RAPID SYNTHESIS OF AUAG ALLOY ON LDHs : HIGHLY ACTIVE CATALYST FOR BENZYL ALCOHOL OXIDATION WENTALIA WIDJAJANTI (M.Sc., NATIONAL CHENG KUNG UNIVERSITY, TAIWAN) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF CHEMICAL AND BIOMOLECULAR ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2012 DECLARATION BY CANDIDATE I hereby declare that this thesis is my own work and effort and that it has not been submitted anywhere for any award. Where other sources of information have been used, they have been acknowledged. Signature: ………………………………………. Name: Wentalia Widjajanti Date: …19 February 2013…………………………………………. Acknowledgements In my course of research work, I have encountered several people whom I am grateful towards. First and foremost, I would like to thank my supervisor for their guidance in my research. Prof Zeng Hua Chun has been enlightening me on the right direction to take for research, and also holding fruitful discussions on the many creative ideas to work on. I am very grateful for the support and mentoring of Prof Zeng Hua Chun as my supervisor during my M.Eng. candidature. My laboratory mates have also been supportive during my work. I would like to thank them (Christopher, Cheng Chao, Dou Jian, Ming Hui, Sheng Yuan, Li Xuan Qi, Li Zheng, and Xi Bao Juan) for guiding me through the use of equipment as well as providing constructive feedback regarding the research topics. Last but not least, I would like to thank my parents, my family, Wainam Fong, Erwin Santoso, Dicky Pranantyo, Yu Nan and all my friends for their care throughout these years and supported me in one way or another during my candidature. i Table of Contents ACKNOWLEDGEMENTS ........................................................................................ I TABLE OF CONTENTS ........................................................................................... II SUMMARY ................................................................................................................. V LIST OF FIGURES .................................................................................................. VI LIST OF TABLES .................................................................................................... IX CHAPTER 1 ................................................................................................................. 1 INTRODUCTION........................................................................................................ 1 1.1 References .............................................................................................................. 4 CHAPTER 2 ................................................................................................................. 7 LITERATURE REVIEW ........................................................................................... 7 2.1 Au-Ag alloy NPs .................................................................................................... 7 2.2 Preparation of Au-Ag alloy NPs .......................................................................... 9 2.3 Layered double hydroxides (LDHs) .................................................................. 11 2.3.1 Structure of hydrotalcites .............................................................................. 12 2.3.2 Preparation methods ...................................................................................... 14 2.4 Attachment of metal NPs onto the support ...................................................... 18 2.5 Alcohol oxidation and green chemistry............................................................. 21 2.6 References ............................................................................................................ 24 CHAPTER 3 ............................................................................................................... 29 EXPERIMENTAL DETAILS .................................................................................. 29 3.1 Characterization techniques .............................................................................. 29 3.1.1 Tranmission electron microscopy ................................................................. 29 3.1.2 Ultraviolet visible absorption spectroscopy .................................................. 31 ii 3.1.3 3.1.4 3.1.5 3.1.6 X-ray diffraction ............................................................................................ 31 Infrared spectrometry .................................................................................... 33 Gas chromatography-flame ionization detector (GC-FID) ........................... 33 Field emission scanning electron microscope (FE-SEM) and energy dispersive X-ray spectroscopy (EDX)........................................................... 34 3.1.7 Thermo gravimetric analyzer (TGA) ............................................................ 35 3.2 Synthesis of Au-Ag alloy NPs ............................................................................. 36 3.3 Synthesis of Au-Ag alloy NPs attached onto LDHs ......................................... 37 3.4 Experimental procedure ..................................................................................... 38 3.4.1 Preparation of Au(I) dodecanthiolate (Au(I) DDT) ...................................... 39 3.4.2 Preparation of Ag(I) dodecanethiolate (Ag(I) DDT) .................................... 39 3.4.3 Preparation of Au-Ag alloy and pure metal NPs using half seeding method 40 3.4.4 Preparation of Au-Ag alloy NPs attached onto LDHs using impregnation method ........................................................................................................... 40 3.4.5 Preparation of Au-Ag alloy NPs attached onto LDHs using MUA and MPTMS as a linkage ..................................................................................... 41 3.4.6 Preparation of catalyst for alcohol oxidation reaction .................................. 42 3.5 References ............................................................................................................ 43 CHAPTER 4 ............................................................................................................... 44 CHARACTERIZATION OF AU-AG/LDHS AS CATALYST ............................. 44 4.1 Results for metal alkanethiolate polymers ....................................................... 44 4.1.1 UV-Visible absorption .................................................................................. 44 4.1.2 NPs structure analysis ................................................................................... 45 4.1.3 Characterization of LDHs NPs ...................................................................... 49 4.1.3.1 Studies on the effect of aging times and temperatures on LDHs ........... 54 4.2 Characterization of Au-Ag alloy NPs/LDH composites .................................. 57 4.2.1 FTIR result for functionalization oleylamine-LDHs (Method 1).................. 63 4.2.2 TGA comparison between NO3-LDHs and Cl-LDHs ................................... 65 4.2.3 XRD results for Au-Ag alloy/LDHs (Method 1) .......................................... 66 4.2.4 Energy dispersive X-ray photospectroscopy ................................................. 68 4.2.5 SEM of LDHs................................................................................................ 73 4.3 Results and discussion for catalytic activity measurement ............................. 73 4.3.1 Catalytic activity testing ................................................................................ 75 4.3.2 Comparison of catalytic performance of catalysts prepared using Method 1 and Method 2 ................................................................................................. 78 4.3.3 Studies on the effect of reaction temperatures of NO3-LDHs ....................... 80 iii 4.3.4 Studies on the effect of calcination temperatures of NO3-LDHs .................. 82 4.3.5 Studies on the effect of Au/Ag ratios of NO3-LDHs ................................... 84 4.3.6 Studies on the effect of overall reaction progress of Au-Ag alloy/NO3-LDHs ....................................................................................................................... 85 4.3.7 Studies on the effect of Au-Ag loading on NO3 LDHs ................................. 86 4.3.8 Studies on the effect of calcination temperatures for Au-Ag alloy/Cl-LDHs ....................................................................................................................... 88 4.3.9 Studies on the effect of Au/Ag alloy ratios on Cl-LDHs .............................. 89 4.3.10 Studies on the effect of alcohol oxidation temperatures on Cl-LDHs .......... 91 4.3.11 Studies of Au-Ag loading effect on Cl-LDHs ............................................... 93 4.3.12 Comparison studies of recyclability of NO3-LDHs and Cl-LDHs ................ 95 4.4 References ............................................................................................................ 99 CHAPTER 5 ............................................................................................................. 101 CONCLUSIONS ...................................................................................................... 101 5.1 Preparation of Au-Ag alloy/LDHs NPs attached on LDHs ........................... 101 5.2 Catalytic activity of catalyst ............................................................................. 102 5.3 Studies comparing the recyclability of NO3-LDHs and Cl-LDHs ................ 103 5.4 Further research ............................................................................................... 104 APPENDIX ............................................................................................................... 106 iv Summary This thesis contains two parts of experimental results and discussion. In the first part (Chapter 3), two methods of preparing the gold-silver alloy attached onto layered double hydroxides (LDHs) are introduced. The results of the two methods were compared in terms of the monodispersity of the particles obtained and the controllability of their properties. The difference in the results obtained was also explained based on the proposed mechanisms of nucleation and growth of the particles. This work is motivated by a desire to develop a fast, efficient and novel method to prepare LDH supported alloy nanoparticles which are known to have many potential applications especially as a catalyst. In Chapter 4, we discuss the catalytic activity of the catalyst contain LDHs as a support and gold-silver alloy as a nanoparticle. We propose two different types of LDHs sources (NO3 and Cl based sources) that showed different levels of catalytic activity. It is observed from previous reports that the activity peaked when the metals were added to the support at the ratio 1:1 for Au-Ag/NO3-LDHs, whereas for AuAg/Cl-LDHs, the highest conversion of benzyl alcohol was obtained when the ratio of Au:Ag was at 3:1. In general, from the catalytic activity result for each parameter, it can be seen that the use of NO3-LDHs resulted in higher benzyl alcohol conversion than Cl-LDHs. This happened because NO3-LDHs support can activate O2 thus leading to faster recovery of the supported Au-Ag catalyst compared to unsupported catalysts. The work presented here was performed using LDHs as a support for heterogeneous catalyst system and the results obtained should make contribution to existing knowledge, since the LDHs might also affect the performance of a catalyst. v List of Figures Figure 2.1. Illustration of the possible structures that alloy NPs can attain: (left) segregated nanoalloy (right) randomly mixed A-B nanoalloy. ..................................... 8 Figure 2.2. Layered structures of LDHs. ..................................................................... 13 Figure 2.3. Formation of Au NPs on the surface of a solid support through adsorption forces. ........................................................................................................................... 18 Figure 2.4. Differences between a stochiometric and a catalytic process for the selective epoxidation of C=C bonds. ........................................................................... 23 Figure 3.1. Transmission electron microscope. ........................................................... 30 Figure 3.2. UV-Visible spectrophotometer. ................................................................. 31 Figure 3.3. X-ray Diffractometer. ................................................................................ 32 Figure 3.4. Gas chromatography-flame ionization detector. ....................................... 34 Figure 3.5. Half seeding method. ................................................................................. 36 Figure 3.6. Method 1 to grow Au-Ag NPs onto LDHs. ............................................... 38 Figure 3.7. Method 2 to grow Au-Ag NPs onto LDHs. ............................................... 38 Figure 4.1. Normalized UV-Vis absorption spectra of pure Au, Au:Ag alloy(1:1), and pure Ag dispersed in ethanol........................................................................................ 44 Figure 4.2. TEM images of LDH NPs with different aging times and temperatures, (a) aging time of 12 hours, reaction temperature of 100oC, (b) aging time of 24 hours, reaction temperature of 100oC, (c) aging time of 48 hours, reaction temperature of 100oC, (d) aging time of 12 hours, reaction temperature of 180oC, (e) aging time of 24 hours, reaction temperature of 180oC, (f) aging time of 48 hours, reaction temperature of 180oC. ...................................................................................................................... 55 Figure 4.3. (a) Illustration of the deposition of Au-Ag alloy NPs onto LDHs surface for catalyst prepared using Method 1, and (b) illustration of the deposition of Au-Ag alloy NPs onto LDHs surface for catalyst prepared with MPTMS (left) and MUA (right) using Method 2. ................................................................................................ 59 Figure 4.4. Au-Ag alloy NPs prepared using the half seeding method. ...................... 60 Figure 4.5. TEM images of LDHs-supported Au-Ag NPs using Method 1. ............... 61 Figure 4.6. TEM images of LDHs-supported Au-Ag NPs using MUA as a linker. .... 62 vi Figure 4.7. TEM images of LDHs-supported Au-Ag NPs using MPTMS as a linker.63 Figure 4.8. FTIR spectra of (a) Ni-Al-Cl LDHs, (b) Ni-Al-NO3 LDHs, and (c) oleylamine-LDHs. ........................................................................................................ 64 Figure 4.9. TGA curves of Ni-Al-Cl LDHs and Ni-Al-NO3 LDHs............................. 65 Figure 4.10. XRD peaks of (a) Ni-Al-Cl LDHs, (b) Ni-Al-NO3 LDHs, (c) NO3-LDHssupported Au-Ag alloy calcinated at 350oC for 4 hours (Metal-Ni-Al-O composite), (d) Cl-LDHs-supported Au-Ag alloy (Metal-Ni-Al-O composite) calcinated at 350oC for 4 hours. ................................................................................................................... 66 Figure 4.11. XRD patterns of enlarged portion of (c) NO3-LDHs-supported Au-Ag alloy (Metal-Ni-Al-O composite), (d) Cl-LDHs-supported Au-Ag alloy (Metal-Ni-AlO composite) with 2θ from 30o to 70o. ........................................................................ 67 Figure 4.12. TEM images for different ratios of Au:Ag alloy deposited onto Ni-Al-Cl LDHs after calcination at 350oC for 4 hours. .............................................................. 71 Figure 4.13. TEM images for different ratios of Au:Ag alloy deposited onto Ni-AlNO3 LDHs after calcination at 350oC for 4 hours. ...................................................... 72 Figure 4.14. SEM images of (a) Cl-LDHs, and (b) NO3-LDHs. ................................. 73 Figure 4.15. GC graphic of benzyl alcohol conversion induced by LDHs-supported Au-Ag alloy NPs. ......................................................................................................... 76 Figure 4.16. (a) TGA curves of LDHs/Au-Ag-MPTMS and LDHs/Au-Ag, (b) TEM images of LDHs/Au-Ag-MPTMS after calcination at 400oC. ..................................... 80 Figure 4.17. Product yield (%) of Au:Ag (1:1) deposited onto NO3-LDHs catalyst for alcohol oxidation reaction with varying different temperatures versus reaction time (hr)................................................................................................................................ 82 Figure 4.18. Product yield (%) of Au:Ag (1:1) deposited onto NO3-LDHs catalyst at 80oC reaction temperature reaction with varying calcination temperatures versus reaction time (hr).......................................................................................................... 83 Figure 4.19. Product yield (%) of Au:Ag deposited onto NO3-LDHs catalyst at 80oC reaction temperature with varying metal ratios versus reaction time (hr). .................. 85 Figure 4.20. Product yield (%) of Au-Ag alloy/NO3-LDHs catalyst for alcohol oxidation reaction based on overall reaction progress. ................................................ 86 Figure 4.21. Product yield (%) of Au-Ag alloy/NO3-LDHs catalyst for alcohol oxidation reaction for different Au-Ag loading on NO3-LDHs. .................................. 87 Figure 4.22. Product yield (%) of Au:Ag (1:1) deposited onto Cl-LDHs catalyst for alcohol oxidation reaction at 110oC with varying calcination temperatures. .............. 89 vii Figure 4.23. Product yield (%) versus reaction time (hr) of Au-Ag NPs deposited onto Cl-LDHs catalyst for alcohol oxidation reaction at 110oC with varying metal ratios. 90 Figure 4.24. Product yield (%) of Au:Ag (1:1) deposited onto Cl-LDHs catalyst for alcohol oxidation reaction with varying reaction temperatures. .................................. 93 Figure 4.25. Product yield (%) of Au:Ag (1:1) deposited onto Cl-LDHs catalyst for alcohol oxidation reaction with varying Au-Ag loading deposited onto Cl-LDHs. .... 95 Figure 4.26. Product yield (%) of Au:Ag (1:1) deposited onto Cl-LDHs and NO3LDHs catalyst for alcohol oxidation reaction with varying number of cycles. ........... 97 Figure 4.27. Product yield (%) of Au:Ag (1:1) deposited onto NO3-LDHs catalyst for alcohol oxidation reaction with varying number of cycles compared with and without washing with NaOH 0.5 M. ......................................................................................... 98 viii List of Tables Table 4.1. TEM images for pure Au, pure Ag, and Au-Ag alloy at different compositions. ............................................................................................................... 