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electrical charges; in fact the total electric potential is the sum of electrostatic and Van der Waals potentials and results in energy barrier. Electrostatic repulsion (Fig. 2-4 left) is generally caused by an electrical double (really multi) layer [131] formed by ions adsorbed on the particle surface (e.g. halides) and the corresponding counter ions (e.g tetraalkylammonium). Small particle sizes lead to enormous surface areas, and this effect is greatly amplified in colloids. In a stable colloid, the mass of a dispersed phase is so low that its buoyancy or kinetic energy is too little to overcome the electrostatic repulsion between charged layers of the dispersing phase. The charge on the dispersed particles can be observed by applying an electric field: all particles migrate to the same electrode and therefore must all have the opposite sign charge with respect to the electrode’s charge. Fig. 2-4. Schematic image of two electrostatically-stabilized nanoparticles (top-left) and plot of electrical potential space distribution (bottom-left). Two sterically-stabilized nanoparticles (top-right) and caused a large energy barrier (bottom-right) against particle interaction. Pictures are adapted from [128, 132]. Steric stabilization (Fig. 2-4-right) is achieved by the coordination of sterically demanded molecules that act as protective shields on the metallic surface. In this way, nanometallic cores are separated from each other and agglomeration is prevented. 2.2 Methods of colloid and supported nanoparticles preparation Nanoparticles can be obtained by two general approaches: “top down” and “bottom up”. In “top down” method bulk materials are mechanically ground to the nanosized scale and stabilized by a suitable molecule [133, 134]. The problem with this method is difficulty in achieving the narrow size distribution and control both shape and average size of the particles. Moreover, bimetallic nanoparticles with core shell structures cannot be obtained by this method. In “bottom up” methods, nanoparticles are obtained by starting with molecular 21
precursors and building-up. This method allows control of size and shape of nanoparticles and their size distribution. Taking into account these reasons we will focus on “bottom up” methods only. The “bottom up” method can be classified conditionally as methods occurring in condensed phase and gas (or vacuum) phase. In condensed phase synthesis, nanoparticles are prepared by means of chemical synthesis; this method is also called as wet chemical preparation. For example, in order to prepare metal nanoparticles, metal salts are usually reduced in the presence of suitable capping agents (stabilizer) that stabilize the nanoparticles from agglomeration. In the gas phase synthesis, metal is vaporized and then vaporized metal atoms are condensed. The same idea can be realized in vacuum where the metal of interest is vaporized with high energy ions or by heating or by laser beam (the latter method can also be used in liquid media) and thus generated metal vapour is deposited on a support. Below we describe methods for preparation metal (due to their particular interest in the current thesis) nanoparticles in condensed and briefly in gas phase. Wet chemical preparation method Wet chemical reduction procedures have been widely used to stabilize the different types of metal nanoparticles with different kinds of organic as well as inorganic stabilizers. As was mentioned above, metal precursors are usually reduced in the presence of a stabilizer in order to prevent the nanoparticles from agglomeration. The choice of metal precursor, stabilizer, reducing agent and reaction conditions (relative precursor/stabilizer concentration, solvent, temperature and even stirring rate) strongly influence in properties of obtained nanoparticles, i.e. average particle size. The main classes of stabilizer predominant in the literature are macromolecules containing P, N, and S (e.g. phosphanes, amines, thioethers) atoms [105, 135-145]. Such solvents as THF [135, 146] or THF/MeOH [147] also can act as stabilizers. Molecules like ionic surfactants stabilize the nanoparticles by means of both electrostatic repulsion and steric hindrance. Their polar group forms an electrostatic layer around them and the long organic moiety gives steric hindrance. In general, lipophilic protecting agents give metal colloids that are soluble in organic media (“organosols”), while hydrophilic agents yield water-soluble colloids (“hydrosols”). Reducing agents play an important role in directing the size and shape of particles. Among different kinds of reducers (NaBH 4 , R-CHO, etc.) we would like to underline hydrogen. In fact, it has been applied in the synthesis of various metal clusters. The advantage of using hydrogen lies in the avoidance of by-products, the disadvantage is a weak reducing agent that is unable to reduce many metal complexes and