nanoparticles by galvanic exchange .... 5 1.3.2. Catalytic applications of supported Pt and Au nanoparticles ........................................... 8 1.4. Model reactions in aqueous solution..................................................................................... 10 1.5. Langmuir-Hinshelwood model ............................................................................................. 13 1.6. Hydroformylation of alkenes using Au and Pt supported mesoporous metal oxides ........... 14 1.7. Diffusion limitations ............................................................................................................. 16 1.8. Aims and objectives .............................................................................................................. 17 1.9. Thesis outline ........................................................................................................................ 17 v.

(8) 1.10. References ........................................................................................................................... 18 Chapter 2: Catalytic applications of Langmuir-Hinshelwood model and role of mass transport limitation on supported mesoporous transition metal oxides .................................................... 27 2.1. Introduction ........................................................................................................................... 28 2.2. Langmuir-Hinshelwood rate constant ................................................................................... 29 2.3. Rates of adsorption and desorption ....................................................................................... 31 2.4. Chemistry of supported mesoporous metal oxides ............................................................... 34 2.4.1. Mesoporous metal oxides in catalysis ............................................................................ 34 2.4.1.1. Mesoporous cerium(IV) oxide………………………………………….….……..35 2.4.1.2. Mesoporous iron(III) oxide………………………………………………...……..35 2.4.1.3. Mesoporous cobalt(II,III) oxide……………………………………….……...….36 2.4.1.4. Mesoporous nickel(II) oxide………………………………………….………….37 2.4.1.5. Mesoporous copper(II) oxide………………………………………………….….37 2.4.1.6. Mesoporous manganese(IV) oxide………………………………..…………..….38 2.4.2. Synthesis and catalytic evaluation of Au and Pt NPs supported on mesoporous metal oxides ............................................................................................................................ 38 2.4.2.1. Pt NPs supported on mesoporous metal oxides………………………………..…38 2.4.2.2. Au NPs supported on mesoporous metal oxides……………………………….…39 2.5. Model reactions ..................................................................................................................... 39 2.5.1. Morin oxidation .............................................................................................................. 41 2.5.2. Rhodamine B degradation .............................................................................................. 48 2.5.3. Nitrophenol reduction .................................................................................................... 49 2.5.4. Methylene blue degradation ........................................................................................... 57 2.6. Catalytic rates vs. mass transport limitation ......................................................................... 63 2.6.1. Damköhler number ......................................................................................................... 63 2.6.2. Bimolecular rate constant ............................................................................................... 67 2.6.3. Fick-Jacobs criterion ...................................................................................................... 71 2.7. Conclusions ........................................................................................................................... 74 vi.

(9) 2.8. References ............................................................................................................................. 75 Chapter 3: Kinetic and catalytic analysis of mesoporous Co3O4 on the oxidation of morin .. 99 3.1. Introduction ......................................................................................................................... 100 3.2. Experimental ....................................................................................................................... 101 3.2.1. Materials ....................................................................................................................... 101 3.2.2. Catalyst synthesis ......................................................................................................... 102 3.2.3. Catalytic evaluation ...................................................................................................... 102 3.2.4. Catalyst characterization .............................................................................................. 102 3.3. Results and discussions ....................................................................................................... 103 3.3.1. Synthesis and characterization of mesoporous Co3O4 catalysts ................................... 103 3.3.2. Catalytic activity and kinetic analysis .......................................................................... 107 3.3.3. Determination of thermodynamic parameters .............................................................. 113 3.3.4. Catalyst reusability and recycling ................................................................................ 115 3.4. Conclusions ......................................................................................................................... 116 3.5. References ........................................................................................................................... 117 Chapter 4: Kinetic and catalytic analysis of mesoporous metal oxides on the oxidation of Rhodamine B ................................................................................................................................ 123 4.1. Introduction ......................................................................................................................... 124 4.2. Experimental ....................................................................................................................... 126 4.2.1. Materials ....................................................................................................................... 126 4.2.2. Synthesis of mesoporous metal oxides......................................................................... 126 4.2.3. Characterization of mesoporous metal oxides ............................................................. 127 4.2.4. Catalytic investigation .................................................................................................. 127 4.3. Results and discussions ....................................................................................................... 128 4.3.1. Synthesis and characterization of mesoporous metal oxides ....................................... 128 4.3.2. Synthesis and characterization of mesoporous CuO .................................................... 132 4.3.3. Catalytic activity of mesoporous metal oxides ............................................................ 135 vii.

(10) 4.3.4. Kinetic and catalytic analysis of mesoporous CuO ...................................................... 137 4.3.5. Thermodynamic parameters and reusability of mesoporous CuO ............................... 144 4.4. Conclusions ......................................................................................................................... 147 4.5. References ........................................................................................................................... 148 Chapter 5: Catalytic activity of different sizes of Ptn/Co3O4 in the oxidative degradation of Methylene Blue with H2O2 .......................................................................................................... 154 5.1. Introduction ......................................................................................................................... 155 5.2. Experimental ....................................................................................................................... 156 5.2.1. Materials ....................................................................................................................... 156 5.2.2. Instrumentation............................................................................................................. 157 5.2.3. Synthesis of Pt NPs ...................................................................................................... 158 5.2.4. Synthesis of Pt NPs supported on mesoporous Co3O4 ................................................. 158 5.2.5. Catalytic evaluation ...................................................................................................... 159 5.3. Results and discussion ........................................................................................................ 160 5.3.1. Synthesis and characterization of Pt NPs supported on mesoporous Co3O4................ 160 5.3.2. Catalytic evaluation of Pt NPs supported mesoporous Co3O4 ..................................... 168 5.3.3. Kinetic evaluation of Pt NPs supported on mesoporous Co3O4 ................................... 171 5.3.4. Thermodynamic parameters and reusability ................................................................ 174 5.4. Conclusions ......................................................................................................................... 178 5.5. References ........................................................................................................................... 179 Chapter 6: Catalytic evaluation of Au and Pt nanoparticles deposited on mesoporous Co3O4 for hydroformylation of 1-octene and oxidation and reduction reactions ............................. 186 6.1. Introduction ......................................................................................................................... 187 6.2. Experimental ....................................................................................................................... 188 6.2.1. Materials and reagents .................................................................................................. 188 6.2.2. Mesoporous Co3O4 synthesis ....................................................................................... 189 6.2.3. Au and Pt deposition on mesoporous Co3O4 ................................................................ 189 viii.

(11) 6.2.4. Instrumentation............................................................................................................. 190 6.2.5. Catalytic evaluation ...................................................................................................... 190 6.3. Results ................................................................................................................................. 191 6.3.1. Synthesis and characterization of Au and Pt supported mesoporous Co3O4................ 191 6.3.2. Catalytic activity and kinetic analysis .......................................................................... 195 6.3.3. Diffusion limitation analysis ........................................................................................ 202 6.4. Discussions ......................................................................................................................... 203 6.5. Conclusions ......................................................................................................................... 204 6.6. References ........................................................................................................................... 205 Chapter 7: Conclusions and recommendations ........................................................................ 210 7.1. Conclusions ......................................................................................................................... 210 7.1.1 Langmuir-Hinshelwood model and diffusion limitations ............................................. 210 7.1.2. Mesoporous cobalt oxide ............................................................................................. 210 7.1.3. Mesoporous copper oxide ............................................................................................ 211 7.1.4. Pt DENs on mesoporous cobalt oxide .......................................................................... 211 7.1.5. Au and Pt NPs deposited on mesoporous cobalt oxide ................................................ 211 7.2. Recommendations ............................................................................................................... 212 Appendix and supplementary information .................................................................................... 212 Chapter 3. ................................................................................................................................... 212 Chapter 4. ................................................................................................................................... 217 Chapter 5. ................................................................................................................................... 221 References .............................................................................................................................. 231 Chapter 6. ................................................................................................................................... 232. ix.