45 Table 4.2. TEM images of LDHs with different ratios of urea to Ni-Al concentration. ...................................................................................................................................... 50 Table 4.3. Amount of elements by weight % for different ratios of Au:Ag alloy deposited onto Ni-Al-Cl LDHs. ................................................................................... 69 Table 4.4. Amount of elements by weight % for different ratios of Au:Ag alloy deposited onto Ni-Al-NO3 LDHs................................................................................. 71 Table 4.5. TON number of each catalyst with different ratios of Au:Ag alloy deposited onto Ni-Al-Cl LDHs. ................................................................................... 78 Table 4.6. TON number of each catalyst with different ratios of Au:Ag alloy deposited onto Ni-Al-NO3 LDHs................................................................................. 78 Table 4.7. Percent conversion and percent selectivity of Au-Ag alloy deposited on the LDHs for 2 hours via benzyl alcohol oxidation with reaction temperature at 80oC. ... 79 ix CHAPTER 1 INTRODUCTION One of the most important processes in the production of fine and specialty chemicals is alcohol oxidation.1 Conventional alcohol oxidation methods involve the use of toxic and expensive stoichiometric metal oxidants, such as chromate and permanganate,2 or harmful organic solvents,3 or require vigorous reaction conditions.4 From both the environmental and economic points of view, there is a strong incentive to develop a green, economic, and efficient alcohol oxidation process.5 The use of heterogeneous solid catalysts in oxidation of alcohols have garnered more attention over homogenous solid catalysts, for reasons such as ease of recovery and recycling, atom utility, as well as enhanced stability in the oxidation reaction. The aforementioned heterogeneous systems can be developed by using noble metal nanoparticles (NPs) supported in liquid phase. Noble metal NPs supported in liquid phase have been identified as potential catalyst for a broad range of hydrogenation and oxidation reactions. Since metal NPs have high tendency to agglomerate and the bulk metal is thermodynamically unstable, organic ligands, surfactants, polymers or inorganic coatings are employed to control the size of NPs and to keep them stable by steric or electrostatic stabilization.5-7 However, stabilization of NPs in the same phase as the reactants might hamper the separation of catalyst from reactants. Strategies to facilitate NPs separation include decantation of biphasic systems, such as the biphasic water/organic solvent system,8-10 or the two-phase ionic liquids system.11-13 In addition, filtration or centrifugation of NPs immobilized with organic and inorganic supports are also effective in separating NP catalysts from the liquid reactants. In 1 general, supported NP catalysts exhibit higher catalytic activities than unsupported NPs regardless of the separation method employed.14,15 Noble metal NPs supported in liquid phase have been extensively studied as catalysts for reactions that are commonly used in the pharmaceutical industry and perfume industry. Tsukuda et al. and Corma et al. reported that Au nanoclusters deposited on metal oxides or polymers are highly effective for the aerobic oxidation of various alcohols.16,17 In addition to that, Ni–Al-based LDH materials have been extensively studied for their application as catalysts. For example, catalytic production of hydrogen by steam reforming of methanol has been carried out using calcined Ni–Al LDH materials, and high selectivity in formation of H2 and CO2 was observed. Moreover, there are various recent literature on Ni-Al LDHs and its various material properties such as crystallinity, porous structure, reducibility, acidity, basicity, catalytic activity and selectivity of ethanal in ethanol oxidation process affected by hydrothermal treatment.18,19 Furthermore, the activation of molecular oxygen on Ni in Ni-Al hydrotalcite-like anion clay was also reported to take place in the oxidation of alcohols.27 Supported metal NPs can be prepared by impregnation methods whereby the metal NPs size and size distribution are finely controlled using organic ligands as capping agent. The organic ligands have to be carefully selected (weakly bound ligands are preferred) or removed to recover activity. On one hand, the presence of protective organic capping ligands or their decomposition products could have a detrimental effect on catalytic activity as the ligands can block catalytically active sites on the surface of NPs.20,21 On the other hand, the capping ligands can also act as spacers between the metal NPs and the support in such a way that beneficial metal– support interactions can be obtained. These interactions can be further tuned and 2 optimized in order to increase the activity of supported metal catalysts.22 For example, one ligand commonly used is dithiol organoalkoxysilane, whereby Au NPs are linked to the ditihiol end24 and LDHs are linked to the organoalkoxysilane end.25,26 In addition to the ligands used in synthetic protocols as stabilizers for NPs, new functional groups can also be anchored on the solid support to assemble NPs; this is a very well-known strategy in other fields of application.23 When a functionalized solid is exposed to a solution of NPs, the terminal groups will enhance the metal–support interaction and attract the metal NPs onto the surface of the solid. In catalytic applications, however, the ligands grafted on the support surface are known to retard catalytic properties of the supported metal NPs. Unfortunately, this possible influence has been underestimated until recently. In this thesis, we summarize some of the emerging approaches for the preparation of noble metal NPs supported by LDHs, with control over variables affecting catalyst activity and selectivity, such as NPs size and size distribution. In addition, special attention was also paid to the use of modified preparation methods that utilize ligands to link metal NPs to LDHs support. It is worth mentioning that our current understanding of how the ligands influence morphology of NPs, metal– support interactions, and catalytic activities is still lacking. After a discussion on the methods used to synthesize metal alloy NPs and ligands grafted onto LDHs support, we explain the methods used in synthesizing catalysts comprising the metal alloy NPs and the support aforementioned. In order to better understand the effectiveness of the prepared catalysts, these catalysts were then applied in alcohol oxidations. Our catalysts can be regenerated with the addition of base unlike other catalysts which require sintering. 3 1.1 References 1. Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed Oxidation of Organic Compounds, New York: Academic Press. 1981. 2. Mijs, W. J.; Jonge, C. R. H. Organic Synthesis by Oxidation with Metal Compounds, New York: Plenum Press. 1986. 3. (a) Hou, Z.; Theyssen, N.; Brinkmann, A.; Leitner, W. Angew. Chem., 44, 1346. Int. Ed. 2005.; (b) Zhan, B. Z.; White, M. A.; Sham, T. K.; Pincock, J. A.; Doucet, R. J.; Robertson, K. N.; Cameron, T. J. Am. Chem. Soc. 2003, 125, 2195; (c) Guan, B. T.; Xing, D.; Cai, G. X.; Wan, X. B.; Yu, N.; Fang, Z.; Shi, J. J. Am. Chem. Soc. 2005, 127, 18004. 4. (a) Zhang, C. X.; Chen, P.; Liu, J.; Zhang, Y. H.; Shen, W.; Xu, H. L.; Tang, Y. Chem. Commun. 2008, 3290; (b) Shen, J.; Shan, W.; Zhang, Y. H.; Du, Y. M.; Xu, H.; Fan, K. N.; Shen, W.; Tang, Y. Chem. Commun. 2004, 2880. 5. Astruc, D.; Lu, F.; Aranzaes, J. R. Angew. Chem. Int. Ed. 2005, 44, 7852. 6. Doyle, A. M.; Shaikhutdinov, S. K.; Jackson, S. D.; Freund, H. J. Angew. Chem. Int. Ed. 2003, 42, 5240. 7. Dahl, J. A.; Maddux, B. L. S.; Hutchison, J. E. Chem. Rev. 2007, 107, 2228. 8. Mevellec, V.; Roucoux, A.; Ramirez, E.; Philippot, K.; Chaudret, B. Adv. Synth. Catal. 2004, 346, 72. 9. Roucoux, A.; Schulz, J.; Patin, H.; Adv. Synth. Catal. 2003, 345, 222. 10. Vasylyev, M. V.; Maayan, G.; Hovav, Y.; Haimov, A.; Neumann, R. Org. Lett. 2006, 8, 5445. 11. Dupont, J.; Fonseca, G. S.; Umpierre, A. P.; Fichtner, P. F. P.; Teixeira, S. R. J. Am. Chem. Soc. 2002, 124, 4228. 4 12. Geldbach, T. J.; Zhao, D. B.; Castillo, N. C.; Laurenczy, G.; Weyershausen, B.; Dyson, P. J. Am. Chem. Soc. 2006, 128, 9773. 13. Polshettiwar, V.; Luque, R.; Fihri, A.; Zhu, H. B.; Bouhrara, M. J. M. Chem. Rev. 2011, 111, 3036. 14. Park, I. S.; Kwon, M. S.; Kim, N.; Lee, J. S.; Kang, K. Y.; Park, J. Chem. Commun. 2005, 45, 5667. 15. Elisson, C. H. P.; Vono, L. L. R.; Hubert, C.; Denicourt, A.; Rossi, L. M.; Roucoux, A. Catal. Today. 2012, 183, 124. 16. Tsunoyama, H.; Sakurai, H.; Negishi, Y.; Tsukuda, T. J. Am. Chem. Soc. 2005, 127, 9374. 17. (a) Abad, A.; Concepcion, P.; Corma, A.; Garcıa, H. Angew. Chem. Int. Ed. 2005, 44, 4066; (b) Abad, A.; Almela, C.; Corma, A.; Garcıa, H. Tetrahedron. 2006, 62, 6666. 18. Qi, C.; Amphlett, J. C.; Peppley, B. A. Appl. Catal. A Gen. 2006, 302, 237. 19. Mikulova, Z.; Cuba, P.; Balabanova, J.; Rojka, T.; Kovanda, F.; Jiratova, K. Chem. Pap. 2007, 61, 103. 20. Kuhn, J. N.; Tsung, C. K.; Huang, W.; Somorjai, G. A. J. Catal. 2009, 265, 209. 21. Stowell, C. A.; Korgel, B. A. Nano Lett. 2005, 5, 1203. 22. Sonstrom, P.; Arndt, D.; Wang, X.; Zielasek, V.; Baumer, M. Angew. Chem. Int. Ed. 2011, 50, 3888. 23. Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221. 24. Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science. 1996, 273, 1690. 5 25. Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. Science. 1995, 267, 1629. 26. Westcott, S. L.; Oldenburg, S. J.; Lee, T. R.; Halas, N. J. Langmuir. 1998, 14, 5396. 27. Rahman, A.; Al-Deyabj, S. Chil. Chem. Soc. 2011, 56, 598. 6 CHAPTER 2 LITERATURE REVIEW 2.1 Au-Ag alloy NPs Bimetallic NPs exhibit significant catalytic properties due to the synergistic effect of the metals. They have attracted great attention because of their changeable composition which leads to their unique size-dependent electronic, optical, and catalytic characteristics, which differ from those of the corresponding pure metal particles.1,2 For example, gold-containing bimetallic NPs show enhanced catalytic activity.3 Au-Ag alloy NPs are catalytically more active than monometallic NPs, Au or Ag NPs, in the oxidation of CO at low temperatures.4 Uniform Au-Ag alloy NPs have been prepared via solution synthetic procedures.5 Numerous chemical methods have been employed for the synthesis of bimetallic NPs using various reducing agents, including sodium borohydride4, citrate6, and hydrazine7. Generally, there are two categories of bimetallic NPs: i) alloy particles wherein the two metals are mixed in the same region of space and ii) core-shell NPs wherein the core and shell material differ. Alloys can be further subdivided into segregated nanoalloys and mixed A-B nanoalloys (see Figure 2.1).8 In the former case, the atoms of one metal are segregated from the atoms of the other metal, combined only at the interface. In the latter case, the atoms of both metals are mixed together, either orderly or randomly. The atomic arrangement in the alloy NPs formed depends on the preparation methods and experimental conditions. Other factors include the relative atomic sizes of the two metals, strength of bonding between the metals and with the surfactants, as well as the surface energies of the metals. 7 The Au-Ag system is expected to be miscible in all proportions due to the similar lattice constants of gold and silver (2.36 Å for Ag and 2.35 Å for Au) and their similar atomic sizes. Furthermore, both silver and gold have the fcc structure. However, due to the similarity in the lattice constants, X-ray diffraction (XRD) could not be used to differentiate the alloys from the pure metals, as the peaks would appear at the same position. In fact, it has been reported that there is no obvious contrast/peak shift in the high resolution transmission electron microscopy (HRTEM) image upon changing from core-shell Ag-Au particles to the alloys.9 Figure 2.1. Illustration of the possible structures that alloy NPs can attain: (left) segregated nanoalloy (right) randomly mixed A-B nanoalloy. Hence, UV-Visible absorption spectroscopy is frequently employed to distinguish Au-Ag alloy NPs from the core-shell particles or a mixture of Au and Ag particles. A mixture of separate Au and Ag NPs would give rise to two absorption peaks. The Plasmon oscillation in the alloy particles is a hybrid resonance that results from the excitation of the conduction d-band electrons.10 Hence for alloy NPs, only one band will be observed in the spectrum between that of pure Au and Ag, which red-shifts when the percentage of gold increases.11-15 Even for Au-Ag alloy particles smaller than 2 nm, a single absorption peak was obtained at a λ max value intermediate between that of pure Au and pure Ag.16 On the other hand, core-shell 8 particles smaller than 3 nm exhibit two absorption peaks: i) one for Au core and ii) Ag shell. Only when there was a large excess of Ag shell was there a disappearance of the Au plasmon and the presence of single peak at the position of Ag absorption. This is consistent with the results for larger core-shell NPs, where two peaks are observed near the energies of the absorption bands of the parent metals.17-20 In fact, the occurrence of a single band in the UV-Visible absorption spectrum and its dependence on the composition of the alloy offers a tuneable optical spectrum. It is known that the peak position of the plasmon absorption of gold NPs cannot be readily correlated to the size of the particles10, and it has been previously shown that the plasmon band position of spherical gold or silver NPs only changes slightly when their size changes to within 1 nm to 100 nm14. Hence should absorption at a particular wavelength be needed, alloy NPs can be used, as the peak position of the plasmon absorption is strongly dependent on the composition of the alloys. The application of such a tuneable plasmon absorbance lies in systems such as bio-labels and biosensors. Absorbance of light at a specific wavelength also finds many applications such as in the production of marker materials which can provide unambiguous identification. 2.2 Preparation of Au-Ag alloy NPs Several methods have been employed to prepare Au-Ag alloy NPs. The most common has been the co-reduction of HAuCl4 and AgNO3 in the presence of hydrazine, citrate in aqueous solution21, and the photochemical method.19 The direct use of the two metal salt precursors has the disadvantages of having to take precaution against the formation of silver chloride, which can lead to a failure in the synthesis. 9 The precipitation of silver chloride has been overcame by using methods such as laser ablation of bulk alloys which also results in the formation of alloy NPs. 22 The particles synthesized are however, not monodispersed. Recently, synthesis of alloy NPs has also been achieved by a replacement reaction between silver NPs and gold metal complex.23 Heating silver and gold precursors in oleylamine also yields alloy NPs.24 However, the silver precursors have to be added in great excess in order to compensate for its slower reduction and this results in an unpredictable composition of the final NPs. Digestive ripening of core-shell Au-Ag NPs in 4-tert-butyltoluene for 8 hours can also lead to the formation of alloys.25 This is similar to a previous work involving the annealing of core–shell Ag-Au NPs in oleylamine at 100oC to obtain alloy particles.9 Small alloy NPs which are less than 5 nm protected with alkanethiolates and dendrimers have also been prepared.13,16 However, the reaction time for these procedures is several hours long. In contrast to alloy particles, the procedure for coreshell particle synthesis involves the deposition of shell metal onto core metal particles.9,17,18,20 This should not be confused with a seeding method26-28 which involves a separation of nucleation and growth, hence resulting in monodispersity. In the seeding method, small particles are first prepared and act as seeds on which further growth of a metal occurs. By inhibiting further nucleation and controlling the growth of the particles, monodispersity can be achieved. 10 2.3 Layered double hydroxides (LDHs) Clays are lamellar solids characterised by charged layers. They may be divided into two broad families: cationic and anionic clays.29,30 The so-called “hydrotalcites” (HT), that is, the solids that have a structure closely related to that of the mineral hydrotalcites, that is, rhombohedral Mg6Al2(OH)16CO3.4H2O, classified as anionic clays. There are 3 important characteristics that make them useful in various applications. First, hydrotalcites have a good anion exchange capacity31,32, and therefore are used as ion-exchangers, adsorbents33,34 or sensors35. Secondly, most hydrotalcites can be used as catalysts for several reactions such as self-condensation and, cross-aldol condensation of aldehydes. Thirdly, hydrotalcites can be prepared with several reducible bivalent (Ni, Cu, Co) and trivalent (Fe, Cr) cations in the structure together with the native cations (Mg, Zn, Al) serving as precursors for the preparation of different mixed oxides. The hydrotalcites are active for alcohol oxidation and hydrogenation/dehydrogenation reactions. Naturally occurring hydrotalcites, and synthetic hydrotalcites-like compounds, also called layered double hydroxides (LDHs), have been investigated for many years. 36,37 The formula of LDHs can be generalized to [M2+1-XM3+X(OH)2]x+[An-x/n.mH2O]x- where M2+ can be Ni2+, Zn2+, Mn2+, Ca2+, and etc.; M3+ can be Al3+, Ga3+, Fe3+, Cr3+, and etc.; and An- can be NO3-, Cl-, CO32-, SO42-, and etc.38 The high anion exchange capacity of LDHs-like materials allow for versatile interlayer anion exchange among inorganic anions as well as among organic anions.39-41 LDHs have been studied extensively for a wide range of catalyst applications,42,43 ceramic precursors,51,44 adsorbents,52,45 bio-organic nanohybrids,46,47 and scavengers of pollutant metals and anions48. Recent research has shown the great flexibility of LDHs-like materials in 11 tailoring chemical and physical properties of materials to be used for specific applications, e.g. molecular recognition, optical storage, batteries, and etc.49-51 Furthermore, researchers have been able to produce catalyst precursors by introducing various transition36 and noble metals52 into the sheets of the LDHs structure. More recently, there has been a large number of new developments using LDHs as a matrix for the storage and delivery of biomedical molecules53,54 and as gene carrier. 