Colloidal method

TABLE 13.3. Characteristics and catalytic activity of samples prepared from the colloidal method and conventional preparation method. 

The preparation and characterization of these colloids have thus motivated a vast amount of work (17). Various colloidal methods are used to control the size and/or the polydispersity of the particles, using reverse (3) and normal (18,19) micelles, Langmuir-BIodgett films (4,5), zeolites (20), two-phase liquid-liquid system (21), or organometallic techniques (22). The achievement of accurate control of the particle size, their stability, and a precisely controllable reactivity of the small particles are required to allow attachment of the particles to the surface of a substrate or to other particles without leading to coalescence and hence losing their size-induced electronic properties. It must be noted that, manipulating nearly monodis-persed nanometer size crystallites with an arbitrary diameter presents a number of difficulties. 

Shaping by axial or isostatic pressing, extrusion, injection moulding, slip casting, tape casting, colloidal methods. 

However, TEM measurements performed on a Pt0 83Sn017/C (Figure 9.15) indicated that the increase of the metal loading on the carbon support led to the formation of a multimodal distribution of the particle size. Then, to overcome this problem, colloidal methods were also developed in our laboratory. 

Although the Bonnemann method is very interesting by allowing to vary and to control easily the composition and the nanostructure of the catalyst and is adapted to the preparation of real fuel cell electrodes, it displays also some limitations. For example, bismuth-containing colloids could not be prepared with the Bonnemann method, and even in presence of platinum salts. Moreover, the presence of bismuth hinders the reduction of platinum salts [59], However, platinum-bismuth is a good catalyst for ethylene glycol electro-oxidation in alkaline medium [59-62], Moreover, colloid of tin alone could not be obtained, and the reaction was only possible by coreduction in the presence of a platinum salt. Then, other colloidal methods should be developed keeping in mind the necessity of a similar flexibility as that of the Bonnemann method. 

The most common method for preparing BaTiOj powder is the conventional solid state reaction [3], alternative methods have been developed [12], such as alkoxide hydrolysis [4], hydrothermal synthesis [5], colloid method [6], and so on. 

Because carbon black is the preferred support material for electrocatalysts, the methods of preparation of (bi)metallic nanoparticles are somewhat more restricted than with the oxide supports widely used in gas-phase heterogeneous catalysis. A further requirement imposed by the reduced mass-transport rates of the reactant molecules in the liquid phase versus the gas phase is that the metal loadings on the carbon support must be very high, e.g., at least lOwt.% versus 0.1-1 wt.% typically used in gas-phase catalysts. The relatively inert character of the carbon black surface plus the high metal loading means that widely practiced methods such as ion exchange [9] are not effective. The preferred methods are based on preparation of colloidal precursors, which are adsorbed onto the carbon black surface and then thermally decomposed or hydrogen-reduced to the (bi)metallic state. This method was pioneered by Petrow and Allen [10], and in the period from about 1970-1995 various colloidal methods are described essentially only in the patent literature. A useful survey of methods described in this literature can be found in the review by Stonehart [11]. Since about 1995, there has been more disclosure of colloidal methods in research journals, such as the papers by Boennemann and co-workers [12]. 

Flatinum is deposited by utilizing the colloidal method. NaHS03 and H2O2 were added to an H2FtCl6 solution to form Ft oxide colloids. The SWNH powder was put into the Ft oxide colloid dispersion, and the Ft oxide colloids were adsorbed on the SWNH surface. After eliminating the Cl, Na, and S ions, the samples were dried and reduced in H2. 

The Ag-NOR staining is performed at room temperature identically to the one-step colloidal method (adapted from Howell and Black, 1980 Ploton et al., 1986) described above for cytological staining of cells and tissue. The reaction is arbitrarily terminated by rinsing the nitrocellulose in distilled,... 

One step methods. PtRuMo/C catalysts obtained by the impregnation manner revealed that the addition of a relatively small amount of Mo results in an electrocatalyst with a higher activity in CO or methanol electrooxidation than with the PtRu/C system. Moreover, Benker et al studied the effect of molybdenum precursor, and the physico-chemical characterization indicated that only traces of molybdenum were present in the samples when Mo (CO) 6 was used for the synthesis, while ammonium molybdate was an appropriate precursor for the synthesis of PtRuMo/C catalysts. On the other hand, a colloidal method developed by Bonnemann et alP was used to prepare carbon supported PtRuMo nanoparticles and established that this method provided a better tool for synthesizing PtRuMo (1 1 1) nanoparticles deposited on a carbon substrate, being more... 

A phosphine-free, low-temperature colloidal method for the eontrolled synthesis of PbSe nanocrystals in both zero-dimension (OD) and one-dimension (ID) was demonstrated by Li et. ID Pb(OH)Cl nanowires ( 80 to 160 nm in diameter) is directly used as a Pb preeursor and reacted with a Se precursor to form monodisperse dot-shaped OD cubie PbSe and ID orthorhombic PbSe nanowires. The authors highlighted the novelty of the method in that a fast nucleation step has been skipped and high quality OD and ID PbSe nanocrystals can undergo controlled synthesis within a relatively slow reaction time (2 to 24 h). The nanowires had a mean diameter of 15-24 nm. 

Very large molecules behave like small pieces of solid. Oxidic colloidal particles are basically strongly cross-linked inorganic polymers. Such compounds can best be made using colloidal methods, such as the sol-gel technique for covalent or ionic metal oxides. These methods will be discussed in Chapters 6 and 8. 

Molten salts (called fluxes) and also water at high pressures and temperatures are excellent solvents for many inorganic solid compounds. Inorganic synthesis from flux is discussed in Section 8.3.2, while water as a solvent for synthesis of oxides under hydrothermal conditions is the subject of Section 8.3.3. Colloidal methods are of increasing importance and the synthesis of mixed oxides by polymerization of hydrolized metal-organic molecules is the fourth subject in the group of synthesis from liquids. 

Regarding the solvent used to prepare the catalyst ink, its properties in catalyst ink should be mentioned as it also plays an important role in determining the microstructure and cataljAic activity of the CL. When ionomer such as Nafion solution is mixed with solvent, the mixture may become a solution, a colloid, or a precipitate due to the different dielectric constants of the solvent. When the dielectric constant is more than 10, a solution is formed between three and 10, a colloidal solution is formed and less than 3, precipitation occurs.If the mixture is a solution (i.e., the solution method ), excessive ionomer may cover the carbon surface, resulting in decreased Pt utilization. However, when the mixture is a colloid (the colloidal method ), ionomer colloids adsorb on the catalyst powder and the size of the catalyst powder agglomerates increases, leading to an increased porosity of the CL. In this case, the mass transfer resistance could be diminished because of the continuous network of ionomers throughout the CL, which then improves the proton transport from the catalyst to the membrane. ... 

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