(12) Publications and conferences presentations a) Publications 1. Nasrin Zohreh, Seyed Hassan Hosseini, Sakineh Alipour, Morena S. Xaba, Reinout Meijboom, Mahdi Fasihi Ramandi, Nazila Gholipour, Mehdi Akhlaghi, Natural salep/PEGylated chitosan double layer towards a more sustainable pH-responsive magnetic nanocarrier for targeted delivery of DOX and hyperthermia application, ACS Applied Nano Materials. 2 (2019) 853– 866. DOI: 10.1021/acsanm.8b02076. 2. Morena S. Xaba, Ji-Hyang Noh, and Reinout Meijboom, Catalytic activity of different sizes of Ptn/Co3O4 in the oxidative degradation of methylene blue with H2O2, Applied Surface Science. 467-468 (2019) 868-880. DOI: 10.1016/j.apsusc.2018.10.259. 3. Morena S. Xaba, Ji-Hyang Noh, Keabetswe Mokgadi and Reinout Meijboom, Kinetic and catalytic analysis of mesoporous metal oxides on the oxidation of rhodamine B, Applied Surface Science. 440 (2018) 1130–1142. DOI: 10.1016/j.apsusc.2018.01.241. 4. Nasrin Zohreh, Seyed Hassan Hosseini, Mahboobeh Jahani, Morena S. Xaba and Reinout Meijboom, Stabilization of Au NPs on symmetrical tridentate NNN-Pincer ligand grafted on magnetic support as water dispersible and recyclable catalyst for coupling reaction of terminal alkyne, Journal of Catalysis. 356 (2017) 255–268. DOI: 10.1016/j.jcat.2017.10.021. 5. Morena S. Xaba and Reinout Meijboom, Kinetic and catalytic analysis of mesoporous Co3O4 on the oxidation of morin, Applied Surface Science. 423 (2017) 53–62. DOI: 10.1016/j.apsusc.2017.06.105. 6. Isiaka A. Lawal, Morena S. Xaba, Reinout Meijboom and Patrick Ndungu, Deposition of palladium on a mesoporous Co3O4 using a solvent free technique and application in the electrooxidation of methanol and ethanol, Electrocatalysis, xxx (2019) xxx-xxx. (Provisionally accepted). 7. Morena S. Xaba*, Reinout Meijboom, Kinetic analysis and catalytic activity of metal nanoparticles and mesoporous metal oxides on selected model reactions, South African Journal of Natural Science, xxx (2019) xxx-xxx. (In press) x.

(13) 8. Morena S. Xaba and Reinout Meijboom, Catalytic applications of Langmuir-Hinshelwood model and role of mass transport limitation on supported mesoporous transition metal oxides: A review, Catalysts (manuscript submitted).. 9. Morena S. Xaba, Oluwatayo Racheal Onisuru, Mpendulo Theophillus Vilakazi and Reinout Meijboom, Catalytic evaluation of Au and Pt nanoparticles deposited on mesoporous Co3O4 for hydroformylation of 1-octene and oxidation and reduction reactions. (manuscript in preparation).. 10. Xolani S. Hlatshwayo, Morena S. Xaba and Reinout Meijboom, Tungsten and molybdenum promoted mesoporous zirconium oxide for the aminolysis of epoxides. (manuscript in preparation).. b) 1.. Conference presentations Kinetic and catalytic analysis of mesoporous metal oxides on the oxidation of rhodamine B, Morena S. Xaba, CATSA 2017, Pilanesberg, South Africa. 19 – 22 November 2017. (Poster presentation). 2.. Kinetiese analise en katalitiese aktiwiteit van mesoporeuse metaaloksiede van geselekteerde modelreaksies, Morena S. Xaba, Die Suid-Afrikaanse Akademie vir Wetenskap en Kuns. University of Pretoria, South Africa, 02 - 03 November 2017. (Oral presentation). 3.. Kinetic and catalytic analysis of mesoporous Co3O4 on the oxidation of morin, Morena S. Xaba, CATSA 2016, Drakensberg, South Africa. 6 – 9 November 2016. (Poster presentation). xi.

(14) Keywords Platinum and gold nanoparticles; galvanic exchange; mesoporous cobalt oxide; reaction rate constant; equilibrium adsorption and desorption rate constant; Freundlich exponents; LangmuirHinshelwood mechanism; generation six hydroxyl terminated poly-amidoamine (PAMAM) dendrimer; platinum dendrimer encapsulated nanoparticles (DENs); nanoparticle size; oxidation and reduction reactions; dye degradation; morin; methylene blue (MB); rhodamine B (RhB); 4nitrophenol; 1-octene; hydrogen temperature programmed reduction (H2-TPR); Ultra-violet visible (UV-vis) absorption spectra; characteristic path length; deposition precipitation method; diffusion limitation; heterogeneous catalysis; Damköhler number; activation energy; thermodynamic parameters; recyclable.. xii.

(15) Abstract This thesis aims at elucidating one of the oldest and biggest obstacle that the 19th and 20th century chemists have been working on in the field of chemical kinetics. Both heterogeneous and homogeneous catalysis investigates this phenomenon using similar and different criteria depending on the type of reaction and catalytic system used. There has been a lot of scientific reports that have demonstrated that the concentration, temperature, viscosity, diffusion and pH; all contribute to the rate constant of reactions occurring in liquid, gas and solid phases. However, the mechanism of reactions in nano-scopic level still remains a subject of controversy. The introduction of wellordered mesoporous transition metal oxide materials, e.g. Co3O4, Fe2O3, MnO2, CuO, NiO, Cr2O3, CeO2 and SiO2, as a control, have contributed significantly to the understanding of kinetics in heterogeneous catalysis. The addition of minute quantities of noble transition metals in the mesoporous metal oxide framework creates a synergy for elevated rate constants.. The oxidation and degradation reaction of environmentally toxic chemicals, such as rhodamin B, methylene blue, morin and 4-nitrophenol were investigated in aqueous media in the presence of an oxidant and the prepared mesoporous catalysts. In this investigation, intrinsic structure activity relationships were obtained and compared to the literature reports. The dependence of the concentration of morin and H2O2 on the rate constant was evident when mesoporous Co3O4 calcined to 150 ºC was used. This was a result of fine-tuned properties such as the total surface area, crystallite size and pore size of the Co3O4-150. The mechanism of this oxidation reaction was modelled and fitted to Langmuir-Hinshelwood mechanism and the surface activity parameters, such as the surface rate constants, equilibrium rate constants and Freundlich exponents were all determined and compared to the literature. Mesoporous CuO as a catalyst was shown to degrade rhodamine B with very high percentage decrease in the maximum wavelength 𝜆𝑚𝑎𝑥 . The dependence of large crystallite sizes of CuO was observed in the rhodamine B oxidation and compared to other mesoporous transition metal oxides. The Langmuir-Hinshelwood mechanism was modeled, and thermodynamic parameters were determined in this study. The recyclability and diffusion limitation studies revealed that the Co3O4-150 and CuO were stable even after the 4th cycle, with no leaching.. The size of dendrimer encapsulated platinum nanoparticles synthesized by galvanic exchange and immobilized on the surface of Co3O4, shows correlation to catalytic oxidation of methylene blue in the presence of H2O2. The nanoparticles with 55-molar ratio to dendrimer Pt55/Co3O4, exhibited a xiii.