2.3.1 Structure of hydrotalcites The basic structure of the clay is closely related to that of brucite, Mg(OH)2. In a typical brucite layer, each Mg2+ is octahedrally surrounded by six OH- ions, resulting in an octahedron that shares its edges with neighbouring Mg(OH)6 octahedra.55 Hydrotalcites are structurally characterized as brucite-like layers in which some of the divalent cations are replaced by trivalent cations, resulting in a net positive charge. This charge is neutralised by the incorporation of exchangeable anions and the water molecules between the layers. The neutrality in hydrotalcite is maintained by carbonate ions. It also contains interlayer water which forms hydrogen bonds with the OH- layer or with the interlayer anions. Hence the 3-D structure of the clay is maintained by the electrostatic interaction and hydrogen bonding between the layer and interlayer anions or molecules.56 The height of each layer of the Mg(OH)6 sheet is 4.77 Å. These sheets are stacked on top of each other and held together with hydrogen bonding. As mentioned earlier, the substitution of Mg2+ ions with Al3+ ions leaves a net positive charge in the interlayer. The carbonate anions counterbalance the positive 12 charge in natural hydrotalcites. However, in the case of their synthetic counterparts, the net positive charge is counterbalanced by various anions and the predominant bonding that exists is electrostatic. In contrast, the dominant interactions in anion exchange involving surfactant anions with long-chain alkyl groups that play an essential role in catalyst applications are hydrophobic interactions instead of electrostatic. Figure 2.2. Layered structures of LDHs. In Figure 2.2, M2+ and M3+ represent the divalent and trivalent metal ions respectively. The interlayer region is composed of hexagonal close-packed sites parallel to the close-packed layers of the hydroxyl groups and metal cations. 13 2.3.2 Preparation methods There are numerous methods by which LDHs may be synthesized. These include electrochemical methods, co-precipitation, sol-gel, hydrothermal crystallisation and urea hydrolysis reaction.57-59 These preparation methods give a wide variety of compositions, M2+:M3+ ratios and metal combinations. The sol-gel method involves the formation of a mobile colloidal suspension that gels due to internal cross-linking. Prinetto et al. prepared Al-Mg and Al-Ni LDHs from the hydrolysis of alkoxides or acetylacetonate precursors with HNO3 and HCl. The principle employed is the hydrolysis and condensation of a metal alkoxides solution. The alkoxides are first dissolved in an organic solvent and thereafter refluxed. Water is added to the refluxed solution, which results in cross linking, hence forming LDHs.60 Ramos et al. prepared LDHs from magnesium ethoxide and various aluminium salts such as acetylacetonate, nitrate, sulphate and chloride of aluminium. It is found that the crystallinity of sol-gel products is dependent on the aluminium salt used; in the order of increasing crystallinity: aluminium acetylcetonate > aluminium chloride > aluminium nitrate > aluminium sulphate. The method was also found to influence the textural properties of LDHs. In addition, the specific area is 3 times greater than that obtained by the co-precipitation method. LDHs from the sol-gel method have the following traits: good homogeneity, good control of M2+:M3+ ratio, high surface area, and porosity features.60 The co-precipitation method is a classical, easy, and convenient method to prepare LDHs in large amounts. The co-precipitation method involves the simultaneous precipitation of cations in predetermined ratios of their starting solution. The method is believed to proceed by means of condensation of hexa-aquo complexes 14 in solution, hence building brucite-like layers with a homogeneous distribution of both metal cations and interlayer anions.61 The first product is obtained by precipitation of the aqueous metal salts in basic solution. The precipitate is then washed and filtered off. Due to the nature of the precipitate, removal of the gel is difficult and hence the yields are small. In addition, some researchers increased the concentration of the individual metal salts in basic solution and reacted the solution with sodium hydroxides and carbonate to increase the yields. Reichle (1985) further concentrated the magnesium and aluminium salt solution and precipitated the hydrotalcite in a very concentrated sodium hydroxide and carbonate solution. The synthesis was followed by crystallization from 65oC to 350oC for 18 hours. The product obtained was well ordered, with a predictable morphology and surface area.69 However, the disadvantage of using such concentrated solution is the repeated washing that have to be carried out to liberate the alkali metal ions, especially when the LDHs is used in catalytic applications. The co-precipitation method is divided into two types: i) low supersaturation and ii) high supersaturation. Supersaturation conditions are reached by physical methods such as evaporation or chemical means such as pH variation. Low supersaturation method entails the slow addition of a mixed metal solution to a second solution containing the anion to be intercalated, with concurrent pH regulation by the addition of the alkali solution. In high supersaturation method, the mixed metal oxide solution is added to an alkali solution of the required anion. Low supersaturation coprecipitation normally results in precipitates with high crystallinity because the rate of crystal growth is higher than the rate of the nucleation. This method allows precise control on the charge density [M2+:M3+ ratio] of the LDHs by means of pH control of the solution. On the other hand, high supersaturation results in a less crystalline 15 product due to the high number of crystallisation nuclei. Constatino et al. prepared a series of Mg-Al compounds by the latter method. There are several drawbacks that arise from this method such as the presence of impurities M(OH)2 and/or M(OH)3 phases, and therefore the LDHs product will have undesirable charge density.61 Generally, co-precipitation products are amorphous with poorly ordered phase crystallites, which are gel-like and require a long drying time of 12 to 24 hours at a temperature range of 60oC to 120oC. The formation of crystallites occurs in two stages: nucleation and aging. Hence, post-preparation treatments such as aging, hydrothermal crystallization, microwave and ultrasound-assisted crystallization or a spray technique should be carried out on them. Aging of the LDHs suspension usually entails heating of the sample to between 25oC and 100oC or to a gentle reflux for several hours/days. Hypothetically, the process occurs through Ostwald Ripening in which larger crystal grow at the expense of smaller ones. This is a thermodynamically driven process in which larger particles are more energetically favoured over smaller particles, and as the process proceeds the overall energy of the system is lowered. In the hydrothermal treatment method, the LDHs suspension is heated in a stainless steel autoclave under high pressure, for example 10 MPa to 150 MPa, and/ or at temperatures exceeding 120oC. The treatment facilitates the dissolution and recrystallisation of LDHs through heating during LDHs formation. Hydrothermal treatment is usually carried out to achieve one of three objectives: i) preparation of LDHs, ii) transformation of small crystallites into large ones, and iii) transformation of amorphous precipitates into crystalline LDHs. Crystallinity of LDHs is essential for characterization purposes. Modification of the co-precipitation method also includes hydrothermal synthesis of Mg-Al LDHs by urea hydrolysis. This method offers the synthesis of 16 LDHs with homogeneous size. Larger particles are formed when a smaller amount of urea is used in the synthesis. The urea hydrolysis reaction results in a better product as compared to the co-precipitation method. Advantages of this method include control of particle size distribution and particle growth. Moreover, effecting urea hydrolysis by hydrothermal treatment or microwave radiation produces highly crystalline Mg-Al LDHs, thereby reducing synthesis time considerably. Hydrothermal treatment at lower temperatures gives larger particle sizes. Co-Al LDHs particles 40 µm in diameter are obtained after 100 days of treatment at 60oC. By adding alcohols or polysaccharides such as chitosan to the starting mixture, the final LDHs particles morphology can be controlled. Although Cr or Cu based LDHs phase cannot be prepared from urea decomposition under normal conditions, Ni-Cr LDHs phase can be prepared by urea hydrolysis using microwave assisted hydrothermal treatment owing to the high temperatures achieved by microwave heating. However, LDHs prepared using urea decomposition usually contain carbonate anions. Lyi et al. developed a procedure to decarbonylate LDHs materials without any morphological changes. Urea possesses the following attributes that collectively make it a desirable precipitating agent: i) it forms a homogeneous solution, ii) it is a weak Bronsted base (pKb = 13.8), iii) it is highly soluble in water, and iv) the hydrolysis rate is controlled by the temperature of the reaction. Therefore, hydrolysis may be conducted slowly, leading to low supersaturation during precipitation as compared to NaOH precipitation. However, the disadvantage of this method is the incorporation of the carbonate anions, which are subsequently very difficult to eliminate.61 17 2.4 Attachment of metal NPs onto the support The active NP catalysts that have been most studied, however, are those of the noble metals Ru, Rh, Pd, Pt, and Au. Au NPs occupy a special place given their great success and present developments.62 The recent interest in using Au NPs as catalysts derive from Haruta’s ground breaking contribution describing the fact that Au NPs are able to promote efficiently the low-temperature CO oxidation and that the catalytic activity of gold decreases as the particle size increases until eventually this activity is lost beyond 20 nm size.63 One strategy to stabilize NPs against their tendency to grow is to support NPs on a solid surface. Figure 2.3 shows that the surface of solids can interact with gold species in solution as the first step in the formation of Au NPs through van der Waals, hydrogen bonds, covalent bonds, and electrostatic forces. These interactions, generally described as adsorption forces, occur mainly with the part of external atoms of the NPs in interfacial contact with the solid surface and reduce the mobility of the NPs, making their aggregation more difficult. Figure 2.3. Formation of Au NPs on the surface of a solid support through adsorption forces. 18 After attachment on the surface, the formation of Au NPs is believed to occur in two steps involving nucleation and growth. Nucleation is not achieved by coalescence of single gold atoms but rather it involves complicated and ill-defined species containing gold atoms and ions smaller than 2 nm. Aurophilicity of gold, i.e. the tendency to form Au–Au bonds plays an important role in this stage. Firstly, the Au atoms are organized into small nuclei. Subsequently, these nuclei grow to form the NPs of the observed final size. Since nucleation requires more energy than growth, the mechanisms can be separated. The greater the difference in the energy requirement between the two mechanisms, the better the size distribution of NPs will be. In addition to this, main parameters that greatly affect the growth of NPs are concentration, gold loading on the surface, and the presence of chloride. The potential utility of LDHs as support was recently demonstrated by Zhang et al. in a report that described the random deposition of AuNPs, prepared by a deposition precipitation technique using urea, on the lateral faces of LDHs platelets. The LDHs used was not exfoliated but was in the form of large crystals with dimensions of several micrometres. Here, we discuss a method to synthesize LDHssupported alloy NPs as a catalyst. This methodology entails the wetting of the solid support with a solution containing the metal precursor. In this method, the metal precursor is dissolved in the minimum quantity of solvent. The metal precursor solution is then added to the support; this allow for metal precursor to be attached to this support, resulting in the formation of a thick paste. The solvent is then centrifuged, filtered out, dried, and calcined.64 The final solid product is used as catalyst. 19 In this work, we report on LDHs-supported Au-Ag alloy NPs synthesized using the impregnation method; in this method, Au/Ag solution via adsorption forces would be adsorbed onto the LDHs surface. This Au-Ag and LDHs combination is active as catalysts for alcohol oxidation when using molecular oxygen as an oxidant, even in the absence of additives or promoters. In this method, the metal precursors form Au DDT (dodecanethiol) or Ag DDT complexes and dissolve in the minimum quantity of solvent. The metal precursor solution is then added to the support using the seeding method which involves a separation of nucleation and growth, hence resulting in monodispersed NPs. In a modified method, small particles are firstly prepared; these particles will act as seeds on which further growth of a metal occurs through drop by drop addition into the support solution. By inhibiting further nucleation and controlling the growth of the particles, monodispersity, and good dispersion of Au-Ag alloy NPs on the LDHs surface can be achieved. Furthermore, we present the facile and simple but successful deposition of alloy NPs onto LDHs under mild conditions in which the LDHs is subsequently exfoliated in oleylamine solution to form nanosheets. Pre-heating treatment followed by rapid stirring of LDHs in oleylamine solution changes the hydrophobicity of the LDHs NPs. Primary advantages of this method include the low cost and abundant supply of LDHs, in addition to the efficient NPs stabilization and the control of the size and morphology of LDHs. 20 2.5 Alcohol oxidation and green chemistry Alcohol oxidation to aldehydes, ketones, or carboxylic derivatives is one of the most important transformations in organic chemistry. Alcohols, being stable compounds and easy to handle and store, play a central role in the preparation of many other functional groups. Also, alcohols are involved as intermediates or as products in many conventional C-C bond forming reactions, such as the Grignard reaction. In spite of the pivotal role alcohols play in organic chemistry, current investigations into alcohol oxidation, although general in scope, are still unsatisfactory from the green chemistry point of view.65 Generally, stoichiometric amounts of transition-metal ions or oxides, oxoacids, or halogenated compounds are used in alcohol oxidations. Also, in the Swern reaction, stoichiometric amounts of sulphides are formed.66 These processes do not conform to the principles of green chemistry, which require minimization of wastes and maximization of atom efficiency.67 In contrast to stoichiometric reactions in which no catalyst is needed, the use of other greener oxidizing reagents requires the development of suitable active and selective catalysts. A paradigmatic case is the selective epoxidation of C=C bonds. This reaction can be carried out in a general way using organic peracids as stoichiometric reagents forming organic acids as side products.65 Alternatively, a more recent process using titanium silicalite (TSI) as catalyst has been developed in which the oxidants can be the environmentally friendly hydrogen peroxide or organic peroxides (Figure 2.4).68 21 The development of a catalytic process is particularly important for the aerobic oxidation of alcohols using molecular oxygen as an oxidant as it is highly efficient for oxidations of alcohol. However, the development of a promising O2-free methodology is particularly interesting both from a practical and environmental point of view because of the following benefits: 1. It eliminates the formation of water, a by-product that deactivates the catalyst and necessitates tedious purification of products from a aqueous reaction mixture, 2. It is tolerant towards alcohols having O2-sensitive functional groups, and 3. It produces hydrogen which is an attractive feedstock for energy generation. One of the major conditions of the required catalyst is that is has to be general for any type of alcohol including primary, secondary, alicyclic, aliphatic, benzylic, allylic, and etc. The ideal catalyst should also be selective in the oxidation of alcohols, leaving other functional groups such as multiple bonds, thioethers, heterocycles, and etc. unaltered. In this contribution, we present the data indicating that we are close to reaching this goal, through the use of Au-Ag NPs. 22 Figure 2.4. Differences between a stochiometric and a catalytic process for the selective epoxidation of C=C bonds. 23 2.6 References 1. Liu, J. H.; Wang, A. Q.; Chi, Y. S.; Lin, H. P.; Mou, C. Y. J. Phys. Chem. B. 2005, 109, 40 - 43. 2. Henglein, A. Chem. Rev. 1989, 89, 1861 - 1873. 3. Bond, G. C. Catal. Today. 2002, 72, 5 - 9. 4. Wang, A. Q.; Liu, J. H.; Lin, S. D.; Lin, T. S.; Mou, C. Y. J. Catal. 2005, 233, 186 - 197. 5. 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These include Fourier transform infrared spectrometry (FT-IR), thermogravimetry (TG), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction and UV-Vis spectroscopy, gas chromatographyflame ionization detector (GC-FID) and X-ray photoelectron spectroscopy (XPS). The results from these characterization techniques allow us to elucidate the structure of the LDHs formed, as well as to explore their potential applications. This chapter describes the characterization techniques employed. 3.1.1 Tranmission electron microscopy Transmission electron microscope (TEM) allows one to characterize the morphology of materials synthesized. In the sample preparation process, a drop of liquid containing NP suspension to be analyzed was casted onto an ultra-thin film such as copper mesh grid coated with formvar. Thermionically emitted electrons obtained by heating a tungsten filament in the microscope would be focused by a series of condenser lenses and further accelerated to the copper grid to produce contrast micrographs of the samples. In high resolution transmission electron microscopy (HRTEM), imaging of the lattices of particles of metals or 29 semiconductors is possible. HRTEM can provide resolution of atomic scale, and thus can be used to analyze and obtain information on the structures of materials. Figure 3.1. Transmission electron microscope. In our experiments, the particle size and the morphology of NPs prepared were characterized by both the TEM and the HRTEM operated at an accelerating voltage of 200 kV. One or two drops of the toluene and water suspension of NPs were casted onto a copper grid and air dried under normal laboratory conditions before the analysis. 