salts. However, the use of hydrogen makes possible to perform reduction under mild condition that might be important for sensitive stabilizers. It is interesting to note, that reduction of metal salts e.g. Na 2 PtCl 4 by microwave radiation, thus in presence of stabilizer gives 20-30 Pt nanoparticles [148]. The decomposition of organometallic complexes with null valence metal atom(s) in presence of stabilizer can also be applied as a method of preparation of nanoparticles. This decomposition is typically initiated by an external force e.g. heat, ultrasonic or by a chemical agent capable to induce decomposition e.g. H 2 or CO. Classical example is in thermal decomposition of iron Fe(CO) 6 [149] or cobalt complex Co 2 (CO) 8 [150] at 130-170 °C in presence of polymer yielding 45 nm Co nanoparticles [151]. 22
Page 1 and 2: Preparation and Characterization of
Page 3 and 4: 5.4 Summary 66 Chapter 6 Cinchonidi
Page 5 and 6: Abstract Studies of the interaction
Page 7 and 8: many compounds are biologically act
Page 9 and 10: Fig. 1-7. Hexahelicene [3]. 7. Chir
Page 11 and 12: R H COOH C OH CH 3 H COO - Bricine-
Page 13 and 14: Catalyst activity, expressed in the
Page 15 and 16: In fact, there are only two heterog
Page 17 and 18: similar way as relative abundance o
Page 19 and 20: surface concentration of cinchonidi
Page 21 and 22: site on Pt surface formed by a adso
Page 23 and 24: Chapter 2 Introduction to the nanos
Page 25: Triangular prism 5 4 Comments: A -
Page 29 and 30: 2.3 Preparation of nanoclusters on
Page 31 and 32: Paris Marseille Ljubljana Synphos Q
Page 33 and 34: Chapter 4 Cinchonidine modified Pt
Page 35 and 36: dihydrocinchonidine (355 mg or 1.2
Page 37 and 38: Table 4-1. Comparison between 5 % P
Page 39 and 40: widening can be used in estimation
Page 41 and 42: 1 Absorbance 0.1 2 1600 1400 1200 1
Page 43 and 44: the focus of these prior experiment
Page 45 and 46: Fig. 4-11. Tilted adsorbed cinchoni
Page 47 and 48: sci., QL. comp. 768 (str) 756 (str)
Page 49 and 50: Conversion, % 100 80 60 40 ITP time
Page 51 and 52: 6A c 10 19.5 B, 20 61 0 3 0.18 0.51
Page 53 and 54: pressure, exposing time, cinchonidi
Page 55 and 56: Fig. 4-16. Linear relationship betw
Page 57 and 58: 1 , 1209 cm -1 and 1382 cm -1 remai
Page 59 and 60: Our results are in line with previo
Page 61 and 62: eaction. The stability of the catal
Page 63 and 64: Table 5-1. Ee and reaction rate ind
Page 65 and 66: Enantiomeric excess, % 85 80 75 A2
Page 67 and 68: 35 30 Before After Percent of parti
Page 69 and 70: catalyst activation seems to indica
Page 71 and 72: Table 5-3. Assignment of some vibra
Page 73 and 74: 6.2 Experimental Materials Cinchoni
Page 75 and 76: Fig. 6-3. TEM image of cinchonidine
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From the results of EP and EB hydro
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enhancement. However the exact fact
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Further, this family of man-made li
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Fig. 7-2. TEM of quiphos modified P
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of quiphos adsorbed on Pt and Pd an
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Fig. 7-8 Flat adsorbed quiphos via
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comp., H 11,12,13,14 2 C b., 2 P 11
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Chapter 8 “Quiphos-spider” modi
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flat crystal surface due to the hig
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Chapter 9 Diphos and diop modified
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Table 9-1. Average crystal size of
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Fig. 9-4. Conformation of diphos mo
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ings. Due to these reasons we can n
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Fig. 9-9. Proposed orientation of a
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The model of adsorption of diop on
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The calculation of energy of formin
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For the preparation of synphos modi
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1 2 20 40 60 80 2Θ Fig. 10-3. XRD
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The DRIFT spectra of synphos (Fig.
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peaks which can be used for analysi
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References [1] G.E. Tranter, J. Che
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[64] A. Szabo, N. Kunzle, T. Mallat
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[128] J.D. Aiken III, R.G. Finke, J
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[189] K. Umezawa, T. Ito, M. Asada,
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I declare that the current Ph.D. th
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