(16) size of 1.2 ± 0.2 nm and had the highest catalytic activity, this behavior was attributed to the large metal surface area to volume as well as high metal dispersion on the surface of Co3O4. The catalyst that gave the highest activity which was Pt55/Co3O4, was subjected to recycling and reusability of up to 8 cycles and showed exponential decline in the activity as the cycles are increased. The experimental data was fitted to Langmuir-Hinshelwood mechanism and thermodynamics and kinetic parameters were calculated and compared to literature reports.. Gold and platinum nanoparticles deposited in the pores of mesoporous Co3O4 using urea created or formed nanoparticles, i.e. Au/Co3O4 and Pt/Co3O4 with sizes of 2.3 ± 0.04 nm and 2.4 ± 0.01 nm, respectively. The as-synthesized nanoparticles were characterised using different analytical techniques. The nanoparticles were catalitically active for 4-NP reduction, methylene blue oxidation, and 1-octene hydroformylation. The stirring speed variation was conducted and it was found that, an increase in the stirring speed results in an increase in the catalytic activity. In the 1octene conversion, the nonanal product formed was monitored using GC-FID and identified by GCMS. Isomerisation of 1-octene was observed with a very low conversion, indicating that both the synthesised catalysts were not 100 % selctive. A probable reaction conversion mechanism of 1octene into nonanal by using the Au/Co3O4 and Pt/Co3O4 catalysts was proposed, describing the hydrogen spill-over on the noble metal to activate the cobalt on the surface, followed by carbon monoxide insertion.. xiv.

(17) T. Abbreviations and nomenclature Absolute temperature, K. ΔS#. Activation entropy, J·mol-1·K-1. Ea. Activation energy of the catalysts system, K·J·mol-1. Ead. Activation energy of adsorption, K·J·mol-1. Edes. Activation energy of desorption, K·J·mol-1. 𝐾𝑖. Adsorption constant of the ith molecule, L·mol-1. 𝐴𝐶8. Area of 1-octene. 𝐴𝑖𝑠. Area of the internal standard. BET. Brunauer-Emmett-Teller. BJH. Barrett-Joyner-Haleda. kB. Boltzmann’s constant, J·K-1. 𝐴𝐵𝐸𝑇. BET Surface area, m2·g-1. kbm. Bimolecular rate constant, M−1·s−1. [A]. Concentration of substrate A, mol·L-1. δ. Characteristic path length, nm. R. Catalyst pellet radius, nm. D. Diffusion coefficient, m2·s−1. 𝑘𝑒𝑡. Electron transport coefficient, dimensionless. re. Effective rate constant, mol·m-2·s-1. De. Effective diffusion coefficient, m2 s−1. ΔH#. Enthalpy of activation, K·J·mol-1. EDX. Energy dispersive X-ray spectroscopy. n. Freundlich exponent, dimensionless. m. Freundlich exponent, dimensionless. GC. Gas chromatography. ΔG#. Gibbs Energy, K·J·mol-1. 𝑅𝑓. GC response factor. H2-TPR. hydrogen-temperature programmed reduction. HR-TEM. Transmission electron microscopy. ICP-MS. Inductively coupled plasma-mass spectrometry. 𝛽. Mass transport coefficient, m·s-1. F. Molecular flux, mol·s-1·m-2 xv.

(18) 𝜆𝑚𝑎𝑥. Maximum absorption wavelength, nm. kobs. Observed rate constant, s-1. 𝐶8. 1-Octene. Dp. Pore diameter, nm. Vp. Pore volume, cm3·g-1. h. Planck’s constant, J·s. p-XRD. Powder X-ray diffraction. rad. Rate of adsorption, mol·s-1. rdes. Rate of desorption, mol·s-1. γ. Relevant geometric quantity, dimensionless. SEM. Scanning electron microscopy. 𝜃𝑖. Surface coverage on the surface by ith molecule, dimensionless. k. Surface rate constant, mol·m-2·s-1. H. Sticking probability, dimensionless. DaII. Second Damkӧhler number, dimensionless. TS. Transition state. S. Total surface area, m2·g-1. TGA. Thermal gravimetric analysis. R. Universal gas constant, J·mol-1·K-1. UV-Vis. Ultra violet-visible. 𝑉𝑖𝑠. Volume of the internal standard at t = 0. 𝑉𝐶8. Volume of 1-octene at t = 0. r. Volume normalized rate constant, mol·m2·L-2·s-1. ϕ. Weisz-Prater parameter, dimensionless. w. Width of the pore, nm. xvi.

(19) List of Figures Chapter 1.. Page. Figure I. 1: Generalized scheme for the synthesis of mesoporous metal oxides [33]. ..................... 4 Figure I. 3: 3rd Generation PAMAM dendrimer with amine terminated periphery structure [63]... 6 Figure I. 4: Generalized synthesis process of dendrimer encapsulated nanoparticles using generation six PAMAM dendrimer by galvanic exchange method. ............................ 8 Figure I. 5: Catalytic activity of the CO oxidation on Pt nanoparticles (10 and 50 pulses) on sputtered and sol gel-prepared TiO2 thin films: (a) turnover frequency (TOF) as a function of increasing temperature and (b) Arrhenius plots derived from Fig 4 (a) [72]. ...................................................................................................................................... 9 Figure I. 6: UV–vis spectrum of the visible light oxidation of orange II in water in the presence of ozone using Bi2O3 as catalyst [80]. ............................................................................ 12 Figure I. 7: Hydroformylation reaction set-up for lab-scale experiment. ...................................... 15 Figure I. 8: Schematic view of the mass-transfer processes in three-phase catalytic reaction [97]. ......................................................................................................................................................... 16. Chapter 2.. Page. Figure II. 1: Schematic illustration of the Langmuir-Hinshelwood mechanism including the transition state (TS) for reactions that proceeds via two steps. ........................... 28 Figure II. 2: Estimation of the adsorbed amount Qs(t), based on the measured forcing (F), mixing (M), and gaseous adsorbate (A) response curves following the switch adsorbate/carrier gas → isotope adsorbate/carrier gas [35]. ..................................32 Figure II. 3: Electron microscopy images. Field emission scanning electron microscope (FESEM) images of mesoporous Mn2O3 after heat treatments at different temperatures: (a) 150, (b) 250, (c) 350 and (d) 450 ºC, (scale bars, 200 nm). HR-TEM images of I Mn2O3250 (scale bar, 20 nm), (f) Mn2O3-250 (scale bar, 10 nm), (g) Mn2O3-250 (scale bar, 5 nm) and (h) NiO-350 (scale bar, 20 nm) [57]. ....................................................36 Figure II. 4: Influence of bromate concentration on the initial rate of aqueous bromate reduction by 2 mgPd·L−1 loading of SiO2@Pd and SiO2@Pd@mSiO2 (pH = 7, T = 20 ± 1 °C, PH2 = 1 atm) [25]. ............................................................................................................40 Figure II. 5: (a) UV-Vis spectrum of the oxidation of morin in the presence of H2O2 using mesoporous Co3O4-150 as a catalyst, indicating the isosbestic points and the decrease. xvii.