30 3.1.2 Ultraviolet visible absorption spectroscopy The absorption properties of alloy NPs were characterized with a ultravioletvisible (UV-Vis) spectrophotometer (Shimadzu UV-2450), operated between 300 nm and 800 nm. Metal alkanethiolate NPs were characterized using Shimadzu UV-3101 PC, in the wavelength range of 300 nm to 500 nm. Figure 3.2. UV-Visible spectrophotometer. 3.1.3 X-ray diffraction Crystalline materials comprise of atoms arranged in regular and periodic fashion, forming atomic planes. As a result of this arrangement, X-ray that strikes upon these atomic planes at an angle will be reflected and constructive interference between the reflected waves may occur depending on the spacing (d) between the planes.1 This gives rise to a signal in the X-ray diffraction (XRD) spectrum. This is represented by the Bragg equation: 2d sinθ = n.λ (3.1) 31 Where λ is the wavelength of the X-rays and n is the number of wavelengths representing the path difference between two waves reflected from the crystal planes. Their path length must differ by an integral number of wavelengths in order for constructive interference to occur. An X-ray diffractrogram shows peaks at different theta (θ) values that correspond to different atomic planes in a sample characterized using X-ray Diffractometer. The crystal structure and composition of the samples can be further analyzed based on the atomic planes identified in the samples. Figure 3.3. X-ray Diffractometer. The metal alkanethiolate polymers prepared in our experiments were characterized by XRD (Shimadzu X-ray diffractometer, Model 6000 with Cu K alpha radiation λ = 1.5406) with a scan speed of of 2° min-1 from θ = 5o to 35o in order to obtain structural information of these polymers. 32 3.1.4 Infrared spectrometry Infrared spectrometry primarily allows one to perform qualitative organic analysis and to determine the structure of molecules.2 Furthermore, IR spectrometry is commonly used to identify functional groups present in materials. The surfactants on the NPs prepared in our experiments were characterized by Fourier transform infrared spectrometer, FTIR (Bio-rad FTS-135). Unlike conventional spectrometry, FTIR provides higher signal-to-noise ratio (SNR) and faster characterization. This is because FTIR measures a wide range of infrared frequencies simultaneously, unlike conventional dispersive spectrometers whereby only a narrow range of infrared frequencies are measured at any time. Higher measurement rate allows for more measurements to be taken and hence can subsequently be used to ratio out noise. Powder samples were prepared from a finely ground mixture consisting 1 miligram of solid sample and 100 miligram of KBr powder. The powder mixture was pressed into transparent pellet which was then analyzed using FTIR at wavenumbers, n = 400 - 4000 cm-1. 3.1.5 Gas chromatography-flame ionization detector (GC-FID) GC samples were diluted with chloroform prior to GC analysis using GC-FID (Model 6890 N from Agilent Technology). At GC inlet, heater was heated up to 200oC and pressure was set at 14.89 psi. Initial oven temperature was set at 50oC and maintained for 5 minutes. Then the oven temperature was increased to 180oC at 33 20oC/min and further raised to 250oC at 5oC/min. The temperature was then maintained for 4 minutes and post run for 2 minutes before the samples were transferred into the detector. The hydrogen and air flowrates were set at 40 ml/min and 450 ml/min respectively. Figure 3.4. Gas chromatography-flame ionization detector. 3.1.6 Field emission scanning electron microscope (FE-SEM) and energy dispersive X-ray spectroscopy (EDX) Information on morphology and structure of samples was obtained using SEM or FE-SEM. A thin layer of the LDHs was mounted on a copper tape, which was then sputter-coated with silver using a silver coater (Cressington 208 HR, High resolution Sputter Coater). The silver coating prevented non-conductive samples from charge build up during characterization in SEM. The silver coated sample was then characterized using secondary electron imaging (SEI) mode in FESEM (JEOL JSM 34 6700F, Field Emission Scanning Electron Microscope). The FE-SEM operating voltage was 5 keV. In addition to that, elemental composition analysis of the samples was performed using the energy-dispersive X-ray spectroscopy module (EDX or EDS, Oxford Instruments, Model 7426). As the electron beam in FE-SEM rastered across the sample surface, it induced X-ray fluorescence irradiation from the sample atoms. Since the energy of each X-ray photon was characteristic of each element, material compositions and elemental ratios were drawn from sorting and ploting X-ray energy.3 3.1.7 Thermo gravimetric analyzer (TGA) The content of organic compounds present in our nanocomposites such as surfactant molecules and polymers were determined by thermogravimetric analysis (TGA, TA instruments, TGA-2050). In our experiments, about 15 miligram of LDHs was heated in a ceramic crucible from room temperature to 800oC at a rate 10oC/min under an air flow rate of 35 ml/min. As the samples underwent heating process in TGA, continuous monitoring on the weight of the samples showed changes in weight of the samples. The weight change in the samples were recorded and attributed to the removal of moisture, solvent, and organic compounds from the samples. 35 3.2 Synthesis of Au-Ag alloy NPs Au-Ag NPs were grown using half seeding method. This method involves heating one of the metal precursors; the heated metal precursor acted as seeds for growth of other metals. It is found that this method gave small and highly monodispersed NPs. Figure 3.5 illustrates the half seeding method.4 By employing alkanethiolates as the precusors, we have successfully prevented the AgCl precipitation by mixing the common precursors HAuCl4 and AgNO3 using simple and facile method. Furthermore, the composition, optical properties, and catalytic properties of the particles can be easily controlled by varying the feed ratio of the two metal precursors; therefore NPs with desired compositions and particular catalytic properties can be easily achieved. Figure 3.5. Half seeding method. 36 3.3 Synthesis of Au-Ag alloy NPs attached onto LDHs One of the methods involves heating the mixtures of LDHs and oleyamine and followed by half seeding method for both Au and Ag metal precursors solution. Oleylamine acts as both surfactant and reducing agent. During the heating process, oleylamine and LDHs formed a linkage. Subsequently, Au(I) DDT and Ag(I) DDT were added drop wise into the mixture of LDHs and oleylamine and became seeds that growed into Au-Ag alloy NPs. It is found that this method gave highly monodispersed NPs with high catalytic activity. To facilitate our discussion, we call this as Method 1. The other LDHs-supported NPs synthesis method, hereby known as Method 2, is by using surfactant to induce the attachment of Au-Ag alloy NPs onto the surface of LDHs. Kim and Osterloh showed that individual sheets of the exfoliated layered perovskite, HCa2Nb3O10, could be decorated with Au NPs by grafting (3-aminopropyl) trimethoxysilane to the sheets.6 In that study, it is shown that the morphology and dispersion of the NPs could be controlled by using surfactant that can bind with the sheets. There are several advantages of ligand coated NPs such as stable and soluble particles that are well dispersed, as well as repeated dissolution and dispersion without the need for thermal treatment nor harsh chemical treatment. Two types of surfactant (Mercaptoundecanoic acid and Mercaptotrimethoxysilane) were used in Method 2 as the linker; it is known that the mercapto group will form strong bonding with the Au-Ag alloy NPs while the hydroxide end groups and the silane end groups will anchor the surfactant to the LDHs. Well-ordered distributions of NPs on LDHs surface were achieved with controllable Au-Ag alloy NPs size. Figure 3.6 and 3.7 illustrates Method 1 and Method 2 in detail. 37 Figure 3.6. Method 1 to grow Au-Ag NPs onto LDHs. Figure 3.7. Method 2 to grow Au-Ag NPs onto LDHs. 3.4 Experimental procedure To facilitate our discussion, the alloy NPs samples will be represented by AuAg(X), where X is the mole fraction of Au in the feed. Hence AuAg(0.3) represents an alloy sample with an Au:Ag ratio 3:7. 38 3.4.1 Preparation of Au(I) dodecanthiolate (Au(I) DDT) HAuCl4.3H2O was dissolved in ethanol to a concentration of 0.1 M. Dodecanethiol(DDT) was also dissolved in ethanol to a concentration of 0.1 M. In this precursor solution, the mole ratio of HAuCl4.3H2O to DDT was kept at 1:5 (i.e.,Au3+:DDT = 1:5).4 Details of a typical experimental procedure were as such: at room temperature, 1 ml of DDT in ethanol (0.1 M) was added to 2.4 ml of ethanol, followed by the addition of 0.2 ml of HAuCl4.3H2O in ethanol (0.1 M). The mixture was stirred for 5 minutes (total 3.6 ml). The resultant light brown precipitate was washed with ethanol twice and then dispersed in ethanol to a concentration of ~0.005 M. 3.4.2 Preparation of Ag(I) dodecanethiolate (Ag(I) DDT) AgNO3 was dissolved in ethanol to a concentration of 0.1 M. DDT was dissolved in ethanol to a concentration of 0.1 M. In this precursor solution, the mole ratio of AgNO3 to DDT was kept at 1:5 (i.e., Ag+:DDT = 1:5).4 Details of a typical experimental procedure were as such: at room temperature, 1 ml of DDT in ethanol (0.1 M) was added to 2.4 ml of ethanol, followed by the addition of 0.2 ml of AgNO3 in ethanol (0.1 M). The mixture was stirred for 5 minutes (total 3.6 ml). The resultant light yellow precipitate was washed with ethanol twice and then dispersed in ethanol to a concentration of ~0.005 M. 39 3.4.3 Preparation of Au-Ag alloy and pure metal NPs using half seeding method The particles synthesized via this method include Au:Ag (1:1), Au:Ag (0.3:0.7). A certain volume of the ethanolic dispersion of the Au(I) DDT (0.005 M) was centrifuged to remove the bulk of ethanol. The precipitate of alkanethiolate was dissolved in 3 ml of oleylamine. Meanwhile, a 10 ml of oleylamine was added to flask 100 ml and heated to 215oC in a silicon oil bath for 8 minutes. After 8 minutes of heating, 3 ml of oleylamine contained Au(I) DDT was added to the flask drop by drop. Immediately upon detecting a slight colour change, a certain volume of Ag(I) DDT in 3 ml oleylamine was added drop by drop. Heating was continued under reflux for a total of 12 minutes.4 The mixture was then cooled down to room temperature and was washed with ethanol twice and centrifuged. The NPs were dried in the oven at 120oC for 12 hours and re-dispersed with toluene for SEM or TEM analysis purpose. 3.4.4 Preparation of Au-Ag alloy NPs attached onto LDHs using impregnation method5 A 0.2 gram of LDHs was added into 20 ml of beaker glass filled with 10 ml oleylamine. By using ultra-sonicator, the dried LDHs were mixed and vibrated with 10 ml oleylamine as a solvent for about 1 hours. This solution was added into flask 100 ml and heated to 215oC for 8 minutes. Subsequently, metal alkanethiolate for Au(I) DDT in 3 ml of oleylamine was added to the flask drop by drop. Immediately upon detecting a slight colour change, a certain volume of Ag(I) DDT in 3 ml 40 oleylamine was added drop by drop. Heating was continued under reflux for a total of 12 minutes. The mixture was cooled down to room temperature. Subsequently, it was washed with ethanol twice and centrifuged. The NPs were dried in the oven at 120oC for 12 hours and re-dispersed with toluene for SEM or TEM analysis purpose. The total volume of the metal solution used for each synthesis is 0.4 ml. The mole ratios of Au to Ag were set as follow: 1. Au:Ag ratio was 1 to 3, corresponding to 0.1 ml and 0.3 ml respectively. 2. Au:Ag ratio was 3 to 1, corresponding to 0.3 ml and 0.1 ml respectively. 3. Au:Ag ratio was 1 to 1, corresponding to 0.2 ml and 0.2 ml respectively. The total number of moles of both metals was kept a constant for all experiments. 3.4.5 Preparation of Au-Ag alloy NPs attached onto LDHs using MUA and MPTMS as a linkage The prepared Au-Ag alloy NPs were dissolved in toluene. 1 ml, 0.1 M MUA and 1 ml, 0.1 M NaOH solution was prepared using ethanol:water (1:1) as a solvent. NaOH was added to neutralize the surfactant solution (pH = 7). 0.1 gram dried LDHs was then dissolved in 10 ml of water and mixed with MUA (neutralized with NaOH) solution. The mixture was stirred for 12 hours at room temperature under N2 flow. Au-Ag alloy in toluene 1.5 ml was added into mixture 5 ml of LDHs and stirred for 41 10 minutes under N2 flow. As a result, Au-Ag alloy NPs were attached to LDHs and brown colour precipitate was formed. The precipitate was then washed with ethanol twice and centrifuged. The NPs were further dried in oven 120oC for 12 hours. The same method was used to link alloy NPs to MPTMS solution. 3.4.6 Preparation of catalyst for alcohol oxidation reaction After the supported NPs were dried in oven for 12 hours, they were futher calcinated at 350oC for 4 hours. For alcohol oxidation reaction, 0.1 gram Au-AgLDHs was added to 0.1 ml of benzyl alcohol dissolved in 10 ml of toluene. Nhexadecane was added as an internal standard for quantitative GC analysis. Upon reaction for 2 hours at 800C, GC was used to determine the yield of benzyl alcohol conversion to benzaldehyde. 42 3.5 References 1. Jenkins, R.; Snyder, R. L. Introduction to X-ray powder diffractometry. 1996. 2. George, W. O.; McIntyre, P. S. Infrared Spectroscopy. London: Wiley. 1987. 3. Scott, V. D.; Love, G.; Reed, S. J. B. Quantitative Electron - Probe Microanalysis. New York: Ellis Horwood. 1995. 4. Ting, T. C.; Qing, H. X.; Wei, J.; Zeng, H. C. Langmuir. 2011, 27, 5633. 5. Chen, X.; Zhu, H. Y.; Zhao, J. C.; Zheng, Z. F.; Gao, X. P. Angew. Chem. 2008, 120, 5433 - 5436. 6. Park, A. Y.; Kwon, H.; Woo, A. J.; Kim, S. J. Advanced Materials. 2005, 17, 106 - 109. 43 CHAPTER 4 CHARACTERIZATION OF Au-Ag/LDHs AS CATALYST 4.1 Results for metal alkanethiolate polymers 4.1.1 UV-Visible absorption The optical properties of pure Au, pure Ag, and Au-Ag alloy NPs have been extensively studied using UV-Vis spectrometry. The UV-Visible absorption spectra of pure Au, Au-Ag alloy and pure Ag are shown in Figure 4.1. Figure 4.1. Normalized UV-Vis absorption spectra of pure Au, Au:Ag alloy(1:1), and pure Ag dispersed in ethanol. 44 As shown in the spectra above, the formation of alloy NPs instead of coreshell particles or structure comprising of a mixture of Au and Ag particles is confirmed by UV-Vis absorption spectrometry; only one distinct peak can be observed in the spectrum for alloy NPs. This peak lies between the absorption peaks of Au and Ag, around 600 - 800 nm. 4.1.2 NPs structure analysis Table 4.1. TEM images for pure Au, pure Ag, and Au-Ag alloy at different compositions. Pure Au (Scale: 50 nm) Au:Ag  1:1 (Scale: 10 nm) 45 Au:Ag  0.3:0.7 (Scale: 20 nm) Au:Ag  0.7:0.3 (Scale: 8 nm) Pure Ag (Scale: 20 nm) TEM images in Table 4.1 show that for all compositions of Au-Ag NPs synthesized, the particles were smaller than 10 nm except for pure Au NPs. TEM images with different ratios of Au:Ag alloy at 1:1, 0.3:0.7, and 0.7:0.3 show that the particle size can be controlled for the same number of atom. The measured particle 46 sizes are 25 ± 2 nm (pure Au), 5 ± 1 nm (Au:Ag  1:1), 6 ± 1nm (Au:Ag  0.3:0.7), 7 ± 1 nm (Au:Ag  0.7:0.3), 9 ± 1 nm (pure Ag). TEM images show that particles with higher ratio of Au to Ag tended to be larger. Furthermore, from TEM images, the Au-Ag alloy NPs are uniform in size and morphology. In addition, the particles dispersion in Au-Ag alloy NPs was better especially for Au:Ag ratio at 0.3:0.7, as compared to single/pure Au or Ag NPs; aggregation occurred in pure Au and Ag NPs and hence the Au or Ag particle size cannot be controlled. It can be concluded that half seeding method is an effective method to synthesize Au-Ag alloy NPs with excellent control over the NPs size. Generally speaking, metal NPs with small particles size (1-3 nm) have high catalytic activity, good light transparency, and highly size-dependant properties.31 It is believed that the monodispersity of Au-Ag alloy NPs achieved in the experiments can be attributed to the staggered addition of the second metal alkanethiolate. During the Au-Ag NPs synthesis using the half-seeding method, the second methal alkanethiolate was added when a colour change in the mixture was observed. The colour change indicated that the Au-Ag alloy NPs were formed and has started growing; these NPs were probably still very small in size and have high surface energy which the system seeked to minimize. The addition of the second metal alkanethiolate helped to control the size of NPs by preventing the Au-Ag alloy NPs growth and agglomeration despite high reaction temperature used. The Au and Ag atoms then diffused and mixed to form Au-Ag alloy. However, the timing of the addition must be accurate. The optimum point of addition of the second metal alkanethiolate was immediately after the colour change in the mixture was observed; 47 agglomeration of NPs will happen if the addition of second metal alkanethiolate is not timed well. At elevated temperature of the synthesis, the alkanethiolate polymers decomposed and oleylamine, being a reducing agent, reduced Au(I) and Ag(I) precursor to Au and Ag NPs. During synthesis, the Au and Ag atoms rapidly interdiffused, leading to the formation of Au-Ag NPs alloy. The diffusivity (D) of an element is dependent on temperature and if the dependence is a Boltzmann-Arrhenius one, it can be expressed by the following equation19: D = D0ex (4.1) Where D0 is the pre-exponential factor of the element and ΔHd is the activation enthalpy of diffusion and k is the Boltzmann constant. This equation shows that diffusivity increases with increasing temperature. This is the reason for obtaining core-shell particles at 50oC but alloy NPs at 100oC as mentioned by Sun et al.19 Hence this hypothesis leads us to believe that at 200oC, alloy NPs were formed instead of core-shell particles. Furthermore, it has been reported that alloy formation from Agcoated Au NPs is due to the presence of vacancy defects at the interface of Au NPs, which further enhanced the mixing of the Au and Ag atoms.19 The defects could be caused by the presence of a stabilizer at the interface. This may be applicable to our case whereby the presence of defects at the surface of the newly formed seed particles could have aided the deposition and mixing of the second metal. Despite the successful synthesis of Ag NPs using the half seeding method, synthesis of Au NPs using the same method has not been equally successful in our 48 experiment, unlike what was reported by Zhang et al., wherein alkanethiolate nanotubes where heated in amine.20 The contrast between results reported by Zhang et al. and our experiments could be attributed to the difference in the preparation of the polymeric precursor; the choice of preparation method plays an important role in determining the final structure of the particles obtained. This is reasonable since many parameters of the nucleation and growth process are strongly affected by the structure of the initial precursor. In the report,20 Au(I) DDT nanotubes formed after many hours of stirring HAuCl4 and DDT together.20 On the other hand, we made use of Au(I) DDT prepared in just 5 minutes; the Au(I) DDT should exist as plate-like aggregates and are structurally different from the Au(I) DDT NTs. 4.1.3 Characterization of LDHs NPs LDHs with smaller lateral sizes offer larger surface area, for more Au-Ag NPs to anchor, and hence are preferred as support. In order to obtain well crystallized LDHs phases, some of the experimental parameters should be particularly controlled and optimized, such as the concentration of both metallic salts, the concentration of the alkaline solution, the addition rate of the reactants, as well as the aging time and the temperature of reaction. Ni-Al LDHs material was prepared using the urea precipitation method at high pressure and high temperature (180oC) for 48 hours. The size and morphology of LDHs can be easily tuned by changing the ratio of urea to NiAl concentration as well as Ni to Al concentration. TEM images of LDHs NPs at different urea concentration are shown in Table 4.2. 