(20) in absorbance at λmax = 410 nm (b) kinetic traces at λmax = 410 nm of the oxidation of morin over the different mesoporous Co3O4 catalysts [135]....................................41 Figure II. 6: The kinetic plot of the product of apparent rate constant, kobs; morin concentration, [morin] versus the product of the surface coverage for morin and H2O2, (a) for constant H2O2 concentration (0.0125 M), and (b) for constant morin concentration (0.0005 M). The solid line is the product of the rate constant k, and surface area S [136]. ......................................................................................................................43 Figure II. 7: (a) UV–vis spectrum of the oxidation of RhB in the presence of H2O2 using mesoporous CuO-250 as a catalyst, (b) kinetic traces at λmax= 554 nm of the oxidation of RhB over the different mesoporous metal oxides [84]. .......................................48 Figure II. 8: The role of dissolved oxygen in determining the induction time for the catalytic reduction of 4-NP to 4-AP [160]............................................................................52 Figure II. 9: Observed rate constants for the Co3O4 doped materials, as a function of (a) the concentration of 4-NP ([NaBH4] = 0.01 M) and (b) the concentration of NaBH4 ([4NP] = 35 μM). Catalyst amount (2 mg) and temperature (25 °C). Data points show the experimental kobs values while the solid lines show the Langmuir-Hinshelwood fitting [167]. ...........................................................................................................57 Figure II. 10: (a) Changes in the UV-vis absorbance spectra of MB dye using Mo-K-OMS-2 for 30 min. (b) Decomposition of MB using different doped K-OMS-2 catalysts under the same conditions [169]. .....................................................................................58 Figure II. 11: The relationship between (a) characteristic path length and equilibrium adsorption constants, (b) characteristic path length and surface rate constant and crystallite size [84]. ........................................................................................................................67 Figure II. 12: Segregation of reactants at the microscale in a porous medium, pure advection case [213]. ......................................................................................................................69 Figure II. 13: Effect of iron oxide concentration and disturbance on the kobs in the presence of granular ferrihydrite and goethite ([H2O2] = 5.88 mM; ▲: ferrihydrite, 225 rpm; ∆: ferrihydrite, 160 rpm; ●: goethite, 225 rpm; ○: goethite, 160 rpm) [221]. ............70 Figure II. 14: The geometry of a pore of length L and width w(x) = w2 – w1. The two walls are represented by the shape functions w1 and w2. The lengths of each wall are l1 and l2, respectively. The coordinates zi run along the two walls [226]. ............................ 72. xviii.

(21) Figure II. 15: Modelling results of n-hexane catalytic combustion over various catalysts and supports; (a) CoOxs1, (b) CoOxg1, (c) CoOxs2, (d) CoOxg2. Structured supports are knitted wire gauze, monolith and woven wire gauze [193]. ..................................73. Chapter 3.. Page. Figure III. 1: Characterization of prepared mesoporous materials: (a) low angle and (b) wide angle p-XRD patterns (c) N2 sorption isotherms and (d) H2-TPR profiles of mesoporous cobalt oxides. .......................................................................................................106 Figure III. 2: TEM images of prepared mesoporous materials (a) Co3O4-150, (b) Co3O4-250, (c) Co3O4-350, (d) Co3O4-450, I Co3O4-550 and (f) commercial Co3O4. .................106 Figure III. 3: (a) UV-Vis spectrum of the oxidation of morin in the presence of H2O2 using mesoporous Co3O4-150 as catalyst, indicating the isosbestic points and the decrease in absorbance at λmax = 410 nm (spectra taken every 3 min), (b) kinetic traces at λmax = 410 nm of the oxidation of morin over the different mesoporous Co3O4 catalysts and, (c) linear fits of the data in Fig. 3(b) showing pseudo-first order kinetics, (d) effect of varying the catalyst concentration on the observed rate constant. ........109 Figure III. 4: Dependence of rate constants to (a) the concentration of morin and (b) the concentration of H2O2. ......................................................................................... 111 Figure III. 5: Dependence of adsorption constants (a) and kinetic parameters (b), to crystallite size. .............................................................................................................................. 112 Figure III. 6: (a) Dependence of the observed rate constant on temperature variation, (b) Arrhenius plots for the determination of activation energies (EA). ......................................114 Figure III. 7: (a) Catalyst recycling bar graph with reaction conditions: [Co3O4-150] = 0.1 mM, [H2O2] = 30 mM, [Morin] = 1.5 mM, [Buffer] = 50 mM, T = 25 °C and (b) TEM image after four recycle runs of Co3O4-150 catalyst. ..........................................116. Chapter 4.. Page. Figure IV. 1: Structure of rhodamine B. ....................................................................................124 Figure IV. 2: (A) Low-angle p-XRD, (B) wide-angle p-XRD, (C) BET isotherm plots and (D) pore size distribution of different mesoporous metal oxides..........................................129 Figure IV. 3: (A) wide-angle p-XRD, (B) pore size distribution, (C) BET isotherms of the mesoporous CuO, and (D) H2-TPR plots of the mesoporous CuO. .......................134. xix.

(22) Figure IV. 4: HR-TEM images of prepared mesoporous materials (A) CuO-250, (B) CuO-350, (C) CuO-450, (D) CuO-550, (scale bar = 50 nm). .....................................................135 Figure IV. 5: (A) UV–vis spectrum of the oxidation of RhB in the presence of H2O2 using mesoporous CuO-250 as catalyst, (B) kinetic traces at λmax= 554 nm of the oxidation of RhB over the different mesoporous metal oxides and, (C) linear fits of the data in Fig. 5(B) showing pseudo-first order kinetics and (D) observed rate constants comparisons of the different mesoporous metal oxides. ........................................136 Figure IV. 6: (A) kinetic traces at λmax= 554 nm of the oxidation of RhB over the four mesoporous CuO and, (B) linear fits of the data in Fig. 6(A) showing pseudo-first order kinetics, (C) kobs comparison bar chart of the different mesoporous CuO and (D) dependence of kobs on catalyst concentration variation. .................................................................138 Figure IV. 7: Influence of (A) Rhodamine B concentration variation, (B) hydrogen peroxide concentration variation on the observed rate constant. ..........................................139 Figure IV. 8: The relationship between (A) characteristic path length and equilibrium adsorption constants, (B) characteristic path length and surface rate constant and crystallite size, (C) activation energy and crystallite size and surface area, (D) surface rate constant and equilibrium adsorption constants. ....................................................................143 Figure IV. 9: (A) Dependence of the observed rate constant on temperature variation, (B) Arrhenius plots for the determination of activation parameters. .............................................145 Figure IV. 10: (A) Catalyst recycling bar graph with reaction conditions: [CuO-550] = 0.55 µM, [H2O2] = 55 mM, [RhB] = 0.275 mM, [Buffer] = 1.98 mM, T = 25 ºC and (B) TEM image after five recycle runs of CuO-550 catalyst, (scale bar = 50 nm). ...............147. Chapter 5.. Page. Figure V. 1: UV-Vis spectra of the synthesis of (a) G6-OH (Pt55) and (b) G6-OH (Pt140) and G6OH (Pt225). ..............................................................................................................161 Figure V. 2: Characterization of the prepared platinum supported nanoparticles: (a) wide-angle pXRD diffraction patterns, (b) N2 sorption isotherms (c) pore size distribution and (d) H2-TPR profiles of mesoporous cobalt oxides nanocomposites. ........................... 163 Figure V. 3: TEM images of (a) G6-OH(Pt55), I G6-OH(Pt140), (i) G6-OH(Pt225), (c) Pt55/Co3O4, (g) Pt140/Co3O4 and (k) Pt225/Co3O4, and size histograms of (b) G6-OH(Pt55), (f) G6OH(Pt140), (j) G6-OH(Pt225), (d) Pt55/Co3O4, (h) Pt140/Co3O4 and (l) Pt225/Co3O4. 166. xx.

(23) Figure V. 4: Thermal gravimetric analyses plots of mesoporous Co3O4 and G6-OH(Ptn)/Co3O4 (n = 55, 140, 225). ......................................................................................................167 Figure V. 5: (a) UV–vis spectrum of the oxidation of MB in the presence of H2O2 using Pt55/Co3O4 as catalyst, (b) kinetic traces at λmax = 664 nm of the oxidation of MB over the mesoporous cobalt oxide and Ptn/Co3O4 catalyst, (c) linear fits of the data in Fig. 5(b) showing pseudo-first order kinetics and (d) kobs as a function of catalyst concentration of the different Ptn/Co3O4 catalyst. .........................................................................169 Figure V. 6: GC–MS spectra of MB oxidation in the presence of Pt55/Co3O4 (a) imine group oxidized (3 min) and (b) dimethylamine groups oxidized (24 min). Reaction conditions: [Pt55/Co3O4] = 0.25 µM, [MB] = 15 µM, [Buffer] = 0.20 M and [H2O2] = 20 mM. ...................................................................................................................170 Figure V. 7: Influence of (a), (c), and (c) MB concentration variations and (b), (d), and (f) H2O2 concentration variations on the kobs. ....................................................................172 Figure V. 8: (a) Arrhenius plots for the determination of activation parameters, and Van’t Hoff plots of (b) Pt55/Co3O4, (c) Pt140/ Co3O4 and (d) Pt225/ Co3O4. Reaction conditions: [catalyst] = 0.25 µM, [MB] = 15 µM, [Buffer] = 0.2 M and [H2O2] = 20 mM. ....177 Figure V. 9: Catalytic recycling bar graph with the following reaction conditions: [Buffer] = 0.2 M, [Pt55/Co3O4] = 1.20 µM, [MB] = 25 µM and [H2O2] = 20 mM. ............................ 178. Chapter 6.. Page. Figure VI. 1: (a) wide angle p-XRD, (b) low angle p-XRD spectra, (c) N2 sorption isotherms and (d) pore size distribution of the Pt/SiO2, Au/SiO2, Pt/Co3O4 and Au/Co3O4. ............... 193 Figure VI. 2: (a) Representative Au/Co3O4 HR-TEM image, (b) representative Pt/Co3O4 HR-TEM image, (c) Au/Co3O4 size distribution histogram, (d) Pt/Co3O4 size distribution histogram. ....................................................................................................................................... 195 Figure VI. 3: EDX spectra of (a) Au/Co3O4 and (b) Pt/Co3O4. .................................................... 196 Figure VI. 4: (a) UV–vis spectrum of the oxidation of MB in the presence of H2O2 using supported mesoporous M/Co3O4 as catalysts (spectra taken every 3 min), (b) kinetic traces at λmax = 664 nm of the oxidation of MB over the different mesoporous M/Co3O4 catalysts, (c) linear fits of the data in Fig. 4(b) showing pseudo-first order kinetics, (d) effect of varying the catalyst concentration on the observed rate constant. ............................................. 197 Figure VI. 5: (a) UV–vis spectrum of the reduction of 4-NP in the presence of NaBH4 using mesoporous M/Co3O4 as catalysts (spectra taken every 3 min), (b) kinetic traces at λmax = xxi.

(24) 400 nm of the reduction of 4-NP over the different mesoporous M/Co3O4 catalysts, (c) linear fits of the data in Fig. 5(b) showing pseudo-first order kinetics, (d) effect of varying the catalyst concentration on the observed rate constant................................................. 199 Figure VI. 6: (a) Catalytic conversion of 1-octene in the presence of the Au/Co3O4 and Pt/Co3O4, (b) time-dependent catalytic conversion of 1-octene in the presence of the Au/Co3O4 and Pt/Co3O4…………………………………………………………………….………….200 Figure VI. 7: Electron ionization mass spectra of the nonanaldehyde product formed………….201 Figure VI. 8: Probable reaction pathway for the hydroformylation of 1-octene over Au/Co3O4 and Pt/Co3O4 catalyst to produce nonanaldehyde…………………………………………..202 Figure VI. 9: Dependence of the observed rate constant and conversion for (a), MB oxidation, (b) 4-NP reduction, and (c) 1-octene conversion on the stirring speed. ............................... 203. List of Tables Chapter 1.. Page. Table I. 1: Normal hydrogen energy (NHE) values of some transition metals. ............................. 7. Chapter 2.. Page. Table II. 1: Langmuir-Hinshelwood parameters of morin oxidation from the experimental data fitted to equation (4). ..................................................................................................45 Table II. 2: Langmuir-Hinshelwood parameters obtained for the oxidation of Rhodamine B over different mesoporous CuO catalysts. ..........................................................................49 Table II. 3: Langmuir-Hinshelwood parameters for 4-NP reduction from the experimental data fitted to equation (4). ................................................................................................ 53 Table II. 4: Langmuir-Hinshelwood parameters for the degradation of MB from the experimental data fitted to equation (4). ........................................................................................61 Table II. 5: Characteristic path length and mass transport coefficient values of some mesoporous metal oxide catalysts. ............................................................................................... 65. Chapter 3.. Page. Table III. 1: BET surface areas, BJH pore sizes, crystallite sizes and lowest temperature H2-TPR peaks of the prepared mesoporous cobalt oxide catalysts. .....................................104. xxii.

(25) Table III. 2: Parameters for the fitting of the experimental kinetics data to the LangmuirHinshelwood equation Eq. (7). ...............................................................................113 Table III. 3: Activation parameters for the four mesoporous cobalt oxide catalysts and comparison with literature values. ............................................................................................. 115. Chapter 4.. Page. Table IV. 1: BET surface areas, BJH pore sizes, pore volumes and crystallite sizes of the prepared mesoporous metal oxides. ......................................................................................130 Table IV. 2: Mass transport coefficient and characteristic path length values of the synthesized mesoporous metal oxide catalysts. .........................................................................131 Table IV. 3: Langmuir-Hinshelwood parameters obtained for the oxidation of Rhodamine B over different mesoporous CuO catalysts.......................................................................141 Table IV. 4: Activation parameters for the four mesoporous copper oxide catalysts and comparison with literature values. ............................................................................................. 146. Chapter 5.. Page. Table V. 1: Physicochemical properties of the prepared mesoporous Co3O4 supported Pt nanoparticles. .......................................................................................................164 Table V. 2: Langmuir-Hinshelwood parameters. .......................................................................174 Table V. 3: Thermodynamic parameters. ...................................................................................176. Chapter 6.. Page. Table VI. 1: BET surface area, pore diameter size, pore volume size, Scherrer crystallite size and ICP-OES metal loading of the prepared nanocomposites. .....................................194. List of Schemes Chapter 6. Page. Scheme 1. M/Co3O4-catalyzed hydroformylation of 1-octene to nonanal (M = Au or Pt).. List of Figures (Supplementary information) Chapter 3. Page. Figure III. S1: EDX spectrum of mesoporous Co3O4-150 confirming the presence of the metallic cobalt and oxygen in the synthesized materials. .................................................. 212 xxiii.