49 Table 4.2. TEM images of LDHs with different ratios of urea to Ni-Al concentration. Conc. of Ni:Al 1 2 0.1 M : 0.05 M A. Urea:(Ni+Al) = 1:1 Urea = 0.15 M 100nm 100nm B. Urea:(Ni+Al) = 5.6:1 Urea = 0.84 M 300nm 300nm 300nm 300nm C. Urea:(Ni+Al) = 10:1 Urea = 1.5 M Conc of Ni:Al 1 2 0.3 M : 0.15 M 50 A. Urea:(Ni+Al) = 1:1 Urea = 0.45 M 400nm 200nm 100nm 100nm B. Urea:(Ni+Al) = 5.6:1 Urea = 2.55 M C. Urea:(Ni+Al) = 10:1 Urea = 4.5 M 200nm 100nm The ratio of the concentration of urea to Ni-Al was varied from 1:1 to 5.6:1, and finally to 10:1, with the Ni:Al concentration kept constant. TEM images show that the morphology and size of the LDHs NPs can be changed by changing the ratio of urea to Ni:Al concentration. For example, when the ratio of urea to reactant was 1:1, less aggregation and larger NPs size were observed; at this ratio, this could be caused by slower nucleation and reduced NPs growth rate. Moreover, slower 51 nucleation and growth rates meant longer growing periods, hence the NPs became bigger. TEM images show that when the ratio of the concentration of urea to Ni-Al was at 1:1 while the Ni-Al concentration was kept constant at 0.1:0.15 M, thick and laterally large plate-like morphology was obtained, and the particle sizes were about 800 - 1000 nm. However, when the ratio of the concentration of urea to Ni:Al was further increased to 5.6:1, TEM images show that round particles were observed and the size of the NPs were about 200 - 400 nm. Therefore, it can be concluded that the urea concentration affects shape and size of NPs. Furthermore, the layered structure is confirmed by their characteristic XRD patterns (refer to appendix). There are slight shifts in major peaks to higher 2θ when the urea concentration was lowered, which indicate decreased interlayer distance and unit cell parameters.21 Consequently, the electrostatic interaction between positive layers and negative interlayers was increased. This further implies that LDH NPs synthesized with different urea concentration were structurally dissimilar. When the ratio of Ni to Al was further increased to 0.3:0.15 M, with the ratio of concentration of urea to Ni-Al maintained at 5.6:1, NPs size and crystallinity decreased. From the XRD data (refer to appendix), it is noteworthy that the solid phase was rather amorphous. The peaks observed for the samples appeared to have shifted and no longer correspond to the LDHs peaks. Based on our findings, the urea concentration and the ratio of Ni to Al concentration that resulted in good shape and size uniformity of LDHs NPs synthesized were 1.5 M and 0.3:0.15 M respectively. This condition was then used later to create the LDH support for Au-Ag alloy NPs synthesized separately. On the other hand, the use of a higher concentration of urea (4.5 M) together with a higher concentration of Ni-AL hydroxides (0.3:0.15 M) obviously influenced the crystallinity of Ni-Al LDHs. Hydrothermal reaction using high concentration of 52 urea and Ni-Al at 180oC over 48 hours resulted in the formation of rod-like particles. Tenne et al.29 reported that many 1-D nanostructures (rod, tube) have been successfully synthesized from 2-D layered structures, such as Ni(OH)2 and Mg(OH)2. Since Ni-Al LDHs is a layered compound, we may suppose that the formation of NiAl LDHs nanorods might be related to the nature of its lamellar 2-D structures. However, the structure of Ni-Al LDHs nanorods is markedly different from the simple layered structures of Ni(OH)2 and Mg(OH)2. The structure of Ni-Al LDHs nanorods can be described as containing brucite (Ni(OH)2)-like layers in which some of the divalent cations (Ni2+) have been replaced by trivalent ions (Al3+) giving positively charged sheets. This charge is balanced by the intercalation of CO32- or SO42- anions in the hydrated interlayer regions. It is obvious that Ni-Al LDHs nanorods possessed complex structures. At a high concentration of Ni-Al hydroxides (0.3:0.15 M) and urea concentration of 4.5 M, nucleation of LDHs occurred and there was an intrinsic tendency for them to grow into rod-like structure due to their anisotropic hexagonal structure. Therefore, LDHs nanorods formed were stable geometrical morphologies in the context of surface chemistry because of low system energy associated with rod structure.30 53 4.1.3.1 Studies on the effect of aging times and temperatures on LDHs The TEM images below show the effect of varying aging times and temperatures on the size and morphology of LDHs. a 100 nm 100 nm b 400 nm 200 nm 200 nm 200 nm c 54 d 200 nm 80 nm e 80 nm 80 nm 300 nm 300 nm f Figure 4.2. TEM images of LDH NPs with different aging times and temperatures, (a) aging time of 12 hours, reaction temperature of 100oC, (b) aging time of 24 hours, reaction temperature of 100oC, (c) aging time of 48 hours, reaction temperature of 100oC, (d) aging time of 12 hours, reaction temperature of 180oC, (e) aging time of 24 hours, reaction temperature of 180oC, (f) aging time of 48 hours, reaction temperature of 180oC. 55 Miyata reported that synthesis temperature has a strong effect on the crystallite size of LDHs, i.e. the crystallite size of LDHs increases when the synthesis temperature is increased from room temperature to 140°C but decreases above 160°C.12 Oh and Kovanda conducted experiments to determine the key parameters that affect crystallite size of LDHs during synthesis and found that they are aging time, reaction temperature, concentration, and etc.13,14 From Figure 4.2.a, the agglomeration of amorphous LDHs was observed at lower temperatures and shorter aging time (100oC and 12 hours). The amorphous LDHs grew with the increase of aging time and temperature. At lower aging temperature (100oC), lamellas were thin and partly irregular on the edges and widely distributed in size. In addition, smaller particles without a fixed structure were found concomitant with the predominant large nanostructures. It is believed that the small particles formed during the initial growth stage of the LDHs. The presence of clusters of NPs at 100oC and 48 hours in the hydrothermal reaction indicated that crystal growth occurred predominantly on the edges. When the temperature was raised from 100 to 180°C, large and rounded hexagonal structures were formed, indicating signs of dissolution during the hydrothermal treatment.15 At prolonged reaction times (48 hours) at 180oC, the amorphous phase gradually turned crystalline with lateral dimensions of about 300 - 400 nm. It is believed that metal oxides have dissolved and the carbonates intercalated into the interlayer space of LDHs, and subsequently, re-crystallization happened.16 In addition, increasing the aging time resulted in the formation of relatively thin hexagonal plate-shaped crystals with rounded edges. Consequently, as the crystal particle sizes became bigger, the particle shapes became more regular. Therefore, it can be concluded that LDHs with 56 high uniformity and high crystallinity were formed at 160 - 180°C with a reaction time of 48 hours. 4.2 Characterization of Au-Ag alloy NPs/LDH composites The morphology of the Au-Ag alloy NPs/LDH composites was examined using TEM; TEM imaging is the most important imaging instrument in establishing the particle size distribution and Au-Ag alloy NPs dispersion on LDHs surface. As shown in Figure 4.4, the TEM images reveal that oleylamine-capped Au-Ag NPs were present in the form of uniform and ultrafine particles. Au-Ag alloy NPs have been synthesized using the half seeding method in the presence of oleylamine; Au-Ag alloy NPs produced have uniform morphology and particle sizes of about 7 - 10 nm. Oleylamine acted both as a reducing agent and as a surfactant which controlled the growth of the Au and Ag precursor solution. The half seeding method, which was developed by Ting Ting22, is believed to play an important role in controlling the metal particle size and the interaction between NPs and the supports, which consequently affect the performance of the supported NPs. This method offers the advantage of the ability to precisely tune the particle size and shape. However, the resultant NPs always exhibit very limited catalytic activity because of the lack of surfactant-free NPs surface. To tackle this problem, LDHs-supported NPs were prepared using two methods (with and without the use of surfactant) to compare the catalytic activity of catalyst. Here, we developed a simple impregnation method for the one pot synthesis of LDHs-supported Au-Ag alloy NPs with controllable sizes of NPs (7 - 10 nm) where LDHs were mixed with 57 oleylamine. Subsequently, Au(I) DDT was added drop by drop, followed by Ag(I) DDT in a heating flask (200oC). Figure 4.3.(a) illustrates how Au-Ag NPs were deposited on LDHs surface. The amine group of oleylamine was easily protonated and it donated its lone electron pair to Lewis acidic atoms. The protonation of amine provided a linkage with LDHs particles. Once protonated, the amine head group with its long alkyl chain tail acted as both ligand and reactor for metal NPs nucleation and growth. Furthermore, this interaction induced the adsorption of Au-Ag on the oxidizable LDHs surface followed by the reduction of AuCl4- to elemental Au and finally end with Ag+ reduction forming Au-Ag alloy NPs. The Au-Ag alloy NPs were subsequently adsorbed on the oxide LDHs. The interaction between Au-Ag NPs and oleylamine was mediated by weak covalent bonds in the case of the amine-functionalized LDHs. These interactions, generally described as adsorption forces, occurred mainly with part of external atoms of the NPs in interfacial contact with the solid surface; these forces reduced the mobility of NPs, thus making their aggregation more difficult. This allowed the immobilization of dense networks of Au-Ag NPs on LDHs, which is of interest for the controlled assembly of nanostructures. There are many different methods for Au deposition on metal oxide supports. Following the successful deposition of Au NPs on TiO2 support,16 the second method was employed in the current experiment for the deposition of alloy NPs onto LDHs. LDHs were stirred with MUA/MPTMS for 12 hours. Then, Au-Ag alloy NPs were added drop wise into the solution of MUA/MPTMS functionalized LDHs. 58 (a) (b) Figure 4.3. (a) Illustration of the deposition of Au-Ag alloy NPs onto LDHs surface for catalyst prepared using Method 1, and (b) illustration of the deposition of Au-Ag alloy NPs onto LDHs surface for catalyst prepared with MPTMS (left) and MUA (right) using Method 2. Figure 4.3.(b) illustrates how Au-Ag NPs were deposited on LDHs surface using Method 2. This method deposited Au-Ag alloy NPs uniformly on the LDHs surface. The main preparation conditions were concentration of Au, pH of reaction, and aging time. The pH of the mixture of the surfactant and LDHs was maintained above 7 in order to bind the hydroxide chain to the LDHs surface. MUA/MPTMS 59 acted as a linker with bifunctional groups; its carbonyl end was attached to the LDHs surface and its thiol tail was attached to the metal Au-Ag alloy NPs. The strong covalent bond that was induced by the surfactant between Au-Ag NPs and LDHs made the NP-LDH bond difficult to break, even with UV irradiation. In addition, the presence of DDT in the initial Au and Ag suspension of Au-Ag alloy NPs prevented the NPs from agglomerating. Thus, Method 2 is the best method to prepare LDHssupported Au-Ag alloy NPs. 50nm Figure 4.4. Au-Ag alloy NPs prepared using the half seeding method. 60 20nm Method 1 : 80 nm 80 nm 80 nm 80 nm Figure 4.5. TEM images of LDHs-supported Au-Ag NPs using Method 1. 61 Method 2 : 100nm 100nm 100nm 50nm Figure 4.6. TEM images of LDHs-supported Au-Ag NPs using MUA as a linker. 60 nm 62 20 nm 60 nm 20 nm 50 nm 80 nm Figure 4.7. TEM images of LDHs-supported Au-Ag NPs using MPTMS as a linker. 4.2.1 FTIR result for functionalization oleylamine-LDHs (Method 1) FTIR spectroscopy was used to characterize LDHs. FTIR spectra of Ni-Al LDHs and oleylamine-LDHs are shown in Figure 4.8. Broad absorption bands between 3300 and 3600 cm-1 (as a result of H-bonding) correspond to a combination of the stretching vibration of the hydroxide groups (O-H) in the brucite sheets and the interlayer water molecules. The bands at 805, 563, and 450 cm-1 can be ascribed to M63 O stretching modes and M-O-H bending vibrations. For Ni-Al-NO3 LDHs, the active absorption band at 1384 cm−1 is assigned to the symmetric stretching vibration of the interlayer carbonate and nitrate anions intercalated in the interlayer gallery, in addition to carbonate anions as suggested by Wang et al.2 c 2927 cm -1 2962 cm 1583 cm % transmittance -1 805 cm 1384 cm -1 -1 -1 b 805 cm -1 1384 cm -1 a 805 cm -1 1384 cm 0 500 1000 1500 -1 2000 2500 3000 3500 4000 -1 Wavelength(cm ) Figure 4.8. FTIR spectra of (a) Ni-Al-Cl LDHs, (b) Ni-Al-NO3 LDHs, and (c) oleylamine-LDHs. The inevitable absorption of CO2 by the basic solution resulted in presence of CO32- that gave rise to absorption band at 1384 cm-1. For oleylamine-intercalated NiAl LDHs, typical C-H antisymmetric stretching mode of CH3 and CH2 can be found at 2962 and 2927 cm−1 respectively. Meanwhile, symmetric stretching of CH3 and CH2 are observed at 2873 and 2852 cm−1. This is a clear indication of oleylamine attached on the LDHs surface. In the spectra of LDHs-oleylamine, the bands at 3373 cm−1 and 1635 cm−1 correspond to the N-H stretching mode. However, the N-H stretching mode of NH2 group of the oleylamine molecule overlaps with the O-H 64 stretching vibration of LDHs to produce a merged band in the range of 3300 to 3500 cm−1. The absorption bands at 1384 cm−1 and 1583 cm−1 can be ascribed to C-N stretching mode and N-H bending mode. All the results abovementioned indicate the presence of oleylamine in the LDHs host. 4.2.2 TGA comparison between NO3-LDHs and Cl-LDHs The thermogravimetric analysis for two sources of LDHs are shown in Figure 4.9. The onset of decomposition of Ni-Al LDHs occurred at 180oC. According to the TGA curves in Figure 4.9, major mass losses of Ni-Al LDHs occurred in three steps : i) loss of adsorbed water from 100 - 180oC, ii) dehydroxylation of hydroxide sheets from 180o - 240oC, and iii) loss of interlayer carbonate, chloride, and nitrate from 240 - 375oC. In addition, it can be seen that Cl-LDHs and NO3-LDHs have almost the same decomposition temperature, indicating that both LDHs have the same thermal stability. 