(26) Figure III. S2: Pore size distributions of (a) Co3O4-150, (b) Co3O4-250, (c) Co3O4-350, (d) Co3O4450, I Co3O4-550 and Commercial Co3O4........................................................... 215 Figure III. S3: Effect of varying the catalyst and morin concentrations on the observed rate constant in the absence of hydrogen peroxide. .................................................................. 215 Figure III. S4: X-Ray Powder Diffraction (XRD) spectrum of the synthesised mesoporous Co3O4150 and the fitted diffraction peak data. ................................................................ 216. Chapter 4. Page. Figure IV. S1: Pore size distribution of (A) CuO-250, (B) CuO-350, (C) CuO-450 and (D) CuO550. ...................................................................................................................... 217 Figure IV. S2: Representative EDX spectra of the mesoporous CuO-550. ................................. 218 Figure IV. S3: Representative SEM image of the mesoporous CuO-550. ................................... 218 Figure IV. S4: Uncatalyzed reaction of Rhodamine B, showing no decrease in the absorbance at a maximum wavelength of 554 nm. ....................................................................... 219 Figure IV. S5: Catalytic reaction of Rhodamine B in the absence of hydrogen peroxide (A) CuO250, (B) CuO-350, (C) CuO-450 and (D) CuO-550, showing no decrease in the absorbance at a maximum wavelength of 554 nm. Reaction cond: [Catalyst] = 0,55 mM, [RhB] = 0,275 mM, [Buffer] = 1,98 mM and T = 25 ºC. ........................... 220. Chapter 5. Page. Figure V. S1: Characterization of the prepared platinum silica supported nanocomposite materials: (a) wide-angle p-XRD patterns, (b) N2 sorption isotherms (c) pore size distribution and (d) H2-TPR profiles of mesoporous cobalt oxides. ......................................... 222 Figure V. S2: SEM images of (a) mesoporous Co3O4, (b) Pt55/Co3O4, (c) Pt140/Co3O4 and (d) Pt225/Co3O4. ............................................................................................................ 223 Figure V. S3: (a) Representative SEM image and (b) EDX spectrum of Co3O4.......................... 223 Figure V. S4: TEM image of mesoporous Co3O4 only. ............................................................... 224 Figure V. S5: TEM images of (a) SiO2 (b) Pt55/SiO2, (c) Pt140/SiO2 and (d) Pt225/SiO2 and their corresponding size distribution histograms (e) Pt55/SiO2, (f) Pt140/SiO2 and (g) Pt225/SiO2................................................................................................................ 225 Figure V. S6: Element mapping profiles of (a) Co3O4 electron image, (b) Co3O4 Cobalt mapping, (c) Co3O4 Oxygen mapping, (d) Pt55/Co3O4 electron image, (e) Pt55/Co3O4 Cobalt xxiv.

(27) mapping, (f) Pt55/Co3O4 Oxygen mapping, (g) Pt55/Co3O4 Platinum mapping, (h) Pt140/Co3O4 electron image, (i) Pt140/Co3O4 Cobalt mapping, (j) Pt140/Co3O4 Oxygen mapping, (k) Pt140/Co3O4 Platinum mapping, (l) Pt225/Co3O4 electron image, (m) Pt225/Co3O4 Cobalt mapping, (n) Pt225/Co3O4 Oxygen mapping and (o) Pt225/Co3O4 Platinum mapping. ................................................................................................. 226 Figure V. S7: Thermal gravimetric analyses plots of Silica and Ptn /SiO2 (n = 55, 140, 225). .... 227 Figure V. S8: Carbonate buffer concentration variation. ............................................................. 227 Figure V. S9: UV-Vis spectra of the uncatalyzed reaction of methylene blue oxidation. ........... 228 Figure V. S10: Methylene blue oxidation mechanism. ................................................................ 229 Figure V. S11: Influence of reaction temperature on the equilibrium adsorption constant (a) Pt55/Co3O4, (b) Pt140/Co3O4, and (c) Pt225/Co3O4. .................................................. 230. Chapter 6. Page. Figure VI. S 1: TEM image of the gold nanoparticles supported on silica, Au/SiO2. ................. 232 Figure VI. S 2: TEM image of the platinum nanoparticles supported on silica, Pt/SiO2. ............ 233 Figure VI. S 3: Typical uncatalyzed absorption spectrum of methylene blue (MB) over a 30 minutes duration, showing no decrease in the maximum absorption wavelength of 664 nm. .................... 234 Figure VI. S 4: Typical uncatalyzed absorption spectrum of 4-nitrophenol over a 30 minutes duration. Showing no decrease in the maximum absorption wavelength of 400 nm. ................... 235 Figure VI. S 5: Electron ionization mass spectra of 2-octene. ..................................................... 235 Figure VI. S 6: Electron ionization mass spectra of 3-octene. ..................................................... 236 Figure VI. S 7: Electron ionization mass spectra of 4-octene. ..................................................... 236 Figure VI. S 8: Total ion chromatogram of the products forming in the hydroformylation of 1octene using Pt/Co3O4 catalyst. Reaction conditions: Pt/Co3O4 = 2 mg, 1-octene = 8.0 mmol, solvent = 14.0 mmol, decane = 102.6 µmol, T = 130 ºC, P = 40 bar, stirring speed = 1200 rpm, time = 6 hours. ............................................................................................................................................. 237. List of Tables (Supplementary information) Chapter 3. Page. Table III. S1: Characteristic path length and mass transport coefficient values of cobalt oxide catalysts. ................................................................................................................. 213. xxv.

(28) Chapter 5. Page. Table V. S1: Theoretical particle size and calculated average particle size comparison.............. 224. xxvi.

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(30) Chapter 1: Synopsis. Chapter 1: Synopsis Nothing in life is to be feared, it is only to be understood. Now is the time to understand more, so that we may fear less. ― Marie Skłodowska Curie. 1.1. Background In this study, non-supported and supported mesoporous transition metal oxides are investigated for their physical and chemical properties, their cause of synergistic effect and their subsequent catalytic activity. Early transition metals are attractive because of their low cost, high natural abundance, thermal stability, diversified composition, high intrinsic activity, and this makes them suitable for heterogeneous catalysis. Some of the drawbacks of the mesoporous transition metal oxides are poor dispersability, low electrical conductivity, low specific surface area, difficulty in controlling the M‒ O energy. We focused on the following transition metals for our catalytic studies; cobalt, nickel, iron, chromium, copper, manganese, silicon and cerium [1]. Silicon is well known to be inactive [2], it is therefore used as a control for colloidal nanoparticles in heterogeneous catalysis. Whilst cerium is a lanthanide element, and it is used as an activity comparison metal because it relates to different isotopes. We also incorporated platinum and gold nanoparticles which have been prepared by the dendritic template method as well as deposition precipitation method. The platinum nanoparticles are immobilized on different mesoporous metal oxides mentioned above for the purpose of enhancing the catalytic activity. Platinum nanoparticles are a very well understood nanoparticles system and are considered a benchmark for the nanoparticle synthesis and catalytic activity testing [3], while ligand-free gold nanoparticles are used as a reference material in the kinetic data fitting of the Langmuir-Hinshelwood model [4].. Metal nanoparticles (NPs) entrapped inside a dendrimer are colloidal metals in a solution (such as in methanol) with a diameter of about less than 1 nm up to 4 – 5 nm [5]. Most metals are inactive in their bulk form, however, when they are in colloidal solution they have proven to be very reactive and exhibit activation energies as low as those of homogeneous organometallic complexes [6,7]. Metal NPs have been shown to be very active catalysts for different kinds of reactions, however, they lack one property which is recovery and recyclability after the reaction is complete [8]. We undertake a step to solve this problem in this study, the immobilization or supporting of colloidal metal NPs in solution onto the solid material for the above-mentioned reason. Supported mesoporous transition metal oxides’ physicochemical characteristics are still widely not understood, and a great potential exists to exploit them for their capability of adsorbing NPs. We particularly. 1.

(31) Chapter 1: Synopsis. use noble metals in colloidal form, gold and platinum are one of the oldest metals to gain great interest among medieval researchers. By permanently immobilizing gold and platinum on the surface and pores of the mesoporous metal oxides, we could have solved one major drawback of all the colloidal NPs.. One aspect of catalytic activity to be understood well when using a solid catalyst for industrial application is the kinetics of a catalytic reaction [9]. In this light, we undertake an investigation to understand and to interpret the mechanism of the reactivity and to elucidate the kinetics involved. There are many proposed mechanisms for surface reactions of interest with colloidal NPs [10–12], however, mechanisms pertaining to organic molecule transformations with immobilized NPs are still a subject of interest for many materials scientists. The Langmuir-Hinshelwood [8,9], Mars-van Krevelen [13–15] and Eley-Rideal [16] mechanisms have been used and reported extensively in the literature, we will use the Langmuir-Hinshelwood model to interpret the mechanism of conversion of substrates on the supported mesoporous metal oxides. Most mechanistic studies favor the surface reactions mechanism following the Langmuir-Hinshelwood mechanism [17].. The industrial sectors are business orientated, with all regulations for all aspects of smooth operation of manufacturing. However, industries such as textiles, cosmetics, pulp, medicinal, food and beverages, are among a few industries which cause a lot of pollution to the environment with and/or without awareness [18]. Reducing these pollution hazardous substances is a key to human sustainability and to global strategic remediation. We investigate the total degradation of these dyes using mesoporous metal oxides and NP supported mesoporous metal oxides in water. By degrading the dyes, we will not only solve one problem, but also that of elucidating the mechanism of the reactions using our synthesized materials as well as the kinetics of reactions.. 1.2. Properties of mesoporous transition metal oxides Mesoporous materials are distinguished from microporous by the diameter size of their pores, where microporous have less than 2 nanometers and mesoporous are in the range, 2 – 50 nanometers and macroporous are larger than 50 nm [24]. The first mesoporous material that was reported in literature was a mesoporous silica M41S series by Mobil oil researchers in 1992 [25,26]. A quarter of a century later, non-silicon ordered mesoporous materials of inorganic nature such as carbon, sulfides, polymers, transition metals have been reported [27–32]. These materials can be synthesized by two 2.

(32) Chapter 1: Synopsis. well-known methods namely, the soft template and hard template method also called nanocasting. Unlike microporous and macroporous, mesoporous materials are suitable for catalytic applications, or substrate conversion on the surface of the metal oxide. They are also very easy to handle, they are non-volatile, they have not been shown to cause any harm to human health whatsoever. They are easy to manufacture in small to medium size laboratories, upscale of the quantities are easy, however, one should be careful not to lose some of the intrinsic properties of these mesoporous transition metal oxides. The use of soft template synthesis method for the preparation of welldefined ordered mesoporous metal oxides offers the possibility to fine tune the properties of these materials [33–35]. Properties such as specific surface area, pore size distribution and pore volume play a crucial role in the catalytic activity of the mesoporous materials.. 1.2.1. Synthetic methods to make mesoporous transition metal oxides Mesoporous metal oxides have chemical applications in gas sensing, electrochemical electrodes, supercapacitors, fuel cells, sorbants for separation, gas storage, magnetic applications, catalysis and in biomedical sciences [27,33,36–40]. Generation of such materials involves the use of inverse micelles, elimination of solvent effects, minimizing the effect of water content and controlling the condensation of inorganic frameworks by NOx decomposition. Nano-size particles are formed in inverse micelles and are randomly packed in a mesoporous structure Fig 1. The hard template synthesis method [41] was first used for their synthesis, and the sol-gel method [42], nano-casting method [43] and finally soft templated method [44] were all introduced. The soft template synthesis method is the most used because of its easy removal of metal clusters. The correct amount of water, nitric acid, solvent, and NOx clusters play a crucial role in the formation of desired physicochemical properties of mesoporous transition metal oxides.. 3.

(33) Chapter 1: Synopsis. Figure 1: Generalized scheme for the synthesis of mesoporous metal oxides [33].. 1.2.2. Mesoporous transition metal oxides in catalysis Mesoporous metal oxides were first used as supports in catalysis to assess their interfacial activity. The main aim of introducing mesoporous metal oxides was to “heterogenise” the most selective homogeneous catalyst systems [45], another factor being separation of the mixture from products. For example, mesoporous zinc oxide [46], copper oxide [47], graphene oxide [48], iron oxide [49], cerium oxide [50] and cobalt oxide [51], have been used as catalysts. These mesoporous metal oxides also give the factor of electron mobility on the support surface, facilitating and enhancing especially redox reactions [52]. The existence of lattice oxygens in the backbone of the mesoporous metal oxides play a critical role in the creation of the vacant site which ultimately becomes the active site for a catalytic reaction. The mesoporous metal oxides such as MCM-41 and SBA-15 have very high surface areas as compared to the transition metal oxides, however the catalytic activity of the latter group is diversified by its M‒O‒M bond which enhances the catalytic activity in many environmental pollution control [53]. Where transition metal oxide materials can be synthesized by nano-casting, sol-gel formation, soft- and the hard-template method, and have been shown to CO oxidation, N2O decomposition, NOx reduction at low temperatures and elimination of organic pollutants [54–56]. One other important factor which controls the catalytic activity of the metal oxide is the oxidation state. Hydrogen temperature programmed reduction (H2-TPR) studies have revealed that the oxidation state of the metal oxides changes with temperature. Reducing lattice oxygens is the first step in TPR analysis, followed by reduction of M3+, M2+ and finally to pure 4.

(34) Chapter 1: Synopsis. metal. Suib and his group have demonstrated that mesoporous iron oxides can degrade Orange II under visible light and neutral pH [33]. Some mesoporous cobalt oxide controlled porosity materials were used to oxidize carbon monoxide which has indeed opened many possibilities to perform oxidation reactions [27,57].. Catalytic reactions occurring at the interface of gases and solids and heterogeneous reactions are evaluated using the mesoporous transition metal oxides as catalyst. MnO2 was used to oxidise morin, 4-NP, methylene blue, methyl orange, on the dye degradations. Co3O4 used in the oxidation of morin in the presence of H2O2, CuO used in the oxidation of rhodamine B. Styrene oxidation and C‒C cross coupling reactions have been investigated using MnO2 and Co3O4.. 1.3. Supported gold and platinum nanoparticles in catalysis 1.3.1. Synthesis of dendrimer encapsulated Pt and Au nanoparticles by galvanic exchange Dendrimers were first discovered by Vögtle and co-workers in the 20th century [58]. The word dendrimer is derived from the Greek word “dendron” meaning a tree. Dendrimers are hyperbranched macromolecules with a core, branches, periphery, and voids [59]. There are two main routes to dendrimer synthesis; the divergent and convergent methods [59,60]. Both these techniques yield similar results in terms of dendrimer properties, both physical and chemical. These structures are used in many industries; pharmaceutical, electronics, drug delivery and many other applications. This is as a result of the ability to functionalise the periphery of these dendrimers, rendering them soluble to different medias such as aquatic, solvent based, supercritical fluids and much more [5]. Dendrimers are very useful in nanoparticle catalysis as they provide a template for the synthesis of highly mono-dispersed, stabilized, and small nanoparticles. The different generations available from different types of dendrimers are the ones that give us the possibility of size control. The surface of the dendrimers can be functionalized, with different groups to facilitate adsorption, and solubility in different media [61]. The poly-amido amine (PAMAM) dendrimers are attractive because their branches are connected by ethyl and amine groups, which makes them flexible to create enough voids for nanoparticle encapsulation Fig 2. Generations from 4th, 5th, and 6th are the widely used from a range of dendrimers for ease of penetration of the hosts on the periphery of the dendrimer [62]. It is also of great interest to use dendrimers as they allow the possibility of investigating the reaction kinetics in the chemical reactions.. 5.