9.5 I 8.5 II III Weight loss (mg) 9.0 NiAlCl (0.3M, 0.15M) NiAlNO3(0.3M, 0.15M) 8.0 7.5 7.0 6.5 6.0 0 200 400 600 800 o Temperature ( C) Figure 4.9. TGA curves of Ni-Al-Cl LDHs and Ni-Al-NO3 LDHs. 65 4.2.3 XRD results for Au-Ag alloy/LDHs (Method 1) 120000 100000 d c Intensity(a.u.) 80000 003 006 b 60000 003 40000 006 012 015 110113 20000 a 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 2 theta(degree) Figure 4.10. XRD peaks of (a) Ni-Al-Cl LDHs, (b) Ni-Al-NO3 LDHs, (c) NO3-LDHssupported Au-Ag alloy calcinated at 350oC for 4 hours (Metal-Ni-Al-O composite), (d) Cl-LDHs-supported Au-Ag alloy (Metal-Ni-Al-O composite) calcinated at 350oC for 4 hours. The first basal diffraction maximum in Ni-Al-Cl and Ni-Al-NO3, i.e. d003 plane peak is responsible for the increase of interlayer spacing. Basal spacing of d003 plane at 2θ = 11o correspond to peak of CO32-. The value obtained is in good agreement with literature.28 From Figure 4.10, there is no shifted peak of basal spacing d003 for both Ni-Al-Cl LDHs and Ni-Al-NO3 LDHs peaks. This concludes that part of LDHs was in unmodified form due to high charge density of CO3-LDHs regardless of LDH sources used. 66 (200) NO3-LDH-Au/Ag Cl=LDH-Au/Ag (111) intensity (a.u) (220) 30 40 50 60 70 2 theta (degree) Figure 4.11. XRD patterns of enlarged portion of (c) NO3-LDHs-supported Au-Ag alloy (Metal-Ni-Al-O composite), (d) Cl-LDHs-supported Au-Ag alloy (Metal-Ni-AlO composite) with 2θ from 30o to 70o. From Figure 4.11 (the enlarged picture of c and d), it can be seen that broad peaks are obtained at 2θ = 38.2o, 2θ = 45o, and 2θ = 65o, indicating that the small particles in the sample were primarily made of Au-Ag alloy NPs which correspond to the (111), (200), and (220) planes of the Au cubic crystal, respectively. Furthermore, for Cl-LDHs-supported Au-Ag alloy peak, there is no crystalline peaks of d003 plane peak due to the degradation of LDHs layer structure. Layer degradation led to the formation of amorphous products such as Al(OH)3, Ni(OH)2. The peaks at 2θ = 36o, 2θ = 45o, and at 2θ = 61o correspond to the oxide phase of hydrotalcite especially for Cl-LDHs (as can be seen in Figure 4.11). For NO3-LDHs-supported Au-Ag alloy, XRD peak at 2θ = 12.5o corresponds to the intercalation of CO32- without layer degradation. No diffraction peak corresponding to crystalline AuOx or AgOx is present, owing to the good uniformity of Au-Ag alloy NPs formation in the oxide matrices. Oleylamine took part in the attachment of Au-Ag alloy NPs onto the LDHs 67 support. Not only did oleylamine acted as a linker, but it also acted as a capping agent where LDHs and oleylamine formed a functionalized complex. Thus, oleylamine reacted with Au-Ag alloy NPs electrostatically. By adding extra oleylamine, there were possibilities of more Au-Ag NPs that being formed and transferred from solution phase to template/surface of LDHs. In the XRD pattern, it can be seen that Au-Ag alloy NPs were supported on the Ni-Al-NO3 external surface, because the XRD peaks of Au-Ag alloy supported on Ni-Al-NO3 LDHs has the same profile as the parent sample. It gives evidence that direct anion exchange did not occur between the Au-Ag alloys and carbonate anions in the interlayer region. However, for Ni-Al-Cl LDHs, it showed different peak as compared parent sample. This is due to layer degradation of Cl-LDH NPs at high temperature. Therefore, Cl-LDH NPs has lower thermal stability than NO3-LDH NPs. 4.2.4 Energy dispersive X-ray photospectroscopy Table 4.3 and 4.4 show the elements in weight % for each catalyst of Au-AgNO3 LDHs and Au-Ag-CL LDHs. Weight % and atomic % in both tables reflect the composition of the Au and Ag metal in the NO3-LDHs and Cl-LDHs. In the pure Au and pure Ag samples, other compounds such as chloride and sulfur exist and performed as precursors for the growth of Au or Ag NPs. It is believed that Au NPs were formed from the Au(I) chloride precursor, and they were reduced from Au3+ in solution using thermal decomposition method and oleylamine as a reductant. On the other hand, Ag NPs were synthesized from thermolysis of a metal-oleate complex in alkanethiol. In Table 4.3 and Table 4.4 for pure Ag NPs, it can be seen that some sulfur compound was generated inside the catalyst. The presence of the sulfur 68 compound shows the possibility of Ag2S (metal sulfide) NPs formation instead of Ag NPs. This Ag2S byproduct might be generated from the thermolysis of metal oleate complexes in dodecanethiol. Several groups have reported the synthesis of Ag2S nanocrystals using various synthetic routes including thermolysis using single source precursors.3 It is said that this method can generate nanocrystals of semiconducting metal sulfides that are uniform in size. Table 4.3. Amount of elements by weight % for different ratios of Au:Ag alloy deposited onto Ni-Al-Cl LDHs. Type Pure Au Element Wt % At % Pure Ag Au:Ag 1:3 Au:Ag 3:1 Au:Ag 1:1 Wt % At % Wt % At % Wt % At % Wt % At % 7.80 10.04 8.9 10.39 9.28 9.82 8.28 9.71 8.26 Al 9.47 Ni 33.18 12.55 37.59 15.30 39.61 16.26 35.35 13.70 36.23 14.17 C 13.72 25.38 6.61 O 38.10 52.89 40.53 60.54 38.66 58.25 39.95 56.84 38.87 55.78 13.16 7.30 14.65 10.55 20.00 10.48 20.04 S - - 0.36 0.26 - - - - - - Cl 1.50 0.94 1.68 1.13 1.6 1.09 1.19 0.76 2.13 1.38 Ag - - 3.18 0.70 1.62 0.36 0.57 0.12 0.80 0.17 Au 4.03 0.45 - - 0.82 0.10 2.58 0.30 1.77 0.21 69 Pure Au 1 um 1 um Pure Ag 1 um 1 um Au:Ag (1:1) 900nm 1 um 70 Au:Ag (3:1) 300 nm 1 um 1 um 1 um Au:Ag (1:3) Figure 4.12. TEM images for different ratios of Au:Ag alloy deposited onto Ni-Al-Cl LDHs after calcination at 350oC for 4 hours. Table 4.4. Amount of elements by weight % for different ratios of Au:Ag alloy deposited onto Ni-Al-NO3 LDHs. Type Pure Au Element Wt % At % Pure Ag Au:Ag 1:3 Au:Ag 3:1 Au:Ag 1:1 Wt % At % Wt % At % Wt % At % Wt % Al 12.15 11.48 11.34 9.52 11.77 9.94 10.07 9.49 13.41 13.25 Ni 44.73 19.42 39.61 15.29 39.94 15.51 36.62 23.87 47.47 21.56 C 12.31 26.12 17.98 33.93 18.77 35.64 15.67 32.58 O 26.54 42.30 28.6 40.52 27.01 38.50 0.43 0.30 S - - - Cl 0.21 0.15 Ag - - 2.03 0.43 1.16 Au 4.07 0.53 - - 1.35 71 - 34.1 6.83 At % 15.18 33.71 29.75 49.59 - - - - - - 0.25 0.6 0.09 0.73 0.18 0.16 2.94 0.26 1.81 0.25 Pure Au Au:Ag 1:3 100nm Au:Ag 1:1 Au:Ag 3:1 100nm 100nm Figure 4.13. TEM images for different ratios of Au:Ag alloy deposited onto Ni-AlNO3 LDHs after calcination at 350oC for 4 hours. 72 4.2.5 SEM of LDHs Figure 4.14 shows SEM images of NO3-LDHs and Cl-LDHs. From the SEM images, it can be seen that these two supports have the same LDH morphology. The structure was a platelet spherical-like shape and the size was about 150 - 200 nm. We synthesized nanosized LDHs because a smaller NPs size will have a larger surface area. As a consequent, a larger number of Au-Ag alloy NPs can be deposited on the LDHs surface, thereby generating higher catalytic activity as compared to NPs deposited on LDHs of larger size. a b Figure 4.14. SEM images of (a) Cl-LDHs, and (b) NO3-LDHs. 4.3 Results and discussion for catalytic activity measurement In this chapter, we present the results of the catalytic activity of Au-Ag alloy on LDHs. Two different sources of support (chloride and nitrate) were used to study the catalytic activity and the loading effect of Au-Ag NPs onto LDHs. Gas chromatography was used to analyze benzaldehyde content and to study how much benzyl alcohol was converted to benzaldehyde using Au-Ag/LDHs as a catalyst. 73 The objective of our study on catalytic activity of Au-Ag/LDHs is twofold : i) to gain insight into the use of simple one pot method to synthesize highly active and selective heterogeneous catalyst to be used in oxidation of alcohols and ii) to propose suitable materials with high conversion rates for selective oxidation of primary alcohols to aldehyde. The major advantages of the one pot method were the recoverability and reusability of heterogeneous metal catalysts and the generation of water as sole by-product when oxygen was used as oxidant. Selective oxidation has gained recognition as an important process in the synthesis of intermediate and fine chemicals. The selective oxidation of primary alcohols to aldehydes provides a direct route to clean and high value perfumery chemicals.4 A lot of researchers have focused on the development of noble metals as active materials which give high conversion in many kinds of alcohol oxidation reactions. However, there are few findings on the use of alloy NPs, especially noble metals, as catalysts. Their characteristics include being highly active catalysts with small particle size and good particle size distribution. We are also concerned with the size of the LDHs as a support in providing a large surface area for the Au-Ag NPs to be attached uniformly on the surface of LDHs. The concept of green chemistry and sustainability are key considerations in current and upcoming industrial practices. Chemical processes that can effectively utilize raw materials, reduce waste, and avoid the use of toxic intermediates under mild reaction conditions are preferred. In order to satisfy the 12 principles of green chemistry, the toxic oxidants traditionally employed in conventional chemistry, such as chromate, or permanganate, should be avoided and much simpler oxidants should be used instead.4 Thus, catalysts based on molecular sieves, mixed oxides, as well as 74 Au, Pd, Ru and Pt supported catalysts have been used for oxidation of alcohols. In addition, it has recently been shown that bimetallic catalysts based on Au and Pd are highly effective in the oxidation of alcohols and polyols. In this chapter, we present Au-Ag alloy NPs impregnated onto the surface of LDHs using the one pot method. Our primary aim is to investigate the catalytic properties of Au-Ag alloy NPs on LDHs with varying metal ratios as well as varying sources of LDHs. 4.3.1 Catalytic activity testing Catalytic activity of Au-Ag/LDHs in the aerobic oxidation of benzyl alcohol under atmospheric pressure using oxygen as oxidant was tested. 0.1 gram of Au-AgLDHs was added to 0.1 ml of benzyl alcohol dissolved in 10 ml of toluene. Nhexadecane was added as an internal standard for quantitative gas chromatography analysis. The mixture was allowed to react for 2 hours at 800C, after which gas chromatography was used to determine the resultant conversion of benzyl alcohol. Mass spectrometry was used to determine the presence of benzaldehyde. 75 Figure 4.15. GC graphic of benzyl alcohol conversion induced by LDHs-supported Au-Ag alloy NPs. If the benzyl alcohol undergoes oxidation process, the conversion of benzyl alcohol can be calculated by the following equation: Conversion = 100% x (4.3) Where A and B are the area of inert standard (n-hexadecane) and substrate (benzyl alcohol) at 0 hour respectively and A1 and B1 are the area of inert standard (nhexadecane) and substrate (benzyl alcohol) at 2 hours respectively. The selectivity of benzaldehyde is calculated based on the peak area percentage of benzaldehyde in gas chromatography as compared to other products formed. Turnover number (TON) and turnover frequency (TOF) Turnover number is the number of moles of substrate that a mole of a catalyst can convert before becoming inactive. 76 TON = (4.4) Table 4.5 and table 4.6 show the TON value of Au-Ag alloy/Cl-LDHs catalyst and Au-Ag alloy/NO3-LDHs catalyst; the highest TON value for Cl-LDHs and NO3LDHs are 32.51 and 41.98 respectively. This shows that NO3-LDHs-supported Au-Ag alloy NPs have higher catalytic activity as compared to Cl-LDHs-supported Au-Ag alloy NPs. The high catalytic activity noted in NO3-LDHs-supported Au-Ag alloy NPs was potentially due to the presence of lattice oxygen vacancies, which were more abundant in NO3-LDHs. It enhanced the activity of the support by favoring the interaction and physisorption of molecular oxygen; consequently, NO3-LDHs were more easily utilized for covalent grafting or physical adsorption as compared to ClLDHs. Moreover, TEM images in Figure 4.12 and 4.13 show that there was a lower number of metal alloy NPs attached onto the surface of Cl-LDHs support. This resulted in more chloride (which is an anion) being generated by Cl-LDHs than by NO3-LDHs. Therefore, more metal halides were formed in the resultant solution than metal alkanethiolate complexes. It is believed that the metal alkanethiolates have a stronger affinity to LDHs than the metal halides; the metal alkanethiolates were attached to the LDHs through electrostatic interaction. It can also be concluded that the interference of Au NPs in the alloy composition resulted in a higher TON value. Furthermore, the higher TON value also proves that Au NPs were more active than Ag NPs, demonstrating that catalyst comprised of Au NPs enhanced the activity of the support due to favourable interaction and physisorption of Au NPs to the support. 77 Table 4.5. TON number of each catalyst with different ratios of Au:Ag alloy deposited onto Ni-Al-Cl LDHs. No. 1 2 3 4 5 Type of catalyst Pure Au Pure Ag Au:Ag(1:3) Au:Ag(3:1) Au:Ag(1:1) Weight % Au 4.03 0.82 2.58 1.77 mmol of catalyst %Conversion (3 hr) TON 0.020 0.029 0.019 0.018 0.016 4.29 2.58 30.24 42.05 55.23 2.02 0.84 15.22 22.09 32.51 Ag 3.18 1.62 0.57 0.80 Table 4.6. TON number of each catalyst with different ratios of Au:Ag alloy deposited onto Ni-Al-NO3 LDHs. No. 1 2 3 4 5 Type of catalyst Pure Au Pure Ag Au:Ag(1:3) Au:Ag(3:1) Au:Ag(1:1) Weight % Au 4.07 1.35 2.944 1.81 mmol of catalyst %Conversion (3 hr) TON 0.021 0.019 0.018 0.021 0.016 89.83 15.38 52.89 80.38 55.84 41.98 7.89 29.00 37.84 33.79 Ag 2.03 1.16 0.60 0.73 4.3.2 Comparison of catalytic performance of catalysts prepared using Method 1 and Method 2 The catalytic activity of catalysts prepared using the two different methods and subsequently calcinated at 350oC for 4 hours is shown in Table 4.7. The catalytic performance was generated by taking gas chromatography samples during the first hour and the second hour. It can be seen that Method 1 resulted in the highest conversion of benzyl alcohol oxidation and 100% of selectivity. 78 Table 4.7. Percent conversion and percent selectivity of Au-Ag alloy deposited on the LDHs for 2 hours via benzyl alcohol oxidation with reaction temperature at 80oC. Preparation method % Conversion (2 hours rxtn) % Selectivity Method 1 24% 100% Method 2 12.81% - Catalyst prepared using Method 1 showed higher conversion than catalyst prepared using Method 2, with a % conversion of 24% and 12.81% respectively after 2 hours of reaction. TEM images of catalyst prepared using Method 2 in Figure 4.6, show that Au-Ag NPs were deposited and dispersed very well on the surface of LDHs. The calcination temperature was raised beyond 350oC to remove surfactant, impurities, and solvent. However, higher calcination temperature induced larger sized NPs because NPs have a tendency to agglomerate at high temperature. The agglomeration of NPs will affect the catalytic activity of the catalysts due to the smaller active area being available in larger Au-Ag NPs. The TGA curves in Figure 4.16.a shows that higher temperature was needed to decompose samples with MPTMS comprised of thiol and silane compounds. The weight loss at the temperature range of 100 - 250oC corresponds to the loss of adsorbed water molecules. The weight loss at temperature range of 250 - 400oC corresponds to the loss of the interlayer region, including nitrate and chloride. This result shows that higher temperature was needed to remove the surfactant when thiol was used as ligand. As a result of high calcination temperature, the NPs agglomerated and became inactive. In addition, AuAg alloy NPs calcinated at 400oC have bigger particle size (10 – 20 nm), as shown in TEM images in Figure 4.16.b. This indicates that catalyst prepared using Method 2 required calcination at a high temperature (400oC) that reduced its catalytic activity. 79 In the next discussion we will focus on the applications of catalysts for benzyl alcohol oxidation reactions prepared using Method 1 as this method showed higher levels of benzaldehyde conversion as compared to Method 2. I Stage 7.0 LDH-Au/Ag-MPTMS LDH-Au/Ag IIStage Weight Loss(mg) 6.5 6.0 5.5 5.0 4.5 0 100 200 300 400 500 600 700 800 o Temperature( C) Figure 4.16. (a) TGA curves of LDHs/Au-Ag-MPTMS and LDHs/Au-Ag, (b) TEM images of LDHs/Au-Ag-MPTMS after calcination at 400oC. 4.3.3 Studies on the effect of reaction temperatures of NO3-LDHs The catalytic activities of catalysts employed in this study are shown in Figure 4.17. This figure shows the varying resultant conversion of benzaldehyde with different reaction time at different reaction temperatures. The graph was generated by taking a gas chromatography sample after every hour, for 3 hours. It shows the temperature profile for alcohol oxidation reaction at 80oC, 90oC, 100oC, 110oC, 120oC, and 130oC where all catalysts were calcinated at a temperature of 350oC. We can conclude that the efficiency of the catalysts increased with an increase in reaction temperature. This is because high reaction temperature favoured a higher reaction rate, thus more benzaldehyde was produced. It is apparent that the conversion rate of the catalyst was higher at 110oC and 120oC than at lower temperatures of 80oC, 90oC, 80 and 100oC. We can conclude that the temperature range of 110 - 120oC provided the optimum conditions for the conversion of benzyl alcohol to benzaldehye and 100% selectivity of benzaldehyde was obtained as a result. However, the conversion of benzaldehyde decreased to 70% of conversion in the 1st hour when the reflux temperature was high (130oC). Although the conversion of benzaldehyde became constant in the second and third hour when the reflux temperature was 130oC, the overall yield decreased by 5% as compared to the yield at temperatures of 110oC and 120oC. This was due to the formation of two major by-products which were observed at high reaction temperature (130oC). The first by-product, as expected, was benzoic acid, which was the result of facile over-oxidation of benzaldehyde at 130oC. Thus, it can be shown that the oxidation of benzyl alcohol did not require intervention from the catalyst; this caused strong deactivation of the Au catalyst. In addition, toluene was also observed at very high selectivity. This is because toluene was formed within a short reaction time and influenced the conversion of benzaldehyde as a main product. We also conclude that most of the reaction became stable at the third hour as the reaction achieved saturation condition. In short, the optimum reflux temperature for catalyst that resulted in the highest efficiency was 110oC. Another factor that might influence the lower conversion rates of the catalyst is the particle size effect. The optimum particle size of Au-Ag alloy NPs as a catalyst for alcohol oxidation reaction is less than 4 nm.5 A smaller particle size will give a larger surface area, and this induces strong binding between more Au-Ag alloy NPs with the substrate (benzyl alcohol). It is believed that large Au-Ag alloy NPs were easily formed at high reaction temperature due to agglomeration, and consequently it influenced the catalytic activity of the catalyst. This explains the lower performance 81 of the catalyst at a higher temperature of 130oC as compared to a lower temperature of 110oC. 100 90 efficiency(%) Yield (%) Product 80 70 60 80C 90C 100C 110C 120C 130C 50 40 30 20 1.0 1.5 2.0 2.5 3.0 Time(hours) Figure 4.17. Product yield (%) of Au:Ag (1:1) deposited onto NO3-LDHs catalyst for alcohol oxidation reaction with varying different temperatures versus reaction time (hr). 4.3.4 Studies on the effect of calcination temperatures of NO3-LDHs Figure 4.18 shows the effect of NO3-LDHs calcination temperatures versus conversion of benzaldehyde. In addition to being primarily a reducing agent in the half seeding method for the synthesis of Au-Ag alloy NPs, oleylamine could also be used as a surfactant. As shown in the synthesis of Au-Ag/LDHs, the presence of a ligand such as oleylamine led to low catalytic activity due to the strong amine bonds between oleylamine and Au-Ag NPs, which covered the active sites of Au-Ag NPs. Thus, calcination was required to remove oleylamine as well as other impurities. However, high calcination temperatures produced noticeable particle agglomeration. 82 In contrast, low calcination temperatures led to incomplete oleylamine elimination with a larger population of Au-Ag alloy NPs. As shown in Figure 4.18, the catalytic activity of Au-Ag LDHs increased as the calcination temperature was increased. The optimum calcination temperature which gave the highest rate of conversion of benzyl alcohol was 350oC. In contrast, the conversion of benzyl alcohol decreased to 35% when the catalyst was calcinated at 400oC. On one hand, higher calcination temperature of catalysts resulted in larger porous-surface area and complete oleylamine removal. On the other hand, NPs agglomerated at higher calcination temperature, resulting in lower catalytic activity as particle size increased. 55 Yield (%) Product Efficiency(%) 50 250C 300C 350C 400C 45 40 35 30 25 1.0 1.5 2.0 2.5 3.0 time(hours) Figure 4.18. Product yield (%) of Au:Ag (1:1) deposited onto NO3-LDHs catalyst at 80oC reaction temperature reaction with varying calcination temperatures versus reaction time (hr). 83 4.3.5 Studies on the effect of Au/Ag ratios of NO3-LDHs We investigate the effect of varying molar ratios of Au-Ag by using impregnation method on NO3-LDHs, whereby the reaction temperature was set at 80oC for 3 hours. Figure 4.19 shows that Ag was not active for this alcohol oxidation reaction. It is observed that if the Ag fraction in the alloy was increased, the conversion of benzaldehyde decreased, and the selectivity increased. In contrast, the addition of Au in the alloy structure significantly enhanced benzyl alcohol conversion, demonstrating a clear synergistic effect for the Au-Ag catalyst as compared to monometallic species. Several mechanisms are believed to be responsible for the enhanced catalytic performance of certain bimetallic system, including a charge transfer phenomenon between the different metals that may favourably change the binding energy of adsorbates, and improve resistance against poisoning of catalyst. 23 Furthermore, we observed that the highest % yield was obtained for the pure AuLDHs catalyst. High catalytic activity shown by Au-LDHs may be due to good stability and uniform particle distribution of Au NPs supported onto LDHs surface. A decrease in conversion of benzaldehyde by pure Ag catalyst may be due to a decrease in LDHs support surface area as well as increased Ag NPs cluster size. Besides, it was noted that the selectivity of benzaldehyde for Au-Ag alloy-LDHs was lower than that of pure Au-LDHs. The lower selectivity of benzaldehyde corresponds to lower conversion of benzaldehyde. The rapid interaction of metal alloys with oxygen and benzyl alcohol induced a surface hydroperoxy intermediate that formed easily and reduced selectivity towards benzaldehyde. 84 95 90 85 80 efficiency(%) Product Yield (%) 75 70 65 60 55 50 45 40 pure Au Au:Ag 3:1 Au:Ag 1:1 Au :Ag 1:3 pure Ag 35 30 25 20 15 10 1.0 1.5 2.0 2.5 3.0 time(hours) Figure 4.19. Product yield (%) of Au:Ag deposited onto NO3-LDHs catalyst at 80oC reaction temperature with varying metal ratios versus reaction time (hr). 4.3.6 Studies on the effect of overall reaction progress of Au-Ag alloy/NO3LDHs Figure 4.20 shows the performance of the alloy as a catalyst over 3 hours of alcohol oxidation reaction. An enhancement in benzyl alcohol oxidation is reflected in the high product yield percentage, especially between the first and the second hour of reaction. This phenomenon occurred due to the accelerated reaction rates between benzyl alcohol and alloy NPs during the second hour of reaction. When the reaction time was extended beyond 2 hours, benzyl alcohol conversion remained relatively constant; it is believed that the saturation point of catalyst has been reached at this point and no additional active sites in the catalyst were available for the reaction to proceed. After 3 hours of oxidation reaction, the product yield of benzaldehyde decreased and the selectivity of benzaldehyde as a product also decreased. In addition, there was a significant increase in the side product yield percentage due to the deactivation of the catalyst after the extended reaction time. 85 100 Yield (%) Product efficiency(%) 80 Side Product Yield(%) Starting Material(%) Product Yield(%) 60 40 20 0 1.0 1.5 2.0 2.5 3.0 time(hours) Figure 4.20. Product yield (%) of Au-Ag alloy/NO3-LDHs catalyst for alcohol oxidation reaction based on overall reaction progress. 4.3.7 Studies on the effect of Au-Ag loading on NO3 LDHs Recent reports highlighting the ability of supported Au NPs to selectively oxidize alkenes and alcohols have drawn attention to the use of Au in the direct activation of benzyl alcohol. One pot synthesis method employed in this experiment created catalysts with high catalytic activity and selectivity using molecular oxygen, whereby the performance of catalysts depended on types of the support used, the amount of Au-Ag loading, and the Au-Ag particle size. Here we investigate the effect of Au-Ag loading on LDHs support. Figure 4.21 shows that lower loading 1.05 wt % of Au-Ag NPs gave lower activity. Meanwhile, the highest conversion of benzyl alcohol at about ~99% was obtained using 2.57 wt % Au-Ag alloy NPs on LDHs. Good catalytic activity (72%) was observed when the Au-Ag alloy loading was 86 further increased to 4.36 wt %. It is believed that when the loading level of Au-Ag alloy NPs was increased, the ratio of the loaded Au to the used Au source decreased because of saturation condition. Therefore when the Au-Ag loading was 2.57 wt %, the optimum condition was reached, resulting in the highest conversion of benzyl alcohol. However, for Au-Ag loading at 1.05 wt %, the lowest conversion was observed. This happened because of the smaller number of Au-Ag NPs formed or attached onto the LDHs support. When the loading level was high (4.36 wt %), large cluster of Au-Ag NPs were observed and the catalyst showed lower activity. This lower activity can be attributed to a reduction in the available metal support sites where both the metal NPs and support were needed for the reaction to occur.6,7(Please refer to the appendix reliable experimental data) 100 1.05 wt% 2.57 wt% 4.36 wt% Product Yield (%) efficiency(%) 90 80 70 60 50 1.0 1.5 2.0 2.5 3.0 time(hours) Figure 4.21. Product yield (%) of Au-Ag alloy/NO3-LDHs catalyst for alcohol oxidation reaction for different Au-Ag loading on NO3-LDHs. 87 4.3.8 Studies on the effect of calcination temperatures for Au-Ag alloy/ClLDHs A variety of metal has been studied recently for selective oxidation reaction of alcohols including Au NPs through a simple one batch approach, in which different catalyst materials were mixed and tested together. In this way, a physical mixture of ceria NPs and silver-impregnated silica was found to be catalytically active due to the cooperative effects of these two compounds.8 Here we investigate NPs of an Au-Ag alloy as a catalyst attached to the Cl-LDHs using the impregnation method. First, we studied the effect of different calcination temperatures on the catalytic activity of benzyl alcohol conversion. It is believed that the presence of a ligand such as oleylamine decreased the catalytic activity of the catalyst. The strong bonding between amine and Au-Ag NPs caused the active sites of Au-Ag NPs to be fully covered by oleylamine. Thus, calcination was required to remove oleylamine (boiling point: 364oC) as well as other impurities. It can be seen that the optimum temperature for calcination was 250oC, which generated about 55% benzyl alcohol conversion. In contrast, the conversion of benzyl alcohol decreased at 400oC because Au-Ag NPs lost their activity at higher temperature that resulted in bigger particle size (10 – 20 nm) due to agglomeration. Temperature programmed desorption (TPD) studies of CO adsorption on several submonolayer Au coverages deposited on Al2O3, FeO (111), and Fe3O4 (111)27 indicated that smaller Au NPs adsorbed onto CO more strongly than larger clusters irrespective of the support used. It is revealed that an enhancement in catalytic activity was dominated by the particle size of NPs. For instance, the surface to volume ratio of NPs increases with decreasing NPs size, resulting in a larger number of low coordinated atoms available for interaction with chemical adsorbates. Thus, the particle size played an important role in the 88 stability of oxides on surface of clusters because this affected the reactivity of NPs. (Please refer to appendix for the reliable experimental data) 55 50 300C 250C 350C 400C 45 efficiency(%) Product Yield (%) 40 35 30 25 20 15 10 5 1.0 1.5 2.0 2.5 3.0 time(hours) Figure 4.22. Product yield (%) of Au:Ag (1:1) deposited onto Cl-LDHs catalyst for alcohol oxidation reaction at 110oC with varying calcination temperatures. 4.3.9 Studies on the effect of Au/Ag alloy ratios on Cl-LDHs We investigate the effect of the Au-Ag molar ratios supported on Cl-LDHs. For this experiment, the reaction was set at 110oC for 3 hours. Figure 4.23 shows that both pure Ag and pure Au were not active for this alcohol oxidation reaction, but AuAg alloy NPs of any composition showed significantly enhanced benzyl alcohol conversion. This demonstrates a clear synergistic effect on the part of the Au-Ag catalyst as compared to monometallic species; the highest conversion was achieved at a Au:Ag ratio of 1:1. Ono et al26 showed that the catalytic activity and stability of Au dispersed on polycrystalline TiC films exhibited a strong dependence on inter-particle 89 distance. The system with the largest average inter-particle distance showed higher stability against agglomeration and has a longer lifetime. Heine9 argued that inter-nuclear distance in Au is rather large because of large repulsive forces between the atoms, while the distance between an electron and nucleus of the Ag atom is smaller than that of Au10, and hence these repulsive forces may be weaker in Ag. This repulsive effect prevented the formation of Au precipitate in solution. Therefore, this effect counteracted the driving force for Ag enrichment due to the lower surface energy of Ag. Consequently, Au-Ag alloy NPs have more uniform particle distribution on the LDHs support unlike their pure counterparts. This contributed to high catalytic activity seen in Au-Ag alloy NPs with respect to benzyl alcohol conversion and longer lifetime for the nanocatalyst. Pure NPs have lower catalytic activity than alloy NPs because of their tendency to agglomerate as well as to degrade Cl-LDHs by forming AuCl4 and AgCl precipitate during synthesis. (Please refer to appendix for the reliable experimental data) 55 50 Product Yield (%) efficiency(%) 45 40 35 30 25 Au:Ag 1:1 Au:Ag 3:1 Au:Ag 1:3 pure Au pure Ag 20 15 10 5 0 1.0 1.5 2.0 2.5 3.0 time(hours) Figure 4.23. Product yield (%) versus reaction time (hr) of Au-Ag NPs deposited onto Cl-LDHs catalyst for alcohol oxidation reaction at 110oC with varying metal ratios. 90 4.3.10 Studies on the effect of alcohol oxidation temperatures on Cl-LDHs Figure 4.24 shows % yield from catalytic conversion of benzaldehyde at different reaction temperatures using catalyst calcinated at 250oC. The graph was generated by taking a gas chromatography sample after the first, second, and third hour. It shows the temperature profile for alcohol oxidation reaction at 80oC, 90oC, 100oC, 110oC, 120oC, and 130oC. The trend of catalytic activity of Cl-LDHs is the same as the trend of catalytic activity for NO3-LDHs. It was found that the optimum reaction temperature for Au-Ag/Cl-LDHs catalyst for conversion of benzyl alcohol to benzaldehye was 110oC. In addition, 100% selectivity for benzaldehyde was observed at this reaction temperature. Therefore, we conclude that the efficiency of the catalyst increased with an increase in reaction temperature because higher reaction rate and higher benzaldehyde conversion were achieved at higher reaction temperatures. However, the conversion of benzaldehyde decreased to about 10% and the selectivity towards benzaldehyde remained at 100% at reflux temperature of 130oC. The lower rate of conversion occurred due to the formation of two major by-products which were observed at 130oC. The first by-product was benzoic acid generated by over-oxidation of benzaldehyde, as expected from a facile process at 130oC. The presence of benzoic acid caused strong deactivation of the Au catalyst. In order to achieve high selectivity, this process has to be limited to low conversion by using a high reaction temperature together with a high catalyst mass to ensure rapid conversion along the primary catalysis pathway. The second by-product was unexpectedly toluene because it was observed at unusually high selectivity. This is because toluene was formed within a short reaction time when the reaction temperature was 110oC. The observation shows that under mild conditions, toluene 91 could be a major product. We also conclude that the reaction became stable at the third hour when it achieved saturation condition. Reflux temperature of 110oC is thus concluded as the optimum temperature of benzaldehyde conversion given high % yield and selectivity obtained. Another factor that could have contributed to the lower % yield of benzaldehyde was the particle size. The optimum particle size for alcohol oxidation reaction is less than 4 nm.5 Smaller particles have a larger active area and induced more bonds between Au-Ag alloy NPs and the substrate (benzyl alcohol). Therefore, it can be concluded that the performance of the catalyst at a higher reaction temperature of 130oC was lower than that at 110oC. Moreover, it can be seen that NO3-LDHs-supported catalyst has higher conversion at its optimum temperature of 110oC as compared to Cl-LDHs-supported catalysts. This is because NO3-LDHs have better dispersity of Au-Ag alloy than Cl-LDHs. From XRD patterns in Figure 4.10, there is a broader peak for NO3-LDHs which indicate that the Au-Ag alloy was not only attached on the surface of NO3-LDHs region, but also strongly interacted with the surface of NO3-LDHs. However, peaks at about θ = 35o and θ = 62o for Cl-LDHs indicate turbostraticity in LDHs rather than the attachment of Au-Ag NPs in LDHs. Finally, it can be concluded that both LDHs sources for NO3- and Cl- have contributed to the alcohol oxidation reaction as a support for the metal NPs attachment as well as to prevent the metal NPs from agglomerating. The type of support used can determine the particle size of metal NPs that grow on the surface of the support. In addition, reaction temperature is also an important parameter for controlling catalytic reaction. (Please refer to the appendix reliable experimental data) 92 55 50 45 Yield (%) Product efficiency(%) 40 110C 80C 90C 100C 120C 130C 35 30 25 20 15 10 5 0 1.0 1.5 2.0 2.5 3.0 time(hours) Figure 4.24. Product yield (%) of Au:Ag (1:1) deposited onto Cl-LDHs catalyst for alcohol oxidation reaction with varying reaction temperatures. 4.3.11 Studies of Au-Ag loading effect on Cl-LDHs Figure 4.25 shows the % product yield versus reaction time with varying AuAg loading on Cl-LDHs. Overall, the total % yield of benzaldehyde for Au-Ag/ClLDHs was lower than Au-Ag/NO3-LDHs. As discussed previously, LDHs support played an important role in the catalytic activity of the catalyst. In the first approximation, we proposed that lattice oxygen vacancies, which were more abundant in NO3-LDHs, could be the factor that enhanced the activity of the support by favouring interaction and physisorption of molecular oxygen. Yan et al. 24 examined size selected Au clusters deposited on MgO and pointed out the importance of oxygen vacancies in the oxide support in controlling the rate of CO oxidation. In addition, 93 Pillay et al.25 showed that the delocalization of electrons from oxygen vacancies in the reduced TiO2 surface drastically altered the adsorption and surface diffusion of small Au particles. It is believed that the high activity exhibited by the Au-Ag/NO3-LDHs catalyst was due to the optimum adsorption of Au-Ag alloy NPs onto the surface of LDHs where efficient mass transfer and preferential adsoption of the reaction substrate were possible. It is found that Au-Ag/alloy Cl-LDHs with Au loading of 2.57 wt % gave the highest conversion of benzaldehyde. Meanwhile, when we further increased the Au loading to 4.36 wt %, the conversion decreased. A reasonable justification for this result is that the activity of the catalyst relied on the free surface of the support; a large quantity of Au-Ag alloy NPs on the support covered the free surface of LDHs, leading to decreased catalytic activity. In other words, both the alloy and LDHs were required in the reaction mechanism. The alcohol oxidation mechanism on the surface of LDHs was complemented by the presence of molecular oxygen. Nanocrystalline LDHs acted as an oxygen pump by means of oxygen physisorption, thus ensuring the oxidation of the co-product, metal hydrides, into water. This function required free surfaces on the LDHs. Moreover, oxygen has been identified as possessing the ability to remove the oxidative surface impurities of the LDHs. This resulted in the Au-Ag alloy having improved resistance against over-oxidation. (Please refer to appendix for reliable experimental data) 94 60 efficiency(%) Product Yield (%) 50 40 1.05 wt% 2.57 wt% 4.36 wt% 30 20 10 0 1.0 1.5 2.0 2.5 3.0 time(hours) Figure 4.25. Product yield (%) of Au:Ag (1:1) deposited onto Cl-LDHs catalyst for alcohol oxidation reaction with varying Au-Ag loading deposited onto Cl-LDHs. 4.3.12 Comparison studies of recyclability of NO3-LDHs and Cl-LDHs The oxidation of alcohol to carbonyl compounds is an important reaction in both the academia and industry.4 The utilisation of heterogeneous catalyst and green oxidants such as molecular oxygen and hydrogen peroxide in place of stochiometric quantities of inorganic oxidants is a desirable approach because a heterogeneous catalyst could be easily recovered and reused. However, there are still practical problems related to catalytic activity and degradation of catalyst in the utilisation of heterogeneous catalyst. The encapsulated Au NPs were tested for aerobic alcohol oxidation in the presence of water and KOH, and it was found that the catalytic activity depended on the particle size of Au NPs. Moreover, the Au NPs encapsulated in the hydrogel were a highly efficient and reusable catalyst.11 Reusability tests show 95 a decrease in activity due to the growth of NPs and change in the morphology of the support. In order to overcome this issue, novel recovery method such as those that utilize the magnetic properties of NPs could be developed. Here we compare the catalytic acitivity versus recyclability of Au-Ag/LDHs between NO3-LDHs and Cl-LDHs as a support. It is known that the support played an important role in the reactivation of the catalyst. The support contributed to the stabilization of metal oxides on the NPs surface and at the NPs/support interface that played an active role as oxygen reservoirs in oxidation reactions. Figure 4.26 shows that Au-Ag/NO3-LDHs catalyst has higher catalytic activity than Au-Ag/Cl-LDHs during the first 3 hours of reaction. It is believed that the NO3-LDHs support provided more reaction sites in the form of oxygen vacancies that contributed to better control on the rate of alcohol oxidation. The more active sites there were, the more active the catalyst was, resulting in enhanced reaction with benzyl alcohol to form benzaldehyde. From the second cycle until the fourth cycle, the catalytic activity decreased to about 20% of conversion. This happened due to the over-oxidation of benzaldehyde to the corresponding carboxylic acid. Consequently, the presence of carboxylic acids in organic solvents in the absence of a base caused strong deactivation of the catalyst. However, the catalyst was successfully reactivated by washing with acetonitrile followed by a 0.5 M aqueous NaOH solution and drying in an oven at 100oC for 12 hours even after significant amount of carboxylic acid was formed. It has been reported that this method allowed recovery of the catalyst to the same level of activity as the first run of the fresh catalyst.5 96 100 90 AuAg LDH-Cl AuAg LDH NO3 Product Yield (%) efficiency(%) 80 70 60 50 40 30 20 10 0 1.0 1.5 2.0 2.5 3.0 3.5 4.0 time(hours) No. of cycle Figure 4.26. Product yield (%) of Au:Ag (1:1) deposited onto Cl-LDHs and NO3LDHs catalyst for alcohol oxidation reaction with varying number of cycles. Here we discuss the catalytic activity of the catalysts after several cycles. The result shows that the catalyst was reactivated after it was washed with acetonitrile and 0.5 M NaOH. It can be seen that in the first cycle, Au-Ag/NO3-LDHs achieved 89% benzyl alcohol conversion, and the catalytic activity at the second cycle was maintained at 88%. The activity began to decrease in the third and fourth cycles, although high conversion was still maintained; the catalytic activity at the third and fourth cycle was 77% and 71% respectively. From this result, it is believed that NaOH reactivated the catalyst and maintained the catalytic activity at a level almost similar to the first cycle. Moreover, in basic medium, the selectivity to benzylaldehyde drastically decreased because of the formation of carboxylate. It is said that the base promoted the over-oxidation of benzylaldehyde to carboxylate. In accordance to the effect of the base on the activity, the selectivity to benzaldehyde and formation of 97 benzoic acid strongly depended on the Au/Pd ratio, i.e. the higher the Au content, the more dominant the over-oxidation of benzaldehyde appeared.18 It can be concluded that the addition of a base enhanced the activity of all catalysts, and the effect on a Ag-rich catalyst was considerably lower than on one with a Au-rich composition. 