(35) Chapter 1: Synopsis. Figure 2: 3rd Generation PAMAM dendrimer with amine terminated periphery structure [58].. Dendrimer encapsulated nanoparticles were first discovered by Crooks and coworker in 1998 [64]. Transition metal ions are used as metal precursors to form nanoparticles within the dendrimer voids or cavities. The metal precursor is first complexed within the dendrimer voids by forming the metal to amine bond through a ligand to metal charge transfer (LMCT). The ions (Mn+) are considered to bond covalently to the amine groups, this could be confirmed by the presence of the LMCT band on the UV-Vis scan. These coordinated metal precursors can now be reduced to a zero charge by using a strong reducing agent such as sodium borohydride (NaBH4). After reduction, the LMCT band disappears and the nanoparticles (NPs) are formed inside the dendrimer. The tuneability of the NPs size is controlled by using the metal to dendrimer molar ratio. The rule of thumb is that, an increase in the metal ion molar ratio to dendrimer, will produce an increased size of NPs. If the metal ion to dendrimer molar ratio exceeds that of the tertiary amines in the dendrimer, then the 6.

(36) Chapter 1: Synopsis. dendrimer stabilized nanoparticles (DSNs) are formed. In this instance, NPs of higher diameters are formed because they can no longer be encapsulated within the dendrimer voids.. Table 1: Normal hydrogen energy (NHE) values of some transition metals. 0. Reaction. E , V vs. NHE. 2+. Cu = Cu + 2e+. Ag = Ag + e 2+. Pd = Pd + 2e 2+. Pt = Pt + 2e 2+. 0,34. -. 0,78 -. 0,83. -. Au = Au + 2e. 1,2 -. 1,5. Because of the normal hydrogen energy (NHE) of the respective metal’s electron transfer, a standard energy value is measured, and it is this NHE value which determines whether a galvanic displacement can occur or not. Among the most used metals in nano-catalysis, copper has the lowest value (0.34) and as such it is normally used as the sacrificial metal to create nanoparticles which a more ‘noble’ Table 1. Platinum and gold have 1.2 and 1.5 respectively, therefore, copper can be displaced by both Pt and Au. The most important part of the galvanic displacement is assuring that the molar equivalent of the more noble metal has a 1:1 ratio with those of the sacrificial metal Fig 3. The other way of obtaining Pt and Au nanoparticles is via the galvanic displacement of Ag, since the NHE values of the more noble metals are larger than those of the Ag. In the case of Pt, this method has shown to create fully reduced nanoparticles as compared to the direct borohydride reduction which have shown to be unable to reduce all the Pt2+ ions to nanoparticles [65].. 7.

(37) Chapter 1: Synopsis. Figure 3: Generalized synthesis process of dendrimer encapsulated nanoparticles using generation six PAMAM dendrimer by galvanic exchange method.. There are three methods of synthesizing dendrimer encapsulated nanoparticles (DENs), the partial displacement, the sequential loading and the co-complexation method. One method of interest is the galvanic exchange of one metal NP by a more noble transition metal [66]. In a galvanic exchange method, metals with higher normal hydrogen energy (NHE.eV) can be formed by displacing those metals with lower NHE values Fig 3 [65,67,68]. The galvanic Exchange is also known as partial displacement method, this is a versatile method for the synthesis of stable monometallic, bimetallic and core@shell nanoparticles using dendrimers as templating agents [23–25]. This method is very quick in terms of synthesis time, taking only a few hours compared to the traditional method of direct borohydride reduction taking three days. In principle, the copper is usually used first as a sacrificial nanoparticle and then displaced by a more noble metal to prepare DENs [67]. Platinum has shown to produce very stable, narrow disperse, and shape selective nanoparticles by this galvanic displacement reaction [69]. 1.3.2. Catalytic applications of supported Pt and Au nanoparticles The use of noble metals in catalysis is known from antiquity, with over thousands of publications in literature. They are known to increase the thermal stability and catalytic synergistic effect of the metal oxide in oxidation and reduction reactions. To fully comprehend the impact of noble metals on any support, its advisable to run the catalytic reaction with and without the noble metal, and to vary the size of the noble metal supported on the mesoporous metal oxide. Chandler and coworkers have investigated the effect of the support and size of different metal NPs on the supports. Hutchings and Astruc focused deeply on the applications of gold atoms as catalyst. Ballauff et al have investigated the immobilization of Pt, Au and AuPt alloy nanoparticles onside the pores of mesoporous silica, SBA-15 and MCM-41. Other researchers have also investigated the immobilization of other noble transition metal NPs such as palladium, silver, nickel, copper, rhodium and iridium nanoparticles on the mesoporous silica. Mesoporous SiO2 has an amorphous structure and described by the broad intensity at 10 ‒ 65 degrees at 2θ, it is therefore inactive. Therefore, chemists take advantage of this fact and support the colloidal noble metal NPs such that the synergistic effect could be elucidated. This application becomes valuable when dealing with mesoporous transition metal oxides, since the M‒O‒M can act as an active site in the catalytic. 8.

(38) Chapter 1: Synopsis. oxidation of some reactions. The difference in the activation energy EA, rate constant k, enthalpy ΔH#, entropy ΔS#, Gibbs free-energy ΔG#, equilibrium adsorption and desorption rate constants KA and KB can be calculated and summarized.. Figure 4: Catalytic activity of the CO oxidation on Pt nanoparticles (10 and 50 pulses) on sputtered and sol gel-prepared TiO2 thin films: (a) turnover frequency (TOF) as a function of increasing temperature and (b) Arrhenius plots derived from Fig 4 (a) [72].. The catalytic activity of Pt and Au NPs supported on mesoporous transition metal oxides ranges from gaseous reactions, homogeneous and heterogeneous. The CO conversion car exhaust, hydrogenation of pinene to pinane, and CO oxidation are examples of gaseous reactions with Pt NPs on support. The synthesis of many pharmaceutical drugs raw materials are obtained by Pt and Au NPs in both batch and flow reactors. Hydroformylation [73], aminolysis of epoxides, polymerization reactions, esterification, biomass conversion, isomerization, styrene and alcohol oxidation, are all at some point carried out using a noble metal on a support as catalyst. Most of the publications reveal that the incorporation of the NPs results in higher conversions and yields, whereas selectivity becomes affected substantially. One method of preparing immobilized nanoparticles is the coaxial vacuum arc plasma deposition (APD) method. This method allows for large scale synthesis of nanoparticles and thus is ideal for industrial applications. In the thesis of Qadir 2015 [72], Pt nanoparticles immobilized on APD Pt/TiO2 prepared by two different methods i.e., sol-gel synthesis and multitarget sputtered TiO2, the latter system exhibited improved catalytic activity with an increasing temperature Fig 4. The turn over frequency (TOF) and turn over numbers (TON) of the 9.

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