100 Yield (%) Product efficiency(%) 80 60 without washing with NaOH with washing NaOH 40 20 0 1 2 3 4 5 no. of cycle Figure 4.27. Product yield (%) of Au:Ag (1:1) deposited onto NO3-LDHs catalyst for alcohol oxidation reaction with varying number of cycles compared with and without washing with NaOH 0.5 M. 98 4.4 References 1. Zanella, R.; Giorgio, S.; Henry, C. R.; Louis, C. J. Phys. Chem. B. 2002, 106, 7634 - 7642. 2. Wang, H.; Xiang, X.; Evans, D. G.; Duan, X. Appl. Surf. Sci. 2009, 255, 6945 - 6952. 3. Sam, H. C.; Kwangjin, A.; Eung, G. K.; Jeong, H. K.; Taeghwan, H. Advanced Functional Materials. 2009, Vol.19, 10, 1645 - 1649. 4. Sheldon, R. A.; Kochi, J. K.; Metal Catalysed Oxidations of Organic Compounds. New York: Academic Press. 1981. 5. Alberto, A.; Avelino, C.; Hermenegildo, G. Chem. Eur. J. 2008, 14, 212 - 222. 6. Murdoch, M.; Nadeem, M. A.; Metson, J. B.; Keane, M. A.; Howe, R. F.; Idriss, H. Nature Chemistry. 2011, 3, 489 – 492. 7. Hiroyuki, M.; Masataka, M.; Takeshi, I.; Shu, K. B. Chem. Soc. Jpn. 2011, 84, 588 - 599. 8. Matthias, J. B.; Thomas, W. H.; Dierk, G. 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Besenbacher, F.; Chorkendorff, I.; Clausen, B. S.; Hammer, B.; Molenbroek, A.M.; Stensgaard, I. Science. 1998, 279, 1913. 24. Yan, Z.; Chinta, S.; Mohamed, A. A.; Fackler, J. P.; Goodman, D. W. J. Am. Chem. Soc. 2005, 127, 1604. 25. Pillay, D.; Hwang, G. S. Phys. Rev. B. 2005, 72, 205422. 26. Ono, L. K.; Roldan, B. Catal. Lett. 2007, 113, 86. 27. Shaikhutdinov, S. K.; Meyer, R.; Naschitzki, M.; Baumer, M.; Freund, H. J. Catal. Lett. 2003, 86, 211. 28. Wang, J.; Song, Y.; Li, Z. Liu, Qi. Energy Fuels. 2010, 24, 6463 - 6467. 29. Tenne, R. Colloids and Surfaces: Physicochemical and Engineering Aspects. 2002, 208, 83. 30. Jeyagowry, T.; Zeng, H. C. J. Am. Chem. Soc. 2002, 124, 6668. 31. Takato, M.; Akifumi, N.; Tomoo, M.; Koichiro. J. Chem. Commun. 2012, 48, 11733 - 11735. 100 CHAPTER 5 CONCLUSIONS 5.1 Preparation of Au-Ag alloy/LDHs NPs attached on LDHs In Method 1, we developed a simple impregnation method for the one pot synthesis of LDHs-supported Au-Ag alloy NPs with controllable sizes of NPs (7 - 10 nm). LDHs were mixed with oleylamine before the drop wise addition of Au DDT followed by Ag DDT in a heating flask at 200oC. The amine group of oleylamine provided a linkage between Au-Ag NPs and LDHs particles. Furthermore, the adsorption of Au-Ag alloy NPs on the oxidizable surface of LDHs was mediated by weak covalent bonds between the Au-Ag NPs and the oleylamine. This allowed the immobilization of dense networks of Au-Ag NPs on LDHs, which is of interest for the controlled assembly of nanoscale architectures. Method 2 was employed in our experiment for the adsorption of alloy NPs onto LDHs. LDHs was stirred with MUA/MPTMS for 12 hours after which Au-Ag alloy NPs were added drop wise into the solution of MUA/MPTMS functionalized LDHs. This method deposited Au-Ag alloy NPs uniformly onto the surfaces of LDHs. The pH of the mixture comprising the surfactant and LDHs was maintained above 7 in order to anchor the hydroxide chain bond to the surface of LDHs. MUA/MPTMS acted as a linker with bifunctional groups; its carbonyl end was attached onto the surface of LDHs and its thiol tail was attached onto the metal Au-Ag alloy NPs. The strong covalent bond that was induced by the surfactant between Au-Ag NPs and LDHs made the NP-LDH bond difficult to break, even with UV irradiation. Thus, Method 2 is the best method to prepare LDHs-supported Au-Ag alloy NPs. 101 LDHs are commonly used as catalysts, adsorbents, and colouring agents. The catalytic activity of LDHs is of great interest due to several attractive properties such as large surface area, ordered structure, good intercalation properties, high stability, and high ion exchange capacity. Here, we synthesized highly crystalline LDHs with a large surface area via urea hydrolysis using the hydrothermal method at low temperatures of 100oC - 180oC, with NPs sizes of around 150 - 200 nm. The ordered hexagonal structure of LDHs with rounded edges synthesized using this method has shown high catalytic activity when Au-Ag alloy NPs were deposited on the external surface of LDHs. LDHs were also easily recycled, as shown by the high rate of recyclability when washed with NaOH. 5.2 Catalytic activity of catalyst Two methods have been introduced to synthesize supported metal-catalyst for their utilisation in the aerobic liquid phase oxidation of alcohol. Method 1 exhibited enhanced efficiency for benzyl-alcohol oxidation using NO3-LDHs-supported Au-Ag alloy NPs prepared by the impregnation method in the one pot synthesis. Meanwhile, the MPTMS/MUA-functionalized LDHs method possessed good uniformity and dispersion of Au-Ag NPs attached onto the surface of LDHs with controllable particle size. Our findings demonstrate the advantages of the Method 1, making it possible to prepare a catalyst with high stability and high catalytic performance in benzyl alcohol oxidation. We proposed two different types of LDHs source (NO3 and Cl based sources) that showed different levels of catalytic activity. Thus, the type of LDHs source will affect the % yield of the intermediate product, benzaldehyde. Based on the 102 observation in our experiment, the conversion of benzyl alcohol peaked when the ratio of Au:Ag NPs used was 1:1 for Au-Ag/NO3-LDHs catalyst, whereas the highest conversion of benzyl alcohol was obtained when the ratio of Au:Ag NPs used was 3:1 for Au-Ag/Cl-LDHs. We believe that the choice of support will determine how fast the catalyst could be recovered. A suitable support can promote recovery of catalytic activity and hence enable the catalyst to operate in milder conditions. In general, it can be seen that the use of NO3-LDHs results in higher benzyl alcohol conversion than Cl-LDHs based on the catalytic activity result for each parameter tested. This is because NO3-LDHs support can activate O2 thus leading to faster recovery of the supported Au-Ag catalyst as compared to unsupported catalysts. 5.3 Studies comparing the recyclability of NO3-LDHs and Cl-LDHs We compare the catalytic activity versus the rate of recyclability of Au-Ag catalyst supported by NO3-LDHs and Cl-LDHs. It is shown that Au-Ag supported by NO3-LDHs has higher catalytic activity during the first 3 hours of the reaction as compared to Au-Ag supported by Cl-LDHs. After both catalysts had been recycled for the second, third and fourth cycle, the catalytic activity decreased to about 20% conversion. This happened due to the over-oxidation of benzaldehyde to the corresponding carboxylic acid; the presence of carboxylic acids in organic solvents in the absence of a base caused strong deactivation of the catalyst. Even though a significant amount of carboxylic acids was formed that resulted in decrease in benzaldehyde conversion, the catalyst was reactivated by washing with acetonitrile followed by 0.5 M NaOH aqueous solution and drying in oven at 100oC for 12 hours. 103 It can be concluded that this method can recover catalyst to the same level of activity as the first run of the fresh catalyst. 5.4 Further research Amide bond linkage is an important structure in pharmaceuticals, chemicals, as well as many natural products and hence, a variety of methods for the synthesis of this functionality have been developed to date. It is usually created via reacting carboxylic acids with amines using coupling reagents or by prior conversion of carboxylic acids to derivatives such as acid chlorides or anhydrides. However, the traditional synthesis methods involve the production of stoichiometric amount of hazardous by-products, the removal of the catalyst, and the utilization of expensive and toxic reagents. Due to recent environmental and economic concerns, several methods which are more desirable have been developed. Milstein discovered and described a reaction between alcohol and amine for amide bond formation catalyzed by ruthenium complexes with hydrogen as the only by-product. However, the limited availability of suitable substrates remains as a disadvantage. Although the catalysts showed excellent activity with sterically unhindered alcohols and amines, limited activity was observed with sterically hindered alcohols and amines. In addition, unsaturated bonds such as C=C were also reported to contribute to lower activity in ruthenium complexes. Continuous efforts have been made in developing a new catalytic system for amide synthesis which can convert aldehydes and amines into amides in a single step without the use of any hazardous reagents except oxidants. Besides the choice of metal as catalyst, another important factor is the choice of oxidant employed in stoichiometric amounts. 104 Hydrogen preoxide (H2O2) is an attractive and inexpensive oxidant that is commonly used in industrial synthesis. The use of H2O2 results in the formation of water as by-product, thus making H2O2 the preferred oxidant. In the absence of H2O2, the reaction did not proceed at all even when a large amount of catalyst was used. Thus, H2O2 is proven to be a key oxidant in the oxidative amidation process. After further development, it was found that the oxidative amidation of benzaldehyde using H2O2 generated moderate yield under acidic conditions without the use of catalyst. However, the presence of water in aq H2O2 prevented the formation of imine, a key intermediate in this reaction. The use of H2O2-urea instead of aqueous H2O2 significantly reduced the amount of benzoic acid formed owing to their catalytic processes and high atom efficiency, it is obvious that the reactions with H2O2-urea as the oxidant have tremendous advantages over traditional amide formation methods which are atom inefficient. The same catalyst, synthesized for oxidation, can be used for a direct amine synthesis reaction. For example, bimetallic NPs (Au-Ag alloy NPs) immobilized on LDHs can be used as a catalyst for oxidative amidation of aldehydes using H2O2 as an oxidant. Conditions may not necessarily be the same as alcohol oxidation but there is a chance that it will work for amidation as well. A new set of experiments would be neccesary to determine the optimum conditions and modifications required in the methods employed in the synthesis of catalyst with high recyclability and catalytic activity such as water soluble bimetallic NPs. Finally, the development of a heterogeneous catalyst is particularly promising, especially with regards to the role of an inorganic catalyst in practical environmental and economic systems. 105 APPENDIX A.1. Reliability of the experiment data for Au-Ag alloy NPs deposited on LDHs-Cl with different temperatures of alcohol oxidation reaction. 106 107 108 A.2. Reliability of the experiment data for Au-Ag alloy NPs deposited on LDHs-Cl with different ratios of Au-Ag alloy NPs. 109 110 A.3. Reliability of the experiment data for Au-Ag alloy NPs deposited on Cl-LDHs with different calcination temperatures. 111 112 A.4. Reliability of the experiment data for Au-Ag alloy NPs deposited on Cl-LDHs with different Au-Ag loading. 113 A.5. Reliability of the experiment data for Au-Ag alloy NPs deposited on NO3-LDHs with different Au-Ag loading. 114 B.1. EDX of NO3-LDHs-supported Au-Ag NPs with ratio Au:Ag = 3:1. B.2. EDX of NO3-LDHs-supported Au-Ag NPs with ratio Au:Ag = 1:1. 115 B.3. EDX of NO3-LDHs-supported Au-Ag NPs for pure Au. B.4. EDX of NO3-LDHs-supported Au-Ag NPs for Au:Ag = 1:3. 116 C.1. XRD data of LDHs with different urea and metal salt concentrations. 15000 Ni:Al=0.3M:0.15M; urea= 0.45M 10000 5000 0 Ni:Al=0.3M:0.15M; urea= 2.55M Intensity 60000 40000 20000 0 Ni:Al=0.3M:0.15M; urea= 4.5M 30000 20000 10000 0 0 30 60 2 theta (degree) 117 90 C.2. XRD data of LDHs with different urea and metal salt concentrations. 25000 0 30 60 90 Ni:Al=0.1:0.05M, urea= 0.15M 20000 15000 10000 5000 Intensity 60000 0 Ni:Al=0.1:0.05M, urea= 0.84M 40000 20000 0 80000 Ni:Al=0.1:0.05M, urea= 1.5M 60000 40000 20000 0 0 30 60 2 theta (degree) 118 90 [...]... Au:Ag alloy deposited onto Ni-Al-NO3 LDHs 71 Table 4.5 TON number of each catalyst with different ratios of Au:Ag alloy deposited onto Ni-Al-Cl LDHs 78 Table 4.6 TON number of each catalyst with different ratios of Au:Ag alloy deposited onto Ni-Al-NO3 LDHs 78 Table 4.7 Percent conversion and percent selectivity of Au-Ag alloy deposited on the LDHs for 2 hours via benzyl alcohol. .. dissolution and recrystallisation of LDHs through heating during LDHs formation Hydrothermal treatment is usually carried out to achieve one of three objectives: i) preparation of LDHs, ii) transformation of small crystallites into large ones, and iii) transformation of amorphous precipitates into crystalline LDHs Crystallinity of LDHs is essential for characterization purposes Modification of the co-precipitation... followed by rapid stirring of LDHs in oleylamine solution changes the hydrophobicity of the LDHs NPs Primary advantages of this method include the low cost and abundant supply of LDHs, in addition to the efficient NPs stabilization and the control of the size and morphology of LDHs 20 2.5 Alcohol oxidation and green chemistry Alcohol oxidation to aldehydes, ketones, or carboxylic derivatives is one of the... environmental and economic points of view, there is a strong incentive to develop a green, economic, and efficient alcohol oxidation process.5 The use of heterogeneous solid catalysts in oxidation of alcohols have garnered more attention over homogenous solid catalysts, for reasons such as ease of recovery and recycling, atom utility, as well as enhanced stability in the oxidation reaction The aforementioned... drop addition into the support solution By inhibiting further nucleation and controlling the growth of the particles, monodispersity, and good dispersion of Au-Ag alloy NPs on the LDHs surface can be achieved Furthermore, we present the facile and simple but successful deposition of alloy NPs onto LDHs under mild conditions in which the LDHs is subsequently exfoliated in oleylamine solution to form nanosheets... means such as pH variation Low supersaturation method entails the slow addition of a mixed metal solution to a second solution containing the anion to be intercalated, with concurrent pH regulation by the addition of the alkali solution In high supersaturation method, the mixed metal oxide solution is added to an alkali solution of the required anion Low supersaturation coprecipitation normally results... alcohol oxidation with reaction temperature at 80oC 79 ix CHAPTER 1 INTRODUCTION One of the most important processes in the production of fine and specialty chemicals is alcohol oxidation. 1 Conventional alcohol oxidation methods involve the use of toxic and expensive stoichiometric metal oxidants, such as chromate and permanganate,2 or harmful organic solvents,3 or require vigorous reaction conditions.4... solution as the first step in the formation of Au NPs through van der Waals, hydrogen bonds, covalent bonds, and electrostatic forces These interactions, generally described as adsorption forces, occur mainly with the part of external atoms of the NPs in interfacial contact with the solid surface and reduce the mobility of the NPs, making their aggregation more difficult Figure 2.3 Formation of Au NPs on. .. final solid product is used as catalyst 19 In this work, we report on LDHs- supported Au-Ag alloy NPs synthesized using the impregnation method; in this method, Au/Ag solution via adsorption forces would be adsorbed onto the LDHs surface This Au-Ag and LDHs combination is active as catalysts for alcohol oxidation when using molecular oxygen as an oxidant, even in the absence of additives or promoters In... should absorption at a particular wavelength be needed, alloy NPs can be used, as the peak position of the plasmon absorption is strongly dependent on the composition of the alloys The application of such a tuneable plasmon absorbance lies in systems such as bio-labels and biosensors Absorbance of light at a specific wavelength also finds many applications such as in the production of marker materials ... of Au-Ag alloy/ NO3 -LDHs catalyst for alcohol oxidation reaction based on overall reaction progress 86 Figure 4.21 Product yield (%) of Au-Ag alloy/ NO3 -LDHs catalyst for alcohol oxidation. .. Cl -LDHs catalyst for alcohol oxidation reaction with varying reaction temperatures 93 Figure 4.25 Product yield (%) of Au:Ag (1:1) deposited onto Cl -LDHs catalyst for alcohol oxidation reaction... temperatures for Au-Ag alloy/ Cl -LDHs 88 4.3.9 Studies on the effect of Au/Ag alloy ratios on Cl -LDHs 89 4.3.10 Studies on the effect of alcohol oxidation temperatures on Cl -LDHs